arXiv subjects

Tobias Stauber

Publications and source records attributed to Tobias Stauber.

32 records · Page 2Linked to original sources

arXiv2020

Nano-photocurrent mapping of local electronic structure in twisted bilayer graphene

We report a combined nano-photocurrent and infrared nanoscopy study of twisted bilayer graphene (TBG) enabling access to the local electronic phenomena at length scales as short as 20 nm. We show that the photocurrent changes sign at carrier densities tracking the local superlattice density of states of TBG. We use this property to identify domains of varying local twist angle by local photo-thermoelectric effect. Consistent with the photocurrent study, infrared nano-imaging experiments reveal optical conductivity features dominated by twist-angle dependent interband transitions. Our results provide a fast and robust method for mapping the electronic structure of TBG and suggest that similar methods can be broadly applied to probe electronic inhomogeneities of moiré superlattices in other van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2020

Plasmons and screening in finite-bandwidth 2D electron gas

The dynamical and nonlocal dielectric function of a two-dimensional electron gas (2DEG) with finite energy bandwidth is computed within random-phase approximation. For large bandwidth, the plasmon dispersion has two separate branches at small and large momenta. The large momenta branch exhibits negative quasi-flat dispersion. The two branches merge with decreasing bandwidth. We discuss how the maximum energy plasmon mode which resides at energies larger than all particle-hole continuum can potentially open a route to low-loss plasmons. Moreover, we discuss the bandwidth effects on the static screening of the charged and magnetic impurities.

cond-mat.mes-hall↗

arXiv2019

Graphene under bichromatic driving: Commensurability and spatio-temporal symmetries

We study the non-linear current response of a Dirac model that is coupled to two time-periodic electro-magnetic fields with different frequencies. We distinguish between incommensurable and commensurable frequencies, the latter characterized by their ratio p/q with co-prime integers p and q. Coupling the (effective) two-level system to a dissipative bath ensures a well-defined long-time solution for the reduced density operator and, thus, the current. We then analyze the spatio-temporal symmetries that force certain current components to vanish and close with conclusions for directed average currents.

cond-mat.mes-hall↗

arXiv2019

Nonretarded edge plasmon-polaritons in anisotropic two-dimensional materials

By an integral equation approach to the time-harmonic classical Maxwell equations, we describe the dispersion in the nonretarded frequency regime of the edge plasmon-polariton (EPP) on a semi-infinite flat sheet. The sheet has an arbitrary, physically admissible, tensor valued and spatially homogeneous conductivity, and serves as a model for a family of two-dimensional conducting materials. We formulate a system of integral equations for the electric field tangential to the sheet in a homogeneous and isotropic ambient medium. We show how this system is simplified via a length scale separation. This view entails the quasi-electrostatic approximation, by which the tangential electric field is replaced by the gradient of a scalar potential, $φ$. By the Wiener-Hopf method, we solve an integral equation for $φ$ in some generality. The EPP dispersion relation comes from the elimination of a divergent limiting Fourier integral for $φ$ at the edge. We connect the existence, or lack thereof, of the EPP dispersion relation to the index for Wiener-Hopf integral equations, an integer of topological character. We indicate that the values of this index may express an asymmetry due to the material anisotropy in the number of wave modes propagating on the sheet away from the edge with respect to the EPP direction of propagation. We discuss extensions such as the setting of two semi-infinite, coplanar sheets. Our theory forms a generalization of the treatment by Volkov and Mikhailov (1988 Sov. Phys. JETP 67 1639).

math-ph↗

arXiv2019

Switchable and unidirectional plasmonic beacons in hyperbolic 2D materials

In hyperbolic 2D materials, energy is channeled to their deep subwavelength polaritonic modes via four narrow beams. Here we consider the launching of surface polaritons in the hyperbolic 2D materials and demonstrate that efficient uni-directional excitation is possible with an elliptically polarized electric dipole, with the optimal choice of dipole ellipticity depending on the materials optical constants. The selection rules afforded by the choice of dipole polarization allow turning off up to two beams, and even three if the dipole is placed close to an edge. This makes the dipole a directionally switchable beacon for the launching of sub-difractional polaritonic beams, a potential logical gate. We develop an analytical approximation of the excitation process which describes the results of the numerical simulations well and affords a simple physical interpretation.

cond-mat.mes-hall↗

arXiv2018

Electro-Optics of Current-carrying Graphene

Electro-optical response of a current-carrying monolayer graphene is studied theoretically. Our calculation takes into account full (diagonal and non-diagonal) conductivity tensor obtained from a particle-conserving out-of-equilibrium distribution function of doped graphene. Our analytical and numerical results indicate that the presence of a moderate DC current throughout a doped graphene channel induces large Kerr rotations within a frequency range which can be tuned up to the mid-infrared frequency range.

cond-mat.mes-hall↗

arXiv2016

Phonon-mediated mid-infrared photoresponse of graphene

The photoresponse of graphene at mid-infrared frequencies is of high technological interest and is governed by fundamentally different underlying physics than the photoresponse at visible frequencies, as the energy of the photons and substrate phonons involved have comparable energies. Here we perform a spectrally resolved study of the graphene photoresponse for mid-infrared light by measuring spatially resolved photocurrent over a broad frequency range (1000-1600 cm$^{-1}$). We unveil the different mechanisms that give rise to photocurrent generation in graphene on a polar substrate. In particular, we find an enhancement of the photoresponse when the light excites bulk or surface phonons of the SiO$_2$ substrate. This work paves the way for the development of graphene-based mid-infrared thermal sensing technology.

cond-mat.mes-hall↗

arXiv2015

Drift-induced modifications to the dynamical polarization of graphene

The response function of graphene is calculated in the presence of a constant current across the sample. For small drift velocities and finite chemical potential, analytic expressions are obtained and consequences on the plasmonic excitations are discussed. For general drift velocities and zero chemical potential, numerical results are presented and a plasmon gain region is identified which is related to interband transitions.

cond-mat.mes-hall↗

arXiv2015

Orbital magnetic susceptibility of graphene and MoS2

We calculate the orbital magnetic susceptibility $χ_{\mathrm{orb}}$ for an 8-band tight-binding model of gapless and gapped graphene using Green's functions. Analogously, we study $χ_{\mathrm{orb}}$ for a $\mathrm{MoS_2}$ 12-band model. For both materials, we unravel the character of the processes involved in the magnetic response by looking at the contribution at each point of the Brillouin zone. By this, a clear distinction between intra- and interband excitations is generally possible and we are able to predict qualitative features of $χ_{\mathrm{orb}}$ only through the knowledge of the band structure. The study is complemented by comparing the magnetic response with that of 2-band lattice Hamiltonians which reduce to the Dirac and Bernevig-Hughes-Zhang (BHZ) models in the continuum limit.

cond-mat.mes-hall↗

arXiv2013

Plasmons and screening in a monolayer of MoS$_2$

We investigate the dynamical dielectric function of a monolayer of molybdenum disulfide within the random phase approximation. While in graphene damping of plasmons is caused by interband transitions, due to the large direct band gap in monolayer MoS$_2$ collective charge excitations enter the intraband electron hole continuum similar to the situation in two-dimensional electron and hole gases. Since there is no electron-hole symmetry in MoS$_2$, the plasmon energies in p- and n-doped samples clearly differ. The breaking of spin degeneracy caused by the large intrinsic spin-orbit interaction leads to a beating of Friedel oscillations for sufficiently large carrier concentrations, for holes as well as for electrons.

cond-mat.mes-hall↗

arXiv2013

Optical conductivity, Drude weight and plasmons in twisted graphene bilayers

We numerically calculate the optical conductivity of twisted graphene bilayers within the continuum model. To obtain the imaginary part, we employ the regularized Kramers-Kronig relation allowing us to discuss arbitrary twist angles, chemical potential and temperature. We find that the Drude weight $D$ as function of the chemical potential $μ$ closely follows the shell structure of twisted bilayer displayed by the density of states. For certain angles, this results in a transport gap D=0 at finite $μ$. We also discuss the loss function which, for low doping, is characterized by acoustic interband "plasmons" and transitions close to the van Hove singularities. For larger doping, the plasmon mode of decoupled graphene bilayer is recovered that is damped especially for small wave numbers.

cond-mat.mes-hall↗

arXiv2012

Dielectric function, screening, and plasmons of graphene in the presence of spin-orbit interactions

We study the dielectric properties of graphene in the presence of Rashba and intrinsic spin-orbit interactions in their most general form, i.e., for arbitrary frequency, wave vector, doping, and spin-orbit coupling (SOC) parameters. The main result consists in the derivation of closed analytical expressions for the imaginary as well as for the real part of the polarization function. Several limiting cases, e.g., the case of purely Rashba or purely intrinsic SOC, and the case of equally large Rashba and intrinsic coupling parameters are discussed. In the static limit the asymptotic behavior of the screened potential due to charged impurities is derived. In the opposite limit ($q=0$, $ω\to0$), an analytical expression for the plasmon dispersion is obtained and afterwards compared to the numerical result. Our result can also be applied to related systems such as bilayer graphene or topological insulators.

cond-mat.mes-hall↗

arXiv2010

Enhancing visibility of graphene on arbitrary substrates by microdroplet condensation

In order to take advantage of the enormous potential of graphene for future electronic micro-circuits and other applications it is necessary to develop reliable, rapid and widely applicable methods to visualize graphene based structures. We report here on a micro-droplet condensation technique, which allows for quick visual identification of graphene on a variety of substrates, including some which were previously considered unsuitable for the visualization of carbon layers. The technique should also be applicable to visualize artificially patterned graphene structures which are expected to be key technologically enabling components in electronic micro-circuits and other applications.

cond-mat.mes-hall↗

arXiv2009

Electronic properties of graphene and graphene nanoribbons with "pseudo-Rashba" spin-orbit coupling

We discuss the electronic properties of graphene and graphene nanoribbons including "pseudo-Rashba" spin-orbit coupling. After summarizing the bulk properties, we first analyze the scattering behavior close to an infinite mass and zigzag boundary. For low energies, we observe strong deviations from the usual spin-conserving behavior at high energies such as reflection acting as spin polarizer or switch. This results in a spin polarization along the direction of the boundary due to the appearance of evanescent modes in the case of non-equilibrium or when there is no coherence between the two one-particle branches. We then discuss the spin and density distribution of graphene nanoribbons.

cond-mat.mes-hall↗