Energy gaps and half-metallicity in β-graphyne nanoribbons

We investigate the energy gaps and half-metallicity of the zigzag-edged β-graphyne nanoribbons via a tight-binding approach. In the presence of on-site Coulomb repulsion and proper transverse electric field strengths, the nanoribbons are forced into a half-metallic state by the electric field. A phase transition from half-metal to insulator is realized by changing the electric field or Coulomb potential. Both the electric field and Coulomb repulsion can open direct band gaps, resulting in a metal-insulator phase transition. The band gaps oscillate with the electric field, contrary to linear change with the Coulomb potential.     

Effect of long range coulomb interactions on the mott transition

We reconsider the Mott transition problem in the presence of long range Coulomb interactions. Using an extended dynamical mean field theory (DMFT) that sums an important class of diagrams absent in ordinary DMFT, we show that in the presence of Coulomb interactions, the zero temperature Mott transition is, as envisioned by Mott, discontinuous in two and three dimensions.     

The quantum control of electron states in a double quantum dot under coulomb blocking conditions

The possibility of providing for a quantum control of electron states by means of a weak electric field (constant or alternating) acting upon a system is studied in a nondissipative approximation for a system of two electrons in a double quantum dot (QD) under Coulomb blocking conditions. It is shown that the Coulomb repulsion facilitates controlled transition of the system from a symmetric (one electron in each QD) to asymmetric (both electrons in one QD) electron configuration under the action of a resonance alternating field or a slowly varying (quasi-constant) field. In the absence of Coulomb repulsion, two electrons can be localized in the same QD only under the action of a strong electric field.     

Orbital-selective mott transitions in the degenerate Hubbard model

We investigate the Mott transitions in two-band Hubbard models with different bandwidths. Applying dynamical mean field theory, we discuss the stability of itinerant quasiparticle states in each band. We demonstrate that separate Mott transitions occur at different Coulomb interaction strengths in general, which merge to a single transition only under special conditions. This kind of behavior may be relevant for the physics of the single-layer ruthenates, Ca2-xSrxRuO4.     

The isospin forbidden 1− → 0+ transition in 40Ca

The isospin forbidden transition 1^? (6.95 MeV) → 0⁺ (g.s.) in ⁴⁰Ca is explained within a model that mixes isospin through single-particle energy differences and the two-body Coulomb interaction. There is no need to introduce an isospin non-conserving part of the nucleon-nucleon interaction to explain this anomalously fast transition.     

Coulomb effects on the Peierls transition

The influence of the intrachain Coulomb interactions on the Peierls transition is examined. The change of the form of the electronic polarizability at twice the Fermi wavevector, to a power law divergence in the presence of Coulomb interactions, is found to enhance the mean field transition temperature for the Peierls transition.     

An improved mean field approach to the phase structure in ZN gauge theory in four dimensions

We extend the mean field approximation scheme to include the effect of fluctuation of the gauge field. As a consequence, we successfully obtain for the Z_N theory (N > 5) the phase transition which separates the Coulomb phase from the ordered phase, as well as that separating the Coulomb and disordered phases. The former transition shows characteristics of higher-order phase transitions.     

Crystal field potential of PrOs4Sb12: consequences for superconductivity

The results of inelastic neutron scattering provide a solution for the crystal field level scheme in PrOs4Sb12, in which the ground state in the cubic crystal field potential of T(h) symmetry is a Gamma(1) singlet. The conduction electron mass enhancement is consistent with inelastic exchange scattering, and we propose that inelastic quadrupolar, or aspherical Coulomb, scattering is responsible for enhancing the superconducting transition temperature. PrOs4Sb12 appears to be the first compound in which aspherical Coulomb scattering is strong enough to overcome magnetic pair breaking and increase T(c).     

Resonant charge exchange involving highly excited atoms

Transition of a classical electron between two Coulomb centers is analyzed on the basis of computer simulations. The contribution to the electron transfer cross section from a tunnel electron transition is evaluated taking into account the strong mixing of highly excited electron states due to motion of Coulomb centers. The rate of transition of a highly excited electron between two Coulomb cores with a fixed separation is evaluated together with the cross section of resonant charge exchange in slow collisions. Typical times of change of the electron momentum as a result of electron motion in the field of two Coulomb centers are determined. The text was submitted by authors in English.     

Universal aspects of coulomb-frustrated phase separation

We study the consequences of Coulomb interactions on a system undergoing a putative first order phase transition. In two dimensions (2D), near the critical density, the system is universally unstable to the formation of new intermediate phases, which we call "electronic microemulsion phases," which consist of an intermediate scale mixture of regions of the two competing phases. A corollary is that there can be no direct transition as a function of density from a 2D Wigner crystal to a uniform electron liquid. In 3D, if the strength of the Coulomb interactions exceeds a critical value, no phase separation occurs, while for the weaker Coulomb strength electronic microemulsions are inevitable. This tendency is considerably more pronounced in anisotropic (quasi-2D or quasi-1D) systems, where a devil's staircase of transitions is possible.     

Structural α-γ transition in iron within the GGA + DMFT method taking into account the rotational invariance of the Coulomb interaction

The structural α-γ transition in iron has been studied in the framework of the GGA + DMFT method using the generalized gradient approximation (GGA) and the dynamical mean field theory (DMFT). The impurity problem in the DMFT has been solved using the recently proposed method based on the Hirsch-Fye algorithm and approximately taking into account the rotational invariance of the Coulomb interaction. It has been shown that a decrease in the calculated Curie-Weiss temperature is accompanied by a decrease in the calculated α-γ transition temperature if one takes into account the rotational invariance of the Coulomb interaction. Moreover, the agreement between the calculated α-γ transition temperature and its experimental value is improved. The results obtained agree with the earlier proposed explanation of the mechanism of this transition, according to which its main driving force is the magnetic correlation energy.     

Photofield ionization of deep centers in semiconductors,taking Coulomb-field effects into account

The electrical-absorption coefficient in semiconductors is calculated for the case of charge-carrier transition from a deep level to the closest level at the base of a Hulthen model potential, including both short-range and long-range (Coulomb) components. The effect of the Coulomb field on the electron wave function in the conduction band is taken into account.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 85–91, December, 1977.     

5d过渡金属氧化物中的奇异量子物性研究

相比于3d和4d过渡金属元素, 5d过渡金属元素既具有很强的自旋轨道耦合相互作用, 同时它们的电子关联作用也不可忽略. 因而5d过渡金属氧化物体系具有许多奇异的量子特性. 这篇综述主要介绍我们在5d过渡金属氧化物中的一些理论进展. 首先介绍烧绿石结构铱氧化物(A₂Ir₂O₇, A=Y或稀土元素)中的Weyl拓扑半金属性. 我们确定出A₂Ir₂O₇这一类具有阻挫结构材料的磁基态, 并预言其是Weyl半金属; 其Weyl 点受到拓扑保护而稳定, 而且它的表面态在费米能级形成特别的费米弧. 其次预言尖晶石结构锇氧化物(AOs₂O₄, A=Ca, Sr)是具有奇异磁电响应的Axion绝缘体; 然后分析了电子关联、自旋轨道耦合对钙钛矿结构的锇氧化物(NaOsO₃)的影响, 并成功定出它的基态磁构型, 最终确定其为Slater绝缘体. 最后介绍了LiOsO₃中铁电金属性的成因.     

Features of transition and Cherenkov radiations and the correlation between them

It is shown that transition radiation arising at the boundary of two media is being emitted as a Cherenkov one, if the phase velocity of transition radiation waves in the medium of transition radiation propagation becomes equal to the velocity of the moving radiating particle (the necessary condition for the Cherenkov radiation). The proof of this statement is based on the analysis of the transition radiation formation zone, which may become large enough and provide interference between the field of transition radiation and the own Coulomb field of the moving particle, in case when the Cherenkov radiation condition is fulfilled. As a result, the transition radiation field transforms into the Cherenkov field. The problem is considered for cases of both a waveguide and free space.     

Interatomic Coulomb interaction influence on the superradiance phase transition

The phase transition to the superradiance state, when the Coulomb interactions between atoms exist, is considered. It is shown that transition temperature decreases and field value threshold increases. The display of this effects in the photon echo observation is discussed.     

Pressure-induced metal-insulator transition in LaMnO3 is not of Mott-Hubbard type

Calculations employing the local density approximation combined with static and dynamical mean field theories (LDA+U and LDA+DMFT) indicate that the metal-insulator transition observed at 32 GPa in paramagnetic LaMnO3 at room temperature is not a Mott-Hubbard transition, but is caused by orbital splitting of the majority-spin eg bands. For LaMnO3 to be insulating at pressures below 32 GPa, both on-site Coulomb repulsion and Jahn-Teller distortion are needed.     

Starspots,stellar cycles and stellar flares: Lessons from solar dynamo models

We theoretically investigate the multistable behavior of a hybrid optomechanical system,in which a charged mechanical resonator is coupled via Coulomb interaction to an optomechanical cavity containing an optical parametric amplifier(OPA).It is shown that the multistable behavior of the mean intracavity photon number can be controlled flexibly by adjusting the nonlinear gain parameter of the OPA,the phase of the field pumping the OPA,the power and frequency of the field driving the cavity,and the Coulomb coupling strength between the two charged mechanical resonators.In particular,the increase of the nonlinear gain parameter can result in a transition from testability to Instability.Moreover,the effect of the Coulomb coupling strength on the bistable behavior of the steady-state positions of the two mechanical resonators is discussed.     

Atomic ionization by ultra—strong laser fields

The problem of multi-photon ionization of hydrogen by ultra-strong electromagnetic fields is solved in the limit where the field energy is greater than the Coulomb energy. The transition rate goes to zero as the reciprocal of the field strength.     

Dielectric enhancement of the exciton energies in laser-dressed near-surface quantum wells

A theoretical study of the intense high-frequency laser field effect on the interband transitions and on the ground (1S-like) and excited (2S-like) exciton states in InGaAs/GaAs near-surface quantum wells is performed within the effective mass approximation. The carrier confinement potentials and image charge contributions to the Coulomb interaction can significantly be modified and controlled by the capped layer thickness and laser field intensity. We found that: (i) the interband and exciton transition energies monotonically enhance with the laser amplitude; (ii) for small capped layers the splitting between the 2S and 1S exciton lines are more sensitive to the dressing laser parameter, and (iii) for high enough laser intensities the dressing effects on both confining potential and Coulomb interactions can yield entirely different exciton emission spectra depending on the cap layer thickness. Our results are compared with the theoretical and experimental data obtained in the absence of the laser field and a good agreement is reached.