We analyze the high-temperature conductivity in one-dimensional integrable models of interacting fermions: the t-V model (anisotropic Heisenberg spin chain) and the Hubbard model, at half-filling in the regime corresponding to insulating ground state. A microcanonical Lanczos method study for finite size systems reveals anomalously large finite-size effects at low frequencies while a frequency-moment analysis indicates a finite d.c. conductivity. This phenomenon also appears in a prototype integrable quantum system of impenetrable particles, representing a strong-coupling limit of both models. In the thermodynamic limit, the two results could converge to a finite d.c. conductivity rather than an ideal conductor or insulator scenario.

We show how to generalize the zero-temperature Lanczos method for calculating dynamical correlation functions to finite temperatures. The key is the microcanonical ensemble which allows us to replace the involved canonical ensemble with a single appropriately chosen state; in the thermodynamic limit it provides the same physics as the canonical ensemble but with the evaluation of a single expectation value. We can employ the same system sizes as for zero temperature but, whereas the statistical fluctuations present in small systems are prohibitive, the spectra of the largest system sizes are surprisingly smooth. We investigate, as a test case, the spin conductivity of the spin-1/2 anisotropic Heisenberg model and in particular we present a comparison of spectra obtained by the canonical and microcanonical ensemble methods.

We report measurements of the magnetic and electrical properties of sintered manganites of the form . We find that the peak in the conductivity moves rapidly to lower temperatures as the chromium content is increased and is completely absent for x > 0.2. The Curie temperature, as determined by magnetization measurements, is much less strongly suppressed, disappearing at . We interpret this behaviour in terms of the very different effects of the chromium in lifting spin and orbital degeneracy.

The Hall constant R H in a tight-binding model of correlated electrons on a ladder at T 0 is expressed in terms of derivatives of the ground state energy with respect to external magnetic and electric fields. This novel method is used for the analysis of the t-J model on finite size ladders. It is found that for a single hole R H is holelike and close to the semiclassical value, while for two holes it can vary with ladder geometry. In odd-leg ladders, R H behaves quite regularly changing sign as a function of doping, the variation being quantitatively close to experimental results in cuprates. PACS numbers: 71.27. + a, 71.10.Fd, 72.15.Gd The Hall response in materials with strongly correlated electrons remains one of the properties least understood theoretically. The subject has been stimulated by experiments in superconducting cuprates [1], revealing anomalous doping and temperature dependence of the Hall constant R H ͑T ͒ in the normal metallic state. For instance, it is well established that the Hall effect is holelike, R H . 0, in materials with a low density of holes n h , introduced by doping the reference antiferromagnetic (AFM) insulator. The clearest realization is La 22x Sr x CuO 4 (LSCO), where the doping x can be directly related to the concentration of mobile holes per unit cell n h x and the semiclassical result R H 1͞n h e 0 seems to be obeyed at lowest T . T c and at low doping [1,2].Theoretical attempts to calculate the Hall effect in models of strongly correlated electrons resulted in quite controversial conclusions. Even for weak correlations [3] or for the problem of a single carrier in a Mott-Hubbard insulator [4], the analysis of the Hall response is fairly involved. In more recent investigations relevant to cuprates, the dynamical Hall constantR H ͑v͒ has been studied within linear response theory for the t-J and Hubbard models, analytically by high-v, T expansion [5] and numerically via exact-diagonalization studies of small systems [6,7]. The obtained results are quite consistent for the high-frequency quantity R ‫ء‬ H R H ͑v !`͒, showing at high T a transition from a holelike, R ‫ء‬ H . 0, to an electronlike, R ‫ء‬ H , 0, at a finite crossover n ‫ء‬ h ϳ 1͞3 [5]. For the most interesting dc limit R H R H ͑v 0͒ the majority of results obtained for 2D systems at low doping and T ! 0 indicate R H , 0 [7], instead of the expected holelike behavior [8]. On the other hand, one of the present authors [9] recently showed that for a single hole doped into a 2D AFM at T 0 the result should be the semiclassical one with R H . 0.From another perspective and stimulated by synthesis and experiments on novel cuprates, models of interacting electrons on ladder systems have also been extensively studied in recent years [10]. The idea is that ladders with a variable number of legs can offer a broader insight into the behavior of correlated electrons and thus can lead to an understanding of the more challenging 2D systems. Again, results for the Hall responseR H ͑v͒ at low doping obtained through linear ...

Hole-doped high-temperature cuprate superconductors below optimum doping have electronlike Fermi surfaces occupying a small fraction of the Brillouin zone. There is strong evidence that this is linked to charge density wave (CDW) order, which reconstructs the large holelike Fermi surfaces predicted by band structure calculations. Recent experiments have revealed the structure of the two CDW components in the benchmark bilayer material YBa 2 Cu 3 O 7-x in high field where quantum oscillation (QO) measurements are performed. We have combined these results with a tight-binding description of the bands in a single bilayer to give a minimal model revealing the essential physics of the situation. Here we show that this approach, combined with the effects of spin-orbit interactions and the pseudogap, gives a good qualitative description of the multiple frequencies seen in the QO observations in this material. Magnetic breakdown through weak CDW splitting of the bands will lead to a field dependence of the QO spectrum and to the observed fourfold symmetry of the results in tilted fields.