No abstract

No abstract

Measurements of the basal-plane resistivity ρ a (T,H) performed on highly oriented pyrolitic graphite, with magnetic field H || c-axis in the temperature interval 2 -300 K and fields up to 8 T, provide evidence for the occurrence of both field-induced and zero-field superconducting instabilities. Additionally, magnetization M(T,H) measurements suggest the occurrence of Fermi surface instabilities which compete with the superconducting correlations. 2 The magnetic field -temperature (H-T) phase diagram of conventional type-II superconductors is well known. In the Meissner state, the surface currents screen the applied magnetic field. Above the lower critical field H c1 (T), the field penetrates the superconductor in the form of a lattice of vortices (Abrikosov lattice). Superconductivity persists up to the upper critical field H c2 (T), described by the Abrikosov-Gor'kov theory [1,2]. On the other hand, it has been proposed [3] that the superconducting state can appear (or reappear) under application to an electron system of high enough magnetic field, such that the Landau quantization of the energy spectrum is important. In particular, when all electrons are in the lowest Landau level, the superconducting transition temperature T c (H) is expected to increase with field increasing, opposite to the T c (H) dependence in the classical low-field-limit [3]. However, superconductivity in the quantum regime has not been identified in experiments so far, remaining the subject of theoretical investigations only.In the present work, we report the results of basal-plane resistivity ρ a (T,H) measurements performed on highly oriented pyrolitic graphite (HOPG), which provide evidence for the occurrence of superconducting correlations in both quantum and classical limits. Besides, magnetization measurements M(T,H) suggest an interplay between superconducting and other Fermi surface instabilities, possibly spin-densitywave (SDW) or charge-density-wave (CDW) type.The HOPG sample was obtained from the Research Institute "GRAPHITE" (Moscow). X-ray diffraction (Θ -2Θ) measurements give the crystal lattice parameters a = 2.48 Å and c = 6.71 Å. The high degree of crystallites orientation along the hexagonal c-axis was confirmed from x-ray rocking curves (FWHM = 1.4 o ). The geometrical sample density was 2.26 ± 0.01 g/cm 3 . A cylindrical specimen with diameter of 5.2 mm, and thickness 3.14 mm, and a parallelepiped 4.9 x 4.3 x 2.5 mm 3 , both made from the 3 same piece of HOPG, were used for magnetization and transport measurements, respectively. The c-axis was along the smallest size of the sample. The studies were performed for H || c-axis. M(T,H) dc magnetization was measured in fields up to 5 T and temperatures between 2 and 300 K by means of SQUID magnetometer MPMS5 (Qunatum Design). Low-frequency (f = 1 Hz) standard four-probe resistance measurements were performed in fields up to 8 T, in the same temperature interval, with PPMS (Physical Properties Measurement System, Quantum Design).Low-temperature portions of the basal-plan...

We develop a phenomenological thermodynamic theory of ferroelectric BaTiO3 (BT) thin films epitaxially grown on cubic substrates using the Landau-Devonshire eight-order potential. The constructed "misfit-temperature" phase diagram is asymmetrical. We found that, overall view of the phase diagram depends on the values of compliances used in calculations and provide two qualitatively different diagrams. A thermodynamic path for BT film grown onto a particular substrate can be found using a plot of the room-temperature tetragonal distortion (c − a)/a as a function of misfit strain.

A near-surface structural phase transition on a SrTiO3 single crystal, occurring at T(*) about 45 K above the bulk cubic-to-tetragonal transition, is observed by means of optical second harmonic generation. The temperature dependence of the second harmonic field in the vicinity of T(*) is described with a phenomenological Landau-type model.