The Pseudogap in La(2-x)Sr(x)CuO(4): A Raman Viewpoint

We report the results of Raman scattering experiments on single crystals of La(2-x)Sr(x)CuO(4) [La214] as a function of temperature and doping. In underdoped compounds low-energy B1g spectral weight is depleted in association with the opening of a pseudogap on regions of the Fermi surface located near (pi, 0) and (0, pi). The magnitude of the depletion increases with decreasing doping, and in the most underdoped samples, with decreasing temperature. The spectral weight that is lost at low-energies (omega < 800 cm-1) is transferred to the higher energy region normally occupied by multi-magnon scattering. From the normal state B2g spectra we have determined the scattering rate Gamma(omega, T) of qausiparticles located near the diagonal directions in k-space, (pi/2, pi/2) regions. In underdoped compounds, Gamma(omega, T) is suppressed at low temperatures for energies less than Eg(x) ~ 800 cm-1. The observed doping dependence of the two-magnon scattering and the scattering rate suppression thus suggest that the pseudogap is characterized by an energy scale Eg ~ J, where J is the antiferromagnetic super-exchange energy. Comparison with the results from other techniques provides a consistent picture of the pseudogap in La214.Comment: 6 pages, 5 figures, minor revisions include correct form of the B2g Raman response function and new figures of the recalculated B2g scattering rate. Presented at the APS March99 Meeting, accepted for publication in the Canadian Journal of Physic

Electron Correlation Driven Heavy-Fermion Formation in LiV2O4

Optical reflectivity measurements were performed on a single crystal of the d-electron heavy-fermion (HF) metal LiV2O4. The results evidence the highly incoherent character of the charge dynamics for all temperatures above T^* \approx 20 K. The spectral weight of the optical conductivity is redistributed over extremely broad energy scales (~ 5 eV) as the quantum coherence of the charge carriers is recovered. This wide redistribution is, in sharp contrast to f-electron Kondo lattice HF systems, characteristic of a metallic system close to a correlation driven insulating state. Our results thus reveal that strong electronic correlation effects dominate the low-energy charge dynamics and heavy quasiparticle formation in LiV2O4. We propose the geometrical frustration, which limits the extension of charge and spin ordering, as an additional key ingredient of the low-temperature heavy-fermion formation in this system.Comment: 5 pages, 3 figure

Observation of Zeeman effect in topological surface state with distinct material dependence

The helical Dirac fermions on the surface of topological insulators host novel relativistic quantum phenomena in solids. Manipulating spins of topological surface state (TSS) represents an essential step towards exploring the theoretically predicted exotic states related to time reversal symmetry (TRS) breaking via magnetism or magnetic field. Understanding Zeeman effect of TSS and determining its g-factor are pivotal for such manipulations in the latter form of TRS breaking. Here, we report those direct experimental observations in Bi2Se3 and Sb2Te2Se by spectroscopic imaging scanning tunneling microscopy. The Zeeman shifting of zero mode Landau level is identified unambiguously by judiciously excluding the extrinsic influences associated with the non-linearity in the TSS band dispersion and the spatially varying potential. The g-factors of TSS in Bi2Se3 and Sb2Te2Se are determined to be 18 and -6, respectively. This remarkable material dependence opens a new route to control the spins in the TSS.Comment: main text: 17 pages, 4 figures; supplementary: 15 pages, 7 figure

Prediction of risk of fracture in the tibia due to altered bone mineral density distribution resulting from disuse : a finite element study

The disuse-related bone loss that results from immobilisation following injury shares characteristics with osteoporosis in postmenopausal women and the aged, with decreases in bone mineral density (BMD) leading to weakening of the bone and increased risk of fracture. The aim of the study was to use the finite element method to: (i) calculate the mechanical response of the tibia under mechanical load and (ii) estimate the risk of fracture; comparing between two groups, an able bodied (AB) group and spinal cord injury (SCI) patients group suffering from varying degree of bone loss. The tibiae of eight male subjects with chronic SCI and those of four able-bodied (AB) age-matched controls were scanned using multi-slice peripheral Quantitative Computed Tomography. Images were used to develop full three-dimensional models of the tibiae in Mimics (Materialise) and exported into Abaqus (Simulia) for calculation of stress distribution and fracture risk in response to specified loading conditions – compression, bending and torsion. The percentage of elements that exceeded a calculated value of the ultimate stress provided an estimate of the risk of fracture for each subject, which differed between SCI subjects and their controls. The differences in BMD distribution along the tibia in different subjects resulted in different regions of the bone being at high risk of fracture under set loading conditions, illustrating the benefit of creating individual material distribution models. A predictive tool can be developed based on these models, to enable clinicians to estimate the amount of loading that can be safely allowed onto the skeletal frame of individual patients who suffer from extensive musculoskeletal degeneration (including SCI, multiple sclerosis and the ageing population). The ultimate aim would be to reduce fracture occurrence in these vulnerable groups