Frustrated ferromagnetic spin-12chain in a magnetic field: The phase diagram and thermodynamic properties

The frustrated ferromagnetic spin-1=2 Heisenberg chain is studied by means of a low-energy eld theory as well as the density-matrix renormalization group and exact diagonalization methods. Firstly, we study the ground-state phase diagram in a magnetic eld and nd an ‘even-odd’ (EO) phase characterized by bound pairs of magnons in the region of two weakly coupled antiferromagnetic chains. A jump in the magnetization curves signals a rst-order transition at the boundary of the EO phase, but otherwise the curves are smooth. Secondly, we discuss thermodynamic properties at zero eld, where we conrm a double-peak structure in the specic heat for moderate frustrating next-nearest neighbor interactions.