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Register a new userURu$_2$Si$_2$ under intense magnetic fields: from hidden order to spin-density wave
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A review of recent state-of-the-art pulsed field experiments performed on URu$_2$Si$_2$ under a magnetic field applied along its easy magnetic axis $mathbf{c}$ is given. Resistivity, magnetization, magnetic susceptibility, Shubnikov-de Haas, and neutron diffraction experiments are presented, permitting to emphasize the relationship between Fermi surface reconstructions, the destruction of the hidden-order and the appearance of a spin-density wave state in a high magnetic field.
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We present measurements of the resistivity $rho_{x,x}$ 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/$costheta$-law, $theta$ being the angle between textbf{H} and textbf{c}. For $mathbf{H}parallelmathbf{c}$, a crossover defined at a kink of $rho_{x,x}$, as initially reported in [Shishido et al., Phys. Rev. Lett. textbf{102}, 156403 (2009)], is found to be strongly sample-dependent: its characteristic field $mu_0H^*$ varies from $simeq20$~T in our best sample with a residual resistivity ratio RRR of $225$ to $simeq25$~T in a sample with a RRR of $90$. A second crossover is defined at the maximum of $rho_{x,x}$ at the sample-independent characteristic field $mu_0H_{rho,max}^{LT}simeq30$~T. Fourier analyzes of SdH oscillations show that $H_{rho,max}^{LT}$ coincides with a sudden modification of the Fermi surface, while $H^*$ lies in a regime where the Fermi surface is smoothly modified. For $mathbf{H}parallelmathbf{a}$, 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 $F_lambdasimeq1400$~T and associated with a low effective mass $m^*_lambda=(1pm0.5)cdot m_0$, and iii) no Fermi surface modification occurs up to 81~T.
A second-order phase transition is associated with emergence of an order parameter and a spontaneous symmetry breaking. For the heavy fermion superconductor URu$_2$Si$_2$, the symmetry of the order parameter associated with its ordered phase below 17.5 K has remained ambiguous despite 30 years of research, and hence is called hidden order (HO). Here we use polarization resolved Raman spectroscopy to specify the symmetry of the low energy excitations above and below the HO transition. These excitations involve transitions between interacting heavy uranium 5f orbitals, responsible for the broken symmetry in the HO phase. From the symmetry analysis of the collective mode, we determine that the HO parameter breaks local vertical and diagonal reflection symmetries at the uranium sites, resulting in crystal field states with distinct chiral properties, which order to a commensurate chirality density wave ground state.
Quantum materials are epitomized by the influence of collective modes upon their macroscopic properties. Relatively few examples exist, however, whereby coherence of the ground-state wavefunction directly contributes to the conductivity. Notable examples include the quantizing effects of high magnetic fields upon the 2D electron gas, the collective sliding of charge density waves subject to high electric fields, and perhaps most notably the macroscopic phase coherence that enables superconductors to carry dissipationless currents. Here we reveal that the low temperature hidden order state of URu$_2$Si$_2$ exhibits just such a connection between the quantum and macroscopic worlds -- under large voltage bias we observe non-linear contributions to the conductivity that are directly analogous to the manifestation of phase slips in one-dimensional superconductors [1], suggesting a complex order parameter for hidden order
The observation of Ising quasiparticles is a signatory feature of the hidden order phase of URu$_2$Si$_2$. In this paper we discuss its nature and the strong constraints it places on current theories of the hidden order. In the hastatic theory such anisotropic quasiparticles are naturally described described by resonant scattering between half-integer spin conduction electrons and integer-spin Ising moments. The hybridization that mixes states of different Kramers parity is spinorial; its role as an symmetry-breaking order parameter is consistent with optical and tunnelling probes that indicate its sudden development at the hidden order transition. We discuss the microscopic origin of hastatic order, identifying it as a fractionalization of three body bound-states into integer spin fermions and half-integer spin bosons. After reviewing key features of hastatic order and their broader implications, we discuss our predictions for experiment and recent measurements. We end with challenges both for hastatic order and more generally for any theory of the hidden order state in URu$_2$Si$_2$.
At T$_0$ = 17.5 K an exotic phase emerges from a heavy fermion state in {ur}. The nature of this hidden order (HO) phase has so far evaded explanation. Formation of an unknown quasiparticle (QP) structure is believed to be responsible for the massive removal of entropy at HO transition, however, experiments and ab-initio calculations have been unable to reveal the essential character of the QP. Here we use femtosecond pump-probe time- and angle-resolved photoemission spectroscopy (tr-ARPES) to elucidate the ultrafast dynamics of the QP. We show how the Fermi surface is renormalized by shifting states away from the Fermi level at specific locations, characterized by vector $q_{<110>} = 0.56 pm 0.08$ {an}. Measurements of the temperature-time response reveal that upon entering the HO the QP lifetime in those locations increases from 42 fs to few hundred fs. The formation of the long-lived QPs is identified here as a principal actor of the HO.
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