Thermoelectric And Thermomagnetic Transport Properties Of Topological Insulator Thin Films

Three-dimensional topological insulators?TIs?represented by Bi₂Te₃ are the best room temperature thermoelectric materials discovered so far.Exploring the effects of the unique properties of TI,especially the topological surface states,on the thermoelectric properties can be very helpful for understanding and optimizing the thermoelectric performance of these materials.Anomalous thermomagnetic transport in a ferromagnetic TI may also bring exotic physical phenomena,thus creating novel applications.This thesis presents experimental studies of thermoelectric and thermomagnetic transport properties of TI thin films grown by molecular beam epitaxy.Quantum confinement introduced by lowering the dimensionality of materials was proposed to be an efficient method for enhancing the thermoelectric power factor.We first study the thermoelectric properties of Bi₂Se₃ TI thin films with varied thickness.However,we find that the thermopower?Seebeck coefficient?and thermoelectric power factor decrease systematically with the reduction of film thickness.Through theoretical calculations based on electronic band structures of the Bi₂Se₃ thin films,we find that this is due to the combined bulk and surface state contribution to the thermoelectric properties of TI.The main origin of the thickness dependent behavior is the increasing contribution of the Dirac-like topological surface state,whose linear dispersion is not optimal to the thermopower.Another factor is the change of the Fermi level position with film thickness,which affects the bulk thermopower value and the relative contribution of the surface states.In TIs,the topological surface states can be removed by a topological phase transition into topological trivial phase.The effect of topological phase transition on thermoelectric properties is investigated in?Bi₁-_xIn_x?₂Se₃ thin films.With the increase of Indium concentration,the resistance of?Bi₁-_xIn_x?₂Se₃ thin film increases dramatically at x where the top and bottom surface states start to hybridize and where the surface states completely disappear.The increases of the thermopower by the reduced and eliminated surface state contribution in the two transitions are minor,as the bulk-channel thermopower is small due to the unfavorable E_F position.Consequently,the power factor decreases with the increase of x.These results show that the topological surface states can be beneficial to thermoelectric performance in terms of increasing the electrical conductivity.We give an analysis on the competition of the two opposite effects of topological surface states on thermopower and on electrical conductivity,providing new clues for optimizing the relative contribution of surface states for further improving the thermoelectric performance of TI-related materials.Besides thermoelectric properties,anomalous thermomagnetic properties in a ferromagnetic TI are also interesting.We perform electromagnetic and thermomagnetic transport studies on a magnetic TI thin film Cr_0.15(Bi_0.1Sb_0.9)_1.85Te₃.The temperature and gate voltage dependence of the anomalous Hall effect exhibits the typical behavior of a quantum anomalous Hall insulator,whereas the anomalous Nernst effect?ANE?shows a sign reversal when the Fermi level is tuned across the charge neutrality point of the surface Dirac cone.We show that the ambi-polar behavior of the ANE can be explained by the semiclassical Mott relation,in conjunction with the ambi-polar Dirac-like band structure of the topological surface states,thus is unique to magnetic TIs.Our results illustrate the crucial role played by the topological surface states on the thermoelectric and thermomagnetic transport properties of TIs,and shed important new lights on further improvement of their thermoelectric performance and potential applications of magnetic TIs.