Spin Orbital Separation

LETTER

doi:10.1038/nature10974

Spin–orbital separation in the
quasi-one-dimensional Mott insulator Sr2CuO3
J. Schlappa1,2, K. Wohlfeld3, K. J. Zhou1{, M. Mourigal4, M. W. Haverkort5, V. N. Strocov1, L. Hozoi3, C. Monney1, S. Nishimoto3,
S. Singh6{, A. Revcolevschi6, J.-S. Caux7, L. Patthey1,8, H. M. Rønnow4, J. van den Brink3 & T. Schmitt1

When viewed as an elementary particle, theelectron has spin and
charge. When binding to the atomic nucleus, it also acquires an
angular momentum quantum number corresponding to the
quantized atomic orbital it occupies. Even if electrons in solids
form bands and delocalize from the nuclei, in Mott insulators they
retain their three fundamental quantum numbers: spin, charge
and orbital1. The hallmark of one-dimensional physics is abreaking up of the elementary electron into its separate degrees of
freedom2. The separation of the electron into independent quasiparticles that carry either spin (spinons) or charge (holons) was
first observed fifteen years ago3. Here we report observation of
the separation of the orbital degree of freedom (orbiton) using
resonant inelastic X-ray scattering on the one-dimensional Mott
insulatorSr2CuO3. We resolve an orbiton separating itself from
spinons and propagating through the lattice as a distinct quasiparticle with a substantial dispersion in energy over momentum, of
about 0.2 electronvolts, over nearly one Brillouin zone.
It was pointed out in the 1970s that in a solid not only the charge and
spin of electrons can become ordered—leading to magnetism—but
also the electrons’orbital degree of freedom1. This observation sparked
a field that has gone on to produce a number of important results.
Although a physical electron combines spin, charge and orbital,
theoretically an electron can be considered a bound state of the three
independent, fundamental quasi-particles: a spinon, carrying the
electron’s spin; a holon (or chargon), carrying its charge; and anorbiton, carrying its orbital degree of freedom.
A remarkable and fundamental property of one-dimensional (1D)
systems is that electronic excitations break up into deconfined spinons
and holons. This was predicted decades ago (ref. 2 and references
therein) and confirmed in the mid 1990s by angle-resolved photoemission spectroscopy experiments3–5. The spin–charge separation is
an example ofparticle fractionalization, a phenomenon in which the
quantum numbers of quasi-particles are not multiples of those of the
elementary particle, but fractions. This effect is one of the most
unusual manifestations of collective quantum physics of interacting
particles and is a profound concept that has found its way into a
number of theories, for example that describing high-temperaturesuperconductivity in copper oxides6,7.
To search for the further fractionalization of the electron, we
consider the excitation of a copper orbital degree of freedom in the
antiferromagnetic spin-chain compound Sr2CuO3. The spin–orbital
separation process that we are looking for is analogous to the spin–
charge separation mechanism (Fig. 1b). The latter occurs, for instance,
when an electron isannihilated, removing a single spin and leaving
behind a hole in the antiferromagnetic chain. This hole can start to
propagate freely only after exciting one spinon (a domain wall in the
antiferromagnetic chain). Subsequently, the spinon can delocalize and

separate itself completely from the holon. When instead of creating a
hole, as typically is done in a photoemission experiment, an electron isexcited from one copper 3d orbital to another, the phenomenon of
spin–orbital separation can in principle occur (Fig. 1a). The orbiton
created in this manner may also deconfine after exciting a spinon, thus
splitting the electron into its orbital and spin degrees of freedom8.
Here we use high-resolution resonant inelastic X-ray scattering
(RIXS) to search experimentally for spin–orbital...