253 research outputs found
Edge instabilities of topological superconductors
Nodal topological superconductors display zero-energy Majorana flat bands at
generic edges. The flatness of these edge bands, which is protected by
time-reversal and translation symmetry, gives rise to an extensive ground-state
degeneracy. Therefore, even arbitrarily weak interactions lead to an
instability of the flat-band edge states towards time-reversal and
translation-symmetry-broken phases, which lift the ground-state degeneracy. We
examine the instabilities of the flat-band edge states of d_{xy}-wave
superconductors by performing a mean-field analysis in the Majorana basis of
the edge states. The leading instabilities are Majorana mass terms, which
correspond to coherent superpositions of particle-particle and particle-hole
channels in the fermionic language. We find that attractive interactions induce
three different mass terms. One is a coherent superposition of imaginary s-wave
pairing and current order, and another combines a charge-density-wave and
finite-momentum singlet pairing. Repulsive interactions, on the other hand,
lead to ferromagnetism together with spin-triplet pairing at the edge. Our
quantum Monte Carlo simulations confirm these findings and demonstrate that
these instabilities occur even in the presence of strong quantum fluctuations.
We discuss the implications of our results for experiments on cuprate
high-temperature superconductors.Comment: 4 pages, 3 figure
Spontaneous particle-hole symmetry breaking of correlated fermions on the Lieb lattice
We study spinless fermions with nearest-neighbor repulsive interactions
(- model) on the two-dimensional three-band Lieb lattice. At
half-filling, the free electronic band structure consists of a flat band at
zero energy and a single cone with linear dispersion. The flat band is expected
to be unstable upon inclusion of electronic correlations, and a natural channel
is charge order. However, due to the three-orbital unit cell, commensurate
charge order implies an imbalance of electron and hole densities and therefore
doping away from half-filling. Our numerical results show that below a
finite-temperature Ising transition a charge density wave with one electron and
two holes per unit cell and its partner under particle-hole transformation are
spontaneously generated. Our calculations are based on recent advances in
auxiliary-field and continuous-time quantum Monte Carlo simulations that allow
sign-free simulations of spinless fermions at half-filling. It is argued that
particle-hole symmetry breaking provides a route to access levels of finite
doping, without introducing a sign problem.Comment: 9 pages, 6 figures, added data for strong Coulomb repulsion and
classical Ising-limi
Laser Improves Biogas Production by Anaerobic Digestion of Cow Dung
This study investigates the digestion of cow dung (CD) for biogas production at laboratory scales. The study was carried out through anaerobic fermentation using cow dung as substrate. The digester was operated at ambient temperatures of 39.5 °C for a period of 10 days. The effect of iron powder in controlling the production of hydrogen sulfide (H2S) has been tested. The optimum concentration of iron powder was 4g/L with the highest biogas production. A Q – swatch Nd:YAG laser has been used to mix and homogenize the components of one of the six digesters and accelerate digestion. At the end of digestion, all digestions effluent was subjected to 5 laser pulses with 250mJ/pules to dispose waste biomass
Dynamic charge correlations near the Peierls transition
The quantum phase transition between a repulsive Luttinger liquid and an
insulating Peierls state is studied in the framework of the one-dimensional
spinless Holstein model. We focus on the adiabatic regime but include the full
quantum dynamics of the phonons. Using continuous-time quantum Monte Carlo
simulations, we track in particular the dynamic charge structure factor and the
single-particle spectrum across the transition. With increasing electron-phonon
coupling, the dynamic charge structure factor reveals the emergence of a charge
gap, and a clear signature of phonon softening at the zone boundary. The
single-particle spectral function evolves continuously across the transition.
Hybridization of the charge and phonon modes of the Luttinger liquid
description leads to two modes, one of which corresponds to the coherent
polaron band. This band acquires a gap upon entering the Peierls phase, whereas
the other mode constitutes the incoherent, high-energy spectrum with backfolded
shadow bands. Coherent polaronic motion is a direct consequence of quantum
lattice fluctuations. In the strong-coupling regime, the spectrum is described
by the static, mean-field limit. Importantly, whereas finite electron density
in general leads to screening of polaron effects, the latter reappear at half
filling due to charge ordering and lattice dimerization.Comment: 8 pages, 7 figures, final versio
Effect of Electron-Phonon Interaction Range for a Half-Filled Band in One Dimension
We demonstrate that fermion-boson models with nonlocal interactions can be
simulated at finite band filling with the continuous-time quantum Monte Carlo
method. We apply this method to explore the influence of the electron-phonon
interaction range for a half-filled band in one dimension, covering the full
range from the Holstein to the Fr\"ohlich regime. The phase diagram contains
metallic, Peierls, and phase-separated regions, which we characterize in terms
of static and dynamical correlation functions. In particular, our results
reveal a suppression of charge correlations with increasing interaction
range, allowing for a power-law decay comparable to the pairing correlations.Comment: 5 pages, 5 figure
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