746 research outputs found
Exotic vs. conventional scaling and universality in a disordered bilayer quantum Heisenberg antiferromagnet
We present large-scale Monte-Carlo simulations of a two-dimensional (2d)
bilayer quantum Heisenberg antiferromagnet with random dimer dilution. In
contrast to the exotic scaling scenarios found in many other random quantum
systems, the quantum phase transition in this system is characterized by a
finite-disorder fixed point with power-law scaling. After accounting for strong
corrections to scaling, characterized by a leading irrelevant exponent of
\omega = 0.48, we find universal, i.e., disorder-independent, critical
exponents z=1.310(6) and \nu=1.16(3). We discuss the consequences of these
findings and suggest new experiments.Comment: 4 pages, 5eps figures included, final version as publishe
Upper-critical dimension in a quantum impurity model: Critical theory of the asymmetric pseudogap Kondo problem
Impurity moments coupled to fermions with a pseudogap density of states
display a quantum phase transition between a screened and a free moment phase
upon variation of the Kondo coupling. We describe the universal theory of this
transition for the experimentally relevant case of particle-hole asymmetry. The
theory takes the form of a crossing between effective singlet and doublet
levels, interacting with low-energy fermions. Depending on the pseudogap
exponent, this interaction is either relevant or irrelevant under
renormalization group transformations, establishing the existence of an
upper-critical "dimension" in this impurity problem. Using perturbative
renormalization group techniques we compute various critical properties and
compare with numerical results.Comment: 4 pages, 2 figs, (v2) title changed, log corrections for r=1 adde
Magnetic excitations in a bond-centered stripe phase: Spin waves far from the semi-classical limit
Using a spin-only model, we compute spin excitation spectra in a
bond-centered stripe state with long-range magnetic order. We employ a bond
operator formalism, which naturally captures both dimerization and broken spin
symmetry in a unified framework. At low energies, the spin excitations resemble
spin waves, but at higher energies they are very similar to spin-1 excitations
of isolated spin ladders. Our theory does well describe neutron scattering data
[J. M. Tranquada et al., Nature 429, 534 (2004)] on LaBaCuO, pointing towards
bond order in this material.Comment: 4 pages, 3 figs, of possible relevance to experiments reported in
cond-mat/0401621; (v2) final version as publishe
Percolation transition in quantum Ising and rotor models with sub-Ohmic dissipation
We investigate the influence of sub-Ohmic dissipation on randomly diluted
quantum Ising and rotor models. The dissipation causes the quantum dynamics of
sufficiently large percolation clusters to freeze completely. As a result, the
zero-temperature quantum phase transition across the lattice percolation
threshold separates an unusual super-paramagnetic cluster phase from an
inhomogeneous ferromagnetic phase. We determine the low-temperature
thermodynamic behavior in both phases which is dominated by large frozen and
slowly fluctuating percolation clusters. We relate our results to the smeared
transition scenario for disordered quantum phase transitions, and we compare
the cases of sub-Ohmic, Ohmic, and super-Ohmic dissipation.Comment: 9 pages, 2 figure
Transport properties in antiferromagnetic quantum Griffiths phases
We study the electrical resistivity in the quantum Griffiths phase associated
with the antiferromagnetic quantum phase transition in a metal. The resistivity
is calculated by means of the semi-classical Boltzmann equation. We show that
the scattering of electrons by locally ordered rare regions leads to a singular
temperature dependence. The rare-region contribution to the resistivity varies
as with temperature where the is the usual Griffiths
exponent which takes the value zero at the critical point and increases with
distance from criticality. We find similar singular contributions to other
transport properties such as thermal resistivity, thermopower and the Peltier
coefficient. We also compare our results with existing experimental data and
suggest new experiments.Comment: 4 pages, 1 figur
Percolation quantum phase transitions in diluted magnets
We show that the interplay of geometric criticality and quantum fluctuations
leads to a novel universality class for the percolation quantum phase
transition in diluted magnets. All critical exponents involving dynamical
correlations are different from the classical percolation values, but in two
dimensions they can nonetheless be determined exactly. We develop a complete
scaling theory of this transition, and we relate it to recent experiments in
LaCu(Zn,Mg)O. Our results are also relevant for
disordered interacting boson systems.Comment: 4 pages, 3 eps figures, final version, as publishe
The quantum phase transition of itinerant helimagnets
We investigate the quantum phase transition of itinerant electrons from a
paramagnet to a state which displays long-period helical structures due to a
Dzyaloshinskii instability of the ferromagnetic state. In particular, we study
how the self-generated effective long-range interaction recently identified in
itinerant quantum ferromagnets is cut-off by the helical ordering. We find that
for a sufficiently strong Dzyaloshinskii instability the helimagnetic quantum
phase transition is of second order with mean-field exponents. In contrast, for
a weak Dzyaloshinskii instability the transition is analogous to that in
itinerant quantum ferromagnets, i.e. it is of first order, as has been observed
in MnSi.Comment: 5 pages RevTe
Breakdown of Landau-Ginzburg-Wilson theory for certain quantum phase transitions
The quantum ferromagnetic transition of itinerant electrons is considered. It
is shown that the Landau-Ginzburg-Wilson theory described by Hertz and others
breaks down due to a singular coupling between fluctuations of the conserved
order parameter. This coupling induces an effective long-range interaction
between the spins of the form 1/r^{2d-1}. It leads to unusual scaling behavior
at the quantum critical point in dimensions, which is determined
exactly.Comment: 4 pp., REVTeX, no figs, final version as publishe
Dissipation effects in random transverse-field Ising chains
We study the effects of Ohmic, super-Ohmic, and sub-Ohmic dissipation on the
zero-temperature quantum phase transition in the random transverse-field Ising
chain by means of an (asymptotically exact) analytical strong-disorder
renormalization-group approach. We find that Ohmic damping destabilizes the
infinite-randomness critical point and the associated quantum Griffiths
singularities of the dissipationless system. The quantum dynamics of large
magnetic clusters freezes completely which destroys the sharp phase transition
by smearing. The effects of sub-Ohmic dissipation are similar and also lead to
a smeared transition. In contrast, super-Ohmic damping is an irrelevant
perturbation; the critical behavior is thus identical to that of the
dissipationless system. We discuss the resulting phase diagrams, the behavior
of various observables, and the implications to higher dimensions and
experiments.Comment: 18 pages, 3 figures; (v2) minor changes, published versio
Nonequilibrium dynamical renormalization group: Dynamical crossover from weak to infinite randomness in the transverse-field Ising chain
In this work we formulate the nonequilibrium dynamical renormalization group
(ndRG). The ndRG represents a general renormalization-group scheme for the
analytical description of the real-time dynamics of complex quantum many-body
systems. In particular, the ndRG incorporates time as an additional scale which
turns out to be important for the description of the long-time dynamics. It can
be applied to both translational invariant and disordered systems. As a
concrete application we study the real-time dynamics after a quench between two
quantum critical points of different universality classes. We achieve this by
switching on weak disorder in a one-dimensional transverse-field Ising model
initially prepared at its clean quantum critical point. By comparing to
numerically exact simulations for large systems we show that the ndRG is
capable of analytically capturing the full crossover from weak to infinite
randomness. We analytically study signatures of localization in both real space
and Fock space.Comment: 15 pages, 4 figures, extended presentation, version as publishe
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