145 research outputs found
Multiferroic and Ferroic Topological Order in Ligand-Functionalized Germanene and Arsenene
Two-dimensional (2D) materials that exhibit ferroelectric, ferromagnetic, or topological order have been a major focal topic of nanomaterials research in recent years. The latest efforts in this field explore 2D quantum materials that host multiferroic or concurrent ferroic and topological order. We present a computational discovery of multiferroic state with coexisting ferroelectric and ferromagnetic order in recently synthesized CH2OCH3-functionalized germanene. We show that an electric-field-induced rotation of the ligand CH2OCH3 molecule can serve as the driving mechanism to switch the electric polarization of the ligand molecule, while unpassivated Ge p(z) orbits generate ferromagnetism. Our study also reveals coexisting ferroelectric and topological order in ligand-functionalized arsenene, which possesses a switchable electric polarization and a Dirac transport channel. These findings offer insights into the fundamental physics underlying these coexisting quantum orders and open avenues for achieving states of matter with multiferroic or ferroic-topological order in 2D-layered materials for innovative memory or logic device implementations
Significant shallow–depth soil warming over Russia during the past 40 years
Knowledge of the spatiotemporal dynamics of the soil temperature in cold environment is key to understanding the effects of climate change on land-atmosphere feedback and ecosystem functions. Here, we quantify the recent thermal status and trends in shallow ground using the most up-to-date data set of over 457 sites in Russia. The data set consists of in situ soil temperatures at multiple depths (0.8, 1.6, and 3.2 m) collected from 1975 to 2016. For the region as a whole, significant soil warming occurred over the period. The mean annual soil temperature at depths of 0.8, 1.6, and 3.2 m increased at the same level, at ca 0.30-0.31 degrees C/decade, whereas the increase in maximum soil temperature ranged from 0.40 degrees C/decade at 0.8 m to 0.31 degrees C/decade at 3.2 m. Unlike the maximum soil temperature, the increases in minimum soil temperature did not vary (ca 0.25 degrees C/decade) with depth. Due to the overall greater increase in maximum soil temperature than minimum soil temperature, the intra-annual variability of soil temperature increased over the decades. Moreover, the soil temperature increased faster in the continuous permafrost area than in the discontinuous permafrost and seasonal frost areas at shallow depths (0.8 and 1.6 m depth), and increased slower at the deeper level (3.2 m). The warming rate of the maximum soil temperature at the shallower depths was less than that at the deeper level over the discontinuous permafrost area but greater over the seasonal frost area. However, the opposite was found regarding the increase in minimum soil temperature. Correlative analyses suggest that the trends in mean and extreme soil temperatures positively relate to the trends in snow cover thickness and duration, which results in the muted response of intra-annual variability of the soil temperature as snow cover changes. This study provides a comprehensive view of the decadal evolutions of the shallow soil temperatures over Russia, revealing that the temporal trends in annual mean and extreme soil temperatures vary with depth and permafrost distribution.Peer reviewe
Decadal Changes in Soil and Atmosphere Temperature Differences Linked With Environment Shifts Over Northern Eurasia
Peer reviewe
Proximity Enhanced Quantum Spin Hall State in Graphene
Graphene is the first model system of two-dimensional topological insulator
(TI), also known as quantum spin Hall (QSH) insulator. The QSH effect in
graphene, however, has eluded direct experimental detection because of its
extremely small energy gap due to the weak spin-orbit coupling. Here we predict
by ab initio calculations a giant (three orders of magnitude) proximity induced
enhancement of the TI energy gap in the graphene layer that is sandwiched
between thin slabs of Sb2Te3 (or MoTe2). This gap (1.5 meV) is accessible by
existing experimental techniques, and it can be further enhanced by tuning the
interlayer distance via compression. We reveal by a tight-binding study that
the QSH state in graphene is driven by the Kane-Mele interaction in competition
with Kekul\'e deformation and symmetry breaking. The present work identifies a
new family of graphene-based TIs with an observable and controllable bulk
energy gap in the graphene layer, thus opening a new avenue for direct
verification and exploration of the long-sought QSH effect in graphene.Comment: 4 figures in Carbon, 201
MPrompt: Exploring Multi-level Prompt Tuning for Machine Reading Comprehension
The large language models have achieved superior performance on various
natural language tasks. One major drawback of such approaches is they are
resource-intensive in fine-tuning new datasets. Soft-prompt tuning presents a
resource-efficient solution to fine-tune the pre-trained language models (PLMs)
while keeping their weight frozen. Existing soft prompt methods mainly focus on
designing the input-independent prompts that steer the model to fit the domain
of the new dataset. Those methods often ignore the fine-grained information
about the task and context of the text. In this paper, we propose a multi-level
prompt tuning (MPrompt) method for machine reading comprehension. It utilizes
prompts at task-specific, domain-specific, and context-specific levels to
enhance the comprehension of input semantics at different granularities. We
also propose an independence constraint to steer each domain-specific prompt to
focus on information within its domain to avoid redundancy. Moreover, we
present a prompt generator that incorporates context-related knowledge in the
prompt generation to enhance contextual relevancy. We conducted extensive
experiments on 12 benchmarks of various QA formats and achieved an average
improvement of 1.94\% over the state-of-the-art methods.Comment: 13 pages, 5 figures, accepted by EMNLP2023-Finding
Anisotropic Ripple Deformation in Phosphorene
Two-dimensional materials tend to become crumpled according to the
Mermin-Wagner theorem, and the resulting ripple deformation may significantly
influence electronic properties as observed in graphene and MoS2. Here we
unveil by first-principles calculations a new, highly anisotropic ripple
pattern in phosphorene, a monolayer black phosphorus, where compression induced
ripple deformation occurs only along the zigzag direction in the strain range
up to 10%, but not the armchair direction. This direction-selective ripple
deformation mode in phosphorene stems from its puckered structure with coupled
hinge-like bonding configurations and the resulting anisotropic Poisson ratio.
We also construct an analytical model using classical elasticity theory for
ripple deformation in phosphorene under arbitrary strain. The present results
offer new insights into the mechanisms governing the structural and electronic
properties of phosphorene crucial to its device applications.Comment: J. Phys. Chem. Lett. 201
Graphene-based topological insulator with an intrinsic bulk band gap above room temperature
Topological insulators (TIs) represent a new quantum state of matter
characterized by robust gapless states inside the insulating bulk gap. The
metallic edge states of a two-dimensional (2D) TI, known as quantum spin Hall
(QSH) effect, are immune to backscattering and carry fully spin-polarized
dissipationless currents. However, existing 2D TIs realized in HgTe and
InAs/GaSb suffer from small bulk gaps (<10 meV) well below room temperature,
thus limiting their application in electronic and spintronic devices. Here, we
report a new 2D TI comprising a graphene layer sandwiched between two Bi2Se3
slabs that exhibits a large intrinsic bulk band gap of 30 to 50 meV, making it
viable for room-temperature applications. Distinct from previous strategies for
enhancing the intrinsic spin-orbit coupling effect of the graphene lattice, the
present graphene-based TI operates on a new mechanism of strong inversion
between graphene Dirac bands and Bi2Se3 conduction bands. Strain engineering
leads to effective control and substantial enhancement of the bulk gap.
Recently reported synthesis of smooth graphene/Bi2Se3 interfaces demonstrates
feasibility of experimental realization of this new 2D TI structure, which
holds great promise for nanoscale device applications.Comment: 3 figures, 1 tabl
Opening Band Gap without Breaking Lattice Symmetry: A New Route toward Robust Graphene-Based Nanoelectronics
Developing graphene-based nanoelectronics hinges on opening a band gap in the
electronic structure of graphene, which is commonly achieved by breaking the
inversion symmetry of the graphene lattice via an electric field (gate bias) or
asymmetric doping of graphene layers. Here we introduce a new design strategy
that places a bilayer graphene sheet sandwiched between two cladding layers of
materials that possess strong spin-orbit coupling (e.g., Bi2Te3). Our ab initio
and tight-binding calculations show that proximity enhanced spin-orbit coupling
effect opens a large (44 meV) band gap in bilayer graphene without breaking its
lattice symmetry, and the band gap can be effectively tuned by interlayer
stacking pattern and significantly enhanced by interlayer compression. The
feasibility of this quantum-well structure is demonstrated by recent
experimental realization of high-quality heterojunctions between graphene and
Bi2Te3, and this design also conforms to existing fabrication techniques in the
semiconductor industry. The proposed quantum-well structure is expected to be
especially robust since it does not require an external power supply to open
and maintain a band gap, and the cladding layers provide protection against
environmental degradation of the graphene layer in its device applications
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