We calculate spectra of magnetic excitations in the spin-spiral state of perovskite manganates. The spectra consist of several branches corresponding to different polarizations and different ways of diffraction from the static magnetic order. Goldsto
ne modes and opening of gaps at zero and non-zero energies due to the crystal field and the Dzyaloshinski-Moriya anisotropies are discussed. Comparing results of the calculation with available experimental data we determine values of effective exchange parameters and anisotropies. To simplify the spin-wave calculation and to get a more clear physical insight in the structure of excitations we use the {sigma}-model-like effective field theory to analyze the Heisenberg Hamiltonian and to derive the spectra.
We argue that the magnetic susceptibility data, Refs. 1-3, for the low-density two-dimensional (2D) silicon-based electron gas indicate that magnetically active electrons are localised in spin-droplets. The droplets exist in both the insulating and m
etallic phases, and interact ferromagnetically, forming an effective 2D Heisenberg ferromagnet. Comparing the data with known analytical and numerical results for a 2D Heisenberg ferromagnet, we determine that JS^2 approx 0.6K, where S is the spin of the droplet and J is the ferromagnetic exchange constant between droplets. We further argue that most likely S=1 with four electrons occupying each droplet on average. We discuss the dependence of the magnetic susceptibility and the specific heat on the external magnetic field, which follows from the model, and hence we suggest further experimental tests of the model.
The side-jump effect is a manifestation of the spin orbit interaction in electron scattering from an atom/ion/impurity. The effect has a broad interest because of its conceptual importance for generic spin-orbital physics, in particular the effect is
widely discussed in spintronics. We reexamine the effect accounting for the exact nonperturbative electron wave function inside the atomic core. We find that value of the effect is much smaller than estimates accepted in literature. The reduction factor is 1/Z^2, where Z is the nucleus charge of the atom/impurity. This implies that the side-jump effect is practically irrelevant for spintronics, the skew scattering and/or the intrinsic mechanism always dominate the anomalous Hall and spin Hall effects.
We propose that ordinary semiconductors with large spin-orbit coupling (SOC), such as GaAs, can host stable, robust, and {it tunable} topological states in the presence of quantum confinement and superimposed potentials with hexagonal symmetry. We sh
ow that the electronic gaps which support chiral spin edge states can be as large as the electronic bandwidth in the heterostructure miniband. The existing lithographic technology can produce a topological insulator (TI) operating at temperature $10- 100K$. Improvement of lithographic techniques will open way to tunable room temperature TI.
We consider a 3-dimensional quantum antiferromagnet which can be driven through a quantum critical point (QCP) by varying a tuning parameter g. Starting from the magnetically ordered phase, the N{e}el temperature will decrease to zero as the QCP is a
pproached. From a generic quantum field theory, together with numerical results from a specific microscopic Heisenberg spin model, we demonstrate the existence of universal behaviour near the QCP. We compare our results with available data for TlCuCl_3
At doping below 6% the bilayer cuprate YBa2Cu3O{6+y} is a collinear antiferromagnet. Independent of doping the value of the staggered magnetization at zero temperature is about 0.6mu_B. This is the maximum value of the magnetization allowed by quantu
m fluctuations of localized spins. In this low doping regime the compound is a normal conductor with a finite resistivity at zero temperature. These experimental observations create a unique opportunity for theory to perform a controlled calculation of the electron spectral function. In the present work we perform this calculation within the framework of the extended t-J model. As one expects the Fermi surface consists of small hole pockets centered at (pi/2,pi/2). The electron spectral function is very strongly anisotropic with maximum of intensity located at the inner parts of the pockets and with very small intensity at the outer parts. We also found that the antiferromagnetic correlations act against the bilayer bonding-antibonding splitting destroying it. The bilayer Fermi surface splitting is practically zero.
The present work addresses YBa$_{2}$Cu$_{3}$O$_{y}$ at doping below x=6% where the compound is a collinear antiferromagnet. In this region YBa$_{2}$Cu$_{3}$O$_{y}$ is a normal conductor with a finite resistivity at zero temperature. The value of the
staggered magnetization at zero temperature is 0.6mu_B, the maximum value allowed by spin quantum fluctuations. The staggered magnetization is almost independent of doping. On the other hand, the Neel temperature decays very quickly from T_N=420K at x=0 to practically zero at x = 0.06. The present paper explains these remarkable properties and demonstrates that the properties result from the physics of a lightly doped Mott insulator with small hole pockets. Nuclear quadrupole resonance data are also discussed. The data shed light on mechanisms of stability of the antiferromagnetic order at x < 6%.
We revisit the problem of a single hole moving in the background of the two dimensional Heisenberg antiferromagnet. The hole is loosely bound by an impurity potential. We show that the bound state is generically a parity doublet: there are parametric
ally close bound states of opposite parity. Due to the degeneracy the bound state readily breaks local symmetries of the square lattice and this leads to formation of the long range spiral distortion of the antiferromagnetic background. A direct analogy with van der Waals forces in atomic physics is discussed.
The role of Coulomb disorder, either of extrinsic origin or introduced by dopant ions in undoped and lightly-doped cuprates, is studied. We demonstrate that charged surface defects in an insulator lead to a Gaussian broadening of the Angle-Resolved P
hotoemisson Spectroscopy (ARPES) lines. The effect is due to the long-range nature of the Coulomb interaction. A tiny surface concentration of defects about a fraction of one per cent is sufficient to explain the line broadening observed in Sr$_2$CuO$_2$Cl$_2$, La$_2$CuO$_{4}$, and Ca$_{2}$CuO$_{2}$Cl$_{2}$. Due to the Coulomb screening, the ARPES spectra evolve dramatically with doping, changing their shape from a broad Gaussian form to narrow Lorentzian ones. To understand the screening mechanism and the lineshape evolution in detail, we perform Hartree-Fock simulations with random positions of surface defects and dopant ions. To check validity of the model we calculate the Nuclear Quadrupole Resonance (NQR) lineshapes as a function of doping and reproduce the experimentally observed NQR spectra. Our study also indicates opening of a substantial Coulomb gap at the chemical potential. For a surface CuO$_2$ layer the value of the gap is of the order of 10 meV while in the bulk it is reduced to the value about a few meV.
Due to the orthorhombic distortion of the lattice, the electronic hopping integrals along the $a$ and $b$ diagonals, the orthorhombic directions, are slightly different. We calculate their difference in the LDA and find $t_{a}^{prime}-t_{b}^{prime}ap
prox 8 $meV. We argue that electron correlations in the insulating phase of La$_{2-x}$Sr$_{x}$CuO$_{4}$, i. e. at doping $xleq 0.055,$ dramatically enhance the $(t_{a}^{prime}-t_{b}^{prime}) $-splitting between the $a$- and $b$-hole valleys. In particular, we predict that the intensity of both angle-resolved photoemission and of optical absorption is very different for the $a$ and $b$ nodal points.
