Fermi surface in the hidden-order state of URu${}_2$Si${}_2$ under intense pulsed magnetic fields up to 81~T

Abstract

We present measurements of the resistivity $\ddot{I}{}_{x,x}$ of URu2Si2 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 \textbf{H} applied along the magnetic easy-axis \textbf{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 1/$\cos \hat{I}¸$-law, $\hat{I}¸$ being the angle between \textbf{H} and \textbf{c}. For $\mathbf{H}\parallel \mathbf{c}$, a crossover defined at a kink of $\ddot{I}{}_{x,x}$, as initially reported in [Shishido et al., Phys. Rev. Lett. \textbf{102}, 156403 (2009)], is found to be strongly sample-dependent: its characteristic field $\hat{I}{¼}_0{H}^{\ast }$ varies from $\simeq 20$~T in our best sample with a residual resistivity ratio RRR of $225$ to $\simeq 25$~T in a sample with a RRR of $90$. A second crossover is defined at the maximum of $\ddot{I}{}_{x,x}$ at the sample-independent characteristic field $\hat{I}{¼}_0{H}_{\ddot{I},max}^{LT}\simeq 30$~T. Fourier analyzes of SdH oscillations show that $H_{\ddot{I},max}^{LT}$ coincides with a sudden modification of the Fermi surface, while $H^{\ast }$ lies in a regime where the Fermi surface is smoothly modified. For $\mathbf{H}\parallel \mathbf{a}$, 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 and almost-spherical orbit - for the first time observed here - at the frequency $F_{\hat{I}}»\simeq 1400$~T and associated with a low effective mass $m_{\hat{I}}^{\ast }»=(1\pm 0.5)\cdot m_0$, and iii) no Fermi surface modification occurs up to 81~T.

11 pages, 8 figures