High-field electron spin resonance (ESR) measurements were performed on the S = 1/2 Ising-like antiferromagnetic chain CsCoCl3 in the frequency range up to 4.4 THz and magnetic fields for H || c up to 53 T. We observed softening of the spinon excitation, which drives the spin chain into the quantum critical state at the critical field Hc = 33.0 T. Above Hc, we observed the string excitation at the wave number q = π, which is characteristic of the S = 1/2 spin chain in the quantum critical state in the magnetic fields. We also observed a crossover of the string excitation to the magnon bound state above the saturation field Hs = 43.9 T upon increasing the magnetic fields. Finally, we discuss how the string excitation and magnon bound state can be excited by the electric dipole transition due to spin quadrupolar resonance.
Quantum few-body systems exhibit a rich variety of phenomena that provide fundamental insights into universality. A paradigmatic example is the Efimov effect, in which an infinite tower of three-body bound states emerges with binding energies forming a geometric sequence. Motivated by advances in trapped-ion systems, recent theoretical work has predicted that the Efimov effect can arise in certain long-range quantum spin chains, where magnons exhibit a general dispersion
εk=u∣k∣z with
z∈(0.52,0.88) and interact through a contact potential. Nevertheless, whether Efimov universality persists in the presence of experimentally relevant long-range interactions between magnons remains unexplored. In this work, we address this issue by investigating the two- and three-magnon problems with a model interaction potential
V(x)=V0/(1+∣x∣β). Our results demonstrate that few-body universality persists for any
β>1: In the two-magnon sector, the low-energy expansion of the scattering matrix near a two-body resonance agrees with the universal form predicted by contact-interaction theory. The three-magnon bound-state spectrum exhibits Efimov scaling, with a scaling factor consistent with the theory. We further highlight the experimental relevance of our model, which can be readily realized in trapped-ion systems. Our results pave the way for quantum simulations of universal few-body physics beyond the traditional paradigm of nonrelativistic particles.