2,900 research outputs found
Diagrammatic perturbation theory and the pseudogap
We study a model of quasiparticles on a two-dimensional square lattice
coupled to Gaussian distributed dynamical fields. The model describes
quasiparticles coupled to spin or charge fluctuations and is solved by a Monte
Carlo sampling of the molecular field distributions. The non-perturbative
solution is compared to various approximations based on diagrammatic
perturbation theory. When the molecular field correlations are sufficiently
weak, the diagrammatic calculations capture the qualitative aspects of the
quasiparticle spectrum. For a range of model parameters near the magnetic
boundary, we find that the quasiparticle spectrum is qualitatively different
from that of a Fermi liquid in that it shows a double peak structure, and that
the diagrammatic approximations we consider fail to reproduce, even
qualitatively, the results of the Monte Carlo calculations. This suggests that
the pseudogap induced by a coupling to antiferromagnetic fluctuations and the
spin-splitting of the quasiparticle peak induced by a coupling to ferromagnetic
spin-fluctuations lie beyond diagrammatic perturbation theory
A factorization of a super-conformal map
A super-conformal map and a minimal surface are factored into a product of
two maps by modeling the Euclidean four-space and the complex Euclidean plane
on the set of all quaternions. One of these two maps is a holomorphic map or a
meromorphic map. These conformal maps adopt properties of a holomorphic
function or a meromorphic function. Analogs of the Liouville theorem, the
Schwarz lemma, the Schwarz-Pick theorem, the Weierstrass factorization theorem,
the Abel-Jacobi theorem, and a relation between zeros of a minimal surface and
branch points of a super-conformal map are obtained.Comment: 21 page
Can Frustration Preserve a Quasi-Two-Dimensional Spin Fluid?
Using spin-wave theory, we show that geometric frustration fails to preserve
a two-dimensional spin fluid. Even though frustration can remove the interlayer
coupling in the ground-state of a classical anti-ferromagnet, spin layers
innevitably develop a quantum-mechanical coupling via the mechanism of ``order
from disorder''. We show how the order from disorder coupling mechanism can be
viewed as a result of magnon pair tunneling, a process closely analogous to
pair tunneling in the Josephson effect. In the spin system, the Josephson
coupling manifests itself as a a biquadratic spin coupling between layers, and
for quantum spins, these coupling terms are as large as the inplane coupling.
An alternative mechanism for decoupling spin layers occurs in classical XY
models in which decoupled "sliding phases" of spin fluid can form in certain
finely tuned conditions. Unfortunately, these finely tuned situations appear
equally susceptible to the strong-coupling effects of quantum tunneling,
forcing us to conclude that in general, geometric frustration cannot preserve a
two-dimensional spin fluid.Comment: 12 pages, 3 figure
Superconductivity and Pseudogap in Quasi-Two-Dimensional Metals around the Antiferromagnetic Quantum Critical Point
Spin fluctuations (SF) and SF-mediated superconductivity (SC) in
quasi-two-dimensional metals around the antiferrromagnetic (AF) quantum
critical point (QCP) are investigated by using the self-consistent
renormalization theory for SF and the strong coupling theory for SC. We
introduce a parameter y0 as a measure for the distance from the AFQCP which is
approximately proportional to (x-xc), x being the electron (e) or hole (h)
doping concentration to the half-filled band and xc being the value at the
AFQCP. We present phase diagrams in the T-y0 plane including contour maps of
the AF correlation length and AF and SC transition temperatures TN and Tc,
respectively. The Tc curve is dome-shaped with a maximum at around the AFQCP.
The calculated one-electron spectral density shows a pseudogap in the
high-density-of-states region near (pi,0) below around a certain temperature T*
and gives a contour map at the Fermi energy reminiscent of the Fermi arc. These
results are discussed in comparison with e- and h-doped high-Tc cuprates.Comment: 5 pages, 3 figure
NMR as a probe of the relaxation of the magnetization in magnetic molecules
We investigate the time autocorrelation of the molecular magnetization
for three classes of magnetic molecules (antiferromagnetic rings, grids and
nanomagnets), in contact with the phonon heat bath. For all three classes, we
find that the exponential decay of the fluctuations of , associated with
the irreversible exchange of energy with the heat bath, is characterized by a
single characteristic time for not too high temperature and
field . This is reflected in a nearly single-lorentzian shape of the
spectral density of the fluctuations. We show that such fluctuations are
effectively probed by NMR, and that our theory explains the recent
phenomenological observation by Baek et al. (PRB70, 134434) that the
Larmor-frequency dependence of data in a large number of AFM rings fits
to a single-lorentzian form.Comment: Published as Phys. Rev. Letters 94, 077203 (2005) in slightly reduced
for
Antiferromagnetic metal to heavy-fermion metal quantum phase transition in the Kondo lattice model: A strong coupling approach
We study the quantum phase transition from an antiferromagnetic metal to a
heavy fermion metal in the Kondo lattice model. Based on the strong coupling
approach we {\it first} diagonalize the Kondo coupling term. Since this strong
coupling approach makes the resulting Kondo term {\it relevant}, the Kondo
hybridization persists even in the antiferromagnetic metal, indicating that
fluctuations of Kondo singlets are not critical in the phase transition. We
find that the quantum transition in our strong coupling approach results from
{\it softening of antiferromagnetic spin fluctuations of localized spins},
driven by the Kondo interaction. Thus, the volume change of Fermi surface
becomes continuous across the transition. .....
Nonlocal Landau theory of the magnetic phase diagram of highly frustrated magnetoelectric CuFeO
A nonlocal Landau-type free energy functional of the spin density is
developed to model the large variety of magnetic states which occur in the
magnetic field-temperature phase diagram of magnetoelectric CuFeO.
Competition among long-range quadratic exchange, biquadratic anti-symmetric
exchange, and trigonal anisotropy terms, consistent with the high-temperature
rhombohedral Rm crystal symmetry, are shown to all play important
roles in stabilizing the unusual combination of commensurate and incommensurate
spin structures in this highly frustrated triangular antiferromagnet. It is
argued that strong magnetoelastic coupling is largely responsible for the
nonlocal nature of the free energy. A key feature of the analysis is that an
electric polarization is induced by a canting of the non-collinear
incommensurate spin structure. Application of the model to ordered spin states
in the triangular antiferromagnets MnBr and NaFeO is also discussed.Comment: 20 pages, 2 figure
Injection locking of a low cost high power laser diode at 461 nm
Stable laser sources at 461 nm are important for optical cooling of strontium
atoms. In most existing experiments this wavelength is obtained by frequency
doubling infrared lasers, since blue laser diodes either have low power or
large emission bandwidths. Here, we show that injecting less than 10 mW of
monomode laser radiation into a blue multimode 500 mW high power laser diode is
capable of slaving at least 50% of the power to the desired frequency. We
verify the emission bandwidth reduction by saturation spectroscopy on a
strontium gas cell and by direct beating of the slave with the master laser. We
also demonstrate that the laser can efficiently be used within the Zeeman
slower for optical cooling of a strontium atomic beam.Comment: 2nd corrected version (minor revisions); Manuscript accepted for
publication in Review of Scientific Instruments; 5 pages, 6 figure
Origin of ferroelectricity in the multiferroic barium fluorides BaMF4
We present a first principles study of the series of multiferroic barium
fluorides with the composition BaMF4, where M is Mn, Fe, Co, or Ni. We discuss
trends in the structural, electronic, and magnetic properties, and we show that
the ferroelectricity in these systems results from the "freezing in" of a
single unstable polar phonon mode. In contrast to the case of the standard
perovskite ferroelectrics, this structural distortion is not accompanied by
charge transfer between cations and anions. Thus, the ferroelectric instability
in the multiferroic barium fluorides arises solely due to size effects and the
special geometrical constraints of the underlying crystal structure.Comment: 8 pages, 6 figures, 3 table
Infinite Layer LaNiO(2): Ni(1+)is not Cu(2+)
The Ni ion in LaNiO has the same formal ionic configuration as
does Cu in isostructural CaCuO, but it is reported to be nonmagnetic and
probably metallic whereas CaCuO is a magnetic insulator. From ab initio
calculations we trace its individualistic behavior to (1) reduced
mixing due to an increase of the separation of site energies () of at least 2 eV, and (2) important Ni mixing with
La states that leads to Fermi surface pockets of La
character that hole-dope the Ni 3d band.Correlation effects do not appear to be
large in LaNiO. However, ad hoc increase of the intraatomic repulsion on
the Ni site (using the LDA+U method) is found to lead to a novel correlated
state: (i) the transition metal and states undergo
consecutive Mott transitions, (ii) their moments are antialigned leading
(ideally) to a "singlet" ion in which there are two polarized orbitals, and
(iii) mixing of the upper Hubbard band with the La
states leaves considerable transition metal 3d character in a band pinned to
the Fermi level. The magnetic configuration is more indicative of a Ni
ion in this limit, although the actual charge changes little with U.Comment: 7 pages, 8 figure
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