67 research outputs found
Large nonreciprocal absorption and emission of radiation in type-I Weyl semimetals with time reversal symmetry breaking
The equality between the spectral, directional emittance and absorptance of
an object under local thermal equilibrium is known as Kirchhoff's law of
radiation. The breakdown of Kirchhoff's law of radiation is physically allowed
by breaking time reversal symmetry and can open opportunities for nonreciprocal
light emitters and absorbers. Large anomalous Hall conductivity and angle
recently observed in topological Weyl semimetals, particularly type-I magnetic
Weyl semimetals and type-II Weyl semimetals, are expected to create large
nonreciprocal electromagnetic wave propagation. In this work, we focus on
type-I magnetic Weyl semimetals and show via modeling and simulation that
nonreciprocal surface plasmons polaritons can result in pronounced
nonreciprocity without an external magnetic field. The modeling in this work
begins with a single pair of Weyl nodes, followed by a more realistic model
with multiple paired Weyl nodes. Fermi-arc surface states are also taken into
account through the surface conductivity. This work points to the promising
applicability of topological Weyl semimetals for magneto-optical and energy
applications.Comment: 24 pages, 4 figure
High-performance chiral all-optical logic gate based on topological edge states of valley photonic crystal
For all-optical communication and information processing, it is necessary to
develop all-optical logic gates based on photonic structures that can directly
perform logic operations. All-optical logic gates have been demonstrated based
on conventional waveguides and interferometry, as well as photonic crystal
structures. Nonetheless, any defects in those structures will introduce high
scattering loss, which compromises the fidelity and contrast ratio of the
information process. Based on the spin-valley locking effect that can achieve
defect-immune unidirectional transmission of topological edge states in valley
photonic crystals (VPCs), we propose a high-performance all-optical logic OR
gate based on a VPC structure. By tuning the working bandwidth of the two input
channels, we prevent interference between the two channels to achieve a stable
and high-fidelity output. The transmittance of both channels is higher than
0.8, and a high contrast ratio of 28.8 dB is achieved. Moreover, the chirality
of the logic gate originated from the spin-valley locking effect allows using
different circularly polarized light as inputs, representing "1" or "0", which
is highly desired in quantum computing. The device's footprint is small,
allowing high-density on-chip integration. In addition, this design can be
experimentally fabricated using current nanofabrication techniques and will
have potential applications in optical communication, information processing,
and quantum computing.Comment: 10 pages, 6 figure
Quantized Thermoelectric Hall Effect Induces Giant Power Factor in a Topological Semimetal
Thermoelectrics are promising by directly generating electricity from waste
heat. However, (sub-)room-temperature thermoelectrics have been a long-standing
challenge due to vanishing electronic entropy at low temperatures. Topological
materials offer a new avenue for energy harvesting applications. Recent
theories predicted that topological semimetals at the quantum limit can lead to
a large, non-saturating thermopower and a quantized thermoelectric Hall
conductivity approaching a universal value. Here, we experimentally demonstrate
the non-saturating thermopower and quantized thermoelectric Hall effect in the
topological Weyl semimetal (WSM) tantalum phosphide (TaP). An ultrahigh
longitudinal thermopower Sxx= 1.1x10^3 muV/K and giant power factor ~525
muW/cm/K^2 are observed at ~40K, which is largely attributed to the quantized
thermoelectric Hall effect. Our work highlights the unique quantized
thermoelectric Hall effect realized in a WSM toward low-temperature energy
harvesting applications.Comment: 54 pages total, 5 main figures + 22 supplementary figures. To appear
in Nature Communications (2020
Through the Lens of Core Competency: Survey on Evaluation of Large Language Models
From pre-trained language model (PLM) to large language model (LLM), the
field of natural language processing (NLP) has witnessed steep performance
gains and wide practical uses. The evaluation of a research field guides its
direction of improvement. However, LLMs are extremely hard to thoroughly
evaluate for two reasons. First of all, traditional NLP tasks become inadequate
due to the excellent performance of LLM. Secondly, existing evaluation tasks
are difficult to keep up with the wide range of applications in real-world
scenarios. To tackle these problems, existing works proposed various benchmarks
to better evaluate LLMs. To clarify the numerous evaluation tasks in both
academia and industry, we investigate multiple papers concerning LLM
evaluations. We summarize 4 core competencies of LLM, including reasoning,
knowledge, reliability, and safety. For every competency, we introduce its
definition, corresponding benchmarks, and metrics. Under this competency
architecture, similar tasks are combined to reflect corresponding ability,
while new tasks can also be easily added into the system. Finally, we give our
suggestions on the future direction of LLM's evaluation
Deep Geophysical Anomalies Beneath the Changbaishan Volcano
Subsurface imaging is key to understanding the origin of intraplate volcanoes. The Changbaishan volcano, located about 2,000 km away from the western Pacific subduction zone, has several debated origins. To investigate this, we compared regional seismic tomography with the electrical resistivity results and obtained high-resolution 1D and quasi-2D velocity-depth profiles. We show that the upper mantle is characterized by two anomalies exhibiting distinct features which cannot be explained by the same mechanism. We document a localized low-velocity anomaly atop the 410-km discontinuity, where the P-wave velocity is reduced more than that of the S-wave (i.e., lower Vp/Vs). We propose that this anomaly is caused by the reduction of the effective moduli during the phase transformation of olivine. The other anomaly, located between 300 and 370 km depth, reveals a significant reduction of the S-wave velocity (i.e., higher Vp/Vs), associated with a reduction of the electrical resistivity, altogether consistent with partial melting
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