Nature Physics. doi:10.1038/nphys1134

Authors: Minoru Yamashita, Norihito Nakata, Yuichi Kasahara, Takahiko Sasaki, Naoki Yoneyama, Norio Kobayashi, Satoshi Fujimoto, Takasada Shibauchi & Yuji Matsuda

The notion of quantum spin-liquids (QSLs), antiferromagnets with quantum fluctuation-driven disordered ground states, is now firmly established in one-dimensional (1D) spin systems as well as in their ladder cousins. The spin-1/2 organic insulator κ-(bis(ethylenedithio)-tetrathiafulvalene)2Cu2(CN)3 (κ-(BEDT-TTF)2Cu2(CN)3; ref. 1) with a 2D triangular lattice structure is very likely to be the first experimental realization of this exotic state in D≥2. Of crucial importance is to unveil the nature of the low-lying elementary spin excitations, particularly the presence/absence of a ‘spin gap’, which will provide vital information on the universality class of this putative QSL. Here, we report on our thermal-transport measurements carried out down to 80 mK. We find, rather unexpectedly, unambiguous evidence for the absence of a gapless excitation, which sharply contradicts recent reports of heat capacity measurements. The low-energy physics of this intriguing system needs be reinterpreted in light of the present results indicating a spin-gapped QSL phase.

Nature Physics. doi:10.1038/nphys1127

Authors: Ophir M. Auslaender, Lan Luan, Eric W. J. Straver, Jennifer E. Hoffman, Nicholas C. Koshnick, Eli Zeldov, Douglas A. Bonn, Ruixing Liang, Walter N. Hardy & Kathryn A. Moler

Superconductors often contain quantized microscopic whirlpools of electrons, called vortices, that can be modelled as one-dimensional elastic objects. Vortices are a diverse area of study for condensed matter because of the interplay between thermal fluctuations, vortex–vortex interactions and the interaction of the vortex core with the three-dimensional disorder landscape. Although vortex matter has been studied extensively, the static and dynamic properties of an individual vortex have not. Here, we use magnetic force microscopy (MFM) to image and manipulate individual vortices in a detwinned YBa2Cu3O6.991 single crystal, directly measuring the interaction of a moving vortex with the local disorder potential. We find an unexpected and marked enhancement of the response of a vortex to pulling when we wiggle it transversely. In addition, we find enhanced vortex pinning anisotropy that suggests clustering of oxygen vacancies in our sample and demonstrates the power of MFM to probe vortex structure and microscopic defects that cause pinning.

Nature Physics. doi:10.1038/nphys1133

Authors: J. S. Lundeen, A. Feito, H. Coldenstrodt-Ronge, K. L. Pregnell, Ch. Silberhorn, T. C. Ralph, J. Eisert, M. B. Plenio & I. A. Walmsley

Measurement connects the world of quantum phenomena to the world of classical events. It has both a passive role—in observing quantum systems—and an active one, in preparing quantum states and controlling them. In view of the central status of measurement in quantum mechanics, it is surprising that there is no general recipe for designing a detector that measures a given observable. Compounding this, the characterization of existing detectors is typically based on partial calibrations or elaborate models. Thus, experimental specification (that is, tomography) of a detector is of fundamental and practical importance. Here, we present the realization of quantum detector tomography. We identify the positive-operator-valued measure describing the detector, with no ancillary assumptions. This result completes the triad, state, process and detector tomography, required to fully specify an experiment. We characterize an avalanche photodiode and a photon-number-resolving detector capable of detecting up to eight photons. This creates a new set of tools for accurately detecting and preparing non-classical light.

Nature Physics. doi:10.1038/nphys1128

Authors: K. Sugawara, T. Sato & T. Takahashi

The discovery of superconductivity in C6Yb and C6Ca (ref. 1) has activated fierce debates on whether it is described within the conventional Bardeen–Cooper–Schrieffer scheme or some other exotic mechanisms are involved, because the superconducting transition temperature (Tc) is significantly higher than that of the alkali-metal graphite intercalation compounds intensively studied in the 1980s (refs 2, 3, 4). The key to understand the mechanism of superconductivity lies in the superconducting energy gap associated with the formation of superconducting pairs. Here, we report the first direct observation of a superconducting gap in C6Ca by high-resolution angle-resolved photoemission spectroscopy. We found that the superconducting gap of 1.8–2.0 meV opens on the intercalant Fermi surface, whereas the gap is very small or absent on the Fermi surface derived from the π* band of graphene layers. These experimental results unambiguously establish that the interlayer band has an essential role for the high-Tc superconductivity in C6Ca.

Nature Physics. doi:10.1038/nphys1109

Authors: R. Daou, Nicolas Doiron-Leyraud, David LeBoeuf, S. Y. Li, Francis Laliberté, Olivier Cyr-Choinière, Y. J. Jo, L. Balicas, J.-Q. Yan, J.-S. Zhou, J. B. Goodenough & Louis Taillefer

A fundamental question for high-temperature superconductors is the nature of the pseudogap phase, which lies between the Mott insulator at zero doping and the Fermi liquid at high doping p (refs 1, 2). Here we report on the behaviour of charge carriers near the zero-temperature onset of this phase, namely at the critical doping p*, where the pseudogap temperature T* goes to zero, accessed by investigating a material in which superconductivity can be fully suppressed by a steady magnetic field. Just below p*, the normal-state resistivity and Hall coefficient of La1.6−xNd0.4SrxCuO4 are found to rise simultaneously as the temperature drops below T*, suggesting a change in the Fermi surface with a large associated drop in conductivity. At p*, the resistivity shows a linear temperature dependence as the temperature approaches zero, a typical signature of a quantum critical point. These findings impose new constraints on the mechanisms responsible for inelastic scattering and Fermi-surface transformation in theories of the pseudogap phase.