Crystal field excitations fromYb3+ions at defective sites in highly stuffedYb2Ti2O7

Abstract

The pyrochlore magnet Yb2Ti2O7 has been proposed as a quantum spin ice candidate, a spin liquid state expected to display emergent quantum electrodynamics with gauge photons among its elementary excitations. However, Yb2Ti2O7's ground state is known to be very sensitive to its precise stoichiometry. Powder samples, produced by solid state synthesis at relatively low temperatures, tend to be stoichiometric, while single crystals grown from the melt tend to display weak "stuffing" wherein ∼ 2% of the Yb 3+ , normally at the A site of the A2B2O7 pyrochlore structure, reside as well at the B site. In such samples Yb 3+ ions should exist in defective environments at low levels, and be subjected to crystalline electric fields (CEFs) very different from those at the stoichiometric A sites. New neutron scattering measurements of Yb 3+ in four compositions of Yb2+xTi2−xO7−y, show the spectroscopic signatures for these defective Yb 3+ ions and explicitly demonstrate that the spin anisotropy of the Yb 3+ moment changes from XY-like for stoichiometric Yb 3+ , to Ising-like for "stuffed" B-site Yb 3+ , or for A-site Yb 3+ in the presence of an oxygen vacancy.Exotic magnetic ground states of cubic pyrochlore magnets, with composition A 2 B 2 O 7 , are of great topical interest, as the pyrochlore lattice is one of the canonical architectures supporting geometrical frustration in three dimensions [1,2]. Magnetism can reside at either the A 3+ site or the B 4+ site, and the magnetic moments' anisotropy and the interactions between the moments conspire to give rise to rich ground state selection. Among the states and materials that have been of recent interest have been the classical spin ice states in Dy and Ho titanate pyrochlores [3][4][5][6][7], spin liquid and spin glass states in molybdate pyrochlores [8], and spin fragmentation in Nd based zirconate pyrochlores [9]. The possibility that a quantum analogue of the spin ice ground state, i.e. quantum spin ice (QSI), may exist in certain low moment pyrochlore magnets, including Yb 2 Ti 2 O 7 and Pr 2 Zr 2 O 7 , has generated much excitement [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24].At low temperatures Yb 2 Ti 2 O 7 displays two magnetic heat capacity anomalies: a broad one near 2 K and a sharp anomaly signifying a thermodynamic phase transition near T C = 0.26 K [25][26][27][28][29]. Below T C , the ordered structure is thought to be a splayed ferromagnet with moments pointing close to the (100) directions [30][31][32][33][34]. However, surprising sample variability has been reported in this phase transition, with some studies not seeing direct evidence for the ferromagnetic ordered state [18,[35][36][37][38][39][40][41]. Using the sharp anomaly in C P as the figure-ofmerit for the phase transition, interesting systematics have been observed [28,29,42,43]. Powder samples grown by solid state synthesis at relatively low temperatures show a sharp C P anomaly and a high T C . usually ∼ 0.26 K [28,29,42,44]; however most single crystal studies display broader the...

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

“…1 (b). It is not clear if this weak inelastic scattering is due to the influence of Ce 3+ or Zr 3+ in defective sites [38], on residual Ce 4+ , or on the possible presence of hybridized phonon -crystal field excitations known as vibronic bound states, as has been recently observed in holmium and terbium pyrochlores [39,40]. In any case, this unidentified contribution to the inelastic scattering yields a small fraction of the spectral weight and we conclude the features at 56 and 112 meV are the CEF excitations corresponding to the main Ce 3+ site.…”

Orientation dependence of the magnetic phase diagram of Yb2Ti2O7

“…Here we report such a study of the quantum magnetism of Yb 2 Ti 2 O 7 , a prototypical pyrochlore magnet [5][6][7][8][9][10][11][12][13], in which we find despite geometric frustration and quantum fluctuations anisotropic ferromagnetism at low temperatures that appears to be deceptively simple at first sight. Early studies of Yb 2 Ti 2 O 7 included a diffuse zerofield neutron spectrum [14][15][16][17], as well as unconventional quasiparticles in the paramagnetic phase [18][19][20] which may be preserved to low temperatures in oxygen-deficient samples [13].…”

Спектральные и магнитные свойства ионов Er-=SUP=-3+-=/SUP=- и Yb-=SUP=-3+-=/SUP=- в кристаллах Y-=SUB=-2-=/SUB=-Ti-=SUB=-2-=/SUB=-O-=SUB=-7-=/SUB=- со структурой пирохлора

“…Структура энергетических спектров и g-факторы основного состояния ионов Er 3+ и Yb 3+ как в магнитоконцентрированных, так и в разбавленных соединениях со структурой пирохлора исследовались ранее методами оптической [5], нейтронной [6][7][8] и мессбауэровской [9] спектроскопии. Однако представленные в литературе наборы параметров кристаллического поля как для ионов Er 3+ , так и для ионов Yb 3+ суще-ственно отличаются друг от друга, а вычисляемые с использованием этих параметров либо магнитные, либо спектральные характеристики не согласуются с данными измерений.…”