Wide Electrical Tunability of the Valley Splitting in a Doubly Gated Silicon-on-Insulator Quantum Well.

The valley splitting of 2D electrons in doubly gated silicon-on-insulator quantum wells is studied by low temperature transport measurements under magnetic fields. At the buried thermal-oxide SiO interface, the valley splitting increases as a function of the electrostatic bias = - (where and are electron densities contributed by back and front gates, respectively) and reaches values as high as 6.3 meV, independent of the total carrier concentration of the channel.

Magnetic Phases and Zone-Folded Phonons in a Frustrated van der Waals Magnet.

2D magnetic materials have attracted extensive research interest due to their potential application in nanospintronics, optospintronics, and magnonics. Ferromagnetic- and antiferromagnetic-layered materials have been demonstrated and successfully inserted into van der Waals heterostructures. However, the effects of magnetic frustration in van der Waals materials and the possibilities offered by spin configurations characterized by nonlinear spin arrangements have not been fully considered yet.

Probing Berry Curvature in Magnetic Topological Insulators through Resonant Infrared Magnetic Circular Dichroism.

Probing the quantum geometry and topology in condensed matter systems has relied heavily on static electronic transport experiments in magnetic fields. Yet, contact-free optical measurements have rarely been explored. Here, we report the observation of resonant magnetic circular dichroism (MCD) in the infrared range in thin film MnBi_{2}Te_{4} exhibiting a spectral intensity that correlates with the anomalous Hall effect.

Structural and electronic features enabling delocalized charge-carriers in CuSbSe.

Inorganic semiconductors based on heavy pnictogen cations (Sb and Bi) have gained significant attention as potential nontoxic and stable alternatives to lead-halide perovskites for solar cell applications. A limitation of these novel materials, which is being increasingly commonly found, is carrier localization, which substantially reduces mobilities and diffusion lengths. Herein, CuSbSe is investigated and discovered to have delocalized free carriers, as shown through optical pump terahertz probe spectroscopy and temperature-dependent mobility measurements.

Magneto-Optical Sensing of the Pressure Driven Magnetic Ground States in Bulk CrSBr.

Competition between exchange interactions and magnetocrystalline anisotropy may bring new magnetic states that are of great current interest. An applied hydrostatic pressure can further be used to tune their balance. In this work, we investigate the magnetization process of a biaxial antiferromagnet in an external magnetic field applied along the easy axis.

Mixing of moiré-surface and bulk states in graphite.

Van der Waals assembly enables the design of electronic states in two-dimensional (2D) materials, often by superimposing a long-wavelength periodic potential on a crystal lattice using moiré superlattices. This twistronics approach has resulted in numerous previously undescribed physics, including strong correlations and superconductivity in twisted bilayer graphene, resonant excitons, charge ordering and Wigner crystallization in transition-metal chalcogenide moiré structures and Hofstadter's butterfly spectra and Brown-Zak quantum oscillations in graphene superlattices. Moreover, twistronics has been used to modify near-surface states at the interface between van der Waals crystals.

Lorentz-Boost-Driven Magneto-Optics in a Dirac Nodal-Line Semimetal.

Optical response of crystalline solids is to a large extent driven by excitations that promote electrons among individual bands. This allows one to apply optical and magneto-optical methods to determine experimentally the energy band gap -a fundamental property crucial to our understanding of any solid-with a great precision. Here it is shown that such conventional methods, applied with great success to many materials in the past, do not work in topological Dirac semimetals with a dispersive nodal line.

Observation of Reentrant Correlated Insulators and Interaction-Driven Fermi-Surface Reconstructions at One Magnetic Flux Quantum per Moiré Unit Cell in Magic-Angle Twisted Bilayer Graphene.

The discovery of flat bands with nontrivial band topology in magic-angle twisted bilayer graphene (MATBG) has provided a unique platform to study strongly correlated phenomena including superconductivity, correlated insulators, Chern insulators, and magnetism. A fundamental feature of the MATBG, so far unexplored, is its high magnetic field Hofstadter spectrum. Here, we report on a detailed magnetotransport study of a MATBG device in external magnetic fields of up to B=31  T, corresponding to one magnetic flux quantum per moiré unit cell Φ_{0}.

Multiple flat bands and topological Hofstadter butterfly in twisted bilayer graphene close to the second magic angle.

Moiré superlattices in two-dimensional van der Waals heterostructures provide an efficient way to engineer electron band properties. The recent discovery of exotic quantum phases and their interplay in twisted bilayer graphene (tBLG) has made this moiré system one of the most renowned condensed matter platforms. So far studies of tBLG have been mostly focused on the lowest two flat moiré bands at the first magic angle θ ∼ 1.

Electronic phase separation in multilayer rhombohedral graphite.

Of the two stable forms of graphite, hexagonal and rhombohedral, the former is more common and has been studied extensively. The latter is less stable, which has so far precluded its detailed investigation, despite many theoretical predictions about the abundance of exotic interaction-induced physics. Advances in van der Waals heterostructure technology have now allowed us to make high-quality rhombohedral graphite films up to 50 graphene layers thick and study their transport properties.

Helical quantum Hall phase in graphene on SrTiO.

The ground state of charge-neutral graphene under perpendicular magnetic field was predicted to be a quantum Hall topological insulator with a ferromagnetic order and spin-filtered, helical edge channels. In most experiments, however, an insulating state is observed that is accounted for by lattice-scale interactions that promote a broken-symmetry state with gapped bulk and edge excitations. We tuned the ground state of the graphene zeroth Landau level to the topological phase through a suitable screening of the Coulomb interaction with the high dielectric constant of a strontium titanate (SrTiO) substrate.

Determining Interaction Enhanced Valley Susceptibility in Spin-Valley-Locked MoS.

Two-dimensional transition metal dichalcogenides (TMDCs) are recently emerged electronic systems with various novel properties, such as spin-valley locking, circular dichroism, valley Hall effect, and superconductivity. The reduced dimensionality and large effective masses further produce unconventional many-body interaction effects. Here we reveal strong interaction effects in the conduction band of MoS by transport experiment.

Electrical switch to the resonant magneto-phonon effect in graphene.

We report a comprehensive study of the tuning with electric fields of the resonant magneto-exciton optical phonon coupling in gated graphene. For magnetic fields around B ∼ 25 T that correspond to the range of the fundamental magneto-phonon resonance, the electron-phonon coupling can be switched on and off by tuning the position of the Fermi level in order to Pauli block the two fundamental inter-Landau level excitations. The effects of such a profound change in the electronic excitation spectrum are traced through investigations of the optical phonon response in polarization resolved magneto-Raman scattering experiments.