Synthesis and characterization of compound crystals with weak phonon couplings




Lee, Hwijong

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Recent advances in experimental and computational techniques have advanced the understanding of phonon transport and phonon couplings to charge and spin degrees of freedom. As an illustrating example, the unusual phonon band features of Boron Arsenide (BAs) result in simultaneously high lattice thermal conductivity and high intrinsic carrier mobility, which make BAs an emerging III-V semiconductor for high-performance electronics devices. Meanwhile, magnon coupling with phonons allows for the thermal generation of spin waves, which can be converted into an electrical signal for readout or vice versa via the spin Hall effect in a normal metal in contact with the magnetic material.

This work seeks to advance the understanding of the coupled transport phenomena in electronic and magnetic materials with unusual phonon-mediated behaviors and to address some of the fundamental questions on the interactions among energy, charge, and spin carriers in the semiconducting BAs, semimetal θ phase tantalum nitride (θ-TaN), and the magnetic insulator yttrium ion garnet (YIG). These questions are addressed through several experimental approaches based on surface electronic state measurements, steady-state bulk thermal and electrical properties measurements, frequency-dependent spin caloritronic measurements, and electron microscopy.

Scanning tunneling spectroscopy measurements of cleaved BAs surfaces show a bulk bandgap of 2.1 eV as well as surface electronic states inside the bulk bandgap. These findings are relevant to the use of BAs as an active layer in future-generation electronic devices. With a similarly large phonon energy gap as semiconducting BAs, θ-TaN is grown via a high-pressure technique for transport measurements to investigate the theoretical prediction of a high thermal conductivity of this semimetal compound with a small electron density of states near the Fermi level. The results show both the effect of grain boundary scattering on suppressing the thermal conductivity and the potential of further increasing the thermal conductivity by increasing the grain size and reducing the defect concentrations. Besides these two investigations of electronic and phononic structures and transport, lock-in measurements are used to investigate the frequency dependence of the spin Seebeck effect (SSE) and detect a spin Peltier magnetoresistance (SPMR) at a heterostructure made of a platinum (Pt) thin film on YIG. The observed frequency dependence of the second harmonic SSE and first harmonic SPMR are analyzed with a model that accounts for both interface and bulk spin Seebeck effects to understand the elusive magnon transport properties.


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