981 research outputs found
Superconducting Puddles and "Colossal'' Effects in Underdoped Cuprates
Phenomenological models for the antiferromagnetic (AF) vs. d-wave
superconductivity competition in cuprates are studied using conventional Monte
Carlo techniques. The analysis suggests that cuprates may show a variety of
different behaviors in the very underdoped regime: local coexistence or
first-order transitions among the competing orders, stripes, or glassy states
with nanoscale superconducting (SC) puddles. The transition from AF to SC does
not seem universal. In particular, the glassy state leads to the possibility of
"colossal'' effects in some cuprates, analog of those in manganites. Under
suitable conditions, non-superconducting Cu-oxides could rapidly become
superconducting by the influence of weak perturbations that align the randomly
oriented phases of the SC puddles in the mixed state. Consequences of these
ideas for thin-film and photoemission experiments are discussed.Comment: RevTeX 4, revised expanded version, 8 pages, 8 figure
Critical behavior of the S=3/2 antiferromagnetic Heisenberg chain
Using the density-matrix renormalization-group technique we study the
long-wavelength properties of the spin S=3/2 nearest-neighbor Heisenberg chain.
We obtain an accurate value for the spin velocity v=3.8+- 0.02, in agreement
with experiment. Our results show conclusively that the model belongs to the
same universality class as the S=1/2 Heisenberg chain, with a conformal central
charge c=1 and critical exponent eta=1Comment: RevTeX (version 3.0), 4 twocolumn pages with 4 embedded figure
Coexistence of Pairing Tendencies and Ferromagnetism in a Doped Two-Orbital Hubbard Model on Two-Leg Ladders
Using the Density Matrix Renormalization Group and two-leg ladders, we
investigate an electronic two-orbital Hubbard model including plaquette
diagonal hopping amplitudes. Our goal is to search for regimes where charges
added to the undoped state form pairs, presumably a precursor of a
superconducting state.For the electronic density , i.e. the undoped
limit, our investigations show a robust antiferromagnetic ground
state, as in previous investigations. Doping away from and for large
values of the Hund coupling , a ferromagnetic region is found to be stable.
Moreover, when the interorbital on-site Hubbard repulsion is smaller than the
Hund coupling, i.e. for in the standard notation of multiorbital Hubbard
models, our results indicate the coexistence of pairing tendencies and
ferromagnetism close to . These results are compatible with previous
investigations using one dimensional systems. Although further research is
needed to clarify if the range of couplings used here is of relevance for real
materials, such as superconducting heavy fermions or pnictides, our theoretical
results address a possible mechanism for pairing that may be active in the
presence of short-range ferromagnetic fluctuations.Comment: 8 pages, 4 Fig
Large-Scale Monte Carlo Study of a Realistic Lattice Model for Ga_(1-x)Mn_xAs
The properties of Mn-doped GaAs are studied at several doping levels and hole
compensations, using a real-space Hamiltonian on an fcc lattice that reproduces
the valence bands of undoped GaAs. Large-scale Monte Carlo (MC) simulations on
a Cray XT3 supercomputer, using up to a thousand nodes, were needed to make
this effort possible. Our analysis considers both the spin-orbit interaction
and the random distribution of the Mn ions. The hopping amplitudes are
functions of the GaAs Luttinger parameters. At the coupling J~1.2eV deduced
from photoemission experiments, the MC Curie temperature and the shape of the
magnetization curves are in agreement with experimental results for annealed
samples. Although there are sizable differences with mean-field predictions,
the system is found to be closer to a hole-fluid regime than to localized
carriers
Transport properties of strongly correlated electrons in quantum dots using a simple circuit model
Numerical calculations are shown to reproduce the main results of recent
experiments involving nonlocal spin control in nanostructures (N. J. Craig et
al., Science 304, 565 (2004)). In particular, the splitting of the
zero-bias-peak discovered experimentally is clearly observed in our studies. To
understand these results, a simple "circuit model" is introduced and shown to
provide a good qualitative description of the experiments. The main idea is
that the splitting originates in a Fano anti-resonance, which is caused by
having one quantum dot side-connected in relation to the current's path. This
scenario provides an explanation of Craig et al.'s results that is alternative
to the RKKY proposal, which is here also addressed.Comment: 5 pages, 5 figure
Unexpected Conductance Dip in the Kondo Regime of Linear Arrays of Quantum Dots
Using exact-diagonalization of small clusters and Dyson equation embedding
techniques, the conductance of linear arrays of quantum dots is
investigated. The Hubbard interaction induces Kondo peaks at low temperatures
for an odd number of dots. Remarkably, the Kondo peak is split in half by a
deep minimum, and the conductance vanishes at one value of the gate voltage.
Tentative explanations for this unusual effect are proposed, including an
interference process between two channels contributing to , with one more
and one less particle than the exactly-solved cluster ground-state. The Hubbard
interaction and fermionic statistics of electrons also appear to be important
to understand this phenomenon. Although most of the calculations used a
particle-hole symmetric Hamiltonian and formalism, results also presented here
show that the conductance dip exists even when this symmetry is broken. The
conductance cancellation effect obtained using numerical techniques is
potentially interesting, and other many-body techniques should be used to
confirm its existence
- …