D. Haug

Electronic Phase Separation in the Slightly Underdoped Iron Pnictide Superconductor Ba1-xKxFe2As2

Here we present a combined study of the slightly underdoped novel pnictide superconductor Ba1-xKxFe2As2 by means of x-ray powder diffraction, neutron scattering, muon-spin rotation (microSR), and magnetic force microscopy (MFM). Static antiferromagnetic order sets in below T{m} approximately 70 K as inferred from the neutron scattering and zero-field-microSR data. Transverse-field microSR below Tc shows a coexistence of magnetically ordered and nonmagnetic states, which is also confirmed by MFM imaging. We explain such coexistence by electronic phase separation into antiferromagnetic and superconducting- or normal-state regions on a lateral scale of several tens of nanometers. Our findings indicate that such mesoscopic phase separation can be considered an intrinsic property of some iron pnictide superconductors.

(pi-pi) electronic order in iron arsenide superconductors

The distribution of valence electrons in metals usually follows the symmetry of the underlying ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron density waves have been observed in many families of superconductors, and are often considered to be essential for superconductivity to exist. Recent measurements seem to show that the properties of the iron pnictides are in good agreement with band structure calculations that do not include additional ordering, implying no relation between density waves and superconductivity in these materials. Here we report that the electronic structure of Ba1-xKxFe2As2 is in sharp disagreement with those band structure calculations, and instead reveals a reconstruction characterized by a (π, π) wavevector. This electronic order coexists with superconductivity and persists up to room temperature (300 K).

Optically induced coherent transport far above Tc in underdoped YBa2Cu3O6+δ

We report on a photo-induced transient state of YBa2Cu2O6+x in which transport perpendicular to the Cu-O planes becomes highly coherent. This effect is achieved by excitation with mid-infrared optical pulses, tuned to the resonant frequency of apical oxygen vibrations, which modulate both lattice and electronic properties. Below the superconducting transition temperature Tc, the equilibrium signatures of superconducting interlayer coupling are enhanced. Most strikingly, the optical excitation induces a new reflectivity edge at higher frequency than the equilibrium Josephson plasma resonance, with a concomitant enhancement of the low frequency imaginary conductivity. Above Tc, the incoherent equilibrium conductivity becomes highly coherent, with the appearance of a reflectivity edge and a positive imaginary conductivity that increases with decreasing frequency. These features are observed up to room temperature in YBa2Cu2O6.45 and YBa2Cu2O6.5. The data above Tc can be fitted by hypothesizing that the light re-establishes a transient superconducting state over only a fraction of the solid, with a lifetime of a few picoseconds. Non-superconducting transport could also explain these observations, although one would have to assume transient carrier mobilities near 10^4 cm^2/(V.sec) at 100 K, with a density of charge carriers similar to the below Tc superfluid density. Our results are indicative of highly unconventional non-equilibrium physics and open new prospects for optical control of complex solids.

Magnetic-field-enhanced incommensurate magnetic order in the underdoped high-temperature superconductor YBa2Cu3O6.45.

We present a neutron-scattering study of the static and dynamic spin correlations in the underdoped high-temperature superconductor YBa2Cu3O6.45 in magnetic fields up to 15 T. The field strongly enhances static incommensurate magnetic order at low temperatures and induces a spectral-weight shift in the magnetic-excitation spectrum. A reconstruction of the Fermi surface driven by the field-enhanced magnetic superstructure may thus be responsible for the unusual Fermi surface topology revealed by recent quantum-oscillation experiments.

Evidence for competing magnetic instabilities in underdoped YBa2Cu3O6+x

We report a polarized neutron scattering study of the orbital-like magnetic order in strongly underdoped

Suppression of the structural phase transition and lattice softening in slightly underdoped Ba1-xKxFe2As2 with electronic phase separation

{\rm YBa_2Cu_3O_{6.45}}

Pseudogap in the chain states of YBa2Cu3O6.6

and

Magnetic-field-enhanced spin freezing on the verge of charge ordering in YBa2Cu3O6.45

{\rm YBa_2(Cu_{0.98}Zn_{0.02})_3O_{6.6}}

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

. Their hole doping levels are located on both sides of the critical doping

Temperature range of superconducting fluctuations above T c in YBa 2 Cu 3 O 7 − δ single crystals

p_{MI}