Nature Physics. doi:10.1038/nphys1452

Author: Denis Jérome

The metal–insulator Mott transition, which has been extensively studied by means of charge transport, is now detected through the electron spins in a two-dimensional organic conductor.

Nature Physics. doi:10.1038/nphys1456

Authors: U. Chatterjee, M. Shi, D. Ai, J. Zhao, A. Kanigel, S. Rosenkranz, H. Raffy, Z. Z. Li, K. Kadowaki, D. G. Hinks, Z. J. Xu, J. S. Wen, G. Gu, C. T. Lin, H. Claus, M. R. Norman, M. Randeria & J. C. Campuzano

A key question in condensed-matter physics is to understand how high-temperature superconductivity emerges on adding mobile charged carriers to an antiferromagnetic Mott insulator. We address this question using angle-resolved photoemission spectroscopy to probe the electronic excitations of the non-superconducting state that exists between the Mott insulator and the d-wave superconductor in Bi2Sr2CaCu2O8+δ. Despite a temperature-dependent resistivity characteristic of an insulator, the excitations in this intermediate state have a highly anisotropic energy gap that vanishes at four points in momentum space. This nodal-liquid state has the same gap structure as that of the d-wave superconductor but no sharp quasiparticle peaks. We observe a smooth evolution of the excitation spectrum, along with the appearance of coherent quasiparticles, as one goes through the insulator-to-superconductor transition as a function of doping. Our results suggest that high-temperature superconductivity emerges when quantum phase coherence is established in a non-superconducting nodal liquid.

Nature Physics. doi:10.1038/nphys1457

Authors: Hidekazu Mimura, Soichiro Handa, Takashi Kimura, Hirokatsu Yumoto, Daisuke Yamakawa, Hikaru Yokoyama, Satoshi Matsuyama, Kouji Inagaki, Kazuya Yamamura, Yasuhisa Sano, Kenji Tamasaku, Yoshinori Nishino, Makina Yabashi, Tetsuya Ishikawa & Kazuto Yamauchi

Hard X-rays have exceptional properties that are useful in the chemical, elemental and structure analysis of matter. Although single-nanometre resolutions in various hard-X-ray analytical methods are theoretically possible with a focused hard-X-ray beam, fabrication of the focusing optics remains the main hurdle. Aberrations owing to imperfections in the optical system degrade the quality of the focused beam. Here, we describe an in situ wavefront-correction approach to overcome this and demonstrate an X-ray beam focused in one direction to a width of 7 nm at 20 keV. We achieved focal spot improvement of the X-ray nanobeam produced by a laterally graded multilayer mirror. A grazing-incidence deformable mirror was used to restore the wavefront shape. Using this system, ideal focusing conditions are achievable even if hard-X-ray focusing elements do not achieve sufficient performance. It is believed that this will ultimately lead to single-nanometre spatial resolution in X-ray analytical methods.

Nature Physics. doi:10.1038/nphys1453

Authors: D. Hanneke, J. P. Home, J. D. Jost, J. M. Amini, D. Leibfried & D. J. Wineland

The universal quantum computer is a device capable of simulating any physical system and represents a major goal for the field of quantum information science. In the context of quantum information, ‘universal’ refers to the ability to carry out arbitrary unitary transformations in the system’s computational space. Combining arbitrary single-quantum-bit (qubit) gates with an entangling two-qubit gate provides a set of gates capable of achieving universal control of any number of qubits, provided that these gates can be carried out repeatedly and between arbitrary pairs of qubits. Although gate sets have been demonstrated in several technologies, they have so far been tailored towards specific tasks, forming a small subset of all unitary operators. Here we demonstrate a quantum processor that can be programmed with 15 classical inputs to realize arbitrary unitary transformations on two qubits, which are stored in trapped atomic ions. Using quantum state and process tomography, we characterize the fidelity of our implementation for 160 randomly chosen operations. This universal control is equivalent to simulating any pairwise interaction between spin-1/2 systems. A programmable multiqubit register could form a core component of a large-scale quantum processor, and the methods used here are suitable for such a device.

Nature Physics. doi:10.1038/nphys1455

Authors: Maciej Sawicki, Daichi Chiba, Anna Korbecka, Yu Nishitani, Jacek A. Majewski, Fumihiro Matsukura, Tomasz Dietl & Hideo Ohno

The question of whether the Anderson–Mott localization enhances or reduces magnetic correlations is central to the physics of magnetic alloys. Particularly intriguing is the case of (Ga, Mn)As and related magnetic semiconductors, for which diverging theoretical scenarios have been proposed. Here, by direct magnetization measurements we demonstrate how magnetism evolves when the density of carriers mediating the spin–spin coupling is diminished by the gate electric field in metal–insulator–semiconductor structures of (Ga, Mn)As. Our findings show that the channel depletion results in a monotonic decrease of the Curie temperature, with no evidence for the maximum expected within the impurity-band models. We find that the transition from the ferromagnetic to the paramagnetic state proceeds by means of the emergence of a superparamagnetic-like spin arrangement. This implies that carrier localization leads to a phase separation into ferromagnetic and non-magnetic regions, which we attribute to critical fluctuations in the local density of states, specific to the Anderson–Mott quantum transition.

Nature Physics. doi:10.1038/nphys1436

Authors: C. Burrowes, A. P. Mihai, D. Ravelosona, J.-V. Kim, C. Chappert, L. Vila, A. Marty, Y. Samson, F. Garcia-Sanchez, L. D. Buda-Prejbeanu, I. Tudosa, E. E. Fullerton & J.-P. Attané

Torques appear between charge carrier spins and local moments in regions of ferromagnetic media where spatial magnetization gradients occur, such as a domain wall, owing to an exchange interaction. This phenomenon has been predicted by different theories and confirmed in a number of experiments on metallic and semiconductor ferromagnets. Understanding the magnitude and orientation of such spin-torques is an important problem for spin-dependent transport and current-driven magnetization dynamics, as domain-wall motion underlies a number of emerging spintronic technologies. One outstanding issue concerns the non-adiabatic spin-torque component β, which has an important role in wall dynamics, but no clear consensus has yet emerged over its origin or magnitude. Here, we report an experimental measurement of β in perpendicularly magnetized films with narrow domain walls (1–10 nm). By studying thermally activated wall depinning, we deduce β from the variation of the Arrhenius transition rate with applied currents. Surprisingly, we find β to be small and relatively insensitive to the wall width, which stands in contrast to predictions from transport theories. In addition, we find β to be close to the Gilbert damping constant α, which, in light of similar results on planar anisotropy systems, suggests a universal origin for the non-adiabatic torque.

Nature Physics. doi:10.1038/nphys1438

Authors: J. H. Eggert, D. G. Hicks, P. M. Celliers, D. K. Bradley, R. S. McWilliams, R. Jeanloz, J. E. Miller, T. R. Boehly & G. W. Collins

Since Ross proposed that there might be ‘diamonds in the sky’ in 1981 (ref. 1), the idea of significant quantities of pure carbon existing in giant planets such as Uranus and Neptune has gained both experimental and theoretical support. It is now accepted that the high-pressure, high-temperature behaviour of carbon is essential to predicting the evolution and structure of such planets. Still, one of the most defining of thermal properties for diamond, the melting temperature, has never been directly measured. This is perhaps understandable, given that diamond is thermodynamically unstable, converting to graphite before melting at ambient pressure, and tightly bonded, being the strongest bulk material known. Shock-compression experiments on diamond reported here reveal the melting temperature of carbon at pressures of 0.6–1.1 TPa (6–11 Mbar), and show that crystalline diamond can be stable deep inside giant planets such as Uranus and Neptune. The data indicate that diamond melts to a denser, metallic fluid—with the melting curve showing a negative Clapeyron slope—between 0.60 and 1.05 TPa, in good agreement with predictions of first-principles calculations. Temperature data at still higher pressures suggest diamond melts to a complex fluid state, which dissociates at shock pressures between 1.1 and 2.5 TPa (11–25 Mbar) as the temperatures increase above 50,000 K.

Nature Physics. doi:10.1038/nphys1437

Authors: Zengwei Zhu, Huan Yang, Benoît Fauqué, Yakov Kopelevich & Kamran Behnia

The Nernst effect has recently emerged as a very sensitive, yet poorly understood, probe of electron organization in solids. Graphene, a single layer of carbon atoms set in a honeycomb lattice, embeds a two-dimensional gas of massless electrons and hosts a particular version of the quantum Hall effect. Recent experimental investigations of its thermoelectric response are in agreement with the theory conceived for a two-dimensional electron system in the quantum Hall regime. Here, we report on a study of graphite, a macroscopic stack of graphene layers, which establishes a fundamental link between the dimensionality of an electronic system and its Nernst response. In striking contrast with the single-layer case, the Nernst signal sharply peaks whenever a Landau level meets the Fermi level. Thus, the degrees of freedom provided by finite interlayer coupling lead to an enhanced thermoelectric response in the vicinity of the quantum limit. As Landau quantization slices a three-dimensional Fermi surface, each intersection of a Landau level with the Fermi level modifies the Fermi-surface topology. According to our results, the most prominent signature of such a topological phase transition emerges in the transverse thermoelectric response.

Nature Physics. doi:10.1038/nphys1429

Authors: C. Altimiras, H. le Sueur, U. Gennser, A. Cavanna, D. Mailly & F. Pierre

The study of heat transport has the potential to reveal new insights into the physics of mesoscopic systems. This is especially true of those that show the integer quantum Hall effect, in which the robust quantization of Hall currents limits the amount of information that can be obtained from charge transport alone. As a consequence, our understanding of gapless edge excitations in these systems is incomplete. Effective edge-state theory describes them as prototypical one-dimensional chiral fermions—a simple picture that explains a large body of observations and suggests the use of quantum point contacts as electronic beam splitters to explore a variety of quantum mechanical phenomena. However, this picture is in apparent disagreement with the prevailing theoretical framework, which predicts in most situations extra gapless edge modes. Here, we present a spectroscopic technique that addresses the question of whether some of the injected energy is captured by the predicted extra states, by probing the distribution function and energy flow in an edge channel operated out-of-equilibrium. Our results show it is not the case and therefore that regarding energy transport, quantum point contacts do indeed behave as optical beam splitters. This demonstrates a useful new tool for heat transport and out-of-equilibrium experiments.

Nature Physics. doi:10.1038/nphys1427

Authors: Se-Chung Oh, Seung-Young Park, Aurélien Manchon, Mairbek Chshiev, Jae-Ho Han, Hyun-Woo Lee, Jang-Eun Lee, Kyung-Tae Nam, Younghun Jo, Yo-Chan Kong, Bernard Dieny & Kyung-Jin Lee

Spin-transfer torque (STT) allows the electrical control of magnetic states in nanostructures. The STT in magnetic tunnel junctions (MTJs) is of particular importance owing to its potential for device applications. It has been demonstrated that the MTJ has a sizable perpendicular STT (nphys1427-m1gif119129, field-like torque), which substantially affects STT-driven magnetization dynamics. In contrast to symmetric MTJs where the bias dependence of nphys1427-m2gif119129 is quadratic, it is theoretically predicted that the symmetry breaking of the system causes an extra linear bias dependence. Here, we report experimental results that are consistent with the predicted linear bias dependence in asymmetric MTJs. The linear contribution is quite significant and its sign changes from positive to negative as the asymmetry is modified. This result opens a way to design the bias dependence of the field-like term, which is useful for device applications by allowing, in particular, the suppression of the abnormal switching-back phenomena.

Nature Physics. doi:10.1038/nphys1433

Authors: Clemens B. Winkelmann, Nicolas Roch, Wolfgang Wernsdorfer, Vincent Bouchiat & Franck Balestro

Single-molecule transistors are currently attracting enormous attention as possible quantum information processing devices. An intrinsic limitation to their prospects, however, is associated with the presence of a small number of quantized conductance channels, each channel with a high access resistance of at best RK/2=h/2e2=12.9 kΩ. However, when the contacting leads become superconducting, long-range correlations can extend throughout the whole system by means of the proximity effect. This not only lifts the resistive limitation of normal-state contacts, but further paves the way to probe electron transport through a single molecule. Here we demonstrate the realization of superconducting single-molecule transistors involving a single C60 fullerene molecule. In the past few years, we have seen gate-controlled Josephson supercurrents induced in the family of low-dimensional carbon structures such as flakes of two-dimensional graphene and portions of one-dimensional carbon nanotubes. The present study, involving a full zero-dimensional fullerene, completes the picture.

Nature Physics. doi:10.1038/nphys1428

Authors: Fumitaka Kagawa, Kazuya Miyagawa & Kazushi Kanoda

Near a Mott transition, which can be tuned by controlling either the charge-carrier density (‘filling’) or the correlation strength (‘bandwidth’), lies fascinating emergent behaviour, such as unconventional superconductivity, and the understanding of the underlying Mott criticality is a longstanding challenge. Recent studies have showed that the bandwidth-controlled Mott criticality (BCMC) involves critical fluctuations in charge and lattice degrees of freedom. Spin is another degree of freedom and its antiferromagnetic fluctuations are ubiquitous in strongly correlated electrons. However, the magnetic aspects of BCMC are unexplored. Here, we report on the magnetic criticality brought about by BCMC. Through NMR investigations on a κ-type organic salt, we observe critical suppression of antiferromagnetic fluctuations accompanied by the critical enhancement of conductance. The two criticalities show the same exponent within experimental error. Site-to-site electron hopping introduces doubly occupied and empty sites, which extinguish stroboscopically the local spins, probably resulting in the identical criticality in charge and spin.

Nature Physics. doi:10.1038/nphys1426

Authors: G. Yu, Y. Li, E. M. Motoyama & M. Greven

Superconductivity involves the formation of electron pairs (Cooper pairs) and their condensation into a macroscopic quantum state. In conventional superconductors, such as Nb3Ge and elemental Hg, weakly interacting electrons pair through the electron–phonon interaction. In contrast, unconventional superconductivity occurs in correlated-electron materials in which electronic interactions are significant and the pairing mechanism may not be phononic. In the cuprates, the superconductivity arises on doping charge carriers into the copper–oxygen layers of antiferromagnetic Mott insulators. Other examples of unconventional superconductors are the heavy-fermion compounds, which are metals with coupled conduction and localized f-shell electrons, and the recently discovered iron–arsenide superconductors. These unconventional superconductors show a magnetic resonance, a prominent collective spin-1 excitation mode in the superconducting state. Here we demonstrate the existence of a universal linear relation, Er∝2Δ, between the magnetic resonance energy (Er) and the superconducting pairing gap (Δ), which spans two orders of magnitude in energy. This relationship is valid for the three different classes of unconventional superconductors, which range from being close to the Mott-insulating limit to being on the border of itinerant magnetism. As the common excitonic picture of the resonance has not led to such universality, our observation suggests a much deeper connection between antiferromagnetic fluctuations and unconventional superconductivity.

Nature Physics. doi:10.1038/nphys1421

Authors: Mark B. Lundeberg & Joshua A. Folk

The unusual electronic properties of single-layer graphene make it a promising materials system for fundamental advances in physics, and an attractive platform for new device technologies. Graphene’s spin-transport properties are expected to be particularly interesting, with predictions for extremely long coherence times and intrinsic spin-polarized states at zero field. To test such predictions, it is necessary to measure the spin polarization of electrical currents in graphene. Here, we resolve spin transport directly from conductance features that are caused by quantum interference. These features split visibly in an in-plane magnetic field, similar to Zeeman splitting in atomic and quantum-dot systems. The spin-polarized conductance features that are the subject of this work may, in the future, lead to the development of graphene devices incorporating interference-based spin filters.

Nature Physics. doi:10.1038/nphys1405

Authors: Andrew C. Walters, Toby G. Perring, Jean-Sébastien Caux, Andrei T. Savici, Genda D. Gu, Chi-Cheng Lee, Wei Ku & Igor A. Zaliznyak

Theories involving highly energetic spin fluctuations are among the leading contenders for explaining high-temperature superconductivity in the cuprates. These theories could be tested by inelastic neutron scattering (INS), as a change in the magnetic scattering intensity that marks the entry into the superconducting state provides a precise quantitative measure of the spin-interaction energy involved in the superconductivity. However, the absolute intensities of spin fluctuations measured in neutron scattering experiments vary widely, and are usually much smaller than expected from fundamental sum rules, resulting in ‘missing’ INS intensity. Here, we solve this problem by studying magnetic excitations in the one-dimensional related compound, Sr2CuO3, for which an exact theory of the dynamical spin response has recently been developed. In this case, the missing INS intensity can be unambiguously identified and associated with the strongly covalent nature of magnetic orbitals. We find that whereas the energies of spin excitations in Sr2CuO3 are well described by the nearest-neighbour spin-1/2 Heisenberg Hamiltonian, the corresponding magnetic INS intensities are modified markedly by the strong 2p–3d hybridization of Cu and O states. Hence, the ionic picture of magnetism, where spins reside on the atomic-like 3d orbitals of Cu2+ ions, fails markedly in the cuprates.

Nature Physics. doi:10.1038/nphys1419

Authors: T. Z. Ward, J. D. Budai, Z. Gai, J. Z. Tischler, Lifeng Yin & J. Shen

The presence of electronic phase separation in complex materials has been linked to many types of exotic behaviour, such as colossal magnetoresistance, the metal–insulator transition and high-temperature superconductivity; however, the mechanisms that drive the formation of coexisting electronic phases are still debated. Here we report transport measurements that show a preferential orientation of electronic phase domains driven by anisotropic long-range elastic coupling between a complex oxide film and substrate. We induce anisotropic electronic-domain formation along one axis of a pseudocubic perovskite single-crystal thin-film manganite by epitaxially locking it to an orthorhombic substrate. Simultaneous temperature-dependent resistivity measurements along the two perpendicular in-plane axes show substantial differences in the metal–insulator transition temperature and extraordinarily high anisotropic resistivities, which indicate that percolative conduction channels open more readily along one axis. These findings suggest that the origin of phase coexistence is much more strongly influenced by strain than by local chemical inhomogeneity.

Nature Physics. doi:10.1038/nphys1406

Authors: Benjamin E. Feldman, Jens Martin & Amir Yacoby

Mono- and bilayer graphene have generated tremendous excitement owing to their unique and potentially useful electronic properties. Suspending single-layer graphene flakes above the substrate has been shown to greatly improve sample quality, yielding high-mobility devices with little charge inhomogeneity. Here we report the fabrication of suspended bilayer graphene devices with very little disorder. We observe quantum Hall states that are fully quantized at a magnetic field of 0.2 T, as well as broken-symmetry states at intermediate filling factors ν=0, ±1, ±2 and ±3. In the ν=0 state, the devices show extremely high magnetoresistance that scales as magnetic field divided by temperature. This resistance is predominantly affected by the perpendicular component of the applied field, and the extracted energy gap is significantly larger than expected for Zeeman splitting. These findings indicate that the broken-symmetry states arise from many-body interactions and underscore the important part that Coulomb interactions play in bilayer graphene.

Nature Physics. doi:10.1038/nphys1420

Authors: F. Guinea, M. I. Katsnelson & A. K. Geim

Among many remarkable qualities of graphene, its electronic properties attract particular interest owing to the chiral character of the charge carriers, which leads to such unusual phenomena as metallic conductivity in the limit of no carriers and the half-integer quantum Hall effect observable even at room temperature. Because graphene is only one atom thick, it is also amenable to external influences, including mechanical deformation. The latter offers a tempting prospect of controlling graphene’s properties by strain and, recently, several reports have examined graphene under uniaxial deformation. Although the strain can induce additional Raman features, no significant changes in graphene’s band structure have been either observed or expected for realistic strains of up to ∼15% (refs 9, 10, 11). Here we show that a designed strain aligned along three main crystallographic directions induces strong gauge fields that effectively act as a uniform magnetic field exceeding 10 T. For a finite doping, the quantizing field results in an insulating bulk and a pair of countercirculating edge states, similar to the case of a topological insulator. We suggest realistic ways of creating this quantum state and observing the pseudomagnetic quantum Hall effect. We also show that strained superlattices can be used to open significant energy gaps in graphene’s electronic spectrum.

Nature Physics. doi:10.1038/nphys1431

Authors: T. Pereg-Barnea, H. Weber, G. Refael & M. Franz

Nature Physics. doi:10.1038/nphys1432

Authors: Jacob D. Stevenson & Peter G. Wolynes

Nature Physics. doi:10.1038/nphys1430

Authors: T. E. Glover, M. P. Hertlein, S. H. Southworth, T. K. Allison, J. van Tilborg, E. P. Kanter, B. Krässig, H. R. Varma, B. Rude, R. Santra, A. Belkacem & L. Young

Nature Physics. doi:10.1038/nphys1423

Authors: Anne Bernand-Mantel, Pierre Seneor, Karim Bouzehouane, Stéphane Fusil, Cyrile Deranlot, Frédéric Petroff & Albert Fert

Nature Physics. doi:10.1038/nphys1425

Authors: G. Anetsberger, O. Arcizet, Q. P. Unterreithmeier, R. Rivière, A. Schliesser, E. M. Weig, J. P. Kotthaus & T. J. Kippenberg

Nature Physics. doi:10.1038/nphys1424

Authors: Sandra Foletti, Hendrik Bluhm, Diana Mahalu, Vladimir Umansky & Amir Yacoby

Nature Physics. doi:10.1038/nphys1422

Authors: Mathieu L. Juan, Reuven Gordon, Yuanjie Pang, Fatima Eftekhari & Romain Quidant