1,238 research outputs found
Quantum criticality of dipolar spin chains
We show that a chain of Heisenberg spins interacting with long-range dipolar
forces in a magnetic field h perpendicular to the chain exhibits a quantum
critical point belonging to the two-dimensional Ising universality class.
Within linear spin-wave theory the magnon dispersion for small momenta k is
[Delta^2 + v_k^2 k^2]^{1/2}, where Delta^2 \propto |h - h_c| and v_k^2 \propto
|ln k|. For fields close to h_c linear spin-wave theory breaks down and we
investigate the system using density-matrix and functional renormalization
group methods. The Ginzburg regime where non-Gaussian fluctuations are
important is found to be rather narrow on the ordered side of the transition,
and very broad on the disordered side.Comment: 6 pages, 5 figure
Contribution of the core to the thermal evolution of sub-Neptunes
Sub-Neptune planets are a very common type of planets. They are inferred to
harbour a primordial (H/He) envelope, on top of a (rocky) core, which dominates
the mass. Here, we investigate the long-term consequences of the core
properties on the planet mass-radius relation. We consider the role of various
core energy sources resulting from core formation, its differentiation, its
solidification (latent heat), core contraction and radioactive decay. We divide
the evolution of the rocky core into three phases: the formation phase, which
sets the initial conditions, the magma ocean phase, characterized by rapid heat
transport, and the solid state phase, where cooling is inefficient. We find
that for typical sub-Neptune planets of ~2-10 Earth masses and envelope mass
fractions of 0.5-10% the magma ocean phase lasts several Gyrs, much longer than
for terrestrial planets. The magma ocean phase effectively erases any signs of
the initial core thermodynamic state. After solidification, the reduced heat
flux from the rocky core causes a significant drop in the rocky core surface
temperature, but its effect on the planet radius is limited. In the long run,
radioactive heating is the most significant core energy source in our model.
Overall, the long term radius uncertainty by core thermal effects is up to 15%.Comment: ApJ Publishe
Modularity clustering is force-directed layout
Two natural and widely used representations for the community structure of
networks are clusterings, which partition the vertex set into disjoint subsets,
and layouts, which assign the vertices to positions in a metric space. This
paper unifies prominent characterizations of layout quality and clustering
quality, by showing that energy models of pairwise attraction and repulsion
subsume Newman and Girvan's modularity measure. Layouts with optimal energy are
relaxations of, and are thus consistent with, clusterings with optimal
modularity, which is of practical relevance because both representations are
complementary and often used together.Comment: 9 pages, 7 figures, see http://code.google.com/p/linloglayout/ for
downloading the graph clustering and layout softwar
Time evolution of Matrix Product States
In this work we develop several new simulation algorithms for 1D many-body
quantum mechanical systems combining the Matrix Product State variational
ansatz with Taylor, Pade and Arnoldi approximations to the evolution operator.
By comparing all methods with previous techniques based on Trotter
decompositions we demonstrate that the Arnoldi method is the best one, reaching
extremely good accuracy with moderate resources. Finally we apply this
algorithm to studying the formation of molecules in an optical lattices when
crossing a Feschbach resonance with a cloud of two-species hard-core bosons.Comment: More extensive comparison with all nearest-neighbor spin s=1/2
models. The results in this manuscript have been superseded by a more
complete work in cond-mat/061021
Quasi-equilibrium states in thermotropic liquid crystals studied by multiple quantum NMR
We study the nature of the quasiinvariants in nematic 5CB and measure their
relaxation times by encoding the multiple quantum coherences of the states
following the JB pulse pair on two orthogonal bases, Z and X. The experiments
were also performed in powder adamantane at 301 K which is used as a reference
compound having only one dipolar quasiinvariant. We show that the evolution of
the quantum states during the build up of the quasi-equilibrium state in 5CB
prepared under the S condition is similar to the case of adamantane and that
their quasi-equilibrium density operators have the same tensor structure. In
contrast, the second constant of motion, whose explicit operator form is not
known, involves a richer composition of multiple quantum coherences on the X
basis of even order, in consistency with the truncation inherent in its
definition. We exploited the exclusive presence coherences 4, 6, 8, besides 0
and 2 under the W condition to measure the spin-lattice relaxation time T_{W}
accurately, so avoiding experimental difficulties that usually impair dipolar
order relaxation measurement such as Zeeman contamination at high fields, and
also superposition of the different quasiinvariants. This procedure opens the
possibility of measuring the spin-lattice relaxation of a quasiinvariant
independent of the Zeeman and S reservoirs, so incorporating a new relaxation
parameter useful for studying the complex molecular dynamics in mesophases. In
fact, we report the first measurement of T_{W} in a liquid crystal at high
magnetic fields. The comparison of the obtained value with the one
corresponding to a lower field (16 MHz) points out that the relaxation of the
W-order strongly depends on the intensity of the external magnetic field,
similarly to the case of the S reservoir, indicating that the relaxation of the
W-quasiinvariant is also governed by the cooperative molecular motions.Comment: 7 figures. http://www.famaf.unc.edu.ar/series/AFis2005.ht
Numerical study of a superconductor-insulator transition in a half-filled Hubbard chain with distant transfers
The ground state of a one-dimensional Hubbard model having the next-nearest
neighbor hopping (t') as well as the nearest-neighbor one (t) is numerically
investigated at half-filling. A quantum Monte Carlo result shows a slowly
decaying pairing correlation for a sizeable interaction strength ,
while the system is shown to become insulating for yet larger
from a direct evaluation of the charge gap with the density-matrix
renormalization group method. The results are consistent with Fabrizio's recent
weak-coupling theory which suggests a transition from a superconductor into an
insulator at a finite U.Comment: 4 pages, RevTeX, uses epsf.sty and multicol.st
Numerical method for non-linear steady-state transport in one-dimensional correlated conductors
We present a method for investigating the steady-state transport properties
of one-dimensional correlated quantum systems. Using a procedure based on our
analysis of finite-size effects in a related classical model (LC line) we show
that stationary currents can be obtained from transient currents in finite
systems driven out of equilibrium. The non-equilibrium dynamics of correlated
quantum systems is calculated using the time-evolving block decimation method.
To demonstrate our method we determine the full I-V characteristic of the
spinless fermion model with nearest-neighbour hopping t_H and interaction V_H
using two different setups to generate currents (turning on/off a potential
bias). Our numerical results agree with exact results for non-interacting
fermions (V_H=0). For interacting fermions we find that in the linear regime eV
<< 4t_H the current I is independent from the setup and our numerical data
agree with the predictions of the Luttinger liquid theory combined with the
Bethe Ansatz solution. For larger potentials V the steady-state current depends
on the current-generating setup and as V increases we find a negative
differential conductance with one setup while the currents saturate at finite
values in the other one. Both effects are due to finite renormalized
bandwidths.Comment: published versio
Reaching out to early-career astrobiologists: AbGradE's actions and perspectives
Astrobiology Graduates in Europe (AbGradE, pronounced ab-grad-ee) is an association of early-career scientists working in fields relevant to astrobiological research. Conceptualized in 2013, it was initially designed as a mini-conference or workshop dedicated to early-career researchers, providing a friendly environment where early-career minds would be able to present their research without being intimidated by the possibility of facing a more traditional audience, composed mainly of senior scientists. Within the last couple of years, AbGradE became the first point of call for European, but also for an increasing number of non-European, early-career astrobiologists. This article aims to present how AbGradE has evolved over the years (in its structure and in its way of organizing events), how it has adapted with the COVID-19 pandemic, and what future developments are considered
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