163 research outputs found
Stripes Disorder and Correlation lengths in doped antiferromagnets
For stripes in doped antiferromagnets, we find that the ratio of spin and
charge correlation lenghts, , provide a sharp criterion for
determining the dominant form of disorder in the system. If stripes disorder is
controlled by topological defects then . In contast,
if stripes correlations are disordered primarily by non-topological elastic
deformations (i.e., a Bragg-Glass type of disorder) then is expected. Therefore, the observation of in and in invariably implies that the stripes
are in a Bragg glass type state, and topological defects are much less relevant
than commonly assumed. Expected spectral properties are discussed. Thus, we
establish the basis for any theoretical analysis of the experimentally
obsereved glassy state in these material.Comment: 4 pages, 2 figure
Transitions from small to large Fermi momenta in a one-dimensional Kondo lattice model
We study a one-dimensional system that consists of an electron gas coupled to
a spin-1/2 chain by Kondo interaction away from half-filling. We show that
zero-temperature transitions between phases with "small" and "large" Fermi
momenta can be continuous. Such a continuous but Fermi-momentum-changing
transition arises in the presence of spin anisotropy, from a Luttinger liquid
with a small Fermi momentum to a Kondo-dimer phase with a large Fermi momentum.
We have also added a frustrating next-nearest-neighbor interaction in the spin
chain to show the possibility of a similar Fermi-momentum-changing transition,
between the Kondo phase and a spin-Peierls phase, in the spin isotropic case.
This transition, however, appears to involve a region in which the two phases
coexist.Comment: The updated version clarifies the definitions of small and large
Fermi momenta, the role of anisotropy, and how Kondo interaction affects
Luttinger liquid phase. 12 pages, 5 figure
Stripes: Why hole rich lines are antiphase domain walls?
For stripes of hole rich lines in doped antiferromagnets, we investigate the
competition between anti-phase and in-phase domain wall ground state
configurations. We argue that a phase transition must occure as a function of
the electron/hole filling fraction of the domain wall. Due to {\em transverse}
kinetic hole fluctuations, empty domain walls are always anti-phase. At
arbitrary electron filling fraction () of the domain wall (and in
particular for as in LaNdSrCuO), it is essential to
account also for the transverse magnetic interactions of the electrons and
their mobility {\em along} the domain wall.
We find that the transition from anti-phase to in-phase stripe domain wall
occurs at a critical filling fraction , for any value of
. We further use our model to estimate the spin-wave
velocity in a stripe system. Finally, relate the results of our microscopic
model to previous Landau theory approach to stripes.Comment: 11 pages, 3 figure
Spin and charge order in the vortex lattice of the cuprates: experiment and theory
I summarize recent results, obtained with E. Demler, K. Park, A. Polkovnikov,
M. Vojta, and Y. Zhang, on spin and charge correlations near a magnetic quantum
phase transition in the cuprates. STM experiments on slightly overdoped BSCCO
(J.E. Hoffman et al., Science 295, 466 (2002)) are consistent with the
nucleation of static charge order coexisting with dynamic spin correlations
around vortices, and neutron scattering experiments have measured the magnetic
field dependence of static spin order in the underdoped regime in LSCO (B. Lake
et al., Nature 415, 299 (2002)) and LaCuO_4+y (B. Khaykovich et al., Phys. Rev.
B 66, 014528 (2002)). Our predictions provide a semi-quantitative description
of these observations, with only a single parameter measuring distance from the
quantum critical point changing with doping level. These results suggest that a
common theory of competing spin, charge and superconducting orders provides a
unified description of all the cuprates.Comment: 18 pages, 7 figures; Proceedings of the Mexican Meeting on
Mathematical and Experimental Physics, Mexico City, September 2001, to be
published by Kluwer Academic/Plenum Press; (v2) added clarifications and
updated reference
Local Moments in an Interacting Environment
We discuss how local moment physics is modified by the presence of
interactions in the conduction sea. Interactions in the conduction sea are
shown to open up new symmetry channels for the exchange of spin with the
localized moment. We illustrate this conclusion in the strong-coupling limit by
carrying out a Schrieffer Wolff transformation for a local moment in an
interacting electron sea, and show that these corrections become very severe in
the approach to a Mott transition. As an example, we show how the Zhang Rice
reduction of a two-band model is modified by these new effects.Comment: Latex file with two postscript figures. Revised version, with more
fully detailed calculation
Topological Excitations of One-Dimensional Correlated Electron Systems
Properties of low-energy excitations in one-dimensional superconductors and
density-wave systems are examined by the bosonization technique. In addition to
the usual spin and charge quantum numbers, a new, independently measurable
attribute is introduced to describe elementary, low-energy excitations. It can
be defined as a number w which determines, in multiple of , how many times
the phase of the order parameter winds as an excitation is transposed from far
left to far right. The winding number is zero for electrons and holes with
conventional quantum numbers, but it acquires a nontrivial value w=1 for
neutral spin-1/2 excitations and for spinless excitations with a unit electron
charge. It may even be irrational, if the charge is irrational. Thus, these
excitations are topological, and they can be viewed as composite particles made
of spin or charge degrees of freedom and dressed by kinks in the order
parameter.Comment: 5 pages. And we are not only splitting point
Localized charged states and phase separation near second order phase transition
Localized charged states and phase segregation are described in the framework
of the phenomenological Ginzburg-Landau theory of phase transitions. The
Coulomb interactions determines the charge distribution and the characteristic
length of the phase separated states. The phase separation with charge
segregation becomes possible because of the large dielectric constant and the
small density of extra charge in the range of charge localization. The phase
diagram is calculated and the energy gain of the phase separated state is
estimated. The role of the Coulomb interaction is elucidated
Relationship between incommensurability and superconductivity in Peierls distorted charge-density-wave systems
We study the pairing potential induced by fluctuations around a
charge-density wave (CDW) with scattering vector Q by means of the Froehlich
transformation. For general commensurability M, defined as |k+M*Q>=|k>, we find
that the intraband pair scattering within the M subbands scales with M whereas
the interband pair scattering becomes suppressed with increasing CDW order
parameter. As a consequence superconductivity is suppressed when the Fermi
energy is located between the subbands as it is usually the case for nesting
induced CDW's, but due to the vertex renormalization it can be substantially
enhanced when the chemical potential is shifted sufficiently inside one of the
subbands. The model can help to understand the experimentally observed
dependence of the superconducting transition temperature from the stripe phase
incommensurability in the lanthanum cuprates.Comment: 6 pages, 3 figure
Aging in a Two-Dimensional Ising Model with Dipolar Interactions
Aging in a two-dimensional Ising spin model with both ferromagnetic exchange
and antiferromagnetic dipolar interactions is established and investigated via
Monte Carlo simulations. The behaviour of the autocorrelation function
is analyzed for different values of the temperature, the waiting
time and the quotient , and being the
strength of exchange and dipolar interactions respectively. Different
behaviours are encountered for at low temperatures as is
varied. Our results show that, depending on the value of , the dynamics
of this non-disordered model is consistent either with a slow domain dynamics
characteristic of ferromagnets or with an activated scenario, like that
proposed for spin glasses.Comment: 4 pages, RevTex, 5 postscript figures; acknowledgment added and some
grammatical corrections in caption
- …