PUB - Publikationen an der Universität Bielefeld

Over the last decade, the spin-1/2 Heisenberg antiferromagnet on the square-kagome (SK) lattice has attracted growing attention as a model system of highly frustrated quantum magnetism. A further motivation for theoreti-cal studies of this model comes from the recent discovery of SK spin-liquid compounds KCu6AlBiO4(SO4)5Cl [M. Fujihala et al., Nat. Commun. 11, 3429 (2020)] and Na6Cu7BiO4(PO4)4[Cl, (OH)]3 [O. V. Yakubovich et al., Inorg. Chem. 60, 11450 (2021)]. The SK antiferromagnet exhibits two nonequivalent nearest-neighbor bonds J1 and J2. One may expect that in SK compounds J1 and J2 are of different strength. Here, we present a numerical study of finite systems of N = 30, 36, and N = 42 sites by means of the finite-temperature Lanczos method. We discuss the temperature dependence of the Wilson ratio P(T), the specific heat C(T), the entropy S(T), and of the susceptibility X(T) of the J1 - J2 SK Heisenberg antiferromagnet varying J2/J1 in a range 0 J2/J1 4. We also discuss the zero-field ground state of the model. We find indications for a magnetically disordered singlet ground state for 0 J2/J1 & LE; 1.65. Beyond J2/J1 & SIM; 1.65 the singlet ground state gives way for a ferrimagnetic ground state, which becomes a stable Lieb ferrimagnet with magnetization M = N/6 (UUD state) for J2/J1 & GE; 1.83. In the region 0.77 & LE; J2/J1 & LE; 1.65 the low-temperature thermodynamics is dominated by a finite singlet-triplet gap filled with low-lying singlet excitations leading to an exponentially activated low-temperature behavior of X(T). On the other hand, the low-lying singlets yield an extra maximum or a shoulderlike profile below the main maximum in the C(T) curve. For J2/J1 & LE; 0.7 the low-temperature thermodynamics is characterized by a large fraction of N/3 weakly coupled spins leading to a sizable amount of entropy at very low temperatures. In an applied magnetic field the magnetization process features plateaus and jumps in a wide range of J2/J1.