59,289 research outputs found
Berryogenesis: self-induced Berry flux and spontaneous non-equilibrium magnetism
Spontaneous symmetry breaking is central to the description of interacting
phases of matter. Here we reveal a new mechanism through which a driven
interacting system subject to a time-reversal symmetric driving field can
spontaneously magnetize. We show that the strong internal ac fields of a metal
driven close to its plasmon resonance may enable Berryogenesis: the spontaneous
generation of a self-induced Bloch band Berry flux. The self-induced Berry flux
supports and is sustained by a circulating plasmonic motion, which may arise
even for a linearly polarized driving field. This non-equilibrium phase
transition occurs above a critical driving amplitude, and may be of either
continuous or discontinuous type. Berryogenesis relies on feedback due to
interband coherences induced by internal fields, and may readily occur in a
wide variety of multiband systems. We anticipate that graphene devices, in
particular, provide a natural platform to achieve Berryogenesis and
plasmon-mediated spontaneous non-equilibrium magnetization in present-day
devices
Energy-driven Drag at Charge Neutrality in Graphene
Coulomb coupling between proximal layers in graphene heterostructures results
in efficient energy transfer between the layers. We predict that, in the
presence of correlated density inhomogeneities in the layers, vertical energy
transfer has a strong impact on lateral charge transport. In particular, for
Coulomb drag it dominates over the conventional momentum drag near zero doping.
The dependence on doping and temperature, which is different for the two drag
mechanisms, can be used to separate these mechanisms in experiment. We predict
distinct features such as a peak at zero doping and a multiple sign reversal,
which provide diagnostics for this new drag mechanism.Comment: 6 pgs, 3 fg
Chiral plasmons without magnetic field
Plasmons, the collective oscillations of interacting electrons, possess
emergent properties that dramatically alter the optical response of metals. We
predict the existence of a new class of plasmons -- chiral Berry plasmons
(CBPs) -- for a wide range of two-dimensional metallic systems including gapped
Dirac materials. As we show, in these materials the interplay between Berry
curvature and electron-electron interactions yields chiral plasmonic modes at
zero magnetic field. The CBP modes are confined to system boundaries, even in
the absence of topological edge states, with chirality manifested in split
energy dispersions for oppositely directed plasmon waves. We unveil a rich CBP
phenomenology and propose setups for realizing them, including in anomalous
Hall metals and optically-pumped 2D Dirac materials. Realization of CBPs will
offer a new paradigm for magnetic field-free, sub-wavelength optical
non-reciprocity, in the mid IR-THz range, with tunable splittings as large as
tens of THz, as well as sensitive all-optical diagnostics of topological bands.Comment: 10 pgs, 3 fg
THEORETICAL STUDIES OF BILIPROTEIN CHROMOPHORES AND RELATED BILE PIGMENTS BY MOLECULAR ORBITAL AND RAMACHANDRAN TYPE CALCULATIONS
Ramachandran calculations have been used to gain insight into steric hindrance in bile
pigments related to biliprotein chromophores. The high optical activity of denatured phycocyanin, as
compared to phycoerythrin, has been related to the asymmetric substitution at ring A, which shifts the
equilibrium towards the P-helical form of the chromophore. Geometric effects on the electronic structures
and transitions have then been studied by molecular orbital calculations for several conjugation
systems including the chromophores of phycocyanin. phytochrome P,, cations, cation radicals and
tautomeric forms. For these different chromophores some general trends can be deduced. For instance,
for a given change in the gross shape (e.g. either unfolding of the molecule from a cyclic-helical to a fully
extended geometry, or upon out-of-plane twists of the pyrrole ring A) of the molecules under study, the
predicted absorption spectra all change in a simikar way. Nonetheless, there are characteristic distinctions
between the different n-systems, both in the transition energies and the charge distribution, which
can be related to their known differences in spectroscopic properties and their reactivity
A new calibration method for time delay standard and its application
A method which is used to measure time delay accurately by using a Type 900-LB slotted line is described. The accuracy for calibrating time delay of a precision coaxial air line Type 900-L is about + or - (0.4 to 0.6)ps, and for coaxial cables with VSWR less than 1.5 and time delay t less than 50ns is about + or - (3 to 5)ps. Theoretical analysis and mathematical derivation of microwave networks in cascade are given. Methods to eliminate the errors which are caused by the discontinuities and the error analysis of the measuring system are presented. Skin effect analysis of the transient characteristic of coaxial transmission line are discussed in detail. Methods to eliminate the errors which result from using the calibrated time delay standard to calibrate time interval measurement instruments are presented. The estimation of errors and formulae for correction of those errors are described
Topological Bloch Bands in Graphene Superlattices
We outline an approach to endow a plain vanilla material with topological
properties by creating topological bands in stacks of manifestly nontopological
atomically thin materials. The approach is illustrated with a model system
comprised of graphene stacked atop hexagonal-boron-nitride. In this case, the
Berry curvature of the electron Bloch bands is highly sensitive to the stacking
configuration. As a result, electron topology can be controlled by crystal axes
alignment, granting a practical route to designer topological materials. Berry
curvature manifests itself in transport via the valley Hall effect and
long-range chargeless valley currents. The non-local electrical response
mediated by such currents provides diagnostics for band topology
Shockley-Ramo theorem and long-range photocurrent response in gapless materials
Scanning photocurrent maps of gapless materials, such as graphene, often
exhibit complex patterns of hot spots positioned far from current-collecting
contacts. We develop a general framework that helps to explain the unusual
features of the observed patterns, such as the directional effect and the
global character of photoresponse. We show that such a response is captured by
a simple Shockley-Ramo-type approach. We examine specific examples and show
that the photoresponse patterns can serve as a powerful tool to extract
information about symmetry breaking, inhomogeneity, chirality, and other local
characteristics of the system.Comment: 7 pgs, 3 fg
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