Search for: All records

Abstract Among various models for estimating interfacial thermal conductance (ITC) across different material interfaces, the diffuse mismatch model (DMM) has been generally evaluated as a reliable approach for material interfaces at high temperatures. The previous works by DMM have indicated the correct order of magnitude of ITC in isotropic material interfaces. However, it cannot accurately reproduce the ITC for low-dimensional anisotropic layered materials that are desired for many potential applications. Also, the inappropriate mode matching process approximation of the phonon dispersion curve tends to overestimate the ITC. In this paper, we revisited and updated the numerical method in our previous work that utilizes a mode-to-mode comparison within the DMM framework to predict ITC with the first-principles accuracy. We employed this model to calculate ITCs between layered materials such as MoS2 and graphite and metals such as Al, Au, and Cr. We then compared our values with previous literature data from calculations of phonon dispersion curve and experimental data from time-domain thermoreflectance measurements. With a better mode matching algorithm, the updated numerical method can predict the ITCs with improved accuracy. Further analysis also confirmed that counting only the three acoustic modes and neglecting the low-frequency optical modes lead to significant underestimation of the ITC using DMM. 

Abstract Nanosized perovskite ferroelectrics are widely employed in several electromechanical, photonics, and thermoelectric applications. Scaling of ferroelectric materials entails a severe reduction in the lattice (phonon) thermal conductivity, particularly at sub‐100 nm length scales. Such thermal conductivity reduction can be accurately predicted using the information of phonon mean free path (MFP) distribution. The current understanding of phonon MFP distribution in perovskite ferroelectrics is still inconclusive despite the critical thermal management implications. Here, high‐quality single‐crystalline barium titanate (BTO) thin films, a representative perovskite ferroelectric material, are grown at several thicknesses. Using experimental thermal conductivity measurements and first‐principles based modeling (including four‐phonon scattering), the phonon MFP distribution is determined in BTO. The simulation results agree with the measured thickness‐dependent thermal conductivity. The results show that the phonons with sub‐100 nm MFP dominate the thermal transport in BTO, and phonons with MFP exceeding 10 nm contribute ≈35% to the total thermal conductivity, in significant contrast to previously published experimental results. The experimentally validated phonon MFP distribution is consistent with the theoretical predictions of other complex crystals with strong anharmonicity. This work paves the way for thermal management in nanostructured and ferroelectric‐domain‐engineered systems for oxide perovskite‐based functional materials. 

Abstract Acting like thermal resistances, ferroelectric domain walls can be manipulated to realize dynamic modulation of thermal conductivity (k), which is essential for developing novel phononic circuits. Despite the interest, little attention has been paid to achieving room‐temperature thermal modulation in bulk materials due to challenges in obtaining a high thermal conductivity switching ratio (k_high/k_low), particularly in commercially viable materials. Here, room‐temperature thermal modulation in 2.5 mm‐thick Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃(PMN–xPT) single crystals is demonstrated. With the use of advanced poling conditions, assisted by the systematic study on composition and orientation dependence of PMN–xPT, a range of thermal conductivity switching ratios with a maximum of ≈1.27 is observed. Simultaneous measurements of piezoelectric coefficient (d₃₃) to characterize the poling state, domain wall density using polarized light microscopy (PLM), and birefringence change using quantitative PLM reveal that compared to the unpoled state, the domain wall density at intermediate poling states (0<d₃₃<d_33,max) is lower due to the enlargement in domain size. At optimized poling conditions (d_33,max), the domain sizes show increased inhomogeneity that leads to enhancement in the domain wall density. This work highlights the potential of commercially available PMN–xPT single crystals among other relaxor‐ferroelectrics for achieving temperature control in solid‐state devices. 

Orbital current has attracted significant attention in recent years due to its potential for energy-efficient magnetization control without the need for materials with strong spin–orbit coupling. However, the fundamental mechanisms governing orbital transport remain elusive. In this study, we systematically explore orbital transport in Ti/Ni bilayers through orbital pumping, drawing an analogy to spin pumping. The orbital current is generated and injected into the Ti layer via the microwave-driven orbital dynamics in Ni, facilitated by its strong spin–orbit correlation. We employed thickness-dependent ferromagnetic resonance measurements and angular-dependent inverse orbital Hall effect (IOHE) detection to probe orbital transport in Ti based on the conventional spin-pumping methodology. The observed enhancement in the damping factor indicates an orbital-diffusion length of ∼5.3 ± 3.7 nm, while IOHE-based estimation suggests a value of around 4.0 ± 1.2 nm, which confirms its short orbital-diffusion length. Furthermore, oblique Hanle measurements in the longitudinal configuration reveal an orbital relaxation time of approximately 16 ps. Our results establish that orbital pumping, analogous to the conventional spin-pumping framework, can serve as a robust technique for elucidating orbital transport mechanisms, paving the way for the design of efficient spin-orbitronic devices. 

The chiral induced spin selectivity (CISS) effect, in which the structural chirality of a material determines the preference for the transmission of electrons with one spin orientation over that of the other, is emerging as a design principle for creating next-generation spintronic devices. CISS implies that the spin preference of chiral structures persists upon injection of pure spin currents and can act as a spin analyzer without the need for a ferromagnet. Here, we report an anomalous spin current absorption in chiral metal oxides that manifests a colossal anisotropic nonlocal Gilbert damping with a maximum-to-minimum ratio of up to 1000%. A twofold symmetry of the damping is shown to result from differential spin transmission and backscattering that arise from chirality-induced spin splitting along the chiral axis. These studies reveal the rich interplay of chirality and spin dynamics and identify how chiral materials can be implemented to direct the transport of spin current.