Spin excitations in the tunable quantum spin ladder
The nature of spin excitations in a spin ladder depends on the ratio of the leg and rung exchange energies. Recently, a new family of spin ladder materials Cu(Qnx)Cl1-xBrx was discovered, where the halogen ions, responsible for the rung interaction, could be gradually exchanged . Using bulk measurements, we have identified how the exchange constants are modified by Cl/Br content . Subsequent Raman spectroscopy measurements revealed changes in the excitation spectrum throughout the doping range .
 Keith et al., Polihedron 30, 3006 (2011)
 Povarov et al., JMMM 370 62 (2014)
 Simutis et al., PRB 93 094412 (2016)
Dielectric response of a low-dimensional gapped quantum magnet
Multiferroics - materials combining magnetic and ferroelectric order are rare in nature. Quantum multiferroics, where magnetic order competes with strong quantum fluctuations, are even more exotic. Nonetheless, they also do exist, and organometallic dielectric Sul-Cu2Cl4 is an interesting example . At zero magnetic field the magnetically disordered ground state of the material is protected by the spin gap and nothing remarkable happens in either electric or magnetic properties. However, moderate magnetic field does induce the spiral ordering of the spins , and this is precisely the moment when ferroelectricity kicks in. While the spin ordering transition itself is shown to be a perfect example of symmetry-protected magnetic Bose-Einstein condensation, the dielectric properties of the ordered phase turn out to be very nontrivial and feature strong nonlinearities. The details of the study can be read in a recent Rapid Communication in Physical Review B .
Universal dynamics in a quantum spin ladder
Although the ground state of the Heisenberg spin ladder is gapped in zero magnetic field, high magnetic field induces a “soft” continuum of excitations at low energies. These excitations are rather exotic fermionic quasiparticles – spinons, and the new field-induced state is the quantum critical Tomonaga-Luttinger spin liquid, endemic to one-dimensional world. The zero-temperature quantum criticality is expected to be reflected in the dynamic properties at finite temperatures just as well, and the low-energy spectrum should have a universal character as function of E/T. The details of this scaling depend on the only phenomenological parameter, describing the interaction between the spinons. Despite the power of this prediction, it has never been verified in a spin ladder material in a high field until the recent inelastic neutron scattering experiment on a strong-leg Heisenberg ladder prototype material DIMPY [1,2]. The spectrum of excitations is indeed found to scale according to the predictions, and the effective interaction between the spinons is confirmed to be attractive (a very unconventional case). The details of this exciting story can be found in the recent Rapid Communication in Physical Review B .