arXiv subjects

Yaguo Wang

Publications and source records attributed to Yaguo Wang.

12 recordsLinked to original sources

arXiv2026

Balanced electron and phonon heat transport in metallic $\varepsilon$-TaN

Most materials with high thermal conductivity belong to one of two classes: metals, where heat is carried predominantly by electrons, and insulators, where heat transport is dominated by the phonon contribution. Materials that combine substantial electronic thermal conductivity and lattice thermal conductivity are rare, because the mechanisms that favor electron transport typically suppress phonon transport, and vice versa. Here, we report the theoretical prediction and experimental realization of such a material, metallic $\varepsilon$-TaN. Our calculations predict a total thermal conductivity at room-temperature of 273$\pm$5Wm$^{-1}$K$^{-1}$ in single crystals and 145$\pm$5Wm$^{-1}$K$^{-1}$ in polycrystals with 0.5$μ$m grains, with an unusually large lattice contribution (79%) for a metal. The latter value is in agreement with our local transient thermoreflectance measurements on polycrystalline samples yielding $\sim$130Wm$^{-1}$K$^{-1}$. We show that the balanced electronic and lattice thermal conductivities of $\varepsilon$-TaN originate from a combination of large Fermi velocity and small Fermi density of states on the electron side, and large speed of sound and wide phonon gap on the lattice side.

cond-mat.mtrl-sci↗

arXiv2026

Stacking-Engineered Thermal Transport and Phonon Filtering in Rhenium Disulfide

Cross-plane heat transport is a critical bottleneck for van der Waals (vdW) electronics, yet its microscopic governing principles remain elusive. We demonstrate that stacking order is an effective control knob for cross-plane phonon transport in multilayer Rhenium Disulfide (ReS2). Thickness-dependent thermal conductivity measurements reveal remarkably long cross-plane phonon mean free paths (MFPs) (>= 200-300 nm) and provide a direct experimental observation of the transition from quasi-ballistic transport to a thickness-independent ballistic limit. AA stacking exhibits nearly double the cross-plane thermal conductivity of AB stacking, driven by longer acoustic phonon lifetimes from a more "coherent" interlayer registry. Integrated deep neural-network molecular dynamics reveals that phonon filtering in ReS2 is fundamentally frequency-selective: weak vdW coupling acts as a low-pass filter, whereas stronger coupling broadens the transmission passband. These results establish ReS2 as a model system where stacking order and interlayer coupling can be engineered to tune heat conduction across diffusive, quasi-ballistic, and ballistic regimes, offering a new framework for thermal management in 2D electronics.

cond-mat.mtrl-sci↗

arXiv2025

Mechanistic Insights into Nonthermal Ablation of Copper Nanoparticles under Femtosecond Laser Irradiation

Femtosecond (fs) laser sintering enables ultrafast and spatially localized energy deposition, making it attractive for additive manufacturing of metal nanoparticles. However, undesired ablation during fs irradiation of copper (Cu) nanoparticles often disrupts uniform sintering, and the underlying ablation mechanisms remain poorly understood. In this work, we investigate the fragmentation and coalescence behavior of Cu nanoparticles subjected to fs laser scanning under fluence conditions relevant to sintering applications. Particle size distributions extracted from scanning electron microscopy reveal a bimodal transformation: emergence of sub-60\,nm debris and formation of large aggregates up to 750 nm. We evaluate two candidate mechanisms -- Coulomb explosion and hot electron blast -- by estimating electron emission, electrostatic pressure, and hot electron temperature using the Richardson--Dushman equation and two-temperature modeling. Our analysis shows that Coulomb explosion is unlikely under the laser fluence used ($\sim$27\,mJ/cm$^2$), as the estimated electrostatic pressure ($\sim$4 kPa) is orders of magnitude below the cohesive strength of Cu. In contrast, hot electron blast is identified as the dominant ablation pathway, with electron temperatures exceeding 5,000 K and resulting blast pressures above 4 GPa. Thermal modeling also suggests moderate lattice heating ($\sim$930\,K), enabling softening and fusion of partially fragmented particles. These results confirm that fs laser-induced ablation in Cu nanoparticles is driven predominantly by nonthermal electron dynamics rather than classical melting or evaporation. Importantly, this work highlights that reducing hot electron temperature -- such as through double-pulse irradiation schemes -- can effectively suppress ablation and expand the sintering window, offering a promising strategy for precision nanoscale additive manufacturing.

physics.optics↗

arXiv2021

Transient Hydrodynamic Lattice Cooling by Picosecond Laser Irradiation of Graphite

Recent theories and experiments have suggested hydrodynamic phonon transport features in graphite at unusually high temperatures. Here, we report a pico-second pump-probe thermal reflectance measurement of heat pulse propagation in graphite. The measurement results reveal transient lattice cooling near the adiabatic center of a 15 $μ$m diameter ring-shape pump beam at temperatures between 80 and 120 K. While such lattice cooling has not been reported in recent diffraction measurements of second sound in graphite, the observation here is consistent with both hydrodynamic phonon transport theory and prior heat pulse measurements of second sound in bulk sodium fluoride.

cond-mat.mtrl-sci↗

arXiv2020

Stacking Order Driven Optical Properties and Carrier Dynamics in ReS2

Two distinct stacking orders in ReS2 are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principle calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm-1 for AA stacking, and 20 cm-1 for AB stacking, making a simple tool for determining the stacking orders in ReS2. Polarized photoluminescence (PL) reveals that AB stacking possesses blue-shifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. Our findings underscore the stacking-order driven optical properties and carrier dynamics of ReS2, mediate many seemingly contradictory results in literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

cond-mat.mtrl-sci↗

arXiv2019

Phonon-Phonon Quantum Coherent Coupling in GaAs/AlAs Superlattice

Quantum coherent coupling between a zone-center phonon and two acoustic phonons was observed in two GaAs/AlAs superlattices (8 nm/8 nm and 5.4 nm/5.4 nm) at ambient temperature. Using degenerate coherent phonon spectroscopy, a multi-cycle oscillation feature appears in the time-resolved phonon amplitudes of both samples, as a result of the coherent energy exchange between a driving phonon mode near first Brillouin zone center and two target acoustic phonon modes. This feature resembles the photon resonant parametric down/up-conversion processes, as well as the reversible coherent energy exchange between the optical field and a mechanical oscillator, suggesting quantum coherent coupling between the driving and target phonon modes. In the 8 nm/8 nm superlattice, the coupling strength increases nonlinearly at high pump fluences, which may eventually reach an extreme state where all three phonon modes share the same coherent state, as predicted by Orbach in the 1960s.

cond-mat.mtrl-sci↗

arXiv2019

Synthesis of clathrate cerium superhydride CeH9 below 100 GPa with atomic hydrogen sublattice

Hydrogen-rich superhydrides are believed to be very promising high-Tc superconductors as they are expected to mimic characteristics of metallic hydrogen. Recent experiments discovered superhydrides at very high pressures, e.g. FeH5 at 130 GPa and LaH10 at 170 GPa. With the motivation of discovering new hydrogen-rich high-Tc superconductors at lowest possible pressure, here we report the prediction and experimental synthesis of cerium superhydride CeH9 below 100 GPa in the laser-heated diamond anvil cell. Ab-initio calculations were carried to evaluate the detailed chemistry of the Ce-H system and to understand the structure, stability and superconductivity of CeH9. CeH9 crystallizes in a P63/mmc clathrate structure with a substantially dense 3-dimensional hydrogen sublattice at 100 GPa. These findings shed a new light on the search for superhydrides in close proximity with atomic hydrogen within a feasible pressure range. Discovery of superhydride CeH9 provides a practical platform to further investigate and understand conventional superconductivity in hydrogen rich superhydrides.

cond-mat.mtrl-sci↗

arXiv2018

Picosecond Transient Thermoreflectance for Thermal Conductivity Characterization

We have developed a transient thermoreflectance technique using picosecond pulsed and cw laser to study thermal conductivity and interface conductance in both thin-films and bulk materials. A real time-resolved system observes a thermal transport along the cross-plane direction of the sample during a single pulse excitation. The suggested TTR technique can measure thermal conductivity in up to a few hundred nm of thin films with a reasonable uncertainty by carefully selecting metal transducer thickness. In this paper, we examine thermal conductivity in several substrates including Si, GaAs, Sapphire, and Glass after depositing Au thin film as metal transducer and compare with reported values to validate our technique. For further study on our method, MoS2 thin-films with different thicknesses are prepared via exfoliating, and their thermal conductivity are measured as average value of 3.4 W/mK. Compared to TDTR technique, TTR is a simpler and inexpensive method to study thermophysical properties and can also measure in-plane thermal property using a grating imaging technique. TTR can be one of the available options for observing thermal transport phenomena in both horizontal and vertical directions with a simple and inexpensive preparation.

cond-mat.mtrl-sci↗

arXiv2018

Comparison between Grating Imaging and Transient Grating Techniques on Measuring Carrier Diffusion in Semiconductor

Optical grating technique, where optical gratings are generated via light inference, has been widely used to measure charge carrier and phonon transport in semiconductors. In this paper, compared are three types of transient optical grating techniques: transient grating diffraction, transient grating heterodyne, and grating imaging, by utilizing them to measure carrier diffusion coefficient in a GaAs/AlAs superlattice. Theoretical models are constructed for each technique to extract the carrier diffusion coefficient, and the results from all three techniques are consistent. Our main findings are: (1) the transient transmission change obtained from transient grating heterodyne and grating imaging techniques are identical, even these two techniques originate from different detection principles; and (2) By adopting detection of transmission change (heterodyne amplification) instead of pure diffraction, the grating imaging technique (transient grating heterodyne) has overwhelming advantage in signal intensity than the transient grating diffraction, with a signal intensity ratio of 315:1 (157:1).

cond-mat.mtrl-sci↗

arXiv2018

Accelerated Carrier Recombination by Grain Boundary/Edge Defects in MBE Grown Transition Metal Dichalcogenides

Defect-carrier interaction in transition metal dichalcogenides (TMDs) play important roles in carrier relaxation dynamics and carrier transport, which determines the performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we investigated the effect of grain boundary/edge defects on the ultrafast dynamics of photoexcited carrier in MBE grown MoTe2 and MoSe2. We found that, comparing with exfoliated samples, carrier recombination rate in MBE grown samples accelerates by about 50 times. We attribute this striking difference to the existence of abundant grain boundary/edge defects in MBE grown samples, which can serve as effective recombination centers for the photoexcited carriers. We also observed coherent acoustic phonons in both exfoliated and MBE grown MoTe2, indicating strong electron-phonon coupling in this materials. Our measured sound velocity agrees well with previously reported result of theoretical calculation. Our findings provide useful reference for the fundamental parameters: carrier lifetime and sound velocity, reveal the undiscovered carrier recombination effect of grain boundary/edge defects, both of which will facilitate the defect engineering in TMD materials for high speed opto-electronics.

cond-mat.mtrl-sci↗

arXiv2017

Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide

Understanding defect effect on carrier dynamics is essential for both fundamental physics and potential applications of transition metal dichalcogenides. Here, the phenomenon of oxygen impurities trapping photo-excited carriers has been studied with ultrafast pump-probe spectroscopy. Oxygen impurities are intentionally created in exfoliated multilayer MoSe2 with Ar+ plasma irradiation and air exposure. After plasma treatment, the signal of transient absorption first increases and then decreases, which is a signature of defect capturing carriers. With larger density of oxygen defects, the trapping effect becomes more prominent. The trapping defect densities are estimated from the transient absorption signal, and its increasing trend in the longer-irradiated sample agrees with the results from X-ray photoelectron spectroscopy. First principle calculations with density functional theory reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states, which are responsible for the carrier trapping. Our findings shed light on the important role of oxygen defects as carrier trappers in transition metal dichalcogenides, and facilitates defect engineering in relevant material and device applications.

cond-mat.mtrl-sci↗

arXiv2017

Giant Thermal Conductivity Enhancement in Multilayer MoS2 under Highly Compressive Strain

Multilayer MoS2 possesses highly anisotropic thermal conductivities along in-plane and cross-plane directions that could hamper heat dissipation in electronics. With about 9% cross-plane compressive strain created by hydrostatic pressure in a diamond anvil cell, we observed about 12 times increase in the cross-plane thermal conductivity of multilayer MoS2. Our experimental and theoretical studies reveal that this drastic change arises from the greatly strengthened interlayer interaction and heavily modified phonon dispersions along cross-plane direction, with negligible contribution from electronic thermal conductivity, despite its enhancement of 4 orders of magnitude. The anisotropic thermal conductivity in the multilayer MoS2 at ambient environment becomes almost isotropic under highly compressive strain, effectively transitioning from 2D to 3D heat dissipation. This strain tuning approach also makes possible parallel tuning of structural, thermal and electrical properties, and can be extended to the whole family of 2D Van der Waals solids, down to two layer systems.

cond-mat.mtrl-sci↗