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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 (through the tools of the ISOTROPY Software Suite) 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.

Symmetry-Mode Structure Solution

Group theory provides the most natural parameter set for describing a distorted crystal structure, since a relatively small number of parameters normally exhibit non-zero values. These symmetry modes are also the ideal parameter set for determining a complicated structural distortion and determining its symmetry without prior assumptions. This was demonstrated in a combined x-ray/neutron study of monoclinic room-temperature tungsten oxide.

Selected Publications

Glass transition in the polaron dynamics of colossal magnetoresistive manganites

B. Campbell (et al.)

Neutron scattering measurements on a bilayer manganite near optimal doping show that the short-range polaron correlations are completely dynamic at high T , but then freeze upon cooling to a temperature T*approximate to310 K . This glass transition suggests that the paramagnetic/insulating state arises from an inherent orbital frustration that inhibits the formation of a long-range orbital- and charge-ordered state. Upon further cooling into the ferromagnetic-metallic state (T-C=114 K) , where the polarons melt, the diffuse scattering quickly develops into a propagating, transverse optic phonon.

Distinct insulating state below the Curie point in Pr_0.7Ba_0.3MnO₃

B. J. Campbell (et al.)

Magnetization, resistivity, thermal expansion, thermopower, and neutron powder diffraction data are presented for the perovskite manganite Pr0.7Ba0.3MnO3. A distinct ferromagnetic insulating state, similar to that observed in the La0.88Sr0.12MnO3 system, is observed in the temperature range between the paramagnetic-ferromagnetic transition at T(FM)approximate to180 K and the broad metal-insulator transition near T-p=120 K. The large A-site cation-size mismatch appears to suppress the ferromagnetic double-exchange interactions relative to the competing ferromagnetic superexchange interactions, thereby giving rise to the ferromagnetic insulating state.

Linear framework defects in zeolite mordenite

B. J. Campbell (et al.)

Framework defect correlations are identified in zeolite mordenite that explain both X-ray powder diffraction anomalies and diffuse intensity features in previously reported electron diffraction data. The c/2 displacement of linear "chains" of framework material parallel to the main [001] channel are shown to selectively suppress and broaden the l = 2n + 1 Bragg reflections, thereby linking these framework defects to observed trends in a number of diffracted intensity ratios. At the same time, these defects are shown to produce thin sheets of diffuse scattering that are restricted to the l = 2n + 1 planes of reciprocal space, features that have been reported in electron diffraction studies of mordenite. An empirical relationship between the defect concentration and the framework aluminum content is also established.

The Structure of Jahn-Teller Polarons in the Colossal Magnetoresistive Manganites

Branton J. Campbell (et al.)

The observation of large negative magnetoresistive effects has generated tremendous scientific and technical interest in the perovskite manganites and related materials. These phenomena are commonly classified under the heading “colossal magnetoresistance” (CMR) to distinguish them from magnetoresistive effects in metals and metal-multilayers. CMR effects are observed in conjunction with the formation of a ferromagnetic metallic (FM) state at low temperatures and/or high magnetic fields. The close connection between the magnetism and the charge transport in these mixed-valence Mn3+n4+ materials is commonly believed to have its origin in the magnetic double-exchange interaction in which itinerant e_aelectrons mediate the ferromagnetic coupling between neighboring Mn core spins. Recent works have shown that double-exchange alone is insufficient to explain the extraordinary magnetoresistance values observed experimentally, so that electron-phonon interactions must also be invoked. Alternatively, a direct Mn-Mn magnetic exchange interaction has been reported to better explain the insensitivity of the Curie temperature (T_c) to the carrier concentration in oxygen-depleted samples. This model also relies on strong electron-lattice coupling, which occurs via the Jahn-Teller (JT) mechanism in which an Mn³⁺O₆octahedron experiences a spontaneous distortion designed to break the degeneracy of its e₅orbital, and thus lower the energy of its extra electron. If the coupling is strong, the electron may induce a significant local distortion of the lattice, which then follows the electron as it hops from site to site, thus forming a polaron.

Structure of nanoscale polaron correlations in La_1.2Sr_1.8Mn₂O₇

B. J. Campbell (et al.)

A system of strongly interacting electron-lattice polarons can exhibit charge and orbital order at sufficiently high polaron concentrations. In this study, the structure of short-range polaron correlations in the layered colossal magnetoresistive perovskite manganite La1.2Sr1.8Mn2O7 has been determined by a cystallographic analysis of broad satellite maxima observed in diffuse x-ray and neutron-scattering data, The resulting q approximate to (0.3,0, +/-1) modulation is a longitudinal octahedral-stretch mode, consistent with incommensurate Jahn-Teller-coupled charge-density-wave fluctuations, that implies an unusual orbital-stripe pattern parallel to the (100) directions.