182 research outputs found
Multiple time scales from hard local constraints: glassiness without disorder
While multiple time scales generally arise in the dynamics of disordered
systems, we find multiple time scales in absence of disorder, in a simple model
with hard local constraints. The dynamics of the model, which consists of local
collective rearrangements of various scales, is not determined by the smallest
scale but by a length that grows at low energies. In real space we find a
hierarchy of fast and slow regions: each slow region is geometrically insulated
from all faster degrees of freedom, which are localized in fast pockets below
percolation thresholds. A tentative analogy with structural glasses is given,
which attributes the slowing down of the dynamics to the growing size of mobile
elementary excitations, rather than to the size of some domains.Comment: 10 pages, 9 figures, v2: pub
Colorings of odd or even chirality on hexagonal lattices
We define two classes of colorings that have odd or even chirality on
hexagonal lattices. This parity is an invariant in the dynamics of all loops,
and explains why standard Monte-Carlo algorithms are nonergodic. We argue that
adding the motion of "stranded" loops allows for parity changes. By
implementing this algorithm, we show that the even and odd classes have the
same entropy. In general, they do not have the same number of states, except
for the special geometry of long strips, where a Z symmetry between even
and odd states occurs in the thermodynamic limit.Comment: 18 pages, 13 figure
Heterogeneous freezing in a geometrically frustrated spin model without disorder: spontaneous generation of two time-scales
By considering the constrained motion of classical spins in a geometrically
frustrated magnet, we find a dynamical freezing temperature below which the
system gets trapped in metastable states with a "frozen" moment and dynamical
heterogeneities. The residual collective degrees of freedom are strongly
correlated, and by spontaneously forming aggregates, they are unable to
reorganize the system. The phase space is then fragmented in a macroscopic
number of disconnected sectors (broken ergodicity), resulting in self-induced
disorder and "thermodynamic" anomalies, measured by the loss of a finite
configurational entropy. We discuss these results in the view of experimental
results on the kagome compounds, SrCr(9p)Ga(12-9p)O19, (H30)Fe3(SO4)2(OH)6,
Cu3V2O7(OH)2.2H2O and Cu3BaV2O8(OH)2.Comment: 17 pages, 14 fi
How to detect weak emergent broken-symmetries of the Kagome antiferromagnet from Raman spectroscopy
We show that the magnetic Raman response of a spin-liquid is independent of
the polarizations of the light for triangular symmetries. In contrast, a
ground-state that has a broken symmetry shows characteristic oscillations when
the polarizations are rotated. This would allow to detect weak broken
symmetries and emergent order-parameters. We focus on the Kagome
antiferromagnet where no conventional long-range order has been found so far,
and present the Raman cross-section of a spin-liquid and a valence bond crystal
(VBC) using a random phase approximation.Comment: 4 pages, 2 figures, v2. intro partially rewritte
Instabilities and Insulator-Metal transitions in Half-Doped Manganites induced by Magnetic-Field and Doping
We discuss the phase diagram of the two-orbital model of half-doped
manganites by calculating self-consistently the Jahn-Teller (JT) distortion
patterns, charge, orbital and magnetic order at zero temperature. We analyse
the instabilities of these phases caused by electron or hole doping away from
half-doping, or by the application of a magnetic-field. For the CE insulating
phase of half-doped manganites, in the intermediate JT coupling regime, we show
that there is a competition between canting of spins (which promotes mobile
carriers) and polaronic self-trapping of carriers by JT defects. This results
in a marked particle-hole asymmetry, with canting winning only on the electron
doped side of half-doping. We also show that the CE phase undergoes a
first-order transition to a ferromagnetic metallic phase when a magnetic-field
is applied, with abrupt changes in the lattice distortion patterns. We discuss
the factors that govern the intriguingly small scale of the transition fields.
We argue that the ferromagnetic metallic phases involved have two types of
charge carriers, localised and band-like, leading to an effective two-fluid
model.Comment: 22 pages, 28 figure
Doping and Field-Induced Insulator-Metal Transitions in Half-Doped Manganites
We argue that many properties of the half-doped manganites may be understood
in terms of a new two-(eg electron)-fluid description, which is energetically
favorable at intermediate Jahn-Teller (JT) coupling. This emerges from a
competition between canting of the core spins of Mn promoting mobile carriers
and polaronic trapping of carriers by JT defects, in the presence of CE,
orbital and charge order. We show that this explains several features of the
doping and magnetic field induced insulator-metal transitions, as the
particle-hole asymmetry and the smallness of the transition fields.Comment: 4 pages, 4 figure
Quantum phase transition induced by Dzyaloshinskii-Moriya in the kagome antiferromagnet
We argue that the S=1/2 kagome antiferromagnet undergoes a quantum phase
transition when the Dzyaloshinskii-Moriya coupling is increased. For
the system is in a moment-free phase and for the system develops
antiferromagnetic long-range order. The quantum critical point is found to be
using exact diagonalizations and finite-size scaling. This
suggests that the kagome compound ZnCu_6_3$ may be in a quantum
critical region controlled by this fixed point.Comment: 5 pages, 4 figures; v2: add. data included, show that D=0.1J is at a
quantum critical poin
The incarnation of the Nersesyan-Tsvelik model in (NO)[Cu(NO3)3]
The topology of the magnetic interactions of the copper spins in the
nitrosonium nitratocuprate (NO)[Cu(NO3)3] suggests that it could be a
realization of the Nersesyan-Tsvelik model, whose ground state was argued to be
either a resonating valence bond (RVB) state or a valence bond crystal (VBC).
The measurement of thermodynamic and resonant properties reveals a behavior
inherent to low dimensional spin S = 1/2 systems and provides indeed no
evidence for the formation of long-range magnetic order down to 1.8 K.Comment: 12 pages, 6 figure
- …