Hybrid AM/FM Mode-Locking of Singly-Resonant OPOs

We investigate a new mode-locking regime in the singly-resonant OPO employing simultaneous amplitude- and frequency-modulation of the intracavity field. This OPO exhibits deterministic, "turn-key" formation of a stable, broadband, chirped frequency comb with high conversion efficiency. Comb-forming dynamics follow a simple phase-space dynamical model, governed by cavity dispersion and modulator chirp, which agrees closely with full numerical simulations. The comb exhibits fast, mode-hop-free tuning over the full gain window of the OPA crystal, controlled by the modulator frequency. Conditions for comb stability, and techniques to enhance comb bandwidth through intentional phase-mismatch and chirping, are investigated.Comment: 20 pages, 15 figures, 4 tables. Appendix: 10 pages, 5 figure

Antiferromagnetism and chiral d-wave superconductivity from an effective t−J−Dt-J-D model for twisted bilayer graphene

Starting from the strong-coupling limit of an extended Hubbard model, we develop a spin-fermion theory to study the insulating phase and pairing symmetry of the superconducting phase in twisted bilayer graphene. Assuming that the insulating phase is an anti-ferromagnetic insulator, we show that fluctuations of the anti-ferromagnetic order in the conducting phase can mediate superconducting pairing. Using a self-consistent mean-field analysis, we find that the pairing wave function has a chiral d-wave symmetry. Consistent with this observation, we show explicitly the existence of chiral Majorana edge modes by diagonalizing our proposed Hamiltonian on a finite-sized system. These results establish twisted bilayer graphene as a promising platform to realize topological superconductivity

Moiré band model and band gaps of graphene on hexagonal boron nitride

Nearly aligned graphene on hexagonal boron nitride (G/BN) can be accurately modeled by a Dirac Hamiltonian perturbed by smoothly varying moir\'e pattern pseudospin fields. Here, we present the moir\'e-band model of G/BN for arbitrary small twist angles under a framework that combines symmetry considerations with input from ab-initio calculations. Our analysis of the band gaps at the primary and secondary Dirac points highlights the role of inversion symmetry breaking contributions of the moir\'e patterns, leading to primary Dirac point gaps when the moir\'e strains give rise to a finite average mass, and to secondary gaps when the moir\'e pseudospin components are mixed appropriately. The pseudomagnetic strain fields which can reach values of up to $\sim$40 Tesla near symmetry points in the moir\'e cell stem almost entirely from virtual hopping and dominate over the contributions arising from bond length distortions due to the moir\'e strains.Comment: 14 pages, 8 figures, 3 table

Moiré band model and band gaps of graphene on hexagonal boron nitride

Nearly aligned graphene on hexagonal boron nitride (G/BN) can be accurately modeled by a Dirac Hamiltonian perturbed by smoothly varying moiré pattern pseudospin fields. Here, we present the moiré-band model of G/BN for arbitrary small twist angles under a framework that combines symmetry considerations with input from ab initio calculations. Our analysis of the band gaps at the primary and secondary Dirac points highlights the role of inversion symmetry breaking contributions of the moiré patterns, leading to primary Dirac point gaps when the moiré strains give rise to a finite average mass, and to secondary gaps when the moiré pseudospin components are mixed appropriately. The pseudomagnetic strain fields, which can reach values of up to <br/

Phonon-Mediated Colossal Magnetoresistance in Graphene/Black Phosphorus Heterostructures

There is a huge demand for magnetoresistance (MR) sensors with high sensitivity, low energy consumption, and room temperature operation. It is well-known that spatial charge inhomogeneity due to impurities or defects introduces mobility fluctuations in monolayer graphene and gives rise to MR in the presence of an externally applied magnetic field. However, to realize a MR sensor based on this effect is hampered by the difficulty in controlling the spatial distribution of impurities and the weak magnetoresistance effect at the monolayer regime. Here, we fabricate a highly stable monolayer graphene-on-black phosphorus (G/BP) heterostructure device that exhibits a giant MR of 775% at 9 T magnetic field and 300 K, exceeding by far the MR effects from devices made from either monolayer graphene or few-layer BP alone. The positive MR of the G/BP device decreases when the temperature is lowered, indicating a phonon-mediated process in addition to scattering by charge impurities. Moreover, a nonlocal MR of >10 000% is achieved for the G/BP device at room temperature due to an enhanced flavor Hall effect induced by the BP channel. Our results show that electron–phonon coupling between 2D material and a suitable substrate can be exploited to create giant MR effects in Dirac semimetals

Giant gate-tunable bandgap renormalization and excitonic effects in a 2D semiconductor

10.1126/sciadv.aaw2347SCIENCE ADVANCES5