Kate Ross has championed a remarkable body of experimental work that established much of what we know about a set of quantum frustrated pyrochlore magnets, in particular Er2Ti2O7, Yb2Ti2O7 and NaA’Co2O7. All three of these pyrochlore magnets correspond to Seff=1/2 XY-like moments decorating a network of corner-sharing tetrahedra – the cubic pyrochlore lattice, one of the archetypes for geometrical frustration in three dimensions. During her PhD at McMaster University, she grew large single crystals of rare earth titanate pyrochlores Yb2Ti2O7 and Er2Ti2O7, appropriate for neutron scattering measurements, and then carried out sophisticated time-of-flight neutron scattering measurements at dilution refrigerator temperatures, and in magnetic fields to 8 T.
This single crystal, inelastic neutron scattering work, carried out from 2008 – 2012, was among the first to fully characterize a four dimensional data set of S(Q, E) (three Q dimensions, one energy dimension) with the purpose of fitting the high magnetic field spin excitation spectrum to linear spin wave theory. The linear spin wave theory utilized a form of anisotropic exchange consistent with the point group symmetry of the rare earth site in the pyrochlore lattice. Doing so meant working closely with theorist Leon Balents at the KITP, UC Santa Barbara and his PhD student at the time, Lucile Savary. This work was carried out for both Yb2Ti2O7 and Er2Ti2O7 single crystals.
Measurements in sufficiently high field and low temperatures are important in this context as the spin excitations from a field-polarized ground state are typically well understood in terms of linear spin wave theory, and thus the underlying spin Hamiltonian is amenable to analysis of a sufficiently comprehensive inelastic neutron scattering data set. For Yb2Ti2O7 the high field, low temperature neutron measurements are crucial, as the zero-field ground state does not support well-defined spin wave excitations at all – an observation established by Kate’s PhD work that remains an outstanding puzzle in the field. It was also among the first experiments discussed within the context of quantum spin ice. For Er2Ti2O7, the determination of a microscopic spin Hamiltonian was the key to unravelling a decade-long mystery as to why Er2Ti2O7 orders into the so-called Psi2 non-coplanar, non-collinear magnetic structure, rather than the coplanar Palmer-Chalker state that had been expected based of isotropic exchange. It also provided a quantitative understanding of Er2Ti2O7’s ground state selection in terms of a quantum order by disorder mechanism. Her later neutron scattering work established the “order-by disorder” gap in this system, consistent with theoretical predictions.
Kate’s PhD thesis also led to the discovery that the thermodynamic phase transition which stoichiometric Yb2Ti2O7 displays at Tc~ 0.26 K is anomalously sensitive to disorder – disorder at the 1% level, which is
difficult to control or even measure. Kate and collaborators carried out very careful neutron crystallographic work that showed that single crystals grown from the melt displayed “weak stuffing” where an excess of 1% Yb resided at the Ti site. This doesn’t occur in polycrystalline materials prepared by solid state synthesis at lower temperatures. This work has been very influential, as “stuffing” is a general phenomena in a wide class of pyrochlores and indeed in other families of materials. During her postdoctoral work with Collin Broholm and continuing until the present day, Kate led a series of neutron scattering experiments on the transition metal pyrochlore magnet NaA’Co2O7, grown by Bob Cava’s group at Princeton, where magnetic Co2+ ions occupy the pyrochlore lattice. The Co2+ moments also take on XY anisotropy and thus this family allows an interesting comparison to the rare earth-based XY pyrochlores that she studied during her PhD. However, transition metal ion-based magnets have intrinsically higher magnetic energy scales compared with rare earth magnets, and this helps to expand the bandwidth of the spin excitation spectrum, and potentially allows exotic magnetic excitations to appear at higher (and hence easier to observe) energies. In fact, this new work is still in progress, but it has already revealed unanticipated richness.
Finally, it is noteworthy to mention that Kate also played a key leadership role in a program of neutron and x-ray diffraction measurements in very high, pulsed magnetic fields. These challenging experiments were carried out in a collaboration between the McMaster group and Hidenori Nojiri (Tohoku University), Garrett Granroth (ORNL) and Zahir Islam (ANL). These experiments involved time-resolved neutron and x-ray diffraction phased with the discharge of a large capacitor bank, passing a large current through a Helmholtz coil. We carried out neutron Laue diffraction on MnWO4, which displays a rich magnetic field, temperature phase diagram, and magnetically ordered phases up to ~ 50 T at low temperatures. We were able to conduct time-resolved Laue neutron diffraction measurements at SNS on MnWO4 up to ~ 30 T. These were difficult measurements as the data sets corresponded to a very low duty cycle: 1 neutron pulse with high field on, 18,000 with field off. And these measurements had to be properly phased with the discharge of the capacitor bank and then identified in the overall data sets. Related, but somewhat easier time-resolved synchrotron x-ray measurements were carried out at the APS on Tb2Ti2O7. Kate played a key role in both the experiments themselves, and, more importantly, in the manipulation of the data sets after the fact. This work resulted in two Phys. Rev. Letts. as well as several instrument development papers. Kate is co-author, but not first author, on these papers, and this work does not appear in her McMaster PhD thesis. Nonetheless, she became the defacto “North American spokesperson” for this work, giving two invited talks on it, at the American Conference on Neutron Scattering and at the 2013 ORNL Neutron Sciences User Meeting.
Her topical body of work has attracted much international interest and significant recognitions. Five years past her PhD, she has 37 lifetime publications and an h-index of 21 (Google Scholar). Her 2011 PRX on
Yb2Ti2O7 has 255 citations, her 2012 PRL on Er2Ti2O7 has 118 citations, while her 2102 PRB on “stuffing” in Yb2Ti2O7 presently has 92 citations. She was awarded the 2017 George Valley Jr. Prize from the APS for outstanding early career contributions in any area of physics, the first female physicist so recognized. Earlier she was awarded the 2014 Alice Wilson Award from the Royal Society of Canada for early career excellence by a Canadian female scientist in any area of science. In 2016, Kate was appointed to a CIfAR Azrieli Global Scholar within the Quantum Materials Program, a most competitive recognition.