Study of the anisotropic hidden order and antiferromagnetic phases of URu2−xOsxSi2 by thermal expansion and magnetoresistance measurements

Thermal expansion and magnetoresistance measurements as a function of temperature (T) and magnetic field (B), respectively, were carried out on Os-substituted URu2Si2 (URu2−xOsxSi2) single crystals with x=0 to 0.28. Mean-field-like peaks at temperature T0 in the linear thermal expansion coefficient vs T curves in both the hidden order (HO) phase and the large-moment antiferromagnetic (LMAFM) phase of URu2−xOsxSi2 exhibit a striking resemblance and a discontinuity in T0(x) at xc=0.15 delineating the boundary between the two phases. Anomalies at xc were also seen in both the calculated effective Grüneisen parameter and the energy gap associated with the HO or LMAFM phase. The hydrostatic pressure derivative of T0 calculated from thermal expansion and specific heat [D. L. Kunwar, Phys. Rev. B105, L041106 (2025) 10.1103/PhysRevB.105.L041106] data agree with the experimental results [C. T. Wolowiec, Proc. Natl Acad. Sci. USA118, e2026591118 (2025) 10.1073/pnas.2026591118], whereas the calculated uniaxial pressure derivatives in different crystallographic directions are anisotropic, as shown in the anisotropic linear thermal expansion. This anisotropy was further reflected in the radically different responses of T0 to B applied in different directions: a few percent change in T0 is induced for B parallel to the c axis, whereas T0 barely changes for B applied in the abplane of the URu2−xOsxSi2 single crystals, regardless of whether the crystals are in the HO or LMAFM phase.