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Field-induced insulating states in a graphene superlattice
摘要： We report on high-field magnetotransport (B up to 35 T) on a gated superlattice based on single-layer graphene aligned on top of hexagonal boron nitride. The large-period moiré modulation (≈15 nm) enables us to access the Hofstadter spectrum in the vicinity of and above one flux quantum per superlattice unit cell (Φ/Φ0 = 1 at B = 22 T). We thereby reveal, in addition to the spin-valley antiferromagnet at ν = 0, two insulating states developing in positive and negative effective magnetic fields from the main ν = 1 and ν = ?2 quantum Hall states, respectively. We investigate the field dependence of the energy gaps associated with these insulating states, which we quantify from the temperature-activated peak resistance. Referring to a simple model of local Landau quantization of third-generation Dirac fermions arising at Φ/Φ0 = 1, we describe the different microscopic origins of the insulating states and experimentally determine the energy-momentum dispersion of the emergent gapped Dirac quasiparticles.
关键词： Hofstadter butterfly,graphene superlattice,Dirac fermions,magnetotransport,insulating states
更新于2025-09-19 17:15:36
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Optical conductivity and transparency in an effective model for graphene
摘要： Motivated by experiments confirming that the optical transparency of graphene is defined through the fine structure constant and that it could be fully explained within the relativistic Dirac fermions in a two-dimensional picture, we investigate in this article how this property is affected by next-to-nearest-neighbor coupling in the low-energy continuum description of graphene. A detailed calculation within the linear response regime allows us to conclude that, somewhat surprisingly, the optical conductivity at zero temperature that determines the transparency remains robust up to this correction.
关键词： graphene,optical conductivity,linear response,transparency,Dirac fermions
更新于2025-09-10 09:29:36
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[IEEE 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz2018) - Nagoya, Japan (2018.9.9-2018.9.14)] 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Spectroscopy of Temperature-Driven Single Valley Dirac Fermions in HgTe/CdHgTe Quantum Wells
摘要： We report on the temperature-dependent magneto-absorption and magnetotransport spectroscopy of HgTe/CdHgTe quantum wells above the critical well thickness dc. Our results, obtained in magnetic fields up to 16 T and temperature range from 1.7 to 150 K, clearly indicate a change in the band-gap energy with temperature. A topological phase transition between quantum spin Hall and trivial insulator states, revealing appearance of single-valley Dirac fermions at T = 27 and 90 K for 6.5 and 8 nm QWs respectively, was clearly observed in our magnetospectroscopy measurements.
关键词： magneto-absorption,topological phase transition,Dirac fermions,HgTe/CdHgTe quantum wells,magnetotransport spectroscopy
更新于2025-09-10 09:29:36
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Optical signatures of Dirac nodal lines in NbAs <sub/>2</sub>
摘要： Using polarized optical and magneto-optical spectroscopy, we have demonstrated universal aspects of electrodynamics associated with Dirac nodal lines that are found in several classes of unconventional intermetallic compounds. We investigated anisotropic electrodynamics of NbAs2 where the spin-orbit coupling (SOC) triggers energy gaps along the nodal lines. These gaps manifest as sharp steps in the optical conductivity spectra σ1(ω). This behavior is followed by the linear power-law scaling of σ1(ω) at higher frequencies, consistent with our theoretical analysis for dispersive Dirac nodal lines. Magneto-optics data affirm the dominant role of nodal lines in the electrodynamics of NbAs2.
关键词： Dirac fermions,nodal-line semimetal,optical conductivity,magneto-optics
更新于2025-09-09 09:28:46