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Register a new userOptically induced Lifshitz transition in bilayer graphene
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It is shown theoretically that the renormalization of the electron energy spectrum of bilayer graphene with a strong high-frequency electromagnetic field (dressing field) results in the Lifshitz transition - the abrupt change in the topology of the Fermi surface near the band edge. This effect substantially depends on the polarization of the field: The linearly polarized dressing field induces the Lifshitz transition from the quadruply-connected Fermi surface to the doubly-connected one, whereas the circularly polarized field induces the multicritical point, where the four different Fermi topologies may coexist. As a consequence, the discussed phenomenon creates physical basis to control the electronic properties of bilayer graphene with light.
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Resolving low-energy features in the density of states (DOS) holds the key to understanding wide variety of rich novel phenomena in graphene based 2D heterostructures. Lifshitz transition in bilayer graphene (BLG) arising from trigonal warping has been established theoretically and experimentally. Nevertheless, the experimental realization of its effects on the transport properties has been challenging because of its relatively low energy scale ($sim 1$ meV). In this work, we demonstrate that the thermoelectric power (TEP) can be used as an effective probe to investigate fine changes in the DOS of BLG. We observe additional entropy features in the vicinity of the charge neutrality point (CNP) in gapped BLG. This apparent violation of Mott formula can be explained quantitatively by considering the effects of trigonal warping, thereby serving as a possible evidence of a Lifshitz transition.
We derive the renormalization group equations describing all the short-range interactions in bilayer graphene allowed by symmetry and the long range Coulomb interaction. For certain range of parameters, we predict the first order phase transition to the uniaxially deformed gapless state accompanied by the change of the topology of the electron spectrum.
We investigate the intraband nonlinear dynamics in doped bilayer graphene in the presence of strong, linearly-polarized, in-plane terahertz fields. We perform degenerate pump-probe experiments with 3.4 THz fields on doped bilayer graphene at low temperature (12 K) and find that when the pump is co-polarized with the probe beam, the differential pump-probe signal is almost double that found in the cross-polarized case. We show that the origin of this pump-induced anisotropy is the difference in the average electron effective mass in the probe direction when carriers are displaced in k-space by the pump either parallel or perpendicular to the direction of the probe polarization. We model the system using both a simple semiclassical model and a Boltzmann equation simulation of the electron dynamics with phenomenological scattering and find good qualitative agreement with experimental results.
In the phenomenon of electromagnetically induced transparency1 (EIT) of a three-level atomic system, the linear susceptibility at the dipole-allowed transition is canceled through destructive interference of the direct transition and an indirect transition pathway involving a meta-stable level, enabled by optical pumping. EIT not only leads to light transmission at otherwise opaque atomic transition frequencies, but also results in the slowing of light group velocity and enhanced optical nonlinearity. In this letter, we report an analogous behavior, denoted as phonon-induced transparency (PIT), in AB-stacked bilayer graphene nanoribbons. Here, light absorption due to the plasmon excitation is suppressed in a narrow window due to the coupling with the infrared active {Gamma}-point optical phonon, whose function here is similar to that of the meta-stable level in EIT of atomic systems. We further show that PIT in bilayer graphene is actively tunable by electrostatic gating, and estimate a maximum slow light factor of around 500 at the phonon frequency of 1580 cm-1, based on the measured spectra. Our demonstration opens an avenue for the exploration of few-photon non-linear optics and slow light in this novel two-dimensional material, without external optical pumping and at room temperature.
We theoretically calculate the impurity-scattering induced resistivity of twisted bilayer graphene at low twist angles where the graphene Fermi velocity is strongly suppressed. We consider, as a function of carrier density, twist angle, and temperature, both long-ranged Coulomb scattering and short-ranged defect scattering within a Boltzmann theory relaxation time approach. For experimentally relevant disorder, impurity scattering contributes a resistivity comparable to (much larger than) the phonon scattering contribution at high (low) temperatures. Decreasing twist angle leads to larger resistivity, and in general, the resistivity increases (decreases) with increasing temperature (carrier density). Inclusion of the van Hove singularity in the theory leads to a strong increase in the resistivity at higher densities, where the chemical potential is close to a van Hove singularity, leading to an apparent density-dependent plateau type structure in the resistivity, which has been observed in recent transport experiments. We also show that the Matthissens rule is strongly violated in twisted bilayer graphene at low twist angles.
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