326 research outputs found
Smearing of the phase transition in Ising systems with planar defects
We show that phase transitions in Ising systems with planar defects, i.e.,
disorder perfectly correlated in two dimensions are destroyed by smearing. This
is caused by effects similar to but stronger than the Griffiths phenomena:
Exponentially rare spatial regions can develop true static long-range order
even when the bulk system is still in its disordered phase. Close to the
smeared transition, the order parameter is very inhomogeneous in space, with
the thermodynamic (average) order parameter depending exponentially on
temperature. We determine the behavior using extremal statistics, and we
illustrate the results by computer simulations.Comment: 15 pages, 5 figures, to appear in J. Phys.
Strong-disorder magnetic quantum phase transitions: Status and new developments
This article reviews the unconventional effects of random disorder on
magnetic quantum phase transitions, focusing on a number of new experimental
and theoretical developments during the last three years. On the theory side,
we address smeared quantum phase transitions tuned by changing the chemical
composition, for example in alloys of the type AB. We also discuss
how the interplay of order parameter conservation and overdamped dynamics leads
to enhanced quantum Griffiths singularities in disordered metallic
ferromagnets. Finally, we discuss a semiclassical theory of transport
properties in quantum Griffiths phases. Experimental examples include the
ruthenates SrCaRuO and (SrCa)RuO as
well as Ba(FeMn)As.Comment: 9 pages, 2 figures, Proceedings of the International Conference on
Recent Progress in Many-Body Theories 17, final version as publishe
Monte-Carlo simulations of the clean and disordered contact process in three dimensions
The absorbing-state transition in the three-dimensional contact process with
and without quenched randomness is investigated by means of Monte-Carlo
simulations. In the clean case, a reweighting technique is combined with a
careful extrapolation of the data to infinite time to determine with high
accuracy the critical behavior in the three-dimensional directed percolation
universality class. In the presence of quenched spatial disorder, our data
demonstrate that the absorbing-state transition is governed by an
unconventional infinite-randomness critical point featuring activated dynamical
scaling. The critical behavior of this transition does not depend on the
disorder strength, i.e., it is universal. Close to the disordered critical
point, the dynamics is characterized by the nonuniversal power laws typical of
a Griffiths phase. We compare our findings to the results of other numerical
methods, and we relate them to a general classification of phase transitions in
disordered systems based on the rare region dimensionality.Comment: 12 pages, 11 eps figures included, applies simulation and data
analysis techniques developed in arXiv:0810.1569 to the 3D contact process,
final version as publishe
Phases and phase transitions in disordered quantum systems
These lecture notes give a pedagogical introduction to phase transitions in
disordered quantum systems and to the exotic Griffiths phases induced in their
vicinity. We first review some fundamental concepts in the physics of phase
transitions. We then derive criteria governing under what conditions spatial
disorder or randomness can change the properties of a phase transition. After
introducing the strong-disorder renormalization group method, we discuss in
detail some of the exotic phenomena arising at phase transitions in disordered
quantum systems. These include infinite-randomness criticality, rare regions
and quantum Griffiths singularities, as well as the smearing of phase
transitions. We also present a number of experimental examples.Comment: Pedagogical introduction to strong disorder physics at quantum phase
transitions. Based on lectures given at the XVII Training Course in the
Physics of Strongly Correlated Systems in Vietri sul Mare, Italy in October
2012. Submitted to the proceedings of this school. 60 pages and 23 figures.
Builds on material reviewed in arXiv:cond-mat/0602312 and arXiv:1005.270
Computing quantum phase transitions
This article first gives a concise introduction to quantum phase transitions,
emphasizing similarities with and differences to classical thermal transitions.
After pointing out the computational challenges posed by quantum phase
transitions, a number of successful computational approaches is discussed. The
focus is on classical and quantum Monte Carlo methods, with the former being
based on the quantum-to classical mapping while the latter directly attack the
quantum problem. These methods are illustrated by several examples of quantum
phase transitions in clean and disordered systems.Comment: 99 pages, 15 figures, submitted to Reviews in Computational Chemistr
Spin excitations in fluctuating stripe phases of doped cuprate superconductors
Using a phenomenological lattice model of coupled spin and charge modes, we
determine the spin susceptibility in the presence of fluctuating stripe charge
order. We assume the charge fluctuations to be slow compared to those of the
spins, and combine Monte Carlo simulations for the charge order parameter with
exact diagonalization of the spin sector. Our calculations unify the spin
dynamics of both static and fluctuating stripe phases and support the notion of
a universal spin excitation spectrum in doped cuprate superconductors.Comment: 4 pages, 4 figs, minor changes, final version as publishe
Ordered droplets in quantum magnets with long-range interactions
A defect coupling to the square of the order parameter in a nearly
quantum-critical magnet can nucleate an ordered droplet while the bulk system
is in the paramagnetic phase. We study the influence of long-range spatial
interactions of the form on the droplet formation. To this
end, we solve a Landau-Ginzburg-Wilson free energy in saddle point
approximation. The long-range interaction causes the droplet to develop an
energetically unfavorable power-law tail. However, for , the free
energy contribution of this tail is subleading in the limit of large droplets;
and the droplet formation is controlled by the defect bulk. Thus, for large
defects, long-range interactions do not hinder the formation of droplets.Comment: 2 pages, 3 eps figures, final version as publishe
Signatures of a quantum Griffiths phase in a d-metal alloy close to its ferromagnetic quantum critical point
We report magnetization () measurements close to the ferromagnetic quantum
phase transition of the d-metal alloy NiV at a vanadium
concentration of . In the diluted regime (), the
temperature () and magnetic field () dependencies of the magnetization
are characterized by nonuniversal power laws and display scaling in a
wide temperature and field range. The exponents vary strongly with and
follow the predictions of a quantum Griffiths phase. We also discuss the
deviations and limits of the quantum Griffiths phase as well as the phase
boundaries due to bulk and cluster physics.Comment: 4 pages, 5 figures, final version as published in the Strongly
Correlated Electron Systems special issue of J. Phys. Condens. Matte
Numerical method for disordered quantum phase transitions in the large limit
We develop an efficient numerical method to study the quantum critical
behavior of disordered systems with order-parameter symmetry
in the large limit. It is based on the iterative solution of the large
saddle-point equations combined with a fast algorithm for inverting the arising
large sparse random matrices. As an example, we consider the
superconductor-metal quantum phase transition in disordered nanowires. We study
the behavior of various observables near the quantum phase transition. Our
results agree with recent renormalization group predictions, i.e., the
transition is governed by an infinite-randomness critical point, accompanied by
quantum Griffiths singularities. Our method is highly efficient because the
numerical effort for each iteration scales linearly with the system size. This
allows us to study larger systems, with up to 1024 sites, than previous
methods. We also discuss generalizations to higher dimensions and other systems
including the itinerant antiferomagnetic transitions in disordered metals.Comment: 8 pages, 6 eps figures, published versio
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