1. frustrated quantum magnetism: the flat-band scenario

Johannes Richter

University of Magdeburg, Germany

29 June 2021 Tue 5 pm

                                      IBS Center for Theoretical Physics of Complex Systems (PCS), Administrative Office (B349), Theory Wing, 3rd floor

                                      Expo-ro 55, Yuseong-gu, Daejeon, South Korea, 34126 Tel: +82-42-878-8633                     

Frustration in magnetic and electronic systems may lead to novel quantum phases such as spin liquids or valence-bond crystals. A particularly interesting class of frustrated systems is the class of so-called flat-band systems. In such systems the frustrated geometry leads to dispersionless (flat) one-particle bands which have a strong influence on the many-body physics of strongly correlated quantum systems. Thus, flat-band systems are receiving a great deal of attention right now, in particular with view of realizing new many-body phases there.

In my talk, first I will give an overview on the low-temperature physics of flat-band Heisenberg spin systems and strongly correlated (Hubbard) electrons. Interestingly for a large variety of such quantum systems a set of exact many-body eigenstates can be constructed. Examples are the 1D sawtooth chain, the 2D kagome lattice, and the 3D pyrochlore lattice. The exact many-particle eigenstates consist of independent magnons (electrons) localized on finite areas of the lattice and become ground states for certain values of total magnetization (electron concentrations).

The very existence of a flat band can lead to several remarkable properties such as electronic ferromagnetism related to a Pauli-correlated percolation problem and a magnon crystallization in 2D spin systems.

Typically, the very existence of a flat band requires fine tuning of the Hamiltonian parameters. Therefore, only a few real-material examples are known being candidates for the experimental observation of such flat-band effects.

In the second part of my talk I consider the quantum spin J1-J2 sawtooth chain, a paradigmatic frustrated spin system that exhibits a flat band  only for two singular ratios of J1 and J2. Applying electric and magnetic fields, the restricting  fine tuning of exchange couplings is dissolved and the system can be driven into a flat-band scenario for a wide region of J1 and J2 values. While the magnetic field acts via the ordinary Zeeman term, the  coupling of the electric field with the spins is given by the Katsura-Nagaosa-Balatsky mechanism, also called inverse  Dzyaloshinskii-Moriya                        

interaction. The resulting novel features are corresponding reciprocal effects: We find a magnetization jump driven by the electric field as well as a jump of the electric polarization driven by the magnetic field, i.e. there is an extraordinarily strong magnetoelectric effect. Analogously to the enhanced magnetocaloric effect the system shows an enhanced electrocaloric effect.