Transport

transport

Transport coefficients are linear responses of charge or heat carriers to external electric or thermal potentials. They come in two basic flavours: longitudinal and transverse. The latter is when the response is perpendicular to the external potential and is measured in the presence of a magnetic field. There are three types of transport coefficients: electrical, thermal, and thermo-electric as summarized in the figure. Electrical resistivity (ρ) is when we apply an electric current through the material and measure the longitudinal electrical voltage. Hall effect is when we measure the transverse electric voltage in the presence of a magnetic field. Thermal conductivity (κ) is when we apply a thermalcurrent through the material and measure the longitudinal temperature difference. Thermal Hall effect is when we measure the transverse temperature difference in the presence of a magnetic field. Seebeck effect (S) is when we apply a thermal current through the material and measure the longitudinal electric voltage. Nernst effect is when we measure the transverse thermo-electric voltage in the presence of a magnetic field.

Typically, we perform these transport measurements at very low temperatures, near zero Kelvin in order to probe the true ground state of a material. For example, measurements of thermal conductivity need to be done at milliKelvin temperature range to reliably extract the residual linear term. Therefore, all the transport measurements above are usually mounted on either a dilution refrigerator or an adiabatic demagnetization fridge.