Jacob Ruff

SEQUOIA: A Newly Operating Chopper Spectrometer at the SNS

A fine resolution chopper spectrometer (SEQUOIA) recently received first neutrons at the SNS. The commissioning phase of the instrument is underway. SEQUOIA is designed to utilize neutrons of an incident energy (Ei) between 10-2000 meV. A monochromatic beam is provided on a sample, 20 m from the decoupled ambient temperature H2O moderator, by filtering the white beam with a Fermi chopper located 18 m from the source. After interacting with the sample, neutrons are detected by an array of 3He linear position sensitive tubes located on a vertical cylinder with a radius of 5.5 m. This contribution presents current results from the commissioning experiments and compares SEQUOIAs actual and predicted performance. These commissioning experiments include characterization of the beam by monitors, determination of the chopper phase offsets, and runs with V and C4H2I2S. The predicted performance is provided by analytical calculations and Monte Carlo simulations.

Order by Quantum Disorder in Er2Ti2O7

Here we establish the systematic existence of a U(1) degeneracy of all symmetry-allowed Hamiltonians quadratic in the spins on the pyrochlore lattice, at the mean-field level. By extracting the Hamiltonian of Er(2)Ti(2)O(7) from inelastic neutron scattering measurements, we then show that the U(1)-degenerate states of Er(2)Ti(2)O(7) are its classical ground states, and unambiguously show that quantum fluctuations break the degeneracy in a way which is confirmed by experiment. The degree of symmetry protection of the classical U(1) degeneracy in Er(2)Ti(2)O(7) is unprecedented in other materials. As a consequence, our observation of order by disorder is unusually definitive. We provide further verifiable consequences of this phenomenon, and several additional comparisons between theory and experiment.

Spin Waves and Quantum Criticality in the Frustrated XY Pyrochlore Antiferromagnet Er2Ti2O7

We report detailed measurements of the low temperature magnetic phase diagram of Er2Ti2O7. Heat capacity and time-of-flight neutron scattering studies of single crystals reveal unconventional low-energy states. Er3+ magnetic ions reside on a pyrochlore lattice in Er2Ti2O7, where local XY anisotropy and antiferromagnetic interactions give rise to a unique frustrated system. In zero field, the ground state exhibits coexisting short and long-range order, accompanied by soft collective spin excitations previously believed to be absent. The application of finite magnetic fields tunes the ground state continuously through a landscape of noncollinear phases, divided by a zero temperature phase transition at micro{0}H{c} approximately 1.5 T. The characteristic energy scale for spin fluctuations is seen to vanish at the critical point, as expected for a second order quantum phase transition driven by quantum fluctuations.

Two-dimensional kagome correlations and field induced order in the ferromagnetic XY pyrochlore Yb2Ti2O7.

Neutron scattering measurements show the ferromagnetic XY pyrochlore Yb2Ti2O7 to display strong quasi-two-dimensional (2D) spin correlations at low temperature, which give way to long range order (LRO) under the application of modest magnetic fields. Rods of scattering along 111 directions due to these 2D spin correlations imply a magnetic decomposition of the cubic pyrochlore system into decoupled kagome planes. A magnetic field of approximately 0.5 T applied along the [110] direction induces a transition to a 3D LRO state characterized by long-lived, dispersive spin waves. Our measurements map out a complex low temperature-field phase diagram for this exotic pyrochlore magnet.

Revisiting static and dynamic spin-ice correlations in Ho₂Ti₂O₇ with neutron scattering

J.P. Clancy, J.P.C. Ruff, S.R. Dunsiger, Y. Zhao, H.A. Dabkowska, J.S. Gardner, 3 Y. Qiu, 4 J.R.D. Copley, T. Jenkins, and B.D. Gaulin 5 Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S 4M1 NIST Center for Neutron Research, NIST, Gaithersburg, Maryland 20899-8102, USA Indiana University, 2401 Milo B. Sampson Lane, Bloomington, Indiana 47408, USA Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA Canadian Institute for Advanced Research, 180 Dundas Street W., Toronto, Ontario, Canada M5G 1Z8

Finite-temperature transitions in dipolar spin ice in a large magnetic field.

We use Monte Carlo simulations to identify the mechanism that allows for phase transitions in dipolar spin ice to occur and survive for applied magnetic field, H, much larger in strength than that of the spin-spin interactions. In the most generic and highest symmetry case, the spins on one out of four sublattices of the pyrochlore decouple from the total local exchange+dipolar+applied field. In the special case where H is aligned perfectly along the [110] crystallographic direction, spin chains perpendicular to H show a transition to q=X long range order, which proceeds via a one to three dimensional crossover. We propose that these transitions are relevant to the origin of specific heat features observed in powder samples of the Dy2Ti2O7 spin ice material for H above 1 Tesla.

Magnetic Excitations from the Exotic Ground State of the Quantum FCC Antiferromagnet Ba2YMoO6

J. P. Carlo, 2 J. P. Clancy, T. Aharen, Z. Yamani, J. P. C. Ruff, J. Wagman, G. J. Van Gastel, H. M. L. Noad, G. E. Granroth, J. E. Greedan, 5 H. A. Dabkowska, and B. D. Gaulin 5, 6 Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1 Canada Canadian Neutron Beam Centre, National Research Council, Chalk River, ON K0J 1J0 Canada Department of Chemistry, McMaster University, Hamilton, ON L8S 4M1 Canada Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA Brockhouse Institute for Materials Research, McMaster University, Hamilton, ON L8S 4M1 Canada Canadian Institute for Advanced Research, Toronto, ON M5G 1Z8 Canada (Dated: January 21, 2013)

Structural Fluctuations in the spin-liquid state of Tb2Ti2O7.

High-resolution x-ray scattering measurements on single crystal Tb2Ti2O7 reveal finite structural correlations at low temperatures. This geometrically frustrated pyrochlore is known to exhibit a spin-liquid or cooperative paramagnetic state at temperatures below approximately 20 K. Parametric studies of structural Bragg peaks appropriate to the Fd3[over ]m space group of Tb2Ti2O7 reveal substantial broadening and peak intensity reduction in the temperature regime 20 K to 300 mK. We also observe a small, anomalous lattice expansion on cooling below a density maximum at approximately 18 K. These measurements are consistent with the development of fluctuations above a cooperative Jahn-Teller, cubic-tetragonal phase transition at very low temperatures.

Field-Induced Order and Spin Waves in the Pyrochlore Antiferromagnet Tb~2Ti~2O~7

High resolution time-of-flight neutron scattering measurements on Tb(2)Ti(2)0(7) reveal a rich low temperature phase diagram in the presence of a magnetic field applied along [110]. In zero field at T = 0.4 K, Tb(2)Ti(2)0(7) is a highly correlated cooperative paramagnet with disordered spins residing on a pyrochlore lattice of corner-sharing tetrahedra. Application of a small field condenses much of the magnetic diffuse scattering, characteristic of the disordered spins, into a new Bragg peak characteristic of a polarized paramagnet. At higher fields, a magnetically ordered phase is induced, which supports spin wave excitations indicative of continuous, rather than Ising-like, spin degrees of freedom.

A portable high-field pulsed-magnet system for single-crystal x-ray scattering studies

We present a portable pulsed-magnet system for x-ray studies of materials in high magnetic fields (up to 30 T). The apparatus consists of a split-pair of minicoils cooled on a closed-cycle cryostat, which is used for x-ray diffraction studies with applied field normal to the scattering plane. A second independent closed-cycle cryostat is used for cooling the sample to near liquid helium temperatures. Pulsed magnetic fields (approximately 1 ms in total duration) are generated by discharging a configurable capacitor bank into the magnet coils. Time-resolved scattering data are collected using a combination of a fast single-photon counting detector, a multichannel scaler, and a high-resolution digital storage oscilloscope. The capabilities of this instrument are used to study a geometrically frustrated system revealing strong magnetostrictive effects in the spin-liquid state.