B. Keimer

Emergent phenomena at oxide interfaces

Recent technical advances in the atomic-scale synthesis of oxide heterostructures have provided a fertile new ground for creating novel states at their interfaces. Different symmetry constraints can be used to design structures exhibiting phenomena not found in the bulk constituents. A characteristic feature is the reconstruction of the charge, spin and orbital states at interfaces on the nanometre scale. Examples such as interface superconductivity, magneto-electric coupling, and the quantum Hall effect in oxide heterostructures are representative of the scientific and technological opportunities in this rapidly emerging field.

From quantum matter to high-temperature superconductivity in copper oxides

The discovery of high-temperature superconductivity in the copper oxides in 1986 triggered a huge amount of innovative scientific inquiry. In the almost three decades since, much has been learned about the novel forms of quantum matter that are exhibited in these strongly correlated electron systems. A qualitative understanding of the nature of the superconducting state itself has been achieved. However, unresolved issues include the astonishing complexity of the phase diagram, the unprecedented prominence of various forms of collective fluctuations, and the simplicity and insensitivity to material details of the ‘normal’ state at elevated temperatures.

Orbital reconstruction and covalent bonding at an oxide interface.

Orbital reconstructions and covalent bonding must be considered as important factors in the rational design of oxide heterostructures with engineered physical properties. We have investigated the interface between high-temperature superconducting (Y,Ca)Ba2Cu3O7 and metallic La0.67Ca0.33MnO3 by resonant x-ray spectroscopy. A charge of about –0.2 electron is transferred from Mn to Cu ions across the interface and induces a major reconstruction of the orbital occupation and orbital symmetry in the interfacial CuO2 layers. In particular, the Cu d3z2–r2 orbital, which is fully occupied and electronically inactive in the bulk, is partially occupied at the interface. Supported by exact-diagonalization calculations, these data indicate the formation of a strong chemical bond between Cu and Mn atoms across the interface. Orbital reconstructions and associated covalent bonding are thus important factors in determining the physical properties of oxide heterostructures.

Dimensionality Control of Electronic Phase Transitions in Nickel-Oxide Superlattices

The structure of metal-oxide superlattices is used to control the electronic order of the system. The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal lanthanum nickelate (LaNiO3) and the wide-gap insulator lanthanum aluminate (LaAlO3) with atomically precise layer sequences. We used optical ellipsometry and low-energy muon spin rotation to show that superlattices with LaNiO3 as thin as two unit cells undergo a sequence of collective metal-insulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO3 layers remain metallic and paramagnetic at all temperatures. Metal-oxide superlattices thus allow control of the dimensionality and collective phase behavior of correlated-electron systems.

Intense paramagnon excitations in a large family of high-temperature superconductors

In the copper oxide superconductors, spin fluctuations might be involved in the electronic pairing mechanism. The case for such magnetically mediated superconductivity is now strengthened by the discovery of high-energy magnetic excitations that are not affected by chemical doping levels within several cuprates.

Strength of the spin-fluctuation-mediated pairing interaction in a high-temperature superconductor

Although spin fluctuations are believed to have an important role in the mechanism responsible for high-temperature superconductivity, it has been unclear whether the strength of their coupling with fermionic quasiparticles is sufficiently strong. Systematic analysis of angle-resolved photoemission and neutron spectra suggests it is. Theories based on the coupling between spin fluctuations and fermionic quasiparticles are among the leading contenders to explain the origin of high-temperature superconductivity, but estimates of the strength of this interaction differ widely1. Here, we analyse the charge- and spin-excitation spectra determined by angle-resolved photoemission and inelastic neutron scattering, respectively, on the same crystals of the high-temperature superconductor YBa2Cu3O6.6. We show that a self-consistent description of both spectra can be obtained by adjusting a single parameter, the spin–fermion coupling constant. In particular, we find a quantitative link between two spectral features that have been established as universal for the cuprates, namely high-energy spin excitations2,3,4,5,6,7 and ‘kinks’ in the fermionic band dispersions along the nodal direction8,9,10,11,12. The superconducting transition temperature computed with this coupling constant exceeds 150u2009K, demonstrating that spin fluctuations have sufficient strength to mediate high-temperature superconductivity.

Orbital reflectometry of oxide heterostructures

The occupation of d orbitals controls the magnitude and anisotropy of the inter-atomic electron transfer in transition-metal oxides and hence exerts a key influence on their chemical bonding and physical properties. Atomic-scale modulations of the orbital occupation at surfaces and interfaces are believed to be responsible for massive variations of the magnetic and transport properties, but could not thus far be probed in a quantitative manner. Here we show that it is possible to derive quantitative, spatially resolved orbital polarization profiles from soft-X-ray reflectivity data, without resorting to model calculations. We demonstrate that the method is sensitive enough to resolve differences of ~3% in the occupation of Ni e(g) orbitals in adjacent atomic layers of a LaNiO(3)-LaAlO(3) superlattice, in good agreement with ab initio electronic-structure calculations. The possibility to quantitatively correlate theory and experiment on the atomic scale opens up many new perspectives for orbital physics in transition-metal oxides.

SUPERCONDUCTIVITY-INDUCED ANOMALIES IN THE SPIN EXCITATION SPECTRA OF UNDERDOPED YBA2CU3O6+X

Polarized and unpolarized neutron scattering have been used to determine the effect of superconductivity on the magnetic excitation spectra of YBa{sub 2}Cu{sub 3}O{sub 6.5} (T{sub c}=52K) and YBa{sub 2}Cu{sub 3}O{sub 6.7} (T{sub c}=67K). Pronounced enhancements of the spectral weight centered around 25 and 33meV, respectively, are observed below T{sub c} in both crystals, compensated predominantly by a loss of spectral weight at {bold {ital higher}} energies. The data provide important clues to the origin of the 40meV magnetic resonance peak in YBa{sub 2}Cu{sub 3}O{sub 7}. {copyright} {ital 1997} {ital The American Physical Society}

Symmetry of charge order in cuprates

Charge-ordered ground states permeate the phenomenology of 3d-based transition metal oxides, and more generally represent a distinctive hallmark of strongly correlated states of matter. The recent discovery of charge order in various cuprate families has fuelled new interest into the role played by this incipient broken symmetry within the complex phase diagram of high-T(c) superconductors. Here, we use resonant X-ray scattering to resolve the main characteristics of the charge-modulated state in two cuprate families: Bi2Sr(2-x)La(x)CuO(6+δ) (Bi2201) and YBa2Cu3O(6+y) (YBCO). We detect no signatures of spatial modulations along the nodal direction in Bi2201, thus clarifying the inter-unit-cell momentum structure of charge order. We also resolve the intra-unit-cell symmetry of the charge-ordered state, which is revealed to be best represented by a bond order with modulated charges on the O-2p orbitals and a prominent d-wave character. These results provide insights into the origin and microscopic description of charge order in cuprates, and its interplay with superconductivity.

Crossover from weak to strong pairing in unconventional superconductors

Superconductors are classified by their pairing mechanism and the coupling strength, measured as the ratio of the energy gap,