N. J. C. Ingle

Missing quasiparticles and the chemical potential puzzle in the doping evolution of the cuprate superconductors.

The evolution of Ca2-xNaxCuO2Cl2 from Mott insulator to superconductor was studied using angle-resolved photoemission spectroscopy. By measuring both the excitations near the Fermi energy as well as nonbonding states, we tracked the doping dependence of the electronic structure and the chemical potential with unprecedented precision. Our work reveals failures in the standard weakly interacting quasiparticle scenario, including the broad line shapes of the insulator and the apparently paradoxical shift of the chemical potential within the Mott gap. To resolve this, we develop a model where the quasiparticle is vanishingly small at half filling and grows upon doping, allowing us to unify properties such as the dispersion and Fermi wave vector with the chemical potential.

Coupling of the B1g Phonon to the Antinodal Electronic States ofBi2Sr2Ca0.92Y0.08Cu2O8+delta

Angle-resolved photoemission spectroscopy on optimally doped Bi(2)Sr(2)Ca(0.92)Y(0.08)Cu(2)O(8+delta) uncovers a coupling of the electronic bands to a 40 meV mode in an extended k-space region away from the nodal direction, leading to a new interpretation of the strong renormalization of the electronic structure seen in Bi2212. Phenomenological agreements with neutron and Raman experiments suggest that this mode is the B(1g) oxygen bond-buckling phonon. A theoretical calculation based on this assignment reproduces the electronic renormalization seen in the data.

Doping dependence of the coupling of electrons to bosonic modes in the single-layer high-temperature Bi2Sr2CuO6 superconductor

A recent highlight in the study of high-T(c) superconductors is the observation of band renormalization or self-energy effects on the quasiparticles. This is seen in the form of kinks in the quasiparticle dispersions as measured by photoemission and interpreted as signatures of collective bosonic modes coupling to the electrons. Here we compare for the first time the self-energies in an optimally doped and strongly overdoped, nonsuperconducting single-layer Bi-cuprate (Bi2Sr2CuO6). In addition to the appearance of a strong overall weakening, we also find that the weight of the self-energy in the overdoped system shifts to higher energies. We present evidence that this is related to a change in the coupling to c-axis phonons due to the rapid change of the c-axis screening in this doping range.

Fermi Surface and Quasiparticle Excitations of Sr2RhO4

The electronic structure of the layered 4d transition metal oxide Sr2RhO4 is investigated by angle resolved photoemission. We find well-defined quasiparticle excitations with a highly anisotropic dispersion, suggesting a quasi-two-dimensional Fermi-liquid-like ground state. Markedly different from the isostructural Sr2RuO4, only two bands with dominant Rh 4dxz,zy character contribute to the Fermi surface. A quantitative analysis of the photoemission quasiparticle band structure is in excellent agreement with bulk data. In contrast, it is found that state-of-the-art density functional calculations in the local density approximation differ significantly from the experimental findings.

Fully gapped single-particle excitations in lightly doped cuprates

The low-energy excitations of the lightly doped cuprates were studied by angle-resolved photoemission spectroscopy. A finite gap was measured over the entire Brillouin zone, including along the d x 2 - y 2 nodal line. This effect was observed to be generic to the normal states of numerous cuprates, including hole-doped La 2 - x Sr x CuO 4 and Ca 2 - x Na x CuO 2 Cl 2 and electron-doped Nd 2 - x Ce x CuO 4 . In all compounds, the gap appears to close with increasing carrier doping. We consider various scenarios to explain our results, including the possible effects of chemical disorder, electronic inhomogeneity, and a competing phase.

Time-resolved optical observation of spin-wave dynamics

We have created a nonequilibrium population of antiferromagnetic spin-waves in Cr2O3, and characterized its dynamics, using frequency- and time-resolved nonlinear optical spectroscopy of the exciton-magnon transition. We observe a time-dependent pump-probe line shape, which results from excitation induced renormalization of the spin-wave band structure. We present a model that reproduces the basic characteristics of the data, in which we postulate the optical nonlinearity to be dominated by interactions with long-wavelength spin-waves, and the dynamics to be due to spin-wave thermalization.

Growth of the Cr oxides via activated oxygen reactive molecular beam epitaxy: Comparison of the Mo and W oxides

The realization of spin polarized tunnel devices made with CrO2, a theorized half-metallic ferromagnet, requires stringent control of surface and interface quality ideally obtainable via molecular beam epitaxy (MBE) growth. We have studied the MBE growth of all the di- and tri-oxides of the group VIB transition metals (Cr, Mo, and W), with the aid of a high flux atomic oxygen source and detection scheme, to help understand which oxidation states are reachable. We find that even though we can reach the +6 oxidation state of Cr (CrO3) we are unable to obtain single phase CrO2, the +4 oxidation state. One interpretation of our results is that the physical effect of pressure, not solely the oxidation potential, is important to the growth of single phase CrO2.

Structural Origin of Apparent Fermi Surface Pockets in Angle-Resolved Photoemission of Bi 2 Sr 2-x La x CuO 6+δ

We observe apparent hole pockets in the Fermi surfaces of single-layer Bi-based cuprate superconductors from angle-resolved photoemission. From detailed low-energy electron diffraction measurements and an analysis of the angle-resolved photoemission polarization dependence, we show that these pockets are not intrinsic but arise from multiple overlapping superstructure replicas of the main and shadow bands. We further demonstrate that the hole pockets reported recently from angle-resolved photoemission [Meng et al., Nature (London) 462, 335 (2009)] have a similar structural origin and are inconsistent with an intrinsic hole pocket associated with the electronic structure of a doped CuO₂ plane.

Sr2RhO4: a new, clean correlated electron metal

We report the image furnace growth of single crystals of a novel correlated electron metal, the rhodate Sr2RhO4. Suitable annealing treatments result in residual resistivity ratios in excess of 100. Bulk specific heat and magnetic susceptibility have been measured, and both the de Haas-van Alphen and Shubnikov-de Haas effects are observable in the best crystals. The quasi-two-dimensional electronic structure has also enabled a comprehensive study of the electronic structure by angle-resolved photoelectron spectroscopy. The implications of our combined spectroscopic results are discussed.

Molecular beam epitaxial growth of SrCu2O3: Metastable structures and the role of epitaxy

Within the study of oxide materials, high pressure bulk growth has generated a number of new and interesting materials. More recently, attention has been paid to using epitaxy to stabilize these high pressure oxide materials as thin films. In this article we report on the molecular beam epitaxial growth of SrCu2O3; a high pressure, highly correlated, model oxide. We find that the choice of substrate can significantly alter not only the structure but also the chemistry of the resulting film. For growth on SrTiO3 substrates the epitaxially stabilized structure for single phase films with a SrCu2O3 composition is based on a tetragonal unit cell. For identical growth conditions, but on a LaAlO3 substrate, a single phase film with the composition and structure of the infinite layer material (SrCuO2) is formed. We also review the literature for the successes and failures of epitaxy to stabilize high pressure structures.