Arkady Shekhter

Bounding the pseudogap with a line of phase transitions in YBa2Cu3O6+δ.

Close to optimal doping, the copper oxide superconductors show ‘strange metal’ behaviour, suggestive of strong fluctuations associated with a quantum critical point. Such a critical point requires a line of classical phase transitions terminating at zero temperature near optimal doping inside the superconducting ‘dome’. The underdoped region of the temperature–doping phase diagram from which superconductivity emerges is referred to as the ‘pseudogap’ because evidence exists for partial gapping of the conduction electrons, but so far there is no compelling thermodynamic evidence as to whether the pseudogap is a distinct phase or a continuous evolution of physical properties on cooling. Here we report that the pseudogap in YBa2Cu3O6+δ is a distinct phase, bounded by a line of phase transitions. The doping dependence of this line is such that it terminates at zero temperature inside the superconducting dome. From this we conclude that quantum criticality drives the strange metallic behaviour and therefore superconductivity in the copper oxide superconductors.

Realization of a three-dimensional spin–anisotropic harmonic honeycomb iridate

The physics of Mott insulators underlies diverse phenomena ranging from high temperature superconductivity to exotic magnetism. Although both the electron spin and the structure of the local orbitals play a key role in this physics, in most systems these are connected only indirectly — via the Pauli exclusion principle and the Coulomb interaction. Iridium-based oxides (iridates) open a further dimension to this problem by introducing strong spin-orbit interactions, such that the Mott physics has a strong orbital character. In the layered honeycomb iridates this is thought to generate highly spin-anisotropic interactions, coupling the spin orientation to a given spatial direction of exchange and leading to strongly frustrated magnetism. The potential for new physics emerging from such interactions has driven much scientific excitement, most recently in the search for a new quantum spin liquid, first discussed by Kitaev [1]. Here we report a new iridate structure that has the same local connectivity as the layered honeycomb, but in a three-dimensional framework. The temperature dependence of the magnetic susceptibility exhibits a striking reordering of the magnetic anisotropy, giving evidence for highly spin-anisotropic exchange interactions. Furthermore, the basic structural units of this material suggest the possibility of a new family of structures, the ‘harmonic honeycomb’ iridates. This compound thus provides a unique and exciting glimpse into the physics of a new class of strongly spin-orbit coupled Mott insulators. ∗ These authors contributed equally to this work.

Scaling between magnetic field and temperature in the high-temperature superconductor BaFe2(As1-xPx)2

I. M. Hayes, Nicholas P. Breznay, 2 Toni Helm, 2 Philip Moll, 2 Mark Wartenbe, Ross D. McDonald, Arkady Shekhter, and James G. Analytis 2 Department of Physics, University of California, Berkeley, California 94720, USA Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Universality of Single-Particle Spectra of Cuprate Superconductors

All the available data for the dispersion and linewidth of the single-particle spectra above the superconducting gap and the pseudogap in metallic cuprates for any doping have universal features. The linewidth is linear in energy below a scale omega(c) and constant above. The cusp in the linewidth at omega(c) mandates, due to causality, a waterfall, i.e., a vertical feature in the dispersion. These features are predicted by a recent microscopic theory. We find that all data can be quantitatively fitted by the theory with a coupling constant lambda(0) and an upper cutoff at omega(c), which vary by less than 50% among the different cuprates and for varying dopings. The microscopic theory also gives these values to within factors of O(2).

Theory of the coupling of quantum-critical fluctuations to fermions and d-wave superconductivity in cuprates

The quantum critical fluctuations of the time-reversal breaking order parameter which is observed in the pseudogap regime of the Cuprates are shown to couple to the lattice equivalent of the local angular momentum of the fermions. Such a coupling favors scattering of fermions through angles close to

Screening of point charge impurities in highly anisotropic metals: application to mu+-spin relaxation in underdoped cuprate superconductors.

\pm \pi/2

Avoided valence transition in a plutonium superconductor

which is unambiguously shown to promote d-wave pairing. The right order of magnitude of both

Robust spin correlations at high magnetic fields in the harmonic honeycomb iridates

T_c

Unfolding the physics of URu2Si2 through silicon to phosphorus substitution.

and the normalized zero temperature gap

Long-wavelength correlations and transport in a marginal Fermi liquid

\Delta/T_c