Showing papers in "Jetp Letters in 2008"

Electron-hole pair condensation in a graphene bilayer

Abstract: The pairing of electrons and holes due to their Coulomb attraction in two parallel, independently gated graphene layers separated by a barrier is considered. At a weak coupling, there exists the BCS-like pair-condensed state. Despite the fact that electrons and holes behave like massless Dirac fermions, the problem of BCS-like electron—hole pairing in the graphene bilayer turns out to be rather similar to that in usual coupled semiconductor quantum wells. The distinctions are due to the Berry phase of electronic wavefunctions and different screening properties. We estimate the values of the gap in a one-particle excitation spectrum for different interlayer distances and carrier concentrations. The influence of the disorder is discussed. At a large enough dielectric susceptibility of the surrounding medium, the weak coupling regime holds at arbitrarily small carrier concentrations. Localized electron—hole pairs are absent in graphene, thus the behavior of the system versus the coupling strength is cardinally different from usual BCS—BEC crossover.

Electronic structure of prototype AFe2As2 and ReOFeAs high-temperature superconductors: A comparison

Abstract: We have performed ab initio LDA calculations of the electronic structure of newly discovered prototype high-temperature superconductors AFe2As2 (A = Ba, Sr) and compared it with the previously calculated electronic spectra of ReOFeAs (Re = La, Ce, Pr, Nd, Sm). In all cases, we obtain almost identical densities of states in a rather wide energy interval (up to 1 eV) around the Fermi level. Energy dispersions are also very similar and almost two dimensional in this energy interval, leading to the same basic (minimal) model of the electronic spectra, determined mainly by Fe d orbitals of the FeAs layers. The other constituents, such as A ions or rare-earth Re (or oxygen states) are more or less irrelevant for superconductivity. LDA Fermi surfaces for AFe2As2 are also very similar to that of ReOFeAs. This makes the more simple AFe2As2 a generic system to study the high-temperature superconductivity in FeAs-layered compounds.

On the formation of freak waves on the surface of deep water

Abstract: Numerical simulation of the fully nonlinear water equations demonstrates the existence of giant breathers on the surface of deep water. The numerical analysis shows that this breather (or soliton of envelope) does not loose energy. The existence of such a breather can explain the appearance of freak waves.

Two-dimensional electron-hole system in a HgTe-based quantum well

Abstract: A two-dimensional electron-hole system consisting of light high-mobility electrons with a density of Ns = (4–7) × 1010 cm−2 and a mobility of μn = (4–6) × 105 cm2/V s and heavier low-mobility holes with a density of Ps = (0.7–1.6) × 1011 cm−2 and a mobility of μp = (3–7) × 104 cm2/V s has been discovered in a quantum well based on mercury telluride with the (013) surface orientation. The system exhibits a number of specific magnetotransport properties in both the classical magnetotransport (positive magnetoresistance and alternating Hall effect) and the quantum Hall effect regime. These properties are associated with the coexistence of two-dimensional electrons and holes.

Muon content of ultrahigh-energy air showers: Yakutsk data versus simulations

Abstract: A sample of 33 extensive air showers (EASs) with estimated primary energies above 2 × 1019 eV and high-quality muon data recorded by the Yakutsk EAS array is analyzed. The observed muon density is compared event-by-event to that expected from CORSIKA simulations for primary protons and iron using SIBYLL and EPOS hadronic interaction models. The study suggests the presence of two distinct hadronic components, “light” and “heavy.” Simulations with EPOS are in good agreement with the expected composition in which the light component corresponds to protons and the heavy component to iron-like nuclei. With SIBYLL, simulated muon densities for iron primaries are a factor of ∼ 1.5 less than those observed for the heavy component for the same electromagnetic signal. Assuming a two-component proton-iron composition and the EPOS model, the fraction of protons with energies E > 1019 eV is 0.52 −0.20 +0.19 at the 95% C.L.

Magnetoelectric interactions and phase transitions in a new class of multiferroics with improper electric polarization

Abstract: Symmetry analysis of magnetoelectric interactions in rare-earth orthoferrites and orthochromites has been performed. It has been shown that symmetry allows the appearance of spontaneous electric polarization or the magnetic-field-induced polarization in the region of antiferromagnetic (centro-asymmetric) ordering of the rare-earth ions. The analysis reveals a number of pronounced anomalies in the behavior of the electric polarization at the metamagnetic and spin-reorientation transitions in DyCrO3 and TbFeO3. This behavior points to a strong sensitivity of the magnetoelectric properties of such magnets to the antiferromagnetic state of the rareearth subsystem and the spin orientation of the d ions.

High energy jets in the Earth’s magnetosheath: Implications for plasma dynamics and anomalous transport

Abstract: High energy density Jets in the magnetosheath near the Earth magnetopause were observed by Interball-1 [1]. In this paper, we continue the investigation of this important physical phenomenon. New data provided by Cluster show that the magnetosheath kinetic energy density during more than one hour exhibits an average level and a series of peaks far exceeding the kinetic energy density in the undisturbed solar wind. This is a surprising finding because the kinetic energy of the upstream solar wind in equilibrium should be significantly diminished downstream in the magnetosheath due to plasma braking and thermalization at the bow shock. We suggest resolving the energy conservation problem by the fact that the nonequilibrium Jets appear to be locally superimposed on the background equilibrium magnetosheath, and, thus, the energy balance should be settled globally on the spatial scales of the entire dayside magnetosheath. We show that both the Cluster and Interball jets are accompanied by plasma superdiffusion and suggest that they are important for the energy dissipation and plasma transport. The character of the Jet-related turbulence strongly differs From that of known standard cascade models. We infer that these Jets may represent the phenomenon of the general physical occurrence observed in other natural systems, such as heliosphere, astrophysical, and fusion plasmas [2-10].

Electronic structure of new LiFeAs high-T c superconductor

Abstract: We present results of ab initio LDA calculations of electronic structure of “next generation” layered iron-pnictide high-T c superconductor LiFeAs (T c = 18 K). Obtained electronic structure of LiFeAs very similar to recently studied ReOFeAs (Re = La, Ce, Pr, Nd, Sm) and AFe2As2 (A = Ba, Sr) compounds. Namely close to the Fermi level its electronic properties are also determined mainly by Fe 3d-orbilats of FeAs4 two-dimensional layers. Band dispersions of LiFeAs are very similar to the LaOFeAs and BaFe2As2 systems as well as the shape of the Fe-3d density of states and Fermi surface.

On the correlation of the highest-energy cosmic rays with nearby extragalactic objects reported by the Pierre Auger Collaboration

Abstract: We argue that the data published by the Pierre Auger Collaboration [1] disfavors, at the 99% confidence level, their hypothesis that most of the highest-energy cosmic rays are protons from nearby astrophysical sources—either active galactic nuclei or other objects with a similar spatial distribution.

Jet quenching with running coupling including radiative and collisional energy losses

Abstract: We calculate the nuclear modification factor for RHIC and LHC conditions accounting for the radiative and collisional parton energy loss with the running coupling constant. We find that the RHIC data can be explained both in the scenario with the chemically equilibrium quark-gluon plasma and purely gluonic plasma with slightly different thermal suppression of the coupling constant. The role of the parton energy gain due to gluon absorption is also investigated. Our results show that the energy gain gives negligible effect.

New mechanism of the formation of the nanorelief on a surface irradiated by a femtosecond laser pulse

Abstract: The kinetics of fast processes induced by an ultrashort laser pulse is considered. The reliefs remaining after the action of a series of ultrashort laser pulses {S. A. Akhmanov, V. I. Emelyanov, N. I. Koroteev, et al., Usp. Fiz. Nauk 147, 675 (1985) [Sov. Phys. Usp. 28, 1084 (1985)]; F. Costache, S. Kouteva-Arguirova, and J. Reif, Appl. Phys. A 79, 1429 (2004)} have been studied. A new mechanism of perturbing the surface of the initially ideal crystal face is described. First, the formation of a relief is induced by a single pulse. Second, the relief scale along the target surface is about the heating depth dT ∼ 10–100 nm rather than the pump-pulse wavelength λpump ∼ 1 μm. Third, the formation of the relief is not attributed to the modulation of the electromagnetic field near the surface due to the interference of the incident light wave with the electromagnetic surface waves on the initial perturbations of the boundary. These three conditions are satisfied for a known instability induced by the interference of the incident and surface waves (see the works cited above [1]). In our case, the nanorelief is formed due to the deformation of the spalled layer by cavitation bubbles owing to the inhomogeneity of the drag force in the target plane. Cavitation is caused by the tension of the substance in the process of the expansion of a heated target. It is similar to the known phenomenon of the cavitation “spallation” in a liquid despite the large difference between the space-time scales of the usual spallation facility and the femtosecond heating. Owing to this difference, usual cavitation does not leave any morphological trace on the outer free surface of the spalled layer.

High-temperature superconductivity in transition metal oxypnictides: A rare-earth puzzle?

Abstract: Extensive ab initio LDA and LSDA + U calculations of an electronic structure of newly discovered high-temperature superconducting series ReO1 − xFxFeAs (Re = La, Ce, Pr, Nd, and Sm and the hypothetical case of Re = Y) have been performed. In all cases, almost identical electronic spectrum (both energy dispersions and the densities of states) has been obtained in a rather wide energy interval (about 2 eV) around the Fermi level. This fact is unlikely to be changed by strong correlations. This leads inevitably to the same critical temperature Tc of a superconducting transition in any theoretical BCS-like mechanism of the Cooper pair formation. The experimentally observed variations of the Tc for different rare-earth substitutions are either due to the disorder effects or less probably because of possible changes in the spin-fluctuation spectrum of FeAs layers caused by magnetic interactions with rare-earth spins in ReO layers.

Spontaneous emission in dielectric nanoparticles

Abstract: An analytical expression is obtained for the radiative-decay rate of an excited optical center in an ellipsoidal dielectric nanoparticle (with sizes much less than the wavelength) surrounded by a dielectric medium. It is found that the ratio of the decay rate A nano of an excited optical center in the nanoparticle to the decay rate A bulk of an excited optical center in the bulk sample is independent of the local-field correction and, therefore, of the adopted local-field model. Moreover, the expression implies that the ratio A nano/A bulk for oblate and prolate ellipsoids depends strongly on the orientation of the dipole moment of the transition with respect to the ellipsoid axes. In the case of spherical nanoparticles, a formula relating the decay rate A nano and the dielectric parameters of the nanocomposite and the volumetric content c of these particles in the nanocomposite is derived. This formula reduces to a known expression for spherical nanoparticles in the limit c ≪ 1, while the ratio A nano/A bulk approaches unity as c tends to unity. The analysis shows that the approach used in a number of papers {H. P. Christensen, D. R. Gabbe, and H. P. Jenssen, Phys. Rev. B 25, 1467 (1982); R. S. Meltzer, S. P. Feofilov, B. Tissue, and H. B. Yuan, Phys. Rev. B 60, R14012 (1999); R. I. Zakharchenya, A. A. Kaplyanskii, A. B. Kulinkin, et al., Fiz. Tverd. Tela 45, 2104 (2003) [Phys. Solid State 45, 2209 (2003)]; G. Manoj Kumar, D. Narayana Rao, and G. S. Agarwal, Phys. Rev. Lett. 91, 203903 (2003); Chang-Kui Duan, Michael F. Reid, and Zhongqing Wang, Phys. Lett. A 343, 474 (2005); K. Dolgaleva, R. W. Boyd, and P. W. Milonni, J. Opt. Soc. Am. B 24, 516 (2007)}, for which the formula for A nano is derived merely by substituting the bulk refractive index by the effective refractive index of the nanocomposite must be revised, because the resulting ratio A nano/A bulk turns out to depend on the local-field model. The formulas for the emission and absorption cross sections σnano for nanoparticles are derived. It is shown that the ratios σnano/σbulk and A nano/A bulk are not equal in general, which can be used to improve the lasing parameters. The experimentally determined and theoretically evaluated decay times of metastable states of dopant rare-earth ions in crystalline YAG and Y2O3 nanoparticles are compared with the corresponding values for bulk crystals of the same structure.

Density ducts formed by heating the Earth’s ionosphere with high-power HF radio waves

Abstract: The parameters of plasma disturbances at altitudes 660 and 840 km, measured by the instruments onboard the French DEMETER satellite and the US DMSP satellites passing through the magnetic flux tube footed at the region of intense modification of the F2 ionospheric layer by the high-power HF radio waves of the Sura heating facility, are presented. The formation of artificial enhanced-density plasma ducts in the outer ionosphere is observed experimentally. Conditions facilitating the formation of such ducts are pointed out.

Electronic structure of new oxygen-free 38-K superconductor Ba1−xKxFe2As2 in comparison with BaFe2As2 from the first principles

Abstract: Based on first-principle FLAPW-GGA calculations, we have investigated the electronic structure of the newly discovered oxygen-free 38-K superconductor Ba1−xKxFe2As2 in comparison with a parent phase—the tetragonal ternary iron arsenide BaFe2As2. The density of states, magnetic properties, near-Fermi band compositions, together with the Sommerfeld coefficients γ and the molar Pauli paramagnetic susceptibility χ have been evaluated. The results obtained allow us to classify these systems as quasi-two-dimensional ionic metals, where the conduction is strongly anisotropic, occurring only in the (Fe-As) layers. According to our calculations, in the case of the hole doping of BaFe2As2, the density of states at the Fermi level grows, which may be a factor promoting the occurrence of superconductivity for Ba1−xKxFe2As2. On the other hand, Ba1−xKxFe2As2 lies at the border of the magnetic instability and the pairing interactions might involve the magnetic or orbital fluctuations.

Kinetic description of the screening of the charge of macroparticles in a nonequilibrium plasma

Abstract: A new approach has been developed for the kinetic description of the screening of the charge of macroparticles and the determination of the distribution of the effective potential in a nonequilibrium plasma. The main concept of the new approach is the description of the absorption of electrons and ions by a macroparticle through introducing effective point sinks in the kinetic equations describing the dynamics of plasma particles. With the use of this approach, explicit relations for the effective potential are derived with allowance for the collisions of electrons and ions with neutrals of the buffer gas and in the presence of an external magnetic field.

Polarization singularities in partially coherent combined beams

Abstract: Vector singularities are predicted and discovered experimentally in partially polarized combined fields formed by incoherent superposition of orthogonally polarized beams. Such singularities are U contours with zero degree of polarization and isolated P points with unit degree of polarization centered at vortices of the orthogonally polarized component of the combined beam. Crossing a U contour switches the polarization state to the orthogonal one. The above-mentioned singularities are adequately described in terms of the complex degree of polarization in the Stokes-space representation. It is shown that the field elements corresponding to the extrema of the complex degree of polarization form the vector skeleton of a partially coherent nonuniformly polarized field.

On superconducting and magnetic properties of iron oxypnictides

Abstract: Pairing symmetry in oxypnictides, a new family of multiband high-Tc superconductors, is partially imposed by the positions of multiple Fermi pockets, which itself can give rise to new order parameters, such as s+,− states or the state of the \( d_{x^2 - y^2 } \) symmetry. Other pairing states may appear on small pockets for long-range interactions, but they are expected to be sensitive to defects. We identify the competing antiferromagnetic order with the triplet exciton transition in the semimetallic background and discuss whether its coexistence with superconductivity explains the doping dependence of Tc.

On the physical meaning of the Unruh effect

Abstract: Simple arguments are presented that detectors moving with constant acceleration (even acceleration for a finite time) should detect particles. The effect is seen to be universal. Moreover, detectors undergoing linear acceleration and uniform circular motion both detect particles for the same physical reason. It is shown that the Unruh effect for a circularly orbiting electron in a constant external magnetic field used as a Unruh-DeWitt detector physically coincides with the experimentally verified Sokolov-Ternov effect.

Generation of spatially coherent radiation in free-electron masers with two-dimensional distributed feedback

Abstract: The results of theoretical and experimental studies on the generation of spatially coherent electromagnetic radiation in a planar free-electron maser with two-dimensional distributed feedback are reported. A two-dimensional Bragg structure is used at the initial part of the interaction space to ensure the transverse synchronization of the radiation. The possibility of the narrowband generation in the 75-GHz frequency band is demonstrated experimentally for a sheet kiloampere electron beam whose width is 20 times larger than the wavelength.

Observation of spin-wave envelope solitons in periodic magnetic film structures

Abstract: The formation and propagation of light and dark microwave spin-wave envelope solitons in a periodic magnetic film structure have been observed. The periodic structure was manufactured on the basis of the single-crystal film of iron-yttrium garnet and a lattice of copper strips placed on the surface of the film perpendicularly to the propagation direction of carrying spin waves. The solitons are generated at frequencies corresponding to the band gap in the spectrum of the spin waves of the periodic structure, which is due to the first Bragg resonance.

Evolution of spin entanglement and an entanglement witness in multiple-quantum NMR experiments

Abstract: We investigate the evolution of entanglement in multiple-quantum (MQ) NMR experiments in crystals with pairs of close nuclear 1/2-spins. The initial thermodynamic equilibrium state of the system in a strong external magnetic field evolves under the nonsecular part of the dipolar Hamiltonian. As a result, MQ coherences of the zeroth and plus/minus second orders appear. A simple condition for the emergence of entanglement is obtained. We show that the measure of the spin-pair entanglement—concurrence—coincides qualitatively with the intensity of MQ coherences of the plus/minus second order and, hence, the entanglement can be studied with MQ NMR methods. We introduce an entanglement witness using MQ NMR coherences of the plus/minus second order.

First observation of a wetting phase transition in low-angle grain boundaries

Abstract: The wetting phase transition at low-angle intercrystallite grain boundaries has been experimentally observed. In contrast to the high-angle grain boundaries with the misorientation angels θ > 15°, the low-angle grain boundaries (θ < 15°) are not continuous two-dimensional defects, but constitute a discrete wall (network) of lattice dislocations (edge and/or helical). The theory predicts that, depending on θ, either a continuous layer of the liquid phase or a wall (network) of microscopic liquid tubes on wetted dislocation nuclei is formed at completely wetted low-angle grain boundaries. It has been shown that the continuous liquid layers at low-angle grain boundaries in the Cu-Ag alloys appear at the temperature TwminL = 970°C, which is 180°C higher than the onset temperature Twmin = 790°C and 50°C lower than the finish temperature of the wetting phase transition at high-angle grain boundaries, Twmax = 1020°C.

Transformation of electromagnetic radiation at moving inhomogeneities of a medium

Abstract: The possibility of a significant up-conversion of the electromagnetic-radiation frequency due to its Doppler shift by ultrarelativistic nonlinear-medium inhomogeneities induced by high-intensity counterpropagating radiation pulses is analyzed. It is shown that the efficiency of parametric redistribution increases resonantly as the velocity of the inhomogeneity approaches the phase velocity of the high-frequency radiation.

From Simplified BLG Action to the First-Quantized M-Theory

Abstract: A concise summary of recent progress in the search for the world-volume action for multiple M2 branes is provided. After the recent discovery of a simplified version of the BLG action, which is based on the ordinary Lie-algebra structure and that does not have coupling constants or extra dynamical fields, attention should be switched to the study of M2 brane dynamics. A viable brane analog of the Polyakov formalism and the Belavin-Knizhnik theorem for strings can probably be provided by the Palatini formalism for a 3d (super-) gravity.

Proximity-induced superconductivity in graphene

Abstract: We propose a way of making graphene superconductive by putting on it small superconductive islands which cover a tiny fraction of graphene area. We show that the critical temperature, T c , can reach several Kelvins at the experimentally accessible range of parameters. At low temperatures, T T c , and zero magnetic field, the density of states is characterized by a small gap E g ≤ T c resulting from the collective proximity effect. Transverse magnetic field H g (T) ∝ E g is expected to destroy the spectral gap driving graphene layer to a kind of a superconductive glass state. Melting of the glass state into a metal occurs at a higher field H g2(T).

Surface enhanced Raman scattering by CdS quantum dots

Abstract: Surface enhanced Raman scattering is studied in nanostructures with CdS quantum dots formed using the Langmuir-Blodgett technology. Features due to quantum dot longitudinal optical phonons are observed in the Raman spectra of both free CdS quantum dots and such dots distributed in an organic matrix. The surface enhanced Raman scattering by nanostructures with CdS quantum dots covered by an Ag cluster film is observed experimentally. Applying Ag clusters onto the nanostructure surfaces results in a sharp (40-fold) increase in the intensity of Raman scattering by optical phonons in the quantum dots. It is shown that the dependence of surface enhanced Raman scattering on the excitation energy is resonant with a maximum at the energy corresponding to the maximum absorption coefficient of Ag clusters.

Influence of the ground state of the Pr3+ ion on magnetic and magnetoelectric properties of the PrFe3(BO3)4 multiferroic

Abstract: The magnetic, magnetoelectric, and magnetoelastic properties of a PrFe 3 (BO 3 ) 4 single crystal and the phase transitions induced in this crystal by the magnetic field are studied both experimentally and theoretically. Unlike the previously investigated ferroborates, this material is characterized by a singlet ground state of the rare-earth ion. It is found that, below T N = 32 K, the magnetic structure of the crystal in the absence of the magnetic field is uniaxial ( l || c ), while, in a strong magnetic field H || c ( H cr ~ 43 kOe at T = 4.2 K), a Fe 3+ spin reorientation to the basal plane takes place. The reorientation is accompanied by anomalies in magnetization, magnetostric- tion, and electric polarization. The threshold field values determined in the temperature interval 2-32 K are used to plot an H - T phase diagram. The contribution of the Pr 3+ ion ground state to the parameters under study is revealed, and the influence of the praseodymium ion on the magnetic and magnetoelectric properties of praseodymium ferroborate is analyzed.

Effect of the Bloch-Siegert shift on the frequency responses of Rabi oscillations in the case of nutation resonance

Abstract: The dynamics of a two-level spin system dressed by bichromatic radiation is studied under the conditions of double resonance when the frequency of one (microwave) field is equal to the Larmor frequency of the spin system and the frequency of the other (radio-frequency) field ωrf is close to the Rabi frequency ω1 in a microwave field. It is shown theoretically that Rabi oscillations between dressed-spin states with the frequency ɛ are accompanied by higher-frequency oscillations at frequencies nωrf and nωrf ± ɛ, where n = 1, 2,.... The most intense among these are the signals corresponding to n = 1. The counter-rotating (antiresonance) components of the RF field give rise to a shift of the dressed-state energy, i.e., to a frequency shift similar to the Bloch-Siegert shift. In particular, this shift is manifested as the dependence of the Rabi-oscillation frequency ɛ on the sign of the detuning ω1 − ωrf from resonance. In the case of double resonance, the oscillation amplitude is asymmetric; i.e., the amplitude at the sum frequency ωrf + ɛ increases, while the amplitude at the difference frequency ωrf − ɛ decreases. The predicted effects are confirmed by observations of the nutation signals of the electron paramagnetic resonance (EPR) of E′1 centers in quartz and should be taken into account to realize qubits with a low Rabi frequency in solids.