Magnetoelastic excitations in the pyrochlore spin liquid Tb2Ti2O7.

At low temperatures, Tb2Ti2O7 enters a spin liquid state, despite expectations of magnetic order and/or a structural distortion. Using neutron scattering, we have discovered that in this spin liquid state an excited crystal field level is coupled to a transverse acoustic phonon, forming a hybrid excitation. Magnetic and phononlike branches with identical dispersion relations can be identified, and the hybridization vanishes in the paramagnetic state. We suggest that Tb2Ti2O7 is aptly named a "magnetoelastic spin liquid" and that the hybridization of the excitations suppresses both magnetic ordering and the structural distortion. The spin liquid phase of Tb2Ti2O7 can now be regarded as a Coulomb phase with propagating bosonic spin excitations.

Power-law spin correlations in the pyrochlore antiferromagnet Tb(2)Ti(2)O(7).

We investigate the low-temperature state of the rare-earth pyrochlore Tb(2)Ti(2)O(7) using polarized neutron scattering. Tb(2)Ti(2)O(7) is often described as an antiferromagnetic spin liquid with spin correlations extending over lengths comparable to individual tetrahedra of the pyrochlore lattice. We confirm this picture at 20 K but find that at 0.05 K the data contain evidence of pinch-point scattering, suggesting that the low temperature state of Tb(2)Ti(2)O(7) has power-law spin correlations.

Cannabinoid receptor 1 (CB1) antagonism enhances glucose utilisation and activates brown adipose tissue in diet-induced obese mice.

AIMS/HYPOTHESIS: We examined the physiological mechanisms by which cannabinoid receptor 1 (CB1) antagonism improves glucose metabolism and insulin sensitivity independent of its anorectic and weight-reducing effects, as well as the effects of CB1 antagonism on brown adipose tissue (BAT) function. METHODS: Three groups of diet-induced obese mice received for 1 month: vehicle; the selective CB1 antagonist SR141716; or vehicle/pair-feeding. After measurements of body composition and energy expenditure, mice underwent euglycaemic-hyperinsulinaemic clamp studies to assess in vivo insulin action. In separate cohorts, we assessed insulin action in weight-reduced mice with diet-induced obesity (DIO), and the effect of CB1 antagonism on BAT thermogenesis. Surgical denervation of interscapular BAT (iBAT) was carried out in order to study the requirement for the sympathetic nervous system in mediating the effects of CB1 antagonism on BAT function. RESULTS: Weight loss associated with chronic CB1 antagonism was accompanied by increased energy expenditure, enhanced insulin-stimulated glucose utilisation, and marked activation of BAT thermogenesis. Insulin-dependent glucose uptake was significantly increased in white adipose tissue and BAT, whereas glycogen synthesis was increased in liver, fat and muscle. Despite marked weight loss in the mice, SR141716 treatment did not improve insulin-mediated suppression of hepatic glucose production nor increase skeletal muscle glucose uptake. Denervation of iBAT blunted the effect of SR141716 on iBAT differentiation and insulin-mediated glucose uptake. CONCLUSIONS/INTERPRETATION: Chronic CB1 antagonism markedly enhances insulin-mediated glucose utilisation in DIO mice, independent of its anorectic and weight-reducing effects. The potent effect on insulin-stimulated BAT glucose uptake reveals a novel role for CB1 receptors as regulators of glucose metabolism.

Orbital dependence of the fermi liquid state in Sr2RuO4.

We have used angle-resolved photoemission spectroscopy to determine the bulk electronic structure of Sr(2)RuO(4) above and below the Fermi liquid crossover near 25 K. Our measurements indicate that the properties of the system are highly orbital dependent. The quasi-2D gamma band displays Fermi liquid behavior while the remaining low energy bands show exotic properties consistent with quasi-1D behavior. In the Fermi liquid state below 25 K, the gamma band dominates the electronic properties, while at higher temperatures the quasi-1D beta and alpha bands become more important.

Giant anharmonicity and nonlinear electron-phonon coupling in MgB2: a combined first-principles calculation and neutron scattering study.

First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB (2) are carried out and found to be in excellent agreement with our inelastic neutron scattering measurements. The numerical results reveal that the E(2g) in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B sigma bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high T(c) and boron isotope effect in MgB (2).

High critical current density and enhanced irreversibility field in superconducting MgB2 thin films.

The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

Superconductivity in the non-oxide perovskite MgCNi3.

The interplay of magnetic interactions, the dimensionality of the crystal structure and electronic correlations in producing superconductivity is one of the dominant themes in the study of the electronic properties of complex materials. Although magnetic interactions and two-dimensional structures were long thought to be detrimental to the formation of a superconducting state, they are actually common features of both the high transition-temperature (Tc) copper oxides and low-Tc material Sr2RuO4, where they appear to be essential contributors to the exotic electronic states of these materials. Here we report that the perovskite-structured compound MgCNi3 is superconducting with a critical temperature of 8 K. This material is the three-dimensional analogue of the LnNi2B2C family of superconductors, which have critical temperatures up to 16 K (ref. 2). The itinerant electrons in both families of materials arise from the partial filling of the nickel d-states, which generally leads to ferromagnetism as is the case in metallic Ni. The high relative proportion of Ni in MgCNi3 suggests that magnetic interactions are important, and the lower Tc of this three-dimensional compound-when compared to the LnNi2B2C family-contrasts with conventional ideas regarding the origins of superconductivity.

Loss of superconductivity with the addition of Al to MgB2 and a structural transition in Mg1-x AlxB2.

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery of superconductivity at 39 K in the simple binary ceramic compound magnesium diboride, MgB2, was therefore surprising. Indeed, this material has been known and structurally characterized since the mid 1950s (ref. 2), and is readily available from chemical suppliers (it is commonly used as a starting material for chemical metathesis reactions). Here we show that the addition of electrons to MgB2, through partial substitution of Al for Mg, results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near an Al content of 10 per cent. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.

Spread of a pharyngeal cancer to the abdominal wall after percutaneous endoscopic gastrostomy.

Percutaneous endoscopic gastrostomy is frequently used in patients with head and neck cancer to establish enteral access for feeding. Spread of head and neck cancer to the gastrostomy site is a rare but increasingly reported complication after percutaneous endoscopic placement. We report the 13th such case in the literature, occurring in a 51-year-old black man with squamous cell carcinoma of the hypopharynx. The mode of tumor spread to the gastrostomy site remains debatable. Evidence exists for hematogenous dissemination and direct implantation. We think percutaneous endoscopic techniques for enteral access in this patient population are contraindicated, and we advocate a laparoscopic approach for gastrostomy placement.

Chromogenic assay for phospholipase D from Streptomyces chromofuscus: application to the evaluation of substrate analogs.

A rapid and convenient chromogenic assay for phospholipase D from Streptomyces chromofuscus (PLDSc) has been developed that converts the choline generated from the enzyme-catalyzed hydrolysis of phospholipids into a chromogenic dye. By quenching the reaction with EDTA at defined times, an initial rate curve is produced from which a kcat and K(m) can be readily derived. This assay has been applied to the biological evaluation of several substrate analogs, all of which appear to be activators rather than substrates or inhibitors of this enzyme. Performing the assay in 96-well microtiter plates allows for the easy screening of potential effectors of this enzyme.

Chromogenic assay for phospholipase C from Bacillus cereus.

A standard chromogenic assay for phosphatidylcholine determination has been modified in the development of a facile method for measuring the rate parameters (Km and Vmax) for phosphatidylcholine-preferring phospholipase C from Bacillus cereus (PLCBc). The assay is applied to the determination of the Ki for a known inhibitor of PLCBc.