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Low-frequency resistance noise spectroscopy is applied to investigate bulk single crystals of the intercalated iron-selenide K x Fe2 – y Se2 superconductors with different iron vacancy orders. Based on a generalized fluctuation model, the well-observed resistance hump above 100 K is interpreted as an insulator–metal phase transition with a characteristic transition energy of 0.1–0.6 eV, indicating a highly inhomogeneous energy distribution configuration. In the superconducting transition regime, we find that the normalized resistance noise scales with resistance R excellently as SR /R 2 ∝ R l rs with the noise exponent l rs ≈ 1.4. With reduced iron vacancy disordering in enhanced superconductivity K x Fe2 – y Se2 crystals, the level of resistance fluctuations is greatly suppressed, suggesting a geometrical phase transition for conduction channel, which is directly related to the microstructure of the crystals.

We develop an experimental tool to investigate the order parameter of superconductors by combining point-contact spectroscopy measurement with high-pressure technique. It is demonstrated for the first time that planar point-contact spectroscopy measurement on noncentrosymmetric superconducting PbTaSe2 single crystals is systematically subjected to hydrostatic pressures up to 12.1 kbar. Under such a high pressure, the normal-state contact resistance is sensitive to the applied pressure, reflecting the underlying variation of contact transparency upon pressures. In a superconducting state, the pressure dependence of the energy gap Δ 0 and the critical temperature T c for gap opening/closing are extracted based on a generalized Blond–Tinkham–Klapwijk model. The gap ratio 2Δ 0/k B T c indicates a crossover from weak coupling to strong coupling in electron pairing strength upon pressure for PbTaSe2. Our experimental results show the accessibility and validity of high-pressure point-contact spectroscopy, offering rich information about high-pressure superconductivity.

Anomalies found in the electronic conductivity and the elastic constants C33 and C44 around the superconducting phase transition in the underdoped BaFe1.93Ni0.07As2 compounds are re-examined. The large anomalies in the elastic constants C33 and C44 modes and the unusual temperature dependence of the real part σ1 of the electronic conductivity found around the superconducting phase transition TC ~ 16 K are attributed to magnetic fluctuations. The establishment of a magnetic order at TM ~ 21 K results in a marked decrease of the scattering of electronic carriers.

Measurements of the real sigma(1) and imaginary sigma(2) part of the conductivity were performed in optimally doped BaFe1.9Ni0.1As2 and overdoped BaFe1.88Ni0.12As2 crystals in the frequency range 20 MHz-1.5 GHz using a single coil technique. The temperature dependence of the London penetration depth follows a T-2 law. The conductivity sigma(1) increases with decreasing temperature below T-c, in agreement with the results obtained for the optimally Co doped BaFe2-xCoxAs2 crystals. The increase of sigma(1) in the superconducting state is attributed to a rapidly decrease of the quasiparticle scattering rate. (C) 2014 Elsevier Ltd. All rights reserved.

We have systematically studied the nematic fluctuations in the electron-doped iron-based superconductor ${\mathrm{BaFe}}_{2\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{As}}_{2}$ by measuring the in-plane resistance change under uniaxial pressure. While the nematic quantum critical point can be identified through the measurements along the (110) direction, as studied previously, quantum and thermal critical fluctuations cannot be distinguished due to similar Curie-Weiss-like behaviors. Here we find that a sizable pressure-dependent resistivity along the (100) direction is present in all doping levels, which is against the simple picture of an Ising-type nematic model. The signal along the (100) direction becomes maximum at optimal doping, suggesting that it is associated with nematic quantum critical fluctuations. Our results indicate that thermal fluctuations from striped antiferromagnetic order dominate the underdoped regime along the (110) direction. We argue that either there is a strong coupling between the quantum critical fluctuations and the fermions, or more exotically, a higher symmetry may be present around optimal doping.

We have systematically studied the low-temperature specific heat of the ${\mathrm{BaFe}}_{2\ensuremath{-}x}{\mathrm{Ni}}_{x}{\mathrm{As}}_{2}$ single crystals covering the whole superconducting dome. Using the nonsuperconducting heavily overdoped $x=0.3$ sample as a reference for the phonon contribution to the specific heat, we find that the normal-state electronic specific heats in the superconducting samples may have a nonlinear temperature dependence, which challenges previous results in the electron-doped Ba-122 iron-based superconductors. A model based on the presence of ferromagnetic spin fluctuations may explain the data between $x=0.1$ and $x=0.15$, suggesting the important role of Fermi-surface topology in understanding the normal-state electronic states.

The mystery of the missing bound states within a superconducting vortex in a pnictide superconductor has been solved. Not only are bound states present, they also provide information on the gap structure of Ba0.6K0.4Fe2As2.

We report detailed measurements of the temperature dependence of the lower critical field H c 1 of the FeAs-based superconductor SmFeAsO 0.9 F 0.1 (Sm-1111) and Ba 0.6 K 0. 4 Fe 2 As 2 (BaK-122) by global and local magnetization measurements. It is found that the obtained H c 1 for both kinds of samples show a weak temperature dependence in low temperature region. We argue that this weak T -dependence of H c 1 of Sm-1111 does not indicate a conventional s -wave state, instead it satisfies a T 2 dependence up to a temperature of (0.5–0.6) T c . In contrast, the data of H c 1 ( T ) of BaK-122 superconductor clearly show a multigap behavior. Excellent fitting to the data can be reached with two s -wave superconducting gaps. Comparison of the absolute values of H c 1 (0) between Sm-1111 and BaK-122 shows a relatively large superfluid density for the latter. This may indicate the distinction between the electron doped and hole-doped FeAs-based superconductors.

Hall effect and magnetoresistance Δ ρ / ρ (0) (MR) in the normal state have been measured on single crystals of Ba 1− x K x Fe 2 As 2 and NdFeAsO 1− x F x . Detailed analysis reveal the following conclusions: (1) For the parent phases of Ba 1− x K x Fe 2 As 2 and NdFeAsO 1− x F x , large Hall effect and MR with strong temperature dependence were observed below a characteristic temperature corresponding to the antiferromagnetic/ structural transition. The field dependence of the Hall resistivity ρ xy exhibits a non-linear behavior, which is accompanied by the violation of the B -square feature of the longitudinal magnetoresistivity Δ ρ xx ( B )/ ρ xx (0). A closer inspection further indicates that they are well related to each other and could be attributed to the multi-band effect or spin-related scattering. (2) The superconducting samples show much smaller Hall coefficient and MR in the normal state. The Hall coefficient shows a weaker temperature dependence compared to the parent phase, while the mean scattering rate 1/ τ H has a power-law like temperature dependence as 1/ τ H ∝ T n ( n = 2–3). (3) For a Ba 1− x K x Fe 2 As 2 sample with T c = 36 K, the field dependence of MR is complicated and the feature varies in different temperature regions. A drastic change of Δ ρ / ρ (0) was found between 80 K and 100 K, which corresponds very well to the maximum of the temperature derivative of the resistivity. This may be attributed to the spin-related scattering of electrons. (4) A comparison between the parent phase and the superconducting sample with T c = 50 K in NdFeAsO 1− x F x suggests that the electronic transport properties in the normal state cannot be easily understood with the simple multi-band model, while a picture concerning a suppression to the quasiparticle density of states at the Fermi energy is more reasonable.

We present specific heat measurements on single crystals of the pnictide superconductors NdFeAsO{sub 1-x}F{sub x} and Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2}. Low-temperature measurements on Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2} reveal that the Sommerfeld coefficient of the quasiparticle specific heat increases linearly with applied magnetic field. This is the signature of a fully gapped superconducting state as is expected for instance in the extended s-wave scenario. The large value of the normal state Sommerfeld coefficient indicates a large electronic density of states and significant mass enhancements. We determine the phase diagram using an entropy conserving construction, which does not require the choice of a resistivity criterion to define the transition temperature. Both materials display clear mean-field steps in the specific heat at T{sub c} of 47 K and 34.6 K, respectively, proving bulk superconductivity. They are characterized by a low superconducting anisotropy near T{sub c} of {Gamma} {approx} 4 for NdFeAsO{sub 1-x}F{sub x} and 2.6 for Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2}, which is promising for potential applications. We observe extraordinarily high upper critical field slopes of {mu}{sub 0}{partial_derivative}H{sub c2}{sup e}/{partial_derivative}T = -6.5 T/K and {mu}{sub 0}{partial_derivative}H{sub c2}{sup ab}/{partial_derivative}T = -17.4 T/K for Ba{sub 1-x}K{sub x}Fe{sub 2}As{sub 2} which points to themore » emergence of paramagnetic limiting effects at low temperatures. A thermodynamic analysis reveals that this material is extreme type-II with K{sub c} {approx} 100 and k{sub ab} {approx} 260.« less

The electronic states of surface adatoms in Na(Fe0.96Co0.03Mn0.01) As have been studied by low-temperature scanning tunneling spectroscopy. The spectra recorded on the adatoms display both superconducting coherence peaks and an asymmetric resonance in a larger energy scale. The Fano-type line shape of the spectra points towards a possible Kondo effect at play. The apparent energy position of the resonance peak shifts about 5 meV to the Fermi level when measured across the critical temperature, supporting that the Bogoliubov quasiparticle is responsible for the Kondo screening in the superconducting state. The tunneling spectra do not show the subgap bound states, which is explained as the weak pair breaking effect given by the weak and broad scattering potential after the Kondo screening.

Measurements of the real R and imaginary X parts of the surface impedance were performed in underdoped BaFe1.93Ni0.07As2 crystals in the frequency range 10 MHz-1.5 GHz. The establishment of the antiferromagnetic order at T-N similar to 50 K gives rise to anomalous increase of electron scattering time. Drude type conductivity yields X and R differ from each other. The increase of the real conductivity sigma(1) in the superconducting state is attributed to a rapid decrease of the quasiparticle scattering time. This result gives evidence of coexistence of superconductivity and antiferromagnetism. (C) 2014 Elsevier Ltd. All rights reserved.

We report on the single crystal growth and superconducting properties of PbTaSe2 with the non-centrosymmetric crystal structure. By using the chemical vapor transport technique, centimeter-size single crystals are successfully obtained. The measurement of temperature dependence of electrical resistivity ρ(T) in both normal and superconducting states indicates a quasi-two-dimensional electronic state in contrast to that of polycrystalline samples. Specific heat C(T) measurement reveals a bulk superconductivity with Tc 3.75 K and a specific heat jump ratio of 1.42. All these results are in agreement with a moderately electron–phonon coupled, type-II Bardeen–Cooper–Schrieffer superconductor.

We present ultrasonic measurements on superconducting BaFe 2− x Ni x As 2 crystals with x = 0.07 and x = 0.15. The elastic constants C 33 and C 44 for the underdoped crystal ( x = 0.07) show a large softening related to the structural phase transition at high temperatures. Anomalies in the sound velocity and the ultrasonic attenuation have been found at the superconducting phase transition T c = 17 K. Ultrasonic attenuation exhibits a peak at the superconducting transition in contrast with the attenuation in conventional superconductors. In the overdoped crystal ( x = 0.15) a minimum of C 66 is found at a temperature just above the superconducting temperature T c = 13 K. Superconducting energy gap values have been tentatively extracted from the longitudinal ultrasonic attenuation. Unconventional behaviour of the ultrasonic attenuation is observed in the superconducting BaFe 2− x Ni x As 2 crystals.

We report an investigation on c-axis point-contact Andreev reflection (PCAR) spectroscopy of optimally doped BaFe1.9Ni0.1As2 single crystals. The PCAR spectrum features the structures of a conductance peak with the energy scale of an electron-boson coupling mode. Quantitative analysis using a generalized Blonder-Tinkham-Klapwijk formalism with two gaps-one isotropic and another angle dependent-resolves a nodeless state in the strong-coupling limit with gap minima on the Fermi surfaces. Using the extracted superconducting energy gap value, a characteristic boson energy Omega(b) is obtained. Our results are consistent with the pairing interaction mediated by the antiferromagnetic spin fluctuations.

We used high-resolution scanning tunneling spectroscopy to study the hole-doped iron pnictide superconductor ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ (${T}_{c}=38\text{ }\text{ }\mathrm{K}$). Features of a bosonic excitation (mode) are observed in the measured quasiparticle density of states. The bosonic features are intimately associated with the superconducting order parameter and have a mode energy of $\ensuremath{\sim}14\text{ }\text{ }\mathrm{meV}$, similar to the spin resonance measured by inelastic neutron scattering. These results indicate a strong electron-spin excitation coupling in iron pnictide superconductors, similar to that in high-${T}_{c}$ copper oxide superconductors.

The vortex states on optimally doped Ba0.6K0.4Fe2As2 and underdoped Ba0.77K0.23Fe2As2 single crystals are imaged by magnetic force microscopy at various magnetic fields below 100 Oe. Local triangular vortex clusters are observed in optimally doped samples. The vortices are more ordered than those in Ba(Fe1-xCox)(2)As-2, and the calculated pinning force per unit length is about 1 order of magnitude weaker than that in optimally Co-doped 122 at the same magnetic field, indicating that the Co doping at the Fe sites induces stronger pinning. The proportion of six-neighbored vortices to the total amount increases quickly with increasing magnetic field, and the estimated value reaches 100% at several tesla. Vortex chains are also found in some local regions, which enhance the pinning force as well as the critical current density. Lines of vortex chains are observed in underdoped samples, and they may have originated from the strong pinning near the twin boundaries arising from the structural transition.

Resistivity, Hall effect, and magnetoresistance have been investigated systematically on single crystals of Ba1-xKxFe2As2 ranging from undoped to optimally doped regions. A systematic evolution of these properties has been observed. It is found that the resistivity in the normal state of Ba1-xKxFe2As2 is insensitive to the doped potassium concentration, which is very different from the electron-doped counterpart Ba(Fe1-xCox)(2)As-2, where the resistivity at 300 K reduces to the half value of the undoped one when the system is optimally doped. In stark contrast, the Hall coefficient R-H changes suddenly from a negative value in the undoped sample to a positive one with slight K doping, and it keeps lowering with further doping. We interpret this dichotomy as being due to the asymmetric scattering rate in the hole and the electron pockets with much higher mobility of the latter. The magnetoresistivity shows also a nonmonotonic doping dependence indicating an anomalous feature at about 80 to 100 K, even in the optimally doped sample, which is associated with a possible pseudogap effect. In the low-temperature region, it seems that the resistivity has similar values when superconductivity sets in, regardless of the different T-c values, which indicates that the influence of impurity scattering on superconductivity is weak. A linear feature of resistivity rho(ab) versus T was observed just above T-c for the optimally doped sample, suggesting a quantum criticality.

We report on low-temperature scanning tunneling microscopy and spectroscopy (STM) studies of the electronic structure of single-crystalline ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. Multiple superconducting gaps are observed in the density of states (DOS) and the sizes of the two dominant gaps ${\ensuremath{\Delta}}_{L}$ and ${\ensuremath{\Delta}}_{S}$ are 7.6 and 3.3 meV, respectively. The flat bottom of the DOS spectra near zero bias indicates the nodeless feature of the gaps, while the global fitting to the spectra definitely requires the anisotropy. The nodeless gaps with finite anisotropy revealed in our STM data agree well with the expectations of an extended $s$-wave superconductivity.

Magnetization and its relaxation have been measured in Ba(Fe1-xCox)(2)As-2 single crystals at various doping levels ranging from very underdoped to very overdoped regimes. A sizable magnetization-relaxation rate has been observed in all samples, indicating a moderate vortex motion and relatively small characteristic pinning energy. Detailed analysis leads to the following conclusions. (1) A prominent second-peak (SP) effect was observed in the samples around the optimal doping level (x approximate to 0.08), but it becomes invisible or very weak in the very underdoped and overdoped samples. (2) The magnetization-relaxation rate is inversely related to the transient superconducting current density revealing the nonmonotonic field and temperature dependence through the SP region. (3) A very sharp magnetization peak was observed near zero field which corresponds to a much reduced relaxation rate. (4) A weak temperature dependence or a plateau of relaxation rate was found in the intermediate-temperature region. Together with the treatment of the generalized inversion scheme, we suggest that the vortex dynamics is describable by the collective pinning model. Finally, vortex phase diagrams were drawn for all the samples showing a systematic evolution of vortex dynamics.

We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of Ba(Fe1-xCox)(2)As-2 over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons in excess of one order of magnitude in the low-doping, low-temperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between antiferromagnetism and superconductivity, while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing but also in the normal transport.

Using high-vacuum annealing treatment, the superconducting transition temperature T-c of the electron-doped Pr0.88LaCe0.12CuO4 +/-delta single crystals was successfully driven to a regime in which the T-c value continuously decreases with oxygen removal. This regime of oxygen-reduction process is hard to be understood according to the previous knowledge of this material. In addition, a remarkable relaxation of T-c over time was observed at room temperature, indicating that the variation in T-c is closely related to the rearrangement of oxygen in the sample. In combination with previous studies, it can be concluded that, in the whole process of oxygen reduction applied on Pr0.88LaCe0.12CuO4 +/-delta, although the strength of antiferromagnetic (fluctuation) correlations does play a role, T-c is dominated by the disorder effect of CuO2 plane either on copper sites or oxygen sites, which leads to a strong depairing effect.

We have measured the magnetic field and temperature dependence of specific heat on Bi2Sr2� xLaxCuO6þ� single crystals in wide doping and temperature regions. The superconductivity related specific-heat coefficientsc and entropy Ssc are determined. It is found thatsc has a humplike anomaly at Tc and behaves as a long tail which persists far into the normal state for the underdoped samples, but for the heavily overdoped samples the anomaly ends sharply just near Tc. Interestingly, we found that the entropy associated with superconductivity is roughly conserved when and only when the long tail part in the normal state is taken into account for the underdoped samples, indicating the residual superconductivity above Tc.

By using solid state reaction method, we have synthesized the Pd-doped superconductor SrFe2-xPdxAs2. The systematic evolution of the lattice constants indicated that the Fe ions were successfully replaced by the Pd. By increasing the doping content of Pd, the antiferromagnetic state of the parent phase is suppressed and superconductivity is induced at a doping level of about x=0.05. Superconductivity with a maximum transition temperature Tc of about 8.7 K was achieved at the doping level of x = 0.15. The general phase diagram of Tc versus x was obtained and found to be similar to the case of Ni and Co doping to the Fe sites. Our results suggest that superconductivity can be easily induced in the FeAs family by adding electrons into the system, regardless with the transition metals of 3d or higher d-orbital electrons.

Low temperature specific heat (SH) was measured on the FeAs-based superconducting single crystals Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ and high pressure synthesized polycrystalline samples SmFeAsO$_{0.9}$F$_{0.1}$. It is found that the sharp SH anomaly $\Delta C/T|_{T_c}$ in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ reaches an unexpected high value of 98 mJ/mol K$^2$, about one order of magnitude larger than that of SmFeAsO$_{0.9}$F$_{0.1}$ ($6\sim8$ mJ/mol K$^2$) samples, suggesting very high normal state quasiparticle density of states in FeAs-122 than in FeAs-1111. Furthermore, we found that the electronic SH coefficient $\gamma_e(T)$ of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ is weakly temperature dependent and increases almost linearly with the magnetic field in low temperature region, which may indicate that the hole-doped FeAs-122 system contains a dominant component with a full superconducting gap, although we cannot rule out the possibility of a small component with anisotropic or nodal gap. A detailed analysis reveals that the $\gamma_e(T)$ of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ cannot be fitted with a single gap of s-wave symmetry probably due to the multigap effect. These results indicate clear difference between the properties of the superconducting state of the holed-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ and the F-doped LnFeAsO (Ln = rare earth elements) systems, which we believe is originated from the complex Fermi surface structures in different systems.

Physical properties have been comprehensively investigated on the noncentrosymmetric superconductor ${\text{Ru}}_{7}{\text{B}}_{3}$ with ${T}_{C}=3.3\text{ }\text{K}$. It is found that the superfluid comprises a major component with a full-gap feature instead of a nodal-like pairing symmetry as revealed by specific-heat and lower-critical-field measurements. Combining with resistivity (${\ensuremath{\rho}}_{xx}$ and ${\ensuremath{\rho}}_{xy}$) and electronic band-structure calculation, the superconducting and normal-state physical parameters were determined by a self-consistent analysis. It is found that ${\text{Ru}}_{7}{\text{B}}_{3}$ may belong to a single band superconductor with energy gap of 0.5 meV and could be categorized into type-II superconductor with weak electron-phonon coupling. An unusual ``kink'' feature is clearly observed in the field induced broadening of the resistivity curves. Possible reasons are given for this unusual two-step transition.

We report a detailed investigation on the lower critical field H{c1} of the superconducting Ba0.6K0.4Fe2As2 (122) single crystals. A pronounced kink is observed on the H{c1}(T) curve, which is attributed to the existence of two superconducting gaps. By fitting the data H{c1}(T) to the two-gap BCS model in the full temperature region, a small gap of Delta{a}(0)=2.0+/-0.3 meV and a large gap of Delta{b}(0)=8.9+/-0.4 meV are obtained. The in-plane penetration depth lambda{ab}(0) is estimated to be 105 nm corresponding to a rather large superfluid density, which points to the breakdown of the Uemura plot in 122 superconductors.

The temperature-dependent resistivity of ${\text{Ba}}_{1\ensuremath{-}x}{\text{K}}_{x}{\text{Fe}}_{2}{\text{As}}_{2}$ ($x=0.23$, 0.25, 0.28, and 0.4) single crystals and the angle-dependent resistivity of superconducting ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$ single crystals were measured in magnetic fields up to 9 T. The data measured on samples with different doping levels revealed very high upper critical fields, which increase with the transition temperature, and a very low superconducting anisotropy ratio $\ensuremath{\Gamma}={H}_{c2}^{ab}/{H}_{c2}^{c}\ensuremath{\approx}2$. By scaling the resistivity within the framework of the anisotropic Ginzburg-Landau theory, the angle-dependent resistivity of the ${\text{Ba}}_{0.6}{\text{K}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$ single crystal measured with different magnetic fields at a certain temperature collapsed onto one curve. As the only scaling parameter, the anisotropy $\ensuremath{\Gamma}$ was alternatively determined for each temperature and was found to be between two and three.

By measuring the dynamic and traditional magnetization relaxations we investigate the vortex dynamics of the recently discovered superconductor SmFeAsO(0.9)F(0.1) with T(c) = 55 K. It is found that the relaxation rate is rather large reflecting a small characteristic pinning energy. Moreover it shows a weak temperature dependence in wide temperature region, which resembles the behavior of the cuprate superconductors. Combined with the resistivity data under different magnetic fields, a vortex phase diagram is obtained. Our results strongly suggest that the model of collective vortex pinning applies to this superconductor very well.

We present point-contact spectroscopy data for junctions between a normal metal and the newly discovered F-doped superconductor LaO$_{0.9}$F$_{0.1-\delta}$FeAs (F-LaOFeAs). A zero-bias conductance peak was observed and its shape and magnitude suggests the presence of Andreev bound states at the surface of F-LaOFeAs, which provides a possible evidence of an unconventional pairing symmetry with a nodal gap function. The maximum gap value $\Delta_0\approx3.9\pm0.7$meV was determined from the measured spectra, in good agreement with the recent experiments on specific heat and lower critical field., Comment: 5 pages, 4 figures

We report measurements of the spin polarization (\textbf{\textit{P}}) of the concentrated magnetic semiconductor EuS using both zero-field and Zeeman-split Andreev reflection spectroscopy (ARS) with EuS/Al planar junctions. The zero-field ARS spectra are well described by the modified (spin-polarized) BTK model with expected superconducting energy gap and actual measurement temperature (no additional spectral broadening). The fittings consistently yield \textbf{\textit{P}} close to 80% regardless of the barrier strength. Moreover, we performed ARS in the presence of a Zeeman-splitting of the quasiparticle density of states in Al. To describe the Zeeman-split ARS spectra, we develop a theoretical model which incorporates the solution to the Maki-Fulde equations into the modified BTK analysis. The method enables the determination of the magnitude as well as the sign of \textbf{\textit{P}} with ARS, and the results are consistent with those from the zero-field ARS. The experiments extend the utility of field-split superconducting spectroscopy from tunnel junctions to Andreev junctions of arbitrary barrier strengths., 6 pages, 4 figures

We have measured ac magnetic susceptibility (chi=chi(')+i chi '') of the electron-doped superconducting Pr0.88LaCe0.12CuO4-delta with applied magnetic field (H) either parallel or perpendicular to the CuO2 plane (H parallel to ab-plane or H parallel to c-axis). For H parallel to ab-plane, a peak in the temperature dependence of the screening current is revealed as a dip in the real part of the ac susceptibility chi. The temperature at which this peak anomaly occurs decreases with increasing field and lies well below the irreversibility line in the H-T phase diagram. This peak effect may arise from the phase transition of Josephson vortices from a quasiordered vortex lattice to a disordered glass phase.

We use in-plane tunneling spectroscopy to study the temperature dependence of the local superconducting gap $\Delta(T)$ in electron-doped copper oxides with various $T_c$'s and Ce-doping concentrations. We show that the temperature dependence of $\Delta(T)$ follows the expectation of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, where $\Delta(0)/k_{B}T_{c}\approx1.72\pm 0.15$ and $\Delta(0)$ is the average superconducting gap across the Fermi surface, for all the doping levels investigated. These results suggest that the electron-doped superconducting copper oxides are weak coupling BCS superconductors., Comment: 5 pages, 4 figures

We report a systematic investigation on c-axis point-contact Andreev reflection (PCAR) in BaFe2−xNixAs2 superconducting single crystals from underdoped to overdoped regions (0.075≤ x ≤0.15). At low temperatures, an in-gap sharp peak at low-bias voltage is observed in PCAR for overdoped samples, in contrast to the case of underdoped junctions, in which an in-gap plateau is observed. The variety of the conductance spectra with doping can be well described by using a generalized Blonder–Tinkham–Klapwijk formalism with an angle-dependent gap. This gap shows a clear crossover from a nodeless in the underdoped side to a nodal feature in the overdoped region. This result provides evidence of the doping-induced evolution of the superconducting order parameter when the inter-pocket and intra-pocket scattering are tuned through doping, as expected in the s± scenario.

We report scanning tunneling microscopy/spectroscopy (STM/STS) studies on iron-based superconductors of Ba1−xKxFe2As2 and nearly optimally doped Fe(Te,Se). Mode-like features were observed universally outside the superconducting gaps in the tunneling spectra, which are similar to our previous observations in other samples and can be ascribed to the interaction between electrons and spin excitations. Furthermore, an almost linear relationship between the superconducting gaps and the superconducting transition temperatures was noted and should also be taken into account in understanding the mechanism of iron-based superconductors.

The point-contact Andreev reflection spectroscopy for a simple BCS type-II superconductor was investigated in pure niobium (Nb). The small broadening factor of Gamma/Delta(0)< 0.09 was achieved by careful preparation of the junctions. Then we attempted to use the recently proposed two-channel model [Phys. Rev. B 72, 012502 (2005)] to explain the spectra measured in various magnetic fields for such low-Gamma case. It was found that this rigid vortex core model is appropriate below a crossover field H-*, above which it is not adequate due to the onset of substantial vortex overlapping effect.

We have fabricated Nb /magnetic tunnel junction (MTJ) structure to investigate superconductivity-induced impact on magnetization states of the proximal ferromagnet. By sweeping temperature in a parallel or antiparallel magnetization state, a thermal hysteresis in MTJ resistance was observed at temperature below the Nb superconducting critical temperature (Tc). In the parallel configuration, a Barkhausen-like jump in junction resistance arises when the magnetization of free layer is close to switching from parallel to antiparallel configuration. We attribute the hysteresis and Barkhausen-like jump to the pinning effect of Nb on the proximal magnetic domains and estimate the magnitude of the moment change of the free layer responsible for the resistance change resulting from Barkhausen-like jump.

In order to understand why the inter- and intra-granular current densities of polycrystalline superconducting oxypnictides differ by three orders of magnitude, we have conducted combined magneto-optical and microstructural examinations of representative randomly oriented polycrystalline Nd and Sm single-layer oxypnictides. Magneto-optical images show that the highest J(c) values are observed within single grains oriented with their c axes perpendicular to the observation plane, implying that the intragranular current is anisotropic. The much lower intergranular J(c) is at least partially due to many extrinsic factors, because cracks and a ubiquitous wetting As-Fe phase are found at many grain boundaries. However, some grain boundaries are structurally clean under high resolution transmission electron microscopy examination. Because the whole-sample global J(c) (5 K) values of the two samples examined are 1000-4000 A cm(-2), some 10-40 times higher than that found in random polycrystalline YBa(2)Cu(3)O(7-x), it appears that the dominant obstruction to intergranular current flow of many present samples is extrinsic, though some intrinsic limitation of current flow across grain boundaries cannot yet be ruled out.

We report the results of angle dependent resistivity of NdFeAsO$_{0.82}$F$_{0.18}$ single crystals in the superconducting state. By doing the scaling of resistivity within the frame of the anisotropic Ginzburg-Landau theory, it is found that the angle dependent resistivity measured under different magnetic fields at a certain temperature can be collapsed onto one curve. As a scaling parameter, the anisotropy $\Gamma$ can be determined for different temperatures. It is found that $\Gamma(T)$ increases slowly with decreasing temperature, varying from $\Gamma \simeq$ 5.48 at T=50 K to $\Gamma \simeq$ 6.24 at T=44 K. This temperature dependence can be understood within the picture of multi-band superconductivity., Comment: 7 pages, 4 figures

By using flux method, we have grown single crystals of NdFeAsO0.82F0.18 at ambient pressures. Resistive measurements reveal a surprising discovery that the anisotropy Γ=(mc∕mab)1∕2 is below 5, which is much smaller than the theoretically calculated value. The data measured up to 400K show a curved feature, which prevents a conjectured linear behavior for an unconventional metal. The upper critical fields determined based on the Werthamer–Helfand–Hohenberg formula [N. R. Werthamer et al., Phys. Rev. 147, 295 (1966)] are Hc2ab(T=0K)≈304T and Hc2c(T=0K)≈62–70T. These high values indicate very encouraging applications of the new superconductors.