840 research outputs found
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
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
Non-dissipative Thermal Transport and Magnetothermal Effect for the Spin-1/2 Heisenberg Chain
Anomalous magnetothermal effects are discussed in the spin-1/2 Heisenberg
chain. The energy current is related to one of the non-trivial conserved
quantities underlying integrability and therefore both the diagonal and off
diagonal dynamical correlations of spin and energy current diverge. The
energy-energy and spin-energy current correlations at finite temperatures are
exactly calculated by a lattice path integral formulation. The low-temperature
behavior of the thermomagnetic (magnetic Seebeck) coefficient is also
discussed. Due to effects of strong correlations, we observe the magnetic
Seebeck coefficient changes sign at certain interaction strengths and magnetic
fields.Comment: 4 pages, references added, typos corrected, Conference proceedings of
SPQS 2004, Sendai, Japa
Analysis of greenhouse gas mitigation performance in UK urban areas
As the threat of irreversible climate change has increased over time, the UK has continued to set increasingly ambitious policies to reduce its carbon emission. An assessment of mitigation progress to date at the local authority level clarifies the factors that have affected greenhouse gas (GHG) emissions on the path to carbon neutrality. This research uses regression analyses between local authorities’ GHG emission redcutions and selected explanatory variables (including population density, household income, and manufacturing employment) identified from the literature to explore mitigation performance over time, focusing on GHG emissions changes between 2005 and 2016. Substantial and relatively consistent GHG emissions reductions were achieved in this time frame, with average total reductions across UK local authorities of 31.2%. Population density was moderately-to-strongly correlated with the success of transportation GHG emissions mitigation, though this sector has seen the smallest percentage declines over this period. Local authorities with densities below 25 inhabitants per hectare were generally among the poorest performers in transportation GHG mitigation. This underscores the need to support remote working and electrification of personal transportation in areas where public/active transportation options are not viable alternatives. Furthermore, consideration of population density in conjunction with domestic and urban planning will allow for future emissions reductions to occur across the UK. Fundamentally, GHG emissions reductions to date are largely driven by historic factors (density), shifting economic structures (deindustrialisation), and centralised initiatives (decarbonisation of electricity generation)
Thermomagnetic Power and Figure of Merit for Spin-1/2 Heisenberg Chain
Transport properties in the presence of magnetic fields are numerically
studied for the spin-1/2 Heisenberg XXZ chain. The breakdown of the
spin-reversal symmetry due to the magnetic field induces the magnetothermal
effect. In analogy with the thermoelectric effect in electron systems, the
thermomagnetic power (magnetic Seebeck coefficient) is provided, and is
numerically evaluated by the exact diagonalization for wide ranges of
temperatures and various magnetic fields. For the antiferromagnetic regime, we
find the magnetic Seebeck coefficient changes sign at certain temperatures,
which is interpreted as an effect of strong correlations. We also compute the
thermomagnetic figure of merit determining the efficiency of the thermomagnetic
devices for cooling or power generation.Comment: 8 page
Non-equilibrium electronic transport in a one-dimensional Mott insulator
We calculate the non-equilibrium electronic transport properties of a
one-dimensional interacting chain at half filling, coupled to non-interacting
leads. The interacting chain is initially in a Mott insulator state that is
driven out of equilibrium by applying a strong bias voltage between the leads.
For bias voltages above a certain threshold we observe the breakdown of the
Mott insulator state and the establishment of a steady-state electronic current
through the system. Based on extensive time-dependent density matrix
renormalization group simulations, we show that this steady-state current
always has the same functional dependence on voltage, independent of the
microscopic details of the model and relate the value of the threshold to the
Lieb-Wu gap. We frame our results in terms of the Landau-Zener dielectric
breakdown picture. Finally, we also discuss the real-time evolution of the
current, and characterize the current-carrying state resulting from the
breakdown of the Mott insulator by computing the double occupancy, the spin
structure factor, and the entanglement entropy.Comment: 12 pages RevTex4, 12 eps figures, as published, minor revision
Transport through quantum dots: A combined DMRG and cluster-embedding study
The numerical analysis of strongly interacting nanostructures requires
powerful techniques. Recently developed methods, such as the time-dependent
density matrix renormalization group (tDMRG) approach or the embedded-cluster
approximation (ECA), rely on the numerical solution of clusters of finite size.
For the interpretation of numerical results, it is therefore crucial to
understand finite-size effects in detail. In this work, we present a careful
finite-size analysis for the examples of one quantum dot, as well as three
serially connected quantum dots. Depending on odd-even effects, physically
quite different results may emerge from clusters that do not differ much in
their size. We provide a solution to a recent controversy over results obtained
with ECA for three quantum dots. In particular, using the optimum clusters
discussed in this paper, the parameter range in which ECA can reliably be
applied is increased, as we show for the case of three quantum dots. As a
practical procedure, we propose that a comparison of results for static
quantities against those of quasi-exact methods, such as the ground-state
density matrix renormalization group (DMRG) method or exact diagonalization,
serves to identify the optimum cluster type. In the examples studied here, we
find that to observe signatures of the Kondo effect in finite systems, the best
clusters involving dots and leads must have a total z-component of the spin
equal to zero.Comment: 16 pages, 14 figures, revised version to appear in Eur. Phys. J. B,
additional reference
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