Abstract
One-dimensional (1D) magnetic insulators have attracted significant interest as a platform for studying quasiparticle fractionalization, quantum criticality, and emergent phenomena. The spin-1/2 Heisenberg chain with antiferromagnetic nearest neighbour interactions is an important reference system; its elementary magnetic excitations are spin-1/2 quasiparticles called spinons that are created in even numbers. However, while the excitation continuum associated with two-spinon states is routinely observed, the study of four-spinon and higher multi-spinon states is an open area of research. Here we show that four-spinon excitations can be accessed directly in Sr2CuO3 using resonant inelastic x-ray scattering (RIXS) in a region of phase space clearly separated from the two-spinon continuum. Our finding is made possible by the fundamental differences in the correlation function probed by RIXS in comparison to other probes. This advance holds promise as a tool in the search for novel quantum states and quantum spin liquids.
Highlights
One-dimensional (1D) magnetic insulators have attracted significant interest as a platform for studying quasiparticle fractionalization, quantum criticality, and emergent phenomena
While the allowed phase space for fourspinon excitations is much larger than for two-spinon excitations[22,24], kinematic constraints on the matrix elements between the spinon manifolds lead a situation where the multi-spinon states only contribute significantly for momentum and energy transfers within the boundaries of the two-spinon continuum. This picture has been confirmed by detailed comparisons between inelastic neutron scattering (INS) experiments[5,12] and exact calculations of the dynamical structure factor (DSF)[5,23,24], which find the twospinon excitations account only for ~73–74% of the total detected spectral weight, while four-spinon excitations exhaust the majority of the remaining sum rule
How can we understand the magnetic excitations in RIXS captured by the t−J model, and why do we see magnetic excitations that are absent in INS? In Fig. 3 we illustrate schematically the magnetic excitation mechanisms in a spin chain with the different scattering techniques: INS (Fig. 3a), Cu L2,3-edge (Fig. 3b), and O K-edge RIXS (Fig. 3c, d)
Summary
One-dimensional (1D) magnetic insulators have attracted significant interest as a platform for studying quasiparticle fractionalization, quantum criticality, and emergent phenomena. When confined to one spatial dimension (1D), systems of interacting electrons host an assortment of macroscopic many-body phenomena, including quantum critical magnetic states with collective excitations carrying fractional quantum numbers. While the allowed phase space for fourspinon excitations (and greater) is much larger than for two-spinon excitations[22,24], kinematic constraints on the matrix elements between the spinon manifolds lead a situation where the multi-spinon states only contribute significantly for momentum and energy transfers within the boundaries of the two-spinon continuum This picture has been confirmed by detailed comparisons between INS experiments[5,12] and exact calculations of the dynamical structure factor (DSF)[5,23,24], which find the twospinon excitations account only for ~73–74% of the total detected spectral weight, while four-spinon excitations exhaust the majority of the remaining sum rule. We show that RIXS at the O K-edge allows for such an observation, a capability that results from the fundamentally different correlation function that it probes compared to, for example, the spin correlation function of the DSF22,26
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