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This laboratory specializes in experimental measurements of the transport of heat, charge and spin or magnetic moment (see inner triangle in Fig. 1).  The experiments focus on the transformation of thermal energy into electrical energy via thermoelectric and spin-Seebeck effects.

Figure 1: Particles (purple), and the extensive thermodynamic quantities (green) they carry.Figure 1: Particles (purple), and the extensive thermodynamic quantities (green) they carry.

The carriers of charge, heat and spin. 

Electrical charge is carried by electrons. The magnetic moment in ferromagnets resides mostly on the spin and the orbital magnetic moments of the atoms that have partially filled electronic d-shells. Conduction electrons also have spins. Heat consists of the microscopic motions of the various particles at finite temperature, which induces a thermal distribution of the energies of the particles. The motion in the positions of atoms in the solid forms waves, called “phonons,” which carry heat (the lattice thermal conductivity KL) and some of which also carry sound. Electron energies are also broadened by heat and give an electronic thermal conductivity KE.  The effect of temperature on the spins is slightly more subtle: at finite temperature the magnetic moments on the atoms in a solid develop a slight precession, and this precession forms waves called “magnons.” These waves also carry heat (the magnon thermal conductivity, KM). The outer triangle in Fig. 1 represents the three particles (electrons, phonons, and magnons) that can carry the extensive thermodynamic quantities of heat, charge, or magnetic moment.

The transport properties

Under an applied voltage, electrons are accelerated and carry an electrical current.  The relation between current density and electric field is the electrical conductivity (sigma in Fig. 1). But, since they also carry heat, their acceleration under an electric field gives rise to the transport of heat, the Peltier effect (denoted pi), one of the thermoelectric effects shown in Fig. 1. When they are spin polarized, the same electrons create a spin current when accelerated, one of the spintronic effects. 

Under an applied temperature gradients, phonons carry heat (the lattice thermal conductivity KL. Electrons are accelerated as well, and carry heat (the electronic thermal conductivity, KE) as well as charge (the Seebeck effect or thermopower (denoted alpha), one of the thermoelectric effects). Finally, thermal gradients drive magnons, resulting in a magnon thermal conductivity (KM) but also in a spin-Seebeck effect (one of the spin-caloritronic effects).

The same reasoning holds in the presence of a gradient in magnetization.  

The Laboratory

The laboratory’s measurements focus on direct (electrical conductivity, phonon thermal conductivity) and mixed transport effects (the diagonal relations in the Fig. 1).  All measurements are made at temperatures covering 2 K to 1100 K, and in externally applied external magnetic fields.  A brief overview of our recent work follows; active research continues along these lines. A brief overview of our recent work is given in Research Details; active research continues along these lines.