123 research outputs found
Quantum-trajectory analysis for charge transfer in solid materials induced by strong laser fields
We investigate the dependence of charge transfer on the intensity of driving
laser field when SiO2 crystal is irradiated by an 800 nm laser. It is
surprising that the direction of charge transfer undergoes a sudden reversal
when the driving laser intensity exceeds critical values with different carrier
envelope phases. By applying quantum-trajectory analysis, we find that the
Bloch oscillation plays an important role in charge transfer in solid. Also, we
study the interaction of strong laser with gallium nitride (GaN) that is widely
used in optoelectronics. A pump-probe scheme is applied to control the quantum
trajectories of the electrons in the conduction band. The signal of charge
transfer is controlled successfully by means of theoretically proposed
approach
QUANTUM TRANSPORT IN SEMICONDUCTOR NANOSTRUCTURES
Advances in complex oxide heterostructures have opened up a novel era in material sciences, and subsequently driven extensive physical research efforts to explore the underlying mechanisms. Material systems show a wide variety of phenomena such as superconductivity, magnetism, ferroelectricity, and spin-orbit coupling, all of which are gate-tunable. This facet demonstrates their significant importance, not only for science and research, but also for their substantial technological applications. One of the most promising systems is the LaAlO3/SrTiO3 heterostructure in which several unit cells of LaAlO3 are deposited on TiO2-terminated SrTiO3, giving rise to a two dimensional electron liquid (2DEL) at the interface region. In the case of 3-unit-cell (3 u.c.) LaAlO3/SrTiO3 interfaces, a conductive atomic force microscope (c-AFM) tip can be utilized to “write” and “erase” nanoscale devices, making LaAlO3/SrTiO3 a highly flexible platform application in novel nanoelectronics. In this thesis, the c-AFM lithography technique was employed to create various nanostructures at the LaAlO3/SrTiO3 interface. In parallel, their corresponding properties were also thoroughly investigated using quantum transport methods. Specifically, the mobility of these created nanostructures was investigated, and it was found that the mobility could be significantly enhanced in a 1D constrain configuration compared to the scenario of 2D structures. Nonlocal transport measurements were also performed on LaAlO3/SrTiO3 Hall bar structures in both normal state, and superconducting state. A novel transport property was revealed, and this property could be due to the spin and quasi-particles. Additionally, it is of note that quantum dot-like and quantum point-contact-like structures can be created via intentionally introducing barriers at the interface in LaAlO3/SrTiO3 nanowires. This can lead to various phenomena, such as electron paring without superconductivity, and conductance plateaus in transport measurements. The demonstrated c-AFM technique was also applied to a similar Al2O3/SrTiO3 material system, in which conducting channels were created at the interface. These channels show extremely high mobility at low temperature(s)
TESTA: Temporal-Spatial Token Aggregation for Long-form Video-Language Understanding
Large-scale video-language pre-training has made remarkable strides in
advancing video-language understanding tasks. However, the heavy computational
burden of video encoding remains a formidable efficiency bottleneck,
particularly for long-form videos. These videos contain massive visual tokens
due to their inherent 3D properties and spatiotemporal redundancy, making it
challenging to capture complex temporal and spatial relationships. To tackle
this issue, we propose an efficient method called TEmporal-Spatial Token
Aggregation (TESTA). TESTA condenses video semantics by adaptively aggregating
similar frames, as well as similar patches within each frame. TESTA can reduce
the number of visual tokens by 75% and thus accelerate video encoding. Building
upon TESTA, we introduce a pre-trained video-language model equipped with a
divided space-time token aggregation module in each video encoder block. We
evaluate our model on five datasets for paragraph-to-video retrieval and
long-form VideoQA tasks. Experimental results show that TESTA improves
computing efficiency by 1.7 times, and achieves significant performance gains
from its scalability in processing longer input frames, e.g., +13.7 R@1 on
QuerYD and +6.5 R@1 on Condensed Movie.Comment: 16 pages, 9 figures, code is available at
https://github.com/RenShuhuai-Andy/TEST
Cell responses to two kinds of nanohydroxyapatite with different sizes and crystallinities
Xiaochen Liu1, Minzhi Zhao1, Jingxiong Lu2, Jian Ma4, Jie Wei2, Shicheng Wei1,31Center for Biomedical Materials and Tissue Engineering, Academy for Advanced Interdisciplinary Studies, Peking University, Beijing, 2Key Laboratory for Ultrafine Materials of Ministry of Education, East China University of Science and Technology, Shanghai, 3Department of Oral and Maxillofacial Surgery, School of Stomatology, Peking University, Beijing, 4Hospital of Stomatology, Tongji University, Shanghai, ChinaIntroduction:Hydroxyapatite (HA) is the principal inorganic constituent of human bone. Due to its good biocompatibility and osteoconductivity, all kinds of HA particles were prepared by different methods. Numerous reports demonstrated that the properties of HA affected its biological effects.Methods: Two kinds of nanohydroxyapatite with different sizes and crystallinities were obtained via a hydrothermal treatment method under different temperatures. It was found that at a temperature of 140°C, a rod-like crystal (n-HA1) with a diameter of 23 ± 5 nm, a length of 47 ± 14 nm, and crystallinity of 85% ± 5% was produced, while at a temperature of 80°C, a rod-like crystal (n-HA2) with a diameter of 16 ± 3 nm, a length of 40 ± 10 nm, and crystallinity of 65% ± 3% was produced. The influence of nanohydroxyapatite size and crystallinity on osteoblast viability was studied by MTT, scanning electron microscopy, and flow cytometry.Results: n-HA1 gave a better biological response than n-HA2 in promoting cell growth and inhibiting cell apoptosis, and also exhibited much more active cell morphology. Alkaline phosphatase activity for both n-HA2 and n-HA1 was obviously higher than for the control, and no significant difference was found between n-HA1 and n-HA2. The same trend was observed on Western blotting for expression of type I collagen and osteopontin. In addition, it was found by transmission electron microscopy that large quantities of n-HA2 entered into the cell and damaged the cellular morphology. Release of tumor necrosis factor alpha from n-HA2 was markedly higher than from n-HA1, indicating that n-HA2 might trigger a severe inflammatory response.Conclusion: This work indicates that not all nanohydroxyapatite should be considered a good biomaterial in future clinical applications.Keywords: nanohydroxyapatite, osteoblast-like cells, cell viability, cell differentiatio
Population Redistribution among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields
We carry out a combined theoretical and experimental investigation on the
population distributions in the ground and excited states of tunnel ionized N2
molecules at various driver wavelengths in the near- and mid-infrared range.
Our results reveal that efficient couplings (i.e., population exchanges)
between the ground state and the excited states occur in strong laser fields.
The couplings result in the population inversion between the ground and the
excited states at the wavelengths near 800 nm, which is verified by our
experiment by observing the amplification of a seed at ~391 nm. The result
provides insight into the mechanism of free-space nitrogen ion lasers generated
in remote air with strong femtosecond laser pulses.Comment: 18 pages, 4 figure
VITATECS: A Diagnostic Dataset for Temporal Concept Understanding of Video-Language Models
The ability to perceive how objects change over time is a crucial ingredient
in human intelligence. However, current benchmarks cannot faithfully reflect
the temporal understanding abilities of video-language models (VidLMs) due to
the existence of static visual shortcuts. To remedy this issue, we present
VITATECS, a diagnostic VIdeo-Text dAtaset for the evaluation of TEmporal
Concept underStanding. Specifically, we first introduce a fine-grained taxonomy
of temporal concepts in natural language in order to diagnose the capability of
VidLMs to comprehend different temporal aspects. Furthermore, to disentangle
the correlation between static and temporal information, we generate
counterfactual video descriptions that differ from the original one only in the
specified temporal aspect. We employ a semi-automatic data collection framework
using large language models and human-in-the-loop annotation to obtain
high-quality counterfactual descriptions efficiently. Evaluation of
representative video-language understanding models confirms their deficiency in
temporal understanding, revealing the need for greater emphasis on the temporal
elements in video-language research.Comment: 23 pages, 6 figures, 18 tables, data is available at
https://github.com/lscpku/VITATEC
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