arXiv subjects

Bolin Liao

Publications and source records attributed to Bolin Liao.

At least 19 recordsLinked to original sources

arXiv2026

Reversible Modulation of Thermal Conductivity in GaN through Strain-Field Screening around Dislocations

Crystalline defects are generally regarded as static phonon scatterers that irreversibly suppress thermal transport. Here we show that elastic strain can reversibly modify dislocation-associated strain fields and strongly alter heat conduction. Using in situ strain-dependent time-domain thermoreflectance measurements, we observe a reversible enhancement of thermal conductivity in GaN by 23% under only 0.21% uniaxial strain. High-resolution X-ray diffraction reveals progressive narrowing of the symmetric (0002) reflection, indicating a reduction in the distribution of lattice rotations and heterogeneous strain. High-resolution electron backscatter diffraction directly shows that the spatial autocorrelations of multiple strain components decay over progressively shorter distances with applied strain, providing real-space evidence for enhanced screening of long-range strain fields. Raman spectroscopy further shows a non-monotonic evolution of the $E_{2}^{\mathrm{high}}$ phonon linewidth near the onset of the thermal-conductivity increase. Together, these results support a picture in which elastic strain reversibly reconfigures pinned dislocation lines and shortens the spatial range of their heterogeneous strain fields, thereby reducing phonon scattering. Our work establishes defect-associated strain correlations as a tunable degree of freedom for controlling thermal transport in crystalline solids.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic Polarons Enable Exceptional Magnetocaloric Response

Magnetocaloric materials are typically limited by a trade-off between magnetic entropy and field responsiveness. Here we show that magnetic polarons provide an intermediate regime that mitigates this constraint and enables an exceptional magnetocaloric response. Using EuB$_6$ as a model system, we combine thermodynamic and magnetic measurements to demonstrate that nanoscale ferromagnetic clusters emerging near the Curie temperature strongly enhance the field-induced entropy collapse. These clusters possess large effective moments that respond efficiently to applied fields while retaining substantial entropy due to their small size and dynamic fluctuations. As a result, EuB$_6$ exhibits a giant cryogenic magnetocaloric response, with both large isothermal entropy change and adiabatic temperature change in the technologically important 10-40 K range. Our results identify magnetic polarons as an underexplored route for optimizing magnetocaloric performance and establish an intermediate magnetic length scale as a design principle for high-performance cryogenic cooling materials.

cond-mat.mtrl-sci↗

arXiv2026

Coupled Spin-lattice Dynamics across a Magnetostructural Phase Transition

First-order magnetostructural phase transitions underpin giant magnetocaloric effects, yet the microscopic role of lattice dynamics in these transitions remains controversial. Here we use first-principles spin-lattice dynamics simulations to investigate the coupled evolution of magnetization and phonon dispersions across the magnetostructural transition in MnAs. Our simulations quantitatively reproduce the experimentally observed Curie temperature, lattice contraction, and free-energy crossing between hexagonal and orthorhombic phases. We show that below the Curie temperature, magnetic-field-induced hardening of soft phonon modes gives rise to a sizable lattice entropy contribution that enhances the total isothermal entropy change by approximately 23% under a 5 T field. In contrast, the lattice entropy change associated with the structural phase transition itself has an opposite sign and partially compensates the lattice contribution due to field-induced phonon hardening. This competition reconciles long-standing discrepancies in the interpretation of magnetocaloric entropy measurements across first-order transitions. In addition, we demonstrate that the strong magnetic-field dependence of the phonon spectrum near the transition enables large tunability of lattice thermal conductivity, highlighting MnAs as a promising platform for magnetic-field-controlled thermal switching. Our results establish a unified microscopic picture of spin-lattice coupling in first-order magnetocaloric materials and provide design principles for engineering enhanced caloric and thermal transport functionalities.

cond-mat.mtrl-sci↗

arXiv2025

Electric-Field-Dependent Thermal Conductivity in Fresh and Aged Bulk Single Crystalline $\mathrm{BaTiO_3}$

Active thermal management requires advances in thermal switching materials, whose thermal conductivity responds to external stimuli. The electric field, as one of the most convenient and effective stimuli, has shown great potential in tuning the thermal conductivity of ferroelectric materials. While previous studies on electric-field-induced ferroelectric thermal switching have primarily focused on thin films and bulk solid solutions with strong extrinsic interface and defect scatterings, bulk single crystals, which can offer clear insights into intrinsic thermal switching mechanisms, have received comparatively less attention. Here, we demonstrate electric-field-induced thermal switching in bulk single-crystalline $\mathrm{BaTiO_3}$ (BTO) at room temperature and elucidate the critical role of domain evolution and aging in governing heat transport. Using a customized steady-state platform with in-situ electric fields up to $\pm$10 kV/cm, we observe a modulation of thermal conductivity up to 35% in fresh BTO driven by polarization reorientation and domain restructuring. First-principles finite-temperature lattice-dynamics calculations confirm that the switching behavior primarily originates from anisotropic phonon transport associated with domain configuration rather than strain-induced changes in phonon velocities. We further reveal that both ambient aging and controlled thermal aging can enhance the switching contrast through the formation and alignment of defect dipoles that modulate phonon-defect scattering. These results establish defect-domain interactions as a powerful design parameter for ferroelectric thermal switches and demonstrate a versatile experimental platform for exploring field-tunable heat transport and phase behavior in bulk functional materials.

cond-mat.mtrl-sci↗

arXiv2025

Lessons from $\alpha$-RuCl3 for pursuing quantum spin liquid physics in atomically thin materials

Quantum spin liquids can arise from Kitaev magnetic interactions, and exhibit fractionalized excitations with the potential for a topological form of quantum computation. This review surveys recent experimental and theoretical progress on the pursuit of phenomena related to Kitaev magnetism in layered and exfoliatable materials, which offer numerous opportunities to apply powerful techniques from the field of atomically thin materials. We primarily focus on the antiferromagnetic Mott insulator $\alpha$-RuCl3, which exhibits Kitaev couplings and is readily exfoliated to single- or few-layer sheets, and thus serves as a test bed for developing probes of Kitaev phenomena in atomically thin materials and devices. We introduce the Kitaev model and how it is realized in $\alpha$-RuCl3 and other material candidates; and cover $\alpha$-RuCl3 synthesis and fabrication into van der Waals heterostructure devices. A key discovery is a work-function-mediated charge transfer that heavily dopes both the $\alpha$-RuCl3 and proximate materials, and can enhance Kitaev interactions by up to 50%. We further discuss a wide range of recent results in electronic transport and optical and tunneling spectroscopies of $\alpha$-RuCl3 devices. The experimental techniques and theoretical insights developed for $\alpha$-RuCl3 establish a framework for discovering and engineering superior two-dimensional Kitaev materials that may ultimately realize elusive quantum spin liquid phases.

cond-mat.str-el↗

arXiv2025

Fast nanothermometry based on direct electron detection of electron backscattering diffraction patterns

Accurate temperature measurement at the nanoscale is crucial for thermal management in next-generation microelectronic devices. Existing optical and scanning-probe thermometry techniques face limitations in spatial resolution, accuracy, or invasiveness. In this work, we demonstrate a fast and non-contact nanothermometry method based on temperature-induced changes in electron backscattering diffraction (EBSD) patterns captured by a high-performance direct electron detector within a scanning electron microscope (SEM). Using dynamical electron simulations, we establish the theoretical temperature sensitivity limits for several semiconductors (Si, Ge, GaAs, and GaN), showing that thermal diffuse scattering (TDS) leads to a measurable smearing of Kikuchi bands in the EBSD patterns. We develop a Fourier analysis method that captures these subtle changes across the full diffraction pattern, achieving a simulated temperature sensitivity of approximately 0.15\% per K. Experimental results on silicon confirm a sensitivity of 0.14\% per K and achieve a 13-K temperature uncertainty with a 10-second acquisition time, and enable spatial temperature mapping under thermal gradients. Our approach offers a pathway toward practical and high-resolution thermal mapping directly in SEMs, expanding the toolbox for device-level thermal diagnostics.

cond-mat.mtrl-sci↗

arXiv2025

Thermal conductivity of boron arsenide above 2100 watts per meter per Kelvin at room temperature

Boron arsenide (BAs) single crystals had been previously reported to have thermal conductivity of 1500 W/mK at room temperature. Now we achieved thermal conductivity above 2100 W/mK at room temperature in BAs crystals due to much lower concentration of impurities Si, C, and O grown from purified arsenic. We also observed a T-1.8 dependence of the thermal conductivity, suggesting a more significant contribution from four-phonon scatterings than suggested by previous theory. We found that our experimental result can be fit with a modified theoretical calculation by tuning down the three-phonon scattering for phonons in the 4-8 THz range, although current phonon transport theory cannot provide a physical explanation. Such an advance will not only attract more effort on growing BAs single crystals and studying their practical applications but also stimulate theoretical work to predict more materials with possibly even higher thermal conductivities.

cond-mat.mtrl-sci↗

arXiv2025

High-throughput Discovery of Anti-gap Semiconductors

Conventional semiconductors typically have bonding states near the valence band maximum (VBM) and antibonding states near the conduction band minimum (CBM). Semiconductors with the opposite electronic configuration, namely an antibonding VBM and a bonding CBM, are here termed ``anti-gap semiconductors". They have been theoretically proposed to exhibit excellent optoelectronic properties because of their strong tolerance to defects. However, no anti-gap semiconductors have been identified so far, despite a known list of semiconductors with an antibonding VBM. Here, we use high-throughput computation to identify over 100 anti-gap semiconductors. From this group, we analyze the transition metal dichalcogenide MX$_2$ (M=Hf, Zr; X=S, Se) family in detail. In addition to verifying their defect tolerance for both electrons and holes using first-principles simulations, we also discovered that photoexcitation of charge carriers can lead to significant lattice stiffening and increased thermal conductivity in anti-gap semiconductors, which can be potentially used as photo-driven thermal switches. Our work analyzes the formation of the anti-gap electronic structure and showcases their unusual photoinduced lattice dynamics that can have a potential impact on their photophysical applications.

cond-mat.mtrl-sci↗

arXiv2025

ALATDYN: a set of Anharmonic LATtice DYNamics codes to compute thermodynamic and thermal transport properties of crystalline solids

We introduce a lattice dynamics package which calculates elastic, thermodynamic and thermal transport properties of crystalline materials from data on their force and potential energy as a function of atomic positions. The data can come from density functional theory (DFT) calculations or classical molecular dynamics runs performed in a supercell. First, the model potential parameters, which are anharmonic force constants are extracted from the latter runs. Then, once the anharmonic model is defined, thermal conductivity and equilibrium properties at finite temperatures can be computed using lattice dynamics, Boltzmann transport theories, and a variational principle respectively. In addition, the software calculates the mechanical properties such as elastic tensor, Gruneisen parameters and the thermal expansion coefficient within the quasi-harmonic approximation (QHA). Phonons, elastic constants and thermodynamic properties results applied to the germanium crystal will be illustrated. Using the force constants as a force field, one may also perform molecular dynamics (MD) simulations in order to investigate the combined effects of anharmonicity and defect scattering beyond perturbation theory.

cond-mat.mtrl-sci↗

arXiv2024

Coupled electron-phonon hydrodynamics in two-dimensional semiconductors

Electronic and thermal transport properties in two-dimensional (2D) semiconductors have been extensively investigated due to their potential to miniaturize transistors. Microscopically, electron-phonon interactions are considered the dominant momentum relaxation mechanism for electrons that limits carrier mobility beyond cryogenic temperatures. However, when electrons and phonons are considered as a single system, electron-phonon interactions conserve the total momentum and energy, leading to the possibility of low-dissipation transport. In this work, we systematically investigate the momentum circulation between electrons and phonons and its impact on carrier transport properties in 2D semiconductors given their strong electron-phonon interactions. We find that, when momentum circulation is taken into account, the total momentum in the coupled electron-phonon system is weakly dissipated, leading to a coupled electron-phonon hydrodynamic transport regime, in which electrons and phonons exhibit a joint drift motion rather than separate diffusive behaviors. In this new transport regime, charge transport properties are significantly enhanced. Contrary to previous belief, our results demonstrate that low-dissipation charge transport can occur despite strong electron-phonon interactions when there is effective momentum circulation between electrons and phonons mediated by the strong interactions. Our work advances fundamental understandings of carrier transport in 2D semiconductors.

cond-mat.mes-hall↗

arXiv2024

Absence of Phonon Softening across a Charge Density Wave Transition due to Quantum Fluctuations

Kagome metals have emerged as a frontier in condensed matter physics due to their potential to host exotic quantum states. Among these, CsV3Sb5 has attracted significant attention for the unusual coexistence of charge density wave (CDW) order and superconductivity, presenting an ideal system for exploring novel electronic and phononic phenomena. The nature of CDW formation in CsV3Sb5 has sparked considerable debate. Previous studies have suggested that the underlying mechanism driving the CDW transition in CsV3Sb5 is distinct from conventional ones, such as electron-phonon coupling and Fermi surface nesting. In this study, we examine the origin of the CDW state via ab initio finite-temperature simulations of the lattice dynamics associated with CDW structures in CsV3Sb5. Through a comparative study of CsV3Sb5 and 2H-NbSe2, we demonstrate that the experimental absence of phonon softening in CsV3Sb5 and the presence of a weakly first order transition can be attributed to quantum zero-point motion of the lattice, which leads to smearing of the CDW landscape and effectively stabilizes the pristine structure even below the CDW transition temperature. We argue that this surprising behavior could cause coexistence of pristine and CDW structures across the transition and lead to a weak first-order transition. We further discuss experimental implications and use the simulation to interpret coherent phonon spectroscopy results in single crystalline CsV3Sb5. These findings not only refine our fundamental understanding of CDW transitions, but also highlight the surprising role of quantum effects in influencing macroscopic properties of relatively heavy-element materials like CsV3Sb5. Our results provide crucial insights into the formation mechanism of CDW materials that exhibit little to no phonon softening, including cuprates, aiding in the understanding of the CDW phase in quantum materials.

cond-mat.mtrl-sci↗

arXiv2024

High-Throughput Search for Photostrictive Materials based on a Thermodynamic Descriptor

Photostriction is a phenomenon that can potentially improve the precision of light-driven actuation, the sensitivity of photodetection, and the efficiency of optical energy harvesting. However, known materials with significant photostriction are limited, and effective guidelines to discover new photostrictive materials are lacking. In this study, we perform a high-throughput computational search for new photostrictive materials based on simple thermodynamic descriptors, namely the band gap pressure and stress coefficients. Using constrained density functional theory simulations, we establish that these descriptors can accurately predict intrinsic photostriction in a wide range of materials. Subsequently, we screen over 4770 stable semiconductors with a band gap below 2 eV from the Materials Project database to search for strongly photostrictive materials. This search identifies PtS$_2$ and Te$_2$I as the most promising ones, with photostriction exceeding 10$^{-4}$ with a moderate photocarrier concentration of 10$^{18}$ cm$^{-3}$. Furthermore, we provide a detailed analysis of factors contributing to strong photostriction, including bulk moduli and band-edge orbital interactions. Our results provide physical insights into photostriction of materials and demonstrate the effectiveness of using simple descriptors in high-throughput searches for new functional materials.

cond-mat.mtrl-sci↗

arXiv2024

Insulator-to-Metal Transition and Anomalously Slow Hot Carrier Cooling in a Photo-doped Mott Insulator

Photo-doped Mott insulators can exhibit novel photocarrier transport and relaxation dynamics and non-equilibrium phases. However, time-resolved real-space imaging of these processes are still lacking. Here, we use scanning ultrafast electron microscopy (SUEM) to directly visualize the spatial-temporal evolution of photoexcited species in a spin-orbit assisted Mott insulator α-RuCl3. At low optical fluences, we observe extremely long hot photocarrier transport time over one nanosecond, almost an order of magnitude longer than any known values in conventional semiconductors. At higher optical fluences, we observe nonlinear features suggesting a photo-induced insulator-to-metal transition, which is unusual in a large-gap Mott insulator. Our results demonstrate the rich physics in a photo-doped Mott insulator that can be extracted from spatial-temporal imaging and showcase the capability of SUEM to sensitively probe photoexcitations in strongly correlated electron systems.

cond-mat.str-el↗

arXiv2024

Atomic-scale tunable phonon transport at tailored grain boundaries and Their Impact on Thermal Conductivity

Grain boundaries (GBs) strongly influence thermal transport in crystalline solids by disrupting lattice periodicity and scattering phonons. Due to the atomic-level disorder and structural complexity, a fundamental understanding of how specific GB geometries regulate nanoscale phonon behavior and macroscopic thermal conductivity has remained elusive. Here, using emerging atomic-resolution vibrational spectroscopy, we directly correlate GB structure, defect-specific vibrational states, and thermal transport in bicrystal strontium titanate with controlled tilt and twist angles. The phonon characterizations and thermal conductivity data reveal two distinct regimes, where low-angle tilt GBs (2deg, 6deg, 10deg) substantially modulate phonon populations and mode frequencies, resulting in pronounced changes in thermal conductivity, whereas high-angle tilt GBs (22deg, 36deg) exhibit weak conductivity variation due to saturated structural disorder and scattering. In contrast, twist GBs introduce periodic defect motifs that only locally tailor phonon transport. Our results suggest tilt and twist angles as complementary knobs for coarse and fine control of phonon propagation and cross-GB thermal transport, providing a predictive framework for thermal engineering in materials.

cond-mat.mes-hall↗

arXiv2024

Imaging Hot Photocarrier Transfer across a Semiconductor Heterojunction with Ultrafast Electron Microscopy

Semiconductor heterojunctions have gained significant attention for efficient optoelectronic devices owing to their unique interfaces and synergistic effects. Interaction between charge carriers with the heterojunction plays a crucial role in determining device performance, while its spatial-temporal mapping remains lacking. In this study, we employ scanning ultrafast electron microscopy (SUEM), an emerging technique that combines high spatial-temporal resolution and surface sensitivity, to investigate photocarrier dynamics across a Si/Ge heterojunction. Charge dynamics are selectively examined across the junction and compared to far bulk areas, through which the impact of the built-in potential, band offsets, and surface effects is directly visualized. In particular, we find that the heterojunction drastically modifies the hot photocarrier diffusivities by up to 300%. These findings are further elucidated with insights from the band structure and surface potential measured by complementary techniques. This work demonstrates the tremendous effect of heterointerfaces on charge dynamics and showcases the potential of SUEM in characterizing realistic devices.

physics.app-ph↗

arXiv2024

Impact of Dimensionality on the Magnetocaloric Effect in Two-dimensional Magnets

Magnetocaloric materials, which exploit reversible temperature changes induced by magnetic field variations, are promising for advancing energy-efficient cooling technologies. The potential integration of two-dimensional materials into magnetocaloric systems represents an emerging opportunity to enhance the magnetocaloric cooling efficiency. In this study, we use atomistic spin dynamics simulations based on first-principles parameters to systematically evaluate how magnetocaloric properties transition from three-dimensional (3D) to two-dimensional (2D) ferromagnetic materials. We find that 2D features such as reduced Curie temperature, sharper magnetic transition, and higher magnetic susceptibility are beneficial for magnetocaloric applications, while the relatively higher lattice heat capacity in 2D can compromise achievable adiabatic temperature changes. We further propose GdSi$_2$ as a promising 2D magnetocaloric material near hydrogen liquefaction temperature. Our analysis offers valuable theoretical insights into the magnetocaloric effect in 2D ferromagnets and demonstrates that 2D ferromagnets hold promise for cooling and thermal management applications in compact and miniaturized nanodevices.

cond-mat.mtrl-sci↗

arXiv2024

Electron Drag Effect on Thermal Conductivity in Two-dimensional Semiconductors

Two-dimensional (2D) materials have shown great potential in applications as transistors, where thermal dissipation becomes crucial because of the increasing energy density. Although thermal conductivity of 2D materials has been extensively studied, interactions between nonequilibrium electrons and phonons, which can be strong when high electric fields and heat current coexist, are not considered. In this work, we systematically study the electron drag effect, where nonequilibrium electrons impart momenta to phonons and influence the thermal conductivity, in 2D semiconductors using ab initio simulations. We find that, at room temperature, electron drag can significantly increase thermal conductivity by decreasing phonon-electron scattering in 2D semiconductors, while its impact in three-dimensional (3D) semiconductors is negligible. We attribute this difference to the large electron-phonon scattering phase space and higher contribution to thermal conductivity by drag-active phonons. Our work elucidates the fundamental physics underlying coupled electron-phonon transport in materials of various dimensionalities.

cond-mat.mtrl-sci↗

arXiv2024

Enhancing Magnetocaloric Material Discovery: A Machine Learning Approach Using an Autogenerated Database by Large Language Models

Magnetic cooling based on the magnetocaloric effect is a promising solid-state refrigeration technology for a wide range of applications in different temperature ranges. Previous studies have mostly focused on near room temperature (300 K) and cryogenic temperature (< 10 K) ranges, while important applications such as hydrogen liquefaction call for efficient magnetic refrigerants for the intermediate temperature 10K to 100 K. For efficient use in this range, new magnetocaloric materials with matching Curie temperatures need to be discovered, while conventional experimental approaches are typically time-consuming and expensive. Here, we report a computational material discovery pipeline based on a materials database containing more than 6000 entries auto-generated by extracting reported material properties from literature using a large language model. We then use this database to train a machine learning model that can efficiently predict magnetocaloric properties of materials based on their chemical composition. We further verify the magnetocaloric properties of predicted compounds using ab initio atomistic spin dynamics simulations to close the loop for computational material discovery. Using this approach, we identify 11 new promising magnetocaloric materials for the target temperature range. Our work demonstrates the potential of combining large language models, machine learning, and ab initio simulations to efficiently discover new functional materials.

cond-mat.mtrl-sci↗