Session Y25: Novel Ordering and Collective Modes in URu2Si2

Novel Electronic Phases and Competing Interactions in the Correlated f-Electron Compound URu2Si2

The correlated f-electron compound URu₂Si₂ undergoes a second-order transition at T_o = 17.5 K into an ordered phase whose identity has eluded researchers for more than three decades. This so-called "hidden order" (HO) phase coexists with unconventional superconductivity (SC) below T_c ≈ 1.5 K. Application of pressure suppresses the T_c of the SC’ing phase and induces a transition from the HO phase to a large-moment antiferromagnetic (LMAFM) phase at a critical pressure P_c ≈ 1.5 GPa. Our research group found that substitution of isoelectronic Fe or Os for Ru suppresses SC and induces a transition from the HO to the LMAFM phase, similar to the behavior of URu₂Si₂ under pressure. The HO-LMAFM phase transition in URu_2−xFe_xSi₂ was attributed to “chemical pressure” associated with the reduction of the unit cell volume upon substitution of smaller Fe atoms for Ru atoms. This allows the HO and LMAFM phases to be studied in single crystals of URu_2−xFe_xSi₂ at atmospheric pressure with techniques that cannot readily be performed on URu₂Si₂ under high pressure (e.g., ARPES, STM, infrared, Raman, neutron scattering, etc.). In this talk, we will review the status of ongoing research on URu_2−xFe_xSi₂ single crystals in the HO and LMAFM phases, such as infrared spectroscopy, elastic and inelastic neutron scattering, electrical resistivity under pressure, and high field (45 T) magnetoresistance measurements. Interestingly, the substitution of Os for Ru in URu₂Si₂, which also induces the HO-LMAFM transition, expands the unit cell and is inconsistent with the "chemical pressure" hypothesis, suggesting the importance of other factors (e.g., spin-orbit coupling). These investigations should be useful in developing an understanding of the underlying physics of URu₂Si₂-based materials and, perhaps, even unmasking the identity of the elusive HO phase!   

Hidden Orders: The Chirality Density Wave and Orbital Antiferromagnetism in URu2Si2

In the uranium-based compound URu₂Si₂, two long range orders compete below a second order phase transition at about 17.5 kelvin: the mysterious non-magnetic “hidden order”, and an unconventional antiferromagnetic phase. The two orders are strikingly similar thermodynamically, yet symmetry-wise different, and the relation between them pose a thirty years old enigma. In this talk, I will introduce, 1) direct evidence of symmetry breaking in the "hidden order" probed by Raman spectroscopy, and 2) a new type of collective mode associated with the two competing orders.
In our study, we tune the "hidden order" into antiferromagnetism by substituting iron for ruthenium. In the “hidden order” phase, we discovered evidence of local vertical and horizontal reflection symmetries breaking at the uranium sites, leading to staggering of chiral 5f electron states, forming a commensurate Chirality Density Wave as the ground state. Above critical Fe concentration, staggering of orbital magnetic moment from the 5f electrons leads to an antiferromagnetic ground state. We drive and detect dynamical oscillations between the "hidden order" and the antiferromagnetic ground states using polarized light, and as such provide direct experimental evidence for an unified order parameter describing the two competing phases. The dynamical oscillations between the two phases present a new type of collective mode, which softens at the first order phase boundary.
Main References:
[1] H.-H. Kung et al., Science 347, 1339 (2015)
[2] H.-H. Kung et al., PRL 117, 227601 (2016)   

Hidden order signatures in the antiferromagnetic phases of URu2Si2 under chemical and hydrostatic pressure

URu₂Si₂ has been an intense area of study for the last 30 years due to a mysterious “hidden order” phase that appears below T₀ = 17.5 K. The hidden order phase has been shown to be extremely sensitive to perturbations, being destroyed quickly by the application of a magnetic field, hydrostatic or uniaxial pressure, and chemical doping. While attempting to understand the properties of URu₂Si₂, neutron scattering has found spin correlations that are intimately related to this hidden order phase and which are also suppressed with these perturbations. Here, I will outline some recent neutron scattering work to study these correlations in two exceptional cases where the hidden order phase is enhanced: hydrostatic pressure and chemical pressure using Fe- and Os-doping. In both of these cases, T₀ increases before an antiferromagnetic phase emerges. By performing a careful analysis of the neutron data, we show that these two phases are much more related than had been previously appreciated, with "signature" excitations of the hidden order phase appearing in the antiferromagnetic phases. This implies that the hidden order is likely compatible with an antiferromagnetic ground state, placing constraints on the nature of the missing order parameter.   

Chiral density wave with local hexadecapole order parameter as the hidden order in URu2Si2

Modern spectroscopic techniques successfully determined the type of long range order in most of solids except in very rare cases. The heavy fermion material URu2Si2 is one of a few materials ordered by the ”hidden mechanism”. The ab-initio Dynamical Mean Field Theory calculations for this material predicted a strange hexadecapolar order back in 2009 as a result of unusual multichannel Kondo effect, which is arrested at low temperature by the crystal field splitting [1,2]. The long range order can be characterized by a complex order parameter Psi, whereby a real Psi describes the hidden order phase, and an imaginary Psi corresponds to the large moment antiferromagnetic phase, providing a unified picture of the broken symmetry phases in this material [1,2]. Recently Raman spectroscopy found a clear evidence for such hexadecapolar order as it necessary involves chirality density modulation picked up by Raman signal in unusual A2g symmetry channel.[3] By studying the Fe- doped sample of URu2Si2, in which the antiferromagnetic phase and the hidden order phase compete, we confirmed that the A2g symmetry collective mode, identified as the order parameter, continuously softens towards the critical point, and then reappears in the other side of the phase diagram.[4]
[1] Arrested Kondo efect and hidden order in URu2Si2, Kristjan Haule and Gabriel Kotliar, Nature Physics 5, 796 - 799 (2009).
[2] Complex Landau Ginzburg Theory of the Hidden Order in URu2Si2, Kristjan Haule, Gabriel Kotliar, Eur. Phys. Lett. 89, 57006 (2010).
[3] Chiral density wave of the hidden order phase in URu2Si2, H.-H. Kung, R. E. Baumbach, E. D. Bauer, V. K. Thorsmølle, W.-L. Zhang, K. Haule, J. A. Mydosh and G. Blumberg, Science 347, 1339 (2015).
[4] Analogy Between the “Hidden Order” and the Orbital Antiferromagnetism in URu2−xFexSi2, H.-H. Kung,1, S. Ran, N. Kanchanavatee, V. Krapivin, A. Lee, J. A. Mydosh, K. Haule, M. B. Maple, and G. Blumberg, Phys. Rev. Lett. 117, 227601 (2016).