Neutron Larmor diffraction investigation of the rare-earth pyrochlores R2Ti2O7 ( R=Tb , Dy, Ho

Abstract

In this work, we present a neutron Larmor diffraction study of the rare-earth pyrochlores R 2 Ti 2 O 7 with R = Tb, Dy, Ho. We measured the temperature dependence of the lattice parameter with precision 10 −5 , between 0.5 and 300 K in each of the three compounds. The lattice parameter of the spin ices Dy 2 Ti 2 O 7 and Ho 2 Ti 2 O 7 enters into the derivation of the charge of the emergent magnetic monopole excitations suggested to exist in these materials. We found that throughout the range of applicability of the theory of emergent monopoles in the spin ices there will be no renormalization of the monopole charge due to lattice contraction. In Tb 2 Ti 2 O 7 , strong magnetoelastic interactions have been reported. We found no sign of the previously reported negative thermal expansion, but did observe anomalies in the thermal expansion that can be correlated with previously observed interactions between phonon and crystal-field excitations. Other features in the thermal expansion of all three compounds can be related to previously observed anomalies of the elastic constants, and explained by the phonon band structure of the rare-earth titanates. The temperature dependence of the lattice strain in all three compounds can be correlated with the thermal population of excited crystal-field levels.

Thermodynamic properties of quadrupolar states in the frustrated pyrochlore magnet Tb2Ti2O7

“…It is thus apparently different from the magnetic dipole order inferred by earlier theories [7,8]. This mysterious, or hidden, order is an important subject for the study of the actual nature of the ground state of Tb 2 Ti 2 O 7 [9][10][11][12][13][14][15][16][17][18].…”

Disorder and Quantum Spin Ice

“…The concentration of defects can be estimated to c = 1.1(5) · 10 −3 , when averaged over the 7 measured Bragg peaks. This yields an average volume change induced by one defect of ∆V /V 1.3 · 10 −3 , namely an order of magnitude hardly measurable by conventional diffraction (but which could be accessed by neutron Larmor diffraction, which allows measuring the intrinsic width of the unit cell volume distribution [34,35]).…”

Magnetoelastic excitation spectrum in the rare-earth pyrochlore Tb2Ti2O7

“…An alternative theory based on a splitting of the groundstate doublet into two singlets by a hypothetical static [57] or dynamical [58] Jahn-Teller distortion has been compared with a comprehensive neutron diffraction experiment, and reproduces well the observed magnetization curve without plateau (at T ≈ 0.07 K) [49], it also quite successfully reproduces the diffuse scattering [38]. However, it has been criticized because there is no clear structural [59][60][61][62][63] or spectroscopic [64,65] evidence for any distortion, and because it cannot reproduce either the large elastic magnetic spectral weight [65] or the hyperfine contribution to the specific heat [22], both of which require a permanent magnetic moment at the terbium site.…”

“…Later, a systematic search for a distortion using xray diffraction down to 0.3 K was made, and although various Bragg peaks were observed to broaden as if approaching a cubic to tetragonal distortion, no distortion actually occurred [60]. Other x-ray diffraction [61,62], neutron Larmor diffraction [63], and thermal expansion [69] experiments have corroborated the absence of a low-temperature distortion down to at least 0.5 K, but show unusual thermal expansion behavior below 20 K.…”