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Gallium Oxide || Electron paramagnetic resonance (EPR) from β-Ga2O3 crystals
摘要： Electron paramagnetic resonance (EPR) is often used to identify and characterize defects and impurities having unpaired spins in semiconductors and insulators [1–4]. Examples include donors, acceptors, vacancies, interstitials, antisites, small polarons, transition-metal ions, and rare-earth ions. Simply stated, an EPR spectrum represents the absorption of microwaves at discrete values of magnetic field. This allows the energy-level scheme of the unpaired spin system to be determined. The importance of EPR arises from its high sensitivity and high resolution. Depending on the line widths, concentrations of defects as low as a few parts per billion can be easily observed. Each paramagnetic defect has a unique EPR spectrum that reflects its g matrix and hyperfine matrices, as well as the zero-field splittings when S is greater than 1/2. Once the identity of the responsible defect has been established, a spectrum can be used to compare concentrations of the defect in different samples and also to monitor changes in the concentration of the defect during photoexcitations and thermal anneals.
关键词： Electron paramagnetic resonance,β-Ga2O3,defects,semiconductors,insulators,impurities
更新于2025-09-09 09:28:46
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Topological states on uneven (Pb,Sn)Se (001) surfaces
摘要： The impact of surface morphology on the electronic structure of topological crystalline insulators is studied theoretically. As an example, the structure of topologically protected electronic states on a (001) (Pb,Sn)Se surface with terraces of atomic height is modeled. Within the envelope function model it is shown that valley mixing, the phenomenon responsible for the peculiar “double Dirac cone” shape of the surface state dispersion, depends crucially on the structure of the surface. By varying the width and the number of atomic layers in the terraces, a comprehensive explanation of recent experimental ?ndings, i.e., the emergence of one-dimensional states bound to odd-height atomic step edges [P. Sessi et al., Science 354, 1269 (2016)] as well as the collapse of “double Dirac cone” structure on a rough surface [C. Polley et al., ACS Nano 12, 617 (2018)], is achieved. This approach allows us also to determine topological indices characterizing terraces and their interfaces. In the (001) surface of (Pb,Sn)Se the adjacent terraces turn out to be described by different values of the winding number topological invariant.
关键词： surface states,double Dirac cone,valley mixing,topological crystalline insulators,winding number topological invariant
更新于2025-09-09 09:28:46
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Surfaces of axion insulators
摘要： Axion insulators are magnetic topological insulators in which the nontrivial Z2 index is protected by inversion symmetry instead of time-reversal symmetry. The naturally gapped surfaces of axion insulators give rise to a half-quantized surface anomalous Hall conductivity (AHC), but the sign of the surface AHC cannot be determined from topological arguments. In this paper, we consider topological phenomena at the surface of an axion insulator. To be explicit, we construct a minimal tight-binding model on the pyrochlore lattice and investigate the all-in-all-out (AIAO) and ferromagnetic (FM) spin configurations. We also implement a recently proposed approach for calculating the surface AHC directly, which allows us to explore how the interplay between surface termination and magnetic ordering determines the sign of the half-quantized surface AHC. In the case of AIAO ordering, we show that it is possible to construct a topological state with no protected metallic states on boundaries of any dimension (surfaces, hinges, or corners), although chiral hinge modes do occur for many surface configurations. In the FM case, rotation of the magnetization by an external field offers promising means of control of chiral hinge modes, which can also appear on surface steps or where bulk domain walls emerge at the surface.
关键词： pyrochlore lattice,all-in-all-out,topological insulators,axion insulators,ferromagnetic,anomalous Hall conductivity
更新于2025-09-04 15:30:14
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Dynamic nuclear polarization from topological insulator helical edge states
摘要： Topological insulators are promising for spintronics and related technologies due to their spin-momentum-locked edge states, which are protected by time-reversal symmetry. In addition to the unique fundamental physics that arises in these systems, the potential technological applications of these protected states have also been driving TI research over the past decade. However, most known topological insulator materials naturally contain spinful nuclei, and their hyperfine coupling to helical edge states intrinsically breaks time-reversal symmetry, removing the topological protection and enabling the buildup of dynamic nuclear spin polarization through hyperfine-assisted backscattering. Here, we calculate the scattering probabilities and nuclear polarization for edge channels containing up to 34 nuclear spins using a numerically exact analysis that exploits the symmetries of the problem to drastically reduce the computational complexity. We then show the emergence of universal scaling properties that allow us to extrapolate our findings to vastly larger and experimentally relevant system sizes. We find that significant nuclear polarization can result from relatively weak helical edge currents, suggesting that it may be an important factor affecting spin transport in topological insulator devices.
关键词： Hyperfine coupling,Dynamic nuclear polarization,Helical edge states,Topological insulators,Spintronics
更新于2025-09-04 15:30:14
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Effects of spin-orbit coupling on the optical response of a material
摘要： We investigate the effects of spin-orbit coupling on the optical response of materials. In particular, we study the effects of the commutator between the spin-orbit coupling part of the potential and the position operator on the optical matrix elements using density functional theory calculations within the generalized gradient approximation. By means of a formalism that separates a fully relativistic Kleinman-Bylander pseudopotential into the scalar-relativistic and spin-orbit-coupling parts, we calculate the contribution of the commutator arising from spin-orbit coupling to the squared optical matrix elements of isolated atoms, monolayer transition-metal dichalcogenides, and topological insulators. In the case of isolated atoms from H (Z = 1) to Bi (Z = 83), the contribution of spin-orbit coupling to the squared matrix elements can be as large as 14%. On the other hand, in the cases of monolayer transition-metal dichalcogenides and topological insulators, we find that this contribution is less than 1% and that it is sufficient to calculate the optical matrix elements and subsequent physical quantities without considering the commutator arising from spin-orbit coupling.
关键词： optical response,spin-orbit coupling,transition-metal dichalcogenides,topological insulators,density functional theory
更新于2025-09-04 15:30:14
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Creating, probing, and manipulating fractionally charged excitations of fractional Chern insulators in optical lattices
摘要： We propose a set of schemes to create and probe fractionally charged excitations of a fractional Chern insulator state in an optical lattice. This includes the creation of localized quasiparticles and quasiholes using both static local defects and the dynamical local insertion of synthetic flux quanta. Simulations of repulsively interacting bosons on a finite square lattice with experimentally relevant open boundary conditions show that already a four-particle system exhibits signatures of charge fractionalization in the quantum-Hall-like state at the filling fraction of 1/2 particle per flux quantum. This result is favorable for the prospects of adiabatic preparation of fractional Chern insulators. Our work is inspired by recent experimental breakthroughs in atomic quantum gases: the realization of strong artificial magnetic fields in optical lattices, the ability of single-site addressing in quantum gas microscopes, and the preparation of low-entropy insulating states by engineering an entropy-absorbing metallic reservoir.
关键词： quasiholes,quantum Hall effect,quasiparticles,optical lattices,fractional Chern insulators
更新于2025-09-04 15:30:14