Field-Induced Spin Mixing in Ultrathin Superconducting Al and Be Films in High Parallel Magnetic Fields

Abstract

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...

Antiparticle to particle ratios and identified hadron spectra in Cu+Cu and Au+Au collisions

“…Different rows correspond to three classes of centrality (expressed as the percentile of the total inelastic Au+Au cross section, starting with the 15% most central collisions in the top row). A smooth evolution of the spectra with centrality can be observed, with the proton spectrum being harder than the meson spectra, similar to results at higher collision energies [4,9]. Figure 2b compares the π + +π − , K + +K − and p+p spectra to the cross sections measured at very low p T .…”

Highlights from STAR

“…(b) baryons Nuclear modification factors from 5% most central Au+Au collisions at √ s NN =200 GeV [14,15,16] Quark coalescence or recombination models [17,18,19,20] for hadron formation offer an elegant explanation for the experimentally observed dependence on the number of constituent quarks. Within some of these models [18], hadrons at intermediate p T are dominantly formed by coalescing quarks stemming from a thermalized parton system, i.e.…”

“…STAR also measured directed flow, v 1 [9] and the higher harmonics v 4 and v 6 [28,29]. For the first time at RHIC, we determined the sign of v 2 to be positive, i.e.…”

Overview of charm production at RHIC

“…Fig.6 in Ref. [20] shows that there is a factor of 3 difference at p T = 1.5 GeV/c between late freeze-out (T f o = 100 MeV, β max = 0.9) [27] and early freeze-out (T f o = 160 MeV, β max = 0.6) [28]. Current measurements of non-photonic electron spectra and direct charm spectra seem to be consistent with a decreasing trend even at p T ≃ 1.0 GeV/c [15,14], which is likely due to multiple collisions and thermalization at low p T with early freeze-out [9] and not due to pQCD energy loss.…”