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 $Γ$ can be determined for different temperatures. It is found that $Γ(T)$ increases slowly with decreasing temperature, varying from $Γ\simeq$ 5.48 at T=50 K to $Γ\simeq$ 6.24 at T=44 K. This temperature dependence can be understood within the picture of multi-band superconductivity.
We use a magnetic force microscope (MFM) to investigate single vortex pinning and penetration depth in NdFeAsO$_{1-x}$F$_x$, one of the highest-$T_c$ iron-based superconductors. In fields up to 20 Gauss, we observe a disordered vortex arrangement, implying that the pinning forces are stronger than the vortex-vortex interactions. We measure the typical force to depin a single vortex, $F_{\mathrm{depin}} \simeq 4.5$ pN, corresponding to a critical current up to $J_c \simeq 7 \times 10^5$ A/cm$^2$. Furthermore, our MFM measurements allow the first local and absolute determination of the supercond…
Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied ballistic superconductor - normal metal - superconductor (SnS) contacts in layered oxypnictide superconductors NdFeAsO$_{0.6}$H$_{0.36}$ with critical temperatures $T_c = 45-48$ K. We directly determined the magnitude of two bulk superconducting order parameters, the large gap $Δ_L \approx 10.4$ meV, and a possible small gap $Δ_S \approx 1.8$ meV, and their temperature dependence. Additionally, a resonant coupling with a characteristic bosonic mode was observed. The boson energy at 4.2 K, $\varepsilon_0 = 10.5…
We use angle resolved photoemission spectroscopy (ARPES) to study the momentum dependence of the superconducting gap in NdFeAsO1-xFx single crystals. We find that the Gamma hole pocket is fully gapped below the superconducting transition temperature. The value of the superconducting gap is 15 +- 1.5 meV and its anisotropy around the hole pocket is smaller than 20% of this value. This is consistent with an isotropic or anisotropic s-wave symmetry of the order parameter or exotic d-wave symmetry with nodes located off the Fermi surface sheets. This is a significant departure from the situation i…
Identifying the symmetry of the superconducting order parameter in the recently-discovered ferro-oxypnictide family of superconductors, RFeAsO$_{1-x}$F$_{y}$, where $R$ is a rare earth, is a high priority. Many of the proposed order parameters have internal $π$ phase shifts, like the d-wave order found in the cuprates, which would result in direction-dependent phase shifts in tunnelling. In dense polycrystalline samples, these phase shifts in turn would result in spontaneous orbital currents and magnetization in the superconducting state. We perform scanning SQUID microscopy on a dense polycry…
The transport behavior and superconducting transition temperature Tc of NdFe1-yRuyAsO0.89F0.11 have been studied for various y values. Because Ru impurities are isoelectronic to host Fe atoms, we basically expect that the number of electrons does not change with y, at least in the region of small y values. The results indicate that the rate of Tc suppression by Ru atoms is too small to be explained by the pair breaking effect of nonmagnetic impurities expected for the S+- symmetry, confirming our previous results for Co doping.
Local structure of NdFeAsO$_{1-x}$F$_{x}$ ($x$=0.0, 0.05, 0.15 and 0.18) high temperature iron pnictide superconductor system is studied using arsenic $K$-edge extended x-ray absorption fine structure measurements as a function of temperature. Fe-As bondlength shows only a weak temperature and F-substitution dependence, consistent with the strong covalent nature of this bond. The temperature dependence of the mean-square relative-displacements of the Fe-As bondlength are well described by the correlated-Einstein model for all the samples, but with different Einstein-temperatures for the superc…
Both DC and AC magnetization measurements were performed on the NdFeAsO0.88F0.12 superconductor to investigate the influence of magnetism on the superconducting properties of this system. The crossover of the ZFC and FC magnetic susceptibility curves under 7.5KOe was observed. The imaginary component of the first harmonics of the AC magnetic susceptibility, increases with the increasing DC field below 10K and shows frequency dependency under 7.5KOe at low temperature. The paramagnetism of Nd3+ ions tilts the magnetic hysteresis loops and broadens the hysteresis width. After correction for the …
High resolution synchrotron X-ray powder diffraction (SXRPD) was used to study the temperature dependence of the oxygen deficient NdFeAsO$_{0.85}$ superconducting compound. By employing a dense temperature sampling we have managed to reveal unnoticed structural modifications that start around $\sim$180K, and disappear at the transition temperature. The data show minor changes of the structural characteristics in the Nd-O charge reservoir layer while in the superconducting Fe-As layer the FeAs$_{4}$ tetrahedron shows gradual modifications below $\sim$180K, which suddenly disappear at T$_{\rm c}…
Among the widely studied superconducting iron-pnictide compounds belonging to the Ln1111 family (with Ln a lanthanide), a systematic investigation of the crossover region between the superconducting and the antiferromagnetic phase for the Ln = Nd case has been missing. We fill this gap by focusing on the intermediate doping regime of NdFeAsO(1-x)F(x) by means of dc-magnetometry and muon-spin spectroscopy measurements. The long-range order we detect at low fluorine doping is replaced by short-range magnetic interactions at x = 0.08, where also superconductivity appears. In this case, longitudin…
The discovery of superconductivity with a critical temperature exceeding 55 K in the iron-oxypnictides and related compounds has quite suddenly given the community a new set of materials - breaking the tyranny of copper. This new class of materials raises fundamental questions related to the origin of the electron pairing in the superconducting state and to the similarity to superconductivity in the cuprates. Here, we report spatially resolved measurements using scanning tunneling microscopy/spectroscopy (STM/STS) of the newly discovered iron-based layered superconductor NdFeAsO0.86F0.14 (Tc =…
The lattice properties at low temperatures of two samples of NdFeAsO1-xFx (x=0.05 and 0.25) have been examined in order to investigate possible structural phase transition that may occur in the optimally doped superconducting sample with respect to the non-superconducting low-F concentration compound. In order to detect small modifications in the ion displacements with temperature micro-Raman and high resolution synchrotron powder diffraction measurements were carried out. No increase of the width of the (220) or (322) tetragonal diffraction peaks and microstrains could be found in the superco…
We investigated transport and magnetic properties of NdFeAsO$_{0.9}$F$_{0.1}$ single crystal under hydrostatic pressures up to 50\,GPa. The ambient pressure superconductivity at $T_{c} \sim$ 45.4\,K is fully suppressed at $P_{c} \sim$ 21 GPa. Upon further increase of the pressure, the ferromagnetism associated with the order of rare-earth subsystem is induced at the border of superconductivity. Our finding is supported by the hysteresis in the magnetization $M$($H$) loops and the strong increase in the field cooled data, $M$($T$), toward low temperatures. We also show that the temperature evol…
The recent realisations of hydrogen doped $Ln$FeAsO ($Ln$=Nd and Sm) superconducting epitaxial thin films call for further investigation of their structural and electrical transport properties. Here, we report on the microstructure of a NdFeAs(O,H) epitaxial thin film and its temperature, field, and orientation dependencies of the resistivity and the critical current density $J_{\rm c}$. The superconducting transition temperature $T_{\rm c}$ is comparable to NdFeAs(O,F). Transmission electron microscopy investigation supported that hydrogen is homogenously substituted for oxygen. A high self-f…
We investigate the temperature dependence of the lower critical field $H_{c1}(T)$, the field at which vortices penetrate into the sample, of a high-quality fluorine-doped NdFeAsO single crystal under static magnetic fields $H$ parallel to the $c$-axis. The temperature dependence of the first vortex penetration field has been experimentally obtained and pronounced changes of the $H_{c1}$(T) curvature are observed, which is attributed to the multiband superconductivity. Using a two-band model with $s$-wave-like gaps, the temperature-dependence of the lower critical field $H_{c1}(T)$ can be well …
Measurements of the in-plane magnetic field penetration depth λ_{ab} in Fe-based superconductors with the nominal composition SmFeAsO_0.85 (T_c\simeq52K) and NdFeAsO_0.85 (T_c\simeq51K) were carried out by means of muon-spin-rotation. The absolute values of λ_{ab} at T=0 were found to be 189(5)nm and 195(5)nm for Sm and Nd substituted samples, respectively. The analysis of the magnetic penetration depth data within the Uemura classification scheme, which considers the correlation between the superconducting transition temperature T_c and the effective Fermi temperature T_F, reveal that both fa…
We present heat capacity measurements of the upper critical fields of single-crystal NdFeAsO1-xFx. In zero-magnetic field a clear step in the heat capacity is observed at Tc = 47K . In fields applied perpendicular to the FeAs-layers the step broadens significantly whereas for the in-plane orientation the field effects are small. This behavior is reminiscent of the CuO2-high-Tc superconductors and is a manifestation of pronounced fluctuation effects. Using an entropy conserving construction we determine the transition temperatures in applied fields and the upper critical field slopes of dHc2,a …
We use angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic properties of the newly discovered oxypnictide superconductor, NdFeAsO_{1-x}F_x. We find a well-defined Fermi surface that consists of a large hole pocket at the Brillouin zone center and a smaller electron pocket in each corner of the Brillouin zone. The overall location and shape of the Fermi surface agrees reasonably well with calculations. The band dispersion is quite complicated with many flat bands located just below the chemical potential. We observe a superconducting gap of 20 meV, which indicates th…
The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at T_S = 142 K and the onset of antiferromagnetic (AFM) Fe order below T_N = 137 K. The Fe moments order AFM in the well-known stripe-like structure in the (ab) plane, but change from AFM to ferromagnetic (FM) arrangement along the c direction below T* = 15 K accompanied by the onset of Nd AFM order b…
The flux pinning mechanism of NdO0.82F0.18FeAs superconductor made under high pressure, with a critical temperature, Tc, of 51 K, has been investigated in detail in this work. The field dependence of the magnetization and the temperature dependence of the magnetoresistivity were measured in fields up to 13 T. The field dependence of the critical current density, Jc(B), was analyzed within the collective pinning model. A crossover field, Bsb, from the single vortex to the small vortex bundle pinning regime was observed. The temperature dependence of Bsb(T) is in good agreement with the delta-l …
We report plane-polarized Raman scattering spectra of iron oxypnictide superconductor NdFeAsO$_{1-x}$F$_x$ single crystals with varying fluorine $x$ content. The spectra exhibit sharp and symmetrical phonon lines with a weak dependence on fluorine doping $x$. The temperature dependence does not show any phonon anomaly at the superconducting transition. The Fe related phonon intensity shows a strong resonant enhancement below 2 eV. We associate the resonant enhancement to the presence of an interband transition around 2 eV observed in optical conductivity. Our results point to a rather weak cou…
Anisotropic superconducting properties including the upper critical field Hc2, thermal activation energy U0, and critical current density Jc are systematically studied in a large Ca1-xLaxFeAs2 single crystal (x ~ 0.18). The obtained Hc2 bears a moderate anisotropy gamma of approximately 2-4.2, located between those of '122' Ba1-xKxFe2As2 (1 < gamma < 2) and '1111' NdFeAsO1-xFx (5 < gamma < 9.2). Both the magnitude of U0 and its field dependence are very similar to those of NdFeAsO1-xFx, also indicating anisotropic superconductivity. Moreover, high and anisotropic Jc's exceeding 10^5 A/cm2 have…
High-quality single crystals of NdFeAsO$_{0.65}$F$_{0.35}$ (the transition temperature $T_{\rm c} \simeq 30.6$~K) were studied in zero-field (ZF) and transverse-field (TF) muon-spin rotation/relaxation ($μ$SR) experiments. An upturn in muon-spin depolarization rate at $T\lesssim 3$~K was observed in ZF-$μ$SR measurements and it was associated with the onset of ordering of Nd electronic moments. Measurements of the magnetic field penetration depth ($λ$) were performed in the TF geometry. By applying the external magnetic field $B_{\rm ex}$ parallel to the crystallographic $c$-axis ($B_{\rm ex}\…
We present results of transport and magnetic properties and heat capacity measurements on polycrystalline CeFeAsO, PrFeAsO, and NdFeAsO. These materials undergo structural phase transitions, spin density wave-like magnetic ordering of small moments on iron, and antiferromagnetic ordering of rare earth moments. The temperature dependence of the electrical resistivity, Seebeck coefficient, thermal conductivity, Hall coefficient, and magnetoresistance are reported. The magnetic behavior of the materials have been investigated using Mossbauer spectroscopy and magnetization measurements. Transport …
Hall effect and magnetoresistance have been measured on single crystals of $NdFeAsO_{1-x}F_{x}$ with x = 0 ($T_c$ = 0 $ $K) and x = 0.18 ($T_c$ = 50 $ $K). For the undoped samples, strong Hall effect and magnetoresistance with strong temperature dependence were found below about 150 K. The magnetoresistance was found to be as large as 30% at 15 K at a magnetic field of 9 T. From the transport data we found that the transition near 155 K was accomplished in two steps: first one occurs at 155 K which may be associated with the structural transition, the second one takes place at about 140 K whic…
We have investigated experimentally how properties of NdFeAsO0.8F0.2 superconductor affected due to the substitution of the Ca2+/Nd3+ doping. Based on the XRD data refinement, various structural parameters such as lattice parameters, bond angles, bond length, and etc. were studied. We have determined the upper limit of the calcium solubility in the NdFeAsO0.8F0.2 phase and it is restricted to x=0.05. Also, we have found that the lattice parameters and the cell volume decreased by increasing the calcium content. According to the XRD data analysis, we have argued that these reductions are due to…
We calculate the flux flow resistivity in a superconductor with multiple s-wave superconducting gaps. Our result agrees well with anomalous field dependence of the resistivity recently observed in the two-gap superconductor MgB$_2$.
Photoemission spectroscopy with low-energy tunable photons on oxygen-deficient iron-based oxypnictide superconductors NdFeAsO0.85 (Tc=52K) reveals a distinct photon-energy dependence of the electronic structure near the Fermi level (EF). A clear shift of the leading-edge can be observed in the superconducting states with 9.5 eV photons, while a clear Fermi cutoff with little leading-edge shift can be observed with 6.0 eV photons. The results are indicative of the superconducting gap opening not on the hole-like ones around Gamma (0,0) point but on the electron-like sheets around M(pi,pi) point…
The crystal structure of LnFeAsO$_{1-y}$ (Ln = La, Nd) has been studied by the powder neutron diffraction technique. The superconducting phase diagram of NdFeAsO$_{1-y}$ is established as a function of oxygen content which is determined by Rietveld refinement. The small As-Fe bond length suggests that As and Fe atoms are connected covalently. FeAs$_4$-tetrahedrons transform toward a regular shape with increasing oxygen deficiency. Superconducting transition temperatures seem to attain maximum values for regular FeAs$_4$-tetrahedrons.
We report the growth of Nb2Pd0.73S5.7 superconducting single crystal fibers via slow cooling solid state reaction method. Superconducting transition temperature (Tc ~ 6.5K) is confirmed from magnetization and transport measurements. A comparative study is performed for determination of superconducting anisotropy, Γ, via conventional method (by taking ration of two superconducting parameters) and scaling approach method. Scaling approach, defined within the framework of the Ginzburg-Landau theory is applied to the angular dependent resistivity measurements to estimate the anisotropy. The value …
The origin of AC resistivity in the high Tc superconductors is not addressed adequately in literature. We found out, Josephson Junction (JJ) decoupling is the main origin of the AC resistivity in high Tc superconductors. We have measured the AC resistivity in the superconductors in the low frequency range by measuring the complex AC impedance of superconducting YBa2Cu3O7 (YBCO) polycrystalline samples. Our data shows that under certain conditions when the number density of Josephson Junctions (JJ) present in the sample and the JJ critical current crosses a threshold value, AC resistivity in th…
Taking advantage of a novel multiprobe setup we have measured, on a unique sample, the ac-magnetic susceptibility, the resistivity, the ac-specific heat and the thermopower of the superconductor heavy fermion CeCu$_{2}$Si$_{2}$ under pressure up to 5.1 GPa. At the superconducting transition temperature $T_c$, the Meissner signal corresponds to that expected for the sample volume and coincides with the specific heat jump and the resistive transition completion temperatures. Differing from previous observations, here the susceptibility measurements did not reveal any anomaly in the vicinity of t…
We have studied the magnetic field dependence of the temperature derivative of the resistivity, dr(H,T)/dT, of a number of Y1-xCaxBa2(Cu1-yZny)3O7-d crystalline thin films over a wide range of sample compositions. From the analysis of the temperature derivative data we have been able to distinguish quite clearly between two characteristic temperature scales, (a) the onset of strong superconducting fluctuation temperature, Tscf and (b) the pseudogap temperature, TPG. Significantly different characteristic features of dr(H,T)/dT at Tscf and at TPG imply that Tscf and TPG have different physical …
Single crystals of K0.80Fe1.76Se2.02 were suscessfully grown from a ternary solution. We show that although crystals form when cooling a near stoichiometric melt, crystals are actually growing out of a ternary solution that remains liquid to at least 850 C. We investigated their chemical composition, anisotropic magnetic susceptibility and resistivity, specific heat, thermoelectric power, London penetration depth and flux penetration via magneto-optical imaging. Whereas the samples appear to be homogeneously superconducting at low temperatures, there appears to be a broadened transtion range c…
The microwave complex surface impedance Z_s of Y(Ni_{1-x}Pt_x)_2B_2C was measured at 0.5 K under magnetic fields H up to 7T. In nominally pure YNi_2B_2C, which is a strongly anisotropic s-wave superconductor, the flux flow resistivity ρ_f calculated from Z_s was twice as large as that expected from the conventional normal-state vortex core model. In Pt-doped samples where the gap anisotropy is smeared out, the enhancement of ρ_f is reduced and ρ_f approaches to the conventional behavior. These results indicate that energy dissipation in the vortex core is strongly affected by the anisotropy of…
We report measurements of the temperature and pressure dependence of the electrical resistivity of single crystalline iron-based chalcogenide Cs0.8Fe2Se2. In this material superconductivity Tc~30K develops from a normal state with extremely large resistivity. At ambient pressure a large "hump" in the resistivity is observed around 200K. Under pressure, the resistivity decreases by two orders of magnitude, concomitant with a sudden Tc suppression around p~8GPa. Even at 9GPa a metallic resistivity state is not recovered, and the ρ(T) "hump" is still detected. A comparison of the data measured up…
Ta$_{2}$S$_{2}$C compound undergoes superconducting transitions at $T_{cl} = 3.60 \pm 0.02$ K and $T_{cu} = 9.0 \pm 0.2$ K. The nature of successive superconducting transitions has been studied from electrical resistivity, linear and nonlinear AC magnetic susceptibilities. The resistivity $ρ$ at $H$ = 0 shows a local maximum near $T_{cu}$, a kink-like behavior around $T_{cl}$, and reduces to zero at below $T_{0}$ = 2.1 K. The $\ln T$ dependence of $ρ$ is observed at $H$ = 50 kOe at low temperatures, which is due to two-dimensional weak-localization effect. Below $T_{cu}$ a two-dimensional supe…
We measured the microwave surface impedances and obtained the superfluid density and flux flow resistivity in single crystals of a phosphor-doped iron-based superconductor SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ single crystals ($x=0.30$, $T_c=25 \mathrm{K}$). At low temperatures, the superfluid density, $n_s (T)/n_s(0)$, obeys a power law, $n_s (T)/n_s (0)=1-C(T/T_c)^n$, with a fractional exponent of $n=1.5$-1.6. The flux flow resistivity was significantly enhanced at low magnetic fields. These features are consistent with the presences of both a gap with line nodes and nodeless gaps with a deep mini…
The temperature dependent resistivity of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (x = 0.23, 0.25, 0.28 and 0.4) single crystals and the angle dependent resistivity of superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystals were measured in magnetic fields up to 9 T. The measurements of temperature dependent resistivity for samples with different doping levels revealed very high upper critical fields which increase with the transition temperature monotonously, and a very low superconducting anisotropy ratio $Γ=H_{c2}^{ab}/H_{c2}^c \approx$ 2. By scaling the resistivity in the frame of the anisotrop…
We present an experimental study of the anisotropic resistivity of superconducting La(2-x)Ba(x)CuO(4) with x=0.095 and transition temperature Tc=32 K. In a magnetic field perpendicular to the CuO(2) layers, H(perp), we observe that the resistivity perpendicular to the layers, ρ(perp), becomes finite at a temperature consistent with previous studies on very similar materials; however, the onset of finite parallel resistivity, ρ(par), occurs at a much higher temperature. This behavior contradicts conventional theory, which predicts that ρ(perp) and ρ(par) should become finite at the same tempera…
We report the precise pressure dependence of FeSe from a resistivity measurement up to 4.15 GPa. Superconducting transition temperature (T_c) increases sensitively under pressure, but shows a plateau between 0.5-1.5 GPa. The maximum T_c, which is determined by zero resistance, is 21 K at approximately 3.5 GPa. The onset value reaches ~37 K at 4.15 GPa. We also measure the nuclear spin-lattice relaxation rate 1/T_1 under pressure using 77Se--NMR measurement. 1/T_1 shows that bulk superconductivity is realized in the zero-resistance state. The pressure dependence of 1/T_1T just above T_c shows a…
The electrical resistivity of polycrystalline Bi2Sr2Ca2Cu3O10-x (Bi-2223) was measured vs. applied magnetic fields up to 0.45 T, applied currents up to 1 A, and temperature from liquid nitrogen temperature (LN2) to room temperature. In the lowest temperature region, the only truly zero resistivity was observed when the magnetic field was zero; otherwise, a quadratic dependence on the magnetic field occurred. Hysteresis was noted at the higher currents. Current vs. voltage curves in this region revealed a non-ohmic resistivity. In the transition region to the mixed state, indications of negativ…
The electrical resistivity (Rxx) and Hall resistivity (Rxy) of LaFeAsO1-xFx have been measured over a wide fluorine doping range 0 =< x =< 0.14 using 60 T pulsed magnets. While the superconducting phase diagram (Tc, x) displays the classic dome-shaped structure, we find that the resistive upper critical field (Hc2) increases monotonically with decreasing fluorine concentration, with the largest Hc2 >= 75 T for x = 0.05. This is reminiscent of the composition dependence in high-Tc cuprates and might correlate with opening of a pseudo-gap in the underdoped region. Further, the temperature depend…
Electrons confined to two dimensions display an unexpected diversity of behaviors as they are cooled to absolute zero. Noninteracting electrons are predicted to eventually "localize" into an insulating ground state, and it has long been supposed that electron correlations stabilize only one other phase: superconductivity. However, many two-dimensional (2D) superconducting materials have shown surprising evidence for metallic behavior, where the electrical resistivity saturates in the zero-temperature limit, the nature of this unexpected metallic state remains under intense scrutiny. We report …
The transport and superconducting properties of Ba_{1-x}K_xFe_2As_2 single crystals with T_c = 31 K were studied. Both in-plane and out-of plane resistivity was measured by modified Montgomery method. The in-plane resistivity for all studied samples, obtained in the course of the same synthesis, is almost the same, unlike to the out-of plane resistivity, which differ considerably. We have found that the resistivity anisotropy γ=ρ_c /ρ_{ab} is almost temperature independent and lies in the range 10-30 for different samples. This, probably, indicates on the extrinsic nature of high out-of-plane …
We report the temperature- and magnetic-field-dependent resistivity of MgB2 sintered at high temperature and high pressure condition. The superconducting transition width for the resistivity measurement was about 0.4 K, and the low-field magnetization showed a sharp superconducting transition with a transition width of about 1 K. The resistivity in the normal state roughly followed T^2 behavior with smaller residual resistivity ratio (RRR) of 3 over broad temperature region above 100 K rather than reported T^3 behavior with larger RRR value of ~ 20 in the samples made at lower pressures. Also,…
The normal state resistivtity, upper critical field $H_{c2}$ and Hall coefficient $R_H$ in superconducting perovskite $MgCNi_3$ ($T_c \approx 8 K$) have been studied. Above 70 K, $ρ(T)$ fits well curve predicted by Bloch-Grüneisen theory consistently with electron-phonon scattering. $H_{c2}(0)$ was estimated to be about 15.0 Tesla within the weak-coupling BCS theory, and the superconducting coherence length $ξ(0)$ is approximately 47 Å. $R_H$ of $MgCNi_3$ is negative for the whole temperature range which definitely indicates that the carrier in $MgCNi_3$ is electron-type. $R_H$ is temperature …
Single crystals of FeSe_1-x_Te_x_ (0.5=<x=<1) have been grown by the Bridgman method. After annealing them at 400 deg for 100 h in vacuum, single crystals of x=0.5-0.9 have exhibited bulk superconductivity. Anisotropic properties of the electrical resistivity and upper critical field, H_c2_, have been investigated for the single-crystal FeSe_1-x_Te_x_ with x=0.6. It has been found that the in-plane resistivity, Rho_ab_, shows a metallic temperature-dependence, while the out-of-plane resistivity, Rho_c_, shows a broad maximum around 100 K. The resistivity ratio, Rho_ab_/Rho_c_, is 44 and 70 at …
Resistivity Rho(T), Hall coefficient RH(T), superconducting transition temperature Tc and slopes of the upper critical field dHc2/dT were studied in (Ba1-xKx)Fe2As2 (x = 0.218, 0.356, 0.531) single crystals irradiated with fast neutrons. It is found that dTc/dRhoSC - the rate of decreasing Tc as a function of the RhoSC (RHoSC is the resistivity at T = Tc) - linearly increases with concentration x. Slow changes in the Hall coefficient RH, as well as the quadratic electronic contribution to the resistivity, show that there are no substantial changes in the topology of the Fermi surface caused by…
Superconducting and structural phase transitions in a pyrochlore oxide Cd2Re2O7 are studied under high pressure by x-ray diffraction and electrical resistivity measurements. A rich P-T phase diagram is obtained, which contains at least two phases with the ideal and slightly distorted pyrochlore structures. It is found that the transition between them is suppressed with increasing pressure and finally disappears at a critical pressure Pc = 3.5 GPa. Remarkable enhancements in the residual resistivity as well as the coefficient A of the AT 2 term in the resistivity are found around the critical p…
Phase slips creation in the thin film in perpendicular magnetic filed with edge superconductivity is studied. These centers are due to thermal activation of the order parameter below superconducting temperature transition leading to the suppression of the superconductivity. The corresponding resistance is calculated. The Alsamazov- Larkin correction to the conductivity above the critical magnetic field destroying the surface superconductivity is studied. Such structures could be applied as a new system for the study of the phase slip phenomenon in one- dimensional superconducting wires.
A linear temperature dependence of the electrical resistivity as T -> 0 is the hallmark of quantum criticality in heavy-fermion metals and the archetypal normal-state property of high-Tc superconductors, yet in both cases it remains unexplained. We report a linear resistivity on the border of spin-density-wave order in the organic superconductor (TMTSF)2X (X = PF6, ClO4), whose strength scales with the superconducting temperature Tc. This scaling, also present in the pnictide superconductors, reveals an intimate connection between linear-T scattering and pairing, shown by renormalization group…
We show results on the vortex core dissipation through current-voltage measurements under applied pressure and magnetic field in the superconducting phase of CeCoIn$_5$. We find that as soon as the system becomes superconducting, the vortex core resistivity increases sharply as the temperature and magnetic field decrease. The sharp increase in flux flow resistivity is due to quasiparticle scattering on critical antiferromagnetic fluctuations. The strength of magnetic fluctuations below the superconducting transition suggests that magnetism is complimentary to superconductivity and therefore mu…
The variation of TC in MgB2 has been systematically investigated for monovalent cation substitution in Mg1-xLixB2 and in Mg1-xCuxB2 and simultaneous cation and Carbon substitution in Mg0.80Li0.20B2-xCx and Mg0.95Cu0.05B2-xCx using dc resistivity and ac susceptibility techniques. TC seems to be uniquely determined by the electron count in the sample, being constant at the MgB2 value for electron counts lower than MgB2 but rapidly decreasing for larger electron counts. The Debye temperature extracted from fitting normal state resistivity to the Bloch-Gruneisen formula shows no systematics with T…
Measurements on resistivity and magnetic susceptibility have been carried out for Bi single crystals under pressures up to 10.5GPa. The temperature dependent resistivity shows a semimetallic behavior at ambient and low pressures (below about 1.6GPa). This is followed by an upturn of resistivity in low temperature region when the pressure is increased, which is explained as a semiconductor behavior. This feature gradually gets enhanced up to a pressure of about 2.52GPa. Then a non-monotonic temperature dependent resistivity appears upon further increasing pressure, which is accompanied by a str…
We report resistivity and magnetization of single crystal MgB2, focusing on the broadening of superconducting (SC) transition in magnetic fields. In-plane and out-of-plane resistivity indicate that the broadening of superconducting transition is independent of Lorentz force and that it is merely dependent on the magnetic field direction. In magnetization, diamagnetic signal begins to appear at almost the same temperature as the onset temperature of resistivity transition. These results suggest that the broadening is attributed not to the surface superconductivity but to the superconducting flu…
