We present a study of transport, thermal, and optical properties of stoichiometric CaB 6 and La-doped CaB 6 . For stoichiometric CaB 6 a strong increase of the resistivity with decreasing temperature and anomalies in the low-temperature behavior of the resistivity and the specific heat have been observed. The application of an external magnetic field at low temperatures is shown to lead to an anomalous magnetoresistance in the case of stoichiometric CaB 6 . The optical conductivity exhibits a strong doping dependence. Rather unexpectedly, small changes in the chemical composition lead to significant changes in the electronic interband transitions in the visible-UV spectral range. The relevance of our results with respect to the recently suggested excitonic scenario for explaining the physical properties of alkaline-earth hexaborides is discussed. PHYSICAL REVIEW B15 OCTOBER 2000-I VOLUME 62, NUMBER 15 PRB 62

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Various recent experimental investigations have revealed unusual magnetic properties of hexaborides with divalent cations M>. EuB is ferromagnetic below 16 K and its low-temperature properties show remarkable similarities to those of manganese oxides, exhibiting the phenomenon of colossal magnetoresistance. Close to the phase transition as well as far below the ordering temperature, EuB exhibits anomalous features, that are brie#y discussed. Alkaline-earth hexaborides are close to a metal}insulator transition and it has been found that, in a narrow range of electron doping, an itinerant-type of ferromagnetic order, stable up to temperatures of the order of 600}900 K, is established. This remarkable phenomenon is suspected to be due to the peculiar electronic band structure of these materials.

Nuclear Magnetic Resonance (NMR) spectroscopy is a wide-spread analytical technique which is used in a large range of different fields, such as quality control, food analysis, material science and structural biology. In the widest sense, NMR is an analytical technique to determine the structure of molecules. At the time of writing this manuscript, commercial NMR spectrometers with a proton resonance frequency ≥ 900 MHz are only available from Bruker. In 2019, Bruker installed the first 1.1 GHz (25.8 T) NMR spectrometer at the St. Jude Children Research Hospital in Memphis, Tennessee, followed by the installation of the first 1.2 GHz (28.2 T) NMR spectrometer at the University of Florence in Italy in 2020. These were the first commercial NMR spectrometers operating at magnetic fields in excess of what can be achieved with conventional low temperature superconductors, and which depend on high temperature superconductors to generate the required magnetic field. In this paper, the requirements on commercial NMR magnets are discussed and the history of high-field NMR magnets is reviewed. Bruker’s R&D program for 1.1 and 1.2 GHz NMR magnets and spectrometers will be described, and some of the key properties of these first commercial NMR magnets with high-temperature superconductors are reported.

We report7 Li pulsed NMR measurements in polycrystalline and single crystal samples of the quasi one-dimensional S = 1 antiferromagnet LiVGe2O6, whose AF transition temperature is TN ≃ 24.5 K. The field (B0) and temperature (T ) ranges covered were 9-44.5 T and 1.7-300 K respectively. The measurements included NMR spectra, the spin-lattice relaxation rate (T −1 1 ), and the spin-phase relaxation rate (T −1 2 ), often as a function of the orientation of the field relative to the crystal axes. The spectra indicate an AF magnetic structure consistent with that obtained from neutron diffraction measurements, but with the moments aligned parallel to the c-axis. The spectra also provide the T -dependence of the AF order parameter and show that the transition is either second order or weakly first order. Both the spectra and the T −1 1 data show that B0 has at most a small effect on the alignment of the AF moment. There is no spin-flop transition up to 44.5 T. These features indicate a very large magnetic anisotropy energy in LiVGe2O6 with orbital degrees of freedom playing an important role. Below 8 K, T −1 1 varies substantially with the orientation of B0 in the plane perpendicular to the c-axis, suggesting a small energy gap for magnetic fluctuations that is very anisotropic.

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27 Al 2D exchange NMR experiments on an icosahedral Al 70 Re 8.6 Pd 21.4 quasicrystal have detected slow atomic motions in the sub-kHz range at temperatures between 0.16 and 130 K. The observed motion is limited in space and can be associated with local phason fluctuations at very low energies, involving quantum tunneling at very low temperatures. It may thus be regarded as an atomic motion acting as a precursor to bulk self-diffusion. [S0031-9007 (98)07456-0] PACS numbers: 61.44.Br, 76.60. -k