820 research outputs found
Superconductivity from doping a spin liquid insulator: a simple one-dimensional example
We study the phase diagram of a one-dimensional Hubbard model where, in
addition to the standard nearest neighbor hopping , we also include a
next-to-nearest neighbor hopping . For strong enough on-site repulsion,
this model has a transition at half filling from a magnetic insulator with
gapless spin excitations at small to a dimerized insulator with a spin
gap at larger . We show that upon doping this model exhibits quite
interesting features, which include the presence of a metallic phase with a
spin gap and dominant superconducting fluctuations, in spite of the repulsive
interaction. More interestingly, we find that this superconducting phase can be
reached upon hole doping the magnetic insulator. The connections between this
model and the two chain models, recently object of intensive investigations,
are also discussed.Comment: 19 pages, plain LaTex using RevTex, 7 postscript figures Modified
version which excludes some LaTex commands giving problems for the previous
versio
Role of Umklapp Processes in Conductivity of Doped Two-Leg Ladders
Recent conductivity measurements performed on the hole-doped two-leg ladder
material reveal an approximately linear
power law regime in the c-axis DC resistivity as a function of temperature for
. In this work, we employ a bosonic model to argue that umklapp processes
are responsible for this feature and for the high spectral weight in the
optical conductivity which occurs beyond the finite frequency Drude-like peak.
Including quenched disorder in our model allows us to reproduce experimental
conductivity and resistivity curves over a wide range of energies. We also
point out the differences between the effect of umklapp processes in a single
chain and in the two-leg ladder.Comment: 10 pages, 2 figure
Friedel Oscillations and Charge Density Waves in Chains and Ladders
The density matrix renormalization group method for ladders works much more
efficiently with open boundary conditions. One consequence of these boundary
conditions is groundstate charge density oscillations that often appear to be
nearly constant in magnitude or to decay only slightly away from the
boundaries. We analyse these using bosonization techniques, relating their
detailed form to the correlation exponent and distinguishing boundary induced
generalized Friedel oscillations from true charge density waves. We also
discuss a different approach to extracting the correlation exponent from the
finite size spectrum which uses exclusively open boundary conditions and can
therefore take advantage of data for much larger system sizes. A general
discussion of the Friedel oscillation wave-vectors is given, and a convenient
Fourier transform technique is used to determine it. DMRG results are analysed
on Hubbard and t-J chains and 2 leg t-J ladders. We present evidence for the
existence of a long-ranged charge density wave state in the t-J ladder at a
filling of n=0.75 and near J/t \approx 0.25.Comment: Revtex, 15 pages, 15 postscript figure
Finite-temperature perturbation theory for quasi-one-dimensional spin-1/2 Heisenberg antiferromagnets
We develop a finite-temperature perturbation theory for quasi-one-dimensional
quantum spin systems, in the manner suggested by H.J. Schulz (1996) and use
this formalism to study their dynamical response. The corrections to the
random-phase approximation formula for the dynamical magnetic susceptibility
obtained with this method involve multi-point correlation functions of the
one-dimensional theory on which the random-phase approximation expansion is
built. This ``anisotropic'' perturbation theory takes the form of a systematic
high-temperature expansion. This formalism is first applied to the estimation
of the N\'eel temperature of S=1/2 cubic lattice Heisenberg antiferromagnets.
It is then applied to the compound CsCuCl, a frustrated S=1/2
antiferromagnet with a Dzyaloshinskii-Moriya anisotropy. Using the next leading
order to the random-phase approximation, we determine the improved values for
the critical temperature and incommensurability. Despite the non-universal
character of these quantities, the calculated values are different by less than
a few percent from the experimental values for both compounds.Comment: 11 pages, 6 figure
Unscreened Coulomb repulsion in the one dimensional electron gas
A tight binding model of electrons interacting via bare Coulomb repulsion is
numerically investigated by use of the Density Matrix Renormalization Group
method which we prove applicable also to very long range potentials. From the
analysis of the elementary excitations, of the spin and charge correlation
functions and of the momentum distribution, a picture consistent with the
formation of a one dimensional "Wigner crystal" emerges, in quantitative
agreement with a previous bosonization study. At finite doping, Umklapp
scattering is shown to be ineffective in the presence of long range forces.Comment: RevTex, 5 pages with 8 eps figures. To be published on Phys. Rev.
Tomonaga-Luttinger features in the resonant Raman spectra of quantum wires
The differential cross section for resonant Raman scattering from the
collective modes in a one dimensional system of interacting electrons is
calculated non-perturbatively using the bosonization method. The results
indicate that resonant Raman spectroscopy is a powerful tool for studying
Tomonaga-Luttinger liquid behaviour in quasi-one dimensional electron systems.Comment: 4 pages, no figur
A Bosonic Model of Hole Pairs
We numerically investigate a bosonic representation for hole pairs on a
two-leg t-J ladder where hard core bosons on a chain represent the hole pairs
on the ladder. The interaction between hole pairs is obtained by fitting the
density profile obtained with the effective model to the one obtained with the
\tj model, taking into account the inner structure of the hole pair given by
the hole-hole correlation function. For these interactions we calculate the
Luttinger liquid parameter, which takes the universal value as
half filling is approached, for values of the rung exchange between strong
coupling and the isotropic case. The long distance behavior of the hole-hole
correlation function is also investigated. Starting from large , the
correlation length first increases as expected, but diminishes significantly as
is reduced and bound holes sit mainly on adjacent rungs. As the isotropic
case is approached, the correlation length increases again. This effect is
related to the different kind of bonds in the region between the two holes of a
hole pair when they move apart.Comment: 11 page
Holons on a meandering stripe: quantum numbers
We attempt to access the regime of strong coupling between charge carriers
and transverse dynamics of an isolated conducting ``stripe'', such as those
found in cuprate superconductors. A stripe is modeled as a partially doped
domain wall in an antiferromagnet (AF), introduced in the context of two
different models: the t-J model with strong Ising anisotropy, and the Hubbard
model in the Hartree-Fock approximation. The domain walls with a given linear
charge density are supported artificially by boundary conditions. In both
models we find a regime of parameters where doped holes lose their spin and
become holons (charge Q=1, spin S_z=0), which can move along the stripe without
frustrating AF environment. One aspect in which the holons on the AF domain
wall differ from those in an ordinary one-dimensional electron gas is their
transverse degree of freedom: a mobile holon always resides on a transverse
kink (or antikink) of the domain wall. This gives rise to two holon flavors and
to a strong coupling between doped charges and transverse fluctuations of a
stripe.Comment: Minor revisions: references update
The transition between hole-pairs and four-hole clusters in four-leg tJ ladders
Holes weakly doped into a four-leg \tj ladder bind in pairs. At dopings
exceeding a critical doping of four hole clusters are
observed to form in DMRG calculations. The symmetry of the ground state
wavefunction does not change and we are able to reproduce this behavior
qualitatively with an effective bosonic model in which the four-leg ladder is
represented as two coupled two-leg ladders and hole-pairs are mapped on hard
core bosons moving along and between these ladders. At lower dopings,
, a one dimensional bosonic representation for hole-pairs
works and allows us to calculate accurately the Luttinger liquid parameter
\krho, which takes the universal value \krho=1 as half-filling is
approached
New species of Entomobrya from Germany (Collembola, Entomobryini)
The systematic study of specimens of Entomobrya from various European museums, private collections and other samplings, allows us to describe several species new of the genus. Specimens from Germany, deposited at the Senckenberg Museum of Natural History Görlitz (SMNG), identified as new species as result of this study, are described: Entomobrya dungeri n. sp., Entomobrya germanica n. sp., Entomobrya saxoniensis n. sp., Entomobrya schulzi Jordana & Baquero n. sp. and Entomobrya dorsolineata n. sp
- …