514 research outputs found
Binding of Holes to Magnetic Impurities in a Strongly Correlated System
The effect of a magnetic (S=1/2) impurity coupled to a 2D system of
correlated electrons (described by the t--J model) is studied by exact
diagonalisations. It is found that, if the exchange coupling of the impurity
with the neighboring spins is ferromagnetic or weakly antiferromagnetic, an
extra hole can form bound states of different spatial symmetries with the
impurity extending to a few lattice spacings. The binding energy is maximum
when the impurity is completely decoupled (vacancy) and vanishes for an
antiferromagnetic coupling exceeding . Several peaks appear in the
single hole spectral function below the lower edge of the quasiparticle band as
signatures of the d-, s- and p-wave boundstates.Comment: Latex 11 pages, postscript files in uuencoded form, report#
LPQTH-94/
The Gap Function Phi(k,w) for a Two-leg t-J Ladder and the Pairing Interaction
The gap function phi(k,omega), determined from a Lanczos calculation for a
doped 2-leg t-J ladder, is used to provide insight into the spatial and
temporal structure of the pairing interaction. It implies that this interaction
is a local near-neighbor coupling which is retarded. The onset frequency of the
interaction is set by the energy of an S=1 magnon-hole-pair and it is spread
out over a frequency region of order the bandwith
Resonant Impurity Scattering in a Strongly Correlated Electron Model
Scattering by a single impurity introduced in a strongly correlated
electronic system is studied by exact diagonalization of small clusters. It is
shown that an inert site which is spinless and unable to accomodate holes can
give rise to strong resonant scattering. A calculation of the local density of
state reveals that, for increasing antiferromagnetic exchange coupling, d, s
and p-wave symmetry bound states in which a mobile hole is trapped by the
impurity potential induced by a local distortion of the antiferromagnetic
background successively pull out from the continuum.Comment: 10 pages, 4 figures available on request, report LPQTH-93-2
Thermodynamic properties of the coupled dimer system Cu(CHN)Cl
We re-examine the thermodynamic properties of the coupled dimer system
Cu(CHN)Cl under magnetic field in the light of
recent NMR experiments [Cl\'emancey {\it et al.}, Phys. Rev. Lett. {\bf 97},
167204 (2006)] suggesting the existence of a finite Dzyaloshinskii-Moriya
interaction. We show that including such a spin anisotropy greatly improves the
fit of the magnetization curve and gives the correct trend of the insofar
unexplained anomalous behavior of the specific heat in magnetic field at low
temperature.Comment: published version with minor change
Phase diagram of the two-dimensional t--J model at low doping
The phase diagram of the planar t--J model at small hole doping is
investigated by finite size scaling of exact diagonalisation data of NXN
clusters (up to 26). Hole-droplet binding energies, compressibility and static
spin and charge correlations are calculated. Short range antiferromagnetic
correlations can produce attractive forces between holes leading to a very rich
phase diagram including a liquid of d-wave hole pairs (for ), a
liquid of hole droplets (quartets) for larger J/t ratios ()
and, at even larger coupling J/t, an instability towards phase separation.Comment: 3 pages, latex, 5 postscript figures, uuencode
Impurity-doped Kagome Antiferromagnet: A Quantum Dimer Model Approach
The doping of quantum Heisenberg antiferromagnets on the kagome lattice by
non-magnetic impurities is investigated within the framework of a generalized
quantum dimer model (QDM) describing a) the valence bond crystal (VBC), b) the
dimer liquid and c) the critical region on equal footing. Following the
approach by Ralko et al. [Phys. Rev. Lett. 101, 117204 (2008)] for the square
and triangular lattices, we introduce the (minimal) extension of the QDM on the
Kagome lattice to account for spontaneous creation of mobile S=1/2 spinons at
finite magnetic field. Modulations of the dimer density (at zero or finite
magnetic field) and of the local field-induced magnetization in the vicinity of
impurities are computed using Lanczos Exact Diagonalization techniques on small
clusters (48 and 75 sites). The VBC is clearly revealed from its pinning by
impurities, while, in the dimer liquid, crystallization around impurities
involves only two neighboring dimers. We also find that a next-nearest-neighbor
ferromagnetic coupling favors VBC order. Unexpectedly, a small size
spinon-impurity bound state appears in some region of the the dimer liquid
phase. In contrast, in the VBC phase the spinon delocalizes within a large
region around the impurity, revealing the weakness of the VBC confining
potential. Lastly, we observe that an impurity concentration as small as 4%
enhances dimerization substantially. These results are confronted to the
Valence Bond Glass scenario [R.R.P. Singh, Phys. Rev. Lett. 104, 177203 (2010)]
and implications to the interpretation of the Nuclear Magnetic Resonance
spectra of the Herbertsmithite compound are outlined.Comment: Extended version. 9 pages, 11 figure
Out-of-equilibrium Correlated Systems : Bipartite Entanglement as a Probe of Thermalization
Thermalization play a central role in out-of-equilibrium physics of ultracold
atoms or electronic transport phenomena. On the other hand, entanglement
concepts have proven to be extremely useful to investigate quantum phases of
matter. Here, it is argued that **bipartite** entanglement measures provide key
information on out-of-equilibrium states and might therefore offer stringent
thermalization criteria. This is illustrated by considering a global quench in
an (extended) XXZ spin-1/2 chain across its (zero-temperature) quantum critical
point. A non-local **bipartition** of the chain **preserving translation
symmetry** is proposed. The time-evolution after the quench of the **reduced**
density matrix of the half-system is computed and its associated
(time-dependent) entanglement spectrum is analyzed. Generically, the
corresponding entanglement entropy quickly reaches a "plateau" after a short
transient regime. However, in the case of the integrable XXZ chain, the
low-energy entanglement spectrum still reveals strong time-fluctuations. In
addition, its infinite-time average shows strong deviations from the spectrum
of a Boltzmann thermal density matrix. In contrast, when the integrability of
the model is broken (by small next-nearest neighbor couplings), the
entanglement spectra of the time-average and thermal density matrices become
remarkably similar.Comment: extended version: 15 pages, 9 figure
Low-frequency current fluctuations in doped ladders
Charge current static and dynamical correlations are computed by exact
diagonalisation methods on a 2-leg t-t'-J ladder which exhibits a sharp
transition between a Luther-Emery (LE) phase of hole pairs and a phase with
deconfined holes. In the LE phase, we find short-range low-energy
incommensurate current fluctuations which are intrinsically connected to the
internal charge dynamics within one hole pair. On the contrary, when holes
unbind, the maximum of the current susceptibility moves abruptly to the
commensurate wavevector and strongly increases for decreasing doping
suggesting an instability towards a staggered flux state at sufficiently small
doping.Comment: 4 pages, 5 figure
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