Polarons in quantum chains with XY exchange and Dzyaloshinsky-Moriya interaction

Excitations of the polaron types are investigated in the spin-1/2 quantum chain with XY exchange and Dzyaloshinsky-Moriya interaction, both coupled to acoustic vibrations of the substrate lattice. The study is carried out via Jordan-Wigner transformation with the help of which the spin chain is mapped onto a chain of spinless fermions. From the resulting effective fermion-lattice Hamiltonian, the discrete equations of motion are derived. These equations are solved in the continuum limit for self-trapped states near the bottom of the fermion spectrum interacting with long-wavelength acoustic lattice modes. The associate polaron solution, which has a pulse shape, is shown to propagate bound to the induced lattice kink distortion by translation along the chain at a constant velocity v. The pair can also experience an additional acceleration ϑ₀ when the free fermion charge is excited above its groundstate. The polaron binding energy is strongly reduced, depending quadratically on the ratio D/J of the Dzyaloshinsky-Moriya interaction strength D to the isotropic XY exchange interaction J. It is also found that polaron parameters depend only on the XY spin-lattice coupling but not on the Dzyaloshinsky-Moriya contribution.     

Antiferromagnetic exchange mechanism of superconductivity in cuprates

The theory of superconducting pairing due to antiferromagnetic exchange is considered. The strong dependence of the superconducting transition temperature T _c on the lattice constant a observed recently in mercury superconductors is explained within the framework of this theory. Calculations have been performed based on the two-band p-d Hubbard model in the strong correlation limit. The large excitation energy Δ_pd for the antiferromagnetic exchange of two particles from different Hubbard subbands results in the suppression of the retardation effects and in the pairing of all the particles in the conduction subband with the Fermi energy E _F ?Δ_pd:T _c ?E _F exp(?1/λ), where λ∝J. The dependence T _c (a) and the isotope effect are explained by the dependence of the exchange interaction J on a and on zero-point vibrations of oxygen ions.     

On the importance of anisotropic exchange coupling on to the structure of spin-polarized clusters in layered cuprates

The low energy spectrum of spin-polarized clusters in layered cuprates is examined. Two types of clusters confining the motion of oxygen holes are discussed. The ground state of the pentanuclear copper cluster has a total spin S = 2, whereas the one of the tetranuclear copper cluster is characterised by S= 3/2. Anisotropic exchange coupling splits the magnetic ground state into a sequence of state, which in a natural way provide e.g. an explanation of EPR data for La₂CuO_4+δ compounds.     

Isotopic fingerprint of electron-phonon coupling in high-Tc cuprates

Angle-resolved photoemission spectroscopy with low-energy tunable photons along the nodal direction of oxygen isotope substituted Bi(2)Sr(2)CaCu(2)O(8+delta) reveals a distinct oxygen isotope shift near the electron-boson coupling "kink" in the electronic dispersion. The magnitude (a few meV) and direction of the kink shift are as expected due to the measured isotopic shift of phonon frequency, and are also in agreement with theoretical expectations. This demonstrates the participation of the phonons as dominant players, as well as pinpointing the most relevant of the phonon branches.     

Pseudogap phase in cuprates: Oxygen orbital moments instead of circulating currents

Circulating current loops within the cuprate unit cell are proposed to play a key role in the physics of the pseudogap phase. However, main experimental observations motivated by this sophisticated proposal and seemingly supporting the circulating current model can be explained within a simple and physically clear microscopic model. It has been argued that, instead of a well-isolated Zhang-Rice (ZR) singlet ¹ A _1g , the ground state of the hole center [CuO₄]^5? (cluster analog of Cu³⁺ ion) in cuprates should be described by a complex ¹ A _1g -^1,3 B _2g -^1,3 E _u multiplet, formed by a competition of conventional hybrid Cu 3d-O 2p $b_{1g} (\sigma ) \propto d_{x^2 - y^2 }$ state and purely oxygen nonbonding O 2p?? states with a _2g (??) and e _{ux, y} (??) symmetry. In contrast to inactive ZR singlet we arrive at several novel competing orbital and spin-orbital order parameters, e.g., Ising-like net orbital magnetic moment, orbital toroidal moment, intra-plaquette??s staggered order of Ising-like oxygen orbital magnetic moments. As a most impressive validation of the non-ZR model we explain fascinating results of recent neutron scattering measurements that revealed novel type of magnetic ordering in pseudogap phase of several hole-doped cuprates.     

Interlayer exchange coupling: Preasymptotic corrections

In the asymptotic limit, the interlayer exchange coupling decays as D^-2, where D is the spacer thickness. A systematic procedure for calculating the preasymptotic corrections, i.e., the terms of order D^-n with ,is presented. The temperature dependence of the preasymptotic corrections is calculated. The results are used to discuss the preasymptotic corrections for the Co/Cu/Co(001) system. Received 7 January 1999     

Why standard estimates of electron-phonon coupling in cuprates do not work

The problem of estimating the coupling strength between oxygen-breathing phonons and electrons in cuprates is discussed. Some of these modes are found in experiments to be strongly coupled, in particular at low doping concentrations. Standard tools, like local density approximation, give a too small coupling. Many-body techniques compare instead much better with experiments. This suggests that electronic correlation effects play a crucial role in the estimate of the electron-phonon coupling from first principles.     

Saturation field of frustrated chain cuprates: broad regions of predominant interchain coupling

A thermodynamic method to extract the interchain coupling (IC) of spatially anisotropic 2D or 3D spin-1/2 systems from their empirical saturation field H(s) (T=0) is proposed. Using modern theoretical methods we study how H(s) is affected by an antiferromagnetic (AFM) IC between frustrated chains described in the J(1)-J(2)-spin model with ferromagnetic 1st and AFM 2nd neighbor in-chain exchange. A complex 3D-phase diagram has been found. For Li(2)CuO(2) and Ca(2)Y(2)Cu(5)O(10), we show that H(s) is solely determined by the IC and predict H(s)≈61 T for the latter. With H(s)≈55 T from magnetization data one reads out a weak IC for Li(2)CuO(2) close to that obtained from inelastic neutron scattering.     

Two-sublattice uniaxial systems with strong antisymmetric exchange: A model for some hexagonal ferrites

We solve rigorously the equilibrium equations for a system of two classical magnetic moments in a transverse field, coupled by Dzialoshinskii antisymmetric exchange, as well as isotropic and anisotropic exchange, in the presence of uniaxial second order anisotropy, whose easy axis is parallel to the Dzialoshinskii D vector. The system allows for planar, axial and conical phases (the latter were never reported before). In all the possible phases the equilibrium equations allow for an exact solution, without introducing any approximation steaming from the assumption of some definite hierarchy of the interaction strengths. We discuss the application of this model to the low field-low temperature transverse magnetization curves reported for some diamagnetically substituted hexagonal ferrites, paying special attention to the conical phases.     

Intermediate coupling model of cuprates: Adding fluctuations to a weak coupling model of pseudogap and superconductivity competition

We demonstrate that many features ascribed to strong correlation effects in various spectroscopies of the cuprates are captured by a calculation of the self-energy incorporating effects of spin and charge fluctuations. The self-energy is calculated over the full doping range from half-filling to the overdoped system. In the normal state, the spectral function reveals four subbands: two widely split incoherent bands representing the remnant of the two Hubbard bands, and two additional coherent, spin- and charge-dressed in-gap bands split by a spin-density wave, which collapses in the overdoped regime. The resulting coherent subbands closely resemble our earlier mean-field results. Here we present an overview of the combined results of our mean-field calculations and the newer extensions into the intermediate coupling regime.     

The incommensurate magnetic structure of a tetragonal antiferromagnet with antisymmetric exchange

Analysis of the incommensurate magnetic structure that emerges for two coexisting types of the antisymmetric Dzyaloshinski-Moriya exchange interaction (the weakly ferromagnetic component of vector D along the tetragonal axis and the helicoidal component distributed in the tetragonal plane) is carried out for the first time for a tetragonal antiferromagnet. The helicoidal component for each pair of interacting spins has a 2D distribution; its direction in the tetragonal plane depends on the direction of the exchange bond in each pair. The Lifshits invariant of the Ginzburg-Landau functional is obtained, which is responsible for the formation of an incommensurate magnetic structure for such a distribution. It is shown in the mean field approximation that the incommensurate magnetic structure that forms in this case is a nonlinear double helicoid with a modulation vector lying in the tetragonal plane and with a varying angle between the polarization planes of quasi-antiferromagnetic sublattices. The ground state of the magnet is degenerate in the orientation of the modulation vector in the tetragonal plane. The rate of variation in the orientations of moments in the polarization planes passing through the tetragonal axis is controlled by the angle between the directions of the moments and the tetragonal axis. The local weakly ferromagnetic moment remaining in the polarization plane varies in magnitude and sign. The relation between the orientations of the modulation and polarization vectors is derived for the cases of simple and inversion tetragonal axes in the space symmetry group of the crystal.     

Ground state of an antiferromagnet with uniform antisymmetric exchange in a magnetic field

A system of two nonlinear differential equations for sublattice angles is proposed to describe the spin orientation distribution in a planar antiferromagnet with uniform antisymmetric exchange in a magnetic field. This system involves the initial symmetry of the problem and is reduced to a single delay differential equation. The solutions of this system are parameterized by the initial condition imposed on the angle of one sublattice at the hyperbolic singular point of the phase space. The numerical analysis of the stability boundary of soliton solutions demonstrates that the transition to the commensurate phase takes place outside the region where the stochastic solutions appear and is accompanied by the magnetization jump Δm ∼ 10⁻¹ m.     

ab plane ac conductivity in the cuprates: pseudogap effects below Tc

Building on our understanding of the superfluid density rho(s)(T), we show how the pseudogap enters the in-plane optical conductivity sigma(omega,T) for temperatures T相似文献   

Dynamical Dzyaloshinsky-Moriya interaction in KCuF3

The spin dynamics of the prototypical quasi-one-dimensional antiferromagnetic Heisenberg spin S=1/2 chain KCuF3 is investigated by electron spin resonance spectroscopy. Our analysis shows that the peculiarities of the spin dynamics require a new dynamical form of the antisymmetric anisotropic spin-spin interaction. This dynamical Dzyaloshinsky-Moriya interaction is related to strong oscillations of the bridging fluorine ions perpendicular to the crystallographic c axis. This new mechanism allows us to resolve consistently the controversies in observation of the magnetic and structural properties of this orbitally ordered perovskite compound.     

Dynamic exchange coupling in magnetic bilayers

A long-range dynamic interaction between ferromagnetic films separated by normal-metal spacers is reported, which is communicated by nonequilibrium spin currents. It is measured by ferromagnetic resonance and explained by an adiabatic spin-pump theory. In such a resonance the spin-pump mechanism of spatially separated magnetic moments leads to an appreciable increase in the resonant linewidth when the resonance fields are well apart, and results in a dramatic linewidth narrowing when the resonant fields approach each other.     

Antiferromagnetic s-d exchange coupling in GaMnAs

Measurements of coherent electron spin dynamics in Ga1-xMnxAs/Al0.4Ga0.6As quantum wells with 0.0006%相似文献