Fermi surface in the hidden-order state of URu_2Si_2 under intense pulsed magnetic fields up to 81 T

摘要

We present measurements of the resistivity p_(x,x) of URu_2Si_2 high-quality single crystals in pulsed high magnetic fields up to 81 T at a temperature of 1.4 K and up to 60 T at temperatures down to 100 mK. For a field H applied along the magnetic easy axis c, a strong sample dependence of the low-temperature resistivity in the hidden-order phase is attributed to a high carrier mobility. The interplay between the magnetic and orbital properties is emphasized by the angle dependence of the phase diagram, where magnetic transition fields and crossover fields related to the Fermi surface properties follow a l/cosθ law, θ being the angle between H and c. For H ‖ c, a crossover defined at a kink of p_(x,x), as initially reported in [Shishido, Phys. Rev. Lett. 102, 156403 (2009)], is found to be strongly sample dependent: its characteristic field μ_0H~* varies from appro~=20 T in our best sample with a residual resistivity ratio RRR =p_(x,x)(300K)/p_(x,x)(2 K) of 225 to appro~=25 T in a sample with a RRR of 90. A second crossover is defined at the maximum of p_(x,x) at the sample-independent low-temperature (LT) characteristic field μ_0H_(p,max)~(LT) appro~=30 T. Fourier analyses of Shubnikov-de Haas oscillations show that H_(p,max)~(LT) coincides with a sudden modification of the Fermi surface, while H~* lies in a regime where the Fermi surface is smoothly modified. For H ‖ a, (ⅰ) no phase transition is observed at low temperature and the system remains in the hidden-order phase up to 81 T, (ⅱ) quantum oscillations surviving up to 7 K are related to a new orbit observed at the frequency F_λ appro~= 1350 T and associated with a low effective mass m_λ~* = (1 ± 0.5)m_0, where m_0 is the free electron mass, and (ⅲ) no Fermi surface modification occurs up to 81 T.