arXiv subjects

M. E. Raikh

Publications and source records attributed to M. E. Raikh.

At least 91 records · Page 5Linked to original sources

arXiv2008

Supergap anomalies in cotunneling between N-S and between S-S leads via a small quantum dot

Cotunneling current through a resonant level coupled to either normal and superconducting or to two superconducting leads is studied for the domain of bias voltages, V, exceeding the superconducting gap, 2Δ. Due to the on-site repulsion in the resonant level, cotunneling of an electron is accompanied by creation of a quasiparticle in a superconducting lead. Energy conservation imposes a threshold for this inelastic transport channel: V_c=3Δfor N-S case and \tilde{V}_c=4Δfor the S-S case. We demonstrate that the behavior of current near the respective thresholds is nonanalytic, namely, δI^{in}(V)\propto (V-V_c)^{3/2}Θ(V-V_c) and δI^{in}(V) \propto (V-\tilde{V}_c)Θ(V-\tilde{V}_c). Stronger anomaly for the S-S leads is the consequence of the enhanced density of states at the edges of the gap. In addition, the enhanced density of states makes the threshold anomalies for two-electron cotunneling processes in the Coulomb-blockaded regions more pronounced than for the N-N leads.

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arXiv2007

Zero-bias tunneling anomaly in a clean 2D electron gas caused by smooth density variations

We show that smooth variations, δn({\bf r}), of the local electron concentration in a clean 2D electron gas give rise to a zero-bias anomaly in the tunnel density of states, ν(ω), even in the absence of scatterers, and thus, without the Friedel oscillations. The energy width, ω_0, of the anomaly scales with the magnitude, δn, and characteristic spatial extent, D, of the fluctuations as (δn/D)^{2/3}, while the relative magnitude δν/νscales as (δn/D). With increasing ω, the averaged δνoscillates with ω. We demonstrate that the origin of the anomaly is a weak curving of the classical electron trajectories due to the smooth inhomogeneity of the gas. This curving suppresses the corrections to the electron self-energy which come from the virtual processes involving two electron-hole pairs

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arXiv2007

Electron Pair Resonance in the Coulomb Blockade

We study many-body corrections to the cotunneling current via a localized state with energy $ε_d$ at large bias voltages $V$. We show that the transfer of {\em electron pairs}, enabled by the Coulomb repulsion in the localized level, results in ionization resonance peaks in the third derivative of the current with respect to $V$, centered at $eV=\pm 2ε_d/3$. Our results predict the existence of previously unnoticed structure within Coulomb-blockade diamonds.

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arXiv2007

Smearing of the 2D Kohn anomaly in a nonquantizing magnetic field: Implications for the interaction effects

Thermodynamic and transport characteristics of a clean two-dimensional interacting electron gas are shown to be sensitive to the weak perpendicular magnetic field even at temperatures much higher than the cyclotron energy, when the quantum oscillations are completely washed out. We demonstrate this sensitivity for two interaction-related characteristics: electron lifetime and the tunnel density of states. The origin of the sensitivity is traced to the field-induced smearing of the Kohn anomaly; this smearing is the result of curving of the semiclassical electron trajectories in magnetic field.

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arXiv2006

Planar array of semiconducting nanotubes in external electric field: Collective screening and polarizability

We study theoretically the charge separation in a planar array of semiconducting nanotubes (NTs) of length, 2h, placed in external electric field, F, parallel to their axes. By employing the conformal mapping, we find analytically the exact distribution of the induced charge density for arbitrary ratio of the gap, E_g, and the voltage drop, eFh, across the NT. We use the result obtained to (i) trace the narrowing of the neutral strip at center of the NT with increasing F; (ii) analyze the polarizability of the array as a function of E_g; (iii) study the field enhancement near the tips.

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arXiv2006

Tunneling between 2D electron layers with correlated disorder: anomalous sensitivity to spin-orbit coupling

Tunneling between two-dimensional electron layers with mutually correlated disorder potentials is studied theoretically. Due to this correlation, the diffusive eigenstates in different layers are almost orthogonal to each other. As a result, a peak in the tunnel I-V characteristics shifts towards small bias, V. This "protects" the peak against the interaction-induced smearing, since the relaxation rate near the Fermi level is low. If the correlation in disorder potentials is complete, the peak position and width are governed by the spin-orbit coupling in the layers; this coupling lifts the orthogonality of the eigenstates. Possibility to use inter-layer tunneling for experimental determination of weak intrinsic spin-orbit splitting of the Fermi surface is discussed.

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arXiv2006

Regimes of correlated hopping via a two-site interacting chain

Inelastic transport of electrons through a two-impurity chain is studied theoretically with account of intersite Coulomb interaction, U. Both limits of ohmic transport (at low bias) and strongly non-ohmic transport (at high bias) are considered. We demonstrate that correlations, induced by a finite U, in conjunction with conventional Hubbard correlations, give rise to a distinct transport regime, with current governed by two-electron hops. This regime realizes when a single-electron hop onto the chain and a single-electron hop out of the chain are both ``blocked'' due to the finite U, so that conventional correlated sequential transport is impossible. The regime of two-electron hops manifests itself in the form of an additional step in the current-voltage characteristics, I(V).

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arXiv2006

Electrostatics of straight and bent nanotubes

Response of a single-walled carbon nanotube to external electric field, F, is calculated analytically within the classical electrostatics. Field-induced charge density distribution is approximately linear along the axis of metallic nanotube and depends rather weakly, as ln(h/r), on the nanotube length, h, (here r is the nanotube radius). In a semiconducting nanotube with a gap, E_g, charge separation occurs as F exceeds the threshold value F_{th}=E_g/eh. For F>F_{th}, positively and negatively charged regions at the ends of nanotube are separated by a neutral strip in the middle. For bent nanotubes the number of neutral strips can be one or two depending on the direction of F.

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arXiv2006

Penetration of external field into regular and random arrays of nanotubes: Implications for field emission

We develop an analytical theory of polarization of a vertically aligned array of carbon nanotubes (NTs) in external electric field. Such arrays are commonly utilized in field-emission devices, due to the known electrostatic effect of strong field enhancement near the tip of an individual NT. A small ratio of the NT radius to the separation between neighboring NTs allows us to obtain asymptotically exact solution for the distribution of induced charge density along the NT axes. For a regular array, this solution allows us to trace the suppression of the field penetration with increasing the density of NTs in the array. We demonstrate that for a random array, fluctuations in the NT density terminate the applicability of our result at distances from the NT tips much larger than the field penetration depth, where the induced charge density is already exponentially small. Our prime conclusion is that, due to collective screening of the external field by the array, the field-emission current decreases drastically for dense arrays compared to an individual NT. We argue that the reason why the strong field emission, described by the Fowler-Nordheim law and observed in realistic arrays, is the strong dispersion in heights of the constituting NTs.

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arXiv2006

Pair tunneling through single molecules

By a polaronic energy shift, the effective charging energy of molecules can become negative, favoring ground states with even numbers of electrons. Here, we show that charge transport through such molecules near ground-state degeneracies is dominated by tunneling of electron pairs which coexists with (featureless) single-electron cotunneling. Due to the restricted phase space for pair tunneling, the current-voltage characteristics exhibits striking differences from the conventional Coulomb blockade. In asymmetric junctions, pair tunneling can be used for gate-controlled current rectification and switching.

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arXiv2006

Two-Electron Linear Intersubband Light Absorption in a Biased Quantum Well

We point out a novel manifestation of many-body correlations in the linear optical response of electrons confined in a quantum well. Namely, we demonstrate that along with conventional absorption peak at frequency close to intersubband energy, there exists an additional peak at double frequency. This new peak is solely due to electron-electron interactions, and can be understood as excitation of two electrons by a single photon. The actual peak lineshape is comprised of a sharp feature, due to excitation of pairs of intersubband plasmons, on top of a broader band due to absorption by two single-particle excitations. The two-plasmon contribution allows to infer intersubband plasmon dispersion from linear absorption experiments.

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arXiv2005

Full counting statistics of strongly non-Ohmic transport through single molecules

We study analytically the full counting statistics of charge transport through single molecules, strongly coupled to a weakly damped vibrational mode. The specifics of transport in this regime - a hierarchical sequence of avalanches of transferred charges, interrupted by "quiet" periods - make the counting statistics strongly non-Gaussian. We support our findings for the counting statistics as well as for the frequency-dependent noise power by numerical simulations, finding excellent agreement.

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arXiv2005

Disorder-Induced Resistive Anomaly Near Ferromagnetic Phase Transitions

We show that the resistivity rho(T) of disordered ferromagnets near, and above, the Curie temperature T_c generically exhibits a stronger anomaly than the scaling-based Fisher-Langer prediction. Treating transport beyond the Boltzmann description, we find that within mean-field theory, dρ/dT exhibits a |T-T_c|^{-1/2} singularity near T_c. Our results, being solely due to impurities, are relevant to ferromagnets with low T_c, such as SrRuO3 or diluted magnetic semiconductors, whose mobility near T_c is limited by disorder.

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arXiv2004

Transmission distribution, P(\ln T), of 1D disordered chain: low-T tail

We demonstrate that the tail of transmission distribution through 1D disordered Anderson chain is a strong function of the correlation radius of the random potential, $a$, even when this radius is much shorter than the de Broglie wavelength, $k_F^{-1}$. The reason is that the correlation radius defines the phase volume of the trapping configurations of the random potential, which are responsible for the low-$T$ tail. To see this, we perform the averaging over the low-$T$ disorder configurations by first introducing a finite lattice spacing $\sim a$, and then demonstrating that the prefactor in the corresponding functional integral is exponentially small and depends on $a$ even as $a \to 0$. Moreover, we demonstrate that this restriction of the phase volume leads to the dramatic change in the shape of the tail of ${\cal P}(\ln T)$ from universal Gaussian in $\ln T $ to a simple exponential (in $\ln T $) with exponent depending on $a$. Severity of the phase-volume restriction affects the shape of the low-$T$ disorder configurations transforming them from almost periodic (Bragg mirrors) to periodically-sign-alternating (loose mirrors).

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arXiv2004

Parametric resonance of a two-dimensional electron gas under bichromatic irradiation

In an ultrahigh mobility 2D electron gas, even a weak nonparabolicity of the electron dispersion, by violating Kohn's theorem, can have a drastic effect on dc magnetotransport under ac drive. We study theoretically the manifestation of this effect in the dc response to the combined action of two driving ac-fields (bichromatic irradiation). Compared to the case of monochromatic irradiation, which is currently intensively studied both experimentally and theoretically, the presence of a second microwave source provides additional insight into the properties of an ac-driven 2D electron gas. In particular, we find that nonparabolicity, being the simplest cause for a violation of Kohn's theorem, gives rise to new qualitative effects specific to bichromatic irradiation. Namely, when the frequencies $ω_1$ and $ω_2$ are well away from the cyclotron frequency, $ω_c$, our simple classical considerations demonstrate that the system becomes parametrically unstable with respect to fluctuations with frequency $(ω_1+ω_2)/2$. As an additional effect of nonparabolicity, this parametric instability can manifest itself in the dc properties of the system. This happens when $ω_1$, $ω_2$ and $ω_c$ are related as 3:1:2, respectively. Even for weak detuning between $ω_1$ and $ω_2$, the effect of the bichromatic irradiation on the dc response in the presence of nonparabolicity can differ dramatically from the monochromatic case. In particular, the equations of motion can acquire multistable solutions. As a result, the diagonal dc-conductivity can assume several stable negative values at the same magnetic field.

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arXiv2004

Effective Drag Between Strongly Inhomogeneous Layers: Exact Results and Applications

We generalize Dykhne's calculation of the effective resistance of a 2D two-component medium to the case of frictional drag between the two parallel two-component layers. The resulting exact expression for the effective transresistance, $ρ^D_{eff}$, is analyzed in the limits when the resistances and transresistances of the constituting components are strongly different - situation generic for the vicinity of the {\em classical} (percolative) metal-insulator transition (MIT). On the basis of this analysis we conclude that the evolution of $ρ^D_{eff}$ across the MIT is determined by the type of correlation between the components, constituting the 2D layers. Depending on this correlation, in the case of two electron layers, $ρ^D_{eff}$ changes either monotonically or exhibits a sharp maximum. For electron-hole layers $ρ^D_{eff}$ is negative and $|ρ^D_{eff}|$ exhibits a sharp minimum at the MIT.

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arXiv2004

Universal Fluctuations of the Random Lasing Threshold in a Sample of a Finite Area

We consider the random lasing from a weakly scattering medium and demonstrate that the distribution of the threshold gain over the ensemble of statistically independent finite-size samples is universal. Universality stems from the facts that: (i) lasing threshold in a given sample is determined by the highest-quality mode of all the random resonators present in the sample, and (ii) the areal {\em density} of the random resonators decays sharply with the quality factor of the mode that they trap. We find analytically the shape of the universal distribution function of the lasing threshold. The shape of this function is governed by a single dimensionless parameter, $β$. This parameter increases as a power law with $\ln S$, where $S$ is the sample area (length, volume), and decreases as a power law with disorder strength. The powers depend on the microscopic mechanism of the light trapping. As a result, the distribution of the thresholds narrows with $S$ and broadens with the disorder strength.

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arXiv2004

Spin-orbit-induced correlations of the local density of states in two-dimensional electron gas

We study the local density of states (LDOS) of two-dimensional electrons in the presence of spin-orbit (SO) coupling. Although SO coupling has no effect on the average density of states, it manifests itself in the correlations of the LDOS. Namely, the correlation function acquires two satellites centered at energy difference equal to the SO splitting, $2ω_{SO}$, of the electron Fermi surface. For a smooth disorder the satellites are well separated from the main peak. Weak Zeeman splitting $ω_{Z} \ll ω_{SO}$ in a parallel magnetic field causes an anomaly in the shape of the satellites. We consider the effect of SO-induced satellites in the LDOS correlations on the shape of the correlation function of resonant-tunneling conductances at different source-drain biases, which can be measured experimentally. This shape is strongly sensitive to the relation between $ω_{SO}$ and $ω_{Z}$.

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