949 research outputs found
Many-body localization characterized from a one-particle perspective
We show that the one-particle density matrix can be used to
characterize the interaction-driven many-body localization transition in closed
fermionic systems. The natural orbitals (the eigenstates of ) are
localized in the many-body localized phase and spread out when one enters the
delocalized phase, while the occupation spectrum (the set of eigenvalues of
) reveals the distinctive Fock-space structure of the many-body
eigenstates, exhibiting a step-like discontinuity in the localized phase. The
associated one-particle occupation entropy is small in the localized phase and
large in the delocalized phase, with diverging fluctuations at the transition.
We analyze the inverse participation ratio of the natural orbitals and find
that it is independent of system size in the localized phase.Comment: 5 pages, 3 figures; v2: added two appendices and a new figure panel
in main text; v3: updated figur
Finite-temperature charge transport in the one-dimensional Hubbard model
We study the charge conductivity of the one-dimensional repulsive Hubbard
model at finite temperature using the method of dynamical quantum typicality,
focusing at half filling. This numerical approach allows us to obtain current
autocorrelation functions from systems with as many as 18 sites, way beyond the
range of standard exact diagonalization. Our data clearly suggest that the
charge Drude weight vanishes with a power law as a function of system size. The
low-frequency dependence of the conductivity is consistent with a finite dc
value and thus with diffusion, despite large finite-size effects. Furthermore,
we consider the mass-imbalanced Hubbard model for which the charge Drude weight
decays exponentially with system size, as expected for a non-integrable model.
We analyze the conductivity and diffusion constant as a function of the mass
imbalance and we observe that the conductivity of the lighter component
decreases exponentially fast with the mass-imbalance ratio. While in the
extreme limit of immobile heavy particles, the Falicov-Kimball model, there is
an effective Anderson-localization mechanism leading to a vanishing
conductivity of the lighter species, we resolve finite conductivities for an
inverse mass ratio of .Comment: 13 pages, 11 figure
Magnetic heat conductivity in : linear temperature dependence
We present experimental results for the thermal conductivity of the
pseudo 2-leg ladder material . The strong buckling of the ladder
rungs renders this material a good approximation to a Heisenberg-chain.
Despite a strong suppression of the thermal conductivity of this material in
all crystal directions due to inherent disorder, we find a dominant magnetic
contribution along the chain direction.
is \textit{linear} in temperature, resembling the
low-temperature limit of the thermal Drude weight of the
Heisenberg chain. The comparison of and
yields a magnetic mean free path of \AA, in good agreement with magnetic measurements.Comment: appears in PR
Frustrated ferromagnetic spin-1/2 chain in a magnetic field: The phase diagram and thermodynamic properties
The frustrated ferromagnetic spin-1/2 Heisenberg chain is studied by means of
a low-energy field theory as well as the density-matrix renormalization group
and exact diagonalization methods. Firstly, we study the ground-state phase
diagram in a magnetic field and find an `even-odd' (EO) phase characterized by
bound pairs of magnons in the region of two weakly coupled antiferromagnetic
chains. A jump in the magnetization curves signals a first-order transition at
the boundary of the EO phase, but otherwise the curves are smooth. Secondly, we
discuss thermodynamic properties at zero field, where we confirm a double-peak
structure in the specific heat for moderate frustrating next-nearest neighbor
interactions.Comment: 4 pages RevTex4, 4 figures. Minor changes, title modified. Additional
material is available here:
http://www.theorie.physik.uni-goettingen.de/~honecker/j1j2-td
Diamond electro-optomechanical resonators integrated in nanophotonic circuits
Diamond integrated photonic devices are promising candidates for emerging
applications in nanophotonics and quantum optics. Here we demonstrate active
modulation of diamond nanophotonic circuits by exploiting mechanical degrees of
freedom in free-standing diamond electro-optomechanical resonators. We obtain
high quality factors up to 9600, allowing us to read out the driven
nanomechanical response with integrated optical interferometers with high
sensitivity. We are able to excite higher order mechanical modes up to 115 MHz
and observe the nanomechanical response also under ambient conditions.Comment: 15 pages, 4 figure
Long-time behavior of the momentum distribution during the sudden expansion of a spin-imbalanced Fermi gas in one dimension
We study the sudden expansion of spin-imbalanced ultracold lattice fermions
with attractive interactions in one dimension after turning off the
longitudinal confining potential. We show that the momentum distribution
functions of majority and minority fermions approach stationary values quickly
due to a quantum distillation mechanism that results in a spatial separation of
pairs and majority fermions. As a consequence, Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) correlations are lost during the expansion. Furthermore, we argue that
the shape of the stationary momentum distribution functions can be understood
by relating them to the integrals of motion in this integrable quantum system.
We discuss our results in the context of proposals to observe FFLO
correlations, related to recent experiments by Liao et al., Nature 467, 567
(2010).Comment: 8 pages including supplementary material, 9 eps figures, revised
version as published, some text moved to the supplemental materia
Universal emergence of the one-third plateau in the magnetization process of frustrated quantum spin chains
We present a numerical study of the magnetization process of frustrated
quantum spin-S chains with S=1, 3/2, 2 as well as the classical limit. Using
the exact diagonalization and density-matrix renormalization techniques, we
provide evidence that a plateau at one third of the saturation magnetization
exists in the magnetization curve of frustrated spin-S chains with S>1/2.
Similar to the case of S=1/2, this plateau state breaks the translational
symmetry of the Hamiltonian and realizes an up-up-down pattern in the spin
component parallel to the external field. Our study further shows that this
plateau exists both in the cases of an isotropic exchange and in the easy-axis
regime for spin-S=1, 3/2, and 2, but is absent in classical frustrated spin
chains with isotropic interactions. We discuss the magnetic phase diagram of
frustrated spin-1 and spin-3/2 chains as well as other emergent features of the
magnetization process such as kink singularities, jumps, and even-odd effects.
A quantitative comparison of the one-third plateau in the easy-axis regime
between spin-1 and spin-3/2 chains on the one hand and the classical frustrated
chain on the other hand indicates that the critical frustration and the phase
boundaries of this state rapidly approach the classical result as the spin S
increases.Comment: 15 pages RevTex4, 13 figure
Monolithic Directly-Modulated Multi-Wavelength- Channel GaInAsP/InP Micro-Ring Laser Array
Two directly-modulated GaInAsP/InP micro-rings with different radii vertically-coupled on a common bus are assessed for both independent and simultaneous operation. A device area <0.12mm^2 per microring allows the generation of 2λx1Gb/s WDM signals with 6nm wavelength separation. These show successful transmission over 25km of single-mode-fibre with < 0.2dB power penalty
- …