141 research outputs found
Group theory of Wannier functions providing the basis for a deeper understanding of magnetism and superconductivity
The paper presents the group theory of best localized and symmetry-adapted
Wannier functions in a crystal of any given space group G or magnetic group M.
Provided that the calculated band structure of the considered material is given
and that the symmetry of the Bloch functions at all the points of symmetry in
the Brillouin zone is known, the paper details whether or not the Bloch
functions of particular energy bands can be unitarily transformed into best
localized Wannier functions symmetry-adapted to the space group G, to the
magnetic group M, or to a subgroup of G or M. In this context, the paper
considers usual as well as spin-dependent Wannier functions, the latter
representing the most general definition of Wannier functions. The presented
group theory is a review of the theory published by one of the authors in
several former papers and is independent of any physical model of magnetism or
superconductivity. However, it is suggested to interpret the special symmetry
of the best localized Wannier functions in the framework of a nonadiabatic
extension of the Heisenberg model, the nonadiabatic Heisenberg model. On the
basis of the symmetry of the Wannier functions, this model of strongly
correlated localized electrons makes clear predictions whether or not the
system can possess superconducting or magnetic eigenstates
Rare earth luminescence: a way to overcome concentration quenching
A model is developed to simulate the rare earth luminescence intensity in dependence of both the excitation rate and the dopant concentration. For low excitation rates, as in the case of photoluminescence investigations, concentration quenching is expected. In contrast for high excitation rates (as generally realized in cathodoluminescence experiments) concentration quenching can be suppressed and thus luminescence intensity increases with increasing dopant concentration. These results reconcile the recent photo- and cathodoluminescence results on GaN:Er presented by Chen et al. (APL 96, 181901, 2010). Further experimental results indicate that the physical basis of the model is adequate
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)]
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
First narrow-band search for continuous gravitational waves from known pulsars in advanced detector data
Spinning neutron stars asymmetric with respect to their rotation axis are potential sources of
continuous gravitational waves for ground-based interferometric detectors. In the case of known pulsars a
fully coherent search, based on matched filtering, which uses the position and rotational parameters
obtained from electromagnetic observations, can be carried out. Matched filtering maximizes the signalto-
noise (SNR) ratio, but a large sensitivity loss is expected in case of even a very small mismatch
between the assumed and the true signal parameters. For this reason, narrow-band analysis methods have
been developed, allowing a fully coherent search for gravitational waves from known pulsars over a
fraction of a hertz and several spin-down values. In this paper we describe a narrow-band search of
11 pulsars using data from Advanced LIGO’s first observing run. Although we have found several initial
outliers, further studies show no significant evidence for the presence of a gravitational wave signal.
Finally, we have placed upper limits on the signal strain amplitude lower than the spin-down limit for 5 of
the 11 targets over the bands searched; in the case of J1813-1749 the spin-down limit has been beaten for
the first time. For an additional 3 targets, the median upper limit across the search bands is below the
spin-down limit. This is the most sensitive narrow-band search for continuous gravitational waves carried
out so far
Erratum: “Searches for Gravitational Waves from Known Pulsars at Two Harmonics in 2015–2017 LIGO Data” (2019, ApJ, 879, 10)
Due to an error at the publisher, in the published article the number of pulsars presented in the paper is incorrect in multiple places throughout the text. Specifically, "222" pulsars should be "221." Additionally, the number of pulsars for which we have EM observations that fully overlap with O1 and O2 changes from "168" to "167." Elsewhere, in the machine-readable table of Table 1 and in Table 2, the row corresponding to pulsar J0952-0607 should be excised as well. Finally, in the caption for Table 2 the number of pulsars changes from "188" to "187.
First measurement of the Hubble Constant from a Dark Standard Siren using the Dark Energy Survey Galaxies and the LIGO/Virgo Binary–Black-hole Merger GW170814
International audienceWe present a multi-messenger measurement of the Hubble constant H 0 using the binary–black-hole merger GW170814 as a standard siren, combined with a photometric redshift catalog from the Dark Energy Survey (DES). The luminosity distance is obtained from the gravitational wave signal detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo Collaboration (LVC) on 2017 August 14, and the redshift information is provided by the DES Year 3 data. Black hole mergers such as GW170814 are expected to lack bright electromagnetic emission to uniquely identify their host galaxies and build an object-by-object Hubble diagram. However, they are suitable for a statistical measurement, provided that a galaxy catalog of adequate depth and redshift completion is available. Here we present the first Hubble parameter measurement using a black hole merger. Our analysis results in , which is consistent with both SN Ia and cosmic microwave background measurements of the Hubble constant. The quoted 68% credible region comprises 60% of the uniform prior range [20, 140] km s−1 Mpc−1, and it depends on the assumed prior range. If we take a broader prior of [10, 220] km s−1 Mpc−1, we find (57% of the prior range). Although a weak constraint on the Hubble constant from a single event is expected using the dark siren method, a multifold increase in the LVC event rate is anticipated in the coming years and combinations of many sirens will lead to improved constraints on H 0
Searches for gravitational waves from known pulsars at two harmonics in 2015-2017 LIGO data
International audienceWe present a search for gravitational waves from 222 pulsars with rotation frequencies ≳10 Hz. We use advanced LIGO data from its first and second observing runs spanning 2015–2017, which provides the highest-sensitivity gravitational-wave data so far obtained. In this search we target emission from both the l = m = 2 mass quadrupole mode, with a frequency at twice that of the pulsar’s rotation, and the l = 2, m = 1 mode, with a frequency at the pulsar rotation frequency. The search finds no evidence for gravitational-wave emission from any pulsar at either frequency. For the l = m = 2 mode search, we provide updated upper limits on the gravitational-wave amplitude, mass quadrupole moment, and fiducial ellipticity for 167 pulsars, and the first such limits for a further 55. For 20 young pulsars these results give limits that are below those inferred from the pulsars’ spin-down. For the Crab and Vela pulsars our results constrain gravitational-wave emission to account for less than 0.017% and 0.18% of the spin-down luminosity, respectively. For the recycled millisecond pulsar J0711−6830 our limits are only a factor of 1.3 above the spin-down limit, assuming the canonical value of 1038 kg m2 for the star’s moment of inertia, and imply a gravitational-wave-derived upper limit on the star’s ellipticity of 1.2 × 10−8. We also place new limits on the emission amplitude at the rotation frequency of the pulsars
GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs
We present the results from three gravitational-wave searches for coalescing compact binaries with component masses above 1 M⊙ during the first and second observing runs of the advanced gravitational-wave detector network. During the first observing run (O1), from September 12, 2015 to January 19, 2016, gravitational waves from three binary black hole mergers were detected. The second observing run (O2), which ran from November 30, 2016 to August 25, 2017, saw the first detection of gravitational waves from a binary neutron star inspiral, in addition to the observation of gravitational waves from a total of seven binary black hole mergers, four of which we report here for the first time: GW170729, GW170809, GW170818, and GW170823. For all significant gravitational-wave events, we provide estimates of the source properties. The detected binary black holes have total masses between 18.6−0.7+3.2 M⊙ and 84.4−11.1+15.8 M⊙ and range in distance between 320−110+120 and 2840−1360+1400 Mpc. No neutron star-black hole mergers were detected. In addition to highly significant gravitational-wave events, we also provide a list of marginal event candidates with an estimated false-alarm rate less than 1 per 30 days. From these results over the first two observing runs, which include approximately one gravitational-wave detection per 15 days of data searched, we infer merger rates at the 90% confidence intervals of 110−3840 Gpc−3 y−1 for binary neutron stars and 9.7−101 Gpc−3 y−1 for binary black holes assuming fixed population distributions and determine a neutron star-black hole merger rate 90% upper limit of 610 Gpc−3 y−1
- …