338 research outputs found
Phase Diagram for Ultracold Bosons in Optical Lattices and Superlattices
We present an analytic description of the finite-temperature phase diagram of
the Bose-Hubbard model, successfully describing the physics of cold bosonic
atoms trapped in optical lattices and superlattices. Based on a standard
statistical mechanics approach, we provide the exact expression for the
boundary between the superfluid and the normal fluid by solving the
self-consistency equations involved in the mean-field approximation to the
Bose-Hubbard model. The zero-temperature limit of such result supplies an
analytic expression for the Mott lobes of superlattices, characterized by a
critical fractional filling.Comment: 8 pages, 6 figures, submitted to Phys. Rev.
DMRG evaluation of the Kubo formula -- Conductance of strongly interacting quantum systems
In this paper we present a novel approach combining linear response theory
(Kubo) for the conductance and the Density Matrix Renormalization Group (DMRG).
The system considered is one-dimensional and consists of non-interacting tight
binding leads coupled to an interacting nanostructure via weak links. Electrons
are treated as spinless fermions and two different correlation functions are
used to evaluate the conductance.
Exact diagonalization calculations in the non-interacting limit serve as a
benchmark for our combined Kubo and DMRG approach in this limit. Including both
weak and strong interaction we present DMRG results for an extended
nanostructure consisting of seven sites. For the strongly interacting structure
a simple explanation of the position of the resonances is given in terms of
hard-core particles moving freely on a lattice of reduced size.Comment: 7 pages, 2 figures. Minor typos correcte
Density Matrices for a Chain of Oscillators
We consider chains with an optical phonon spectrum and study the reduced
density matrices which occur in density-matrix renormalization group (DMRG)
calculations. Both for one site and for half of the chain, these are found to
be exponentials of bosonic operators. Their spectra, which are correspondingly
exponential, are determined and discussed. The results for large systems are
obtained from the relation to a two-dimensional Gaussian model.Comment: 15 pages,8 figure
The Density Matrix Renormalization Group applied to single-particle Quantum Mechanics
A simplified version of White's Density Matrix Renormalization Group (DMRG)
algorithm has been used to find the ground state of the free particle on a
tight-binding lattice. We generalize this algorithm to treat the tight-binding
particle in an arbitrary potential and to find excited states. We thereby solve
a discretized version of the single-particle Schr\"odinger equation, which we
can then take to the continuum limit. This allows us to obtain very accurate
results for the lowest energy levels of the quantum harmonic oscillator,
anharmonic oscillator and double-well potential. We compare the DMRG results
thus obtained with those achieved by other methods.Comment: REVTEX file, 21 pages, 3 Tables, 4 eps Figure
Commensurate-incommensurate transition of cold atoms in an optical lattice
An atomic gas subject to a commensurate periodic potential generated by an
optical lattice undergoes a superfluid--Mott insulator transition. Confining a
strongly interacting gas to one dimension generates an instability where an
arbitrary weak potential is sufficient to pin the atoms into the Mott state;
here, we derive the corresponding phase diagram. The commensurate pinned state
may be detected via its finite excitation gap and the Bragg peaks in the static
structure factor.Comment: 4 pages, 2 figure
Planar fiber-chip-coupling using angle-polished polarization maintaining fibers
We report on our latest developments of a planar fiber-chip-coupling scheme, using angle polished, polarization maintaining (PM) fibers. Most integrated photonic chip components are polarization sensitive and a suitable way to launch several wavelength channels with the same polarization to the chip is the use of PM fibers. Those impose several challenges at processing and handling to achieve a stable, permanent, and low-loss coupling. We present the processing of the fibers in detail and experimental results for our planar and compact fiber-chip-coupling technique
First 10Gb/s Transmission with radiation-hardened Silicon Photonic Mach-Zehnder Modulators in a Full Transmission System
We present standard and radiation-hardened pn depletion-type Mach-Zehnder modulators (MZM) to be used in high-bandwidth, fully-integrated wavelength division multiplexing (WDM) transmitter designs, aiming to upgrade the optical data transmission of future detector systems. A detailed characterization of the modulators with respect to modulation efficiency and RF response was carried out. Based on this work, a first optical link with a data rate of 11.3 Gb/s was set up
A high-performance Echelle grating de-multiplexer based on two stigmatic points and its flat-top solution
Building blocks of a scalable and radiation-hardened integrated transmitter unit based on 250 nm SOI
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