The influence of a static magnetic field on the fluctuation superconductivity

We study the influence of an external static magnetic field on the additional conductivity due to fluctuation superconductivity for bulk material and for thin films with the magnetic field normal to the film surfaces. Only a weak-coupling superconductor in the dirty limit is considered. This additional conductivity turns out to be anisotropic in general.     

Heat conduction in the vortex state of NbSe2: evidence for multiband superconductivity

The thermal conductivity kappa of the layered s-wave superconductor NbSe2 was measured down to T(c)/100 throughout the vortex state. With increasing field, we identify two regimes: one with localized states at fields very near H(c1) and one with highly delocalized quasiparticle excitations at higher fields. The two associated length scales are naturally explained as multiband superconductivity, with distinct small and large superconducting gaps on different sheets of the Fermi surface. This behavior is compared to that of the multiband superconductor MgB2 and the conventional superconductor V3Si.     

Field and temperature dependence of the superfluid density in LaFeAsO1-xFx superconductors: a muon spin relaxation study

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (x=0.075 and x=0.1). The temperature dependence of the deduced superfluid density is consistent with a BCS s-wave or a dirty d-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of lambda ab(0)=254(2) nm for x=0.1 and lambda ab(0)=364(8) nm for x=0.075. Further evidence for unconventional superconductivity is provided by the ratio of Tc versus the superfluid density, which is close to the Uemura line of high-Tc cuprates.     

Superconductivity with two-fold symmetry in topological superconductor Srx Bi2Se3

Topological superconductivity is the quantum condensate of paired electrons with an odd parity of the pairing function. By using a Corbino-shape like electrode configuration, we measure the c-axis resistivity of the recently discovered superconductor SrxBi_2Se_3 with the magnetic field rotating within the basal planes, and find clear evidence of two-fold superconductivity. The Laue diffraction measurements on these samples show that the maximum gap direction is either parallel or perpendicular to the main crystallographic axis. This observation is consistent with the theoretical prediction and strongly suggests that SrxBi_2Se_3 is a topological superconductor.     

用两带Ginzburg-Landau理论分析两带超导体Lu2Fe3Si5的表面临界磁场

用两带Ginzburg-Landau(GL)理论分析了Lu₂Fe₃Si₅的表面临界磁场,当超导体的表面与任一主晶面重合,且外磁场平行于超导体的表面时,比值H_c3/H_c2(H_C2是Lu₂Fe₃Si₅的上临界磁场)强烈依赖于温度. 当超导体的表面是bc平面,且 关键词： 两带超导体 GL理论 2Fe₃Si₅')" href="#">Lu₂Fe₃Si₅ 表面临界磁场     

Multiband superconductivity in the heavy fermion compound PrOs4Sb12

The thermal conductivity of the heavy fermion superconductor Pr(Os(4)Sb(12) was measured down to T(c)/40 throughout the vortex state. At lowest temperatures and for magnetic fields H approximately 0.07H(c2), already 40% of the normal state thermal conductivity is restored. This behavior (similar to that observed in MgB2) is a clear signature of multiband superconductivity in this compound.     

PdTe: a strongly coupled superconductor

We report the electrical transport, magnetic, and thermodynamic properties of polycrystalline PdTe which exhibits superconductivity below 4.5 K. Using the measured values for the lower (H(c1)) and upper (H(c2)) critical fields, and the specific heat C(p), we estimate the thermodynamic critical field H(c)(0), coherence length ξ(0), penetration depth λ(0), and the Ginzburg-Landau parameter κ. Compared with band structure calculations, the density of states at the Fermi level is enhanced due to electron-phonon coupling with λ(ep) = 1.4. Furthermore, the large values of ΔC(p)/γ(n)T(c) and 2Δ(0)/k(B)T(c) suggest that PdTe is a strongly coupled superconductor.     

Superconductivity induced by longitudinal ferromagnetic fluctuations in UCoGe

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T(Curie)～2.5 K and T(SC)～0.6 K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H∥c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by H∥c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.     

Angular position of nodes in the superconducting gap of quasi-2D heavy-fermion superconductor CeCoIn5

The thermal conductivity of the heavy-fermion superconductor CeCoIn5 has been studied in a magnetic field rotating within the 2D planes. A clear fourfold symmetry of the thermal conductivity which is characteristic of a superconducting gap with nodes along the ( +/- pi,+/- pi) directions is resolved. The thermal conductivity measurement also reveals a first-order transition at H(c2), indicating a Pauli limited superconducting state. These results indicate that the symmetry most likely belongs to d(x(2)-y(2)), implying that the anisotropic antiferromagnetic fluctuation is relevant to the superconductivity.     

Surface Superconductivity¶in Applied Magnetic Fields Above HC2

In this paper we study the surface superconductivity phenomenon of type 2 superconductors under applied magnetic fields above the critical field . We show that, for a cylindrical superconductor of infinite height placed in a homogeneous applied magnetic field, H e ₃, with H lying in between and keeping away from and , superconductivity persists uniformly at a thin sheath surrounding the entire lateral surface of the sample. As the applied field decreases gradually, superconductivity in the surface sheath becomes strong and develops into a surface superconducting state, while the interior of the sample remains close to the normal state. Received: 21 May 2001 / Accepted: 11 February 2002     

Scanning tunneling spectroscopy in the superconducting state and vortex cores of the beta-pyrochlore KOs2O6

We performed the first scanning tunneling spectroscopy measurements on the pyrochlore superconductor KOs2O6 (T(c)=9.6 K) in both zero magnetic field and the vortex state at several temperatures above 1.95 K. This material presents atomically flat surfaces, yielding spatially homogeneous spectra which reveal fully gapped superconductivity with a gap anisotropy of 30%. Measurements performed at fields of 2 and 6 T display a hexagonal Abrikosov flux line lattice. From the shape of the vortex cores, we extract a coherence length of 31-40 A, in agreement with the value derived from the upper critical field H(c2). We observe a reduction in size of the vortex cores (and hence the coherence length) with increasing field which is consistent with the unexpectedly high and unsaturated upper critical field reported.     

Doping dependent evolution of magnetism and superconductivity in Eu(1-x)K(x)Fe2As2 (x = 0-1) and temperature dependence of the lower critical field H(c1)

We have synthesized polycrystalline samples of Eu(1-x)K(x)Fe2As2 (x = 0-1) and carried out systematic characterization using x-ray diffraction, ac and dc magnetic susceptibility, and electrical resistivity measurements. A clear signature of the coexistence of a superconducting transition (T(c) = 5.5 K) with spin density wave (SDW) ordering is observed in our underdoped sample with x = 0.15. The SDW transition disappears completely for the x = 0.3 sample and superconductivity arises below 20 K. The superconducting transition temperature Tc increases with increase in the K content and a maximum Tc = 33 K is reached for x = 0.5, beyond which it decreases again. The doping dependent Tx phase diagram is extracted from the magnetic and electrical transport data. It is found that magnetic ordering of Eu moments coexists with the superconductivity up to x = 0.6. The isothermal magnetization data taken at 2 K for the doped samples suggest the 2+ valence state of the Eu ions. We also present the temperature dependence of the lower critical field H(c1) of the superconducting polycrystalline samples. The values of H(c1)(0) obtained for x = 0.3, 0.5, and 0.7 after taking the demagnetization factor into account are 202, 330, and 212 Oe, respectively. The London penetration depth λ(T) calculated from the lower critical field does not show exponential dependence at low temperature, as would be expected for a fully gapped clean s-wave superconductor. In contrast, it shows a T2 power law feature up to T = 0.3Tc, as observed in Ba(1-x)K(x)Fe2As2 and BaFe(2-x)Co(x)As2.     

Superconductivity and field-induced magnetism in Pr2-xCexCuO4 single crystals

We report muon-spin rotation and relaxation (muSR) measurements on single crystals of the electron-doped high-T(c) superconductor Pr2-xCexCuO4. In a zero external magnetic field, superconductivity is found to coexist with dilute Cu spins that are static on the muSR time scale. In an applied field, we observe a mu(+)-Knight shift that is primarily due to the magnetic moment induced on the Pr ions. Below the superconducting transition temperature T(c), an additional source of local magnetic field appears throughout the volume of the sample. This finding is shown to be consistent with field-induced antiferromagnetic ordering of the Cu spins. Measurements of the temperature dependence of the in-plane magnetic penetration depth lambda(ab) in the vortex state are also presented.     

Magnetic determination of H(c2) under accurate alignment in (TMTSF)2ClO4

Cantilever magnetometry has been used to measure the upper critical magnetic field H(c2) of the quasi-one-dimensional molecular organic superconductor (TMTSF)2ClO4. From simultaneous resistivity and torque magnetization experiments conducted under precise field alignment, H(c2) at low temperature is shown to reach 5 T, nearly twice the Pauli paramagnetic limit imposed on spin singlet superconductors. These results constitute the first thermodynamic evidence for a large H(c2) in this system and provide support for spin triplet pairing in this unconventional superconductor.     

Anisotropy of magnetothermal conductivity in Sr2RuO4

The dependence of in-plane and interplane thermal conductivities of Sr2RuO4 on temperature, as well as magnetic field strength and orientation, is reported. We found no notable anisotropy in the thermal conductivity for the magnetic field rotation parallel to the conducting plane in the whole range of experimental temperatures and fields, except in the vicinity of the upper critical field H(c2), where the anisotropy of the H(c2) itself plays a dominant role. This finding imposes strong constraints on the possible models of superconductivity in Sr2RuO4 and supports the existence of a superconducting gap with a line of nodes running orthogonal to the Fermi surface cylinder.     

Single-gap s-Wave superconductivity near the charge-density-wave quantum critical point in CuxTiSe2

The in-plane thermal conductivity kappa of the layered superconductor CuxTiSe2 was measured down to temperatures as low as Tc/40, at x=0.06 near where the charge-density-wave order vanishes. The absence of a residual linear term at T-->0 is strong evidence for conventional s-wave superconductivity in this system. This is further supported by the slow magnetic field dependence, also consistent with a single gap, of uniform magnitude across the Fermi surface. Comparison with the closely related material NbSe2, where the superconducting gap is 3 times larger on the Nb 4d band than on the Se 4p band, suggests that in Cu0.06TiSe2 the Se 4p band is below the Fermi level and Cu doping into the Ti 3d band is responsible for the superconductivity.     

Evidence for a percolation-driven transition to coherent surface superconductivity

Above the upper critical field we have investigated the field dependences of the surface conductance, G'-iG" and the critical current J(c) of an electropolished pure niobium cylinder. The low frequency limits of G', G", and J(c) display power-law singularities, defining a transition to coherent surface superconductivity at H(c)(c3). The critical exponents as well as the dynamical scaling of G'-iG" are consistent with predictions for a two-dimensional percolation transition. Relating H(c)(c3) to the conventional onset field, we find H(c)(c3)/H(c3)=0.81, and, surprisingly, this ratio turns out to be independent of significant variations of H(c3) due to differently structured surfaces.     

Distribution of a local magnetic field in superconductors with an uncorrelated random lattice of abrikosov vortices

The distribution of a local magnetic field near the surface of a uniaxial anisotropic type-II superconductor is determined in the framework of the London model in the case when the Abrikosov vortices are randomly distributed in the superconductor. The distribution of a local magnetic field is obtained as a function of the distance from the surface of the superconductor. It is demonstrated that the shape of the distribution of the local magnetic field near the surface differs substantially from that in the bulk of the superconductor. This difference should be taken into account in interpreting experimental data on the local magnetic field in the surface region of the superconductor and in thin superconducting films (with a thickness of less than or equal to λ, where λ is the depth of penetration of the magnetic field into the superconductor). It is shown that, as in the case of a regular lattice of vortices, the value of λ can be determined from observations of the distribution of the local magnetic field in type-II superconductors with an uncorrelated random lattice of vortices.     

Direct observation of the scaling relation between density of states and pairing gap in a dirty superconductor

Theories and experiments on dirty superconductors are complex but important in terms of both theoretical fundamentals and practical applications. These activities are even more challenging when magnetic fields are present because the field distribution, electron density of states, and superconducting pairing potentials become nonuniform. Here, we present tunneling microspectroscopic experiments on Nb C single crystals and demonstrate that Nb C is a homogeneous dirty superconductor. When applying magnetic fields to the samples, we found that the zero-energy local density of states and the pairing energy gap followed the explicit scaling relation proposed by de Gennes for homogeneous dirty superconductors in high magnetic fields. More significantly, our experimental findings indicate that the validity of the scaling relation extends to magnetic field strengths far below the upper critical field, calling for a new nonperturbative understanding of this fundamental property in dirty superconductors. On the practical side, we used the observed scaling relation to derive a simple and straightforward experimental scheme for estimating the superconducting coherence length of a dirty superconductor in magnetic fields.