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Register a new userHigh temperature electron-hole superfluidity with strong anisotropic gaps in double phosphorene monolayers
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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.
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We study direct and indirect excitons in Rydberg states in phosphorene monolayers, bilayer and van der Waals (vdW) heterostructure in an external magnetic field, applied perpendicular to the monolayer or heterostructure within the framework of the effective mass approximation. Binding energies of magnetoexcitons are calculated by a numerical integration of the Schrodinger equation using the Rytova-Keldysh potential for direct magnetoexcitons and both the Rytova-Keldysh and Coulomb potentials for indirect one. The latter allows to understand the role of screening in phosphorene. We report the magnetic field energy contribution to the binding energies and diamagnetic coefficients (DMCs) for magnetoexcitons that strongly depend on the effective mass of electron and hole and their anisotropy and can be tuned by the external magnetic field. We demonstrate theoretically that the vdW phosphorene heterostructure is a novel category of 2D semiconductor offering a tunability of the binding energies of magnetoexcitons by mean of external magnetic field and control the binding energies and DMCs by the number of hBN layers separated two phosphorene sheets. Such tunability is potentially useful for the devices design.
Spontaneous orbital magnetism observed in twisted bilayer graphene (tBG) on nearly aligned hexagonal boron nitride (BN) substrate builds on top of the electronic structure resulting from combined G/G and G/BN double moire interfaces. Here we show that tBG/BN commensurate double moire patterns can be classified into two types, each favoring the narrowing of either the conduction or valence bands on average, and obtain the evolution of the bands as a function of the interlayer sliding vectors and electric fields. Finite valley Chern numbers $pm 1$ are found in a wide range of parameter space when the moire bands are isolated through gaps, while the local density of states associated to the flat bands are weakly affected by the BN substrate invariably concentrating around the AA-stacked regions of tBG. We illustrate the impact of the BN substrate for a particularly pronounced electron-hole asymmetric band structure by calculating the optical conductivities of twisted bilayer graphene near the magic angle as a function of carrier density. The band structures corresponding to other $N$-multiple commensurate moire period ratios indicate it is possible to achieve narrow width $W lesssim 30$ meV isolated folded band bundles for tBG angles $theta lesssim 1^{circ}$.
Moire superlattices of van der Waals heterostructures provide a powerful new way to engineer the electronic structures of two-dimensional (2D) materials. Many novel quantum phenomena have emerged in different moire heterostructures, such as correlated insulators, superconductors, and Chern insulators in graphene systems and moire excitons in transition metal dichalcogenide (TMDC) systems. Twisted phosphorene offers another attractive system to explore moire physics because phosphorene features an anisotropic rectangular lattice, different from the isotropic hexagonal lattice in graphene and TMDC. Here we report emerging anisotropic moire optical transitions in twisted monolayer/bilayer phosphorene. The optical resonances in phosphorene moire superlattice depend sensitively on the twist angle between the monolayer and bilayer. Surprisingly, even for a twist angle as large as 19{deg} the moire heterostructure exhibits optical resonances completely different from those in the constituent monolayer and bilayer phosphorene. The new moire optical resonances exhibit strong linear polarization, with the principal axis lying close to but different from the optical axis of bilayer phosphorene. Our ab initio calculations reveal that the {Gamma}-point direct bandgap and the rectangular lattice of phosphorene, unlike the K-point bandgap of hexagonal lattice in graphene and TMDC, give rise to the remarkably strong moire physics in large-twist-angle phosphorene heterostructures. Our results highlight the exciting opportunities to explore moire physics in phosphorene and other van der Waals heterostructures with different lattice configurations.
We perform systematic investigation on the geometric, energetic and electronic properties of group IV-VI binary monolayers (XY), which are the counterparts of phosphorene, by employing density functional theory based electronic structure calculations. For this purpose, we choose the binary systems XY consisting of equal numbers of group IV (X = C, Si, Ge, Sn) and group VI elements (Y = O, S, Se, Te) in three geometrical configurations, the puckered, buckled and planar structures. The results of binding energy calculations show that all the binary systems studied are energetically stable. It is observed that, the puckered structure, similar to that of phosphorene, is the energetically most stable geometric configuration. Our results of electronic band structure predict that puckered SiO and CSe are direct band semiconductors with gaps of 1.449 and 0.905 eV, respectively. Band structure of CSe closely resembles that of phosphorene. Remaining group IV-VI binary monolayers in the puckered configuration and all the buckled monolayers are also semiconductors, but with indirect band gaps. Importantly, we find that the difference between indirect and direct band gaps is very small for many puckered monolayers. Thus, there is a possibility of making these systems undergo transition from indirect to direct band gap semiconducting state by a suitable external influence. Indeed, we show in the present work that seven binary monolayers namely SnS, SiSe, GeSe, SnSe, SiTe, GeTe and SnTe become direct band gap semiconductors when they are subjected to a small mechanical strain (<= 3 %). This makes nine out of sixteen binary monolayers studied in the present work direct band gap semiconductors. Thus, there is a possibility of utilizing these binary counterparts of phosphorene in future light-emitting diodes and solar cells.
We predict the occurrence of Bose-Einstein condensation and superfluidity of dipolar magnetoexcitons for a pair of quasi-two-dimensional spatially separated $alpha$-${cal T}_3$ layers. We have solved a two-body problem for an electron and a hole for the model Hamiltonian for the $alpha$-${cal T}_3$ double layer in a magnetic field. The energy dispersion of collective excitations, the spectrum of sound velocity, and the effective magnetic mass of magnetoexcitons are obtained in the integer quantum Hall regime for high magnetic fields. The superfluid density and the temperature of the Kosterlitz-Thouless phase transition are probed as functions of the excitonic density, magnetic field, and the inter-layer separation.
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