Semiclassical theory of potential scattering for massless Dirac fermions

In this paper we study scattering of two-dimensional massless Dirac fermions by a potential that depends on a single Cartesian variable. Depending on the energy of the incoming particle and its angle of incidence, there are three different regimes of scattering. To find the reflection and transmission coefficients in these regimes, we apply the Wentzel–Kramers–Brillouin (WKB), also called semiclassical, approximation. We use the method of comparison equations to extend our prediction to nearly normal incidence, where the conventional WKB method should be modified due to the degeneracy of turning points. We compare our results to numerical calculations and find good agreement.     

Effect of nanoparticles on the structure and properties of an aluminum alloy poured into a mould with the use of a magnetohydrodynamic disk pump

The influence of two nanomodifiers with different compositions during their homogenization in the AL7 aluminum melt and moulding on the properties of the modified aluminum alloy is studied. Experiments are performed with the use of a centrifugal conductive magnetohydrodynamic pump. The melt is poured into a graphite mould with three cylindrical channels 38 mm in diameter and 160 mm long, which are designed for a metal mass of 500 g. Two compositions are used as modifying agents: nano-scale particles of the aluminum nitride powder 40-100 nm in size and metallized carbon nanotubes smaller than 25 nm, which are clad with aluminum to improve wetting of their surface. The analysis of the structure of the experimental and reference samples shows that the use of modifiers leads to refinement of the grain structure of the cast metal. According to the Hall-Petch theory, this effect may result in improvement of mechanical characteristics of the cast metal.     

Photo-induced bias of the hysteresis loop in ferrites at room temperature with long-time memory

New photo-magnetic effects with an indefinitely long-time memory are found at room temperature in the epitaxial Mg_0.75Mn_0.21Co_0.04Fe₂O₄ ferrite film. Illumination of the ferrimagnetic material with low-intensity (0.4 W cm⁻²) circularly polarized light with or without a static magnetic field in the Faraday effect geometry results in a number of nonlinear effects in both space and time. In a uniform crystal with cubic symmetry, the long-lived photo-induced magnetization (PIM) with a unidirectional anisotropy appears along the direction of the incoming light. The effects depend on a combination of magnetic field H and the helicity of circularly polarized light σ. Two combinations H⁺,σ⁺ and H⁻,σ⁻ lead to a photo-induced unidirectional anisotropy with a shift of the hysteresis loop along an applied field and a change in loop parameters. The loop contracts by a factor of two, the shift of the mid-point H_sh increases by factor of five surpassing the coercivity H_c, the coercivity H_c1 and remanence M_r1 (for decreasing applied field) reverse the sign, increasing by 9 Oe and reducing by a factor of 4.5, respectively. The effects cannot be erased by a conventional demagnetization (using an AC current that is reduced to zero amplitude), but can be removed using an illumination with two other combinations (H⁺,σ⁻ and H⁻,σ⁺) as well as by heating at temperatures higher than the Curie temperature. This long-lived room-temperature memory effect may arise from the formation of complex photo-induced defects including photo-induced magnetic polarons. The possible mechanisms responsible for the appearance of a room-temperature photo-induced unidirectional anisotropy with a long-lived memory are discussed. These new photo-magnetic effects may find an application in magneto-optical memory devices.     

Finite-temperature magnetism of transition metals: an ab initio dynamical mean-field theory

We present an ab initio quantum theory of the finite-temperature magnetism of iron and nickel. A recently developed technique which combines dynamical mean-field theory with realistic electronic structure methods successfully describes the many-body features of the one electron spectra and the observed magnetic moments below and above the Curie temperature.     

Quantum oscillations without quantum coherence

We study numerically the damping of quantum oscillations and the dynamics of the density matrix in model many-spin systems decohered by a spin bath. We show that oscillations of some density matrix elements can persist with considerable amplitude long after other elements, along with the entropy, have come close to saturation, i.e., when the system has been decohered almost completely. The oscillations exhibit very slow decay, and may be observable in experiments.     

Kondo resonance for orbitally degenerate systems

Formation of the Kondo state in the general two-band Anderson model has been investigated within the numerical renormalization group calculations. The Abrikosov-Suhl resonance is essentially asymmetric for the model with one electron per impurity (quarter filling case) in contrast with the one-band case. An external magnetic (pseudomagnetic) field breaking spin (orbital) degeneracy leads to asymmetric splitting and essential broadening of the many-body resonance. Unlike the standard Anderson model, the "spin-up" Kondo peak is pinned against the Fermi level, but not suppressed by the magnetic field.     

Experimental Study of an MHD Source of Light with a TC Layer

The possibility of using a high-temperature current layer initiated in a plasma flow as a powerful source of light with a wide range of emission is demonstrated. The study was performed on a setup with a disk MHD channel in the regime of TC layer generation in argon and sodium plasma flows.     

Quantum-Hall activation gaps in graphene

We have measured the quantum-Hall activation gaps in graphene at filling factors nu=2 and nu=6 for magnetic fields up to 32 T and temperatures from 4 to 300 K. The nu=6 gap can be described by thermal excitation to broadened Landau levels with a width of 400 K. In contrast, the gap measured at nu=2 is strongly temperature and field dependent and approaches the expected value for sharp Landau levels for fields B>20 T and temperatures T>100 K. We explain this surprising behavior by a narrowing of the lowest Landau level.