8,116 research outputs found
Bose and Mott Glass Phases in Dimerized Quantum Antiferromagnets
We examine the effects of disorder on dimerized quantum antiferromagnets in a
magnetic field, using the mapping to a lattice gas of hard-core bosons with
finite-range interactions. Combining a strong-coupling expansion, the replica
method, and a one-loop renormalization group analysis, we investigate the
nature of the glass phases formed. We find that away from the tips of the Mott
lobes, the transition is from a Mott insulator to a compressible Bose glass,
however the compressibility at the tips is strongly suppressed. We identify
this finding with the presence of a rare Mott glass phase not previously
described by any analytic theory for this model and demonstrate that the
inclusion of replica symmetry breaking is vital to correctly describe the
glassy phases. This result suggests that the formation of Bose and Mott glass
phases is not simply a weak localization phenomenon but is indicative of much
richer physics. We discuss our results in the context of both ultracold atomic
gases and spin-dimer materials.Comment: 10 pages (including supplementary material), 3 figure
The reason why doping causes superconductivity in LaFeAsO
The experimental observation of superconductivity in LaFeAsO appearing on
doping is analyzed with the group-theoretical approach that evidently led in a
foregoing paper (J. Supercond 24:2103, 2011) to an understanding of the cause
of both the antiferromagnetic state and the accompanying structural distortion
in this material. Doping, like the structural distortions, means also a
reduction of the symmetry of the pure perfect crystal. In the present paper we
show that this reduction modifies the correlated motion of the electrons in a
special narrow half-filled band of LaFeAsO in such a way that these electrons
produce a stable superconducting state
Editorial: Thinking (with) the Unconscious in Media and Communication Studies: Introduction to the Special Issue - Digital Media, Psychoanalysis and the Subject
This is the first-ever special issue of a media and communication journal that addresses questions of subjectivity, digital media and the Internet with a focus on psychoanalytic theory.
The contributing authors seek to reassess and reinvigorate psychoanalytic thinking in media and communication studies. They undertake this reassessment with a particular focus on the question of what psychoanalytic concepts, theories and modes of inquiry can contribute to the study of contemporary digital media
Density profiles of a colloidal liquid at a wall under shear flow
Using a dynamical density functional theory we analyze the density profile of
a colloidal liquid near a wall under shear flow. Due to the symmetries of the
system considered, the naive application of dynamical density functional theory
does not lead to a shear induced modification of the equilibrium density
profile, which would be expected on physical grounds. By introducing a
physically motivated dynamic mean field correction we incorporate the missing
shear induced interparticle forces into the theory. We find that the shear flow
tends to enhance the oscillations in the density profile of hard-spheres at a
hard-wall and, at sufficiently high shear rates, induces a nonequilibrium
transition to a steady state characterized by planes of particles parallel to
the wall. Under gravity, we find that the center-of-mass of the density
distribution increases with shear rate, i.e., shear increases the potential
energy of the particles
Quantum Phase Transitions in Spin Systems
We discuss the influence of strong quantum fluctuations on zero-temperature
phase transitions in a two-dimensional spin-half Heisenberg system. Using a
high-order coupled cluster treatment, we study competition of magnetic bonds
with and without frustration. We find that the coupled cluster treatment is
able to describe the zero-temperature transitions in a qualitatively correct
way, even if frustration is present and other methods such as quantum Monte
Carlo fail.Comment: 8 pages, 12 Postscipt figures; Accepted for publication in World
Scientifi
The structural distortion in antiferromagnetic LaFeAsO investigated by a group-theoretical approach
As experimentally well established, undoped LaFeAsO is antiferromagnetic
below 137K with the magnetic moments lying on the Fe sites. We determine the
orthorhombic body-centered group Imma (74) as the space group of the
experimentally observed magnetic structure in the undistorted lattice, i.e., in
a lattice possessing no structural distortions in addition to the
magnetostriction. We show that LaFeAsO possesses a partly filled "magnetic
band" with Bloch functions that can be unitarily transformed into optimally
localized Wannier functions adapted to the space group Imma. This finding is
interpreted in the framework of a nonadiabatic extension of the Heisenberg
model of magnetism, the nonadiabatic Heisenberg model. Within this model,
however, the magnetic structure with the space group Imma is not stable but can
be stabilized by a (slight) distortion of the crystal turning the space group
Imma into the space group Pnn2 (34). This group-theoretical result is in
accordance with the experimentally observed displacements of the Fe and O atoms
in LaFeAsO as reported by Clarina de la Cruz et al. [nature 453, 899 (2008)]
Die plek van Empedokles in die metafisies-mistieke tradisie
The place of Empedocles in metaphysical-mystical traditionThis article argues that Empedocles was more than a pre-Socratic philosopher. His thinking was also essentially mystical and should be situated on a large map of metaphysical-mystical continuities with the following dimensions: A historically discernable cultural and religious pool, encompassing not only South-Eastern Europe, Asia Minor and Mediterranean Africa, but also the north-eastern Eurasian shamanic tradition, and India; an historically largely inaccesible esoteric tradition; a set of structural elements of the human psyche, running under and across historical religions through time; and the development of a new convergence of previously historically unconnected mystical traditions in the social and cultural circumstances of today. In particular, the article investigates similarities and differences between Empedocles and Indian (specifically Buddhist) views on various issues, such as the four roots and the cyclical dialectic of love and strife. In that context the article notes the remarkable interpretation of Empedocles by Peter Kingsley which seems to draw Empedocles closer to Buddhism, but without explicating this implication of his reception
Triplon mean-field analysis of an antiferromagnet with degenerate Shastry-Sutherland ground states
We look into the quantum phase diagram of a spin-
antiferromagnet on the square lattice with degenerate Shastry-Sutherland ground
states, for which only a schematic phase diagram is known so far. Many exotic
phases were proposed in the schematic phase diagram by the use of exact
diagonalization on very small system sizes. In our present work, an important
extension of this antiferromagnet is introduced and investigated in the
thermodynamic limit using triplon mean-field theory. Remarkably, this
antiferromagnet shows a stable plaquette spin-gapped phase like the original
Shastry-Sutherland antiferromagnet, although both of these antiferromagnets
differ in the Hamiltonian construction and ground state degeneracy. We propose
a sublattice columnar dimer phase which is stabilized by the second and third
neighbor antiferromagnetic Heisenberg exchange interactions. There are also
some commensurate and incommensurate magnetically ordered phases, and other
spin-gapped phases which find their places in the quantum phase diagram.
Mean-field results suggest that there is always a level-crossing phase
transition between two spin gapped phases, whereas in other situations, either
a level-crossing or a continuous phase transition happens
Pacifying the Fermi-liquid: battling the devious fermion signs
The fermion sign problem is studied in the path integral formalism. The
standard picture of Fermi liquids is first critically analyzed, pointing out
some of its rather peculiar properties. The insightful work of Ceperley in
constructing fermionic path integrals in terms of constrained world-lines is
then reviewed. In this representation, the minus signs associated with
Fermi-Dirac statistics are self consistently translated into a geometrical
constraint structure (the {\em nodal hypersurface}) acting on an effective
bosonic dynamics. As an illustrative example we use this formalism to study
1+1-dimensional systems, where statistics are irrelevant, and hence the sign
problem can be circumvented. In this low-dimensional example, the structure of
the nodal constraints leads to a lucid picture of the entropic interaction
essential to one-dimensional physics. Working with the path integral in
momentum space, we then show that the Fermi gas can be understood by analogy to
a Mott insulator in a harmonic trap. Going back to real space, we discuss the
topological properties of the nodal cells, and suggest a new holographic
conjecture relating Fermi liquids in higher dimensions to soft-core bosons in
one dimension. We also discuss some possible connections between mixed
Bose/Fermi systems and supersymmetry.Comment: 28 pages, 5 figure
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