Effect of Hund's rule coupling on SU(4) spin-orbital system

We investigate the ground-state property of a one-dimensional two-orbital Hubbard model at quarter filling by numerical techniques such as the density-matrix renormalization group method and the exact diagonalization. When the Hund's rule coupling $J$ is zero, the model is SU(4) symmetric. In fact, both spin and orbital correlations have a peak at $q=\pi/2$, indicating an SU(4) singlet state with a four-site periodicity. On the other hand, with increasing $J$, it is found that the peak position of the orbital correlation changes to $q=\pi$, while that of the spin correlation remains at $q=\pi/2$. We briefly discuss how the SU(4) symmetry is broken by $J$.Comment: 2 pages, 2 figures, Proceedings of ICM2006 (August 20-25, 2006, Kyoto

Low-Energy Properties of Antiferromagnetic Spin-1/2 Heisenberg Ladders with an Odd Number of Legs

An effective low-energy description for multi-leg spin-1/2 Heisenberg ladders with an odd number of legs is proposed. Using a newly developed Monte Carlo loop algorithm and exact diagonalization techniques, the uniform and staggered magnetic susceptibility and the entropy are calculated for ladders with 1, 3, and 5 legs. These systems show a low-temperature scaling behavior similar to spin-1/2 chains with longer ranged unfrustrated exchange interactions. The spinon velocity does not change as the number of legs increases, but the energy scale parameter decreases markedly.Comment: 4 pages and 5 figure

Phase diagram of three-leg ladders at strong coupling along the rungs

A phase diagram of the t-J three-leg ladder as a function of hole dopping is derived in the limit where the coupling parameters along the rungs, $t_{\perp}$ and $J_{\perp}$, are taken to be much larger than those along the legs, $t_{||}$ and $J_{||}$ At large exchange coupling along the rungs, $J_{\perp}/t_{\perp}> 3 / \sqrt{2}$, there is a transition from a low-dopping Luttinger liquid phase into a Luther-Emery liquid at a critical hole concentration $n_{crit}\approx 1/3$. In the opposite case, $J_{\perp}/t_{\perp}< 3 / \sqrt{2}$, there as a sequence of three Luttinger liquid phases (LLI, LLII and LLIII) as a function of hole dopping.Comment: 9 pages, 15 figure

Ist ein kommunales Konjunkturprogramm sinnvoll?

In seiner Regierungserklärung vom 14. März 2003 kündigte Bundeskanzler Gerhard Schröder unter anderem zinsgünstige Kreditprogramme für bestimmte kommunale Investitionsvorhaben an. Nach Ansicht von Prof. Dr. Helmut Seitz, Universität Frankfurt/Oder, handelt es sich hierbei »um ein reines Zinsverbilligungsprogramm für kommunale Infrastrukturprojekte ... Die Erwartung zu hegen, dass sich mit diesem Programm bei einem Subventionswert von 2 q je Einwohner in den nächsten Jahren eine spürbare Belebung der kommunalen Investitionsaktivität, geschweige denn der Konjunktur, bewerkstelligen lässt, ist sicherlich völlig überzogen.« Prof. Dr. Gunnar Schwarting, Städtetag Rheinland-Pfalz, sieht die Gefahr, dass das Programm nur zu einer Umfinanzierung bestehender Kreditrahmen und damit lediglich zu Mitnahmeeffekten führt. Auch Dr. Birgit Frischmuth, Deutscher Städtetag, äußert sich skeptisch: »Mit einem Kreditprogramm der Kreditanstalt für Wiederaufbau wird das Ziel einer wirksamen Soforthilfe für die Kommunen verfehlt. ... Es gibt keine Alternative zu einer Gemeindefinanzreform.« Für Prof. Dr. Horst Zimmermann, Universität Marburg, dagegen »ist eine kurzfristige Stärkung der Einnahmen der Städte sinnvoll. ... Das Fazit lautet daher, dass ein kommunales Konjunkturprogramm derzeit durchaus Sinn macht.

Ist ein kommunales Konjunkturprogramm sinnvoll?

In seiner Regierungserklärung vom 14. März 2003 kündigte Bundeskanzler Gerhard Schröder unter anderem zinsgünstige Kreditprogramme für bestimmte kommunale Investitionsvorhaben an. Nach Ansicht von Prof. Dr. Helmut Seitz, Universität Frankfurt/Oder, handelt es sich hierbei »um ein reines Zinsverbilligungsprogramm für kommunale Infrastrukturprojekte ... Die Erwartung zu hegen, dass sich mit diesem Programm bei einem Subventionswert von 2 q je Einwohner in den nächsten Jahren eine spürbare Belebung der kommunalen Investitionsaktivität, geschweige denn der Konjunktur, bewerkstelligen lässt, ist sicherlich völlig überzogen.« Prof. Dr. Gunnar Schwarting, Städtetag Rheinland-Pfalz, sieht die Gefahr, dass das Programm nur zu einer Umfinanzierung bestehender Kreditrahmen und damit lediglich zu Mitnahmeeffekten führt. Auch Dr. Birgit Frischmuth, Deutscher Städtetag, äußert sich skeptisch: »Mit einem Kreditprogramm der Kreditanstalt für Wiederaufbau wird das Ziel einer wirksamen Soforthilfe für die Kommunen verfehlt. ... Es gibt keine Alternative zu einer Gemeindefinanzreform.« Für Prof. Dr. Horst Zimmermann, Universität Marburg, dagegen »ist eine kurzfristige Stärkung der Einnahmen der Städte sinnvoll. ... Das Fazit lautet daher, dass ein kommunales Konjunkturprogramm derzeit durchaus Sinn macht.«Konjunkturpolitik, Kommunale Politik, Kommunale Infrastruktur

Low-lying magnon excitations in integer-spin ladders and tubes

We consider low-energy excitation structures of N-leg integer-spin ladder and tube systems with an antiferromagnetic (AF) intrachain coupling and a uniform external field. The tube means the ladder with the periodic boundary condition along the interchain (rung) direction. Odd-leg AF-rung tubes have the frustration. In order to analyze all systems including frustrated tubes, we apply a field-theoretical method based on the nonlinear sigma model. We mainly focus on the systems without any external fields. In this case, it is shown that the lowest bands of frustrated tubes always consist of six-fold degenerate magnon excitations, while those of all other systems are triply degenerate. This result implies that the ground states of frustrated tubes (all non-frustrated systems) become a two (one)-component Tomonaga-Luttinger liquid, when a sufficiently strong uniform field is applied.Comment: 4 pages, 5 figures, published version (Proceedings of ISSP-9). See also cond-mat/050604

Elementary excitations of the symmetric spin-orbital model: The XY limit

The elementary excitations of the 1D, symmetric, spin-orbital model are investigated by studying two anisotropic versions of the model, the pure XY and the dimerized XXZ case, with analytical and numerical methods. While they preserve the symmetry between spin and orbital degrees of freedom, these models allow for a simple and transparent picture of the low--lying excitations: In the pure XY case, a phase separation takes place between two phases with free--fermion like, gapless excitations, while in the dimerized case, the low-energy effective Hamiltonian reduces to the 1D Ising model with gapped excitations. In both cases, all the elementary excitations involve simultaneous flips of the spin and orbital degrees of freedom, a clear indication of the breakdown of the traditional mean-field theory.Comment: Revtex, two figure

Quantum Monte Carlo Loop Algorithm for the t-J Model

We propose a generalization of the Quantum Monte Carlo loop algorithm to the t-J model by a mapping to three coupled six-vertex models. The autocorrelation times are reduced by orders of magnitude compared to the conventional local algorithms. The method is completely ergodic and can be formulated directly in continuous time. We introduce improved estimators for simulations with a local sign problem. Some first results of finite temperature simulations are presented for a t-J chain, a frustrated Heisenberg chain, and t-J ladder models.Comment: 22 pages, including 12 figures. RevTex v3.0, uses psf.te

Lightly Doped t-J Three-Leg Ladders - an Analog for the Underdoped Cuprates

The three-leg ladder has one odd-parity and two even-parity channels. At low doping these behave quite differently. Numerical calculations for a t-J model show that the initial phase upon hole doping has two components - a conducting Luttinger liquid in the odd-parity channel, coexisting with an insulating (i.e. undoped) spin liquid phase in the even-parity channels. This phase has a partially truncated Fermi surface and violates the Luttinger theorem. This coexistence of conducting fermionic and insulating paired bosonic degrees of freedom is similar to the recent proposal of Geshkenbein, Ioffe, and Larkin for the underdoped spin-gap normal phase of the cuprates. A mean field approximation is derived which has many similarities to the numerical results. One difference however is an induced hole pairing in the odd-parity channel at arbitrary small dopings, similar to that proposed by Geshkenbein, Ioffe, and Larkin for the two-dimensional case. At higher dopings, we propose that a quantum phase transition will occur as holes enter the even-parity channels, resulting in a Luther-Emery liquid with hole pairing with essentially d-wave character. In the mean field approximation a crossover occurs which we interpret as a reflection of this quantum phase transition deduced from the numerical results.Comment: RevTex, 36 pages with 16 figure