405 research outputs found
Characterization of a correlated topological Kondo insulator in one dimension
We investigate the ground-state of a p-wave Kondo-Heisenberg model introduced
by Alexandrov and Coleman with an Ising-type anisotropy in the Kondo
interaction and correlated conduction electrons. Our aim is to understand how
they affect the stability of the Haldane state obtained in the SU(2) symmetric
case without the Hubbard interaction. By applying the density-matrix
renormalization group algorithm and calculating the entanglement entropy we
show that in the anisotropic case a phase transition occurs and a N\'eel state
emerges above a critical value of the Coulomb interaction. These findings are
also corroborated by the examination of the entanglement spectrum and the spin
profile of the system which clarify the structure of each phase.Comment: 6 pages, 9 figure
Entanglement, excitations and correlation effects in narrow zigzag graphene nanoribbons
We investigate the low-lying excitation spectrum and ground-state properties
of narrow graphene nanoribbons with zigzag edge configurations. Nanoribbons of
comparable widths have been synthesized very recently [P. Ruffieux, \emph{et
al.} Nature \textbf{531}, 489 (2016)], and their descriptions require more
sophisticated methods since in this regime conventional methods, like
mean-field or density-functional theory with local density approximation, fail
to capture the enhanced quantum fluctuations. Using the unbiased density-matrix
renormalization group algorithm we calculate the charge gaps with high accuracy
for different widths and interaction strengths and compare them with mean-field
results. It turns out that the gaps are much smaller in the former case due to
the proper treatment of quantum fluctuations. Applying the elements of quantum
information theory we also reveal the entanglement structure inside a ribbon
and examine the spectrum of subsystem density matrices to understand the origin
of entanglement. We examine the possibility of magnetic ordering and the effect
of magnetic field. Our findings are relevant for understanding the gap values
in different recent experiments and the deviations between them.Comment: 8 pages, 7 figures, revised version, accepted for publication in PR
Optimizing momentum space DMRG using quantum information entropy
In order to optimize the ordering of the lattice sites in the momentum space
and quantum chemistry versions of the density matrix renormalization group
(DMRG) method we have studied the separability and entanglement of the target
state for the 1-D Hubbard model and various molecules. By analyzing the
behavior of von Neumann and Neumann-Renyi entropies we have found criteria that
help to fasten convergence. A new initialization procedure has been developed
which maximizes the Kullback-Leibler entropy and extends the active space (AS)
in a dynamical fashion. The dynamically extended active space (DEAS) procedure
reduces significantly the effective system size during the first half sweep and
accelerates the speed of convergence of momentum space DMRG and quantum
chemistry DMRG to a great extent. The effect of lattice site ordering on the
number of block states to be kept during the RG procedure is also investigated.Comment: 15 pages, 15 figure
Competition between Hund's coupling and Kondo effect in a one-dimensional extended periodic Anderson model
We study the ground-state properties of an extended periodic Anderson model
to understand the role of Hund's coupling between localized and itinerant
electrons using the density-matrix renormalization group algorithm. By
calculating the von Neumann entropies we show that two phase transitions occur
and two new phases appear as the hybridization is increased in the symmetric
half-filled case due to the competition between Kondo-effect and Hund's
coupling. In the intermediate phase, which is bounded by two critical points,
we found a dimerized ground state, while in the other spatially homogeneous
phases the ground state is Haldane-like and Kondo-singlet-like, respectively.
We also determine the entanglement spectrum and the entanglement diagram of the
system by calculating the mutual information thereby clarifying the structure
of each phase.Comment: 9 pages, 9 figures, revised version, accepted for publication in PR
Analysis of two-orbital correlations in wavefunctions restricted to electron-pair states
Wavefunctions constructed from electron-pair states can accurately model
strong electron correlation effects and are promising approaches especially for
larger many-body systems. In this article, we analyze the nature and the type
of electron correlation effects that can be captured by wavefunctions
restricted to electron-pair states. We focus on the Antisymmetric Product of
1-reference orbital Geminal (AP1roG) method combined with an orbital
optimization protocol presented in [Phys. Rev. B, 89, 201106(R), 2014] whose
performance is assessed against electronic structures obtained form DMRG
reference data. Our numerical analysis covers model systems for strong
correlation: the one-dimensional Hubbard model with periodic boundary condition
as well as metallic and molecular hydrogen rings. Specifically, the accuracy of
AP1roG is benchmarked using the single-orbital entropy, the orbital-pair mutual
information as well as the eigenvalue spectrum of the one-orbital and
two-orbital reduced density matrices. Our study indicates that contributions
from singly occupied states become important in the strong correlation regime
which highlights the limitations of the AP1roG method. Furthermore, we examine
the effect of orbital rotations within the AP1roG model on correlations between
orbital pairs.Comment: 15 pages, 8 figure
Unified phase diagram of models exhibiting neutral-ionic transition
We have studied the neutral-ionic transition in organic mixed-stack
compounds. A unified model has been derived which, in limiting cases, is
equivalent to the models proposed earlier, the donor-acceptor model and the
ionic Hubbard model. Detailed numerical calculations have been performed on
this unified model with the help of the density-matrix renormalization-group
(DMRG) procedure calculating excitation gaps, ionicity, lattice site entropy,
two-site entropy, and the dimer order parameter on long chains and the unified
phase diagram has been determined.Comment: 11 pages, 8 figure
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