In this work, we determine the effects of temperature on the magnetoplasmon spectrum of an electrically modulated graphene monolayer as well as the two-dimensional electron gas (2DEG). The intra-Landau-band magnetoplasmon spectrum within the Self Consistent Field (SCF) approach is investigated for both the aforementioned systems. Results obtained not only exhibit Shubnikov-de Hass (SdH) oscillations but also commensurability oscillations (Weiss oscillations). These oscillations are periodic as a function of inverse magnetic field. We find that both the magnetic oscillations, SdH and Weiss, have a greater amplitude and are more robust against temperature in graphene compared to conventional 2DEG. Furthermore, there is a $π$ phase shift between the magnetoplasmon oscillations in the two systems which can be attributed to Dirac electrons in graphene acquiring a Berry's phase as they traverse a closed path in a magnetic field.
The magnetoplasmon spectrum of a magnetically modulated two-dimensional electron gas (MM2DEG) is investigated. We derive the inter and intra Landau band magnetoplasmon spectrum within the self consistent field approach. The derivation is performed at zero temperature as well as at finite temperature. Results are presented for the inter and intra Landau band magnetoplasmon spectrum as a function of the inverse magnetic field. As a result of magnetic modulation, magnetic Weiss oscillations are found to occur in the magnetoplasmon spectrum. Furthermore, our finite temperature theory facilitates a…
We present a theoretical study of magnetoplasmon spectrum of a two-dimensional electron gas in the presence of Rashba spin-orbit interaction (RSOI), one-dimensional weak electric modulation and a perpendicular magnetic field. The intra-Landau-band magnetoplasmon spectrum is determined in the presence of spin-orbit interaction within the self consistent field approach at finite temperature. Due to Rashba effect, the spin of finite-momentum electrons feels a magnetic field perpendicular to the electron momentum in the inversion plane. The magnetoplasmon spectrum of the modulated two-dimensional …
We theoretically study electrically tunable magnetoplasmons in a monolayer of silicene or germanene. We derive the dynamical response function and take into account the effects of strong spin-orbit coupling (SOC) and of an external electric filed $E_z$ perpendicular to the plane of the buckled silicene/germanene. Employing the random-phase approximation we analyze the magnetoplasmon spectrum. The dispersion relation has the same form as in a two-dimensional electron gas with the cyclotron and plasma frequencies modified due to the SOC and the field $E_z$. In the absence of SOC and $E_z$, our r…
Motivated by the recent experiment of Hochgraefe et al., we have investigated the magnetoplasmon excitations in a periodic array of quantum wires with a periodic modulation along the wire direction. The equilibrium and dynamic properties of the system are treated self-consistently within the Thomas-Fermi-Dirac-von Weizsaecker approximation. A calculation of the dynamical response of the system to a far-infrared radiation field reveals a resonant anticrossing between the Kohn mode and a finite-wavevector longitudinal excitation which is induced by the density modulation along the wires. Our the…
We determine the collective excitations spectrum and discuss the numerical results for a parabolically confined density modulated quasi-one dimensional electron gas (1DEG) in the presence of an external magnetic field. We derive the inter-and intra-band magnetoplasmon spectrum within the Self Consistent Field (SCF) approach. In this work we focus on magnetoplasmon oscillations in this system and as such results are presented for the intra-Landau-band magnetoplasmon spectrum that exhibits oscillatory behavior, these oscillations are not with constant period in 1/B and are significantly effected…
We show that in graphene epitaxially grown on SiC the Drude absorption is transformed into a strong terahertz plasmonic peak due to natural nanoscale inhomogeneities, such as substrate terraces and wrinkles. The excitation of the plasmon modifies dramatically the magneto-optical response and in particular the Faraday rotation. This makes graphene a unique playground for plasmon-controlled magneto-optical phenomena thanks to a cyclotron mass 2 orders of magnitude smaller than in conventional plasmonic materials such as noble metals.
We present the collective excitations spectrum (magnetoplasmon spectrum) of Dirac electrons in a weakly modulated single graphene layer in the presence of a uniform magnetic field. We consider electric modulation in one-dimension and the magnetic field applied perpendicular to graphene.We derive analytical results for the intra-Landau band plasmon spectrum within the self-consistent-field approach. We find Weiss oscillations in the magnetoplasmon spectrum which is the primary focus of this work. Results are presented for the intra-Landau band magnetoplasmon spectrum as a function of inverse ma…
Two-dimensional materials supporting deep-subwavelength plasmonic modes can also exhibit strong magneto-optical responses. Here, we theoretically investigate magnetoplasmons (MPs) in monolayer black phosphorus (BP) structures under moderate static magnetic fields. We consider three different structures, namely, a continuous BP monolayer, an edge formed by a semi-infinite sheet, and finally, a triangular wedge configuration. Each of these structures shows strongly anisotropic magneto-optical responses induced both by the external magnetic field and by the intrinsic anisotropy of the BP lattice.…
We report on a tunable - by magnetic field and gate voltage - conversion of terahertz radiation into a dc current in spatially modulated bilayer graphene. We experimentally demonstrate that the underlying physics is related to the so-called ratchet effect. Our key findings are the direct observation of a sharp cyclotron resonance in the photocurrent and the demonstration of two effects caused by electron-electron interaction: the plasmonic splitting of the resonance due to long-range Coulomb coupling and the partial suppression of its second harmonic due to fast interparticle collisions. We de…
Theoretical investigation of thermodynamic properties of electrically modulated monolayer graphene in the presence of a perpendicular magnetic field B is presented.This work is aimed at determining the modulation induced effects on the thermodynamic properties of electrically modulated graphene.The results obtained are compared with those of the conventional 2DEG. The one-dimensional periodic potential due to electric modulation lifts the degeneracy of the Landau Levels and converts them into bands whose width oscillates as a function of B. We find Weiss type oscillations for small values of B…
We have performed a detailed investigation of the electrical properties of a graphene monolayer which is modulated by a weak one dimensional periodic potential in the presence of a perpendicular magnetic field. The periodic modulation broadens the Landau Levels into bands which oscillate with B. The electronic conduction in this system can take place through either diffusive scattering or collisional scattering off impurities. Both these contributions to electronic transport are taken into account in this work. In addition to the appearance of commensurability oscillations in both the collisio…
We have investigated the electrical transport properties of Dirac electrons in a monolayer graphene sheet in the presence of both electric and magnetic modulations. The effects of the modulations on quantum transport when they are in phase and out of phase are considered. We present the energy spectrum and the bandwidth of the Dirac electrons in the presence of both the modulations. We determine the $σ_{yy}$ component of the magnetoconductivity tensor for this system which is shown to exhibit Weiss oscillations.Asymptotic expressions for $σ_{yy}$ are also calculated to better illustrate the ef…
We report on the modulation of tunneling resistance in MoS2 monolayers by nano-indentation using an atomic force microscope (AFM). The resistance between the conductive AFM tip and the bottom electrode separated by a monolayer MoS2 is reversibly reduced by up to 4 orders of magnitude, which is attributed to enhanced quantum tunneling when the monolayer is compressed by the tip force. Under the WKB approximation, the experimental data is quantitatively explained by using the metal-insulator-metal tunneling diode model. As an ideal tunneling medium, the defect-free, nanometer-thick MoS2 monolaye…
A monolayer graphene exists in an environment where a uniform magnetic field interacts a spatially modulated magnetic field. The spatially modulated magnetic field could affect Landau levels due to a uniform magnetic field. The modulation effects on Landau levels are investigated through the Peierl's tight-binding model. The magneto-electronic properties are dominated by the period, the strength, and the direction of a spatially modulated magnetic field. Such a field could induce the growth in dimensionality, the change of energy dispersions, the destroy of state degeneracy, and the creation o…
The canalization of terahertz surface plasmon polaritons using a modulated graphene monolayer is investigated for subwavelength imaging. An anisotropic surface conductivity formed by a set of parallel nanoribbons with alternating positive and negative imaginary conductivities is used to realize the canalization regime required for hyperlensing. The ribbons are narrow compared to the wavelength, and are created electronically by gating a graphene layer over a corrugated ground plane. Good quality canalization of surface plasmon polaritons is shown in the terahertz even in the presence of realis…
We draw motivation from recent experimental studies and present a comprehensive study of magnetothermoelectric transport in a graphene monolayer within the linear response regime. We employ the modified Kubo formalism developed for thermal transport in a magnetic field. Thermopower as well as thermal conductivity as a function of the gate voltage of a graphene monolayer in the presence of a magnetic field perpendicular to the graphene plane is determined for low magnetic fields (~1 Tesla) as well as high fields (~8 Tesla). We include the effects of screened charged impurities on thermal transp…
We present a theoretical study of Weiss oscillations in magnetoconductivity of bilayer graphene. Bilayer graphene in the presence of a perpendicular magnetic field and a unidirectional weak electric modulation is considered.We determine the $σ_{yy}$ component of the magnetoconductivity tensor for this system which is shown to exhibit Weiss oscillations. We show that Weiss oscillations in the magnetoconductivity of bilayer graphene are enhanced and more robust with temperature compared to those in conventional two-dimensional electron gas systems whereas they are less robust with temperature co…
We study the electronic transport properties of a monolayer graphene with a one-dimensional modulated height profile caused, for instance, by substrate ondulations. We show that the combined effect of the resulting strain fields induce modulated scalar and vector potentials that give rise to Weiss oscillations in the magnetoconductivity. We also find that similar effects can be obtained by applying a parallel magnetic field to the graphene-substrate interface. The parameters of an experimental set-up for a physical realization of these findings in graphene systems are discussed.
We present a theoretical study of the electronic structure of magnetically modulated graphene. We consider monolayer graphene in the presence of a perpendicular magnetic field and a unidirectional weak magnetic modulation. The density of states and the bandwidth of the Dirac electrons in this system are determined. We have found magnetic Weiss oscillations in the bandwidth and the density of states. These oscillations are out of phase and larger in amplitude than the ones in the electrically modulated graphene. In addition, these oscillations are in phase and smaller in amplitude to those of m…
We applied the Floquet scattering-matrix formalism to studying the electronic transport properties in a mesoscopic Dirac system. Using the method, we investigate theoretically quantum pumping driven by a series of time-periodic potentials in graphene monolayer both in the adiabatic and non-adiabatic regimes. Our numerical results demonstrate that adding harmonic modulated potentials can break the time reversal symmetry when no voltage bias is applied to the graphene monolayer. Thus, when the system is pumped with proper dynamic parameters, these scatterers can produce a nonzero dc pumped curre…
A vertical transistor based on a double gated, atomically thin heterojunction is theoretically examined. Both p-type and n-type transistor operations can be conveniently achieved by using one of the two gates as the switching gate. The transistor shows excellent saturation of output I-V characteristics due to drain-induced depletion and lack of tunneling barrier layers. The subthreshold slope could be below the thermionic limit due to band filtering as the switching mechanism. The atomically thin vertical PN heterojunction can be electrostatically modulated from a type II heterojunction to a b…
We investigate the Goos-Hänchen-like shifts for Dirac fermions in transmission through a monolayer graphene barrier. The lateral shifts, as the functions of the barrier's width and the incidence angle, can be negative and positive in Klein tunneling and classical motion, respectively. Due to their relations to the transmission gap, the lateral shifts can be enhanced by the transmission resonances when the incidence angle is less than the critical angle for total reflection, while their magnitudes become only the order of Fermi wavelength when the incidence angle is larger than the critical ang…
The stacking and bending of graphene are trivial but extremely powerful agents of control over graphene's manifold physics. By changing the twist angle, one can drive the system over a plethora of exotic states via strong electron correlation, thanks to the moiré superlattice potentials, while the periodic or triaxial strains induce discretization of the band structure into Landau levels without the need for an external magnetic field. We fabricated a hybrid system comprising both the stacking and bending tuning knobs. We have grown the graphene monolayers by chemical vapor deposition, using $…
We report a first principles theoretical investigation of quantum transport in monolayer WSe2 field effect transistor (FET). Due to a strong spin-orbit interaction (SOI) and the atomic structure of the two-dimensional (2D) lattice, monolayer WSe2 has an interesting electronic structure that exhibits Zeeman-like up-down spin texture near the K and K' points of the Brillouin zone. In a FET, the gate electric field induces an extra, externally tunable SOI that re-orients the spins into a Rashba-like texture thereby realizing electric control of the spin. Quantum transport is modulated by the spin…
We have investigated the electrical transport properties of Dirac electrons in a monolayer graphene sheet in the presence of a perpendicular magnetic field that is modulated weakly and periodically along one direction.We find that the Landau levels broaden into bands and their width oscillates as a function of the band index and the magnetic field.We determine the $σ_{yy}$ component of the magnetoconductivity tensor for this system which is shown to exhibit Weiss oscillations.We also determine analytically the asymptotic expressions for $σ_{yy}$.We compare these results with recently obtained …
We investigate the FIR-absorption of short period parallel quantum wires in a perpendicular quantizing magnetic field. The external time-dependent electric field is linearly polarized along the wire modulation. The mutual Coulomb interaction of the electrons is treated self-consistently in the ground state and in the absorption calculation within the Hartree approximation. We consider the effects of a metal gate grating coupler, with the same or with a different period as the wire modulation, on the absorption. The evolution of the magnetoplasmon in the nonlocal region where it is split into s…
Inter-Landau-level transitions in the bilayer graphene at high perpendicular magnetic field at the filling-factor v<<1 have been studied. The next-nearest-neighbor transitions, energy difference between dimer and non-dimer sites and layer asymmetry are included. The influence of Coulomb interaction is taken into account. The magnetoplasmon excitations in bilayer graphene at small momenta are considered in the frame of the Hartree-Fock approximation. It is shown that asymmetry in cyclotron resonance of clean bilayer graphene depends on magnetic field. At lower magnetic fields the energy splitti…
We present theoretical calculations for the cyclotron resonance and various magnetoplasmon modes of a Coulomb interacting two-dimensional GaAs electron gas (2DEG) modulated as a lateral superlattice of quantum dots subjected to an external perpendicular constant magnetic field. We use a real-time excitation approach based on the Liouville-von Neumann equation for the density operator, that can go beyond linear response delivering information of all longitudinal and transverse collective modes of interest to the same order. We perform an extensive analysis of the coexisting collective modes due…
We investigate theoretically the effects of modulated periodic perpendicular magnetic fields on the electronic states and optical absorption spectrum in a monolayer black phosphorus (phosphorene). We demonstrate that different phosphorene magnetic superlattice (PMS) orientations can give rise to distinct energy spectra, i.e., tuning the intrinsic electronic anisotropy. The Rashba spin-orbit coupling (RSOC) will develop a spin-splitting energy dispersion in this phosphorene magnetic supperlattice. Anisotropic momentum-dependent carrier distributions along/perpendicular to the magnetic strips ar…
It has been shown that Bernal stacked bilayer graphene (BLG) in a uniform magnetic field demonstrates integer quantum Hall effect with a zero Landau-level anomaly \cite{Geimbilayer}. In this article we consider such system in a two dimensional periodic magnetic modulation with square lattice symmetry. It is shown algebraically that the resulting Hofstadter spectrum can be expressed in terms of the corresponding spectrum of monolayer graphene in a similar magnetic modulation. In the weak-field limit, using the tight-binding model, we also derive the Harper-Hofstadter equation for such BLG syste…
We have derived the adsorption potential of $^4$He atoms on fluorographene (GF), on graphane and on hexagonal boron nitride (hBN) by a recently developed ab initio method that incorporates the van der Waals interaction. The $^4$He monolayer on GF and on hBN is studied by state-of-the-art quantum simulations at T=0 K. With our adsorption potentials we find that in both cases the ground state of $^4$He monolayer is a fluid and not an ordered state with localized atoms as on graphite and on graphene. In the case of GF the present result is in qualitative agreement with the superfluid phase that w…
We report the first observation of gate-controlled field emission current from a tungsten diselenide (WSe2) monolayer, synthesized by chemical-vapour deposition on SiO2/Si substrate. Ni contacted WSe2 monolayer back-gated transistors, under high vacuum, exhibit n-type conduction and drain-bias dependent transfer characteristics, which are attributed to oxygen/water desorption and drain induced Schottky barrier lowering, respectively. The gate-tuned n-type conduction enables field emission, i.e. the extraction of electrons by quantum tunnelling, even from the flat part of the WSe2 monolayers. E…
Heavy metal-based two-dimensional van der Waals materials have a large, coupled spin and valley Hall effect (SVHE) that has potential use in spintronics and valleytronics. Optical measurements of the SVHE have largely been performed below 30 K and understanding of the SVHE-induced spin/valley polarizations that can be electrically generated is limited. Here, we study the SVHE in monolayer p-type tungsten diselenide (WSe2). Kerr rotation (KR) measurements show the spatial distribution of the SVHE at different temperatures, its persistence up to 160 K, and that it can be electrically modulated v…
We apply a circularly and linearly polarized terahertz field on a monolayer of graphene taking into account spin-orbit interactions of the intrinsic and Rashba type. It turns out that the field can not only be used to induce a gap in the energy spectrum, but also to close an existing gap due to the different reaction of the spin components on circularly polarized light. Signatures of spin-orbit coupling on the density of states of the driven system can be observed even for energies where the static density of states is independent of spin-orbit interactions. Furthermore it is shown that the ti…
Giant Anisotropic Magnetoresistance in Magnetic Monolayers CrPX3 (X = S, Se, Te) due to symmetry breaking between the in-plane and out-of-plane crystallographic axes
The effect of magnetic modulation on thermodynamic properties of a graphene monolayer in presence of a constant perpendicular magnetic field is reported here. One-dimensional spatial electric or magnetic modulation lifts the degeneracy of the Landau levels and converts into bands and their band width oscillates with magnetic field leading to Weiss-type oscillation in the thermodynamic properties. The effect of magnetic modulation on thermodynamic properties of a graphene sheet is studied and then compared with electrically modulated graphene and magnetically modulated conventional two-dimensio…
We propose a device concept, based on monolayer stanene, able to provide highly polarized spin currents (up to a $98\%$) with voltage-controlled spin polarization operating at room temperature and with small operating voltage ($0.3$ V). The concept exploits the presence of spin-polarized edge states in a stanene nanoribbon. The spin polarization of the total current can be modulated by a differential tuning of the transmission properties, and of the occupation of edge states of different spin, via the application of an in-plane electric field. We demonstrate device operation using ab-initio an…
The electronic topology is generally related to the Berry curvature, which can induce the anomalous Hall effect in time-reversal symmetry breaking systems. Intrinsic monolayer transition metal dichalcogenides possesses two nonequivalent K and K' valleys, having Berry curvatures with opposite signs, and thus vanishing anomalous Hall effect in this system. Here we report the experimental realization of asymmetrical distribution of Berry curvature in a single valley in monolayer WSe2 through applying uniaxial strain to break C3v symmetry. As a result, although the Berry curvature itself is still …
Recent observation of quantum emitters in monolayers of hexagonal boron nitride (h-BN) has provided a novel platform for optomechanical experiments where the single-photon emitters can couple to the motion of freely suspended h-BN membrane. Here, we propose a scheme where the electronic degree of freedom of an embedded color center is coupled to the motion of the hosting h-BN resonator via dispersive forces. We show that the coupling of membrane vibrations to the electronic degree of freedom of the emitter can reach the strong regime. By suitable driving of a three-level $Λ$-system composed of…
Two-dimensional (2D) MoSi$_2$N$_4$ monolayer is an emerging class of air-stable 2D semiconductor possessing exceptional electrical and mechanical properties. Despite intensive recent research efforts devoted to uncover the material properties of MoSi$_2$N$_4$, the physics of electrical contacts to MoSi$_2$N$_4$ remains largely unexplored thus far. In this work, we study the van der Waals heterostructures composed of MoSi$_2$N$_4$ contacted by graphene and NbS$_2$ monolayers using first-principle density functional theory calculations. We show that the MoSi$_2$N$_4$/NbS$_2$ contact exhibits an …
The band convergence strategy, which improves Seebeck coefficient by inducing multi-valley in bandstructures, has been widely used in thermoelectric performance (TE) enhancing. However, the phonon-assisted intervalley scattering effect is neglected and the mode-selection rules remain unclear. In this work, TE properties for $α$-, $β$- and $γ$-PbP are intestigated under the consideration of full mode-, energy- and momentum-resolved electron-phonon interactions (EPI). The group theory is used to analyze the selection rules for EPI matrix elements. Our calculations reveal that, the intervalley sc…
We investigate the resonance energy transfer (RET) rate between two quantum emitters near a suspended graphene sheet in vacuum under the influence of an external magnetic field. We perform the analysis for low and room temperatures and show that, due to the extraordinary magneto-optical response of graphene, it allows for an active control and tunability of the RET even in the case of room temperature. We also demonstrate that the RET rate is extremely sensitive to small variations of the applied magnetic field, and can be tuned up to a striking six orders of magnitude for quite realistic valu…
We report on transport properties of monolayer graphene with a laterally modulated potential profile, employing striped top gate electrodes with spacings of 100 nm to 200 nm. Tuning of top and back gate voltages gives rise to local charge carrier density disparities, enabling the investigation of transport properties either in the unipolar (nn') or the bipolar (np') regime. In the latter pronounced single- and multibarrier Fabry-Perot (FP) resonances occur. We present measurements of different devices with different numbers of top gate stripes and spacings. The data are highly consistent with …
Conductance switching has been reported in many molecular junction devices, but in most cases has not been convincingly explained. We investigate conductance switching in Pt/stearic acid monolayer/Ti devices using pressure-modulated conductance microscopy. For devices with conductance G>>G_Q or G<<G_Q, where GQ=2e^2/h is the conductance quantum, pressure-induced conductance peaks <30 nm in diameter are observed, indicating the formation of nanoscale conducting pathways between the electrodes. For devices with G~ 1- 2 G_Q, in addition to conductance peaks we also observed conductance dips and o…
The generalized tight-binding model with exact diagonalization method is developed to calculate the optical properties of monolayer graphene in the presence of composite magnetic fields. The ratio of the uniform magnetic field and the modulated one accounts for a strong influence on the structure, number, intensity and frequency of absorption peaks, and thus the extra selection rules that are subsequently induced can be explained. When the modulated field increases, each symmetric peak, under a uniform magnetic field, splits into a pair of asymmetric peaks with lower intensities. The threshold…
We investigate the magnetic proximity effect in van der Waals heterostructure formed by a monolayer semiconductor stacked on a 2D ferromagnet, where the lattice mismatch and twisting between the layers typically lead to the formation of moiré pattern. We find that the magnetic proximity effect arising from the spin dependent interlayer coupling depends sensitively on the interlayer atomic registry. Consequently, in the moiré pattern, the spatial variation of the atomic registry leads to a lateral modulation of magnetic proximity field. Such moiré modulated magnetic proximity effect manifests a…
Silicene is a competitive and promising 2D material, possessing interesting topological, electronic and optical properties. The presence of strong spin orbit interaction in silicene and its analogues, germanene and tinene, leads to the opening of a gap in the energy spectrum and spin-splitting of the bands in each valley. We develop a general method to determine the magneto-optic response of silicene, when a Gaussian beam is incident on silicene grown on a dielectric substrate in the presence of a static magnetic field. We use a semiclassical treatment of silicene monolayer to describe the Far…
