2,276 research outputs found
Experimental determination of dipole moments for molecular ions: Improved measurements for ArH^+
An improved value for the dipole moment of ArH^+ has been obtained from new measurements of the rotational g factors of ArH^+ and ArD^+ made with tunable farâIR laser spectroscopy. Systematic errors present in earlier measurements have been eliminated. The new result (ÎŒ=3.0±0.6 D) is slightly higher than the ab initio value of Rosmus (2.2 D) at the 2Ï limits of precision
Fractional Quantum Hall Effect and vortex lattices
It is demonstrated that all observed fractions at moderate Landau level
fillings for the quantum Hall effect can be obtained without recourse to the
phenomenological concept of composite fermions. The possibility to have the
special topologically nontrivial many-electron wave functions is considered.
Their group classification indicates the special values of of electron density
in the ground states separated by a gap from excited states
Energy-efficient coding with discrete stochastic events
We investigate the energy efficiency of signaling mechanisms that transfer information by means of discrete stochastic events, such as the opening or closing of an ion channel. Using a simple model for the generation of graded electrical signals by sodium and potassium channels, we find optimum numbers of channels that maximize energy efficiency. The optima depend on several factors: the relative magnitudes of the signaling cost (current flow through channels), the fixed cost of maintaining the system, the reliability of the input, additional sources of noise, and the relative costs of upstream and downstream mechanisms. We also analyze how the statistics of input signals influence energy efficiency. We find that energy-efficient signal ensembles favor a bimodal distribution of channel activations and contain only a very small fraction of large inputs when energy is scarce. We conclude that when energy use is a significant constraint, trade-offs between information transfer and energy can strongly influence the number of signaling molecules and synapses used by neurons and the manner in which these mechanisms represent information
Hierarchy wave functions--from conformal correlators to Tao-Thouless states
Laughlin's wave functions, describing the fractional quantum Hall effect at
filling factors , can be obtained as correlation functions in
conformal field theory, and recently this construction was extended to Jain's
composite fermion wave functions at filling factors . Here we
generalize this latter construction and present ground state wave functions for
all quantum Hall hierarchy states that are obtained by successive condensation
of quasielectrons (as opposed to quasiholes) in the original hierarchy
construction. By considering these wave functions on a cylinder, we show that
they approach the exact ground states, the Tao-Thouless states, when the
cylinder becomes thin. We also present wave functions for the multi-hole
states, make the connection to Wen's general classification of abelian quantum
Hall fluids, and discuss whether the fractional statistics of the
quasiparticles can be analytically determined. Finally we discuss to what
extent our wave functions can be described in the language of composite
fermions.Comment: 9 page
Band Structure of the Fractional Quantum Hall Effect
The eigenstates of interacting electrons in the fractional quantum Hall phase
typically form fairly well defined bands in the energy space. We show that the
composite fermion theory gives insight into the origin of these bands and
provides an accurate and complete microscopic description of the strongly
correlated many-body states in the low-energy bands. Thus, somewhat like in
Landau's fermi liquid theory, there is a one-to-one correspondence between the
low energy Hilbert space of strongly interacting electrons in the fractinal
quantum Hall regime and that of weakly interacting electrons in the integer
quantum Hall regime.Comment: 10 page
A First-Landau-Level Laughlin/Jain Wave Function for the Fractional Quantum Hall Effect
We show that the introduction of a more general closed-shell operator allows
one to extend Laughlin's wave function to account for the richer hierarchies
(1/3, 2/5, 3/7 ...; 1/5, 2/9, 3/13, ..., etc.) found experimentally. The
construction identifies the special hierarchy states with condensates of
correlated electron clusters. This clustering implies a single-particle (ls)j
algebra within the first Landau level (LL) identical to that of multiply filled
LLs in the integer quantum Hall effect. The end result is a simple generalized
wave function that reproduces the results of both Laughlin and Jain, without
reference to higher LLs or projection.Comment: Revtex. In this replacement we show how to generate the Jain wave
function explicitly, by acting with the generalized ls closed-shell operator
discussed in the original version. We also walk the reader through a
classical 1d caricature of this problem so that he/she can better understand
why 2s+1, where s is the spin, should be associated with the number of
electrons associated with the underlying clusters or composites. 11 page
Laboratory measurement of the pure rotational spectrum of vibrationally excited HCO^+ (v_2 = 1) by far-infrared laser sideband spectroscopy
Laboratory observations of the pure rotational spectrum of HCO^+ in its lowest excited bending state
(v_1, v^l_2 v_3)_= (0,1^1,0) are reported. Because of their severe excitation requirements, such vibrational satellites
and the high-J ground-state lines also measured here sample only hot, dense regions of matter in active molecular
cloud cores and circumstellar envelopes. As the HCO^+ abundance is tied directly to the gas fractional ionization, it is probable that the vibrationally excited formyl ion transitions will provide high-contrast
observations of shocked molecular material, rather than the more quiescent, radiatively heated gas surrounding
stellar sources detected with the few vibrationally excited neutral species observed to date
Quantum Hall quasielectron operators in conformal field theory
In the conformal field theory (CFT) approach to the quantum Hall effect, the
multi-electron wave functions are expressed as correlation functions in certain
rational CFTs. While this approach has led to a well-understood description of
the fractionally charged quasihole excitations, the quasielectrons have turned
out to be much harder to handle. In particular, forming quasielectron states
requires non-local operators, in sharp contrast to quasiholes that can be
created by local chiral vertex operators. In both cases, the operators are
strongly constrained by general requirements of symmetry, braiding and fusion.
Here we construct a quasielectron operator satisfying these demands and show
that it reproduces known good quasiparticle wave functions, as well as predicts
new ones. In particular we propose explicit wave functions for quasielectron
excitations of the Moore-Read Pfaffian state. Further, this operator allows us
to explicitly express the composite fermion wave functions in the positive Jain
series in hierarchical form, thus settling a longtime controversy. We also
critically discuss the status of the fractional statistics of quasiparticles in
the Abelian hierarchical quantum Hall states, and argue that our construction
of localized quasielectron states sheds new light on their statistics. At the
technical level we introduce a generalized normal ordering, that allows us to
"fuse" an electron operator with the inverse of an hole operator, and also an
alternative approach to the background charge needed to neutralize CFT
correlators. As a result we get a fully holomorphic CFT representation of a
large set of quantum Hall wave functions.Comment: minor changes, publishe
Statistical Interparticle Potential between Two Anyons
The density matrix of a two-anyon system is evaluated and used to investigate
the "statistical interparticle potential" following the theory of Uhlenbeck.
The main purpose is to see how the statistical potential will depend on the
fractional statistical parameter . The result shows that the
statistical potential for a two-anyon system with is
always repulsive. For the system with , the potential is
repulsive at short distances and becomes attractive at long distances. It
remains only in the boson system () that the repulsive potential
arising from the exclusion principle can disappear and lead to an attractive
potential at all distances.Comment: Latex 5 pages, correct typos and figur
Tunable far-infrared laser spectroscopy of hydrogen bonds: The K_a = O(u)â1(g) rotation-tunneling spectrum of the HCI dimer
The ground state K_a =0(u)â1(g) bâtype subband of the rotationâtunneling spectrum of the symmetric ^(35)Clâ^(35)Cl,^(37)Clâ^(37)Cl, and the mixed ^(35)Clâ^(37)Cl hydrogen chloride dimers have been recorded near 26.3 cm^(â1) with subâDoppler resolution in a continuous twoâdimensional supersonic jet with a tunable farâinfrared laser spectrometer. Quadrupole hyperfine structure from the chlorine nuclei has been resolved. From the fitted rotational constants a (H^(35)Cl)_2 centerâofâmass separation of 3.81 Ă
is derived for the K_a =1(g) levels, while the nuclear quadrupole coupling constants yield a vibrationally averaged angular structure for both tunneling states of approximately 20â25 deg for the hydrogen bonded proton and at least 70â75 deg for the external proton. This nearly orthogonal structure agrees well with that predicted by ab initio theoretical calculations, but the observed splittings and intensity alterations of the lines indicate that the chlorine nuclei are made equivalent by a large amplitude tunneling motion of the HCl monomers. A similar geared internal rotation tunneling motion has been found for the HF dimer, but here the effect is much greater. The ground state tunneling splittings are estimated to lie between 15â18 cm^(â1), and the selection rules observed indicate that the trans tunneling path dominates the large amplitude motion, as expected, provided the dimer remains planar. From the observed hyperfine constants, we judge the dimer and its associated tunneling motion to be planar to within 10°
- âŠ