Showing papers by "David Broido published in 2022"
TL;DR: Yue et al. as discussed by the authors used the optical transient grating technique to measure the ambipolar mobility of cubic boron arsenide (c-BAs) samples at room temperature despite spatial variations.
Abstract: Semiconductors with high thermal conductivity and electron-hole mobility are of great importance for electronic and photonic devices as well as for fundamental studies. Among the ultrahigh–thermal conductivity materials, cubic boron arsenide (c-BAs) is predicted to exhibit simultaneously high electron and hole mobilities of >1000 centimeters squared per volt per second. Using the optical transient grating technique, we experimentally measured thermal conductivity of 1200 watts per meter per kelvin and ambipolar mobility of 1600 centimeters squared per volt per second at the same locations on c-BAs samples at room temperature despite spatial variations. Ab initio calculations show that lowering ionized and neutral impurity concentrations is key to achieving high mobility and high thermal conductivity, respectively. The high ambipolar mobilities combined with the ultrahigh thermal conductivity make c-BAs a promising candidate for next-generation electronics. Description Swift carriers Boron arsenide is a semiconductor with several interesting properties, including a high thermal conductivity. Theoretical calculations also suggest that it has high ambipolar mobility, a measure of the mobility of electrons and holes. Yue et al. and Shin et al. used different types of measurements to observe a high ambipolar mobility in very pure cubic boron arsenide. Shin et al. were able to simultaneously measure the high thermal and electrical transport properties in the same place in their samples. Yue et al. found even higher ambipolar mobility than the theoretical estimates at a few locations. Boron arsenide’s combination of transport properties could make it an attractive semiconductor for various applications. —BG Boron arsenide is a semiconductor with high thermal conductivity and electron-hole mobility.
20 citations
TL;DR: In this article , the authors used a self-consistent ab initio theory of coupled electron-phonon transport to reveal a huge drag enhancement of the thermopower of lightly doped diamond.
3 citations
TL;DR: In this article , temperature dependent magnetization measurements of the studied material were integrated into a first principles computational scheme to calculate phonon modes and investigate the anomalously low thermal expansion of the classical Invar alloy.
Abstract: Theoretical approaches to study thermal properties of magnetic materials typically require accurate models of magnetic interactions in order to define the entropy. Here we introduce a complementary approach for examining thermal properties in magnetic systems where an accepted model for such interactions does not exist. In place of a specific model for magnetic interactions, the approach integrates measurements of temperature dependent magnetization of the studied material into a first principles computational scheme. The approach calculates system pressure from thermally disordered microstates that properly incorporate vibrational and spin subsystems at each temperature as well as the coupling between these subsystems. We apply the approach to calculate phonon modes and to investigate the anomalously low thermal expansion of the classical Invar alloy ${\mathrm{Fe}}_{0.65}{\mathrm{Ni}}_{0.35}$. The calculated phonon dispersions for Invar are in excellent agreement with measured data. The Invar thermal expansion is shown to remain small between 50 K and room temperature, consistent with the experimentally observed low thermal expansion value in this same temperature range. This anomalously small thermal expansion is directly connected to a small positive contribution from lattice thermal disorder that is nearly canceled by a large negative magnetic disorder contribution. By contrast, calculations for bcc Fe show a much larger thermal expansion, consistent with experiment, which is dominated by a large contribution from lattice thermal disorder that is reduced only slightly by a small negative contribution from that of magnetism. These findings give insight into the unusual nature of magnetism and spin-lattice coupling in Invar and Fe. In addition, they give promising preliminary support to the presented methodology as a complementary way to investigate thermal properties of magnetic materials. The success achieved on Invar and Fe motivates future testing of the approach on other magnetic materials.
2 citations
TL;DR: In this article , the authors demonstrate the high-sensitivity, sub-micron resolved imaging of vector photocurrent using ac magnetometry with a nitrogen-vacancy center spin ensemble.
Abstract: Photocurrent measurements are incisive probes of crystal symmetry, electronic band structure, and material interfaces. However, conventional scanning photocurrent microscopy (SPCM) con-volves the processes for photocurrent generation and collection, which can obscure the intrinsic light-matter interaction. Here, by using ac magnetometry with a nitrogen-vacancy center spin ensemble, we demonstrate the high-sensitivity, sub-micron resolved imaging of vector photocurrent flow. Our imaging reveals that in anisotropic semimetals WTe 2 and TaIrTe 4 , the photoexcited electron carriers propagate outward along the zigzag chains and inward perpendicular to the chains. This circulating pattern is explained by our theoretical modeling to emerge from an anisotropic photothermoelectric effect (APTE) caused by a direction-dependent thermopower. Through simultaneous SPCM and magnetic imaging, we directly visualize how local APTE photocurrents stim-ulate long-range photocurrents at symmetry-breaking edges and contacts. These results uniquely validate the Shockley-Ramo process for photocurrent collection in gapless materials and identify the overlooked APTE as the primary origin of robust photocurrents in anisotropic semimetal devices. Our work highlights quantum-enabled photocurrent flow microscopy as a clarifying perspective for complex optoelectronic
2 citations
TL;DR: In this article , the authors measured the peak thermal conductivity of boron arsenide (BAs) at temperatures between 120 and 150 K and reported that the peak temperature of BAs is at most 4.16m in two samples and 5m in another.
Abstract: Recent experiments have validated prior theories of unusual high thermal conductivity $(\ensuremath{\kappa})$ in boron arsenide (BAs) and revealed large $\ensuremath{\kappa}$ variation associated with extended and point defects in the samples. The peak $\ensuremath{\kappa}$ provides valuable insights into the competition between intrinsic phonon-phonon scattering processes and extrinsic boundary and defect scattering processes in BAs. However, prior measurement methods have not been able to measure the peak $\ensuremath{\kappa}$ because of fundamental and technical limitations. Here, we report peak $\ensuremath{\kappa}$ measurements of BAs crystals synthesized under different conditions with source materials of different purities via a vapor transport method. For three representative samples, the measured $\ensuremath{\kappa}$ peak appears at temperatures between 120 and 150 K and varies from $410\ifmmode\pm\else\textpm\fi{}60\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ to $830\ifmmode\pm\else\textpm\fi{}100\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. The measured thermal conductivities agree with theoretical calculations across the full temperature range. The similar calculated and measured peak temperatures helps to narrow down the boundary scattering mean free path to be around 4 \ensuremath{\mu}m in two samples and 5 \ensuremath{\mu}m in another, while the variation of the peak magnitude reveals a one-order-of-magnitude difference in the strength of point defect scattering. The phonon-defect scattering behavior correlates well with the measured electronic Raman scattering background, the impurity concentrations revealed by secondary ion mass spectroscopy, and the Hall concentration and mobility of the $p$-type samples except for an anomalously high hole concentration that appears in one sample, which indicates nonuniform impurity distribution. The observed correlation clarifies the origins of extrinsic phonon scattering mechanisms in BAs crystals.
1 citations
TL;DR: In this article , an unknown mechanism for bulk photocurrent generation termed the anisotropic photothermoelectric effect (APTE) was elucidated, where unequal thermopowers along different crystal axes drive intricate circulations of photocurrent around the photoexcitation.
Abstract: Materials that rectify light into current in their bulk are desired for optoelectronic applications. In inversion-breaking Weyl semimetals, bulk photocurrents may arise due to nonlinear optical processes that are enhanced near the Weyl nodes. However, the photoresponse of these materials is commonly studied by scanning photocurrent microscopy (SPCM), which convolves the effects of photocurrent generation and collection. Here, we directly image the photocurrent flow inside the type-II Weyl semimetals WTe2 and TaIrTe4 using high-sensitivity quantum magnetometry with nitrogen-vacancy center spins. We elucidate an unknown mechanism for bulk photocurrent generation termed the anisotropic photothermoelectric effect (APTE), where unequal thermopowers along different crystal axes drive intricate circulations of photocurrent around the photoexcitation. Using simultaneous SPCM and magnetic imaging at the sample's interior and edges, we visualize how the APTE stimulates the long-range photocurrent collected in our Weyl semimetal devices through the Shockley-Ramo theorem. Our results highlight an overlooked, but widely relevant source of current flow and inspire novel photodetectors using homogeneous materials with anisotropy.
1 citations
11 May 2022
TL;DR: In this paper , a hybrid ab initio theoretical approach for examining thermal properties in magnetic systems of unknown entropy is presented, which integrates measurements of temperature dependent magnetization of the studied material.
Abstract: -A hybrid ab initio theoretical approach for examining thermal properties in magnetic systems of unknown entropy is presented. Commonly used theoretical approaches interrogate thermal properties from Gibbs/Helmholtz free energies, which require an accurate model of magnetic interactions. The present approach avoids this requirement by instead calculating system pressure from thermally disordered microstates that properly incorporate vibrational and spin subsystems at each temperature as well as the coupling between these subsystems. In place of a specific model for magnetic interactions, the approach integrates measurements of temperature dependent magnetization of the studied material. We apply the approach to calculate phonon modes and to investigate the anomalously low thermal expansion of the classical Invar alloy, Fe 0.65 Ni 0.35 . The calculated phonon dispersions for Invar are in excellent agreement with measured data. The Invar thermal expansion is shown to remain small between 50 K and room temperature, consistent with the experimentally observed low thermal expansion value in this same temperature range. This anomalously small thermal expansion is directly connected to a small positive contribution from lattice thermal disorder that is nearly canceled by a large negative magnetic disorder contribution. By contrast, calculations for bcc Fe show a much larger thermal expansion, consistent with experiment, which is dominated by a large contribution from lattice thermal disorder that is reduced only slightly by a small negative contribution from that of magnetism. These findings give insights into the unusual nature of magnetism and spin-lattice coupling in Invar and Fe, and