arXiv subjects

R. Eder

Publications and source records attributed to R. Eder.

At least 73 records · Page 4Linked to original sources

arXiv1997

Photoemission spectra of the t-J model in 1 and 2D: similarities and differences

We present an exact diagonalization study of the single particle spectral function in the 1D and 2D t-J model. By studying the scaling properties with J and t we find a simple building pattern in 1D and show that every spectral feature can be uniquely assigned by a spinon and holon momentum. We find two types of low energy excitations: a band with energy scale J and high spectral weight disperses upwards in the interior part of the Brillouin zone and reaches E_F at k_F, and a band with energy scale t and low spectral weight disperses downwards in the outer part of the zone, touching E_F at 3k_F. An analogous analysis of the 2D case at half filling shows that the t-band exists also in this case, but is diffuse and never reaches the Fermi energy. For the doped case in 2D the picture is more reminiscent of 1D, in particular the `main-band' with a dispersion proportional J and the `shadow band' with energy scale t can be identified also in this case. This leads us to propose that the shadow bands discovered by Aebi et al. in Bi2212 are the 2D analogue of the 3k_F singularity in 1D systems and unrelated to antiferromagnetic spin fluctuations.

cond-mat.str-el↗

arXiv1997

Hole Photoproduction in Insulating Copper Oxide

To explain the experimental spectra for angle resolved photoemission we consider a modified t-J model. The modified model includes next nearest (t') and next next nearest (t'') hopping as well as Hubbard model corrections to the spectral weights. A Dyson equation which relates the single hole Green's functions for a given pseudospin and given spin is derived and is compared to experimental results.

cond-mat↗

arXiv1997

Single particle excitations of the Kondo lattice

We present a simple theory for the description of the single particle excitations in the Kondo lattice model. Thereby we derive an `effective Hamiltonian' which describes the coherent propagation of single particle-like fluctuations on a strong coupling background. Even for f-electrons which are replaced by Kondo-spins, the resulting spectral function obeys the Luttinger theorem including the f-electrons, and our calculation fully reproduces the nontrivial variation of the spectral function and momentum distribution with electron density and parameter values as seen in exact diagonalization and density matrix renormalization group studies.

cond-mat.str-el↗

arXiv1996

Doping dependent quasiparticle band structure in cuprate superconductors

We present an exact diagonalization study of the single particle spectral function in the so-called t-t'-t''-J model in 2D. As a key result, we find that unlike the `pure' t-J model, hole doping leads to a major reconstruction of the quasiparticle band structure near (pi,0): whereas for the undoped system the quasiparticle states near (pi,0) are deep below the top of the band at (pi/2,pi/2), hole doping shifts these states up to E_F, resulting in extended flat band regions close to E_F and around (pi,0). This strong doping-induced deformation can be directly compared to angle resolved photoemission results on Sr_2 Cu Cl_2 O_2, underdoped Bi2212 and optimally doped Bi2212. We propose the interplay of long range hopping and decreasing spin correlations as the mechanism of this deformation.

cond-mat.supr-con↗

arXiv1996

Optical conductivity of strongly correlated electron systems

We present an exact diagonalization study of the frequency and wave vector dependent conductivity $σ(q,ω)$ in small clusters of $2D$ $t$$-$$J$ model. Unlike the related dynamical density correlation function, $σ(q=0,ω)$ in the underdoped regime has the exchange constant $J$ as its characteristic energy scale and is dominated by a resonance-like excitation with frequency $\sim 1.7J$. We interpret this as transition to a $p$-like excited state of a spin-bag type quasiparticle (or, alternatively, a tightly bound spinon-holon pair) and show that a simple calculation based on the string picture explains the numerical results semiquantitatively. For doping levels $\ge 25% t$ remains the only energy scale of $σ(q=0,ω)$.

cond-mat↗

arXiv1996

Dynamics of the 1D Heisenberg model and optical absorption of spinons in cuprate antiferromagnetic chains

We use numerical and analytical results to construct a simple ansatz for the energy dynamical correlation function of the 1D antiferromagnetic Heisenberg model. This is applied to compute the phonon assisted absorption spectra of magnetic excitations (spinons) in quasi-one dimensional spin 1/2 insulators and show to reproduce very well recent infrared measurements in Sr$_2$CuO$_3$.

cond-mat↗

arXiv1996

Low energy excitations of the t-J model in 1D and 2D

We present an exact diagonalization study of the low energy singlet and triplet states for both 1D and 2D t-J model. A scan of the parameter ratio J/t shows that for nearly all low energy states in both 1D and 2D the excitation energy takes the linear form E(t,J) = a t + b J. In 1D this is the natural consequence of the factorization of the low energy wave functions, i.e. spin-charge separation. Examination of the low energy eigenstates in 2D shows that in the singlet sector the lowest states are collective modes, which for larger J correspond to `dynamical hole columns', i.e. a periodic modulation of the hole density. This modulation is well reproduced by treating holes as hard-core Bosons with an attractive interaction.

cond-mat↗

arXiv1996

Inter-site Coulomb interaction and Heisenberg exchange

Based on exact diagonalization results for small clusters we discuss the effect of inter-site Coulomb repulsion in Mott-Hubbard or charge transfer insulators. Whereas the exchange constant J for direct exchange is substantially enhanced by inter-site Coulomb interaction, that for superexchange is suppressed. The enhancement of J in the single-band models holds up to the critical value for the charge density wave (CDW) instability, thus opening the way for large values of J. Single-band Hubbard models with sufficiently strong inter-site repulsion to be near a CDW instability thus may provide `physical' realizations of t-J like models with the `unphysical' parameter ratio J/t=1.

cond-mat↗

arXiv1995

Heavy Quasiparticles in the Anderson Lattice Model

An exact-diagonalization technique on small clusters is used to study the dynamics of the one-dimensional symmetric Anderson lattice model. Our calculated excitation spectra reproduce key features expected for an infinite Kondo lattice such as nearly localized low-energy spin excitations and extended regions of `heavy-quasiparticle' bands. We show that, in contrast to the hybridization picture, low-energy spin excitations of the nearly-localized $f$-electron system play a key role in the formation of an almost dispersionless low-energy band of heavy quasiparticles.

cond-mat↗

arXiv1995

Inverse photoemission in strongly correlated electron systems

Based on exact results for small clusters of 2D t-J model we demonstrate the existence of several distinct `channels' in its inverse photoemission (IPES) spectrum. Hole-like quasiparticles can either be annihilated completely, or leave behind a variable number of spin excitations, which formed the `dressing cloud' of the annihilated hole. In the physical parameter regime the latter processes carry the bulk of IPES weight and although the Fermi surface takes the form of hole pockets, the distribution of spectal weight including these `magnon-bands' in the IPES spectrum is reminiscent of free electrons. The emerging scenario for Fermiology and spectral weight distribution is shown to be consistent with photoemission, inverse photemission and de Haas--van Alphen experiments on cuprate superconductors.

cond-mat↗

arXiv1995

Excitons in C_60 studied by Temperature Dependent Optical Second- Harmonic Generation

The electric dipole forbidden T_1g excitonic state of solid C_60 at omega=1.81 eV can be probed with a Second-Harmonic Generation (SHG) experiment. We show that the SHG line shape depends strongly on the degree of rotational order. We observe a splitting into two peaks below the rotational ordering phase transition temperature of 260 K. The origin of this splitting is discussed in terms of a possible Jahn-Teller effect, a possible Davydov splitting due to the four molecules per unit cell in the low temperature phase, and a mixing of the nearly degenerate T_1g and G_g free molecule states because of the lower symmetry in the solid. The exciton band structure is calculated with a charge transfer mediated propagation mechanism as suggested by Lof et al. and with one-electron (-hole) transfer integrals determined from band structure calculations. Comparison with our experimental SHG data leads to a reasonable agreement and shows that a mixing of T_1g and G_g states may explain the splitting at low temperature.

cond-mat↗

arXiv1995

Brownian motion of Massive Particle in a Space with Curvature and Torsion and Crystals with Defects

We develop a theory of Brownian motion of a massive particle, including the effects of inertia (Kramers' problem), in spaces with curvature and torsion. This is done by invoking the recently discovered generalized equivalence principle, according to which the equations of motion of a point particle in such spaces can be obtained from the Newton equation in euclidean space by means of a nonholonomic mapping. By this principle, the known Langevin equation in euclidean space goes over into the correct Langevin equation in the Cartan space. This, in turn, serves to derive the Kubo and Fokker-Planck equations satisfied by the particle distribution as a function of time in such a space. The theory can be applied to classical diffusion processes in crystals with defects.

cond-mat↗

arXiv1995

Ground state properties and dynamics of the bilayer t-J model

We present an exact diagonalization study of bilayer clusters of t-J model. Our results indicate a crossover between two markedly different regimes which occurs when the ratio J_perp/J between inter-layer and intra-layer exchange constants increases: for small J_perp/J the data suggest the development of 3D antiferromagnetic correlations without appreciable degradation of the intra-layer spin order and the d_(x2-y2) hole pairs within the planes persist. For larger values of J_perp/J local singlets along the inter-layer bonds dominate, leading to an almost complete suppression of the intra-layer spin correlation and the breaking of the intra-layer pairs. The ground state with two holes in this regime has s-like symmetry. The data suggest that the crossover may occur for values of J_perp/J as small as 0.2. We present data for static spin correlations, spin gap, and electron momentum distribution and spectral function of the `inter-layer RVB state' realized for large J_perp/J. The latter deviates from the single layer ground state, making it an implausible candidate for modelling high-temperature superconductors.

cond-mat↗

arXiv1995

Anomalous Spin and Charge Dynamics of the 2D t-J Model at low doping

We present an exact diagonalization study of the dynamical spin and density correlation function of the 2D t-J model for hole doping < 25%. Both correlation functions show a remarkably regular, but completely different scaling behaviour with both hole concentration and parameter values: the density correlation function is consistent with that of bosons corresponding to the doped holes and condensed into the lowest state of the noninteracting band of width 8t, the spin correlation function is consistent with Fermions in a band of width J. We show that the spin bag picture gives a natural explanation for this unusual behaviour.

cond-mat↗

arXiv1994

Exact Diagonalization Study of Strongly Correlated Electron Models: Hole pockets and shadow bands in the doped t-J model

A detailed exact-diagonalization study is made for the doping dependence of the single-particle spectral function $A({\bf k},ω)$ and momentum distribution function $n({\bf k})$ of the two-dimensional $t$$-$$J$ model as a representative model for doped Mott insulators. The results for $A({\bf k},ω)$ show unambiguously that the rigid-band behavior is realized in the small-cluster $t$$-$$J$ model: upon doping, the uppermost states of the quasiparticle band observed at half filling simply cross the Fermi level and reappear as the lowermost states of the inverse photoemission spectrum, while the photoemission side of the band remains essentially unaffected. We discuss problems in directly determining the Fermi surface from $n({\bf k})$ and make a situation where they are largely avoided; we then find clear signatures of a Fermi surface which takes the form of small hole pockets. The identical scaling with $t/J$ of the quasiparticle weight $Z_h$ and difference in $n({\bf k})$ between neighboring ${\bf k}$-points suggests the existence of such a Fermi surface in the physical regime of parameters. We construct spin-bag operators which describe the holes dressed by the antiferromagnetic spin fluctuations and find that elementary electronic excitations of the system can be described well in terms of weakly-interacting spin-1/2 Fermionic quasiparticles corresponding to the doped holes. We make a comparison with other numerical calculations and recent angle-resolved photoemission experiment and argue that, adopting this rather conventional Fermi-liquid scenario with non-Luttinger Fermi surface, one would explain many quasiparticle-relating properties of doped cuprates in a very simple and natural way. We also show that the dynamical spin and charge excitations deviate from the particle-hole excitations of this Fermi liquid: e.g., the dominant low-energy spin excitation at momentum transfer $(π,π)$ reflects excitations of the incoherent spin background and is identified as a collective mode comparable to spin waves in the Heisenberg antiferromagnet.

cond-mat↗

arXiv1994

Spin and charge dynamics of the t-J model at intermediate electron densities: absence of spin-charge separation

We present an exact diagonalization study of the dynamical spin and density correlation functions in small clusters of 2D t-J model for intermediate and low electron densities, rho<0.7. Both correlation functions agree remarkably well with the convolution of the single-particle spectral function, i.e. the simplest estimate within the Fermi liquid picture. Deviations from the convolution are shown to originate from symmetry-related selection rules, which are unaccounted for in the convolution estimate. Comparing the momentum distribution in the final states of the low lying spin and charge excitations to that of the ground state we establish that for all fillings under consideration the low energy peaks in both correlation functions originate from particle-hole excitations, as expected in a Fermi liquid.

cond-mat↗

arXiv1994

Spin dynamics of the doped t-J model

We present a new method for studying the low energy excitations in finite cluster systems and apply it to clarify the nature of the low energy spin excitations in the moderately doped t-J model. We find that the character of the low energy spin excitations changes within the Brillouin zone, with the dominant excitation for momentum transfer (pi,pi) being a spin wave-like collective mode. Spin excitations with momentum transfer different from (pi,pi) have particle-hole character. We give an explicit construction of a low energy collective spin mode in a system with `almost broken symmetry' and demonstrate its validity by explicit numerical check.

cond-mat↗

arXiv1994

Hole pockets in the t-J model

We present an exact diagonalization study of the electron momentum distribution n(k) in small clusters of t-J model for different hole concentrations and t/J. Structures in n(k) which were previously interpreted as a `large' Fermi surface are identified as originating from the well known many-body backflow. To obtain reliable information about the true Fermi surface, we focus on the regime t= J.

cond-mat↗