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Hybridization-driven gap in U3Bi4Ni3: a 209Bi NMR/NQR study

Description: We report {sup 209}Bi nuclear-magnetic-resonance and nuclear-quadrupole-resonance measurements on a single crystal of the Kondo insulator U{sub 3}Bi{sub 4}Ni{sub 3}. The {sup 209}Bi nuclear-spin-lattice relaxation rate (T{sub 1}{sup -1}) shows activated behavior and is well fit by a spin gap of 220 K. The {sup 209}Bi Knight shift (K) exhibits a strong temperature dependence arising from 5f electrons, in which K is negative at high temperatures and increases as the temperature is lowered. Below 50 K, K shows a broad maximum and decreases slightly upon further cooling. Our data provide insight into the evolution of the hyperfine fields in a fully gapped Kondo insulator based on 5f electron hybridization.
Date: January 1, 2009
Creator: Baek, Seung H
Partner: UNT Libraries Government Documents Department

THEORY OF THE KNIGHT SHIFT AND FLUX QUANTIZATION IN SUPERCONDUCTORS

Description: Consequences of a generalization of the theory of superconductivity that yields a finite Knight shift are presented. In this theory, by introducing an electron-electron interaction that is not spatially invariant, the pairing of electrons with varying total momentum is made possible. An expression for Xs (the spin susceptibility in the superconducting state) is derived. In general Xs is smaller than Xn, but is not necessarily zero. The precise magnitude of Xs will vary from sample to sample and will depend on the nonuniformity of the samples. There should be no marked size dependence and no marked dependence on the strength of the magnetic field; this is in accord with observation. The basic superconducting properties are retained, but there are modifications in the various electromagnetic and thermal properties since the electrons paired are not time sequences of this generalized theory on flux quantization arguments are presented. (auth)
Date: May 1, 1962
Creator: Cooper, L.N.; Lee, H.J.; Schwartz, B.B. & Silvert, W.
Partner: UNT Libraries Government Documents Department

Theory of exotic superconductivity and normal states of heavy electron and high temperature superconductivity materials. Progress report, February 15, 1994--February 14, 1995

Description: This is a progress report for the DOE project covering the period 2/15/94 to 2/14/95. The PI had a fruitful sabbatical during this period, and had some important new results, particularly in the area of new phenomenology for heavy fermion superconductivity. Significant new research accomplishments are in the area of odd-in-time-reversal pairing states/staggered superconductivity, the two-channel Kondo lattice, and a general model for Ce impurities which admits one-, two-, and three-channel Kondo effects. Papers submitted touch on these areas: staggered superconductivity - a new phenomenology for UPt{sub 3}; theory of the two-channel Kondo lattice in infinite dimensions; general model of a Ce{sup 3+} impurity. Other work was done in the areas: Knight shift in heavy fermion alloys and compounds; symmetry analysis of singular pairing correlations for the two-channel Kondo impurity model.
Date: February 1, 1995
Creator: Cox, D. L.
Partner: UNT Libraries Government Documents Department

The Search for Magnetic Order in delta-Pu metal using muon spin relaxation

Description: We review results from previous muon spin relaxation ({mu}SR) measurements in applied fields of H{sub 0} = 0 and 0.25 T which established an upper limit for the ordered or disordered frozen spin moment above T = 4 K in {delta}-Pu (4.3 at. % Ga) of {micro}{sub ord} {le} 10{sup -3} {mu}{sub B}. In addition, we present new data in H{sub 0} = 0.25 T and 2 T applied field on a highly annealed {delta}-Pu (4.3 at. % Ga) sample. Neither the muon Knight shift (H{sub 0} = 2 T) nor the inhomogeneous linewidths in the new sample show appreciable temperature dependence below about T = 60 K, also consistent with no spin freezing. Recent theoretical arguments advanced to explain these results are mentioned.
Date: October 16, 2006
Creator: Heffner, R; Ohishi, K; Fluss, M; Morris, G; MacLaughlin, D; Shu, L et al.
Partner: UNT Libraries Government Documents Department

The selective hydrogenation of crotonaldehyde over bimetallic catalysts

Description: The selective hydrogenation of crotonaldehyde has been investigated over a monometallic Pt/SiO{sub 2} catalyst and platinum bimetallic catalysts where the second metal was either silver, copper, or tin. The effects of addition of a second metal to the Pt/SiO{sub 2} system on the selectivity to crotyl alcohol were investigated. The Pt-Sn bimetallic catalysts were characterized by hydrogen chemisorption, {sup 1}H NMR and microcalorimetry. The Pt-Ag/SiO{sub 2} and Pt-Cu/SiO{sub 2} catalysts were characterized by hydrogen chemisorption. Pt-Sn/SiO{sub 2} catalysts selectively hydrogenated crotonaldehyde to crotyl alcohol and the method of preparation of these catalysts affected the selectivity. The most selective Pt-Sn/SiO{sub 2} catalysts for the hydrogenation of crotonaldehyde to crotyl alcohol were those in which the Sn precursor was dissolved in a HCl solution. Sn increased both the rate of formation of butyraldehyde and the rate of formation of crotyl alcohol. The Pt/SiO{sub 2}, Pt-Ag/SiO{sub 2} and Pt-Cu/SiO{sub 2} catalysts produced only butyraldehyde. Initial heats of adsorption ({approximately}90 kJ/mol) measured using microcalorimetry were not affected by the presence of Sn on Pt. We can conclude that there is no through metal electronic interaction between Pt and Sn at least with respect to hydrogen surface bonds since the Pt and Pt-Sn at least with respect to hydrogen surface bonds since the Pt and Pt-Sn had similar initial heats of adsorption coupled with the invariance of the {sup 1}H NMR Knight shift.
Date: February 1, 1997
Creator: Schoeb, A.M.
Partner: UNT Libraries Government Documents Department

Muon spin relaxation and Knight shift in the heavy-fermion superconductor UPt/sub 3/

Description: Positive muon spin relaxation experiments have been conducted on the heavy-fermion superconductor UPt/sub 3/ in both the normal and superconducting states for zero, transverse, and longitudinally applied magnetic fields. Below 6 K in zero applied field, the ..mu../sup +/ relaxation rate is approximately twice that expected from /sup 195/Pt nuclear dipolar relaxation alone. Transverse- and longitudinal-field measurements show that the observed relaxation rate depends on magnetic field and is quasistatic in origin. It is suggested that the onset of very weak (approx.10/sup -3/ ..mu../sub B//U atom) magnetic ordering below approximately 6 K is responsible for the observed increase in the relaxation rate. ..mu../sup +/ Knight shift measurements in the normal state of UPt/sub 3/ show a temperature dependent shift K/sub ..mu../ which tracks the bulk susceptibility chi. From the K/sub ..mu../ vs chi plot, a ..mu../sup +/ hyperfine field of approximately 100 Oe/..mu../sub B/ is extracted.
Date: January 1, 1986
Creator: Cooke, D.W.; Heffner, R.H.; Hutson, R.L.; Schillaci, M.E.; Smith, J.L.; Willis, J.O. et al.
Partner: UNT Libraries Government Documents Department

Muon spin relaxation in CeCu/sub 2/Si/sub 2/ and muon Knight shift in various heavy-fermion systems

Description: Positive muon spin precession has been observed in varius heavy-fermion systems in the transverse external magnetic field. In the superconductor CeCu/sub 2.1/Si/sub 2/, the relaxation rate of muon spins increases rapidly with decreasing temperature below T/sub c/. This is interpreted as the results of the inhomogeneous fields due to the imperfect penetration of the external field into the type-II superconducting state. The magnetic-field penetration depth lambda is derived from the observed muon spin relaxation rate. lambda is about 1200 A at T approx. 0.5T/sub c/, and the temperature dependence of lambda is consistent with the relation expected for a BCS superconductor. We have also measured the muon Knight shift K/sub ..mu../ in the normal (or paramagnetic) state of various heavy-fermion systems. K/sub ..mu../ is large and negative (about -1000 approx. -3000 ppM at T = 10 K) for CeCu/sub 2/Si/sub 2/, UPt/sub 3/ and CeAl/sub 3/, while more complicated signals are measured in CePb/sub 3/ and CeB/sub 6/. The negative muon Knight shift in the non-magnetic heavy-fermion systems is discussed in terms of the Kondo-coupling between the conduction- and f-electrons.
Date: January 1, 1986
Creator: Uemura, Y.J.; Kossler, W.J.; Hitti, B.; Kempton, J.R.; Schone, H.E.; Yu, X.H. et al.
Partner: UNT Libraries Government Documents Department

Copper and oxygen NMR studies on the magnetic properties of YBa sub 2 Cu sub 3 O sub 7-y

Description: Microscopic magnetic properties of the CuO{sub 2} layers in YBa{sub 2}Cu{sub 3}O{sub 7-y} have been investigated from Cu and O NMR experiments on the y{approx equal}0 (T{sub c}=92K) and y=0.37 (T{sub c}=62K) materials. The Knight shift at the planar Cu and the planar oxygen sites are found to be proportional to a common spin susceptibility {chi}{sub s} which depends on temperature and oxygen-content, strongly supporting a single component spin model for the CuO{sub 2} planes. In the y=0.37 material, {chi}{sub s} shows a significant reduction with decreasing temperature in the normal state. The nuclear relaxation rate (1/T{sub 1}) in the y{approx equal}0 material, particularly different behaviors at Cu and O sites, can be accounted for by the hyperfine coupling of Cu and O nuclei to an antiferromagnetically correlated single spin system. Quite different behaviors of 1/T{sub 1} were observed in the y=0.37 material, which might be due to combined effects of antiferromagnetic correlations and a temperature-dependent spin susceptibility. 38 refs., 7 figs.
Date: January 1, 1989
Creator: Takigawa, Masashi.
Partner: UNT Libraries Government Documents Department

{sup 209}Bi NMR in heavy-electron system YbBiPt

Description: Bismuth NMR Knight shift and spin lattice relaxation rate 1/T{sub 1} are reported between 35--325K in the low-carrier heavy fermion system YbBiPt. The Knight shift is strongly temperature dependent and negative. Its temperature dependence tracks the bulk susceptibility with a hyperfine coupling constant A{sub hf} = {minus}7.89 kOe/{mu}B. At low temperatures 1/T{sub 1} exhibits a dramatic increase, such that the average 4f spin correlation time {tau}{sub f} shows a crossover behavior at about 75K. The rate 1/{tau}{sub f} is proportional to temperature, but with a different proportionality constant above and below about 75K. The linear temperature dependence is consistent with non-interacting 4f local moments which are relaxed via Korringa-type scattering with the conduction electrons. Below 75K, we infer that the reduced thermal excitation of a higher crystal-field multiplet is responsible for the dramatic decrease in the rate of 4f relaxation.
Date: June 1, 1994
Creator: Reyes, A. P.; Le, L. P.; Heffner, R. H.; Ahrens, E. T.; Fisk, Z. & Canfield, P. C.
Partner: UNT Libraries Government Documents Department

/sup 17/O NMR study of YBa/sub 2/Cu/sub 3/O/sub 7/minus/delta/

Description: We present our /sup 17/O NMR experimental results in a magnetically aligned powder sample of YBa/sub 2/Cu/sub 3/O/sub 7/minus/delta/ (T/sub c/ = 93 K). The sign of the anisotropic Knight Shift shows that the spin density resides mainly on the p/sub sigma/ orbitals at the planar O(2,3) and bridging O(4) sites. About 30 % of the total spin susceptibility is attributed to the oxygen 2p/sub sigma/ states. The Knight shift at the O(2,3) sites decreases more rapidly than that at the chain O(1) sites, indicating a larger gap in the planes. The nuclear relaxation rate at the O(2,3) sites shows linear-T (Korringa) behavior above T/sub c/, in contrast to the much temperature dependence at the planar Cu(2) sites. However, these two rates show identical temperature dependence below about 110 K, indicating that an important change in the spin dynamics takes place above T/sub c/. 13 refs., 3 figs.
Date: January 1, 1989
Creator: Takigawa, M.; Hammel, P.C.; Heffner, R.H.; Fisk, Z.; Ott, K.C. & Thompson, J.D.
Partner: UNT Libraries Government Documents Department