arXiv subjects

S. -L. Drechsler

Publications and source records attributed to S. -L. Drechsler.

At least 109 records · Page 6Linked to original sources

arXiv2002

Study of the electron-phonon interaction in metal diborides MeB_2 (Me=Zr, Nb, Ta, Mg) by point-contact spectroscopy

We review investigations of the electron-phonon interaction (EPI) in metal diborides MeB_2 (Me=Zr, Nb, Ta, Mg) by point-contact (PC) spectroscopy. For transition metal compounds the PC EPI functions were recovered and EPI parameter $λ\lesssim 0.1$ were estimated. The data are consistent with the measured surface phonon dispersion curves. The low $λ$ value questions some reports about superconductivity in these compounds. Contrary, EPI in superconducting MgB_2 films manifests also in the PC spectra itself by virtue of an elastic EPI contribution to the excess current determined by the energy dependence of the superconducting order parameter. To analyse the phonon features in the PC spectra of MgB$_2$ a two-band model is exploited and the proximity effect in the {\bf k}-space is suggested.

cond-mat.supr-con↗

arXiv2002

Electron-phonon interaction in transition metal diborides TB_2 (T=Zr, Nb, Ta) studied by point-contact spectroscopy

The electron-phonon interaction (EPI) in transition metal diborides TB_2 (T=Zr, Nb, Ta) is investigated by point-contact (PC) spectroscopy. The PC EPI functions were recovered and the EPI parameters lambda<0.1 were estimated for all three compounds. Common and distinctive features between the EPI functions for those diborides are discussed also in connection with the superconductivity in MgB_2.

cond-mat.supr-con↗

arXiv2002

Analysis of possible field-induced superconductivity in anthracene, other polyacenes, and C60

We consider electronic structure and superconductivity aspects in field-doped polyacenes (PA) and C60. Within a modified Thomas-Fermi approach for typical experimental values of the surface charge density the injected charge is confined to a monolayer. The electron-phonon coupling constant for internal modes is estimated using the work of Devos et al. (Phys. Rev. B 58, 8236 (1998)) and the density of states N(0) estimated from a 2D-one-band model derived from a full potential LDA band structure calculation for bulk anthracene. The large differences in the reported Tc-values for n-doped PA and C60 are ascribed to enhanced empirical Coulomb pseudopotentials for polyacenes.

cond-mat.supr-con↗

arXiv2002

On the correllation effect in Peierls-Hubbard chains

We reexamine the dimerization, the charge and the spin gaps of a half-filled Peierls-Hubbard chain by means of the incremental expansion technique. Our numerical findings are in significant quantitative conflict with recently obtained results by M. Sugiura and Y. Suzumura [J. Phys. Soc. Jpn. v. 71 (2002) 697] based on a bosonization and a renormalization group method, especially with respect to the charge gap. Their approach seems to be valid only in the weakly correlated case.

cond-mat.str-el↗

arXiv2002

Influence of the van Hove singularity on the specific heat jump in BCS superconductors

Within the weak-coupling BCS scheme we derive a general form of the coefficients in the Ginzburg-Landau expansion of the free energy of a superconductor for the case of a Fermi level close to a van Hove singularity (VHS). A simple expression for the influence of the VHS on the specific heat jump is then obtained for the case where gaps for different bands are distinct but nearly constant at the corresponding sheets of the Fermi surface.

cond-mat.supr-con↗

arXiv2002

MgCNi3 : Complex behavior in a strongly coupled superconductor

Polycrystalline samples of the recently discovered MgCNi3 superconductor were investigated by transport, ac susceptibility, dc magnetization and specific heat measurements in magnetic fields up to 16 T. Consistent results were obtained for the temperature dependence of the upper critical field H_c2(T) from resistance, ac susceptibility and specific heat measurements. A WHH like temperature dependence of H_c2(T) and the quadratic relationship H_c2(0) ~ T_c^2 point to an effective predominant single band behavior near the quasi clean limit. Evidence for strong electron-phonon and electron-paramagnon coupling was found analyzing the specific heat data. The corresponding s- and p-wave scenarios are briefly discussed using calculated densities of states of different Fermi surface sheets.

cond-mat.supr-con↗

arXiv2002

The unusual electronic structure of the "pseudo-ladder" compound CaCu2O3

Experimental and theoretical studies of the unoccupied electronic structure of CaCu2O3 single crystals have been performed using polarization-dependent x-ray absorption spectroscopy and band structure calculations. The measured hole distribution shows an unusual large number of holes in orbitals parallel to the interlayer direction which is in agreement with the theoretical analysis. CaCu2O3 deviates significantly from the standard pd-sigma cuprate picture. The corresponding strong interlayer exchange is responsible for the missing spin gap generic for other two-leg ladder cuprates.

cond-mat.str-el↗

arXiv2002

Fermi Surfaces of Diborides: MgB2 and ZrB2

We provide a comparison of accurate full potential band calculations of the Fermi surfaces areas and masses of MgB2 and ZrB2 with the de Haas-van Alphen date of Yelland et al. and Tanaka et al., respectively. The discrepancies in areas in MgB2 can be removed by a shift of sigma-bands downward with respect to pi-bands by 0.24 eV. Comparison of effective masses lead to orbit averaged electron-phonon coupling constants lambda(sigma)=1.3 (both orbits), lambda(pi)=0.5. The required band shifts, which we interpret as an exchange attraction for sigma states beyond local density band theory, reduces the number of holes from 0.15 to 0.11 holes per cell. This makes the occurrence of superconductivity in MgB2 a somewhat closer call than previously recognized, and increases the likelihood that additional holes can lead to an increased Tc.

cond-mat.supr-con↗

arXiv2002

Low temperature value of the upper critical field Hc2(0) of isotropic single band metals

The strong coupling Schossmann-Schachinger theory for the upper critical field is considered methodically from the user point of view. We check the accuracy of approximate formulas for the upper critical field. In particular, we explain in detail as a recently proposed convenient expression (Shulga et al. cond-mat/0103154) can be justified. The connection of Hc2(0) with the critical temperature Tc is considered and the similarity with the Allen-Dynes Tc-formula is shown explicitly.

cond-mat.supr-con↗

arXiv2002

Theoretical de Haas-van Alphen Data and Plasma Frequencies of MgB2 and TaB2

The de Haas-van Alphen-frequencies as well as the effective masses for a magnetic field parallel to the crystallographic c-axis are calculated within the local spin density approximation (LSDA) for MgB2 and TaB2. In addition, we analyze the plasma frequencies computed for each Fermi surface sheet. We find a large anisotropy of Fermi velocities in MgB2 in difference to the nearly isotropic behavior in TaB2. We compare calculations performed within the relativistic non-full potential augmented-spherical-wave (ASW) scheme and the scalar-relativistic full potential local orbital (FPLO) scheme. A significant dependence for small cross sections on the bandstructure method is found. The comparison with the first available experimental de Haas-van Alphen-data by Yelland et al. (Ref. 19) shows deviations from the electronic structure calculated within both L(S)DA approaches although the cross section predicted by FPLO are closer to the experimental data. The elucidation of the relevant many-body effects beyond the standard LDA is considered as a possible key problem to understand the superconductivity in MgB2.

cond-mat.supr-con↗

arXiv2001

Surface states and their possible role in the superconductivity of MgB2

We report layer-Korringa-Kohn-Rostocker calculations for bulk and surface states as well as the corresponding angle resolved photoemission (ARPES) intensities of MgB2. Our theoretical results reproduce very well the recent ARPES data by Uchiyama et al., cond-mat/0111152. At least two surface states are assigned. Consequences of SFS on the anisotropy of the upper critical fields and other properties in the superconducting state of small grains in micropowder samples are briefly discussed.

cond-mat↗

arXiv2001

Localisation of doped holes in edge-shared CuO_2 chain cuprates: consequences for dynamic spectral weight transfer

We present a joint experimental and theoretical study of the electronic structure of the cuprate chain systems A_(1-x)CuO_2 (A=Ca,Sr,Ba), as measured using O-K and Cu-L_3 x-ray absorption spectroscopy. The doping-dependent behaviour in these systems is radically different to that in conventional 2D cuprate networks formed from corner sharing CuO_4 plaquettes, and follows from the strongly suppressed inter-plaquette hybridisation resulting from the 90 degree Cu-O-Cu interaction pathway in the chain systems. Spectroscopically, this results in (a) a classical mixed-valent scenario whereby the different final states in the Cu-L_3 spectra can be used directly to 'read-off' the Cu valency and (b) a drastic reduction in the dynamic spectral weight transfer from the upper Hubbard band to the low energy scale in the O-K spectra. The final picture emerges of localisation of the doped holes, with the chain then comprised of a mixture of pure Cu(II)O_4 and Cu(III)O_4 plaquettes.

cond-mat.str-el↗

arXiv2001

Electronic structure and weak electron-phonon coupling in TaB$_2$

We present electronic structure calculations together with resistivity, susceptibility, and specific heat measurements for TaB$_2$ to search for the recently contradictorily reported superconductivity and to study related normal state properties. We ascribe the absence of superconductivity down to 1.5 K for our TaB$_2$ samples to the generally weak electron phonon coupling derived from comparison of the calculated and measured specific heat constants. For the E$_{2g}$ and the B$_{1g}$ $Γ$ point phonons we derive from the calculated deformation potentials very small electron phonon couplings for these modes, opposite to the strong coupling of the E$_{2g}$ mode in MgB$_2$, probably responsible for its high $T_c$. In comparison to MgB$_2$, we discuss the origin of the quite different features in the density of states and of the Fermi surfaces. The differences are mainly due to the strong hybridization between Ta 5$d$ and B 2$p$ states outside the hexagonal basis plane.

cond-mat.supr-con↗

arXiv2001

Specific heat and disorder in the mixed state of non-magnetic borocarbides

The temperature and magnetic field dependence of the specific heat cp(T,H) in the superconducting mixed state as well as the upper critical field Hc2(T) have been measured for polycrystalline Y_xLu_{1-x}Ni_2B_2C and Y(Ni_{1-y}Pt_y)_2B_2C samples. The linear-in-T electronic specific heat contribution gamma(H)T exhibits significant deviations from the usual linear-in-H law for all x and y the transition metal site (T) resulting in a disorder dependent negative curvature of gamma(H). The deviations from that linear behaviour of our unsubstituted samples are the largest reported so far for any superconductor. The H_c2(T) data point to the quasi-clean limit for (Y,Lu)-substitutions and to a transition to the quasi-dirty limit for (Ni,Pt)-substitutions. The gamma(H) dependence is discussed in the unitary d-wave as well as in the quasi-clean s-wave limits. From a consideration of gamma(H) data only, d-wave pairing cannot be ruled out.

cond-mat.supr-con↗

arXiv2001

The upper critical field problem in MgB2

The upper critical field Hc2(T) for MgB2 is analyzed in terms of single and multi-band Eliashberg models. The relatively high value of Hc2(0) in the range of 14 to 18 Tesla can be understood, if a strongly coupled subgroup of heavy quasiparticles is involved in the superconductivity. The sizable coupling to both low- and high-frequency bosons is essential. This picture is supported by LDA calculations of Fermi velocity distribution over different sheets of the Fermi surface with special emphasis on the hole tubes. The possible origin of anharmonic soft modes is briefly discussed. Similarities and differences with transition metal borocarbides are figured out.

cond-mat.supr-con↗

arXiv2000

Specific heat of (Y,Lu)Ni2B2C in the mixed state

The temperature and magnetic field dependences of the electronic specific heat in the superconducting mixed state as well as the upper critical field have been measured on a (Y,Lu)Ni2B2C series to study the influence of disorder. The electronic specific heat in the vortex state shows deviations from the usual gamma(H) proportional to H-law for conventional superconductors for all samples resulting in a disorder dependent negative curvature of gamma proportional to H sup(1-beta). For samples with disorder at the R site beta and the positive curvature exponent alpha of H sub(c2) are reduced but alpha, beta remain always positive, as distinct from much stronger disorder due to Pt substions at the Ni site where alpha, beta rapidly tend to 0. Alpha shows a minimum at equal Y and Lu concentration. H sub(c2)(00, T sub(c), the curvatures of H sub(c2) and gamma(h), as well as the normal state Sommerfeld constant gamma sub(N) are reduced similarly.

cond-mat↗

arXiv2000

Electron-Phonon Coupling Origin of the resistivity in YNi_{2}B_{2}C Single Crystals

Resistivity measurements from 4.2 K up to 300 K were made on YNi_{2}B_{2}C single crystals with Tc=15.5 K. The resulting rho(T) curve shows a perfect Bloch-Grueneisen (BG) behavior, with a very small residual resistivity which indicates the low impurity content and the high cristallographic quality of the samples. The value lambda_{tr}=0.53 for the transport electron-phonon coupling constant was obtained by using the high-temperature constant value of d(rho)/dT and the plasma frequency reported in literature. The BG expression for the phononic part of the resistivity rho_{ph}(T) was then used to fit the data in the whole temperature range, by approximating alpha^{2}_{tr}F(Omega) with the experimental phonon spectral density G(Omega) multiplied by a two-step weighting function to be determined by the fit. The resulting fitting curve perfectly agrees with the experimental points. We also solved the real-axis Eliashberg equations in both s- and d-wave symmetries under the approximation alpha^{2}F(Omega)= alpha^{2}_{tr}F(Omega). We found that the value of lambda_{tr} here determined in single-band approximation is quite compatible with Tc and the gap Delta experimentally observed. Finally, we calculated the normalized tunneling conductance, whose comparison with break-junction tunnel data gives indication of the possible s-wave symmetry for the order parameter in YNi_{2}B_{2}C.

cond-mat.supr-con↗

arXiv2000

The electronic structure of CuSiO3 - a possible candidate for a new inorganic spin-Peierls compound ?

Electronic structure calculations are presented for the well-known CuGeO3 and the recently discovered isostructural CuSiO3 compounds. The magnitude of the dispersion in chain direction is considerably smaller for CuSiO3, whereas the main interchain couplings are rather similar in both compounds. Starting from extended one-band tight-binding models fitted to the bandstructures, the exchange integrals were estimated for both compounds in terms of a spatially anisotropic Heisenberg model. Remarkable frustrating second neighbor couplings are found both for intra- and inter-chain interactions. A magnetic moment of about 0.35 Bohr-magneton is predicted for CuSiO3 in the Neel state.

cond-mat.str-el↗