Quantum Spin Ice Dynamics in the Dipole-Octupole Pyrochlore Magnet Ce2Zr2O7

Abstract

Neutron scattering measurements on the pyrochlore magnet Ce2Zr2O7 reveal an unusual crystal field splitting of its lowest J = 5/2 multiplet, such that its ground state doublet is composed of mJ = ± 3/2, giving these doublets a dipole -octupole (DO) character with local Ising anisotropy. Its magnetic susceptibility shows weak antiferromagnetic correlations with θCW = -0.4(2) K, leading to a naive expectation of an All-In, All-Out ordered state at low temperatures. Instead our low energy inelastic neutron scattering measurements show a dynamic quantum spin ice state, with suppressed scattering near |Q| = 0, and no long range order at low temperatures. This is consistent with recent theory predicting symmetry enriched U(1) quantum spin liquids for such DO doublets decorating the pyrochlore lattice. Finally, we show that disorder, especially oxidation of powder samples, is important in Ce2Zr2O7 and could play an important role in the low temperature behaviour of this material.The rare-earth pyrochlore oxides R 2 B 2 O 7 , where R 3+ and B 4+ consist generally of rare earth and transitionmetal ions respectively, display a wealth of both exotic and conventional magnetic ground states. Their R 3+ ions decorate a network of corner-sharing tetrahedra, one of the archetypes for geometrical frustration in three dimensions. Due to strong crystal electric field (CEF) effects, the nature of the magnetic interactions in such materials are strongly influenced by their single-ion physics [1][2][3]. A naive theoretical description of the magnetic interactions in rare-earth pyrochlores is generally performed by introducing an ad hoc effective single-ion term in addition to Heisenberg exchange interactions. For example, Heisenberg antiferromagnetism with an effective Ising anisotropy leads to non-frustrated All-In, All-Out (AIAO) magnetic order, as seen in several heavy rare earth iridate pyrochlores [4,5] and illustrated in the insert to Fig.1(a). Heisenberg ferromagnetism and an effective Ising anisotropy give rise to a classical spin ice ground state [6], as seen in (Ho,Dy) 2 Ti 2 O 7 [7, 8] and illustrated as the 2I2O local structure in the inset to Fig.1(a). However, to capture all the physics that can arise at low temperatures, the magnetic interactions should be projected into pseudo-spin operators acting solely on the low energy CEF states [3,[9][10][11][12][13]. This procedure has been applied for example in the Yb 3+ [11,14,15] and Er 3+ [12, 16-18] XY pyrochlores where CEF effects give rise to effective S = 1/2 quantum degrees of freedom that interact via anisotropic exchange interactions.More recently, it has been realized that the precise composition of the ground state crystal field doublets in rare-earth pyrochlores is crucial in determining the form of the microscopic Hamiltonian, and in itself, diversifies the possibility of quantum magnetic states [3,19]. This has been appreciated for some time in the case of non-Kramers doublets, based on magnetic ions with an even number of electrons such as the 4f 2 configuratio...

Long-range magnetic order in the ${\tilde S}=1/2$ triangular lattice antiferromagnet KCeS$_2$

“…It can possibly originate from a minority of Ce ions in a different surrounding, for instance in the vicinity of crystal defects such as stacking faults that are typical for the layered delafossite structure [56][57][58]. Similar additional CEF lines have been previously observed, for instance, in the isostructural NaYbO 2 [25], NaYbS 2 [19], NaYbSe 2 [28], and in the Ce pyrochlore Ce 2 Zr 2 O 7 [59].…”

Spectroscopic insight into the interplay between structural disorder and oxidation degree in melt‐grown Ce0.5Zr0.5O2‐y compounds

“…However, the Raman spectra show additional broad bands at ~810 and 720 cm −1 . In fact, similar features were observed by inelastic neutron scattering in Ref . and attributed to vibronic (phonon + CF transition) combinations.…”

“…Refs . show that two CF transitions are expected within the 2 F 5/2 manifold, the already mentioned transition at 448 cm −1 to the first excited doublet and another one at 880 cm −1 to the second excited doublet, in very good agreement with the highest wavenumber band of Figure .…”

“…However, the band does not fulfil polarisation rules for a T 2g mode. In a remarkable coincidence, a crystal field (CF) transition within the 2 F 5/2 ground state multiplet of Ce 3+ has been found by inelastic neutron scattering at the very same energy . It is strongly appealing to see whether the two kinds of excitations are superposed in Raman spectra and whether they can be resolved in low temperature measurements.…”

“…Polarisation measurements at low temperature (77 K) have enabled the identification of electronic Raman transitions between crystal field split levels of Ce 3+ not hitherto studied. Although far from the scope of this work, we note that Ce 2 Zr 2 O 7 has received interest in recent times because of its magnetic properties as a “quantum spin ice” or “quantum spin liquid,” so that providing an accurate analysis of vibrational, electronic, and mixed character excitations may contribute to a deeper understanding of the physical properties of this compound at low temperatures.…”