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Branton Campbell in the lab

Branton J. Campbell

Professor

Department of Physics & Astronomy

Brigham Young University

Provo, UT 84602, USA

Tel: 801-422-5758, Fax: 801-422-0553

Email: branton_campbell[at]byu.edu

//physics.byu.edu/faculty/campbell/

Research Interests

I apply state-of-the-art x-ray and neutron scattering techniques to study local and long-range structures in a variety of complex solids, including fast-ion conductors, ferroelectric relaxors, high-temperature superconductors, and colossal magnetoresistive manganites, where nanoscale structural features influence macroscopic physical properties. This includes the development of symmetry-mode analysis as a tool for the determination, refinement and interpretation of distorted structures involving lattice strains, atomic displacements, magnetic moments and occupational orderings at both commensurate and incommensurate wavevectors.

Gamma-Alumina Nanoparticles

Nanoparticles of gamma-alumina obtained from a novel synthetic solvent-deficient method show promise as improved industrial catalyst-supports. X-ray PDF analysis reveals that they are born with a high concentration of defects that locally resemble boehmite. As the nanoparticles are annealed to successively higher temperatures, the boehmite-like defects heal gradually rather than disappearing in an abrupt phase transition, which explains several previously misunderstood of properties of the gamma phase.

Gd₅Si_4-xBi_x Structures: Novel Slab Sequences Achieved by Turning off the Directionality of Nearest-Slab Interactions

By Branton J. Campbell (et al.)

Abstract:

Substitution of Bi for Si leads to the complete cleavage of the interslab dimers T-T in the Gd5Si4-xBix system with x = 1.58 - 2.42 (T is a mixture of Si and Bi). Equivalence of the interslab T ... T contacts, achieved through combination of the electronic and geometrical parameters, removes directionality of nearest-slab interactions and allows for a novel slab stacking. Two new slab sequences, ABCDABCD (x = 2.07, I4(1)/acd space group) and ABADABAD (x = 2.42, P4(2)bc), have been discovered in Gd5Si4-xBix in addition to the known one, ABAB, that is dominant among the RE5X4 phases (RE is a rare-earth element, X is a p-element). The slab stacking for x = 2.07 and x = 2.42 is dictated by the second-nearest slab interactions which promote an origin shift either for the entire slab sequence as in ABCDABCD or for every other second-nearest slab pair as in ABADABAD. The loss of the directionality of the nearest-slab bonding allows for extensive stacking faults and leads to diffuse scattering.

The flexible embedded-fiber neutron detector

By T. K. McKnight, J. B. Czirr, and B. J. Campbell (et al.)

Abstract: We present a novel area-detector design, the flexible embedded-fiber detector (FEFD), which combines high-efficiency, low-cost, and very simple signal processing. It consists of wavelength-shifting fibers embedded in a zinc-sulfide lithium-fluoride-based scintillator and a physically flexible binder that allows the detecting surface to be wrapped into circular paths, so that each fiber is concentric with a single Debye-Scherrer cone. The FEFD design has been investigated via Monte Carlo simulations and by efficiency measurements performed using the CHEX instrument at the Intense Pulsed Neutron Source. (C) 2008 Elsevier B.V. All rights reserved.

Rietveld refinement of structural distortion-mode amplitudes

By B. J. Campbell and H. T. Stokes (et al.)

Abstract: For any crystal structure that can be viewed as a low-symmetry distortion of some higher-symmetry parent structure, one can represent the details of the distorted structure with a list of distortion-mode amplitudes rather than the traditional list of xyz atomic coordinates. After importing mode definitions from the ISODISPLACE software, TOPAS Academic can now directly refine structural distortion-mode amplitudes. In this article, we demonstrate that the distortion-mode basis is well-suited to a Rietveld refinement of the room-temperature structure of WO3, which has 24 displacive atomic-coordinate degrees of freedom. Only five distortion-mode amplitudes are needed to capture the essential features of this structure.

Order parameters for phase transitions to structures with one-dimensional incommensurate modulations

By Harold T. Stokes, Branton J. Campbell, and Dorian M. Hatch

Abstract: Phase transitions that result in incommensurate structural modulations are widely observed in crystalline solids and are relevant to a broad range of physical phenomena in magnetic, electronic, optical and structural materials. While the (3+1)-dimensional superspace-group symmetries associated with one-dimensional modulations have been tabulated, the order parameters that produce these modulations have not been explored in detail. Here, using group-theoretical methods, we present a unique and exhaustive enumeration of the isotropy subgroups (and their corresponding order-parameter directions) belonging to irreducible representations of the (3+1)-dimensional superspace extensions of the 230 crystallographic space groups at all incommensurate k points. The vast majority of experimentally observed incommensurately modulated structures have order parameters belonging to one of these subgroups.

Microscopic annealing process and its impact on superconductivity in T '-structure electron-doped copper oxides

By Branton J. Campbell (et al.)

Abstract: High-transition-temperature superconductivity arises in copper oxides when holes or electrons are doped into the CuO2 planes of their insulating parent compounds. Whereas hole doping quickly induces metallic behaviour and superconductivity in many cuprates, electron doping alone is insuffcient in materials such as R2CuO4 (R is Nd, Pr, La, Ce and so on), where it is necessary to anneal an as grown sample in a low-oxygen environment to remove a tiny amount of oxygen in order to induce superconductivity. Here we show that the microscopic process of oxygen reduction repairs Cu deficiencies in the as-grown materials and creates oxygen vacancies in the stoichiometric CuO2 planes, effectively reducing disorder and providing itinerant carriers for superconductivity. The resolution of this long-standing materials issue suggests that the fundamental mechanism for superconductivity is the same for electron- and hole-doped copper oxides.