267 research outputs found
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-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 (existence of a small interaction stabilizing
the long range order at 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 . 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 within SWA, acquires new terms proportional to
and . 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 rather than ,
where 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
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 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 of localized orbitals
with a conduction-electron band in the 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 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 within
the t-J model is developed. An exact representation for 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
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