Triple-Axis Spectrometer
TAX | HB-3 | HFIR
The HB-3 is a high-intensity triple axis spectrometer that is ideal for measuring magnetic excitations in solids, up to 100 meV and for measuring structural and magnetic order parameters in bulk materials. This instrument is most beneficial to the condensed matter and materials science communities. Due to its versatility and easy access this instrument can be used for parametric studies using a variety of ancillary sample environments to provide a complete control of thermodynamic variables such as temperature, magnetic field, and pressure. During the last few years most of the demand for this instrument has been focused in studies of unconventional superconductors, quantum magnets, thermoelectrics, ferroelectrics and multiferroics.
Examples of typical experiments carried out at HB-3 over the last few years are listed below:
- An inelastic neutron scattering experiment on single crystals of superconducting BaFe1.84Co0.16As2 revealed a magnetic excitation located at wave vectors (1/2 1/2 L) in tetragonal notation. On cooling below TC, a clear resonance peak is observed at this wave vector with an energy of 8.6 meV, corresponding to 4.5 kBTC. This is in good agreement with the canonical value of 5 kBTC observed in the cuprate high TC superconductors. The spectrum shows strong dispersion in the tetragonal plane but weak dispersion along the c axis, indicating that the magnetic fluctuations are 2-dimensional in nature. This is in sharp contrast to the anisotropic 3-dimensional spin excitations seen in the undoped parent compounds. Five crystals with a total mass of 2 g were used for this study which took 7 days of HB-3 beam time (see M. D. Lumsden, A. D. Christianson, D. Parshall, M. B. Stone, S. E. Nagler, G.J. MacDougall, H. A. Mook, K. Lokshin, T. Egami, D. L. Abernathy, E. A. Goremychkin, R. Osborn, M. A. McGuire, A. S. Sefat, R. Jin, B. C. Sales, D. Mandrus, “Two-dimensional resonant magnetic excitation in BaFe1.84Co0.16As2”, Phys. Rev. Lett. 102, 107005 (2009)).
- A high- pressure study on a 0.5 g of the unconventional superconductor FeTe0.6Se0.4 was performed up to 1.5 GPa by inelastic neutron scattering at HB-3. Both the neutron spin resonance energy wr and the superconducting transition Tc increase with pressure up to a maximum at 1.3 GPa. This is evidence of the correlation between these two fundamental parameters of unconventional superconductivity. Surprisingly Tc and wr do not scale linearly and thus a simple relationship between these energies does not exist even in a single sample. This result is consistent with a pressure-induced weakening of the coupling strength associated with the fundamental pairing mechanism. These measurements took 14 days of HB-3 beam time (K. Marty, A. D. Christianson, A. M. dos Santos, B. Sipos, K. Matsubayashi, Y. Uwatoko, J. A. Fernandez-Baca, C. A. Tulk, T. A. Maier, B. C. Sales, and M. D. Lumsden, “Pressure induced renormalization of energy scales in the unconventional superconductor FeTe0.6Se0.4”, unpublished).
- An inelastic neutron scattering experiment was performed at HB-3 to study the temperature dependence of the low-energy spin excitations in single crystals of superconducting FeTe0.6Se0.4 (Tc = 14 K). In the low-temperature superconducting state, the imaginary part of the dynamic susceptibility at the electron and hole Fermi-surfaces nesting wave vector Q = (0.5,0.5), χ’’(Q,ω), has a small spin gap, a two-dimensional neutron spin resonance above the spin gap, and increases linearly with increasing energy above the resonance. While the intensity of the resonance decreases like an order parameter with increasing temperature and disappears at temperature slightly above Tc, the energy of the mode is weakly temperature dependent and vanishes concurrently above Tc. This suggests that in spite of its similarities with the resonance in electron-doped superconducting BaFe2−x(Co,Ni)xAs2, the mode in FeTe0.6Se0.4 is not directly associated with the superconducting electronic gap (see Leland W. Harriger, O. J. Lipscombe, Chenglin Zhang, Hui-qian Luo, Karol Marty, M. D. Lumsden, and Pengcheng Dai, “Temperature dependence of the resonance and low spin excitations in superconducting FeTe0.6Se0.4”, Phys. Rev. B 85, 054511 (2012).
- The phonon-dispersion relations for cubic PbTiO3 have been determined along the [1,0,0], [1,1,0] and [1,1,1] directions at T = 793 K in the cubic paraelectric phase. Selected branches were measured up to T=1173K. The TO Δ5, Σ3, Σ4, and Λ3 branches drop significantly toward the Γ point, this softening is largest as T approaches TC, suggesting that this softening dominates the paraelectric to ferroelectric transition. The TA Λ3 branch along [ξ, ξ, ξ] shows significant softening around ξ = 0.25 and 0.5. This softening is weakly T dependent. For this study a 30´30´6 mm3 crystal of PBTiO3 was used , the measurements took 12 days (see Izumi Tomeno, Jaime A Fernandez-Baca, Karol J Marty, Yorihiko Tsunoda and Kunihiko Oka, “Lattice dynamics of PbTiO3”, Journal of Physics: Conference Series 340, 012054 (2012) and in preparation).
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