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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 materials. 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 transition. Recent evidence for charge modulation and its associated fluctuations 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. [ABSTRACT FROM AUTHOR]

In underdoped cuprate superconductors, phase stiffness is low and long-range superconducting order is destroyed readily by thermally generated vortices (and anti-vortices), giving rise to a broad temperature regime above the zero-resistive state in which the superconducting phase is incoherent. It has often been suggested that these vortex-like excitations are related to the normal-state pseudogap or some interaction between the pseudogap state and the superconducting state. However, to elucidate the precise relationship between the pseudogap and superconductivity, it is important to establish whether this broad phase-fluctuation regime vanishes, along with the pseudogap, in the slightly overdoped region of the phase diagram where the superfluid pair density and correlation energy are both maximal. Here we show, by tracking the restoration of the normal-state magnetoresistance in overdoped La2−xSrxCuO4, that the phase-fluctuation regime remains broad across the entire superconducting composition range. The universal low phase stiffness is shown to be correlated with a low superfluid density, a characteristic of both underdoped and overdoped cuprates. The formation of the pseudogap, by inference, is therefore both independent of and distinct from superconductivity. [ABSTRACT FROM AUTHOR]

Measurements of quantum oscillations in the cuprate superconductors afford an opportunity to assess the extent to which their electronic properties yield to a description rooted in Fermi-liquid theory. However, such an analysis is hampered by the small number of oscillatory periods observed in the accessible magnetic field range. Here we employ a genetic algorithm to globally model the field, angular and temperature dependence of the quantum oscillations observed in the resistivity of YBa2Cu3O6.59. This approach successfully fits an entire data set to a Fermi surface consisting of two small, quasi-two-dimensional cylinders, rather than the large Fermi surface predicted by conventional electronic-structure calculations. A key result is the first identification of the effects of Zeeman splitting, indicating that the quasiparticles behave as nearly free spins, constraining the source of the Fermi-surface reconstruction to something other than a spin-density wave with moments perpendicular to the magnetic field. [ABSTRACT FROM AUTHOR]

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 materials. 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 transition. Recent evidence for charge modulation and its associated fluctuations 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. [ABSTRACT FROM AUTHOR]

In underdoped cuprate superconductors, phase stiffness is low and long-range superconducting order is destroyed readily by thermally generated vortices (and anti-vortices), giving rise to a broad temperature regime above the zero-resistive state in which the superconducting phase is incoherent. It has often been suggested that these vortex-like excitations are related to the normal-state pseudogap or some interaction between the pseudogap state and the superconducting state. However, to elucidate the precise relationship between the pseudogap and superconductivity, it is important to establish whether this broad phase-fluctuation regime vanishes, along with the pseudogap, in the slightly overdoped region of the phase diagram where the superfluid pair density and correlation energy are both maximal. Here we show, by tracking the restoration of the normal-state magnetoresistance in overdoped La2−xSrxCuO4, that the phase-fluctuation regime remains broad across the entire superconducting composition range. The universal low phase stiffness is shown to be correlated with a low superfluid density, a characteristic of both underdoped and overdoped cuprates. The formation of the pseudogap, by inference, is therefore both independent of and distinct from superconductivity. [ABSTRACT FROM AUTHOR]