554 research outputs found
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
Magnetic Domains and Stripes in the Spin-Fermion Model for Cuprates
Monte Carlo simulations applied to the Spin-Fermion model for cuprates show
the existence of antiferromagnetic spin domains and charge stripes upon doping.
The stripes are partially filled, with a filling of approximately 1/2 hole per
site, and they separate spin domains with a phase shift among them. The
stripes observed run either along the x or y axes and they are separated by a
large energy barrier. No special boundary conditions or external fields are
needed to stabilize these structures at low temperatures. When magnetic
incommensurate peaks are observed at momentum and symmetrical
points, charge incommensurate peaks appear at and symmetrical
points, as experimentally observed. The strong charge fluctuations responsible
for the formation of the stripes also induce a pseudogap in the density of
states.Comment: Four pages with four figures embedded in tex
Aspects of the FM Kondo Model: From Unbiased MC Simulations to Back-of-an-Envelope Explanations
Effective models are derived from the ferromagnetic Kondo lattice model with
classical corespins, which greatly reduce the numerical effort. Results for
these models are presented. They indicate that double exchange gives the
correct order of magnitude and the correct doping dependence of the Curie
temperature. Furthermore, we find that the jump in the particle density
previously interpreted as phase separation is rather explained by ferromagnetic
polarons.Comment: Proceedings of Wandlitz Days of Magnetism 200
Temperature Derivative of the Superfluid Density in the Attractive Hubbard model
Based on extensions of the grand-canonical Quantum Monte-Carlo algorithm to
incorporate magnetic fields, we provide numerical data confirming the existence
of a Kosterlitz-Thouless transition in the attractive Hubbard model. Here, we
calculate the temperature derivative of the superfluid density, to pin down the
transition. Away from half-band filling, the above quantity, shows a response
which increases with lattice size at the transition temperature. In contrast,
such a signal is not observed for the case of a half-band filling.Comment: Latex 8 pages, 3 figures (in postscript format) appendded at the end
of the fil
Influence of next-nearest-neighbor electron hopping on the static and dynamical properties of the 2D Hubbard model
Comparing experimental data for high temperature cuprate superconductors with
numerical results for electronic models, it is becoming apparent that a hopping
along the plaquette diagonals has to be included to obtain a quantitative
agreement. According to recent estimations the value of the diagonal hopping
appears to be material dependent. However, the values for discussed
in the literature were obtained comparing theoretical results in the weak
coupling limit with experimental photoemission data and band structure
calculations. The goal of this paper is to study how gets renormalized as
the interaction between electrons, , increases. For this purpose, the effect
of adding a bare diagonal hopping to the fully interacting two dimensional
Hubbard model Hamiltonian is investigated using numerical techniques. Positive
and negative values of are analyzed. Spin-spin correlations, ,
vs , and local magnetic moments are studied for values
of ranging from 0 to 6, and as a function of the electronic density. The
influence of the diagonal hopping in the spectral function
is also discussed, and the changes in the gap present in the density of states
at half-filling are studied. We introduce a new criterion to determine probable
locations of Fermi surfaces at zero temperature from data obtained
at finite temperature. It appears that hole pockets at
may be induced for negative while a positive produces similar
features at and . Comparisons with the standard 2D
Hubbard () model indicate that a negative hopping amplitude appears
to be dynamically generated. In general, we conclude that it is very dangerous
to extract a bare parameter of the Hamiltonian from PES data whereComment: 9 pages (RevTex 3.0), 12 figures (postscript), files packed with
uufile
Interference Effects in the Conductance of Multi-Level Quantum Dots
Using exact-diagonalization techniques supplemented by a Dyson equation
embedding procedure, the transport properties of multilevel quantum dots are
investigated in the Kondo regime. The conductance can be decomposed into the
contributions of each level. It is shown that these channels can carry a
different phase, and destructive interference processes are observed when the
phase difference between them is . This effect is very different from
those observed in bulk metals with magnetic impurities, where the phase
differences play no significant role. The effect is also different from other
recent studies of interference processes in dots, as discussed in the text. In
particular, no external magnetic field is here introduced, and the hopping
amplitudes dot-leads for all levels are the same. However, conductance
cancellations induced by interactions are still observed. Another interesting
effect reported here is the formation of localized states that do not
participate in the transport. When one of these states crosses the Fermi level,
the electronic occupation of the quantum dot changes, modifying the many-body
physics of the system and indirectly affecting the transport properties. Novel
discontinuities between two finite conductance values can occur as the gate
voltage is varied, as discussed here
Quasiparticle dispersion of the t-J and Hubbard models
The spectral weight of the two dimensional and Hubbard models has been calculated using exact diagonalization and
quantum Monte Carlo techniques, at several densities . The photoemission region contains two
dominant distinct features, namely a low-energy quasiparticle peak with
bandwidth of order J, and a broad valence band peak at energies of order t.
This behavior away from half-filling, as long as the
antiferromagnetic (AF) correlations are robust. The results give support to
theories of the copper oxide materials based on the behavior of holes in
antiferromagnets, and it also provides theoretical guidance for the
interpretation of experimental photoemission data for the cuprates.Comment: (minor changes) RevTeX, 4 figures available on reques
Optical conductivity of the Hubbard model at finite temperature
The optical conductivity, , of the two dimensional one-band
Hubbard model is calculated at finite temperature using exact diagonalization
techniques on finite clusters. The in-plane d.c. resistivity, , is
also evaluated. We find that at large U/t and temperature T, is
approximately linear with temperature, in reasonable agreement with experiments
on high-T superconductors. Moreover, we note that displays
charge excitations, a mid-infrared (MIR) band and a Drude peak, also as
observed experimentally. The combination of the Drude peak and the MIR
oscillator strengths leads to a conductivity that decays slower than
at energies smaller than the insulator gap near half-filling.Comment: 12 pages, 3 figures appended, Revtex version 2.0, preprin
CORE and the Haldane Conjecture
The Contractor Renormalization group formalism (CORE) is a real-space
renormalization group method which is the Hamiltonian analogue of the Wilson
exact renormalization group equations. In an earlier paper\cite{QGAF} I showed
that the Contractor Renormalization group (CORE) method could be used to map a
theory of free quarks, and quarks interacting with gluons, into a generalized
frustrated Heisenberg antiferromagnet (HAF) and proposed using CORE methods to
study these theories. Since generalizations of HAF's exhibit all sorts of
subtle behavior which, from a continuum point of view, are related to
topological properties of the theory, it is important to know that CORE can be
used to extract this physics. In this paper I show that despite the folklore
which asserts that all real-space renormalization group schemes are necessarily
inaccurate, simple Contractor Renormalization group (CORE) computations can
give highly accurate results even if one only keeps a small number of states
per block and a few terms in the cluster expansion. In addition I argue that
even very simple CORE computations give a much better qualitative understanding
of the physics than naive renormalization group methods. In particular I show
that the simplest CORE computation yields a first principles understanding of
how the famous Haldane conjecture works for the case of the spin-1/2 and spin-1
HAF.Comment: 36 pages, 4 figures, 5 tables, latex; extensive additions to conten
Binding of holes and pair spectral function in the t-J model
Clusters of the two-dimensionnal t--J model with 2 holes and up to 26 sites
are diagonalized using a Lanczos algorithm. The behaviour of the binding energy
with system size suggests the existence of a finite critical value of J above
which binding occurs in the bulk. Only the d-wave pair field operator acting on
the Heisenberg GS has a finite overlap with the 2 hole ground state for all the
clusters considered. The related spectral function associated with the
propagation of a d-wave (spin singlet) pair of holes in the antiferromagnetic
background is calculated. The quasiparticle peak at the bottom of the spectrum
as well as some structure appearing above the peak survive with increasing
cluster size. Although no simple scaling law was found for the quasiparticle
weight the data strongly suggest that this weight is finite in the bulk limit
and is roughly proportional to the antiferromagnetic coupling J (for J<1).Comment: Report LPQTH-93/01, 18 pages (REVTEX), 8 postscript files include
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