Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session Y68: Multipolar Quantum Spin Liquids in Pyrochlore LatticeInvited
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Sponsoring Units: GMAG Chair: Bella Lake, Helmholtz Zentrum Berlin for Materialien und Energie Room: Room 420 |
Friday, March 10, 2023 8:00AM - 8:36AM |
Y68.00001: Ground State Selection in the Dipole-Octupole Pyrochlores Ce2Zr2O7 and Ce2Sn2O7 Invited Speaker: Bruce D Gaulin The Ce2X2O7 pyrochlores, with X = Zr and Sn, are insulators with Ce3+ degrees of freedom decorating a pyrochlore lattice, an archetype for geometrical frustration in three dimensions. High energy neutron spectroscopy reveals that the crystal field ground state originating from J=5/2 Ce3+ is comprised exclusively of mJ=±3/2, which gives the resulting Seffective=1/2 degree of freedom both a dipole and and an octupole character [1, 2, 3]. It also imprints a near-neighbour XYZ spin Hamiltonian onto this system, and such interacting Seffective=1/2 degrees of freedom on the pyrochlore lattice are theoertically known to allow for both U(1) spin liquid ground states, and ordered All-In All Out (AIAO) ground states, both of which can have either dipolar or octupolar character. I will review our recent experimental work on both large single crystals of Ce2Zr2O7 and on small single crystals and powder samples of Ce2Sn2O7. Our measurements on Ce2Zr2O7 above T=0.06 K show no evidence of an ordered state [4]. Theoretical modelling of heat capacity data, magnetic susceptibility data and neutron scattering allows for estimates of the terms in the near-neighbour XYZ Hamiltonian, and these strongly support a U(1) quantum spin liquid ground state for Ce2Zr2O7 [4]. A similar analysis on related experimental data on Ce2Sn2O7 shows it to display a dipolar spin ice state at T∼ 0.1 K, proximate to an AIAO dipolar Neel ground state [5]. I will review these recent results in the context of related experimental studies [2,6]. |
Friday, March 10, 2023 8:36AM - 9:12AM |
Y68.00002: Sleuthing out exotic quantum spin liquidity in the spin-orbit coupled pyrochlore magnet Ce2Zr2O7 Invited Speaker: Hitesh J Changlani The search for quantum spin liquids (QSLs) -- topological magnets with fractionalized excitations -- has been a central theme in condensed matter and materials physics. While theories are plentiful, tracking them down in materials has turned out to be tricky because of the difficulty to diagnose experimentally a state with only topological, rather than conventional, forms of order. Materials with pyrochlore geometries have proven particularly promising, hosting a classical Coulomb phase in the spin ices Dy/Ho2Ti2O7, with subsequent proposals of candidate QSLs in other pyrochlores (e.g. NaCaNi2F7). On this front, connecting experiments with detailed theory to demonstrate the existence of a robust QSL has remained a central challenge. I will present work motivated by recent thermodynamic and neutron scattering experiments on the pseudo spin-1/2 pyrochlore Ce2Zr2O7 where we identified its microscopic effective Hamiltonian through a combination of finite temperature Lanczos, Monte Carlo and spin dynamics calculations [1]. The magnetic properties of Ce2Zr2O7 emerge from interactions between cerium (Ce3+) ions, whose ground state doublet (with J = 5/2,mJ = ±3/2), with a dipole-octupole character, arises from strong spin orbit coupling and crystal field effects. The Hamiltonian parameter values suggest a previously theorized but unobserved exotic phase, a π-flux U(1) QSL, and it allows us to predict its response to an applied magnetic field. The continuum seen in the dynamical structure factor, consistent with the existence of a gapless QSL, is largely suppressed on the introduction of a magnetic field, giving way to Bragg peaks. However, the absence of any dispersive modes is strongly reflective of the octupolar nature of the low energy modes, a finding that can be directly tested in neutron experiments. We suggest that the octupolar nature of the moments makes them less prone to be affected by crystal imperfections or magnetic impurities, while also hiding some otherwise characteristic signatures from neutrons, making this QSL arguably more stable than its more conventional counterparts. |
Friday, March 10, 2023 9:12AM - 9:48AM |
Y68.00003: Octupolar correlations and spinon spectrum in Ce2Sn2O7 quantum spin ice Invited Speaker: Romain Sibille A correlated liquid state was reported in the cerium stannate pyrochlore Ce2Sn2O7 [1]. Its true nature remained elusive, but recent works on Ce2Sn2O7 [2] and Ce2Zr2O7 [3-4] have further investigated the case of cerium pyrochlores based on degrees of freedom having both magnetic dipole and magnetic octupole components. Although these works vary in their details, they [2,4-6] agree towards a quantum spin ice (QSI) based on a manifold of ice-rule correlated octupoles. Earlier theoretical works [7-8] had conceptualised this octupolar QSI, where quantum dynamics is endowed by weaker couplings between other components of the ‘dipole-octupole’ pseudo-spins. |
Friday, March 10, 2023 9:48AM - 10:24AM |
Y68.00004: Unveiling dynamical signatures of dipolar-octupolar quantum spin ice Invited Speaker: Yong-Baek Kim Recent experiments on Ce2Zr2O7 suggest that this material may host a novel form of quantum spin ice, a three-dimensional quantum spin liquid with an emergent photon. The Ce3+ local moments on the pyrochlore lattice are described by pseudospin 1/2 degrees of freedom, whose components transform as dipolar and octupolar moments under symmetry operations. In principle, there exist four possible quantum spin ice regimes, depending on whether the Ising component is in the dipolar/octupolar channel, and two possible flux configurations of the emergent gauge field. We theoretically investigate dynamical spin structure factors in all four quantum spin ice regimes using computations on quantum and classical models, which include exact diagonalization, molecular dynamics, and gauge mean-field theory. Contrasting the distinct signatures of quantum and classical results for the four possible quantum spin ice regimes and elucidating the role of quantum fluctuations, we show that the quantum dynamical structure factor computed for the π-flux octupolar quantum spin ice regime is most compatible with the neutron scattering results on Ce2Zr2O7. We also suggest an ultrasound experiment that can detect the emergent photon excitations in this system. |
Friday, March 10, 2023 10:24AM - 11:00AM |
Y68.00005: Cerium pyrochlores as multipolar spin liquid candidates Invited Speaker: Owen M Benton Rare earth pyrochlore oxides R2M2O7 have long been thought of as likely candidates to realize three-dimensional quantum spin liquids (QSLs), with highly entangled ground states and fractionalised excitations. Recent experiments have suggested that such a realization may have been achieved in the cerium pyrochlores, Ce2Sn2O7 and Ce2Zr2O7. One reason for the interest in these materials is that the exchange Hamiltonian describing their low energy physics is unusually simple and unusually supportive of QSL ground states. The physics is further enriched by the role of multipolar moments, which allows the existence of octupolar QSLs in which the dominant correlations are those of the cerium octupole moments. In this talk, I will review theoretical and experimental progress in understanding Ce pyrochlores and related materials. I will discuss the ground state phase diagram of the minimal exchange Hamiltonian, and how real materials can be located on this phase diagram by comparing experimental measurements to numerical calculations, leading to the inference that Ce2Zr2O7 realises an exotic π-flux U(1) QSL. This inference nevertheless stops short of an unambiguous demonstration that the QSL state has been realised, and I will discuss how such an experimental demonstration can be achieved. |
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