Nuclear magnetic resonance studies of pseudospin fluctuations in URu2Si2

Abstract

We report 29 Si NMR measurements in single crystals and aligned powders of URu2Si2 in the hidden order and paramagnetic phases. The spin-lattice-relaxation data reveal evidence of pseudospin fluctuations of U moments in the paramagnetic phase. We find evidence for partial suppression of the density of states below 30 K, and analyze the data in terms of a two component spin-fermion model. We propose that this behavior is a realization of a pseudogap between the hidden order transition THO and 30 K. This behavior is then compared to other materials that demonstrate precursor fluctuations in a pseudogap regime above a ground state with long-range order. 75.30.Mb, 75.25.Dk, 76.60.Es Despite more than twenty years since its discovery, URu 2 Si 2 continues to attract considerable interest in the condensed matter community. 1,2 This heavy fermion system develops a "hidden" order phase below T HO = 17.5 K, and an unconventional superconducting state below 1.5 K. 1,3 The nature of the hidden order (HO) phase remains controversial, but it clearly does not involve magnetic ordering of dipole moments. 4 It may involve order of higher order multipoles, 5 exotic spin, orbital or spinorbital density waves, 6-10 or hybridization between local moments and conduction electrons. 11-13 Extensive neutron scattering and angle-resolved photoemission work has suggested that it has an itinerant nature and involves some type of Fermi surface instability. 4, [14][15][16][17][18] Recent evidence has suggested that the hidden order is intimately connected with the onset of coherence of the Kondo lattice. [19][20][21][22] In Kondo lattice systems the 5f electrons of the U are partially screened by the conduction electrons, leading to a renormalization of the electronic dispersion near the Fermi level. At high temperatures the 5f electrons remain localized and scatter the itinerant conduction electrons. [19][20][21][22] Below a coherence temperature, T coh , the f electrons hybridize with the conduction electrons, and the electronic dispersion reflects renormalized heavy quasiparticles. In other Kondo lattice systems coherence emerges as a crossover, but recent scanning tunneling microscopy (STM) results suggest that T coh coincides with T HO , and thus the HO parameter is in fact the hybridization gap. 13,21,22 A re-examination and re-analysis of previous thermodynamic and neutron scattering measurements under the context of HO gap fluctuations have revealed the possible existence of a pseudogap occurring before the HO state and starting around 30 K. 23 This analysis highlighted the presence of anomalies in magnetic susceptibility, 6 point contact spectroscopy (PCS), 24 and neutron scattering measurements. 14,25 Other thermodynamic probes (heat capacity, thermal expansion and ultrasound velocity) also register an anomaly between 25-30 K, where these are sensitive to changes in the elastic constants of the crystal lattice. 26-29 A similar temperature scale has been observed in ultrafast and conventional optical spectroscopy, which found a ...

Evidence for a nematic component to the hidden-order parameter in URu2Si2 from differential elastoresistance measurements

“…Extrapolation of the best fit function to lower temperatures (shown as a green line in Fig. 2) reveals significant deviation from CurieWeiss-like behaviour for temperatures below B30 K. The onset of this deviation at 30 K is coincident with a suppression in (T 1 T) À 1 seen in recent NMR experiments 21 , suggestive of a common origin, possibly associated with the progressive development of strongly fluctuating order.…”

Symmetry and correlations underlying hidden order inURu2Si2

“…The carrier density decrease and absence of this larger pocket in the HO phase suggests that the large Z FS is gapped at low temperatures, whereas the experimental similarity of the quantum oscillations, bulk properties, and point contact spectroscopy 79 between HO and AFM phases implies that it is gapped in both HO and AFM states. A charge gap is detected at T > T HO 4,7,9 , which suggests that it is not directly responsible for the entropy change at T HO . Figure 7a shows a model FS inferred from the previous discussion that consists of an electron pocket (green) at Γ Γ Γ and several hole pockets (orange).…”

“…The shape of the specific heat anomaly at T HO resembles the second-order Bardeen-Cooper-Schriefer superconducting transition, suggesting that an energy gap is created in the itinerant electron states 2,3 . Energy gaps in the electronic states are also inferred from many other measurements [4][5][6][7][8][9] , but recent findings suggest that these features really develop at temperatures greater than T HO , so it is likely that these gaps are associated with the development of local-itinerant electron correlations starting at much higher temperatures. These correlations are further associated with magnetic excitations that develop at high temperature but become gapped and dispersing in the HO phase 10,11 .…”

Optical study of hybridization and hidden order in URu2Si2

“…These features have been attributed to a "pseudogap" in analogy with the cuprate superconductors where similar signatures are seen. NMR (T 1 T ) −1 [29] shows a depression growing in depth below 30 K. No new optical May 2, 2018 Philosophical Magazine…”