25 research outputs found
Quantum Fluctuations and Hydrodynamic Noise in Low Dimensions
Our theoretical work is organized in two independent parts: Part I belongs to the field of condensed matter theory and deals with the spectral signatures of collective states in one dimensional (1D) metals: Electrons in 1D metals are expected to fractionalize into collective spin and charge degrees of freedom. A recent candidate material for the realization of a 1D metal are mirror-twin boundaries in monolayer MoS2. Scanning tunneling spectroscopy was used to record the local density of states along these 1D line defects. In our purely theoretical work, we calculate the local density of states as predicted by Tomonaga-Luttinger-liquid theory in order to reveal the nature of the 1D states spectroscopically. The comparison of measured and theoretical spectra allows us to identify the observed doubling of the energy levels as signature of emergent spin and charge excitations. Part II belongs to the field of non-equilibrium physics and deals with the macroscopic description of equilibration: Equilibration of closed systems is hampered by the diffusive transport of locally conserved quantities as described by fluctuating hydrodynamics. After a sudden perturbation, the buildup of equilibrium fluctuations occurs only algebraically slowly, giving rise to hydrodynamic long-time tails. However, the standard tool in transport theory, the Boltzmann equation, fails to describe equilibration. Adding a noise term restores the missing correlations, resulting in a stochastic Langevin-Boltzmann equation. In our work, we derive a simplified version: a fluctuating relaxation-time approximation. We also set up a stable integration scheme for this type of equation and demonstrate that the numerical solution is in agreement with the predictions of fluctuating hydrodynamics. As an addition, we discuss slow changes of state. We show that the entropy production vanishes algebraically slowly in the adiabatic limit due to the presence of hydrodynamic slow modes
Interference of quantum critical excitations and soft diffusive modes in a disordered antiferromagnetic metal
We study the temperature-dependent quantum correction to conductivity due to
the interplay of spin density fluctuations and weak disorder for a
two-dimensional metal near an antiferromagnetic (AFM) quantum critical point.
AFM spin density fluctuations carry large momenta around the ordering vector
and, at lowest order of the spin-fermion coupling, only scatter
electrons between "hot spots" of the Fermi surface which are connected by
. Earlier, it was seen that the quantum interference between AFM
spin density fluctuations and soft diffusive modes of the disordered metal is
suppressed, a consequence of the large-momentum scattering. The suppression of
this interference results in a non-singular temperature dependence of the
corresponding interaction correction to conductivity. However, at higher order
of the spin-fermion coupling, electrons on the entire Fermi surface can be
scattered successively by two spin density fluctuations and, in total, suffer a
small momentum transfer. This higher-order process can be described by
composite modes which carry small momenta. We show that the interference
between formally subleading composite modes and diffusive modes generates
singular interaction corrections which ultimately dominate over the
non-singular first-order correction at low temperatures. We derive an effective
low-energy theory from the spin-fermion model which includes the
above-mentioned higher-order process implicitly and show that for weak
spin-fermion coupling the small-momentum transfer is mediated by a composite
propagator. Employing the conventional diagrammatic approach to impurity
scattering, we find the correction for
temperatures above an exponentially small crossover scale.Comment: 13 pages, 7 figures. Published versio
Excitation of the molecular gas in the nuclear region of M82
We present high resolution HIFI spectroscopy of the nucleus of the
archetypical starburst galaxy M82. Six 12CO lines, 2 13CO lines and 4
fine-structure lines are detected. Besides showing the effects of the overall
velocity structure of the nuclear region, the line profiles also indicate the
presence of multiple components with different optical depths, temperatures and
densities in the observing beam. The data have been interpreted using a grid of
PDR models. It is found that the majority of the molecular gas is in low
density (n=10^3.5 cm^-3) clouds, with column densities of N_H=10^21.5 cm^-2 and
a relatively low UV radiation field (GO = 10^2). The remaining gas is
predominantly found in clouds with higher densities (n=10^5 cm^-3) and
radiation fields (GO = 10^2.75), but somewhat lower column densities
(N_H=10^21.2 cm^-2). The highest J CO lines are dominated by a small (1%
relative surface filling) component, with an even higher density (n=10^6 cm^-3)
and UV field (GO = 10^3.25). These results show the strength of multi-component
modeling for the interpretation of the integrated properties of galaxies.Comment: Accepted for publication in A&A Letter
HIFI spectroscopy of low-level water transitions in M82
We present observations of the rotational ortho-water ground transition, the
two lowest para-water transitions, and the ground transition of ionised
ortho-water in the archetypal starburst galaxy M82, performed with the HIFI
instrument on the Herschel Space Observatory. These observations are the first
detections of the para-H2O(111-000) (1113\,GHz) and ortho-H2O+(111-000)
(1115\,GHz) lines in an extragalactic source. All three water lines show
different spectral line profiles, underlining the need for high spectral
resolution in interpreting line formation processes. Using the line shape of
the para-H2O(111-000) and ortho-H2O+(111-000) absorption profile in conjunction
with high spatial resolution CO observations, we show that the (ionised) water
absorption arises from a ~2000 pc^2 region within the HIFI beam located about
~50 pc east of the dynamical centre of the galaxy. This region does not
coincide with any of the known line emission peaks that have been identified in
other molecular tracers, with the exception of HCO. Our data suggest that water
and ionised water within this region have high (up to 75%) area-covering
factors of the underlying continuum. This indicates that water is not
associated with small, dense cores within the ISM of M82 but arises from a more
widespread diffuse gas component.Comment: 5 pages, 4 figures. Accepted for publication in A&
CO(4-3) and CO(7-6) maps of the nucleus of NGC 253
Context.Molecular line excitation studies of the nuclei of nearby starburst galaxies yield
important information on the starburst phenomena, in particular on the temperature and
density of the star-forming gas. Such studies also
provide templates for high redshift galaxies with even more extreme star formation.
Aims.Fundamental constraints on the physical properties
in the nuclear regions of external galaxies can be derived from the spectral
energy distribution (i.e., integrated flux density
vs. rotational quantum number) of
CO rotational emission arising from warm gas.
Methods.The resolution and sensitivity of the APEX telescope
makes it feasible to perform
spatially resolved studies of submillimeter (submm) CO emission from
the warm, dense gas in nearby starburst
nuclei. Using the FLASH dual-channel heterodyne receiver we mapped
emission in the CO and 7-6 lines toward the archetypical nuclear
starburst galaxy NGC 253.
Results.Combining our new observations
with data from the literature, we derive the CO line SED in the central 250 pc of
NGC 253, which peaks near the 6-5 transition and has a shape
very similar to that of M 82. All CO transitions in the central
region can well be fitted with a single temperature/density
Large Velocity Gradient (LVG) model.
A good match to the observations is found by assuming kinetic gas temperatures
that are comparable to the dust temperature (Tkin K) and
a H2 density of order 104 cm-3.
Conclusions.Our very first APEX submm study of a
nearby starburst nucleus (NGC 253)
meaningfully constrains the physical
properties of the star-forming molecular gas it contains.
With broader band spectrometers and a chopping
secondary coming soon, the impact of APEX on extragalactic
astrophysics will be foreseeably significant
Understanding the photoemission distribution of strongly interacting two-dimensional overlayers
Photoemission tomography (PT), the analysis of the photoemission intensity distribution within the plane wave final-state approximation, is being established as a useful tool for extracting the electronic and geometric structure of weakly interacting organic overlayers. Here we present a simple method for extending PT, which until now has been based on the calculations of isolated molecules. By including the substrate and a damped plane-wave final state, we are able to simulate the photoemission intensity distribution of two-dimensional molecular overlayers with both strong intermolecular and molecule-substrate interactions, here demonstrated for the model system 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) on Cu(100). It is shown that the interaction and hybridization of the lowest unoccupied molecular orbital of PTCDA with substrate states leads to its occupation and the formation of a strongly dispersing intermolecular band, whose experimental magnitude of 1.1 eV and k-space periodicity is well reproduced theoretically