Superconductivity at 250 K in lanthanum hydride under high pressures.

With the discovery1 of superconductivity at 203 kelvin in H3S, attention returned to conventional superconductors with properties that can be described by the Bardeen-Cooper-Schrieffer and the Migdal-Eliashberg theories. Although these theories predict the possibility of room-temperature superconductivity in metals that have certain favourable properties-such as lattice vibrations at high frequencies-they are not sufficient to guide the design or predict the properties of new superconducting materials. First-principles calculations based on density functional theory have enabled such predictions, and have suggested a new family of superconducting hydrides that possess a clathrate-like structure in which the host atom (calcium, yttrium, lanthanum) is at the centre of a cage formed by hydrogen atoms2-4. For LaH10 and YH10, the onset of superconductivity is predicted to occur at critical temperatures between 240 and 320 kelvin at megabar pressures3-6. Here we report superconductivity with a critical temperature of around 250 kelvin within the [Formula: see text] structure of LaH10 at a pressure of about 170 gigapascals. This is, to our knowledge, the highest critical temperature that has been confirmed so far in a superconducting material. Superconductivity was evidenced by the observation of zero resistance, an isotope effect, and a decrease in critical temperature under an external magnetic field, which suggested an upper critical magnetic field of about 136 tesla at zero temperature. The increase of around 50 kelvin compared with the previous highest critical temperature1 is an encouraging step towards the goal of achieving room-temperature superconductivity in the near future.

Electronic in-plane symmetry breaking at field-tuned quantum criticality in CeRhIn5.

Electronic nematic materials are characterized by a lowered symmetry of the electronic system compared to the underlying lattice, in analogy to the directional alignment without translational order in nematic liquid crystals. Such nematic phases appear in the copper- and iron-based high-temperature superconductors, and their role in establishing superconductivity remains an open question. Nematicity may take an active part, cooperating or competing with superconductivity, or may appear accidentally in such systems. Here we present experimental evidence for a phase of fluctuating nematic character in a heavy-fermion superconductor, CeRhIn5 (ref. 5). We observe a magnetic-field-induced state in the vicinity of a field-tuned antiferromagnetic quantum critical point at Hc ≈ 50 tesla. This phase appears above an out-of-plane critical field H* ≈ 28 tesla and is characterized by a substantial in-plane resistivity anisotropy in the presence of a small in-plane field component. The in-plane symmetry breaking has little apparent connection to the underlying lattice, as evidenced by the small magnitude of the magnetostriction anomaly at H*. Furthermore, no anomalies appear in the magnetic torque, suggesting the absence of metamagnetism in this field range. The appearance of nematic behaviour in a prototypical heavy-fermion superconductor highlights the interrelation of nematicity and unconventional superconductivity, suggesting nematicity to be common among correlated materials.

Experimental Observation and Spin Texture of Dirac Node Arcs in Tetradymite Topological Metals.

We report the observation of a nontrivial spin texture in Dirac node arcs, i.e., novel topological objects formed when Dirac cones of massless particles extend along an open one-dimensional line in momentum space. We find that such states are present in all the compounds of the tetradymite M_{2}Te_{2}X family (M=Ti, Zr, or Hf and X=P or As) regardless of the weak or strong character of the topological invariant. The Dirac node arcs in tetradymites are thus the simplest possible textbook example of a type-I Dirac system with a single spin-polarized node arc.

Bulk Fermi Surfaces of the Dirac Type-II Semimetallic Candidates MAl_{3} (Where M=V, Nb, and Ta).

We report a de Haas-van Alphen (dHvA) effect study on the Dirac type-II semimetallic candidates MAl_{3} (where, M=V, Nb and Ta). The angular dependence of their Fermi surface (FS) cross-sectional areas reveals a remarkably good agreement with our first-principles calculations. Therefore, dHvA supports the existence of tilted Dirac cones with Dirac type-II nodes located at 100, 230 and 250 meV above the Fermi level ϵ_{F} for VAl_{3}, NbAl_{3} and TaAl_{3} respectively, in agreement with the prediction of broken Lorentz invariance in these compounds. However, for all three compounds we find that the cyclotron orbits on their FSs, including an orbit nearly enclosing the Dirac type-II node, yield trivial Berry phases. We explain this via an analysis of the Berry phase where the position of this orbit, relative to the Dirac node, is adjusted within the error implied by the small disagreement between our calculations and the experiments. We suggest that a very small amount of doping could displace ϵ_{F} to produce topologically nontrivial orbits encircling their Dirac node(s).

Engineering the Structural and Electronic Phases of MoTe2 through W Substitution.

MoTe2 is an exfoliable transition metal dichalcogenide (TMD) that crystallizes in three symmetries: the semiconducting trigonal-prismatic 2H- or α-phase, the semimetallic and monoclinic 1T'- or ß-phase, and the semimetallic orthorhombic γ-structure. The 2H-phase displays a band gap of ∼1 eV making it appealing for flexible and transparent optoelectronics. The γ-phase is predicted to possess unique topological properties that might lead to topologically protected nondissipative transport channels. Recently, it was argued that it is possible to locally induce phase-transformations in TMDs, through chemical doping, local heating, or electric-field to achieve ohmic contacts or to induce useful functionalities such as electronic phase-change memory elements. The combination of semiconducting and topological elements based upon the same compound might produce a new generation of high performance, low dissipation optoelectronic elements. Here, we show that it is possible to engineer the phases of MoTe2 through W substitution by unveiling the phase-diagram of the Mo1-xWxTe2 solid solution, which displays a semiconducting to semimetallic transition as a function of x. We find that a small critical W concentration xc ∼ 8% stabilizes the γ-phase at room temperature. This suggests that crystals with x close to xc might be particularly susceptible to phase transformations induced by an external perturbation, for example, an electric field. Photoemission spectroscopy, indicates that the γ-phase possesses a Fermi surface akin to that of WTe2.

Atypical Exciton-Phonon Interactions in WS2 and WSe2 Monolayers Revealed by Resonance Raman Spectroscopy.

Resonant Raman spectroscopy is a powerful tool for providing information about excitons and exciton-phonon coupling in two-dimensional materials. We present here resonant Raman experiments of single-layered WS2 and WSe2 using more than 25 laser lines. The Raman excitation profiles of both materials show unexpected differences. All Raman features of WS2 monolayers are enhanced by the first-optical excitations (with an asymmetric response for the spin-orbit related XA and XB excitons), whereas Raman bands of WSe2 are not enhanced at XA/B energies. Such an intriguing phenomenon is addressed by DFT calculations and by solving the Bethe-Salpeter equation. These two materials are very similar. They prefer the same crystal arrangement, and their electronic structure is akin, with comparable spin-orbit coupling. However, we reveal that WS2 and WSe2 exhibit quite different exciton-phonon interactions. In this sense, we demonstrate that the interaction between XC and XA excitons with phonons explains the different Raman responses of WS2 and WSe2, and the absence of Raman enhancement for the WSe2 modes at XA/B energies. These results reveal unusual exciton-phonon interactions and open new avenues for understanding the two-dimensional materials physics, where weak interactions play a key role coupling different degrees of freedom (spin, optic, and electronic).

Electron pockets in the Fermi surface of hole-doped high-Tc superconductors.

High-temperature superconductivity in copper oxides occurs when the materials are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping. The underlying evolution of the electron system in the absence of superconductivity is still unclear, and a question of central importance is whether it involves any intermediate phase with broken symmetry. The Fermi surface of the electronic states in the underdoped 'YBCO' materials YBa2Cu3O(y) and YBa2Cu4O8 was recently shown to include small pockets, in contrast with the large cylinder that characterizes the overdoped regime, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic-field-induced normal state of YBa2Cu3O(y) and YBa2Cu4O8, which reveals that these pockets are electron-like rather than hole-like. We propose that these electron pockets most probably arise from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped copper oxides near the onset of antiferromagnetic order. Comparison with materials of the La2CuO4 family that exhibit spin/charge density-wave order suggests that a Fermi surface reconstruction also occurs in those materials, pointing to a generic property of high-transition-temperature (T(c)) superconductors.

Superconducting pairs with extreme uniaxial anisotropy in URu2Si2.

We report magnetic field orientation-dependent measurements of the superconducting upper critical field in high quality single crystals of URu(2)Si(2) and find the effective g factor estimated from the Pauli limit to agree remarkably well with that found in quantum oscillation experiments, both quantitatively and in the extreme anisotropy (≈10(3)) of the spin susceptibility. Rather than a strictly itinerant or purely local f-electron picture being applicable, the latter suggests the quasiparticles subject to pairing in URu(2)Si(2) to be "composite heavy fermions" formed from bound states between conduction electrons and local moments with a protected Ising behavior. Non-Kramers doublet local magnetic degrees of freedom suggested by the extreme anisotropy favor a local pairing mechanism.

Chemical pressure effects on pyrochlore spin ice.

A comparison among the two sets of studied pyrochlore spin ices, Ho2Sn2O7, Ho2Ti2O7, and Ho2Ge2O7 with Ho3+ spins and Dy2Sn2O7, Dy2Ti2O7, and Dy2Ge2O7 with Dy3+ spins, shows that the application of chemical pressure through each set drives the system toward the antiferromagnetic phase boundary from the spin ice region, which agrees with the prediction of the "dipolar spin ice" model of den Hertog and Gingras. Among all the studied pyrochlore spin ices, Dy2Ge2O7 has the smallest ratio of Jnn/Dnn=-0.73.

Dimensional reduction at a quantum critical point.

Competition between electronic ground states near a quantum critical point (QCP)--the location of a zero-temperature phase transition driven solely by quantum-mechanical fluctuations--is expected to lead to unconventional behaviour in low-dimensional systems. New electronic phases of matter have been predicted to occur in the vicinity of a QCP by two-dimensional theories, and explanations based on these ideas have been proposed for significant unsolved problems in condensed-matter physics, such as non-Fermi-liquid behaviour and high-temperature superconductivity. But the real materials to which these ideas have been applied are usually rendered three-dimensional by a finite electronic coupling between their component layers; a two-dimensional QCP has not been experimentally observed in any bulk three-dimensional system, and mechanisms for dimensional reduction have remained the subject of theoretical conjecture. Here we show evidence that the Bose-Einstein condensate of spin triplets in the three-dimensional Mott insulator BaCuSi2O6 (refs 12-16) provides an experimentally verifiable example of dimensional reduction at a QCP. The interplay of correlations on a geometrically frustrated lattice causes the individual two-dimensional layers of spin-(1/2) Cu2+ pairs (spin dimers) to become decoupled at the QCP, giving rise to a two-dimensional QCP characterized by linear power law scaling distinctly different from that of its three-dimensional counterpart. Thus the very notion of dimensionality can be said to acquire an 'emergent' nature: although the individual particles move on a three-dimensional lattice, their collective behaviour occurs in lower-dimensional space.

Anisotropic hysteretic Hall effect and magnetic control of chiral domains in the chiral spin states of Pr2Ir2O7.

We uncover a strong anisotropy in both the anomalous Hall effect (AHE) and the magnetoresistance of the chiral spin states of Pr(2)Ir(2)O(7). The AHE appearing below 1.5 K at a zero magnetic field shows hysteresis which is most pronounced for fields cycled along the [111] direction. This hysteresis is compatible with the field-induced growth of domains composed by the 3-in 1-out spin states which remain coexisting with the 2-in 2-out spin ice manifold once the field is removed. Only for fields applied along the [111] direction, we observe a large positive magnetoresistance and Shubnikov-de Haas oscillations above a metamagnetic critical field. These observations suggest the reconstruction of the electronic structure of the conduction electrons by the field-induced spin texture.

Sequential spin polarization of the Fermi surface pockets in URu2Si2 and its implications for the hidden order.

Using Shubnikov-de Haas oscillations measured in URu2Si2 over a broad range in a magnetic field of 11-45 T, we find a cascade of field-induced Fermi surface changes within the hidden order phase I and further signatures of oscillations within field-induced phases III and V [previously discovered by Kim et al., [Phys. Rev. Lett. 91, 256401 (2003)]. A comparison of kinetic and Zeeman energies indicates a pocket-by-pocket polarization of the Fermi surface leading up to the destruction of the hidden order phase I at ≈35 T. The anisotropy of the Zeeman energy driving the transitions in URu2Si2 points to an itinerant hidden order parameter involving quasiparticles whose spin degrees of freedom depart significantly from those of free electrons.

Spin liquid state in the S = 1/2 triangular lattice Ba3CuSb2O9.

The synthesis and characterization of Ba3CuSb2O9, which has a layered array of Cu2+ spins in a triangular lattice, are reported. The magnetic susceptibility and neutron scattering experiments of this material show no magnetic ordering down to 0.2 K with a θ(CW) = -55 K. The magnetic specific heat reveals a T-linear dependence with a γ = 43.4 mJ K(-2) mol(-1) below 1.4 K. These observations suggest that Ba3CuSb2O9 is a new quantum spin liquid candidate with a S = 1/2 triangular lattice.

High-pressure sequence of Ba3NiSb2O9 structural phases: new S = 1 quantum spin liquids based on Ni2+.

Two new gapless quantum spin-liquid candidates with S = 1 (Ni(2+)) moments: the 6H-B phase of Ba(3)NiSb(2)O(9) with a Ni(2+)-triangular lattice and the 3C phase with a Ni(2/3)Sb(1/3)-three-dimensional edge-shared tetrahedral lattice were obtained under high pressure. Both compounds show no magnetic order down to 0.35 K despite Curie-Weiss temperatures θ(CW) of -75.5 (6H-B) and -182.5 K (3C), respectively. Below ~25 K, the magnetic susceptibility of the 6H-B phase saturates to a constant value χ(0) = 0.013 emu/mol, which is followed below 7 K by a linear-temperature-dependent magnetic specific heat (C(M)) displaying a giant coefficient γ = 168 mJ/mol K(2). Both observations suggest the development of a Fermi-liquid-like ground state. For the 3C phase, the C(M) perpendicular T(2) behavior indicates a unique S = 1, 3D quantum spin-liquid ground state.

A coherent three-dimensional Fermi surface in a high-transition-temperature superconductor.

All conventional metals are known to possess a three-dimensional Fermi surface, which is the locus in reciprocal space of the long-lived electronic excitations that govern their electronic properties at low temperatures. These excitations should have well-defined momenta with components in all three dimensions. The high-transition-temperature (high-T(c)) copper oxide superconductors have unusual, highly two-dimensional properties above the superconducting transition. This, coupled with a lack of unambiguous evidence for a three-dimensional Fermi surface, has led to many new and exotic models for the underlying electronic ground state. Here we report the observation of polar angular magnetoresistance oscillations in the overdoped superconductor Tl2Ba2CuO6+delta in high magnetic fields, which firmly establishes the existence of a coherent three-dimensional Fermi surface. Analysis of the oscillations reveals that at certain symmetry points, however, this surface is strictly two-dimensional. This striking form of the Fermi surface topography, long-predicted by electronic band structure calculations, provides a natural explanation for a wide range of anisotropic properties both in the normal and superconducting states. Our data reveal that, despite their extreme electrical anisotropy, the high-T(c) materials at high doping levels can be understood within a framework of conventional three-dimensional metal physics.

Itinerant spin excitations near the hidden order transition in URu(2)Si(2).

By means of neutron scattering we show that the high temperature precursor to the hidden order state of the heavy fermion superconductor URu(2)Si(2) exhibits heavily damped incommensurate paramagnons whose strong energy dispersion is very similar to that of the long-lived longitudinal f spin excitations that appear below T(0). This suggests that there is a strongly hybridized character to the itinerant excitations observed previously above the hidden order transition. Here we present evidence that the itinerant excitations, like those in chromium, are due to Fermi surface nesting of hole and electron pockets; hence the hidden order phase probably originates from a Fermi surface instability. We identify wavevectors that span nested regions of a f-d hybridized band calculation and that match the neutron spin crossover from incommensurate to commensurate on approach to the hidden order phase.

Evidence for negative thermal expansion in the superconducting precursor phase SmFeAsO.

The fluorine-doped rare-earth iron oxypnictide series SmFeAsO1-x F x (0 [Formula: see text] 0.10) was investigated with high resolution powder x-ray scattering. In agreement with previous studies (Margadonna et al 2009 Phys. Rev. B. 79 014503), the parent compound SmFeAsO exhibits a tetragonal-to-orthorhombic structural distortion at [Formula: see text] = 130 K which is rapidly suppressed by [Formula: see text] 0.10 deep within the superconducting dome. The change in unit cell symmetry is followed by a previously unreported magnetoelastic distortion at 120 K. The temperature dependence of the thermal expansion coefficient [Formula: see text] reveals a rich phase diagram for SmFeAsO: (i) a global minimum at 125 K corresponds to the opening of a spin-density wave instability as measured by pump-probe femtosecond spectroscopy (Mertelj et al 2010 Phys. Rev. B 81 224504) whilst (ii) a global maximum at 110 K corresponds to magnetic ordering of the Sm and Fe sublattices as measured by magnetic x-ray scattering (Nandi et al 2011 Phys. Rev. B 84 055419). At much lower temperatures than [Formula: see text], SmFeAsO exhibits a significant negative thermal expansion on the order of -40 ppm · K-1 in contrast to the behaviour of other rare-earth oxypnictides such as PrFeAsO (Kimber et al 2008 Phys. Rev. B 78 140503) and the actinide oxypnictide NpFeAsO (Klimczuk et al 2012 Phys. Rev. B 85 174506) where the onset of [Formula: see text] 0 only appears in the vicinity of magnetic ordering. Correlating this feature with the temperature and doping dependence of the resistivity and the unit cell parameters, we interpret the negative thermal expansion as being indicative of the possible condensation of itinerant electrons accompanying the opening of a SDW gap, consistent with transport measurements (Tropeano et al 2009 Supercond. Sci. Technol. 22 034004).

Uncovering the behavior of Hf2Te2P and the candidate Dirac metal Zr2Te2P.

Results are reported for single crystal specimens of Hf2Te2P and compared to its structural analogue Zr2Te2P, which was recently proposed to be a potential reservoir for Dirac physics [1]. Both materials are produced using the iodine vapor phase transport method and the resulting crystals are exfoliable. The bulk electrical transport and thermodynamic properties indicate Fermi liquid behavior at low temperature for both compounds. Quantum oscillations are observed in magnetization measurements for fields applied parallel but not perpendicular to the c-axis, suggesting that the Fermi surfaces are quasi-two dimensional. Frequencies are determined from quantum oscillations for several parts of the Fermi surfaces. Lifshitz-Kosevich fits to the temperature dependent amplitudes of the oscillations reveal small effective masses, with a particularly small value [Formula: see text] for the α branch of Zr2Te2P. Electronic structure calculations are in good agreement with quantum oscillation results and illustrate the effect of a stronger spin-orbit interaction going from Zr to Hf. These results suggest that by using appropriate tuning parameters this class of materials may deepen the pool of novel Dirac phenomena.

Interplanar coupling-dependent magnetoresistivity in high-purity layered metals.

The magnetic field-induced changes in the conductivity of metals are the subject of intense interest, both for revealing new phenomena and as a valuable tool for determining their Fermi surface. Here we report a hitherto unobserved magnetoresistive effect in ultra-clean layered metals, namely a negative longitudinal magnetoresistance that is capable of overcoming their very pronounced orbital one. This effect is correlated with the interlayer coupling disappearing for fields applied along the so-called Yamaji angles where the interlayer coupling vanishes. Therefore, it is intrinsically associated with the Fermi points in the field-induced quasi-one-dimensional electronic dispersion, implying that it results from the axial anomaly among these Fermi points. In its original formulation, the anomaly is predicted to violate separate number conservation laws for left- and right-handed chiral (for example, Weyl) fermions. Its observation in PdCoO2, PtCoO2 and Sr2RuO4 suggests that the anomaly affects the transport of clean conductors, in particular near the quantum limit.

Composite fermions and broken symmetries in graphene.

The electronic properties of graphene are described by a Dirac Hamiltonian with a four-fold symmetry of spin and valley. This symmetry may yield novel fractional quantum Hall (FQH) states at high magnetic field depending on the relative strength of symmetry-breaking interactions. However, observing such states in transport remains challenging in graphene, as they are easily destroyed by disorder. In this work, we observe in the first two Landau levels the two-flux composite-fermion sequences of FQH states between each integer filling factor. In particular, the odd-numerator fractions appear between filling factors 1 and 2, suggesting a broken-valley symmetry, consistent with our observation of a gap at charge neutrality and zero field. Contrary to our expectations, the evolution of gaps in a parallel magnetic field suggests that states in the first Landau level are not spin-polarized even up to very large out-of-plane fields.