323 research outputs found
Local density of states of the one-dimensional spinless fermion model
We investigate the local density of states of the one-dimensional half-filled
spinless fermion model with nearest-neighbor hopping t>0 and interaction V in
its Luttinger liquid phase -2t < V <= 2t. The bulk density of states and the
local density of states in open chains are calculated over the full band width
4t with an energy resolution <= 0.08t using the dynamical density-matrix
renormalization group (DDMRG) method. We also perform DDMRG simulations with a
resolution of 0.01t around the Fermi energy to reveal the power-law behaviour
predicted by the Luttinger liquid theory for bulk and boundary density of
states. The exponents are determined using a finite-size scaling analysis of
DDMRG data for lattices with up to 3200 sites. The results agree with the exact
exponents given by the Luttinger liquid theory combined with the Bethe Ansatz
solution. The crossover from boundary to bulk density of states is analyzed. We
have found that boundary effects can be seen in the local density of states at
all energies even far away from the chain edges
Phase separation in the Edwards model
The nature of charge transport within a correlated background medium can be
described by spinless fermions coupled to bosons in the model introduced by
Edwards. Combining numerical density matrix renormalization group and
analytical projector-based renormalization methods we explore the ground-state
phase diagram of the Edwards model in one dimension. Below a critical boson
frequency any long-range order disappears and the system becomes metallic. If
the charge carriers are coupled to slow quantum bosons the Tomonaga-Luttinger
liquid is attractive and finally makes room for a phase separated state, just
as in the t-J model. The phase boundary separating repulsive from the
attractive Tomonaga-Luttinger liquid is determined from long-wavelength charge
correlations, whereas fermion segregation is indicated by a vanishing inverse
compressibility. On approaching phase separation the photoemission spectra
develop strong anomalies.Comment: 6 pages, 5 figures, final versio
Characterization of Mott-insulating and superfluid phases in the one-dimensional Bose--Hubbard model
We use strong-coupling perturbation theory, the variational cluster approach
(VCA), and the dynamical density-matrix renormalization group (DDMRG) method to
investigate static and dynamical properties of the one-dimensional
Bose--Hubbard model in both the Mott-insulating and superfluid phases. From the
von Neumann entanglement entropy we determine the central charge and the
transition points for the first two Mott lobes. Our DMRG results for the
ground-state energy, momentum distribution function, boson correlation function
decay, Mott gap, and single particle-spectral function are reproduced very well
by the strong-coupling expansion to fifth order, and by VCA with clusters up to
12 sites as long as the ratio between the hopping amplitude and on-site
repulsion, t/U, is smaller than 0.15 and 0.25, respectively. In addition, in
the superfluid phase VCA captures well the ground-state energy and the sound
velocity of the linear phonon modes. This comparison provides an authoritative
estimate for the range of applicability of these methods. In strong-coupling
theory for the Mott phase, the dynamical structure factor is obtained from the
solution of an effective single-particle problem with an attractive potential.
The resulting resonances show up as double-peak structure close to the
Brillouin zone boundary. These high-energy features also appear in the
superfluid phase which is characterized by a pronounced phonon mode at small
momenta and energies, as predicted by Bogoliubov and field theory. In one
dimension, there are no traces of an amplitude mode in the dynamical
single-particle and two-particle correlation functions.Comment: 15 pages, 12 figure
Local spectral properties of Luttinger liquids: scaling versus nonuniversal energy scales
Motivated by recent scanning tunneling and photoemission spectroscopy
measurements on self-organized gold chains on a germanium surface we
reinvestigate the local single-particle spectral properties of Luttinger
liquids. In the first part we use the bosonization approach to exactly compute
the local spectral function of a simplified field theoretical low-energy model
and take a closer look at scaling properties as a function of the ratio of
energy and temperature. Translational invariant Luttinger liquids as well as
those with an open boundary (cut chain geometry) are considered. We explicitly
show that the scaling functions of both setups have the same analytic form. The
scaling behavior suggests a variety of consistency checks which can be
performed on measured data to experimentally verify Luttinger liquid behavior.
In a second part we approximately compute the local spectral function of a
microscopic lattice model---the extended Hubbard model---close to an open
boundary using the functional renormalization group. We show that as a function
of energy and temperature it follows the field theoretical prediction in the
low-energy regime and point out the importance of nonuniversal energy scales
inherent to any microscopic model. The spatial dependence of this spectral
function is characterized by oscillatory behavior and an envelope function
which follows a power law both in accordance with the field theoretical
continuum model. Interestingly, for the lattice model we find a phase shift
which is proportional to the two-particle interaction and not accounted for in
the standard bosonization approach to Luttinger liquids with an open boundary.
We briefly comment on the effects of several one-dimensional branches cutting
the Fermi energy and Rashba spin-orbit interaction.Comment: 19 pages, 5 figures, version as accepted for publication in J.
Phys.:Condensed Matte
Strong coupling expansion for the Bose-Hubbard and the Jaynes-Cummings lattice model
A strong coupling expansion, based on the Kato-Bloch perturbation theory,
which has recently been proposed by Eckardt et al. [Phys. Rev. B 79, 195131]
and Teichmann et al. [Phys. Rev. B 79, 224515] is implemented in order to study
various aspects of the Bose-Hubbard and the Jaynes-Cummings lattice model. The
approach, which allows to generate numerically all diagrams up to a desired
order in the interaction strength is generalized for disordered systems and for
the Jaynes-Cummings lattice model. Results for the Bose-Hubbard and the
Jaynes-Cummings lattice model will be presented and compared with results from
VCA and DMRG. Our focus will be on the Mott insulator to superfluid transition.Comment: 29 pages, 21 figure
Anharmonicity in one-dimensional electron-phonon system
We investigate the effect of anharmonicity on the one-dimensional half-filled
Holstein model by using the determinant quantum Monte Carlo method. By
calculating the order parameters we find that with and without anharmonicity
there is always an transition from a disorder phase to a dimerized phase.
Moreover, in the dimerized phase a lattice dimerization and a charge density
wave coexist. The anharmonicity represented by the quartic term suppresses the
dimerization as well as the charge density wave, while a double-well potential
favors the dimerization. In addition, by calculating the correlation exponents
we show that the disorder phase is metallic with gapless charge excitations and
gapful spin excitations while in the dimerized phase both excitations are
gapful.Comment: 5 page
Toward fulfilling the aspirational goal of science as self-correcting: A call for editorial courage and diligence for error correction
Science is often described as ‘self-correcting’. Correction of scientific errors is vital, but it does not occur spontaneously. Rather, correction depends on individual scientists behaving in accordance with the self-correction norm. Some authors have suggested that failure to correct certain errors be considered scientific misconduct. But when serious errors are found, our experience suggests that corrections are not always expeditious, thorough, clear, and open. Herein, we address journals’ distinctive roles in correcting peer-reviewed scientific literature. The scientific community needs key individuals, including journal editors, to facilitate the correction process and to adjudicate disagreements in the field
Discrete Symmetry Breaking Transitions Between Paired Superfluids
We explore the zero-temperature phase diagram of bosons interacting via
Feshbach resonant pairing interactions in one dimension. Using DMRG (Density
Matrix Renormalization Group) and field theory techniques we characterize the
phases and quantum phase transitions in this low-dimensional setting. We
provide a broad range of evidence in support of an Ising quantum phase
transition separating distinct paired superfluids, including results for the
energy gaps, correlation functions and entanglement entropy. In particular, we
show that the Ising correlation length, order parameter and critical properties
are directly accessible from a ratio of the atomic and molecular two-point
functions. We further demonstrate that both the zero-momentum occupation
numbers and the visibility are in accordance with the absence of a purely
atomic superfluid phase. We comment on the connection to recent studies of
boson pairing in a generalized classical XY model.Comment: 18 pages, 22 figure
Courtship Initiation Is Stimulated by Acoustic Signals in Drosophila melanogaster
Finding a mating partner is a critical task for many organisms. It is in the interest of males to employ multiple sensory modalities to search for females. In Drosophila melanogaster, vision is thought to be the most important courtship stimulating cue at long distance, while chemosensory cues are used at relatively short distance. In this report, we show that when visual cues are not available, sounds produced by the female allow the male to detect her presence in a large arena. When the target female was artificially immobilized, the male spent a prolonged time searching before starting courtship. This delay in courtship initiation was completely rescued by playing either white noise or recorded fly movement sounds to the male, indicating that the acoustic and/or seismic stimulus produced by movement stimulates courtship initiation, most likely by increasing the general arousal state of the male. Mutant males expressing tetanus toxin (TNT) under the control of Gr68a-GAL4 had a defect in finding active females and a delay in courtship initiation in a large arena, but not in a small arena. Gr68a-GAL4 was found to be expressed pleiotropically not only in putative gustatory pheromone receptor neurons but also in mechanosensory neurons, suggesting that Gr68a-positive mechanosensory neurons, not gustatory neurons, provide motion detection necessary for courtship initiation. TNT/Gr68a males were capable of discriminating the copulation status and age of target females in courtship conditioning, indicating that female discrimination and formation of olfactory courtship memory are independent of the Gr68a-expressing neurons that subserve gustation and mechanosensation. This study suggests for the first time that mechanical signals generated by a female fly have a prominent effect on males' courtship in the dark and leads the way to studying how multimodal sensory information and arousal are integrated in behavioral decision making
Association of Increased Serum S100B Levels With High School Football Subconcussive Head Impacts
Astrocyte-enriched marker, S100B, shows promise for gauging the severity of acute brain trauma, and understanding subconcussive effects will advance its utility in tracking real-time acute brain damage. The aim of the study was to investigate whether serum S100B elevations were associated with frequency and magnitude of subconcussive head impacts in adolescents. This prospective cohort study of 17 high-school football players consisted of the following 12 time points: pre-season baseline, 5 in-season pre-post games, and post-season. A sensor-installed mouthguard recorded the number of head impacts, peak linear (PLA) and peak rotational (PRA) head accelerations from every practice and game. During the 5 games, players wore chest-strap heart-rate monitors to estimate players' excess post-exercise oxygen consumption (EPOC), accounting for physical exertion effects. At each time point, blood samples were obtained and assessed for S100B and creatine kinase levels to account for astrocyte damage/activation and muscle damage, respectively. Using k-means clustering on the impact data, players were categorized into high- or low-impact group. Two players withdrew during the first month of the study. A total of 156 blood samples from 15 players were assessed for S100B and creatine kinase levels and included in the analysis. A median value of 596 head impacts from 15 players were recorded during all practices and games in a season. S100B levels were significantly elevated in all post-game measures compared with the respective pre-game values (median-increase, 0.022 μg/L; interquartile-range, 0.011–0.043 μg/L, p < 0.05 for all games). Greater acute S100B increases were significantly associated with greater impact frequency, sum of PLA and PRA, with negligible contributions from physical exertion and muscle damage effects. The high-impact group exhibited greater increases in serum S100B levels at post-games than the low-impact group (high vs. low, 0.043 ± 0.035 μg/L vs. 0.019 ± 0.017 μg/L, p = 0.002). The degree of acute S100B increases was correlated with subconcussive head impact exposure, suggesting that acute astrocyte damage may be induced in an impact-dependent manner. Acute changes in serum S100B levels may become a useful tool in monitoring real-time brain damage in sports
- …