23,674 research outputs found
Surprises on the Way from 1D to 2D Quantum Magnets: the Novel Ladder Materials
One way of making the transition between the quasi-long range order in a
chain of S=1/2 spins coupled antiferromagnetically and the true long range
order that occurs in a plane, is by assembling chains to make ladders of
increasing width. Surprisingly this crossover between one and two dimensions is
not at all smooth. Ladders with an even number of legs have purely short range
magnetic order and a finite energy gap to all magnetic excitations. Predictions
of this novel groundstate have now been verified experimentally. Holes doped
into these ladders are predicted to pair, and possibly superconduct.Comment: Review Article, Science, TeX file, 18 pages, 6 figures available upon
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A Simple Model for the Checkerboard Pattern of Modulated Hole Densities in Underdoped Cuprates
A simple model is proposed as a possible explanation for the checkerboard
pattern of modulations in the hole density observed in recent tunneling
experiments on underdoped cuprates. Two assumptions are made; first, an
enhanced hole density near the acceptor dopants and secondly short range
correlations in the positions of these dopants caused by their electrostatic
and anisotropic elastic interactions. Together these can lead to a structure
factor in qualitative agreement with experiment.Comment: 4 pages, 4 figures; Fig.3 and Fig.4(c) added, typos corrected,
references adde
Interband proximity effect and nodes of superconducting gap in Sr2RuO4
The power-law temperature dependences of the specific heat, the nuclear
relaxation rate, and the thermal conductivity suggest the presence of line
nodes in the superconducting gap of Sr2RuO4. These recent experimental
observations contradict the scenario of a nodeless (k_x+ik_y)-type
superconducting order parameter. We propose that interaction of superconducting
order parameters on different sheets of the Fermi surface is a key to
understanding the above discrepancy. A full gap exists in the active band,
which drives the superconducting instability, while line nodes develop in
passive bands by interband proximity effect.Comment: 4 pages, 1 figur
Multiexciton molecules in the hexaborides
We investigate multiexciton bound states in a semiconducting phase of
divalent hexaborides. Due to three degenerate valleys in both the conduction
and valence bands the binding energy of a 6-exciton molecule is greatly
enhanced by the shell effect. The ground state energies of multiexciton
molecules are calculated using the density functional formalism. We also show
that charged impurities stabilize multiexciton complexes leading to
condensation of localized excitons. These complexes can act as nucleation
centers of local moments.Comment: RevTEX, 7 pages with 3 figure
Pairing and Excitation Spectrum in doped t-J Ladders
Exact diagonalization studies for a doped t-J ladder (or double chain) show
hole pairing in the ground state. The excitation spectrum separates into a
limited number of quasiparticles which carry charge and spin and a triplet mode. At half-filling the former vanish but the latter
evolves continuously into the triplet band of the spin liquid. At low doping
the quasiparticles form a dilute Fermi gas with a strong attraction but
simultaneously the Fermi wavevector, as would be measured in photoemission, is
large.Comment: 10 pages and 4 PostScript figures, RevTeX 3.0, ETH-TH/94-0
Charge profile of surface doped C60
We study the charge profile of a C60-FET (field effect transistor) as used in
the experiments of Schoen, Kloc and Batlogg. Using a tight-binding model, we
calculate the charge profile treating the Coulomb interaction in a mean-field
approximation. The charge profile behaves similarly to the case of a continuous
space-charge layer, in particular it is confined to a single interface layer
for doping higher than ~0.3 electron (or hole) per C60 molecule. The morahedral
disorder of the C60 molecules smoothens the structure in the density of states.Comment: 6 pages, 9 figure
Flow to strong coupling in the two-dimensional Hubbard model
We extend the analysis of the renormalization group flow in the
two-dimensional Hubbard model close to half-filling using the recently
developed temperature flow formalism. We investigate the interplay of d-density
wave and Fermi surface deformation tendencies with those towards d-wave pairing
and antiferromagnetism. For a ratio of next nearest to nearest neighbor
hoppings, t'/t=-0.25, and band fillings where the Fermi surface is inside the
Umklapp surface, only the d-pairing susceptibility diverges at low
temperatures. When the Fermi surface intersects the Umklapp surface close to
the saddle points, d-wave pairing, d-density wave, antiferromagnetic and, to a
weaker extent, d-wave Fermi surface deformation susceptibilities grow together
when the interactions flow to strong coupling. We interpret these findings as
indications for a non-trivial strongly coupled phase with short-ranged
superconducting and antiferromagnetic correlations, in close analogy with the
spin liquid ground state in the well-understood two-leg Hubbard ladder.Comment: 8 pages, to appear in European Physical Journal
How fast do Jupiters grow? Signatures of the snowline and growth rate in the distribution of gas giant planets
We present here observational evidence that the snowline plays a significant
role in the formation and evolution of gas giant planets. When considering the
population of observed exoplanets, we find a boundary in mass-semimajor axis
space that suggests planets are preferentially found beyond the snowline prior
to undergoing gap-opening inward migration and associated gas accretion. This
is consistent with theoretical models suggesting that sudden changes in opacity
-- as would occur at the snowline -- can influence core migration. Furthermore,
population synthesis modelling suggests that this boundary implies that gas
giant planets accrete ~ 70 % of the inward flowing gas, allowing ~ 30$ %
through to the inner disc. This is qualitatively consistent with observations
of transition discs suggesting the presence of inner holes, despite there being
ongoing gas accretion.Comment: 7 pages, 6 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
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