Journal of Applied Physics is an influential international journal publishing significant new experimental and theoretical results of applied physics research. Topics covered in Journal of Applied Physics are diverse, reflecting the most current applied physics research, and include areas of particular emerging interest. Content is published online daily and collected into weekly online and printed issues (48 issues per year).
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This year the discovery of femtosecond demagnetization by laser pulses is 20 years old. For the first time, this milestone work by Bigot and coworkers gave insight directly into the time scales of microscopic interactions that connect the spin and electron system. While intense discussions in the field were fueled by the complexity of the processes in the past, it now became evident that it is a puzzle of many different parts. Rather than providing an overview that has been presented in previous reviews on ultrafast processes in ferromagnets, this perspective will show that with our current depth of knowledge the first applications are developed: THz spintronics and alloptical spin manipulation are becoming more and more feasible. The aim of this perspective is to point out where we can connect the different puzzle pieces of understanding gathered over 20 years to develop novel applications. Based on many observations in a large number of experiments. Differences in the theoretical models arise from the localized and delocalized nature of ferromagnetism. Transport effects are intrinsically nonlocal in spintronic devices and at interfaces. We review the need for multiscale modeling to address the processes starting from electronic excitation of the spin system on the picometer length scale and subfemtosecond time scale, to spin wave generation, and towards the modeling of ultrafast phase transitions that altogether determine the response time of the ferromagnetic system. Today, our current understanding gives rise to the first usage of ultrafast spin physics for ultrafast magnetism control: THz spintronic devices. This makes the field of ultrafast spindynamics an emerging topic open for many researchers right now.

Despite recent progress in the firstprinciples calculations and measurements of phonon meanfreepaths (ℓ), contribution of lowenergy phonons to heat conduction in silicon is still inconclusive, as exemplified by the discrepancies as large as 30% between different firstprinciples calculations. Here, we investigate the contribution of lowenergy phonons with ℓ > 0.8 μm by accurately measuring the crossplane thermal conductivity (Λcross) of crystalline silicon films by timedomain thermoreflectance (TDTR), over a wide range of film thicknesses 1 ≤ h f ≤ 10 μm and temperatures 100 ≤ T ≤ 300 K. We employ a dualfrequency TDTR approach to improve the accuracy of our Λcross measurements. We find from our Λcross measurements that phonons with ℓ > 0.8 μm contribute 53 W m^{−1} K^{−1} (37%) to heat conduction in natural Si at 300 K, while phonons with ℓ > 3 μm contribute 523 W m^{−1} K^{−1} (61%) at 100 K, >20% lower than firstprinciples predictions of 68 W m^{−1} K^{−1} (47%) and 717 W m^{−1} K^{−1} (76%), respectively. Using a relaxation time approximation model, we demonstrate that macroscopic damping (e.g., Akhieser's damping) eliminates the contribution of phonons with meanfreepaths >20 μm at 300 K, which contributes 15 W m^{−1} K^{−1} (10%) to calculated heat conduction in Si. Thus, we propose that omission of the macroscopic damping for lowenergy phonons in the firstprinciples calculations could be one of the possible explanations for the observed differences between our measurements and calculations. Our work provides an important benchmark for future measurements and calculations of the distribution of phonon meanfreepaths in crystalline silicon.

The structural, morphological, optical, and electrical transport characteristics of a metamorphic, brokengap InAs/GaSb pin tunnel diode structure, grown by molecular beam epitaxy on GaAs, were demonstrated. Precise shutter sequences were implemented for the strainbalanced InAs/GaSb active layer growth on GaAs, as corroborated by highresolution Xray analysis. Crosssectional transmission electron microscopy and detailed micrograph analysis demonstrated strain relaxation primarily via the formation of 90° Lomer misfit dislocations (MDs) exhibiting a 5.6 nm spacing and intermittent 60° MDs at the GaSb/GaAs heterointerface, which was further supported by a minimal lattice tilt of 180 arc sec observed during Xray analysis. Selective area diffraction and Fast Fourier Transform patterns confirmed the full relaxation of the GaSb buffer layer and quasiideal, strainbalanced InAs/GaSb heteroepitaxy. Temperaturedependent photoluminescence measurements demonstrated the optical band gap of the GaSb layer. Strong optical signal at room temperature from this structure supports a highquality material synthesis. Current–voltage characteristics of fabricated InAs/GaSb pin tunnel diodes measured at 77 K and 290 K demonstrated two biasdependent transport mechanisms. The Shockley–Read–Hall generation–recombination mechanism at low bias and bandtoband tunneling transport at high bias confirmed the pin tunnel diode operation. This elucidated the importance of defect control in metamorphic InAs/GaSb tunnel diodes for the implementation of lowvoltage and highperformance tunnel field effect transistor applications.

We investigate the resonant tunneling in a single layer graphene superlattice with modulated energy gap and Fermi velocity via an effective Diraclike Hamiltonian. We calculate the transmission coefficient with the transfer matrix method and analyze the effect of a Fermi velocity modulation on the electronic transmission, in the case of normal and oblique incidence. We find it is possible to manipulate the electronic transmission in graphene by Fermi velocity engineering, and show that it is possible to tune the transmitivity from 0 to 1. We also analyze how a Fermi velocity modulation influences the total conductance and the Fano factor. Our results are relevant for the development of novel graphenebased electronic devices.

When acquiring accurate ultrasonic images, we must precisely estimate the mechanical properties of the soft tissue. This study investigates and estimates the viscoelastic properties of the tissue by analyzing shear waves generated through an acoustic radiation force. The shear waves are sourced from a localized pushing force acting for a certain duration, and the generated waves travel horizontally. The wave velocities depend on the mechanical properties of the tissue such as the shear modulus and viscoelastic properties; therefore, we can inversely calculate the properties of the tissue through parametric studies.