arXiv subjects

D. Poilblanc

Publications and source records attributed to D. Poilblanc.

71 records · Page 4Linked to original sources

arXiv1996

Multimode electron-phonon interactions in the 2-dimensional t-J-Holstein model

Polaronic effects are investigated in the two dimensional t--J model (describing Cu spins and holes of the planes in high-$T_c$ cuprates) in the presence of in-plane dynamic oxygen breathing modes. We focus on the effect of dynamic multi-phonon modes. Spin and charge structure factors are computed. We found evidence for self-localisation of the holes and {\it increased} short range antiferromagnetic spin-spin correlations between the remaining spins. This effect persists when all the in-plane oxygen vibration degrees of freedom are taken into account. This suggests that dynamic phonons could play a key role in stabilizing inhomogeneous stripe phases or domain wall structures.

cond-mat↗

arXiv1995

Optical Conductivity of a t--J Ladder

The optical conductivity $σ(ω)$ of a doped two-leg t--J ladder is calculated for an electric field polarized parallel to the legs of the ladder. The conductivity has a Drude weight proportional to the hole doping and an apparent threshold for absorption (a pseudo gap) which may be associated with the energy to break a pair. This pseudogap in $σ(ω)$ is present even though the pairs have a modified $d_{x^2-y^2}$-like wave function because the geometry of the ladder leads to quasi-particle states which probe the gap along an antinode.

cond-mat↗

arXiv1995

Dynamical Properties of Two Coupled Hubbard Chains at Half-filling

Using grand canonical Quantum Monte Carlo (QMC) simulations combined with Maximum Entropy analytic continuation, as well as analytical methods, we examine the one- and two-particle dynamical properties of the Hubbard model on two coupled chains at half-filling. The one-particle spectral weight function, $A({\bf k},ω)$, undergoes a qualitative change with interchain hopping $t_\perp$ associated with a transition from a four-band insulator to a two-band insulator. A simple analytical model based on the propagation of exact rung singlet states gives a good description of the features at large $t_\perp$. For smaller $t_\perp$, $A({\bf k}, ω)$ is similar to that of the one-dimensional model, with a coherent band of width the effective antiferromagnetic exchange $J$ reasonably well-described by renormalized spin-wave theory. The coherent band rides on a broad background of width several times the parallel hopping integral $t$, an incoherent structure similar to that found in calculations on both the one- and two-dimensional models. We also present QMC results for the two-particle spin and charge excitation spectra, and relate their behavior to the rung singlet picture for large $t_\perp$ and to the results of spin-wave theory for small $t_\perp$.

cond-mat↗

arXiv1995

T-Matrix Formulation of Impurity Scattering in Correlated Systems

Using a generalized T-matrix description which, in principle, exactly includes Coulomb correlations and potential scattering events, resonant and bound impurity states are discussed. Like in the non-interacting case, the effects of the scattering potential can be divided into different partial wave channels, exploiting the symmetry of the underlying lattice. Due to Coulomb correlations bare local (i.e.\ s-wave) potentials become dynamic and extended, being responsible also for p-, d-wave etc. scattering effects. Numerically exact results for both the two-dimensional t--J and Hubbard models are used to construct a simple (static) approximation to the effective impurity potential which is shown to reproduce the exact resonant scattering and bound states in the relevant symmetry channels.

cond-mat↗

arXiv1995

Pair breaking by impurities in the two-dimensional t-J model

Pair breaking mechanisms by impurities are investigated in the two-dimensional t-J model by exact diagonalization techniques. Analysis of binding energies, pairing correlations, dynamical spin and pair susceptibilities shows that non-magnetic impurities are more effective in suppressing pairing than magnetic ones in agreement with experimental studies of Zn- and Ni- substituted High-Tc superconductors.

cond-mat↗

arXiv1995

Luttinger liquid behaviour and superconducting correlations in t-J ladders

The low energy behaviour of the isotropic t--J ladder system is investigated using exact diagonalization techniques, specifically finding the Drude weight, the charge velocity and the compressibility. By applying the ideas of Luttinger liquid theory, we determine the correlation exponent $K_ρ$ which defines the behaviour of the long range correlations in the system. The boundary to phase separation is determined and a phase diagram is presented. At low electron density, a Tomonaga-Luttinger-like phase is stabilized whilst at higher electron densities a gapped phase with power law pairing correlations is stabilized: A large region of this gapped phase is found to exhibit dominant superconducting correlations.

cond-mat↗

arXiv1995

Evidence for composite nature of quasiparticles in the 2D t-J model

It is shown that the dynamics of a single hole in a quantum antiferromagnet (described by the t--J model) can be simply understood in terms of a composite quasiparticle. This description provides naturally two different energy scales t and J corresponding to the inverse masses of the charge (holon) and spin (spinon) elementary excitations respectively. This picture is consistent with the exact results obtained on small clusters for the single hole spectral function and optical conductivity providing that one assumes the existence of a string-like force of magnitude J between the holon and the spinon. Then the hole quasiparticle can be interpreted as a bound state of its two constituents.

cond-mat↗

arXiv1995

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 $J/t\gtrsim 0.2$), a liquid of hole droplets (quartets) for larger J/t ratios ($J/t\gtrsim 0.5$) and, at even larger coupling J/t, an instability towards phase separation.

cond-mat↗

arXiv1995

Evidence for a superfluid density in t--J ladders

Applying three independent techniques, we give numerical evidence for a finite superfluid density in isotropic hole-doped t--J ladders: We show the existence of anomalous flux quantization, emphasising the contrasting behaviour to that found in the `Luttinger liquid' regime stabilised at low electron densities; We consider the nature of the low-lying excitation modes, finding the 1-D analog of the superconducting state; And using a density matrix renormalization group approach, we find long range pairing correlations and exponentially decaying spin-spin correlations.

cond-mat↗

arXiv1995

SPIN AND CHARGE MODES OF THE t-J LADDER

The spin and charge excitations of the t--J ladder are studied by exact diagonalization techniques for several electron densities. The various modes are classified according to their spin (singlet or triplet excitations) and their parity under a reflection with respect to the symmetry axis along the chains and a finite size scaling of the related gaps is performed. At low doping, formation of hole pairs leads to a spin gap for all $J/t$ ratios. This phase is characterized by (at least) one vanishing energy mode {\it only} in the charge bonding channel when $K_x\rightarrow 0$ consistent with the existence of superconducting pairing correlations. At larger doping the spin gap disappears. Although the anti-bonding modes remain gapped, low energy $K_x\sim 0$ and $K_x\sim 2k_F$ spin and charge bonding modes are found consistent with a single band Luttinger scenario. At sufficient low electron density and above a critical value of J/t we also expect another phase of electron pairs with gapped spin excitations.

cond-mat↗

arXiv1995

Large physical spin approach for strongly correlated electrons

We present a novel approach for a systematic large--spin expansion of the $t$-$J$ Hamiltonian which enables us to work without the constraint of no double occupancy. In our scheme we can perform the large--spin limit ensuring that the low energy spin excitations are in {\em exact} correspondence with the physical excitations of the $s={1\over2}$ Hilbert space. As a consequence, we expect a smooth dependence of the physical quantities on the expansion parameter $1/ s$. As a first application of the method we study the case of a single hole in a Néel background. A systematic expansion in fluctuations about this stable solution indicates that by increasing $t/J$ the quasiparticle weight strongly depends on the momentum carried by the hole. Results, obtained on small lattice sizes, are found in excellent agreement with exact diagonalization data.

cond-mat↗

arXiv1994

Spin Gaps in Coupled t-J Ladders

Spin gaps in coupled $t$-$J$ ladders are investigated by exact diagonalization of small clusters up to 4$\times$8 sites. At half-filling, the numerical results for the triplet excitation spectrum are in very good agreement with a second order perturbation expansion in term of small inter-ladder and intra-ladder exchange couplings between rungs ($J/J^\prime$$<$$0.25$). The band of local triplet excitations moving coherently along the ladder (with momenta close to $π$) is split by the inter-ladder coupling. For intermediate couplings finite size scaling is used to estimate the spin gap. In the isotropic infinite 4-chain system (two coupled ladders) we find a spin gap of $0.245 J$, roughly half of the single ladder spin gap. When the system is hole doped, bonding and anti-bonding bound pairs of holes can propagate coherently along the chains and the spin gap remains finite.

cond-mat↗

arXiv1994

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 $\sim 0.3 J$. 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.

cond-mat↗

arXiv1994

Magnetic order and disorder in the frustrated quantum Heisenberg antiferromagnet in two dimensions

We have performed a numerical investigation of the ground state properties of the frustrated quantum Heisenberg antiferromagnet on the square lattice (``$J_1-J_2$ model''), using exact diagonalization of finite clusters with 16, 20, 32, and 36 sites. Using a finite-size scaling analysis we obtain results for a number of physical properties: magnetic order parameters, ground state energy, and magnetic susceptibility (at $q=0$). For the unfrustrated case these results agree with series expansions and quantum Monte Carlo calculations to within a percent or better. In order to assess the reliability of our calculations, we also investigate regions of parameter space with well-established magnetic order, in particular the non-frustrated case $J_2<0$. We find that in many cases, in particular for the intermediate region $0.3 < J_2/J_1 < 0.7$, the 16 site cluster shows anomalous finite size effects. Omitting this cluster from the analysis, our principal result is that there is Néel type order for $J_2/J_1 < 0.34$ and collinear magnetic order (wavevector $\bbox{Q}=(0,π)$) for $J_2/J_1 > 0.68$. There thus is a region in parameter space without any form of magnetic order. Including the 16 site cluster, or analyzing the independently calculated magnetic susceptibility we arrive at the same conclusion, but with modified values for the range of existence of the nonmagnetic region. We also find numerical values for the spin-wave velocity and the spin stiffness. The spin-wave velocity remains finite at the magnetic-nonmagnetic transition, as expected from the nonlinear sigma

cond-mat↗

arXiv1993

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.

cond-mat↗

arXiv1993

Persistent Currents in 1D Disordered Rings of Interacting Electrons

We calculate the persistent current of 1D rings of spinless fermions with short-range interactions on a lattice with up to 20 sites, and in the presence of disorder, for various band fillings. We find that {\it both} disorder and interactions always decrease the persistent current by localizing the electrons. Away from half-filling, the interaction has a much stronger influence in the presence of disorder than in the pure case.

cond-mat↗

arXiv1993

Bound states of holes in an antiferromagnet

The formation of bound states of holes in an antiferromagnetic spin-1/2 background is studied using numerical techniques applied to the ${\rm t-J}$ Hamiltonian on clusters with up to 26 sites. An analysis of the binding energy as a function of cluster size suggests that a two hole bound state is formed for couplings larger than a ``critical'' value ${\rm J/t]_c}$. The symmetry of the bound state is $\dx2y2$. We also observed that its ``quasiparticle'' weight ${\rm Z_{2h}}$ (defined in the text), is finite for all values of the coupling ${\rm J/t}$. Thus, in the region ${\rm J/t \geq J/t]_c}$ the bound state of two holes behaves like a quasiparticle with charge $Q=2e$, spin $S=0$, and $\dx2y2$ internal symmetry. The relation with recent ideas that have suggested the possibility of d-wave pairing in the high temperature cuprate superconductors is briefly discussed.

cond-mat↗