Electrical resistivity and tunneling anomalies in CeCuAs2

The compound CeCuAs2 is found to exhibit negative temperature (T) coefficient of electrical resistivity (rho) under ambient pressure conditions in the entire T-range of investigation (45 mK to 300 K), even in the presence of high magnetic fields. Preliminary tunneling spectroscopic measurements indicate the existence of a psuedo-gap at least at low temperatures, thereby implying that this compound could be classified as a Kondo semi-conductor, though rho(T) interestingly is not found to be of an activated type.Comment: To appear in the proceedings of SCES200

Scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy of FeSe₁–xTex

The iron-chalcogenide superconductor FeSe₁–xTex (0.5 < x < 1) was investigated by scanning-tunneling microscopy/ spectroscopy (STM/STS) and break-junction techniques. In the STM topography of the samples, randomly distributed Te and Se surface atomic structure patterns correlate well with the bulk composition, demonstrating that nanoscale surface features directly reflect bulk properties. The high-bias STS measurements clarified the gap-like structure at ≈ 100–300 meV, which is consistent with the break-junction data. These highenergy structures were also found in sulfur substituted FeS₀.₁Te₀.₉. Possible origin of such spectral peculiarities is discussed. The superconducting gap 2Δ ≈ 3.4 ± 0.2 meV at temperature T = 4.2 K was found in the break junction of FeSe₁–xTex with the critical temperature Tc ≈ 10 K. The corresponding characteristic gap to Tc ratio 2Δ/kBTc ≈ 4 ± 0.2 indicates moderate superconducting coupling (kB is the Boltzmann constant)

Paramagnetic effect of the magnetic field on superconductors with charge-density waves

The limiting field Hp for spin-singlet superconductors with charge-density waves (CDWs), which paramagnetically destroys the ordered phase, possessing coexisting superconducting and CDW order parameters, is calculated self-consistently. It is shown that Hp always exceeds the Pauli limits both for pure superconducting and pure CDW phases. Relevant experimental data for inorganic and organic superconductors with high upper critical magnetic fields are analyzed and are shown to be in qualitative agreement with the proposed theory

Spin-polarized electron tunneling between charge-density-wave metals

For junctions between metals partially gapped by charge density waves (CDWs), the quasiparticle tunnel currents J(V) and conductances G(V) in external magnetic fields H are calculated as functions of H, the bias voltage V, temperature T, the dielectric gaps ∑, and the gapped portions μ of the Fermi surface (FS). The paramagnetic effect of H is taken into account, whereas orbital effects are neglected. General expressions are obtained for different CDW metal electrodes. Analytical formulas are obtained for T = 0. Explicit numerical calculations are carried out for symmetrical junctions. The results are substantially unlike those for junctions between superconductors. It is shown that due to the interplay between quasiparticles from nested and non-nested FS sections the junction properties involve features appropriate to both symmetrical and asymmetrical setups. In particular, for H = 0 discontinuities at eV = ±2∑ and square-root singularities at eV = ±∑ should coexist. Here e is the elementary charge. For H ≠ 0 the former remain intact, while the latter split. It is suggested to use the splitting as a verification of the CDW nature of the pseudogap in high-Tc superconducting oxides

Analysis of the pseudogap-related structure in tunnel spectra of the superconducting Bi₂Sr₂CaCu₂O₈₊δ revealed by break-junction technique

Tunnel conductance G(V)for break-junctions made of as-grown single-crystal Bi₂Sr₂CaCu₂O₈₊δ samples with Tc≈86-89 K was measured and clear-cut dip-hump structures (DHSs) were found in the range 80-120mV of the bias voltage V. The theory of tunneling in symmetric junctions between inhomogeneous charge-density-wave (CDW) superconductors, considered in the framework of the s-pairing model, has been developed. CDWs have been shown to be responsible for the appearance of the DHS in the tunnel current- voltage characteristics and properly describes experimental results

Influence of Fermi surface topology on the quasiparticle spectrum in the vortex state

We study the influence of Fermi surface topology on the quasiparticle density of states in the vortex state of type II superconductors. We observe that the field dependence and the shape of the momentum and spatially averaged density of states is affected significantly by the topology of the Fermi surface. We show that this behavior can be understood in terms of characteristic Fermi surface functions and that an important role is played by the number of points on the Fermi surface at which the Fermi velocity is directed parallel to the magnetic field. A critical comparison is made with a broadened BCS type density of states, that has been used frequently in analysis of tunneling data. We suggest a new formula as a replacement for the broadened BCS model for the special case of a cylindrical Fermi surface. We apply our results to the two gap superconductor MgB$_2$ and show that in this particular case the field dependence of the partial densities of states of the two gaps behaves very differently due to the different topologies of the corresponding Fermi surfaces, in qualitative agreement with recent tunneling experiments.Comment: 12 pages 12 figure

Phase diagram of YBa2_2Cu3_3O7−y_{7-y} at T<<Tc_c based on Cu(2) transverse nuclear relaxation

Two maxima in transverse relaxation rate of Cu(2) nuclei in YBa$_2$Cu$_3$O$_{7-y}$ are observed, at T = 35 K and T = 47 K. Comparison of the $^{63}$Cu(2) and $^{65}$Cu(2) rates at T = 47 K indicates the magnetic character of relaxation. The enhancement at T = 47 K of fluctuating local magnetic fields perpendicular to the CuO$_2$ planes is connected with the critical fluctuations of orbital currents. Maximum at T = 35 K is connected with the appearance of inhomogeneous supeconducting phase. Together with data published to date, our experimental results allow to suggest a qualitatively new phase diagram of the superconducting phase.Comment: 4 LaTEX pages + 3 figures in *.ps forma

Resistivity, Hall effect and Shubnikov-de Haas oscillations in CeNiSn

The resistivity and Hall effect in CeNiSn are measured at temperatures down to 35 mK and in magnetic fields up to 20 T with the current applied along the {\it b} axis. The resistivity at zero field exhibits quadratic temperature dependence below $\sim$0.16 K with a huge coefficient of the $T^2$ term (54 $\mu$$\Omega$cm/K$^2$). The resistivity as a function of field shows an anomalous maximum and dip, the positions of which vary with field directions. Shubnikov-de Haas (SdH) oscillations with a frequency {\it F} of $\sim$100 T are observed for a wide range of field directions in the {\it ac} and {\it bc} planes, and the quasiparticle mass is determined to be $\sim$10-20 {\it m}$_e$. The carrier density is estimated to be $\sim10^{-3}$ electron/Ce. In a narrow range of field directions in the {\it ac} plane, where the magnetoresistance-dip anomaly manifests itself clearer than in other field directions, a higher-frequency ($F=300\sim400\text{T}$) SdH oscillation is found at high fields above the anomaly. This observation is discussed in terms of possible field-induced changes in the electronic structure.Comment: 15 pages, 5 figures, to appear in Phys. Rev. B (15 Sept. 2002 issue

Superconducting gap and pair breaking in CeRu2 studied by point contacts

The superconducting gap in a CeRu$_{2}$ single crystal is investigated by point contacts. BCS-like behavior of the gap $\Delta$ in the temperature range below T$_{c}^{*}<$T$_{c}$, where T$_{c}$ is the critical temperature, is established, indicating the presence of a gapless superconductivity region (between T$_{c}^{*}$ and T$_{c}$). The pair-breaking effect of paramagnetic impurities, supposedly Ce ions, is taken into consideration using the Scalski-Betbeder-Matibet-Weiss approach based on Abrikosov-Gorkov theory. It allows us to recalculate the superconducting order parameter $\Delta ^{\alpha}$ (in the presence of paramagnetic impurities) and the gap $\Delta ^{P}$ (in the pure case) for the single crystal and for the previously studied polycrystalline CeRu$_{2}$. The value 2$\Delta^{P}$(0)$\approx$2 meV, with 2$\Delta ^{P}$(0)$/$k$_{B}$T$_{c} \approx$3.75, is found in both cases, indicating that CeRu$_{2}$ is a ``moderate'' strong-coupling superconductor.Comment: 4 pages incl. 3 figs., publ. in Fiz. Nizk. Temp. (http://fnte.ilt.kharkov.ua/list.html