Evidence of the Berezinskii-Kosterlitz-Thouless phase in a frustrated magnet

Abstract

The Berezinskii-Kosterlitz-Thouless (BKT) mechanism, building upon proliferation of topological defects in 2D systems, is the first example of phase transition beyond the Landau-Ginzburg paradigm of symmetry breaking. Such a topological phase transition has long been sought yet undiscovered directly in magnetic materials. Here, we pin down two transitions that bound a BKT phase in an ideal 2D frustrated magnet TmMgGaO4, via nuclear magnetic resonance under in-plane magnetic fields, which do not disturb the low-energy electronic states and allow BKT fluctuations to be detected sensitively. Moreover, by applying out-of-plane fields, we find a critical scaling behavior of the magnetic susceptibility expected for the BKT transition. The experimental findings can be explained by quantum Monte Carlo simulations applied on an accurate triangular-lattice Ising model of the compound which hosts a BKT phase. These results provide a concrete example for the BKT phase and offer an ideal platform for future investigations on the BKT physics in magnetic materials.

Spin Supersolidity in Nearly Ideal Easy-axis Triangular Quantum Antiferromagnet Na$_2$BaCo(PO$_4$)$_2$

“…The model hosts a very rich phase diagram in both temperature and magnetic field, which are yet to be fully uncovered by experiments. In particular, the model shows a second BKT transition at ∼ 60 mK in zero field; in finite field B a, the model shows two subtle transitions at B a1 ≈ 0.07 T and B a2 ≈ 0.75 T. These transitions may be detected by nuclear magnetic resonance [46], magneto-torque measurements [47], and magnetocaloric measurements [48][49][50]. On the theory front, while the S = 1/2 easy-axis TLAF and its classical counterpart share similar features in their finite-temperature phase diagrams, it was realized early on that the quantum model also possess unique traits that are not fully captured by the classical model [44].…”

“…As a powerful thermodynamic solver, XTRG has been successfully applied in solving triangular-lattice spin models [25] and related compounds [46,52], Kitaev model [53] and materials [54], correlated fermions in ultra-cold quantum gas [55], and even moiré quantum materials [56].…”

Emergent Rokhsar-Kivelson point in realistic quantum Ising models

“…To our knowledge, our generalised TFIM the first experimental relevant system to simulate QDM physics that accesses both the RK-DQCP and the 'Cantor deconfinement' regime. The TFIM is a realistic model in a variety of contexts, such as rare-earth frustrated magnets [59][60][61][62][63][64][65][66], dielectric materials [67], cold atom systems [68] and even superconducting qubits [69]. In addition, the RK-DQCP that appears in our system exhibit clear experimental signatures.…”

Hierarchical single-ion anisotropies in spin-1 Heisenberg antiferromagnets on the honeycomb lattice

“…In recent years, the search for solid-state realizations of Berezinskii-Kosterlitz-Thouless (BKT) topological phase transitions [1][2][3] in magnetic compounds has lead to the identification of several quasi two-dimensional (2D) antiferromagnetic candidate materials [4][5][6]. While in most systems, the BKT behavior is obstructed by the presence of residual interlayer couplings, these were found to be negligible for the specific Ni 2+ based compound BaNi 2 V 2 O 8 , in which spin-1 degrees of freedom reside in effectively decoupled 2D honeycomb lattice layers [7,8].…”