19 research outputs found
Dual current anomalies and quantum transport within extended reservoir simulations
Quantum transport simulations are rapidly evolving and now encompass
well-controlled tensor network techniques for many-body transport. One powerful
approach combines matrix product states with extended reservoirs. In this
method, continuous reservoirs are represented by explicit, discretized
counterparts where a chemical potential or temperature drop is maintained by
relaxation. Currents are strongly influenced by relaxation when it is very weak
or strong, resulting in a simulation-analog of Kramers' turnover in
solution-phase chemical reactions. At intermediate relaxation, the intrinsic
conductance-that given by the Landauer or Meir-Wingreen expressions-moderates
the current. We demonstrate that strong impurity scattering (i.e., a small
steady-state current) reveals anomalous transport regimes within this
methodology at weak-to-moderate and moderate-to-strong relaxation. The former
is due to virtual transitions and the latter to unphysical broadening of the
populated density of states. The Kramers' turnover analog thus has five
standard transport regimes, further constraining the parameters that lead to
the intrinsic conductance. In particular, a relaxation strength proportional to
the reservoir level spacing-the commonly assumed strategy-can prevent
convergence to the continuum limit. This underscores that the turnover profiles
enable identification of simulation parameters that achieve proper physical
behavior.Comment: 16 pages, 5 figure
A GGA plus U approach to effective electronic correlations in thiolate-ligated iron-oxo (IV) porphyrin
High-valent oxo-metal complexes exhibit correlated electronic behavior on dense, low-lying electronic state manifolds, presenting challenging systems for electronic structure methods. Among these species, the iron-oxo (IV) porphyrin denoted Compound I occupies a privileged position, serving a broad spectrum of catalytic roles. The most reactive members of this family bear a thiolate axial ligand, exhibiting high activity toward molecular oxygen activation and substrate oxidation. The default approach to such systems has entailed the use of hybrid density functionals or multi-configurational/multireference methods to treat electronic correlation. An alternative approach is presented based on the GGA+U approximation to density functional theory, in which a generalized gradient approximation (GGA) functional is supplemented with a localization correction to treat on-site correlation as inspired by the Hubbard model. The electronic structure of thiolate-ligated iron-oxo (IV) porphyrin and corresponding Coulomb repulsion U are determined both empirically and self-consistently, yielding spin-distributions, state level splittings, and electronic densities of states consistent with prior hybrid functional calculations. Comparison of this detailed electronic structure with model Hamiltonian calculations suggests that the localized 3d iron moments induce correlation in the surrounding electron gas, strengthening local moment formation. This behavior is analogous to strongly correlated electronic systems such as Mott insulators, in which the GGA+U scheme serves as an effective single-particle representation for the full, correlated many-body problem
Performance of reservoir discretizations in quantum transport simulations
Quantum transport simulations often use explicit, yet finite, electronic
reservoirs. These should converge to the correct continuum limit, albeit with a
trade-off between discretization and computational cost. Here, we study this
interplay for extended reservoir simulations, where relaxation maintains a bias
or temperature drop across the system. Our analysis begins in the
non-interacting limit, where we parameterize different discretizations to
compare them on an even footing. For many-body systems, we develop a method to
estimate the relaxation that best approximates the continuum by controlling
virtual transitions in Kramers' turnover for the current. While some
discretizations are more efficient for calculating currents, there is little
benefit with regard to the overall state of the system. Any gains become
marginal for many-body, tensor network simulations, where the relative
performance of discretizations varies when sweeping other numerical controls.
These results indicate that a given reservoir discretization may have little
impact on numerical efficiency for certain computational tools. The choice of a
relaxation parameter, however, is crucial, and the method we develop provides a
reliable estimate of the optimal relaxation for finite reservoirs.Comment: Manuscript + S
Open System Tensor Networks and Kramers' Crossover for Quantum Transport
Tensor networks are a powerful tool for many-body ground states with limited
entanglement. These methods can nonetheless fail for certain time-dependent
processes - such as quantum transport or quenches - where entanglement growth
is linear in time. Matrix-product-state decompositions of the resulting
out-of-equilibrium states require a bond dimension that grows exponentially,
imposing a hard limit on simulation timescales. However, in the case of
transport, if the reservoir modes of a closed system are arranged according to
their scattering structure, the entanglement growth can be made logarithmic.
Here, we apply this ansatz to open systems via extended reservoirs that have
explicit relaxation. This enables transport calculations that can access steady
states, time dynamics and noise, and periodic driving (e.g., Floquet states).
We demonstrate the approach by calculating the transport characteristics of an
open, interacting system. These results open a path to scalable and numerically
systematic many-body transport calculations with tensor networks.Comment: 7 pages, 4 figure