Diffuse scattering from correlated electron systems.

The role of inhomegeneity in determining the properties of correlated electron systems is poorly understood because of the dearth of structural probes of disorder at the nanoscale. Advances in both neutron and x-ray scattering instrumentation now allow comprehensive measurements of diffuse scattering in single crystals over large volumes of reciprocal space, enabling structural correlations to be characterized over a range of length scales from 5 to 200 angstroms or more. When combined with new analysis tools, such as three-dimensional difference pair-distribution functions, these advanced capabilities have produced fresh insights into the interplay of structural fluctuations and electronic properties in a broad range of correlated electron materials.

Multiferroicity in plastically deformed SrTiO.

Quantum materials have a fascinating tendency to manifest novel and unexpected electronic states upon proper manipulation. Ideally, such manipulation should induce strong and irreversible changes and lead to new relevant length scales. Plastic deformation introduces large numbers of dislocations into a material, which can organize into extended structures and give rise to qualitatively new physics as a result of the huge localized strains.

Mechanisms of Hysteresis and Reversibility across the Voltage-Driven Perovskite-Brownmillerite Transformation in Electrolyte-Gated Ultrathin LaSrCoO.

Perovskite cobaltites have emerged as archetypes for electrochemical control of materials properties in electrolyte-gate devices. Voltage-driven redox cycling can be performed between fully oxygenated perovskite and oxygen-vacancy-ordered brownmillerite phases, enabling exceptional modulation of the crystal structure, electronic transport, thermal transport, magnetism, and optical properties. The vast majority of studies, however, have focused heavily on the perovskite and brownmillerite end points.

Optically Probing Unconventional Superconductivity in Atomically Thin BiSrCaYCuO.

Atomically thin cuprates exhibiting a superconducting phase transition temperature similar to that of the bulk have recently been realized, although the device fabrication remains a challenge and limits the potential for many novel studies and applications. Here, we use an optical pump-probe approach to noninvasively study the unconventional superconductivity in atomically thin BiSrCaYCuO (Y-Bi2212). Apart from finding an optical response due to the superconducting phase transition that is similar to that of bulk Y-Bi2212, we observe that the sign and amplitude of the pump-probe signal in atomically thin flakes vary significantly in different dielectric environments depending on the nature of the optical excitation.

Doping- and Strain-Dependent Electrolyte-Gate-Induced Perovskite to Brownmillerite Transformation in Epitaxial LaSrCoO Films.

Much recent attention has focused on the voltage-driven reversible topotactic transformation between the ferromagnetic metallic perovskite (P) SrCoO and oxygen-vacancy-ordered antiferromagnetic insulating brownmillerite (BM) SrCoO. This is emerging as a paradigmatic example of the power of electrochemical gating (using, , ionic liquids/gels), the wide modulation of electronic, magnetic, and optical properties generating clear application potential. SrCoO films are challenging with respect to stability, however, and there has been little exploration of alternate compositions.

Emergence of superconductivity in the cuprates via a universal percolation process.

A pivotal step toward understanding unconventional superconductors would be to decipher how superconductivity emerges from the unusual normal state. In the cuprates, traces of superconducting pairing appear above the macroscopic transition temperature T, yet extensive investigation has led to disparate conclusions. The main difficulty has been to separate superconducting contributions from complex normal-state behaviour.

Dynamics of correlation-frozen antinodal quasiparticles in superconducting cuprates.

Many puzzling properties of high-critical temperature () superconducting (HTSC) copper oxides have deep roots in the nature of the antinodal quasiparticles, the elementary excitations with wave vector parallel to the Cu-O bonds. These electronic states are most affected by the onset of antiferromagnetic correlations and charge instabilities, and they host the maximum of the anisotropic superconducting gap and pseudogap. We use time-resolved extreme-ultraviolet photoemission with proper photon energy (18 eV) and time resolution (50 fs) to disclose the ultrafast dynamics of the antinodal states in a prototypical HTSC cuprate.

Doping-dependent charge order correlations in electron-doped cuprates.

Understanding the interplay between charge order (CO) and other phenomena (for example, pseudogap, antiferromagnetism, and superconductivity) is one of the central questions in the cuprate high-temperature superconductors. The discovery that similar forms of CO exist in both hole- and electron-doped cuprates opened a path to determine what subset of the CO phenomenology is universal to all the cuprates. We use resonant x-ray scattering to measure the CO correlations in electron-doped cuprates (La2-x Ce x CuO4 and Nd2-x Ce x CuO4) and their relationship to antiferromagnetism, pseudogap, and superconductivity.

Universal sheet resistance and revised phase diagram of the cuprate high-temperature superconductors.

Upon introducing charge carriers into the copper-oxygen sheets of the enigmatic lamellar cuprates, the ground state evolves from an insulator to a superconductor and eventually to a seemingly conventional metal (a Fermi liquid). Much has remained elusive about the nature of this evolution and about the peculiar metallic state at intermediate hole-carrier concentrations (p). The planar resistivity of this unconventional metal exhibits a linear temperature dependence (ρ ∝ T) that is disrupted upon cooling toward the superconducting state by the opening of a partial gap (the pseudogap) on the Fermi surface.

Revealing the high-energy electronic excitations underlying the onset of high-temperature superconductivity in cuprates.

In strongly correlated systems the electronic properties at the Fermi energy (E(F)) are intertwined with those at high-energy scales. One of the pivotal challenges in the field of high-temperature superconductivity (HTSC) is to understand whether and how the high-energy scale physics associated with Mott-like excitations (|E-E(F)|>1 eV) is involved in the condensate formation. Here, we report the interplay between the many-body high-energy CuO(2) excitations at 1.