arXiv subjects

Karlo Penc

Publications and source records attributed to Karlo Penc.

71 records · Page 4Linked to original sources

arXiv2002

Quantum Phase Transition in the SU(4) Spin-Orbital Model on the Triangular Lattice

Motivated by the absence of cooperative Jahn-Teller effect in LiNiO2 and BaVS3, two layered oxides with triangular planes, we study the SU(4) symmetric spin-orbital model on the triangular lattice. Upon reducing the next-nearest neighbour coupling, we show that the system undergoes a quantum phase transition to a liquid phase. A variational approach to this liquid phase shows that simple types of long-range correlations are suppressed, suggesting that it is stable against lattice distortions.

cond-mat.str-el↗

arXiv2002

Fractional charges in pyrochlore lattices

A pyrochlore lattice is considered where the average electron number of electrons per site is half--integer, concentrating on the case of exactly half an electron per site. Strong on-site repulsions are assumed, so that all sites are either empty or singly occupied. Where there are in addition strong nearest--neighbour repulsions, a tetrahedron rule comes into effect, as previously suggested for magnetite. We show that in this case, there exist excitations with fractional charge (+/-) e/2. These are intimately connected with the high degeneracy of the ground state in the absence of kinetic energy terms. When an additional electron is inserted into the system, it decays into two point like excitations with charge -e/2, connected by a Heisenberg spin chain which carries the electron's spin.

cond-mat.str-el↗

arXiv2001

Exact Spectral Functions of a Non Fermi Liquid in 1 Dimension

We study the exact one electron propagator and spectral function of a solvable model of interacting electrons due to Schulz and Shastry. The solution previously found for the energies and wave functions is extended to give the spectral functions, which turn out to be computable, interesting and non trivial. They provide one of the few examples of cases where the spectral functions are known asymptotically as well as exactly.

cond-mat.str-el↗

arXiv2000

Dynamical correlations in one-dimensional charge-transfer insulators

The single-particle spectral function and the density response of a two band Emery model for CuO chains is calculated for large on-site Cu repulsion U and large on-site energy difference Δ. For U>>U-Δ>>t the eigenfunctions are products of charge and spin parts, which allows analytical calculation of spectral functions in that limit. For other parameters numerical diagonalization is used. The low energy hole carriers are shown to be the one-dimensional analogs of the Zhang-Rice singlets. The validity of the one-band model is discussed. The results are relevant to the interpretation of photoemission and EELS experiments on SrCuO2 and Sr2CuO3 .

cond-mat.str-el↗

arXiv2000

spl(2,1) dynamical supersymmetry and suppression of ferromagnetism in flat band double-exchange models

The low energy spectrum of the ferromagnetic Kondo lattice model on a N-site complete graph extended with on-site repulsion is obtained from the underlying spl(2,1) algebra properties in the strong coupling limit. The ferromagnetic ground state is realized for 1 and N+1 electrons only. We identify the large density of states to be responsible for the suppression of the ferromagnetic state and argue that a similar situation is encountered in the Kagome, pyrochlore, and other lattices with flat bands in their one-particle density of states.

cond-mat.str-el↗

arXiv1998

Solution of quantum double exchange on the complete graph from spl(2,1) dynamical supersymmetry

The exact and analytical low energy spectrum of the ferromagnetic Kondo lattice model on a complete graph extended with on-site repulsion $U$ is obtained in terms of a dynamical spl(2,1) supersymmetry in the limit of infinitely strong Hund's coupling (double exchange). Furthermore, we show that for the particular value of $U=J_H/2$ the supersymmetry is not constrained to infinite $J_H/t$ only and we calculate the energy including the $t^2/J_H$ corrections analytically and we give numerical evidence which suggest that the Kondo Hamiltonian is itself supersymmetric for any $J_H/t$. On a $N$ site graph, the ferromagnetic ground state is realized for 1 and $N$+1 electrons only. In the leading order in the value of the core spin, the quantum and semiclassical spectra are identical.

cond-mat.str-el↗

arXiv1997

Finite temperature spectral-functions of strongly correlated one-dimensional electron systems

The spectral functions of tJ and tJ_{XY} models in the limit of J/t-> 0 and at finite temperatures T>>t are calculated using the spin-charge factorized wave function. We find that the Luttinger-liquid like scaling behavior for a finite system with L sites is restricted below temperatures of the order T = J/L. We also observe weight redistribution in the photoemission spectral function in the energy range t, which is much larger than the temperature.

cond-mat.str-el↗

arXiv1997

Spectral Function and Self-Energy of the One-Dimensional Hubbard Model in the U --> infinity Limit

The interpretation of the k dependent spectral functions of the one-dimensional, infinite U Hubbard model obtained by using the factorized wave-function of Ogata and Shiba is revisited. The well defined feature which appears in addition to low energy features typical of Luttinger liquids, and which, close to the Fermi energy, can be interpreted as the shadow band resulting from $2k_F$ spin fluctuations, is further investigated. A calculation of the self-energy shows that, not too close to the Fermi energy, this feature corresponds to a band, i.e. to a solution of the Dyson equation $ω- ε(k) - Re Σ(k,ω) =0$.

cond-mat.str-el↗

arXiv1997

Spectral functions of the 1D Hubbard model in the U -> \infty limit: How to use the factorized wave-function

We give the details of the calculation of the spectral functions of the 1D Hubbard model using the spin-charge factorized wave-function for several versions of the U -> +\infty limit. The spectral functions are expressed as a convolution of charge and spin dynamical correlation functions. A procedure to evaluate these correlation functions very accurately for large systems is developed, and analytical results are presented for the low energy region. These results are fully consistent with the conformal field theory. We also propose a direct method of extracting the exponents from the matrix elements in more general cases.

cond-mat.str-el↗

arXiv1996

Spectral Functions of One-dimensional Models of Correlated Electrons

Using the Ogata-Shiba wave function, the spectral functions of the one-dimensional infinite U Hubbard model are calculated for various concentrations. It is shown that the ``shadow band'' feature due to 2k_F fluctuations becomes more intense close to half-filling. Comparing these results with exact diagonalization data obtained on finite clusters for the finite U Hubbard model and for the t-J model, it is also shown that this feature remains well-defined for physically reasonable values of the parameters (U/t\simeq 10, J/t\simeq 0.4). The ``shadow'' structure in the spectral functions should thus be observable in angle-resolved photoemission experiments for a variety of quasi-one dimensional compounds.

cond-mat.str-el↗

arXiv1996

Dynamical density-density correlations in one-dimensional Mott insulators

The dynamical density-density correlation function is calculated for the one-dimensional, half-filled Hubbard model extended with nearest neighbor repulsion using the Lanczos algorithm for finite size systems and analytically for large on site repulsion compared to hopping amplitudes. At the zone boundary an excitonic feature exists for any finite nearest neighbor repulsion and exhausts most of the spectral weight, even for parameters where no exciton is visible at zero momentum.

cond-mat↗

arXiv1996

Ferromagnetism in multi--band Hubbard models: From weak to strong Coulomb repulsion

We propose a new mechanism which can lead to ferromagnetism in Hubbard models containing triangles with different on-site energies. It is based on an effective Hamiltonian that we derive in the strong coupling limit. Considering a one-dimensional realization of the model, we show that in the quarter-filled, insulating case the ground-state is actually ferromagnetic in a very large parameter range going from Tasaki's flat-band limit to the strong coupling limit of the effective Hamiltonian. This result has been obtained using a variety of analytical and numerical techniques. Finally, the same results are shown to apply away from quarter-filling, in the metallic case.

cond-mat↗

arXiv1995

Shadow band in the one-dimensional large $U$ Hubbard model

We show that the factorized wave-function of Ogata and Shiba can be used to calculate the $k$ dependent spectral functions of the one-dimensional, infinite $U$ Hubbard model, and of some extensions to finite $U$. The resulting spectral function is remarkably rich: In addition to low energy features typical of Luttinger liquids, there is a well defined band, which we identify as the shadow band resulting from $2k_F$ spin fluctuations. This band should be detectable experimentally because its intensity is comparable to that of the main band for a large range of momenta.

cond-mat↗

arXiv1995

Propagating S=1/2 particles in S=1 Haldane gap systems

Inspired by the recent experiments on Y$_{2-x}$Ca$_x$BaNiO$_5$, (J. F. DiTusa {\it et al.}, Phys. Rev. Lett. 73, 1857 (1994)), we discuss the dispersion relation of the $s$=1/2 particles in the $s$=1 Heisenberg and VBS model in the limit of small hopping amplitudes. The effective $s$=1/2 edge spins mix with the spin of the impurity resulting in one four--fold and two two--fold degenarate bands. We briefly discuss the interaction between the $s$=1/2 particles arising from the background.

cond-mat↗

arXiv1995

Susceptibility of the one-dimensional, dimerized Hubbard model

We show that the zero temperature susceptibility of the one-dimensional, dimerized Hubbard model at quarter-filling can be accurately determined on the basis of exact diagonalization of small clusters. The best procedure is to perform a finite-size scaling of the spin velocity $u_σ$, and to calculate the susceptibility from the Luttinger liquid relation $χ=2/πu_σ$. We show that these results are reliable by comparing them with the analytical results that can be obtained in the weak and strong coupling limits. We have also used quantum Monte Carlo simulations to calculate the temperature dependence of the susceptibility for parameters that should be relevant to the Bechgaard salts. This shows that, used together, these numerical techniques are able to give precise estimates of the low temperature susceptibility of realistic one-dimensional models of correlated electrons.

cond-mat↗

arXiv1995

Spectral function of the 1D Hubbard model in the $U\to +\infty $ limit

We show that the one-particle spectral functions of the one-dimensional Hubbard model diverge at the Fermi energy like $|ω-\varepsilon_F|^{-3/8}$ in the $U\to +\infty $ limit. The Luttinger liquid behaviour $|ω-\varepsilon_F|^α$, where $α\to 1/8$ as $U\to +\infty $, should be limited to $|ω-\varepsilon_F| \sim t^2/U$ (for $U$ large but finite), which shrinks to a single point, $ω=\varepsilon_F$,in that limit. The consequences for the observation of the Luttinger liquid behaviour in photoemission and inverse photoemission experiments are discussed.

cond-mat↗

arXiv1993

Phase diagram of the one-dimensional extended Hubbard model with attractive and/or repulsive interactions at quarter filling

We study the phase diagram of the one dimensional (1D) $U-V$ model at quarter filling in the most general case where the on-site and first-neighbour interactions $U$ and $V$ can be both attractive and repulsive. The results have been obtained using exact diagonalization of small clusters and variational techniques, as well as exact results in various limits. We have analyzed four properties of the groundstate: i)~whether it is insulating or metallic; \hbox{ii)~whether} it is homogenous or phase separated; iii)~whether it has a spin gap; iv)~whether it has dominant superconducting fluctuations. With eight phases, the resulting phase diagram is unexpectedly rich. The four phases not found in the weak coupling limit are: i) an insulating phase when $U$ and $V$ are large enough; ii) a region of phase separation when $V$ is attractive; iii) another region of phase separation when $V$ is large enough and $U$ small; iv) a region with dominant superconducting fluctuations when $V$ is intermediate and $U$ small. The actual nature of this last phase, which has pairs but no spin gap, is not fully clear yet.

cond-mat↗