arXiv subjects
Publications and source records attributed to N. M. Chtchelkatchev.
The energy relaxation in the spin-polarized disordered electron systems is studied in the diffusive regime. We derived the quantum kinetic equation in which the kernel of electron-electron collision integral explicitly depends on the electron magnetization. As the consequence, the inelastic scattering rate is found to have non-monotonic dependence on the spin polarization of the electron system.
We study the magnetotransport in small hybrid junctions formed by high-mobility GaInAs/InP heterostructures coupled to superconducting (S) and normal metal (N) terminals. Highly transmissive superconducting contacts to a two-dimensional electron gas (2DEG) located in a GaInAs/InP heterostructure are realized by using a Au/NbN layer system. The magnetoresistance of the S/2DEG/N structures is studied as a function of dc bias current and temperature. At bias currents below a critical value, the resistance of the S/2DEG/N structures develops a strong oscillatory dependence on the magnetic field, with an amplitude of the oscillations considerably larger than that of the reference N/2DEG/N structures. The experimental results are qualitatively explained by taking Andreev reflection in high magnetic fields into account.
We present analysis of the cross-over behavior of disordered interacting two-dimensional electron systems in the parallel magnetic field. Using the so-called cross-over one-loop renormalization group equations for the resistance and electron-electron interaction amplitudes we qualitatively explain experimentally observed transformation of the temperature dependence of the resistance from a reentrant (nonmonotonic) behavior in relatively weak fields into an insulating-type behavior in stronger fields.
We construct a model for qualitative description of the orientational glass transition in C_60 on the spin-glass theory basis. The physical origin of the frustration and the disorder is discussed.
Josephson effect in short ballistic SINIS and SIFIS double-tunnel junctions, consisting of clean superconductors (S), a normal metal (N) or metallic ferromagnet (F), and insulating interfaces (I) is studied. For SINIS double-tunnel junctions, sharp peaks in the critical Josephson current as a function of the junction width result from the resonant amplification of the Andreev process when the quasi-bound states enter the superconducting gap. For SIFIS double-tunnel junctions spin split quasi-bound states partially amplify the supercurrent and trigger the transitions between 0 and $π$ states of the junction. Instead of the critical current reaching a peak value (which happens when the Andreev states cross the Fermi surface in SINIS junctions), a narrow dip opens up exactly at the peak due to the compensation of partial currents flowing in opposite directions. This is related to the spin polarization of Andreev states and explains the coexistence of stable and metastable 0 and $π$ states in the vicinity of the transition. With increased barrier transparency, the described mechanism of $0-π$ transitions is modified by the broadening and overlapping of quasi-bound states (transmission resonances). Temperature-induced transitions both from 0 to $π$ and from $π$ to 0 states are studied by computing the phase diagram (temperature vs. junction width) for different interfacial transparencies varying from metallic (transparent) to the tunnel limit.
Vortex states in the mixture of ultracold atomic clouds of bosons and fermions are investigated using the effective Hamiltonian for the Bose subsystem. A stability of the Bose system in the case of attractive interaction between components is studied in the framework of variational Bose wave function and Thomas-Fermi approximation. It is shown that the critical number of bosons increases in the presence of the vortex.
We investigate how domain structure of the ferromagnet in superconductor-ferromagnet heterostructures may change their transport properties. We calculate the distribution of current in the superconductor induced by magnetic field of Bloch domain walls, find the ``lower critical'' magnetization of the ferromagnet that provides vortices in the superconductor.
Experimental and theoretical study of superconductor (S) -- ferromagnet (F) -- superconductor junctions showed that in certain range of parameters (e.g., the length of the ferromagnet $d_F$, the exchange field, $E_{\rm ex}$) the ground state of a SFS junction corresponds to superconducting phase difference $π$ or 0. The phase diagram of a SFS junction with the respect to $π$ and 0 states is investigated in this letter in $E_{\rm ex}, d_{F}, T$ space. It is shown that the phase diagram is very sensitive to the geometry of the system, in particular, to the amount of disorder.
We consider a single-electron transistor (SET) whose central island is a nanomechanical oscillator. The gate capacitance of the SET depends on the mechanical displacement, thus, the vibrations of the island vibrations may strongly influence the current-voltage characteristics, current noise, and higher cumulants of the current. Harmonic oscillations of the island and oscillations with random amplitude (e.g., due to the thermal activation) change the transport characteristics in a different way. The noise spectrum has a peak at the frequency of the island oscillations; when the island oscillates harmonically, the peak reduces to a $δ$-peak. We show that knowledge of the SET transport properties helps to determine in what way the island oscillates, to estimate the amplitude, and the frequency of the oscillations.
Consider two normal leads coupled to a superconductor; the first lead is biased while the second one and the superconductor are grounded. In general, a finite current $I_2(V_1,0)$ is induced in the grounded lead 2; its magnitude depends on the competition between processes of Andreev and normal quasiparticle transmission from the lead 1 to the lead 2. It is known that in the tunneling limit, when normal leads are weakly coupled to the superconductor, $I_2(V_1,0)=0$, if $|V_1|<Δ$ and the system is in the clean limit. In other words, Andreev and normal tunneling processes compensate each-other. We consider the general case: the voltages are below the gap, the system is either dirty or clean. It is shown that $I_2(V_1,0)=0$ for general configuration of the normal leads; if the first lead injects spin polarized current then $I_2=0$, but spin current in the lead-2 is finite. XISIN structure, where X is a source of the spin polarized current could be applied as a filter separating spin current from charge current. We do an analytical progress calculating $I_1(V_1,V_2), I_2(V_1,V_2)$.
On the basis of microscopic statistical mechanics of simple liquids the orientational interaction between clusters consisting of a particle and its nearest neighbors is estimated. It is shown that there are ranges of density and temperature where the interaction changes sign as a function of a radius of a cluster. The model of interacting cubic and icosahedral clusters is proposed and solved in mean-field replica symmetric approximation. It is shown that the glass order parameter grows smoothly upon cooling, the transition temperature being identified with the temperature of the replica symmetry breaking. It is shown that upon cooling a Lennard-Jones system, cubic clusters freeze first. The transition temperature for icosahedral clusters is about ten per cent lower. So the local structure of Lennard-Jones glass in the vicinity of glass transition should be most probably cubic.
We discuss controversial results for the statistics of charge transport through coherent conductors. Two distribution functions for the charge transmitted was obtained previously, first by L.Levitov and G.Lesovik, [JETP Letters Vol.55 p.555 (1992)] and the other initially by the same authors [ibid. Vol.58 p.230 (1993)], and later the result was reproduced by several authors. The latter distribution functions actually coincides with classical binomial distribution (though obtained purely quantum mechanically) former (result of 1992) is different and we call it here quantum binomial distribution. The two distribution function represent two opposite universal limits - one is purely quantum, where interference is important, and the other is semiclassical, where interference is smeared out. We show, that high order charge correlators, determined by the either distribution functions, can all be measured in different setups. The high order current correlators, starting the third order, reveal (missed in previous studies) special oscillating frequency dependence on the scale of the inverted time flight from the obstacle to the measuring point. Depending on setup, the oscillating terms give substantially different contributions.
Andreev quantum dot (AQD) is a superconducting junction where Bogolyubov quasiparticles can be trapped in discrete Andreev levels. We investigate the feasibility of manipulating individual spin in an AQD. We show that AQD can be brought into spin-1/2 state. The coupling between spin and superconducting current facilitate manipulation and measurement of this state in comparison with common semiconductor quantum dots. We demonstrate that AQD's coupled inductively can serve as a solid-state base for universal quantum computing.
The critical temperature T_c of a superconductor/ferromagnet (SF) bilayer can exhibit nonmonotonic dependence on the thickness d_f of the F layer. SF systems have been studied for a long time; according to the experimental situation, the "dirty" limit is often considered which implies that the mean free path in the layers is the second smallest spatial scale after the Fermi wavelength. However, all calculations reported for the dirty limit were done with some additional assumptions, which can be violated in actual experiments. Therefore, we develop a general method (to be exact, two independent methods) for investigating T_c as a function of the bilayer's parameters in the dirty case. Comparing our theory with experiment, we obtain good agreement. In the general case, we observe three characteristic types of T_c(d_f) behavior: 1) nonmonotonic decay of T_c to a finite value exhibiting a minimum at particular d_f, 2) reentrant behavior, characterized by vanishing of T_c in a certain interval of d_f and finite values otherwise, 3) monotonic decay of T_c and vanishing at finite d_f. Qualitatively, the nonmonotonic behavior of T_c(d_f) is explained by the interference of quasiparticles in the F layer, which can be either constructive or destructive depending on the value of d_f.
We investigate the Josephson effect in S$_{\rm F}$XS$_{\rm F}$ junctions, where S$_{\rm F}$ is a superconducting material with a ferromagnetic exchange field, and X a weak link. The critical current $I_c$ increases with the (antiparallel) exchange fields if the distribution of transmission eigenvalues of the X-layer has its maximum weight at small values. This exchange field enhancement of the supercurrent does not exist if X is a diffusive normal metal. At low temperatures, there is a correspondence between the critical current in an S$_{\rm F}$IS$_{\rm F}$ junction with collinear orientations of the two exchange fields, and the AC supercurrent amplitude in an SIS tunnel junction. The difference of the exchange fields $h_1-h_2$ in an S$_{\rm F}$IS$_{\rm F}$ junction corresponds to the potential difference $V_1-V_2$ in an SIS junction; i.e., the singularity in $I_c$ [in an S$_{\rm F}$IS$_{\rm F}$ junction] at $|h_1-h_2|=Δ_1+Δ_2$ is the analogue of the Riedel peak. We also discuss the AC Josephson effect in S$_{\rm F}$IS$_{\rm F}$ junctions.
Superconductor-Ferromagnetic-Superconductor (SFS) Josephson junctions are known to exhibit a transition between $π$ and 0 states. In this note we find the $π-0$ phase diagram of an SFS junction depending on the transparency of an intermediate insulating layer (I). We show that in general, the Josephson critical current is nonzero at the $π-0$ transition temperature. Contributions to the current from the two spin channels nearly compensate each other and the first harmonic of the Josephson current as a function of phase difference is suppressed. However, higher harmonics give a nonzero contribution to the supercurrent.
Superconductor/ferromagnet bilayers are known to exhibit nontrivial dependence of the critical temperature T_c on the thickness d_f of the ferromagnetic layer. We develop a general method for investigation of T_c as a function of the bilayer's parameters. It is shown that interference of quasiparticles makes T_c(d_f) a nonmonotonic function. The results are in good agreement with experiment. Our method also applies to multilayered structures.
Conductance $G$ of a 2DEG-Superconductor (S) device in a high magnetic field is studied: $G(ν)$ is calculated. When the cyclotron diameter in 2DEG is larger than the width of the 2DEG-S surface then $G(ν)$ becomes nonmonotonous function due to the Aharonov--Bohm type interference of quasiparticles at the surface. At certain parameters of the junction the conductance oscillates with $ν$.