Upper critical field in {Ba1−x_{1-x}Kx_xBiO3_3}: magnetotransport versus magnetotunneling

Elastic tunneling is used as a powerful direct tool to determine the upper critical field $H_{c2}(T)$ in the high-$T_c$ oxide Ba$_{1-x}$K$_x$BiO$_3$. The temperature dependence of $H_{c2}$ inferred from the tunneling follows the Werthamer-Helfand-Hohenberg prediction for type-II superconductors. A comparison will be made with resistively determined critical field data.Comment: 4 pages incl. 5 figure

Disentanglement of the electronic and lattice parts of the order parameter in a 1D Charge Density Wave system probed by femtosecond spectroscopy

We report on the high resolution studies of the temperature (T) dependence of the q=0 phonon spectrum in the quasi one-dimensional charge density wave (CDW) compound K0.3MoO3 utilizing time-resolved optical spectroscopy. Numerous modes that appear below Tc show pronounced T-dependences of their amplitudes, frequencies and dampings. Utilizing the time-dependent Ginzburg-Landau theory we show that these modes result from linear coupling of the electronic part of the order parameter to the 2kF phonons, while the (electronic) CDW amplitude mode is overdamped.Comment: 4 pages, 3 figures + supplementary material, accepted for publication in Phys. Rev. Let

Investigations of the magnetic and lattice vibrations in CoBr 2 using neutron scattering

Inelastic neutron scattering has been used to investigate both the acoustic phonons and the magnetic excitations in CoBr2. The acoustic phonon dispersion curves were measured in three principal symmetry directions at room temperature. From the initial slopes the sound velocities have been derivated and the elastic constants calculated. The two dimensional nature of the crystallographic structure is apparent in the difference between the sound velocities parallel and perpendicular to the closed packed layers although the difference is less than in other halides. Furthermore, the order parameter of the magnetic lattice was found to be β = 0.3 a value not inconsistent with that expected for a 3 D Ising model. The magnon dispersion was investigated in the (100) and (001) directions at 5 K. The observed dispersion could be explained using a model containing an isotropic exchange within the ground state doublet of the 4T 1 term. Renormalization of the spin waves revealed a strong magnon-phonon coupling in the (001) direction.Les phonons acoustiques et les magnons ont été étudiés dans CoBr2 par la diffusion inélastique des neutrons. Les courbes de dispersion des phonons acoustiques ont été mesurées dans les trois principales directions de symétrie, à la température ambiante. Les vitesses du son et les constantes élastiques en ont été déduites. Le caractère bidimensionnel de la structure cristallographique est mis en évidence par la différence entre les vitesses du son se propageant parallèlement et perpendiculairement aux plans. La valeur obtenue pour les paramètres d'ordre du réseau magnétique (β = 0,3) est en accord avec celle attendue dans le cas d'un modèle d'Ising à 3 dimensions. La dispersion des magnons a été étudiée dans les directions (100) et (001) à 5 K. On rend compte de la dispersion observée en utilisant un échange isotrope à l'intérieur du doublet fondamental issu du terme 4T1. La renormalisation des ondes de spins met en évidence un fort couplage magnon-phonon dans la direction (001)

Evidence for collective δT

The nature of pinning has been investigated in the three-dimensional high-Tc superconductor $\rm (K,Ba)BiO_3$. We show that the magnetic-field dependence of the critical current deduced from pulsed-field magnetization measurements can be well described by the collective-pinning theory on a large temperature range $0.06 < T/T_{\rm c} < 0.62$. This permits us to construct a nonequilibrium H-T phase diagram including both the single-vortex and vortex bundle regimes. Furthermore, the temperature dependence of the collective-pinning length Lc is in excellent agreement with a model of flux pinning induced by spatial fluctuations of the critical temperature ($\delta T_{\rm c}$ pinning model)

PHONON DISPERSION AND TRANSVERSE MODE SOFTENING IN RbFeCl3

Phonon dispersion curves of RbFeCl3 have been measured by neutron scattering. For wave vectors in the hexagonal plane the transverse phonons polarized along Z are found to have very low energies. Furthermore the shape of this branch changes drastically as the temperature is lowered from room temperature to 20K, the zone boundary energy at the K point being decreased by a factor of two. However no Bragg reflection indicative of a phase transition has been detected at this point. These features are interpreted as showing evidence that RbFeCl3 is at the border of a phase transition for which a microscopic mechanism will be discussed

Relaxation and anomalous

Ac shielding and classical dc relaxation experiments have been used to study the flux creep phenomena in the cubic ${\rm (K,Ba)BiO_3}$ superconductor ($T_{\rm c} \sim 30\;{\rm K}$). The relaxation rate is found to be constant ($S \sim 1.5\%$) at low temperature and magnetic field and increases sharply as the vortex-glass transition line is approached. This behavior can be attributed to an anomalous decrease of the μ exponent ($U(J) = U_0(J_0/J)^{\mu}$) close to $T_{\rm g}(H)$. In this regime, the temperature dependence of the apparent critical current J is then directly related to $\mu(T)$ as $J(T) = J_0/[kT/U_0\cdot\ln(1/\omega \tau)]^{\mu (T)}$. A similar analysis can be made on the J(B) data recently published by Abulafia et al. (Phys. Rev. Lett., 77 (1996) 1597) on $\rm YBaCuO$ single crystals

Cross-sectional TEM investigations of the microstructure and film-substrate substrate interface in Y-Ba-Cu-O thin films

International audienc