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Register a new userLayer Correlated Phase in Double Layer Graphene at $Delta u$ = 0
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In the recent advancement in Graphene heterostructures, it is possible to create a double layer tunnel decoupled Graphene system which has strong interlayer electronic interaction. In this work, we restrict the parameters in the Hamiltonian using simple symmetry arguments. We study the ground state of this system in the Hartree-Fock approximation at $ u_1= u_2=0$. In addition to the phases found in monolayer Graphene, we found the existence of layer correlated phase which breaks the layer $U(1)$ symmetry. At non-zero Zeeman coupling strength ($E_z$) this layer correlated state has a small magnetization, which vanishes as $E_z$ goes to zero. We discuss the bulk gapless modes using the Goldstone theorem. We also comment on the edge structure for the layer correlated phase.
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We induce surface carrier densities up to $sim7cdot 10^{14}$cm$^{-2}$ in few-layer graphene devices by electric double layer gating with a polymeric electrolyte. In 3-, 4- and 5-layer graphene below 20-30K we observe a logarithmic upturn of resistance that we attribute to weak localization in the diffusive regime. By studying this effect as a function of carrier density and with ab-initio calculations we derive the dependence of transport, intervalley and phase coherence scattering lifetimes on total carrier density. We find that electron-electron scattering in the Nyquist regime is the main source of dephasing at temperatures lower than 30K in the $sim10^{13}$cm$^{-2}$ to $sim7 cdot 10^{14}$cm$^{-2}$ range of carrier densities. With the increase of gate voltage, transport elastic scattering is dominated by the competing effects due to the increase in both carrier density and charged scattering centers at the surface. We also tune our devices into a crossover regime between weak and strong localization, indicating that simultaneous tunability of both carrier and defect density at the surface of electric double layer gated materials is possible.
We analyze the effect of screening provided by the additional graphene layer in double layer graphene heterostructures (DLGs) on transport characteristics of DLG devices in the metallic regime. The effect of gate-tunable charge density in the additional layer is two-fold: it provides screening of the long-range potential of charged defects in the system, and screens out Coulomb interactions between charge carriers. We find that the efficiency of defect charge screening is strongly dependent on the concentration and location of defects within the DLG. In particular, only a moderate suppression of electron-hole puddles around the Dirac point induced by the high concentration of remote impurities in the silicon oxide substrate could be achieved. A stronger effect is found on the elastic relaxation rate due to charged defects resulting in mobility strongly dependent on the electron denisty in the additional layer of DLG. We find that the quantum interference correction to the resistivity of graphene is also strongly affected by screening in DLG. In particular, the dephasing rate is strongly suppressed by the additional screening that supresses the amplitude of electron-electron interaction and reduces the diffusion time that electrons spend in proximity of each other. The latter effect combined with screening of elastic relaxation rates results in a peculiar gate tunable weak-localization magnetoresistance and quantum correction to resistivity. We propose suitable experiments to test our theory and discuss the possible relevance of our results to exisiting data.
It is by now well established that high-quality graphene enables a gate-tunable low-loss plasmonic platform for the efficient confinement, enhancement, and manipulation of optical fields spanning a broad range of frequencies, from the mid infrared to the Terahertz domain. While all-electrical detection of graphene plasmons has been demonstrated, electrical plasmon injection (EPI), which is crucial to operate nanoplasmonic devices without the encumbrance of a far-field optical apparatus, remains elusive. In this work, we present a theory of EPI in double-layer graphene, where a vertical tunnel current excites acoustic and optical plasmon modes. We first calculate the power delivered by the applied inter-layer voltage bias into these collective modes. We then show that this system works also as a spectrally-resolved molecular sensor.
We study the dynamic effects in the double graphene-layer (GL) structures with the resonant-tunneling (RT) and the negative differential inter-GL conductivity. Using the developed model, which accounts for the excitation of self-consistent oscillations of the electron and hole densities and the ac electric field between GLs (plasma oscillations), we calculate the admittance of the double-GL RT structures as a function of the signal frequency and applied voltages, and the spectrum and increment/decrement of plasma oscillations. Our results show that the electron-hole plasma in the double-GL RT structures with realistic parameters is stable with respect to the self-excitation of plasma oscillations and aperiodic perturbations. The stability of the electron-hole plasma at the bias voltages corresponding to the inter-GL RT and strong nonlinearity of the RT current-voltage characteristics enable using the double-GL RT structures for detection of teraherz (THz) radiation. The excitation of plasma oscillations by the incoming THz radiation can result in a sharp resonant dependence of detector responsivity on radiation frequency and the bias voltage. Due to a strong nonlinearity of the current-voltage characteristics of the double-GL structures at RT and the resonant excitation of plasma oscillations, the maximum responsivity, $R_V^{max}$, can markedly exceed the values $(10^4 - 10^5)$~V/W at room temperature.
Two-dimensional systems that host one-dimensional helical states are exciting from the perspective of scalable topological quantum computation when coupled with a superconductor. Graphene is particularly promising for its high electronic quality, versatility in van der Waals heterostructures and its electron and hole-like degenerate 0$th$ Landau level. Here, we study a compact double layer graphene SQUID (superconducting quantum interference device), where the superconducting loop is reduced to the superconducting contacts, connecting two parallel graphene Josephson junctions. Despite the small size of the SQUID, it is fully tunable by independent gate control of the Fermi energies in both layers. Furthermore, both Josephson junctions show a skewed current phase relationship, indicating the presence of superconducting modes with high transparency. In the quantum Hall regime we measure a well defined conductance plateau of 2$e^2/h$ an indicative of counter propagating edge channels in the two layers. Our work opens a way for engineering topological superconductivity by coupling helical edge states, from graphenes electron-hole degenerate 0$th$ Landau level via superconducting contacts.
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