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Evidence for strong electron-lattice coupling in La{sub 2{minus}x}Sr{sub x}NiO{sub 4}

Description: The inelastic neutron scattering spectra were measured for several Sr concentrations of polycrystalline La{sub 2{minus}x}Sr{sub x}NiO{sub 4}. The authors find that the generalized phonon density-of-states is identical for x = 0 and x = 1/8. For x = 1/3 and x = 1/2, the band of phonons corresponding to the in-plane oxygen vibrations (> 65 meV) splits into two subbands centered at 75 meV and 85 meV. The lower frequency band increases in amplitude for the x = 1/2 sample, indicating that it is directly related to the hole concentration. These changes are associated with the coupling of oxygen vibrations to doped holes which reside in the NiO{sub 2} planes and are a signature of strong electron-lattice coupling. Comparison of La{sub 1.9}Sr{sub 0.1}CuO{sub 4} and La{sub 1.875}Sr{sub 0.125}NiO{sub 4} demonstrates that much stronger electron-lattice coupling occurs for particular modes in the cuprate for modest doping and is likely related to the metallic nature of the cuprate.
Date: April 1, 1999
Creator: McQueeney, R.J. & Sarrao, J.L.
Partner: UNT Libraries Government Documents Department

Magnesium substitutions in rare-earth metal germanides with the Gd5Si4 type. Synthesis, structure determination and magnetic properties of RE5-xMgxGe4 (RE=Gd-Tm, Lu and Y)

Description: A series of magnesium-substituted rare-earth metal germanides with a general formula RE{sub 5-x}Mg{sub x}Ge{sub 4} (x {approx} 1.0-2.3; RE =Gd-Tm, Lu, Y) have been synthesized by high-temperature reactions and structurally characterized by single-crystal X-ray diffraction. These compounds crystallize with the common Gd{sub 5}Si{sub 4} type in the orthorhombic space group Pnma (No. 62; Z =4; Pearson's code oP36) and do not appear to undergo temperature-induced crystallographic phase transitions down to 120 K. Replacing rare-earth metal atoms with Mg, up to nearly 45 % at., reduces the valence electron count and is clearly expressed in the subtle changes of the Ge-Ge and metal-metal bonding. Magnetization measurements as a function of the temperature and the applied field reveal complex magnetic structures at cryogenic temperatures, and Curie-Weiss paramagnetic behavior at higher temperatures. The observed local moment magnetism is consistent with RE+ ground states in all cases. In the magnetically ordered phases, the magnetization cannot reach saturation in fields up to 50 kOe. The structural trends across the series and the variations of hte magnetic properties as a function of the Mg content are also discussed. KEYWORDS: Rare-earth intermetallics, germanides, crystal structure,Gd{sub 5}Si{sub 4} type.
Date: January 1, 2009
Creator: Sarrao, J L; Thompson, Joe D; Tobash, P H & Bobev, S
Partner: UNT Libraries Government Documents Department

Kondo insulators

Description: The Kondo insulating materials present a particularly simple limiting case of the strongly correlated electron lattice problem: one occupied f-state interacting with a single half-filled conduction band. Experiment shows that the solution to this problem has some remarkably simple aspects. Optical conductivity data display the strong coupling nature of this physics.
Date: October 1, 1994
Creator: Fisk, Z.; Sarrao, J. L. & Thompson, J. D.
Partner: UNT Libraries Government Documents Department

The physics and chemistry of heavy Fermions

Description: The heavy Fermions are a subset of the f-element intermetallics straddling the magnetic/non-magnetic boundary. Their low temperature properties are characterized by an electronic energy scale of order 1--10 K. Among the low temperature ground states observed in heavy Fermion compounds are exotic superconductors and magnets, as well as unusual semiconductors. We review here the current experimental and theoretical understanding of these systems.
Date: December 31, 1994
Creator: Fisk, Z.; Sarrao, J. L.; Smith, J. L. & Thompson, J. D.
Partner: UNT Libraries Government Documents Department

Photoelectron Spectroscopy of YbInCu{sub 4}: Direct Testing of Correlated Electron Models

Description: The electronic properties of single crystal YbInCu{sub 4} have been investigated by means of high resolution photoelectron spectroscopy. A first order, isostructural phase transition for YbInCu{sub 4} at T{sub v}=42 K leads to changes in the Kondo temperature of more than an order of magnitude (27 K vs. 400 K). This phase transition and accompanying Kondo temperature change provide the most direct test of the single impurity model (SIM) to date. Particle hole symmetry allows the SIM to be used for Yb compounds as well as Ce heavy fermions with the great advantage that the predicted Kondo resonance is found on the occupied side of the spectral weight function for Yb materials and is thus directly observable in photoemission. The photoemission results are incongruous with the single impurity model predictions for temperature dependence, binding energy and 4f occupancy, encouraging a reevaluation of the single impurity model. The experiments were conducted using the PGM undulator and 4 meter NIM beamlines at SRC. The spectra were taken at photon energies of 40 eV and 90 eV and the combined energy resolution of the analyzer and monochromator was 45- 85 meV.
Date: December 31, 1997
Creator: Joyce, J.J.; Arko, A.J.; Sarrao, J.L. & Fisk, Z.
Partner: UNT Libraries Government Documents Department

Electron diffraction study of La{sub 2}Li{sub O.5}Cu{sub O.5}O{sub 4}

Description: La{sub 2}Li{sub 0.5}Cu{sub 0.5}O{sub 4} has been investigated by electron diffraction and high resolution electron microscopy at room temperature. Ordering between Li and Cu cations is fairly perfect within the ({anti a}, {anti c}) plane but there is disorder between Cu and Li cations along the {anti b} axis. The detection of weak reflections which violate the C centering of the Cmmm space group (initially proposed for this compound), indicates that the crystal symmetry allows lattice distortions. The evolution of the structure as a function of the temperature may lead to determine whether the low energy singlet-triplet excitation, found in NMR/NQR studies, is coupled with lattice distortions.
Date: March 1, 1998
Creator: Moshopoulou, E.; Thompson, J.D.; Sarrao, J.L. & Fisk, Z.
Partner: UNT Libraries Government Documents Department

The local atomic structure of La{sub 1{minus}x}Sr{sub x}CoO{sub 3}: Effects induced by the spin-state and non-metal to metal transitions

Description: The pair density function (PDF) used in the analysis of pulsed neutron diffraction data of La{sub 1{minus}x}Sr{sub x}CoO{sub 3} revealed new structural effects which are correlated to the susceptibility and transport transitions. The transition in the spin configuration of the Co ions from the low-spin (LS) to the high-spin (HS) state in the Co perovskite oxides can potentially induce structural distortions due to the coupling of the spin to the lattice and charge. The ground state of the pure compound, LaCoO{sub 3}, is in the LS state and is non-magnetic. A transition occurs to the HS state at {approximately} 50 K as indicated from the susceptibility measurements due to the thermal excitation of electrons to the e{sub g} level. The Co{sub LS}O{sub 6} octahedra associated with the Co ions in the LS configuration are distinguished from the Co{sub HS}O{sub 6} octahedra with the Co in the HS configuration because the Co{sub LS}-O bond length is shorter than the Co{sub HS}-O distance due to the different size of the corresponding Co ions. Such bond lengths are clearly identified in the local structure between 15--300 K. This finding is in contrast to the average structure which shows only one type of bond length in this temperature range but two types of bond lengths are suggested at considerably higher temperatures. This suggests that whereas the LS and HS CoO{sub 6}-octahedra coexist, they are randomly distributed in the crystal lattice at lower temperatures and become ordered at higher temperatures.
Date: March 1, 1998
Creator: Louca, D.; Sarrao, J.L. & Kwei, G.H.
Partner: UNT Libraries Government Documents Department

Techniques and processes for the measurement of the resonances of small single crystals

Description: The mechanical resonances of small oriented single crystals of materials of interest to basic science and engineering can be used to determine all the elastic moduli and the ultrasonic attenuation of these materials. To measure the resonances of the samples without introducing the resonances of the measuring system requires that the transducers be non-resonant at the frequencies of interest, and that they be well isolated from their mounts. However, for samples near 1 mm in the largest dimension, the transducer design problem becomes sever, and the signals become weak. In addition, no resonances can be missed, and, often, the symmetry class of the resonances must be known. We outline here appropriate transducer, electronics, and system designs to circumvent these problems. 10 refs., 4 figs.
Date: January 1, 1991
Creator: Migliori, A.; Stekel, A.; Sarrao, J.L.; Visscher, W.M.; Bell, T. & Lei, M.
Partner: UNT Libraries Government Documents Department

Thermodynamics and electrodynamics of unusual narrow-gap semiconductors

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL) that has led to a fully funded DOE program to continue this work. The project was directed toward exploring the Ettingshausen effect, which is the direct extension of the familiar Peltier-effect refrigerator (the process used in popular coolers that run off automotive electrical power) in which a magnetic field is used to enhance refrigeration effects at temperatures well below room temperature. Such refrigeration processes are all-solid-state and are of potentially great commercial importance, but essentially no work has been done since the early 1970s. Using modern experimental and theoretical techniques, the authors have advanced the state-of-the-art significantly, laying the groundwork for commercial cryogenic solid-state refrigeration.
Date: December 1998
Creator: Migliori, A.; Darling, T. W.; Trugman, S. A.; Freibert, F.; Moshopoulou, E. & Sarrao, J. L.
Partner: UNT Libraries Government Documents Department

Charge inhomogeneities in the colossal magnetoresistant manganites from the local atomic structure

Description: The authors have measured atomic pair distribution functions (PDF) of La{sub 1{minus}x}Ca{sub x}MnO{sub 3} using high energy x-ray diffraction. This approach yields accurate PDFs with very high real-space resolution. It also avoids potential pitfalls from the more usual neutron measurements that magnetic scattering is present in the measurement, that the neutron scattering length of manganese is negative leading to partial cancellation of PDF peaks, and that inelasticity effects might distort the resulting PDF. They have used this to address the following questions which do not have a satisfactory answer: (1) What are the amplitudes and natures of the local Jahn-Teller and polaronic distortions in the CMR region. (2) Is the ground-state of the ferromagnetic metallic phase delocalized or polaronic. (3) As one moves away from the ground-state, by raising temperature or decreasing doping, towards the metal insulator transition, how does the state of the material evolve?
Date: January 12, 2000
Creator: Billinge, S. J. L.; Petkov, V.; Proffen, T.; Kwei, G. H.; Sarrao, J. L.; Shastri, S. D. et al.
Partner: UNT Libraries Government Documents Department

Heat Capacity Measurements in Pulsed Magnetic Fields

Description: The new NHMFL 60T quasi-continuous magnet produces a flat-top field for a period of 100 ms at 60 Tesla, and for longer time at lower fields, e.g. 0.5 s at 45 Tesla. We have developed for the first time the capability to measure heat capacity at very high magnetic fields in the NHMFL 60T quasi-continuous magnet at LANL, using a probe built out of various plastic materials. The field plateau allows us to utilize a heat-pulse method to obtain heat capacity data. Proof-of-principle heat capacity experiments were performed on a variety of correlated electron systems. Both magnet performance characteristics and physical properties of various materials studied hold out a promise of wide application of this new tool.
Date: October 23, 1998
Creator: Jaime, M.; Movshovich, R.; Sarrao, J.L.; Kim, J.; Stewart, G.; Beyermann, W.P. et al.
Partner: UNT Libraries Government Documents Department

New approaches to thermoelectric cooling effects in magnetic fields

Description: The authors review thermoelectric effects in a magnetic field at a phenomenological level. Discussions of the limiting performance and problems with its computation for both Peltier and Ettingshausen coolers are presented. New principles are discussed to guide the materials scientist in the search for better Ettingshausen materials, and a brief review of the subtle measurement problems is presented.
Date: August 1, 1997
Creator: Migliori, A.; Darling, T.W.; Freibert, F.; Trugman, S.A.; Moshopoulou, E. & Sarrao, J.L.
Partner: UNT Libraries Government Documents Department

Spin Freezing and Recovery of Sublattice Magnetization in Lightly Doped Lanthanum Cuprate

Description: {sup 139}La NQR studies in lightly doped La{sub 2}Cu{sub 1-x}Li{sub x}O{sub 4} and La{sub 1.8-x}Eu{sub 0.2}Sr{sub x}CuO{sub 4} are reviewed. A strong enhancement of the {sup 139}La relaxation rate with a peak accompanied by a sudden increase of the local field at low T has been observed similarly to La{sub 2-x}Sr{sub x}CuO{sub 4}. The anomalous magnetic properties are discussed in the light of the microscopic segregation of doped holes into hole-rich domain walls separating undoped AF domains.
Date: October 24, 1998
Creator: Suh, B. J.; Hammel, P. C.; Sarrao, J. L.; Thompson, J. D.; Fisk, Z.; Hucker, M. et al.
Partner: UNT Libraries Government Documents Department

Magnetotransport measurements in magnetic fields up to 50T in SmB{sub 6} and FeSi

Description: We have measured the transverse magnetoresistance in high-quality single-crystalline samples of SmB{sub 6} and FeSi at 4K in applied magnetic fields to 50T and in the temperature range 4K < T < 150K in magnetic fields to 18T. The magnetoresistance in SmB6 at 4K decreases quadratically and reaches about {minus}45% at 50T. The compound FeSi reveals three distinct features in the magnetoresistance: a negative magnetoresistance for fields below 5T, a kink at around 10T, and a broad maximum near 3OT.
Date: December 31, 1993
Creator: Lacerda, A.; Graf, T.; Sarrao, J. L.; Mandrus, D.; Hundley, M. F.; Thompson, J. D. et al.
Partner: UNT Libraries Government Documents Department

Magnetism in BaCoS{sub 2}

Description: BaCoS{sub 2} is a layered Mott-Hubbard insulator that orders antiferromagnetically near 300 K. We report magnetic susceptibility measurements on polycrystalline BaCoS{sub 2} from 77 K to 800 K, and in- and out-of-plane measurements on single crystals from 2 K to 350 K. We also report a powder neutron refinement of the magnetic structure of BaCoS{sub 2}. The neutron measurements indicate that the moments lie in the plane, and that the magnetic unit cell is the same as the (orthorhombic) chemical cell. The ordered moment obtained from the neutron refinement is close to 3 {mu}{sub B} indicating that the Co ions are in the high-spin (S = 3/2) configuration. The effective moment inferred from the high temperature susceptibility, p = 5.26 {mu}{sub B} is also consistent with high-spin Co{sup +2}.
Date: August 1996
Creator: Mandrus, D.; Chakoumakos, B. C.; Fernandez-Baca, J. A.; Nagler, S. E.; Sales, B. C. & Sarrao, J. L.
Partner: UNT Libraries Government Documents Department

Cerium heavy-fermion compounds near their T = 0 magnetic-non-magnetic boundary

Description: Measurements of the temperature-dependent specific heat and thermal expansion coefficient near a T = 0 magnetic-non-magnetic boundary, accessed in CeRh{sub 2}Si{sub 2} by application of pressure and in CeRh{sub 2{minus}x}Ru{sub x}Si{sub 2} at ambient pressure by chemical substitution, emphasize the role of disorder in producing non-Fermi-liquid behavior. Interestingly, superconductivity also develops near this boundary in some crystallographically-ordered Ce-based heavy-fermion compounds.
Date: December 1, 1997
Creator: Thompson, J.D.; Hundley, M.F.; Movshovich, R.; Sarrao, J.L.; Graf, T.; Uwatoko, Y. et al.
Partner: UNT Libraries Government Documents Department

Local and average crystal structure and displacements of La{sup 11}B{sub 6} and EuB{sub 6} as a function of temperature

Description: Measurements of both the average crystal structure from Rietveld refinement of neutron powder diffraction (NPD) data and the local structure from La L{sub III}-edge x-ray-absorption fine-structure (XAFS) are presented for a La{sup 11}B{sub 6} sample as a function of temperature ({approx}10-320 K). These data are compared to XAFS results on a EuB{sub 6} sample. The single-site La and B positional distribution widths and the La-B and La-La bond length distribution widths and their temperature dependence are compared. This comparison allows an estimate of the La and B site displacements, and we find that these sublattices are only slightly correlated with each other. Moreover, while the temperature dependence of the displacement parameters of the average sites from diffraction fit an Einstein model well, the temperature dependence of the La-B bond length distribution width requires at least two vibrational frequencies, corresponding to the La and B frequencies of the individual sites. XAFS data on EuB{sub 6} indicate that the situation is the same in the Eu compound. In addition, comparisons between data taken below and above the ferromagnetic transition temperature for EuB{sub 6} place stringent limits on the lattice involvement in the associated metal-insulator transition and the ensuing large magnetoresistance effect. This lack of lattice involvement in the magnetoresistance transition is in sharp contrast to the strong lattice involvement observed in the colossal magnetoresistance lanthanum manganese perovskites.
Date: January 30, 2001
Creator: Booth, C.H.; Sarrao, J.L.; Hundley, M.F.; Cornelius, A.L.; Kwei, G.H.; Bianchi, A. et al.
Partner: UNT Libraries Government Documents Department

{mu}SR studies of Li-doped La{sub 2}CuO{sub 4}

Description: Magnetic properties of the Li-doped cuprates La{sub 2}Cu{sub 1{minus}x}Li{sub x}O{sub 4} (where x = 0.01, 0.05, 0.10, 0.45, and 0.50) have been studied by {mu}SR. For low Li concentrations (x {le} 0.10) the authors find a rapid suppression of T{sub N} as x increases, but little change in the magnitude and temperature dependence of the AFM order parameter. This indicates that Li doping effectively destroys AFM without strongly affecting the onsite Cu moments and the shape of the spinwave excitation spectrum. For high Li concentrations they find magnetic clusters in about 15% of the sample volume; the remaining volume is non-magnetic, suggesting possible single-state formation.
Date: June 1996
Creator: Le, L. P.; Heffner, R. H.; MacLaughlin, D. E.; Kojima, K.; Luke, G. M.; Nachumi, B. et al.
Partner: UNT Libraries Government Documents Department

Specific heat of CeRhIn5: Pressure-driven evolution of the ground state from antiferromagnetism to superconductivity

Description: Measurements of the specific heat of antiferromagnetic CeRhIn{sub 5}, to 21 kbar, and for 21 kbar to 70 kOe, show a discontinuous change from an antiferromagnetic ground state below 15 kbar to a superconducting ground state above, and suggest that it is accompanied by a weak thermodynamic first-order transition. Bulk superconductivity appears, apparently with d-wave electron pairing, at the critical pressure, 15 kbar; with further increase in pressure a residual temperature-proportional term in the specific heat disappears.
Date: September 1, 2001
Creator: Fisher, R.A.; Bouquet, F.; Phillips, N.E.; Hundley, M.F.; Pagliuso, P.G.; Sarrao, J.L. et al.
Partner: UNT Libraries Government Documents Department

Lattice disorder and magnetism in f-electron intermetallics

Description: Real materials can have real differences compared to ideal systems. For instance, non-Fermi liquid (NFL) behavior was initially thought to be due to chemical disorder, since the first such materials were all substituted. Although several nominally well-ordered NFL's have been discovered and extensively studied, the effect of disorder on the magnetic properties of f-electron intermetallic systems remains poorly understood. Disorder in NFL systems is reviewed from an experimental, local structure point of view, including a discussion of results on the nominally ordered U{sub 3}Ni{sub 3}Sn{sub 4} and CeCoIn{sub 5} systems, and the chemically disordered UCu{sub 4}Pd and CeRhRuSi{sub 2} systems.
Date: July 29, 2004
Creator: Booth, C.H.; Han, S.-W.; Skanthakumar, S. & Sarrao, J.L.
Partner: UNT Libraries Government Documents Department

Two energy scales and slow crossover in YbAl3

Description: We present results for YbAl{sub 3} which show that the susceptibility {chi}(T), 4f occupation number n{sub f}(T) and the entropy S(T) exhibit a slow crossover between the Fermi liquid and local moment regimes. In addition both {chi}(T) and the linear specific heat coefficient {gamma} = C/T exhibit low temperature peaks that imply that in addition to the Kondo scale (T{sub K} {approx} 500K) there is a second low temperature scale (T{sub coh} {approx} 50K) for the onset of coherence. We discuss these results in the context of Nozieres' exhaustion in the Anderson lattice.
Date: January 15, 2002
Creator: Lawrence, J.M.; Ebihara, T.; Riseborough, P.S.; Booth, C.H.; Pagliuso, P.G.; Sarrao, J.L. et al.
Partner: UNT Libraries Government Documents Department

Effects of self-irradiation on local crystal structure and 5flocalization in PuCoGa5

Description: The 18.5 K superconductor PuCoGa{sub 5} has many unusual properties, including those due to damage induced by self-irradiation. The superconducting transition temperature decreases sharply with time, suggesting a radiation-induced Frenkel defect concentration much larger than predicted by current radiation damage theories. Extended x-ray absorption fine-structure measurements demonstrate that while the local crystal structure in fresh material is well ordered, aged material is disordered much more strongly than expected from simple defects, consistent with strong disorder throughout the damage cascade region. These data highlight the potential impact of local lattice distortions relative to defects on the properties of irradiated materials and underscore the need for more atomic-resolution structural comparisons between radiation damage experiments and theory.
Date: October 20, 2006
Creator: Booth, C.H.; Daniel, M.; Wilson, R.E.; Bauer, E.D.; Mitchell,J.N.; Moreno, N.O. et al.
Partner: UNT Libraries Government Documents Department