Quantum fluid dynamics based current-density functional study of a helium atom in a strong time-dependent magnetic field

Evolution of the helium atom in a strong time-dependent (TD) magnetic field (B) of strength up to 10¹¹ G is investigated through a quantum fluid dynamics (QFD) based current-density functional theory (CDFT). The TD-QFD-CDFT computations are performed through numerical solution of a single generalized nonlinear Schr?dinger equation employing vector exchange-correlation potentials and scalar exchange-correlation density functionals that depend both on the electronic charge-density and the current-density. The results are compared with that obtained from a B-TD-QFD-DFT approach (based on conventional TD-DFT) under similar numerical constraints but employing only scalar exchange-correlation potential dependent on electronic charge-density only. The B-TD-QFD-DFT approach, at a particular TD magnetic field-strength, yields electronic charge- and current-densities as well as exchange-correlation potential resembling with that obtained from the time-independent studies involving static (time-independent) magnetic fields. However, TD-QFD-CDFT electronic charge- and current-densities along with the exchange-correlation potential and energy differ significantly from that obtained using B-TD-QFD-DFT approach, particularly at field-strengths >10⁹ G, representing dynamical effects of a TD field. The work concludes that when a helium atom is subjected to a strong TD magnetic field of order >10⁹ G, the conventional TD-DFT based approach differs “dynamically” from the CDFT based approach under similar computational constraints.     

Current-density functional study of the Neon atom in a strong ultrashort magnetic field

A Ne atom exposed to a strong ultrashort magnetic field of the order of ¹⁰⁹ G${10}^9\text{G}$ that lasts for a few femtoseconds is investigated through a current-density functional theory (CDFT) based approach employing a vector exchange–correlation potential that depends on the electronic charge-density as well as on the current-density of the atom. The CDFT based approach yields time-dependent electronic charge- and current-density, along with the exchange–correlation potential and energy, significantly different from that obtained using a current-density independent approach, in particular, at the field-strengths >¹⁰⁹ G$>{10}^9\text{G}$.     

Electron density changes and high harmonics generation in H2 molecule under intense laser fields

Under interaction with a high-intensity laser field, the real-time femtosecond dynamics of the electron density in the H₂ molecule has been studied quantum mechanically. For this purpose, a time-dependent generalized nonlinear Schr?dinger equation of motion, developed earlier in our laboratory by combining density functional theory and quantum fluid dynamics in real space, is solved numerically at the equilibrium internuclear distance of the molecule. By employing various time-dependent calculated properties as probes, information and insight are obtained about the phenomena of excitation, ionization, bond-softening, dipole formation and high-harmonics generation. The present approach goes beyond the linear response formalism.     

Vibrationally elastice − — H2O scattering at intermediate energies

Vibrationally elastic total cross-sections ofe ⁻—H₂O scattering are calculated at intermediate energiesE _i=10–300 eV. The interaction potentials are treated in spherical models. The dipole rotational excitation, which is significant but not dominant above 10 eV, is treated incoherently. Effects of electronic excitation-ionization, significant above 30 eV or so, are considered through a complex optical potential. A dynamically distorted charge-density is employed to calculate the imaginary part of the complex potential. Comparisons are made with recent theoretical and experimental data. The mutual agreement is better in total cross-sections than in differential cross-sections.     

GeTe和GeSe 分子在外电场下的特性研究

对Ge原子采用6-311++G^**基函数,Te和Se原子采用SDB-cc-pVTZ基函数,利用密度泛函理论的局域自旋密度近似方法优化得到了GeTe和GeSe分子的稳定构型,并计算了外电场作用下GeTe和GeSe基态分子的平衡核间距、总能量、最高已占据分子轨道能量E_H、最低未占分子轨道能量E_L、能隙、谐振频率和红外谱强度. 在上述计算的基础上利用单激发组态相互作用-局域自旋密度近似方法研究了GeTe和GeSe分子在外电场下的激发特性. 结果表明:随着正向电场强度的增大,分子核间距逐渐增大,分子总能量逐渐降低,谐振频率逐渐减小,红外谱强度则逐渐增大. 在0-2.0569×10¹⁰ V·m^-1的电场范围内,GeTe分子的E_H 均高于GeSe分子的E_H;随着正向电场的增大,GeTe与GeSe的E_H差逐渐变大,GeTe的E_L低于GeSe的E_L,它们的E_L均随正向电场的增大而增大. 无外场时,GeTe分子的能隙比GeSe分子的能隙要小;在外电场反向增大的过程中, GeTe和GeSe的分子能隙始终减小. 外电场的大小和方向对GeTe和GeSe分子的激发能、振子强度及跃迁的波长均有较大影响. 关键词： GeTe GeSe 外电场 激发态     

Energies of the H+2 ion in strong magnetic fields

Variational calculations of the energy values and the equilibrium internuclear separations of the H⁺₂ ion in the 1σ_g and 1π_g states are carried out to confirm our previous results based on the Monte Carlo method; in the absence of an external magnetic field the antibonding 1π_g state changes to a bonding state with increasing magnetic field. As a byproduct, a simple variational form is proposed for the wave function of the H atom in strong magnetic fields.     

Quantum oscillations of the resistivity and hall coefficient and the quantum limit in Bi<Subscript>0.93</Subscript>Sb<Subscript>0.07</Subscript> alloys in a magnetic field along the trigonal axis

The dependences of the electrical resistivity ρ and the Hall coefficient R on the magnetic field have been measured for single-crystal samples of the n-Bi_0.93Sb_0.07 semiconductor alloys with electron concentrations in the range 1 × 10¹⁶ cm⁻³ < n < 2 × 10¹⁸ cm⁻³. It has been found that the measured dependences exhibit Shubnikov-de Haas quantum oscillations. The magnetic fields corresponding to the maxima of the quantum oscillations of the electrical resistivity are in good agreement with the calculated values of the magnetic fields in which the Landau quantum level with the number N intersects the Fermi level. The quantum oscillations of the Hall coefficient with small numbers are characterized by a significant spin splitting. In a magnetic field directed along the trigonal axis, the quantum oscillations of the resistivity ρ and the Hall coefficient R are associated with electrons of the three-valley semiconductor and are in phase with the magnetic field. In the case of a magnetic field directed parallel to the binary axis, the quantum oscillations associated both with electrons of the secondary ellipsoids in weaker magnetic fields and with electrons of the main ellipsoid in strong magnetic fields (after the overflow of electrons from the secondary ellipsoids to the main ellipsoid) are also in phase. In magnetic fields of the quantum limit ħω _c /2 ≥ E _F, the electrical conductivity increases with an increase in the magnetic field: σ₂₂(H) ∼ H ^k . A theoretical evaluation of the exponent in this expression for a nonparabolic semiconductor leads to values of k close to the experimental values in the range 4 ≤ k ≤ 4.6, which were obtained for samples of the semiconductor alloys with different electron concentrations. A further increase in the magnetic field results in a decrease of the exponent k and in the transition to the inequality σ₂₂(H) ≤ σ₂₁(H).     

Einstein<Emphasis Type="Italic">A</Emphasis>-coefficients for rotational transitions in the ring-chain isomer of C<Subscript>5</Subscript>H<Subscript>2</Subscript>

Laboratory formation of four isomers of C⁵H² molecule is reported and detection of the ring-chain isomer (isomer 1) of C₅H₂ in cosmic objects has been suggested. For identification of a molecule in cosmic objects, one of the required input data is EinsteinA-coefficients (radiative transition probabilities) for the molecule. Here, we report EinsteinA-coefficients for electric dipole transitions in the ring-chain isomer of C₅H₂ among the rotational levels of the ground electronic and ground vibrational states up to 21 cm⁻¹.     

Ferromagnetic resonance and bistability field in a uniaxial magnetic film

Peculiarities of ferromagnetic resonance (FMR) corresponding to bias along the “hard” magnetic axis of a film with 2D uniaxial anisotropy are studied based on numerical solution of magnetic moment dynamics equations. It is shown that an additional resonance peak is formed in the FMR spectrum in the vicinity of “bistability field” H _b . The dependence of this field on the amplitude of the microwave field and damping parameters is analyzed.     

Contribution of superconducting grains and grain boundaries to the magnetoresistance of ceramic YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> high-temperature superconductors in weak magnetic fields

The current-voltage characteristics of granular YBa₂Cu₃O_6.95 high-temperature superconductor samples have been measured at a temperature of 77.3 K in external transverse magnetic fields H _ext with a strength of up to H _ext ≈ 500 Oe for low transport current densities (0.1 A/cm² ≤ j ≤ 0.6 A/cm²). The current-voltage characteristics obtained have been used to construct dependences of the magnetoresistance ρ on the quantities j (ρ(j) _Hext=const) and H _ext(ρ(H _ext) _{j = const}). It has been revealed that the current and field dependences of the magnetoresistance exhibit anomalies at H _ext ≥ H _c1g , where H _c1g is the lower critical field of superconducting grains. A comparative analysis of the dependences ρ(j)H _{ext = const} and ρ(H _ext) _{j = const} has made it possible to develop concepts regarding the influence of the processes of redistribution of the magnetic field between grain boundaries and superconducting grains on the transport and galvanomagnetic properties of granular high-temperature superconductors. It has been established that the field dependences of the magnetoresistance exhibit specific features associated with the beginning of penetration of Josephson vortices into grain boundaries in the magnetic field H _c1J and with the breaking of a continuous chain of Josephson junctions in the magnetic field H _c2J .     

An expansion method for calculating molecular properties IV.

The energies of the 2pσ and 2sσ states of H₂ ⁺ and HeH²⁺ and those of the a ³, A ¹, b ³, B ¹Σ⁺ states of HeH⁺ are calculated up to second order in the heavy nuclear charge. For the one-electron systems the first-order equation is solved analytically employing a special separation technique for degeneracies. The consideration of screening enables a recently proposed interchange result for the degenerate case to be tested. The results for the 2sσ states are good while those for the 2pσ states are only good at small internuclear distances.

For the two-electron systems the method employed is a single perturbation approach for two perturbations in the presence of degeneracy. The results for HeH⁺ appear to be good only at small internuclear distances. The corresponding results for the excited states of H₂ are omitted since an interesting problem arises at first order.     

Isotope effect in charge transport of LuB12

The galvanomagnetic properties of single-crystal samples with various isotopic boron compositions have been investigated for the first time for the normal state of superconductor LuB₁₂ (T c ≈ 0.44 K). Precision measurements of the resistivity, Hall coefficient, and magnetic susceptibility have been performed over a wide temperature range of 2–300 K in magnetic fields up to 80 kOe. A change of the charge transport regime in this nonmagnetic compound with metallic conduction is shown to occur near T* ≈ 50−70 K. As a result, a sharp peak with significantly different amplitudes for Lu¹⁰B₁₂ and Lu¹¹B₁₂ is recorded in the temperature dependences of the Hall coefficient R _H(T) near T*. A significant (about 10%) difference (in absolute value) of the Hall coefficients R _H for the Lu¹⁰B₁₂ and Lu¹¹B₁₂ compounds at helium and intermediate temperatures has been found and the patterns of behavior of the dependence R _H(H) for T < T* in an external magnetic field H ≤ 80 kOe for Lu¹⁰B₁₂ and Lu¹¹B₁₂ are shown to differ significantly. Analysis of the Curie-Weiss contribution to the magnetic susceptibility χ(T) leads to the conclusion about the formation of magnetic moments μ_eff ≈ (0.13−0.19)μ_B in each unit cell of the fcc structure of LuB₁₂ compounds with various isotopic compositions. The possibility of the realization of an electronic topological 2.5-order transition near T* and the influence of correlation effects in the 5d-band on the formation of a spin polarization near the rare-earth ions in LuB₁₂ is discussed.     

Study of the NMR spectra of hydrogen isotopic analogs and estimation of the magnetic moment of the triton

Nuclear magnetic resonance spectra of the hydrogen isotopic analogs TH and TD on the triton resonance and HT, HD, and H₂ on the proton resonance in the gas phase have been studied. The triton and proton spectra from a single sample have been recorded simultaneously by using the inductivity of a common receiver LC circuit. The energies of the interaction between the magnetic moments of the nuclei of the hydrogen isotopic analogs have been determined: J _tp = 299.3(1) Hz, J _td = 45.5(1) Hz, and J _pd = 43.3(1) Hz. The ratio of the resonance frequencies of the HT molecule nuclei: F _t (TH)/F _p (HT) = 1.066693898(2), which is equal to the ratio of the magnetic moments of the nuclei in the bound state, has been obtained. If the value Δσ(TH) = 2.04 × 10⁻⁸ calculated previously is used for the difference in the screening of nuclei in the HT molecule, then the ratio of the magnetic moment of the triton to the magnetic moment of the proton is μ _t /μ _p = 1.066693920(2), where the statistical standard deviation of the data is given in the parentheses in the units of the last digit.     

Decay of an electronic vortex of a collisionless shock wave as a possible mechanism of a “Coulomb explosion”

A new mechanism of a “Coulomb explosion,” where ions are accelerated by the electric field separating charges at the magnetic Debye radius r _B∼B/4πen _e, is proposed on the basis of a nonquasineutral model of electronic vortices in a magnetic field. It is shown by means of numerical calculations that in the process of acceleration of the ions a collisionless shock wave, whose front has an effective width of the order of δ∼r _B, determined by the breakdown of quasineutrality, is formed in a time of the order of ω _pi ⁻¹ , where ω_pi is the ion plasma frequency. The origin of such explosive dynamics is the formation of “holes” in the electron density at characteristic times of the order of ω _pe ⁻¹ (ω_pe is the electronic plasma frequency) as a result of the generation of electronic vorticity by the Weibel instability of an electromagnetic wave. Calculations for a laser pulse with intensity J∼6×10¹⁸ W/cm² show that the ions expand in the radial direction with velocities up to 3.5×10⁸ cm/s. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 10, 669–674 (25 November 1999)     

Effect of conduction electrons on the law of approach to saturation of a metallic ferromagnet with surface pinning of the magnetic moment

The effect of conduction electrons on the magnetization curve of a metallic ferromagnet with surface pinning of the magnetic moment is investigated theoretically. The electronic contribution is due to the rearrangement of the discrete spectrum of charge carriers trapped by the nonuniform magnetic induction of such a ferromagnet, and it is a kind of diamagnetic effect that appreciably decreases the volume-averaged magnetization of the ferromagnet. A powerlaw dependence H ^−3/4 on the external magnetic field H is obtained according to the law of magnetization approach to saturation. This dependence is due to the contribution of the conduction electrons. Fiz. Tverd. Tela (St. Petersburg) 41, 647–653 (April 1999)     

Thermodynamic properties of the most stable gaseous small silicon-carbon clusters in their ground states

The most stable structures of gaseous Si _m C _n (3 ⩽ n+m ⩽ 6) clusters in their ground electronic states are determined with the high level electronic correlation method QCISD(T)/g3large. Thermodynamic properties on heat capacity (C _p,m ^Θ), entropy (S _m ^Θ), Gibbs energy function (−[G ^Θ −H ^Θ(T _r )]/T) and enthalpy function (H ^Θ−H ^Θ(T _r )) are predicted with standard statistical thermodynamics using the structure parameters and vibrational frequencies obtained with B3PW91/6-31G(d) method combined with the electronic excitation energies determined with time dependent density functional (TD DFT) method at B3PW91/6-31G(d) level. The electronic energies are calculated with the accurate model chemistry method at G3(QCI) level of theory and the Δ _f H _m ^Θ (0 K), Δ _f H _m ^Θ (298.15 K) and Δ _f G _m ^Θ (298.15 K) values are predicted. The heat capacities C _p,m ^Θ(T) as a function of temperature within 298.15-2000 K are fitted into analytical equations. The thermodynamic functions at higher temperatures are determined classically by using these equations. Most of the results obtained in this work are consistent with the available experiments.     

Zero-field muon spin rotation in monocrystalline chromium

Spin precession of positive muons in chromium in zero applied magnetic field is reported for the first time. The observations cover the temperature range from about 2.5 K to 10 K and thus pertain to the so-called longitudinal spin-density wave (LSDW) state of antiferromagnetic Cr. The conclusions that may be drawn from the existence of one rather sharp spin precession line are discussed, among them the estimateD _μ=2.4·10⁻¹⁴ m² s⁻¹ for the muon diffusivity at 4 K. Considerable evidence exists for a strong interactions of μ⁺ with the charge-density waves that are likely to accompany the LSDWs in Cr.     

Dissociation of molecular hydrogen ions by an IR laser pulse

Dissociation dynamics of the simplest molecular systems, such as H₂ ⁺, D₂ ⁺ and HD⁺ ions, in an intense IR laser field has been investigated by numerical modeling. An n-term approximation has been developed to describe the molecular system dynamics in an intense electromagnetic field. Calculations by the n-term approximation have been compared to an accurate numerical solution of the two-particle problem. The dissociation probability as a function of the frequency and intensity of radiation for different isotopes in a molecular hydrogen ion is discussed. A quasistatic model of molecule dissociation in an IR field has been suggested, and limits of its applicability have been determined. Zh. éksp. Teor. Fiz. 113, 128–143 (January 1998)     

Trapping of a magnetic flux in granular YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> high-temperature superconductors under the action of an external magnetic field and transport current

Magnetic flux trapping (MFT) in granular YBa₂Cu₃O_{7 − δ} high-temperature superconductors (HTSCs) is studied. At T = 77.36 K, the dependence of the hysteresis of the transverse magnetoresistance on transport current I and the maximum value of external magnetic field H _ext is found in the measurement cycle 0 → H _ext^max → 0. The dependences of the parameters characterizing MFT, namely, residual magnetoresistance, field H _min at which the magnetoresistance is minimal, and the magnetoresistance at H _ext = H _min, on I and H _ext^maxare determined. MFT is found to occur in HTSC granules under the action of an external magnetic field exceeding the lower critical field of superconducting granules H _c1A, and the transport current only weakly affects the magnitude of MFT.     

Non-analytical angular dependence of the upper critical magnetic field in a quasi-one-dimensional superconductor

We have derived the so-called gap equation, which determines the upper critical magnetic field, perpendicular to conducting chains of a quasi-one-dimensional superconductor. By analyzing this equation at low temperatures, we have found that the calculated angular dependence of the upper critical magnetic field is qualitatively different than that in the so-called effective mass model. In particular, our theory predicts a non-analytical angular dependence of the upper critical magnetic field, H _c2(0) − H _c2(α) ∼ α^3/2, when magnetic field is close to some special crystallographic axis and makes an angle α with it. We discuss possible experiments on the superconductor (DMET)₂I₃ to discover this non-analytical dependence.