One of the leading issues in high-T(c) superconductors is the origin of the pseudogap phase in underdoped cuprates. Using polarized elastic neutron diffraction, we identify a novel magnetic order in the YB(2)Cu(3)O(6+) system. The observed magnetic order preserves translational symmetry of the lattice as proposed for orbital moments in the circulating current theory of the pseudogap state. To date, it is the first direct evidence of a hidden order parameter characterizing the pseudogap phase in high-T(c) cuprates.

Electronic phases with symmetry properties matching those of conventional liquid crystals have recently been discovered in transport experiments on semiconductor heterostructures and metal oxides at milli-Kelvin temperatures. We report the spontaneous onset of a onedimensional, incommensurate modulation of the spin system in the high-temperature superconductor YBa 2 Cu 3 O 6.45 upon cooling below ~150 K, while static magnetic order is absent above 2 K. The evolution of this modulation with temperature and doping parallels that of the in-plane anisotropy of the resistivity, indicating an electronic nematic phase that is stable over a wide temperature range. The results suggest that soft spin fluctuations are a microscopic route towards electronic liquid crystals, and nematic order can coexist with high-temperature superconductivity in underdoped cuprates.The electronic states near the Fermi level of high-temperature superconductors derive from the hybridized d-and p-orbitals of copper and oxygen ions in a square-planar network. At a doping level of 1/8 hole per Cu ion, experimental work on a specific superconducting cuprate family, (La,Nd) 2-x (Sr,Ba) x CuO 4 (La214), has shown that the two-dimensional electron system in the CuO 2 layers can support a state with uniaxial spin (1-3) and charge (1,4,5) order ("stripes"). Static stripe order implies that both translational and rotational symmetries of the copper-oxide square lattice are spontaneously broken. More unusual "electronic liquid crystal" states (6) that break the rotational symmetry of the lattice while at least partially preserving its translational symmetry can arise from quantum fluctuations of stripes(6-8), or from Fermi surface instabilities (9-11). Electronic nematic states have recently been discovered in semiconductor heterostructures (12) and in the bulk transition metal oxide Sr 3 Ru 2 O 7 (13). In both cases, however, they are stable only at milli-Kelvin temperatures and in high magnetic fields, and have thus far only been probed by transport measurements. We use neutron scattering to address the role of magnetic degrees of freedom in driving the formation of electronic liquid crystals, and to explore the presence of liquid-crystalline order in the cuprates.In the prior experiments on semiconductor heterostructures and on Sr 3 Ru 2 O 7 , the nematic director was aligned by external magnetic fields, resulting in a strong macroscopic anisotropy of 2 the current flow (12,13), analogous to the alignment of nematic domains in conventional liquid crystals by electric fields or confining walls. In the cuprates, subtle crystallographic distortions can serve as aligning fields for symmetry-broken electronic phases. They reduce the fourfold rotational symmetry of the CuO 2 layer to a twofold rotational or mirror symmetry by introducing a slight (~1%) difference between the in-plane lattice parameters. In stripe-ordered La214, the stripe domains in every CuO 2 layer are aligned by such a twofold axis, but the layers are stacked in such a way that the glob...

The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high-transition-temperature (high-T(c)) superconductivity, characterized by anomalous physical properties. There are open questions about the number of distinct phases and the possible presence of a quantum-critical point underneath the superconducting dome. The picture has remained unclear because there has not been conclusive evidence for a new type of order. Neutron scattering measurements for YBa(2)Cu(3)O(6+delta) (YBCO) resulted in contradictory claims of no and weak magnetic order, and the interpretation of muon spin relaxation measurements on YBCO and of circularly polarized photoemission experiments on Bi(2)Sr(2)CaCu(2)O(8+delta)(refs 12, 13) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa(2)CuO(4+delta) (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order. Together with recent results for YBCO, this observation constitutes a demonstration of the universal existence of such a state. The findings appear to rule out theories that regard T* as a crossover temperature rather than a phase transition temperature. Instead, they are consistent with a variant of previously proposed charge-current-loop order that involves apical oxygen orbitals, and with the notion that many of the unusual properties arise from the presence of a quantum-critical point.

The origin of superconductivity in bulk SrTiO3 is a mystery, since the nonmonotonous variation of the critical transition with carrier concentration defies the expectations of the crudest version of the BCS theory. Here, employing the Nernst effect, an extremely sensitive probe of tiny bulk Fermi surfaces, we show that down to concentrations as low as 5.5 × 10 17 cm −3 , the system has both a sharp Fermi surface and a superconducting ground state. The most dilute superconductor currently known has therefore a metallic normal state with a Fermi energy as little as 1.1 meV on top of a band gap as large as 3 eV. Occurrence of a superconducting instability in an extremely small, single-component and barely anisotropic Fermi surface implies strong constraints for the identification of the pairing mechanism.

We present a study of angle-resolved quantum oscillations of electric and thermoelectric transport coefficients in semi-metallic WTe2, which has the particularity of displaying a large B 2 magnetoresistance. The Fermi surface consists of two pairs of electron-like and hole-like pockets of equal volumes in a "Russian doll" structure. Carrier density, Fermi energy, mobility and the mean-freepath of the system are quantified. An additional frequency is observed above a threshold field and attributed to magnetic breakdown across two orbits. In contrast to all other dilute metals, the Nernst signal remains linear in magnetic field even in the high-field (ωcτ ≫ 1) regime. Surprisingly, none of the pockets extend across the c-axis of the first Brillouin zone, making the system a three-dimensional metal with moderate anisotropy in Fermi velocity yet a large anisotropy in mean-free-path. 2 ) was reported with no sign of saturation up to 60 T[6]. This is the expected behavior of a perfectly-compensated semi-metal [7], but has not been seen in bismuth [8] or graphite [9], two compensated semi-metals whose Fermi surface is accurately known. A first step towards uncovering the ultimate reason behind the quadratic magnetoresistance of WTe 2 is a quantitative determination of the structure of the Fermi surface and the components of the mobility tensor.In this letter, we report on a study of quantum oscillations of resistivity, Seebeck and Nernst coefficients in high-quality single crystals of WTe 2 and find that the Fermi surface consists of two pairs of electron-like and hole-like pockets. Each pair is concentric with identical structure like a set of Russian dolls. The anisotropy is much smaller than one would naively expect in a layered system. The longer axis of the pockets is much shorter than the height of the Brillouin zone, in contrast to the theoretical expectations. Moreover, we find another distinctive feature of this semi-metal in addition to quadratic magnetoresistance, which is a Nernst response linear in magnetic field deep inside the high-field limit. Our results quantify carrier concentration of the system and set plausible quantitative windows for mobilities and Fermi energies leading to the huge quadratic magnetoresistance and large field-linear Nernst signal.

Polarized beam neutron-scattering measurements on a highly perfect crystal of YBa 2 Cu 3 O 6.6 show a distinct magnetic transition to a different phase with an onset at about 235 K, the temperature expected for the pseudogap transition. The moment is found to be about 0.1 B for each sublattice and has a correlation length of at least 75 Å. We found the critical exponent for the magnetic neutron intensity to be 2␤ = 0.37Ϯ 0.12. This is the proper range for the class of transition that has no specific-heat divergence possibly explaining why none is found at the pseudogap transition.Superconductivity produces a gap in the quasiparticle spectrum which for conventional materials disappears as the temperature is increased to T c , where the superconducting electron pairs are no longer bound together. However, for the underdoped cuprate materials a pseudogap measured by a number of techniques appears at a temperature T ‫ء‬ Ͼ T c , with T ‫ء‬ increasing as T c gets smaller. This pseudogap associated with T ‫ء‬ is one of the most puzzling and important attributes of the cuprate superconductors. 1,2 Indeed since superconductivity originates from the pseudogap state, it is this state that has to be understood to determine the mechanism for hightemperature superconductivity. One possible origin of the pseudogap is to postulate that phase incoherent pairs are established as the material is cooled through the pseudogap temperature T ‫ء‬ , with superconductivity developing at a lower temperature T c when phase coherence is established. 3 This picture has been studied by the Nernst effect, 4 which shows the presence of vortexlike excitations in the pseudogap phase. However, upon warming the Nernst effect 4 disappears well before T ‫ء‬ is reached, suggesting that well-defined preformed pairs may not be present at temperatures as high as T ‫ء‬ .Completely different approaches 5-7 to the problem postulated a state with broken symmetry that displayed a pattern of circulating currents ͑CC phase͒ in the a-b plane. As the sample is cooled, this state appears at T ‫ء‬ and for near optimal doping ends at a quantum critical point. The present study considers a magnetic signal found at certain ͑H , K , L͒ positions in the reciprocal lattice. These would correspond to the phases considered by Varma 5,6 which preserve the translational symmetry of the lattice. Since translational symmetry is not broken, the signal to observe the phases occurs at the Bragg positions of the unit cell which means they sit on top of the much more intense nuclear scattering. A very sensitive polarized beam experiment is thus needed to observe them. Fauqué et al. 8 have undertaken polarized neutron-scattering measurements to search for this phase at the expected positions for several underdoped YBa 2 Cu 3 O 6+x ͑YBCO͒ samples. They observed an increase in polarized neutron scattering upon cooling, which represents a magnetic signal, which could be associated with the pseudogap temperature T ‫ء‬ . We have undertaken similar polarized neutron measurements on a well ...

In doped SrTiO3 superconductivity persists down to an exceptionally low concentration of mobile electrons. This restricts the relevant energy window and possible pairing scenarios. We present a study of quantum oscillations and superconducting transition temperature, Tc as the carrier density is tuned from 10 17 to 10 20 cm −3 and identify two critical doping levels corresponding to the filling thresholds of the upper bands. At the first critical doping, which separates the single-band and the two-band superconducting regimes in oxygen-deficient samples, the steady increase of Tc with carrier concentration suddenly stops. Near this doping level, the energy dispersion in the lowest band displays a downward deviation from parabolic behavior. The results impose new constraints for microscopic pairing scenarios.Superconductiviy is induced in insulating SrTiO 3 by introducing n-type charge carriers through chemical doping[1] and survives over three orders of magnitude of carrier concentration. The transition temperature, T c , peaks to 0.45 K around a carrier density of n H ∼ 10 20 cm −3 [2]. A superconducting dome has also been detected in the metallic interfaces of SrTiO 3 [3] when the carrier density is modulated by a gate voltage bias [4]. In unconventional superconductors, such as high-T c cuprates, superconducting domes are often attributed to the proximity of a magnetic order or a Mott insulator. The recent discovery of superconducting dome in gate-tuned MoS 2 [5] in absence of a competing order, however, highlights the limits of our current understanding of the interplay between carrier concentration and superconductivity and motivates a fresh reexamination of superconducting domes. In the specific case of SrTiO 3 , superconductivity occurs in the vicinity of an aborted ferroelectric order[6] and survives deep inside the dilute metallic regime when the Fermi temperature becomes more than one order of magnitude lower than the Debye temperature [7]. This is a second puzzle in addition to the one raised by the drop in T c on the overdoped side. These two questions, raised at the opposite limits of the superconducting dome, remain unsettled.According to band calculations[9-11], doping SrTiO 3 with n-type carriers can fill three bands one after the other. Once the critical threshold for the occupation of a band is attained, a new Fermi surface concentric with the previous one emerges. Previous studies of quantum oscillations in bulk doped SrTiO 3 [7, 12-14] have detected both multiple-frequency [7,13,14] and singlefrequency [7,14] oscillations at different doping levels, but did not determine these critical doping levels. Moreover, according to tunneling experiments, doped SrTiO 3 beyond a carrier density of 10 19 cm −3 is a multi-gap superconductor [8]. The interplay between multi-band occupation in the normal state and multi-gap superconductivity has been a subject of recent theoretical attention [20].We present a systematic study of quantum oscillations and superconducting transition as a function of carrier con...

Electrons are offered a valley degree of freedom in presence of particular lattice structures. Manipulating valley degeneracy is the subject matter of an emerging field of investigation, mostly focused on charge transport in graphene[1-4]. In bulk bismuth, electrons are known to present a threefold valley degeneracy and a Dirac dispersion in each valley. Here we show that because of their huge in-plane mass anisotropy, a flow of Dirac electrons along the trigonal axis is extremely sensitive to the orientation of in-plane magnetic field. Thus, a rotatable magnetic field can be used as a valley valve to tune the contribution of each valley to the total conductivity.According to our measurements, charge conductivity by carriers of a single valley can exceed four-fifth of the total conductivity in a wide range of temperature and magnetic field. At high temperature and low magnetic field, the three valleys are interchangeable and the three-fold symmetry of the underlying lattice is respected.As the temperature lowers and/or the magnetic field increases, this symmetry is spontaneously lost. The latter may be an experimental manifestation of the recently proposed valley-nematic Fermi liquid state [9]. arXiv:1109.2774v1 [cond-mat.str-el]