396 research outputs found
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
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
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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