Conductivity in the Square Lattice Hubbard Model at High Temperatures: Importance of Vertex Corrections.

Recent experiments on cold atoms in optical lattices allow for a quantitative comparison of the measurements to the conductivity calculations in the square lattice Hubbard model. However, the available calculations do not give consistent results, and the question of the exact solution for the conductivity in the Hubbard model remained open. In this Letter, we employ several complementary state-of-the-art numerical methods to disentangle various contributions to conductivity and identify the best available result to be compared to experiment. We find that, at relevant (high) temperatures, the self-energy is practically local, yet the vertex corrections remain rather important, contrary to expectations. The finite-size effects are small even at the lattice size 4×4, and the corresponding Lanczos diagonalization result is, therefore, close to the exact result in the thermodynamic limit.

Bilateral Distal Tibial Stress Fracture in a Triathlete.

Stress fractures are common injuries in athletes. They result from excessive, repetitive loads on the normal bone which can cause an imbalance between bone resorption and formation, because during periods of intense exercise, bone formation lags behind bone resorption. The most common locations for stress fractures are weight-bearing bones of the lower extremities. This kind of injuries can easily be missed because they usually do not follow an acute injury in a fit young athlete, and plain radiographs usually appear normal. In this paper, a case of an amateur triathlete who suffered from a bilateral distal tibial stress fracture is presented. Key words: stress fracture, tibia, bilateral, distal tibia.

Magnetic inhomogeneity on a triangular lattice: the magnetic-exchange versus the elastic energy and the role of disorder.

Inhomogeneity in the ground state is an intriguing, emergent phenomenon in magnetism. Recently, it has been observed in the magnetostructural channel of the geometrically frustrated α-NaMnO2, for the first time in the absence of active charge degrees of freedom. Here we report an in-depth numerical and local-probe experimental study of the isostructural sister compound CuMnO2 that emphasizes and provides an explanation for the crucial differences between the two systems. The experimentally verified, much more homogeneous, ground state of the stoichiometric CuMnO2 is attributed to the reduced magnetoelastic competition between the counteracting magnetic-exchange and elastic-energy contributions. The comparison of the two systems additionally highlights the role of disorder and allows the understanding of the puzzling phenomenon of phase separation in uniform antiferromagnets.

Local spin relaxation within the random Heisenberg chain.

Finite-temperature local dynamical spin correlations S(nn)(ω) are studied numerically within the random spin-1/2 antiferromagnetic Heisenberg chain. The aim is to explain measured NMR spin-lattice relaxation times in BaCu(2)(Si(0.5)Ge(0.5))(2)O(7), which is the realization of a random spin chain. In agreement with experiments we find that the distribution of relaxation times within the model shows a very large span similar to the stretched-exponential form. The distribution is strongly reduced with increasing T, but stays finite also in the high-T limit. Anomalous dynamical correlations can be associated with the random singlet concept but not directly with static quantities. Our results also reveal the crucial role of the spin anisotropy (interaction), since the behavior is in contrast with the ones for the XX model, where we do not find any significant T dependence of the distribution.

Thermodynamics of a bad metal-Mott insulator transition in the presence of frustration.

Thermodynamic properties of the Hubbard model on the anisotropic triangular lattice at half filling are calculated by the finite-temperature Lanczos method. The charge susceptibility exhibits clear signatures of a Mott metal-insulator transition. The metallic phase is characterized by a small charge susceptibility, large entropy, large renormalized quasiparticle mass, and large spin susceptibility. The fluctuating local magnetic moment in the metallic phase is large and comparable to that in the insulating phase. These bad metallic characteristics occur above a relatively low coherence temperature, as seen in organic charge transfer salts.

Consistent description of the metallic phase of overdoped cuprate superconductors as an anisotropic marginal Fermi liquid.

We consider a model self-energy consisting of an isotropic Fermi liquid term and a marginal Fermi liquid term which is anisotropic over the Fermi surface, vanishing in the same directions as the superconducting gap and the pseudogap. This model self-energy gives a consistent description of experimental results from angle-dependent magnetoresistance, specific heat, de Haas-van Alphen, and measurements of the quasiparticle dispersion near the Fermi surface from photoemission. In particular, we reconcile the strongly doping-dependent anomalous scattering rate observed in angle-dependent magnetoresistance with the almost doping-independent specific heat.