Phonon excitations of composite-fermion landau levels

Phonon excitations of fractional quantum Hall states at filling factors nu = 1/3, 2/5, 4/7, 3/5, 4/3, and 5/3 are experimentally shown to be based on Landau-level transitions of composite fermions. At filling factor nu = 2/3, however, a linear field dependence of the excitation energy in the high-field regime rather hints towards a spin transition excited by the phonons. We propose to explain this surprising observation by an only partially polarized 2/3 ground state, making the energetically lower lying spin transition also allowed for phonon excitations.     

Observation of multiple magnetorotons in the fractional quantum Hall effect

Magnetorotons in the dispersions of collective gap excitation modes of fractional quantum Hall liquids are measured in resonant inelastic light scattering experiments. Two deep magnetoroton minima are observed at nu = 2/5, while a single deep minimum is resolved at nu = 1/3. The observations are the first evidence of multiple roton minima in gap excitations of the quantum liquids. The results support Chern-Simons and composite fermion calculations that predict multiple roton minima for states with nu>1/3.     

New phase transition between partially and fully polarized quantum Hall states with charge and spin gaps at nu = 2/3

The average electron spin polarization Rho of a two-dimensional electron gas confined in GaAs/GaAlAs multiple quantum wells was measured by NMR near the fractional quantum Hall state with filling factor nu = 2/3. Above this filling factor (2/3< or = nu < 0.85), a strong depolarization is observed corresponding to two spin flips per additional flux quantum. The most remarkable behavior of the polarization is observed at nu = 2/3, where a quantum phase transition from a partially polarized (Rho approximately 3/4) to a fully polarized (Rho = 1) state can be driven by increasing the ratio between the Zeeman and the Coulomb energy above a critical value eta(c) = Delta(Z)/Delta(C) = 0.0185.     

Quantum Hall fractions in rotating Bose-Einstein condensates

We study the quantum Hall phases that appear in the dilute limit of rotating Bose-Einstein condensates. By exact diagonalization in a spherical geometry we obtain the ground state and low-lying excited states of a small number of bosons as a function of the filling fraction nu, the ratio of the number of bosons to the number of vortices. We show the occurrence of the Jain principal sequence of incompressible liquids for nu=2/3,3/4,4/3,5/4 in addition to the Laughlin state nu=1/2 as well as the Pfaffian state for nu=1. We give gap estimates by finite-size scaling of both charged and neutral excitations.     

Flavor altering excitations of composite fermions

Past theoretical studies have considered excitations of a given flavor of composite fermions across composite-fermion quasi-Landau levels. We show that in addition there exists a ladder of flavor changing excitations in which composite fermions are partially dissociated by a removal of some or all of their vortices, and suggest that these are relevant to the intriguing experimental discovery of Hirjibehedin et al. [Phys. Rev. Lett. 91, 186802 (2003)] of coexisting excitation modes of composite fermions of different flavors in the filling factor range 1/3 > nu > or = 1/5.     

Observation of quantum Hall "valley Skyrmions"

We report measurements of the interaction-induced quantum Hall effect in a spin-polarized AlAs two-dimensional electron system where the electrons occupy two in-plane conduction band valleys. Via the application of in-plane strain, we tune the energies of these valleys and measure the energy gap of the quantum Hall state at filling factor nu = 1. The gap has a finite value even at zero strain and, with strain, rises much faster than expected from a single-particle picture, suggesting that the lowest energy charged excitations at nu = 1 are "valley Skyrmions."     

Stochastization of Long Living Spin-Cyclotron Excitations in a Spin-Unpolarised Quantum Hall System

JETP Letters - We address the kinetics of long-lived excitations at zero temperature in an electronic quantum Hall system with filling factor $$\nu = 2$$ . The initial coherent state of...     

Evidence of Landau levels and interactions in low-lying excitations of composite fermions at 1/3<or= nu <or=2/5

Excitation modes in the range 2/5>or=nu>or=1/3 of the fractional quantum Hall regime are observed by resonant inelastic light scattering. Spectra of spin-reversed excitations suggest a structure of lowest spin-split Landau levels of composite fermions that is similar to that of electrons. Spin-flip energies determined from spectra reveal significant composite fermion interactions. The filling factor dependence of mode energies displays an abrupt change in the middle of the range when there is partial population of a composite fermion level.     

Light scattering observations of spin reversal excitations in the fractional quantum Hall regime

Resonant inelastic light scattering experiments access the low lying excitations of electron liquids in the fractional quantum Hall regime in the range 2/5≥ν≥1/3. Modes associated with changes in the charge and spin degrees of freedom are measured. Spectra of spin reversed excitations at filling factor ν?1/3 and at ν?2/5 identify a structure of lowest spin-split Landau levels of composite fermions (CFs) that is similar to that of electrons. Observations of spin wave excitations enable determinations of energies required to reverse spin. The spin reversal energies obtained from the spectra illustrate the significant residual interactions of composite fermions. At ν=1/3 energies of spin reversal modes are larger but relatively close to spin conserving excitations that are linked to activated transport. Predictions of composite fermion theory are in good quantitative agreement with experimental results.     

Observation of collapse of pseudospin order in bilayer quantum Hall ferromagnets

The Hartree-Fock paradigm of bilayer quantum Hall states with finite tunneling at filling factor nu=1 has full pseudospin ferromagnetic order with all the electrons in the lowest symmetric Landau level. Inelastic light scattering measurements of low energy spin excitations reveal major departures from the paradigm at relatively large tunneling gaps. The results indicate the emergence of a novel correlated quantum Hall state at nu=1 characterized by reduced pseudospin order. Marked anomalies occur in spin excitations when pseudospin polarization collapses by application of in-plane magnetic fields.     

Observation of an interedge magnetoplasmon mode in a degenerate two-dimensional electron gas

We study the propagation of edge magnetoplasmons by time-resolved current measurements in a sample which allows for selective detection of edge states in the quantum Hall regime. At filling factors close to nu=3 we observe two decoupled modes of edge excitations, one of which is related to the innermost compressible strip and is identified as an interedge magnetoplasmon mode. From the analysis of the propagation velocities of each mode the internal spatial parameters of the edge structure are derived.     

Edge excitations and non-Abelian statistics in the moore-read state: A numerical study in the presence of coulomb interaction and edge confinement

We study the ground state and low-energy excitations of fractional quantum Hall systems on a disk at a filling fraction nu = 5/2, with Coulomb interaction and a background confining potential. We find the Moore-Read ground state is stable within a finite but narrow window in parameter space. The corresponding low-energy excitations contain a fermionic branch and a bosonic branch, with widely different velocities. A short-range repulsive potential can stabilize a charge +e/4 quasihole at the center, leading to a different edge excitation spectrum due to the change of boundary conditions for Majorana fermions, clearly indicating the non-Abelian nature of the quasihole.     

Splitting of long-wavelength modes of the fractional quantum hall liquid at nu = 1/3

Resonant inelastic light scattering experiments at nu = 1/3 reveal a novel splitting of the long-wavelength modes in the low energy spectrum of quasi-particle excitations in the charge degree of freedom. We find a single peak at small wave vectors that splits into two distinct modes at larger wave vectors. The evidence of well-defined dispersive behavior at small wave vectors indicates a coherence of the quantum fluid in the micron length scale. We evaluate interpretations of long-wavelength modes of the electron liquid.     

Transition from an electron solid to the sequence of fractional quantum Hall states at very low Landau level filling factor

At low Landau level filling of a two-dimensional electron system, typically associated with the formation of an electron crystal, we observe local minima in Rxx at filling factors nu = 2/11, 3/17, 3/19, 2/13, 1/7, 2/15, 2/17, and 1/9. Each of these developing fractional quantum Hall (FQHE) states appears only above a filling-factor-specific temperature. This can be interpreted as the melting of an electron crystal and subsequent FQHE liquid formation. The observed sequence of FQHE states follows the series of composite fermion states emanating from nu = 1/6 and nu = 1/8.     

Partition noise and statistics in the fractional quantum hall effect

A microscopic theory of current partition in fractional quantum Hall liquids, described by chiral Luttinger liquids, is developed to compute the noise correlations, using the Keldysh technique. In this Hanbury-Brown and Twiss geometry, at Laughlin filling factors nu = 1/3, the real time noise correlator exhibits oscillations which persist over larger time scales than that of an uncorrelated Hall fluid. The zero frequency noise correlations are negative at filling factor 1/3 as for bare electrons (antibunching), but are strongly reduced in amplitude. These correlations become positive (bunching) for nu < or = 1/5, suggesting a tendency towards bosonic behavior.     

Spin texture and magnetoroton excitations at nu=1/3

Neutral spin texture (ST) excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin flips. At low magnetic fields, the ST energy is below that of the magnetoroton minimum. With increasing in-plane magnetic field these mode energies cross at a critical ratio of the Zeeman and Coulomb energies of eta(c)=0.020+/-0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magnetoroton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3.     

Evolving dark energy with w not = -1

Theories of evolving quintessence are constructed that generically lead to deviations from the w = -1 prediction of nonevolving dark energy. The small mass scale that governs evolution, m(phi) approximately = 10(-33) eV, is radiatively stable, and the "Why now?" problem is solved. These results rest on seesaw cosmology: Fundamental physics and cosmology can be broadly understood from only two mass scales, the weak scale nu and the Planck scale M. Requiring a scale of dark energy rho(DE)(1/4) governed by nu2/M and a radiatively stable evolution rate m(phi) given by nu4/M3 leads to a distinctive form for the equation of state w(z). Dark energy resides in the potential of a hidden axion field that is generated by a new QCD-like force that gets strong at the scale lambda approximately = nu2/M approximately = rho(DE)(1/4). The evolution rate is given by a second seesaw that leads to the axion mass m(phi) approximately = lambda2/f, with f approximately = M.     

Electron correlation in the second Landau level: a competition between many nearly degenerate quantum phases

At a very low-temperature of 9 mK, electrons in the second Landau level of an extremely high-mobility two-dimensional electron system exhibit a very complex electronic behavior. With a varying filling factor, quantum liquids of different origins compete with several insulating phases leading to an irregular pattern in the transport parameters. We observe a fully developed nu=2+2/5 state separated from the even-denominator nu=2+1/2 state by an insulating phase and a nu=2+2/7 and nu=2+1/5 state surrounded by such phases. A developing plateau at nu=2+3/8 points to the existence of other even-denominator states.     

Multichannel Kondo models in non-Abelian quantum Hall droplets

We study the coupling between a quantum dot and the edge of a non-Abelian fractional quantum Hall state which is spatially separated from it by an integer quantum Hall state. Near a resonance, the physics at energy scales below the level spacing of the edge states of the dot is governed by a k-channel Kondo model when the quantum Hall state is a Read-Rezayi state at filling fraction nu=2+k/(k+2) or its particle-hole conjugate at nu=2+2/(k+2). The k-channel Kondo model is channel isotropic even without fine-tuning in the former state; in the latter, it is generically channel anisotropic. In the special case of k=2, our results provide a new venue, realized in a mesoscopic context, to distinguish between the Pfaffian and anti-Pfaffian states at filling fraction nu=5/2.