Jeffrey W. Lynn

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).

Spin chirality on a two-dimensional frustrated lattice

The collective behaviour of interacting magnetic moments can be strongly influenced by the topology of the underlying lattice. In geometrically frustrated spin systems, interesting chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. We report a study of the spin chirality on a two-dimensional geometrically frustrated lattice. Our new chemical synthesis methods allow us to produce large single-crystal samples of KFe3(OH)6(SO4)2, an ideal Kagomé lattice antiferromagnet. Combined thermodynamic and neutron scattering measurements reveal that the phase transition to the ordered ground-state is unusual. At low temperatures, application of a magnetic field induces a transition between states with different non-trivial spin-textures.

Direct visualization of the Jahn–Teller effect coupled to Na ordering in Na5/8MnO2

The cooperative Jahn-Teller effect (CJTE) refers to the correlation of distortions arising from individual Jahn-Teller centres in complex compounds. The effect usually induces strong coupling between the static or dynamic charge, orbital and magnetic ordering, which has been related to many important phenomena such as colossal magnetoresistance and superconductivity. Here we report a Na5/8MnO2 superstructure with a pronounced static CJTE that is coupled to an unusual Na vacancy ordering. We visualize this coupled distortion and Na ordering down to the atomic scale. The Mn planes are periodically distorted by a charge modulation on the Mn stripes, which in turn drives an unusually large displacement of some Na ions through long-ranged Na-O-Mn(3+)-O-Na interactions into a highly distorted octahedral site. At lower temperatures, magnetic order appears, in which Mn atomic stripes with different magnetic couplings are interwoven with each other. Our work demonstrates the strong interaction between alkali ordering, displacement, and electronic and magnetic structure, and underlines the important role that structural details play in determining electronic behaviour.

Observation of Magnetic Monopoles in Spin Ice

Excitations from a strongly frustrated system, the kagome ice state of the spin ice Dy 2 Ti 2 O 7 under magnetic fields along a [111] direction, have been studied. They are theoretically proposed to be regarded as magnetic monopoles. Neutron scattering measurements of spin correlations show that close to the critical point the monopoles are fluctuating between high- and low-density states, supporting that the magnetic Coulomb force acts between them. Specific heat measurements show that monopole-pair creation obeys an Arrhenius law, indicating that the density of monopoles can be controlled by temperature and magnetic field.

Structural collapse and superconductivity in rare-earth-doped CaFe2As2

Aliovalent rare-earth substitution into the alkaline-earth site of CaFe

Dzyaloshinskii-Moriya interaction and spin reorientation transition in the frustrated kagome lattice antiferromagnet

{}_{2}

A comparison of united atom, explicit atom, and coarse-grained simulation models for poly(ethylene oxide).

As

Low Energy Spin Waves and Magnetic Interactions in SrFe2As2

{}_{2}

Phonon localization drives polar nanoregions in a relaxor ferroelectric

single crystals is used to fine tune structural, magnetic, and electronic properties of this iron-based superconducting system. Neutron and single-crystal x-ray scattering experiments indicate that an isostructural collapse of the tetragonal unit cell can be controllably induced at ambient pressures by the choice of substituent ion size. This instability is driven by the interlayer As-As anion separation, resulting in an unprecedented thermal expansion coefficient of

Intrinsic Properties of Stoichiometric LaFePo

180\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}