2,812 research outputs found
Wigner crystals in two-dimensional transition-metal dichalcogenides: Spin physics and readout
Wigner crystals are prime candidates for the realization of regular electron
lattices under minimal requirements on external control and electronics.
However, several technical challenges have prevented their detailed
experimental investigation and applications to date. We propose an
implementation of two-dimensional electron lattices for quantum simulation of
Ising spin systems based on self-assembled Wigner crystals in transition-metal
dichalcogenides. We show that these semiconductors allow for minimally invasive
all-optical detection schemes of charge ordering and total spin. For incident
light with optimally chosen beam parameters and polarization, we predict a
strong dependence of the transmitted and reflected signals on the underlying
lattice periodicity, thus revealing the charge order inherent in Wigner
crystals. At the same time, the selection rules in transition-metal
dichalcogenides provide direct access to the spin degree of freedom via Faraday
rotation measurements.Comment: 15 pages, 12 figure
Effects of laser parameters and scanning strategy on structural and mechanical properties of 3D NiTi implants fabricated with selective laser melting
Non-locality of non-Abelian anyons
Topological systems, such as fractional quantum Hall liquids, promise to
successfully combat environmental decoherence while performing quantum
computation. These highly correlated systems can support non-Abelian anyonic
quasiparticles that can encode exotic entangled states. To reveal the non-local
character of these encoded states we demonstrate the violation of suitable Bell
inequalities. We provide an explicit recipe for the preparation, manipulation
and measurement of the desired correlations for a large class of topological
models. This proposal gives an operational measure of non-locality for anyonic
states and it opens up the possibility to violate the Bell inequalities in
quantum Hall liquids or spin lattices.Comment: 7 pages, 3 figure
Defect mediated melting and the breaking of quantum double symmetries
In this paper, we apply the method of breaking quantum double symmetries to
some cases of defect mediated melting. The formalism allows for a systematic
classification of possible defect condensates and the subsequent confinement
and/or liberation of other degrees of freedom. We also show that the breaking
of a double symmetry may well involve a (partial) restoration of an original
symmetry. A detailed analysis of a number of simple but representative examples
is given, where we focus on systems with global internal and external (space)
symmetries. We start by rephrasing some of the well known cases involving an
Abelian defect condensate, such as the Kosterlitz-Thouless transition and
one-dimensional melting, in our language. Then we proceed to the non-Abelian
case of a hexagonal crystal, where the hexatic phase is realized if
translational defects condense in a particular rotationally invariant state.
Other conceivable phases are also described in our framework.Comment: 10 pages, 4 figures, updated reference
Non-Abelian Chern-Simons models with discrete gauge groups on a lattice
We construct the local Hamiltonian description of the Chern-Simons theory
with discrete non-Abelian gauge group on a lattice. We show that the theory is
fully determined by the phase factors associated with gauge transformations and
classify all possible non-equivalent phase factors. We also construct the gauge
invariant electric field operators that move fluxons around and
create/anihilate them. We compute the resulting braiding properties of the
fluxons. We apply our general results to the simplest class of non-Abelian
groups, dihedral groups D_n.Comment: 16 pages, 7 figure
GMRT mini-survey to search for 21-cm absorption in Quasar-Galaxy Pairs at z~0.1
We present the results from our 21-cm absorption survey of a sample of 5
quasar-galaxy pairs (QGPs), with the redshift of the galaxies in the range
0.03<zg<0.18, selected from the SDSS. The HI 21-cm absorption was searched
towards the 9 sight lines with impact parameters ranging from 10 to 55 kpc
using GMRT. 21-cm absorption was detected only in one case i.e. towards the
Quasar (zq=2.625 SDSS J124157.54+633241.6)-galaxy (zg=0.143 SDSS
J124157.26+633237.6) pair with the impact parameter 11 kpc. The quasar sight
line in this case pierces through the stellar disk of a galaxy having near
solar metallicity (i.e (O/H)+12=8.7) and star formation rate uncorrected for
dust attenuation of 0.1 M_odot/yr. The quasar spectrum reddened by the
foreground galaxy is well fitted with the Milky Way extinction curve (with an
Av of 0.44) and the estimated HI column density is similar to the value
obtained from 21-cm absorption assuming spin temperature of 100K. Combining our
sample with the z<0.1 data available in the literature, we find the
detectability of 21-cm absorption with integrated optical depth greater than
0.1 km\s to be 50% for the impact parameter less than 20 kpc. Using the surface
brightness profiles and relationship between the optical size and extent of the
HI disk known for nearby galaxies, we conclude that in most of the cases of
21-cm absorption non-detection, the sight lines may not be passing through the
HI gas. We also find that in comparison to the absorption systems associated
with these QGPs, z<1 DLAs with 21-cm absorption detections have lower CaII
equivalent widths despite having higher 21-cm optical depths and smaller impact
parameters. This suggests that the current sample of DLAs may be a biased
population that avoids sight lines through dusty star-forming galaxies. A
systematic survey of QGPs is needed to confirm these findings and understand
the nature of 21-cm absorbers.Comment: 17 pages, 5 tables, 19 figures, accepted for publication in MNRAS
(abstract abridged
Qudit surface codes and gauge theory with finite cyclic groups
Surface codes describe quantum memory stored as a global property of
interacting spins on a surface. The state space is fixed by a complete set of
quasi-local stabilizer operators and the code dimension depends on the first
homology group of the surface complex. These code states can be actively
stabilized by measurements or, alternatively, can be prepared by cooling to the
ground subspace of a quasi-local spin Hamiltonian. In the case of spin-1/2
(qubit) lattices, such ground states have been proposed as topologically
protected memory for qubits. We extend these constructions to lattices or more
generally cell complexes with qudits, either of prime level or of level
for prime and , and therefore under tensor
decomposition, to arbitrary finite levels. The Hamiltonian describes an exact
gauge theory whose excitations
correspond to abelian anyons. We provide protocols for qudit storage and
retrieval and propose an interferometric verification of topological order by
measuring quasi-particle statistics.Comment: 26 pages, 5 figure
The nature of LINER galaxies: Ubiquitous hot old stars and rare accreting black holes
Galaxies, which often contain ionised gas, sometimes also exhibit a so-called
low-ionisation nuclear emission line region (LINER). For 30 years this was
attributed to a central mass-accreting supermassive black hole (AGN) of low
luminosity, making LINER galaxies the largest AGN-sub-population, dominating in
numbers over higher luminosity Seyfert galaxies and quasars. This, however,
poses a serious problem. While the inferred energy balance is plausible, many
LINERs clearly do not contain any other independent signatures of an AGN. Using
integral field spectroscopic data from the CALIFA survey, we aim at comparing
the observed radial surface brightness profiles with what is expected from
illumination by an AGN. Essential for this analysis is a proper extraction of
emission-lines, especially weak lines such as the Balmer Hb line which is
superposed on an absorption trough. To accomplish this, we use the GANDALF code
which simultaneously fits the underlying stellar continuum and emission lines.
We show for 48 galaxies with LINER-like emission, that the radial emission-line
surface brightness profiles are inconsistent with ionisation by a central
point-source and hence cannot be due to an AGN alone. The most probable
explanation for the excess LINER-like emission is ionisation by evolved stars
during the short but very hot and energetic phase known as post-AGB. This leads
us to an entirely new interpretation. Post-AGB stars are ubiquitous and their
ionising effect should be potentially observable in every galaxy with gas
present and stars older than ~1 Gyr, unless a stronger radiation field from
young hot stars or an AGN outshines them. This means that galaxies with
LINER-like emission are in fact not a class defined by a property, but rather
by the absence of a property. It also explains why LINER emission is observed
mostly in massive galaxies with old stars and little star formation.Comment: 8 pages, 7 figure
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