Luttinger-Liquid Physics and Bose-Einstein Condensation in Quantum Magnets

The phase diagram of quasi-1D arrays of quantum spins in temperature and magnetic field is particularly rich: quantum disordered, quantum critical, spin Luttinger-liquid, BEC, and classically saturated phases can in principle be studied [1-3]. However, model materials are rare, where these phases can be explored experimentally with the exception of the novel metal-organic compound (C5H12N)2CuBr4, a quasi-1D two-leg spin ladder. We determined the complete phase diagram of this material [2,3], and measured by inelastic neutron scattering the spin excitation spectra as a function of applied magnetic field across the two intrinsic quantum critical points [4]. 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 [4].