arXiv subjects

R. Cote

Publications and source records attributed to R. Cote.

33 records · Page 2Linked to original sources

arXiv2002

Commensurate-incommensurate transitions of quantum Hall stripe states in double-quantum-well systems

In higher Landau levels (N>0) and around filling factors nu =4N+1, a two-dimensional electron gas in a double-quantum-well system supports a stripe groundstate in which the electron density in each well is spatially modulated. When a parallel magnetic field is added in the plane of the wells, tunneling between the wells acts as a spatially rotating effective Zeeman field coupled to the ``pseudospins'' describing the well index of the electron states. For small parallel fields, these pseudospins follow this rotation, but at larger fields they do not, and a commensurate-incommensurate transition results. Working in the Hartree-Fock approximation, we show that the combination of stripes and commensuration in this system leads to a very rich phase diagram. The parallel magnetic field is responsible for oscillations in the tunneling matrix element that induce a complex sequence of transitions between commensurate and incommensurate liquid or stripe states. The homogeneous and stripe states we find can be distinguished by their collective excitations and tunneling I-V, which we compute within the time-dependent Hartree-Fock approximation.

cond-mat.mes-hall↗

arXiv2001

Dynamics of quantum Hall stripes in double-quantum-well systems

The collective modes of stripes in double layer quantum Hall systems are computed using the time-dependent Hartree-Fock approximation. It is found that, when the system possesses spontaneous interlayer coherence, there are two gapless modes, one a phonon associated with broken translational invariance, the other a pseudospin-wave associated with a broken U(1) symmetry. For large layer separations the modes disperse weakly for wavevectors perpendicular to the stripe orientation, indicating the system becomes akin to an array of weakly coupled one-dimensional XY systems. At higher wavevectors the collective modes develop a roton minimum associated with a transition out of the coherent state with further increasing layer separation. A spin wave model of the system is developed, and it is shown that the collective modes may be described as those of a system with helimagnetic ordering.

cond-mat.mes-hall↗

arXiv2001

Optical absorption in the soliton-lattice state of a double-quantum-well system

When the separation between layers in a double-quantum-well system is sufficiently small, the ground state of the two-dimensional electron gas at filling factor 1 has an interwell phase coherence even in the absence of tunneling. For non-zero tunneling, this coherent state goes through a commensurate-incommensurate transition as the sample is tilted with respect to the quantizing magnetic field at filling factor 1. In this article, we compute the optical (infrared) absorption spectrum of the coherent state from the commensurate state at small tilt angle to the soliton-lattice state at larger tilt angle and comment on the possibility of observing experimentally the distinctive signature of the soliton lattice.

cond-mat.mes-hall↗

arXiv2000

Dipole Blockade and Quantum Information Processing in Mesoscopic Atomic Ensembles

We describe a technique for manipulating quantum information stored in collective states of mesoscopic ensembles. Quantum processing is accomplished by optical excitation into states with strong dipole-dipole interactions. The resulting ``dipole blockade'' can be used to inhibit transitions into all but singly excited collective states. This can be employed for a controlled generation of collective atomic spin states as well as non-classical photonic states and for scalable quantum logic gates. An example involving a cold Rydberg gas is analyzed.

quant-ph↗

arXiv2000

Fast quantum gates for neutral atoms

We propose several schemes for implementing a fast two-qubit quantum gate for neutral atoms with the gate operation time much faster than the time scales associated with the external motion of the atoms in the trapping potential. In our example, the large interaction energy required to perform fast gate operations is provided by the dipole-dipole interaction of atoms excited to low-lying Rydberg states in constant electric fields. A detailed analysis of imperfections of the gate operation is given.

quant-ph↗

arXiv2000

Stability of the Smectic Quantum Hall State: A Quantitative Study

We present an effective elastic theory which {\em quantitatively} describes the stripe phase of the two-dimensional electron gas in high Landau levels ($N\geq2$). The dynamical matrix is obtained with remarkably high precision from the density-density correlation function in the time-dependent Hartree-Fock approximation. A renormalization group analysis shows that at T=0, as the partial filling factor $Δν\equivν-\lfloorν\rfloor$ moves away from 1/2, the anisotropic conducting state may undergo quantum phase transitions: stripes may get pinned along their conducting direction by disorder, or may lock into one another to form a two-dimensional crystal. The model predicts values of $Δν$ for each transition. The transitions should be reflected in the temperature dependence of the dissipative conductivity.

cond-mat.mes-hall↗

arXiv2000

Collective Modes of Quantum Hall Stripes

The collective modes of striped phases in a quantum Hall system are computed using the time-dependent Hartree-Fock approximation. Uniform stripe phases are shown to be unstable to the formation of modulations along the stripes, so that within the Hartree-Fock approximation the groundstate is a stripe crystal. Such crystalline states are generically gapped at any finite wavevector; however, in the quantum Hall system the interactions of modulations among different stripes is found to be remarkably weak, leading to an infinite collection of collective modes with immeasurably small gaps. The resulting long wavelength behavior is derivable from an elastic theory for smectic liquid crystals. Collective modes for the phonon branch are computed throughout the Brillouin zone, as are spin wave and magnetoplasmon modes. A soft mode in the phonon spectrum is identified for partial filling factors sufficiently far from 1/2, indicating a second order phase transition. The modes contain several other signatures that should be experimentally observable.

cond-mat.mes-hall↗

arXiv1997

Superfluidity in sympathetic cooling with atomic Bose condensates

The dynamical structure of an atomic Bose-Einstein condensate limits the efficiency of the condensate in cooling slow impurity atoms. To illustrate the point, we show that an impurity atom moving in a homogeneous zero-temperature condensate is not scattered incoherently if its velocity is lower than the condensate sound velocity $c$, limiting cooling to velocities $v \geq c$. This striking effect is an expression of superfluidity and provides a direct means to detect the fundamental property of superfluidity in atomic condensates. Furthermore, we show that the fermionic lithium-isotope, $^{6}$Li, is a reasonable candidate for sympathetic cooling by a $^{23}$Na-condensate.

cond-mat↗

arXiv1997

Collective Excitations, NMR, and Phase Transitions in Skyrme Crystals

At Landau level filling factors near nu =1, quantum Hall ferromagnets form a Skyrme crystal state with quasi-long-range translational and non-collinear magnetic order. We develop an effective low energy theory which explains the presence in these systems of magnetic excitations at low energies below the Larmor gap (Delta) and which predicts a dramatic enhancement of the nuclear spin relaxation rate by a factor of 1000. The effective theory predicts a rich set of quantum and classical phase transitions. Based in part on accurate time-dependent Hartree-Fock calculations of the ordered state collective excitation spectrum, we discuss aspects of the T-nu-Delta crystal phase diagram.

cond-mat.mes-hall↗

arXiv1996

Hartree-Fock Theory of Skyrmions in Quantum Hall Ferromagnets

We report on a study of the charged-skyrmion or spin-texture excitations which occur in quantum Hall ferromagnets near odd Landau level filling factors. Particle-hole symmetry is used to relate the spin-quantum numbers of charged particle and hole excitations and neutral particle-hole pair excitations. Hartree-Fock theory is used to provide quantitative estimates of the energies of these excitations and their dependence on Zeeman coupling strength, Landau level quantum numbers, and the thicknesses of the two-dimensional electron layers. For the case of $ν$ near three we suggest the possibility of first order phase transitions with increasing Zeeman coupling strength from a many skyrmion state to one with many maximally spin-polarized quasiparticles.

cond-mat.mes-hall↗

arXiv1996

Internal Excitations and Dissipative Damping of Quantum Hall Skyrmions

We propose an intrinsic maximum speed for dissipationless E cross B drift of Skyrmion quasiparticles in quantum Hall ferromagnets. When this speed is exceeded, Skyrmions can radiate spin-waves by making internal excitations which allow total spin to be conserved. Our proposal is illustrated by a time-dependent Hartree-Fock approximation calculation of the excitation spectrum for a Skyrmion bound to an impurity.

cond-mat↗

arXiv1995

Spin-ordering and magnon collective modes for two-dimensional electron lattices in strong magnetic fields

We study the spin-ordering and the magnon collective modes of the two-dimensional Wigner crystal state at strong magnetic fields. Our work is based on the Hartree-Fock approximation for the ground state and the time-dependent Hartree-Fock approximation for the collective modes. We find that the ground state is ferromagnetic, i.e that all spins are aligned at T=0 even when the electronic g-factor is negligibly small. The magnon calculations show that the spin-stiffness is much smaller in the crystal state than in fluid states which occur at nearby Landau level filling factors.

cond-mat↗

arXiv1995

Skyrme Crystal In A Two-Dimensional Electron Gas

The ground state of a two-dimensional electron gas at Landau level filling factors near $ν=1$ is a Skyrme crystal with long range order in the positions and orientations of the topologically and electrically charged elementary excitations of the $ν=1$ ferromagnetic ground state. The lowest energy Skyrme crystal is a square lattice with opposing postures for topological excitations on opposite sublattices. The filling factor dependence of the electron spin-polarization, calculated for the square lattice Skyrme crystal, is in excellent agreement with recent experiments.

cond-mat↗

arXiv1994

Collective Modes of Soliton-Lattice States in Double-Quantum-Well Systems

In strong perpendicular magnetic fields double-quantum-well systems can sometimes occur in unusual broken symmetry states which have interwell phase coherence in the absence of interwell hopping. When hopping is present in such systems and the magnetic field is tilted away from the normal to the quantum well planes, a related soliton-lattice state can occur which has kinks in the dependence of the relative phase between electrons in opposite layers on the coordinate perpendicular to the in-plane component of the magnetic field. In this article we evaluate the collective modes of this soliton-lattice state in the generalized random-phase aproximation. We find that, in addition to the Goldstone modes associated with the broken translational symmetry of the soliton-lattice state, higher energy collective modes occur which are closely related to the Goldstone modes present in the spontaneously phase-coherent state. We study the evolution of these collective modes as a function of the strength of the in-plane magnetic field and comment on the possibility of using the in-plane field to generate a finite wave probe of the spontaneously phase-coherent state.

cond-mat↗

arXiv1994

Spiral Magnets as Gapless Mott Insulators

In the large $U$ limit, the ground state of the half-filled, nearest-neighbor Hubbard model on the triangular lattice is the three-sublattice antiferromagnet. In sharp contrast with the square-lattice case, where transverse spin-waves and charge excitations remain decoupled to all orders in $t/U$, it is shown that beyond leading order in $t/U$ the three Goldstone modes on the triangular lattice are a linear combination of spin and charge. This leads to non-vanishing conductivity at any finite frequency, even though the magnet remains insulating at zero frequency. More generally, non-collinear spin order should lead to such gapless insulating behavior.

cond-mat↗