Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F05: Field Induced Phenomena in Alpha-RuCl3Invited
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Sponsoring Units: DCMP DMP GMAG Chair: George Jackeli, Univ Stuttgart Room: LACC 152 |
Tuesday, March 6, 2018 11:15AM - 11:51AM |
F05.00001: Excitations in the field-induced quantum spin liquid state of α-RuCl3 Invited Speaker: Paula Kelley The quasi-2D honeycomb magnetic insulator RuCl3 is a promising candidate to exhibit Kitaev interactions and exotic phenomena related to its proximity to the Kitaev quantum spin liquid (QSL) ground state. A moderate magnetic field applied in the honeycomb plane suppresses the fragile magnetic order in RuCl3 that arises due to additional, non-Kitaev, interactions. Here we present time-of-flight inelastic neutron scattering results as a function of magnetic field, demonstrating the disappearance of the magnetic Bragg peaks and spin waves of the ordered state with increasing field. Above the critical field, only a gapped continuum spectrum of magnetic excitations remains in the order-suppressed limit. The scattering in this field-induced spin liquid state is compared to calculations of the response function for the ideal Kitaev model in a field. |
Tuesday, March 6, 2018 11:51AM - 12:27PM |
F05.00002: Magnetic field and pressure induced magnetism of the Kitaev system α-RuCl3 Invited Speaker: Anja Wolter The Kitaev model on a honeycomb lattice has caused an abiding fascination due to its quantum spin liquid ground state. Here, α-RuCl3 is believed to be the prime material to-date to harbor such a quantum spin liquid phase. Despite its antiferromagnetic ground state below TN = 7 K, spin-liquid-like behavior may be observed if the magnetic order is suppressed under various external conditions, such as under the application of a magnetic field or pressure. We thoroughly investigated changes of the magnetic ground state of α-RuCl3 under applied magnetic fields by means of different experimental techniques [1,2]. For the in-plane direction, we find a suppression of the zero-field antiferromagnetic order with increasing field up to μ0Hc ≈ 7 T. In the disordered state above Hc, the magnetic excitations are strongly suppressed, implying the opening of a spin-excitation gap. Remarkably, our low-temperature specific heat data point toward a field-induced quantum critical point (QCP) at Hc; this is supported by universal scaling behavior near Hc. We relate the data to theoretical calculations based on a J1–K1–Γ1–J3 honeycomb model. As a second route towards the realization of a quantum spin-liquid state, we tuned the magnetic properties of α-RuCl3 by the application of hydrostatic pressure. Its influence on the Kitaev-like interactions, and on the magnetic ground state was studied by means of magnetization studies under hydrostatic pressure. Surprisingly, a new high-pressure phase emerges above ~ 0.5 GPa with a collapse of the magnetic susceptibility. Via both experimental and theoretical investigations of pressure-induced changes of the crystallographic structure of α-RuCl3 we resolve the origin and the nature of this new unconventional magnetic state. |
Tuesday, March 6, 2018 12:27PM - 1:03PM |
F05.00003: Phase Diagram of α-RuCl3 in an in-plane Magnetic Field Invited Speaker: Jennifer Sears The layered honeycomb material α-RuCl3 is thought to be a material with significant Kitaev magnetic interactions, an unusual bond-dependent Hamiltonian that can support a spin liquid ground state. Although α-RuCl3 orders magnetically and is thus not a realization of the Kitaev spin liquid, there have been some indications that the material is in some sense close to the spin liquid phase. Remarkably, it has been found that the application of an external magnetic field destabilizes the zigzag magnetic order in this material. We will present magnetic neutron diffraction measurements of the magnetic order parameter, showing the disappearance of the zigzag magnetic order. These measurements are combined with bulk magnetization and heat capacity measurements to map out the low temperature phase diagram in the presence of an in-plane magnetic field, and characterize the different phases observed. |
Tuesday, March 6, 2018 1:03PM - 1:39PM |
F05.00004: Dynamics of Kitaev spin liquids and other two-dimensional quantum spin models Invited Speaker: Ruben Verresen Obtaining dynamical response functions for topological phases of quantum matter is a challenging but important problem, as it encodes the characteristics of its exotic excitations. To this purpose, we introduce a matrix-product state based method for two-dimensional microscopic Hamiltonians. This is applied to the different phases of the Kitaev-Heisenberg model, where we identify characteristic dynamical features. In the ordered phases proximate to the spin liquid, we find significant broad high energy features beyond spin-wave theory, which resemble those of the Kitaev model. This establishes the concept of a proximate spin liquid, which was recently invoked in the context of inelastic neutron scattering experiments on $\alpha$-RuCl$_3$. We also discuss spectral features of other two-dimensional model magnets. |
Tuesday, March 6, 2018 1:39PM - 2:15PM |
F05.00005: Heisenberg-Kitaev physics in magnetic fields Invited Speaker: Lukas Janssen α-RuCl3 is a Mott insulator in the regime of strong spin-orbit coupling. In this material, the effective degrees of freedom are localized spins on a two-dimensional honeycomb lattice. They interact via strongly direction-dependent exchange couplings and, under a suitable external perturbation, may realize Kitaev's celebrated honeycomb spin model. When subject to a magnetic field, recent experiments in fact appear to suggest a field-induced quantum spin liquid characterized by fractionalized Majorana excitations. I will describe and explain the behavior of the relevant extended Heisenberg-Kitaev spin models in applied magnetic fields. Our results reveal surprisingly rich phase diagrams, with non-trivial intermediate phases including vortex crystals and other multi-Q states. I will discuss possible origins of large magnetic anisotropies as observed experimentally, and highlight different mechanisms to stabilize zigzag magnetic order and their distinct field responses. |
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