Chalk River Laboratories

About: Chalk River Laboratories is a based out in . It is known for research contribution in the topics: Neutron diffraction & Neutron scattering. The organization has 2297 authors who have published 2700 publications receiving 73287 citations.

Dynamic frustrated magnetism in Tb 2 Ti 2 O 7 at 50 mK

Abstract: The low temperature $(Tl1\mathrm{K})$ properties of the cooperative paramagnet ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ have been studied by ac susceptibility, neutron diffraction and neutron spin echo techniques. Like several other frustrated magnets, ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7},$ is believed to remain paramagnetic down to $\ensuremath{\sim}0.07\mathrm{K}.$ However, recent studies [Yasui et al., J. Phys. Soc. Jpn. 71, 599 (2002), for example] suggest that ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ enters an ordered, albeit glassy, state at a relatively high temperature, $\ensuremath{\sim}1.5\mathrm{K}.$ Our results confirm that the majority of the spins in ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ fluctuate very rapidly, even at 50 mK and that static, spatial correlations do not develop beyond nearest neighbor at similar temperatures. We suggest that the observation of a partial freezing of this magnetic system, at finite temperature, is a result of a small fraction of spins freezing around defects in the stoichiometric crystal structure.

Magnetic properties of the geometrically frustrated S=(1)/(2) antiferromagnets, La 2 LiMoO 6 and Ba 2 YMoO 6 , with the B-site ordered double perovskite structure: Evidence for a collective spin-singlet ground state

Abstract: Two B-site ordered double perovskites, ${\text{La}}_{2}{\text{LiMoO}}_{6}$ and ${\text{Ba}}_{2}{\text{YMoO}}_{6}$, based on the $S=\frac{1}{2}$ ion, ${\text{Mo}}^{5+}$, have been investigated in the context of geometric magnetic frustration. Powder neutron diffraction, heat capacity, susceptibility, muon-spin relaxation $(\ensuremath{\mu}\text{SR})$, and $^{89}\text{Y}$ NMR-including magic-angle spinning (MAS) NMR data have been collected. ${\text{La}}_{2}{\text{LiMoO}}_{6}$ crystallizes in $P{2}_{1}/n$ with $a=5.59392(19)\text{ }\text{\AA{}}$, $b=5.69241(16)\text{ }\text{\AA{}}$, $c=7.88029(22)\text{ }\text{\AA{}}$, and $\ensuremath{\beta}=90.2601(30)\ifmmode^\circ\else\textdegree\fi{}$ at 299.7 K, while ${\text{Ba}}_{2}{\text{YMoO}}_{6}$ is cubic, $Fm3m$, with $a=8.39199(65)\text{ }\text{\AA{}}$ at 297.8 K. ${\text{Ba}}_{2}{\text{YMoO}}_{6}$ shows no distortion from cubic symmetry even at 2.8 K in apparent violation of the Jahn-Teller theorem for a ${t}_{2\text{g}}^{1}$ ion. $^{89}\text{Y}$ NMR MAS data indicate about a 3% level of Y/Mo site mixing. ${\text{La}}_{2}{\text{LiMoO}}_{6}$ deviates strongly from simple Curie-Weiss (C-W) paramagnetic behavior below 150 K and zero-field-cooled/field-cooled (ZFC/FC) irreversibility occurs below 20 K with a weak, broad susceptibility maximum near 5 K in the ZFC data. A Curie-Weiss fit shows a reduced ${\ensuremath{\mu}}_{\text{eff}}=1.42{\ensuremath{\mu}}_{\text{B}}$, (spin $\text{only}=1.73{\ensuremath{\mu}}_{\text{B}}$) and a Weiss temperature, ${\ensuremath{\theta}}_{\text{C}}$, which depends strongly on the temperature range of the fit. Powder neutron diffraction and heat capacity show no evidence for long-range magnetic order to 2 K. On the other hand oscillations develop below 20 K in $\ensuremath{\mu}\text{SR}$ indicating at least short-range magnetic correlations. Susceptibility data for ${\text{Ba}}_{2}{\text{YMoO}}_{6}$ also deviate strongly from the C-W law below 150 K with a nearly spin only ${\ensuremath{\mu}}_{\text{eff}}=1.72{\ensuremath{\mu}}_{\text{B}}$ and ${\ensuremath{\theta}}_{\text{C}}=\ensuremath{-}219(1)\text{ }\text{K}$. There is no discernible ZFC/FC irreversibility to 2 K. Heat capacity, neutron powder diffraction, and $\ensuremath{\mu}\text{SR}$ data show no evidence for long-range order to 2 K but a very broad, weak maximum appears in the heat capacity. The $^{89}\text{Y}$ NMR paramagnetic Knight shift shows a remarkable local spin susceptibility behavior below about 70 K with two components from roughly equal sample volumes, one indicating a singlet state and the other a strongly fluctuating paramagnetic state. Further evidence for a singlet state comes from the behavior of the relaxation rate, $1/{T}_{1}$. These results are discussed and compared with those from other isostructural $S=\frac{1}{2}$ materials and those based on $S=3/2$ and $S=1$.

Structure, thermal, and transport properties of the clathrates Sr 8 Zn 8 Ge 38 , Sr 8 Ga 16 Ge 30 , and Ba 8 Ga 16 Si 30

Abstract: The structural parameters, thermal properties, and transport properties of three type I clathrates, namely ${\mathrm{Sr}}_{8}{\mathrm{Zn}}_{8}{\mathrm{Ge}}_{38}$, ${\mathrm{Sr}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$, and ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Si}}_{30}$, have been determined at or below room temperature. The structural parameters of these clathrates were determined by powder neutron diffraction. Their lattice thermal expansion is two to four times greater than that of the diamond phases of silicon and germanium, consistent with more anharmonic lattice vibrations. From the temperature dependence of the isotropic atomic displacement parameters, the estimated rattling frequencies of guests in the large cages of these clathrates are in the range $50--60\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The heat capacities of these three clathrate materials increase smoothly with increasing temperatures and approach the Dulong--Petit value around room temperature. The Gr\"uneisen parameter of these materials is constant between $100$ and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ but increases below $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, due to the dominance of the low-frequency guest-rattling modes. The room-temperature electrical resistivity and the Seebeck coefficient show that these materials are metallic. The temperature profile of the thermal conductivities and calculated phonon mean free paths of these materials show glasslike behavior, although they are crystalline materials, indicating strong resonant scattering of heat-carrying acoustic phonons via the rattling of the guests in the clathrate cages.