D. V. Fil

Interlayer tunneling and the problem of superfluidity in bilayer quantum Hall systems

A possibility of nondissipative transmission of electrical current from the source to the load using superfluid electron-hole pairs in bilayers is studied. The problem is considered with reference to quantum Hall bilayers with the total filling factor VT=1. At nonzero interlayer tunneling the current pattern looks as a sum of uniform planar counterflow currents and Josephson vortices. The difference of electrochemical potentials of the layers (that is required to support the current in the load circuit) causes the motion of the Josephson vortices. In such a situation the second superfluid viscosity comes into play and results in dissipation of energy. It is found that the loss power is proportional to the square of the matrix element of the interlayer tunneling and depends nonlinearly on the load resistance.

Collective excitations of a two-dimensional interacting Bose gas in external fields

We present a method of finding approximate analytical solutions for the spectra and eigenvectors of collective modes in a two-dimensional system of interacting bosons subjected to a linear external potential or the potential of a special form u(x,y)={mu}-u cosh{sup 2} x/l, where {mu} is the chemical potential. The eigenvalue problem is solved analytically for an artificial model allowing the unbounded density of the particles. The spectra of collective modes are calculated numerically for the stripe, the rare density valley, and the edge geometry and compared with the analytical results. It is shown that the energies of the modes localized at the rare density region and at the edge are well approximated by the analytical expressions. We discuss Bose-Einstein condensation (BEC) in the systems under investigations at T{ne}0 and find that in case of a finite number of the particles the regime of BEC can be realized, whereas the condensate disappears in the thermodynamic limit.

Superconductivity of electron–hole pairs in a bilayer graphene system in a quantizing magnetic field

A state with spontaneous interlayer phase coherence in a bilayer quantum Hall system based on graphene is studied. This state can be regarded as a gas of superfluid electron–hole pairs whose components belong to different layers. A superfluid flow of such pairs is equivalent to two electric supercurrents in the layers. It is shown that in a graphene system a state with interlayer phase coherence arises if a definite unbalance of the filling factors of the Landau levels in neighboring layers is created. The temperature of the transition into a superfluid state, the maximum interlayer distance for which phase coherence is possible, and the critical values of the supercurrent are found. The advantages of using graphene systems instead of GaAs heterostructures to realize bilayer electron–hole superconductivity are discussed.

Critical Velocities in Two-Component Superfluid Bose Gases

Abstract On the ground of the Landau criterion we study the behavior of critical velocities in a superfluid two-component Bose gas. It is found that under motion of the components with different velocities the velocity of each component should not be lower than a minimum phase velocity of elementary excitations (s−). The Landau criterion yields a relation between the critical velocities of the components (vc1, vc2). The velocity of one or even both components may exceed s−. The maximum value of the critical velocity of a given component can be reached when the other component does not move. The approach is generalized for a two-component condensate confined in a cylindrical harmonic potential.

Charge ordering and interlayer phase coherence in quantum Hall superlattices

The possibility of the existence of states with a spontaneous interlayer phase coherence in multilayer electron systems in a high perpendicular magnetic field is investigated. It is shown that phase coherence can be established in such systems only within individual pairs of adjacent layers, while such coherence does not exist between layers of different pairs. The conditions for stability of a state with interlayer phase coherence against transition to a charge-ordered state are determined. It is shown that in a system with N⩽10 layers there is stability at any value of the interlayer distance d. For N>10 there are two intervals of stability: at sufficiently large and at sufficiently small d. For N→∞ the stability interval in the region of small d vanishes.

Superfluid state of magnetoexcitons in double layer graphene structures

The possibility of realizing a superfluid state of bound electron‐hole pairs (magnetoexcitons) with spatially separated components in a graphene double layer structure (two graphene layers separated by a dielectric layer) subjected to a strong perpendicular to the layers magnetic field is analyzed. We show that the superfluid state of magnetoexcitons may emerge only under certain imbalance of filling factors of the layers. The imbalance can be created by an electrostatic field (external gate voltage). The spectrum of elementary excitations is found and the dependence of the Berezinskii‐Kosterlitz‐Thouless transition temperature on the interlayer distance is obtained. The advantages of using the graphene double layer systems instead of double quantum well GaAs heterostructures are discussed.

Mass of an Abrikosov vortex

In the excitation of a vortex lattice in the mixed phase of Yb6 single crystals by an elastic wave, the dynamic response is found to have a negative component quadratic in the frequency; we associate this component with the vortex mass. The value of the effect is in catastrophic contradiction with the existing theoretical estimates.

Drag of superfluid current in bilayer Bose systems

An effect of nondissipative drag of a superfluid flow in a system of two Bose gases confined in two parallel quasi-two-dimensional traps is studied. Using an approach based on the introduction of density and phase operators, we compute the drag current at zero and finite temperatures for arbitrary ratio of particle densities in the adjacent layers. We demonstrate that in a system of two ring-shaped traps the “drag force” influences the drag trap in the same way as an external magnetic flux influences a superconducting ring. This allows one to use the drag effect to control persistent current states in superfluids and opens up the possibility of implementing a Bose analog of the superconducting Josephson flux qubit.

Piezoelectric mechanism of orientation of a bilayer Wigner crystal in a GaAs matrix

A mechanism for orientation of bilayer classical Wigner crystals in a piezoelectric medium is considered. For the GaAs system the piezoelectric correction to the electrostatic interaction between electrons is calculated. It is shown that taking into account the correction due to the piezoelectric effect leads to a dependence of the total energy of the electron crystal on its orientation with respect to the crystallographic axes of the surrounding matrix. A generalization of Ewald’s method is obtained for calculating the anisotropic interaction between electrons in a Wigner crystal. The method is used to calculate the energy of bilayer Wigner crystals in electron layers parallel to the crystallographic planes (001), (0–11), and (111) as a function of their orientation and the distance between layers, and the energetically most favorable orientation for all types of electron lattices in a bilayer system is found. It is shown that phase transitions between structures with different lattice symmetry in a Wigner crystal can be accompanied by a change of its orientation.

Magnetoelastic effects inNd 2 CuO 4 single crystals

We have measured the anisotropy of the spin reorientation fields inNd2CuO4 single crystals from the magnetoelastic effects. ForT>1.5K our experimental data can be understood it terms of the four-sublattice antiferromagnet model and allow to estimate the relation between the phenomenological constants of the model. ForT<1.5K we have found a manifestation of new spin-reorientation field stimulated phase transition. These transitions exist only in narrow range of the angles between the magnetic field and [100] directions and are connected most likely with theNd spins.