Charge Dynamics in the Planar t-J Model

The finite-temperature optical conductivity $\sigma(\omega)$ in the planar $t-J$ model is analysed using recently introduced numerical method based on the Lanczos diagonalization of small systems (up to 20 sites), as well as by analytical approaches, including the method of frequency moments and the retraceable-path approximation. Results for a dynamical mobility of a single hole at elevated temperatures $T>t$ reveal a Gaussian-like $\mu(\omega)$ spectra, however with a nonanalytical behavior at low $\omega$. In the single hole response a difference between the ferromagnetic (J=0) and the antiferromagnetic ($J>0$) polaron shows up at $T<J$. At larger dopings numerical results in studied systems are consistent with the thermodynamical behavior for $T>T^*\ge 0.1~t$. $\sigma(\omega)$ spectra show a non-Drude falloff at large frequencies. In particular for `optimum' doping $n_h \sim 0.2$ we obtain in the low-$\omega,T$ regime the relaxation rate $\tau^{-1} \sim 0.6 (\omega+\xi T)$ with $\xi \sim 3$, being consistent with the marginal Fermi liquid concept and experiments. Within the same regime we reproduce the nearly linear variation of dc resistivity $\rho$ with $T$. This behavior is weakly dependent on $J$, provided that $J<t$.Comment: 21 pages of text plus 17 figures, postscrip

A Review of the fossil record of turtles of the clade Baenidae

The fossil record of the turtle clade Baenidae ranges from the Early Cretaceous (Aptian—Albian) to the Eocene. The group is present throughout North America during the Early Cretaceous, but is restricted to the western portions of the continents in the Late Cretaceous and Paleogene. No credible remains of the clade have been reported outside of North America to date. Baenids were warmadapted freshwater aquatic turtles that supported high levels of diversity at times through niche partitioning, particularly by adapting to a broad range of dietary preferences ranging from omnivorous to molluscivorous. Current phylogenies place Baenidae near the split of crown-group Testudines. Within Baenidae three more inclusive, named clades are recognized: Baenodda, Palatobaeninae and Eubaeninae. A taxonomic review of the group concludes that of 49 named taxa, 30 are nomina valida, 12 are nomina invalida and 7 are nomina dubia

A large marine eosauropterygian reptile with affinities to nothosauroid diapsids from the Early Triassic of British Columbia, Canada

Sauropterygia, one of the main clades of Mesozoic marine reptiles, diversified shortly after the Permo-Triassic biotic crisis and afterwards remained one of the major components of Early Triassic and later Mesozoic marine ecosystems. On the other hand, actual specimens of marine reptiles of Olenekian age are still rare in the fossil record, coming only from a few localities worldwide. Here we describe associated remains of a larger marine reptile of around 4 m body length, with nothosauroidean affinities from the Sulphur Mountain Formation exposed at the L cirque locality of Wapiti Lake area in British Columbia. Although the specimen records only scattered parts of the posterior vertebral column, some gastral ribs and most notably, the proximal portion of one hind limb together with a fan-shaped ischium, it represents one of the oldest records of Sauropterygia and larger representatives of aff. Nothosauroidea specifically, as well as the northernmost occurrence of such animals in the Triassic. As such, the new specimen is important for understanding the biogeography and early evolution of the group and that of Sauropterygia, in general