Spinon signatures in the critical phase of the (1,1/2) ferrimagnet in a magnetic field

We propose an effective theory for the critical phase of a quantum ferrimag netic chain with alternating spins 1 and 1/2 in an external magnetic field. With the help of the matrix product variational approach, the system is ma pped to a spin-1/2 XXZ chain in an (effective) magnetic field; as a by-prod uct, we obtain an excellent description of the optical magnon branch in the gapped phase. Recent finite-temperature density matrix renormalization gro up results for the low-temperature part of the specific heat are well descr ibed by the present approach, and the "pop-up" peaks, developing near the c ritical field values and in the middle of the critical phase, are identifie d with the contributions from two different spinon bands of the effective s pin-1/2 chain. The effect should be observable as well in other spin-gap sy stems in an external field, particularly in spin ladders.