arXiv subjects

Benjamin F. Mead

Publications and source records attributed to Benjamin F. Mead.

3 recordsLinked to original sources

arXiv2026

Spontaneous Circular Dichroism and Memory Effect from Surface Defects in Altermagnetic MnTe

Altermagnetism has emerged as a new class of magnetism with vanishing magnetization and macroscopic time reversal symmetry breaking. Hexagonal MnTe is a g-wave altermagnet with a 307 K Néel temperature, large spin splitting, and an anomalous Hall effect. However, the relationship between altermagnetism and the anomalous Hall effect in MnTe remains debated. Here, using optical magnetic circular dichroism, we observe hysteresis and a memory effect in a MnTe bulk crystal below 250 K. No hysteresis is seen after polishing the bulk crystal and in thin films. We conclude that the hysteresis in magnetic circular dichroism in the bulk crystal is from surface defects. The similar temperature dependence and a sign change between circular dichroism and the anomalous Hall effect indicate that the origin of the anomalous Hall effect in transport is likely not intrinsic.

cond-mat.mtrl-sci↗

arXiv2026

Weak localization and universal conductance fluctuations in large area twisted bilayer graphene

We study diffusive magnetotransport in highly p-doped large area twisted bilayer graphene in 1°, 7°, 9° and 20° samples. We report weak localization in twisted bilayer graphene for the first time. All samples exhibit weak localization, from which we extract the phase coherence length and intervalley scattering lengths, and from that determine that dephasing is caused by electron-electron scattering and intervalley scattering is caused by point defects. We observe signatures of universal conductance fluctuations in the 9° sample, which has high mobility and is near the van Hove singularity. Further improvements in sample quality and applications to large area moire materials will open new avenues to observe quantum interference effects.

cond-mat.mes-hall↗

arXiv2025

Terahertz Landau level spectroscopy of Dirac fermions in millimeter-scale twisted bilayer graphene

Exotic electronic physics including correlated insulating states and fractional Chern insulators have been observed in twisted bilayer graphene in a magnetic field when the Fermi velocity vanishes, however a question remains as to the stability of these states which is controlled by the gap to the first excited state. Free-space terahertz magneto-optics can directly probe the gap to charge excitations which bounds the stability of electronic states, but this measurement has thus-far been inaccessible due to the micron size of twisted bilayer graphene samples, while the wavelength of terahertz light is up to a millimeter. Here we leverage advances in fabrication to create twisted bilayer graphene samples over 5 mm x 5 mm in size with a uniform twist angle and study the magnetic field dependence of the cyclotron resonance by a complex Faraday rotation experiment in p-doped large angle twisted bilayer graphene. These measurements directly probe charge excitations in inter-Landau level transitions and determine the Fermi velocity as a function of twist angle.

cond-mat.mes-hall↗