1,166 research outputs found
Spin conductivity in almost integrable spin chains
The spin conductivity in the integrable spin-1/2 XXZ-chain is known to be
infinite at finite temperatures T for anisotropies -1 < Delta < 1.
Perturbations which break integrability, e.g. a next-nearest neighbor coupling
J', render the conductivity finite. We construct numerically a non-local
conserved operator J_parallel which is responsible for the finite spin Drude
weight of the integrable model and calculate its decay rate for small J'. This
allows us to obtain a lower bound for the spin conductivity sigma_s >= c(T) /
J'^2, where c(T) is finite for J' to 0. We discuss the implication of our
result for the general question how non-local conservation laws affect
transport properties.Comment: 6 pages, 5 figure
Temperature, inocula and substrate: contrasting electroactive consortia, diversity and performance in microbial fuel cells
The factors that affect microbial community assembly and its effects on the performance of bioelectrochemical systems are poorly understood. Sixteen microbial fuel cell (MFC) reactors were set up to test the importance of inoculum, temperature and substrate: Arctic soil versus wastewater as inoculum; warm (26.5°C) versus cold (7.5°C) temperature; and acetate versus wastewater as substrate. Substrate was the dominant factor in determining performance and diversity: unexpectedly the simple electrogenic substrate delivered a higher diversity than a complex wastewater. Furthermore, in acetate fed reactors, diversity did not correlate with performance, yet in wastewater fed ones it did, with greater diversity sustaining higher power densities and coulombic efficiencies. Temperature had only a minor effect on power density, (Q10: 2 and 1.2 for acetate and wastewater respectively): this is surprising given the well-known temperature sensitivity of anaerobic bioreactors. Reactors were able to operate at low temperature with real wastewater without the need for specialised inocula; it is speculated that MFC biofilms may have a self-heating effect. Importantly, the warm acetate fed reactors in this study did not act as direct model for cold wastewater fed systems. Application of this technology will encompass use of real wastewater at ambient temperatures
Signatures of integrability in charge and thermal transport in 1D quantum systems
Integrable and non-integrable systems have very different transport
properties. In this work, we highlight these differences for specific one
dimensional models of interacting lattice fermions using numerical exact
diagonalization. We calculate the finite temperature adiabatic stiffness (or
Drude weight) and isothermal stiffness (or ``Meissner'' stiffness) in
electrical and thermal transport and also compute the complete momentum and
frequency dependent dynamical conductivities and
. The Meissner stiffness goes to zero rapidly with system
size for both integrable and non-integrable systems. The Drude weight shows
signs of diffusion in the non-integrable system and ballistic behavior in the
integrable system. The dynamical conductivities are also consistent with
ballistic and diffusive behavior in the integrable and non-integrable systems
respectively.Comment: 4 pages, 4 figure
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
Phase diagram of an anisotropic frustrated ferromagnetic spin-1/2 chain in a magnetic field: a density matrix renormalization group study
We study the phase diagram of a frustrated spin-1/2 ferromagnetic chain with
anisotropic exchange interactions in an external magnetic field, using the
density matrix renormalization group method. We show that an easy-axis
anisotropy enhances the tendency towards multimagnon bound states, while an
easy-plane anisotropy favors chirally ordered phases. In particular, a moderate
easy-plane anisotropy gives rise to a quantum phase transition at intermediate
magnetization. We argue that this transition is related to the finite-field
phase transition experimentally observed in the spin-1/2 compound LiCuVO_4.Comment: The final published versio
Lower bounds for the conductivities of correlated quantum systems
We show how one can obtain a lower bound for the electrical, spin or heat
conductivity of correlated quantum systems described by Hamiltonians of the
form H = H0 + g H1. Here H0 is an interacting Hamiltonian characterized by
conservation laws which lead to an infinite conductivity for g=0. The small
perturbation g H1, however, renders the conductivity finite at finite
temperatures. For example, H0 could be a continuum field theory, where momentum
is conserved, or an integrable one-dimensional model while H1 might describe
the effects of weak disorder. In the limit g to 0, we derive lower bounds for
the relevant conductivities and show how they can be improved systematically
using the memory matrix formalism. Furthermore, we discuss various applications
and investigate under what conditions our lower bound may become exact.Comment: Title changed; 9 pages, 2 figure
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