1,164 research outputs found
Charge Nematicity and Electronic Raman Scattering in Iron-based Superconductors
We review the recent developments in electronic Raman scattering measurements
of charge nematic fluctuations in iron-based superconductors. A simple
theoretical framework of a -wave Pomeranchuk transition is proposed in order
to capture the salient features of the spectra. We discuss the available Raman
data in the normal state of 122 iron-based systems, particularly Co doped
BaFeAs, and we show that the low energy quasi-elastic peak, the
extracted nematic susceptibility and the scattering rates are consistent with
an electronic driven structural phase transition. In the superconducting state
with a full gap the quasi-elastic peak transforms into a finite frequency
nematic resonance, evidences for which are particularly strong in the electron
doped systems. A crucial feature of the analysis is the fact that the
electronic Raman signal is unaffected by the acoustic phonons. This makes Raman
spectroscopy a unique probe of electronic nematicity.Comment: Invited review to a special issue in Iron-based superconductor
Quantum critical elasticity
We discuss elastic instabilities of the atomic crystal lattice at zero
temperature. Due to long-range shear forces of the solid, at such transitions
the phonon velocities vanish, if at all, only along certain crystallographic
directions, and, consequently, the critical phonon fluctuations are suppressed
to a lower dimensional manifold and governed by a Gaussian fixed-point. In case
of symmetry-breaking elastic transitions, a characteristic critical phonon
thermodynamics arises that is found, e.g., to violate Debye's -law for the
specific heat. We point out that quantum critical elasticity is triggered
whenever a critical soft mode couples linearly to the strain tensor. In
particular, this is relevant for the electronic Ising-nematic quantum phase
transition in a tetragonal crystal as discussed in the context of certain
cuprates, ruthenates and iron-based superconductors.Comment: 5 pages, 3 figures; (v2) rewritten introduction, supplement included;
(v3) minor changes, published versio
Nucleation-accumulation Mode Trade-off in Non-volatile Particle Emissions From a Small Non-road Small Diesel Engine
Small (\u3c 8 kW) non-road engines are a significant source of pollutants such as particle number (PN) emissions. Many small non-road engines do not have diesel particulate filters (DPFs). They are so designed that air–fuel ratio (AFR) can be adjusted to control visible diesel smoke and particulate matter (PM) resulting from larger accumulation mode particles. However, the effect of AFR variation on smaller nucleation mode nanoparticle emissions is not well understood. Several studies on larger engines have reported a trade-off between smaller and larger particles. In this study, AFR was independently varied over the entire engine map of a naturally aspirated (NA) non-road small diesel engine using forced induction (FI) of externally compressed air. AFR’s ranged from 57 to 239 compared to the design range of 23–92 for the engine, including unusually high AFR’s at full-load operation, not previously reported for conventional combustion. As expected, larger accumulation mode particles were lowered (up to 15 times) for FI operation. However, the smaller nucleation mode nanoparticles increased up to 15 times. Accumulation mode particles stopped decreasing above an AFR threshold while nucleation particles continuously increased. In-cylinder combustion analysis showed a slightly smaller ignition delay and higher burn rate for FI cases relative to NA operation. Much higher peak cylinder pressures were accompanied by much lower combustion and exhaust gas temperatures (EGT), due to higher in-cylinder mass during FI operation. Peak nucleation mode emissions were shown to be negatively correlated to EGT for all the data, collapsing on a single curve. This is consistent with some other studies reporting increased nucleation mode emissions (and higher accumulation mode particles) with decreased load, lower speed, lower EGR, advanced combustion phasing, and higher injection pressure, all of which reduce EGT. The nucleation-accumulation trade-off has been explained by the ‘adsorption hypothesis’ by some investigators. In the current work, an alternative/supplemental argument has been made for the possibility that lower cylinder temperatures during the late-burning phase (correlated to lower EGT) phase hampers oxidation of nucleation mode particles and increases nucleation mode emissions
Prethermal fragmentation in a periodically driven Fermionic chain
We study a Fermionic chain with nearest-neighbor hopping and density-density
interactions, where the nearest-neighbor interaction term is driven
periodically. We show that such a driven chain exhibits prethermal strong
Hilbert space fragmentation (HSF) in the high drive amplitude regime at
specific drive frequencies . This constitutes the first
realization of HSF for out-of-equilibrium systems. We obtain analytic
expressions of using a Floquet perturbation theory and
provide exact numerical computation of entanglement entropy, equal-time
correlation functions, and the density autocorrelation of Fermions for finite
chains. All of these quantities indicate clear signatures of strong HSF. We
study the fate of the HSF as one tunes away from and discuss
the extent of the prethermal regime as a function of the drive amplitude.Comment: v2: 3 figs, 4 pages + appendices. Refs added; minor change
Effects of Lifshitz Transition on Charge Transport in Magnetic Phases of Fe-Based Superconductors
The unusual temperature dependence of the resistivity and its in-plane
anisotropy observed in the Fe-based superconducting materials, particularly
Ba(FeCo)As, has been a longstanding puzzle. Here we
consider the effect of impurity scattering on the temperature dependence of the
average resistivity within a simple two-band model of a dirty spin density wave
metal. The sharp drop in resistivity below the N\'eel temperature in the
parent compound can only be understood in terms of a Lifshitz transition
following Fermi surface reconstruction upon magnetic ordering. We show that the
observed resistivity anisotropy in this phase, arising from nematic defect
structures, is affected by the Lifshitz transition as well.Comment: 8 pages, 5 figure
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