An ultracold graphene analog

Two highly active fields of physics have merged in recent years, as researchers work to build models of condensed matter systems using ultracold atoms suspended in optical lattices. Graphene provides an environment for many intriguing physics problems, with its massless fermions, unusually high carrier mobility, and anomalous quantum Hall behavior. Now, Kean Loon Lee and colleagues at the National University of Singapore, and at Ecole Normale Supérieure and Institut Non Linéaire de Nice in France, report inPhysical Review A their theoretical studies of ultracold atoms arranged in a hexagonal graphenelike optical lattice.

When atoms are loaded into optical traps researchers can control their position and the strength of their interactions. The authors model a two-dimensional honeycomb lattice of traps created by the interference of three laser beams. They then carry out tight-binding calculations of the band structure to show that a signature of graphene—transport of massless excitations—could indeed exist in this analogous system. Lee et al. also study the hopping of nearest-neighbor atoms and the influence of lattice distortions, providing a useful guidepost to future experimental efforts. – David Voss

Phys. Rev. A 80, 043411

Drilling for tunable photons in a nanohole

When free electrons interact with a periodic structured environment, such as the surface of a metal grating, they emit photons [1]. The same principle guides the operation of the free-electron laser, whereby a beam of relativistic electrons passes through a spatially periodic transverse magnetic field, generating tunable, coherent, high-power radiation [2].

Writing in Physical Review Letters, Giorgio Adamo and colleagues from the University of Southampton in the UK, and collaborators in Taiwan and Spain, take the concept of tunable light sources into the realm of the nanoscale. Adamo et al. fire an electron beam through a 700-nm-diameter hole in a stack of alternating silica and gold layers, each200 nm thick. As the electrons travel through the periodically layered structure, they emit near-infrared photons whose frequency can be tuned by adjusting the electron energies in the 20–40 keV range. The tunability of this “light well,” together with its compact size, makes this device potentially interesting as an on-chip light source for nanophotonic circuits, or in optical memory and display applications. Scaling the concept from the THz range to the UV appears within reach by varying the periodicity of the structure.

At this proof-of-concept stage there are caveats: The emitted light is incoherent and the photon conversion process is hampered by losses, with only 2–4 photons emitted per 100 000 electrons at maximum intensity. If the technical challenges presented in this demonstration can be surpassed, Adamo et al.’s results could pave the way for a new generation of on-chip tunable light sources. – Manolis Antonoyiannakis

[1] S. J. Smith and E. M. Purcell, Phys. Rev. 92, 1069 (1953).

[2] L. R. Elias et al., Phys. Rev. Lett. 36, 717 (1976); D. A. G. Deacon et al., Phys. Rev. Lett. 38, 892 (1977).

Simulating nuclear pasta

In the collapsing core of a supernova, nuclei get squeezed together so tightly that they lose their individual identities and merge into a giant mass of nucleons. But according to theories going back almost 40 years, at slightly lower densities the nuclei will connect up to form what are called “pasta” phases—rods (“spaghetti”), flat slabs (“lasagna”), and even volumes of nucleons with spherical or rod-shaped voids (“cheese” and “anti-spaghetti”).

To see if these phases can really form in a supernova, Gentaro Watanabe of RIKEN at Wako in Japan and Hidetaka Sonoda of the University of Tokyo, along with colleagues from those and other Japanese institutions, have performed ab initio simulations of nuclei being squeezed toward the conditions of the pasta phases, as they report inPhysical Review Letters.

They find that straight rods form once the density of the nuclei reaches about 30% that of a nucleus, but the formation process is surprising. As the initially spherical nuclei are squeezed, they begin to stick together in pairs at right angles to one another, forming a giant herringbone pattern, quite different from the parallel connections that were expected. As the density increases, the pairs fuse into “zig-zag” rods, which eventually straighten out.

Watanabe et al. explain that the nuclei initially link up because of the strong-force attraction between nucleons in neighboring nuclei. This picture contradicts the conventional view that the connections result from the so-called fission instability, which can cause a nucleus to deform into an ellipsoid that would touch and ultimately join with its neighbors. There is evidence that pasta phases should have large effects on neutrino transport, which is a major focus of supernova research, so Watanabe et al.recommend incorporating nuclear pasta into future supernova simulations. – David Ehrenstein

The narrowing search for the Higgs boson

In the Standard Model mass of a particle arises from spontaneous symmetry breaking. A byproduct of this mechanism is a particle called the Higgs boson. In a paper appearing in Phys. Rev. Lett. 102, 021802 (Published January 15, 2009), the CDF collaboration reports a search for the Higgs boson, via its expected decay to two W bosons. Based on a large amount of data (3 inverse femtobarns), they see no sign of the Higgs boson, and constrain its production cross section to less than ten times the prediction of the Standard Model. In fact, for the resonance energy of twice the W mass, or 160 GeV, they put an upper bound on the cross section for the decay to be less than twice the Standard Model prediction.