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Volume 39, Issue 6, June 2013

Interaction between quantum twolevel systems (qubits) and electromagnetic fields can provide additional coupling channels to qubit states. In particular, the interwell relaxation or Rabi oscillations, resulting, respectively, from the multi or singlemode interaction, can produce effective crossovers, leading to electromagnetically induced interference in microwave driven qubits. The environment is modeled by a multimode thermal bath, generating the interwell relaxation. Relaxation induced interference, independent of the tunnel coupling, provides deeper understanding to the interaction between the qubits and their environment. It also supplies a useful tool to characterize the relaxation strength as well as the characteristic frequency of the bath. In addition, we demonstrate the relaxation can generate population inversion in a strongly driving twolevel system. On the other hand, different from Rabi oscillations, Rabioscillationinduced interference involves more complicated and modulated photon exchange thus offers an alternative means to manipulate the qubit, with more controllable parameters including the strength and position of the tunnel coupling. It also provides a testing ground for exploring nonlinear quantum phenomena and quantum state manipulation in qubits either with or without crossover structure.

Kinetics of nucleation during the bcchcp structural transition in solid helium
View Description Hide DescriptionThe nucleation kinetics of the hcp phase in the supercooled bcc phase of solid 4He and a 3He4He solution containing 1% 3He is studied experimentally. Precision measurements of the pressure in the crystal are made as it is cooled along an isochore near the bcchcp phase equilibrium curve. The lifetime of the metastable bcc phase in 4He is found to have a complicated dependence on the level of supercooling which can be described in terms of two nucleation mechanisms—homogeneous and heterogeneous. The homogeneous mechanism occurs for the highly supercooled bcc phase in carefully annealed crystals, while the heterogeneous mechanism, for which the activation coefficients are several orders of magnitude lower than for the homogeneous mechanism, occurs with low supercooling. The influence of defects and impurities on the nucleation process is also studied. In unannealed (defective) 4He crystals the nucleation rate is found to have an enormous scatter, which is indicative of poor reproducibility from one crystal to another. For annealed samples of 3He4He solution the nucleation kinetics data can be described only by the heterogeneous nucleation mechanism.

Pseudogap in CdBa_{2}Cu_{3}O_{7−δ} HTSC material
View Description Hide DescriptionLonglived minima are detected at T min1 = 275 K and T min2 = 252 K in the ρ(T) curve of polycrystalline CdBa2 Cu 3O7−δ, which has a superconducting transition at 86.8 K. It is shown that the excess conductivity Δσ(T) near T min obeys the classical AslamazovLarkin fluctuation theory. This suggests that fluctuation Cooper pairs may develop in CdBa2 Cu 3O7−δ at T ≫ Tc . The experimental data are used to calculate the coherence length ξ c (0), the 2D3D crossover temperature T 0, and the interplanar pairing constants. The magnitude and temperature dependence of the pseudogap for a sample without the minima are found to be typical of cuprate HTSC materials.

Effect of screening on the phononpolaron states in diluted magnetic semiconductor nanotubes
View Description Hide DescriptionWe calculate the effect of screening on the weakly coupled Fröhlich polaron binding energy of the ground state and the polaron contribution to the mass for the lowest spindown subband of the conduction band in diluted magnetic semiconductor nanotubes. Expressions for the polaron energy and the polaron contribution to the effective mass are obtained as functions of the magnetic field and the radius of the nanotube, taking into account the screening effect as well as virtual transitions from the ground state with n = 0 to sizequantized subbands with n′ = 1, 2, …, 10. Numerical calculations show that screening reduces the polaron binding energy and polaron contribution to the effective mass by about 35% and 25%, respectively, for a magnetic field of 0.1 T. Screening reduces the polaron binding energy of the ground state by approximately 30%15% and also reduces the polaron contribution to the effective mass by 25%10% when the nanotube radius is increased by 1040 nm.

Electromagnetically induced interference in a superconducting flux qubit
View Description Hide DescriptionInteraction between quantum twolevel systems (qubits) and electromagnetic fields can provide additional coupling channels to qubit states. In particular, the interwell relaxation or Rabi oscillations, resulting, respectively, from the multi or singlemode interaction, can produce effective crossovers, leading to electromagnetically induced interference in microwave driven qubits. The environment is modeled by a multimode thermal bath, generating the interwell relaxation. Relaxation induced interference, independent of the tunnel coupling, provides deeper understanding to the interaction between the qubits and their environment. It also supplies a useful tool to characterize the relaxation strength as well as the characteristic frequency of the bath. In addition, we demonstrate the relaxation can generate population inversion in a strongly driving twolevel system. On the other hand, different from Rabi oscillations, Rabioscillationinduced interference involves more complicated and modulated photon exchange thus offers an alternative means to manipulate the qubit, with more controllable parameters including the strength and position of the tunnel coupling. It also provides a testing ground for exploring nonlinear quantum phenomena and quantum state manipulation in qubits either with or without crossover structure.

Quadrupole phases and phase transitions in uniaxial magnets with tensor interactions
View Description Hide DescriptionUniaxial magnets with spin one, singleion anisotropy, and anisotropic biquadratic exchange interaction are investigated in the absence of magnetic field. Possible types of quadrupole ordering in the system are discussed for the case in which the anisotropy of biquadratic exchange interaction has the most general form compatible with the axial symmetry. Besides the two phases with the stationary planes of quadrupole ordering (QO Z and QO X phases), there may appear a phase (QO< phase) in which the orientation of the quadrupolarordering plane changes continuously as the Hamiltonian parameters are varied. For the QO X phase, two branches of spin excitations have been found, one of which includes the Goldstone mode. The QO< phase boundary with the other two quadrupolar phases at finite temperatures has been determined, the expression for the QO<–QO Z phase boundary has been derived analytically.

Currentvoltage characteristics of lowresistance tunnel structures ferromagnetinsulatorsuperconductor Co_{2}CrAl–I–Pb
View Description Hide DescriptionA physical model of tunneling processes leading to a nonequilibrium superconducting state in ferromagnetsuperconductor junctions is proposed and confirmed experimentally. The model is based on the analysis of experimental currentvoltage characteristics and the determination of junction voltage dependences of the energy gap, the effective chemical potential and the effective temperature of quasiparticles in the modified energy distribution of quasiparticles as well as on taking into account the change in the density of electron states due to destruction of the Cooper pairs, caused by the proximity effect.

Nonlinear dynamics and solitons of a chain with spherical magnetic nanoparticles
View Description Hide DescriptionFreemoving magnetic particles in the presence of a magnetic dipole interaction between their magnetic moments can be arranged in dense phases, the simplest of which is a linear chain. The nonlinear dynamics associated with the displacement of particles in such a chain is studied with taking into account the magnetic dipole interaction and the molecular Van der Waals interaction. It is shown that this chain has interesting properties of a nonlinear dynamical system. For it one can construct exact solutions which describe propagating nonlinear waves of a constant amplitude with a wavelength comparable with a distance between particles, as well as soliton solutions. A role of the nonlinearity of oscillations turns out to be very important for a problem of stability of a chain with respect to the transition to a dense threedimensional cluster.

Electrical resistance relaxation induced by high pressure in single crystals of YBa_{2}Cu_{3}O_{7−δ}
View Description Hide DescriptionAn effect of high hydrostatic pressure on the electrical resistance in the abplane of single crystals of YBa2 Cu 3O7−δ with oxygen deficiency is investigated. It is found that the temperature dependence of the electrical resistance is determined by the fluctuation conductivity near Tc and by the scattering of electrons by phonons in a normal state. A high pressure causes the redistribution of labile oxygen, increasing a phase separation. The depressuriazation is accompanied by relaxation processes in both the phonon and the electronic subsystems, the characteristic times of which are significantly different from each other.

The mechanics of 2D crystals: A change from the atomiclattice description to equations of the elasticity theory
View Description Hide DescriptionA twodimensional (2D) crystal formed by a system of identical atoms with a pair centrosymmetric interaction between them is considered. It is assumed that in the initial state of equilibrium atoms occupy sites of a flat translationsymmetrical mesh, and the deformed state appears as a result of their displacements in the crystal plane (longitudinal deformations) and in the direction perpendicular to it (flexural deformations). It is shown that in the continuum description an infinitely thin anisotropic film with a finite mass density, which is capable of elastic longitudinal and flexural deformations, corresponds to this crystal. In the framework of classical mechanics we derive the basic relations and equations for atomic displacements and corresponding to them equations of the elasticity theory, describing both modes of deformation of a 2D crystal in the linear approximation as well as with taking into account anharmonicities. The explicit expressions which relates moduli of linear and nonlinear elasticity of the crystal with the potential of interatomic interaction and geometrical characteristics of the flat crystal lattice are obtained.

Fermi resonance in Ne, Ar and Krmatrix infrared spectra of 5bromouracil
View Description Hide DescriptionLowtemperature matrix isolation Fouriertransform infrared spectroscopy and quantumchemical calculations with DFT/B3LYP and MP2 methods were used for investigation of isolated 5bromouracil (BrU) molecules. Only one tautomeric form of BrU was dominated in the lowtemperature Ne, Ar, and Kr matrices. It was revealed that population of minor hydroxytautomers did not exceed 0.2%. Appearance of additional absorption bands in the region of stretching vibrations νCO (about 1710 cm−1) as well as of deformation ones (1297, 1093, 901 cm−1) was explained by Fermi resonance. In Ne matrices the peak intensities of absorption bands assigned to the outofplane vibrations of the ring and exocyclic atoms were decreased sharply. For the first time, least square method with the using of polynomial was proposed for the corrective scaling of calculated frequencies of vibrations. It is shown that the correction of calculated frequencies with the polynomial of degree two permits to decrease the rootmeansquare discrepancy between the calculated and experimental ones to 4–5 cm−1 in the region of 1500–500 cm−1. The same polynomial may be applied for the correction of spectra of molecules with a similar structure.

Relaxation phenomena in cryogenic electrolytes
View Description Hide DescriptionProposed is a scenario for the development of observed relaxation phenomena in a cryogenic electrolyte with the structure of “liquid hydrogen + injected ions.” Ions of one sign are generated in the bulk of liquid hydrogen in the presence of external field E ⊥ by a stationary radioactive source of ± ions at the bottom of a vessel. After accumulation near the free surface of the liquid with a finite density ns the ions can break its stability producing a pulse of ion current to the collector located above the liquid surface. The outlined process is periodically repeated. Its period contains information on the ion mobility and, which is most interesting, on dissociation (association) processes occurring in a system of charged particles placed in an external field. The cryogenic problem is a good model for dissociation in the presence of external field occurring in normal electrolytes without any external ion sources.

Elastic properties of compressed cryocrystals in a deformed atom model
View Description Hide DescriptionA model with deformed atom shells was built to investigate the elastic properties of raregas Ne and Kr crystals under high pressure. It is shown that the observed deviation from the Cauchy relation δ cannot be adequately reproduced when taking into account only the manybody interaction. The individual pressure dependence of δ is the result of competition of the manybody interaction and the quadrupole interaction associated with the quadrupoletype deformation of electron shells of the atoms during the displacement of the nuclei. Each kind of interaction makes a strongly pressure dependent contribution to δ. In the case of Ne and Kr, contributions of these interactions are compensated to a good precision, providing δ being almost constant against pressure.

Diffusion model of the thermal conductivity plateau of weak solid solutions of neon in parahydrogen
View Description Hide DescriptionThe diffusion kinetic of classic impurity particles has been investigated in the framework of the onedimensional twolevel model and applied for the explanation of solid hydrogen thermal conductivity data with extremely low concentrations of neon impurity in samples grown at different crystallization rates, at which the plateau effect was observed. The main idea is that heavy isotopic impurities could segregate into thin long chains near dislocation cores if the growth rate is slow. Neon impurity chains can persist for a long time. Such rigid linear objects ensure inelastic scattering of phonons. The diffusion coefficient of neon atoms in (pH2)1− c Ne c mixtures was estimated for the experimental conditions with c = 0.0001 at. % and c = 0.0002 at. %.

Reactivity of free radical intermediates that form spontaneously during molecular chlorine action on acetylene and vinyl monomers at low temperatures
View Description Hide DescriptionIn this study, ESR, UV, VIS and IR spectroscopy, as well as chromatography, calorimetry, and elemental analysis were employed to show the spontaneous formation of free radicals under lowtemperature action of molecular chlorine on acetylene monomer pdiethynylbenzene and on vinyl monomer acrylamide. These radicals are able to initiate a chain reaction of polymerization of monomers at low temperatures. This reaction results in a completely soluble polymer with a 25% yield, while an insoluble crosslinked polymer has been obtained during polymerization of pdiethynylbenzene initiated by other means. For example, under radiolysis by 1000 kGy the soluble fraction of obtained polymer was ≈1%, while the overall yield was ≈10%. pDiethynylbenzene polymerizes at temperatures close to the melting temperature of chlorine (170 K). Polymerization of acrylamide takes place in the temperature range of 180–210 K with a ≈10% yield of polymer. The lowtemperature chlorination of a 20% solution of acrylamide in glycerol enables a twofold increase of the polymer yield and a tenfold decrease of the chlorine content in it.