1,784 research outputs found
Thermal conductivity of the one-dimensional Fermi-Hubbard model
We study the thermal conductivity of the one-dimensional Fermi-Hubbard model
at finite temperature using a density matrix renormalization group approach.
The integrability of this model gives rise to ballistic thermal transport. We
calculate the temperature dependence of the thermal Drude weight at half
filling for various interactions and moreover, we compute its filling
dependence at infinite temperature. The finite-frequency contributions
originating from the fact that the energy current is not a conserved quantity
are investigated as well. We report evidence that breaking the integrability
through a nearest-neighbor interaction leads to vanishing Drude weights and
diffusive energy transport. Moreover, we demonstrate that energy spreads
ballistically in local quenches with initially inhomogeneous energy density
profiles in the integrable case. We discuss the relevance of our results for
thermalization in ultra-cold quantum gas experiments and for transport
measurements with quasi-one dimensional materials
Full-depth Coadds of the WISE and First-year NEOWISE-Reactivation Images
The Near Earth Object Wide-field Infrared Survey Explorer (NEOWISE)
Reactivation mission released data from its first full year of observations in
2015. This data set includes ~2.5 million exposures in each of W1 and W2,
effectively doubling the amount of WISE imaging available at 3.4 and 4.6
microns relative to the AllWISE release. We have created the first ever
full-sky set of coadds combining all publicly available W1 and W2 exposures
from both the AllWISE and NEOWISE-Reactivation (NEOWISER) mission phases. We
employ an adaptation of the unWISE image coaddition framework (Lang 2014),
which preserves the native WISE angular resolution and is optimized for forced
photometry. By incorporating two additional scans of the entire sky, we not
only improve the W1/W2 depths, but also largely eliminate time-dependent
artifacts such as off-axis scattered moonlight. We anticipate that our new
coadds will have a broad range of applications, including target selection for
upcoming spectroscopic cosmology surveys, identification of distant/massive
galaxy clusters, and discovery of high-redshift quasars. In particular, our
full-depth AllWISE+NEOWISER coadds will be an important input for the Dark
Energy Spectroscopic Instrument (DESI) selection of luminous red galaxy and
quasar targets. Our full-depth W1/W2 coadds are already in use within the DECam
Legacy Survey (DECaLS) and Mayall z-band Legacy Survey (MzLS) reduction
pipelines. Much more work still remains in order to fully leverage NEOWISER
imaging for astrophysical applications beyond the solar system.Comment: coadds available at http://unwise.me, zoomable full-sky rendering at
http://legacysurvey.org/viewe
The Metallicity of the Monoceros Stream
We present low-resolution MMT Hectospec spectroscopy of 594 candidate
Monoceros stream member stars. Based on strong color-magnitude diagram
overdensities, we targeted three fields within the stream's footprint, with 178
deg < l < 203 deg and -25 deg < b < 25 deg. By comparing the measured iron
abundances with those expected from smooth Galactic components alone, we
measure, for the first time, the spectroscopic metallicity distribution
function for Monoceros. We find the stream to be chemically distinct from both
the thick disk and halo, with [Fe/H] = -1, and do not detect a trend in the
stream's metallicity with Galactic longitude. Passing from b = +25 deg to b =
-25 deg the median Monoceros metallicity trends upward by 0.1 dex, though
uncertainties in modeling sample contamination by the disk and halo make this a
marginal detection. In each field, we find Monoceros to have an intrinsic
[Fe/H] dispersion of 0.10-0.22 dex. From the CaII K line, we measure [Ca/Fe]
for a subsample of metal poor program stars with -1.1 < [Fe/H] < -0.5. In two
of three fields, we find calcium deficiencies qualitatively similar to
previously reported [Ti/Fe] underabundances in Monoceros and the Sagittarius
tidal stream. Further, using 90 spectra of thick disk stars in the Monoceros
pointings with b ~ +/-25 deg, we detect a 0.22 dex north/south metallicity
asymmetry coincident with known stellar density asymmetry at R_GC ~ 12 kpc and
|Z| ~ 1.7 kpc. Our median Monoceros [Fe/H] = -1.0 and its relatively low
dispersion naturally fit the expectation for an appropriately luminous M_V ~
-13 dwarf galaxy progenitor.Comment: accepted for publication in Ap
Strongly interacting bosons on a three-leg ladder in the presence of a homogeneous flux
We perform a density-matrix renormalization-group study of strongly
interacting bosons on a three-leg ladder in the presence of a homogeneous flux.
Focusing on one-third filling, we explore the phase diagram in dependence of
the magnetic flux and the inter-leg tunneling strength. We find several phases
including a Meissner phase, vortex liquids, a vortex lattice, as well as a
staggered-current phase. Moreover, there are regions where the chiral current
reverses its direction, both in the Meissner and in the staggered-current
phase. While the reversal in the latter case can be ascribed to spontaneous
breaking of translational invariance, in the first it stems from an effective
flux increase in the rung direction. Interactions are a necessary ingredient to
realize either type of chiral-current reversal
Comparative study of theoretical methods for nonequilibrium quantum transport
We present a detailed comparison of three different methods designed to
tackle nonequilibrium quantum transport, namely the functional renormalization
group (fRG), the time-dependent density matrix renormalization group (tDMRG),
and the iterative summation of real-time path integrals (ISPI). For the
nonequilibrium single-impurity Anderson model (including a Zeeman term at the
impurity site), we demonstrate that the three methods are in quantitative
agreement over a wide range of parameters at the particle-hole symmetric point
as well as in the mixed-valence regime. We further compare these techniques
with two quantum Monte Carlo approaches and the time-dependent numerical
renormalization group method.Comment: 19 pages, 7 figures; published versio
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