Paul Goddard

Abstract:

An outstanding problem in the field of high-transition-temperature (high-Tc) superconductivity is the identification of the normal state out of which superconductivity emerges in the mysterious underdoped regime. The normal state uncomplicated by thermal fluctuations can be studied using applied magnetic fields that are sufficiently strong to suppress long-range superconductivity at low temperatures. Proposals in which the normal ground state is characterized by small Fermi surface pockets that exist in the absence of symmetry breaking have been superseded by models based on the existence of a superlattice that breaks the translational symmetry of the underlying lattice. Recently, a charge superlattice model that positions a small electron-like Fermi pocket in the vicinity of the nodes (where the superconducting gap is minimum) has been proposed as a replacement for the prevalent superlattice models that position the Fermi pocket in the vicinity of the pseudogap at the antinodes (where the superconducting gap is maximum). Although some ingredients of symmetry breaking have been recently revealed by crystallographic studies, their relevance to the electronic structure remains unresolved. Here we report angle-resolved quantum oscillation measurements in the underdoped copper oxide YBa2Cu3O6 + x. These measurements reveal a normal ground state comprising electron-like Fermi surface pockets located in the vicinity of the nodes, and also point to an underlying superlattice structure of low frequency and long wavelength with features in common with the charge order identified recently by complementary spectroscopic techniques.

Abstract:

The reaction of MnCl2·4H2O with t-BuPO 3H2 in the presence of triethylamine afforded the tridecanuclear cage, [MnIIMnIII12(μ 4-O)6(μ-OH)6(O3P-t-Bu) 10(OH2)2(DMF)4] ·[2MeOH·4DMF] (1). The structural analysis of 1 reveals that it is a mixed-valent complex containing a [MnIIMnIII12(μ4-O)6] core. The centre of the core is occupied by a MnII ion which is surrounded by 12 MnIII ions. The latter are connected to each other by six μ-OH- and 10 t-BuPO32- ligands. The vacant coordination sites of six MnIII ions situated in the periphery are occupied by four DMF and two water molecules. Magnetic studies on 1 reveal a frequency-dependent response which is characteristic of single-molecule magnets. © 2014 Elsevier Ltd. All rights reserved.

Abstract:

Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. The synthesis, structure, and magnetism of a nonanuclear iron(III) phosphonate, [Fe 9 III (μ 3 -O) 4 (O 3 PC 5 H 9 ) 3 (O 2 CCMe 3 ) 13 ]·(EtOH) 0.5 ·(Et 2 O) 0.5 (1), is described. Compound 1 was obtained in the direct reaction of anhydrous ferric chloride, cyclopentylphosphonic acid, and pivalic acid in the presence of triethylamine. Compound 1 possesses a distorted icosahedral structure; nine of the vertices are occupied by Fe III , whereas three others are occupied by the phosphorus atoms of the cyclopentylphosphonate ligand. Mössbauer spectroscopy of 1 confirms the presence of high-spin Fe III . Low-temperature magnetic studies reveal the presence of strong antiferromagnetic coupling between the Fe 3+ ions. The reaction of anhydrous ferric chloride with cyclopentylphosphonic acid and pivalic acid in the presence of triethylamine at room temperature afforded a nonanuclear iron(III) phosphonate. It possesses a cage structure with a distorted icosahedral core. Mössbauer studies confirm high-spin Fe III , whereas magnetic measurements suggest strong antiferromagnetic couplings between the Fe 3+ ions.