Browsing by Subject "Phonons"

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Effect of molecular Berry curvature on the dynamics of phonons
(2022-08-31) Saparov, Daniyar; Niu, Qian; MacDonald, Allan H; Tutuc, Emanuel; Shi, Li
Under the Born-Oppenheimer approximation, the electronic ground state evolves adiabatically and can accumulate geometrical phases characterized by the molecular Berry curvature. In this work, we study the effect of the molecular Berry curvature on the lattice dynamics in a system with broken time-reversal symmetry. The molecular Berry curvature is formulated based on the single-particle electronic Bloch states. It manifests as a non-local effective magnetic field in the equations of motion of the ions that are beyond the widely adopted Raman spin-lattice coupling model. We employ the Bogoliubov transformation to solve the quantized equations of motion and to obtain phonon polarization vectors. We apply our formula to the Haldane model on a honeycomb lattice and find a large molecular Berry curvature around the Brillouin zone center. As a result, the degeneracy of the optical branches at this point is lifted intrinsically. The lifted optical phonons show circular polarizations, possess large phonon Berry curvature, and have a nearly quantized angular momentum that modifies the Einstein-de Haas effect.
Experimental investigations of energy carrier interactions with atomic disorders and artificial long-range orders
(2022-10-06) Smith, Brandon Paul; Shi, Li, Ph. D.; Orbach, Raymond L; Akinwande, Deji; Bank, Seth R; Wang, Yaguo
The field of nanoscale energy transport, conversion, and storage is at an exciting time with next-generation devices manipulating discrete energy carriers, e.g. phonons, photons, and electrons, in confined dimensions arriving closer to commercialization, such as solid-state flexible electronics and optoelectronics utilizing one dimension (1D) and two dimensional (2D) nanomaterials. The transport dynamics of quasiparticles and their coupling are modified, notably in low dimensional nanomaterials, with the inclusion of disorder and artificial long-range order. Through this lens, it is possible to probe interesting physics and draw out intrinsic properties of the nanomaterials. This is especially important for electronic systems and energy conversion & storage devices where heat generation and dissipation within nano- and microscale locations of nonequilibrium impedes continued advancement. This thesis examines outstanding questions concerning nanoscale thermal and thermoelectric transport in low-dimensional materials to further understanding of crystal disorder and artificial long-range order. Specifically, the material systems investigated are alloy disorder and surface roughness in semiconducting silicon germanium (SiGe) nanowires, microscale rippling in layered molybdenum disulfide (MoS2) flakes, intra- and interlayer interactions in bulk and monolayer MoS2, and artificially created, long-range domain walls in twisted bilayer graphene (TBG). The fundamental questions are addressed through electrothermal, optothermal, and scanning probe metrology techniques. First, eight-probe thermal conductivity measurements of SiGe nanowires show that alloying suppresses thermal transport, and the mean-free-paths of low-frequency phonons are suppressed by diffuse surface roughness scattering in nanowires. The diffuse surface scattering results in length-independent thermal conductivity for lengths over two micrometers. Similarly, four-probe thermal conductivity measurements reveal that microscale ripples have negligible effects on phonon transport in 2D layers as the ripple wavelengths and curvatures are much larger than the phonon mean free paths and wavelengths. The peak thermal conductivity is found to increase with decreasing Raman scattering intensity in the frequency range with vanishing phonon density of states in MoS2 indicating an important role of point defect scattering. In addition, this dissertation presents an experimental effort to employ micro-Raman spectroscopy to investigate local nonequilibrium among different phonon polarizations in MoS2 inside the focused laser spot. It also describes an exploration of ultra-high vacuum scanning probe microscopy for probing the local thermoelectric property of twisted bilayer graphene moiré superstructures.
Optical excitation and probing of nonequilibrium phonons in molybdenum disulfide
(2021-08-13) Sokalski, Peter Cyprian; Shi, Li, Ph. D.; Wang, Yaguo
Layered transition metal dichalognides (TMDs) are being explored as next-generation electronic and optoelectronic materials due to their thickness-dependent electronic bandstructure, high mobility, and mechanical flexibility. In electronic and optoelectronic devices, electrons are energized by high electric fields and dissipate their energy via scattering with phonons, which are the energy quanta of lattice vibration. It is known that the very different coupling strengths between electrons and different phonon polarizations can result in local nonequilibrium among the different optical and acoustic phonon polarizations in both cubic phase group IV and V semiconductors and hexagonal phase monolayer graphene. In comparison, it is unclear whether similar hot phonon phenomena can occur in TMDs. This thesis reports Raman spectroscopy measurements of the electron-phonon energy dissipation processes in bulk Molybdenum Disulfide (MoS₂) excited by the Raman laser beam. The population and temperature of two Raman-active optical phonon modes are obtained from the measured intensity ratios between the Stokes and anti-Stokes bands, whereas the Raman peak shift is used to determine the population and equivalent temperature of different phonon polarizations that are anharmonically coupled to the Raman-active modes. The measurement results reveal preferential heating of the optical phonons. The non-equilibrium measured with different laser beam spot sizes is used to the extract the thermalization lengths between the optical phonons and the lattice.
Phonon and magnon thermometry using light scattering techniques
(2019-01-24) Olsson, Kevin Somerville; Li, Elaine; Shi, Li, Ph. D.; Chelikowsky, James R; MacDonald, Allan H; Tsoi, Maxim
Technology has advanced to the point where large thermal gradients are produced on the scale of nanometers. These scales are smaller than the interaction length of the thermal energy carriers, and the standard descriptions of thermal transport, such as Fourier’s law, no longer apply. Beyond changing the thermal transport, nonequilibrium in magnets between energy carriers such as magnons and phonons, leads to a host of new phenomena known as spin caloritronics. Accurate descriptions of the carrier nonequilibrium are crucial for further understanding these systems. Advances in technology have also lead to thermometry techniques offering temperature sensing on micrometer or smaller length scales. However, few techniques are able to distinguish between energy carriers. Inelastic light scattering is an optical technique that is non-contact, flexible, and, most importantly, able to individually probe different energy carriers. There are two inelastic light scattering techniques corresponding to different frequency regimes: Raman scattering for THz frequencies and Brillouin light scattering (BLS) for GHz frequencies. This work examines the use of these techniques for detecting carriers nonequilibrium in semiconductors and magnetic insulators. In semiconductors, nonequilibrium between optical phonons and the dominant energy carrier, acoustic phonons, can occur due to different relaxation rates. Raman scattering is an establish technique for measuring THz frequency optical phonon temperatures. BLS has the same ability to measure GHz frequency acoustic phonon temperatures. A combination of these techniques is presented to simultaneously measure optical and acoustic phonon temperatures in silicon. For spin caloritronic phenomena, magnons are also of interest. BLS is an established technique for studying magnon dynamics in ferromagnets. Its ability to measure magnon temperatures in equilibrium, or magnon density in nonequilibrium, is presented and evaluated. Lastly, BLS is used to probe acoustic phonons and magnons in the ferrimagnet insulator YIG under local laser heating. A nonequilibrium is observed between the magnons and phonons, which is analyzed with a magnon diffusion model. This analysis yields a magnon diffusion length and chemical potential, and a local measurement of the thermally generated spin current. These result demonstrate the capability of inelastic light scattering for measure carrier nonequilibrium important to thermal transport and spin caloritronic phenomena.
Thermal and thermoelectric measurements of silicon nanoconstrictions, supported graphene, and indium antimonide nanowires
(2009-12) Seol, Jae Hun; Shi, Li, Ph. D.
This dissertation presents thermal and thermoelectric measurements of nanostructures. Because the characteristic size of these nanostructures is comparable to and even smaller than the mean free paths or wavelengths of electrons and phonons, the classical constitutive laws such as the Fourier’s law cannot be applied. Three types of nanostructures have been investigated, including nanoscale constrictions patterned in a sub-100 nm thick silicon film, monatomic thick graphene ribbons supported on a silicon dioxide (SiO₂) beam, and indium antimonide (InSb) nanowires. A suspended measurement device has been developed to measure the thermal resistance of 48-174 nm wide constrictions etched in 35-65 nm thick suspended silicon membranes. The measured thermal resistance is more than ten times larger than the diffusive thermal resistance calculated from the Fourier’s law. The discrepancy is attributed to the ballistic thermal resistance component as a result of the smaller constriction width than the phonon-phonon scattering mean free path. Because of diffuse phonon scattering by the side walls of the constriction with a finite length, the phonon transmission coefficient is 0.015 and 0.2 for two constrictions of 35 nm x 174 nm x220 nm and 65 nm x 48 nm x 50 nm size. Another suspended device has been developed for measuring the thermal conductivity of single-layer graphene ribbons supported on a suspended SiO₂ beam. The obtained room-temperature thermal conductivity of the supported graphene is about 600 W/m-K, which is about three times smaller than the basal plane values of high-quality pyrolytic graphite because of phonon-substrate scattering, but still considerably higher than for common thin film electronic materials. The measured thermal conductivity is in agreement with a theoretical result based on quantum mechanical calculation of the threephonon scattering processes in graphene, which finds a large contribution to the thermal conductivity from the flexural vibration modes. A device has been developed to measure the Seebeck coefficients (S) and electrical conductivities ([sigma]) of InSb nanowires grown by a vapor-liquid-solid process. The obtained Seebeck coefficient is considerably lower than the literature values for bulk InSb crystals. It was further found that decreasing the base pressure during the VLS growth results in an increase in the Seebeck coefficient and a decrease in the electrical conductivity, except for a nanowire with the smallest diameter of 15 nm. This trend is attributed to preferential oxidation of indium by residual oxygen in the growth environment, which could cause increased n-type Sb doping of the nanowires with increasing base pressure. The deviation in the smallest diameter nanowire from this trend indicates a large contribution from the surface charge states in the nanowire. The results suggest that better control of the chemical composition and surface states is required for improving the power factor of InSb nanowires. On approach is to use Indium-rich source materials for the growth to compensate for the loss of indium due to oxidation by residual oxygen.