The high-pressure-induced spin-reorientation transition in a ferromagnetic semiconductor Cd<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>GeAs<Subscript>2</Subscript>

The dependences of the longitudinal magnetoresistance (Δρ _zz /ρ₀)(P), transverse magnetoresistance (Δρ _xx /ρ₀)(P), and magnetic susceptibility (χ/χ₀(P)) on hydrostatic pressure P ≤ 7 GPa in the ferromagnetic semiconductor Cd_0.7Mn_0.3GeAs₂ at room temperature were investigated.     

Spin-dependent correction to the relativistic-electron mass in QED in the presence of an external electric field

A new expression is found for the spin-dependent contribution Δm _s to the self-energy of an electron moving with a transverse momentum p⊥ in an electric field. The structure of an asymptotic expansion of Δm _s (r, χ) as a function of two dynamical invariants r = γ _⊥⁻² and χ = γ _⊥|ɛ|/ɛ _c (γ _⊥² ≡ 1 + p _⊥²/m ² c ² and ɛ _c ≡ m ² c ³/|e|ℏ) is clarified with the aid of this expression. The expansion of Δm _s (r, χ) can be represented in the form of a Taylor series in r, the coefficients in this series, F ₀(χ), F ₁(χ), etc., being integrals of the Mellin type. The major coefficient F ₀(χ) is universal and, in the case of an appropriate interpretation of χ, describes well-known spin-dependent corrections to the mass in three different cases of a constant external field (for r → 0). The asymptotic properties of the function F ₁(χ) are studied in detail, but only order-of-magnitude estimates are obtained for F ₂(χ) and F ₃(χ). A comparison of these contributions revealed that, in the semiclassical region χ ≪ 1, r is indeed the parameter of the aforementioned expansion, while, for χ ≫ 1, the true parameter is rχ ² ≡ β ². In particular, the anomalous magnetic moment develops, owing to F ₁(χ), a term that grows logarithmically for χ ≫ 1, but which does not violate the hierarchy of terms in the Taylor series being considered, provided that β remains smaller than unity.     

Pressure-induced metamagnetic transition in the Cd<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>GeAs<Subscript>2</Subscript> ferromagnetic semiconductor

The magnetic susceptibility χ/χ₀ and the longitudinal Δρ _zz /ρ₀ and transverse Δρ _xx /ρ₀ magnetoresistances have been measured as functions of the hydrostatic pressure P ≤ 7 GPa at room temperature in the high-temperature ferromagnetic semiconductor Cd_0.7Mn_0.3GeAs₂ with a chalcopyrite structure and the Curie temperature T _c = 355 K. A pressure-induced metamagnetic transition from the low-magnetization state to the high-magnetization state has been observed in Cd_0.7Mn_0.3GeAs₂ near the magnetic ordering temperature. This transition is accompanied by the hysteresis of the magnetic susceptibility and magnetoresistance.     

The potts glass in uniform and random fields: a monte carlo investigation

As a simple model of an anisotropic orientational glass with short range forces, the 3-state Potts model on the simple cubic lattice with nearest neighbor interactions drawn from a Gaussian distribution is considered. With Monte Carlo methods we study the response of the system to a uniform “field” which favors one of the states. This is motivated by experiments which apply stress that favors one molecular orientation of the quadrupolar glass. The responsem to that fieldh=H/k _BT is analyzed in terms of an expansionm= χ₁ h+χ₁ h ²+χ₁ h ³+..., where χ₁ is the linear susceptibility, and χ₂,χ₁₃ are nonlinear susceptibilities. Unlike the case of spin glasses, where the spin inversion symmetry of the system in the absence of fields implies χ₂≡0,χ₂ is nonzero here and diverges to −∞ at the zero temperature transition of the model, while χ₃ diverges to +∞ as in spin glasses. At inifinite temperature, however, χ₁=1/3, χ₂=1/18 and χ₃=-1/54, i.e. the nonlinear susceptibilities have a different sign as at low temperature. In contrast, a random field does not induce a uniform order parameterm but only a glass order parameterq. The temperature dependence of this glass order parameterq(T) shows for intermediate field strength order parameterq(T) shows for intermediate field strength a maximum of the slopedq(T)/dT very similar to corresponding experiments.     

Manganese energy level in <Emphasis Type="Italic">p</Emphasis>-InAs〈Mn〉 under pressure

Resistivity ρ and the Hall coefficient R _H at atmospheric pressure in the temperature range of 77–400 K and the dependences of these parameters (ρ(P) and R _H(P)) and magnetic susceptibility (χ(P)) on hydrostatic pressures of up to P ≤ 7 GPa at 300 K in p-InAs〈Mn〉 single crystals was investigated. The baric coefficients of the ionization energy of Mn impurity centers and the pressure dependence of the dielectric constant ɛ(P) were determined.     

A Kind Tripartite Entangled State Representation and?Its Application in Quantum Teleportation

We discuss entanglement for a tripartite system by setting up a new state vector representation |p,χ ₁,χ ₂〉 in three-mode Fock space. The Schmidt decomposition of |p,χ ₁,χ ₂〉 is presented and its application in teleporting a bipartite entangled state or a two-mode squeezed state to a pair of receivers is analyzed.     

Leading-twist contribution to χ<Subscript><Emphasis Type="Italic">c</Emphasis>0,2</Subscript> → ωω decays

The leading twist contribution to χ _c0,2 → ωω decays in the color-singlet approximation is considered. It is shown that the prediction for Br(χ _c0 → ωω) is in good agreement with the experimental data, while Br(χ _c2 → ωω) differs from the experiment significantly. The text was submitted by the author in English.     

Energy and critical ionic-bond parameter of a 3D large-radius bipolaron

A theory of a strong-coupling large-radius bipolaron has been developed. The possibility of the formation of 3D bipolarons in high-temperature superconductors is discussed. For the bipolaron energy, the lowest variational estimate has been obtained at α > 8, where α is the electron-phonon coupling constant. The critical ionic-bond parameter η _c = ɛ_∞/ɛ₀, where ɛ_∞ and ɛ₀ are the high-frequency and static dielectric constants, has been found to be η _c = 0.2496.     

Strong dynamic magnetoelectric coupling in metamaterial

Magnetoelectric susceptibility (ξ) of classical magnetoelectrics is always substantially smaller that the geometric average of the electric (χ _e ) and magnetic (χ _m ) susceptibilities. Materials reaching the equality ξ ² = χ _e χ _m as a limiting value may thus be considered as ideal magnetoelectrics. Here we investigate the magnetoelectric susceptibility of a metamaterial built from split ring resonators both experimentally and within an equivalent circuit model. Due to a direct proportionality between electric polarization and magnetization, the magnetoelectric susceptibility of this metamaterial fulfils the equality ξ ² = χ _e χ _m at least in the dynamic regime.     

Search for theχ′ c charmonium states as solution to the CDFψ′ puzzle

The efforts of Roy-Sridhar-Close-Cho-Wise-Trivedi to resolve the CDFψ′ anomaly with cascades from above-thresholdχ′ _c states require well defined signatures [a small total width and a large branching fraction forχ′ _cJ →γ+ψ′] for the solution to be viable. Here we estimate the production of such states from BR(B→χ′ _cJ +X)BR(χ′ _cJ →γψ′) andγγ production ofχ′ _c2 at CLEO II, and comment on the feasibility of testing the hypothesis in terms of current experimental capabilities.     

Synthesis and Mössbauer Study of Maghemite Nanowire Arrays

An analytical formula for the distance dependence of the electric field gradient produced by a Gaussian charge density distribution n(r) is derived. This charge density is displaced by z ₀ along the z-axis. The system has cylindrical symmetry; hence it suffices to calculate V _zz(0). It turns out that V _zz(0) is always smaller than the value with the total charge shrunk into a point. For distances larger than about four times the Gaussian width σ the expression approaches the point charge value. For z₀ → 0, i.e. a spherically symmetric charge distribution around the origin, V _zz(0) vanishes quadratically, as required by symmetry. A slab-wise calculation in cylindrical coordinates is presented which shows the contribution to V _zz(0) for infinitesimally thin slabs as a function of distance from the origin. This analytical formula allows for a fast computation of electric field gradients from a given charge density distribution for Gaussian expansions of Slater-type orbitals. An example for a hydrogen atom will be given.     

Synthesis, structure, and dielectric properties of KH2PO4 fractal aggregates

Macroscopic fractal aggregates of KH₂PH₄ (KDP) measuring up to 500 μm have been obtained. The fractal structure forms as a result of the precipitation of KDP particles from a supersaturated aqueous solution in the presence of a temperature gradient followed by a diffusioncontrolled mechanism of aggregation. The electron-microscopic analysis performed has shown that the fractals are formed predominantly from crystallites of the tetragonal modification measuring ∼1 μm. The dielectric constant (ɛ) of fractal KH₂PO₄ has been measured in the temperature range 80–300 K. A characteristic anomaly has been discovered on the ɛ(T) curve in the vicinity of 122 K, which attests to a ferroelectric phase transition. The absolute value of ɛ is significantly smaller than the components ɛ ₁₁ and ɛ ₃₃ for KH₂PO₄. Fiz. Tverd. Tela (St. Petersburg) 41, 2059–2061 (November 1999)     

Slow Motion of Charges¶Interacting Through the Maxwell Field

We study the Abraham model for N charges interacting with the Maxwell field. On the scale of the charge diameter, R _ϕ, the charges are a distance ɛ^-1 R _ϕ apart and have a velocity with ɛ a small dimensionless parameter. We follow the motion of the charges over times of the order ɛ^-3/2 R _ϕ/c and prove that on this time scale their motion is well approximated by the Darwin Lagrangian. The mass is renormalized. The interaction is dominated by the instantaneous Coulomb forces, which are of the order ɛ². The magnetic fields and first order retardation generate the Darwin correction of the order ɛ³. Radiation damping would be of the order ɛ^7/2. Received: 13 January 2000 / Accepted: 4 February 2000     

Analysis on configuration spaces and Gibbs cluster ensembles

The distribution μ of a Gibbs cluster point process in χ = ℝ^d (with n-point clusters) is studied via the projection of an auxiliary Gibbs measure defined on the space of configurations in χ × χ ⁿ. We show that μ is quasi-invariant with respect to the group Diff₀(χ) of compactly supported diffeomorphisms of χ and prove an integration-by-parts formula for μ. The corresponding equilibrium stochastic dynamics is then constructed by using the method of Dirichlet forms. Dedicated to the memory of Vladimir Geyler Research supported in part by DFG Grant 436 RUS 113/722.     

Photophoresis of coarse aerosol particles with nonuniform thermal conductivity

The photophoresis of a coarse solid spherical aerosol particle in a one-component gas of nonuniform temperature is examined with consideration of the inertial effects in the hydrodynamic equations and the temperature jump in the Knudsen layer. The problem is solved in the spherical coordinates r, Θ, and ϕ. The photophoresis of a homogeneous particle is considered first. Then the results are generalized to an inhomogeneous particle. A particle whose thermal conductivity χ _i varies as a function of r is chosen as a model which describes a broad class of natural and artificially produced aerosol particles. It is shown that the error can be significant if the variable internal thermal conductivity χ _i =χ _i (r) of the particle is ignored and only the value of the thermal conductivity on its surface χ _i (a) is considered, on the assumption that the particle is homogeneous. It is also shown that a particle with a variable internal thermal conductivity χ _i =χ _i (r) and a density of heat sources within it q _i (r,Θ) can be regarded as a homogeneous particle with a thermal conductivity γχ _i (a) and a heat-source density m(r)q _i (r,Θ). Recurrence formulas for gand m(r) in the general case are presented. Analytical expressions for γ and m(r) are found for a model particle with pronounced inhomogeneity. Zh. Tekh. Fiz. 68, 1–6 (April 1998)     

Even harmonics of AC susceptibility in some sintered pellets of high temperature superconductors

The real (χ′ _n) and imaginary (χ″ _n) parts of even harmonic susceptibility (n⩽6) are measured as a function of external DC field (H _dc) in the field increasing (H↑) and decreasing (H↑) cycle. Hysteresis is observed betweenH↑ andH↓ cycles. In theH↓ cycle, at a field,H _comp=4·2G, bothχ′ andχ″ of all the even harmonics vanish indicating a true cancellation of fields in the intergrain region (H _eff≈0) caused by the balance betweenH _dc and the remanent magnetization of the grains. The position of the extrema and the zero of the various harmonics undergo a shift proportional to the remanent magnetization of the grains at that particular field.     

Field localization and particle confinement effects on pair creation by Schwinger’s mechanism

Solutions of the Dirac equation in the presence of a static uniform electric fieldɛ in thez-direction and a linear confining potentialAz, are obtained. Generalized reflection and transmission coefficients are derived for such divergent potentials forɛ >A/e. The eigenspectrum and corresponding localized eigenfunctions forɛ <A/e are obtained from the reflection coefficient and the continuum solutions respectively. The rate for the electric field to decay into pairs is derived from the transmission coefficient. Neglecting nonabelian effects in quantum chromodynamics we identify the fieldɛ with a colour electric field and the produced particles with a quark and an antiquark. By considering a cylindrical geometry, we thus obtain a generalization of Schwinger’s formula, for the fieldɛ in a finite spatial region with the quark (antiquark) being confined in thez direction by the linear potentialAz and in the perpendicular direction by the MIT bag boundary condition. The result is used to qualitatively study Schwinger’s mechanism of quark-gluon plasma (QGP) formation in ultrarelativistic heavy ion collisions. It is found that the critical strength of the field required to create pairs is enhanced,ɛ _c(A) >ɛ _c(A = 0). The rate of pair creation for constantɛ, decreases for non-zeroA, implying longer QGP formation times. Because ofɛ _c(A) >ɛ _c(0), QGP is predicted to be formed in the early stages of the nuclear collision. The finite size effects and the MIT bag boundary condition effects on QGP formation are also discussed.     

Thermoelectric power in quasi-two-dimensional systems with scattering of current carriers by phonons

The thermopower α in electron systems with a quasi-two-dimensional energy spectrum is investigated in the relaxation-time tensor approximation. The longitudinal and transverse components of the thermopower are calculated for scattering of the current carriers by different types of phonons. It is shown that the anisotropy of the thermopower in such systems is substantial. The dependence of a on the ratio of the Fermi level ɛ _F to the half-width ɛ ₀ of the one-dimensional conduction band is considered. For scattering by acoustical and nonpolar optical phonons, the thermopower changes sign: α becomes positive for ɛ _F<ɛ ₀. Comparison of the theory with published experimental data demonstrates good qualitative agreement. Fiz. Tverd. Tela (St. Petersburg) 39, 1857–1858 (October 1997)     

Quark-antiquark states and their radiative transitions in terms of the spectral integral equation: Charmonia

Earlier by the authors (Yad. Fiz. 70, 68 (2007)), the bƃ states were treated in the framework of the spectral integral equation, together with simultaneous calculations of radiative decays of the considered bottomonia. In the present paper, such a study is carried out for the charmonium states. We reconstruct the interaction in the c-c sector on the basis of the data for the charmonium levels with J _PC = 0⁻⁺, 1⁻⁻, 0⁺⁺, 1⁺⁺, 2⁺⁺, 1⁺⁻ and radiative transitions ψ(2S) → γχ _c0(1P), γχ _c1(1P), γχ _c2(1P), γχ _c(1S) and χ _c0(1P), χ _c1(1P), χ _c2(1P) → γJ/ψ. The c-c levels and their wave functions are calculated for the radial excitations with n ≤ 6. Also, we determine the c-c component of the photon wave function using the e ⁺ e ⁻-annihilation data: e ⁺ e ⁻ → J/ψ(3097), ψ(3686), ψ(3770), ψ(4040), ψ(4160), ψ(4415) and perform the calculations of the partial widths of the two-photon decays for the n = 1 states η _c0(1S), χ _c0(1P), χ _c2(1P) → γγ and n = 2 states η _c0(2S) → γγ, χ _c0(2P) → γγ. We discuss the status of the recently observed c-c states X(3872) and Y(3941): according to our results, the X(3872) can be either χ _c1(2P) or η _c2(1D), while Y(3941) is χ _c2(2P). The text was submitted by the authors in English.