Magnetic frustration usually leads to highly competing states, and therefore frustrated magnets can usually be tuned by external magnetic fields and give rise to rich quantum phase transitions, which opens the window for looking for exotic states and excitations. Condensed-matter NMR (nuclear magnetic resonance), taking advantage of the strong hyperfine couplings between electrons and the nuclei, is a sensitive probe of magnetic states and low-energy excitations. Here I report our recent NMR studies which reveal novel field-induced quantum phase transitions in several low-dimensional, frustrated magnetic materials.
For a honeycomb lattice α-RuCl3 which contains Kitaev interactions, the magnetic field suppresses the magnetic ordering and leads to a disordered phase. The disordered phase demonstrates characters of gapless, fractional excitations, and is consistent with a proximate Kitaev spin liquid . For a quasi-1D XXZ chain compound with Ising anisotropy, SrCo2V2O8, we establish a strong suppression of the magnetic ordering by an effective transverse field, and resolve two quantum critical points (QCPs). The low-field one is consistent with a 3D QCP, and the high-field one is consistent with a 1D QCP in the (1+1) Ising universality class . For a triangular lattice antiferromagnet Ba8CoNb6O24, we show that this material approaches a real 2D, spin-1/2, Heisenberg limit. A Quantum spin liquid is suggested at zero field. The magnetic field first induces magnetic ordering, and then leads to a fully polarized state at finite temperatures, with a distinctive (pressure, temperature) phase diagram compared to the classical Heisenberg model .
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