943 research outputs found
Quantum detection of electronic flying qubits
We consider a model of a detector of ballistic electrons at the edge of a
two-dimensional electron gas in the integer quantum Hall regime. The electron
is detected by capacitive coupling to a gate which is also coupled to a passive
RC circuit. Using a quantum description of this circuit, we determine the
signal over noise ratio of the detector in term of the detector
characteristics. The back-action of the detector on the incident wavepacket is
then computed using a Feynman-Vernon influence functional approach. Using
information theory, we define the appropriate notion of quantum limit for such
an "on the fly" detector. We show that our particular detector can approach the
quantum limit up to logarithms in the ratio of the measurement time over the RC
relaxation time. We argue that such a weak logarithmic effect is of no
practical significance. Finally we show that a two-electron interference
experiment can be used to probe the detector induced decoherence.Comment: 15 pages, 7 figures, published versio
Quantum Hall fractions for spinless Bosons
We study the Quantum Hall phases that appear in the fast rotation limit for
Bose-Einstein condensates of spinless bosonic atoms. We use exact
diagonalization in a spherical geometry to obtain low-lying states of a small
number of bosons as a function of the angular momentum. This allows to
understand or guess the physics at a given filling fraction nu, ratio of the
number of bosons to the number of vortices. This is also the filling factor of
the lowest Landau level. In addition to the well-known Bose Laughlin state at
nu =1/2 we give evidence for the Jain principal sequence of incompressible
states at nu =p/(p+- 1) for a few values of p. There is a collective mode in
these states whose phenomenology is in agreement with standard arguments coming
e.g. from the composite fermion picture. At filling factor one, the potential
Fermi sea of composite fermions is replaced by a paired state, the Moore-Read
state. This is most clearly seen from the half-flux nature of elementary
excitations. We find that the hierarchy picture does not extend up to the point
of transition towards a vortex lattice. While we cannot conclude, we
investigate the clustered Read-Rezayi states and show evidence for
incompressible states at the expected ratio of flux vs number of Bose
particles.Comment: RevTeX 4, 11 pages, 13 figure
The spin-1 two-dimensional J1-J2 Heisenberg antiferromagnet on a triangular lattice
The spin-1 Heisenberg antiferromagnet on a triangular lattice with the
nearest- and next-nearest-neighbor couplings, and , ,
is studied in the entire range of the parameter . Mori's projection operator
technique is used as a method which retains the rotation symmetry of spin
components and does not anticipate any magnetic ordering. For zero temperature
four second-order phase transitions are observed. At the
ground state is transformed from the long-range ordered spin
structure into a state with short-range ordering, which in its turn is changed
to a long-range ordered state with the ordering vector at . For
a new transition to a state with a short-range order occurs.
This state has a large correlation length which continuously grows with
until the establishment of a long-range order happens at . In
the range , the ordering vector is incommensurate. With growing
it moves along the line to the point which is reached at . The obtained state with a long-range order can be conceived as three
interpenetrating sublattices with the spin structure on each of
them.Comment: 13 pages, 5 figures, accepted for publication in Physics Letters
quantum Heisenberg antiferromagnet on the triangular lattice: a group symmetry analysis of order by disorder
On the triangular lattice, for between and , the classical
Heisenberg model with first and second neighbor interactions presents
four-sublattice ordered ground-states. Spin-wave calculations of Chubukov and
Jolicoeur\cite{cj92} and Korshunov\cite{k93} suggest that quantum fluctuations
select amongst these states a colinear two-sublattice order. From theoretical
requirements, we develop the full symmetry analysis of the low lying levels of
the spin-1/2 Hamiltonian in the hypotheses of either a four or a two-sublattice
order. We show on the exact spectra of periodic samples ( and )
how quantum fluctuations select the colinear order from the four-sublattice
order.Comment: 15 pages, 4 figures (available upon request), Revte
Collinear N\'eel-type ordering in partially frustrated lattices
We consider two partially frustrated S = 1/2 antiferromagnetic spin systems
on the triangular and pentagonal lattices. In an elementary plaquette of the
two lattices, one bond has exchange interaction strength () whereas all other bonds have exchange interaction strength unity. We show
that for less than a critical value , collinear
N\'eel-type ordering is possible in the ground state. The ground state energy
and the excitation spectrum have been determined using linear spin wave theory
based on the Holstein-Primakoff transformation.Comment: Four pages, LaTeX, Four postscripts figures, Phys. Rev. B58, 73
(1998
- …