arXiv subjects

L. Yu. Kravchenko

Publications and source records attributed to L. Yu. Kravchenko.

8 recordsLinked to original sources

arXiv2023

Energetics of point defects in yttrium aluminum garnet doped with Mg and Si

An influence of Mg and Si dopants on the formation energies and the concentration of point defects in yttrium aluminum garnet (YAG) is studied using the density functional approach. The formation energies of Mg and Si substitutional and interstitial defects, native point defects and defect complexes versus the oxygen chemical potential are obtained. It is shown that in YAG doped with Mg, negatively charged Mg substitutional defects are compensated by free carriers (holes) and positively charged oxygen vacancies, whereas interstitial Mg ions play a minor role. The concentration of oxygen vacancies increases under an increase in the concentration of Mg ions. In YAG doped with Si, positively charged Si substitutional defects are compensated by negatively changed isolated cation vacancies and complexes of Si ions and cation vacancies. Under an increase in the concentration of Si ions most of Al and Y vacancies bind in complexes with Si ions. As a result, the concentration of isolated cation vacancies depends nonmonotonically on the concentration of Si ions. The maximum of the concentration of isolated cation vacancies is reached at $0.02 - 0.04$ at. % of Si, depending on sintering conditions. Mg - Si complexes have very low formation energies. Due to formation of such complexes, Si and Mg increases the solubility of each other in YAG. At the same time Mg - Si complexes do not influence the concentration of anion and cation vacancies. The overall concentration of vacancies in YAG codoped with Mg and Si in equal atomic concentrations is low. At an excess concentration of Si or Mg the concentration of vacancies increases by orders of magnitude.

cond-mat.mtrl-sci↗

arXiv2020

Control of charge state of dopants in insulating crystals: Case study of Ti-doped sapphire

We study mechanisms of control of charge state and concentration of different point defects in doped insulating crystals. The approach is based on the density functional theory calculations. We apply it to the problem of obtaining of Ti-doped sapphire crystals with high figure-of-merit (FOM). The FOM of a given sample is defined as the ratio of the coefficient of absorption at the pump frequency to the coefficient of absorption at the working frequency of Ti:sapphire laser. It is believed that FOM is proportional to the ratio of the concentration of isolated Ti$^{3+}$ ions to the concentration of Ti$^{3+}$-Ti$^{4+}$ pairs. We find that generally this ratio is in inverse proportion to the concentration of Ti$^{4+}$ isolated substitutional defects with the coefficient of proportionality that depends on the temperature at which the thermodynamically equilibrium concentration of defects is reached. We argue that in certain cases the inverse proportion between concentrations of Ti$^{3+}$-Ti$^{4+}$ and Ti$^{4+}$ may be violated. We show that codopants that form positively (negatively) charged defects may decrease (increase) the concentration of positively charged defects formed by the main dopants. To evaluate the effect of codoping it is important to take into account not only isolated defects but defect complexes formed by codopants, as well. In particular, we show that codoping of Ti:sapphire with nitrogen results in an essential increase of the concentration of Ti$^{4+}$ and in a decrease of the FOM, and, consequently, growth or annealing in the presence of nitrogen or its compounds is unfavorable for producing Ti:sapphire laser crystals. The approach developed can be used for determining appropriate growth and annealing conditions for obtaining doped crystals with the required characteristics.

cond-mat.mtrl-sci↗

arXiv2018

Defect complexes in Ti-doped sapphire: A first principles study

First-principles calculations have been performed to study the formation of defect complexes in Ti doped alpha-Al2O3 crystals. The formation energies of isolated Ti3+ and Ti4+ defects, pairs, triples and quadruples of Ti ions and Al vacancies are computed under different equilibrium conditions of Al-Ti-O related phases. Taking into account charge neutrality of the whole system we determine the equilibrium concentrations of simple and complex defects as well as the total equilibrium concentration of Ti in an alpha-Al2O3 crystal. It is shown that the equilibrium concentration of complex defects can be on the same order of or even larger than the concentrations of isolated substitutional Ti3+ and Ti4+ defects. It is found that in Ti-deficient conditions the relative fraction of isolated defects increases and the balance is shifted towards Ti4+ defects. A universal relation between equilibrium concentrations of isolated and complex defects is obtained. The band structure of the system with complex defects is calculated and extra levels inside the band gap caused by such defects are found.

cond-mat.mtrl-sci↗

arXiv2009

Superconductivity of electron-hole pairs in a bilayer graphene system in a quantizing magnetic field

The state with a spontaneous interlayer phase coherence in a graphene based bilayer quantum Hall system is studied. This state can be considered as a gas of superfluid electron-hole pairs with the components of the pair belonging to different layers. Superfluid flux of such pairs is equivalent to two electrical supercurrents in the layers. It is shown that the state with the interlayer phase coherence emerges in the graphene system if a certain imbalance of the Landau level filling factors of the layers is created. We obtain the temperature of transition into the superfluid state, the maximum interlayer distance at which the phase coherence is possible, and the critical values of the supercurrent. The advantages of use of graphene systems instead of GaAs heterostructures for the realization of the bilayer electron-hole superconductivity is discussed.

cond-mat.supr-con↗

arXiv2009

Superfluid state of magnetoexcitons in double layer graphene structures

The possibility of realization of a superfluid state of bound electron-hole pairs (magnetoexcitons) with spatially separated components in a graphene double layer structure (two graphene layers separated by a dielectric layer) subjected by a strong perpendicular to the layers magnetic field is analyzed. We show that the superfluid state of magnetoexcitons may emerge only under certain imbalance of filling factors of the layers. The imbalance can be created by an electrostatic field (external gate voltage). The spectrum of elementary excitations is found and the dependence of the Berezinskii-Kosterlitz-Thouless transition temperature on the interlayer distance is obtained. The advantages of use graphene double layer systems instead of double quantum well GaAs heterostructures are discussed.

cond-mat.str-el↗

arXiv2009

Stationary waves in a supersonic flow of a two-component Bose gas

A stationary wave pattern occurring in a flow of a two-component Bose-Einstein condensate past an obstacle is studied. We consider the general case of unequal velocities of two superfluid components. The Landau criterium applied to the two-component system determines a certain region in the velocity space in which superfluidity may take place. Stationary waves arise out of this region, but under the additional condition that the relative velocity of the components does not exceed some critical value. Under increase of the relative velocity the spectrum of the excitations becomes complex valued and the stationary wave pattern is broken. In case of equal velocities two sets of stationary waves that correspond to the lower and the upper Bogolyubov mode can arise. If one component flows and the other is at rest only one set of waves may emerge. Two or even three interfere sets of waves may arise if the velocities approximately of equal value and the angle between the velocities is close to pi/2. In two latter cases the stationary waves correspond to the lower mode and the densities of the components oscillate out-of-phase. The ratio of amplitudes of the components in the stationary waves is computed. This quantity depends on the relative velocity, is different for different sets of waves, and varies along the crests of the waves. For the cases where two or three waves interfere the density images are obtained.

cond-mat.other↗

arXiv2008

Critical currents and giant non-dissipative drag for superfluid electron-hole pairs in quantum Hall multilayers

Superfluid properties of electron-hole pairs in a quantum Hall four-layer system are investigated. The system is considered as a solid state realization of a two-component superfluid Bose gas with dipole-dipole interaction. One superfluid component is formed in the top bilayer and the other component - in the bottom one. We obtain the dispersion equation for the collective mode spectrum and compute the critical parameters (the critical interlayer distance and the critical currents) versus the filling factor. We find that the critical currents of the components depend on each other. The maximum critical current of a given component can be reached if the current of the other component is equal to zero. The non-dissipative drag effect between the components is studied. It is shown that in the system considered the drag factor is very large. Under appropriate conditions it can be about 10 per sent, that is at least in three order larder than one predicted for two-component atomic Bose gases.

cond-mat.mes-hall↗

arXiv2008

Critical velocities in two-component superfluid Bose gases

On the ground of the Landau criterion we study the behavior of critical velocities in a superfluid two-component Bose gas. It is found that under motion of the components with different velocities the velocity of each component should not be lower than a minimum phase velocity of elementary excitations (s_). The Landau criterion yields a relation between the critical velocities of the components (v_{c1}, v_{c2}). The velocity of one or even both components may exceed s_. The maximum value of the critical velocity of a given component can be reached when the other component does not move. The approach is generalized for a two-component condensate confined in a cylindrical harmonic potential. PACS numbers: 03.75.Kk,03.75.Mn

cond-mat.other↗