We report spin-dependent electron density of states (DOS) studies of ultra-thin superconducting Al and Be films in high parallel magnetic fields. Superconductor-insulator-superconductor (SIS) tunneling spectra are presented in which both the film and the counterelectrode are in the paramagnetic limit. This SIS configuration is exquisitely sensitive to spin mixing and/or spin flip processes which are manifest as DOS singularities at eV = 2∆o ± eVz. Both our Al and Be data show a well defined subgap peak whose magnitude grows dramatically as the parallel critical field is approached. Though this feature has previously been attributed to spin-orbit scattering, it is more consistent with fluctuations into a field induced mixed-spin state. [3,4], research on systems exhibiting a nontrivial interplay between magnetism and superconductivity has moved to the forefront on condensed matter physics. In this Letter we probe the spin states of superconducting Al and Be films in high parallel magnetic fields via spin polarized electron tunneling measurements [5]. The films are sufficiently thin so as to restrict the transverse motion of electrons, thus allowing us to access the high field regime while maintaining time reversal symmetry [6] up to the Clogston-Chandrasekhar critical field, where g is the Landé g-factor, µ B is the Bohr magneton, and ∆ o is the superconducting gap [7]. Though the films are too disordered to support a FFLO phase [8], they are a model system for studying the spin states of BCS superconductivity in the presence of a non-negligible Zeeman field that ultimately drives the first-order spin-paramagnetic transition associated with H c|| [8,9] and the long conjectured FFLO regime just above H c|| [8]. Tunneling measurements in fields H || 1 2 H c|| reveal a subgap peak in the DOS spectrum, shifted down from the primary BCS peak by the Zeeman energy. The magnitude of the satellite peak varies as the square root of the reduced field. Though this peak has previously been attributed to spin-orbit (SO) scattering in Al [10], it is also manifest in the much lighter element Be, suggesting that it is a property of the high field condensate.In the mid 1970's Tedrow, Meservey, and coworkers conducted a series of tunneling experiments on paramagnetically limited Al films. They showed that the tunneling spectrum of a superconductor-insulator-superconductor (S-I-S) junction, in which both the film and the counter-electrode are thin, will not exhibit a Zeeman splitting so long as there is no spin mixing or spin flip processes [8]. Assuming that the gap is ∆ o on either side of the junction, then the tunneling spectrum has a single BCS peak at the usual |eV | = 2∆ o , independent of the Zeeman energy eV z = gµ B H || , where e is the electron charge. If, however, there is spin flip during the tunneling, then satellite peaks will appear at energies |eV | = 2∆ o ± eV z [8,10]. Similarly, if there is a mechanism by which the spin eigenstates are partially mixed, then there will be a minority-spin satellite peak in the sp...

PACS number(s): 74.70.AdWe report the synthesis and measurements of magnetic, transport, and thermal properties of polycrystalline Nb 0.18 Re 0.82 , which has a superconducting transition at T c ~ 8.8 K. The noncentrosymmetric α-Mn structure of the compound is confirmed by X-ray diffraction. Using the measured values for the lower critical field H c1 , upper critical field H c2 , and the specific heat C, we estimate the thermodynamic critical field H c (0), coherence length ξ(0), penetration depth λ(0), and the Ginzburg-Landau parameter κ(0). The specific heat jump at T c , ΔC/γT c = 1.86, suggests that Nb 0.18 Re 0.82 is a moderately coupled superconductor. Below T c the electronic specific heat decays exponentially, suggesting that the gap is isotropic. Our data suggest that the triplet admixture is weak in the polycrystalline form of compound. However, the estimated value of the upper critical field H c2 (0) is close to the calculated Pauli limit.

We have synthesized polycrystalline samples of the noncentrosymmetric superconductor Mo 3 Al 2 C by arc and RF melting, measured its transport, magnetic and thermodynamic properties, and computed its band structure.Experimental results indicate a bulk superconducting transition at T c ~ 9.2 K, while the density of states at the Fermi surface is found to be dominated by Mo d-orbitals. Using the measured values for the lower critical field H c1 , upper critical field H c2 , and the specific heat C, we estimated the thermodynamic critical field H c (0), coherence length ξ(0), penetration depth λ(0), and the Ginzburg-Landau parameter κ(0). The specific heat jump at T c , ΔC/γT c = 2.14, suggests that Mo 3 Al 2 C is moderately-to-strongly coupled, consistent with the fast opening of the gap, as evidenced by the rapid release of entropy below T c from our electronic specific heat measurements. Above 2K the electronic specific heat exhibits the power law behavior, suggesting that synthesis of single crystals and measurements at lower temperature are needed to establish whether the gap is anisotropic. The estimated value of the upper critical field H c2 (0) is close to the calculated Pauli limit, therefore further studies are needed to determine whether the absence of an inversion center results in a significant admixture of the triplet component of the order parameter.

We present magnetization, specific heat, resistivity, and Hall effect measurements on the cubic B20 phase of MnGe and CoGe and compare to measurements of isostructural FeGe and electronic-structure calculations. In MnGe, we observe a transition to a magnetic state at T c = 275 K as identified by a sharp peak in the ac magnetic susceptibility, as well as second phase transition at lower temperature that becomes apparent only at finite magnetic field. We discover two phase transitions in the specific heat at temperatures much below the Curie temperature, one of which we associate with changes to the magnetic structure. A magnetic field reduces the temperature of this transition which corresponds closely to the sharp peak observed in the ac susceptibility at fields above 5 kOe. The second of these transitions is not affected by the application of field and has no signature in the magnetic properties or our crystal-structure parameters. Transport measurements indicate that MnGe is metallic with a negative magnetoresistance similar to that seen in isostructural FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about 0.5 carriers per formula unit, also similar to that found in FeGe and MnSi. CoGe is shown to be a low carrier density metal with a very small, nearly temperature-independent diamagnetic susceptibility.

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