Articles in 2011

The ability to switch a semiconductor into a topological insulator would produce topological states on demand. Applying a time-dependent field to well-studied semiconductor quantum wells may lead to this kind of control.

As the pnictide superconductors have metallic ground states, the Heisenberg model has not been successful in describing the magnetic behaviour. But the addition of a biquadratic interaction term to the usual Heisenberg Hamiltonian leads to a description of many experimental observations.

Permeable membranes play a vital role in many biological processes. Simulations of Brownian motion now show that the spatially correlated disorder of these membranes unexpectedly modifies diffusion rates.

Quantum oscillations in a copper-oxide superconductor are observed using a thermodynamic probe. Surprisingly, these oscillations lie on a background signal that is consistent with d-wave superconductivity in the vortex state, in a magnetic field up to 45 T.

One of the fundamental questions of nanoscale spin-transport is how a spin-polarized current interacts with a ferromagnet under finite bias. So far, experimental results have remained inconclusive, but a novel technique allows direct and quantitative measurements of the spin-transfer torque in magnetic tunnel junctions and should provide a basis for understanding and modelling the phenomenon.

Quantum simulations, where one quantum system is used to emulate another, are starting to become experimentally feasible. Here, four-photon states are used to simulate spin tetramers, which are important in the description of certain solid-state systems. Emerging frustration within the tetramer is observed, as well as evolution of the ground state from a localized to a resonating-valence-bond state.

The atom-like electronic structure of semiconductor quantum dots makes them ideal for storing well-defined numbers of electrons. This in turn can be used in the development of standards for current, to independently define the ampere.

Magnetic monopoles have recently been discovered in so-called spin-ice materials. The measurement of magnetic current in a spin-ice crystal now demonstrates the macroscopic consequences of these free, magnetically charged particles, and establishes a perfect equivalence between the bulk electrical properties of a conducting fluid and the bulk magnetic properties of spin ice in the magnetic-monopole regime.

Topological quantum computation schemes — where quantum information is stored non-locally — provide, in theory, an elegant way of avoiding the deleterious effects of decoherence, but they have proved difficult to realize experimentally. A proposal to engineer topological phases into networks of one-dimensional semiconducting wires should bring topological quantum computers a step closer.

Ultracold quantum gases in optical lattices have been used to study a wide range of many-body effects. Nearly all experiments so far, however, have been performed in cubic optical lattice structures. Now a ‘honeycomb’ lattice structure has been realized. The approach promises insight into materials with hexagonal crystal symmetries, such as graphene or carbon nanotubes.

The magnetic properties of GaMnAs could be useful in the development of spintronic devices. Yet the precise origin of these properties has been hotly debated. Resonant-tunnelling spectra obtained from GaMnAs devices of superlative quality could finally resolve this issue.

A thermal Casimir force—an attraction between two metal surfaces caused by thermal, rather than quantum, fluctuations in the electromagnetic field—has now been identified experimentally between a flat and a spherical gold plate.

The mystery of the missing bound states within a superconducting vortex in a pnictide superconductor has been solved. Not only are bound states present, they also provide information on the gap structure of Ba_0.6K_0.4Fe₂As₂.

Most of the notable properties of graphene are a result of the cone-like nature of the points in its electronic structure where its conduction and valance bands meet. Similar structures arise in 2D HgTe quantum wells, but without the spin- and valley-degeneracy of graphene; their properties are also likely to be easier to control.

Magnetic reconnection has long been implicated in the acceleration of electrons to relativistic speeds in the Earth’s magnetosphere. Satellite observations and simulations indicate it is just part of the story, the rest of which involves a second process known as betatron acceleration.

Microwave radiation has a dramatic effect on the magneto-resistance of two-dimensional electron systems, even reducing it below zero. It is thought that this is the result of the formation of distinct current domains. Direct experimental evidence for these domains is now presented for the first time.

Disorder leads to localization of electrons at low temperatures, changing metals to insulators. In a superconductor the electrons are paired up, and scanning tunnelling microscopy shows that the pairs localize together rather than breaking up and forming localized single electrons in the insulating state.

The coupling of spin and orbital motion of electrons in carbon nanotubes has been demonstrated before, but a study now shows that the strength and sign of the spin–orbit coupling can be tuned by a gate voltage, and that, importantly for future applications, the effect survives in the presence of disorder.

A micrometre-scale device that exploits the piezoresistive characteristics of silicon acts like an engine, converting heat into mechanical work in one mode of operation, and, in another, like a refrigerator, suppressing mechanical fluctuations.

The nature of the percolation transition—how links add to a system until it is extensively connected—crucially underlies the structure and function of virtually all growing complex networks. Percolation transitions have long been thought to be continuous, but recent numerical work suggests that certain percolating systems exhibit discontinuous phase transitions. This study explains the key microscopic mechanisms underlying such ‘explosive percolation’.