PUB - Publikationen an der Universität Bielefeld

Over the last decade, the interest in the spin-1/2 Heisenberg antiferromagnet (HAF) on the square kagome (also called shuriken) lattice has been growing as a model system of quantum magnetism with a quantum paramagnetic ground state, flat-band physics near the saturation field, and quantum scars. A further motivation to study this model comes from the recent discovery of a gapless spin liquid in the square kagome magnet numerical investigations of the specific heat C(T ), the entropy S(T ), as well as the susceptibility ??? (T ) by means of the finite-temperature Lanczos method for system sizes of N = 18, 24, 30, 36, 42, 48, and N = 54. We find that the specific heat exhibits a low-temperature shoulder below the major maximum which can be attributed to low-lying singlet excitations filling the singlet-triplet gap, which is significantly larger than the singlet-singlet gap. This observation is further supported by the behavior of the entropy S(T ), where a change in curvature is present just at about T/J = 0.2, the same temperature where the shoulder in C sets in. For the susceptibility the low-lying singlet excitations are irrelevant, and the singlet-triplet gap leads to an exponentially activated low-temperature behavior. The maximum in ??? (T ) is found at a pretty low temperature Tmax/J = 0.146 (for N = 42) compared to Tmax/J = 0.935 for the unfrustrated square-lattice HAF signaling the crucial role of frustration also for the susceptibility. We find a striking similarity of our square kagome data with the corresponding ones for the kagome HAF down to very low T. The magnetization process featuring plateaus and jumps and the field dependence of the specific heat that exhibits characteristic peculiarities attributed to the existence of a flat one-magnon band are discussed as well.