In this work, we review single mode SiO2 fiber Bragg grating techniques for dilatometry studies of small single-crystalline samples in the extreme environments of very high, continuous, and pulsed magnetic fields of up to 150 T and at cryogenic temperatures down to <1 K. Distinct millimeter-long materials are measured as part of the technique development, including metallic, insulating, and radioactive compounds. Experimental strategies are discussed for the observation and analysis of the related thermal expansion and magnetostriction of materials, which can achieve a strain sensitivity (ΔL/L) as low as a few parts in one hundred million (≈10−8). The impact of experimental artifacts, such as those originating in the temperature dependence of the fiber’s index of diffraction, light polarization rotation in magnetic fields, and reduced strain transfer from millimeter-long specimens, is analyzed quantitatively using analytic models available in the literature. We compare the experimental results with model predictions in the small-sample limit, and discuss the uncovered discrepancies.

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Time reversal symmetry and magnetoelastic correlations are probed by means of high-resolution volume dilatometry in URu2Si2 at cryogenic temperatures, and magnetic fields sufficient to suppress the hidden order state at HHO(T = 0.66 K) 35 T. We report a significant magnetoelastic volume expansion at and above HHO(T ), and even above THO, possibly a consequence of field-induced f-electron localization. We investigate in detail the magnetostriction and magnetization as the temperature is reduced across two decades in temperature from 30 K where the system is paramagnetic, to 0.5 K in the realm of the hidden order state. We find a dominant quadratic-in-field dependence ∆L/L ∝ H 2 , a result consistent with a state that is symmetric under time reversal. The data shows, however, an incipient yet unmistakable asymptotic approach to linear (∆L/L ∝ 1 − H/H0) for 15 T < H < HHO(0.66 K) ∼ 40 T at the lowest temperatures. We discuss these results in the framework of a Ginzburg-Landau formalism that proposes a complex order parameter for the HO phase to model the (H,T,p) phase diagram. arXiv:1812.02798v2 [cond-mat.str-el]

51Cr and 60Co diffusion along grain boundary (GB) in polycrystalline Zr and -Zr-20%Nb were measured by means of the radiotracer technique in an overall temperature range [380-1000] K. The use of Harrison´s C and B kinetics resulted in direct data of the GB diffusivity (Dgb) and the apparent GB diffusivity (Pgb). The analyzed temperatures involved those of power reactors service. The GB segregation factors s were determined or evaluated in the limit of very dilute solute concentration.

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