We analyze the transport properties of bilayer quantum Hall systems at total filling factor ν = 1 in drag geometries as a function of interlayer bias, in the limit where the disorder is sufficiently strong to unbind meron-antimeron pairs, the charged topological defects of the system. We compute the typical energy barrier for these objects to cross incompressible regions within the disordered system using a Hartree-Fock approach, and show how this leads to multiple activation energies when the system is biased. We then demonstrate using a bosonic Chern-Simons theory that in drag geometries, current in a single layer directly leads to forces on only two of the four types of merons, inducing dissipation only in the drive layer. Dissipation in the drag layer results from interactions among the merons, resulting in very different temperature dependences for the drag and drive layers, in qualitative agreement with experiment.PACS numbers: 73. 03.75.Lm, Double layer quantum Hall systems at filling factor ν = 1 display many properties akin to those of superfluids [1]. This behavior results from the pairing of electrons in one layer with holes in the other, producing excitons that condense into a state with interlayer coherence even in the absence of tunneling [2]. In experiments these systems display a strong interlayer tunneling peak at zero bias, reminiscent of the DC-Josephson effect [3], and vanishing single-layer resistances as temperature T → 0 in counterflow experiments [4]. Nevertheless, the "superfluidity" in this system remains imperfect: there is no truly dissipationless transport at low but finite temperature in either of these types of experiments. Recently, it has been demonstrated that this behavior may be qualitatively understood if one assumes that disorder produces unpaired merons -the analog of vortices in a thin film superfluidthat remain weakly mobile at any finite temperature [5].While these experiments strongly suggest the nearcoherence of the two layers in this system, one class of experiments has so far defied explanation in these terms. These are drag measurements, in which current is injected and removed in a single layer, and the voltage drop measured in either layer. The resulting resistances when measured as a function of temperature have roughly activated behaviors. The activation energies for the drive and drag layers behave very differently with interlayer bias: the former are asymmetric with respect to the bias direction, while the latter are roughly symmetric [6,7]. Naïvely this suggests that each layer has separate quasiparticles with different activation energies, and interlayer coherence essentially plays no role. Yet such a picture is very difficult to reconcile with the experiments described above, in which imperfect superfluidity is manifest.In this paper, we will propose a solution to this puzzle. Our approach involves a transport theory for this system in which disorder is incorporated [5] via a slowly-varying random potential (such as results from a remote doping layer), pro...

Recent experiments on strongly correlated bilayer quantum Hall systems strongly suggest that, contrary to the usual assumption, the electron spin degree of freedom is not completely frozen either in the quantum Hall or in the compressibles states that occur at filling factor ν = 1. These experiments imply that the quasiparticles at ν = 1 could have both spin and pseudospin textures i.e. they could be CP 3 skyrmions. Using a microscopic unrestricted Hartree-Fock approximation, we compute the energy of several crystal states with spin, pseudospin and mixed spin-pseudospin textures around ν = 1 as a function of interlayer separation d for different values of tunneling (∆SAS) , Zeeman (∆Z ), and bias (∆ b ) energies. We show that in some range of these parameters, crystal states involving a certain amount of spin depolarization have lower energy than the fully spin polarized crystals. We study this depolarization dependence on d, ∆SAS, ∆Z and ∆ b and discuss how it can lead to the fast NMR relaxation rate observed experimentally.

Periodic nanostructures can display the dynamics of arrays of atoms while enabling the tuning of interactions in ways not normally possible in nature. We examine one-dimensional ͑1D͒ arrays of a "synthetic atom," a one-dimensional ring with a nearest-neighbor Coulomb interaction. We consider the classical limit first, finding that arrays of singly charged rings possess antiferroelectric order at low temperatures when the charge is discrete, but that they do not order when the charge is treated as a continuous classical fluid. In the quantum limit Monte Carlo simulation suggests that the system undergoes a quantum phase transition as the interaction strength is increased. This is supported by mapping the system to the 1D transverse field Ising model. Finally, we examine the effect of magnetic fields. We find that a magnetic field can alter the electrostatic phase transition producing a ferroelectric ground state, solely through its effect of shifting the eigenenergies of the quantum problem. D R 1 1 FIG. 1. ͑Color online͒ A schematic picture of the ground state of classical point electrons for 1D array of rings. The ring radius is R and the separation is D. The 1D ordering is antiferroelectric for the infinite size system and thus has a double degenerate ground state. PHYSICAL REVIEW B 78, 075411 ͑2008͒

Domain walls formed by one dimensional array of vortex lines have been recently predicted to exist in disordered helical magnets and multiferroics. These systems are on one hand analogues to the vortex line lattices in type-II superconductors while on the other hand they propagate in the magnetic medium as a domain boundary. Using a long wavelength approach supported by numerical optimization we lay out detailed theory for dynamics and structure of such topological fluctuations at zero temperature in presence of weak disorder. We show the interaction between vortex lines is weak. This is the direct consequence of the screening of the vorticity by helical background in the system. We explain how one can use this result to understand the elasticity of the wall with a vicinal surface approach. Also we show the internal degree of freedom of this array leads to the enhancement of its mobility. We present estimates for the interaction and mobility enhancements using the microscopic parameters of the system. Finally we determine the range of velocities/force densities in which the internal movement of the vortex wall can be effective in its dynamics.