430 research outputs found
Cluster Dynamical Mean-Field Theory of the density-driven Mott transition in the one-dimensional Hubbard model
The one-dimensional Hubbard model is investigated by means of two different
cluster schemes suited to introduce short-range spatial correlations beyond the
single-site Dynamical Mean-Field Theory, namely the Cluster-Dynamical
Mean-Field Theory and its periodized version. It is shown that both cluster
schemes are able to describe with extreme accuracy the evolution of the density
as a function of the chemical potential from the Mott insulator to the metallic
state. Using exact diagonalization to solve the cluster impurity model, we
discuss the role of the truncation of the Hilbert space of the bath, and
propose an algorithm that gives higher weights to the low frequency
hybridization matrix elements and improves the speed of the convergence of the
algorithm.Comment: 6 pages, 4 figures, minor corrections in v
Pseudogap induced by short-range spin correlations in a doped Mott insulator
We study the evolution of a Mott-Hubbard insulator into a correlated metal
upon doping in the two-dimensional Hubbard model using the Cellular Dynamical
Mean Field Theory. Short-range spin correlations create two additional bands
apart from the familiar Hubbard bands in the spectral function. Even a tiny
doping into this insulator causes a jump of the Fermi energy to one of these
additional bands and an immediate momentum dependent suppression of the
spectral weight at this Fermi energy. The pseudogap is closely tied to the
existence of these bands. This suggests a strong-coupling mechanism that arises
from short-range spin correlations and large scattering rates for the pseudogap
phenomenon seen in several cuprates.Comment: 6 pages, 6 figure
Unconventional high-energy-state contribution to the Cooper pairing in under-doped copper-oxide superconductor HgBaCaCuO
We study the temperature-dependent electronic B1g Raman response of a
slightly under-doped single crystal HgBaCaCuO with a
superconducting critical temperature Tc=122 K. Our main finding is that the
superconducting pair-breaking peak is associated with a dip on its
higher-energy side, disappearing together at Tc. This result hints at an
unconventional pairing mechanism, whereas spectral weight lost in the dip is
transferred to the pair-breaking peak at lower energies. This conclusion is
supported by cellular dynamical mean-field theory on the Hubbard model, which
is able to reproduce all the main features of the B1g Raman response and
explain the peak-dip behavior in terms of a nontrivial relationship between the
superconducting and the pseudo gaps.Comment: 7 pages 4 figure
Nodal/Antinodal Dichotomy and the Two Gaps of a Superconducting Doped Mott Insulator
We study the superconducting state of the hole-doped two-dimensional Hubbard
model using Cellular Dynamical Mean Field Theory, with the Lanczos method as
impurity solver. In the under-doped regime, we find a natural decomposition of
the one-particle (photoemission) energy-gap into two components. The gap in the
nodal regions, stemming from the anomalous self-energy, decreases with
decreasing doping. The antinodal gap has an additional contribution from the
normal component of the self-energy, inherited from the normal-state pseudogap,
and it increases as the Mott insulating phase is approached.Comment: Corrected typos, 4.5 pages, 4 figure
On the Nonlinear Shaping Gain with Probabilistic Shaping and Carrier Phase Recovery
The performance of different probabilistic amplitude shaping (PAS)techniques in the nonlinear regime is investigated, highlighting its dependence on the PAS block length and the interaction with carrier phase recovery (CPR). Different PAS implementations are considered, based on different distribution matching (DM) techniques—namely, sphere shaping, shell mapping with different number of shells, and constant composition DM—and amplitude-to-symbol maps. When CPR is not included, PAS with optimal block length provides a nonlinear shaping gain with respect to a linearly optimized PAS (with infinite block length); among the considered DM techniques, the largest gain is obtained with sphere shaping. On the other hand, the nonlinear shaping gain becomes smaller, or completely vanishes, when CPR is included, meaning that in this case all the considered implementations achieve a similar performance for a sufficiently long block length. Similar results are obtained in different link configurations ( km, km, and km single-mode-fiber links), and also including laser phase noise, except when in-line dispersion compensation is used. Furthermore, we define a new metric, the nonlinear phase noise (NPN) metric, which is based on the frequency resolved logarithmic perturbation models and explains the interaction of CPR and PAS. We show that the NPN metric is highly correlated with the performance of the system. Our results suggest that, in general, the optimization of PAS in the nonlinear regime should always account for the presence of a CPR algorithm. In this case, the reduction of the rate loss (obtained by using sphere shaping and increasing the DM block length) turns out to be more important than the mitigation of the nonlinear phase noise (obtained by using constant-energy DMs and reducing the block length), the latter being already granted by the CPR algorithm
Anomalous superconductivity and its competition with antiferromagnetism in doped Mott insulators
Proximity to a Mott insulating phase is likely to be an important physical
ingredient of a theory that aims to describe high-temperature superconductivity
in the cuprates. Quantum cluster methods are well suited to describe the Mott
phase. Hence, as a step towards a quantitative theory of the competition
between antiferromagnetism (AFM) and d-wave superconductivity (SC) in the
cuprates, we use Cellular Dynamical Mean Field Theory to compute zero
temperature properties of the two-dimensional square lattice Hubbard model. The
d-wave order parameter is found to scale like the superexchange coupling J for
on-site interaction U comparable to or larger than the bandwidth. The order
parameter also assumes a dome shape as a function of doping while, by contrast,
the gap in the single-particle density of states decreases monotonically with
increasing doping. In the presence of a finite second-neighbor hopping t', the
zero temperature phase diagram displays the electron-hole asymmetric
competition between antiferromagnetism and superconductivity that is observed
experimentally in the cuprates. Adding realistic third-neighbor hopping t''
improves the overall agreement with the experimental phase diagram. Since band
parameters can vary depending on the specific cuprate considered, the
sensitivity of the theoretical phase diagram to band parameters challenges the
commonly held assumption that the doping vs T_{c}/T_{c}^{max} phase diagram of
the cuprates is universal. The calculated ARPES spectrum displays the observed
electron-hole asymmetry. Our calculations reproduce important features of
d-wave superconductivity in the cuprates that would otherwise be considered
anomalous from the point of view of the standard BCS approach.Comment: 13 pages, 7 figure
Interplay of magnetic and structural transitions in Fe-based pnictide superconductors
The interplay between the structural and magnetic phase transitions occurring
in the Fe-based pnictide superconductors is studied within a Ginzburg-Landau
approach. We show that the magnetoelastic coupling between the corresponding
order parameters is behind the salient features observed in the phase diagram
of these systems. This naturally explains the coincidence of transition
temperatures observed in some cases as well as the character (first or
second-order) of the transitions. We also show that magnetoelastic coupling is
the key ingredient determining the collinearity of the magnetic ordering, and
we propose an experimental criterion to distinguish between a pure elastic from
a spin-nematic-driven structural transition.Comment: 5 pages, 3 figures. v2: Fig. 1 improved, references added
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