Thermodynamic functions within the van Hove BCS model: Symmetry mixing effects

All the new layer perovskite superconductors seem to show a phenomenon of symmetry mixing with repect to the order parameter. An analysis of the different alternative of mixing and how far could them be presented is carried out. For the particular case of s+id symmetry of the gap, the temperature dependence of the specific heat ($C_{es}$) and the thermodynamic critical magnetic field ($H_{c}$) are calculated. A double peak transition is observed on $C_{es}(T)$ in the mixed regime while the single peak behavior is recovered for a purely symmetric state (s or d). $C_{es}$ presents a quadratic law at low tempeartures for a d-wave gap and for the s-wave one the typical exponential attenuation. The temperature dependence of $H_{c}$ shows a clear phase transition of second order at temperatures where the d-wave component becomes negligible. A comparison with other results and experiments is done.Comment: Revised version and accepted to be published in Physica

Observation of Andreev bound states in bicrystal grain-boundary Josephson junctions of the electron doped superconductor LaCeCuO

We observe a zero-bias conductance peak (ZBCP) in the ab-plane quasiparticle tunneling spectra of thin film grain-boundary Josephson junctions made of the electron doped cuprate superconductor LaCeCuO. An applied magnetic field reduces the spectral weight around zero energy and shifts it non-linearly to higher energies consistent with a Doppler shift of the Andreev bound states (ABS) energy. For all magnetic fields the ZBCP appears simultaneously with the onset of superconductivity. These observations strongly suggest that the ZBCP results from the formation of ABS at the junction interfaces, and, consequently, that there is a sign change in the symmetry of the superconducting order parameter of this compound consistent with a d-wave symmetry.Comment: 9 pages, 7 figures; December 2004, accepted for publication in Phys. Rev.

c-axis Josephson Tunnelling in Twinned and Untwinned YBCO-Pb Junctions

Within a microscopic two band model of planes and chains with a pairing potential in the planes and off diagonal pairing between planes and chains we find that the chains make the largest contribution to the Josephson tunnelling current and that through them the d-wave part of the gap contributes to the current. This is contrary to the usual assumption that for a d-wave tetragonal superconductor the c-axis Josephson current for incoherent tunnelling into an s-wave superconductor is zero while that of a d-wave orthorhombic superconductor with a small s-wave component to its gap it is small but non-zero. Nevertheless it has been argued that the effect of twins in YBCO would lead to cancellation between pairs of twins and so the observation of a current in c-axis YBCO-Pb experiments is evidence against a d-wave type order parameter. We argue that both theory and experiment give evidence that the two twin orientations are not necessarily equally abundant and that the ratio of tunnelling currents in twinned and untwinned materials should be related to the relative abundance of the two twin orientations.Comment: 6 pages, RevTeX 3.0, 15 PostScript figur

Core-Level Photoemission and Work-Function Investigation of Na on Cu(110)

Core-level photoemission, low-energy electron diffraction (LEED), and work-function change measurements have been carried out to study the coverage dependence of Na/Cu(110) at room temperature. The results of LEED and work-function measurements are qualitatively similar to most other investigations of alkali-metal adsorption on fcc(110) metal surfaces. With LEED, we observed an alkali-metal-induced (1×2) reconstruction at intermediate coverage. We have performed a simple calculation to account for the work-function differences between Na/fcc(110) and Na/fcc(111) metal surfaces. The comparison of coverage-dependent core-level binding-energy shifts between Na/Cu(110) and Na/Cu(111) reveals that a low-coverage plateau in the curve of binding energy vs Na coverage for Na/Cu(110) is associated with the Na-induced reconstruction, and can be accounted for within a localized picture of the reconstruction

Fermi Liquid Properties of a Two Dimensional Electron System With the Fermi Level Near a van Hove Singularity

We use a diagrammatic approach to study low energy physics of a two dimensional electron system where the Fermi level is near van-Hove singularies in the energy spectrum. We find that in most regions of the $\epsilon_F-T$ phase diagram the system behaves as a normal Fermi liquid rather than a marginal Fermi liquid. Particularly, the imaginary part of the self energy is much smaller than the excitation energy, which implies well defined quasiparticle excitations, and single particle properties are only weakly affected by the presence of the van-Hove singularities. The relevance to high temperature superconductivity is also discussed.Comment: 10 pages, 4 postscript figure

Contiguous 3d and 4f magnetism: towards strongly correlated 3d electrons in YbFe2Al10

We present magnetization, specific heat, and 27Al NMR investigations on YbFe2Al10 over a wide range in temperature and magnetic field. The magnetic susceptibility at low temperatures is strongly enhanced at weak magnetic fields, accompanied by a ln(T0/T) divergence of the low-T specific heat coefficient in zero field, which indicates a ground state of correlated electrons. From our hard X-ray photo emission spectroscopy (HAXPES) study, the Yb valence at 50 K is evaluated to be 2.38. The system displays valence fluctuating behavior in the low to intermediate temperature range, whereas above 400 K, Yb3+ carries a full and stable moment, and Fe carries a moment of about 3.1 mB. The enhanced value of the Sommerfeld Wilson ratio and the dynamic scaling of spin-lattice relaxation rate divided by T [27(1/T1T)] with static susceptibility suggests admixed ferromagnetic correlations. 27(1/T1T) simultaneously tracks the valence fluctuations from the 4f -Yb ions in the high temperature range and field dependent antiferromagnetic correlations among partially Kondo screened Fe 3d moments at low temperature, the latter evolve out of an Yb 4f admixed conduction band.Comment: To appear in Phys. Rev. Let

Phase-sensitive Evidence for d-wave Pairing Symmetry in Electron-doped Cuprate Superconductors

We present phase-sensitive evidence that the electron-doped cuprates Nd1.85Ce0.15CuO4-y (NCCO) and Pr1.85Ce0.15CuO4-y (PCCO) have d-wave pairing symmetry. This evidence was obtained by observing the half-flux quantum effect, using a scanning SQUID microscope, in c-axis oriented films of NCCO or PCCO epitaxially grown on tricrystal [100] SrTiO3 substrates designed to be frustrated for a d(x2-y2) order parameter. Samples with two other configurations, designed to b unfrustrated for a d-wave superconductor, do not show the half-flux quantum effect.Comment: 4 pages, Latex, 4 figure

Bulk and surface electronic properties of SmB6: a hard x-ray photoelectron spectroscopy study

We have carried out bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES) measurements on in-situ cleaved and ex-situ polished SmB6 single crystals. Using the multiplet-structure in the Sm 3d core level spectra, we determined reliably that the valence of Sm in bulk SmB6 is close to 2.55 at ~5 K. Temperature dependent measurements revealed that the Sm valence gradually increases to 2.64 at 300 K. From a detailed line shape analysis we can clearly observe that not only the J=0 but also the J=1 state of the Sm 4f 6 configuration becomes occupied at elevated temperatures. Making use of the polarization dependence, we were able to identify and extract the Sm 4f spectral weight of the bulk material. Finally, we revealed that the oxidized or chemically damaged surface region of the ex-situ polished SmB6 single crystal is surprisingly thin, about 1 nm only.Comment: 11 pages, 8 figure