15 research outputs found
Fractional Quantum Hall States of Clustered Composite Fermions
The energy spectra and wavefunctions of up to 14 interacting quasielectrons
(QE's) in the Laughlin nu=1/3 fractional quantum Hall (FQH) state are
investigated using exact numerical diagonalization. It is shown that at
sufficiently high density the QE's form pairs or larger clusters. This
behavior, opposite to Laughlin correlations, invalidates the (sometimes
invoked) reapplication of the composite fermion picture to the individual QE's.
The series of finite-size incompressible ground states are identified at the QE
filling factors nu_QE=1/2, 1/3, 2/3, corresponding to the electron fillings
nu=3/8, 4/11, 5/13. The equivalent quasihole (QH) states occur at nu_QH=1/4,
1/5, 2/7, corresponding to nu=3/10, 4/13, 5/17. All these six novel FQH states
were recently discovered experimentally. Detailed analysis indicates that QE or
QH correlations in these states are different from those of well-known FQH
electron states (e.g., Laughlin or Moore-Read states), leaving the origin of
their incompressibility uncertain. Halperin's idea of Laughlin states of QP
pairs is also explored, but is does not seem adequate.Comment: 14 pages, 9 figures; revision: 1 new figure, some new references,
some new data, title chang
Crystallization of a classical two-dimensional electron system: Positional and orientational orders
Crystallization of a classical two-dimensional one-component plasma
(electrons interacting with the Coulomb repulsion in a uniform neutralizing
positive background) is investigated with a molecular dynamics simulation. The
positional and the orientational correlation functions are calculated for the
first time. We have found an indication that the solid phase has a
quasi-long-range (power-law) positional order along with a long-range
orientational order. This indicates that, although the long-range Coulomb
interaction is outside the scope of Mermin's theorem, the absence of ordinary
crystalline order at finite temperatures applies to the electron system as
well. The `hexatic' phase, which is predicted between the liquid and the solid
phases by the Kosterlitz-Thouless-Halperin-Nelson-Young theory, is also
discussed.Comment: 3 pages, 4 figures; Corrected typos; Double columne
Vertical Confinement and Evolution of Reentrant Insulating Transition in the Fractional Quantum Hall Regime
We have observed an anomalous shift of the high field reentrant insulating
phases in a two-dimensional electron system (2DES) tightly confined within a
narrow GaAs/AlGaAs quantum well. Instead of the well-known transitions into the
high field insulating states centered around , the 2DES confined
within an 80\AA-wide quantum well exhibits the transition at .
Comparably large quantum lifetime of the 2DES in narrow well discounts the
effect of disorder and points to confinement as the primary driving force
behind the evolution of the reentrant transition.Comment: 5 pages, 4 figure
Persistent edge currents for paired quantum hall states
We study the behavior of the persistent edge current for paired quantum Hall
states on the cylinder. We show that the currents are periodic with the unit
flux . At low temperatures, they exhibit anomalous oscillations in
their flux dependence.The shape of the functions converges to the sawtooth
function periodic with .Comment: RevTex 8 pages. one figure. to appear in Phys.Rev.
Anisotropic transport in unidirectional lateral superlattice around half-filling of the second Landau level
We have observed marked transport anisotropy in short period (a=92 nm)
unidirectional lateral superlattices around filling factors nu=5/2 and 7/2:
magnetoresistance shows a sharp peak for current along the modulation grating
while a dip appears for current across the grating. By altering the ratio a/l
(with l=sqrt{hbar/eB_perp} the magnetic length) via changing the electron
density n_e, it is shown that the nu=5/2 anisotropic features appear in the
range 6.6 alt a/l alt 7.2 varying their intensities, becoming most conspicuous
at a/l simeq 6.7. The peak/dip broadens with temperature roughly preserving its
height/depth up to 250 mK. Tilt experiments reveal that the structures are
slightly enhanced by an in-plane magnetic field B_| perpendicular to the
grating but are almost completely destroyed by B_| parallel to the grating. The
observations suggest the stabilization of a unidirectional charge-density-wave
or stripe phase by weak external periodic modulation at the second Landau
level.Comment: REVTeX, 5 pages, 3 figures, Some minor revisions, Added notes and
reference
Beyond paired quantum Hall states: parafermions and incompressible states in the first excited Landau level
The Pfaffian quantum Hall states, which can be viewed as involving pairing
either of spin-polarized electrons or of composite fermions, are generalized by
finding the exact ground states of certain Hamiltonians with k+1-body
interactions, for all integers k > 0. The remarkably simple wavefunctions of
these states involve clusters of k particles, and are related to correlators of
parafermion currents in two-dimensional conformal field theory. The k=2 case is
the Pfaffian. For k > 1, the quasiparticle excitations of these systems are
expected to possess nonabelian statistics, like those of the Pfaffian. For k=3,
these ground states have large overlaps with the ground states of the (2-body)
Coulomb-interaction Hamiltonian for electrons in the first excited Landau level
at total filling factors \nu=2+3/5, 2+2/5.Comment: 11 pages Revtex in two column format with 4 eps figures included in
the M
Structures for Interacting Composite Fermions: Stripes, Bubbles, and Fractional Quantum Hall Effect
Much of the present day qualitative phenomenology of the fractional quantum
Hall effect can be understood by neglecting the interactions between composite
fermions altogether. For example the fractional quantum Hall effect at
corresponds to filled composite-fermion Landau levels,and
the compressible state at to the Fermi sea of composite fermions.
Away from these filling factors, the residual interactions between composite
fermions will determine the nature of the ground state. In this article, a
model is constructed for the residual interaction between composite fermions,
and various possible states are considered in a variational approach. Our study
suggests formation of composite-fermion stripes, bubble crystals, as well as
fractional quantum Hall states for appropriate situations.Comment: 16 pages, 7 figure
Enhanced stability of the square lattice of a classical bilayer Wigner crystal
The stability and melting transition of a single layer and a bilayer crystal
consisting of charged particles interacting through a Coulomb or a screened
Coulomb potential is studied using the Monte-Carlo technique. A new melting
criterion is formulated which we show to be universal for bilayer as well as
for single layer crystals in the case of (screened) Coulomb, Lennard--Jones and
1/r^{12} repulsive inter-particle interactions. The melting temperature for the
five different lattice structures of the bilayer Wigner crystal is obtained,
and a phase diagram is constructed as a function of the interlayer distance. We
found the surprising result that the square lattice has a substantial larger
melting temperature as compared to the other lattice structures. This is a
consequence of the specific topology of the defects which are created with
increasing temperature and which have a larger energy as compared to the
defects in e.g. a hexagonal lattice.Comment: Accepted for publication in Physical Review
Linear temperature dependence of conductivity in the "insulating" regime of dilute two-dimensional holes in GaAs
The conductivity of extremely high mobility dilute two-dimensional holes in
GaAs changes linearly with temperature in the insulating side of the
metal-insulator transition. Hopping conduction, characterized by an
exponentially decreasing conductivity with decreasing temperature, is not
observed when the conductivity is smaller than . We suggest that
strong interactions in a regime close to the Wigner crystallization must be
playing a role in the unusual transport.Comment: 3 pages, 2 figure