Predictive Design Of Advanced Energy Materials With Topological Or Superconducting Properties

The ever-increasing energy demand in human life emphasizes the need of clean and renewable energy resources,and the decrease of energy dispassion as much as possible.Thermoelectricity and superconductivity are two representative physical phenomena promising for these energy-related applications.The former converts waste heat into clean electric power and the latter enables dispassion-less electric transport.When joining a thermoelectric and a superconductor together,the conceptually even more appealing superconductive thermo-generator,which can transfer heat into power without resistive losses,can be expected.In the past decade,the participation of topology into these two topics boosted discovery of more exotic physics,such as back-scattering-free boundary states in topological insulators which usually possess excellent thermoelectric bulk states,and Majorana states in topological superconductors with non-Abelian statistics essential for powerful topological quantum computations.Considering these broad applications,there has been a resurgence in the search for new class of materials to design high-efficiency energy conversion systems.In this thesis,a comprehensive study has been carried out for prediction,synthesis,and characterization of advanced energy materials which can help push these cutting-edge concepts towards practical applications.The thesis is organized as follows:The introductory Chapter 1 consists of three parts.In the first part,we give a brief introduction to the developments of the topological concept in condensed matter physics.In the following two parts,we briefly overview the role of nontrivial topology in thermoelectricity and superconductivity.The second chapter deals with the methodology used for the present thesis,which explains the introduction of many-body theory and several established methods to solve the many-body equations and the role of density functional theory.At the end of this chapter,we also introduce the theoretical concept of the Debye-Callaway model to estimate the lattice thermal conductivity.Chapter 3 is devoted to discuss our work on investigating the potential impact of topologically nontrivial surface states on thermoelectricity.In this work,we establish a missing link between the theoretically studied model systems and experimentally investigated materials by explicitly considering the effect of physical dopants,and show that the preservation of nontrivial topology during the inevitable step of doping will result in enhanced thermoelectric performance.We first computationally demonstrate that SnTe doping substantially engineers the low energy bands rather than rigidly shifting the Fermi level,giving rise to significantly enhance thermoelectric performance.Next,we further confirm that such intrinsic SnTe doping preserves the nontrivial topology,which in turn favors high electrical conductivity and thermoelectricity.At the end of this chapter,we also discuss the central findings in connection with the existing experiments.Bismuth telluride-based alloys are well-known thermoelectric materials for room temperature refrigeration.However,the strong intrinsic thermal excitations above 400 K severely degrade their thermoelectric performance,which limit their practical applications.In Chapter 4,we demonstrate that a tiny amount of Ca doping suppresses the detrimental bipolar effect by increasing the hole concentrations.Furthermore,by electron microscopy experiments,together with theoretical analysis on phonon transports,we reveal that an ultra-low thermal conductivity is attributed to the strong phonon scatterings in a wide frequency range via tuning the multiscale microstructures,such as dislocations and stacking faults caused by Ca doping.As a result,a peak ZT value of 1.3 at 400 K was achieved,with an average ZT value of 1.21 between 300-500 K in Bi0.48Ca0.22Sb1.5Te3 sample.Chapter 5 mainly focuses on the epitaxial growth of black phosphorene(BlackP),which is still a daunting challenge in the 2D growth field.Here we propose a straightforward yet conceptually intuitive approach to fabricating BlackP via epitaxial growth.Our substrates of choice are the BlackP-like group ?-? substrates that possess both structural and chemical affinities.We first show that the BlackP is energetically and thermodynamically stable on SnSe(001)as the representative substrate.We further study the atomistic growth mechanisms of the grown structure and show that the mutual interaction between two phosphorus adatoms is attractive on SnSe(001,leading to the formation of dimers which is essentially favorable to achieve large-area growth.Furthermore,we reveal a type-? band alignment of the BlackP/SnSe(001)Janus heterobilayer,with novel catalytic and optoelectronic properties.The FeSe monolayer has been widely explored to achieve high transition temperature due to its exotic pairing mechanism and its simple crystal structure,which is flexible to tune the superconductivity.In Chapter 6,we use first-principles approaches to identify a chemically isovalent and structurally identical counterpart of(Li1-xFexOH)FeSe,namely,(Li1-xCoxOH)CoSb,which not only is an attractive candidate to harbor high-Tc superconductivity,but also exhibits two distinctly new features surrounding topology and magnetism.We first show that superlattice structures of(Li1-xCoxOH)CoSb are dynamically and thermodynamically stable,thereby feasible to fabricate.Next,we demonstrate that(Li1-xCoxOH)CoSb exhibits superconducting properties comparable with or superior to(Li1-xFexOH)FeSe,which can be attributed to the isovalent nature between these two.More strikingly,we reveal that(LiOH)CoSb is topologically nontrivial even without any extrinsic doping in the spacer layers,offering an intrinsically cleaner and more ideal candidate system for realizing topological superconductivity.Finally,in Chapter 7 we present a short summary of this thesis.