Holes in a Quantum Spin Liquid

Magnetic neutron scattering provides evidence for nucleation of antiferromagnetic droplets around impurities in a doped nickel oxide-based quantum magnet. The undoped parent compound contains a spin liquid with a cooperative singlet ground state and a gap in the magnetic excitation spectrum. Calcium doping creates excitations below the gap with an incommensurate structure factor. We show that weakly interacting antiferromagnetic droplets with a central phase shift of pi and a size controlled by the correlation length of the quantum liquid can account for the data. The experiment provides a quantitative impression of the magnetic polarization cloud associated with holes in a doped transition metal oxide.     

Guest-dependent spin crossover in a nanoporous molecular framework material

The nanoporous metal-organic framework Fe2(azpy)4(NCS)4.(guest) (azpy is trans-4,4'-azopyridine) displays reversible uptake and release of guest molecules and contains electronic switching centers that are sensitive to the nature of the sorbed guests. The switching of this material arises from the presence of iron(II) spin crossover centers within the framework lattice, the sorbed phases undergoing "half-spin" crossovers, and the desorbed phase showing no switching property. The interpenetrated framework structure displays a considerable flexibility with guest uptake and release, causing substantial changes in the local geometry of the iron(II) centers. The generation of a host lattice that interacts with exchangeable guest species in a switchable fashion has implications for the generation of previously undeveloped advanced materials with applications in areas such as molecular sensing.     

Spin disorder on a triangular lattice

As liquids crystallize into solids on cooling, spins in magnets generally form periodic order. However, three decades ago, it was theoretically proposed that spins on a triangular lattice form a liquidlike disordered state at low temperatures. Whether or not a spin liquid is stabilized by geometrical frustration has remained an active point of inquiry ever since. Our thermodynamic and neutron measurements on NiGa2S4, a rare example of a two-dimensional triangular lattice antiferromagnet, demonstrate that geometrical frustration stabilizes a low-temperature spin-disordered state with coherence beyond the two-spin correlation length. Spin liquid formation may be an origin of such behavior.     

Rotation of a single molecule within a supramolecular bearing

Experimental visualization and verification of a single-molecule rotor operating within a supramolecular bearing is reported. Using a scanning tunneling microscope, single molecules were observed to exist in one of two spatially defined states laterally separated by 0.26 nanometers. One was identified as a rotating state and the other as an immobilized state. Calculations of the energy barrier for rotation of these two states show that it is below the thermal energy at room temperature for the rotating state and above it for the immobilized state.     

Anatomy and dynamics of a supramolecular membrane protein cluster

Most plasmalemmal proteins organize in submicrometer-sized clusters whose architecture and dynamics are still enigmatic. With syntaxin 1 as an example, we applied a combination of far-field optical nanoscopy, biochemistry, fluorescence recovery after photobleaching (FRAP) analysis, and simulations to show that clustering can be explained by self-organization based on simple physical principles. On average, the syntaxin clusters exhibit a diameter of 50 to 60 nanometers and contain 75 densely crowded syntaxins that dynamically exchange with freely diffusing molecules. Self-association depends on weak homophilic protein-protein interactions. Simulations suggest that clustering immobilizes and conformationally constrains the molecules. Moreover, a balance between self-association and crowding-induced steric repulsions is sufficient to explain both the size and dynamics of syntaxin clusters and likely of many oligomerizing membrane proteins that form supramolecular structures.     

Acid catalysis in basic solution: a supramolecular host promotes orthoformate hydrolysis

Although many enzymes can promote chemical reactions by tuning substrate properties purely through the electrostatic environment of a docking cavity, this strategy has proven challenging to mimic in synthetic host-guest systems. Here, we report a highly charged, water-soluble, metal-ligand assembly with a hydrophobic interior cavity that thermodynamically stabilizes protonated substrates and consequently catalyzes the normally acidic hydrolysis of orthoformates in basic solution, with rate accelerations of up to 890-fold. The catalysis reaction obeys Michaelis-Menten kinetics and exhibits competitive inhibition, and the substrate scope displays size selectivity, consistent with the constrained binding environment of the molecular host.     

Spin label studies in chlamydomonas

When spin label is added to Chlamydomonas the organism is apparently unaffected, but the paramagnetic resonance signal of the spin label decreases. Irradiation with visible light greatly accelerates this decrease, which is partially reversible. If the cells are grown in the presence of the spin label and washed well, no spin label signal is detectable. However, such cells can no longer catalyze the destruction of added spin label in the light. This finding sug- gests that the molecule is bound to a specific site, which undergoes a change in conformation with illumination.     

Spin susceptibility scaling in high-temperature superconductors

The spin response of a nested Fermi surface represented by a tight binding energy band is found to exhibit scaling in frequency divided by temperature within a restricted regime close to half-filling of the band. Computations of the spin susceptibility reveal a surprising momentum variation at various temperatures and frequencies. Neutron scattering data on the high-temperature superconductor YBa(2)Cu(3)O(6+x) are analyzed for scaling near a momentum vector that spans nested regions of the orbit. Changes in the Fermi energy remove the scaling properties and reduce the susceptibility to the conventional Fermi liquid behavior of ordinary metals. These results imply that pairing mechanisms of superconductivity need to cope with competing spin density wave and charge density wave instabilities.     

Spin coupling in engineered atomic structures

We used a scanning tunneling microscope to probe the interactions between spins in individual atomic-scale magnetic structures. Linear chains of 1 to 10 manganese atoms were assembled one atom at a time on a thin insulating layer, and the spin excitation spectra of these structures were measured with inelastic electron tunneling spectroscopy. We observed excitations of the coupled atomic spins that can change both the total spin and its orientation. Comparison with a model spin-interaction Hamiltonian yielded the collective spin configuration and the strength of the coupling between the atomic spins.     

MATERIALS SCIENCE: Putting a New Spin on Carbon Nanotubes

Nanotube fibers are expected to have a wide range of applications from energy storage to high-strength mechanical devices. But as Baughman explains in his Perspective, methods for making such fibers have been of limited success. In contrast, the process reported by Vigolo et al. shows great promise. Together with a recently reported, more economically viable nanotube production process, this method may open the door to large-scale devices and materials based on carbon nanotubes.     

Spin transition zone in Earth's lower mantle

Mineral properties in Earth's lower mantle are affected by iron electronic states, but representative pressures and temperatures have not yet been probed. Spin states of iron in lower-mantle ferropericlase have been measured up to 95 gigapascals and 2000 kelvin with x-ray emission in a laser-heated diamond cell. A gradual spin transition of iron occurs over a pressure-temperature range extending from about 1000 kilometers in depth and 1900 kelvin to 2200 kilometers and 2300 kelvin in the lower mantle. Because low-spin ferropericlase exhibits higher density and faster sound velocities relative to the high-spin ferropericlase, the observed increase in low-spin (Mg,Fe)O at mid-lower mantle conditions would manifest seismically as a lower-mantle spin transition zone characterized by a steeper-than-normal density gradient.     

Spin chirality, Berry phase, and anomalous Hall effect in a frustrated ferromagnet

An electron hopping on non-coplanar spin sites with spin chirality obtains a complex phase factor (Berry phase) in its quantum mechanical amplitude that acts as an internal magnetic field, and is predicted to manifest itself in the Hall effect when it is not cancelled. The present combined work of transport measurement, neutron scattering, and theoretical calculation provides evidence that the gigantic anomalous Hall effect observed in Nd2Mo2O7, a pyrochlore ferromagnet with geometrically frustrated lattice structure, is mostly due to the spin chirality and the associated Berry phase originating from the Mo spin tilting.     

A crossover in the mechanical response of nanocrystalline ceramics

Multimillion-atom molecular dynamics simulation of indentation of nanocrystalline silicon carbide reveals unusual deformation mechanisms in brittle nanophase materials, resulting from the coexistence of brittle grains and soft amorphous grain boundary phases. Simulations predict a crossover from intergranular continuous deformation to intragrain discrete deformation at a critical indentation depth. The crossover arises from the interplay between cooperative grain sliding, grain rotations, and intergranular dislocation formation similar to stick-slip behavior. The crossover is also manifested in switching from deformation dominated by indentation-induced crystallization to deformation dominated by disordering, leading to amorphization. This interplay between deformation mechanisms is critical for the design of ceramics with superior mechanical properties.     

Spin transfer torques in MnSi at ultralow current densities

Spin manipulation using electric currents is one of the most promising directions in the field of spintronics. We used neutron scattering to observe the influence of an electric current on the magnetic structure in a bulk material. In the skyrmion lattice of manganese silicon, where the spins form a lattice of magnetic vortices similar to the vortex lattice in type II superconductors, we observe the rotation of the diffraction pattern in response to currents that are over five orders of magnitude smaller than those typically applied in experimental studies on current-driven magnetization dynamics in nanostructures. We attribute our observations to an extremely efficient coupling of inhomogeneous spin currents to topologically stable knots in spin structures.     

Forming supramolecular networks from nanoscale rods in binary, phase-separating mixtures

Simulations show that when low-volume fractions of nanoscale rods are immersed in a binary, phase-separating blend, the rods self-assemble into needle-like, percolating networks. The interconnected network arises through the dynamic interplay of phase-separation between the fluids, through preferential adsorption of the minority component onto the mobile rods, and through rod-rod repulsion. Such cooperative effects provide a means of manipulating the motion of nanoscopic objects and directing their association into supramolecular structures. Increasing the rod concentration beyond the effective percolation threshold drives the system to self-assemble into a lamellar morphology, with layers of wetted rods alternating with layers of the majority-component fluid. This approach can potentially yield organic/inorganic composites that are ordered on nanometer scales and exhibit electrical or structural integrity.     

Association of crossover points with topoisomerase I cleavage sites: a model for nonhomologous recombination

Nonhomologous DNA recombination is frequently observed in somatic cells upon the introduction of DNA into cells or in chromosomal events involving sequences already stably carried by the genome. In this report, the DNA sequences at the crossover points for excision of SV40 from chromosomes were shown to be associated with eukaryotic topoisomerase I cleavage sites in vitro. The precise location of the cleavage sites relative to the crossover points has suggested a general model for nonhomologous recombination mediated by topoisomerase I.     

磁性半导体隧道结中自旋输运的偏压影响

利用量子隧穿理论研究了磁性半导体隧道结中自旋输运的偏压影响,分别讨论了自旋注入源是铁磁金属和铁磁半导体两种情况下,磁性半导体隧道单结中自旋注入效率对偏压的依赖关系。数值计算表明,增加偏压对于提高单结的自旋注入效率是有利的,但会降低单结或由两个磁性隧道单结组成的双结的隧道磁阻(TMR)。数值计算还发现对于磁性隧道双结在顺序隧穿和两边势垒对称的情况下成立的关系式TMR=η2(η为单结自旋注入效率)在有限偏压下不再成立,这些数值结果与实验一致。     

Solitons in a reaction-diffusion system

Solitary waves in reaction-diffusion systems usually annihilate on collision. A nonlinear system of reaction-diffusion equations has been constructed which has solitons: solitary waves whose interaction in a collision results in the emergence of two solitary waves identical to the colliding waves.