Recent years remarkable progress was achieved in fundamental understanding of quantum phenomena in condensed matter physics. A wide range of exotic magnetic excitations and unconventional states of matter have been theoretically predicted and then experimentally observed in materials which are found to be physical realization for theoretical models. However, the experimental studies of many intriguing phenomena are still limited by lack of the ideal physical realizations for certain physical models. Therefore, searching for a possibility to control and change physical properties of the materials in the demanded way is an important scientific task.
Recent studies of the effects of pressure on the magnetic properties of solid-state magnets reveal that the external pressure can change the magnetic exchange interactions and induce quantum phase transitions giving experimental access to novel states of matter. This means that pressure can be used as a tool to customize the magnetic properties of solid-state compounds by changing the underlying Hamiltonian, potentially generating new and unconventional phenomena.
The PRESSMAG project is dedicated to exploring the hydrostatic pressure effects on the magnetic structure, Hamiltonian and critical behaviour of novel quasi-two-dimensional planar antiferromagnet BaNi2V2O8 at base and finite temperatures where the signatures for the rare Berezinskii-Kosterlitz-Thouless (BKT) criticality have been recently experimentally observed at ambient conditions.
In the conventional picture the long-rage magnetic order is ruled out by the Mermin-Wagner theorem in two-dimensional planar (2D XY) magnets at finite temperatures. However, Kosterlitz and Thouless, independently with Berezinskii, predicted a topological phase transition in 2D XY systems from disordered to quasi-ordered state at finite transition temperature TBKT brought by the pairing of the topological defects, spin-vortices, into spin-vortex/antivortex pairs. Although initially the BKT phenomena was expected only in 2D XY model, later theoretical studies suggest that it can also appear in quasi- two-dimensional magnets with planar anisotropy extending the class of compounds where BKT behaviour can be experimentally observed. This attracts strong attention to the search and investigation of the physical realizations of such systems.
Recent studies found BaNi2V2O8 to be an ideal physical realization for the quasi-two-dimensional magnet. Despite conventional long range magnetic order below TN, this compound exhibits magnetic properties of various 2D models over a wide temperature range at ambient pressure. In particular, BaNi2V2O8 behaves as 2D XY, 2D XXZ or 2D Haeisenberg magnet depending on temperature regime. In the planar 2D XY regime at ambient pressure BaNi2V2O8 reveals signatures of Berezinskii – Kosterlitz – Thousless criticality as seen experimentally and confirmed by theoretical computations. Because the estimated T_BKT is lower than TN, the quasi-ordered state of spin-vortex/antivortex-pairs in BaNi2V2O8 is hidden by conventional long range magnetic so that only signatures for the decomposed pairs are experimentally observed just above TN in ambient pressure.
The aim of PRESSMAG project is to study the effects of hydrostatic pressure on the overall magnetic properties such as Hamiltonian, magnetic structure and critical behaviour of model quasi-2D magnetic system BaNi2V2O8 and probing the hydrostatic pressure as a tool to customize TN, TBKT and planar 2D XY regime in BaNi2V2O8 making BKT phenomena more experimentally accessible. The results of PRESSMAG project are expected to be general and can be applied for the other quasi-2D magnets with similar Hamiltonian.
