5,745 research outputs found
The two dimensional Antiferromagnetic Heisenberg model with next nearest neighbour Ising exchange
We have considered the antiferromagnetic Heisenberg model in two
dimensions, with an additional Ising \nnn interaction. Antiferromagnetic \nnn
interactions will lead to frustration, and the system responds with flipping
the spins down in the plane. For large next nearest neighbour coupling the
system will order in a striped phase along the z axis, this phase is reached
through a first order transition. We have considered two generalizations of
this model, one with random \nnn interactions, and one with an enlarged unit
cell, where only half of the atoms have \nnn interactions. In both cases the
transition is softened to a second order transition separating two ordered
states. In the latter case we have estimated the quantum critical exponent
. These two cases then represent candidate examples of
deconfined quantum criticality.Comment: Extensive revisions. Two new models with contious quantum phase
transitio
Spin dynamics across the superfluid-insulator transition of spinful bosons
Bosons with non-zero spin exhibit a rich variety of superfluid and insulating
phases. Most phases support coherent spin oscillations, which have been the
focus of numerous recent experiments. These spin oscillations are Rabi
oscillations between discrete levels deep in the insulator, while deep in the
superfluid they can be oscillations in the orientation of a spinful condensate.
We describe the evolution of spin oscillations across the superfluid-insulator
quantum phase transition. For transitions with an order parameter carrying
spin, the damping of such oscillations is determined by the scaling dimension
of the composite spin operator. For transitions with a spinless order parameter
and gapped spin excitations, we demonstrate that the damping is determined by
an associated quantum impurity problem of a localized spin excitation
interacting with the bulk critical modes. We present a renormalization group
analysis of the quantum impurity problem, and discuss the relationship of our
results to experiments on ultracold atoms in optical lattices.Comment: 43 pages (single-column format), 8 figures; v2: corrected discussion
of fixed points in Section V
Holographic Quantum Critical Transport without Self-Duality
We describe general features of frequency-dependent charge transport near
strongly interacting quantum critical points in 2+1 dimensions. The simplest
description using the AdS/CFT correspondence leads to a self-dual
Einstein-Maxwell theory on AdS_4, which fixes the conductivity at a
frequency-independent self-dual value. We describe the general structure of
higher-derivative corrections to the Einstein-Maxwell theory, and compute their
implications for the frequency dependence of the quantum-critical conductivity.
We show that physical consistency conditions on the higher-derivative terms
allow only a limited frequency dependence in the conductivity. The frequency
dependence is amenable to a physical interpretation using transport of either
particle-like or vortex-like excitations.Comment: 42 pages, 7 figures. A new figure showing the frequency dependence of
EM dual conductivity and few references added. Abstract, introduction,
section 5 and discussion extended. To appear in Phys.Rev.
Metallic spin glasses
Recent work on the zero temperature phases and phase transitions of strongly
random electronic system is reviewed. The transition between the spin glass and
quantum paramagnet is examined, for both metallic and insulating systems.
Insight gained from the solution of infinite range models leads to a quantum
field theory for the transition between a metallic quantum paramagnetic and a
metallic spin glass. The finite temperature phase diagram is described and
crossover functions are computed in mean field theory. A study of fluctuations
about mean field leads to the formulation of scaling hypotheses.Comment: Contribution to the Proceedings of the ITP Santa Barbara conference
on Non-Fermi liquids, 25 pages, requires IOP style file
Interface ordering and phase competition in a model Mott-insulator--band-insulator heterostructure
The phase diagram of model Mott-insulator--band-insulator heterostructures is
studied using the semiclassical approximation to the dynamical-mean-field
method as a function of thickness, coupling constant, and charge confinement.
An interface-stabilized ferromagnetic phase is found, allow the study of its
competition and possible coexistence with the antiferromagnetic order
characteristic of the bulk Mott insulator.Comment: 5 pages, 3 figures, manuscript revised, results unchange
Dynamical properties of a nonequilibrium quantum dot close to localized-delocalized quantum phase transitions
We calculate the dynamical decoherence rate and susceptibility of a
nonequilibrium quantum dot close to the delocalized-to-localized quantum phase
transitions. The setup concerns a resonance-level coupled to two spinless
fermionic baths with a finite bias voltage and an Ohmic bosonic bath
representing the dissipative environment. The system is equivalent to an
anisotropic Kondo model.
As the dissipation strength increases, the system at zero temperature and
zero bias show quantum phase transition between a conducting delocalized phase
to an insulating localized phase. Within the nonequilibrium functional
Renormalization Group (FRG) approach, we address the finite bias crossover in
dynamical decoherence rate and charge susceptibility close to the phase
transition. We find the dynamical decoherence rate increases with increasing
frequency. In the delocalized phase, it shows a singularity at frequencies
equal to positive or negative bias voltage. As the system crossovers to the
localized phase, the decoherence rate at low frequencies get progressively
smaller and this sharp feature is gradually smeared out, leading to a single
linear frequency dependence. The dynamical charge susceptibility shows a
dip-to-peak crossover across the delocalized-to-localized transition. Relevance
of our results to the experiments is discussed.Comment: 7 pages, 7 figure
Competition between spin density wave order and superconductivity in the underdoped cuprates
We describe the interplay between d-wave superconductivity and spin density
wave (SDW) order in a theory of the hole-doped cuprates at hole densities below
optimal doping. The theory assumes local SDW order, and associated electron and
hole pocket Fermi surfaces of charge carriers in the normal state. We describe
quantum and thermal fluctuations in the orientation of the local SDW order,
which lead to d-wave superconductivity: we compute the superconducting critical
temperature and magnetic field in a `minimal' universal theory. We also
describe the back-action of the superconductivity on the SDW order, showing
that SDW order is more stable in the metal. Our results capture key aspects of
the phase diagram of Demler et al. (cond-mat/0103192) obtained in a
phenomenological quantum theory of competing orders. Finally, we propose a
finite temperature crossover phase diagram for the cuprates. In the metallic
state, these are controlled by a `hidden' quantum critical point near optimal
doping involving the onset of SDW order in a metal. However, the onset of
superconductivity results in a decrease in stability of the SDW order, and
consequently the actual SDW quantum critical point appears at a significantly
lower doping.
All our analysis is placed in the context of recent experimental results.Comment: 27 pages, 11 figures; (v2) added clarifications and refs, and
corrected numerical errors (thanks to A. Chubukov
Specific heat of the S=1/2 Heisenberg model on the kagome lattice: high-temperature series expansion analysis
We compute specific heat of the antiferromagnetic spin-1/2 Heisenberg model
on the kagome lattice. We use a recently introduced technique to analyze
high-temperature series expansion based on the knowledge of high-temperature
series expansions, the total entropy of the system and the low-temperature
expected behavior of the specific heat as well as the ground-state energy. In
the case of kagome-lattice antiferromagnet, this method predicts a
low-temperature peak at T/J<0.1.Comment: 6 pages, 5 color figures (.eps), Revtex 4. Change in version 3: Fig.
5 has been corrected (it now shows data for 3 different ground-state
energies). The text is unchanged. v4: corrected an error in the temperature
scale of Fig. 5. (text unchanged
Engineering correlation and entanglement dynamics in spin systems
We show that the correlation and entanglement dynamics of spin systems can be
understood in terms of propagation of spin waves. This gives a simple, physical
explanation of the behaviour seen in a number of recent works, in which a
localised, low-energy excitation is created and allowed to evolve. But it also
extends to the scenario of translationally invariant systems in states far from
equilibrium, which require less local control to prepare. Spin-wave evolution
is completely determined by the system's dispersion relation, and the latter
typically depends on a small number of external, physical parameters.
Therefore, this new insight into correlation dynamics opens up the possibility
not only of predicting but also of controlling the propagation velocity and
dispersion rate, by manipulating these parameters. We demonstrate this
analytically in a simple, example system.Comment: 4 pages, 4 figures, REVTeX4 forma
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