Angle-resolved Photoemission Spectroscopy (ARPES) Studies On Low-dimensional Topological Quantum Materials

After decades of development,the development of silicon-based semiconductor industry has approached the physical limit,and the characteristic size of integrated cir-cuits has reached the width of dozens of atoms.In order to solve a series of problems such as current leakage,power consumption and heating caused by quantum size ef-fect,we need to find new materials.Among them,Dirac Fermionic materials,including graphene,topological insulators and topological semimetals,have attracted extensive attention in recent years.For Dirac materials,the low energy excited quasiparticles can be described by the relativistic covariant Dirac equation,and there are linearly disper-sive cross bands（Dirac cones）in the band structure.The effective mass of electrons near the cones is zero,and thus information can be transmitted more quickly and with lower power consumption.Just as semiconductors revolutionized technology in the last century,it is reasonable to expect that Dirac materials will be used more widely in the real world in coming decades.In this paper,we used Angle-resolved Photoelectron Spectroscopy（ARPES）to study the electronic structures of various low-dimensional topological quantum materi-als,and obtained the following results:1.Dirac cones in one-dimensional materials were discovered.Large area ordered silene nanoribbons were successfully prepared on Ag（110）substrate by molecular beam epitaxy technique,and the quality of our film was verified by scanning tunneling micro-scope and low energy electron diffraction.Combining angle-resolved photoelectron spectroscopy and first-principles calculations,we found that there existences an one-dimensional Dirac cone in SiNRs with a slightly higher Fermi velocity than graphene,demonstrating that SiNRs have high carrier mobility.By varying the photon energies,we confirmed that the Dirac cones are derived from silene nanoribbons rather than sub-strates,and that there is no band gap at the Dirac point within the resolution range of our instrument,proving that SiNRs is a 1D Dirac semi-metal.First-principles calculations show that there is an 1D zero-gap Dirac cone in the silicon nanoribbons with alternating five-membered ring atomic structure,which is in good agreement with the experimen-tal results.The main contribution to the Dirac cone band is from the_zorbital of Si_satoms.The model analysis shows that the Dirac cone bands are mainly derived from the contribution of armchair Si chains,and the Su-Schrieffer-Heeger model can be used to describe SiNRs structure well,which is the reason why SiNRs bands have topological properties.Our results provide a new platform for studying and regulating the rich phys-ical properties of 1D Dirac materials.For example,under an external magnetic field,time inversion and specular symmetry will be broken,which will lead to topological phase transitions in Si NR:the 1D Dirac cone will split into two pairs of 1d Weyl cones.These interesting properties have not been explored theoretically or experimentally.2.The molecular artificial graphene system was constructed.Large area ordered monolayers of C₆₀were successfully prepared on Cu（111）and Au（111）substrates by molecular beam epitaxy technique.The quality of the monolayer was verified by scan-ning tunneling microscope and low energy electron diffraction.It is found that the C₆₀/（111）system is an artificial graphene system,that is,the Dirac cone exists in every K（K′）in brillouin region.The C₆₀molecule acts as an electron barrier to modulate the free electron gas on the surface of（111）metal,confining the electrons in the free elec-tron gas to a honeycomb lattice structure to obtain an electronic band structure similar to graphene.We confirmed that C₆₀/Cu（111）–4×4 and C₆₀/Au（111）–2√3×2√3R30°are two different types of superlattice system of artificial graphene,This demon-strates the universality of our approach to fabricate artificial graphene using molecular self-assembly.Large area artificial graphene constructed by molecular self-assembly provides a new platform for fabrication of novel molecular quantum devices.3.The topological electronic structure of antiferromagnetic weak topological in-sulator HoSbTe was studied.From the theoretical calculation results,HoSbTe is a Dirac nodal semi-metal without considering SOC,and its electronic structure is similar to that of ZrSiS.When SOC is considered,the location of the Dirac nodal line opens an energy gap and makes the system a weak topological insulator,that is,each layer of HoSbTe is a two-dimensional topological insulator,and the HoSbTe crystal can be regarded as a stack of two-dimensional topological insulators.Experimentally,our ARPES measure-ments were in good agreement with the theoretical calculation,and a>100 me V gap was detected in the directionΓM.An energy gap of several hundred me V is measurable for most experimental techniques.Weak topological insulators are very rare and only a few materials have been experimentally confirmed.The existence of magnetic ordered and weak topological insulating states makes HoSbTe a promising material for basic research and device applications.