Hybridization-driven orthorhombic lattice instability in URu2Si2

Abstract

We have measured the elastic constant (C 11 -C 12 )/2 in URu 2 Si 2 by means of high-frequency ultrasonic measurements in pulsed magnetic fields H [001] up to 61.8 T in a wide temperature range from 1.5 to 116 K. We found a reduction of (C 11 -C 12 )/2 that appears only in the temperature and magnetic field region in which URu 2 Si 2 exhibits a heavy-electron state and hidden order. This change in (C 11 -C 12 )/2 appears to be a response of the 5f electrons to an orthorhombic and volume conservative strain field xx -yy with 3 symmetry. This lattice instability is likely related to a symmetry-breaking band instability that arises due to the hybridization of the localized f electrons with the conduction electrons and is probably linked to the hidden-order parameter of this compound. DOI: 10.1103/PhysRevB.88.195150 PACS number(s): 71.27.+a, 62.20.de, 62.65.+k The heavy-fermion compound URu 2 Si 2 exhibits a secondorder phase transition involving uranium's 5f -electron state at T o = 17.5 K, and also exhibits unconventional superconductivity at T c ∼ 1.4 K. Though the transition at T o shows clear anomalies in several thermodynamic quantities, 1-3 the order parameter has not been identified microscopically for more than a quarter of a century; thus, the unknown transition has been called "hidden order (HO)." 4In high magnetic fields (using static or pulsed magnetic field) for H [001] at low temperatures, URu 2 Si 2 undergoes three metamagnetic transitions in the range 35 and 39 T with a collapse of the HO phase, [5][6][7] where the c-axis magnetization increases in three steps and reaches a value of ∼1.5 μ B /U at 60 T and 1.5 K, which is approximately half of the value expected from the localized 5f -electron state of U 3+ (3.62 μ B ) or U 4+ (3.58 μ B ). In addition, the electrical resistivity shows an abrupt decrease above 40 T. This suggests that tuning URu 2 Si 2 by means of a magnetic field decreases the hybridization between 5f and conduction electrons, and, therefore, leads to a reduced effective electron mass. 6,7 Given this situation, it is plausible that the hybridized-electron state observed at low-temperature and in low-magnetic-field regions will change to a relatively light Fermi-liquid state when a high magnetic field is applied along the [001] axis as indicated by the disappearance of the heavy band.7 Thus, the collapse of the HO and the drastic change of the 5f electronic state appear to be strongly connected and suggest that the order parameter of the HO phase is veiled in the screening of the localized 5f electronic state via their strong hybridization with conduction electrons states.In contrast, several recent theoretical models predict that the order parameter of the HO phase is of local nature and based on higher-rank electric and magnetic multipoles. [8][9][10] In particular, the xy(x 2 -y 2 ) electric hexadecapole model, which has been proposed by Kusunose and Harima and also partly related to the theory of Haue and Kotliar, 11 suggests that electric quadrupole moments O xy (=J x J y +...

Symmetry and correlations underlying hidden order inURu2Si2

“…In the high-temperature paramagnetic phase, an underlying symmetry-breaking tendency is inferred from softening in the Γ 3 ultrasonic mode below the coherence temperature 85 , which disappears along with the HO phase in applied high magnetic fields 86 . We observe no signatures of this in the phonon dispersions (Fig.…”

“…6) is due to the significant overlap of strong paramagnetic scattering. Furthermore, the softening observed in ultrasound studies 85,86 appears to be limited to very small Q and does not produce any dramatic effects at higher Q in the acoustic phonons. In contrast, the optic phonons do show some unusual behavior.…”

“…The energies of Raman-active optic phonons show minimal temperature dependence but signs of low-temperature electronphonon coupling 83,84 . Ultrasound studies, which probe the very low-E acoustic phonons, show tendencies toward symmetry-breaking: softening is observed in a volume- conserving, symmetry-breaking mode below 70 K 85 , and this softening disappears when high field destabilizes the HO phase 86 . There is also an increase in thermal conductivity at T HO 87 that has been argued to arise from the electrostatic coupling of the HO parameter to the lattice 88 .…”

Unfolding the physics of URu2Si2 through silicon to phosphorus substitution

“…The development of new experimental techniques and access to ultra-high-purity single crystal specimens has recently advanced our understanding of the ordered states in this compound. For instance, excitations in the A 2 g channel have been identified by electronic Raman spectroscopy as a signature of the hidden order11, while elastoresistance12, resonant ultrasound13 (Ramshaw, B.J., Private communication.) and spectroscopic measurements14 suggest the presence of fluctuations in the B 2 g , B 1 g and E g channels, respectively.…”

Single Crystal Growth of URu2Si2 by the Modified Bridgman Technique

“…The fact that it also hosts unconventional superconductivity makes this material even more attractive. Recent success in producing high quality specimens [5][6][7] has resulted in a surge of experimental work, including electronic Raman spectroscopy [8], elastoresistance [9], resonant ultrasound [10], Kerr rotation [11], X-ray scattering [12], and quantum oscillation measurements [13][14][15], which have provided unprecedented insight into the electronic, ordered state, and superconducting behavior. However, progress is limited by restricted access to high quality specimens, particularly due to challenges that are inherent to the mainstream growth techniques.…”