Non-adiabatic potentials for H2 +

In principle exact non-adiabatic pseudo-potentials for H₂ ⁺ and D₂ ⁺ are formulated in terms of the theory of conditional probability amplitudes in wave mechanics. Numerical results for the v = 0 and v = 1 (J = 0) states of H₂ ⁺ are derived from previously computed accurate non-adiabatic wave-functions. These results indicate that the non-adiabatic pseudo-potentials only differ from the adiabatic potential by small energies of the same magnitude as the non-adiabatic corrections to the adiabatic energy levels.     

Magnetic-field-induced valence transition in rare-earth systems

The magnetic-field-induced valence transition in rare-earth systems has been investigated using the periodic Anderson model supplemented by the Falicov-Kimball term. This model has been solved by first decoupling the Falicov-Kimball term as proposed by Khomskii and Koharjan and then taking the limit of infinite intra-site Coulomb repulsion. The valence transition both in the absence and in the presence of magnetic field as a function of temperature is studied. It has been found that the system makes transition from non-magnetic to magnetic state when the magnetic field increases beyond a critical value H _c. The phase boundary defined in terms of reduced field H _c(T)/H _c(0) and reduced temperature T/T _v (T _v being the valence transition temperature in the absence of field) is almost independent of the position of the localized level. The results are in qualitative agreement with experimental observations in Yb- and Eu-compounds.     

Relaxation times and low-temperature specific heat of the quadrupolar glass

Computer simulations of a simplified model for molecular orientational order in certain solids provide results consistent with glass-like linear specific heat c_v at low temperatures. The temperature dependence of the quadrupolar relaxation time is found to be well described by the expression 1/τ～exp [-A/(T?T₀)], with T₀ well below the onset of the linear c_v regime. The relationship to experiments on solid hydrogen is discussed.     

Validation of the Eriksen method for the exact Foldy-Wouthuysen representation

The Eriksen method is proven to yield a correct and exact result when a sufficient condition of exact transformation to the Foldy-Wouthuysen (FW) representation is satisfied. Therefore, the Eriksen method is confirmed as valid. This makes it possible to establish the limits within which the approximate “step-by-step” methods are applicable. The latter is done by comparing the relativistic formulas for a Hamiltonian operator in FW representation (obtained using those methods) and the known expression for the first terms of a series, which defines the expansion of this operator in powers of v/c as found by applying the Eriksen method.     

A quantum-mechanical model for the ionization and excitation of atoms during sputtering

A quantum-mechanical model is developed for the process by which an atom is excited or ionized as it is sputtered from a metal surface. The probability of excitation is given by R = (A/ΔE)²(hv/aΔE)ⁿ, where A is the binding energy of a surface atom before sputtering, v is its average velocity after sputtering, a is the thickness of the surface, and Δ E the excitation energy. For ionization, ΔE = I?φ, with I the ionization energy of the sputtered atom, and φ the work function of tke surface. Available experimental data for ionization are fitted best with a = (1.4 ± 0.3)A?, and n = 2.5 ± 0.3. The model is expected to be applicable to bombarding energies up to about 100 keV.     

Superconductivity,magnetic susceptibility and thermal relaxation in amorphous CuZr

From measurements of the superconducting transition temperature and valence magnetic susceptibility ξ_v, of amorphous Cu_100?xZr_x (30? x ? 80), we find that ln T_c varies as (ξ_v)^?1 implying that with changing x, T_c is controlled only by the electronic density of states. By contrast, thermal relaxation reduces T_c substantially, leaving ξ_v almost unaltered, implying that T_c is changed only by change in the effective electron-electron coupling.     

Electrical resistivity study of some lead-borate glasses

We present the results of electrical resistivity measurements performed on 20Fe₂O₃80[3B₂O₃(1?x)PbOxA_uO_v] glasses, with AuOv=K₂O, Ca, As₂O₃ and GeO₂. For a given temperature the electrical conductivity is linearly dependent on the fraction of ferrous ions. The experimental data are well described by using a model involving small polarons.     

Theory of order-disorder transformation in binary systems of the Cu-Au type-I

The phase diagram of f.c.c. alloys which order with formation of L1₂ and L1₀ superstructures is calculated by using the Kikuchi approximation. For both superstructures the order-disorder transformation is a first-order transition. There are three regions of ordered phases. The phase region boundaries are significantly different from those found by Shockley and Li. The maximum of the order-disorder transition temperature in the case of the L1₂ superstructure is displaced from the stoichiometric composition AB₃ to a composition having a greater concentration of A. The following correlation was found between the ordering energy v=2v_AB-v_AA-v_BB and the critical order-disorder transition temperature T_c: for the L1₂ superstructure, v=2·059kT_c; for the L1₀ superstructure, v=2·113kT_c (k=the Boltzmann constant).     

The influence of atomic steps on the photoelectric properties of clean cleaved silicon as derived from the surface photovoltage

The correlation of structural and electrical properties of clean silicon surfaces cleaved in UHV was investigated quantitatively by the surface photovoltage, using light with an energy larger than the band gap of silicon. The surface photovoltage, which is a function of band bending and recombination probability, depends strongly on the appearance of atomic steps. The additional surface states vary with density and crystallographic orientation of the steps as well as with adsorption of oxygen. The experimental facts can be explained by accepting a shift of the Fermi level at the surface towards the valence band due to edge atoms. By measuring the change of sign of the surface photovoltage of crystals with various dopings an exponential temperature dependence of the ratio of the recombination probabilities r_v/r_c for transitions from and into the Surface states has been derived.     

The early stage of phase separation for the system polydimethylsiloxane-diethyl carbonate

The time evolution of a ring formed by the forward scattered light in the phase separation process was found to be divided into an early and a late stage. The effective concentration diffusivity D¹ estimated within the linearized approximation in the unstable region was found to be approximately represented by -D¹∝?^v1 with v¹ ≈0.6₂, where ?=| (T-T_c)/T_c|.     

Silver atom center in α-quartz

A new paramagnetic center, Afs_Ag?Si, in silver-diffused α-quartz is described and its spin-Hamiltonian parameters are presented. The center is formulated as a silver atom $\text{(S =}\text{1}\text{2}\text{)}$ situated in a c-axis channel, bonded weakly to a normal silicon cation, with four oxygen anions as neighbors. A_Ag?Si is created by X-irradiation and is unstable at room temperature.     

Quantum theory of nonresonant electromagnetic and ultrasonic absorption in glasses

We develop quantum theory of nonresonant ultrasonic and electromagnetic absorption in glasses at low temperatures. In the quantum region where ?ω?kT the nonresonant absorption coefficients are proportional to ω³ which seems to be in agreement with the existing experimental data.The existence of characteristic temperature T_c (or characteristic energy E_c = kT_c) of the order of 10 + 20 K is established. At higher interlevel spacing E the concept of two-level systems in their conventional form is not applicable because of their strong coupling to the phonons. Neither the perturbation theory is applicable for calculation of absorption in the frequency interval ?ω?_c or at temperature interval T?T_c = E_c/k.     

Measurement of the spin-structure function g 2 and the semi-inclusive double-spin asymmetries at HERMES

A measurement of the virtual-photon asymmetry A ₂ and of the spin-structure function g ₂ of the proton are presented for the kinematic range of the HERMES experiment carried out at the HERA electron storage ring at DESY. Semi-inclusive longitudinal double-spin asymmetries provide new insight into the valence quark helicity distributions Δu _v and Δd _v.     

On the cΠ (v = 0) state of carbon monoxide

A full analysis of the near infrared c³Π-b³Σ⁺ (0-0) band is given and term values for both states determined. The c³Π (v = 0) state was jointly analysed with the perturbing k³Π (v = 2) state and data from the c³Π-X¹Σ⁺ (0-0) transition and 3A band system were included. It is shown that the available data are consistent with the c³Π (v = 0) state having near Hund’s case b coupling with a spin-orbit constant of A = 0.45 ± 0.02 cm⁻¹, a homogeneous perturbation with the k³Π (v = 2) state, and Λ-type doubling arising predominantly from its interaction with the j³Σ⁺ state. A discrepancy with a more recent report of the 3A band system is identified and discussed. The perturbed b³Σ⁺ state term values are consistent with a previously reported five state interaction model.     

Piezospectroscopic investigation of the nature of the subsidiary valence band extrema in TiO2

In order to assess the localization in k-space of the secondary valence band extrema, we have measured the stress dependence of the indirect transition of TiO₂, at 1.6 K, for static uniaxial compression along the [100] and [110] directions. From the stress-induced splittings we localize the secondary valence-band extrema in [100] direction (Δ direction) and associate the indirect gap of TiO₂ with a Δ_4v → Γ_1c phonon-assisted transition. The shear deformation potential of the Δ_4v valence band has been determined from the stress-induced splitting of the TA phonon-assited transition. We find |C_Δ4v| = 0.75 eV.     

Temperature dependence of the diffuse streak system in K2SnCl6

We have investigated the system of diffuse streaks in K₂SnCl₆ single crystals in the temperature interval from 4 to 640 K by the photographic registration of the scattered neutron intensity. The streak intensity decreases from T_{c¹ = 261 K} with increasing as well as with In the region of the phase transition at T_{c¹ = 261 K} and T_{c² = 255 K} the intensity decreases with falling temperature to one third. The high temperature behaviour is explained by a soft A_2g librational mode. Below T_{c² = 255 K} the intensity is proportional to ΔT. The streaks are detectable down to 230 K.     

Behavior of the Lorenz number in the light heavy-fermion system YbInCu4

The electrical resistivity and thermal conductivity of two polycrystalline YbInCu₄ samples prepared by different techniques at the Ioffe Physicotechnical Institute, RAS (St. Petersburg, Russia), and the Goethe University (Frankfurt-am-Main, Germany) are studied within the temperature range 4.2–300 K. At T _v～75–78 K, these samples exhibited an isostructural phase transition from a state with an integer valence (at T>T _v) to a state with an intermediate valence (at T<T _v) of the Yb ions. It is shown that at T<T _v; i.e., in the temperature range where YbInCu₄ is assumed to be a light heavy-fermion compound, the Lorenz number behaves as it should in a classical heavy-fermion system. At T>T _v, where YbInCu₄ is a semimetal, the Lorenz number has a value characteristic of standard metals.     

Multiphoton interband transitions assisted by photoexcited free carriers

The rate of generation of electron-hole pairs by intense light is calculated within a three-band model of an insulator including a valence band v and conduction bands c and c ₁. It is assumed that the photon energy ?ω is small in comparison with the band gap E _g and generation occurs due to the transitions involving free electrons in the upper conduction band c ₁. It is shown that, at a light intensity j ～ 10¹¹ W/cm² and an electron concentration in the c ₁ band exceeding 10¹⁷ cm^?3, the rates of the transitions c ₁ +l ?ω → ccv (l = 2 or 3) exceed the rates of direct five-photon transitions between the v and the c bands. In this case, a multiphoton avalanche can be initiated.