"The experimental discovery of fractional Chern insulators (FCIs) in rhombohedral pentalayer graphene twisted on hexagonal boron nitride (hBN) has preceded theoretical prediction. Supported by large-scale first -principles relaxation calculations at the experimental twist angle of 0.77 degrees, we obtain an accurate continuum model of n = 3, 4, 5, 6, 7 layer rhombohedral graphene-hBN moiré systems. Focusing on the pentalayer case, we analytically explain the robust |C| = 0, 5 Chern numbers seen in the low -energy single -particle bands and their flattening with displacement field, making use of a minimal two -flavor continuum Hamiltonian derived from the full model. We then predict nonzero valley Chern numbers at the nu = -4, 0 insulators observed in experiment. Our analysis makes clear the importance of displacement field and the moiré potential in producing localized ""heavy fermion"" charge density in the top valence band, in addition to the nearly free conduction band. Lastly, we study doubly aligned devices as additional platforms for moiré FCIs with higher Chern number bands."

We propose minimal transport experiments in the coherent regime that can probe the chirality of twisted moire structures. We show that only with a third contact and in the presence of an in-plane magnetic field (or another time-reversal symmetry breaking effect) a chiral system may display nonreciprocal transport in the linear regime. We then propose to use the third lead as a voltage probe and show that opposite enantiomers give rise to different voltage drops on the third lead. Additionally, in the scenario of layer-discriminating contacts, the third lead can serve as a current probe capable of detecting different handedness even in the absence of a magnetic field. In a complementary configuration, applying opposite voltages on the two layers of the third lead gives rise to a chiral (super)current in the absence of a source-drain voltage whose direction is determined by its chirality.

Understanding electron-phonon interactions is fundamentally important and has crucial implications for device applications. However, in twisted bilayer graphene near the magic angle, this understanding is currently lacking. Here, we study electron-phonon coupling using time- and frequency-resolved photovoltage measurements as direct and complementary probes of phonon-mediated hot-electron cooling. We find a remarkable speedup in cooling of twisted bilayer graphene near the magic angle: The cooling time is a few picoseconds from room temperature down to 5 kelvin, whereas in pristine bilayer graphene, cooling to phonons becomes much slower for lower temperatures. Our experimental and theoretical analysis indicates that this ultrafast cooling is a combined effect of superlattice formation with low-energy moiré,. phonons, spatially compressed electronic Wannier orbitals, and a reduced superlattice Brillouin zone. This enables efficient electron-phonon Umklapp scattering that overcomes electron-phonon momentum mismatch. These results establish twist angle as an effective way to control energy relaxation and electronic heat flow.

The discovery of magic angle twisted bilayer graphene (MATBG), in which two sheets of monolayer graphene are precisely stacked at a specific angle, has opened up a plethora of grand new opportunities in the field of topology, superconductivity, strange metal, and other strongly correlated effects. This review will focus on the various forms of quantum phases in MATBG revealed through quantum transport measurements. The goal is to highlight the uniqueness and current understanding of the various phases, especially how electronic interaction plays a role in them, as well as open questions in regard to the phase diagram.

Detecting light at the single-photon level is one of the pillars of emergent photonic technologies. This is realized through state-of-the-art superconducting detectors that offer efficient, broadband and fast response. However, the use of low TC superconducting thin films limits their operation temperature to approximately 4 K and below. Here, we demonstrate proof-of-concept nanodetectors based on exfoliated, two-dimensional cuprate superconductor Bi2Sr2CaCu2O8-delta that exhibit single-photon sensitivity at telecom wavelength at a record temperature of T = 20 K. These non-optimized devices exhibit a slow (similar to ms) reset time and a low detection efficiency (-10(-4)). We realize the elusive prospect of single-photon sensitivity on a high-TC nanodetector thanks to a novel approach, combining van der Waals fabrication techniques and a non-invasive nanopatterning based on light ion irradiation. This result paves the way for broader application of single-photon technologies, relaxing the cryogenic constraints for single-photon detection at telecom wavelength.

Detecting light at the single-photon level is one of the pillars of emergent photonic technologies. This is realized through state-of-the-art superconducting detectors that offer efficient, broadband and fast response. However, the use of low TC superconducting thin films limits their operation temperature to approximately 4 K and below. Here, we demonstrate proof-of-concept nanodetectors based on exfoliated, two-dimensional cuprate superconductor Bi(2)Sr(2)CaCu(2)O(8-delta )that exhibit single-photon sensitivity at telecom wavelength at a record temperature of T = 20 K. These non-optimized devices exhibit a slow (similar to ms) reset time and a low detection efficiency (-10(-4)). We realize the elusive prospect of single-photon sensitivity on a high-TC nanodetector thanks to a novel approach, combining van der Waals fabrication techniques and a non-invasive nanopatterning based on light ion irradiation. This result paves the way for broader application of single-photon technologies, relaxing the cryogenic constraints for single-photon detection at telecom wavelength.

We report on observation of the infrared photoresistance of twisted bilayer graphene (tBLG) under continuous quantum cascade laser illumination at a frequency of 57.1 THz. The photoresistance shows an intricate sign-alternating behavior under variations of temperature and back gate voltage, and exhibits giant resonance-like enhancements at certain gate voltages. The structure of the photoresponse correlates with weaker features in the dark dc resistance reflecting the complex band structure of tBLG. It is shown that the observed photoresistance is well captured by a bolometric model describing the electron and hole gas heating, which implies an ultrafast thermalization of the photoexcited electron-hole pairs in the whole range of studied temperatures and back gate voltages. We establish that photoresistance can serve a highly sensitive probe of the temperature variations of electronic transport in tBLG.

Magic-angle twisted trilayer graphene (MATTG) hosts flat electronic bands, and exhibits correlated quantum phases with electrical tunability. In this work, we demonstrate a spectroscopy technique that allows for dissociation of intertwined bands and quantification of the energy gaps and Chern numbers C of the correlated states in MATTG by driving band crossings between Dirac cone Landau levels and energy gaps in the flat bands. We uncover hard correlated gaps with C = 0 at integer moiré unit cell fillings of v = 2 and 3 and reveal charge density wave states originating from van Hove singularities at fractional fillings v = 5/3 and 11/3. In addition, we demonstrate displacement-field-driven first-order phase transitions at charge neutrality and v = 2, which are consistent with a theoretical strong-coupling analysis, implying C(2)Tsymmetry breaking. Overall, these properties establish a diverse electrically tunable phase diagram of MATTG and provide an avenue for investigating other related systems hosting both steep and flat bands.