Phonon Thermal Transport ofURu2Si2: Broken Translational Symmetry and Strong-Coupling of the “Hidden Order” to the Lattice

Abstract

A dramatic increase in the total thermal conductivity ( ) is observed in the hidden order (HO) state of single crystal URu 2 Si 2 . Through measurements of the thermal Hall conductivity, we explicitly show that the electronic contribution to is extremely small, so that this large increase in is dominated by phonon conduction. An itinerant BCS or mean-field model describes this behavior well: the increase in is associated with the opening of a large energy gap at the Fermi surface, thereby decreasing electronphonon scattering. Our analysis implies that the ''hidden order'' parameter is strongly coupled to the lattice, suggestive of a broken symmetry involving charge degrees of freedom. DOI: 10.1103/PhysRevLett.97.156401 PACS numbers: 71.27.+a, 71.28.+d, 72.15.Eb At symmetry breaking (2nd order) phase transitions, an ''order parameter'' must characterize the nonsymmetric, low temperature state [1]. The heavy-fermion compound URu 2 Si 2 undergoes a 2nd order phase transition at 17:5 K, yet the order parameter has remained hidden for over 20 years. This is an example of one of the most fundamental problems in solid state physics: the symmetries and associated order parameters are not obvious in many interesting systems. In fact, the order parameters of antiferromagnets (AF, staggered magnetization) and superconductors (SC, condensate wave function phase) were similarly hidden for many years. New discoveries of such nontrivial hidden-order systems (e.g., 2D XY magnets exhibit order of a purely topological character) thus continually expand our understanding of symmetry in solids.Most conceivable experimental probes have been utilized in an attempt to uncover the HO phase. Yet still, this diverse body of experimental data does not seem to convincingly converge to any existing model [2 -4]. For example, some characteristics of AF ordering occur: a large peak appears in the specific heat. However, the tiny observed staggered moment is inconsistent with the entropy lost at the transition [4], instead more likely due to a small AF impurity-phase [5]. Moreover, due to strong hybridization between the conduction electrons and nearly localized U-5f 2 moments, neither a purely local nor itinerantelectron model can be applied. Indeed, there is strong evidence for a Fermi-liquid comprised of ÿ 5 nonKramers doublet degrees of freedom [6,7].We present another route towards uncovering the HO in URu 2 Si 2 via thermal conductivity ( ) measurements. T undergoes a very large increase at the HO transition. Although this feature has been observed before [8,9], a model for T has not been obtained in a way that elucidates the HO. First, we explicitly show that T is dominated by phonons. We then apply a mean-field or BCS framework to describe ph whereby electron-phonon (e-ph) scattering decreases as a gap ( ) in the electronic density of states (DOS) opens at the HO transition. An unusually large 2 0 =k B T C ratio of 8 is needed in order to best describe the data, in agreement with point contact spectroscopy (PCS) estimates of , that...

Magnetotransport properties of CeRu2Al10 : Similarities to URu2Si2

“…As already mentioned, our observations on κ(T ) of CeRu 2 Al 10 show striking resemblance to those of URu 2 Si 2 : In the hidden-order phase of the latter compound, a large enhancement of κ as well as a field-induced suppression have been observed 5,6 . At T >T h , κ(T ) is insensitive to field, similar to our observations made for CeRu 2 Al 10 .…”

“…Special attention is put on the excess thermal conductance emerging in the ordered phase below T 0 and its strong suppression upon applying magnetic field. Note that, strikingly similar behaviors have ever been observed for another heavy-fermion system, URu 2 Si 2 , in the hidden-order phase below T h = 17.5 K 5,6 . Enhanced phononic thermal conductance arising from a freezing out of relevant scattering centers below T h has been argued to be the origin 5,6 .…”

“…Note that, strikingly similar behaviors have ever been observed for another heavy-fermion system, URu 2 Si 2 , in the hidden-order phase below T h = 17.5 K 5,6 . Enhanced phononic thermal conductance arising from a freezing out of relevant scattering centers below T h has been argued to be the origin 5,6 . This argument, however, is questionable in view of the strong field dependence and because of recent detailed experimental investigations on phonon dynamics by inelastic x-ray scattering measurements 7 .…”

Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

“…Although the microscopic order parameter has not yet been definitively established, a multitude of bulk property measurements indicate that a partial gapping of the Fermi surface is involved in the HO transition. In addition to the BCS-like specific heat anomaly, there are anomalies in the electrical resistivity, magnetic susceptibility [77][78][79], ultrasound [86], thermal expansion [87], and an increase in lattice thermal conductivity [88]. Additional evidence points to the opening of a large gap in the tunneling spectrum [89], the incommensurate spin excitation spectrum [90], and the crossing of the chemical potential by a heavy electron band [91].…”

Hydrostaticity and hidden order: effects of experimental conditions on the temperature–pressure phase diagram of URu2Si2

“…URu 2 Si 2 crystallizes in the tetragonal ThCr 2 Si 2 structure with lattice parameters of 4.13Å and 9.58Å; this structural anisotropy is reflected in the bulk transport and magnetic properties [1,2]. The transition from the paramagnetic (PM) to HO phase is marked by a large specific heat anomaly at 17 K and appears to be associated with a Fermi surface instability [3,4,5]. In the HO phase, a small staggered moment is observed, but its magnitude cannot account for the large entropy liberated by the HO transition [6], leading to speculation that the entropy is instead associated with some hidden order parameter, whose nature still remains elusive.…”