Resonant scattering of three-level Rydberg atoms in a microwave field

We examine the theory of potential scattering of Rydberg atoms in a microwave field. The model of a three-level atom is employed to calculate the radiative force emerging in the resonant coherent interaction with the microwave field for the case of a two-photon resonance and high intensities, using the method of quasienergies of the system consisting of the atom and the field. We determine the probabilities of Landau-Zener transitions in the spatial regions where under two-photon resonance conditions the quasienergies of the atoms approach one another by a small quantity. We also study the dynamics of the variation of the spatial profile of a beam of Rydberg atoms caused by resonant scattering. Finally, we give the results of the first experimental observation of the variation of the transverse beam profile when Rydberg atoms pass through a nonuniform microwave field formed in a rectangular waveguide and in resonance with the two-photon 36P–37P transition. Zh. éksp. Teor. Fiz. 111, 796–815 (March 1997)     

Precision Stark spectroscopy of the fine structure of the microwave transition 37P-37S in sodium Rydberg atoms

The results of experimental and theoretical investigations of the fine structure of the spectrum of the microwave transition 37P-37S in sodium Rydberg atoms in a weak electric field are reported. New data are obtained on the experimental values of the scalar and tensor polarizabilities of the 37P level. A numerical simulation of the Stark diagram, taking into account the influence of the anticrossings of the Rydberg levels, is performed, and the results are found to be in good agreement with experiment. Pis'ma Zh. éksp. Teor. Fiz. 68, No. 12, 853–857 (25 December 1998)     

Mott-insulator-superfluid-liquid transition in a one-dimensional bosonic Hubbard model: Quantum Monte Carlo method

The values of the insulator gap Δ in one-dimensional systems of interacting bosons described by the Hubbard Hamiltonian are calculated at low temperatures by the quantum world-line Monte Carlo algorithm. The dependence of Δ on the size of the system, the temperature, and the parameters of the model is investigated. It is shown that a chain with N _a=50 sites is already sufficient to estimate the thermodynamic value of the critical quantity (t/U)_c for which a transition from the insulator into the superfluid state occurs in a commensurate system. To within the computational error, this value, (t/U)_c=0.300±0.005, agrees with the value (t/U)_c=0.304±0.002 obtained previously by the combined “exact diagonalization + renormalization-group analysis” method. The characteristic Kosterlitz-Thouless behavior of the insulator gap is demonstrated near the critical region: Δ∼exp[−b(1−t/t _c)^−1/2]. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 92–96 (25 July 1996)     

Ferroelectric phase transition in LaBSiO5 crystals from results of thermal and dielectric measurements

The specific heat of the ceramic and the permittivity of a single-crystal sample of LaBSiO₅, a new ferroelectric in the stilwellite family, were measured in a temperature range which includes the phase transition point (T _C=140 °C). The excess entropy of the phase transition ΔS=1.05 J/mol · K and the Curie-Weiss constant C _C-W=3.2×10³ K were determined. The results indicate that the phase transition in this crystal is of a “mixed” nature and exhibits features of a displacement-type transition and an order-disorder transition. Fiz. Tverd. Tela (St. Petersburg) 40, 1310–1312 (July 1998)     

Quantum-nondemolition measurement of the number of photons in a microcavity

A new scheme for quantum-nondemolition measurement of the number and statistics of photons in a microwave cavity on the basis of interferometry of strongly-controlled V atoms which interact by a dispersion interaction with a cavity mode at a different transition is proposed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 58–63 (10 July 1997)     

Molecular Stark effect spectroscopy of diflavonol and inhomogeneous broadening of its electronic spectra in erythrocyte membranes

Using spectroscopy of the molecular Stark effect and fluorescence spectroscopy, we study the characteristics of diflavonol 3,7-dihydroxy-2,8-di(4-dimethylaminophenyl)-4H,6H-pyrano[3,2-g]chromene-4,6-dione (DFME), which demonstrates intramolecular charge and proton phototransfer. In the ground state, this dye has only one form and, in the excited state, it has two forms, i.e., normal and phototautomeric. We found that, for the normal form of DFME, the transition dipole moment that is responsible for the absorption (m _a ), the dipole moment in the equilibrium ground state (μ _g ), and the change of the dipole moment upon transition of the molecule in the excited Franck-Condon state (Δ ^a μ) are parallel. In the ground equilibrium state, the dipole moments in 1,4-dioxane and cyclohexane are equal to μg = 12.2 × 10⁻³⁰ C m and μ _g = 11.0 × 10⁻³⁰ C m, respectively. Upon excitation, they increase by Δ ^a μ = 61 × 10⁻³⁰ C m and Δ ^a μ = 50.2 × 10⁻³⁰ C m in these solvents. We study the spectral characteristics of DFME in organic solvents and erythrocyte membranes. A spectral inhomogeneity of DFME in erythrocyte ghosts is found. The inhomogeneous broadening of fluorescence spectra is manifested as a long-wavelength shift of the band of the normal form of DFME by 1640 cm⁻¹ upon excitation at the red edge of the absorption spectrum.     

Realization of a heavily doped and fully compensated semiconductor state in a crystalline semiconductor with a deep impurity band

We use the data on the pressure (up to P=1.5 GPa) and field (up to H=17 kOe) dependence of the Hall coefficient and the resistivity at 77.6 and 300 K in p-CdSnAs₂〈Cu〉 to calculate the effective kinetic characteristics of the charge carriers, the density and mobility of the conduction electrons and the holes of the deep acceptor and valence bands, in an interval of excess-acceptor densities N _ext ranging from 10¹⁰–10¹⁷ cm⁻³. We establish that in a heavily doped semiconductor with a deep impurity band at the tail of the density of states of the intrinsic band, with unequal donor and acceptor densities, a a heavily doped and fully compensated semiconductor state is realized under hydrostatic compression. The threshold value of the pressure that initiates the transition into such a state, P _c, depends on the extent to which the impurity band is populated. In p-CdSnAs₂〈Cu〉 at N _ext=N _A, where N _A is the density of deep acceptors, and T⩽77.6 K the value of P _c amounts to 10⁻⁴ GPa. As the population of the deep acceptor band grows, P _c increases and in the limit becomes infinite. We discuss the special features of the electrophysical properties of p-CdSnAs₂〈Cu〉 arising from the absence of an energy gap between the states of the conduction band and those of the deep acceptor band. Zh. éksp. Teor. Fiz. 111, 562–574 (February 1997)     

Synchronization of high-frequency vibrations of slipping phase centers in a tin whisker under microwave radiation

Current-voltage characteristics of a system with a variable number of slipping phase centers resulting from phase separation in a tin whisker under external microwave field with a frequency Ω/2π≅35–45 GHz have been studied experimentally. Emergence and disappearance of steps with zero slope in a whisker’s current-voltage characteristic at U _m/n =(m/n)U _Ω, where m and n are integers and U _Ω is determined by Josephson’s formula _ℏΩ=2eU _Ω, have been investigated. Microwave field generated by slipping phase centers is nonharmonic, and the system of slipping phase centers permits synchronization of internal oscillations at a microwave frequency by an external field with a frequency which is the n-th harmonic of internal oscillations. The estimated microwave power generated by a whisker is 10⁻⁸ W. Stimulation of superconductivity in a current-carrying whisker has been detected. Zh. éksp. Teor. Fiz. 113, 1364–1375 (April 1998)     

Interaction of a modulated electron beam with a magnetoactive plasma

Experimental results concerning the interaction of a modulated electron beam with a magnetoactive plasma in the whistler frequency range are reported. It was shown experimentally that when a beam is injected into the plasma, waves can be generated by two possible mechanisms: Cherenkov emission of whistlers by the modulated beam, and transition radiation from the beam injection point. In the case of weak beam currents (N _b/N ₀)≪⁻⁴) the Cherenkov resonance radiation is more than an order of magnitude stronger than the transition radiation; the Cherenkov emission efficiency decreases at high beam currents. The transformation of the distribution function of the beam is investigated for the case of weak beam currents. It is shown that in the case of the Cherenkov interaction with whistlers the beam is retarded and the beam distribution function becomes wider and acquires a plateau region. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 378–382 (25 March 1998)     

Luminescence of short-lived color centers induced in LiF crystals by a pulsed microwave discharge

A new phenomenon — intense luminescence of noncolored lithium fluoride (LiF) crystals excited by an electrodeless pulsed microwave discharge at the prebreakdown stage of development — is observed. This luminescence consists of the luminescence of short-lived aggregate F₂ and F ₃ ⁺ color centers at room temperature. It is shown that the density of short-lived color centers induced in the surface layer of LiF crystals by a microsecond microwave discharge reaches values of ∼10¹⁹−10²⁰ cm⁻³. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 163–167 (10 August 1997)     

Modeling a dimer state in CuGeO3 in the two-dimensional anisotropic Heisenberg model with alternated exchange interaction

The two-dimensional Heisenberg spin-1/2 model with alternated exchange interaction along the c axis and an anisotropic distribution of the exchange interaction in the lattice, J _b/J _c=0.1, is examined. A quantum Monte Carlo method is used to calculate the phase diagrams of the antiferromagnet, the dimer state in a plane, the value of the alternation δ of the exchange interaction, and the anisotropy Δ=1−J ^xy/J ^z of the exchange interaction, Δ∼δ ^0.58(6). The following characteristics are calculated for Δ=0.25: the dependence of the temperature of the dimer-state-paramagnet transition on the alternation of the exchange interaction, T _c(δ)=0.55(4)(δ−0.082(6))^0.50(3), the singlet-triplet energy gap, and the dependence of the magnetization on the external field for some values of δ. The value of the exchange interaction, J _c=127 K, the alternation of the exchange interaction, δ=0.11J _c, and the correlation radius along the c axis, ξ _c≈28c, are determined. Finally, it is found that the temperature dependence of the susceptibility and the specific heat are in good agreement with the experimental data. Zh. éksp. Teor. Fiz. 112, 2184–2197 (December 1997)     

229m Th(3/2+, 3.5 eV) and a check of the exponentiality of the decay law

A new object is proposed for checking the exponentiality of the decay law of an isolated metastable state at long times — the anomalously low-lying level 3/2⁺ (3.5±0.5 eV) in ²²⁹Th. Highly efficient excitation of this level by laser radiation in combination with optimal collection of optical photons emitted in an isomeric transition make it possible to achieve measurement times longer than 50T _1/2. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 521–525 (25 April 1998)     

Spin susceptibility of neutron-irradiation-disordered YBa2Cu3O6.9 in the normal and superconducting states

The spin-spin relaxation rate ⁶³ T ₂ ⁻¹ of ⁶³Cu nuclei in CuO₂ layers is measured in the normal and superconducting states of the compound YBa₂Cu₃O_6.9 (T _c ^onset =94 K) subjected to radiation-induced disordering by a fast-neutron flux Φ to T _c ^onset =68 K (Φ=7×10¹⁸ cm⁻²) and T _c ^onset <4 K (Φ=12×10¹⁸ cm⁻²). It is found that as the structural disorder increases, the contribution of the indirect spin-spin interaction ⁶³ T _2G ⁻¹ , which is related to the value of the spin susceptibility at the boundary of the Brillouin zone of the copper planes χ_s(q={π/a; π/a}), decreases slightly at the transition to the superconducting state for the initial sample and remains unchanged for the weakly disordered sample. This behavior of the short-wavelength contribution to the spin susceptibility attests to the stability of the x ²−y ² symmetry of the energy gap against structural disorder, in accordance with proposed theoretical models of Cooper pairing for high-T _c cuprates. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 172–177 (10 February 1998)     

Matrix element of the anomalously low-energy (3.5±0.5 eV) transition in 229Th and the isomer lifetime

The matrix element of the anomalously low-energy (3.5±0.5 eV) nuclear M1 transition between the first excited state and the ground state of the ²²⁹Th nucleus is determined with allowance for the Coriolis mixing of the rotational bands. The upper and lower limits on the lifetime of the level with respect to an isomeric transition are given. A method is proposed for measuring the half-life of the low-lying isomer ^229m Th directly in a ²³³U sample. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 233–238 (25 February 1998)     

Kinetics of crystal growth in superfluid helium at high temperatures

The growth rate of ⁴He crystals from superfluid is measured in the temperature range 1.2–1.75 K at supersaturations up to 40 mbar. The growth rate is observed to decrease at high supersaturations: above 5 mbar in the bcc phase and above 20 mbar in the hcp phase. The temperature dependence of the kinetic growth factor K is measured in the low-supersaturation limit. In the vicinity of the superfluid transition the kinetic growth factor exhibits critical behavior: K ∝ (Tλ−T)^ε with the exponent ε=0.743±0.123. A jump in the growth factor is observed at the bcc-hcp transition point. The crystal growth kinetics problem is solved in the hydrodynamic approximation, explaining both the temperature behavior of K and the existence of the jump in the modification of the crystal structure. Zh. éksp. Teor. Fiz. 114, 1313–1328 (October 1998)     

Observation of fine structure in the photoluminescence spectrum of an Er3+ ion in an amorphous silicon matrix

A multicomponent Stark structure corresponding to a 4I _13/2→4I _15/2 transition in the 4f ¹¹ shell of Er³⁺ ions is observed in hydrogenated amorphous silicon (a-Si:H) subjected to low-temperature (150 °C) anneal. The observation of narrow, strong components indicates that the erbium ions form a highly ordered local surrounding (Er-O-Si nanoclusters) in the labile, disordered structural network of a-Si:H. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 12, 780–783 (25 December 1999) Deceased.     

Giant negative magnetoresistance in a nonmagnetic semiconductor

Studies of a classical III–V semiconductor (InSb) doped with 3d magnetic ions (Mn²⁺, having a localized spin S=55/2) reveal some unexpected transport properties. It is found that the transition from the metallic to the low-temperature insulator phase occurs at an impurity concentration N _Mn∼N _cr=2× 10¹⁷ cm⁻³ and a temperature T<T _cr∼1 K. Under these conditions a giant negative magnetoresistance arises. The experimental results can be explained in terms of the onset of a hard Mott-Hubbard gap Δ in the impurity band formed by the shallow manganese acceptor in InSb at N _Mn∼N _cr. A model describing the gap formation is proposed. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 358–362 (10 March 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

EPR of trivalent iron ions in a LiCaAlF6 crystal

The orientational dependences of the EPR spectra of Fe³⁺-doped LiCaAlF₆ single crystals (space group P31c, Z=2), grown at the Laboratory of Magnetic Radio Spectroscopy at Kazan’ State University, have been investigated in detail. The spectrum is described by a trigonal spin Hamiltonian with the following parameters: B ₂₀=40.072×10⁻⁴ cm⁻¹, B ₄₀=−5.799×10⁻⁴ cm⁻¹, B ₄₃=−4.281×10⁻⁴ cm⁻¹, A _s=24.33±1, A _p=6.13±1, g _∥=g _⊥=2.00217±0.0003. A theoretical calculation of the hyperfine structure parameters shows that they are described quite well when allowance is made for the overlapping of the wave functions of the paramagnetic center and the ligands (F⁻). Fiz. Tverd. Tela (St. Petersburg) 39, 488–490 (March 1997)     

Self-energy contribution to the ground state hyperfine splitting of Bi82+

The self-energy contribution to the hyperfine splitting of the ground state of Bi⁸²⁺ is calculated for a point nucleus. It is found that the theoretical value of the wavelength of the ground-state hyperfine-splitting transition of ²⁰⁹Bi⁸²⁺ is λ=244(1) nm and is in good agreement with experiment. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 309–310 (10 March 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Optical nutation at a Raman-active transition

Optical nutation at the Raman-active transition 6P _1/2−6P _3/2 of thallium atoms (ω _R /2πc=7793 cm ⁻¹) under resonant Raman excitation by a biharmonic picosecond pulsed field, giving rise to substantial motion of the population, is detected. Optical nutation appears as an oscillatory behavior of the energy of the anti-Stokes scattering of probe pulses, which follow with a fixed delay, as a function of the product of the energies of the excitation pulses. As a result of the dynamic Stark effect, which decreases the frequency of the transition under study, resonance excitation conditions are satisfied for negative initial detunings of the Raman excitation frequency from resonance. The Raman scattering cross section for the transition under study is estimated by comparing the experimental data with the calculations. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 1, 7–12 (10 July 1999)