arXiv subjects

Vladimir I. Fal'ko

Publications and source records attributed to Vladimir I. Fal'ko.

141 records · Page 8Linked to original sources

arXiv2000

Magnon-assisted Andreev reflection in a ferromagnet-superconductor junction

We study subgap transport in a ferromagnet-superconductor junction at low temperature due to Andreev reflection. The mismatch of spin polarized current in the ferromagnet and spinless current in the superconductor results in an additional contact resistance which can be reduced by magnon emission in the ferromagnet. Using the s-f model and focusing on half-metallic ferromagnets, we calculate the corresponding nonlinear contribution to the current which is asymmetric with respect to the sign of the bias voltage and is related to the density of states of magnons in the ferromagnet.

cond-mat.mes-hall↗

arXiv2000

Electron - hole asymmetry and activation energy in quantum Hall ferromagnets

We argue that the dissipative transport in ferromagnetic quantum Hall effect liquids at $ν=2N+1$ is dominated by the thermal activation of pairs consisting of an electron and an antiskyrmion (topological texture which represents a hole with 'screened' exchange interaction), thus manifesting the lack of electron-hole symmetry in quantum Hall ferromagnets. We find that the activation energy of such a pair is not the exchange energy, but is determined by the interplay between the excess Zeeman energy of a skyrmion and the charging energy of its topological texture: $${\cal E}=aε_{\rm Z}^{1/3}E_{\rm C}^{2/3}\ln ^{1/3}(\frac{\Im_{i}}{E_{\rm C}^{2/3}ε_{\rm Z}^{1/3}}), E_{\rm C}=\frac{e^{2}}{χλ}, $$ with $ a\approx 1.75$.

cond-mat.mes-hall↗

arXiv1999

Phonon-mediated thermal conductance of mesoscopic wires with rough edges

We present an analysis of acoustic phonon propagation through long, free-standing, insulating wires with rough surfaces. Due to a crossover from ballistic propagation of the lowest-frequency phonon mode at $ω<ω_{1}=πc/W$ to a diffusive (or even localized) behavior upon the increase of phonon frequency, followed by re-entrance into the quasi-ballistic regime, the heat conductance of a wire acquires an intermediate tendency to saturate within the temperature range $T\sim \hbar ω_{1}/k_{B}$.

cond-mat↗

arXiv1999

Correlations in optical phonon spectra of complex solids

Spectral correlations in the optical phonon spectrum of a solid with a complex unit cell are analysed using the Wigner-Dyson statistical approach. Despite the fact that all force constants are real, we find that the statistics are predominantly of the GUE type depending on the location within the Brillouin zone of a crystal and the unit cell symmetry. Analytic and numerical results for the crossover from GOE to GUE statistics are presented.

cond-mat.mes-hall↗

arXiv1999

Spin-orbit coupling effect on quantum Hall ferromagnets with vanishing Zeeman energy

We present the phase diagram of a ferromagnetic quantum Hall effect liquid in a narrow quantum well with vanishing single-particle Zeeman splitting, $ε_{\rm Z}$ and pronounced spin-orbit coupling. Upon decreasing $ε_{\rm Z}$, the spin-polarization field of a liquid takes, first, the easy-axis configuration, followed by the formation of a helical state, which affects the transport and NMR properties of a liquid and the form of topological defects in it. The analysis is extended over high odd integer filling factors.

cond-mat.mes-hall↗

arXiv1999

Enhanced fluctuations of the tunneling density of states near bottoms of Landau bands measured by a local spectrometer

We have found that the local density of states fluctuations (LDOSF) in a disordered metal, detected using an impurity in the barrier as a spectrometer, undergo enhanced (with respect to SdH and dHvA effects) oscillations in strong magnetic fields, omega _cτ> 1. We attribute this to the dominant role of the states near bottoms of Landau bands which give the major contribution to the LDOSF and are most strongly affected by disorder. We also demonstrate that in intermediate fields the LDOSF increase with B in accordance with the results obtained in the diffusion approximation.

cond-mat.mes-hall↗

arXiv1999

Interplay between spin-relaxation and Andreev reflection in ferromagnetic wires with superconducting contacts

We analyze the change in the resistance of a junction between a diffusive ferromagnetic (F) wire and normal metal electrode, due to the onset of superconductivity (S) in the latter and a double Andreev scattering process leading to a complete internal reflection of a large fraction of the spin-polarized electrons back into the ferromagnet. The superconducting transition results in an additional contact resistance arising from the necessity to match spin-polarized current in F-wire to spin-less current in S-reservoir, which is comparable to the resistance of a piece of a F-wire with the length equal to the spin-relaxation length.

cond-mat.mes-hall↗

arXiv1999

Topological defects and Goldstone excitations in domain walls between ferromagnetic quantum Hall effect liquids

It is shown that the low-energy spectrum of a ferromagnetic quantum Hall effect liquid in a system with a multi-domain structure generated by an inhomogeneous bare Zeeman splitting $ε_{Z}$ is formed by excitations localized at the walls between domains. For a step-like $ε_Z(r)$, the domain wall spectrum includes a spin-wave with a linear dispersion and a small gap due to spin-orbit coupling, and a low-energy topological defects. The latter are charged and may dominate in the transport under conditions that the percolation through the network of domain walls is provided.

cond-mat.mes-hall↗

arXiv1998

Universal correlations in vibrational spectra of complex crystals

We study spectral statistics of lattice modes in a disordered crystal and in a crystal with a complex unit cell. The correlations of the eigenmode frequencies of a block of a disordered solid is found to obey the GOE Wigner-Dyson statistics. In contrast, the set of eigenfrequencies of a crystal with a complex unit cell taken at the same point ${\bf Q}\not=0$ in the Brillouin zone exhibit correlations specific to the GUE universality class.

cond-mat↗

arXiv1996

Magneto-optical evidence of the percolation nature of the metal-insulator transition in the 2D electron system

We compare the results of the transport and time-resolved magneto-luminescence measurements in disordered 2D electron systems in GaAs-AlGaAs heterostructures in the extreme quantum limit, in particular, in the vicinity of the metal-insulator transition (MIT). At filling factors $ν<1$, the optical signal has two components: the single-rate exponentially decaying part attributed to a uniform liquid and a power-law long-living tail specific to a microscopically inhomogeneous state of electrons. We interprete this result as a separation of the 2D electron system into a liquid and localized phases, especially because the MIT occurs strikingly close to those filling factors where the liquid occupies ${1\over 2}$ of the sample area (the percollation threshold condition in two-component media).

cond-mat↗

arXiv1996

Long-range correlations in the wave functions of chaotic systems

We study correlations of the amplitudes of wave functions of a chaotic system at large distances. For this purpose, a joint distribution function of the amplitudes at two distant points in a sample is calculated analytically using the supersymmetry technique. The result shows that, although in the limit of the orthogonal and unitary symmetry classes the correlations vanish, they are finite through the entire crossover regime and may be reduced only by localization effects.

cond-mat↗

arXiv1995

Statistics of pre-localized states in disordered conductors

The distribution function of local amplitudes of single-particle states in disordered conductors is calculated on the basis of the supersymmetric $σ$-model approach using a saddle-point solution of its reduced version. Although the distribution of relatively small amplitudes can be approximated by the universal Porter-Thomas formulae known from the random matrix theory, the statistics of large amplitudes is strongly modified by localization effects. In particular, we find a multifractal behavior of eigenstates in 2D conductors which follows from the non-integer power-law scaling for the inverse participation numbers (IPN) with the size of the system. This result is valid for all fundamental symmetry classes (unitary, orthogonal and symplectic). The multifractality is due to the existence of pre-localized states which are characterized by power-law envelopes of wave functions, $|ψ_t(r)|^2\propto r^{-2μ}$, $μ<1$. The pre-localized states in short quasi-1D wires have the power-law tails $|ψ(x)|^2\propto x^{-2}$, too, although their IPN's indicate no fractal behavior. The distribution function of the largest-amplitude fluctuations of wave functions in 2D and 3D conductors has logarithmically-normal asymptotics.

cond-mat↗

arXiv1995

Quantum conductance fluctuations in 3D ballistic adiabatic wires.

Quantum conductance of 3D ballistic wires with idealy flat boundaries obeys fluctuations with the properties quite distinguishable from those of universal conductance fluctuations: Both their amplitude and the sensitivity to the magnetic field flux $Φ=HS$ penetrated into the sample cross-sectional area $S$ are different and depend on details of the cross-sectioanl shape of the wire. When the latter is integrable, conductance fluctuations have the enlarged amplitude $δG\sim\left[(e^2/h)^3G\right]^{1/4}$. When the cross-sectional shape of a wire is non-integrable, the irregular part of a conductance has the $e^ 2/h$ scale, whereas the correlation field is reduced to the value of $H_S\sim (λ_F/\sqrt S)^{1/2}(Φ_0/S)$ and the correlation voltage of the nonlinear conductance fluctuations has the scale of $eV_c\sim\hbar^2/mS\sim E_F/(S/λ_F)$, where $λ_F=1/p_F$ is the Fermi wavelength.

cond-mat↗

arXiv1995

Conductance fluctuations due to a single bistable scatterer in a weakly connected conductor.

We study the effect of a single bistable scatterer on conductance of mesoscopic conductors in the crossover regime from open to closed systems. After the resistance of the contact between the metallic grain and bulk electrode exceeds the resistance of a grain, the effect of a bistability is enhanced and takes the universal form. Its efficiency scales with the cross-section of the bistable impurity and of conductances of contacts but is independent of this impurity position inside a sample. Mutual correlations between magneto-conductance fluctuations at two conductance levels also look universal. Enhancement of the conductance sensitivity to variations of impurity configurations in samples with highly resistive contacts transforms the spectral density of a quantum excess noise toward a white-noise behavior and requires higher stability of the sample in order to observe mesoscopic effects in direct-current measurements.

cond-mat↗

arXiv1995

Multifractality: generic property of eigenstates of 2D disordered metals.

The distribution function of local amplitudes of eigenstates of a two-dimensional disordered metal is calculated. Although the distribution of comparatively small amplitudes is governed by laws similar to those known from the random matrix theory, its decay at larger amplitudes is non-universal and much slower. This leads to the multifractal behavior of inverse participation numbers at any disorder. From the formal point of view, the multifractality originates from non-trivial saddle-point solutions of supersymmetric $σ$-model used in calculations.

cond-mat↗