14 research outputs found
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
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 K with onset carrier densities
as high as cm. 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
Plasmon modes in monolayer and double-layer black phosphorus under applied uniaxial strain
We study the effects of an applied in-plane uniaxial strain on the plasmon dispersions of monolayer, bilayer, and double-layer black phosphorus structures in the long-wavelength limit within the linear elasticity theory. In the low-energy limit, these effects can be modeled through the change in the curvature of the anisotropic energy band along the armchair and zigzag directions. We derive analytical relations of the plasmon modes under uniaxial strain and show that the direction of the applied strain is important. Moreover, we observe that along the armchair direction, the changes of the plasmon dispersion with strain are different and larger than those along the zigzag direction. Using the analytical relations of two-layer phosphorene systems, we found that the strain-dependent orientation factor of layers could be considered as a means to control the variations of the plasmon energy. Furthermore, our study shows that the plasmonic collective modes are more affected when the strain is applied equally to the layers compared to the case in which the strain is applied asymmetrically to the layers. We also calculate the effect of strain on the drag resistivity in a double-layer black phosphorus structure and obtain that the changes in the plasmonic excitations, due to an applied strain, are mainly responsible for the predicted results. This study can be readily extended to other anisotropic two-dimensional materials
Electron–hole superfluidity in strained Si/Ge type II heterojunctions
Excitons are promising candidates for generating superfluidity and Bose–Einstein condensation (BEC) in solid-state devices, but an enabling material platform with in-built band structure advantages and scaling compatibility with industrial semiconductor technology is lacking. Here we predict that spatially indirect excitons in a lattice-matched strained Si/Ge bilayer embedded into a germanium-rich SiGe crystal would lead to observable mass-imbalanced electron–hole superfluidity and BEC. Holes would be confined in a compressively strained Ge quantum well and electrons in a lattice-matched tensile strained Si quantum well. We envision a device architecture that does not require an insulating barrier at the Si/Ge interface, since this interface offers a type II band alignment. Thus the electrons and holes can be kept very close but strictly separate, strengthening the electron–hole pairing attraction while preventing fast electron–hole recombination. The band alignment also allows a one-step procedure for making independent contacts to the electron and hole layers, overcoming a significant obstacle to device fabrication. We predict superfluidity at experimentally accessible temperatures of a few Kelvin and carrier densities up to ~6 × 1010 cm−2, while the large imbalance of the electron and hole effective masses can lead to exotic superfluid phases