Neutron Scattering and Magnetism
Laboratory for Solid State Physics · ETH Zurich

Neutron Scattering

Neutron scattering spectra in the quantum paramagnet (left) and Tomonaga-Luttinger spin liquid (right) phases of the quantum spin ladder material DIMPY

Neutron scattering spectra in the quantum paramagnet (left) and Tomonaga-Luttinger spin liquid (right) phases of the quantum spin ladder material DIMPY. From D. Schmidiger, P. Bouillot, T. Guidi, R. Bewley, C. Kollath, T. Giamarchi, A. Zheludev, Spectrum of a magnetized strong-leg quantum spin ladder , Phys. Rev. Lett. 111, 107202 (2013); arXiv:1306.4216.

Our main experimental tool in studying quantum magnetic materials is neutron scattering. This technique directly and quantitatively measures spin correlations in both space and time. This, in turn, provides detailed information on the magnetic ground state of the system and on the magnetic excitations.

Sufficiently intense neutron beams cannot be produced in the lab. Instead, we perform our experiments at dedicated large-scale facilities such as reactors and accelerator-based sources. This is still "small physics" though: we are not involved in the "nuclear" business of producing the neutrons. We come to such facilities with our samples, mount them on the appropriate instrument, open the beam shutter and use the incoming neutrons to take scattering data. The experiment itself typically lasts a few days, and our students take part in these beamtimes as full team members, semester students included. At the facilities we are assisted by local scientists who may not be experts in our physics, but know the workings of the corresponding spectrometer in every detail. The "real" experiment, however, begins when we come back to ETH and start analyzing the data.

Neutron measurements require careful planning many months in advance and precise, disciplined execution at the instrument. The data analysis may take many weeks or months. The entire research cycle may span over a year or even a couple of years. For this reason, neutron scattering data are rarely the first to be published for a new exciting quantum magnetic material. But in the end we get momentum- and frequency-resolved data that are simply impossible to obtain with any other technique. These measurements are so comprehensive and informative that neutron scattering is often the last experiment that needs to be carried out.

Swiss Intense Spallation Source SINQ (at Paul Scherrer Institut)

psi area

Bird's eye view over the PSI area

SINQ is the source closest to home: the Paul Scherrer Institut is a mere 35 km from the Hönggerberg campus. Neutrons are produced by spallation, with a 590 MeV proton beam from the world's most powerful proton cyclotron knocking neutrons out of the nuclei of a lead target. At the turn of the century SINQ became the first accelerator-based source to deliver a continuous neutron beam, just like a reactor; it remains the only one of its kind to this day. SINQ also pioneered the use of "supermirror" neutron guides as the baseline design ingredient for all of its scattering instruments, and the entire guide system has very recently been rebuilt, increasing the useful neutron flux at the sample position severalfold. Our workhorse instrument there is the multiplexing crystal-analyzer spectrometer CAMEA, designed specifically for measurements in extreme sample environments. It collects neutrons at eight final energies over a 60° range of scattering angles simultaneously, mapping out entire regions of momentum-energy space in a single scan, even with the sample buried inside a dilution refrigerator or a high-field magnet.

INSPSICAMEA

Inelastic neutron scattering spectra of the S = 1 antiferromagnet Cs2RuO4 in zero field, measured on the CAMEA spectrometer. From S. D. Nabi, M. Zhu, K. Yu. Povarov, D. G. Mazzone, J. Lass, Y. Wu, Z. Yan, S. Gvasaliya, A. Zheludev, Spin-flop-like transition as quantum critical point in Cs2RuO4 , Phys. Rev. B 112, 134436 (2025); arXiv:2507.19853.

Institut Laue-Langevin, Grenoble, France

ILL-Grenoble

Night view over the European Photon and Neutron science campus at Grenoble. ILL is on the left

The most famous neutron source of all is the high-flux research reactor at the Institut Laue-Langevin (ILL). Founded in 1967 by France and Germany, soon joined by the United Kingdom, it has been the flagship of neutron science for half a century, and its 58 MW reactor still produces the most intense continuous neutron flux in the world. Many if not most technological advances in neutron instrumentation originated here. Today some 40 instruments, continuously modernized, serve well over a thousand experiments every year. Our group is a frequent user of the IN5 time-of-flight spectrometer, the IN12 and IN22 3-axis spectrometers and the rather unique D23 lifting-counter diffractometer. An added attraction is the setting: the science campus occupies the tip of a peninsula at the confluence of the Drac and Isère rivers, with the French Alps rising on all sides.

Data from IN5: dispersion of magnon excitations in Ni(Cl1−xBrx)2·4SC(NH2)2 (DTNX), measured just on the ordered side of the composition-induced quantum phase transition. Replacing a few percent of chlorine with bromine on the non-magnetic site closes the spin gap of the parent quantum paramagnet DTN and induces magnetic order: quantum criticality at zero field and ambient pressure. From A. Mannig, K. Yu. Povarov, J. Ollivier, A. Zheludev, Spin waves near the edge of halogen substitution induced magnetic order in Ni(Cl1−xBrx)2·4SC(NH2)2 , Phys. Rev. B 98, 214419 (2018); arXiv:1810.10912.

ISIS neutron source at Rutherford Appleton Laboratory, Oxfordshire, UK

isis-hall

General view over the ISIS Neutron Source hall

ISIS is the UK's national pulsed neutron source: an 800 MeV proton synchrotron feeds two spallation target stations, and the neutrons arrive in short, sharp bursts fifty times a second. The time-averaged flux is modest by reactor standards, but the peak flux within each pulse is enormous, which is exactly what time-of-flight instruments thrive on; ISIS operates some of the best such instruments in the world. Our group regularly uses two of them. LET is a cold-neutron multi-chopper spectrometer that records complete scattering maps for several incident neutron energies simultaneously. WISH is a long-wavelength diffractometer on the second target station, purpose-built for determining magnetic structures with large unit cells and small ordered moments, and works beautifully even on tiny single crystals at dilution-refrigerator temperatures.

Magnetic diffraction measured on WISH

Magnetic neutron diffraction from a 55 mg single crystal of the triangular-lattice antiferromagnet (CD3ND3)2NaRuCl6, measured on WISH at 50 mK in magnetic fields applied along the c axis. The incommensurate Bragg peaks in zero field are part of the fingerprint of a multi-q ground state, our candidate for the long-sought ℤ2 vortex crystal; the field then drives the system through a whole cascade of ordered phases. From J. Nagl, K. Yu. Povarov, B. Duncan, C. Näppi, D. Khalyavin, P. Manuel, F. Orlandi, J. Sourd, B. V. Schwarze, F. Husstedt, S. A. Zvyagin, O. Zaharko, P. Steffens, A. Hiess, D. R. Allan, S. A. Barnett, Z. Yan, S. Gvasaliya, A. Zheludev, ℤ2 vortex crystal candidate in the triangular S = 1/2 quantum antiferromagnet , npj Quantum Mater. (2026); arXiv:2512.01793.

NIST Center for Neutron Research (NCNR), Gaithersburg, Maryland, USA

NCNR experiment

Florian Landolt, Simon Bettler and AZ during an experiment on MACS in 2018

The NIST Center for Neutron Research (NCNR) near Washington, DC is one of the leading reactor-based facilities in the US, and arguably the leading one when it comes to cold neutrons. Its 20 MW reactor is not particularly powerful by world standards. The strength of the NCNR lies elsewhere: in an excellent cold source, a large guide hall, and a park of instruments superbly matched to the low energy scales of quantum magnetism. Just as important, the NCNR has a long tradition of hosting outside users and training students; a remarkable fraction of the American neutron scattering community learned the trade there. Our favorite instrument is the Multi-Axis Crystal Spectrometer (MACS), a third-generation cold-neutron 3-axis spectrometer. Its doubly focusing monochromator views a huge solid angle of the cold source and delivers one of the most intense monochromatic cold beams anywhere, while twenty analyzer-detector channels sweep large areas of reciprocal space in parallel.

The MACS spectrometer is particularly well suited for measuring constant-energy slices of the excitation spectrum. Here is a comparison between measured (left) and calculated (right) spin wave excitations in the frustrated ferro-antiferromagnet Pb2VO(PO4)2. From S. Bettler, F. Landolt, Ö. M. Aksoy, Z. Yan, S. Gvasaliya, Y. Qiu, E. Ressouche, K. Beauvois, S. Raymond, A. N. Ponomaryov, S. A. Zvyagin, A. Zheludev, Magnetic structure and spin waves in the frustrated ferro-antiferromagnet Pb2VO(PO4)2 , Phys. Rev. B 99, 184437 (2019); arXiv:1902.11172.

Spallation Neutron Source (SNS) and High Flux Isotope Reactor (HFIR) at Oak Ridge, Tennessee, USA

Enlarged view: SNS

Oak Ridge National Laboratory is home to two complementary neutron sources. The Spallation Neutron Source is the most powerful accelerator-based neutron source in the world: a superconducting linear accelerator slams protons into a liquid-mercury target sixty times a second, and a recently completed upgrade has pushed the proton beam power towards the 2 MW mark. The pulsed beam is tailor-made for time-of-flight spectroscopy. Our instrument of choice is the Cold Neutron Chopper Spectrometer CNCS, on which several of our key experiments on triangular-lattice and spin-chain materials were carried out.

The High Flux Isotope Reactor, on a hill a few miles away, is the exact complement: an 85 MW reactor delivering one of the most intense continuous neutron fluxes on the planet. A continuous beam is what conventional crystal spectrometers want, and we use the cold-neutron 3-axis instrument CTAX for targeted high-resolution studies of individual excitations, of the kind where time-of-flight mapping would be wasteful.

Spin waves in RbFeCl3 measured on CNCS

Spin waves in the easy-plane triangular-lattice antiferromagnet RbFeCl3, measured on the CNCS spectrometer (top row) and calculated (bottom row) along three reciprocal-space directions (schematic on the right). At first glance a textbook magnon spectrum; in truth, the measured intensities cannot be reproduced without invoking long-range magnetic dipolar interactions, which are almost universally neglected in this class of materials. From L. Stoppel, S. Hayashida, Z. Yan, A. Podlesnyak, A. Zheludev, Anomalous spin waves in CsFeCl3 and RbFeCl3 , Phys. Rev. B 104, 094422 (2021); arXiv:2106.09447.

The Materials and Life Science Experimental Facility (MLF) at Japan Proton Accelerator Research Complex (J-PARC), Tokai, Japan

The AMATERAS spectrometer in the MLF experimental hall

The second experimental hall of the MLF, with the AMATERAS time-of-flight spectrometer (BL14) highlighted

The Materials and Life Science Experimental Facility (MLF) at the J-PARC accelerator complex is the world's second most powerful accelerator-based neutron source, surpassed only by the SNS. A 3 GeV proton synchrotron drives a mercury spallation target 25 times a second, and the facility, built in the 2000s from a clean slate, is thoroughly modern from moderators to detectors. The instrument we keep coming back to is AMATERAS, a cold-neutron disk-chopper spectrometer named after the Shinto sun goddess. It was on AMATERAS that we mapped out the excitation continuum of the spin supersolid K2Co(SeO3)2. On top of that, Tokai sits right on the Pacific coast, and the sushi restaurants in the area are amazing: our students are always happy to travel to Japan for an experiment.

jparc

Magnetic excitation spectrum of the triangular-lattice spin supersolid K2Co(SeO3)2, measured on the AMATERAS spectrometer [1,2]. In place of sharp spin waves, the response is a broad dispersive continuum, of the kind one would sooner expect of a quantum spin liquid.