Triplet and in-gap magnetic states in the ground state of the quantum frustrated FCC antiferromagnet Ba 2 YMoO 6 J. P. Carlo, 1, 2 J. P. Clancy, 1 T. Aharen, 3 Z. Yamani, 2 J. P. C. Ruff, 1 J. Wagman, 1 G. J. Van Gastel, 1 H. M. L. Noad, 1 G. E. Granroth, 4 J. E. Greedan, 3, 5 H. A. Dabkowska, 5 and B. D. Gaulin 1, 5, 6 1 Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1 Canada 2 Canadian Neutron Beam Centre, National Research Council, Chalk River, ON K0J 1J0 Canada 3 Department of Chemistry, McMaster University, Hamilton, ON L8S 4M1 Canada 4 Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA 5 Brockhouse Institute for Materials Research, McMaster University, Hamilton, ON L8S 4M1 Canada 6 Canadian Institute for Advanced Research, Toronto, ON M5G 1Z8 Canada (Dated: August 25, 2011) The geometrically frustrated double perovskite Ba2YMoO6 is characterized by quantum s=1/2 spins at the Mo 5+ sites of an undistorted face-centered cubic (FCC) lattice. Previous low- temperature characterization revealed an absence of static long-range magnetic order and suggested a non-magnetic spin-singlet ground state. We report new time-of-flight and triple-axis neutron spec- troscopy of Ba2YMoO6 that shows a 28 meV spin excitation with a bandwidth of ∼4 meV, which vanishes above ∼125 K. We identify this as the singlet-triplet excitation out of a singlet ground state, and further identify a weaker continuum of magnetic states within the gap, reminiscent of spin-polaron states arising due to weak disorder. Geometrically frustrated magnetic materials [1, 2] are of great topical interest due to the complex interplay between competing interactions resulting in rich phase diagrams, including spin-glass, spin-ice and spin-liquid ground states. Triangular and tetrahedral architectures are most often associated with geometric frustration, al- though the phenomenon occurs in diverse systems with various lattices, magnetic interactions, and anisotropies. In two dimensions (2D), networks of edge- and corner- sharing triangles give rise to the triangular and kagome lattices, respectively, while in three dimensions (3D), tetrahedral networks form the face-centered cubic (FCC) and pyrochlore lattices. Frustrated lattices of antiferromagnetically (AF) cou- pled moments have been studied in a variety of mate- rials. Well-studied 2D systems, consisting of loosely- coupled stacks of planes, include the triangular magnets NaCrO 2 [3] and VCl 2 [4], Kagome magnets such as Her- bertsmithite [5] and several Jarosite AFs (e.g. [6]). Other quasi-2D magnetic materials and models which possess competing interactions exist, with resulting physics very similar to that originating from geometrical frustration, including the so-called J 1 -J 2 systems [7], square planar lattices decorated by magnetic moments with opposing nearest-neighbor and next-nearest-neighbor interactions. One such system of topical interest is SrCu 2 (BO 3 ) 2 [8, 9], an experimental realization of the Shastry-Sutherland s= 1 / 2 Heisenberg model [10], with moments on a pla- nar lattice of orthogonally oriented dimers. Each dimer, composed of two s= 1 / 2 Cu 2+ moments, exhibits a sin- glet ground state with an s=1 triplet excitation above aΔ ∼ 3 meV gap. In 3D, well-studied frustrated sys- tems include the rare earth titanates, in which magnetic moments reside on essentially perfect pyrochlore lattices [11], and exhibit a wide variety of ground states including spin ice [12, 13], long-range order (LRO) [14, 15], field- induced order [16] and spin liquid [17, 18]. Both classical and quantum spins decorating these lattices have been, and are, of interest. But the quantum versions can give rise to exotic disordered spin-liquid states, as may be rel- evant to resonating valence bond states [19]. While experimental and theoretical works on classical and quantum quasi-2D triangular and kagome magnets and 3D pyrochlore magnets abound, there are very few studies of quantum FCC frustrated systems. In rock-salt ordered double perovskites [20] (Fig. 1(a)) the magnetic moments comprise an edge-sharing tetrahedral network (Fig. 1(c)). While most are not perfect s= 1 / 2 FCC sys- tems (e.g. [21, 22]), experimental studies have revealed a wealth of ground states. The s= 3 / 2 systems La 2 LiRuO 6 and Ba 2 YRuO 6 exhibit AF LRO [23]. Analogous s=1 systems show spin freezing without LRO in Ba 2 YReO 6 , and a collective singlet state in La 2 LiReO 6 [24]. The ex- treme quantum s= 1 / 2 case is realized in Sr 2 CaReO 6 [25], La 2 LiMoO 6 and Ba 2 YMoO 6 [26]. While the first two exhibit short-range magnetic correlations without LRO, only Ba 2 YMoO 6 maintains cubic symmetry to 2 K and shows no signs of magnetic order in NMR, muon spin relaxation, neutron diffraction or susceptibility measure- ments [26–28], making it an excellent realization of a quantum FCC antiferromagnet. Ba 2 YMoO 6 was characterized in depth by Aharen et al. [26] Y and Mo ions lie on alternate B sites in an NaCl-like arrangement with only ∼3% B-site disor- der, so that the magnetic Mo 5+ ions form a lattice of edge-sharing tetrahedra. Bulk susceptibility measure- ments show high-temperature AF C-W behavior with Θ W = -219 K, and some deviation from C-W at lower temperatures. However, susceptibility, heat capacity and muon spin relaxation measurements found no evidence arXiv:1105.3457v3 [cond-mat.str-el] 23 Aug 2011