118 research outputs found
Towards superfluidity of dipolar excitons in a TMDC double layer
We study formation and superfluidity of dipolar excitons in double layer
heterostructures formed by two transition metal dichalcogenide (TMDC)
atomically thin layers. Considering screening effects for an electron-hole
interaction via the harmonic oscillator approximation for the Keldysh
potential, the analytical expressions for the exciton energy spectrum and the
mean field critical temperature for the superfluidity are obtained. It
is shown that binding energies of A excitons are larger than for B excitons.
The mean field critical temperature for a two-component dilute exciton system
in a TMDC double layer is analyzed and shown that latter is an increasing
function of the factor , determined by the effective masses of A and B
excitons and their reduced mass. Comparison of the calculations for
performed by employing the Coulomb and Keldysh interactions demonstrates the
importance of screening effects in TMDC.Comment: 14 pages, 5 figure
High-temperature superfuidity of the two-component Bose gas in a TMDC bilayer
The high-temperature superfluidity of two-dimensional dipolar excitons in two
parallel TMDC layers is predicted. We study Bose-Einstein condensation in the
two-component system of dipolar A and B excitons. The effective mass, energy
spectrum of the collective excitations, the sound velocity and critical
temperature are obtained for different TMDC materials. It is shown that in the
Bogolubov approximation the sound velocity in the two-component dilute exciton
Bose gas is always larger than in any one-component. The difference between the
sound velocities for two-component and one-component dilute gases is caused by
the fact that the sound velocity for two-component system depends on the
reduced mass of A and B excitons, which is always smaller than the individual
mass of A or B exciton. Due to this fact, the critical temperature Tc for
superfluidity for the two-component exciton system in TMDC bilayer is about one
order of magnitude higher than Tc in any one-component exciton system. We
propose to observe the superfluidity of two-dimensional dipolar excitons in two
parallel TMDC layers, which causes two opposite superconducting currents in
each TMDC layer.Comment: 18 pages, 4 figure
Superfluidity of Dipolar Excitons in a Black Phosphorene Double Layer
We study the formation of dipolar excitons and their superfluidity in a black
phosphorene double layer. The analytical expressions for the single dipolar
exciton energy spectrum and wave function are obtained. It is predicted that a
weakly interacting gas of dipolar excitons in a double layer of black
phosphorus exhibits superfluidity due to the dipole-dipole repulsion between
the dipolar excitons. In calculations are employed the Keldysh and Coulomb
potentials for the interaction between the charge carriers to analyze the
influence of the screening effects on the studied phenomena. It is shown that
the critical velocity of superfluidity, the spectrum of collective excitations,
concentrations of the superfluid and normal component, and mean field critical
temperature for superfluidity are anisotropic and demonstrate the dependence on
the direction of motion of dipolar excitons. The critical temperature for
superfluidity increases if the exciton concentration and the interlayer
separation increase. It is shown that the dipolar exciton binding energy and
mean field critical temperature for superfluidity are sensitive to the electron
and hole effective masses. The proposed experiment to observe a directional
superfluidity of excitons is addressed.Comment: 19 pages, 7 figure
Laplace transform approach for the dynamics of N qubits coupled to a resonator
An approach to use the method of Laplace transform for the perturbative
solution of the Schr\"{o}dinger equation at any order of the perturbation for a
system of qubits coupled to a cavity with photons is suggested. We
investigate the dynamics of a system of superconducting qubits coupled to a
common resonator with time-dependent coupling. To account for the contribution
of the dynamical Lamb effect to the probability of excitation of the qubit, we
consider counter-rotating terms in the qubit-photon interaction Hamiltonian. As
an example, we illustrate the method for the case of two qubits coupled to a
common cavity. The perturbative solutions for the probability of excitation of
the qubit show excellent agreement with the numerical calculations.Comment: 8 pages, 1 figur
The electron-hole superfluidity in two coaxial nanotubes
The superfluid phase and Coulomb drag effect caused by the pairing in the
system of spatially separated electrons and holes in two coaxial cylindrical
nanotubes are predicted. It is found that the drag resistance as a function of
temperature experiences a jump at the critical temperature and can be used for
the manifestation of the superfluid transition. It is demonstrated that at
sufficiently low temperatures the order parameter and free energy density
exhibit a kink due to the electron-hole asymmetry that is controlled by the
radii of the nanotubes
Time evolution of the quantum entanglement between qubits due to dynamical Lamb effect in the presence of dissipation
A theoretical framework to investigate the time evolution of the quantum
entanglement due to the dynamical Lamb effect between superconducting
qubits coupled to a coplanar waveguide in the presence of different sources of
dissipation is developed. We quantitatively analyze the case of and
qubits under the assumptions of single switching of the coupling and absence of
dissipation within a perturbative approach. The same systems are analyzed for
the general case of periodic switching of the coupling in the presence of
dissipation via numerical calculations. Different measures of entanglement
compatible with mixed states are adopted. It is demonstrated that the different
measures show different level of details of the latter. The concurrence and the
negativity are obtained in the two qubits case, the three- and the
negativity in the three qubits case. It is shown that time-dependent
Greenberger-Horne-Zeilinger states can be created even in presence of
dissipation. To maximize the quantum entanglement between the qubits, the
effects of tuning several parameters of the system are investigated.Comment: 21 pages, 7 figure
Tunable quantum entanglement of three qubits in a non-stationary cavity
We investigate the tunable quantum entanglement and the probabilities of
excitations in a system of three qubits in a non-stationary cavity due to the
dynamical Lamb effect, caused by non-adiabatic fast change of the boundary
conditions of the cavity. The transition amplitudes and the probabilities of
excitation of qubits due to the dynamical Lamb effect have been evaluated. The
conditional concurrence and the conditional residual tangle for each fixed
amount of created photons are introduced and calculated as measures of the
pairwise or three-way dynamical quantum entanglement of the qubits. We also
give a prescription on how to increase the values of those quantities by
controlling the frequency of the cavity photons. A physical realization of the
system with three superconducting qubits, coupled to a coplanar waveguide
entangled due to the non-adiabatic fast change of boundary conditions of the
cavity is proposed.Comment: 11 page
Phase transitions in the two-dimensional electron-hole gas
A first order phase transition between a BCS phase and an insulating Mott
phase for a gas of spatially separated electrons and holes with tunable Coulomb
interaction and variable density is predicted. In the framework of a BCS-like
mean-field approach and a Landau expansion in terms of the pairing order
parameter the phase diagram is studied. This indicates several phases and phase
transitions, including an electron-hole plasma at low density and weak
interaction, an intermediate BCS phase with Cooper pairs and an electron-hole
plasma at high density and weak interaction. The insulating Mott phase appears
for the strong interaction and low temperatures. We briefly discuss the
possibilities to realize these phases in realistic systems such as coupled
quantum wells and graphene double layers.Comment: 10 pages, 4 figure
Bose-Einstein condensation of polaritons in graphene in a high magnetic field
The Bose-Einstein condensation (BEC) of magnetoexcitonic polaritons in a
graphene layer embedded in a optical microcavity in a high magnetic field
is predicted. The essential property of this system (in contrast, e.g., to a
quantum well embedded in a cavity) is stronger influence of magnetic field and
weaker influence of disorder. A two-dimensional (2D) magnetoexcitonic
polaritons gas is considered in a planar harmonic electric field potential
applied to excitons or a parabolic shape of the optical cavity causing the
trapping of microcavity photons. It is shown that the effective polariton mass
increases with magnetic field as . The BEC critical
temperature decreases as and increases with the spring
constant of the parabolic trap. The Rabi splitting related to the creation of a
magnetoexciton in a high magnetic field in graphene is obtained.Comment: 7 pages, 1 figur
Drag effects in the system of electrons and microcavity polaritons
The theory of the drag effects in the system of spatially separated electrons
and excitons in coupled quantum wells (QW) embedded in an optical microcavity
is developed. It is shown that at low temperature an electron current induces
the (normal component) polariton flow, therefore, a transport of photons along
the cavity. However, the electron current dragged by the polariton flow is
strongly suppressed below polariton superfluid transition temperature and
hence, the strong suppression of the induced electron current indicates the
superfluidity of polaritons. Therefore, the transport properties of polaritons
can be investigated by measuring the current or voltage in the electron
subsystem. At high temperatures we study the exciton-electron drag effects. At
high temperatures regime, from one hand, the existence of the electric current
in an electron QW induces the exciton flow in the other QW, from the other
hand, the electron current in one QW induces the exciton flow in the other QW
via the drag of excitons by the electrons. The drag coefficients for the
polariton-electron systems are calculated and analyzed. We discuss the possible
experimental observation of the drag effects in the system of electrons and
microcavity polaritons, that also allow to observe the cavity polaritons
superfluidity.Comment: 16 pages, 7 figures, Physical Review B, in press (2010
- β¦