Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F05: Field Induced Phenomena in AlphaRuCl3Invited

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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 fieldinduced quantum spin liquid state of αRuCl_{3} Invited Speaker: Paula Kelley The quasi2D honeycomb magnetic insulator RuCl_{3} 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 RuCl_{3} that arises due to additional, nonKitaev, interactions. Here we present timeofflight 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 ordersuppressed limit. The scattering in this fieldinduced 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 αRuCl_{3} 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, αRuCl_{3} is believed to be the prime material todate to harbor such a quantum spin liquid phase. Despite its antiferromagnetic ground state below T_{N }= 7 K, spinliquidlike 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 αRuCl_{3} under applied magnetic fields by means of different experimental techniques [1,2]. For the inplane direction, we find a suppression of the zerofield antiferromagnetic order with increasing field up to μ_{0}H_{c} ≈ 7 T. In the disordered state above H_{c}, the magnetic excitations are strongly suppressed, implying the opening of a spinexcitation gap. Remarkably, our lowtemperature specific heat data point toward a fieldinduced quantum critical point (QCP) at H_{c}; this is supported by universal scaling behavior near H_{c}. We relate the data to theoretical calculations based on a J_{1}–K_{1}–Γ_{1}–J_{3} honeycomb model. As a second route towards the realization of a quantum spinliquid state, we tuned the magnetic properties of αRuCl_{3 }by the application of hydrostatic pressure. Its influence on the Kitaevlike interactions, and on the magnetic ground state was studied by means of magnetization studies under hydrostatic pressure. Surprisingly, a new highpressure phase emerges above ~ 0.5 GPa with a collapse of the magnetic susceptibility. Via both experimental and theoretical investigations of pressureinduced changes of the crystallographic structure of αRuCl_{3} 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 αRuCl_{3} in an inplane Magnetic Field Invited Speaker: Jennifer Sears The layered honeycomb material αRuCl_{3} is thought to be a material with significant Kitaev magnetic interactions, an unusual bonddependent Hamiltonian that can support a spin liquid ground state. Although αRuCl_{3} 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 inplane 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 twodimensional 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 matrixproduct state based method for twodimensional microscopic Hamiltonians. This is applied to the different phases of the KitaevHeisenberg model, where we identify characteristic dynamical features. In the ordered phases proximate to the spin liquid, we find significant broad high energy features beyond spinwave 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 twodimensional model magnets. 
Tuesday, March 6, 2018 1:39PM  2:15PM 
F05.00005: HeisenbergKitaev physics in magnetic fields Invited Speaker: Lukas Janssen αRuCl_{3} is a Mott insulator in the regime of strong spinorbit coupling. In this material, the effective degrees of freedom are localized spins on a twodimensional honeycomb lattice. They interact via strongly directiondependent 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 fieldinduced quantum spin liquid characterized by fractionalized Majorana excitations. I will describe and explain the behavior of the relevant extended HeisenbergKitaev spin models in applied magnetic fields. Our results reveal surprisingly rich phase diagrams, with nontrivial intermediate phases including vortex crystals and other multiQ 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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