  Lifshitz transition, a change in Fermi surface topology, is likely to greatly
influence exotic correlated phenomena in solids, such as high-temperature
superconductivity and complex magnetism. However, since the observation of
Fermi surfaces is generally difficult in the strongly correlated systems, a
direct link between the Lifshitz transition and quantum phenomena has been
elusive so far. Here, we report a marked impact of the pressure-induced
Lifshitz transition on thermoelectric performance for SnSe, a promising
thermoelectric material without strong electron correlation. By applying
pressure up to 1.6 GPa, we have observed a large enhancement of thermoelectric
power factor by more than 100% over a wide temperature range (10-300 K).
Furthermore, the high carrier mobility enables the detection of quantum
oscillations of resistivity, revealing the emergence of new Fermi pockets at
~0.86 GPa. The observed thermoelectric properties linked to the multi-valley
band structure are quantitatively reproduced by first-principles calculations,
providing novel insight into designing the SnSe-related materials for potential
valleytronic as well as thermoelectric applications.
