arXiv subjects

Jung-Fu Lin

Publications and source records attributed to Jung-Fu Lin.

18 recordsLinked to original sources

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↗

arXiv2024

Spherically symmetric Earth models yield no net electron spin

Terrestrial experiments that use electrons in Earth as a spin-polarized source have been demonstrated to provide strong bounds on exotic long-range spin-spin and spin-velocity interactions. These bounds constrain the coupling strength of many proposed ultralight bosonic dark-matter candidates. Recently, it was pointed out that a monopole-dipole coupling between the Sun and the spin-polarized electrons of Earth would result in a modification of the precession of the perihelion of Earth. Using an estimate for the net spin-polarization of Earth and experimental bounds on Earth's perihelion precession, interesting constraints were placed on the magnitude of this monopole-dipole coupling. Here we investigate the spin associated with Earth's electrons. We find that there are about $6 \times 10^{41}$ spin-polarized electrons in the mantle and crust of Earth oriented anti-parallel to their local magnetic field. However, when integrated over any spherically-symmetric Earth model, we find that the vector sum of these spins is zero. In order to establish a lower bound on the magnitude of the net spin along Earth's rotation axis we have investigated three of the largest breakdowns of Earth's spherical symmetry: the large low shear-velocity provinces of the mantle, the crustal composition, and the oblate spheroid of Earth. From these investigations we conclude that there are at least $5 \times 10^{38}$ spin-polarized electrons aligned anti-parallel to Earth's rotation axis. This analysis suggests that the bounds on the monopole-dipole coupling that were extracted from Earth's perihelion precession need to be relaxed by a factor of about 2000.

hep-ph↗

arXiv2022

Quantitative determination of interlayer electronic coupling at various critical points in bilayer MoS2

Tailoring interlayer coupling has emerged as a powerful tool to tune the electronic structure of van der Waals (vdW) bilayers. One example is the usage of the moire pattern to create controllable two-dimensional electronic superlattices through the configurational dependence of interlayer electronic couplings. This approach has led to some remarkable discoveries in twisted graphene bilayers, and transition metal dichalcogenide (TMD) homo- and hetero-bilayers. However, a largely unexplored factor is the interlayer distance, d, which can impact the interlayer coupling strength exponentially. In this letter, we quantitatively determine the coupling strengths as a function of interlayer spacing at various critical points of the Brillouin zone in bilayer MoS2. The exponential dependence of the coupling parameter on the gap distance is demonstrated. Most significantly, we achieved a 280% enhancement of K-valley coupling strength with an 8% reduction of the vdW gap, pointing to a new strategy in designing a novel electronic system in vdW bilayers.

cond-mat.mtrl-sci↗

arXiv2021

Iron-rich Fe-O compounds with closest-packed layers at core pressures

Oxygen solubility in solid iron is extremely low, even at high pressures and temperatures. Thus far, no Fe-O compounds between Fe and FeO endmembers have been reported experimentally. We observed chemical reactions of Fe with FeO or Fe$_2$O$_3$ $in\ situ$ x-ray diffraction experiments at 220-260 GPa and 3,000-3,500 K. The refined diffraction patterns are consistent with a series of Fe$_n$O (n $>$ 1) compounds (e.g., Fe$_{25}$O$_{13}$ and Fe$_{28}$O$_{14}$) identified using the adaptive genetic algorithm. Like $\epsilon$-Fe in the hexagonal close-packed (hcp) structure, the structures of Fe$_n$O compounds consist of oxygen-only close-packed monolayers distributed between iron-only layers. $Ab\ initio$ calculations show systematic electronic properties of these compounds that have ramifications for the physical properties of Earth's inner core.

cond-mat.mtrl-sci↗

arXiv2021

Strain-Modulated Interlayer Charge and Energy Transfers in MoS2/WS2 Heterobilayer

Excitonic properties in 2D heterobilayers are closely governed by charge transfer (CT) and excitonic energy transfer (ET) at van der Waals interfaces. Various means have been employed to modulate the interlayer CT and ET, including electrical gating and modifying interlayer spacing, but with limited extent in their controllability. Here, we report a novel method to modulate these transfers in MoS2/WS2 heterobilayer by applying compressive strain under hydrostatic pressure. Raman and photoluminescence measurements, combined with density functional theory calculations show pressure-enhanced interlayer interaction of the heterobilayer. Photoluminescence enhancement factor {\eta} of WS2 in heterobilayer decreases by five times up to ~4 GPa, suggesting a strong ET, whereas it increases by an order of magnitude at higher pressures and reaches almost unity, indicating enhanced CT. Theoretical calculations show that orbital switching in the conduction bands is responsible for the modulation of the transfers. Our findings provide a compelling approach towards effective mechanical control of CT and ET in 2D excitonic devices.

cond-mat.mtrl-sci↗

arXiv2020

Low thermal conductivity of iron-silicon alloys at Earth core conditions with implications for the geodynamo

Earth core is composed of iron (Fe) alloyed with light elements, e.g., silicon (Si). Its thermal conductivity critically affects Earth thermal structure, evolution, and dynamics, as it controls the magnitude of thermal and compositional sources required to sustain a geodynamo over Earth history. Here we directly measured thermal conductivities of solid Fe and Fe-Si alloys up to 144 GPa and 3300 K. 15 at% Si alloyed in Fe substantially reduces its conductivity by about 2 folds at 132 GPa and 3000 K. An outer core with 15 at% Si would have a conductivity of about 20 W m-1 K-1, lower than pure Fe at similar pressure-temperature conditions. This suggests a lower minimum heat flow, around 3 TW, across the core-mantle boundary than previously expected, and thus less thermal energy needed to operate the geodynamo. Our results provide key constraints on inner core age that could be older than two billion-years.

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

Blocked radiative heat transport in the hot pyrolitic lower mantle

The heat flux across the core-mantle boundary (QCMB) is the key parameter to understand the Earth/s thermal history and evolution. Mineralogical constraints of the QCMB require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (krad) of a realistic lower mantle (pyrolite) in situ using an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to ~3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from ~0.8 W/m/K at 1000 km to ~0.35 W/m/K at the CMB, the latter being ~30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of ~8.5 TW, at odds with present estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo.

physics.geo-ph↗

arXiv2019

Opaque Lowermost Mantle

Earth/s lowermost mantle displays complex geological structures that likely result from heterogeneous thermal and electromagnetic interaction with the core. Geophysical models of the core-mantle boundary (CMB) region rely on the thermal and electrical conductivities of appropriate geomaterials which, however, have never been probed at representative pressure and temperature (P-T) conditions. Here we report on the opacity of single crystalline bridgmanite and ferropericlase, which is linked to both their radiative and electrical conductivity, measured in dynamically- and statically-heated diamond anvil cells as well as computed from first-principles at CMB conditions. Our results show that light absorption in the visible spectral range is enhanced upon heating in both minerals but the rate of change in opacity with temperature is a factor of six higher in ferropericlase. As a result, bridgmanite in the lowermost mantle is moderately transparent while ferropericlase is highly opaque. Our measurements suggest a very low (< 1 W/m/K) and largely temperature-independent radiative conductivity in the lowermost mantle, at odds with previous studies. This implies that the radiative mechanism has not contributed significantly to cooling the Earth/s core throughout the geologic time and points to a present-day CMB heat flow of 9-11 TW. Opaque ferropericlase is electrically conducting and mediates strong core-mantle electromagnetic coupling, explaining the intradecadal oscillations in the length of day, low secular geomagnetic variations in Central Pacific, and the preferred paths of geomagnetic pole reversals.

physics.geo-ph↗

arXiv2019

Fe alloy slurry and a compacting cumulate pile across Earth's inner-core boundary

Seismic observations show a reduced compressional-wave gradient at the base of the outer core relative to the preliminary reference Earth model and seismic wave asymmetry between the east-west hemispheres at the top of the inner core. Here, we propose a model for the inner core boundary (ICB), where a slurry layer forms through fractional crystallization of an Fe alloy at the base of the outer core (F layer) above a compacting cumulate pile at the top of the inner core (F' layer). Using recent mineral physics data, we show that fractional crystallization of an Fe alloy (e.g., Fe-Si-O) with light element partitioning can explain the observed reduced velocity gradient in the F layer, in cases with a solid fraction of ~15(5)% in liquid with a compositional gradient due to preferential light element partitioning into liquid. The compacting cumulate pile in the F' layer may exhibit lateral variations in thickness between the east-west hemispheres due to lateral variations of large-scale heat flow in the outer core, which may explain the east-west asymmetry observed in the seismic velocity. Our interpretations suggest that the inner core with solid Fe alloy has a high shear viscosity of ~10^23 Pa s.

physics.geo-ph↗

arXiv2018

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↗

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↗

arXiv2017

Iron Partitioning between Ferropericlase and Bridgmanite in the Earth's Lower Mantle

Earth's lower mantle is generally believed to be seismically and chemically homogeneous because most of the key seismic parameters can be explained using a simplified mineralogical model at expected press-temperature conditions. However, recent high-resolution tomographic images have revealed seismic and chemical stratification in the middle-to-lower parts of the lower mantle. Thus far, the mechanism for the compositional stratification and seismic inhomogeneity, especially their relationship with the speciation of iron in the lower mantle, remains poorly understood. We have built a complete and integrated thermodynamic model of iron and aluminum chemistry for lower mantle conditions, and from this model has emerged a stratified picture of the valence, spin and composition profile in the lower mantle. Within this picture the lower mantle has an upper region with Fe3+ enriched bridgmanite with high-spin ferropericlase and metallic Fe, and a lower region with low-spin, iron-enriched ferropericlase coexisting with iron-depleted bridgmanite and almost no metallic Fe. The transition between the regions occurs at a depth of around 1600km and is driven by the spin transition in ferropericlase, which significantly changes the iron partitioning and speciation to one that favors Fe2+ in ferropericlase and suppresses Fe3+ and metallic iron formation These changes lead to lowered bulk sound velocity by 3-4% around the mid-lower mantle and enhanced density by ~1% toward the lowermost mantle. The predicted chemically and seismically stratified lower mantle differs dramatically from the traditional homogeneous model.

cond-mat.mtrl-sci↗

arXiv2016

Optical signatures of low spin Fe3+: a new probe for the spin state of bridgmanite and post-perovskite

Iron spin transition directly affects properties of lower mantle minerals and can thus alter geophysical and geochemical characteristics of the deep Earth. While the spin transition in ferropericlase has been vigorously established at P ~ 60 GPa and 300 K, experimental evidence for spin transitions in other rock-forming minerals, such as bridgmanite and post-perovskite, remains controversial. Multiple valence, spin, and coordination states of iron in bridgmanite and post-perovskite are difficult to resolve with conventional spin-probing techniques. Optical spectroscopy, on the other hand, is sensitive to high/low spin ferrous/ferric iron at different sites; thus, it can be a powerful probe for spin transitions. Here we establish the optical signature of low spin Fe3+O6, a plausible low spin unit in bridgmanite and post-perovskite, by optical absorption experiments in diamond anvil cells. We show that the optical absorption of Fe3+O6 in NAL (new aluminous phase) is very sensitive to the iron spin state and represents a model behavior of bridgmanite and post-perovskite in the deep lower mantle across a spin transition. Specifically, an absorption band centered at ~ 19000 cm-1 is characteristic of the 2T2g to 2T1g (2A2g) transition in low spin Fe3+ in NAL at 40 GPa. This new spectroscopic information constrains the crystal field splitting energy of low spin Fe3+ to ~ 22200 cm-1 which we also independently confirm by our first-principles calculations. Together with available information on the electronic structure of Fe3+O6-compounds, we constrain the spin-pairing energy of Fe3+ in an octahedral field to ~ 20000-23000 cm-1. This implies that octahedrally-coordinated Fe3+ in bridgmanite is low spin at P > ~ 40 GPa.

physics.geo-ph↗

arXiv2016

Radiative conductivity and abundance of post-perovskite in the lowermost mantle

Thermal conductivity of the lowermost mantle governs the heat flow out of the core energizing planetary-scale geological processes. Yet, there are no direct experimental measurements of thermal conductivity at relevant pressure-temperature conditions of Earth's core-mantle boundary. Here we determine the radiative conductivity of post-perovskite at near core-mantle boundary conditions by optical absorption measurements in a laser-heated diamond anvil cell. Our results show that the radiative conductivity of Mg0.9Fe0.1SiO3 post-perovskite (< 1.2 W/m/K) is ~ 40% smaller than bridgmanite at the base of the mantle. By combining this result with the present-day core-mantle heat flow and available estimations on the lattice thermal conductivity we conclude that post-perovskite is as abundant as bridgmanite in the lowermost mantle which has profound implications for the dynamics of the deep Earth.

physics.geo-ph↗

arXiv2016

Origin of Pressure-induced Superconducting Phase in K$_{x}$Fe$_{2-y}$Se$_{2}$ studied by Synchrotron X-ray Diffraction and Spectroscopy

Pressure dependence of the electronic and crystal structures of K$_{x}$Fe$_{2-y}$Se$_{2}$, which has pressure-induced two superconducting domes of SC I and SC II, was investigated by x-ray emission spectroscopy and diffraction. X-ray diffraction data show that compressibility along the c-axis changes around 12 GPa, where a new superconducting phase of SC II appears. This suggests a possible tetragonal to collapsed tetragonal phase transition. X-ray emission spectroscopy data also shows the change in the electronic structure around 12 GPa. These results can be explained by the scenario that the two SC domes under pressure originate from the change of Fermi surface topology. Present results here show that the nesting condition plays a key role in stabilizing the superconducting state helping to address outstanding fundamental question as to why the SC II appears under pressure.

cond-mat.supr-con↗

arXiv2015

Origin of the superconductivity of WTe2 under pressure

Tungsten ditelluride (WTe2) has attracted significant attention due to its interesting electronic properties, such as the unsaturated magnetoresistance and superconductivity. Recently, it has been proposed to be a new type of Weyl semimetal, which is distinguished from other transition metal dichalcogenides (TMDs) from a topological prospective. Here, we study the structure of WTe2 under pressure with a crystal structure prediction and ab initio calculations combined with high pressure synchrotron X-ray diffraction and Raman spectroscopy measurements. We find that the ambient orthorhombic structure (Td) transforms into a monoclinic structure (1T') at around 4-5 GPa. As the transition pressure is very close to the critical point in recent high-pressure electrical transport measurements, the emergence of superconductivity in WTe2 under pressure is attributed to the Td-1T' structure phase transition, which associates with a sliding mechanism of the TMD layers and results in a shorter Te-Te interlayer distance compared to the intralayer ones. These results highlight the critical role of the interlayer stacking and chalcogen interactions on the electronic and superconducting properties of multilayered TMDs under hydrostatic strain environments.

cond-mat.mtrl-sci↗

arXiv2013

Unified understanding of the valence transition in the rare-earth monochalcogenides under pressure

Valence instability is a key ingredient of the unusual properties of f electron materials, yet a clear understanding is lacking as it involves a complex interplay between f electrons and conduc- tion states. Here we propose a unified picture of pressure-induced valence transition in Sm and Yb monochalcogenides, considered as model system for mixed valent 4f-electron materials. Using high-resolution x-ray absorption spectroscopy, we show that the valence transition is driven by the promotion of a 4f electron specifically into the lowest unoccupied (LU) 5d t2g band. We demonstrate with a promotional model that the nature of the transition at low pressures is intimately related to the density of states of the LU band, while at high pressures it is governed by the hybridization strength. These results set a new standard for the generic understanding of valence fluctuations in f-electron materials.

cond-mat.str-el↗