arXiv subjects

Xinping Shi

Publications and source records attributed to Xinping Shi.

3 recordsLinked to original sources

arXiv2024

Strong Temperature Dependence of Thermal Conductivity in High-Purity Cubic Boron Arsenide

Materials with high thermal conductivity are needed to conduct heat away from hot spots in high power electronics and optoelectronic devices. Cubic boron arsenide (c-BAs) has a high thermal conductivity due to its special phonon dispersion relation. Previous experimental studies of c-BAs report a room-temperature thermal conductivity between 1000 and 1300 W m-1 K-1. We synthesized high purity isotopically enriched c-BAs single crystals with room-temperature thermal conductivity of around 1500 W m-1 K-1. Using time-domain thermoreflectance (TDTR), we measured thermal conductivity and found a 1/T2 temperature dependence between 300 K and 600 K - slightly stronger than predictions from state-of-the-art theoretical models. Brillouin and Raman scattering revealed minimal changes in phonon frequencies over the same temperature range, suggesting that the observed 1/T2 dependence is not caused by temperature dependent changes in phonon dispersion. To probe defect densities in the BAs crystals we studied, we conducted transient reflectivity microscopy (TRM) measurements of absorption at sub-bandgap photon energies. We observe a correlation between TRM signal intensity and thermal conductivity. Notably, samples with thermal conductivity near 1500 W m-1 K-1 still exhibited nonzero TRM signals, suggesting the presence of defects despite the high thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2021

Differentiating Contributions of Electrons and Phonons to the Thermoreflectance Spectra of Gold

To better understand the many effects of temperature on the optical properties of metals, we experimentally and theoretically quantify the electron vs. phonon contributions to the thermoreflectance spectra of gold. We perform a series of pump/probe measurements on nanoscale Pt/Au bilayers at wavelengths between 400 and 1000 nm. At all wavelengths, we find that changes in phonon temperature, not electron temperature, are the primary contributor to the thermoreflectance of Au. The thermoreflectance is most sensitive to the electron temperature at wavelength of ~480 nm due to interband transitions between d-states and the Fermi-level. In the near infrared, the electron temperature is responsible for only ~2% of the total thermoreflectance. We also compute the thermoreflectance spectra of Au from first principles. Our calculations further confirm that phonon temperature dominates thermoreflectance of Au. Most of Au's thermoreflectance is due to the effect of the phonon population on electron lifetime.

cond-mat.mtrl-sci↗

arXiv2020

Picosecond Spin Orbit Torque Switching

Reducing energy dissipation while increasing speed in computation and memory is a long-standing challenge for spintronics research. In the last 20 years, femtosecond lasers have emerged as a tool to control the magnetization in specific magnetic materials at the picosecond timescale. However, the use of ultrafast optics in integrated circuits and memories would require a major paradigm shift. An ultrafast electrical control of the magnetization is far preferable for integrated systems. Here we demonstrate reliable and deterministic control of the out-of-plane magnetization of a 1 nm-thick Co layer with single 6 ps-wide electrical pulses that induce spin-orbit torques on the magnetization. We can monitor the ultrafast magnetization dynamics due to the spin-orbit torques on sub-picosecond timescales, thus far accessible only by numerical simulations. Due to the short duration of our pulses, we enter a counter-intuitive regime of switching where heat dissipation assists the reversal. Moreover, we estimate a low energy cost to switch the magnetization, projecting to below 1fJ for a (20 nm)^3 cell. These experiments prove that spintronic phenomena can be exploited on picosecond time-scales for full magnetic control and should launch a new regime of ultrafast spin torque studies and applications.

physics.app-ph↗