736 research outputs found
Crossover of magnetoconductance autocorrelation for a ballistic chaotic quantum dot
The autocorrelation function C_{\varphi,\eps}(\Delta\varphi,\,\Delta \eps)=
\langle \delta g(\varphi,\,\eps)\, \delta
g(\varphi+\Delta\varphi,\,\eps+\Delta \eps)\rangle ( and \eps are
rescaled magnetic flux and energy) for the magnetoconductance of a ballistic
chaotic quantum dot is calculated in the framework of the supersymmetric
non-linear -model. The Hamiltonian of the quantum dot is modelled by a
Gaussian random matrix. The particular form of the symmetry breaking matrix is
found to be relevant for the autocorrelation function but not for the average
conductance. Our results are valid for the complete crossover from orthogonal
to unitary symmetry and their relation with semiclassical theory and an
-matrix Brownian motion ensemble is discussed.Comment: 7 pages, no figures, accepted for publication in Europhysics Letter
In vivo magnetic resonance imaging: insights into structure and function of the central nervous system
Correlation functions of one-dimensional Bose-Fermi mixtures
We calculate the asymptotic behaviour of correlation functions as a function
of the microscopic parameters for a Bose-Fermi mixture with repulsive
interaction in one dimension. For two cases, namely polarized and unpolarized
fermions the singularities of the momentum distribution functions are
characterized as a function of the coupling constant and the relative density
of bosons.Comment: RevTeX 4, 10 pages, 2 figure
Breit-Wigner width for two interacting particles in one-dimensional random potential
For two interacting particles (TIP) in one-dimensional random potential the
dependence of the Breit-Wigner width , the local density of states and
the TIP localization length on system parameters is determined analytically.
The theoretical predictions for are confirmed by numerical
simulations.Comment: 10 pages Latex, 4 figures included. New version with extended
numerical results and discussions of earlier result
Analytical Results for Random Band Matrices with Preferential Basis
Using the supersymmetry method we analytically calculate the local density of
states, the localiztion length, the generalized inverse participation ratios,
and the distribution function of eigenvector components for the superposition
of a random band matrix with a strongly fluctuating diagonal matrix. In this
way we extend previously known results for ordinary band matrices to the class
of random band matrices with preferential basis. Our analytical results are in
good agreement with (but more general than) recent numerical findings by
Jacquod and Shepelyansky.Comment: 8 pages RevTex and 1 Figure, both uuencode
Anderson-like impurity in the one-dimensional t-J model: formation of local states and magnetic behaviour
We consider an integrable model describing an Anderson-like impurity coupled
to an open -- chain. Both the hybridization (i.e. its coupling to bulk
chain) and the local spectrum can be controlled without breaking the
integrability of the model. As the hybridization is varied, holon and spinon
bound states appear in the many body ground state. Based on the exact solution
we study the state of the impurity and its contribution to thermodynamic
quantities as a function of an applied magnetic field. Kondo behaviour in the
magnetic response of the impurity can be observed provided that its parameters
have been adjusted properly to the energy scales of the holon and spinon
excitations of the one-dimensional bulk.Comment: 32 pages, 11 figure
Theory of quasi-one dimensional imbalanced Fermi gases
We present a theory for a lattice array of weakly coupled one-dimensional
ultracold attractive Fermi gases (1D `tubes') with spin imbalance, where strong
intratube quantum fluctuations invalidate mean field theory. We first construct
an effective field theory, which treats spin-charge mixing exactly, based on
the Bethe ansatz solution of the 1D single tube problem. We show that the 1D
Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state is a two-component Luttinger
liquid, and its elementary excitations are fractional states carrying both
charge and spin. We analyze the instability of the 1D FFLO state against
inter-tube tunneling by renormalization group analysis, and find that it flows
into either a polarized Fermi liquid or a FFLO superfluid, depending on the
magnitude of interaction strength and spin imbalance. We obtain the phase
diagram of the quasi-1D system and further determine the scaling of the
superfluid transition temperature with intertube coupling.Comment: new expanded version, 8 pages, updated reference
Conductance length autocorrelation in quasi one-dimensional disordered wires
Employing techniques recently developed in the context of the Fokker--Planck
approach to electron transport in disordered systems we calculate the
conductance length correlation function
for quasi 1d wires. Our result is valid for arbitrary lengths L and .
In the metallic limit the correlation function is given by a squared
Lorentzian. In the localized regime it decays exponentially in both L and
. The correlation length is proportional to L in the metallic regime
and saturates at a value approximately given by the localization length
as .Comment: 23 pages, Revtex, two figure
Diffusion-weighted magnetic resonance imaging (MRI) without susceptibility artifacts: single-shot stimulated echo acquisition mode (STEAM) MRI with iterative reconstruction and spatial regularization
This work describes a new method for diffusion-weighted (DW) magnetic resonance imaging (MRI) without susceptibility artifacts. The technique combines a DW spin-echo module and a single-shot stimulated echo acquisition mode (STEAM) MRI readout with undersampled radial trajectories and covers a volume by a gapless series of cross-sectional slices. In a first step, optimal coil sensitivities for all slices are obtained from a series of non-DW acquisitions by nonlinear inverse reconstruction with regularization to the image and coil sensitivities of a directly neighboring slice. In a second step, these coil sensitivities are used to compute all series of non-DW and DW images by linear inverse reconstruction with spatial regularization to a neighboring image. Proof-of-principle applications to the brain (51 sections) and prostate (31 sections) of healthy subjects were realized for a protocol with two b-values and 6 gradient directions at 3 T. Including averaging the measuring times for studies of the brain at 1.0×1.0×3.0 mm3 resolution (b =1,000 s mm−2) and prostate at 1.4×1.4×3.0 mm3 resolution (b =600 s mm-2) were 2.5 min and 4.5 min, respectively. All reconstructions were accomplished online with use of a multi-GPU computer integrated into the MRI system. The resulting non-DW images, mean DW images averaged across directions and maps of the apparent diffusion coefficient confirm the absence of geometric distortions or false signal alterations and demonstrate diagnostic image quality. The novel method for DW STEAM MRI of a volume without susceptibility artifacts warrants extended clinical trials
Localized to extended states transition for two interacting particles in a two-dimensional random potential
We show by a numerical procedure that a short-range interaction induces
extended two-particle states in a two-dimensional random potential. Our
procedure treats the interaction as a perturbation and solve Dyson's equation
exactly in the subspace of doubly occupied sites. We consider long bars of
several widths and extract the macroscopic localization and correlation lengths
by an scaling analysis of the renormalized decay length of the bars. For ,
the critical disorder found is , and the critical
exponent . For two non-interacting particles we do not find any
transition and the localization length is roughly half the one-particle value,
as expected.Comment: 4 two-column pages, 4 eps figures, Revtex, to be published in
Europhys. Let
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