Articles in 2014

The conducting surface states of 3D topological insulators are two-dimensional. In an analogous way, the edge states of 2D topological insulators are one-dimensional. Direct evidence of this one-dimensionality is now presented, by means of scanning tunnelling spectroscopy, for bismuth bilayers—one of the first theoretically predicted 2D topological insulators.

The shakti lattice describes a new type of frustration not found in naturally occurring materials. Fabrication of the first artificial spin-ice array displaying shakti dynamics confirms the locally ordered, globally degenerate nature of these exotic lattice structures.

Wound repair is thought to involve cell migration and the contraction of a tissue-level biopolymer ring—invoking analogy with the pulling of purse strings. Traction-force measurements now show that this ring engages the tissue's surroundings to steer migration, prompting revision of the purse-string mechanism.

How do flocks of birds remain cohesive while dodging predators? A study tracking up to 400 starlings reveals that information propagates in a linear fashion and with no attenuation, meaning that the language of phase transitions in correlated materials can be used to describe flocking behaviour.

Intense lasers can both ionize atoms and subsequently drive the recollision of the released electrons with their ionized parents. Holography experiments now show that the orientation of the parent can change the recollision process, requiring a refinement of the commonly used strong-field approximation.

When a bubble bursts on reaching a surface, mass transfer from the liquid to the gas phase can occur—aerosol dispersion. Now, the inverse transport process is reported: submicrometre-sized oil droplets, formed during bubble-bursting, are zipped across the interface to the liquid phase.

Two-dimensional electronic spectroscopic data and theoretical simulations provide the most convincing evidence so far that organisms exploit quantum coherence for efficient energy conversion during photosynthesis.

Laser-assisted tunnelling allows quantum gases in optical lattices to be exposed to tunable artificial magnetic fields. Using such fields to confine a bosonic gas to an array of one-dimensional ladders, a low-dimensional equivalent of the Meissner effect has been observed.

Isotope production is usually associated with nuclear reactors, but there are alternative approaches. One such proposal is based on the well-known atomic physics experimental techniques of optical pumping and magnetic guiding, and its viability for isotope separation is now experimentally demonstrated.

A proposed network of atomic clocks—using non-local entangled states—could achieve unprecedented stability and accuracy in time-keeping, as well as being secure against internal or external attack.

Graphene on boron nitride gives rise to a moiré superlattice displaying the Hofstadter butterfly: a fractal dependence of energy bands on external magnetic fields. Now, by means of capacitance spectroscopy, further aspects of this system are revealed—most notably, suppression of quantum Hall antiferromagnetism at particular commensurate magnetic fluxes.

Frequency changes in the partial tones of a sound can affect the way we perceive it, a phenomenon generally understood to be cortical in origin. A mesoscopic model now attributes perceived pitch to a physical mechanism linked to the presence of the cochlear fluid.

Magnetoresistance measurements on underdoped La_2−xSr_xCuO₄, a cuprate superconductor, reveal quantum-critical behaviour of the resistivity—the signature of a superconductor–insulator transition. The magnetic-field-driven transition involves an intermediate state, which is only superconducting at zero temperature.

The spontaneous breaking of a system’s symmetry results in an entropy decrease. Now, an experiment involving a particle subject to a potential with a shape that changes from a single- to a double-well demonstrates that the associated entropy changed is detectable. Moreover, the experimental setup enables the realization of a Szilard engine.

A single layer of graphene on top of a hexagonal boron-nitride sheet can stretch to form a commensurate structure, or not — depending on the rotation angle between the two layers. In the case of commensurability, strain gets concentrated in domain walls, resulting in soliton-like structures.

Spin polarization due to spin–orbit coupling requires broken inversion symmetry. Now, calculations show that the effect arises from local site-asymmetry rather than global space-group asymmetry, and that a hitherto overlooked form of spin polarization should also exist in centrosymmetric structures.

The effect of structural disorder on superconductivity can be subtle: for two crystalline arrangements of superconducting lead monolayers deposited on silicon, there are unexpected spatial variations that result in macroscopically different behaviour.

It is now shown that coupled optical microcavities bear all the hallmarks of parity–time symmetry; that is, the system’s dynamics are unchanged by both time-reversal and mirror transformations. The resonant nature of microcavities results in unusual effects not seen in previous photonic analogues of parity–time-symmetric systems: for example, light travelling in one direction is resonantly enhanced but there are no resonance peaks going the other way.

The thermal and quantum fluctuations around a quantum critical point can be studied independently by mapping the evolution of the spin dynamics in the critical region of a dimerized quantum magnet using neutron scattering.

Quantum criticality is often found in metallic compounds that are close to being magnetic. What about insulators in which the electric moments are fluctuating? These too can be described by the same framework—over a wider temperature range than in quantum critical metals.