Triplet and in-gap magnetic states in the ground state of the quantum frustrated fcc antiferromagnet Ba${}_2$YMoO${}_6$

The geometrically frustrated double perovskite Ba${}_2$YMoO${}_6$ is characterized by quantum $s=1/2$ spins at the Mo${}^{5+}$ sites of an undistorted fcc lattice. Previous low-temperature characterization revealed an absence of static long-range magnetic order and suggested a nonmagnetic spin-singlet ground state. We report unique time-of-flight and triple-axis neutron spectroscopy of Ba${}_2$YMoO${}_6$ that shows a 28 meV spin excitation with a bandwidth of $\sim$4 meV, which vanishes above $\sim$125 K. We identify this as the singlet-triplet excitation that arises 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.

Figure 1

(a) Unit cell of Ba${}_2$YMoO${}_6$, with Y and Mo occupying alternate $B$ sites. (b) Schematic diagram depicting frustration of AF-coupled moments on a tetrahedron, and possible formation of orthogonal singlet dimers. (c) Mo${}^{5+}$ ions in the lattice in (a), forming a network of edge-sharing tetrahedra.

Figure 2

(a), (c), (d) Dynamic susceptibility $\Delta {\chi }^{\prime \prime }(Q,\hslash \omega )$ at $T=6$, 70, and 125 K, where ${\chi }^{\prime \prime }$($Q,\hslash \omega$) at $T=175$ K has been subtracted from each to isolate the magnetic scattering, as described in the text. (b) shows $\Delta {\chi }^{\prime \prime }(Q,\hslash \omega )$ at $T=6$ K with $T=175$ K subtracted, but with the plotted intensity scale range restricted to $>$0 only, thus highlighting where ${\chi }^{\prime \prime }(Q,\hslash \omega )$ at 6 K exceeds that at 175 K. The lower intensity scale refers to (a), (c), and (d), and the upper refers to (b).

Figure 3

(a) ${\chi }^{\prime \prime }$($Q,\hslash \omega$) plotted versus $Q$ for six temperatures, integrated in energy between 26 and 31 meV. (b) ${\chi }^{\prime \prime }$($Q,\hslash \omega$) plotted versus energy for six temperatures, integrated in $Q$ over the range 1.5 Å${}^{-1}<Q<1.8$ Å${}^{-1}$. The scattering centered on $\sim$28 meV exists only at low $Q<2.5$ Å${}^{-1}$ and at low $T<125$ K, and is therefore magnetic in origin and consistent with a weakly dispersive spin-triplet excitation.

Figure 4

(a) Temperature dependence of the background-subtracted scattering intensity at $Q=1.7$ Å${}^{-1}$ at the average of 27.5 and 30.5 meV, collected with the C5 triple-axis spectrometer, showing a characteristic fall-off of the triplet intensity toward zero at $\sim$125 K; normalized SEQUOIA (SNS) data at 26–31 meV is included for reference. (b) Temperature dependence of the background-subtracted intensity at 7 meV and a 16.5–17 meV energy transfer. The solid lines represent fits of the $T>200$ K data to the thermal occupancy factor. Excess low-temperature scattering is attributed to either (a) the triplet excitation, or (b) magnetic states within the gap.