arXiv subjects

Qichen Song

Publications and source records attributed to Qichen Song.

14 recordsLinked to original sources

arXiv2026

Observation of microscopic domain effects in the metal-insulator transition of thin-film NdNiO$_3$

Perovskite oxides display correlated electrical, magnetic, and thermal properties that can be further tuned in the thin-film limit, making them contenders for next-generation electronics. Measuring thermal transport in thin films is challenging, because traditional techniques are dominated by the substrate. Here, frequency-domain thermoreflectance (FDTR) of an epitaxial NdNiO$_3$ thin film reveals a sharp change in out-of-plane thermal conductivity across the metal-insulator transition. Complementary frequency-domain photoreflectance (FDPR) reveals a large change in ambipolar diffusivity of photoexcited carriers. While the in-plane electrical resistance shows large hysteresis, out-of-plane thermal and charge transport shows negligible hysteresis. We attribute this discrepancy to anisotropy in the percolation of nanoscale domains across the transition as the film thickness approaches the domain length scale. We establish FDTR and FDPR as sensitive probes of quantum material phase transitions and highlight NdNiO$_3$ for thermal control and memory applications.

cond-mat.mes-hall↗

arXiv2023

Probing carrier and phonon transport in semiconductors all at once through frequency-domain photo-reflectance

Semiconductor devices favor high carrier mobility for reduced Joule heating and high thermal conductivity for rapid heat dissipation. The ability to accurately characterize the motion of charge carriers and heat carriers is necessary to improve the performance of electronic devices. However, the conventional approaches of measuring carrier mobility and thermal conductivity require separate and independent measurement techniques. These techniques often involve invasive probing, such as depositing thin metal films on the sample as Ohmic contacts for characterizing electrical transport or as optical transducers for characterizing thermal transport, which becomes more cumbersome as the geometry of the semiconductor devices becomes small and complicated. Here we demonstrate a non-contact frequency-domain pump-probe method that requires no sample pretreatment to simultaneously probe carrier and phonon transport. We find that the optical reflectance depends on both excess carriers and phonons in response to exposure to a modulated continuous-wave pump laser source. By modeling the ambipolar diffusion of photo-induced excess carriers, energy transfer between electrons and phonons, and phonon diffusion, we are able to extract temperature-dependent electrical and thermal transport coefficients in Si, Ge, SiGe and GaAs. The intrinsically weak perturbation from the continuous-wave pump laser avoids invoking the highly nonequilibrium transport regime that may arise when using a pulsed laser and allows for accurate assessment of electrical transport in actual devices. Our approach provides a convenient and accurate platform for the study of the charge transport and energy dissipation in semiconductors.

cond-mat.mes-hall↗

arXiv2023

Virtual Node Graph Neural Network for Full Phonon Prediction

The structure-property relationship plays a central role in materials science. Understanding the structure-property relationship in solid-state materials is crucial for structure design with optimized properties. The past few years witnessed remarkable progress in correlating structures with properties in crystalline materials, such as machine learning methods and particularly graph neural networks as a natural representation of crystal structures. However, significant challenges remain, including predicting properties with complex unit cells input and material-dependent, variable-length output. Here we present the virtual node graph neural network to address the challenges. By developing three types of virtual node approaches - the vector, matrix, and momentum-dependent matrix virtual nodes, we achieve direct prediction of $\Gamma$-phonon spectra and full dispersion only using atomic coordinates as input. We validate the phonon bandstructures on various alloy systems, and further build a $\Gamma$-phonon database containing over 146,000 materials in the Materials Project. Our work provides an avenue for rapid and high-quality prediction of phonon spectra and bandstructures in complex materials, and enables materials design with superior phonon properties for energy applications. The virtual node augmentation of graph neural networks also sheds light on designing other functional properties with a new level of flexibility.

cond-mat.dis-nn↗

arXiv2022

Panoramic mapping of phonon transport from ultrafast electron diffraction and machine learning

One central challenge in understanding phonon thermal transport is a lack of experimental tools to investigate mode-based transport information. Although recent advances in computation lead to mode-based information, it is hindered by unknown defects in bulk region and at interfaces. Here we present a framework that can reveal microscopic phonon transport information in heterostructures, integrating state-of-the-art ultrafast electron diffraction (UED) with advanced scientific machine learning. Taking advantage of the dual temporal and reciprocal-space resolution in UED, we are able to reliably recover the frequency-dependent interfacial transmittance with possible extension to frequency-dependent relaxation times of the heterostructure. This enables a direct reconstruction of real-space, real-time, frequency-resolved phonon dynamics across an interface. Our work provides a new pathway to experimentally probe phonon transport mechanisms with unprecedented details.

cond-mat.mtrl-sci↗

arXiv2021

Significant reduction in semiconductor interface resistance via interfacial atomic mixing

The contact resistance between two dissimilar semiconductors is determined by the carrier transmission through their interface. Despite the ubiquitous presence of interfaces, quantitative simulation of charge transport across such interfaces is difficult, limiting the understanding of interfacial charge transport. This work employs Green's functions to study the charge transport across representative Si/Ge interfaces. For perfect interfaces, it is found that the transmittance is small and the contact resistance is high, not only because the mismatch of carrier pockets makes it hard to meet the momentum conservation requirement, but also because of the incompatible symmetries of the Bloch wave functions of the two sides. In contrast, atomic mixing at the interface increases the carrier transmittance as the interface roughness opens many nonspecular transmission channels, which greatly reduces the contact resistance compared with the perfect interface. Specifically, we show that disordered interfaces with certain symmetries create more nonspecular transmission. The insights from our study will benefit the future design of high-performance heterostructures with low contact resistance.

cond-mat.mes-hall↗

arXiv2021

Evaluation of diffuse mismatch model for phonon scattering at disordered interfaces

Diffuse phonon scattering strongly affects the phonon transport through a disordered interface. The often-used diffuse mismatch model assumes that phonons lose memory of their origin after being scattered by the interface. Using mode-resolved atomic Green's function simulation, we demonstrate that diffuse phonon scattering by a single disordered interface cannot make a phonon lose its memory and thus the applicability of diffusive mismatch model is limited. An analytical expression for diffuse scattering probability based on the continuum approximation is also derived and shown to work reasonably well at low frequencies.

cond-mat.mes-hall↗

arXiv2020

Signature of Many-Body Localization of Phonons in Strongly Disordered Superlattices

Many-body localization (MBL) has attracted significant attention due to its immunity to thermalization, role in logarithmic entanglement entropy growth, and opportunities to reach exotic quantum orders. However, experimental realization of MBL in solid-state systems has remained challenging. Here we report evidence of a possible phonon MBL phase in disordered GaAs/AlAs superlattices. Through grazing-incidence inelastic X-ray scattering, we observe a strong deviation of the phonon population from equilibrium in samples doped with ErAs nanodots at low temperature, signaling a departure from thermalization. This behavior occurs within finite phonon energy and wavevector windows, suggesting a localization-thermalization crossover. We support our observation by proposing a theoretical model for the effective phonon Hamiltonian in disordered superlattices, and showing that it can be mapped exactly to a disordered 1D Bose-Hubbard model with a known MBL phase. Our work provides momentum-resolved experimental evidence of phonon localization, extending the scope of MBL to disordered solid-state systems.

cond-mat.mtrl-sci↗

arXiv2018

Simultaneously high electron and hole mobilities in cubic boron-V compounds: BP, BAs and BSb

Through first-principles calculations, the phonon-limited transport properties of cubic boron-V compounds (BP, BAs and BSb) are studied. We find that the high optical phonon frequency in these compounds leads to the substantial suppression of polar scattering and the reduction of inter-valley transition mediated by large-wavevector optical phonons, both of which significantly facilitate charge transport. We also discover that BAs simultaneously has a high hole mobility (2110 cm2/V-s) and electron mobility (1400 cm2/V-s) at room temperature, which is rare in semiconductors. Our findings present a new insight in searching high mobility polar semiconductors, and point to BAs as a promising material for electronic and photovoltaic devices in addition to its predicted high thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2017

$\textit{Ab initio}$ study of electron mean free paths and thermoelectric properties of lead telluride

Last few years have witnessed significant enhancement of thermoelectric figure of merit of lead telluride (PbTe) via nanostructuring. Despite the experimental progress, current understanding of the electron transport in PbTe is based on either band structure calculation using first principles with constant relaxation time approximation or empirical models, both relying on adjustable parameters obtained by fitting experimental data. Here, we report parameter-free first-principles calculation of electron and phonon transport properties of PbTe, including mode-by-mode electron-phonon scattering analysis, leading to detailed information on electron mean free paths and the contributions of electrons and phonons with different mean free paths to thermoelectric transport properties in PbTe. Such information will help to rationalize the use and optimization of nanosctructures to achieve high thermoelectric figure of merit.

cond-mat.mes-hall↗

arXiv2017

Dirac-Electrons-Mediated Magnetic Proximity Effect in Topological Insulator / Magnetic Insulator Heterostructures

The possible realization of dissipationless chiral edge current in a topological insulator / magnetic insulator heterostructure is based on the condition that the magnetic proximity exchange coupling at the interface is dominated by the Dirac surface states of the topological insulator. Here we report a polarized neutron reflectometry observation of Dirac electrons mediated magnetic proximity effect in a bulk-insulating topological insulator (Bi$_{0.2}$Sb$_{0.8}$)$_{2}$Te$_{3}$ / magnetic insulator EuS heterostructure. We are able to maximize the proximity induced magnetism by applying an electrical back gate to tune the Fermi level of topological insulator to be close to the charge neutral point. A phenomenological model based on diamagnetic screening is developed to explain the suppressed proximity induced magnetism at high carrier density. Our work paves the way to utilize the magnetic proximity effect at the topological insulator/magnetic insulator hetero-interface for low-power spintronic applications.

cond-mat.mes-hall↗

arXiv2017

Generalized two-temperature model for coupled phonons

The design of graphene-based composite with high thermal conductivity requires a comprehensive understanding of phonon coupling in graphene. We extended the two-temperature model to coupled groups of phonon. The study give new physical quantities, the phonon-phonon coupling factor and length, to characterize the couplings quantitatively. Besides, our proposed coupling length has an obvious dependence on system size. Our studies can not only observe the nonequilibrium between different groups of phonon, but explain theoretically the thermal resistance inside graphene.

cond-mat.mes-hall↗

arXiv2016

The Effect of Shallow vs. Deep Level Doping on the Performance of Thermoelectric Materials

It is well known that the efficiency of a good thermoelectric material should be optimized with respect to doping concentration. However, much less attention has been paid to the optimization of the dopant's energy level. Thermoelectric materials doped with shallow levels may experience a dramatic reduction in their figures of merit at high temperatures due to the excitation of minority carriers that reduces the Seebeck coefficient and increases bipolar heat conduction. Doping with deep level impurities can delay the excitation of minority carriers as it requires a higher temperature to ionize all dopants. We find through modeling that, depending on the material type and temperature range of operation, different impurity levels (shallow or deep) will be desired to optimize the efficiency of a thermoelectric material. For different materials, we further clarify where the most preferable position of the impurity level within the band gap falls. Our research provides insights in choosing the most appropriate dopants for a thermoelectric material in order to maximize the device efficiency.

cond-mat.mtrl-sci↗

arXiv2016

Three-dimensional non-Bosonic non-Fermionic quasiparticle through a quantized topological defect of crystal dislocation

It is a fundamental postulate that quasiparticles in 3D space obey either Bosonic or Fermionic statistics, satisfying either canonical commutation or anti-commutation relation. However, under certain constraints, such as the 2D dimensional constraint, canonical quantization algebra is allowed to break down, and quasiparticles can obey other statistics, such as anyonic statistics. In this study, we show that dislons- the quasiparticles in 3D due to quantized displacement field of a dislocation- can also obey neither Bosonic nor Fermionic statistics due to the topological constraint of the dislocation. With this theory, an effective electron field theory based on the electron-dislon interaction is obtained, which consists of two types of interactions. One classical-type of interaction is reducible to the well-known deformation potential scattering, and the other quantum-type of interaction indicates an effective attraction between electrons. The role of dislocations in superconductivity is clarified as the competition between the classical and quantum interactions, showing excellent agreement with experiments.

cond-mat.mes-hall↗

arXiv2016

The adjustable thermal resistor by reversibly folding a graphene sheet

Phononic (thermal) devices are studied such as thermal diode, thermal transistors, thermal logic gates, and thermal memories. However, the thermal resistor has not been demonstrated yet. Here, we propose an instantaneously adjustable thermal resistor by folded graphene. Through theoretical analysis and molecular dynamics simulations, we studied the phonon folding effect and the dependent of thermal resistivity on the length between two folds and the overall length. Further, we discuss on the possibility to realize the instantaneously adjustable thermal resistor in experiment. Our studies bring insights in designing thermal resistor and understanding thermal modulation of 2D materials by adjusting its basic structure parameters.

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