124 research outputs found
Disorder- and correlation-driven metal-insulator transitions
Metal-insulator transitions driven by disorder (Delta) and/or by electron
correlations (U) are investigated within the Anderson-Hubbard model with local
binary-alloy disorder using a simple but consistent mean-field approach. The
Delta-U phase diagram is derived and discussed for T=0 and finite temperatures.Comment: 2 pages, 2 figures, submitted to the SCES'04, Ref.4 update
Krylov-space approach to the equilibrium and the nonequilibrium single-particle Green's function
The zero-temperature single-particle Green's function of correlated fermion
models with moderately large Hilbert-space dimensions can be calculated by
means of Krylov-space techniques. The conventional Lanczos approach consists of
finding the ground state in a first step, followed by an approximation for the
resolvent of the Hamiltonian in a second step. We analyze the character of this
approximation and discuss a numerically exact variant of the Lanczos method
which is formulated in the time domain. This method is extended to get the
nonequilibrium single-particle Green's function defined on the
Keldysh-Matsubara contour in the complex time plane. The proposed method will
be important as an exact-diagonalization solver in the context of
self-consistent or variational cluster-embedding schemes. For the recently
developed nonequilibrium cluster-perturbation theory, we discuss the efficient
implementation and demonstrate the feasibility of the Krylov-based solver. The
dissipation of a strong local magnetic excitation into a non-interacting bath
is considered as an example for applications.Comment: 20 pages, 5 figures, v2 with minor corrections, JPCM in pres
Non-equilibrium cluster-perturbation theory
The cluster perturbation theory (CPT) is one of the simplest but systematic
quantum cluster approaches to lattice models of strongly correlated electrons
with local interactions. By treating the inter-cluster potential, in addition
to the interactions, as a perturbation, it is shown that the CPT can be
reformulated as an all-order re-summation of diagrams within standard
weak-coupling perturbation theory where vertex corrections are neglected. This
reformulation is shown to allow for a straightforward generalization of the CPT
to the general non-equilibrium case using contour-ordered Green's functions.
Solving the resulting generalized CPT equation on the discretized
Keldysh-Matsubara time contour, the transient dynamics of an essentially
arbitrary initial pure or mixed state can be traced. In this way, the
time-dependent expectation values of one-particle observables can be obtained
within an approximation that neglects spatial correlations beyond the extension
of the reference cluster. The necessary computational effort is very moderate.
A detailed discussion and simple test calculations are presented to demonstrate
the strengths and the shortcomings of the proposed approach. The
non-equilibrium CPT is systematic and is controlled in principle by the inverse
cluster size. It interpolates between the non-interacting and the atomic or
decoupled-cluster limit which are recovered exactly and is found to predict the
correct dynamics at very short times in a general non-trivial case. The effects
of initial-state correlations on the subsequent dynamics and the necessity to
extend the Keldysh contour by the imaginary Matsubara branch are analyzed
carefully and demonstrated numerically. It is furthermore shown that the
approach can describe the dissipation of spin and charge to an uncorrelated
bath with an essentially arbitrary number of degrees of freedom.Comment: 14 pages, 9 figure
On a Dual Pair of Spaces of Smooth and Generalized Random Variables
A dual pair G and G* of smooth and generalized random variables, respectively, over the white noise probability space is studied. G is constructed by norms involving exponentials of the Ornstein-Uhlenbeck operator, G* is its dual. Sufficient criteria are proved for when a function on S(IR) is the S-transform of an element in G or G*
Mott transition in one dimension: Benchmarking dynamical cluster approaches
The variational cluster approach (VCA) is applied to the one-dimensional
Hubbard model at zero temperature using clusters (chains) of up to ten sites
with full diagonalization and the Lanczos method as cluster solver. Within the
framework of the self-energy-functional theory (SFT), different cluster
reference systems with and without bath degrees of freedom, in different
topologies and with different sets of variational parameters are considered.
Static and one-particle dynamical quantities are calculated for half-filling as
a function of U as well as for fixed U as a function of the chemical potential
to study the interaction- and filling-dependent metal-insulator (Mott)
transition. The recently developed Q-matrix technique is used to compute the
SFT grand potential. For benchmarking purposes we compare the VCA results with
exact results available from the Bethe ansatz, with essentially exact dynamical
DMRG data, with (cellular) dynamical mean-field theory and full diagonalization
of isolated Hubbard chains. Several issues are discussed including convergence
of the results with cluster size, the ability of cluster approaches to access
the critical regime of the Mott transition, efficiency in the optimization of
correlated-site vs. bath-site parameters and of multi-dimensional parameter
optimization. We also study the role of bath sites for the description of
excitation properties and as charge reservoirs for the description of filling
dependencies. The VCA turns out to be a computationally cheap method which is
competitive with established cluster approaches.Comment: 19 pages, 19 figures, v3 with minor corrections, extended discussio
Microbial iron reduction during passive in situ remediation of an acidic mine pit lake mesocosm
AbstractFerric iron reduction was studied in a pilot-scale enclosure experiment for passive biological remediation of an acidic mine pit lake in Lusatia, Germany. The metabolic properties of prokaryotes involved in Fe(III) reduction may be important for the outcome of biological remediation, as chemolithotrophic Fe(III) reduction can counteract the desired pH increase, but heterotrophic Fe(III) reduction will provide the necessary Fe(II) for precipitation of sulfide minerals following sulfate reduction. Therefore, vertical profiles of sediment parameters related to iron and sulfur cycling were determined in conjunction with viable counts of different ferric iron-reducing micro-organisms using selective media. Findings were compared to an untreated reference site. The addition of organic matter stimulated ferric iron reduction and sulfate reduction in the enclosure and led to elevated pH and accumulations of ferrous iron and reduced sulfur compounds. Numbers of neutrophilic heterotrophic Fe(III) reducers increased during treatment, those of acidophilic heterotrophic Fe(III) reducers remained similar, and those of acidophilic chemolithotrophic Fe(III) reducers decreased. Zones of ferric iron-reducing activity corresponded well with microbial depth profiles; however, viable counts of neutrophilic or acid-tolerant Fe(III) reducers must have been underestimated based on the corresponding observed activity levels. Ferric iron reduction by chemolithotrophic acidophiles seemed to be of minor importance, so a lowering of pH values due to Fe(III) reducing activity is unlikely
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