Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A7: Unconventional Transport and Magnetic Properties of Hexaborides |
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Sponsoring Units: DCMP Chair: John Sarrao, Los Alamos National Laboratory Room: Colorado Convention Center Korbel 4A-4B |
Monday, March 5, 2007 8:00AM - 8:36AM |
A7.00001: Magneto-optical evidence of double exchange in a percolating lattice Invited Speaker: Because of the potential technological applications, materials exhibiting colossal magnetoresistive (CMR) effects are of high current interest in solid state physics. Europium hexaboride ($EuB_{6}$) and the well known manganites, for which the onset of ferromagnetism is accompanied by a dramatic reduction of the electrical resistivity, are primary examples, that have intensively been studied. We concentrate on the series of cubic $Eu_{1-x}Ca_{x}B_{6}$, which displays interesting correlations between magnetic, transport and optical properties. Substituting $Eu$ by $Ca$ in ferromagnetic $EuB_6$ leads to a percolation limited magnetic ordering. We present and discuss magneto- optical data of the $Eu_{1-x}Ca_{x}B_6$ series, based on measurements of the reflectivity $R(\omega)$ from the far infrared up to the ultraviolet, as a function of temperature and magnetic field. Via the Kramers-Kronig transformation of $R$ we extract the complete absorption spectra of samples with different values of $x$. The change of the spectral weight in the Drude component by increasing the magnetic field agrees with a scenario based on the double exchange model, and suggests a crossover from a ferromagnetic metal to a ferromagnetic Anderson insulator upon increasing $Ca$-content at low temperatures. This work appeared in Phys. Rev. Lett. 96, 016403 (2006) [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 9:12AM |
A7.00002: Exploring Competing Interactions in the Hexaborides with Field- and Pressure-Tuned Optical Spectroscopy Invited Speaker: |
Monday, March 5, 2007 9:12AM - 9:48AM |
A7.00003: Disentangling Surface and Bulk Electronic Structures of EuB$_6$ Invited Speaker: By means of angle-resolved photoemission, the surface and bulk electronic structures of UHV-cleaved ferromagnetic hexaboride EuB$_6$ have been disentangled to reveal both a variable density surface 2D electron gas and an exchange splitting of the boron p-bands below the bulk ferromagnetic ordering temperature of the localized Eu 4f moments. Surface-slab LDA calculations find (i) a distinct surface-related band residing in the bulk-projected bandgap along X-M, (ii) a 2D X-point electron pocket, and (iii) energy-shifted surface-atom Eu 4f states resulting from an electric dipole at the highly ionic surface. These surface-related features manifest in experiment as time dependent effects, the filling of a small X-point electron pocket of Eu 4d-character, correlated to changes in the overall B-p band structure and a splitting of the Eu 4f states. The time-dependent behavior is explained in terms of clustering of mobile surface Eu atoms on the freshly cleaved surface. Understanding and control of these surface effects then allows the bulk electronic structure to be discerned, including a pinning of the Fermi level to the bulk X-point valence band maximum and the magnitude of the valence band exchange splitting below T$_c$. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:24AM |
A7.00004: Double exchange model and magnetic polarons in Eu-based hexaborides Invited Speaker: In this presentation, important details of the Double Exchange model at low densities and their pertinence to the physics of ferromagnetic hexaborides will be addressed. After a brief survey of some key experiments and signatures of these compounds, it will be shown that in such systems, where itinerant electrons at extremely reduced densities interact with a dense local spin subsystem, effects of Anderson localization are paramount, providing a consistent picture of the remarkable and unusual response of materials like Eu$_{1-x}$Ca$_x$B$_6$ in magnetic, optical and transport measurements. In this context we will show how one can understand the blue shift in the plasma frequency, the enhancement of carrier density and the CMR effect upon entering the ferromagnetic phase of EuB$_6$ as effects stemming from Anderson localization mechanisms. We will provide an interpretation for the metal insulator transition that occurs upon doping, under the light of recent magneto-optical experiments. In addition, the region of stability of magnetic polarons in the low density Double Exchange Model is discussed, and shown to be consistent with experimental hints of a polaronic phase mediating the PM-FM transition in the vicinity of T$_C$. [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 11:00AM |
A7.00005: Magnetic polarons in EuB$_6$ and other low carrier density ferromagnets. Invited Speaker: Magnetic polarons are formed in systems with low carrier density and large local exchange coupling between the spin background and the spin of the carrier. Magnetic polarons can be free, as in manganese pyrochlores, or bound by Coulomb interaction to impurities in the lattice, as in diluted magnetic semiconductors. EuB$_6$ is a likely candidate for the formation of magnetic polarons as revealed by spin flip Raman scattering and muon-spin rotation measurements. I will give a general perspective on magnetic polarons with special emphasis on the evaluation of the experimental evidence for magnetic polarons in EuB$_6$[1]. This work was supported by Churchill College (University of Cambridge), LPS, and NSF. \newline \newline [1] M.J. Calderon, L.G.L. Wegener, and P.B. Littlewood, Physical Review B 70, 092408 (2004). [Preview Abstract] |
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