A. V. Boris

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.

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.

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.

Magnetism, Charge Order, and Giant Magnetoresistance in SrFeO~3~-~d~e~l~t~a Single Crystals

The electronic and magnetic properties of SrFeO(3-delta) single crystals with controlled oxygen content (0< or =delta< or =0.19) have been studied systematically by susceptibility, transport, and spectroscopic techniques. An intimate correlation between the spin-charge ordering and the electronic transport behavior is found. Giant negative as well as positive magnetoresistance are observed.

Spin-controlled Mott-Hubbard bands in LaMnO3 probed by optical ellipsometry

Spectral ellipsometry is used to determine the dielectric function of an untwinned crystal of LaMnO3 in the range 0.5-5.6 eV at temperatures 50<or=T<or=300 K. A pronounced redistribution of spectral weight is found at the Ne el temperature T(N)=140 K. The anisotropy of the spectral weight transfer matches the magnetic ordering pattern. A superexchange model quantitatively describes spectral weight transfer induced by spin correlations. This analysis implies that the lowest-energy transitions around 2 eV are intersite d-d transitions, and that LaMnO3 is a Mott-Hubbard insulator.

Orbital control of noncollinear magnetic order in nickel oxide heterostructures.

We have used resonant x-ray diffraction to develop a detailed description of antiferromagnetic ordering in epitaxial superlattices based on two-unit-cell thick layers of the strongly correlated metal LaNiO3. We also report reference experiments on thin films of PrNiO3 and NdNiO3. The resulting data indicate a spiral state whose polarization plane can be controlled by adjusting the Ni d-orbital occupation via two independent mechanisms: epitaxial strain and spatial confinement of the valence electrons. The data are discussed in light of recent theoretical predictions.

Evidence for two separate energy gaps in underdoped high-temperature cuprate superconductors from broadband infrared ellipsometry

Li Yu, D. Munzar, A.V. Boris, P. Yordanov, J. Chaloupka, Th. Wolf, C.T. Lin, B. Keimer, and C. Bernhard 1.) University of Fribourg, Department of Physics and Fribourg, Center for Nanomaterials (FriMat), Chemin du Musee 3, CH-1700 Fribourg, Switzerland 2.) Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany 3.) Institute of Condensed Matter Physics, Masaryk University, Kotlárská 2, CZ-61137 Brno, Czech Republic 4.) Forschungszentrum Karlsruhe, IFP, D-76021 Karlsruhe, Germany (Dated: February 5, 2008)

Nanoscale Layering of Antiferromagnetic and Superconducting Phases in Rb 2 Fe 4 Se 5 Single Crystals

We studied phase separation in the single-crystalline antiferromagnetic superconductor Rb(2)Fe(4)Se(5) (RFS) using a combination of scattering-type scanning near-field optical microscopy and low-energy muon spin rotation (LE-μSR). We demonstrate that the antiferromagnetic and superconducting phases segregate into nanometer-thick layers perpendicular to the iron-selenide planes, while the characteristic in-plane size of the metallic domains reaches 10  μm. By means of LE-μSR we further show that in a 40-nm thick surface layer the ordered antiferromagnetic moment is drastically reduced, while the volume fraction of the paramagnetic phase is significantly enhanced over its bulk value. Self-organization into a quasiregular heterostructure indicates an intimate connection between the modulated superconducting and antiferromagnetic phases.

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,

Charge ordering and magnetopolarons in Na0.82CoO2.

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