Yu. B. Kudasov

Semiconductor-metal transition in FeSi in ultrahigh magnetic fields up to 450 T

The magnetic susceptibility and conductivity of single-crystal iron monosilicide are investigated in ultrahigh magnetic fields up to 450 T at a temperature of 77 K. It is found that the conductivity of iron monosilicide increases continuously by two orders of magnitude as the magnetic field increases. The results obtained can be interpreted as a semiconductor-metal transition induced by the magnetic field. The dependence of the conductivity on the magnetic field is described well on the basis of the spin-fluctuation theory.

Semiconductor-metal transition in FeSi in an ultrahigh magnetic field

We study the conductivity and magnetic susceptibility of single-crystal iron monosilicide in ultrahigh magnetic fields (up to 500 T) at low temperatures. The experimental methods used in measuring the conductivity and magnetic susceptibility are discussed. At 77K we detect a gradual increase in the conductivity of iron monosilicide by more than a factor of 100 as the magnetic field gets stronger. At 4.2K we detect a first-order phase transition in a field of 355 T accompanied by a sudden change in the value of the magnetic moment by 0.95 μB per iron atom and a transition to a phase with high conductivity. The results are discussed within the scope of the spin-fluctuation theory.

Temperature-dependent magnetospectroscopy of HgTe quantum wells

We report on magnetospectroscopy of HgTe quantum wells in magnetic fields up to 45 T in temperature range from 4.2 K up to 185 K. We observe intra- and inter-band transitions from zero-mode Landau levels, which split from the bottom conduction and upper valence subbands, and merge under the applied magnetic field. To describe experimental results, realistic temperature-dependent calculations of Landau levels have been performed. We show that although our samples are topological insulators at low temperatures only, the signature of such phase persists in optical transitions at high temperatures and high magnetic fields. Our results demonstrate that temperature-dependent magnetospectroscopy is a powerful tool to discriminate trivial and topological insulator phases in HgTe quantum wells.

Nearest-neighbor correlations in the Hubbard model

Abstract The Hubbard Hamiltonian is investigated by means of a new variational trial wave function. The trial wave function includes either intrasite and nearest-neighbor correlations in an explicit form. To calculate density matrices Kikuchis pseudoensemble method is used. The case of a half-filled fermionic band is carefully investigated in the limit of a large number of lattice sites. The ground state energy and correlation functions are determined for Bethe lattices with z = 2,4 and 6 nearest neighbors.The Hubbard Hamiltonian is investigated by means of a variational trial wave function of Gutzwillers type. The wave function includes nearest - neighbor correlations in an explicit form. To calculate density matrices the method of Kikuchis pseudoensemble is used. The case of half-filled fermionic band carefully investigated in the limit of a large number of lattice sites. The ground state energy and correlation functions are determined for lattices with z=2,4 and 6 nearest neighbors.

Induced Rashba splitting of electronic states in monolayers of Au, Cu on a W(110) substrate

The paper sums up a theoretical and experimental investigation of the influence of the spin–orbit coupling in W(110) on the spin structure of electronic states in deposited Au and Cu monolayers. Angle-resolved photoemission spectroscopy reveals that in the case of monolayers of Au and Cu spin–orbit split bands are formed in a surface-projected gap of W(110). Spin resolution shows that these states are spin polarized and that, therefore, the spin–orbit splitting is of Rashba type. The states evolve from hybridization of W 5d, 6p-derived states with the s, p states of the deposited metal. Interaction with Au and Cu shifts the original W 5d-derived states from the edges toward the center of the surface-projected gap. The size of the spin–orbit splitting of the formed states does not correlate with the atomic number of the deposited metal and is even higher for Cu than for Au. These states can be described as W-derived surface resonances modified by hybridization with the p, d states of the adsorbed metal. Our electronic structure calculations performed in the framework of the density functional theory correlate well with the experiment and demonstrate the crucial role of the W top layer for the spin–orbit splitting. It is shown that the contributions of the spin–orbit interaction from W and Au act in opposite directions which leads to a decrease of the resulting spin–orbit splitting in the Au monolayer on W(110). For the Cu monolayer with lower spin–orbit interaction the resulting spin splitting is higher and mainly determined by the W.

Lattice dynamics and phase diagram of aluminum at high temperatures

The dispersion of phonons in the fcc, hcp, and bcc phases of aluminum is calculated at ultrahigh pressures by the method of small displacements in a supercell. The stability of the phonon subsystem is studied. The thermodynamic characteristics are calculated in the quasi-harmonic approximation, and a phase diagram of aluminum is plotted. As compared to the Debye model, the use of a phonon spectrum calculated in the quasi-harmonic approximation significantly broadens the hcp phase field and strongly shifts the phase boundary between the fcc and bcc phases. The normal isentrope is calculated at megabar pressures. It is shown to intersect the fcc-hcp and hcp-bcc phase boundaries. The sound velocity along the normal isentrope is calculated. It is shown to have a nonmonotonic character.

Investigation of magnetoabsorption at different temperatures in HgTe/CdHgTe quantum-well heterostructures in pulsed magnetic fields

The magnetoabsorption in magnetic fields as high as 40 T is investigated at T > 77 K in HgTe/CdHgTe quantum-well heterostructures (dQW ≈ 8 nm). The spectra reveal two lines associated both with intraband transition from the lower Landau level in the conduction band and with interband transition. It is shown that the band structure in these systems changes from inverted to normal with increasing temperature.

Magnetic structure and phase diagram of Sr5Rh4O12 frustrated Ising magnet

The magnetic structure of Sr5Rh4O12 is based on Ising chains of rhodium ions with a variable valence, Rh3+-Rh4+. The ordering in the chains is assumed to be ferromagnetic. It has been shown that the magnetic structure and phase diagram of Sr5Rh4O12 are well described in a model taking into account weak antiferromagnetic interactions between the nearest and next-nearest neighbors on the triangular lattice of ferromagnetic Ising chains. The ground state at low temperatures is the two-sublattice stripe phase; this phase in the magnetic field is transformed to the ferrimagnetic phase and, then, to the ferromagnetic phase. Small plateaus can be observed in the region of the transition from the ferrimagnetic phase to the ferromagnetic one.

Measurements of High-Frequency Complex Impedances in Fast Processes

A fast-acting complex impedance measuring device for conducting investigations in pulse electrophysical experiments is described. The device features a high noise immunity. The working frequency of the measuring device varies from 20 to 80 MHz, and the time constant is <100 ns.

Megagauss magnetization measurements

Techniques of magnetization measurements in ultrahigh magnetic fields are discussed. A high rate of magnetic field growth and electromagnetic noises impose severe restriction on an experimental set-up. It is shown that a magnetization step (at a metamagnetic transition) and the conductivity can be estimated from magnetization data. Magnetic oscillations are also observable in ultrahigh magnetic fields. Experimental results on FeSi, high-temperature superconductors, rare-earth intermetallic compounds, and high-spin molecular clusters are briefly reviewed.