Self-compensation in manganese-doped ferromagnetic semiconductors

We present a theory of interstitial Mn in Mn-doped ferromagnetic semiconductors. Using density-functional theory, we show that under the non-equilibrium conditions of growth, interstitial Mn is easily formed near the surface by a simple low-energy adsorption pathway. In GaAs, isolated interstitial Mn is an electron donor, each compensating two substitutional Mn acceptors. Within an impurity-band model, partial compensation promotes ferromagnetic order below the metal-insulator transition, with the highest Curie temperature occurring for 0.5 holes per substitutional Mn.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let

Exact Ground States of the Periodic Anderson Model in D=3 Dimensions

We construct a class of exact ground states of three-dimensional periodic Anderson models (PAMs) -- including the conventional PAM -- on regular Bravais lattices at and above 3/4 filling, and discuss their physical properties. In general, the f electrons can have a (weak) dispersion, and the hopping and the non-local hybridization of the d and f electrons extend over the unit cell. The construction is performed in two steps. First the Hamiltonian is cast into positive semi-definite form using composite operators in combination with coupled non-linear matching conditions. This may be achieved in several ways, thus leading to solutions in different regions of the phase diagram. In a second step, a non-local product wave function in position space is constructed which allows one to identify various stability regions corresponding to insulating and conducting states. The compressibility of the insulating state is shown to diverge at the boundary of its stability regime. The metallic phase is a non-Fermi liquid with one dispersing and one flat band. This state is also an exact ground state of the conventional PAM and has the following properties: (i) it is non-magnetic with spin-spin correlations disappearing in the thermodynamic limit, (ii) density-density correlations are short-ranged, and (iii) the momentum distributions of the interacting electrons are analytic functions, i.e., have no discontinuities even in their derivatives. The stability regions of the ground states extend through a large region of parameter space, e.g., from weak to strong on-site interaction U. Exact itinerant, ferromagnetic ground states are found at and below 1/4 filling.Comment: 47 pages, 10 eps figure

Transverse instabilities of multiple vortex chains in superconductor-ferromagnet bilayers

Using scanning tunneling microscopy and Ginzburg-Landau simulations we explore vortex configurations in magnetically coupled NbSe$_2$-Permalloy superconductor-ferromagnet bilayer. The Permalloy film with stripe domain structure induces periodic local magnetic induction in the superconductor creating a series of pinning-antipinning channels for externally added magnetic flux quanta. Such laterally confined Abrikosov vortices form quasi-1D arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. At high fields we show that the system becomes non-linear due to a change in both the number of vortices and the confining potential. The longitudinal instabilities of the resulting vortex structures lead to vortices `levitating' in the anti-pinning channels.Comment: accepted in PRB-Rapid

Frustrated impurity spins in ordered two-dimensional quantum antiferromagnets

Dynamical properties of an impurity spin coupled symmetrically to sublattices of ordered 2D Heisenberg quantum antiferromagnet (i.e., frustrated impurity spin) are discussed at $T\ge0$ (existence of a small interaction stabilizing the long range order at $T\ne0$ is implied). We continue our study on this subject started in Phys.Rev.B 72, 174419 (2005), where spin-1/2 defect is discussed and the host spins fluctuations are considered within the spin-wave approximation (SWA). In the present paper we i) go beyond SWA and ii) study impurities with spins $S\ge1/2$. It is demonstrated that in contrast to defects coupled to sublattices asymmetrically longitudinal host spins fluctuations play important role in the frustrated impurity dynamics. The spectral function, that is proportional to $\omega^2$ within SWA, acquires new terms proportional to $\omega^2$ and $\omega T^2$. It is observed that the spin-1/2 impurity susceptibility has the same structure as that obtained within SWA: the Lorenz peak and the non-resonant term. The difference is that the width of the peak becomes larger being proportional to $f^2(T/J)^3$ rather than $f^4(T/J)^3$, where $f$ is the dimensionless coupling parameter. We show that transverse static susceptibility acquires a new negative logarithmic contribution. In accordance with previous works we find that host spins fluctuations lead to an effective one-ion anisotropy on the impurity site. Then defects with $S>1/2$ appears to be split. We observe strong reduction of the value of the splitting due to longitudinal host spins fluctuations. We demonstrate that the dynamical impurity susceptibility contains $2S$ Lorenz peaks corresponding to transitions between the levels, and the non-resonant term.Comment: 17 pages, 7 figures, to appear in PR

Wick's Theorem and a New Perturbation Theory Around the Atomic Limit of Strongly Correlated Electron Systems

A new type of perturbation expansion in the mixing $V$ of localized orbitals with a conduction-electron band in the $U\to\infty$ Anderson model is presented. It is built on Feynman diagrams obeying standard rules. The local correlations of the unperturbed system (the atomic limit) are included exactly, no auxiliary particles are introduced. As a test, an infinite-order ladder-type resummation is analytically treated in the Kondo regime, recovering the correct energy scale. An extension to the Anderson-lattice model is obtained via an effective-site approximation through a cumulant expansion in $V$ on the lattice. Relation to treatments in infinite spatial dimensions are indicated.Comment: selfextracting postscript file containing entire paper (10 pages) including 3 figures, in case of trouble contact author for LaTeX-source or hard copies (prep0994

Optical and dc conductivities of cuprates: Spin-fluctuation scattering in the t-J model

A microscopic theory of the electrical conductivity $\sigma(\omega)$ within the t-J model is developed. An exact representation for $\sigma(\omega)$ is obtained using the memory-function technique for the relaxation function in terms of the Hubbard operators, and the generalized Drude law is derived. The relaxation rate due to the decay of charge excitations into particle-hole pairs assisted by antiferromagnetic spin fluctuations is calculated in the mode-coupling approximation. Using results for the spectral function of spin excitations calculated previously, the relaxation rate and the optical and dc conductivities are calculated in a broad region of doping and temperatures. The reasonable agreement of the theory with experimental data for cuprates proves the important role of spin-fluctuation scattering in the charge dynamics.Comment: 13 pages,15 figures, v.2, publication referenc

On the theory of pseudogap anisotropy in the cuprate superconductors

We show by means of the theory of the order parameter phase fluctuations that the temperature of "closing" (or "opening") of the gap (and pseudogap) in the electron spectra of superconductors with anisotropic order parameter takes place within a finite temperature range. Every Fourier-component of the order parameter has its own critical temperature

Disentanglement of the electronic and lattice parts of the order parameter in a 1D Charge Density Wave system probed by femtosecond spectroscopy

We report on the high resolution studies of the temperature (T) dependence of the q=0 phonon spectrum in the quasi one-dimensional charge density wave (CDW) compound K0.3MoO3 utilizing time-resolved optical spectroscopy. Numerous modes that appear below Tc show pronounced T-dependences of their amplitudes, frequencies and dampings. Utilizing the time-dependent Ginzburg-Landau theory we show that these modes result from linear coupling of the electronic part of the order parameter to the 2kF phonons, while the (electronic) CDW amplitude mode is overdamped.Comment: 4 pages, 3 figures + supplementary material, accepted for publication in Phys. Rev. Let

Spiral magnetic structure in spin-5/2 frustrated trimerized chains in SrMn3P4O14

We study a spin-5/2 antiferromagnetic trimerized chain substance SrMn3P4O14 using neutron powder diffraction experiments. The coplanar spiral magnetic structure appears below T_N1 = 2.2(1) K. Values of several magnetic structure parameters change rapidly at T_N2 = 1.75(5) K, indicating another phase transition, although the magnetic structures above and below T_N2 are the qualitatively same. The spiral magnetic structure can be explained by frustration between nearest-neighbor and next-nearest-neighbor exchange interactions in the trimerized chains.Comment: submitted to Phys. Rev.

Conductance through a potential barrier embedded in a Luttinger liquid: nonuniversal scaling at strong coupling

We calculate the linear response conductance of electrons in a Luttinger liquid with arbitrary interaction g_2, and subject to a potential barrier of arbitrary strength, as a function of temperature. We map the Hamiltonian in the basis of scattering states into an effective low energy Hamiltonian in current algebra form. First the renormalization group (RG) equation for weak interaction is derived in the current operator language both using the operator product expansion and the equation of motion method. To access the strong coupling regime, two methods of deducing the RG equation from perturbation theory, based on the scaling hypothesis and on the Callan-Symanzik formulation, are discussed. The important role of scale independent terms is emphasized. The latter depend on the regulaization scheme used (length versus temperature cutoff). Analyzing the perturbation theory in the fermionic representation, the diagrams contributing to the renormalization group beta-function are identified. A universal part of the beta-function is given by a ladder series and summed to all orders in g_2. First non-universal corrections beyond the ladder series are discussed and are shown to differ from the exact solutions obtained within conformal field theory which use a different regularization scheme. The RG equation for the temperature dependent conductance is solved analytically. Our result agrees with known limiting cases.Comment: 24 pages, 5 figures, REVTEX, final versio