691,121 research outputs found
Defects as a reason of continuity of normal-incommensurate phase transitions
Almost all normal-incommensurate phase transitions observed experimentally
are continuous. We show that there is not any theoretical reason for this
general behaviour in perfect crystals. A normal-incommensurate phase transition
that is not too far from the mean-field tricritical point should be
discontinuous and it is highly improbable that so far reported
normal-incommensurate phase transitions lie very far from this point. To
understand this behaviour we study influence of defects on a hypothetical
first-order normal-incommensurate phase transition in a pure material. We have
found that this influence is strikingly different from that on other kinds of
first-order phase transitions. The change of the discontinuity of the order
parameter at the transition is negative and formally diverges within our
approximate theory. At the same time the diminishing of the phase transition
temperature remains finite. We interpret these results as an indication that at
least some of the observed seemingly second-order normal-incommensurate
transitions would be first-order transitions in defectless crystals.Comment: 17 pages, 1 figur
Quantum phase transitions of the diluted O(3) rotor model
We study the phase diagram and the quantum phase transitions of a
site-diluted two-dimensional O(3) quantum rotor model by means of large-scale
Monte-Carlo simulations. This system has two quantum phase transitions, a
generic one for small dilutions, and a percolation transition across the
lattice percolation threshold. We determine the critical behavior for both
transitions and for the multicritical point that separates them. In contrast to
the exotic scaling scenarios found in other random quantum systems, all these
transitions are characterized by finite-disorder fixed points with power-law
scaling. We relate our findings to a recent classification of phase transitions
with quenched disorder according to the rare region dimensionality, and we
discuss experiments in disordered quantum magnets.Comment: 11 pages, 14 eps figures, final version as publishe
Phase structure of excited baryonic matter in the relativistic mean field theory
We analyze the phase structure of the nonlinear mean-field meson theory of baryonic matter (nucleons plus delta resonances). Depending on the choice of the coupling constants, we find three physically distinct phase transitions in this theory: a nucleonic liquid-gas transition in the low temperature, Tc2ρ0 and Tc<50 MeV. All three phase transitions are of first order. It is shown that the occurrence of these different phase transitions depends critically on the coupling constants. Since the production of pions also depends strongly on the coupling constants, it is seen that the equation of state cannot be derived unambiguously from pion data
Nonequilibrium Quantum Phase Transitions in the XY model: comparison of unitary time evolution and reduced density matrix approaches
We study nonequilibrium quantum phase transitions in XY spin 1/2 chain using
the algebra. We show that the well-known quantum phase transition at
magnetic field persists also in the nonequilibrium setting as long as
one of the reservoirs is set to absolute zero temperature. In addition, we find
nonequilibrium phase transitions associated to imaginary part of the
correlation matrix for any two different temperatures of the reservoirs at and , where is the anisotropy and
the magnetic field strength. In particular, two nonequilibrium quantum
phase transitions coexist at . In addition we also study the quantum
mutual information in all regimes and find a logarithmic correction of the area
law in the nonequilibrium steady state independent of the system parameters. We
use these nonequilibrium phase transitions to test the utility of two models of
reduced density operator, namely Lindblad mesoreservoir and modified Redfield
equation. We show that the nonequilibrium quantum phase transition at
related to the divergence of magnetic susceptibility is recovered in the
mesoreservoir approach, whereas it is not recovered using the Redfield master
equation formalism. However none of the reduced density operator approaches
could recover all the transitions observed by the algebra. We also study
thermalization properties of the mesoreservoir approach.Comment: 25 pages, 10 figure
Monte Carlo studies of antiferromagnetic spin models in three dimensions
We study several antiferromagnetic formulations of the O(3) spin model in
three dimensions by means of Monte Carlo simulations. We discuss about the
vacua properties and analyze the phase transitions. Using Finite Size Scaling
analysis we conclude that all phase transitions found are of first orderComment: 4 pages, 2 Postscript figures. Contribution to Lattice '9
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