Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J55: Phonons and Thermal Transport at the Nanoscale IIFocus Live
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Sponsoring Units: DMP Chair: Zhiting Tian, Cornell University |
Tuesday, March 16, 2021 3:00PM - 3:36PM Live |
J55.00001: Nanotube mechanical resonators – tiny electron forces and few quanta of vibrations Invited Speaker: Adrian Bachtold Mechanical resonators based on carbon nanotubes feature a series of truly exceptional properties. Carbon nanotubes are the lightest resonators fabricated thus far. The mechanical vibrations are sensitive to the tiny forces associated with the electrons flowing through the nanotube, and vice versa. In this talk, I will discuss our efforts to cool the amplitude of the thermal vibrations to a few quanta [1]. Cooling is achieved using a simple yet powerful method, which consists in applying a constant (DC) current of electrons through the suspended nanotube in a dilution fridge. The origin of the observed cooling is attributed to an electrothermal effect. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J55.00002: Nanoscale Vibrational Mapping of Single SiGe Quantum Dots in the Electron Microscope Chaitanya Gadre, Xingxu Yan, Qichen Song, Jie Li, Lei Guo, Toshihiro Aoki, Sheng-Wei Lee, Gang Chen, Ruqian Wu, Xiaoqing Pan Through the engineering of complex structures such as alloys, nanostructures, and superlattice interfaces, the propagation of phonons can be manipulated to suppress material thermal conductivity1. Due to the lack of spatial resolution of conventional optical methods, experimental study of phonon behavior at nanostructure interfaces has been impossible until now. Recent developments in electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) have made it possible to study phonons at nanometer resolution2. Here we demonstrate the two-dimensional nanoscale vibrational mapping of a single SiGe quantum dot (QD) using an atom-sized probe in the electron microscope. For the first time, we experimentally reveal an enhancement in the Si optical mode intensity below the abrupt interface of the QD suggesting increased phonon population due to phonon reflection. We have also developed a new technique to map the differential momentum and flux of optical phonon modes at the nanoscale to directly reveal the reflection of propagating Si optical modes. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J55.00003: Generation and Detection of Coherent GHz Surface Phonons in Superlattices Changxiu Li, Nikolay Chigarev, Théo Thréard, Kedong Zhang, Artem Husiev, Frederick Niepceron, Vincent Tournat, Samuel Raetz, Hong Lu, Vitalyi E Gusev The development of transducers to excite/detect coherent surface acoustic waves (SAWs) with picosecond periods and nanometer-level wavelengths and localization depths would be very useful for nanometrology, nanoimaging, sensing and manipulations in fundamental and applied research. Earlier, the control of SAWs and Lamb waves up to the record frequencies of about 100 and 200 GHz, respectively, was achieved by applying fs pump-probe laser pulses to metallic gratings deposited on a substrate. To reach 1 THz phonons, the grating period should be shorter than 3 nm with a velocity of 3 km/s, which is hardly achievable with conventional techniques. We suggest to engineer unconventional transducers based on cleaved nanostructured bulk materials to address this issue [1]. Currently, bulk superlattices (SLs) with a period of several atomic layers can be grown by epitaxy. They can be cleaved along the direction normal to their layers, producing a periodically nanopatterned surface. We report here first experimental monitoring by lasers of coherent phonons up to 70 GHz on the cleaved cross sections of Ga1-x AlxAs/Ga1-yAlyAs SLs. [1] V.E. Gusev, In: D. Bićanić (eds) Photoacoustic and Photothermal Phenomena III. Springer Series in Optical Sciences, 69, 323 (Springer, Berlin, Heidelberg, 1992). |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J55.00004: Superlattice as a phonon trap in zincblende quantum wells Hua Wang, Mario Borunda, Kieran Mullen In hot carrier photovoltaic devices, photoexcited electrons and holes are spatially separated in "Type II" quantum wells to reduce their recombination rate. But there are well-understood mechanisms by which the optical phonons created in electron-phonon interaction can decay into acoustic phonons and escape the quantum well, cooling down the lattice and carriers. To enhance performance, we have to manage heat transport without compromising the photoelectronic conversion rate. We build a realistic model of a Type II quantum well superlattice and calculate its phonon structure. We use DFT to determine the parameters needed to represent several typical zincblende structures in the form of Tersoff potentials. We then examine how superlattice design can affect phonon transport as desired to improve efficiency. We explore heat management for different designs and seek to engineer the phonon structure to obtain the desired heat dissipation rate. Finding the optimal quantum well design is then a complex interplay between electronic and phononic properties. We use our potentials to go beyond the harmonic approximation and calculate phonon scattering for more realistic predictions. In addition, these potentials will establish a parameter library useful for future device design. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J55.00005: Phonon coherence in GaN/InN Superlattices Natalia Bedoya, Tobias Spitaler, René Hammer The diffusive picture of phonon transport is replaced by the so-called ballistic one at nanoscale lengths, where interfaces and boundaries play an important role. A phonon approaching an interface can be specularly (coherent) reflected or diffusively (incoherent) scattered in all directions. The wave interference effects of phonons yield phonon band gaps and modified phonon dispersion relations. These materials are known as phononic crystals and could be used as phonon sources, filters, and detectors.1 Superlattices (SL) have been proven ideal for realizing phononic crystals. These SLs would serve as a source of coherent phonons, as long as the phonons can propagate coherently within these structures. We use molecular dynamics simulations to estimate the phonon coherence length in GaN/InN SLs of different layer thicknesses to investigate this effect. We also study the impact of such coherence on the thermal transport properties of the studied systems. |
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