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Low-temperature measurements of the Hall effect in cuprate materials in which superconductivity is suppressed by high magnetic fields show that the pseudogap is not related to the charge ordering that has been seen at intermediate doping levels, but is instead linked to the antiferromagnetic Mott insulator at low doping. Bridging the pseudogap phase The possible origin of the enigmatic 'pseudogap' phase in the high-temperature superconductors comes into sharper focus in light of some new low-temperature Hall measurements at magnetic fields high enough to suppress the confounding effects of superconductivity. Louis Taillefer and colleagues are able to show that the psudogap is not, as some have suspected, related to the charge-ordering that has been seen at intermediate doping levels, but is instead linked to the Mott insulator state at low doping. 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 1 and its relation to the charge order 2 , 3 , 4 that reconstructs the Fermi surface 5 , 6 at intermediate doping is still unclear 7 , 8 , 9 , 10 . 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 YBa 2 Cu 3 O y 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.

Salt crystallization is a major cause of weathering of rocks, artworks and monuments. Damage can only occur if crystals continue to grow in confinement, i.e. within the pore space of these materials, thus generating mechanical stress. We report the direct measurement, at the microscale, of the force exerted by growing alkali halide salt crystals while visualizing their spontaneous nucleation and growth. The experiments reveal the crucial role of the wetting films between the growing crystal and the confining walls for the development of the pressure. Our results suggest that the measured force originates from repulsion between the similarly charged confining wall and the salt crystal separated by a ~1.5 nm liquid film. Indeed, if the walls are made hydrophobic, no film is observed and no repulsive forces are detected. We also show that the magnitude of the induced pressure is system specific explaining why different salts lead to different amounts of damage to porous materials.

(Ba,K)BiO 3 constitute an interesting class of superconductors, where the remarkably high superconducting transition temperature T c of 30 K arises in proximity to charge density wave order. However, the precise mechanism behind these phases remains unclear. Here, enabled by high-pressure synthesis, we report superconductivity in (Ba,K)SbO 3 with a positive oxygen–metal charge transfer energy in contrast to (Ba,K)BiO 3 . The parent compound BaSbO 3− δ shows a larger charge density wave gap compared to BaBiO 3 . As the charge density wave order is suppressed via potassium substitution up to 65%, superconductivity emerges, rising up to T c  = 15 K. This value is lower than the maximum T c of (Ba,K)BiO 3 , but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced charge density wave gap and superconductivity in (Ba,K)SbO 3 indicates that strong oxygen–metal covalency may be more essential than the sign of the charge transfer energy in the main-group perovskite superconductors. High-pressure synthesis is used to stabilize superconducting (Ba,K)SbO 3 , whose properties provide a fresh perspective on the origin of superconductivity in these types of materials.

The ability to make decisions based on data, with its inherent uncertainties and variability, is a complex and vital skill in the modern world. The need for such quantitative critical thinking occurs in many different contexts, and although it is an important goal of education, that goal is seldom being achieved. We argue that the key element for developing this ability is repeated practice in making decisions based on data, with feedback on those decisions. We demonstrate a structure for providing suitable practice that can be applied in any instructional setting that involves the acquisition of data and relating that data to scientific models. This study reports the results of applying that structure in an introductory physics laboratory course. Students in an experimental condition were repeatedly instructed to make and act on quantitative comparisons between datasets, and between data and models, an approach that is common to all science disciplines. These instructions were slowly faded across the course. After the instructions had been removed, students in the experimental condition were 12 times more likely to spontaneously propose or make changes to improve their experimental methods than a control group, who performed traditional experimental activities. The students in the experimental condition were also four times more likely to identify and explain a limitation of a physical model using their data. Students in the experimental condition also showed much more sophisticated reasoning about their data. These differences between the groups were seen to persist into a subsequent course taken the following year.

When pushed out of a syringe, polymer solutions form droplets attached by long and slender cylindrical filaments whose diameter decreases exponentially with time before eventually breaking. In the last stages of this process, a striking feature is the self-similarity of the interface shape near the end of the filament. This means that shapes at different times, if properly rescaled, collapse onto a single universal shape. A theoretical description based on the Oldroyd-B model was recently shown to disagree with existing experimental results. By revisiting these measurements and analysing the interface profiles of very diluted polyethylene oxide solutions at high temporal and spatial resolution, we show that they are very well described by the model.

Amontons’ law defines the friction coefficient as the ratio between friction force and normal force, and assumes that both these forces depend linearly on the real contact area between the two sliding surfaces. However, experimental testing of frictional contact models has proven difficult, because few in situ experiments are able to resolve this real contact area. Here, we present a contact detection method with molecular-level sensitivity. We find that while the friction force is proportional to the real contact area, the real contact area does not increase linearly with normal force. Contact simulations show that this is due to both elastic interactions between asperities on the surface and contact plasticity of the asperities. We reproduce the contact area and fine details of the measured contact geometry by including plastic hardening into the simulations. These new insights will pave the way for a quantitative microscopic understanding of contact mechanics and tribology. Amontons’ law assumes that friction and normal forces depend linearly on the contact area. Here, the authors use a new contact detection method to show that the law is broken because asperities interact and deform in the contact area to change it, thereby also changing the friction force.

After the discovery of stripelike order in lanthanum-based copper oxide superconductors, charge-ordering instabilities were observed in all cuprate families. However, it has proven difficult to distinguish between unidirectional (stripes) and bidirectional (checkerboard) charge order in yttrium- and bismuth-based materials. We used resonant x-ray scattering to measure the two-dimensional structure factor in the superconductor YBa2Cu3O6+y in reciprocal space. Our data reveal the presence of charge stripe order (i.e., locally unidirectional density waves), which may represent the true microscopic nature of charge modulation in cuprates. At the same time, we find that the well-established competition between charge order and superconductivity is stronger for charge correlations across the stripes than along them, which provides additional evidence for the intrinsic unidirectional nature of the charge order.

In order to identify the mechanism responsible for the formation of charge-density waves (CDW) in cuprate superconductors, it is important to understand which aspects of the CDW’s microscopic structure are generic and which are material-dependent. Here, we show that, at the local scale probed by NMR, long-range CDW order in YBa 2 Cu 3 O y is unidirectional with a commensurate period of three unit cells ( λ  = 3 b ), implying that the incommensurability found in X-ray scattering is ensured by phase slips (discommensurations). Furthermore, NMR spectra reveal a predominant oxygen character of the CDW with an out-of-phase relationship between certain lattice sites but no specific signature of a secondary CDW with λ  = 6 b associated with a putative pair-density wave. These results shed light on universal aspects of the cuprate CDW. In particular, its spatial profile appears to generically result from the interplay between an incommensurate tendency at long length scales, possibly related to properties of the Fermi surface, and local commensuration effects, due to electron-electron interactions or lock-in to the lattice. Understanding cuprate superconductors requires better knowledge of the microscopic structure of their charge-density waves (CDW). Here, the authors report evidence that the long-range CDW order in YBa 2 Cu 3 O y has a local commensurate period of three unit cells.

Charge-stripe order and superconductivity Nuclear magnetic resonance measurements of the model high-temperature copper oxide superconductor YBa 2 Cu 3 O y demonstrate that high magnetic fields induce charge order, without spin order, within the material's CuO 2 planes. The observed charge order has characteristics similar to those of stripe-ordered copper oxides, in which electronic charges spontaneously organize themselves into 'stripes'. The charge order develops only when superconductivity fades away. This work suggests that stripes are more common objects in the cuprates than was thought. They seem to compete with superconductivity, although the tendency to form stripes may be a necessary ingredient of high temperature superconductivity. 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 1 . 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 2 , 3 . To uncover underlying electronic order, magnetic fields strong enough to destabilize superconductivity can be used. Such experiments, including quantum oscillations 4 , 5 , 6 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 7 , 8 , 9 . 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 4 a -periodic modulation as in stripe-ordered copper oxides 1 . 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.

Charge density wave (CDW) correlations have been shown to universally exist in cuprate superconductors. However, their nature at high fields inferred from nuclear magnetic resonance is distinct from that measured with x-ray scattering at zero and low fields. We combined a pulsed magnet with an x-ray free-electron laser to characterize the CDW in YBa2Cu3O6.67 via x-ray scattering in fields of up to 28 tesla. While the zero-field CDW order, which develops at temperatures below ~150 kelvin, is essentially two dimensional, at lower temperature and beyond 15 tesla, another three-dimensionally ordered CDW emerges. The field-induced CDW appears around the zero-field superconducting transition temperature; in contrast, the incommensurate in-plane ordering vector is field-independent. This implies that the two forms of CDW and high-temperature superconductivity are intimately linked.