The pseudogap is a partial gap in the electronic density of states that opens in the normal (non-superconducting) state of cuprate superconductors and whose origin is a long-standing puzzle. Its ...connection to the Mott insulator phase at low doping (hole concentration, p) remains ambiguous and its relation to the charge order that reconstructs the Fermi surface at intermediate doping is still unclear. Here we use measurements of the Hall coefficient in magnetic fields up to 88 tesla to show that Fermi-surface reconstruction by charge order in the cuprate YBa2Cu3Oy ends sharply at a critical doping p = 0.16 that is distinctly lower than the pseudogap critical point p* = 0.19 (ref. 11). This shows that the pseudogap and charge order are separate phenomena. We find that the change in carrier density n from n = 1 + p in the conventional metal at high doping (ref. 12) to n = p at low doping (ref. 13) starts at the pseudogap critical point. This shows that the pseudogap and the antiferromagnetic Mott insulator are linked.
Charge-ordered ground states permeate the phenomenology of 3d-based transition metal oxides, and more generally represent a distinctive hallmark of strongly correlated states of matter. The recent ...discovery of charge order in various cuprate families has fuelled new interest into the role played by this incipient broken symmetry within the complex phase diagram of high-T(c) superconductors. Here, we use resonant X-ray scattering to resolve the main characteristics of the charge-modulated state in two cuprate families: Bi2Sr(2-x)La(x)CuO(6+δ) (Bi2201) and YBa2Cu3O(6+y) (YBCO). We detect no signatures of spatial modulations along the nodal direction in Bi2201, thus clarifying the inter-unit-cell momentum structure of charge order. We also resolve the intra-unit-cell symmetry of the charge-ordered state, which is revealed to be best represented by a bond order with modulated charges on the O-2p orbitals and a prominent d-wave character. These results provide insights into the origin and microscopic description of charge order in cuprates, and its interplay with superconductivity.
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IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, UL, UM, UPUK
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.
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DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Background. Marked increases in Clostridium difficile infection (CDI) incidence, driven by epidemic strain spread, is a global phenomenon. Methods. The Clostridium difficile Ribotyping Network (CDRN) ...was established in 2007 as part of enhanced CDI surveillance in England, to facilitate the recognition and control of epidemic strains. We report on changes in CDI epidemiology in England in the first 3 years of CDRN. Results. CDRN received 12 603 fecal specimens, comprising significantly (P < .05) increasing numbers and proportions of national CDI cases in 2007–2008 (n = 2109, 3.8%), 2008–2009 (n = 4774, 13.2%), and 2009–2010 (n = 5720, 22.3%). The C. difficile recovery rate was 90%, yielding 11 294 isolates for ribotyping. Rates of 9 of the 10 most common ribotypes changed significantly (P < .05) during 2007–2010. Clostridium difficile ribotype 027 predominated, but decreased markedly from 55% to 36% and 21% in 2007–2008, 2008–2009, and 2009–2010, respectively. The largest regional variations in prevalence occurred for ribotypes 027, 002, 015, and 078. Cephalosporin and fluoroquinolone use in CDI cases was reported significantly (P < .05) less frequently during 2007–2010. Mortality data were subject to potential reporting bias, but there was a significant decrease in CDI-associated deaths during 2007–2010, which may have been due to multiple factors, including reduced prevalence of ribotype 027. Conclusions. Access to C. difficile ribotyping was associated with significant changes in the prevalence of epidemic strains, especially ribotype 027. These changes coincided with markedly reduced CDI incidence and related mortality in England. CDI control programs should include prospective access to C. difficile typing and analysis of risk factors for CDI and outcomes.
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In the quest to increase the critical temperature Tc of cuprate superconductors, it is essential to identify the factors that limit the strength of superconductivity. The upper critical field Hc2 is ...a fundamental measure of that strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. Here we show that the thermal conductivity can be used to directly detect Hc2 in the cuprates YBa2Cu3Oy, YBa2Cu4O8 and Tl2Ba2CuO6+δ, allowing us to map out Hc2 across the doping phase diagram. It exhibits two peaks, each located at a critical point where the Fermi surface of YBa2Cu3Oy is known to undergo a transformation. Below the higher critical point, the condensation energy, obtained directly from Hc2, suffers a sudden 20-fold collapse. This reveals that phase competition-associated with Fermi-surface reconstruction and charge-density-wave order-is a key limiting factor in the superconductivity of cuprates.
The pseudogap regime of high-temperature cuprates harbours diverse manifestations of electronic ordering whose exact nature and universality remain debated. Here, we show that the short-ranged charge ...order recently reported in the normal state of YBa2Cu3Oy corresponds to a truly static modulation of the charge density. We also show that this modulation impacts on most electronic properties, that it appears jointly with intra-unit-cell nematic, but not magnetic, order, and that it exhibits differences with the charge density wave observed at lower temperatures in high magnetic fields. These observations prove mostly universal, they place new constraints on the origin of the charge density wave and they reveal that the charge modulation is pinned by native defects. Similarities with results in layered metals such as NbSe2, in which defects nucleate halos of incipient charge density wave at temperatures above the ordering transition, raise the possibility that order-parameter fluctuations, but no static order, would be observed in the normal state of most cuprates if disorder were absent.
Superconductivity often emerges in the proximity of, or in competition with, symmetry-breaking ground states such as antiferromagnetism or charge density waves15 (CDW). A number of materials in the ...cuprate family, which includes the high transition-temperature (high-Tc) superconductors, show spin and charge density wave order.
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Electronic charges introduced in copper-oxide (CuO(2)) planes generate high-transition-temperature (T(c)) superconductivity but, under special circumstances, they can also order into filaments called ...stripes. Whether an underlying tendency towards charge order is present in all copper oxides and whether this has any relationship with superconductivity are, however, two highly controversial issues. To uncover underlying electronic order, magnetic fields strong enough to destabilize superconductivity can be used. Such experiments, including quantum oscillations in YBa(2)Cu(3)O(y) (an extremely clean copper oxide in which charge order has not until now been observed) have suggested that superconductivity competes with spin, rather than charge, order. Here we report nuclear magnetic resonance measurements showing that high magnetic fields actually induce charge order, without spin order, in the CuO(2) planes of YBa(2)Cu(3)O(y). The observed static, unidirectional, modulation of the charge density breaks translational symmetry, thus explaining quantum oscillation results, and we argue that it is most probably the same 4a-periodic modulation as in stripe-ordered copper oxides. That it develops only when superconductivity fades away and near the same 1/8 hole doping as in La(2-x)Ba(x)CuO(4) (ref. 1) suggests that charge order, although visibly pinned by CuO chains in YBa(2)Cu(3)O(y), is an intrinsic propensity of the superconducting planes of high-T(c) copper oxides.
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The application of magnetic fields to layered cuprates suppresses their high-temperature superconducting behaviour and reveals competing ground states. In widely studied underdoped YBa2Cu3O6+x ...(YBCO), the microscopic nature of field-induced electronic and structural changes at low temperatures remains unclear. Here we report an X-ray study of the high-field charge density wave (CDW) in YBCO. For hole dopings ∼0.123, we find that a field (B∼10 T) induces additional CDW correlations along the CuO chain (b-direction) only, leading to a three-dimensional (3D) ordered state along this direction at B∼15 T. The CDW signal along the a-direction is also enhanced by field, but does not develop an additional pattern of correlations. Magnetic field modifies the coupling between the CuO2 bilayers in the YBCO structure, and causes the sudden appearance of the 3D CDW order. The mirror symmetry of individual bilayers is broken by the CDW at low and high fields, allowing Fermi surface reconstruction, as recently suggested.
The interplay between superconductivity and any other competing order is an essential part of the long-standing debate on the origin of high-temperature superconductivity in cuprate materials1, 2. ...Akin to the situation in the heavy fermions, organic superconductors and pnictides, it has been proposed that the pairing mechanism in the cuprates comes from fluctuations of a nearby quantum phase transition3. Recent evidence for charge modulation4 and its associated fluctuations5, 6, 7 in the pseudogap phase of YBa2Cu3Oy makes charge order a likely candidate for a competing order. However, a thermodynamic signature of the charge-ordering phase transition is still lacking. Moreover, whether the charge modulation is uniaxial or biaxial remains controversial. Here we address both issues by measuring sound velocities in YBa2Cu3O6.55 in high magnetic fields. We provide the first thermodynamic signature of the competing charge-order phase transition in YBa2Cu3Oy and construct a field-temperature phase diagram. The comparison of different acoustic modes indicates that the charge modulation is biaxial, which differs from a uniaxial stripe charge order. PUBLICATION ABSTRACT
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