arXiv subjects

A. N. Rubtsov

Publications and source records attributed to A. N. Rubtsov.

60 records · Page 4Linked to original sources

arXiv2002

Quantum discrete $ϕ^4$ model at finite temperatures: the mean-field phase diagram

The quantum discrete $ϕ^4$ model at finite temperature is studied in the mean-field approximation. The phase diagrams are obtained for a wide range of the model parameters. The domains of applicability for the classical, quantum, and order-disorder limits are determined. It is shown that a remarkable part of the phase diagram is reproduced by the first quantum correction to the classical result. Parameters of Sn$_2$P$_2$S$_6$, SrTiO$_3$, and KH$_2$PO$_4$ are placed in the phase diagram of the model.

cond-mat.stat-mech↗

arXiv2002

Is the mean-field approximation so bad? A simple generalization yelding realistic critical indices for 3D Ising-class systems

Modification of the renormalization-group approach, invoking Stratonovich transformation at each step, is proposed to describe phase transitions in 3D Ising-class systems. The proposed method is closely related to the mean-field approximation. The low-order scheme works well for a wide thermal range, is consistent with a scaling hypothesis and predicts very reasonable values of critical indices.

cond-mat.stat-mech↗

arXiv2000

Analytical model for a crossover between uncorrelated and fractal behaviour of a self-repulsive chain

The thermodynamics of a long self-repulsive chain is studied. In $D<4$ dimensions it shows two distinctly different regimes, corresponding to weak and strong correlations in the system. A simple microscopic analytical model is presented which successfully describes both the regimes. The self-consistent scheme is used, in which the center of mass of a chain is fixed explicitly. This allows to take correlations into account in an indirect manner.

cond-mat.stat-mech↗

arXiv1999

Oscillatory screening of the dc electric field in the Si-SiO$_2$ multiple quantum wells probed by second-harmonic generation

DC-electric field, being screened in 3D semiconductors, normally decays monotonically in space. Experimental studies of the DC electric field screening in Si-SiO$_2$ multiple quantum wells by electric field induced optical second-harmonic generation show a non-monotonic, oscillatory-like decay. The model of electrons localized inside quantum wells, with the first subband occupied, allows a description of the phenomenon. Interwell Coulomb interaction, a finite value of the electron charge and strong effective-mass anisotropy result in a crucial difference from 3D Fermi liquid.

cond-mat↗

arXiv1999

Crossover between displacive and order-disorder phase transition

The phase transition in a 3D array of classical anharmonic oscillators with harmonic nearest-neighbour coupling (discrete $ϕ^4$ model) is studied by Monte Carlo (MC) simulations and by analytical methods. The model allows to choose a single dimensionless parameter a determining completely the behaviour of the system. Changing a from 0 to $+\infty$ allows to go continuously from the displacive to the order-disorder limit. We calculate the transition temperature $T_c$ and the temperature dependence of the order parameter down to T=0 for a wide range of the parameter a. The $T_c$ from MC calculations shows an excellent agreement with the known asymptotic values for small and large a. The obtained MC results are further compared with predictions of the mean-field and independent-mode approximations as well as with predictions of our own approximation scheme. In this approximation, we introduce an auxiliary system, which yields approximately the same temperature behaviour of the order parameter, but allows the decoupling of the phonon modes. Our approximation gives the value of $T_c$ within an error of 5% and satisfactorily describes the temperature dependence of the order parameter for all values of a.

cond-mat.stat-mech↗

arXiv1998

DC-electric-field-induced and low-frequency electromodulation second-harmonic generation spectroscopy of Si(001)-SiO$_2$ interfaces

The mechanism of DC-Electric-Field-Induced Second-Harmonic (EFISH) generation at weakly nonlinear buried Si(001)-SiO$_2$ interfaces is studied experimentally in planar Si(001)-SiO$_2$-Cr MOS structures by optical second-harmonic generation (SHG) spectroscopy with a tunable Ti:sapphire femtosecond laser. The spectral dependence of the EFISH contribution near the direct two-photon $E_1$ transition of silicon is extracted. A systematic phenomenological model of the EFISH phenomenon, including a detailed description of the space charge region (SCR) at the semiconductor-dielectric interface in accumulation, depletion, and inversion regimes, has been developed. The influence of surface quantization effects, interface states, charge traps in the oxide layer, doping concentration and oxide thickness on nonlocal screening of the DC-electric field and on breaking of inversion symmetry in the SCR is considered. The model describes EFISH generation in the SCR using a Green function formalism which takes into account all retardation and absorption effects of the fundamental and second harmonic (SH) waves, optical interference between field-dependent and field-independent contributions to the SH field and multiple reflection interference in the SiO$_2$ layer. Good agreement between the phenomenological model and our recent and new EFISH spectroscopic results is demonstrated. Finally, low-frequency electromodulated EFISH is demonstrated as a useful differential spectroscopic technique for studies of the Si-SiO$_2$ interface in silicon-based MOS structures.

physics.optics↗