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March 2025 ESRFnews

I

T is one of the most famous experiments in

physics. Light illuminates a pair of slits in a wall,

generating an array of bright and dark patches

on a screen. The British physicist Thomas Young

first performed the experiment at the turn of the

19th century to demonstrate that light can interfere

with itself, behaving as a wave. Much later, quantum

versions of the experiment would demonstrate

something far more mysterious: that photons, electrons

and other particles can exhibit wave-like interference

patterns but apparently only when noone is watching

The experiment has in it the heart of quantum

mechanics wrote the American physicist and Nobel

laureate Richard Feynman In reality it contains the

only mystery

Today few scientists doubt the merits of quantum

mechanics It has proved itself through mindboggling

predictive power not to mention a host of practical

applications semiconductor electronics lasers

superconducting magnets quantum cryptography

and quantum computing to name but a few Yet it

is still a subject ripe with puzzles both in its basic

interpretation and in its role in condensed matter

where each material can serve as a quantum playground

(see Insight, p10). One puzzle is the existence of peculiar

types of magnetism, as studied by ESRF users such as

Markus Grüninger from the University of Cologne

in Germany. Unravelling these phenomena has led

Grüninger and his colleagues to shift the boundaries

of an X-ray technique – amazingly, in such a way as to

recall Young’s famous experiment once again.

“Our experiments rely on the excellent beam quality at

the ESRF, the outstanding performance of the set-up at

beamline ID20 and the fruitful collaboration with the

beamline staff says Grüninger

The technique in question is resonant inelastic Xray

scattering RIXS This begins with an Xray photon

knocking a tightly bound electron up to a higher atomic

energy level Almost instantaneously an electron from

another high energy level relaxes into the resultant hole

releasing a new photon By measuring the difference in

energy between the incoming and outgoing photons

users can learn how the process has changed the solid in

collective excitations of electron charge and spin the

latter being the basis of magnetism The ESRF has helped

develop RIXS since the 1990s and currently offers it at

S H U T T E R S T O C K/ S O L A R S E V E N

Novel quantum

magnets of fer a

fascinating window

into the fundamental

interactions of

matter, but are highly

mysterious. To unravel

them, ESRF users

have had to push the

boundaries of an

X-ray technique.

EXOTIC MAGNETISM

All in a spin