1,195 research outputs found
"Spin-Disentangled" Exact Diagonalization of Repulsive Hubbard Systems: Superconducting Pair Propagation
By a novel exact diagonalization technique we show that bound pairs propagate
between repulsive Hubbard clusters in a superconducting fashion. The size of
the matrices that must be handled depends on the number of fermion
configurations {\em per spin}, which is of the order of the square root of the
overall size of the Hilbert space. We use CuO units connected by weak O-O
links to model interplanar coupling and c-axis superconductivity in Cuprates.
The numerical evidence on CuO and CuO prompts a new
analytic scheme describing the propagation of bound pairs and also the
superconducting flux quantization in a 3-d geometry.Comment: 5 pages, 3 figure
Three-Body and One-Body Channels of the Auger Core-Valence-Valence decay: Simplified Approach
We propose a computationally simple model of Auger and APECS line shapes from
open-band solids. Part of the intensity comes from the decay of unscreened
core-holes and is obtained by the two-body Green's function ,
as in the case of filled bands. The rest of the intensity arises from screened
core-holes and is derived using a variational description of the relaxed ground
state; this involves the two-holes-one-electron propagator , which
also contains one-hole contributions. For many transition metals, the two-hole
Green's function can be well described by the Ladder
Approximation, but the three-body Green's function poses serious further
problems. To calculate , treating electrons and holes on equal
footing, we propose a practical approach to sum the series to all orders. We
achieve that by formally rewriting the problem in terms of a fictitious
three-body interaction. Our method grants non-negative densities of states,
explains the apparent negative-U behavior of the spectra of early transition
metals and interpolates well between weak and strong coupling, as we
demonstrate by test model calculations.Comment: AMS-LaTeX file, 23 pages, 8 eps and 3 ps figures embedded in the text
with epsfig.sty and float.sty, submitted to Phys. Rev.
Time-dependent transport in graphene nanoribbons
We theoretically investigate the time-dependent ballistic transport in
metallic graphene nanoribbons after the sudden switch-on of a bias voltage .
The ribbon is divided in three different regions, namely two semi-infinite
graphenic leads and a central part of length , across which the bias drops
linearly and where the current is calculated. We show that during the early
transient time the system behaves like a graphene bulk under the influence of a
uniform electric field . In the undoped system the current does not grow
linearly in time but remarkably reaches a temporary plateau with dc
conductivity , which coincides with the minimal
conductivity of two-dimensional graphene. After a time of order
( being the Fermi velocity) the current departs from the first plateau
and saturates at its final steady state value with conductivity
typical of metallic nanoribbons of finite width.Comment: 5 pages, 5 figure
Antiferromagnetism of the 2D Hubbard Model at Half Filling: Analytic Ground State at Weak Coupling
We introduce a local formalism to deal with the Hubbard model on a N times N
square lattice (for even N) in terms of eigenstates of number operators, having
well defined point symmetry. For U -> 0, the low lying shells of the kinetic
energy are filled in the ground state. At half filling, using the 2N-2 one-body
states of the partially occupied shell S_{hf}, we build a set of (2N-2 N-1)^{2}
degenerate unperturbed ground states with S_{z}=0 which are then resolved by
the Hubbard interaction \hat{W}=U\sum_{r}\hat{n}_{r\ua}\hat{n}_{r\da}. In
S_{hf} we study the many-body eigenstates of the kinetic energy with vanishing
eigenvalue of the Hubbard repulsion (W=0 states). In the S_{z}=0 sector, this
is a N times degenerate multiplet. From the singlet component one obtains the
ground state of the Hubbard model for U=0^{+}, which is unique in agreement
with a theorem by Lieb. The wave function demonstrates an antiferromagnetic
order, a lattice step translation being equivalent to a spin flip. We show that
the total momentum vanishes, while the point symmetry is s or d for even or odd
N/2, respectively.Comment: 13 pages, no figure
Pairing in Cu-O Models: Clues of Joint Electron-Phonon and Electron-Electron Interactions
We discuss a many-electron Hamiltonian with Hubbard-like repulsive
interaction and linear coupling to the phonon branches, having the Cu-O plane
of the superconducting cuprates as a paradigm. A canonical transformation
extracts an effective two-body problem from the many-body theory. As a
prototype system we study the \cu cluster, which yields electronic pairing in
the Hubbard model; moreover, a standard treatment of the Jahn-Teller effect
predicts distortions that destroy electronic pairing. Remarkably, calculations
that keep all the electronic spectrum into account show that vibrations are
likely to be synergic with electronic pairing, if the coupling to
half-breathing modes predominates, as experiments suggest.Comment: 4 pages, 3 figures, accepted by Phys. Rev.
The global and persistent millennial-scale variability in the thermoluminescence profiles of shallow and deep Mediterranean sea cores
In this paper we present the thermoluminescence (TL) profile in the last 7500 y, measured in the upper part of the deep Tyrrhenian sea core CT85-5. This core was dated with tephroanalysis and radiocarbon techniques: a constant
sedimentation rate (10 cm/ky) was found up to 200 cm. The sampling interval adopted for obtaining the TL profile is 2.5 mm, corresponding to 25 y. Using different
spectral-analysis methods, we show the presence of a millennial-scale variability, corresponding to an average period of about 1315 y. This oscillation has been noted
also in other climatic indices measured in North Atlantic sea sediment cores and in the Greenland GISP2 ice core. This result indicates that this millennial oscillation is
the expression of climate changes of worldwide extent. We show that this millennial periodicity persisted during the last deglaciation. The transition to Holocene was
determined in our core by the oxygen isotope ratio d 18O measured in Globigerina bulloides. The fact that the observed TL changes do not have a local character is also
suggested by the excellent agreement between this deep sea TL profile of the uppermost part of the core and the TL profile measured in the shallow Ionian sea GT89-3 core over the last 2500 y, with a time resolution of 3.096 y
The sunspot cycle recorded in the thermoluminescence profile of the GT89/3 Ionian sea core
We measured the thermoluminescence (TL) depth profile of the
GT89O3 shallow-water Ionian sea core. This profile has been transformed into a time series using the accurate sedimentation rate previously determined by radiometric and tephroanalysis methods. The TL measurements were performed in samples of equal thickness of 2 mm, corresponding to a time interval of 3.096 y. The TL time series spans A1800 y. The DFT power spectral densities in the decadal periodicity
range of this TL series show significant periodicities at 10.7, 11.3 and 12 y closely similar to the periodicities present in the sunspot number series. These results confirm that the TL signal in recent sea sediments faithfully records the solar variability, as we previously proposed
W=0 pairing in Hubbard and related models of low-dimensional superconductors
Lattice Hamiltonians with on-site interaction have W=0 solutions, that
is, many-body {\em singlet} eigenstates without double occupation. In
particular, W=0 pairs give a clue to understand the pairing force in repulsive
Hubbard models. These eigenstates are found in systems with high enough
symmetry, like the square, hexagonal or triangular lattices. By a general
theorem, we propose a systematic way to construct all the W=0 pairs of a given
Hamiltonian. We also introduce a canonical transformation to calculate the
effective interaction between the particles of such pairs. In geometries
appropriate for the CuO planes of cuprate superconductors, armchair
Carbon nanotubes or Cobalt Oxides planes, the dressed pair becomes a bound
state in a physically relevant range of parameters. We also show that W=0 pairs
quantize the magnetic flux like superconducting pairs do. The pairing mechanism
breaks down in the presence of strong distortions. The W=0 pairs are also the
building blocks for the antiferromagnetic ground state of the half-filled
Hubbard model at weak coupling. Our analytical results for the
Hubbard square lattice, compared to available numerical data, demonstrate that
the method, besides providing intuitive grasp on pairing, also has quantitative
predictive power. We also consider including phonon effects in this scenario.
Preliminary calculations with small clusters indicate that vector phonons
hinder pairing while half-breathing modes are synergic with the W=0 pairing
mechanism both at weak coupling and in the polaronic regime.Comment: 42 pages, Topical Review to appear in Journal of Physics C: Condensed
Matte
22 year cycle in the planktonic 18 of a shallow-water Ionian sea core
The d18 O profile of Globigerinoides ruber was measured in the GT90/3 Ionian sea core between 1205 and 1898 AD. The high temporal resolution of 3.87 y allowed us to determine the presence in the time series of an 11 y component with an
amplitude of 0.07‰, at significance level of 99% (by Monte Carlo singular spectrum analysis, MC-SSA). Here we focus attention on the 22 y periodicity in the time series and we show that SSA principal components (PCs) 15 and 16 carry this oscillation, in phase with the Hale solar cycle, obtained by inverting the odd cycles of the sunspot number series. This result shows that the even and odd Schwabe cycles do not have the same influence on this climatic record
Interacting resonant-level model with long-range interactions: Fast screening and suppression of the zero-bias conductance
The effects of long-range interactions in quantum transport are still largely unexplored, mainly due to
the difficulty of devising efficient embedding schemes. In this work we present a substantial progress
in the interacting resonant level model by reducing the problem to the solution of Kadanoff-Baym-like
equations with a correlated embedding self-energy. The method allows us to deal with short- and long-range
interactions and is applicable from the transient to the steady-state regime. Furthermore, memory effects are
consistently incorporated and the results are not plagued by negative densities or nonconservation of the electric
charge. We employ the method to calculate densities and currents with long-range interactions appropriate to
low-dimensional leads, and show the occurrence of a jamming effect, which drastically reduces the screening
time and suppresses the zero-bias conductance. None of these effects are captured by short-range model
interactions
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