Letters in 2009

Graphene is expected to possess characteristics that are particularly useful for transporting and manipulating electronic spin. The discovery of spin-dependent interference features in its electrical characteristics could be useful in the development of graphene spintronics.

A study of a one-dimensional system may have finally resolved the long-standing discrepancy between the expected and measured inelastic neutron scattering intensities in the high-temperature cuprate superconductors.

Complex oxide films are highly anisotropic in the way they conduct electricity, which is due to phase separation. However, the origin of this metal–insulator phase coexistence has been unclear. Transport measurements now show that strain, rather than chemical inhomogeneity, is mainly responsible.

The ‘transmon’ design for superconducting qubits is particularly promising, owing to the long coherence times that it enables. Now, high-fidelity single-shot readout of such qubits — necessary for operating a quantum processor — has been demonstrated

High-intensity X-ray sources such as synchrotrons and free-electron lasers need large particle accelerators to drive them. The demonstration of a synchrotron X-ray source that uses a laser-driven particle accelerator could widen the availability of intense X-rays for research in physics, materials science and biology.

The presence of disorder makes it difficult to determine the intrinsic properties of graphene in its ideal form. Measurements of high-quality bilayer graphene flakes suspended above a substrate identify the persistence of quantum Hall behaviour at magnetic fields an order of magnitude lower than seen before, and previously unseen symmetry breaking of the lowest Landau level is also observed.

Owing to the fact that graphene is just one atom thick, it has been suggested that it might be possible to control its properties by subjecting it to mechanical strain. New analysis indicates not only this, but that pseudomagnetic behaviour and even zero-field quantum Hall effects could be induced in graphene under realistic amounts of strain.

Similar to atoms in cold gases, exciton–polaritons in semiconductor microcavities can undergo Bose–Einstein condensation. A striking consequence of the appearance of macroscopic coherence in these systems is superfluidity. Now, clear evidence for such behaviour has been found in an exciton–polariton condensate.

Frequency combs have revolutionized frequency metrology. High-harmonic generation in atoms has led to fast sources of short-wavelength photons. Combining these two technologies enables the transfer of frequency combs to the vacuum-ultraviolet with potential applications in spectroscopy.

As well as providing subatomic-scale real-space images of metals, the scanning tunnelling microscope also reveals momentum–space information. Now it is possible to use this technique to image a heavy-electron liquid and obtain information on orbital structures.

The so-called hidden-order state in URu₂Si₂ is further obscured by conflicting experimental observations. A first-principles calculation shows that an order parameter with real and imaginary parts can explain many of these conflicts.

More efficient solar-energy conversion is possible if a single high-energy photon can be made to generate two electron–hole pairs in a cell, rather than a single pair plus heat. It is now shown that, contrary to expectation, this carrier multiplication is better in bulk semiconductor materials than in quantum dots.

One of the many unusual characteristics of graphene is that it shows ‘puddles’ of positive and negative charge throughout. A systematic scanning tunnelling microscope study shows that these puddles are not a consequence of ripples in graphene’s structure as originally thought, but are due to charged impurities below its surface.

When two identical photons hit a half-silvered mirror, quantum mechanics requires that both pass through or both be reflected in the same direction. Previously, this effect had only been demonstrated with photons from similar light sources. It has now been repeated with photons generated by two completely different physical processes.

Scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy are complementary probes, and yet the results of recent studies using these techniques on quasiparticle excitations in the copper oxide superconductors seem to be contradictory. In fact, there is no contradiction.

Using arguments from computational complexity theory, fundamental limitations are found for how efficient it is to calculate the ground-state energy of many-electron systems using density functional theory.

Magnetic switching is typically accomplished by using a driving field that stays on until the magnetization is rotated to its final position. An experiment demonstrates that, in antiferromagnets, inertial effects can be harnessed, such that only a short ‘kick’ is required to transfer sufficient momentum to the spin system for it to reorient.

Radiofrequency spectroscopy provides a microscopic probe of fermionic pairing in ultracold Fermi gases. Calculations now suggest that there is a one-to-one correspondence between the theory of these spectra and the theory of paraconductivity fluctuations in superconductors, that is, the effect of enhanced conductivity even before the system enters the superconducting state.

A two-dimensional lattice of vortices melts into an isotropic liquid with increasing temperature. A microscopic view of the melting transition reveals that this actually occurs in three steps, one of which is an unusual liquid-crystal-like vortex phase.

Electromagnetically induced transparency in an atomic gas can slow the propagation of images. It is now shown that the diffraction of such images as they propagate can be controlled and even eliminated. This is achieved by using atomic diffusion to influence the spreading of the image.