Giant magnetoacoustic effect in KMnF3 due to nuclear spin waves

An explanation is proposed for the gigantic magnetoacoustic effect that we observed in KMnF₃ in previous work {Kh. G. Bogdanova, V. A. Golenishchev-Kutuzov, M. I. Kurkin et al., Zh. éksp. Teor. Fiz. 112, 1830 (1997) [JETP 85, 1001 (1997)]}. The effect entails a tenfold amplitude reduction of an acoustic pulse in a magnetic field that varies over the range 0–8 kOe. It is shown that this effect is due to the interference of two nuclear magnetoelastic waves propagating in the sample under magnetoacoustic resonance conditions, if this resonance occurs in the region of strong spatial dispersion of nuclear spin waves. The effect is said to be gigantic because it exceeds in magnitude the magnetoacoustic effects observed previously in magnetically ordered materials even though it is due to nuclear magnetism, which is 10⁵ times weaker than electronic magnetism. We observe a concomitant anomalous dependence of the dispersion of the velocity of sound on the external magnetic field. Zh. éksp. Teor. Fiz. 115, 1727–1739 (May 1999)     

Effect of helical edge states on the tunneling conductance in ferromagnet/noncentrosymmetric superconductor junction

Helical edge states exist in the mixed spin-singlet and spin-triplet phase of a noncentrosymmetric (NCS) superconductor [Y. Tanaka, T. Yokoyama, A.V. Balatsky, N. Nagaosa, Phys. Rev. B 79, 060505(R) (2009)]. In this article we have considered a planar ferromagnetic metal/NCS superconductor tunnel junction and have studied the effect of these helical edge states which manifests itself through the charge and spin tunneling conductance across the junction. We have shown the behavior of conductance for the entire range of variation of γ = Δ _-/Δ ₊ where Δ _± are the order parameters in the positive and negative helicity bands of the NCS superconductor. There exists a competition between the Rashba parameter α and the exchange energy E _ex which is crucial for determining the variation of the conductance with the applied bias voltage across the junction. We have found a nonzero spin current across the junction which appears due to the exchange energy in the Ferromagnet and modulates with the bias voltage. It also changes its profile when the strength of the exchange energy is varied.     

Charge ordering in crystals of Pr1? x Sr x MnO3 ( x = 0.22, 0.24) ferromagnetic manganites

The structural and magnetic states of Pr_1−x Sr _x MnO₃ (x = 0.22, 0.24) manganite crystals were studied over a wide temperature range. Measurements of the magnetic susceptibility and electrical resistivity demonstrated that these manganites belong to the class of ferromagnetic semiconductors. Thermal neutron elastic scattering patterns revealed that, in the temperature range 4.2–350 K, the manganites have an orthorhombic structure (space group Pnma) with a well-pronounced cooperative Jahn-Teller effect. Major emphasis was placed on the nuclear magnetic superstructure with a wave vector q = (2π/2a, 0, 2π/2c). It was shown that this superstructure suggests 1/4-type charge ordering in the manganites under investigation. Original Russian Text ? S.F. Dubinin, S.G. Teploukhov, V.E. Arkhipov, V.D. Parkhomenko, é.A. Neĭfel’d, A.V. Korolev, N.A. Ugryumova, Ya.M. Mukovskiĭ, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 4, pp. 704–710.     

Imperfection of the nanostructure, phase transitions, 55Mn NMR, and magnetoresistive properties of La 0.73+ Ca 0.3 ? x2+ Sr x2+ MnO3 ± δ ceramics

Magnetoresistive ceramic samples La_0.7Ca_{0.3 − x} Sr _x MnO_{3 ± δ} sintered at temperatures of 1150 and 1350°C are investigated using X-ray diffraction, microscopic, resistance, and magnetic (χ, ⁵⁵Mn NMR) measurements. The specific features of the influence of the composition on the type and parameters of the perovskite structure, its imperfection, the porous crystallite structure, the metal-semiconductor and ferromagnetic-paramagnetic phase transitions, the ⁵⁵Mn NMR spectra, and the magnetoresistance effect are established. The magnetic phase diagram is constructed. The conclusions are drawn regarding the nonuniformity of the distribution of ions and vacancies around manganese involved in the high-frequency electron-hole exchange (Mn³⁺ ai Mn⁴⁺) and the nanostructured separation of the perovskite structure containing anion and cation vacancies, with the concentrations and magnetoresistance effect decreasing and the lattice parameters and phase transition temperatures increasing as calcium is replaced by strontium. Original Russian Text ? A.V. Pashchenko, A.A. Shemyakov, V.P. Pashchenko, V.A. Turchenko, V.K. Prokopenko, Yu.F. Revenko, Yu.V. Medvedev, B.M. éfros, G.G. Levchenko, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 6, pp. 1127–1136.     

Main parameters of a coaxial electrostatic lens

Coaxial electrostatic lenses are used in combination with a magnetic field in plasma-optical and manometric devices [A. I. Morozov and S. V. Lebedev, in Reviews of Plasma Physics, Vol. 8, M. A. Leontovich (ed.), Consultants Bureau, New York (1980); V. P. Afanas’ev, A. A. Vydrik, L. P. Ovsyannikova, and E. V. Shpak, Sov. Phys. Tech. Phys. 25, 1049 (1980)]. Their employment in systems for transporting beams of high-energy charged particles is also known [P. Krejcik, B. V. King, and I. C. Kelly, Optik (Stuttgart) 55, 385 (1980)], since, along with quadrupole lenses, they have a large optical strength. Expressions for the trajectories of charged low-energy particles were obtained by L. P. Ovsyannikova and S. V. Pasovets [Sov. Phys. Tech. Phys. 32, 1422 (1987)] in the approximation of a rectangular field model with consideration of the variation of the energy at the entrance and exit from the field. The possibility, in principle, of focusing a parallel beam in a ring is demonstrated. Zh. Tekh. Fiz. 67, 81–82 (December 1997)     

Photogalvanic effect in an asymmetric system of three quantum wells in a strong magnetic field

The photogalvanic effect (PGE) in an asymmetric undoped system of three GaAs/AlGaAs quantum wells illuminated with white light of various intensities is investigated in magnetic fields up to 75 kOe at temperatures ranging from 4.2 K up to 300 K. A maximum of the spontaneous photogalvanic current J ^PGE as a function of the magnetic field predicted by A. A. Gorbatsevich et al., JETP Lett. 57, 580 (1993), is observed. Analysis of the experimental data shows that the main initial characteristic of the PGE is not the spontaneous current but rather the electromotive force E ^PGE arising in the direction perpendicular to the applied magnetic field. It is determined that this emf is independent of the intensity of the incident light, increases linearly with the size d of the illuminated region, and decreases slowly with temperature: E _max ^PGE ∼0.8 V at 300 K and ∼0.1 V at 4.2 K for d∼3 mm. The curve E ^PGE(H) at room temperature is determined with allowance for the strong transverse magnetoresistance of the nanostructure. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 197–202 (10 February 1996)     

Electron and phonon effects in superconducting tunnel detectors of x-radiation

Nb/Al/AlO_x/Nb superconducting tunnel junctions were investigated in the role of x-ray detectors. Amplitude spectra of pulses arising upon irradiation of tunnel junctions of different sizes by ⁵⁵Mn x-radiation were recorded at a temperature T=1.4 K. We also analyzed the temporal shape of the pulses. We considered the influence of diffuse motion of nonequilibrium quasiparticles, the inverse tunneling effect, and exchange of 2Δ phonons between electrodes, on the characteristics of the tunnel detectors. It is shown that phonon processes can bring about changes in the amplitude, duration, and polarity of the signal. Fiz. Tverd. Tela (St. Petersburg) 41, 1168–1175 (July 1999)     

Solving the Bethe-Salpeter equation for two fermions in Minkowski space

The method of solving the Bethe-Salpeter equation in Minkowski space, developed previously for spinless particles (V.A. Karmanov, J. Carbonell, Eur. Phys. J. A 27, 1 (2006)), is extended to a system of two fermions. The method is based on the Nakanishi integral representation of the amplitude and on projecting the equation on the light-front plane. The singularities in the projected two-fermion kernel are regularized without modifying the original BS amplitudes. The numerical solutions for the J = 0 bound state with the scalar, pseudoscalar and massless vector exchange kernels are found. The stability of the scalar and positronium states without vertex form factor is discussed. Binding energies are in close agreement with the Euclidean results. Corresponding amplitudes in Minkowski space are obtained.     

Transient photoimpedance response of YBa2Cu3O7−0δ epitaxial films in the mixed state

This paper reports on an investigation of transient photoimpedance response to radiation, in other words, real-time variations of the impedance induced by femtosecond optical pulses in superconducting films transferred to the mixed state by an external magnetic field applied parallel to the c-axis. When the films were in a state characterized by the absence of dc resistivity, the response amplitude increased with the magnetic field faster than expected owing to the contribution of magnetic vortices to the impedance of a superconductor with s-wave pairing of electrons. It turned out that the effect is due to a growth in the effective density of quasiparticle states in the mixed state of a d-wave superconductor. In the absence of magnetic field, however, the response amplitude was higher at lower temperatures, which contradicts the predictions of both models. Possible reasons for this feature in the response as a function of temperature are discussed. Zh. éksp. Teor. Fiz. 116, 1035–1047 (September 1999)     

De Haas-van Alphen effect in unconventional superconductors

A theory of the de Haas-van Alphen effect in type-II p-wave and D-wave superconductors (the latter corresponds to the B _1g one-dimensional representation of group D _4h ) has been developed. Solutions for the order parameter and density of quasiparticle states near the upper critical field have been calculated. If the curve enclosing the extremal cross section of the Fermi surface in the plane perpendicular to the external magnetic field coincides with the line of nodes of the superconducting order parameter, the effect of the transition to the superconducting state on the amplitude of magnetization oscillations is negligible. If the line of nodes is oriented differently with respect to the applied magnetic field, the de Haas-van Alphen oscillations are suppressed in a manner qualitatively similar to the case of conventional superconductors. Zh. éksp. Teor. Fiz. 113, 2174–2192 (June 1998)     

Optical and thermal orientation of localized excitons in solid solutions under resonant excitation in a longitudinal magnetic field

A theoretical and experimental study of the effect of a longitudinal magnetic field on optical orientation and magneto-circular polarization of the luminescence of localized excitons in semiconducting solid solutions is reported. It is shown that recombination takes place through two types of emitting states differing substantially in the degree of anisotropy, g factor, and spin relaxation time. Estimates are made of the g factors, anisotropic and exchange splittings, lifetime, and spin relaxation time of localized states in a CdS_0.96Se_0.04/GaAs solidsolution epitaxial layer. Fiz. Tverd. Tela (St. Petersburg) 40, 900–902 (May 1998)     

Magnetic phase diagram of spinel spin-glasses

The article by Villain [Z. Phys. B — Condensed Matter33, 31 (1979)] is discussed and a modified magnetic phase diagram is suggested for the spinel system (AB₂O₄) in which theA andB sites are partially (or completely) occupied by magnetic atoms. This diagram takes into account the antiferromagnetic exchange interactionsJ _AA,J _BB andJ _AB between nearest neighbor cations of various types. Regions of paramagnetic, antiferromagnetic, ferrimagnetic and possible spin glass behaviour are indicated on the diagram.Supported by the National Science Foundation under Grant ISP-80-11451     

Photon splitting in an ultrastrong magnetic field

We analyze the splitting of a photon with energy ω below the e ⁺ e ⁻ pair-production threshold in an ultrastrong magnetic field. We use the amplitudes found by employing the operator diagrammatic technique. In a field considerably above the critical values the process amplitudes become independent of the field strength. A study of the polarization operator of a photon in an external field of arbitrary strength in the energy range considered in the present investigation shows that there is only one set of polarizations of the initial and final photons for which the splitting amplitude is nonzero. Zh. éksp. Teor. Fiz. 111, 52–62 (January 1997)     

Composition,temperature and geometric dependent hysteresis behaviours in Ising-type segmented nanowire with magnetic and diluted magnetic,and its soft/hard magnetic characteristics

Abstract

In this paper, a theoretical approach to evaluate the hysteresis behaviours of Ising-type segmented nanowire (ISN) comprising magnetic and diluted magnetic segments is defined. The dependency to the system parameters are calculated for optimising their performance in applications like magnetic sensors or recording media. The effects of the composition (p) and temperature (T) as well as crystal field on the hysteresis behaviours are investigated in detail. We studied the effect of the segment dimensions obtained from the exchange interactions. The coercivity (H_C) and remanence (M_r) of the ISN are derived from hysteresis loops. The phase diagrams are presented in the different planes as function of H_C and M_r to investigate the magnetic characteristics of the ISN. Under certain conditions, namely p = 0 and J_D = 0, we also examined the effect of temperature on the nanowire with magnetic and non-magnetic segments. The distinct hysteresis properties and soft/hard the magnetic characteristics depending upon these factors are observed. We found that the magnetic hardness decreases case as the temperatures increase as well as p and crystal field (Δ) decrease. Moreover, when the p increase and Δ decrease the triple hysteresis loop behaviour occurs in the system. Comparisons between the observed theoretical results and some experimental works of nanowire with hysteresis behaviours are made and a very good agreement is obtained.     

Influence of spin–orbit interaction,Zeeman effect and light polarisation on the electronic and optical properties of pseudo-elliptic quantum rings under magnetic field

ABSTRACT

We theoretically investigated the influences of the magnetic field and light polarisation on the electronic and optical properties of a GaAs/GaAlAs pseudo-elliptic quantum ring, modelled by an outer ellipsis and an inner circle, in the presence of the Rashba and Dresselhaus spin–orbit interactions and Zeeman effect. We show that Aharonov-Bohm oscillations of the energy spectrum are not affected by the presence of the Zeeman effect alone but, in the presence of Rashba and Dresselhaus spin–orbit couplings, the periodicity of certain levels becomes hardly definite. The Zeeman effect generally enhances/diminishes the separation levels produced by Rashba/Dresselhaus interactions (SOI) and when both types of SOI are considered, the effect depends on their relative strength. The magnetic field can trigger spin-flip for each type of spin–orbit interaction and Zeeman effect or their combination through anticrossings in the energy spectra. Our results reveal that the absorption spectra are very sensitive to the magnetic field and light polarisation. For all polarisations considered, the magnetic field increment leads to the redshift or blueshift of some particular peaks (an effect of this ring geometry) and a better separation of the peaks. The x-polarised light determines spectra with many small, but separated peaks while the circular polarised light leads to spectra with large peaks of high amplitude.     

Toward a nonlinear theory of acceleration of charged particles in a linearly polarized EH-ubitron field

The motion of charged particles in a linearly polarized EH-ubitron field with sinusoidal time dependence of the variable component of the magnetic field is investigated. It is shown that, depending on the parameters of the system and the initial conditions of entry of the particle into the EH-field region, three different types of motion are possible: ballistic motion, capture in the vicinity of one of the maxima of the magnetic field, and escape of particles from the EH-field region. An analytical expression is found for the criterion determining the type of motion of the particles, along with analytical solutions for the energy of the particles in the approximation of slow variation of the magnetic field amplitude in time. Peculiarities of the motion of the particles in EH fields with arbitrary rate of change of the magnetic field amplitude in time are investigated numerically. Zh. Tekh. Fiz. 69, 87–93 (January 1999)     

Magnetic structure of Ce<Subscript>2</Subscript>Fe<Subscript>17 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intermetallic compounds

The magnetic structure of intermetallic compounds Ce₂Fe_{17 − x} Mn _x (0 ≤ x ≤ 3) was studied using neutron diffraction. The neutron diffraction patterns measured at 4.2 K contain satellites indicating a modulated structure with the wave vector k = [0, 0, τ]. As the concentration x increases, the value of τ increases, while the average magnetic moment of Fe/Mn atoms decreases. A change in the magnitudes of the average magnetic moment and wave vector k is explained by competition between exchange interactions at distances of nearest neighbor transition element atoms.     

Manifestation of ferromagnetism in lightly oxygen-doped La2CuO4 single crystals in magnetic fields H<50 Oe

The temperature and field dependences χ(T,H) in La₂CuO_4+δ single crystals with δ<0.015 have been investigated in magnetic fields 0.1<H<450 Oe by the differential magnetic susceptibility method. It was found that under oxygen doping conditions ferromagnetic regions are formed. These regions produce a characteristic curve of the magnetic susceptibility χ(T,H), which is observed only in magnetic fields of less than 50 Oe. This can be explained by the formation of ferrons [A. Aharony et al. Phys. Rev. Lett. 60, 1330 (1988); L. I. Glazman and A. S. Ioselevich, Z. Phys. B 80, 268 (1990)] in an antiferromagnetic matrix. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 152–155 (10 August 1996)     

The excitonic spectrum of germanium in a high magnetic field

In recently reported experiments with uniaxially deformed germanium in a magnetic field [V. B. Timofeev and A. V. Chernenko, JETP Lett. 61, 617 (1995)], it was found that applying a magnetic field of sufficiently high intensity results in the appearance of a new line in the optical spectrum of the excitons. In the present paper a mechanism is proposed which can provide an explanation for this experimentally observed spectral feature. The new spectral line may be attributed to the formation of strongly bound biexcitonic molecules in the quantum state ³Π_u. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 405–409 (25 March 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Anomalous burn-through of thin foils by high-intensity laser radiation

The paper presents results of experiments performed on the Pico facility in which foils were heated by laser radiation, and anomalously fast burn-through of foils by a structured laser beam was detected. Comparison with two-dimensional calculations has allowed us to suggest a tentative mechanism for the effect under investigation. The targets in the experiments were thin aluminum foils of thickness 3 to 40 μm. The flux density of laser radiation on the target surface varied between 10¹³ and 10¹⁴ W/cm². We detected a strong dependence of the transmitted energy on the foil thickness and the shortening of the transmitted laser pulse. Penetration of laser radiation through foils with thicknesses considerably larger than 3 μm has been observed, although it was stated in earlier publications [V. V. Ivanov, A. K. Knyazev, A. V. Kutsenko, et al., Kratk. Soobshch. Fiz. FIAN No. 7–8, 37 (1997)]; A. é. Bugrov, I. N. Burdonskii, V. V. Gol’tsov et al., Zh. éksp. Teor. Fiz. 111, 903 (1997) [JETP 84, 903 (1997)] that, at the laser radiation parameters used in our experiment, the evaporated layer of the foil could not be thicker than 2 μm. Two-dimensional calculations have allowed us to interpret this effect in terms of local “piercing” of the target at spots on the target surface where the radiation intensity has its peaks. The possibility of reducing these peaks by using a symmetrizing prepulse is discussed in the paper. Zh. éksp. Teor. Fiz. 116, 1287–1299 (October 1999)