Direct Observation of Magnon Fractionalization in the Quantum Spin   Ladder
  We measure by inelastic neutron scattering the spin excitation spectra as a function of applied magnetic field in the quantum spin-ladder material (C5H12N)2CuBr4. Discrete magnon modes at low fields in the quantum disordered phase and at high fields in the saturated phase contrast sharply with a spinon continuum at intermediate fields characteristic of the Luttinger-liquid phase. By tuning the magnetic field, we drive the fractionalization of magnons into spinons and, in this deconfined regime, observe both commensurate and incommensurate continua. 
B. Thielemann, Ch. R\"uegg, H. M. R{\o}nnow, A. M. L\"auchli, J.-S.   Caux, B. Normand, D. Biner, K. W. Kr\"amer, H.-U. G\"u...