402 research outputs found
Probing topology by "heating": Quantized circular dichroism in ultracold atoms
We reveal an intriguing manifestation of topology, which appears in the
depletion rate of topological states of matter in response to an external
drive. This phenomenon is presented by analyzing the response of a generic 2D
Chern insulator subjected to a circular time-periodic perturbation: due to the
system's chiral nature, the depletion rate is shown to depend on the
orientation of the circular shake. Most importantly, taking the difference
between the rates obtained from two opposite orientations of the drive, and
integrating over a proper drive-frequency range, provides a direct measure of
the topological Chern number of the populated band (): this "differential
integrated rate" is directly related to the strength of the driving field
through the quantized coefficient . Contrary to the
integer quantum Hall effect, this quantized response is found to be non-linear
with respect to the strength of the driving field and it explicitly involves
inter-band transitions. We investigate the possibility of probing this
phenomenon in ultracold gases and highlight the crucial role played by edge
states in this effect. We extend our results to 3D lattices, establishing a
link between depletion rates and the non-linear photogalvanic effect predicted
for Weyl semimetals. The quantized circular dichroism revealed in this work
designates depletion-rate measurements as a universal probe for topological
order in quantum matter.Comment: 10 pages, 5 figures (including Sup. Mat.). Revised version, accepted
for publicatio
Effect of Coulomb interactions on the optical properties of doped graphene
Recent optical conductivity experiments of doped graphene in the infrared
regime reveal a strong background in the energy region between the intra and
interband transitions difficult to explain within conventional pictures. We
propose a phenomenological model taking into account the marginal Fermi liquid
nature of the quasiparticles in graphene near the neutrality point that can
explain qualitatively the observed features. We also study the electronic Raman
signal and suggest that it will also be anomalous.Comment: 4 pages, 2 figure
Second-harmonic generation in the topological multifold semimetal RhSi
Recent experiments in the topological Weyl semimetal TaAs have observed record-breaking second-harmonic generation (SHG), a nonlinear optical response at 2? generated by an incoming light source at ?. However, whether SHG is enhanced in topological semimetals in general is a challenging open question because their band structure entangles the contributions arising from trivial bands and topological band crossings. In this work, we circumvent this problem by studying RhSi, a chiral topological semimetal with a simple band structure with topological multifold fermions close to the Fermi energy. We measure SHG in a wide frequency window, ? [0.27,1.5]eV and, using first-principles calculations, we establish that, due to their linear dispersion, the contribution of multifold fermions to SHG is subdominant as compared with other regions in the Brillouin zone. Our calculations suggest that parts of the bands where the dispersion is relatively flat contribute significantly to SHG. As a whole, our results suggest avenues to enhance SHG responses. © 2022 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society
Magnetism and anomalous transport in the Weyl semimetal PrAlGe: Possible route to axial gauge fields
In magnetic Weyl semimetals, where magnetism breaks time-reversal symmetry,
large magnetically sensitive anomalous transport responses are anticipated that
could be useful for topological spintronics. The identification of new magnetic
Weyl semimetals is therefore in high demand, particularly since in these
systems Weyl node configurations may be easily modified using magnetic fields.
Here we explore experimentally the magnetic semimetal PrAlGe, and unveil a
direct correspondence between easy-axis Pr ferromagnetism and anomalous Hall
and Nernst effects. With sizes of both the anomalous Hall conductivity and
Nernst effect in good quantitative agreement with first principles
calculations, we identify PrAlGe as a system where magnetic fields can connect
directly to Weyl nodes via the Pr magnetization. Furthermore, we find the
predominantly easy-axis ferromagnetic ground state co-exists with a low density
of nanoscale textured magnetic domain walls. We describe how such nanoscale
magnetic textures could serve as a local platform for tunable axial gauge
fields of Weyl fermions.Comment: 42 pages, 5 figure
Determination of the E2/M1 Ratio in the \gamma N \to \Delta(1232) Transition from a Simultaneous Measurement of p(\vec{\gamma},p)\pi^0 and p(\vec{\gamma},\pi^+)n
Tagged linearly polarized photons have been used at the Mainz Microtron MAMI
for simultaneous measurements of the p(\vec{\gamma},p)\pi^0 and
p(\vec{\gamma},\pi^+)n reaction channels to study the \gamma N \to \Delta(1232)
transition. The energy dependence of the magnetic dipole M_{1+}^{3/2} and
electric quadrupole E_{1+}^{3/2} amplitudes have been extracted from these data
in the photon energy range from 270 to 420 MeV. The E2/M1 ratio for the \gamma
N \to \Delta(1232) transition has been determined to be -
(2.5+-0.1_{stat}+-0.2_{sys}) % at the resonance position delta_{33}=90^0.Comment: 25 pages Latex including 13 postscript figures submitted for
publication in Phys. Rev.
Gate-tuning of graphene plasmons revealed by infrared nano-imaging
Surface plasmons are collective oscillations of electrons in metals or
semiconductors enabling confinement and control of electromagnetic energy at
subwavelength scales. Rapid progress in plasmonics has largely relied on
advances in device nano-fabrication, whereas less attention has been paid to
the tunable properties of plasmonic media. One such medium-graphene-is amenable
to convenient tuning of its electronic and optical properties with gate
voltage. Through infrared nano-imaging we explicitly show that common
graphene/SiO2/Si back-gated structures support propagating surface plasmons.
The wavelength of graphene plasmons is of the order of 200 nm at
technologically relevant infrared frequencies, and they can propagate several
times this distance. We have succeeded in altering both the amplitude and
wavelength of these plasmons by gate voltage. We investigated losses in
graphene using plasmon interferometry: by exploring real space profiles of
plasmon standing waves formed between the tip of our nano-probe and edges of
the samples. Plasmon dissipation quantified through this analysis is linked to
the exotic electrodynamics of graphene. Standard plasmonic figures of merits of
our tunable graphene devices surpass that of common metal-based structures.Comment: 15 pages, 3 figure
The formation of human populations in South and Central Asia
By sequencing 523 ancient humans, we show that the primary source of ancestry in modern South Asians is a prehistoric genetic gradient between people related to early hunter-gatherers of Iran and Southeast Asia. After the Indus Valley Civilization’s decline, its people mixed with individuals in the southeast to form one of the two main ancestral populations of South Asia, whose direct descendants live in southern India. Simultaneously, they mixed with descendants of Steppe pastoralists who, starting around 4000 years ago, spread via Central Asia to form the other main ancestral population. The Steppe ancestry in South Asia has the same profile as that in Bronze Age Eastern Europe, tracking a movement of people that affected both regions and that likely spread the distinctive features shared between Indo-Iranian and Balto-Slavic languages
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