J. Porras

Resonant x-ray scattering study of charge-density wave correlations inYBa2Cu3O6+x

We report the results a comprehensive study of charge density wave (CDW) correlations in untwinned YBCO6+x single crystals with 0.4 0.09) an enhancement of the signal when an external magnetic field up to 6 T is applied in the superconducting state. For samples with p~0.08, where prior work has revealed a field-enhancement of incommensurate magnetic order, the RXS signal is field-independent. This supports a previously suggested scenario in which incommensurate charge and spin orders compete against each other, in addition to individually competing against. We discuss the relationship of these results to stripe order 214, the pseudogap phenomenon, superconducting fluctuations, and quantum oscillations.

Magnetic phase diagram of underdoped YBa2Cu3Oy inferred from torque magnetization and thermal conductivity

Significance We obtain the magnetic phase diagram in the underdoped cuprate YBa2Cu3Oy using torque magnetometry at temperatures 0.3–70 K and magnetic fields up to 45 T. At low fields, vortices (quantized flux tubes) form a vortex solid that is strongly pinned to the lattice. At large fields, melting of the solid to a vortex liquid produces nonzero dissipation. However, the vortex liquid persists to fields above 41 T. We have also mapped out the “transition” fields at which the charge-density–wave state (observed in X-ray diffraction experiments) becomes stable. Our results show that, in intense fields, superconductivity adjusts to coexist with the charge-density wave, but the Cooper pairs, which define the superconducting fluid, survive to fields well above 41 T. Strong evidence for charge-density correlation in the underdoped phase of the cuprate YBa2Cu3Oy was obtained by NMR and resonant X-ray scattering. The fluctuations were found to be enhanced in strong magnetic fields. Recently, 3D charge-density–wave (CDW) formation with long-range order (LRO) was observed by X-ray diffraction in H> 15 T. To elucidate how the CDW transition impacts the pair condensate, we have used torque magnetization to 45 T and thermal conductivity κxx to construct the magnetic phase diagram in untwinned crystals with hole density p = 0.11. We show that the 3D CDW transitions appear as sharp features in the susceptibility and κxx at the fields HK and Hp, which define phase boundaries in agreement with spectroscopic techniques. From measurements of the melting field Hm(T) of the vortex solid, we obtain evidence for two vortex solid states below 8 K. At 0.5 K, the pair condensate appears to adjust to the 3D CDW by a sharp transition at 24 T between two vortex solids with very different shear moduli. At even higher H (41 T), the second vortex solid melts to a vortex liquid which survives to fields well above 41 T. de Haas–van Alphen oscillations appear at fields 24–28 T, below the lower bound for the upper critical field Hc2.

Coherent modulation of the YBa2Cu3O6+x atomic structure by displacive stimulated ionic Raman scattering

Midinfrared light pulses can be used to resonantly excite optically active lattice vibrations in solids to amplitudes as high as several percent of interatomic distances. This technique has been shown to induce changes in the collective magnetic and electronic properties of many materials [1–3]. Integral to this optical control mechanism is the anharmonic coupling between the directly driven, optically active mode and other lattice vibrations [4–6]. The coupling is typically dominated by cubic anharmonicities and involves a displacive force that acts onto the crystal lattice in two ways. First, a phononic analog to rectification in nonlinear optics causes a quasistatic displacement along the normal mode coordinates of all coupled Raman modes. Secondly, whenever the displacive force rises promptly compared to the period of any of the anharmonically coupled modes, coherent oscillatory motions of these modes are excited. This second effect is the stimulated equivalent of ionic Raman scattering. Here, we study the coherent optical response of YBa2Cu3O6+x. Infrared-active apical oxygen motions are driven resonantly with midinfrared pulses at 20 THz, under the conditions for which superconducting transport is transiently enhanced [7–9]. We find that for excitation with pulses of 140 fs duration, for which only modes with frequency < 6T Hz can be driven coherently, oscillations of four Raman modes are stimulated, involving displacements of the copper atoms along the crystallographic c axis. This motion induces periodic changes in the in-plane O-Cu bond buckling and leads to an oscillatory transfer of charges between the CuO2 planes and the Cu-O chains [9], effectively modifying the doping of the planes; an effect that may be part of the puzzle of optically enhanced superconductivity in this compound [7–9]. We next discuss the process of stimulated ionic Raman scattering in more detail. The indirect excitation of coherent Raman modes by resonant excitation of large amplitude infrared-active (IR) vibrations in a solid is described to lowest order by the lattice Hamiltonian

Persistent Paramagnons Deep in the Metallic Phase of Sr_{2-x}La_{x}IrO_{4}.

We have studied the magnetic excitations of electron-doped Sr_{2-x}La_{x}IrO_{4} (0≤x≤0.10) using resonant inelastic x-ray scattering at the Ir L_{3} edge. The long-range magnetic order is rapidly lost with increasing x, but two-dimensional short-range order (SRO) and dispersive magnon excitations with nearly undiminished spectral weight persist well into the metallic part of the phase diagram. The magnons in the SRO phase are heavily damped and exhibit anisotropic softening. Their dispersions are well described by a pseudospin-1/2 Heisenberg model with exchange interactions whose spatial range increases with doping. We also find a doping-independent high-energy magnetic continuum, which is not described by this model. The spin-orbit excitons arising from the pseudospin-3/2 manifold of the Ir ions broaden substantially in the SRO phase, but remain largely separated from the low-energy magnons. Pseudospin-1/2 models are therefore a good starting point for the theoretical description of the low-energy magnetic dynamics of doped iridates.

Accessing the entire overdoped regime in pristine YBa2Cu3O6+x by application of pressure

We attain the previously unaccessed full superconducting dome in a pristine high temperature cuprate superconductor by applying pressures up to 280 kbar to samples of near stoichiometric YBa2Cu3O7. The obtained superconducting phase boundary as a function of hole doping closely follows the superconducting dome in La2-xSrxCuO4. Measurements are now enabled to trace the evolution of various entangled phases and the Fermi surface from the underdoped to overdoped regime in a single high purity cuprate superconducting family of materials.

Rapid suppression of the charge density wave in YBa2Cu3O6.6 under hydrostatic pressure

We report on the effects of hydrostatic pressure (HP) on the charge density wave observed in underdoped cuprates. We studied YBa

Cu 63 -NMR study of oxygen disorder in ortho-II YBa 2 Cu 3 O y

_2

Dynamical decoherence of the light induced interlayer coupling in YBa2Cu3O6+δ

Cu

Crystal growth and intrinsic magnetic behaviour of Sr2IrO4

_3

Light-Induced Metastable State in Charge-Ordered YBa

O