Neutron spectroscopic study of crystal field excitations in Tb2Ti2O7 and Tb2Sn2O7

We present time-of-flight inelastic neutron scattering measurements at low temperature on powder samples of the magnetic pyrochlore oxides Tb2Ti2O7 and Tb2Sn2O7. These two materials possess related, but different ground states, with Tb2Sn2O7 displaying "soft" spin ice order below Tn~0.87 K, while Tb2Ti2O7 enters a hybrid, glassy spin ice state below Tg~0.2 K. Our neutron measurements, performed at T=1.5 K and 30 K, probe the crystal field states associated with the J=6 states of Tb3+ within the appropriate Fd\bar{3}m pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb3+ magnetic moment in each material's ground state, information that is an essential starting point for any description of the low-temperature phase behavior and spin dynamics in Tb2Ti2O7 and Tb2Sn2O7. While these two materials have much in common, the cubic stanate lattice is expanded compared to the cubic titanate lattice. As our measurements show, this translates into a factor of ~2 increase in the crystal field bandwidth of the 2J+1=13 states in Tb2Ti2O7 compared with Tb2Sn2O7. Our results are consistent with previous measurements on crystal field states in Tb2Sn2O7, wherein the ground state doublet corresponds primarily to m_J=|\pm 5> and the first excited state doublet to mJ=|\pm 4>. In contrast, our results on Tb2Ti2O7 differ markedly from earlier studies, showing that the ground state doublet corresponds to a significant mixture of mJ=|\pm 5>, |\mp 4>, and |\pm 2>, while the first excited state doublet corresponds to a mixture of mJ=|\pm 4>, |\mp 5>, and |\pm 1>. We discuss these results in the context of proposed mechanisms for the failure of Tb2Ti2O7 to develop conventional long-range order down to 50 mK.Comment: 12 pages, 6 figures. Version is the same as the published one, except for figure placement on page

Quantum spin correlations in an organometallic alternating sign chain

High resolution inelastic neutron scattering is used to study excitations in the organometallic magnet DMACuCl$_3$. The correct magnetic Hamiltonian describing this material has been debated for many years. Combined with high field bulk magnetization and susceptibility studies, the new results imply that DMACuCl$_3$ is a realization of the $S=1/2$ alternating antiferromagnetic-ferromagnetic (AFM-FM) chain. Coupled-cluster calculations are used to derive exchange parameters, showing that the AFM and FM interactions have nearly the same strength. Analysis of the scattering intensities shows clear evidence for inter-dimer spin correlations, in contrast to existing results for conventional alternating chains. The results are discussed in the context of recent ideas concerning quantum entanglement.Comment: 5 pages, 4 figures included in text. Submitted to APS Journal

Singlet-Triplet Excitations in the Unconventional Spin-Peierls System TiOBr

We have performed time-of-flight neutron scattering measurements on powder samples of the unconventional spin-Peierls compound TiOBr using the fine-resolution Fermi chopper spectrometer (SEQUOIA) at the SNS. These measurements reveal two branches of magnetic excitations within the commensurate and incommensurate spin-Peierls phases, which we associate with n = 1 and n = 2 triplet excitations out of the singlet ground state. These measurements represent the first direct measure of the singlet-triplet energy gap in TiOBr, which is determined to be Eg = 21.2 +/- 1.0 meV.Comment: 5 pages, 4 figures, submitted for publicatio

Electron doping evolution of the magnetic excitations in NaFe1−x_{1-x}Cox_xAs

We use time-of-flight (ToF) inelastic neutron scattering (INS) spectroscopy to investigate the doping dependence of magnetic excitations across the phase diagram of NaFe$_{1-x}$Co$_x$As with $x=0, 0.0175, 0.0215, 0.05,$ and $0.11$. The effect of electron-doping by partially substituting Fe by Co is to form resonances that couple with superconductivity, broaden and suppress low energy ($E\le 80$ meV) spin excitations compared with spin waves in undoped NaFeAs. However, high energy ($E> 80$ meV) spin excitations are weakly Co-doping dependent. Integration of the local spin dynamic susceptibility $\chi^{\prime\prime}(\omega)$ of NaFe$_{1-x}$Co$_x$As reveals a total fluctuating moment of 3.6 $\mu_B^2$/Fe and a small but systematic reduction with electron doping. The presence of a large spin gap in the Co-overdoped nonsuperconducting NaFe$_{0.89}$Co$_{0.11}$As suggests that Fermi surface nesting is responsible for low-energy spin excitations. These results parallel Ni-doping evolution of spin excitations in BaFe$_{2-x}$Ni$_x$As$_2$, confirming the notion that low-energy spin excitations coupling with itinerant electrons are important for superconductivity, while weakly doping dependent high-energy spin excitations result from localized moments.Comment: 14 pages, 16 figure

Unexpected Enhancement of Three-Dimensional Low-Energy Spin Correlations in Quasi-Two-Dimensional Fe1+y_{1+y}Te1−x_{1-x}Sex_{x} System at High Temperature

We report inelastic neutron scattering measurements of low energy ($\hbar \omega < 10$ meV) magnetic excitations in the "11" system Fe$_{1+y}$Te$_{1-x}$Se$_{x}$. The spin correlations are two-dimensional (2D) in the superconducting samples at low temperature, but appear much more three-dimensional when the temperature rises well above $T_c \sim 15$ K, with a clear increase of the (dynamic) spin correlation length perpendicular to the Fe planes. The spontaneous change of dynamic spin correlations from 2D to 3D on warming is unexpected and cannot be naturally explained when only the spin degree of freedom is considered. Our results suggest that the low temperature physics in the "11" system, in particular the evolution of low energy spin excitations towards %better satisfying the nesting condition for mediating superconducting pairing, is driven by changes in orbital correlations