396 research outputs found
Composite Fermions and quantum Hall systems: Role of the Coulomb pseudopotential
The mean field composite Fermion (CF) picture successfully predicts angular
momenta of multiplets forming the lowest energy band in fractional quantum Hall
(FQH) systems. This success cannot be attributed to a cancellation between
Coulomb and Chern-Simons interactions beyond the mean field, because these
interactions have totally different energy scales. Rather, it results from the
behavior of the Coulomb pseudopotential V(L) (pair energy as a function of pair
angular momentum) in the lowest Landau level (LL). The class of short range
repulsive pseudopotentials is defined that lead to short range Laughlin like
correlations in many body systems and to which the CF model can be applied.
These Laughlin correlations are described quantitatively using the formalism of
fractional parentage. The discussion is illustrated with an analysis of the
energy spectra obtained in numerical diagonalization of up to eleven electrons
in the lowest and excited LL's. The qualitative difference in the behavior of
V(L) is shown to sometimes invalidate the mean field CF picture when applied to
higher LL's. For example, the nu=7/3 state is not a Laughlin nu=1/3 state in
the first excited LL. The analysis of the involved pseudopotentials also
explains the success or failure of the CF picture when applied to other systems
of charged Fermions with Coulomb repulsion, such as the Laughlin quasiparticles
in the FQH hierarchy or charged excitons in an electron-hole plasma.Comment: 27 pages, 23 figures, revised version (significant changes in text
and figures), submitted to Phil. Mag.
Spin phase diagram of the nu_e=4/11 composite fermion liquid
Spin polarization of the "second generation" nu_e=4/11 fractional quantum
Hall state (corresponding to an incompressible liquid in a one-third-filled
composite fermion Landau level) is studied by exact diagonalization. Spin phase
diagram is determined for GaAs structures of different width and electron
concentration. Transition between the polarized and partially unpolarized
states with distinct composite fermion correlations is predicted for realistic
parameters.Comment: 5 pages, 3 figure
Magnon Localization in Mattis Glass
We study the spectral and transport properties of magnons in a model of a
disordered magnet called Mattis glass, at vanishing average magnetization. We
find that in two dimensional space, the magnons are localized with the
localization length which diverges as a power of frequency at small
frequencies. In three dimensional space, the long wavelength magnons are
delocalized. In the delocalized regime in 3d (and also in 2d in a box whose
size is smaller than the relevant localization length scale) the magnons move
diffusively. The diffusion constant diverges at small frequencies. However, the
divergence is slow enough so that the thermal conductivity of a Mattis glass is
finite, and we evaluate it in this paper. This situation can be contrasted with
that of phonons in structural glasses whose contribution to thermal
conductivity is known to diverge (when inelastic scattering is neglected).Comment: 11 page
Residual interactions and correlations among Laughlin quasiparticles: Novel hierarchy states
The residual interactions between Laughlin quasiparticles can be obtained
from exact numerical diagonalization studies of small systems. The
pseudopotentials V_QP(R)$ describing the energy of interaction of QE's (or
QH's) as a function of their "relative angular momentum" R cannot support
Laughlin correlations at certain QP filling factors (e.g., nu_QE}=1/3 and
nu_QH=1/5). Because of this the novel condensed quantum fluid states observed
at nu=4/11, 4/13 and other filling fractions cannot possibly be spin polarized
Laughlin correlated QP states of the composite Fermion hierarchy. Pairing of
the QP's clearly must occur, but the exact nature of the incompressible ground
states is not completely clear.Comment: 5 pages, 2 figures, accepted for Solid State Commu
Strong Correlation to Weak Correlation Phase Transition in Bilayer Quantum Hall Systems
At small layer separations, the ground state of a nu=1 bilayer quantum Hall
system exhibits spontaneous interlayer phase coherence and has a
charged-excitation gap E_g. The evolution of this state with increasing layer
separation d has been a matter of controversy. In this letter we report on
small system exact diagonalization calculations which suggest that a single
phase transition, likely of first order, separates coherent incompressible (E_g
>0) states with strong interlayer correlations from incoherent compressible
states with weak interlayer correlations. We find a dependence of the phase
boundary on d and interlayer tunneling amplitude that is in very good agreement
with recent experiments.Comment: 4 pages, 4 figures included, version to appear in Phys. Rev. Let
Two Experimental Tests of the Halperin-Lubensky-Ma Effect at the Nematic-Smectic-A Phase Transition
We have conducted two quantitative tests of predictions based on the
Halperin-Lubensky-Ma (HLM) theory of fluctuation-induced first-order phase
transitions. First, we explore the effect of an external magnetic field on the
nematic-smectic-A (NA) transition in a liquid crystal. Second, we examine the
dependence of the first-order discontinuity as a function of mixture
concentration in pure 8CB and three 8CB-10CB mixtures. We find the first
quantitative evidence for deviations from the HLM theory.Comment: 4 pages, 2 figure
Strong enhancement of drag and dissipation at the weak- to strong- coupling phase transition in a bi-layer system at a total Landau level filling nu=1
We consider a bi-layer electronic system at a total Landau level filling
factor nu =1, and focus on the transition from the regime of weak inter-layer
coupling to that of the strongly coupled (1,1,1) phase (or ''quantum Hall
ferromagnet''). Making the assumption that in the transition region the system
is made of puddles of the (1,1,1) phase embedded in a bulk of the weakly
coupled state, we show that the transition is accompanied by a strong increase
in longitudinal Coulomb drag, that reaches a maximum of approximately
. In that regime the longitudinal drag is increased with decreasing
temperature.Comment: four pages, one included figur
Dynamic Scaling of Magnetic Flux Noise Near the KTB Transition in Overdamped Josephson Junction Arrays
We have used a dc Superconducting QUantum Interference Device to measure the
magnetic flux noise generated by the equilibrium vortex density fluctuations
associated with the Kosterlitz-Thouless-Berezinskii (KTB) transition in an
overdamped Josephson junction array. At temperatures slightly above the KTB
transition temperature, the noise is white for and scales as
for . Here , where is the correlation
length and is the dynamic exponent. Moreover, when all frequencies are
scaled by , data for different temperatures and frequencies collapse on
to a single curve. In addition, we have extracted the dynamic exponent and
found .Comment: 5 pages, LaTeX (REVTeX) format, requires epsfig and amstex style
files. 3 figures included. Tentatively scheduled for publication in Physical
Review Letters, 18 March, 199
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