arXiv subjects

Chunlong Hu

Publications and source records attributed to Chunlong Hu.

7 recordsLinked to original sources

arXiv2026

Dynamics of Long-lived Carriers in Molybdenum Carbide Nanosheets

Molybdenum carbide (MoC) is a promising candidate for substituting expensive platinum-group metals in many applications owing to its low cost and excellent properties, therefore a comprehensive understanding of its carrier dynamics is demanded for advancing the synthesis strategy and implementation of MoC. In this work, the carrier relaxation in MoC nanosheets is investigated by combining the femtosecond transient reflection spectroscopy with the first-principles calculations. The measured processes of electron-electron, electron-phonon, and phonon-phonon scattering show significantly longer lifetimes compared to those of other transition metal carbides. Through the analysis of calculated phonon dispersion, the nanosecond carrier lifetime is explained by the restricted phonon decay pathways, which is induced by the large mass difference between Mo and C atoms. The observed slow carrier cooling rate in our study offers a straightforward approach for effectively utilizing hot carriers, which is expected to improve the performance of photothermal and photovoltaic devices.

cond-mat.mtrl-sci↗

arXiv2025

Polaronic-Distortion-Driven Enhancement of Excitonic Auger Recombination in ZnO Nanoparticles

The surface effect and quantum confinement render nanomaterials the optoelectronic properties more susceptible to nonradiative processes than their bulk counterparts. These nonradiative processes usually contain a series of interwoven and competing sub-processes, which are challenging to disentangle. Here, we investigate the structural origin of Auger recombination in ZnO nanoparticles using transient absorption spectroscopy and ultrafast electron diffraction. The photogenerated hot holes are captured by oxygen vacancies through an Auger mechanism, inducing significant local structural distortions around the oxygen vacancy and its neighboring zinc tetrahedron on a sub-picosecond timescale. The recombination of trapped holes accelerates the lattice thermalization and stabilizes the formed small hole polarons. Subsequently, the recombination of localized polarons forms a confined exciton-polaron complex that may account for the long-lived (>7 ns) visible luminescence observed in ZnO nanoparticles. Our findings are potentially applicable to other transition metal oxide nanomaterials, bringing insights for the optimization of their functional properties.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast atomic dimerization of Peierls distortion in semimetal molybdenum ditelluride

Semimetal molybdenum ditelluride (1T'-MoTe$_2$) possess diverse phase transitions enriching its application prospects. The structural response during these transitions is crucial to understanding the underlying mechanisms, but the desired details of pathway and time span are still insufficient. Here, we investigate the lattice evolution in few-layer 1T'-MoTe$_2$ after photoexcitation, using ultrafast electron diffraction and density functional theory (DFT) calculations. The observed complex lattice responses with unintuitively evolving Bragg peak intensity and interplanar spacing, are best interpreted as the combination of shear displacement and Mo-Mo bond shortening in a few picoseconds, and a metastable structure in nanoseconds, basing on the analyses of structure factor and pair distribution function. The DFT calculations reveal that, the photodoped electrons induced population change of the antibonding states close to Fermi level, lead to the shear displacement and the dimerization of Mo pairs. Our findings present new insights for elucidating the picture of Peierls distortion in 1T'-MoTe$_2$.

cond-mat.mtrl-sci↗

arXiv2025

Valley resolved dynamics of phonon bottleneck in semiconductor molybdenum ditelluride

Semiconductor molybdenum ditelluride (2H-MoTe2) possess multiple valleys in the band structure, enriching its physical properties and potentials in applications. The understanding of its multivalley nature of fundamental processes involving population and relaxation of carriers and phonons is still evolving; particularly, the possible phonon bottleneck has not yet been addressed. Here, we investigate the carrier intra- and intervalley scattering and the phonon dynamics in different valleys in photoexcited few-layer 2H-MoTe2, by using the time resolved measurements of optical absorption and electron diffraction, together with the density functional theory calculation and molecular dynamics simulation. The pathways and timescales of carrier relaxation, accompanied with the emissions of optical phonons at the Brillouin zone center and acoustic phonons at the zone border are revealed. We present a couple of approaches to estimate the population of different phonon modes based on the results of optical and electron diffraction measurements, hence quantitatively identify the occurrences of phonon bottleneck located in different valleys. Our findings make possible to construct a comprehensive picture of the complex interactions between carriers and phonons in 2H-MoTe2 with the valley degree of freedom resolved.

cond-mat.mtrl-sci↗

arXiv2024

Formation of Anisotropic Polarons in Antimony Selenide

Antimony Selenide (Sb$_2$Se$_3$) is an attractive candidate of photovoltaics with not yet satisfying efficiency. Beside defects, polaron formation originated from lattice distortion was proposed to account for trapping free carriers, and the subsequent photoexcitation dynamics and optoelectronic properties, but such a mechanism is still lack of structural observations. Here we directly track the pathways of carrier and lattice evolutions after photoexcitation through optical and electron diffraction pump-probe methods, revealing the temporal correlations between dynamics of both degrees of freedom. The observed opposite separation changes of Se2-Sb2 and Sb2-Sb1 atom pairs in a few picoseconds, and the intermediate state induced by local structural distortions lasting several tens of picoseconds, coinciding with the optical phonons population and coupling, and the trapping process of carriers, respectively, together with the analyses of modulation on diffuse scattering by the atomic displacement fields of polaron model, indicate the formation of anisotropic polarons with large size. Our findings provide carrier and structural information for helping the elucidation of polaron scenario in Sb2Se3, and probably in materials with anisotropic structure and soft lattice which are popular in developing novel optoelectronics.

cond-mat.mtrl-sci↗

arXiv2024

Time-resolved optical assessment of exciton formation in mixed two-dimensional perovskite films

We report the observation of exciton formation from the cooled band-edge carriers in mixed two-dimensional hybrid organic-inorganic perovskites using femtosecond transient absorption spectroscopy. By monitoring the changes of bleach signal upon excitations with various photon energy, we are able to extract the values of exciton binding energy and the occupancies of carriers of free and bound states for each two-dimensional phase. We also confirm the existence of Mahan exciton when injected carrier density is above the Mott criterion.

physics.optics↗

arXiv2017

Lie Algebrized Gaussians for Image Representation

We present an image representation method which is derived from analyzing Gaussian probability density function (\emph{pdf}) space using Lie group theory. In our proposed method, images are modeled by Gaussian mixture models (GMMs) which are adapted from a globally trained GMM called universal background model (UBM). Then we vectorize the GMMs based on two facts: (1) components of image-specific GMMs are closely grouped together around their corresponding component of the UBM due to the characteristic of the UBM adaption procedure; (2) Gaussian \emph{pdf}s form a Lie group, which is a differentiable manifold rather than a vector space. We map each Gaussian component to the tangent vector space (named Lie algebra) of Lie group at the manifold position of UBM. The final feature vector, named Lie algebrized Gaussians (LAG) is then constructed by combining the Lie algebrized Gaussian components with mixture weights. We apply LAG features to scene category recognition problem and observe state-of-the-art performance on 15Scenes benchmark.

cs.CV↗