Anisotropy of a hole magnetic polaron in a semimagnetic semiconductor

It is shown that the shape of a hole magnetic polaron in a semimagnetic semiconductor with the sphalerite structure is anisotropic: The polaron is strongly oblate in the direction of its magnetic moment. When the anisotropy of the hole spectrum is taken into account, the properties of the polaron depend on the orientation of its spin with respect to the crystallographic axes. Specifically, the binding energy of the polaron is maximum when the spin is oriented along the [111] axis. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 209–213 (10 February 1996)     

Redistribution of the hole spectral weight due to longrange spin correlations in the three-band Hubbard model

The spectrum of the spin-polaron hole exitation is investigated in the framework of the three-band model for the CuO₂ plane in hightemperature superconductors. The problem is treated taking into account the coupling of a local polaron with the antiferromagnetic spin wave with Q=(≈,≈). This leads to fundamental changes in the lowest polaron band ε₁(k) and to a strong redistribution of the bare electron filling. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 173–177 (10 August 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Magnetoacoustic resonance on nuclear spin waves in the cubic antiferromagnet RbMnF3

Magnetoacoustic resonance on nuclear spin waves is measured in the cubic antiferromagnet RbMnF₃. A resonance change with respect to a constant magnetic field H ₀ with maximum damping at H ₀≈4×10³ Oe is observed in the amplitude of an acoustic pulse passing through a sample owing to excitation of nuclear spin waves under nuclear magnetoacoustic resonance conditions. A study of the angular dependence of the damping revealed a 90° periodicity consistent with the fact that the [001] direction, around which the rotation takes place, is a four-fold axis of the crystal. An analysis of the dispersion law for nuclear spin waves shows that longitudinal ultrasound propagating along the [001] axis perpendicular to H ₀ excites a branch of nuclear spin waves whose frequency depends on the magnitude of the constant magnetic field. Fiz. Tverd. Tela (St. Petersburg) 41, 297–300 (February 1999)     

Longitudinal nuclear spin relaxation in solids in a triply rotating coordinate system: Selective observation of the multispin dipole contribution

The longitudinal nuclear spin relaxation in an effective magnetic field H _e3 acting in a triply rotating coordinate system is recorded. Rotating and doubly rotating coordinate systems are employed for strong suppression of the secular nuclear dipole interactions in the first two orders and for separation of higher-order interactions (four-and five-spin). Experiments on protons in polycrystalline benzene showed that the contribution of such multispin dipole interactions to this relaxation can be observed selectively as a pronounced local minimum in the temperature dependence of the relaxation time. This contribution correponds to ultraslow molecular motions with rates ≃ γH _e3≃2π(10¹−10³) s⁻¹ and can be employed to study such motions in detail, including for purposes of identification of the form of the motion. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 335–340 (10 September 1996)     

Insulator-metal transition shift related to magnetic polarons in La<Subscript>0.67-x</Subscript>Y<Subscript>x</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

The magnetic transport properties have been measured for La_0.67-xY_xCa_0.33MnO₃ ( 0 ⩽ x ⩽ 0.14) system. It was found that the transition temperature T _p almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above T _p , there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of T _p vs. H was understood, and it was estimated that the size of the magnetic polaron is of 9.7 ∼ 15.4 ? which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The transport properties at temperatures higher than T _p conform to the variable-range hopping mechanism. Received 27 August 2002 / Received in final form 2 December 2002 Published online 14 March 2003     

Spin fluctuations in the vicinity of a charged particle in solid3He

At low temperature the positively charged muon (μ⁺) in solid³He is localized in a polaron which, unlike a chemically bound complex, is held together with polarizational attraction (van der Waals force). The dynamic effects in the muon relaxation are determined by the spin exchange of the³He atoms in the polaron. In crystals with large molar volumes (melting pressureP _m<60 bar) the rate of magnetic field fluctuations at the muon site is at least one order of magnitude lower than in an unperturbed crystal. In an external electric field the μ⁺ produces an anisotropic local distortion which reduces the rate of the local³He spin exchange and leads to an increase of the muon spin relaxation rate.     

The transient EPR spectra and spin dynamics of coupled three-spin systems in photosynthetic reaction centres

The concept of introducing an additional, stable paramagnetic species into photosynthetic reaction centres to increase the information content of their spin polarized transient EPR spectra is investigated theoretically. The light-induced electron transfer in such systems generates a series of coupled three-spin states consisting of sequential photoinduced radical pairs coupled to the stable spin which acts as an “observer”. The spin polarized transient EPR spectra are investigated using the coupled three-spin system P⁺I⁻Q⁻ _A in pre-reduced bacterial reaction centres as a specific example which has been studied experimentally. The evolution of the spin system and the spin polarized EPR spectra of P⁺I⁻Q⁻ _A and Q⁻ _A following recombination of the radical pair (P = primary donor, I = primary acceptor, Q_A = quinone acceptor) are calculated numerically by solving the equations of motion for the density matrix. The net polarization of the observer spin is also calculated analytically by perturbation theory for the case of a single, short-lived, charge-separated state. The result bears a close resemblance to the chemically induced nuclear polarization (CIDNP) generated in photolysis reactions in which a nuclear spin plays the role of the observer interacting with the radical pair intermediates. However, because the Zeeman frequencies of the three electron spins involved are usually quite similar, the polarization of the electron observer spin in strong magnetic fields can reflect features of the CIDNP effect in both, high and low magnetic fields. The dependence of the quinone spin polarization on the exchange couplings in the three-spin system is investigated by numerical simulations, and it is shown that the observed emissive polarization pattern is compatible with either sign, positive or negative, for a range of exchange couplings, J_PI, in the primary pair. The microwave frequency and orientation dependence of the spectra are discussed as two of several possible criteria for determining the sign of J_PI.     

Submillimeter and infrared spectroscopy on La0.85Sr0.15MnO3

The optical conductivity of La_0.85Sr_0.15MnO₃ single crystals was studied by means of submillimeter and infrared spectroscopy for frequencies cm^-1 and temperatures 10 K < T <300 K. The submillimeter conductivity follows the temperature dependence of the dc-data. The phonon spectrum of La_0.85Sr_0.15MnO₃ changes considerably below K revealing a structural phase transition induced by charge or orbital order. At T =10 K a number of phonon modes can be identified in addition to the room-temperature spectrum. The optical conductivity () in the mid-infrared reveals the characteristics of small polaron absorption. Below the magnetic ordering temperature the polaron binding energy is highly reduced, but the onset of charge order interrupts the formation of free charge carriers with a Drude-like behavior. The frequency and temperature dependence of in this regime qualitatively resembles the small polaron predictions by Millis et al. (Phys. Rev. B 54, 5405 (1996)). Received 5 November 1999     

Structure of spin polarons in the t-t′ -t″ -Jz model

We calculate the Green function in the t-t '-t ”-J^z model and analyze the deformation of the quantum Néel state in the presence of a moving hole. Solving the problem in a self-consistent Born approximation and using Reiter's wave function we have found various spin correlation functions. We show that the different sign of hopping elements between the hole- and electron-doped system of high- cuprates is responsible for the sharp difference of the polaron structure between the two systems with antiferromagnetism stabilized in the electron-doped case by carriers moving mainly on one sublattice. Received 11 January 2000     

Muonium quantum diffusion and spin dynamics in solid xenon

We present measurements of the transverse (T ₂ ^–1 ) and longitudinal (T ₁ ^–1 ) spin relaxation rates of muonium (Mu) atoms in solid natural xenon (n-Xe) as well as pure¹³⁶Xe (which has no nuclear moments). The temperature dependences ofT ₂ ^–1 andT ₁ ^–1 in natural Xe belowT 115 K demonstrate the quantum character of Mu diffusion governed by one-phonon interactions. Taking into account both the polaron effect (PE) and the effect of fluctuational preparation of the barrier (FPB) makes it possible to consistently describe Mu diffusion in Xe. Mu spin relaxation in¹³⁶Xe at high temperatures is not due to nuclear hyperfine (NHF) interactions.     

Relaxation and shift of the precession frequency of the spin of a negative muon in n-type silicon

The residual polarization of negative muons in n-type silicon with impurity density (1.6±0.2) · 10¹³ cm⁻³ is investigated as a function of temperature in the range 10–300 K. The measurements are performed in an external magnetic field of 0.08 T oriented transversely to the spin of the muons. Relaxation of the muon spin and a shift of the precession frequency are observed at temperatures below 30 K. The relaxation rate at 30 K equals 0.25±0.08 μs⁻¹. The shift of the precession frequency at 20 K equals 7 · 10⁻³. Both the relaxation rate and the shift of the precession frequency increase as the temperature decreases. At temperatures below 30 K the relaxation rate is described well by the relation Λ=bT ^−q , where q=2.8±0.2. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 539–543 (10 April 1996)     

Combined polaron states in magnetooptical effects in a quantum well

Combined polaron states in a rectangular quantum well in a strong magnetic field perpendicular to the well plane are discussed. These states are due to interaction between two discrete electron levels with different Landau quantum numbers (n and n ₁) and different size-quantization quantum numbers (m and m ₁) on the one hand and a confined LO phonon on the other under conditions of low temperature when the energy difference between the electronic levelsis equal or close to the energy of the confined LO phonon. The expression for the resonant magnetic field H _res at which a combined polaron is formed contains the energy difference between size-quantized levels, so it is a function of quantum well parameters. The separation ΔE _res between branches in the energy spectrum of a combined polaron and H _res has been calculated as a function of the quantum well width d. The resonant field H _res can be reduced dramatically in comparison with the case m=m ₁. The case of size-quantization with n=n ₁ has been analyzed. The energy difference ΔE _res is in the range (1–5)· 10⁻³ eV. The damping of combined polaron states due to the effect of anharmonicity on the LO phonon has been studied. Interband absorption and features in the reflection spectrum due to interband transitions have been calculated for an arbitrary ratio between the radiative and “phonon” lifetime of a combined polaron have been investigated. Zh. éksp. Teor. Fiz. 116, 1419–1439 (October 1999)     

Singlet-triplet model of cuprate magnetism

A model for describing the spin subsystem of cuprates within a model of polar singlet-triplet Jahn-Teller centers is proposed. In this model spin ordering is described by two vector order parameters S (the total spin of a CuO₄ cluster) and V (the operator of the change in spin multiplicity per cluster). It is shown within a modified mean-field approximation that the formation of a noncollinear magnetic structure characterized by an antiferromagnetic wave vector, which, however, has a nontrivial temperature dependence of the order parameters, is possible along with the formation of ordinary collinear structures of the (anti)ferromagnetic type. The temperature dependences of the order parameters and the principal equilibrium thermodynamic characteristics, viz., the static susceptibility and the specific heat, are obtained. Fiz. Tverd. Tela (St. Petersburg) 40, 1785–1792 (October 1998)     

EPR spectrum of donors in 6H SiC in a broad temperature range

An EPR study of donors in 6H SiC crystals with an uncompensated donor concentration (N _D−N _A) of 2×10¹⁸ to 1×10¹⁶ cm⁻³ performed in the temperature range 4.2 to 160 K at frequencies of 9 and 140 GHz showed that 6H n-SiC samples have two donor states in the gap. One of them originates from nitrogen occupying three inequivalent lattice sites with ionization energies of 150 and 80 meV, and the second is connected with a structural defect lying deeper in the gap than nitrogen. The temperature dependences of donor EPR line intensities have been found to deviate from the Curie law. The observed EPR line-intensity peaks of donors are produced in a temperature-driven successive redistribution of donor electrons between the donor levels. The temperature dependences of EPR line intensities obtained from samples with low donor concentrations were used to determine the valley-orbit splitting of nitrogen in cubic sites. Fiz. Tverd. Tela (St. Petersburg) 40, 1824–1828 (October 1998)     

Effective hyperfine temperature in frustrated Gd 2Sn 2O 7: two level model and 155Gd Mössbauer measurements

Using ¹⁵⁵Gd M?ssbauer spectroscopy down to 27 mK, we show that, in the geometrically frustrated pyrochlore Gd₂Sn₂O₇, the Gd³⁺ hyperfine levels are populated out of equilibrium. From this, we deduce that the hyperfine field, and the correlated Gd³⁺ moments which produce this field, continue to fluctuate as T ↦ 0. With a model of a spin 1/2 system experiencing a magnetic field which reverses randomly in time, we obtain an analytical expression for the steady state probability distribution of the level populations. This distribution is a simple function of the ratio of the nuclear spin relaxation time to the average electronic spin-flip time. In Gd₂Sn₂O₇, we find the two time scales are of the same order of magnitude. We discuss the mechanism giving rise to the nuclear spin relaxation and the influence of the electronic spin fluctuations on the hyperfine specific heat. The corresponding low temperature measurements in Gd₂Ti₂O₇ are presented and discussed. Received 17 October 2001 Published online 6 June 2002     

Electron spin resonance and nuclear magnetic resonance of sodium macrostructures in strongly irradiated NaCl-K crystals: Manifestation of quasi-one-dimensional behavior of electrons

Data from an investigation of electron spin resonance and nuclear magnetic resonance of NaCl-K (∼1 mole%) crystals strongly irradiated with electrons imply the observation of a metal-insulator transition with decreasing temperature and the manifestation of quasi-one-dimensional electron motion in sodium macrostructures. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 178–183 (10 February 1998)     

Magnetic properties of co-doped SnO2 diluted magnetic semiconductors

Polycrystalline samples of Sn_1−xCo_xO₂ are prepared for x=0.02, 0.05 and 0.10. They all exhibit room temperature ferromagnetism with decrease in saturation magnetization with increase in doping. The magnetization data recorded at 85 K, 295 K and 400 K for x ≥ 0.05 could be analyzed in terms of bound magnetic polaron model and the typical polaron concentration at room temperature is found to be in the order of 10²¹ per m⁻³.     

Microstructure of pair centers of Cr3+-Cr2+ ions in the KZnF3 crystal

Studies involving the piezospectroscopy method have shown that the symmetry of the pair centers of Cr³⁺-Cr²⁺ ions in the KZnF₃ crystal is tetragonal. In this paper we develop a microscopic model of a pair center. We use the temperature dependence of the integrated intensity of the absorption line to find the effective hopping integral for an e _g electron, t _σσ=205 ± 10 cm⁻¹, and the polaron reduction factor, equal to 0.11. By analyzing the selection rules for exchange-induced electric dipole transitions under double-exchange conditions we identify all the absorption lines of Cr³⁺-Cr²⁺ pairs. Zh. éksp. Teor. Fiz. 114, 1421–1429 (October 1998)     

Spin Polarization of Fluorine Nuclei in the Si/CaF<Subscript>2</Subscript> Nanostructure Under Optical Excitation

The laws of fluorine nuclear spin polarization in the Si/CaF₂ nanostructure under optical excitation of the charge carriers in it has been considered theoretically. It has been shown that maximum values (up to 3% of the concentration of nuclei in the lattice) are attained under a high rate of optical excitation (>10⁻⁹ sec⁻¹) when the nuclear spin diffusion process and the Auger recombination prevail. In this case, the nuclear relaxation time in an individual layer reaches 100–300 sec and the spin diffusion radius decreases to 0.3 nm. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 397–403, May–June, 2005.