Growth-induced anisotropy in Ba0.6K0.4BiO3 superconducting single crystal

0.6 K_0.4BiO₃ single crystal have been performed to study the nature of the growth-induced anisotropy revealed using chemical etching technique. It was found that this anisotropy is responsible for differences in low-field susceptibility and in hysteresis loops observed for different magnetic field orientations. Received: 9 December 1997/Accepted: 28 May 1998     

Anharmonicity and superconductivity in Ba0.6K0.4BiO3

The temperature dependence of the x-ray absorption spectra above the L ₃ absorption edge of bismuth in the superconducting oxide Ba_0.6K_0.4BiO₃ are investigated. It is found that the local structure is different from the simple cubic structure indicated by x-ray and neutron-diffraction data. It is shown that the oxygen atoms move in an anharmonic double-well potential arising as a result of the existence of two nonequivalent types of octahedral environments of bismuth. Vibrations in such potential modulate the Bi-O bond lengths at the low frequency of the rotational (“tilting” type) mode of the oxygen octahedra and thus give rise to a strong electron-phonon interaction, which explains the quite high superconducting transition temperatures T _c ∼30 K. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 12, 977–982 (25 June 1998)     

Soft phonons and the central mode at structural phase transitions

Applying Landau's quasi-particle picture to an interacting phonon system it is shown that the softening of phonon modes is connected with an enhancement of the quasi-particle interaction. This in turn leads to a critical slowing-down of fluctuations around local equilibrium which may give rise to an additional central peak in the scattering function.     

Direct transition from Bose glass to normal state in the (K,Ba)BiO3 superconductor

The introduction of columnar defects in (K,Ba )Bi O3 single crystals shifts both the irreversibility and thermodynamic transition lines, respectively, deduced from ac susceptibility (and/or transport) and specific heat measurements, upwards. This shift can be attributed to the defect-induced decrease of the difference (Delta F) between the free energies in the superconducting and the normal states, assuming that the position of the superconducting transition is given by the condition absolute value Delta F approximately k(B )T/xi(3 ). This criterion also perfectly reproduces the influence of the angle between the tracks and the external field. This result suggests that no vortex liquid phase exists in this system.     

Anomalous magnetic field dependence of the thermodynamic transition line in the isotropic superconductor (K,Ba)BiO3

Thermodynamic (specific heat, reversible magnetization, tunneling spectroscopy) and transport measurements have been performed on high quality (K,Ba)BiO3 single crystals. The temperature dependence of the magnetic field H(C(p)) corresponding to the onset of the specific heat anomaly presents a clear positive curvature. H(C(p)) is significantly smaller than the field H(Delta) for which the superconducting gap vanishes but is closely related to the irreversibility line deduced from transport data. Moreover, the temperature dependence of the reversible magnetization presents a strong deviation from the Ginzburg-Landau theory emphasizing the peculiar nature of the superconducting transition in this material.     

Anisotropic enhancement of superconductivity in heavy-ion irradiated (K,Ba)BiO3

We have measured the specific heat, resistivity, and ac susceptibility of (K,Ba)BiO3 single crystals before and after introduction of either point or columnar defects by electron (EI) or heavy-ion irradiation (HII). While the magnetic field dependence of these properties remains mainly unaffected by EI, the irreversibility line and the location of the specific heat anomaly are both shifted up in temperature after HII. The shift is apparent only if the magnetic field is applied parallel to the ion tracks. For perpendicularly applied fields, both lines lie at the same field as in the pristine sample. These experiments call the nature of the vortex liquid state into question.     

Soft mode at an incommensurate structural phase transition

We show within the RPA approximation that a soft mode is associated with an incommensurate structural phase transition in a simple microscopic model of two level ions interacting with the phonons of a crystal.     

Increase in the magnetization loop width in the Ba0.6K0.4BiO3 superconductor: Possible manifestation of phase separation

The magnetization of Ba_0.6K_0.4BiO₃ samples in fields up to 90 kOe in the temperature range from 2 to 30 K is investigated. It is shown that the observed increase in the width of the magnetization loop can be explained by a decrease in the phase nonuniformity upon an increase in the magnetic field. The asymmetric hysteretic dependence of magnetization with the secondary peak was successfully described by the extended critical state model taking into account the phase separation in the superconductor.     

Anharmonic phonons and structural phase transitions

Exact results for the order parameter and the meansquare displacement as functions of temperature are given for a quantum interacting phonon Hamiltonian with quartic anharmonicities. Upper bounds for the transition temperature are also derived. Approximate theories including the mean field approximation, the random phase approximation (or quasiharmonic approximation) and the self consistent approach (using Blume-Hubbard scheme) are compared with our exact results. The mean field approximation for the meansquare displacement is found to violate our bounds.The classical value is shown to form a lower bound for the kinetic energy. Upper bounds for the kinetic energy are obtained showing the region of temperature in which the use of the high temperature expansion of the fluctuation-dissipation theorem is justified. Comparison of the Hamiltonian and our results with electron-paramagnetic-resonance measurements are discussed.     

Microwave impedance of Ba0.6K0.4BiO3 crystals: Comparison with Nb

The surface impedance Z _s=R _s+iX _s of samples of Ba_0.6K_0.4BiO₃ in the temperature range 4<T<50 K is measured at 9.42 GHz. The BCS theory completely describes the electrodynamic properties of Nb in the dirty limit, and its application to Ba_0.6K_0.4BiO₃ allows determination of the London penetration depth λ _L(0)=3100±100 Å. Zh. éksp. Teor. Fiz. 111, 696–704 (February 1997)     

Ab initio prediction of pressure-induced structural phase transition of superconducting FeSe

External pressure driven phase transitions of FeSe are predicted using ab initio calculations. The calculations reveal that α-FeSe makes transitions to NiAs-type, MnP-type, and CsCl-type FeSe. Transitions from NiAs-type to MnP-type and CsCl-type FeSe are also predicted. MnP-type FeSe is also found to be able to transform to CsCl-type FeSe, which is easier from α-FeSe than the transition to MnP-type FeSe, but comparable to the transition from NiAs-type FeSe. The calculated electronic structures show that all phases of FeSe are metallic, but the ionic interaction between Fe-Se bonds becomes stronger and the covalent interaction becomes weaker when the structural phase transition occurs from α-FeSe to the other phases of FeSe. The experimentally observed decrease in T(c) of superconducting α-FeSe at high pressure may be due to a structural/magnetic instability, which exists at high pressure. The results suggest an increase of the T(c) of α-FeSe if such phase transitions are frustrated by suitable methods.     

Nonlinear ac response of the vortex system in the cubic (K, Ba)BiO3 superconductor

We present ac susceptibility measurements performed on single crystals with different geometries: thick films, bars and hollow cylinders. We show that the dependence of the real () and imaginary () parts of the ac susceptibility is in very good agreement with Brandt's numerical calculations (Phys. Rev. B 58, 6523 (1998)) in the modified Bean critical state. Creep effects (at ) are investigated by studying the frequency dependence of the current density deduced from the temperature scans of the ac susceptibility over a large frequency range (). The relaxation rate is temperature independent and very similar to the one usually obtained in high cuprates. Received 17 March 1999     

On a possible superconducting phase transition in Fe2Te3

A superconducting phase transition is predicted for Fe₂Te₃ on the basis of an empirical determination of the electron-phonon coupling constant λ. This result is shown to be consistent with recent calculations of the superconducting state parameters.     

Hydrostatic-pressure induced phase transition of phonons in single-walled nanotubes

We study the effect of the hydrostatic pressure on the phonons in single-walled carbon nanotubes (SWNTs) in a magnetic field. We calculate the magnetic moments of the phonons using a functional integral technique, and find that the phonons in SWNTs undergo a pressure-induced phase transition from the paramagnetic phase to the diamagnetic phase under hydrostatic pressure 2 GPa. We explain the mechanism of generating this phase transition.     

Linear low-temperature dependence of the surface impedance of a Ba0.6K0.4BiO3 single crystal

The temperature dependences of the real part R _s and the imaginary part X _s of the surface impedance Z _s =R _s +iX _s of the superconductor Ba_0.6K_0.4BiO₃ (T _c ≃30 K) are measured at a frequency of 9.4 GHz. Its temperature dependence _{Z s} (T) and that of the complex conductivity σ _s (T) can be described on the basis of a two-fluid model under two assumptions: The density of superconducting carriers increases linearly, and the relaxation time increases as a power law (∝1/T ⁵), with decreasing temperature T<T _c . This model also describes well the curves Z _s (T) and σs (T) recently measured for YBa₂Cu₃O_6.95 and Bi₂Sr₂CaCu₂O₈ single crystals. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 783–788 (10 December 1996)