⁷³Ge-NMR Investigation of the Hidden Order in URu₂Si₂

Abstract

In order to investigate the local symmetry at the Si(4e)-site above and below hidden order temperature T O = 17.5 K in URu 2 Si 2 , we have carried out 73 Ge-nuclear magnetic resonance (NMR) measurement on 10%73 Ge-substituted sample URu 2 (Si 0.9 Ge 0.1 ) 2 . The specific heat measurement on the present sample shows a clear second-order transition, indicating that the hidden order is still a bulk property. Besides, equally split 73 Ge-NMR lines due to the nuclear quadrupole interaction gives evidence for fairly uniform electric-field gradient at the Ge-substituted sites, Although this is the first attempt to investigate the local symmetry at the 4e-site by proving electric interactions, no evidence for symmetry lowering below T O has been obtained at present.The mysterious order at T O = 17.5 K in URu 2 Si 2 , so called the "hidden order", has been a topic of interest and controversy for more than two decades. Recently a new aspect from the space group analysis has been presented, 1) in which the authors indicated symmetry lowing of the system below T O is characterized by changes in local charge distribution without causing any kind of lattice distortion. From experimental point of view, in order to examine theoretical predictions, one needs information about how symmetry of the system changes at the second-order phase transition. As for detecting modifications of the local charge distributions, measurements of the nuclear quadrupole interaction is useful, because the nuclear spin I (restricted to I ≥ 1) interacts with the electricfield gradient (EFG) that surrounding charges cause at the observing nuclear position. This approach has been successfully employed in the determination of an ordered structure in the filled Skutterudite compound SmRu 4 P 12 . Namely 101 Runuclear quadrupole resonance (NQR) line shape is clearly split into two peaks below the second-order phase transition, implying the crystal structure changes to one of the maximal subgroups (R3) of the filled Skutterudite structure. 2)In this paper, we present the first results of 73 Ge-nuclear magnetic resonance (NMR) measurements on 10%73 Gesubstituted sample URu 2 (Si 0.9 Ge 0.1 ) 2 . Here, motivated by the above-mentioned space group analysis, the local symmetry at the Si(4e)-site will be discussed. There are actually many reports of NMR study on URu 2 Si 2 so far (see Ref. [3][4][5] and references therein). However, none of them studied the nuclear quadrupole interactions at the 4e-site, because 29 Si nuclear spin I = 1/2 does not couple to the EFG. In this context, the NMR experiment of isotope 73 Ge (I = 9/2) on the Si-site opens a way to obtain more detailed information about the local symmetry at this site. Note that, according to the prediction of Ref. [1], the possible symmetry lowering at T O in URu 2 Si 2 does not accompanied with any splitting of the three sites, but does with just breakdown of 4-fold symmetry at the U(2a)-and 4e-sites. This might be more difficult to observe * E-mail address: mito@sci.u-hyogo.ac.jp directly, compare...