Anisotropic hybrid excitation modes in monolayer and double-layer phosphorene on polar substrates

We investigate the anisotropic hybrid plasmon-SO phonon dispersion relations in monolayer and double-layer phosphorene systems located on the polar substrates, such as SiO2, h-BN and Al2O3. We calculate these hybrid modes with using the dynamical dielectric function in the RPA by considering the electron-electron interaction and long-range electric field generated by the substrate SO phonons via Frohlich interaction. In the long-wavelength limit, we obtain some analytical expressions for the hybrid plasmon-SO phonon dispersion relations which represent the behavior of these modes akin to the modes obtaining from the loss function. Our results indicate a strong anisotropy in plasmon-SO phonon modes, whereas they are stronger along the light-mass direction in our heterostructures. Furthermore, we find that the type of substrate has a significant effect on the dispersion relations of the coupled modes. Also, by tuning the misalignment and separation between layers in double-layer phosphorene on polar substrates, we can engineer the hybrid modes.Comment: 10 pages, 7 figure

Phononic thermal conductivity in silicene: the role of vacancy defects and boundary scattering

We calculate the thermal conductivity of free-standing silicene using the phonon Boltzmann transport equation within the relaxation time approximation. In this calculation, we investigate the effects of sample size and different scattering mechanisms such as phonon-phonon, phonon-boundary, phonon-isotope and phonon-vacancy defect. Moreover, the role of different phonon modes is examined. We show that, in contrast to graphene, the dominant contribution to the thermal conductivity of silicene originates from the in-plane acoustic branches, which is about 70\% at room temperature and this contribution becomes larger by considering vacancy defects. Our results indicate that while the thermal conductivity of silicene is significantly suppressed by the vacancy defects, the effect of isotopes on the phononic transport is small. Our calculations demonstrate that by removing only one of every 400 silicon atoms, a substantial reduction of about 58\% in thermal conductivity is achieved. Furthermore, we find that the phonon-boundary scattering is important in defectless and small-size silicene samples, specially at low temperatures.Comment: 9 pages, 11 figure

Tuning electronic properties and contact type in van der Waals heterostructures of bilayer SnS and graphene

Using first-principles calculations, we study the structural and electronic properties of the bilayer SnS/graphene, bilayer SnS/bilayer graphene (AA-stacked), bilayer SnS/bilayer graphene (AB-stacked) and monolayer SnS/graphene/monolayer SnS van der Waals (vdW) heterostructures. Electronic properties of all components of the vdW heterostructures are well preserved, which reflects the weakness of the vdW interaction. In the cases of bilayer SnS/graphene and bilayer SnS/bilayer graphene (AA-stacked), an Ohmic contact is formed which can be turned first into p-type and then into n-type Schottky contacts via application of an external electric field. Calculations show that an Ohmic contact is also formed at the interface of bilayer SnS/bilayer graphene (AB-stacked) heterostructure, but interestingly, by applying the perpendicular electric field a transition from semimetal/semiconductor contact to semiconductor/semiconductor one occurs which can enhance its optical properties. Alternatively, in the monolayer SnS/graphene/monolayer SnS vdW heterosructure, a p-type Schottky contact is established that changes into Ohmic contact under an applied electric field. Our results clearly indicate that the electronic properties of the vdW heterostructures can be tuned efficiently by external electric field, which is important in designing of new nanoelectronic devices.Comment: 12 pages, 11 figure

Many-body effects due to the electron-electron interaction in silicene under an applied exchange field:The case of valley-spin coupling

We investigate the many-body effects induced by the electron–electron interaction in a valley–spin-polarized silicene under a perpendicularly applied exchange field. We calculate the real and imaginary parts of the self-energy within the leading order dynamical screening approximation where the screened interaction is obtained from the random phase approximation. Our study on the valley- and spin-dependent real and imaginary parts of the self-energy indicates that the different coupled valley–spin subbands may exhibit distinct characteristics. Moreover, we obtain the corresponding spectral functions and find that the plasmaron and quasiparticle peaks have different spectral weights and broadenings in all states. Interestingly, it seems that there are clear dependencies for the position and broadening of the peaks on valley–spin indexes. In addition, we study the effect of the electron–electron interaction on the renormalized velocity in the on-shell approximation and show that the renormalized velocity in gapped states becomes greater, and in gapless states, it becomes smaller as the wave vector grows

Energy Transfer Rate in Double-Layer Graphene Systems: Linear Regime

We investigate theoretically the energy transfer phenomenon in a double-layer graphene (DLG) system in which two layers are coupled due to the Coulomb interlayer interaction without appreciable interlayer tunneling. We use the balance equation approach and the dynamic and temperature dependent random phase approximation (RPA) screening function in our calculations to obtain the rates of energy transfer between two graphene layers at different layer electron temperatures, densities and interlayer spacings and compare the results with those calculated for the conventional double-layer two-dimensional electron gas (2DEG) systems. In addition, we study the effect of changing substrate dielectric constant on the rate of energy transfer. The general behavior of the energy transfer rate in the DLG is qualitatively similar to that obtained in the double-layer 2DEG but quantitatively its DLG values are an order of magnitude greater. Also, at large electron temperature differences between two layers, the electron density dependence of the energy transfer for the DLG system is significantly different from that found for the double-layer 2DEG system, particularly in case of unequal layer electron densities.Comment: 12 pages,4 figure

High temperature electron-hole superfluidity with strong anisotropic gaps in double phosphorene monolayers

Excitonic superfluidity in double phosphorene monolayers is investigated using the BCS mean-field equations. Highly anisotropic superfluidity is predicted where we found that the maximum superfluid gap is in the BEC regime along the armchair direction and in the BCS-BEC crossover regime along the zigzag direction. We estimate the highest Kosterlitz-Thouless transition temperature with maximum value up to $\sim 90$ K with onset carrier densities as high as $4 \times 10^{12}$ cm$^{-2}$. This transition temperature is significantly larger than what is found in double electron-hole few-layers of graphene. Our results can guide experimental research towards the realization of anisotropic condensate states in electron-hole phosphorene monolayers.Comment: 7 pages, 4 figure

Coulomb drag in anisotropic systems: a theoretical study on a double-layer phosphorene

We theoretically study the Coulomb drag resistivity in a double-layer electron system with highly anisotropic parabolic band structure using Boltzmann transport theory. As an example, we consider a double-layer phosphorene on which we apply our formalism. This approach, in principle, can be tuned for other double-layered systems with paraboloidal band structures. Our calculations show the rotation of one layer with respect to another layer can be considered a way of controlling the drag resistivity in such systems. As a result of rotation, the off-diagonal elements of drag resistivity tensor have non-zero values at any temperature. In addition, we show that the anisotropic drag resistivity is very sensitive to the direction of momentum transfer between two layers due to highly anisotropic inter-layer electron-electron interaction and also the plasmon modes. In particular, the drag anisotropy ratio, \r{ho}yy/\r{ho}xx, can reach up to ~ 3 by changing the temperature. Furthermore,our calculations suggest that including the local field correction in dielectric function changes the results significantly. Finally, We examine the dependence of drag resistivity and its anisotropy ratio on various parameters like inter-layer separation, electron density, short-range interaction and insulating substrate/spacer.Comment: 10 pages, 9 figure

Effect of finite-temperature local field corrections on many-body properties of quantum wires

The effect of low temperature on the many-body properties of an n-type GaAs-based quasi one-dimensional electron gas (quantum wire) has been investigated in the RPA, Hubbard and STLS approximations. Numerical results for the finite-temperature static structure factors, local field corrections, pair correlation functions, plasmon dispersion relations and inverse static dielectric functions of the system have been obtained and compared with the zero-temperature values. As a general result, the behavior of the system at finite-temperature does not change significantly at small values of wave vectors. Also, it has been found out that while applying the temperature dependent Hubbard and STLS to the quantum wire yields different results for local field corrections and pair correlation functions, the increase in temperature within all approximations causes the plasmon modes to shift upward in energy and the sharp peaks in the inverse static dielectric curve to become less pronounced