Showing papers by "Moscow Institute of Physics and Technology published in 2012"

A real time system for dynamic hand gesture recognition with a depth sensor

Abstract: Recent advances in depth sensing provide exciting opportunities for the development of new methods for human activity understanding. Yet, little work has been done in the area of hand gesture recognition which has many practical applications. In this paper we propose a real-time system for dynamic hand gesture recognition. It is fully automatic and robust to variations in speed and style as well as in hand orientations. Our approach is based on action graph, which shares similar robust properties with standard HMM but requires less training data by allowing states shared among different gestures. To deal with hand orientations, we have developed a new technique for hand segmentation and orientation normalization. The proposed system is evaluated on a challenging dataset of twelve dynamic American Sign Language (ASL) gestures.

Field-induced polar order at the Néel temperature of chromium in rare-earth orthochromites: Interplay of rare-earth and Cr magnetism

Abstract: We report field-induced switchable polarization ($P$ \ensuremath{\sim} 0.2--0.8 \ensuremath{\mu}C/cm${}^{2}$) below the N\'eel temperature of chromium (${T}_{N}$${}^{\mathrm{Cr}}$) in weakly ferromagnetic rare-earth orthochromites, $R$CrO${}_{3}$ ($R$ $=$ rare earth) but only when the rare-earth ion is magnetic. Intriguingly, the polarization in ErCrO${}_{3}$ (${T}_{C}$ $=$ 133 K) disappears at a spin-reorientation (Morin) transition (${T}_{SR}\ensuremath{\sim}22$ K) below which the weak ferromagnetism associated with the Cr sublattice also disappears, demonstrating the crucial role of weak ferromagnetism in inducing the polar order. Further, the polarization ($P$) is strongly influenced by an applied magnetic field, indicating a strong magnetoelectric effect. We suggest that the polar order occurs in $R$CrO${}_{3}$, due to the combined effect of the poling field that breaks the symmetry and the exchange field on the R ion from the Cr sublattice that stabilizes the polar state. We propose that a similar mechanism could work in the isostructural rare-earth orthoferrites $R$FeO${}_{3}$ as well.

Where is the supercritical fluid on the phase diagram

Abstract: We discuss the fluid state of matter at high temperature and pressure. We review the existing ways in which the boundary between a liquid and a quasigas fluid above the critical point are discussed. We show that the proposed 'thermodynamic' continuation of the boiling line, the 'Widom line', exists as a line near the critical point only, but becomes a bunch of short lines at a higher temperature. We subsequently propose a new 'dynamic' line separating a liquid and a gas-like fluid. The dynamic line is related to different types of particle trajectories and different diffusion mechanisms in liquids and dense gases. The location of the line on the phase diagram is determined by the equality of the liquid relaxation time and the minimal period of transverse acoustic excitations. Crossing the line results in the disappearance of transverse waves at all frequencies, the diffusion coefficient acquiring a value close to that at the critical point, the speed of sound becoming twice the particle thermal speed, and the specific heat reaching 2. In the high-pressure limit, the temperature on the dynamic line depends on pressure in the same way as does the melting temperature. In contrast to the Widom line, the proposed dynamic line separates liquid and gas-like fluids above the critical point at arbitrarily high pressure and temperature. We propose calling the new dynamic line the 'Frenkel line'.

Light propagation in composite materials with gain layers

Abstract: Light propagation through a single gain layer and a multilayer system with gain layers is studied. Results obtained using the Fresnel formulas, Airy's series summation, and the numerical solution of the nonlinear Maxwell-Bloch equations by the finite difference time domain (FDTD) method are analyzed and compared. Normal and oblique propagation of a wave through a gain layer and a slab of a photonic crystal are examined. For the latter problem, the gain line may be situated in either the pass or stop band of the photonic crystal. It is shown that the monochromatic plane-wave approximation is generally inapplicable for active media, because it leads to results that violate causality. But the problem becomes physically meaningful and correct results can be obtained for all three approaches once the structure of the wavefront and the finite aperture of the beam are taken into account.

Evidence for triplet superconductivity in a superconductor-ferromagnet spin valve.

Abstract: We have studied the dependence of the superconducting (SC) transition temperature on the mutual orientation of magnetizations of Fe1 and Fe2 layers in the spin valve system CoO(x)/Fe1/Cu/Fe2/Pb. We find that this dependence is nonmonotonic when passing from the parallel to the antiparallel case and reveals a distinct minimum near the orthogonal configuration. The analysis of the data in the framework of the SC triplet spin valve theory gives direct evidence for the long-range triplet superconductivity arising due to noncollinearity of the two magnetizations.

Global models of runaway accretion in white dwarf debris discs

Abstract: A growing number of young white dwarfs (WDs) with metal-enriched atmospheres are accompanied by excess infrared (IR) emission, indicating that they are encircled by a compact dusty disc of solid debris. Such ‘WD debris discs’ are thought to originate from the tidal disruption of asteroids or other minor bodies. However, the precise mechanism responsible for transporting matter from the disruption radius to the WD surface remains unclear, especially in systems with the highest inferred metal accretion rates g s−1, which cannot be explained by Poynting–Robertson (PR) drag alone. Here we present global time-dependent calculations of the coupled evolution of the gaseous and solid components of WD debris discs. Solids transported inwards (initially due to PR drag) sublimate at tens of WD radii, producing a source of gas that both accretes on to the WD surface and viscously spreads outwards in radius, where it overlaps with the solid disc. Our calculations show that if the aerodynamic coupling between the solids and gaseous discs is sufficiently strong (and/or the gas viscosity sufficiently weak), then gas builds up near the sublimation radius faster than it can viscously spread away. Since the rate of drag-induced solid accretion increases with gas density, this results in a runaway accretion process, as predicted by Rafikov, during which the WD accretion rate reaches values orders of magnitude higher than can be achieved by PR drag alone, consistent with the highest measured values of . We explore the evolution of WD debris discs across a wide range of physical conditions and describe the stages of the runaway process in detail. We also calculate the predicted distribution of observed accretion rates , finding reasonable agreement with the current sample. We use our disc evolution model to show that the steady-state assumption commonly adopted to calculate WD metal accretion rates is inaccurate when the metal settling time in the WD atmosphere is long compared to the viscous time-scale; a long metal settling phase following a runaway accretion event may explain some metal-polluted WDs with no current IR excess. Although the conditions necessary for runaway accretion are at best marginally satisfied given the minimal level of aerodynamic drag between circular gaseous and solid discs, the presence of other stronger forms of solid–gas coupling – such as would result if the gaseous disc is only mildly eccentric – substantially increase the likelihood of runaway accretion.

Simulation of indivisible qubit channels in collision models

Abstract: A sequence of controlled collisions between a quantum system and its environment (composed of a set of quantum objects) naturally simulates (with arbitrary precision) any Markovian quantum dynamics of the system under consideration. In this paper we propose and study the problem of simulation of an arbitrary quantum channel via collision models. We show that a correlated environment is capable to simulate non-Markovian evolutions leading to any indivisible qubit channel. In particular, we derive the corresponding master equation generating a continuous time non-Markovian dynamics implementing the universal NOT gate being an example of the most non-Markovian quantum channels. (Some figures may appear in colour only in the online journal)

Minkowski Polynomials and Mutations

Abstract: Given a Laurent polynomial f, one can form the period of f: this is a function of one complex variable that plays an important role in mirror symmetry for Fano manifolds. Mutations are a particular class of birational transformations acting on Laurent polynomials in two variables; they preserve the period and are closely connected with cluster algebras. We propose a higher-dimensional analog of mutation acting on Laurent polynomials f in n variables. In particular we give a combinatorial description of mutation acting on the Newton polytope P of f, and use this to establish many basic facts about mutations. Mutations can be understood combinatorially in terms of Minkowski rearrangements of slices of P, or in terms of piecewise-linear transformations acting on the dual polytope P* (much like cluster transformations). Mutations map Fano polytopes to Fano polytopes, preserve the Ehrhart series of the dual polytope, and preserve the period of f. Finally we use our results to show that Minkowski polynomials, which are a family of Laurent polynomials that give mirror partners to many three-dimensional Fano manifolds, are connected by a sequence of mutations if and only if they have the same period.

Mechanical transition from α-helical coiled coils to β-sheets in fibrin(ogen).

Abstract: We characterized the α-to-β transition in α-helical coiled-coil connectors of the human fibrin(ogen) molecule using biomolecular simulations of their forced elongation and theoretical modeling. The force (F)-extension (X) profiles show three distinct regimes: (1) the elastic regime, in which the coiled coils act as entropic springs (F 175-200 pN; X > 40-50 nm). In the plastic regime, the three-stranded α-helices undergo a noncooperative phase transition to form parallel three-stranded β-sheets. The critical extension of the α-helices is 0.25 nm, and the energy difference between the α-helices and β-sheets is 4.9 kcal/mol per helical pitch. The soft α-to-β phase transition in coiled coils might be a universal mechanism underlying mechanical properties of filamentous α-helical proteins.

Spatially resolved observations of a split-band coronal type II radio burst

Abstract: Context. The origin of coronal type II radio bursts and the nature of their band splitting are still not fully understood, though a number of scenarios have been proposed to explain them. This is largely due to the lack of detailed spatially resolved observations of type II burst sources and of their relations to magnetoplasma structure dynamics in parental active regions. Aims. To make progress in solving this problem on the basis of one extremely well observed solar eruptive event. Methods. The relative dynamics of multithermal eruptive plasmas, observed in detail by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, and of harmonic type II burst sources, observed by the Nancay Radioheliograph at ten frequencies from 445 to 151 MHz, was studied for the 3 November 2010 event arising from an active region behind the east solar limb. Special attention was given to the band splitting of the burst. Analysis was supplemented by investigation of coronal hard X-ray (HXR) sources observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager. Results. We found that the flare impulsive phase was accompanied by the formation of a double coronal HXR source, whose upper part coincided with the hot (T ≈ 10 MK) eruptive plasma blob. The leading edge (LE) of the eruptive plasmas (T ≈ 1−2 MK) moved upward from the flare region with a speed of v ≈ 900−1400 km s-1 . The type II burst source initially appeared just above the LE apex and moved with the same speed and in the same direction. After ≈ 20 s, it started to move about twice as fast, but still in the same direction. At any given moment, the low-frequency component (LFC) source of the splitted type II burst was situated above the high-frequency component (HFC) source, which in turn was situated above the LE. We also found that at a given frequency the HFC source was located slightly closer to the photosphere than the LFC source.Conclusions. Based on the set of established observational facts, we conclude that the shock wave, which could be responsible for the observed type II radio burst, was initially driven by the multi-temperature eruptive plasmas, but later transformed to a freely propagating blast shock wave. The preferable interpretation of the type II burst splitting is that its LFC was emitted from the upstream region of the shock, whereas the HFC was emitted from the downstream region. The shock wave in this case could be subcritical.

Surface Plasmon Polariton Amplification upon Electrical Injection in Highly Integrated Plasmonic Circuits

Abstract: We propose a very efficient approach for amplification of surface plasmon polaritons (SPPs) in a nanoscale waveguiding geometry with strong (∼λ/10) mode confinement. The implemented scheme of electric pumping is based on a single-heterostructure Schottky-barrier diode and has been numerically shown to ensure full compensation of the SPP propagation losses at wavelengths around 3 μm and, moreover, to provide net SPP gain. The presented concept creates the backbone for the implementation of highly integrated large-scale hybrid electronic-plasmonic circuits operating at extremely high speeds and opens the prospects for the realization of integrated coherent SPP sources.

A nanosecond surface dielectric barrier discharge at elevated pressures: time-resolved electric field and efficiency of initiation of combustion

Abstract: We study a nanosecond surface dielectric barrier discharge (SDBD) initiated by negative or positive polarity pulses 10?15?kV in amplitude in a cable, 25?30?ns FWHM, 5?ns rise time, in the regime of a single shot or 3?Hz repetitive frequency. Discharge parameters, namely spatial structure of the discharge and time- and space-resolved electric field are studied in a N2?:?O2?=?4?:?1 mixture for P?=?1?5?atm. The possibility of igniting a combustible mixture with the help of an SDBD is demonstrated using the example of a stoichiometric C2H6?:?O2 mixture at ambient initial temperature and at 1?atm pressure. Flame propagation and ignited volume as a function of time are compared experimentally for two discharge geometries: SDBD and pin-to-pin configurations at the same shape and amplitude of the incident pulse. It is shown that the SDBD can be considered as a multi-point ignition system with maximum energy release near the high-voltage electrode. Numerical modeling of the discharge and subsequent combustion kinetics for the SDBD conditions is performed. The discharge action leads to the production of atoms and radicals as well as to fast gas heating, due to the relaxation of electronic and vibrational degrees of freedom. The calculated ignition delay time is in reasonable agreement with the experimental results.

The algebraic Bethe ansatz for scalar products in SU(3)-invariant integrable models

Abstract: We study SU(3)-invariant integrable models solvable by nested algebraic Bethe ansatz. We obtain a determinant representation for particular case of scalar products of Bethe vectors. This representation can be used for the calculation of form factors and correlation functions of XXX SU(3)-invariant Heisenberg chain.

First Observation of eta(1405) Decays into f(0)(980)pi(0)

Abstract: The decays J/psi -> gamma pi(+)pi(-)pi(0) and J/psi ->gamma pi(0)pi(0)pi(0) are analyzed using a sample of 225 X 10(6) J/psi events collected with the BESIII detector. The decay of eta(1405) -> f(0)(980)pi(0) with a large isospin violation is observed for the first time. The width of the f(0)(980) observed in the dipion mass spectra is anomalously narrower than the world average. Decay rates for three-pion decays of the eta' are also measured precisely.

Spin-Parity Analysis of pp̄ Mass Threshold Structure in J/ψ and ψ(3686) Radiative Decays

Abstract: A partial wave analysis of the p (p) over bar mass-threshold enhancement in the reaction J/psi -> gamma p (p) over bar is used to determine its J(PC) quantum numbers to be 0(-+), its peak mass to be below threshold at M 1832(-5)(+19)(stat)(-17)(+18)(syst) +/- 19(model) MeV/c(2), and its total width to be Gamma gamma X(p)]BR[X(p (p) over bar) -> p (p) over bar] = [9:0(-1.1)(+0.4)(stat)(-5.0)(+1:5)(syst) +/- 2: 3(model)] x 10(-5). A similar analysis performed on psi(3686) -> gamma p (p) over bar decays shows, for the first time, the presence of a corresponding enhancement with a production rate relative to that for J/psi decays of R = [5.08(-0.45)(+0: 71)(stat)(-058)(+0: 67) (syst) +/- 0: 12(model)%.

Atomistic simulation of laser ablation of gold: Effect of pressure relaxation

Abstract: The process of ablation of a gold target by femto- and picosecond laser radiation pulses has been studied by numerical simulations using an atomistic model with allowance for the electron subsystem and the dependence of the ion-ion interaction potential on the electron temperature. Using this potential, it is possible to take into account the change in the physical properties of the ion subsystem as a result of heating of the electron subsystem. The results of simulations reveal a significant difference between the characteristics of metal ablation by laser pulses of various durations. For ablation with subpicosecond pulses, two mechanisms of metal fracture related to the evolution of electronic pressure in the system are established.

Condensation of electron-hole pairs in a two-layer graphene system: Correlation effects

Abstract: Condensation of pairs formed by spatially separated electrons and holes in a system of two isolated graphene layers is studied beyond the mean-field approximation. Suppression of the screening of the pairing interaction at large distances, caused by the appearance of the gap, is considered self-consistently. A mutual positive feedback between the appearance of the gap and the enlargement of the interaction leads to a sharp transition to a correlated state with a greatly increased gap above some critical value of the coupling strength. At a coupling strength below the critical value, this correlation effect increases the gap approximately by a factor of 2. The maximal coupling strength achievable in experiments is close to the critical value. This indicates the importance of correlation effects in closely spaced graphene bilayers at weak substrate dielectric screening. Another effect beyond the mean-field approximation considered is the influence of vertex corrections on the pairing, which is shown to be very weak.

Spectral Duality Between Heisenberg Chain and Gaudin Model

Abstract: In our recent paper we described relationships between integrable systems inspired by the AGT conjecture. On the gauge theory side an integrable spin chain naturally emerges while on the conformal field theory side one obtains some special reduced Gaudin model. Two types of integrable systems were shown to be related by the spectral duality. In this paper we extend the spectral duality to the case of higher spin chains. It is proved that the N-site GL(k) Heisenberg chain is dual to the special reduced k+2-points gl(N) Gaudin model. Moreover, we construct an explicit Poisson map between the models at the classical level by performing the Dirac reduction procedure and applying the AHH duality transformation.

Towards a proof of agt conjecture by methods of matrix models

Abstract: A matrix model approach to proof of the AGT relation is briefly reviewed. It starts from the substitution of conformal blocks by the Dotsenko–Fateev β-ensemble averages and Nekrasov functions by a double deformation of the exponentiated Seiberg–Witten prepotential in β≠1 and gs≠0 directions. Establishing the equality of these two quantities is a typical matrix model problem.

On the mean profiles of radio pulsars – I. Theory of propagation effects

Abstract: We study the influence of propagation effects on the mean profiles of radio pulsars using the method of wave propagation in inhomogeneous media described by Kravtsov & Orlov. This approach allows us first to take into consideration the transition from geometrical optics to vacuum propagation, the cyclotron absorption and the wave refraction simultaneously. In addition, the non-dipole magnetic field configuration, the drift motion of plasma particles and their realistic energy distribution are taken into account. It is confirmed that, for ordinary pulsars (period P ∼ 1 s, surface magnetic field B0 ∼ 1012 G) and typical plasma generation near the magnetic poles (multiplicity parameter λ = ne/nGJ ∼ 103), the polarization is formed inside the light cylinder at a distance resc ∼ 1000 R from the neutron star, the circular polarization being 5–20 per cent which is in agreement with observational data. A one-to-one correspondence between the signs of circular polarization and position angle (PA) derivative along the profile for both ordinary and extraordinary waves is predicted. Using numerical integration we now can model the mean profiles of radio pulsars. It is shown that the standard S-shape form of the PA swing can be realized for small enough multiplicity λ and large enough bulk Lorentz factor γ only. It is also shown that the value of the maximum derivative of PA, which is often used for determination of the angle between magnetic dipole and rotation axis, depends on the plasma parameters and could differ from the rotation vector model (RVM) prediction.

Interatomic potential for uranium in a wide range of pressures and temperatures

Abstract: Using the force-matching method we develop an interatomic potential that allows us to study the structure and properties of α-U, γ-U and liquid uranium. The potential is fitted to the forces, energies and stresses obtained from ab initio calculations. The model gives a good comparison with the experimental and ab initio data for the lattice constants of α-U and γ-U, the elastic constants, the room-temperature isotherm, the normal density isochore, the bond-angle distribution functions and the vacancy formation energies. The calculated melting line of uranium at pressures up to 80 GPa and the temperature of the α-γ transition at 3 GPa agree well with the experimental phase diagram of uranium.

Multistate organization of transmembrane helical protein dimers governed by the host membrane.

Abstract: Association of transmembrane (TM) helices taking place in the cell membrane has an important contribution to the biological function of bitopic proteins, among which receptor tyrosine kinases represent a typical example and a potent target for medical applications. Since this process depends on a complex interplay of different factors (primary structures of TM domains and juxtamembrane regions, composition and phase of the local membrane environment, etc.), it is still far from being fully understood. Here, we present a computational modeling framework, which we have applied to systematically analyze dimerization of 18 TM helical homo- and heterodimers of different bitopic proteins, including the family of epidermal growth factor receptors (ErbBs). For this purpose, we have developed a novel surface-based modeling approach, which not only is able to predict particular conformations of TM dimers in good agreement with experiment but also provides screening of their conformational heterogeneity. Using all-atom molecular dynamics simulations of several of the predicted dimers in different model membranes, we have elucidated a putative role of the environment in selection of particular conformations. Simulation results clearly show that each particular bilayer preferentially stabilizes one of possible dimer conformations, and that the energy gain depends on the interplay between structural properties of the protein and the membrane. Moreover, the character of protein-driven perturbations of the bilayer is reflected in the contribution of a particular membrane to the free energy gain. We have found that the approximated dimerization strength for ErbBs family can be related to their oncogenic ability.

Parafermionic Liouville field theory and instantons on ALE spaces

Abstract: In this paper we study the correspondence between the $ \widehat{\text{su}}{(n)_k} \oplus \widehat{\text{su}}{(n)_p}/\widehat{\text{su}}{(n)_{{k + p}}} $ coset conformal field theories and $ \mathcal{N} = {2} $ SU(n) gauge theories on $ {\mathbb{R}^4}/{\mathbb{Z}_p} $ . Namely we check the correspondence between the SU(2) Nekrasov partition function on $ {\mathbb{R}^4}/{\mathbb{Z}_4} $ and the conformal blocks of the S 3 parafermion algebra (in S and D modules). We find that they are equal up to the U(1)-factor as it was in all cases of AGT-like relations. Studying the structure of the instanton partition function on $ {\mathbb{R}^4}/{\mathbb{Z}_p} $ we also find some evidence that this correspondence with arbitrary p takes place up to the U(1)-factor.

IR divergences and kinetic equation in de Sitter space. (Poincare patch; principal series)

Abstract: We explicitly show that the one loop IR correction to the two-point function in de Sitter space scalar QFT does not reduce just to the mass renormalization. The proper interpretation of the loop corrections is via particle creation revealing itself through the generation of the quantum averages $$ \left\langle {a_p^{ + }{a_p}} \right\rangle, \,\left\langle {{a_p}{a_{{ - p}}}} \right\rangle $$ and $$ \left\langle {a_p^{ + }a_{{ - p}}^{ + }} \right\rangle $$ , which slowly change in time. We show that this observation in particular means that loop corrections to correlation functions in de Sitter space can not be obtained via analytical continuation of those calculated on the sphere. We find harmonics for which the particle number $$ \left\langle {a_p^{ + }{a_p}} \right\rangle $$ dominates over the anomalous expectation values $$ \left\langle {{a_p}{a_{{ - p}}}} \right\rangle $$ and $$ \left\langle {a_p^{ + }a_{{ - p}}^{ + }} \right\rangle $$ . For these harmonics the Dyson-Schwinger equation reduces in the IR limit to the kinetic equation. We solve the latter equation, which allows us to sum up all loop leading IR contributions to the Whiteman function. We perform the calculation for the principle series real scalar fields both in expanding and contracting Poincare patches.

The algebraic Bethe ansatz for scalar products in SU(3)-invariant integrable models

Abstract: We study SU(3)-invariant integrable models solvable by nested algebraic Bethe ansatz. We obtain a determinant representation for particular case of scalar products of Bethe vectors. This representation can be used for the calculation of form factors and correlation functions of XXX SU(3)-invariant Heisenberg chain.