arXiv subjects

V. M. Fomin

Publications and source records attributed to V. M. Fomin.

At least 37 records · Page 2Linked to original sources

arXiv2003

Quantum Transport in a Nanosize Silicon-on-Insulator Metal-Oxide-Semiconductor

An approach is developed for the determination of the current flowing through a nanosize silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFET). The quantum mechanical features of the electron transport are extracted from the numerical solution of the quantum Liouville equation in the Wigner function representation. Accounting for electron scattering due to ionized impurities, acoustic phonons and surface roughness at the Si/SiO2 interface, device characteristics are obtained as a function of a channel length. From the Wigner function distributions, the coexistence of the diffusive and the ballistic transport naturally emerges. It is shown that the scattering mechanisms tend to reduce the ballistic component of the transport. The ballistic component increases with decreasing the channel length.

cond-mat.mes-hall↗

arXiv2002

Superconductivity in mesoscopic high-$T_{c}$ superconducting particles

Based on the Hubbard model in the framework of non-phonon kinematical mechanism and taking into account the discreetness of an electronic energy spectrum, the superconducting critical temperature of a mesoscopic high-$T_{c}$ sphere is analyzed as a function of doping and as a function of a particle's radius. The critical temperature $T_{c}$ is found to be an oscillating function of the radius of a particle. The size-dependent doping regime is revealed in high-$T_{c}$ nanoparticles. Our analysis shows that each oscillation in $T_{c}$ corresponds to the increase of a number of the energy levels in the sphere by one. The amplitude of oscillations of $T_{c}$ increases with decreasing $R$ and can reach a value of 6 K for nanopartilces with sizes about 25 nm, in a good agreement with experimental studies of YBa$_{2}$Cu$_{3}$O$_{7-δ}$ nanoparticles.

cond-mat.supr-con↗

arXiv2002

Nucleation of superconductivity in mesoscopic star-shaped superconductors

We study the phase transition of a star-shaped superconductor, which covers smoothly the range from zero to two dimensions with respect to the superconducting coherence length. Detailed measurements and numerical calculations show that the nucleation of superconductivity in this device is very inhomogeneous, resulting in rich structure in the superconducting transition as a function of temperature and magnetic field. The superconducting order parameter is strongly enhanced and mostly robust in regions close to multiple boundaries.

cond-mat.supr-con↗

arXiv2002

Reply to the comment by E. Menendez-Proupin on "Multi-phonon Raman scattering in semiconductor nanocrystals: Importance of non-adiabatic transitions"

In our recent paper [Phys. Rev. B 65, 075316 (2002)], the selection rules for the Raman scattering in spherical nanocrystals are analyzed. Here we show, that a physically expedient choice of the coordinate system, related to the polarization vector of the incoming or scattered light, simplifies and clarifies the analysis.

cond-mat.mes-hall↗

arXiv2002

Stable vortex-antivortex molecules in mesoscopic superconducting triangles

A thermodynamically stable vortex-antivortex pattern has been revealed in mesoscopic type I superconducting triangles, contrary to type II superconductors where similar patterns are unstable. The stable vortex-antivortex "molecule" appears due to the interplay between two factors: a repulsive vortex-antivortex interaction in type I superconductors and the vortex confinement in the triangle.

cond-mat.supr-con↗

arXiv2001

Shape of nanosize superconducting grains: Does it influence pairing characteristics?

The Richardson exact solution for the reduced BCS Hamiltonian is applied to examine how sensitive are the pairing characteristics (condensation energy, spectroscopic gap, parity gap) to a specific configuration of single-electron energy levels in nanosize metallic grains. Using single-electron energy spectra in parallelepiped-shaped potential boxes with various volumes and aspect ratios as a model of energy levels in grains, we show that this sensitivity is extremely high. Just due to such an extreme sensitivity, the detailed shape of grains cannot be detected through the pairing characteristics, averaged over an ensemble of grains, even in the case of relatively small size dispersion within this ensemble. We analyse the effect of the pairing interaction on the excited-level spacings in superconducting grains and comment on the influence of shape-dependent fluctuations in single-electron energy spectra on the possibility to reveal this effect through tunnelling measurements.

cond-mat.supr-con↗

arXiv2001

Vortex states in a mesoscopic superconducting triangle

The set of the nonlinear Ginzburg-Landau equations is solved for an Al mesoscopic superconducting triangle of finite thickness. We calculate the distributions of the superconducting phase in the triangle and of the magnetic field in and near the triangle. The distribution of the superconducting phase in the triangle is studied as a function of the applied magnetic field. Possible scenarios of penetration of the magnetic field into the triangle are analyzed. We consider two different states: a single vortex state and a state in the form of a symmetric combination of three vortices and an antivortex with vorticity L_a = - 2 ("3 - 2" combination). The free energy calculations show that a single vortex penetrates the triangle through a midpoint of one side. The "3 - 2" combination turns out to be thermodynamically preferable when the vortices are close to the center of the triangle. Equilibrium is achieved when a single vortex (or each component of the "3 - 2" combination) is in the center of the triangle.

cond-mat.supr-con↗

arXiv2001

Multi-phonon Raman scattering in semiconductor nanocrystals: importance of non-adiabatic transitions

Multi-phonon Raman scattering in semiconductor nanocrystals is treated taking into account both adiabatic and non-adiabatic phonon-assisted optical transitions. Because phonons of various symmetries are involved in scattering processes, there is a considerable enhancement of intensities of multi-phonon peaks in nanocrystal Raman spectra. Cases of strong and weak band mixing are considered in detail. In the first case, fundamental scattering takes place via internal electron-hole states and is participated by s- and d-phonons, while in the second case, when the intensity of the one-phonon Raman peak is strongly influenced by the interaction of an electron and of a hole with interface imperfections (e. g., with trapped charge), p-phonons are most active. Calculations of Raman scattering spectra for CdSe and PbS nanocrystals give a good quantitative agreement with recent experimental results.

cond-mat.mes-hall↗

arXiv2001

Development of an eight-band theory for quantum-dot heterostructures

We derive a nonsymmetrized 8-band effective-mass Hamiltonian for quantum-dot heterostructures (QDHs) in Burt's envelope-function representation. The 8x8 radial Hamiltonian and the boundary conditions for the Schroedinger equation are obtained for spherical QDHs. Boundary conditions for symmetrized and nonsymmetrized radial Hamiltonians are compared with each other and with connection rules that are commonly used to match the wave functions found from the bulk kp Hamiltonians of two adjacent materials. Electron and hole energy spectra in three spherical QDHs: HgS/CdS, InAs/GaAs, and GaAs/AlAs are calculated as a function of the quantum dot radius within the approximate symmetrized and exact nonsymmetrized 8x8 models. The parameters of dissymmetry are shown to influence the energy levels and the wave functions of an electron and a hole and, consequently, the energies of both intraband and interband transitions.

cond-mat.mes-hall↗

arXiv2001

Electron and hole states in quantum-dot quantum wells within a spherical 8-band model

In order to study heterostructures composed both of materials with strongly different parameters and of materials with narrow band gaps, we have developed an approach, which combines the spherical 8-band effective-mass Hamiltonian and the Burt's envelope function representation. Using this method, electron and hole states are calculated in CdS/HgS/CdS/H_2O and CdTe/HgTe/CdTe/H_2O quantum-dot quantum-well heterostructures. Radial components of the wave functions of the lowest S and P electron and hole states in typical quantum-dot quantum wells (QDQWs) are presented as a function of radius. The 6-band-hole components of the radial wave functions of an electron in the 8-band model have amplitudes comparable with the amplitude of the corresponding 2-band-electron component. This is a consequence of the coupling between the conduction and valence bands, which gives a strong nonparabolicity of the conduction band. At the same time, the 2-band-electron component of the radial wave functions of a hole in the 8-band model is small compared with the amplitudes of the corresponding 6-band-hole components. It is shown that in the CdS/HgS/CdS/H_2O QDQW holes in the lowest states are strongly localized in the well region (HgS). On the contrary, electrons in this QDQW and both electron and holes in the CdTe/HgTe/CdTe/H_2O QDQW are distributed through the entire dot. The importance of the developed theory for QDQWs is proven by the fact that in contrast to our rigorous 8-band model, there appear spurious states within the commonly used symmetrized 8-band model.

cond-mat.mes-hall↗

arXiv2001

Strong enhancement of superconductivity in a nanosized Pb bridge

In recent experiments with a superconducting nanosized Pb bridge formed between a scanning tunneling microscope tip and a substrate, superconductivity has been detected at magnetic fields, which are few times larger than the third (surface) critical field. We describe the observed phenomenon on the basis of a numerical solution of the Ginzburg-Landau equations in a model structure consisting of six conoids. The spatial distribution of the superconducting phase is shown to be strongly inhomogeneous, with concentration of the superconducting phase near the narrowest part (the ``neck'') of the bridge. We show that suppression of superconductivity in the bridge by applied magnetic field or by temperature first occurs near the bases and then in the neck region, what leads to a continuous superconducting-to-normal resistive transition. A position of the transition midpoint depends on temperature and, typically, is by one order of magnitude higher than the second critical field H_c2. We find that the vortex states can be realized in the bridge at low temperatures T/T_c < 0.6. The vortex states lead to a fine structure of the superconducting-to-normal resistive transition. We also analyze vortex states in the bridge, which are characterized by a varying vorticity as a function of the bridge's height.

cond-mat.supr-con↗

arXiv2001

Photoluminescence Spectra of Quantum Dots: Enhanced Efficiency of the Electron-Phonon Interaction

A theory of photoluminescence in semiconductor quantum dots is developed which relies on two key ingredients. First, it takes into account non-adiabaticity of the exciton-phonon system. Second, it includes the multimode dielectric model of LO-phonons and of the electron-phonon interaction in confined systems. The role of non-adiabaticity is shown to be of paramount importance in spherical quantum dots, where the lowest one-exciton state can be degenerate or quasi-degenerate. For various quantum dot structures, the calculated intensities of phonon satellites as a function of temperature, excitation energy and size of quantum dots are in a fair agreement with experimental data on photoluminescence.

cond-mat.mes-hall↗

arXiv2001

Surface-induced magnetic anisotropy for impurity spins in granular AuFe films

The theory of the surface-induced anisotropy is extended to the case of granular films of dilute magnetic alloys. Since the surface-induced blocking of a magnetic-impurity spin appears to be very sensitive to the specific polycrystalline structure, we speculate that the apparent discrepancy between the experimental results of different groups for the size dependence of the Kondo resistivity can be linked to different microstructure of the samples. We apply our model to calculate the magnetization of impurity spins in small AuFe grains and to interpret the experimental data on the anomalous Hall effect in thin Fe doped Au films.

cond-mat.mes-hall↗

arXiv2001

Temperature Dependence of the Cyclotron Mass in n-Type CdS

Recent cyclotron resonance experiments in n-type CdS at ultra-high magnetic fields have revealed a pronounced maximum of the electron cyclotron mass as a function of temperature. In order to interpret these data, we calculate the magneto-absorption spectra of polarons in n-CdS using the arbitrary-coupling approach. We show that in high magnetic fields the polaron effects beyond the weak-coupling approximation clearly reveal themselves in the magneto-optical absorption even at relatively small values of the Froehlich coupling constant. In particular, those effects result in a non-monotonous behaviour of the cyclotron mass as a function of temperature. We extend the theory to take into account a combined effect of several scattering mechanisms on the magneto-absorption spectra. The extended theory allows us to interpret quantitatively the experimentally observed behaviour of the cyclotron mass in CdS.

cond-mat.stat-mech↗

arXiv2001

Theory of bound polarons in oxide compounds

We present a multilateral theoretical study of bound polarons in oxide compounds MgO and α-Al_2O_3 (corundum). A continuum theory at arbitrary electron-phonon coupling is used for calculation of the energies of thermal dissociation, photoionization (optically induced release of an electron (hole) from the ground self-consistent state), as well as optical absorption to the non-relaxed excited states. Unlike the case of free strong-coupling polarons, where the ratio κof the photoionization energy to the thermal dissociation energy was shown to be always equal to 3, here this ratio depends on the Froehlich coupling constant αand the screened Coulomb interaction strength β. Reasonable variation of these two parameters has demonstrated that the magnitude of κremains usually in the narrow interval from 1 to 2.5. This is in agreement with atomistic calculations and experimental data for hole O^- polarons bound to the cation vacancy in MgO. The thermal dissociation energy for the ground self-consistent state and the energy of the optically induced charge transfer process (hops of a hole between O^{2-} ions) have been calculated using the quantum-chemical method INDO. Results obtained within the two approaches for hole O$^-$ polarons bound by the cation vacancies (V^-) in MgO and by the Mg^{2+} impurity (V_{Mg}) in corundum are compared to experimental data and to each other. We discuss a surprising closeness of the results obtained on the basis of independent models and their agreement with experiment.

cond-mat.str-el↗

arXiv2001

Optical Absorption Spectra of Bipolarons

The absorption of large bipolarons is investigated using the path-integral method. The response of a bipolaron to an external electromagnetic field is derived in the framework of the memory-function approach. The bipolaron optical absorption spectrum consists of a series of relatively narrow peaks. The peculiarities of the bipolaron optical absorption as a function of the frequency of the electromagnetic field may be attributed to the transitions involving relaxed excited states and scattering states of a bipolaron.

cond-mat.str-el↗

arXiv2000

Shape-induced magnetic anisotropy in dilute magnetic alloys

We extend the theory of the surface-induced magnetic anisotropy to mesoscopic samples with arbitrary geometry. The shape-induced anisotropy of impurity spins in small brick-shaped grains of dilute magnetic alloys is studied in detail. The surface-induced blocking of a magnetic-impurity spin is shown to be very sensitive to geometric parameters of a grain. This implies that the apparent discrepancy between the experimental data of different groups on the size dependence of the Kondo resistivity can result from different microstructure of the used samples. In order to interpret recent experimental data on the anomalous Hall effect in thin polycrystalline Fe doped Au films, we analyse the magnetisation of impurity spins as a function of the impurity position and of the grain shape.

cond-mat.mes-hall↗

arXiv2000

Many-polaron system confined to a quantum dot: ground-state energy and optical absorption

We find for the first time the ground state energy and the optical absorption spectra for N electrons (holes) interacting with each other and with the longitudinal optical (LO) phonons at an arbitrary electron-phonon coupling strength $α$ in a parabolic confinement potential. A recently developed path integral formalism for identical particles is used in order to account for the fermion statistics. The approach is applicable to closed and open shells. Using an extension of the Jensen-Feynman variational principle, the ground state energy of the N-polarons system in a parabolic confinement potential is analyzed as a function of N and $α$. A ferromagnetic-to-nonmagnetic transition is shown to occur between states with different total spin of the system in the case of strong electron-phonon interaction. This transition is manifested through the optical absorption spectra and should be experimentally observable. Strong mixing between zero-phonon and one-phonon states is revealed in the optical absorption spectrum, when the confinement frequency parameter is in resonance with the LO phonon frequency ("confinement-phonon resonance"). Moments of the optical absorption spectra are calculated for a N-polaron system in a parabolic quantum dot.

cond-mat.str-el↗