Formation of a nuclear spin polaron under optical orientation in GaAs-type semiconductors

A theory is developed for the formation of a nuclear spin polaron under optical cooling of nuclear spins in the vicinity of donor centers. It is shown that the polaron does not form above a certain limiting nuclear-spin temperature. For a shallow donor in GaAs, this temperature is about 10⁻⁷ K. The formation of a nuclear spin polaron should manifest itself in an anomalous increase of the spin relaxation time of the total spin of its component nuclei. Fiz. Tverd. Tela (St. Petersburg) 40, 1018–1021 (June 1998)     

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.     

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     

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     

Luminescence polarization and spontaneous lowering of symmetry caused by magnetic-polaron formation in semimagnetic-semiconductor quantum wells

An experimental and theoretical study of the magnetic polaron states of two-dimensional excitons in quantum wells based on semimagnetic semiconductors (Cd,Mn)Te is reported. It is shown that magnetic-polaron formation in in-plane magnetic fields leads to a lowering of the system symmetry, provided the fields are not too strong. The magnetic moment of the polaron thus formed is not parallel to the external magnetic field and contains a component normal to the quantum-well plane. This spontaneous lowering of the symmetry results in a change of the polarization characteristics of the luminescence from magnetic polaron states and in a weakening (compared to the three-dimensional case) in the efficiency of magnetic field-induced polaron suppression. Fiz. Tverd. Tela (St. Petersburg) 39, 2079–2084 (November 1997)     

Unitary transformation treatment of a single hole in an Ising antiferromagnet

The behavior of a single hole in a two-dimensional Ising antiferromagnet (t-J ^z model), is studied in the generalized Dyson-Maleev representation, where the spins are mapped on boson operators and the hole is described as a spinless fermion. The formal similarity with Fröhlich's polaron Hamiltonian suggests that thet-J ^z model can be approximately diagonalized by means of two successive unitary transformations, analogous to those used by Lee, Low, and Pines in their intermediate-coupling treatment of the polaron. Our approach yields an upper bound to the exact ground state energy, as well as the corresponding ground state eigenvector. Fork=0 our energy bound is remarkably close to the result of the self-consistent Born approximation over a wide range of the coupling parameter, which includes the range typically assumed for the high-T _c materials. The ground state eigenvector is used to calculate the spatial distribution of bosons (spin deviations) surrounding the hole. Here our results are qualitatively very similar to those obtained in previous work, showing that our ground state eigenvector accounts quite well for the small size of the “spin polaron” in thet-J ^z model.     

Magnetic polarons in semimagnetic-semiconductor-based heterostructures

Specific features in the behavior of localized magnetic polarons formed under optical excitation in heterostructures based on semimagnetic semiconductors are considered. These features are due to the strong anisotropy of the hole g factor in low-dimensional systems based on zincblende crystals. The anisotropy is due to the strong spin-orbit coupling in the valence band which, in quantum confinement conditions, results in quadrupole splitting of the hole spin levels. The g factor anisotropy manifests itself in a strong anisotropy of the magnetic and magneto-optical characteristics of localized magnetic polarons. Fiz. Tverd. Tela (St. Petersburg) 40, 800–802 (May 1998)     

Optical orientation of particles with a random g-factor in semiconductor nanostructures

In real quasi-two-dimensional semiconductor nanostructures (quantum wells, quantum dots), the transverse g-factor of holes is a stochastic quantity. This fact should be taken into account in analyzing the optical orientation and Hanle effect of holes. The Hall effect for an ensemble of particles with a “random” g-factor has been treated theoretically. In the case where the spin relaxation time of a hole with a characteristic g-factor is shorter than the hole lifetime, there can occur a narrowing of the depolarization contour and an increase in its amplitude. In the opposite case of long spin relaxation times (trions in quantum dots), a formula has been derived, which generalizes the previously obtained result to the case of an arbitrary tilt angle of the magnetic field with respect to the plane of the layer (Hanle effect in the tilted form).     

The mass of a large polaron

We show that, beginning with the works of L. D. Landau and S. I. Pekar, the effective mass of a large polaron has been determined with a crucial error. Since all such research ignored the spatial dispersion of the lattice polarizability, the maximum group velocity of phonons is found to be zero, so that the phonon “cloud” of a polaron is unable to follow the polaron. We allow for the spatial dispersion of the lattice polarizability and derive an expression for the effective polaron mass valid over the entire velocity range in which a polaron can exist: from zero to the maximum group velocity of phonons. According to this expression, the polaron mass depends not only on the phonon frequency, reciprocal effective dielectric constant, and the carrier mass but also on the maximum group velocity of phonons interacting with the carrier and on the polaron velocity. Zh. éksp. Teor. Fiz. 112, 278–283 (July 1997)     

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)     

A spin polaron in a two-dimensional antiferromagnet: From a local singlet to a complex quasiparticle

We set forth basic theoretical ideas concerning the spin-polaron scenario for charge excitations in a two-dimensional antiferromagnet. A distinctive feature of the approach being developed consists in considering a local polaron (rather than a bare hole) as a zero approximation for the quasiparticle. At the next step, this excitation is dressed in antiferromagnetic spin waves to form a polaron of intermediate (or infinite) radius. The method allows us to continuously describe the transition from zero to finite temperatures and to consider a wide doping range. Our approach accounts for the main results of ARPES experiments in a CuO₂ plane.     

Two band model for superconducting copper oxides

Assuming that predominant charge carriers in superconducting cooper oxides are holes on O sites, we derive a microscopic model representing a coupled system of spin degrees of freedom on Cu sites and a low concentration of mobile holes. The analysis of a single magnetic polaron shows a large enhancement of the hole effective mass and a strong sensitivity of the antiferromagnetic ground state on doping. Possible mechanisms for the hole pairing are also discussed.     

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.     

Oscillations of the thermodynamic properties of a one-dimensional mesoscopic ring caused by Zeeman splitting

It is shown that the interrelationship of spin splitting in a magnetic field and spatial quantization in a one-dimensional ballistic ring coupled with a reservoir results in mesoscopic oscillations of a new type which vanish with increasing temperature. The period of such oscillations is inversely proportional to the density of states in the spin subsystem in the ring. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 595–600 (10 November 1999)     

Cooper instability of nonlocal spin polarons in the CuO<Subscript>2</Subscript> plane of high-temperature superconductors

The energy structure of nonlocal spin polarons has been obtained for the real structure of the CuO₂ plane of cuprate superconductors in the ensemble of such Fermi quasiparticles. A nonlocal spin polaron is formed due to the exchange interaction of the spin of an oxygen hole with the spins of the two nearest copper ions. The scattering amplitude of nonlocal spin polarons in the cooper channel calculated using the diagrammatic technique indicates that the spin and charge degrees of freedom are strongly correlated.     

Equations of motion of a spinning relativistic particle in external fields

We consider the motion of a spinning relativistic particle in external electromagnetic and gravitational fields to first order in the external field but to arbitrary order in the spin. The influence of the spin on the particle trajectory is properly accounted for by describing the spin noncovariantly. Specific calculations are performed through second order in the spin. A simple derivation is presented for the gravitational spin-orbit and spin-spin interactions of a relativistic particle. We discuss the gravimagnetic moment (GM), a particular spin effect in general relativity. We show that for a Kerr black hole the gravimagnetic ratio, i.e., the coefficient of the GM, equals unity (just as the gyromagnetic ratio equals 2 for a charged Kerr hole). The equations of motion obtained for a spinning relativistic particle in an external gravitational field differ substantially from the Papapetrou equations. Zh. éksp. Teor. Fiz. 113, 1537–1557 (May 1998)     

μSR on an induced‐moment spin glass system: Hg1-xFexSe

Zero and longitudinal field μSR have been used to examine the singlet ground state magnetism of the diluted magnetic semiconductor Hg_1-xFe_xSe. For x\geq 0.05\ μSR experiments indicate spin glass behaviour which is limited to sample regions with locally enhanced Fe²⁺ content. Longitudinal field spectra in the glassy state reveal a static local field \sim 50 mT due to induced moments of Fe²⁺. These are proposed to originate from a distribution of bound magnetic polaron energies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Superexchange pairing and magnetic ordering in cuprate superconductors

The first analytic study of superexchange ion interaction in the cuprate layer of high-T _c superconductors is reported. It is shown that the superexchange nonadditive contributions are dominant in the onset of long-range magnetic order in a system, as well as are responsible for hole pairing to produce an energetically preferable, stable spin configuration. The Heisenberg parameter and the pair binding energy obtained ab initio are in a good quantitative agreement with experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 2127–2131 (December 1999)