568,271 research outputs found
Quasiclassical frustration
We study the dissipative properties of a harmonic oscillator subject to two
independent heat baths, one of which couples to its position and the other one
to its momentum. This model describes a large spin impurity in a ferromagnet.
We find that some effects of the two heat baths partially cancel each other.
Most notably, oscillations may remain underdamped for arbitrarily strong
coupling. This effect is a direct consequence of the mutually conjugate
character of position and momentum. For a single dissipative bath coupled
linearly to both position and momentum, no underdamped regime is possible for
strong coupling. The dynamics of purity loss for one and two wave packets is
also investigated
Frustration in Biomolecules
Biomolecules are the prime information processing elements of living matter.
Most of these inanimate systems are polymers that compute their structures and
dynamics using as input seemingly random character strings of their sequence,
following which they coalesce and perform integrated cellular functions. In
large computational systems with a finite interaction-codes, the appearance of
conflicting goals is inevitable. Simple conflicting forces can lead to quite
complex structures and behaviors, leading to the concept of "frustration" in
condensed matter. We present here some basic ideas about frustration in
biomolecules and how the frustration concept leads to a better appreciation of
many aspects of the architecture of biomolecules, and how structure connects to
function. These ideas are simultaneously both seductively simple and perilously
subtle to grasp completely. The energy landscape theory of protein folding
provides a framework for quantifying frustration in large systems and has been
implemented at many levels of description. We first review the notion of
frustration from the areas of abstract logic and its uses in simple condensed
matter systems. We discuss then how the frustration concept applies
specifically to heteropolymers, testing folding landscape theory in computer
simulations of protein models and in experimentally accessible systems.
Studying the aspects of frustration averaged over many proteins provides ways
to infer energy functions useful for reliable structure prediction. We discuss
how frustration affects folding, how a large part of the biological functions
of proteins are related to subtle local frustration effects and how frustration
influences the appearance of metastable states, the nature of binding
processes, catalysis and allosteric transitions. We hope to illustrate how
Frustration is a fundamental concept in relating function to structural
biology.Comment: 97 pages, 30 figure
Glass transition in models with controlled frustration
A class of models with self-generated disorder and controlled frustration is
studied. Between the trivial case, where frustration is not present at all, and
the limit case, where frustration is present over every length scale, a region
with local frustration is found where glassy dynamics appears. We suggest that
in this region, the mean field model might undergo a p-spin like transition,
and increasing the range of frustration, a crossover from a 1-step replica
symmetry breaking to a continuous one might be observed.Comment: 4 pages, 6 figure
Overfrustrated and Underfrustrated Spin-Glasses in d=3 and 2: Evolution of Phase Diagrams and Chaos Including Spin-Glass Order in d=2
In spin-glass systems, frustration can be adjusted continuously and
considerably, without changing the antiferromagnetic bond probability p, by
using locally correlated quenched randomness, as we demonstrate here on
hypercubic lattices and hierarchical lattices. Such overfrustrated and
underfrustrated Ising systems on hierarchical lattices in d=3 and 2 are
studied. With the removal of just 51 % of frustration, a spin-glass phase
occurs in d=2. With the addition of just 33 % frustration, the spin-glass phase
disappears in d=3. Sequences of 18 different phase diagrams for different
levels of frustration are calculated in both dimensions. In general,
frustration lowers the spin-glass ordering temperature. At low temperatures,
increased frustration favors the spin-glass phase (before it disappears) over
the ferromagnetic phase and symmetrically the antiferromagnetic phase. When any
amount, including infinitesimal, frustration is introduced, the chaotic
rescaling of local interactions occurs in the spin-glass phase. Chaos increases
with increasing frustration, as seen from the increased positive value of the
calculated Lyapunov exponent , starting from when
frustration is absent. The calculated runaway exponent of the
renormalization-group flows decreases with increasing frustration to
when the spin-glass phase disappears. From our calculations of entropy and
specific heat curves in d=3, it is seen that frustration lowers in temperature
the onset of both long- and short-range order in spin-glass phases, but is more
effective on the former. From calculations of the entropy as a function of
antiferromagnetic bond concentration p, it is seen that the ground-state and
low-temperature entropy already mostly sets in within the ferromagnetic and
antiferromagnetic phases, before the spin-glass phase is reached.Comment: Published version, 18 phase diagrams, 12 figures, 10 page
Supersolidity, entropy and frustration
We study the properties of t-t'-V model of hard-core bosons on the triangular
lattice that can be realized in optical lattices. By mapping to the spin-1/2
XXZ model in a field, we determine the phase diagram of the t-V model where the
supersolid characterized by the ordering pattern (x,x,-2x') ("ferrimagnetic" or
SS A) is a ground state for chemical potential \mu >3V. By turning on either
temperature or t' at half-filling \mu =3V, we find a first order transition
from SS A to the elusive supersolid characterized by the (x,-x,0) ordering
pattern ("antiferromagnetic" or SS C). In addition, we find a large region
where a superfluid phase becomes a solid upon raising temperature at fixed
chemical potential. This is an analog of the Pomeranchuk effect driven by the
large entropic effects associated with geometric frustration on the triangular
lattice.Comment: 4 pages, igures, LaTe
Combined effect of frustration and dimerization in ferrimagnetic chains and square lattice
Within the zero-temperature linear spin-wave theory we have investigated the
effect of frustration and dimerization of a Heisenberg system with alternating
spins and on one- and two-dimensional lattices. The combined
effect most visibly appears in the elementary excitation spectra. In contrast
to the ground state energy that decreases with dimerization and increases with
frustration, the excitation energies are shown to be suppressed in energy by
both dimerization and frustration. The threshold value of frustration that
signals a transition from a classical ferrimagnetic state to a spiral state,
decreases with dimerization, showing that dimerization further helps in the
phase transition. The correlation length and sublattice magnetization decrease
with both dimerization and frustration indicating the destruction of the
long-range classical ferrimagnetic. The linear spin wave theory shows that in
the case of a square lattice, dimerization initially opposes the
frustration-led transition to a spiral magnetic state, but then higher
magnitudes of lattice deformation facilitate the transition. It also shows that
the transition to spiral state is inhibited in a square lattice beyond a
certain value of dimerization.Comment: 8 pages, latex, 12 postscript figure
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