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Article Dans Une Revue Nature Physics Année : 2020

Destabilization of hidden order in URu2Si2 under magnetic field and pressure

Résumé

T he focus of much research on correlated-electron quantum materials is the competition between ordered and disordered electronic states, their tuning via quantum phase transitions and their possible relationship with unconventional superconductivity 1-6. Amongst these materials, URu 2 Si 2 occupies a special place, due to the richness of its quantum electronic properties and the challenge of understanding them 7. Discussions and controversies still exist about the nature of the 'hidden-order' (HO) phase, which is established below the temperature T 0 = 17.5 K. Efforts have been devoted to understand its order parameter, but so far none has been successful in determining it unambiguously. The transition at T 0 leads to strong physical responses, such as a large jump in the heat capacity 8 , an enhancement and a sudden gapping of intersite magnetic fluctuations 9,10 , a boost of the carrier mobility and a modification of the Fermi surface 11-14 and the opening of an electronic gap 15,16. While neutron diffraction experiments 17 showed a small antiferromagnetic (AF) moment below T 0 , most now accept that ambient-pressure antiferromagnetism is an extrinsic property induced by crystal defects and distortions 18. One proposal, motivated by Fermi-surface studies under pressure, is that the HO has the same periodicity of wavevector k 0 = (0 0 1) as the AF phase stabilized under pressure 19. The symmetry of the HO has also been considered: torque 20 and diffrac-tion 21 experiments lead to the proposal of a nematic behaviour in the HO phase. However, this picture faces a series of contradictory diffraction 22,23 , NMR 24 and thermodynamic 25 experiments, and the question of the symmetry remains open. Beyond experiments , many theories have tried to describe the HO within different approaches: localized, itinerant or dual (localized/itinerant) f-electrons, multipolar order, nematicity, spin liquid, 'hastatic' order and so on 26-30. It is difficult to find experimental evidence, in a direct and unambiguous manner, for the order parameters proposed by these models. To reveal the physics of the HO, an alternative route is to study how it can be destabilized by tuning external parameters 5. In the HO state, gapped low-energy magnetic fluctuations peaked at the wavevectors k 0 and k 1 = (0.6 0 0) (refs. 9,10) can be seen as precursors of long-range magnetic ordering with the same wavevectors and indicate nearby quantum phase transitions. In heavy-fermion paramagnets, such intersite magnetic fluctuations often indicate the proximity of a magnetically ordered phase. For instance, in the textbook CeRu 2 Si 2 case quantum magnetic fluctuations with wave-vector k 1 = (0.31 0 0) can be transformed into long-range magnetic order with the same wavevector by La doping 31. As well as doping, pressure and magnetic field are ideal tools to tune the electronic interactions. In URu 2 Si 2 , hydrostatic pressure 32-34 leads to a phase transition from the HO phase towards an AF state, with the wave-vector k 0 and the moments aligned along the c axis of the tetrago-nal crystal. At ambient pressure, under a magnetic field from 35 to 39 T applied along c, the HO is replaced by a spin-density wave (SDW), where the magnetic moments are ordered with the wave-vector k 1 and aligned along c (refs. 35,36). Remarkably, experiments under combined magnetic field and pressure, in the limited ranges 16 T/2 GPa (ref. 37) and 45 T/1 GPa (ref. 38), showed that a magnetic field applied to the pressure-induced AF phase induces a return to the HO phase 37 , and that the SDW phase shrinks and moves to higher fields under pressure 38. The relative stability of these different types of order is clearly complicated, and a full description of how URu 2 Si 2 evolves under combined pressure and magnetic field is needed. Here, we present the three-dimensional (3D) magnetic-field-pressure-temperature phase diagram of URu 2 Si 2. Its magnetoresis-tivity was measured in magnetic fields up to 60 T combined with pressures up to 4 GPa. We find a rich phase diagram indicating a subtle competition between the electronic interactions. The main features are the disappearance of the field-induced SDW phase and a squeezing out of the HO phase under high pressure. We emphasize that many of the boundaries of the 3D phase diagram are controlled by the field and pressure dependences of a single parameter characterizing the electronic correlations. The mystery of the hidden-order phase in the correlated electron paramagnet URu 2 Si 2 is still unsolved. To address this problem, one strategy is to search for clues in the subtle competition between this state and neighbouring magnetically ordered states. It is now well established that long-range antiferromagnetic order can be stabilized in this metal when it is under pressure and that a spin-density wave manifests when a magnetic field is applied along the easy magnetic axis c. However, the full boundaries of the hidden-order phase in the pressure-magnetic-field plane have not been determined so far. Here we present a systematic investigation of URu 2 Si 2 under combined high pressures and intense magnetic fields. The boundaries of the hidden-order, antiferromagnetic and spin-density-wave phases are mapped out, indicating an intricate three-dimensional phase diagram. We show that the field-induced spin-density-wave and hidden-order phases disappear in favour of antiferromagnetism at high pressure. Interestingly, a large number of phase boundaries are controlled by the field and pressure dependences of a single parameter. This gives new constraints for theories that model the electronic correlations and ordered phases in URu 2 Si 2. NAtURe PhySicS | VOL 16 | SEPTEMbER 2020 | 942-948 | www.nature.com/naturephysics 942
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hal-02988470 , version 1 (24-11-2020)

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William Knafo, S. Araki, G. Lapertot, D. Aoki, G. Knebel, et al.. Destabilization of hidden order in URu2Si2 under magnetic field and pressure. Nature Physics, 2020, 16 (9), pp.942-948. ⟨10.1038/s41567-020-0927-4⟩. ⟨hal-02988470⟩
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