|Abstract:||Understanding thermal transport properties of materials is essential for both device applications and materials physics. Thermal conductivity and interface thermal conductance are important engineering parameters for small-scale device applications. In addition, microscopic quantities of how different types of heat carriers interact each other are of crucial importance that determine dynamics of charges and spins.
In this thesis, I use ultrafast pump-probe metrology to experimentally investigate thermal transport properties in various materials systems. The first subject is two-dimensional materials having in-plane anisotropies, i.e., black phosphorus, WTe2, and ReS2, which were first prepared in 2D structures all in 2014. These materials are promising candidates for next-generation electronics and optoelectronics, and the knowledge of thermal transport properties is needed for engineering heat dissipation in devices. However, the low crystal symmetries complicate experimental evaluation of the properties.
To determine thermal conductivity of the three materials along the three coordination axes, I use time-domain thermoreflectance (TDTR) of conventional geometry, where pump and probe beams are co-aligned, and TDTR of beam-offset geometry, where pump and probe beams on sample surface are spatially separated. The beam-offset TDTR allows to measure in-plane thermal conductivity along any arbitrary direction on sample planes but requires a significantly large thickness of a sample. I report the three-dimensional thermal conductivity of BP, WTe2, and ReS2, and their interface thermal conductance with metals. The results are discussed in terms of crystal structure, constituent elements, and atomic bonding strength, and compared with other high-symmetry two-dimensional materials.
The second topic is non-equilibrium heat transport in Pt and Co. Laser-induced non-equilibrium in metals have been extensively studied for noble metals using transient reflectance. However, the interpretation of transient reflectance is not straightforward as reflectance is affected by temperatures of electrons and phonons, and lattice strains. Transition metals also behave differently from noble metals and thus require different theoretical explanations.
I propose to use a four-atomic-layers-thick Co layer as a thermometer to probe non-equilibrium dynamics in metals. I first characterize the properties of Co, e.g., carrier coupling parameters between electrons, phonons, and magnons, by using time-resolved quadratic and linear magneto-optical Kerr effects (TR-QMOKE and TR-MOKE, respectively) on 10-nm-thick and sub-nm-thick Co layers, respectively. Then I use sub-nm-thick Co layers embedded in much thicker Pt layers to investigate non-equilibrium heat transport in Pt. The fast magnetization dynamics of the ultrathin Co layers allows to isolate the electronic temperature at a precise location in Pt/Co/Pt trilayers with sub-picosecond time-resolution. I demonstrate that a model based on the diffusive transport of heat by electrons and the exchange of heat between excitations of electrons, phonons, and magnons consistently explains the temperature evolutions in Pt with different thicknesses, 2−42 nm.
Lastly, I study thermal transport properties of magnetic tunnel junctions (MTJs). MTJs show interesting charge and spin dynamics when they are subject to temperature gradient, such as the tunnel-magneto Seebeck effect and spin Seebeck tunneling. To harness the thermally induced spin behaviors, it is essential to accurately describe the temperature profiles across an MTJ structure while the challenge is to determine the temperature drop across an oxide tunnel barrier. In this work, I use a Co or CoFeB electrode layer in an MTJ as a thermometer and determine the effective thermal conductivity across oxide tunnel barriers, MgO and MgAl2O4.