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Register a new userSuperfluidity of dipolar magnetoexcitons in doped double-layered $alpha$-${cal T}_3$ lattice in a strong magnetic field
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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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The interaction energy for the indirect-exchange or Ruderman-Kittel-Kasuva-Yosida (RKKY) interaction between magnetic spins localized on lattice sites of the $alpha$-${cal T}_3$ model is calculated using linear response theory. In this model, the $texttt{AB}$-honeycomb lattice structure is supplemented with $texttt{C}$ atoms at the centers of the hexagonal lattice. This introduces a parameter $alpha$ for the ratio of the hopping integral from hub-to-rim and that around the rim of the hexagonal lattice. A valley and $alpha$-dependent retarded Greens function matrix is used to form the susceptibility. Analytic and numerical results are obtained for undoped $alpha$-${cal T}_3$, when the chemical potential is finite and also in the presence of an applied magnetic field. We demonstrate the anisotropy of these results when the magnetic impurities are placed on the $texttt{A,B}$ and $texttt{C}$ sublattice sites. Additionally, comparison of the behavior of the susceptibility of $alpha$-${cal T}_3$ with graphene shows that there is a phase transition at $alpha=0$.
We address the electronic properties of quantum dots in the two-dimensional $alpha-mathcal{T}_3$ lattice when subjected to a perpendicular magnetic field. Implementing an infinite mass boundary condition, we first solve the eigenvalue problem for an isolated quantum dot in the low-energy, long-wavelength approximation where the system is described by an effective Dirac-like Hamiltonian that interpolates between the graphene (pseudospin 1/2) and Dice (pseudospin 1) limits. Results are compared to a full numerical (finite-mass) tight-binding lattice calculation. In a second step we analyse charge transport through a contacted $alpha-mathcal{T}_3$ quantum dot in a magnetic field by calculating the local density of states and the conductance within the kernel polynomial and Landauer-Buttiker approaches. Thereby the influence of a disordered environment is discussed as well.
We study the importance of interband effects on the orbital susceptibility of three bands $alpha$-${cal T}_3$ tight-binding models. The particularity of these models is that the coupling between the three energy bands (which is encoded in the wavefunctions properties) can be tuned (by a parameter $alpha$) without any modification of the energy spectrum. Using the gauge-invariant perturbative formalism that we have recently developped, we obtain a generic formula of the orbital susceptibility of $alpha$-${cal T}_3$ tight-binding models. Considering then three characteristic examples that exhibit either Dirac, semi-Dirac or quadratic band touching, we show that by varying the parameter $alpha$ and thus the wavefunctions interband couplings, it is possible to drive a transition from a diamagnetic to a paramagnetic peak of the orbital susceptibility at the band touching. In the presence of a gap separating the dispersive bands, we show that the susceptibility inside the gap exhibits a similar dia to paramagnetic transition.
We have discovered two-dimensional zero-gap material with a layered structure in the organic conductor $alpha$-(BEDT-TTF)$_2$I$_3$ under high hydrostatic pressure. In contrast to graphene, the electron-hole symmetry is not good except at the vicinity of the Dirac points. Thus, temperature dependence of the chemical potential, $mu$, plays an important role in the transport in this system. The experimental formula of $mu$ is revealed. We succeeded in detecting the inter-band effects of a magnetic field on the Hall conductivity when $mu$ passes the Dirac point.
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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