Research

Interests

Phonon dynamics is crucial for understanding many material properties such as thermal transportation, thermal expansion, and phase stability. Lattice anharmonicity reduces the phonon-related thermal transportation in materials which are important in thermoelectrics, geophysics, and high temperature applications. Phonon anharmonicity of the metals and alloys is a key to understand the phonon-phonon and phonon-electron coupling, which are the fundamental problems of metal systems. Phonon-induced negative thermal expansion (NTE) of materials is a shortcut for engineering materials with desired thermal expansion properties. Also, phonon vibrational entropy plays important roles in stabilizing metal alloys phases. The research on thermal barriers, thermoelectric materials, fuel cells, and energy storage materials all involves studying temperature-induced phonons behaviors.



Projects

Phonon anomalies in thermoelectrics
    Thermoelectric materials such as PbTe can directly convert waste heat into electricity. To be efficient, thermoelectric materials must have a low thermal conductivity. This requires disrupting the phonon propagation to suppress the lattice contribution, and thus one seeks short phonon lifetimes, and anharmonic systems. Detailed measurements of phonon linewidths, and the temperature dependence of phonon dispersions, are crucial to benchmark microscopic theories of thermal conductivity, and develop more efficient materials.

Phonons and vibrational entropy in copper-zirconium alloys
    The role of vibrational entropy in stabilizing or destabilizing the amorphous phase was determined by comparing the amorphous material above and below the glass transition temperature, in its equilibrium two-phase crystalline state at low temperatures, and at high temperatures in the B2 phase. 

Phonon-induced negative thermal expansion (NTE) 
    NTE sometime originates from the phonon dynamics, for example in zirconium tungstate. It was recently discovered that the scandium fluoride (ScF3) exhibits wide-temperature-range, isotropic NTE from a few to 1000 K. Inelastic neutron scattering experiment was performed to analyze the phonon density of states of this material. Phonon anharmonicity and vibrational entropy also show unusual behavior and their connections to the NTE are studied.

Anharmonicity of metal oxides
    Metal oxides have a huge variety in structures and their phonon anharmonicity is usually poorly studies. Raman spectroscopy and inelastic neutron scattering are used to study the phonon lifetime and phonon-phonon coupling of ZrO2, HfO2, yttrium-stabilized zirconia (YSZ), TiO2, SnO2, Fe2O3, MgO, CaO, and etc.

Phonon-phonon coupling in metal and alloys
    Ph(onon)-ph(onon) coupling in metals and alloys are very important because phonon lifetime in metals usually involve both ph-ph coupling and ph-e(lectron) coupling. The latter may be significant at elevated temperatures. The ph-ph coupling has to be carefully measured or calculated and subtracted from the total phonon linewidth in order to study the ph-e coupling. Inelastic neutron scattering and triple-axis spectrometer measurements are compared with first-principles calculations to understand phonon lifetime.

Time-resolved excited-state study
    Materials in excited-state are key components of solar cells and fuel cells. Many of the processes have a lifetime in the scale of nanosecond to microsecond. This time scale is too fast to be studied by traditional static state methods but too slow by femtosecond methods. Nanosecond time-resolved Raman spectroscopy could measure the dynamical process in excited state and with the help of optimization algorithm, calculate the structure in real space.    


Experimental Techniques

Neutron scattering has been the workhorse for studying phonons for decades. The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory is currently the most intense pulsed neutron beams in the world. We perform time-of-flight inelastic neutron scattering experiments at ARCS(Wide Angular-Range Chopper Spectrometer), CNCS (Cold Neutron Chopper Spectrometer)SEQUOIA (Fine-Resolution Fermi Chopper Spectrometer), and HYSPEC (Hybrid Spectrometer). We also perform neutron diffraction experiment at POWGEN (Powder Diffractometer)

Triple-axis spectrometer uses neutrons to study the phonon dispersion in samples. The High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory is the highest flux reactor-based source of neutrons for condensed matter research in the United States. We perform experiment at HB-3, CG-4C(CTAX), and HB-1 instruments at HIFR. We also do experiments at NIST Center for Neutron Research (NCNR).

Nuclear resonance inelastic X-ray scattering (NRIX) experiments are perform on sector 3 and sector 16 at Advanced Photon Source (APS) at Argonne National Laboratory. High resolution inelastic X-ray scattering (HERIX) experiments are performed at sector 30.

Raman scattering spectroscopy is one of the major techniques used by chemists and geologists for identifying materials. In phonon dynamics study, the high resolution, the flexibility of samples environment, and fast turn-around time make Raman the ideal technique to study energy and lifetime of phonons at Brillouin zone center in non-metal materials. l had set up a Raman system at Caltech, which is able to perform ground state measurement from 80~1200 K. A pump-prob time-resolved system for studying materials at excited-states with nanosecond lifetime using pockels cell gating is in development. High pressure Raman system using diamond anvil cell is also being tested.


Computational Techniques

First-principles density functional phonon calculations using VASP and QuantumEspresso. Phonon properties through ab initio molecular dynamics (MD).

Classical lattice dynamics and molecular dynamics of large systems using GULP.

Neutron instruments simulations using Monte Carlo methods. 
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