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A model with two channels of electrical transport (TCET) for perovskite manganites is proposed, and it is described by an equivalent device with two current-carrier channels. In one channel, there is a spin-independent resistor (R3) with an equivalent resistivity of ρ3. In the other channel, there are two spin-dependent resistors in series (R1 and R2) with an equivalent resistivity of ρ1 + ρ2. The component ρ1 includes residual resistivity and the resistivity contributed by crystal-lattice scattering. The other component of the equivalent resistivity, ρ2, originates from the spin orientations of the itinerant electrons and the local electrons of the outer O 2p and Mn 3d orbits that deviate from the orientation of their ground states when the test temperature is close to the Curie temperature. Using this model, we fitted the experimental curves of the resistivity versus test temperature for single-crystalline La1−xSrxMnO3 (0.00 ≤ x ≤ 0.40) and polycrystalline La0.6Sr0.4Mn1−xFexO3 (0.00 ≤ x ≤ 0.30). In addition, we investigated the effects of the fraction of the antiferromagnetic phase, scattering at the crystallite interfaces, and the crystal-cell constants on the samples' resistivity. The physical mechanism of the TCET model was explained using an O 2p itinerant-electron model, which has been used in other studies to explain the magnetic ordering of several series of spinel ferrites and perovskite manganites.

Investigations of phase relations in the ternary system Ti-Fe-Sb show that the single-phase region of the Heusler phase is significantly shifted from stoichiometric TiFeSb (reported previously in the literature) to the Fe-rich composition TiFe1.33Sb. This compound also exhibits Fe/Ti substitution according to Ti1+xFe1.33-xSb (-0.17 ≤ x ≤ 0.25 at 800 °C). Its stability, crystal symmetry and site preference were established by using X-ray powder techniques and were backed by DFT calculations. The ab initio modeling revealed TiFe1.375Sb to be the most stable composition and established the mechanisms behind Fe/Ti substitution for the region Ti1+xFe1.33-xSb, and of the Fe/Co substitution within the isopleth TiFe1.33Sb-TiCoSb. The calculated residual resistivity of Ti1+xFe1.33-xSb, as well as of the isopleths TiFe1.33Sb-TiCoSb, TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.33Co0.75Sb-TiCoSb0.75Sn0.25, are in a good correlation with the experimental data. From magnetic measurements and 57Fe Mossbauer spectrometry, a paramagnetic behavior down to 4.2 K was observed for TiFe1.33Sb, with a paramagnetic Curie-Weiss temperature of -8 K and an effective moment of 1.11μB per Fe. Thermoelectric (TE) properties were obtained for the four isopleths Ti1+xFe1.33-xSb, TiFe1.33Sb-TiCoSb, TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.29Co0.78Sb-TiCoSb0.75Sn0.25 by measurements of electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (λ) at temperatures from 300 K to 823 K allowing the calculation of the dimensionless figure of merit (ZT). Although p-type Ti1+xFe1.33-xSb indicates a semi-conducting behavior for the Fe rich composition (x = -0.133), the conductivity changes to a metallic type with increasing Ti content. The highest ZT = 0.3 at 800 K was found for the composition TiFe1.33Sb. The TE performance also increases with Fe/Co substitution and reaches ZT = 0.42 for TiCo0.5Fe0.665Sb. No further increase of the TE performance was observed for the Sb/Sn substituted compounds within the sections TiFe0.665Co0.5Sb-TiCoSb0.75Sn0.25 and TiFe0.33Co0.75Sb-TiCoSb0.75Sn0.25. However, ZT-values could be enhanced by about 12% via the optimization of the preparation route (ball-mill conditions and heat treatments).

In this work, we have investigated structural, transport, magnetic, magnetocaloric (MC) properties and critical exponents analysis of the (Ni 2.1− x Co x )Mn 0.9 Ga ( x = 0, 0.04, 0.12 and 0.2) Heusler alloys. For all compositions, cubic austenite (A) phase with metallic character is observed at room temperature (RT). With increasing of Co content, magnitude of resistivity decreases, whereas residual resistivity ( ρ 0 ) and electron scattering factor ( A ) increases linearly. Magnetic measurements exhibit that ferromagnetic (FM) Curie temperature ( T C A ) increases towards RT by increasing Co concentration. All samples show conventional MC and maximum magnetic entropy change (Δ S M peak ) of −2.8 Jkg −1 K −1 is observed for x = 0.12 at 147 K under 5 T. Further, hysteresis is observed between cooling and warming cycles around FM-PM ( T C A ) transition in x = 0, 0.04 samples, which suggests that first order nature of transition. However, there is no hysteresis across T C A for x = 0.12 and 0.2 samples suggesting second-order nature of the transition. The critical exponents are calculated for x = 0.12 sample around T C A using Arrott plot and Kouvel-Fisher method, the estimated critical exponents are found closer to the mean-field model reveals the long range ferromagnetic ordering in this composition.

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.

We investigate a unique material system, Ba0.6K0.4Fe2As2 thin film, using the optical spectroscopic technique. The Ba0.6K0.4Fe2As2 thin film was prepared on an Al2O3 (0001) substrate using the pulsed laser deposition (PLD) method and a subsequent post-annealing process. The Ba0.6K0.4Fe2As2 thin film sample shows a much larger residual resistivity as compared to that of a single crystal of the same compound. We applied the well-known multilayer thin film analysis to obtain optical constants of Ba0.6K0.4Fe2As2 thin film layer from the measured reflectance spectra of the Ba0.6K0.4Fe2As2/Al2O3 sample. The overall temperature-dependent optical properties were found to be similar to those of a corresponding single crystal. From our analysis, we also roughly estimated the superfluid density of 3860 cm−1 and the corresponding London penetration depth of 412 nm of the Ba0.6K0.4Fe2As2 thin film. Our results indicate that this Ba0.6K0.4Fe2As2 thin film prepared by PLD does retain some of the intrinsic physical properties of single crystal samples.

The quadrupole and dipole magnets for the high-luminosity large hadron collider (HL-LHC) upgrade will be based on Nb 3 Sn Rutherford cables that operate at 1.9 K and experience magnetic fields of up to about 12 T. An important step in the design of these magnets is the development of the high-aspect-ratio Nb 3 Sn cables to achieve the nominal field with sufficient margin. The strong plastic deformation of unreacted Nb 3 Sn strands during the Rutherford cabling process may induce non-negligible I c and Residual Resistivity Ratio degradation. In this paper, the cabling degradation is investigated as a function of the cable geometry for both Powder-in-Tube and Restack Rod Process conductors. Based on this analysis, new baseline geometries for both 11 T and quadrupole magnets of HL-LHC are proposed.

The multiple solutions of conduction and vapor cooled copper leads modeling current delivery to a superconducting magnet have been numerically calculated. Both ideal convection and convection with a finite heat transfer coefficient for an imposed coolant mass flow rate have been considered. Because of the nonlinearities introduced by the temperature dependent material properties, two solutions exist, one stable and one unstable regardless of the cooling method. The limit points separating the stable form the unstable steady states form the blow-up threshold beyond which, any further increase in the operating current results in a thermal runway. An interesting finding is that the multiplicity persists even when the cold end temperature is raised above the liquid nitrogen temperature. The effect of various parameters such as the residual resistivity ratio, the overcurrent and the variable conductor cross section on the bifurcation structure and their stabilization effect on the blow-up threshold is also evaluated.

We report the observation of quantum Hall effect (QHE) in a Bi 2 Se 3 single crystal having carrier concentration ( n ) ∼ 1.13 × 10 19 cm −3 , three dimensional Fermi surface and bulk transport characteristics. The plateaus in Hall resistivity coincide with minima of Shubnikov de Haas oscillations in resistivity. Our results demonstrate that the presence of perfect two dimensional transport is not an essential condition for QHE in Bi 2 Se 3 . The results of high resolution X-ray diffraction (HRXRD), energy-dispersive X-ray spectroscopy (EDX), and residual resistivity measurements show the presence of enhanced crystalline defects and microstrain. We discuss the possible origin of the observed QHE and correlate its existence to the crystalline defects.

Single-layer vanadium nitride (VN) and bilayer Pd0.96Fe0.04/VN and VN/Pd0.92Fe0.08 thin-film heterostructures for possible spintronics applications were synthesized on (001)-oriented single-crystalline magnesium oxide (MgO) substrates utilizing a four-chamber ultrahigh vacuum deposition and analysis system. The VN layers were reactively magnetron sputtered from a metallic vanadium target in Ar/N2 plasma, while the Pd1−xFex layers were deposited by co-evaporation of metallic Pd and Fe pellets from calibrated effusion cells in a molecular beam epitaxy chamber. The VN stoichiometry and Pd1−xFex composition were controlled by X-ray photoelectron spectroscopy. In situ low-energy electron diffraction and ex situ X-ray diffraction show that the 30 nm thick single-layer VN as well as the double-layer VN(30 nm)/Pd0.92Fe0.08(12 nm) and Pd0.96Fe0.04(20 nm)/VN(30 nm) structures have grown cube-on-cube epitaxially. Electric resistance measurements demonstrate a metallic-type temperature dependence for the VN film with a small residual resistivity of 9 μΩ·cm at 10 K, indicating high purity and structural quality of the film. The transition to the superconducting state was observed at 7.7 K for the VN film, at 7.2 K for the Pd0.96Fe0.04/VN structure and at 6.1 K for the VN/Pd0.92Fe0.08 structure with the critical temperature decreasing due to the proximity effect. Contrary to expectations, all transitions were very sharp with the width ranging from 25 mK for the VN film to 50 mK for the VN/Pd0.92Fe0.08 structure. We propose epitaxial single-crystalline thin films of VN and heteroepitaxial Pd1−xFex/VN and VN/Pd1−xFex (x ≤ 0.08) structures grown on MgO(001) as the materials of a choice for the improvement of superconducting magnetic random access memory characteristics.

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 show that the Fermi surface can survive the presence of extreme compositional disorder in the equiatomic alloy \udNi\ud0.25\udFe\ud0.25\udCo\ud0.25\udCr\ud0.25\ud. Our high-resolution Compton scattering experiments reveal a Fermi surface which is smeared across a significant fraction of the Brillouin zone (up to 40% of \ud2\udπ\ud/\uda\ud). The extent of this smearing and its variation on and between different sheets of the Fermi surface have been determined, and estimates of the electron mean free path and residual resistivity have been made by connecting this smearing with the coherence length of the quasiparticle states.

In this chapter, we expose the new state of matter exhibited by strongly correlated Fermi systems represented by various HF compounds. The quantum physics of different HF compounds is universal, and emerges regardless of the microscopic structure of the compounds. This uniform behavior allows us to view it as the main characteristic of the new state of matter exhibited by HF compounds. In this chapter, we reveal and explain the universal scaling behavior of the thermopower S/T in such different heavy-fermion (HF) compounds as \(\mathrm {YbRh_2Si_2}\), \(\beta \)-\(\mathrm{YbAlB_4}\), and the strongly correlated layered cobalt oxide \(\mathrm{[BiBa_{0.66}K_{0.36}O_{2}]CoO_{2}}\). Using the archetypical HF metal \(\mathrm {YbRh_2Si_2}\) as an example, we demonstrate that the scaling behavior of S/T is violated by the antiferromagnetic phase transition, since both the residual resistivity \(\rho _0\) and the density of states N experience jumps at the phase transition, making the thermopower experience two jumps and change its sign. Our elucidation is based on flattening of the single-particle spectrum that profoundly affects \(\rho _0\) and N. To depict the main features of the S/T behavior, we construct the \(T-B\) schematic phase diagram. Our calculated S/T for the HF compounds are in good agreement with experimental facts and support our observations.

The recent observation of Weyl fermions in the itinerant 4d ferromagnetic perovskite SrRuO3 points to this material being a good platform for exploring novel physics related to a pair of Weyl nodes in epitaxial heterostructures. In this Letter, we report the thickness-dependent magnetotransport properties of ultra-high-quality epitaxial SrRuO3 films grown under optimized conditions on SrTiO3 substrates. Signatures of Weyl fermion transport, i.e., unsaturated linear positive magnetoresistance accompanied by a quantum oscillation having a high quantum mobility of 10 000 cm2/V s and a π Berry phase, were observed in films with thicknesses as small as 10 nm. Residual resistivity increased with the decreasing film thickness indicating disorder near the interface between SrRuO3 and the SrTiO3 substrate. Since this disorder affects the magnetic and electrical properties of the films, the Curie temperature decreases and the coercive field increases with the decreasing thickness. Thickness-dependent magnetotransport measurements revealed that the threshold residual resistivity ratio to observe Weyl fermion transport is 21. These results provide guidelines for realizing quantum transport of Weyl fermions in SrRuO3 near heterointerfaces.

We have synthesized single crystal of layered ternary compound having composition TaIrTe4 using tellurium flux method. Herein, we report experimental study of magneto-transport properties on Weyl semimetal TaIrTe4 in the temperature regime 2.5K to 300K. TaIrTe4 show the linear magneto-resistance 4.1 % at 3.5K in a magnetic field of 6 T. The residual resistivity ration (RRR) is around 2.4. HR-TEM and SEAD patterns reveal the formation of single crystal TaIrTe4.

Electrical conductivity/resistivity of elemental fcc metals, such as Al and Cu, has been investigated intensively for decades, both theoretically and experimentally. Since these metals are of great practical importance for electrical wiring, reducing their resistivity even by a few percent may have very strong impact on their application effectiveness. In this paper, we report on electrical resistivity measurements in Al single crystals grown by the Bridgman method. We found that their resistivity at room temperature decreases by 11.5% upon heat treatment in a boron environment at 600 °C, i.e., well below the melting temperature of Al (Tm = 660 °C). The residual resistivity indeed reaches its lower limit dictated by electron–phonon interaction at room temperature. We explain this effect by the boron-induced formation of distorted regions at the surface of the Al crystals. These regions are 30–50 μm in size and comprise finer grains with an average size of 5 μm, separated by low-angle grain boundaries. Resistivity reduction is mainly due to the getter effect, i.e., the removal of the impurity atoms from the crystal bulk by the outward diffusion to the distorted surface regions.

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 renormalization of the Wiedemann-Franz (WF) ratio in strongly correlated electron systems is analyzed. We demonstrate that the WF law is violated at the quantum-critical point, where the effective mass diverges, and beyond point of fermion condensation (FC), where the single-particle spectrum\(\epsilon (p)\) becomes flat. Experimental observations of the much-studied compounds CeCoIn\(_5\) and YbRh\(_2\)Si\(_2\) at vanishing temperatures carefully probe the nature of their magnetic-field-tuned QCPs. The violation of Wiedemann-Franz (WF) law, along with jumps revealed both in the residual resistivity \(\rho _0\) and the Hall resistivity \(R_H\), provides vital clues to the origin of their non-Fermi liquid behavior. The empirical facts point unambiguously to association of the observed QCP with FC forming flat bands.

Polar metals are an intriguing class of materials that simultaneously host free carriers and polar structural distortions. Despite the name "polar metal," however, most well-studied polar metals are poor electrical conductors. Here, we demonstrate the molecular beam epitaxial (MBE) growth of LaPtSb and LaAuGe, two polar metal compounds whose electrical resistivity is an order of magnitude lower than the well studied oxide polar metals. These materials belong to a broad family of $ABC$ intermetallics adopting the stuffed wurtzite structure, also known as hexagonal Heusler compounds. Scanning transmission electron microscopy (STEM) reveals a polar structure with unidirectionally buckled $BC$ (PtSb, AuGe) planes. Magnetotransport measurements demonstrate good metallic behavior with low residual resistivity ($\rho_{LaAuGe}=59.05$ $\mu\Omega\cdot$cm and $\rho_{LaAPtSb}=27.81$ $\mu\Omega\cdot$cm at 2K) and high carrier density ($n_h\sim 10^{21}$ cm$^{-3}$). Photoemission spectroscopy measurements confirm the band metallicity and are in quantitative agreement with density functional theory (DFT) calculations. Through DFT-Chemical Pressure and Crystal Orbital Hamilton Population analyses, the atomic packing factor is found to support the polar buckling of the structure, though the degree of direct interlayer $B-C$ bonding is limited by repulsion at the $A-C$ contacts. When combined with insulating hexagonal Heuslers, these materials provide a new platform for fully epitaxial, multiferroic heterostructures.

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

Most interesting examples of violations of the Mott-Ioffe-Regel (MIR) resistivity limit are found in materials with strong electronic correlations that are not well understood by theory. We demonstrate that first principles theory can predict the experimentally observed frequency dependence of the optical conductivity for a novel class of metals where the residual resistivity is near or above the MIR limit, which we define as a ``bad metal.'' The predicted optical conductivity of a NiCoCr alloy is in good agreement with experiment. It is demonstrated that the width of the Drude peak describing the low-frequency part of optical conductivity is comparable to the Fermi energy. The latter, together with a mean free path comparable to the interatomic distance, indicates the absence of well-defined quasiparticles. In contrast to traditional bad metals with strong electron-electron interactions, both the high resistivity and the large width of the Drude peak in these alloys result from strong scattering on disordered atomic potentials that can be understood using modern density functionals.

We study the quantum phase transitions in the nickel pnctides, CeNi2−δ(As1−xPx)2 (δ ≈ 0.07–0.22) polycrystalline samples. This series displays the distinct heavy fermion behavior in the rarely studied parameter regime of dilute carrier limit. We systematically investigate the magnetization, specific heat and electrical transport down to low temperatures. Upon increasing the P-content, the antiferromagnetic order of the Ce-4f moment is suppressed continuously and vanishes at xc ~ 0.55. At this doping, the temperature dependences of the specific heat and longitudinal resistivity display non-Fermi liquid behavior. Both the residual resistivity ρ0 and the Sommerfeld coefficient γ0 are sharply peaked around xc. When the P-content reaches close to 100%, we observe a clear low-temperature crossover into the Fermi liquid regime. In contrast to what happens in the parent compound x = 0.0 as a function of pressure, we find a surprising result that the non-Fermi liquid behavior persists over a nonzero range of doping concentration, xc x 2−δ(As1−xPx)2 in comparison with those of its 1111 counterpart, CeNi(As1−xPx)O. Our results indicate a non-Fermi liquid phase in the global phase diagram of heavy fermion metals.

The high-entropy alloys Al$_{x}$CrFeCoNi exist over a broad range of Al concentrations ($0 < x < 2$). With increasing Al content their structure is changed from the fcc to bcc phase. We investigate the effect of such structural changes on transport properties including the residual resistivity and the anomalous Hall resistivity. We have performed a detailed comparison of the first-principles simulations with available experimental data. We show that the calculated residual resistivities for all studied alloy compositions are in a fair agreement with available experimental data as concerns both the resistivity values and concentration trends. We emphasize that a good agreement with experiment was obtained also for the anomalous Hall resistivity. We have completed study by estimation of the anisotropic magnetoresistance, spin-disorder resistivity, and Gilbert damping. The obtained results prove that the main scattering mechanism is due to the intrinsic chemical disorder whereas the effect of spin polarization on the residual resistivity is appreciably weaker., 8 pages, 3 figures, accepted in Phys. Rev. B

The strain irreversibility cliff (SIC), marking the abrupt change of the intrinsic irreversible strain limit εirr,0 as a function of heat-treatment (HT) temperature θ in Nb3Sn superconducting wires made by the restacked-rod process (RRP®), is confirmed in various wire designs. It adds to the complexity of reconciling conflicting requirements on conductors for fabricating magnets. Those intended for the high-luminosity upgrade of the Large Hardon Collider (LHC) at the European Organization for Nuclear Research (CERN) facility require maintaining the residual resistivity ratio RRR of conductors above 150 to ensure stability of magnets against quenching. This benchmark may compromise the conductors’ mechanical integrity if their εirr,0 is within or at the bottom of SIC. In this coupled investigation of strain and RRR properties to fully assess the implications of SIC, we introduce an electro-mechanical stability criterion that takes into account both aspects. For standard-Sn billets, this requires a strikingly narrow HT temperature window that is impractical. On the other hand, reduced-Sn billets offer a significantly wider choice of θ, not only for ensuring that εirr,0 is located at the SIC plateau while RRR ≥ 150, but also for containing the strain-induced irreversible degradation of the conductor’s critical-current beyond εirr,0. This study suggests that HT of LHC magnets, made of reduced-Sn wires having a Nb/Sn ratio of 3.6 and 108/127 restacking architecture, be operated at θ in the range of 680 to 695 °C (when the dwell time is 48 hours).

Incorporation of the secondary oxide phase into the manganite composite capable of enhancing low-field magnetoresistance (LFMR) for viability in high-performance spintronic applications. Polycrystalline La0.67Ca0.33MnO3 (LCMO) was prepared via the sol–gel route in this study. The structural, microstructural, magnetic, electrical, and magneto-transport properties of (1−x) LCMO: x NiO, x = 0.00, 0.05, 0.10, 0.15 and 0.20 were investigated in detail. The X-ray diffraction (XRD) patterns showed the coexistence of LCMO and NiO in the composites. The microstructural analysis indicated the amount of NiO nanoparticles segregated at the grain boundaries or on the surface of LCMO grains increased with the increasing secondary phase content. LCMO and NiO still retained their individual magnetic phase as observed from AC susceptibility (ACS) measurement. This further confirmed that there is no interfacial diffusion reaction between these two compounds. The NiO nanoparticle acted as a barrier to charge transport and caused an increase in resistivity for composite samples. The residual resistivity due to the grain/domain boundary is responsible for the scattering mechanism in the metallic region as suggested by the theoretical model fitting, ρ(T)=ρ0+ρ2T2+ρ4.5T4.5. The magnetoresistance values of LCMO and its composites were found to increase monotonically with the decrease in temperature. Hence, the LFMR was observed. Nonetheless, the slight reduction of LFMR in composites was attributed to the thick boundary layer created by NiO and impaired the spin polarised tunnelling process.

We developed a soft and high thermal conductive thermal link for cryogenic applications. The measured maximum thermal conductivity was approximately 18500 W / m / K at 10 K. This spring constant was 1/43 of that of a single thick wire with the same cross-sectional area at room temperature. We reached these performances by utilizing high purity aluminum (99.9999 % , 6 N) and by stranding many thin wires that are 0.15 mm in diameter. The electrical Residual Resistivity Ratio was also measured and used to estimate the thermal conductivity by the Wiedemann–Franz law. We observed the size effect on the electrical conductivity, and evaluated the effect of mechanical deformation to the RRR. These evaluations allow us to design a pragmatic thermal link for cryogenic application.

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.

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 effect of the Ni/Cu substitution on the electrical resistivity and microstructure of the polycrystalline GdNi5–xCux series was studied. The value of temperature of phase transition (Tph) estimated from temperature dependence of electrical resistance varied non-linearly across copper doping from 32.5 K (x=0.0) to 29.1 K (x=5.0). The value of residual resistivity (ρo) estimated at low temperature range decreased from 27.28 μΩcm (x=0.0) to 9.44 μΩcm (x=5.0), which was discussed as the influence of microstructure. In order to describe the temperature dependence of resistivity ρ(T) a variety of approaches were applied due to different scattering mechanisms occurring at high and low temperature ranges. The change within ρ(T) curvature was evidenced at low temperature range across copper doping. The temperature variation of the resistivity was quite peculiar for Cu-rich compounds (x=4.8, x=5.0), which might be correlated with the incommensurate magnetic structure derived from the weakly negative interaction between the nearest neighbours of Gd. The correlation between microstructure and resistivity was observed.

Isochronal annealing followed by residual resistivity measurements at 12 K was performed in Fe-0.6Ni-0.6Si and Fe-1.5Mn-0.6Si alloys irradiated with 1 MeV proton ions below 70 K, and recovery stages were compared with those of Fe–0.6Ni and Fe–1.5Mn. The effects of silicon addition in the Fe-Ni alloy was observed as the appearance of a new recovery stage at 282–372 K, presumably corresponding to clustering of solute atoms in matrix, and as a change in mixed dumbbell migration at 122–142 K. Silicon addition mitigated the manganese effect in Fe–Mn alloy that is obstructing the recovery of radiation defects. Reduction of resistivity in Fe-Mn-Si alloy also suggested formation of small solute atom clusters.

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.

A combination of High Energy Ball Milling in the planetary mills (HEBM) with Spark Plasma Sintering (SPS) allows production of dense (>97%) nano-composites of immiscible metals, Cu and Cr, so-called pseudo-alloys. New results on electrical resistance of the pseudo-alloys have been obtained from cryogenic to elevated temperatures, in comparison with the Cu-Cr micro-composites and pure Cu. The results show that duration of HEBM influences hardness and electric resistivity of the materials in a wide temperature range, including cryogenic residual resistivity. This influence remains after SPS. Increasing of the electrical resistivity and hardness with increasing of the mechanical impact is explained by significant increase of specific surface of grain boundaries due to grains refinement. At the same time, some regimes of HEBM results in increasing of the electrical conductivity (decreasing resistance) due to mechanical boundaries purification. These findings allow optimization of the process for preparation of the alloys with desired electrical conductivity and mechanical properties, specifically for application as electrode materials.

Magneto-conductive mechanisms and properties of La 1.2 Re 0.2 Ca 1.6 Mn 2 O 7 (Re=La, Pr, Y, Gd and Eu) have been investigated. In the low temperature range, combining residual resistivity, weak localization effect, electron–electron and electron–phonon interactions in a model, fit well the resistivity curves of undoped, Pr-doped and Y-doped samples. The Gd-doped and Eu-doped ones require the introduction of the small polaron metallic conduction. In the high temperature range, 3D-Mott's variable range hopping (3D-VRH) is the best model to fit resistivity of the undoped, Pr-doped and Y-doped samples, while Effros-Shklovskii model (ES-VRH) is the best one for Gd-doped and Eu-doped samples. In the entire temperature range, the percolation model fit well the resistivity. Using 3D-VRH model, the density of state (DOS) decreases with doping by Gd and Eu, whereas mean hopping distance R h ( T ) and mean hopping energy E h ( T ) increase. This may explain the resistivity increase and the crossover to the ES-VRH model. Values of R h ( T ) vary between 1.811 and 1.030 nm, which allow us to suggest 3D-VRH as the best model fitting current results. E h ( T ) values range from 0.1157 to 0.2635 eV. The DOS increases as increasing magnetic field while R h ( T ) and E h ( T ) decrease, which is in agreement with the observed decrease of resistivity.

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.

In the spin-valve Co/Pt/[Co/Pt]2 multilayers with perpendicular magnetic anisotropy, it has been found that magnetoresistance, anomalous Hall resistivity, and longitudinal resistivity demonstrate oscillatory behaviors with the increase in Pt spacer thickness. Moreover, the parameter b oscillates as a function of Pt spacer thickness, which is experimentally determined by fitting the equation $\rho _{AH} =a^{\prime }\rho _{xx0} +b\rho _{xx}^{2} $ (ρ x x0 is the residual resistivity, a ′ is the skew scattering parameter, and b consists of the side jump and the intrinsic contributions) via the temperature dependence of the resistivity. These observed oscillatory behaviors can be mainly ascribed to the polarization of Pt spacer, while the polarization in the Co/Pt multilayers depends on the Pt spacer thickness.

Herein we systematically investigated the alloying effects on structural, magnetic, and electrical/thermal transport properties in MAX-phase Cr2−xMxGeC (M = Ti, V, Mn, Fe, and Mo). The alloying of M with the larger covalent radius than that of Cr increases lattice constants (a and c) as well as unit cell volume (V) of Cr2−xMxGeC, and vice versa. However, the c/a ratio monotonously decreases with increasing alloying level x, which is due to a larger change of a than that of c. The Pauli paramagnetic ground state of Cr2GeC is confirmed by magnetic measurements and low-temperature specific heat analysis. Interestingly, ferromagnetism can be introduced in Cr2−xMxGeC by doping magnetic elements (Mn and Fe) and non-magnetic elements (Ti and Mo), which may be due to a reconstruction of the Fermi surface caused by chemical doping. All our samples show a metal-like electrical transport behavior, and the residual resistivity ratio decreases with increasing alloying concentration, which are mainly attributed to the disorders induced by alloying. The change of electron specific heat coefficient is consistent with the change of density state of Fermi surface in Cr2−xMxGeC. In addition, solid-solution scattering is the dominant factor for the behavior of thermal conductivity k(T) in Cr2−xVxGeC, while enhanced phonon scattering induced by alloying is the decisive factor for the change of k(T) in Cr2−xMoxGeC. The positive Seebeck coefficient of Cr2−xVxGeC and Cr2−xMoxGeC may be close related to the decrease of structural anisotropy.

MgB 2 films and MgB 2 /BN bilayer films have been fabricated in the same reactor. The MgB 2 films and the bottom MgB 2 layers of the MgB 2 /BN bilayer films were synthesized by hybrid physical-chemical vapor deposition. Furthermore, the BN layers were synthesized by chemical vapor deposition. The degradation of MgB 2 films and MgB 2 /BN bilayer films that are submerged into deionized water at room temperature has been studied. To clarify the protection of BN for MgB 2 films against degradation in water, we analyzed the physical and magnetic properties, including the superconducting critical temperature T c , the critical current density J c , and the residual resistivity ratio. In addition, scanning electron microscopy and X-ray diffraction measurements were taken. The results indicate that the BN films can protect MgB 2 films from degradation in water, which would be beneficial for the device application of MgB 2 films.

Accepted: 2016-03-02, 資料番号: SA1150324000

Antiperovskite intermetallic Mn3−xCrxZnN (0≤x≤0.5) compounds have been prepared to study the Cr doping effect on the electronic transport properties. The temperature dependent resistivity ρ(T) for x≤0.3 shows notable and broader slope change. Meanwhile, with increasing Cr doping, the slope change shifts to lower temperatures and is gradually suppressed for x>0.3. For x=0.0 and 0.5, the measurements of magnetization from 10 K to 350 K reveal that Mn3−xCrxZnN compounds go through a transition from the dominant AFM interactions to one where the FM and AFM interactions coexist. For low-temperature region (15–80 K), the electron–electron scattering is dominant, and the number of phonons abruptly increases with increasing temperature. Small polaron hopping model (SPH) is used to understand the temperature dependence of electrical resistivity in the high-temperature range of 200–300 K and a clear decrease of the polaron activation energy was observed for x≤0.2. The residual resistivity ratio (RRR) decreases while the residual resistivity increases (ρ0) with increasing Cr-doping level. The maximum total entropy change (ΔSmax) decreases by increasing the Cr doping from x=0.0 to 0.2.

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.

We performed combined experimental and theoretical studies of the effect of Cr substitution for Fe on the structural, magnetic, transport, electronic, and mechanical properties of Fe 3 - x Cr x Ge ( 0 ⩽ x ⩽ 1 ) intermetallic alloys. Single phase microstructures are observed for x ⩽ 0.70 . Higher Cr concentrations x > 0.70 are multi-phased. A hexagonal D0 19 structure is found for all Cr concentrations, with the lattice parameters increasing systematically with an increasing Cr content. All the alloys in the series are found to be ferromagnets with large magnetization values of about 6 μ B / f . u . and high Curie temperature above room temperature. The low-temperature saturation magnetic moments agree fairly well with our theoretical results and also obey the Slater-Pauling rule. The density functional theory calculation reveals that Cr substitution energetically favours one of the Fe sites in Fe3Ge. The electrical resistivity measured over the temperature range from 5 K to 400 K shows metallic behavior, with a residual resistivity ratio that decreases with Cr content. Vicker’s hardness values are observed to increase with increasing Cr content to approximately 5 GPa.

The Hall Effect and magnetization of Heusler alloys Co2YSi (Y = Ti, V, Cr, Mn, Fe, Co, Ni) were measured at T = 4.2 K and 300 K in magnetic fields of up to 100 kOe as well as the temperature dependence of the electroresistivity from 4.2 to 300 K. The normal and anomalous Hall coefficients, saturation magnetization, residual resistivity, type and concentration of current carriers and their mobility were obtained. It was demonstrated that there is a clear correlation between the electronic and magnetic parameters obtained, depending on the number of valence electrons z, at the transition from Co2TiSi (z=26) to Co2NiSi (z=32). The observed peculiarities of electronic and magnetic parameters may be due to the appearance of the states of the half-metallic ferromagnet and/or spin gapless semiconductor. © Published under licence by IOP Publishing Ltd. This work was carried out as part of the state task of the Russian Ministry of Education and Science (themes “Spin”,No. AAAA-A18-118020290104-2) with partial support from the Russian Foundation for Basic Research (projects Nos. 18-32-00686 and 18-02-00739) and the Government of the Russian Federation (Act No. 211, contract No. 02.A03.21.0006).

Noncollinear magnetic moments in antiferromagnets (AFM) lead to a complex behavior of electrical transport, even to a decreasing resistivity due to an increasing temperature. Proper treatment of such phenomena is required for understanding AFM systems at finite temperatures; however first-principles description of these effects is complicated. With ab initio techniques, we investigate three models of spin fluctuations (magnons) influencing the transport in AFM CuMnAs; the models are numerically feasible and easily implementable to other studies. We numerically justified a fully relativistic collinear disordered local moment approach and we present its uncompensated generalization. A saturation or a decrease of resistivity caused by magnons, phonons, and their combination (above approx. 400 K) was observed and explained by changes in electronic structure. Within the coherent potential approximation, our finite-temperature approaches may be applied also to systems with impurities, which are found to have a large impact not only on residual resistivity, but also on canting of magnetic moments from the AFM to the ferromagnetic (FM) state., 25 pages, 8 figures, 2 tables; accepted version to JMMM

Single crystals of SrCo2Al and BaCo2Al9 were grown using a self-flux method. A LeBail analysis of the powder X-ray diffraction patterns show that both compounds crystallize in a hexagonal (P6/mmm) crystal structure with lattice parameters: a ​= ​7.8995(1) A, c ​= ​3.9159(1) A for SrCo2Al9, and a ​= ​7.9162(2) A, c ​= ​3.9702(1) A for BaCo2Al9 aluminide. The low temperature analysis of the heat capacity measurements give a Sommerfeld coefficient γ ​= ​4.99(6) mJ mol−1 K−2 for SrCo2Al9 and almost twice larger γ ​= ​7.94(9) mJ mol−1 K−2 for BaCo2Al9. Resistivity measurements show metallic-like behavior, with reasonably large residual resistivity ratio RRR ​= ​6 and 10 for SrCo2Al9 and BaCo2Al9, respectively. Neither heat capacity nor resistivity measurements reveal any phase transition down to 1.8 ​K.

Electroresistance (ER) has been intensively studied in low- and intermediate-bandwidth manganites, which possess phase separation characteristics. As for the Sr- and Ba-doped large-bandwidth manganites, however, few results about ER have been reported so far. Here we report ER effect in oxygen-deficient La0.8Ba0.2MnO3-δ thin films, which were obtained by applying a large electric current (33 mA) to the pristine films in vacuum. While the pristine film displays a negligible change in resistivity with respect to the test current, the oxygen-deficient film shows significant ER effect, i.e. ER ratio of -22% at 260 K under a test current of 0.3 mA. By gradually restoring oxygen content in the films, it is found that the ER effect is closely related to the residual resistivity at low temperatures, demonstrating the key role of grain boundaries. Furthermore, the residual resistivity can readily be tuned by heating the oxygen-deficient films in air, suggesting strong oxygen activity in the grain boundaries. The magnetoresistance (MR) data show current dependent feature, also revealing the role of grain boundaries. At 40 K, the MR ratio of the 100 °C restored film under 30 kOe increases from -15% to -25% when decreasing the test current from 1 to 10-3 mA. The large ER effect in the oxygen-deficient films is discussed based upon the conductive filament picture in grain boundaries. Our approach to controlling the ER effect through oxygen deficiency makes oxide films more promising for potential applications in the memristive devices and neuomorphic computing.