| Plasmon describes the collective oscillation of itinerate electrons in solid systems,which can be excited by light or high-energy electrons.Possessing both the optical and electrical properties,plasmons can be widely used in photonics and electronics.On the one hand,plasmons are manifested as surface plasmon on the surfaces of metals or nanoparticles and can confine the light within the dimension of subwavelength,which enables the manipulation of light in the nanoscale.On the other hand,the highfrequency character of plasmons can be exploited to design optical and electrical circuits.Besides,plasmons are very sensitive to the molecular adsorption and are already used to make hypersensitive molecular sensor.With the rise of the two-dimensional layered materials,the quantum confinement effect is nonnegligible due to the low dimension and small size.Therefore,the plasmon within the classical electromagnetic filed model is insufficient,calling for the quantum description of the plasmons.In this thesis,we perform a series of theoretical and numerical studies surrounding the novel functionalities and properties in low-dimensional quantum materials,including the unconventional high-temperature superconductor of monolayer FeSe grown on oxide substrate and the heterostructure of graphene/topological insulator.The main achievements and the structure of this thesis are as follows:In Chapter 1,the concept of plasmon is introduced and the dispersions of volume plasmon and surface plasmon are derived from the well-known Maxwell’s equations.Next,we introduce the superconductivity and the well-established theoretical models,including the Bardeen-Cooper-Schrieffer(BCS)model and Migdal-Eliashberg theory.Beyond the conventional superconductors,the more intriguing thing is the discovery of unconventional superconductors,including the most famous Cu-based superconductors and Fe-based superconductors.In particular,the recently discovered FeSe-based superconductors possess simpler Fermi surface and the similar tunable nature by charge doping as Cu-based superconductors,which provide an easier platform to study the underlying electron pairing mechanism.Finally,we introduce the quantum topological materials which emerge recently and have been widely studied in the field of condensed matter physics,in which we will focus on the frequently used topological band theory.In Chapter 2,a phenomenological model of polaronic plasmon is constructed to study the pairing mechanism of the FeSe-based superconductors.First,in the system of one unit-cell FeSe grown on SrTiO3(1uc-FeSe/STO),the collective excitation of electrons is experimentally identified to be polaronic plasmon,which means the electrons in the system are of polaronic nature.The polaron is composed of the electrons in the FeSe layer and the distorted STO lattice.Given the system is non-adiabatic(EF/Ωph<1),we propose a phenomenological interfacial dynamical polaron model,which reveals the substrate phonon can strengthen the electron pairing in luc-FeSe/STO system.Next,we further apply the polaron model to the(Li1-xFexOH)FeSe thin film systems,which share the similar nonadiabatic feature.Exploiting the two-fluids model of polaron and bipolaron,we can well explain the experimentally observed electric transport data,which implies the vital role of polaron in the(Li1-xFexOH)FeSe system.In Chapter 3,the universal feature of the unconventional superconductors is that the carrier densities are relatively low compared to the general good metals,and the superconductivity can be easily tuned by charge doping within a reasonable range.Such systems are usually nonadiabatic that results in the break down of the Eliashberg theory.However,we find another boson-plasmon,with similar energy scale as phonon,could play an important role in mediating electron pairing in the nonadiabatic systems.Given that we have proven the substrate phonon can strengthen the electron pairing in 1uc-FeSe/STO,we propose the multichannel pairing model of "phonon+plasmon".Applying the "phonon+plasmon" model to the 1uc-FeSe/STO system,we can explain the experimentally observed high superconducting transition temperature and the tunable superconductivity by charge doping.Moreover,we further predict the isotope effect of multichannel pairing and the replica band induced by plasmon.The "phonon+plasmon" model can be easily applied to other superconductors with relatively low carrier densities,such as other FeSe-based superconductors,Cu-based superconductors,and alkali-metal doped fullerenes.In Chapter 4,due to the intrinsic impurities or artificial surface charge doping,the trivial two-dimensional electron gas(2DEG)coexists with the topologically protected surface state at the surface of three-dimensional topological insulators.Given their close spatial vicinity,it is natural to expect the hybridization between the 2DEG and the surface states.Taking the Bi2Se3 system as a case study,we propose a theoretical model to describe the hybridization between the 2DEG and the nontrivial surface states.After hybridization,the topology of the latter are unchanged and can further propagates into the former,which exhibits the Rashba-type spin splitting.On the other hand,in the combined system,we also predict a new optical interband plasmon,which is rooted in the band inversion between the 2DEG and the nontrivial surface states.Meanwhile,the widely-studied Dirac plasmon now opens a gap,attributed to the enhanced interband eletron-hole pairs.We also show the tunability of the new interband plamson mode,which can be dismissed or strengthened by hole or electron doping.In Chapter 5,a comprehensive study of the plasmon excitations in the heterostructure of graphene/topological insulator is discussed here.Both graphene and topological insulators have received tremendous attention due to their Dirac nature electrons.Recently,many studies reveal the proximate-induced or-improved functionalities in the heterostructure of graphene/topological insulators.Exploiting the first-principles calculation,we find two characteristic bands crossing the Fermi level of the heterostructure.Then we construct an effective Hamiltonian to capture the low-energy band dispersions based on first-principles results and explore the collective excitations of the heterostructure.Through band-resolved calculations,we identify two plasmon modes originating from the intraband electrons transition within the two characteristic bands,respectively.More importantly,we can artificially select the wanted plasmon mode in the heterostructure through doping carriers with different type and concentrations.In Chapter 6,the brief summary and outlook of this thesis are given.Based on the characteristics of the quantum plasmon,we study its new functionalities in unconventional superconductors and quantum topological materials,which provides new thoughts to resolve the mechanism of high-temperature superconductivity and the application of topological insulators.Moreover,we also propose some personal views on the the development of the superconductivity.。... |
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