1,025 research outputs found
Multivariate side-band subtraction using probabilistic event weights
A common situation in experimental physics is to have a signal which can not
be separated from a non-interfering background through the use of any cut. In
this paper, we describe a procedure for determining, on an event-by-event
basis, a quality factor (-factor) that a given event originated from the
signal distribution. This procedure generalizes the "side-band" subtraction
method to higher dimensions without requiring the data to be divided into bins.
The -factors can then be used as event weights in subsequent analysis
procedures, allowing one to more directly access the true spectrum of the
signal.Comment: 17 pages, 9 figure
Collective edge modes in fractional quantum Hall systems
Over the past few years one of us (Murthy) in collaboration with R. Shankar
has developed an extended Hamiltonian formalism capable of describing the
ground state and low energy excitations in the fractional quantum Hall regime.
The Hamiltonian, expressed in terms of Composite Fermion operators,
incorporates all the nonperturbative features of the fractional Hall regime, so
that conventional many-body approximations such as Hartree-Fock and
time-dependent Hartree-Fock are applicable. We apply this formalism to develop
a microscopic theory of the collective edge modes in fractional quantum Hall
regime. We present the results for edge mode dispersions at principal filling
factors and for systems with unreconstructed edges. The
primary advantage of the method is that one works in the thermodynamic limit
right from the beginning, thus avoiding the finite-size effects which
ultimately limit exact diagonalization studies.Comment: 12 pages, 9 figures, See cond-mat/0303359 for related result
Hamiltonian Description of Composite Fermions: Magnetoexciton Dispersions
A microscopic Hamiltonian theory of the FQHE, developed by Shankar and myself
based on the fermionic Chern-Simons approach, has recently been quite
successful in calculating gaps in Fractional Quantum Hall states, and in
predicting approximate scaling relations between the gaps of different
fractions. I now apply this formalism towards computing magnetoexciton
dispersions (including spin-flip dispersions) in the , 2/5, and 3/7
gapped fractions, and find approximate agreement with numerical results. I also
analyse the evolution of these dispersions with increasing sample thickness,
modelled by a potential soft at high momenta. New results are obtained for
instabilities as a function of thickness for 2/5 and 3/7, and it is shown that
the spin-polarized 2/5 state, in contrast to the spin-polarized 1/3 state,
cannot be described as a simple quantum ferromagnet.Comment: 18 pages, 18 encapsulated ps figure
Hamiltonian theory of gaps, masses and polarization in quantum Hall states: full disclosure
I furnish details of the hamiltonian theory of the FQHE developed with Murthy
for the infrared, which I subsequently extended to all distances and apply it
to Jain fractions \nu = p/(2ps + 1). The explicit operator description in terms
of the CF allows one to answer quantitative and qualitative issues, some of
which cannot even be posed otherwise. I compute activation gaps for several
potentials, exhibit their particle hole symmetry, the profiles of charge
density in states with a quasiparticles or hole, (all in closed form) and
compare to results from trial wavefunctions and exact diagonalization. The
Hartree-Fock approximation is used since much of the nonperturbative physics is
built in at tree level. I compare the gaps to experiment and comment on the
rough equality of normalized masses near half and quarter filling. I compute
the critical fields at which the Hall system will jump from one quantized value
of polarization to another, and the polarization and relaxation rates for half
filling as a function of temperature and propose a Korringa like law. After
providing some plausibility arguments, I explore the possibility of describing
several magnetic phenomena in dirty systems with an effective potential, by
extracting a free parameter describing the potential from one data point and
then using it to predict all the others from that sample. This works to the
accuracy typical of this theory (10 -20 percent). I explain why the CF behaves
like free particle in some magnetic experiments when it is not, what exactly
the CF is made of, what one means by its dipole moment, and how the comparison
of theory to experiment must be modified to fit the peculiarities of the
quantized Hall problem
Fermionic SK-models with Hubbard interaction: Magnetism and electronic structure
Models with range-free frustrated Ising spin- and Hubbard interaction are
treated exactly by means of the discrete time slicing method. Critical and
tricritical points, correlations, and the fermion propagator, are derived as a
function of temperature T, chemical potential \mu, Hubbard coupling U, and spin
glass energy J. The phase diagram is obtained. Replica symmetry breaking
(RSB)-effects are evaluated up to four-step order (4RSB). The use of exact
relations together with the 4RSB-solutions allow to model exact solutions by
interpolation. For T=0, our numerical results provide strong evidence that the
exact density of states in the spin glass pseudogap regime obeys \rho(E)=const
|E-E_F| for energies close to the Fermi level. Rapid convergence of \rho'(E_F)
under increasing order of RSB is observed. The leading term resembles the
Efros-Shklovskii Coulomb pseudogap of localized disordered fermionic systems in
2D. Beyond half filling we obtain a quadratic dependence of the fermion filling
factor on the chemical potential. We find a half filling transition between a
phase for U>\mu, where the Fermi level lies inside the Hubbard gap, into a
phase where \mu(>U) is located at the center of the upper spin glass pseudogap
(SG-gap). For \mu>U the Hubbard gap combines with the lower one of two SG-gaps
(phase I), while for \mu<U it joins the sole SG-gap of the half-filling regime
(phase II). We predict scaling behaviour at the continuous half filling
transition. Implications of the half-filling transition between the deeper
insulating phase II and phase I for delocalization due to hopping processes in
itinerant model extensions are discussed and metal-insulator transition
scenarios described.Comment: 29 pages, 26 Figures, 4 jpeg- and 3 gif-Fig-files include
Kondo effect in coupled quantum dots: a Non-crossing approximation study
The out-of-equilibrium transport properties of a double quantum dot system in
the Kondo regime are studied theoretically by means of a two-impurity Anderson
Hamiltonian with inter-impurity hopping. The Hamiltonian, formulated in
slave-boson language, is solved by means of a generalization of the
non-crossing approximation (NCA) to the present problem. We provide benchmark
calculations of the predictions of the NCA for the linear and nonlinear
transport properties of coupled quantum dots in the Kondo regime. We give a
series of predictions that can be observed experimentally in linear and
nonlinear transport measurements through coupled quantum dots. Importantly, it
is demonstrated that measurements of the differential conductance , for the appropriate values of voltages and inter-dot tunneling
couplings, can give a direct observation of the coherent superposition between
the many-body Kondo states of each dot. This coherence can be also detected in
the linear transport through the system: the curve linear conductance vs
temperature is non-monotonic, with a maximum at a temperature
characterizing quantum coherence between both Kondo states.Comment: 20 pages, 17 figure
Theory of the first-order isostructural valence phase transitions in mixed valence compounds YbIn_{x}Ag_{1-x}Cu_{4}
For describing the first-order isostructural valence phase transition in
mixed valence compounds we develop a new approach based on the lattice Anderson
model. We take into account the Coulomb interaction between localized f and
conduction band electrons and two mechanisms of electron-lattice coupling. One
is related to the volume dependence of the hybridization. The other is related
to local deformations produced by f- shell size fluctuations accompanying
valence fluctuations. The large f -state degeneracy allows us to use the 1/N
expansion method. Within the model we develop a mean-field theory for the
first-order valence phase transition in YbInCu_{4}. It is shown that the
Coulomb interaction enhances the exchange interaction between f and conduction
band electron spins and is the driving force of the phase transition. A
comparison between the theoretical calculations and experimental measurements
of the valence change, susceptibility, specific heat, entropy, elastic
constants and volume change in YbInCu_{4} and YbAgCu_{4} are presented, and a
good quantitative agreement is found. On the basis of the model we describe the
evolution from the first-order valence phase transition to the continuous
transition into the heavy-fermion ground state in the series of compounds
YbIn_{1-x}Ag_{x}Cu_{4}. The effect of pressure on physical properties of
YbInCu_{4} is studied and the H-T phase diagram is found.Comment: 17 pages RevTeX, 9 Postscript figures, to be submitted to Phys.Rev.
Implications of LHC Searches on SUSY Particle Spectra: The pMSSM Parameter Space with Neutralino Dark Matter
We study the implications of LHC searches on SUSY particle spectra using flat
scans of the 19-parameter pMSSM phase space. We apply constraints from flavour
physics, g_mu-2, dark matter and earlier LEP and Tevatron searches. The
sensitivity of the LHC SUSY searches with jets, leptons and missing energy is
assessed by reproducing with fast simulation the recent CMS analyses after
validation on benchmark points. We present results in terms of the fraction of
pMSSM points compatible with all the constraints which are excluded by the LHC
searches with 1 fb^{-1} and 15 fb^{-1} as a function of the mass of strongly
and weakly interacting SUSY particles. We also discuss the suppression of Higgs
production cross sections for the MSSM points not excluded and contrast the
region of parameter space tested by the LHC data with the constraints from dark
matter direct detection experiments.Comment: 14 pages, 13 figures. v2: increased statistics, to appear in EPJ
Spin-Charge Separation in the Model: Magnetic and Transport Anomalies
A real spin-charge separation scheme is found based on a saddle-point state
of the model. In the one-dimensional (1D) case, such a saddle-point
reproduces the correct asymptotic correlations at the strong-coupling
fixed-point of the model. In the two-dimensional (2D) case, the transverse
gauge field confining spinon and holon is shown to be gapped at {\em finite
doping} so that a spin-charge deconfinement is obtained for its first time in
2D. The gap in the gauge fluctuation disappears at half-filling limit, where a
long-range antiferromagnetic order is recovered at zero temperature and spinons
become confined. The most interesting features of spin dynamics and transport
are exhibited at finite doping where exotic {\em residual} couplings between
spin and charge degrees of freedom lead to systematic anomalies with regard to
a Fermi-liquid system. In spin dynamics, a commensurate antiferromagnetic
fluctuation with a small, doping-dependent energy scale is found, which is
characterized in momentum space by a Gaussian peak at (, ) with
a doping-dependent width (, is the doping
concentration). This commensurate magnetic fluctuation contributes a
non-Korringa behavior for the NMR spin-lattice relaxation rate. There also
exits a characteristic temperature scale below which a pseudogap behavior
appears in the spin dynamics. Furthermore, an incommensurate magnetic
fluctuation is also obtained at a {\em finite} energy regime. In transport, a
strong short-range phase interference leads to an effective holon Lagrangian
which can give rise to a series of interesting phenomena including linear-
resistivity and Hall-angle. We discuss the striking similarities of these
theoretical features with those found in the high- cuprates and give aComment: 70 pages, RevTex, hard copies of 7 figures available upon request;
minor revisions in the text and references have been made; To be published in
July 1 issue of Phys. Rev. B52, (1995
Integrating group Delphi, fuzzy logic and expert systems for marketing strategy development:the hybridisation and its effectiveness
A hybrid approach for integrating group Delphi, fuzzy logic and expert systems for developing marketing strategies is proposed in this paper. Within this approach, the group Delphi method is employed to help groups of managers undertake SWOT analysis. Fuzzy logic is applied to fuzzify the results of SWOT analysis. Expert systems are utilised to formulate marketing strategies based upon the fuzzified strategic inputs. In addition, guidelines are also provided to help users link the hybrid approach with managerial judgement and intuition. The effectiveness of the hybrid approach has been validated with MBA and MA marketing students. It is concluded that the hybrid approach is more effective in terms of decision confidence, group consensus, helping to understand strategic factors, helping strategic thinking, and coupling analysis with judgement, etc
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