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The Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) has announced a project to design and manufacture a high field MRI magnet with a 900 mm warm bore and comprised of Nb3Sn Rutherford cables in 2017. The maximum magnetic induction of magnet system is 14.3 T and magnetic field homogeneity in a spherical volume with 22 cm diameter is better than 0.5 ppm. The high performance Nb3Sn strand is the potential options to use for such high field magnet system. ASIPP cooperates with Western Superconducting Technologies Company (WST) to develop high performance Nb3Sn strand whose critical current density Jc is higher than 2000 A/mm2 at 4.2 K, 12 T. To obtain the actual performance of WST high performance Nb3Sn strand, three short samples are prepared with different non-copper ratio or different heat treatment, the critical current Ic or quench current of all sample is measured at different field by the voltage-current (V-I) method, the residual resistivity ratio (RRR) of all sample is also measured. To study the strand whether it will be affected by low field instability, a small coil is built with WST strand, and the experimental result is presented and discussed in this paper.

In this work, a series of FeS1-xTex (0 ≤ x ≤ 0.15) single crystals were successfully synthesized by a hydrothermal method for the first time. According to the measurement of in-plane resistivity, Hall effect, and magnetoresistance (MR), we find that the superconducting transition temperature Tc is rapidly suppressed with the increasing Te substitution, and finally the superconductivity disappears when x > 0.05. With the substitution of Te for S, the residual resistivity ρ0 increases while the residual resistivity ratio (RRR) decreases monotonously. Meanwhile, the MR of FeS1-xTex is also reduced by Te doping. All these results reveal that the Te substitution introduces more impurity scattering. In consequence, the non-linear field-dependent of Hall resistivity ρxy at low temperature region is suppressed and a linear behavior is restored upon Te doping. The negative Hall coefficients RH for all the FeS1-xTex samples suggest that the electron-type carrier dominates the electrical conduction. Moreover, the MR of FeS1-xTex obviously follows Kohler’s law, indicating the isotropic scattering rates in the Fermi surface.

For developing a 14 Tesla whole-body superconducting magnetic resonance imaging (MRI) magnet, a new kind of a Nb3Sn composite conductor with Rutherford cable-in-channel (RIC) structure is adopted. This structure of the conductor has a high current carrying capacity with an increased thermal and mechanical stability. The Rutherford cable is the core component of the RIC conductor, and controlling the deformation and performance degradation of Nb3Sn strand are the key points in Rutherford cable development. The strand would experience plastic deformation during the compaction process, which causes sub-elements damage and degrades the transport performance and residual resistivity ratio (RRR). In this paper, the impact of transverse compression on the sub-element of RRP 84/91 Nb3Sn strand before heat treatment was studied with experiments and finite element model (FEM), the experiment results and FEM results were compared and analyzed. Experiment results show that the transverse deformation of the strand should be kept below 20% to avoid the Nb barrier from breaking. The deformation analysis result of the FEM simulation shows good agreement with experimental results.

A project aiming to fabricate a full-length 2.8-m long Nb3Sn superconducting undulator for the storage ring was started last year at the ANL APS. These Nb3Sn undulators operate at a maximum magnetic field on the conductor of about 5 T. To address instabilities at this field, two Nb3Sn wires with small subelement size were used. Specifically, Restacked Rod Processed wires of 0.6 mm in diameter and with 144 and 150 superconducting subelements respectively, over 169 total. The equivalent subelement diameter, DS , of these wires is ∼35 μm. At these small DS values, the critical current density is known to deteriorate, and the Residual Resistivity Ratio is very sensitive to heat treatment. A delicate balance has therefore to be found to obtain parameters within operation specifications. In this paper we show performance results from different heat treatments.

We describe the effect of nonmagnetic impurity on the superconductivity behavior of the NdFeAsO0.8F0.2 iron-based superconductor. The resistivity measurements show that the superconductivity is suppressed upon increasing the low amounts of calcium impurity (x ≤ 0.05). Also, the TC decreases with the increase in the residual resistivity. Such behavior is qualitatively described by the Abrikosov–Gorkov theory and confirms that these impurities act as scattering centers. Moreover, we present the phase diagram of our synthesized samples for the various calcium dopings. We find that according to the increase in the calcium impurities and the decrease in the spin-density wave transition tempeθrature (TSDW), Fe ions are arranged stripe-antiferromagnetic at lower temperatures and also the superconducting transition temperature (TC) declines. Based on our results and in agreement with the available theories as is explained in the text, since the S++ state has no effect on the impurity-doped samples, and for low amounts of calcium, the S± state that is attributed to the spin-fluctuations causes the superconductivity suppression. So, it confirms the role of the spin-fluctuations as a dominant pairing mechanism in our synthesized samples.

Superconductivity induced by a magnetic field near metamagnetism is a striking manifestation of magnetically-mediated superconducting pairing. After being observed in itinerant ferromagnets, this phenomenon was recently reported in the orthorhombic paramagnet UTe$_2$. Under a magnetic field applied along the hard magnetization axis b, superconductivity is reinforced on approaching metamagnetism at $\mu_0H_m$ = 35 T, but it abruptly disappears beyond $H_m$. On the contrary, field-induced superconductivity was reported beyond $\mu_0H_m$ = 40-50 T in a magnetic field tilted by $\simeq25-30\deg$ from b in the (b,c) plane. Here we explore the phase diagram of UTe2 under these two magnetic-field directions. Zero-resistance measurements permit to confirm unambiguously that superconductivity is established beyond Hm in the tilted-field direction. While superconductivity is locked exactly at fields either smaller (for a H || b), or larger (for H tilted by $\simeq27\deg$ from b to c), than Hm, the variations of the Fermi-liquid coefficient in the electrical resistivity and of the residual resistivity are surprisingly similar for the two field directions. The resemblance of the normal states for the two field directions puts constraints for theoretical models of superconductivity and implies that some subtle ingredients must be in play., Comment: 18 pages, 5 Figures, includes Supplementary Information (9 pages, 8 Figures)

We revisit a superconducting phase diagram and electronic structures across the charge density wave (CDW) phase transition of Cu-underdoped $1T\text{\ensuremath{-}}{\mathrm{Cu}}_{\ensuremath{\delta}}\mathrm{Ti}{\mathrm{Se}}_{2}$ ($\ensuremath{\delta}\ensuremath{\sim}0.03$) under hydrostatic pressure. Superconductivity appears right after the complete collapse of the long-range CDW at a critical pressure of ${P}_{\mathrm{c}}\ensuremath{\sim}2.48\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, apart from the reported superconducting phase diagrams; it is found that the superconducting transition temperature shows a domelike pressure dependence covering a narrow pressure range with a maximum of ${{T}_{\mathrm{c}}}^{\mathrm{max}}\ensuremath{\sim}2.80\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at 4.80 GPa. Accordingly, the residual resistivity ${\ensuremath{\rho}}_{0}$ and temperature exponent $n$ of normal-state resistivity (from \ensuremath{\sim}3.30 at ambient pressure to \ensuremath{\sim}2.38 at ${P}_{\mathrm{c}}$ and \ensuremath{\sim}4.0 at 6.50 GPa) reduce sizably while the quadratic temperature coefficient $A$ of normal-state resistivity is enhanced by one order in magnitude; these results indicate the importance of CDW quantum fluctuation in superconducting pairing; low-$T$ resistivity upwarps with a \ensuremath{-}ln$T$ dependence below a characteristic temperature ${T}^{*}$ which has a domelike shape in the pressure range of 2.82--4.80 GPa. Based on two-band analysis of Hall conductivity and Kohler-fitting of magnetotransport (MR), energy bands are dominated by electron-type carriers across the CDW phase transition for $Pl{P}_{\mathrm{c}}$, and they reverse into hole-type for $Pg{P}_{\mathrm{c}}$; interestingly, the mobility of carriers increases up to five times at ${P}_{\mathrm{c}}$, but carrier concentration shows a weak pressure dependence. The MR value increases with the pressure for $Pl{P}_{\mathrm{c}}$ and then jump up to a saturated value after the collapse of the CDW. Our results show that the collapse of the CDW is accompanied by the reconstruction of the Fermi surface, and the enhancement in MR can be mainly attributed to the change of mobility. Possible mechanisms are discussed.

The transition metal dichalcogenide $\mathrm{Ti}{\mathrm{Se}}_{2}$ is an ideal correlated system for studying the interplay between superconductivity (SC) and a charge density wave (CDW) because both symmetry-breaking phases can be easily controlled by either Cu intercalation or physical pressure. SC appears in proximity to a CDW quantum critical point (QCP) induced by both Cu intercalation and applied pressure, raising the possibility of CDW-driven SC. Here, we report tuning the CDW QCP by simultaneously controlling Cu intercalation and external pressure and the appearance of a SC dome centered on the tunable QCP. When subjected to pressure, CDW ordering of Cu-intercalated ${\mathrm{Cu}}_{0.025}\mathrm{Ti}{\mathrm{Se}}_{2}$ is completely suppressed at 2.3 GPa, where the residual resistivity and the resistivity-temperature exponent decrease sharply, indicating the presence of the CDW QCP. The upper critical field of ${\mathrm{Cu}}_{0.025}\mathrm{Ti}{\mathrm{Se}}_{2}$ is 3.51 kOe, 16 times larger than that of pristine $\mathrm{Ti}{\mathrm{Se}}_{2}$, and its temperature dependence is linear, indicating that SC of $\mathrm{Ti}{\mathrm{Se}}_{2}$ is switched from the two-dimensional- to anisotropic three-dimensional-like by Cu intercalation. These discoveries show that the simultaneous application of Cu intercalation and pressure move the CDW QCP and that the highest SC transition temperature is pinned to the QCP, suggesting that the SC in $\mathrm{Ti}{\mathrm{Se}}_{2}$ is strongly correlated with CDW quantum criticality.

Detailed analysis reveals that an incorporation of stabilized defect complexes within metallic thin films, though a highly disordering and nonequilibrium process, gives rise to superconductivity, Fermi-liquid (FL) transport, and a universal correlation among them. This remarkable manifestation of correlated macroscopic quantum effects is attributed to a phonon-mediated electron-electron, e-e, scattering channel which encompasses both Koshino-Taylor and Bergmann's pseudo-Umklapp processes. This channel---denoted below as pseudo-Umklapp e-e scattering channel---is distinctly different from traditional ones in that disorder leads to a breakdown of lattice momentum conservation (significantly enlarging available phase space), to a spectral weight transfer towards lower frequencies (modifying electron-phonon coupling constant $\ensuremath{\lambda}$), and to a relaxation of kinematic constraints (all phonic polarization modes become available for mediation). On modeling the distorted structure in terms of Hosemann's paracrystal and using standard quantum many-body techniques, we demonstrate the role of distortion and softening in establishing this pseudo-Umklapp channel and, consequently, the surge of superconductivity, the FL transport, and the correlation of their parameters. This unifying approach allows us to derive analytical expressions for ${T}_{c}({\ensuremath{\rho}}_{\ensuremath{\circ}})$ (hallmark of superconductivity), the coefficient $A({\ensuremath{\rho}}_{\ensuremath{\circ}})$ (hallmark of FL transport), and the universal kinematic scaling relation $\mathrm{ln}(\frac{{T}_{c}}{\ensuremath{\theta}})\ensuremath{\propto}{A}^{\frac{\ensuremath{-}1}{2}}$: All are in satisfactory agreement with experiments ($\ensuremath{\theta}$ is an energy scale; residual resistivity ${\ensuremath{\rho}}_{\ensuremath{\circ}}$ measures the extent of disorder).

In this study, we determined the direct current DC isothermal magnetization (M − H and M-T), global critical current density (Jc), intra-grain critical current density (Jcintra), and excess conductivity for samples of CoFe2O4 (CFO; 1.0, 2.0, and 3.0 wt%) combined with Y1Ba2Cu3O7-δ (YBCO). X-ray diffraction analysis and Rietveld refinement suggested that all of the composite samples belonged to the Pmmm space group. No significant variations in the lattice parameters were observed after the inclusion of CFO in the YBCO matrix. The crystallite size decreased after CFO was added to the YBCO matrix according to field emission scanning electron microscopy. The addition of CFO to the YBCO matrix changed the flux pinning ability, thereby reducing the global critical current densities (Jc) and intra-grain critical current density (Jcintra) as the CFO concentration increased, but Jc and Jcintra both increased at the higher concentration., Jc and Jcintra were obtained from DC isothermal magnetization measurements (M − H) using Bean's critical state model. The critical temperature (TC) decreased and the residual resistivity (ρ0) increased as the wt.% of CFO increased, thereby indicating that the trapping of charge carriers occurred as well as grain boundary resistivity enhancement. Excess conductivity analysis was performed to investigate the thermal fluctuations near the critical temperature and various crossover temperatures were obtained using the Aslamazov–Larkin model.

The variations of superconducting and mechanical properties of La0.67Sr0.33MnO3 (LSMO) [x = 0.0, 0.1, and 0.2] doped YBa2Cu3O7−δ (YBCO) composite thick films were examined. All the composite films were synthesized by diffusion reaction technique. The structural and morphological analysis were investigated through X-ray powder diffraction along with Rietveld refinement and Field emission scanning electron microscopy (FESEM) respectively. The transport measurement suggested that the inclusion of ferromagnetic LSMO decreases the superconducting transition temperature ( $$ T_{C}^{on} $$ ) and enhances the residual resistivity (ρ0). The broadening of resistive transition occurs below the onset transition temperature and shows the dissipative flux pinning. Various superconducting parameters were obtained from the excess conductivity analysis of the composite films and were in good agreement with the experimental findings. The mechanical strength of all the composite films was explained through different models like Hays–Kendall model, Elastic/Plastic deformation model and Proportional specimen resistance model.

In this paper, the results of the superconductivity evaluation of MgB2 intermetallic-based superconducting materials prepared through mechanical alloying are reported. The formation of the MgB2 intermetallic phase took place through an intermediate phase reaction at the solid-state reaction temperature, which tends to favor single-phase materials. The annealing of mechanically milled Mg and B powders resulted in the highest mass fraction of MgB2 observed in this study, reaching more than 90% after annealing at 900 °C for 1 h. At that point, there were still second phases present consisting of Mg, MgO, and Fe2B. Improved superconducting properties were obtained in samples sintered at temperatures of 850 °C and 900 °C that had a low resistivity, low critical temperature gap values, and a high residual resistivity ratio (RRR).

We study the effect of addition of Ag on normal, paracoherent and superconducting coherent state of YBa2Cu3O7-δ (YBCO). We find that both room temperature resistivity and residual resistivity decrease with the addition of Ag in YBCO. In absence of magnetic field, the addition of Ag in YBCO shift various crossover temperatures of paracoherent state and 3D fluctuations dominate the mean-field region. However, the paracoherent state remains nearly unaffected in presence of 12 T magnetic field and 2D superconducting fluctuations dominate the mean-field region. The coherent state of YBCO improves by the addition of Ag in absence as well as in presence of 12 T magnetic field.

Understanding the competition between superconductivity and other ordered states (such as antiferromagnetic or charge-density-wave (CDW) state) is a central issue in condensed matter physics. The recently discovered layered kagome metal AV3Sb5 (A = K, Rb, and Cs) provides us a new playground to study the interplay of superconductivity and CDW state by involving nontrivial topology of band structures. Here, we conduct high-pressure electrical transport and magnetic susceptibility measurements to study CsV3Sb5 with the highest Tc of 2.7 K in AV3Sb5 family. While the CDW transition is monotonically suppressed by pressure, superconductivity is enhanced with increasing pressure up to P1 ≈ 0.7 GPa, then an unexpected suppression on superconductivity happens until pressure around 1.1 GPa, after that, Tc is enhanced with increasing pressure again. The CDW is completely suppressed at a critical pressure P2 ≈ 2 GPa together with a maximum Tc of about 8 K. In contrast to a common dome-like behavior, the pressure-dependent Tc shows an unexpected double-peak behavior. The unusual suppression of Tc at P1 is concomitant with the rapidly damping of quantum oscillations, sudden enhancement of the residual resistivity and rapid decrease of magnetoresistance. Our discoveries indicate an unusual competition between superconductivity and CDW state in pressurized kagome lattice., The recently discovered kagome metal AV3Sb5 is a new playground to study the interplay between superconductivity and charge-density-wave (CDW) state. Here, the authors report pressure-dependent evolution of CDW and superconductivity in CsV3Sb5, suggesting an unusual competition between the two phases.

Hot Universe Baryon Surveyor (HUBS)1 is being conceptualized in China as a high throughput and highresolution spectroscopic X-ray mission dedicated to studying cosmic “missing” baryons, which are thought to exist in the gas of very low density and temperature of roughly one million degrees in the halo of galaxies or in large-scale structures. To detect weak emission from the “missing” baryons, HUBS will employ an X-ray microcalorimeter based on transition-edge sensors (TES) array that operates at very low temperatures. The key characteristics of the detector technology are excellent energy resolution and high quantum efficiency, which makes it an ideal choice for constructing a non-dispersive X-ray imaging spectrometer. We are developing X-ray microcalorimeters for HUBS, based on superconducting Mo/Cu bilayer films. In this work, we present results on characterization of the Mo/Cu films and TES devices at temperatures below 200 mK, including their I − V characteristics, pulse signals and energy resolutions. We have also studied correlations between the superconductivity and other properties of the films (including residual resistivity ratio, stress, crystalline structure, interface properties, etc.). Preliminary results are presented in this work.

The magnet system of 14 $T$ magnetic resonance imaging (MRI) is under design at Institute of Plasma Physics Chinses Academy of Sciences (ASIPP), Hefei, China. In the latest design, the maximum magnetic induction field of the magnet system is 14.3 T. High performance Nb3Sn strand is the best candidate superconducting material to build such high field magnet device. To meet the requirement, ASIPP and Western Superconducting Technologies Company (WST) are cooperating to develop high performance Nb3Sn strand, the critical current density J c is higher than 2000A/mm2 at 4.2 K, 12 T. To study WST high performance Nb3Sn strand, the critical current Ic or quench current was measured at different field using the voltage-current (V-I) method. Besides, the residual resistivity ratio (RRR) were also measured. Furthermore, a small coil was designed and fabricated to study the strand whether it will be affected by low field instability. The preliminary investigation is presented and discussed in here.

We investigate the effect of Ni doping on the Fe site in single crystals of the magnetic superconductor $\mathrm{Rb}\mathrm{Eu}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$ for doping concentrations of up to 4%. A clear suppression in the superconducting transition temperature is observed in specific-heat, resistivity, and magnetization measurements. Upon Ni doping, the resistivity curves shift up in a parallel fashion, indicating a strong increase of the residual resistivity due to scattering by charged dopant atoms while the shape of the curve and thus the electronic structure appear largely unchanged. The observed step $\mathrm{\ensuremath{\Delta}}C/{T}_{c}$ at the superconducting transition decreases strongly for increasing Ni doping in agreement with expectations based on a model of multiband superconductivity and strong interband pairing. The upper critical field slopes are reduced upon Ni doping for in-plane as well as out-of-plane fields, leading to a small reduction in the superconducting anisotropy. The specific-heat measurements of the magnetic transition reveal the same Berezinskii-Kosterlitz-Thouless behavior close to the transition temperature ${T}_{m}$ for all doping levels. The transition temperature is essentially unchanged upon doping. The in-plane to out-of-plane anisotropy of Eu magnetism observed at small magnetic fields is unaltered as compared to the undoped compound. All of these observations indicate a decoupling of the Eu magnetism from superconductivity and essentially no influence of Ni doping on the Eu magnetism in this compound.

We have investigated the structural, magnetic and superconduction properties of [Nb(1.5 nm)/Fe(x)]10 superlattices deposited on a thick Nb(50 nm) layer. Our investigation showed that the Nb(50 nm) layer grows epitaxially at 800 °C on the Al2O3(1−102) substrate. Samples grown at this condition possess a high residual resistivity ratio of 15–20. By using neutron reflectometry we show that Fe/Nb superlattices with x < 4 nm form a depth-modulated FeNb alloy with concentration of iron varying between 60% and 90%. This alloy has weak ferromagnetic properties. The proximity of this weak ferromagnetic layer to a thick superconductor leads to an intermediate phase that is characterized by a suppressed but still finite resistance of structure in a temperature interval of about 1 K below the superconducting transition of thick Nb. By increasing the thickness of the Fe layer to x = 4 nm the intermediate phase disappears. We attribute the intermediate state to proximity induced non-homogeneous superconductivity in the structure.

The Advanced Technology Centre (ATC) of National Astronomical Observatory of Japan is developing microwave kinetic inductance detectors (MKIDs) for large-array pixel cameras for millimeter and sub-millimeter astronomy. We investigated single-crystal Nb thin layers to form superconducting microresonators. We compared the performances of MKIDs based on crystalline Nb structure and those based on polycrystalline Nb. We carried out the entire manufacture of the detectors in the ATC clean room. DC magnetron sputtering is used to grow single-crystal Nb films on r-plane sapphire substrates at an elevated temperature of 800 °C. The residual resistivity ratio (RRR) measured on these single-crystal Nb layers reached values ranging from 40 to 80. We made MKIDs with this crystalline Nb layer, and we measured internal quality factors of the detectors up to 106. The measurement of the noise power spectral density of these MKIDs gave a low value of − 95 dBc/Hz from 100 Hz to 100 kHz. The internal quality factor Qi and the fractional resonance frequency change δfr/fr of MKIDs with respect to the temperature variation are usually following the extended Mattis–Bardeen equations. However, we noticed a deviation from the theoretical prediction for temperature lower than 1 K (in our case). This deviation has already been observed on Al MKIDs and explained by a theory taking into account the Kondo effect and the kinetic inductance contribution. We demonstrated that our measurements on single-crystal Nb MKIDs are also in agreement with the same theory.

In this work, we show that significant enhancement of superconducting properties of MgB2 thin films prepared by vacuum evaporation and ex-situ annealing can be obtained when the annealing is performed in O2 atmosphere instead of the usually employed inert atmosphere (Ar in the current study). Rapid annealing in O2 significantly improves the critical transition temperature (Tc), critical current density (jc), and upper critical magnetic field (Hc2) compared to the control sample prepared in the same way, but annealed in Ar. Structural and composition analyses of the films demonstrate that the annealing in O2 atmosphere produces a sufficiently thick MgO layer on the surface of MgB2 thin films, which acts as a protective barrier preventing out-diffusion of Mg from the films during the high-temperature annealing. This consequently leads to better stoichiometry (B/Mg ratio), increased MgB2 grain size and enhanced intergrain connectivity. The higher residual resistivity of the films annealed in O2 atmosphere also indicates higher concentration of intragrain impurities, which causes a further increase of the jc and Hc.

We have investigated the current (I) - voltage (V) characteristics and vortex dynamics of Ba0.54K0.46Fe2As2 single crystal (BaK122) in H ‖ a b -plane and H ‖ c -axis orientations. The single crystal has been prepared by conventional growth method using self flux and characterized by onset Tc = 38.5 K, ΔTc = 1.0 K (offset-onset criterion) and residual resistivity ratio (RRR) ≈ 8. Usually for high-Tc superconductors, I - V curve shows exponential behavior due to flux creep at low magnetic field and it changes to near-linear nature with increase of magnetic field due to flux flow. We have shown that I –V characteristics differ significantly in H ‖ a b orientations compared to H ‖ c orientation which might be due to the presence of second magnetization peak (SMP). We have also estimated the activation energy using the Arrhenius plot in the thermally activated flux flow (TAFF) regime from the temperature dependent resistivity curves in both field orientation and have shown the variation of activation energy with magnetic field.

Chemical doping effects on structure and superconductivity were studied by introducing Cr at Fe sites in Ba2Ti2Fe2As4O, where Cr2+ can play dual role as impurities and hole dopants. Ba2Ti2(Fe1-xCrx)2As4O (0 ≤ x ≤ 8%) compounds are synthesized by a solid-state reaction and characterized with x-ray diffraction, electric resistivity and magnetization measurements. It is observed that the Cr dopants induce lattice expansion and local structure distortion, and significantly suppress the superconducting transition temperature Tc. In the meantime, the residual resistivity is found steadily increased with Cr doping. Comprehensive analyses show that Tc suppression is strongly correlated with the impurity-induced scattering to electrons, which can be fitted with the Abrikosov-Gor'kov-like scaling function.

In this work, the effect of Ca2+ iso-valence substitution for Sr2+ on properties of Bi2Sr1.6La0.4CuO6+δ superconductors is presented. Samples series with nominal composition of Bi2Sr1.6-xCaxLa0.4CuO6+δ (x= 0, 0.2, 0.4, 0.6 and 0.8) are prepared by a solid-state reaction method. When Ca content is increased, the X-ray diffraction technique shows that the cell parameters a and c decrease, while b one is almost constant. The scanning electron microscopy analysis reveals that the substitution has no significant effect on the porosity and the grain size of the samples. The physical properties of the samples are studied by the analysis of the magneto-resistivity curves measured under magnetic fields in the range 0-1 T. As Ca is added, the results show that the high temperature transition appears and is pushed up to 94.87 K for x=0.8. The substitution also improves the bulk onset critical transition temperature, the transition width, the residual resistivity, the activation energy of vortices and the irreversibility field. The best results are seen for x=0.4 of Ca content.

Temperature dependences of the basal-plane electrical resistance of underdoped YBa 2 Cu 3 O t − δ single crystals are investigated in a broad temperature range down to the superconducting transition, after a fast cooldown from 650 ° C as well as after a room-temperature annealing. A stepped behavior of the superconducting transition has been observed, pointing to an inhomogeneity of the samples. Annealing has been revealed to lead to a narrowing of the superconducting transition and a reduction of both, the residual resistivity and the phonon scattering coefficient. This is accompanied by a slight increase of the Debye temperature attributed to a decrease of the lattice parameter caused, in turn, by a redistribution of the labile oxygen. The revealed changes of the parameters of the temperature dependence of the electrical resistance point to an equalization of the concentration of the labile oxygen in the layers and its ordering.

Potassium tungsten bronzes $$\hbox {K}_{x}\hbox {WO}_{3}$$ ( $$x=0.20$$ and 0.22) with the coexistence of charge density wave (CDW) and superconductivity (SC) were prepared from $$\hbox {K}_{2}\hbox {WO}_{4},\, \hbox {WO}_{3}$$ and W powders using a hybrid microwave method. The structure refinement confirmed that all samples had a pure hexagonal phase with the space group of $$P6_{3}$$ /mcm. The distortion degree of W–O octahedron declines with x and is independent of synthesis condition for the same x (=0.20). The CDW transition is studied as a function of residual resistivity ratio. By increasing the crystallinity of sample, this transition can be suppressed, which is probably attributed to the interaction between CDW and defects in crystallites. The CDW transition temperature increases with x, which may be related to the decline of the distortion degree of W–O octahedron. The competition between CDW and SC is observed according to the resistivity and magnetization measurements.

The ground states of bosons have been classified into superfluid, Mott insulator, and bose glass. Recent experiments in two-dimensional superconductors strongly suggest the existence of the fourth quantum state of Cooper pairs, i.e., bose metal or quantum metal, where the resistivity remains constant at lowest temperature. However, its theoretical understanding remains unsettled. In this paper, we show theoretically that the bosons in the dilute limit subject to dissipation can lose the superfluidity and remain metallic, utilizing the Feynman's picture of superfluidity in the first quantized formulation. This result is relevant to the quantum vortices under an external magnetic field in two-dimensional superconductors with the finite resistivity of the normal core as the source of dissipation., 5+2 pages, 4+3 figures; v2: added references and clarified the difference from previous study

This multidisciplinary study paves way to investigate the crucial of fundamental characteristic properties including the bulk density, electrical, superconducting, flux pinning mechanism, crystal structure quality and strength quality of interaction between the superconducting grains in the Bi(2.1)Sr(2.0)Ca(1.1)Cu(2.0)Oy (Bi-2212) superconducting materials with the partial replacement of gallium foreign impurity by bismuth nanoparticles in the crystal structure. Characterizations of polycrystalline ceramic materials prepared by standard ceramic route in the atmospheric air are performed by means of conventional experimental measurement methods such as powder X-ray diffraction, Archimedes water displacement, dc electrical resistivity versus temperature and critical current density examinations. All the bulk Bi-site Ga partial replaced materials exhibit the Bi-2212 superconducting phase within the different fraction levels (%73.1 -94.8), moderate self-field critical current densities 54-96 A/cm(2) and wide-ranging offset and onset critical transition temperature range of 45.65 K-84.52 K and 70.06 K-85.00 K. As for the experimental findings of bulk density and related degrees of granularity (porosity) parameters, the bulk density parameter is found to be between 5.76 g/cm(3) and 6.12 g/cm(3) when the corresponding residual porosity value is also obtained to be in a range of 8.57 % -2.86%. Moreover, the mobile hole carrier concentrations in the short-range-ordered antiferromagnetic Cu-O-2 layers are found to be in the range from 0.085 until 0.152. Additionally, the role of Ga/Bi partial substitution in the crystal lattice on the normal state resistivity, residual resistivity, residual resistivity ratio, vibrational mode intensities, texturing, superconducting volume fractions, mobile hole carrier concentrations, average crystallite sizes, Lotgering indices and cell parameters are discussed in details. All the experimental results and theoretical approaches show that the characteristic properties tend to improve regularly with the increment in the Ga foreign impurity level until x = 0.05 due to the increment in the crystal structure quality and interaction between the superconducting grains. After the critical Ga/Bi substitution level of x = 0.05, every feature degrades considerably. (C) 2018 Elsevier B.V. All rights reserved. https://doi.org/10.1016/j.jallcom.2018.09.071

The improved mechanical properties of the Ni-alloy reinforced Bi 2-x Pb x Sr 2 Ca 2 Cu 3 O 10-x (Bi-2223)-released on 2016 by the Sumitomo Electric Industries, Osaka, Japan, (HT-NX type)-renewed the interest on this material for high-field applications. Whether Bi-2223 can be preferred to other high-temperature superconductors for very-high-field magnets depends also on the solutions found to protect the coil in the case of quenches. A comprehensive study of the quench propagation requires information about the temperature and field dependence of the critical current (I C ), the thermal conductivity (κ) of the conductor, and the electrical resistivity of the normal matrix (ρ norm ). Most of these data are unavailable in the literature for the HT-NX tape. This paper intends to fill this gap, reporting the results of an extensive measurement campaign of its electrical and thermo-physical properties. We have found that I C decreases linearly with T in the investigated range of fields 1-19 T. The well-known magnetic hysteresis of I C (B) has been examined in detail. We have observed that the value of I C at a given field varies upon repeated I-V measurements, with a consequent reduction of the hysteresis amplitude. The longitudinal κ has been investigated in magnetic fields up to 19 T. ρ norm has been measured between 4.2 K and 300 K and a residual resistivity ratio of ~40 has been found. The experimental study of the I C (T,B) surface, of the thermal conduction properties κ(T,B), and of the matrix electrical resistivity ρ norm provides fundamental elements for a quench propagation study, which is critical in view of high-field applications of this conductor.

We report the enhanced superconducting properties of double-chain based superconductor Pr 2 Ba 4 Cu 7 O 15 − δ synthesized by the citrate pyrolysis technique. The reduction heat treatment in vacuum results in the appearance of superconducting state with T c =22-24 K, accompanied by the higher residual resistivity ratios. The superconducting volume fractions are estimated from the ZFC data to be 50 ∼ 55 % , indicating the bulk superconductivity. We evaluate from the magneto-transport data the temperature dependence of the superconducting critical field, to establish the superconducting phase diagram. The upper critical magnetic field is estimated to be about 35 T at low temperatures from the resistive transition data using the Werthamer-Helfand-Hohenberg formula. The Hall coefficient R H of the 48-h-reduced superconducting sample is determined to be − 0.5 × 10 − 3 cm 3 /C at 30 K, suggesting higher electron concentration. These findings have a close relationship with homogeneous distributions of the superconducting grains and improved weak links between their superconducting grains in the present synthesis process.

Improvement of superconducting properties of MgB2 superconductor becomes necessary in several medium and large scale commercial practical applications where generation of high magnetic field is of significant interest. High power cables, energy storage devices, wind turbines and magnetic resonance imaging are the potential areas of application where MgB2 superconductor with improved property is indispensable. In order to facilitate quick determination of transition temperature of MgB2 superconductor and to determine the conditions at which this superconducting property is optimal for the desired application, this present contribution hybridizes genetic algorithm (HGA) with support vector regression (SVR) machine learning to model the transition temperature of doped MgB2 superconductor using ambient room temperature resistivity (RTR), residual resistivity ratio (RRR) and structural lattice distortion (SLD) due to the incorporated dopants as descriptors. The developed HGA-SVR-RTR model that utilizes RTR as descriptor shows better performance than the proposed HGA-SVR-RRR model (that employs RRR as descriptor) and HGA-SVR-SLD model (that implements SLD as descriptors) with improvement in performance by 88.45% and 71.41%, respectively using mean absolute error (MAE) as a parameter that evaluates the model performance. The developed HGA-SVR-RTR model also outperforms the existing models such as GPR-prediction (2020), STTE model (2016) and STTE model (2014) with performance improvement of 74.85%, 74.76% and 92.96%, respectively using MAE as a yardstick for performance comparison. The developed HGA-SVR-RRR model performs better than HGA-SVR-SLD model with percentage improvement of 59.6% on the basis of MAE. The performance of the developed models would definitely facilitate cost effective search for doped MgB2 superconductor for desired application without experimental stress.

We study the effect of addition of BaZrO 3 (BZO) on normal and superconducting state of YBa 2 Cu 3 O 7-δ (YBCO). We find that in general both room temperature and residual resistivity increase with the addition of BZO except at low concentration of BZO. The temperature dependence of resistivity in presence of magnetic field also shows less resistivity broadening in composites containing low concentration of BZO below transition temperature (T C ). The zero temperature upper critical field (H c2 (0)), estimated by using Werthamer, Helfand and Hohenberg theory and Ginzburg Landau theory, shows an increase by the finite addition of BZO in YBCO. Further, the activation energy (U 0 ) determined from Arrhenius plots and vortex glass transition temperature (T g ) also increase with the limited addition of BZO. Such an enhancement in H c2 (0), U o and T g has been attributed to the increase in grain connectivity of YBCO . We conclude that the limited addition of BZO in YBCO significantly improves its superconducting performance in magnetic environment.

We present a study of parameter optimization for synthesizing truly stoichiometric polycrystalline LiFeAs. Stoichiometric LiFeAs has been prepared in a very broad range of synthesis temperature (200–900 °C) under otherwise exactly the same conditions, and has been characterized by structural, magnetic, transport, nuclear quadrupole resonance (NQR), and specific heat measurements. Our study showed that the LiFeAs phase is formed at 200 °C with a large amount of impurity phases. The amount of these impurity phases reduces with increasing synthesis temperature and the clean LiFeAs phase is obtained at a synthesis temperature of 600 °C. Magnetic susceptibility and resistivity measurements confirmed that the superconducting properties such as the critical temperature T c , and the upper critical field H c2 do not depend on the synthesis temperature (≤ 700 °C), remaining at almost the same value of ∼19 K and ∼40 T, respectively. However, the width ΔT c of the transition and the NQR line width decrease with increasing the synthesis temperature and reached to minimum value for the synthesis temperature of 600 °C. Our careful analysis suggests that the best sample obtained at 600 °C is optimal concerning the low resistivity, high residual resistivity ratio ( RRR ), low ΔT c , high T c and H c2 , and a small NQR line width with values which are comparable to that reported for LiFeAs single crystals. Specific heat measurements confirmed the bulk superconducting nature of the samples. The H c2 value estimated from the specific heat is consistent with that of the resistivity measurements. Concisely, 600 °C synthesis temperature yields optimal high quality polycrystalline LiFeAs bulk samples. Further improvement of the quality of the sample prepared at 600 °C could be obtained by a controlled slow cooling process. Microstructural analysis reveals that the abundance of micro-cracks becomes strongly reduced by the slow cooling process, resulting in an increase in clean and well-connected grain boundaries.

Two types of high-purity aluminum-stabilized superconducting conductor were developed and produced for use in large-aperture high-field superconducting solenoids. One (DS1) has transverse rectangular dimensions of 5.27 mm × 20.1 mm, whereas the other (PS) has 5.52 mm × 30.1 mm. The superconductor for both cables is copper-stabilized NbTi strand woven into a Rutherford cable. The stabilizer for the DS1 conductor is 5N aluminum, whereas the stabilizer for PS is an aluminum alloy of 5N aluminum with a 0.1wt% Ni addition. Continuous unit lengths up to 1700 m are required for production. In total, over 18 000 m are required for this contract. A major goal of the R&D is to produce a cable having high shear strength between Cu and Al boundary while maintaining high residual resistivity ratio (RRR) and strength in the Al parts. We report on the cable mechanical properties, RRR of aluminum, sharing stress between Cu and Al, and critical current $(I_{C})$ of the cable, as well as the progress toward the fabrication of long unit lengths for production.

In this study, the magnetotransport measurements are performed in the field range of 0–7 T to determine the effect of Nd nanoparticles inserted in the Y-123 crystal structure on the superconducting characteristics and flux pinning mechanism of the YBa2−yNdyCu3O7 materials (0.000 ≤ y ≤ 0.500). Moreover, the experimental measurement results enable us to evaluate the crucial parameters as regards critical transition temperatures ( $$T_{c}^{onset}$$ and $$T_{c}^{offset}$$ ), room temperature resistivities (ρ300), residual resistivity ratios (RRR), thermodynamic critical fields (μ0Hc), lower critical fields (μ0Hc1), upper critical fields (μ0Hc2), critical fields (μ0Hc3), irreversibility fields (μ0Hirr), coherence lengths (ξ), penetration depths (λ) and Ginzburg–Landau ( $$\kappa$$ ) parameters. All the values obtained show the considerable improvement with the increment of the Nd nanoparticles in the Y-123 crystal system up to the critical value of y = 0.250 beyond which the prompt degradation begins in the crystal structure due to new induced artificial dislocations and permanent disorders in the Cu–O2 consecutively stacked layers. Hence, the excess Nd inclusions lead to the thermal motion of the vortices easily. This fact is completely confirmed by the magnetotransport measurements performed in the external magnetic fields. Namely, the optimum level of Nd dopant seriously increases the coupled regions as a result of the degradation of pancake positional fluctuations, and pinning of two-dimensional pancake vortices enhances with the recoupling of the adjacent layers. However in case of the maximum Nd inclusions, the flattened pancakes with more interplane interaction energy never travel from layer to layer due to the alteration of the coupled vortices into the discrete pancakelike (recoupling linelike) nature. In other words, the excess Nd nanoparticles inserted in the Y-123 crystal system reduce significantly the interlayer Josephson coupling length, elastic moduli of the vortex lattice and magnetic field carrying capacity. Furthermore, the flux pinning energy (U0) is calculated from thermally activated flux flow model. The activation energy computed increases regularly with the increment of Nd decorations until y = 0.250 after which the value significantly decreases towards to the global minimum point. However, the pinning energy values always decrease with the increase of the external magnetic field strength. In case of external magnetic fields of 7 T the maximum activation energy is found to be about 477 K. As for the Y-123 material doped with maximum Nd additives of y = 0.500, the flux pinning energy values are not calculated at any applied magnetic field which is >0.1 T due to the cooper pair-breaking in Ba-site Nd substituted Y-123 systems. At the same time, the evaluated parameters follow the same trend: firstly enhancement up to the critical Nd dopant value of y = 0.250 and then the rapid regression towards the global minimum point. Finally, the optimum Nd impurity level is found to be y = 0.250 in the Y-123 system at even higher applied magnetic field strengths.

CRESST-II is a direct dark matter experiment that uses scintillating calorimeters to detect WIMP-induced nuclear scatter processes. Heat and light signals are read out with tungsten transition edge sensors (TESs) that are optimized toward their sensitivity to non-thermal phonons. The usage of superconducting thin film structures (e.g., aluminum) serving as phonon collectors to increase the collection area for this signal component is an approach to improve the sensitivity of the TES. The performance of the phonon collectors depends on the material properties and the quality achieved in the production process. We optimized the size of the phonon collectors for the given quality of CRESST-II light detectors. The diffusion lengths measured in this work are $$\mathcal {O}$$ (1 mm) and show a strong correlation to the Residual Resistivity Ratio of the respective films. First tests of CRESST-II light detectors with larger as well as thicker phonon collectors individually show improvements in the measured pulse height of 30 %.

Epitaxial superconducting δ-MoN and δ-NbN thin films, exhibiting a superconducting transition temperature ( T c onset ) of 13 K and 10.3 K, respectively, were prepared by a simple chemical solution deposition on c-cut Al2O3 substrates. A sharp transition and the large residual resistivity ratio (RRR) ρ300K/ρonset of δ-MoN and δ-NbN thin films suggest high quality of the prepared epitaxial thin films. The critical current density (Jc) under 100 Oe at 5 K is of 1.37 and 0.74 MA/cm2 for the δ-MoN and δ-NbN thin films, respectively. The results will provide a facile route to prepare epitaxial superconducting metal nitride thin films.

Controlled pointlike disorder introduced by 2.5-MeV electron irradiation was used to probe the superconducting state of single crystals of $\mathrm{CaK}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{4}{\mathrm{As}}_{4}$ superconductor at $x=0$ and 0.05 doping levels. Both compositions show an increase of the residual resistivity and a decrease of the superconducting transition temperature, ${T}_{c}$, at the rate of $d{T}_{c}/d\ensuremath{\rho}({T}_{c})\ensuremath{\approx}0.19$ K/($\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$) for $x=0$ and 0.38 K/($\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$) for $x=0.05$, respectively. In the Ni-doped compound ($x=0.05$), the coexisting spin-vortex crystal (SVC) magnetic phase is suppressed at the rate of $d{T}_{N}/d\ensuremath{\rho}({T}_{N})\ensuremath{\approx}$ 0.16 K/($\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$). The low-temperature variation of London penetration depth is well approximated by the power-law function, $\mathrm{\ensuremath{\Delta}}\ensuremath{\lambda}(T)=A{T}^{n}$, with $n\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}2.5$ for $x=0$ and $n\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}1.9$ for $x=0.05$ in the pristine state. Detailed analysis of $\ensuremath{\lambda}(T)$ and ${T}_{c}$ evolution with disorder is consistent with two effective nodeless energy gaps in the density of states due to robust ${s}_{\ifmmode\pm\else\textpm\fi{}}$ pairing. Overall the behavior of $\mathrm{CaK}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{4}{\mathrm{As}}_{4}$ at $x=0$ is similar to a slightly overdoped ${\mathrm{Ba}}_{1\ensuremath{-}y}{\mathrm{K}}_{y}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ at $y\ensuremath{\approx}$ 0.5, and at $x=0.05$ to an underdoped composition at $y\ensuremath{\approx}$ 0.2.

Superconducting Mg-doped TlBa 2 (Ca 2− y Mg y )Cu 3 O 10− δ ( y =0, 0.5, 1.0, 1.5) samples were irradiated with gold (Au +4 ) ions. The zero resistivity critical temperature { T c (0)} was decreased and residual resistivity was increased after irradiation. The irradiation-induced suppression of T c (0) can be related with several factors, such as reduction of superconducting volume fraction caused by damaging and deterioration of samples quality after Au +4 ions irradiation. A pronounced fishtail was observed in all the samples with the applied magnetic field and all these features qualitatively agreed with the predictions of the order–disorder theory of vortex matter. The significant broadening in transition widths as function of external applied field was observed after ions irradiation. The activation energy was increased with the increasing Mg contents and Au +4 ions irradiation. But the activation energy of irradiated samples remained higher as compared to un-irradiated samples for all the values of external applied magnetic field. Almost similar decreasing trend in activation energy was observed with increasing magnitude of external applied magnetic field in all the samples. The ions irradiation hardening was observed in Mg-doped TlBa 2 (Ca 2− y Mg y )Cu 3 O 10− δ samples.

Temperature dependence of the normal state electrical resistivity of polycrystalline MgB 2 added with 0, 0.5, 1, 3 and 5 wt. % of (Bi, Pb)-2223 (Bi 1.8 Pb 0.26 Sr 2 Ca 2 Cu 3 O 10+ x ) superconducting powder have been investigated in the light of two band approach based on π and σ bands of MgB 2 superconductor. The scattering rates ( γ σ , γ π ) and residual resistivity ( ρ 0 σ , ρ 0 π ) of each band are estimated for the investigated samples. Our observation for pure MgB 2 shows much higher scattering rate in π bands, as compared to σ bands and hence indicates ‘dirty’ nature of the samples. However, the addition of 2223 in MgB 2 is found to enhance the scattering rate in both bands, but the enhancement is more pronounced in π bands as compared to σ bands. Contribution of each individual band towards the total electrical conductivity of 2223 added MgB 2 pellets are separated. Our analysis confirms that σ band contribution shows a small increase with 2223 addition and reaches nearly 89% for 5 wt. % 2223 added MgB 2 polycrystalline pellets. The electron–phonon coupling constant ( λ ) of pure and 2223 added MgB 2 pellets calculated using Mc-Millan expression is found to be nearly invariant with 2223 addition.

We investigated the occurrence and nature of superconductivity in single crystals of YFe$_2$Ge$_2$ grown out of Sn flux by employing x-ray diffraction, electrical resistivity, and specific heat measurements. We found that the residual resistivity ratio (RRR) of single crystals can be greatly improved, reaching as high as $\sim$60, by decanting the crystals from the molten Sn at $\sim$350$^\circ$C and/or by annealing at temperatures between 550$^\circ$C and 600$^\circ$C. We found that samples with RRR $\gtrsim$ 34 showed resistive signatures of superconductivity with the onset of the superconducting transition $T_c\approx1.4$ K. RRR values vary between 35 and 65 with, on average, no systematic change in $T_c$ value, indicating that systematic changes in RRR do not lead to comparable changes in $T_c$. Specific heat measurements on samples that showed clear resistive signatures of a superconducting transition did not show any signature of a superconducting phase transition, which suggests that the superconductivity observed in this compound is either some sort of filamentary, strain stabilized superconductivity associated with small amounts of stressed YFe$_2$Ge$_2$ (perhaps at twin boundaries or dislocations) or is a second crystallographic phase present at levels below detection capability of conventional powder x-ray techniques., Comment: 8 pages, 11 figures

The main scope of this comprehensive study is to determine the critical dopant level of metal to insulator transition (MIT) for Bi-site La substituted Bi-2212 superconducting ceramics by means of bulk density, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron dispersive X-ray (EDX), dc resistivity ( ρ – T ) and transport critical current density ( J c ) experimental methods. The samples produced in this work are prepared by the conventional solid state reaction technique at the constant temperature of 840 °C for the annealing duration of 60 h. All the experimental evidences obtained indicate that the microstructural, electrical and superconducting properties suppress with the increment of the La dopant level in the Bi-2212 superconducting matrix up to x = 0.20 beyond which the crucial characteristics diminish rapidly due to not only the deformation between the Bi-2212 layers and grain boundaries but also the disorder and hole localization (filling) problem in the crystal structure. As for the numerical values, onset and offset critical temperatures systematically decrease from 85.67 K and 84.49 K to 82.74 K and 59.38 K with the impurity content until x = 0.15. However, the sample doped with the content level of x = 0.20 exhibits further pseudo-critical transition (related to Bi-2201 phase) at about 23 K as a consequence of the existence of inhomogeneities in the oxidation state of superconducting grains. This fact is even verified by the degree of the broadening. Moreover, residual resistivity (temperature-independent) value tends to increase rapidly because of the impurity scattering and defect interfaces such as grain boundaries, stacking faults, voids, planar and micro defects, being favored by the bulk density and porosity analyses. Similarly, critical current density is measured to be in the range of 771 A/cm 2 (for the pure sample)–78 A/cm 2 (for the compound doped with x = 0.20). The critical current densities and temperatures are not measurable for the compounds substituted by the excess La concentration level ( x > 0.20) due to the beginning point of the metal to insulator transition (nonsuperconducting behavior). In other words, the disappearance of the superconductivity results from the destruction of the phase coherence (phase breaking). Additionally, surface morphology and interaction between the superconducting grains retrograde significantly with the La concentration. EDX measurement evidences also show that during the sample preparation procedure the elements each successfully enter into the Bi-2212 crystal structure and the observed peaks of Cu and especially Bi degrade regularly with the La inclusions, meaning that the foreign particles may substitute for the elements (especially Bi). Based on the XRD measurement results, a axis length is observed to elongate regularly due to both the induced electrons into anti-bonding orbital and ionic radius of La 3+ > that of Bi 3+ whereas the cell parameter c is obtained to shrink considerably as a result of the required extra energy for the bond formation in the orbital hybrid process. Moreover, the electrical conduction transition from the 2D variable range hopping (VRH) to phonon-assisted 3D VRH mechanism is theoretically discussed with the aid localization length, probable jump distance, average hopping energy and inelastic diffusion length parameters.

Single crystals of ${\mathrm{HfTe}}_{3}$ were successfully grown using a chemical transport reaction in an extremely narrow temperature range. Here, we report a comparative study of polycrystalline and single-crystal samples. The electrical resistivity $\ensuremath{\rho}(T)$ measured on polycrystalline samples shows a broad hump and clear drop at 80 and 1.7 K, which correspond to the formation of the charge density wave (CDW) and superconducting (SC) transition, respectively. For the single crystals, $\ensuremath{\rho}(T)$ shows a sharp change at ${T}_{\mathrm{CDW}}=93$ K, and the superconductivity is absent, in contrast to the polycrystalline samples. With the current flowing along the $a$ and $b$ directions, the coincidence of the linear temperature dependence of $\ensuremath{\rho}(T)/\ensuremath{\rho}(300\phantom{\rule{4pt}{0ex}}\mathrm{K})$ above ${T}_{\mathrm{CDW}}$ strongly implies that the electron-electron scattering mechanism dominates the transport properties in a quasi-one-dimensional chain. Furthermore, a metal-semiconductor-like transition is confirmed below ${T}_{\mathrm{CDW}}$ in ${\ensuremath{\rho}}_{c}$. The drop observed at 4.3 K in ${\ensuremath{\rho}}_{b}(T)$ for the single crystal with more defects (small residual resistivity ratio, large ${\ensuremath{\rho}}_{0}$, and weak drop) provides direct evidence of a disorder-related SC fluctuation in the CDW system. With temperature decreasing, the carrier density exhibits a similar and rapid decrease below ${T}_{\mathrm{CDW}}$ for flowing current in both the $a$ and $b$ directions, whereas an obvious enhancement of carrier mobility appears as $I\ensuremath{\parallel}b$. An analysis of x-ray photoelectron spectroscopy spectra suggests that the mixed-valence states of Hf and Te could be related to the CDW formation in the multichain system of ${\mathrm{HfTe}}_{3}$.

We report a study on the superconducting pairing mechanism of FeSe via the pair-breaking effects induced by ${\mathrm{H}}^{+}$ irradiation combined with low-temperature specific heat measurements. A multigap structure with nodes or gap minima is suggested in a clean FeSe by the specific heat results. The suppression of critical temperature ${T}_{c}$ with increasing the defect density manifests a two-step behavior. When the increase in the residual resistivity is small, $\mathrm{\ensuremath{\Delta}}{\ensuremath{\rho}}_{0}\ensuremath{\lesssim}4.5\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$, ${T}_{c}$ is gradually suppressed with increasing the density of the scattering centers, suggesting the presence of symmetry-unprotected nodes or gap minima. However, for $\mathrm{\ensuremath{\Delta}}{\ensuremath{\rho}}_{0}\ensuremath{\gtrsim}4.5\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$, ${T}_{c}$ is almost independent of the scattering, which indicates that the nodes or gap minima are lifted and the order parameter becomes almost isotropic without a sign change. Thus, the superconductivity in FeSe is found to be realized in a symmetry-unprotected nodal or highly anisotropic ${s}_{++}$ state.

We present a $^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) and resistivity study of the effect of 5.5 MeV proton irradiation on the optimal electron doped $(x=0.068)$ and overdoped $(x=0.107)$ $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x})}_{2}{\mathrm{As}}_{2}$ iron based superconductors. While the proton induced defects only mildly suppress the critical temperature and increase residual resistivity in both compositions, sizable broadening of the NMR spectra was observed in all the irradiated samples at low temperature. The effect is significantly stronger in the optimally doped sample where the Curie Weiss temperature dependence of the line width suggests the onset of ferromagnetic correlations coexisting with superconductivity at the nanoscale. $1/{T}_{2}$ measurements revealed that the energy barrier characterizing the low energy spin fluctuations of these compounds is enhanced upon proton irradiation, suggesting that the defects are likely slowing down the fluctuations between $(0,\ensuremath{\pi})$ and $(\ensuremath{\pi}$,0) nematic ground states.

We investigated several FeSe single crystals grown by two different methods by utilizing experimental techniques namely, resistivity, magnetoresistance, specific heat, scanning tunneling microscopy, and spectroscopy. The residual resistivity ratio (RRR) shows systematic differences between samples grown by chemical vapor transport and flux vapor transport, indicating variance in the amount of scattering centers. Although the superconducting transition temperature $T_c$ is not directly related to RRR, our study evidences subtle differences in the features of an incipient ordering mode related to a depletion of density of states at the Fermi level. For instance, the onset temperature of anisotropic spin-fluctuations at $T^* \approx 75$ K, and the temperature of the opening-up of a partial gap in the density of states at $T^{**} \approx 30$ K are not discernible in the samples with lower RRR. Further, we show that the functional dependence of the electronic specific heat below 2 K, which allows to determine the nodal features as well as the small superconducting gap, differs significantly in crystals grown by these two different methods. Our investigation suggests that some of the controversies about the driving mechanism for the superconducting gap or its structure and symmetry is related to minute differences in the crystals arising due to the growth techniques used and the total amount of scattering centers present in the sample., Comment: 10 pages, 9 figures

We have prepared thin films of FeSe by Pulsed Laser Deposition (PLD) technique and carried out characterization and investigation of superconducting and normal state properties as a function of thickness. Our studies on the as-prepared films indicate a possible disorder induced metal to superconductor transition in the system with increasing thickness. The residual resistivity ratio of the thickest film (around 635nm) is found to be 11.95 indicating its high quality. With the aim to synthesize (Li0.8Fe0.2)OHFeSe, the only air-stable intercalate of FeSe, in thin film form by bilayer intercalation of FeSe and LiFeO2, we have synthesized LiFeO2 which is also a battery cathode material. We have also prepared and characterized LiFeO2/FeSe bi-layers

Polycrystalline MgB2 pellets with and without addition of superconducting Bi1.8Pb0.26Sr2Ca2Cu3O10+x powder has been synthesized by solid reaction. The effect of (Bi,Pb)-2223 addition on structural and critical current density of MgB2 superconductor has been investigated. XRD data reveals no substitution for Mg or B. The superconducting transition temperature (Tc0) of MgB2 (∼37.97 K) remains almost invariant with 2223 addition. Temperature dependence of the normal state electrical resistivity of 2223 added MgB2 is well explained by modified Rowell’s model and it suggests that 2223 addition in MgB2 marginally influences the percentage connectivity and intragrain residual resistivity. Critical current density (Jc) has been estimated at 4, 10, 20 and 30 K from M–H curves using Bean’s model. MgB2 sample with 1 wt.% of 2223 addition gives the best performance and display nearly two times enhancement in Jc compared to that of pure MgB2 in the field range between 0 and ±6 T. Our analysis confirms that the observed magnetic field dependence of Jc of the present system is explained well in terms of the collective pinning model. Using the best fitted parameters (Jc (0) and H0) of collective pinning model, an excellent scaling is established between the reduced critical current density, Jn (=Jc/Jc (0)) and the reduced field hn (=H/H0) at different temperatures for both pure and 2223 added MgB2 polycrystalline pellets. It is also confirmed that δTc type pinning dominates in pure MgB2 and for 2223 added MgB2 major contribution originates from δl type pinning at lower temperatures.

In this work, a comparative study on the effect of doping by low content cerium at Ca site of Bi(Pb)-2212 phases is presented. Superconducting ceramics of nominal compositions Bi2Sr2Ca1−xCexCu2O8+δ (called CC series) and Bi1.6Pb0.4Sr2Ca1−xCexCu2O8+δ (called PCC series) with x=0, 0.025, 0.05, 0.075 and 0.1 are successfully synthesized in air by conventional solid state reaction. They are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and resistivity measurements. In both series, the doping reduces the cell volume of samples. SEM photographs show that the samples are composed of grains with a flat shape that characterizes Bi-based cuprates. Quantitative EDS analysis confirms the reduction of Ca content and the increase of Ce one. The variation of resistivity with temperature shows that all samples exhibit a metallic-like character in the normal state. For all doping levels, the PCC series present highest onset critical transition temperatures. The highest value of 90.84 K is obtained for x=0.075 of Ce content, a result which is in line with that obtained in Y doped series. Also, samples of PCC series are more homogeneous. The residual resistivity increases with cerium content in both series, suggesting that doping introduces structural defects and disorder into the samples.

In this study, the superconducting properties of 4 wt% coronene (C24H12) added polycrystalline bulk MgB2 samples prepared by hot pressing and cold pressing, were reported in detail. The results were compared to determine one of which pressing methods is more useful to substitute of carbon released from C24H12 into MgB2 structure and to increase the superconducting properties of MgB2. The lattice strain, crystallite size, residual resistivity, superconducting transition temperature (T-c), critical current density (J(c)), flux pinning force density (F-p) and vertical/lateral levitation force (F-z,F-x) were analysed for the hot and cold pressed samples. The a-lattice parameter and T-c value reduced sharply for 4 wt% C(24)H(12)added cold pressed sample, resulting in a notable decrease of increment ratio of J(c) and F-p values, while the depressing of T-c is lower for 4 wt% C24H12 added hot pressed sample. The cold pressing strongly increased the lattice strain, confirming more C substitution, while the hot pressing ensured smaller crystallite size. The J(c) and F-p values were respectively found as 4.6 x 10(3) A/cm(2), 1.3 x 10(4) A/cm(2), 5.8 x 10(3) A/cm(2) and 1.8 x 10(8) N/m(3), 5.1 x 10(8) N/m(3), 2.3 x 10(8) N/m(3) for the pure sample and, 4 wt% C24H12 added hot and cold pressed samples at 20 K, 4 T. The maximum repulsive and attractive levitation force values at 20 K in ZFC and FC regimes were obtained for the 4 wt% C24H12 added hot pressed sample. Finally, the hot pressing provides an effective way to substitute C into MgB2 lattice without a remarkable decrease of T-c value, ensuring more homogenous and compacted structure, more electron-phonon interaction and strong flux pinning force.