Anharmonic effects in the A15 compounds induced by sublattice distortions

We demonstrate that elastic anomalies and lattice instabilities in the the A15 compounds are describable in terms of first-principles LDA electronic structure calculations. We show that at T=0 V_3Si, V_3Ge, and Nb_3Sn are intrinsically unstable against shears with elastic moduli C_11-C_12 and C_44, and that the zone center phonons, Gamma_2 and Gamma_12, are either unstable or extremely soft. We demonstrate that sublattice relaxation (internal strain) effects are key to understanding the behavior of the A15 materials.Comment: 5 pages, RevTex, 3 postscript figures, Submitted to Phys. Rev. Lett. Apr. 23, 1997 July 7, 1997: minor corrections, final accepted versio

Thermodynamic stabilities of ternary metal borides: An ab initio guide for synthesizing layered superconductors

Density functional theory calculations have been used to identify stable layered Li-$M$-B crystal structure phases derived from a recently proposed binary metal-sandwich (MS) lithium monoboride superconductor. We show that the MS lithium monoboride gains in stability when alloyed with electron-rich metal diborides; the resulting ordered Li$_{2(1-x)}M_x$B$_2$ ternary phases may form under normal synthesis conditions in a wide concentration range of $x$ for a number of group-III-V metals $M$. In an effort to pre-select compounds with the strongest electron-phonon coupling we examine the softening of the in-plane boron phonon mode at $\Gamma$ in a large class of metal borides. Our results reveal interesting general trends for the frequency of the in-plane boron phonon modes as a function of the boron-boron bond length and the valence of the metal. One of the candidates with a promise to be an MgB$_2$-type superconductor, Li$_2$AlB$_4$, has been examined in more detail: according to our {\it ab initio} calculations of the phonon dispersion and the electron-phonon coupling $\lambda$, the compound should have a critical temperature of $\sim4$ K.Comment: 10 pages, 9 figures, submitted to PR

Elastic anomalies in HoNi2B2C single crystals

We have measured temperature and magnetic field dependencies of the sound velocities and the sound attenuation in HoNi2B2C single crystals. The main result is a huge softening the velocity of C66 mode due to a cooperative Jahn-Teller effect, resulting in a tetragonal-orthorhombic structural phase transition. Anomalies in the behavior of the C66 mode through various magnetic phase transitions permit us to revise the low temperature H-T phase diagrams of this compound.Comment: v2: a discussion of the C44 mode with the comparison to Y borocarbide was adde

Experimental observation of Frohlich superconductivity in high magnetic fields

Resistivity and irreversible magnetisation data taken within the high-magnetic-field CDWx phase of the quasi-two-dimensional organic metal alpha-(BEDT-TTF)2KHg(SCN)4 are shown to be consistent with a field-induced inhomogeneous superconducting phase. In-plane skin-depth measurements show that the resistive transition on entering the CDWx phase is both isotropic and representative of the bulk.Comment: ten pages, four figure

Mixed pairing symmetry in \kappa-(BEDT-TTF)_2 X organic superconductors from ultrasonic velocity measurements

Discontinuities in elastic constants are detected at the superconducting transition of layered organic conductors \kappa-(BEDT-TTF)_{2}X by longitudinal and transverse ultrasonic velocity measurements. Symmetry arguments show that discontinuities in shear elastic constants can be explained in the orthorhombic compound only if the superconducting order parameter has a mixed character that can be of two types, either A_{1g}+B_{1g} or B_{2g}+B_{3g} in the classification of irreducible representations of the orthorhombic point group D_{2h}. Consistency with other measurements suggests that the A_{1g}+B_{1g} (d_{xy}+d_{z(x+y)}) possibility is realized. Such clear symmetry-imposed signatures of mixed order parameters have not been observed in other superconducting compounds.Comment: 5 pages, LaTeX,3 figure

Gap to Transition Temperature Ratio in Density Wave Ordering: a Dynamical Mean Field Study

We use the dynamical mean-field method to determine the origin of the large ratio of the zero temperature gap to the transition temperature observed in most charge density wave materials. The method is useful because it allows an exact treatment of thermal fluctuations. We establish the relation of the dynamical mean-field results to conventional diagrammatics and thereby determine that in the physically relevant regime the origin of the large ratio is a strong inelastic scattering.Comment: 4 figure

Coexistence of Band Jahn Teller Distortion and superconductivity in correlated systems

The co-existence of band Jahn-Teller (BJT) effect with superconductivity (SC) is studied for correlated systems, with orbitally degenerate bands using a simple model. The Hubbard model for a doubly degenerate orbital with the on-site intraorbital Coulomb repulsion treated in the slave boson formalism and the interorbital Coulomb repulsion treated in the Hartree-Fock mean field approximation, describes the correlated system. The model further incorporates the BJT interaction and a pairing term to account for the lattice distortion and superconductivity respectively. It is found that structural distortion tends to suppress superconductivity and when SC sets in at low temperatures, the growth of the lattice distortion is arrested. The phase diagram comprising of the SC and structural transition temperatures $T_c$ and $T_s$ versus the dopant concentration $\delta$ reveals that the highest obtainable $T_c$ for an optimum doping is limited by structural transition. The dependence of the occupation probabilities of the different bands as well as the density of states (DOS) in the distorted-superconducting phase, on electron correlation has been discussed.Comment: RevTex, 4 pages, 4 figuers (postscript files attached) Journal Reference : Phys. Rev. B (accepted for publication

Strong diamagnetic response and specific heat anomaly above T_c in underdoped La_(2-x)Sr_xCuO_4

By measuring AC susceptibility using a very low amplitude of the AC field (<1 mG) it is shown that underdoped samples of La_(2-x)Sr_xCuO_4 (LASCO), are diamagnetic in a temperature region above T_c up to a temperature T^*. This behavior is only observed with AC fields along the c-axis whereas for fields in the ab-plane no diamagnetism above Tc was detected. The diamagnetism is almost frequency independent in the frequency range 0.1-10 kHz. At T* a broad step anomaly in the specific heat is inferred through measurements of the elastic constant c33. We suggest that the observed diamagnetism and the anomaly in the elastic constant are associated with the existence of phase incoherent Cooper pairs between Tc and T*.Comment: 5 pages 7 figures, to appear in Phys. rev

Gap ratio in anharmonic charge-density-wave systems

Many experimental systems exist that possess charge-density-wave order in their ground state. While this order should be able to be described with models similar to those used for superconductivity, nearly all systems have a ratio of the charge-density-wave order parameter to the transition temperature that is too high for conventional theories. Recent work explained how this can happen in harmonic systems, but when the lattice distortion gets large, anharmonic effects must play an increasingly important role. Here we study the gap ratio for anharmonic charge-density wave systems to see whether the low-temperature properties possess universality as was seen previously in the transition temperature and to see whether the explanation for the large gap ratios survives for anharmonic systems as well.Comment: (5 pages, 3 figures, ReVTeX

Weak Localization Effect in Superconductors by Radiation Damage

Large reductions of the superconducting transition temperature $T_{c}$ and the accompanying loss of the thermal electrical resistivity (electron-phonon interaction) due to radiation damage have been observed for several A15 compounds, Chevrel phase and Ternary superconductors, and $\rm{NbSe_{2}}$ in the high fluence regime. We examine these behaviors based on the recent theory of weak localization effect in superconductors. We find a good fitting to the experimental data. In particular, weak localization correction to the phonon-mediated interaction is derived from the density correlation function. It is shown that weak localization has a strong influence on both the phonon-mediated interaction and the electron-phonon interaction, which leads to the universal correlation of $T_{c}$ and resistance ratio.Comment: 16 pages plus 3 figures, revtex, 76 references, For more information, Plesse see http://www.fen.bilkent.edu.tr/~yjki