Collapse of Spin Excitations in Quantum Hall States of Coupled Electron Double Layers

Abstract

Remarkable softenings of long wavelength intersubband spin excitations of dilute electron double layers are observed at even integer quantum Hall states. These excitations in coupled GaAs double quantum wells were probed by resonant inelastic light scattering. Their softening is attributed to enhanced exchange vertex corrections (excitonic binding) in the quantum Hall states. The collapse of the spin-density mode with dS z 0 to an energy close to the Zeeman splitting suggests the existence of unstable spin-flip intersubband excitations with dS z 1. [S0031-9007(96)02143-6] PACS numbers: 73.20.Dx, 73.20.Mf, 78.30.Fs Studies of dispersive collective excitations in the integer and fractional quantum Hall regimes offer unique insights into fundamental effects and new phenomena in electron systems of reduced dimensionality. The presence of magnetoroton minima in the dispersions of charge-density excitations (CDE) is one of many characteristic manifestations of electron-electron interactions [1,2]. The minima occur at wave vectors q Ӎ 1͞l 0 , where l 0 ͑hc͞eB͒ 1͞2 is the magnetic length and B is the magnetic field. The roton minima result from vertex corrections due to exchange Coulomb interactions in the neutral quasiparticle-quasihole pairs of the excitations. These interactions are excitonic effects that reduce the collective mode energies.In electron double layers, the introduction of the additional degree of freedom associated with layer index creates remarkable new electron correlation effects. For example, at filling factor n 1 electron bilayers show intriguing and rich phase diagrams determined by the magnitude of the symmetric-antisymmetric gap D SAS , the distance d between the layers, and the in-plane magnetic field [3][4][5]. In magnetotransport experiments, these quantum phase transitions appear as the suppression of the n 1 state [4,6]. Several theoretical works have linked the disappearance of the quantum Hall state to a vertex-correction-driven collapse of the symmetric to antisymmetric intersubband CDE energy at wave vectors q Ӎ 1͞l 0 [7]. Although not yet directly observed, the predicted CDE instability highlights the major role of excitonic effects on the dispersive collective excitations of the electron gas in the quantum Hall regime.Inelastic light scattering experiments have previously shown that vertex corrections also have a major impact on the intersubband spin-density excitation (SDE) modes of electron bilayers in double quantum wells (DQW) [8]. For such excitations a different class of vertexcorrection-driven quantum phase transitions has been predicted to occur at zero magnetic field [9]. Here the signature of the instability is the collapse of the energy of the long wavelength (q 0) intersubband SDE mode of the symmetric to antisymmetric transitions.While this zero-field instability is not observed in GaAs DQW [10], inelastic light scattering measurements in perpendicular magnetic field have uncovered marked softenings of the intersubband SDE at even n quantum Hall states [11]. A recent H...

Interlayer coherence inν=1andν=2bilayer quantum Hall states

“…The phase transition occurs seemingly between the ν = 1 + 1 compound state and the ν = 2 "coherent state". Phase transitions have also been observed at ν = 2 in optical experiments by Pellegrini et al: first they used samples with different densities [8]; second they tilted samples in the magnetic field [9]. In each of them, they have found two distinctive phases with respect to spin-excitation modes.…”

“…They have concluded a phase transition between spin polarized and unpolarized states at ν = 2. It is important to relate the phase transitions found in the magnetotransport [6] and optical [8,9] experiments, if any.In this paper, we report the results of experiments on the ν = 1 and 2 BLQH states, where we have measured the Hall-plateau width and the activation energy by changing the density in each quantum well and simultaneously tilting the sample in a magnetic field. In this way we control both the density difference and the conjugate phase difference simultaneously in a BLQH state.…”

“…They have concluded a phase transition between spin polarized and unpolarized states at ν = 2. It is important to relate the phase transitions found in the magnetotransport [6] and optical [8,9] experiments, if any.…”

SU(4) coherent effects to the canted antiferromagnetic phase in bilayer quantum Hall systems at ν=2

“…Second one is a spin singlet phase, which we call the ppin phase because it is also a fully pseudospin polarized one. They have been observed experimentally [9,10,11]. The last one is a canted antiferromagnetic phase (C-phase), which is predicted [12,13,14,15,16,17] between the spin and ppin phase in a phase diagram.…”

Dynamics and Transport Near Quantum-Critical Points

“…For technical reasons not explored here, the rotor models also describe integer spin chains, and a large class of Heisenberg antiferromagnets in higher dimensions. Finally, rotor models are also useful in double-layer quantum Hall systems [33], where superexchange effects lead to an antiferromagnetic coupling between the electrons in the two layers [34]. These systems (excluding the doublelayer quantum Hall system for which the reader is referred to the literature) will be described in more detail in the following subsections.…”