Research Highlights

Nanomechanical Shuttling in Coulomb Blockade Structures 
Mechanical degrees of freedom take on a primary role in the charge transfer process in many single-electron devices, where transport is controlled by quantum-mechanical tunneling and Coulomb interactions and where parameters of a tunnel barrier can be modified by mechanical motion. A typical system of this kind is a single electron transistor (SET) with a deformable tunnel barrier, the so called Nano-Electro-Mechanical SET (NEM-SET). A new kind of electron transport in this and other types of nanodevices is referred to as "shuttle transport" of electrons and means that electrons are transferred between the metallic leads via a movable small-sized island. [more information.....]

Microscopic theory of thermal phase slips in clean narrow superconducting wires 
Finite resistivity in superconducting wires appears due to phase slip events: local abrupt switches of the system between its metastable states accompanied by phase jumps discontinuously by 2π, as illustrated by the animation. The resistivity is mainly determined by the energy barrier for such an event. For a clean one-channel wire it is possible to find exact analytical solution for the energy barrier valid in the whole temperature and current range. [more information.....]

Self-Assembled Magnetic Surface Swimmers 
We report studies of a novel type of self-assembled magnetically actuated surface swimmers (magnetic snakes), formed from a dispersion of magnetic microparticles on the liquid/air interface and energized by an alternating magnetic field. We show that under certain conditions the snakes spontaneously break the symmetry of surface flows and turn into self-propelled objects. Parameters of the driving magnetic field tune the propulsion velocity of these snake-like swimmers. We find that the surface flows symmetry can be also broken in a controlled fashion by attaching a large bead to a magnetic snake (bead-snake hybrid), transforming it into a robust self-locomoting entity. Observed phenomena have been successfully described by developed phenomenological model based on the amplitude equation for surface waves coupled to the large-scale hydrodynamic mean flow equation. [more information.....]

[1] Driven Magnetic Particles on a Fluid Surface: Pattern Assisted Surface Flows,
M. Belkin, A. Snezhko, I. S. Aranson, and W.-K. Kwok, Phys. Rev. Lett., 99, 158301 (2007).

Collective alignment of polar filaments by molecular motors 
We study the alignment of polar biofilaments, such as microtubules and actin, subject to the action of multiple molecular motors attached simultaneously to more than one filament. Focusing on a paradigm model of only two filaments interacting with multiple motors, we were able to investigate in detail the alignment dynamics. While almost no alignment occurs in the case of a single motor, the filaments become rapidly aligned due to the collective action of the motors. Our analysis shows that the alignment time is governed by the number of bound motors and the magnitude of the motor force fluctuations. We demonstrate that the timescale of alignment is in the order of seconds, much faster than for passive crosslink-induced bundling. in vitro experiments on multiple-motor alignment of microtubules are in good agreement with these results. [more information.....]

Hopping transport in granular metals and superconductors 
Arrays of metallic granules, granular metals, are in the insulating phase at low temperatures if the coupling between granules is weak and exhibit stretched exponential conductivity behavior σ∝exp(-(T0/T)1/2), resembling Efros-Shklovskii hopping conductivity in semiconductors. This temperature dependence observed not only in numerous experiments on disordered granular metals but, strikingly, also in perfectly periodic arrays of metallic granules and periodic arrays of semiconductor quantum dots without visible traces of disorder posed a fundamental problem that resisted almost three decade long intense attacks of theorists. In our recent work [1] we explain the observed dependence in terms of the Mott-Efros-Shklovskii-like variable range hopping mechanism. Our approach includes two key ingredients: (i) electrostatic disorder induced on the granules by the charged centers imbedded in dielectric matrix of granular conductors and (ii) the theory of coherent cotunneling through the chain of granules providing mechanism for the electron hopping over several granules. [more information.....]

[1] Granular electronic systems,
I. S. Beloborodov, A. V. Lopatin, V. M. Vinokur, and K. B. Efetov, Rev. Mod. Phys., 79, 469 (2007).
[2] Coulomb effects and hopping transport in granular metals ,
I. S. Beloborodov, A. V. Lopatin, and V. M. Vinokur, Phys. Rev. B, 72, 125121 (2005).

(click to play animations)

A new method for random number generation 
Many researches in a rich variety of sciences, including quantum, statistical, and nuclear physics, quantum chemistry, material science, and many others, rely heavily in their research on the use of random numbers. The most efficient way of generating sequences of random numbers is based on deterministic recursive rules, which produce pseudorandom numbers. The design of the incrementally powerful random number generators (RNG) that behave as realizations of independent uniformly distributed random variables and approximate "true randomness" [1] remains one of the major challenges for computational science. A new method for constructing high-quality pseudorandom numbers generators (RNG) developed by Lev Barash and Lev Shchur [2] marks a breakthrough in the field. Traditionally, random numbers (RN) are the coordinates of some periodic trajectory, and correlations exist both along the trajectory and between bits of the given number. The proposed approach to making numbers less correlated is to generate an ensemble of uncorrelated trajectories, and use one bit (represented by two digits 0 and 1) from the position of each trajectory to construct s-bit number from s coordinates. Generation of the ensemble of trajectories is naturally parallel computation. The series of generators are developed using new method with the efficient realizations for Pentium processors, making our RNGs competitive with the best known generators. Generators were tested with the battery of calibrated sets of tests. [more information.....]

[1] The art of the computer programming, Vol. 2 ,
D. Knuth, (Addison-Wesley, Cambridge, 1981)
[2] L. Barash and L.N. Shchur,
Periodic orbits of the ensemble of Sinai-Arnold cat maps and pseudorandom number generation,
Phys. Rev. E 73, 036701 (2006). [physics/0409069.]

Nondemolition measurements of a single quantum spin using Josephson oscillations 
Quantum measurements in mesoscopic systems by use of tunneling attracted recently great interest, due to the challenges of both the single spin detection and quantum computing where the final state of a qubit after computation must be measured. We showed that quantum nondemolition (QND) measurements of the spin projection on the direction of the applied magnetic field are possible with the use of spin-dependent Josephson tunneling at low temperatures when effects of quasiparticles are negligible. [more information.....]

Full counting statistics of a charge shuttle 
Current flowing in a nanograin that can oscillate between two leads induces vibration of the grain itself. The resulting current voltage characteristics can differ substantially from the static case. In particular the oscillations of the grain can become so large and regular that electrons can jump in or out of the central grain only when the grain is near one of the two leads. In this regime the current flowing through the system becomes proportional to the frequency of oscillation. In this work not only the average current transmitted, but all current cumulants have been considered (Full counting statistics). Its knowledge allows to understand better charge transfer dynamics. A strong reduction of the Fano factor together with an asymmetric distribution of transmitted charges have been found. [more information.....]

[1] Full Counting Statistics of a charge shuttle,
F. Pistolesi, Phys. Rev. B, 69, 245409 (2004).

Transport Properties of Granular Metals 
Granular metals exhibit a wealth of behaviors generic to strongly interacting disordered electronic systems and offer a unique experimental tool for studying the interplay between the effects of disorder and interactions. Depending on the strength of coupling between the grains these systems can assume either insulating- or metallic phases. [more information.....]

[1] Transport Properties of Granular Metals at Low Temperatures,
I. S. Beloborodov, K. B. Efetov, A. V. Lopatin, and V. M. Vinokur, Phys. Rev. Lett., 91, 246801 (2003).

Josephson vortices and solitons inside pancake vortex lattice in layered superconductors 
In very anisotropic layered superconductors tilted magnetic field generates two interpenetrating vortex sublattices. This set of crossing lattices contains a sublattice of Josephson vortices and a sublattice of pancake-vortex stacks. Due to competing energy and length scales crossing vortex lattices have very rich spectrum of interesting properties. In particular, the pancake-vortex sublattice modifies structure of the Josephson vortex in a very unusual way. [more information.....]

[1] Josephson vortices and solitons inside pancake vortex lattice in layered superconductors,
A. E. Koshelev, Phys. Rev. B 68, 094520 (2003).

Vortex state excitations in soft magnetic submicron-size elements 
Submicron magnetic elements with regular shapes (rectangular, cylindrical etc.) attracted much attention during the last years due to considerable progress in the elements' preparation and characterization. Spatial confinement and finite-size effects alter essentially magnetic properties of the elements. These effects are critically important for new technological applications, such as patterned magnetic media, memory cells, spintronics devices etc. Thus, it is necessary to have an adequate theoretical description of magnetic properties of individual patterned elements, which will be combined into more complex structures such as two-dimensional arrays, multilayer elements etc. The magnetization distribution within a magnetic element in zero field depends on its size and shape. The interplay of finite size and shape-induced modifications of magnetic behavior of small elements leads to variety of their static and dynamic properties. The competition between different energies results, in general, in some non-uniform magnetic states. The curling-type or vortex magnetization state is a ground state of ferromagnetic disk-shaped sub-micron elements. [more information.....]

[1] Eigenfrequencies of vortex state excitations in magnetic submicron-size disks,
K.Y. Guslienko, B.A. Ivanov, V. Novosad et al., J. Appl. Phys. 91, 8037 (2002).
[2] Spin excitations of magnetic vortices in ferromagnetic nanodots,
V. Novosad, M. Grimsditch, K.Y. Guslienko et al., Phys. Rev. B 66, 052407 (2002).

Domain wall superconductivity in hybrid superconductor-ferromagnet structures 
The hybrid superconductor-ferromagnet (S/F) systems give an interesting example of magnetism and superconductivity interplay with an important potential for practical applications. The presence of the domain wall is inherent to all ferromagnets of macroscopical sizes, and so the question of influence of the domain structure on the superconducting characteristic of S/F structures arises. Depending on the parameters of domain structure superconductivity may be enhanced or suppressed in the region near the domain wall. The displacement of the domain wall by the external magnetic field could produce the motion of the superconducting channel in S layer. [more information.....]

Quantum-level resolved Coulomb blockade 
The phenomenon of the Coulomb blockade has been ubiquitous in solid state physics. It is a manifestation of the fact that electron occupying a restricted region in space ( e.g. a quantum dot) blockades the passage of other electrons, owing to electrostatic interaction. We have found that transport and spectral properties of quantum dots depend crucially on the relative signs of the couplings of (consecutive) dot's levels to the external leads. [more information.....]

Nanoscale superconductor as a ballistic quantum switch 
A rich variety of vortex structures appear in mesoscopic superconductors in which only a few quanta of magnetic flux are trapped. The quasiparticle excitations in the vortices form coherent quantum-mechanical states that offer the possibility of controlling the phase-coherent transport through the sample by changing the number of trapped flux quanta and their configuration. [more information.....]

Magnetic flux instabilities in type-II superconductors 
The formation of a macroscopic current-carrying critical state in type II superconductors occurs via penetration of the magnetic flux front of pinned vortices from the surface of the sample. Recent advances in the magneto-optical imaging have revealed puzzling instabilities of the critical state, including magnetic macro-turbulence, kinetic front roughening, magnetic avalanches and dendritic-type instabilities. These phenomena display remarkable similarities with other dendritic structures in crystal growth, nonequilibrium chemical and biological systems, and crack propagation. [more information.....]

Large-scale flow formation in ensembles of swimming bacteria 
We conduct experimental and theoretical studies of self-organization of concentrated ensembles of swimming bacteria Bacilus Subtilis. Experiments are performed in a very thin (of the order of 1 bacterium diameter) fluid film spanned between four supporting fibers. Small amplitude electric field is used to adjust dynamically the density of bacteria inside the experimental cell. We also develop mathematical model of hydrodynamically-induced self-organization of concentrated ensembles of swimming bacteria in thin fluid film. [more information.....]

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