Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J47: Kitaev Magnetism: TheoryFocus

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Sponsoring Units: GMAG DMP Chair: Stephen Winter, Goethe University Frankfurt Room: 710/712 
Tuesday, March 3, 2020 2:30PM  2:42PM 
J47.00001: Understanding the MagneticField Anisotropy of Kitaev Materials Ahmed Rayyan, Jacob Gordon, HaeYoung Kee The Kitaev spin liquid remains the most theoretically wellunderstood quantum spin liquid due to the exact solvability of the Kitaev model. However, other interactions such as the offdiagonal spin interactions render the model nonintegrable, but give rise to the magnetic order seen at low temperatures in candidate materials. Recently it was shown that when a magnetic field is applied along a certain direction, the magnetic order melts into an intermediatefield phase before the spins are fully polarized, A thorough understanding of the interplay between field and spin exchange anisotropy is required to understand fieldinduced phases. To this end, we present the classical and quantum phase diagrams of the extended anisotropic Kitaev model under different magnetic field directions. 
Tuesday, March 3, 2020 2:42PM  2:54PM 
J47.00002: Majoranamagnon crossover by a magnetic field in the Kitaev model Yukitoshi Motome, Junki Yoshitake, Joji Nasu, Yasuyuki Kato Kitaev quantum spin liquids host Majorana fermions via the fractionalization of spins. In a magnetic field, the Majorana fermions were predicted to comprise a topological state with anionic excitations, which has attracted great attention by the recent discovery of the halfquantized thermal Hall conductivity. Nevertheless, a reliable theory remains elusive for the field effect, especially at finite temperature. Here we present unbiased largescale numerical results for the Kitaev model in a wide range of magnetic field and temperature, obtained by continuoustime quantum Monte Carlo simulations. We find that the unconventional paramagnetic region showing fractional spin dynamics extends at finite temperature, far beyond the field range where the topological state is expected at zero temperature. Our results show the confinementdeconfinement behavior between the fractional Majorana excitations and the conventional magnons. 
Tuesday, March 3, 2020 2:54PM  3:06PM 
J47.00003: Spinone Kitaev model, same but different? Ilia Khait, HaeYoung Kee, YongBaek Kim We study various properties of the spinone Kitaev model on honeycomb lattices consisting of two and threeleg ladder geometries using density matrix renormalization group. We discuss similarities to the spinhalf model, and outline differences. Our results suggest that the spinone model is a bona fide spin liquid candidate. 
Tuesday, March 3, 2020 3:06PM  3:18PM 
J47.00004: Vacancyinduced Lowenergy States in the Kitaev Model WenHan Kao, Johannes Knolle, Roderich Moessner, Natalia Perkins Since 2006, the Kitaev honeycomb model has attracted significant attention due to the exactly solvable spinliquid ground state with fractionalized Majorana excitations [1] and the possible materialization in magnetic Mott insulators with strong spinorbit couplings [2]. Recently, the 5delectron compound H_{3}LiIr_{2}O_{6} has shown to be a strong candidate of Kitaev physics considering the absence of longrange ordered magnetic state [3]. In this work, we demonstrate that a finite density of random vacancies gives rise to a remarkable pile up of lowenergy states and possibly explains the experimental findings in H_{3}LiIr_{2}O_{6}. We study both the freeflux and the vacancyinduced boundflux background and their responses to additional timereversal symmetrybreaking term, which imitates the magnetic field in real experiments. 
Tuesday, March 3, 2020 3:18PM  3:30PM 
J47.00005: Field induced phases of the KitaevΓ ladder Erik Sorensen, Andrei Catuneanu, Jacob Gordon, HaeYoung Kee The Kitaev spin model on honeycomb lattice has attracted significant attention since the emergence of αRuCl_{3} as a promising Kitaev spin liquid candidate in the presence of a magnetic field. While the mechanism of such a fieldinduced Kitaev spin liquid is not yet fully understood, theoretical studies have shown that the bonddependent Γ and Kitaev interactions are equally significant in αRuCl_{3} leading to a minimal spin½ KitaevΓ (KG) model. In the pure antiferromagnetic Kitaev limit, previous numerical studies have shown gapless states for a range of intermediate field strengths, leading to the suggestion of a U(1) spin liquid phase. However, a possibility of incommensurate magnetic orderings cannot be excluded. Employing largescale numerical techniques, we study the KG model in a twoleg ladder system in the presence of a magnetic field for the entire phase space of the KG model, finding both disordered and incommensurate ordered phases near the antiferromagnetic Kitaev region. We discuss the models limitations and relation to the twodimensional honeycomb lattice. 
Tuesday, March 3, 2020 3:30PM  3:42PM 
J47.00006: FieldDriven Phenomena in 2d and 3d Kitaev Magnets Ciaran Hickey, Simon Trebst, Matthias Gohlke, Christoph Berke Kitaev's honeycomb model is an exactly solvable spin model that realises a quantum spin liquid ground state, with fractionalised excitations in the form of Majorana fermions and plaquette flux excitations. Recent studies have shown that applying an external magnetic field can give rise to a rich set of fielddriven phenomena, including the appearance of a gapless U(1) spin liquid. However, the Kitaev model is not unique to the honeycomb lattice, the model can in fact be defined, and exactly solved, on a range of tricoordinated lattices in two and three dimensions. This naturally provides an enormous playground within which to study fielddriven phenomena in quantum spin liquids. We will discuss a number of examples and construct a generic phase diagram for the Kitaev model in the presence of a magnetic field. 
Tuesday, March 3, 2020 3:42PM  3:54PM 
J47.00007: Magnetic Field Induced Competing Phases in SpinOrbital Entangled Kitaev Magnets Li Ern Chern, Ryui Kaneko, HyunYong Lee, YongBaek Kim There has been a great interest in magnetic field induced quantum spin liquids in Kitaev magnets after the discovery of neutron scattering continuum and half quantized thermal Hall conductivity in the material $\alpha$RuCl$_3$. In this work, we provide a semiclassical analysis of the relevant theoretical models, which enable us to treat large system sizes approximating the thermodynamic limit. We find a series of competing magnetic orders with fairly large unit cells at intermediate magnetic fields, which are mostly missed by previous studies. We show that quantum fluctuations are typically strong in these large unit cell orders, while the spin wave dispersion resembles a scattering continuum. The huge quantity of magnon bands with finite Chern numbers also gives rise to an unusually large thermal Hall conductivity. Given the highly frustrated nature of the spin model, the large unit cell orders are likely to melt into the putative spin liquid in the quantum limit. Our work provides an important basis for a thorough investigation of emergent spin liquids and competing phases in Kitaev magnets. [arXiv:1905.11408] 
Tuesday, March 3, 2020 3:54PM  4:06PM 
J47.00008: Numerical Studies of the KitaevGamma Model Under a Magnetic Field Jacob Gordon, Andrei Catuneanu, HaeYoung Kee Recently, there has been excitement generated around αRuCl_{3} as a candidate for the material realization of the Kitaev spin liquid (KSL). Beyond the dominant ferromagnetic (FM) Kitaev interaction, subleading spin interactions are required to explain the zigzag (ZZ) magnetic ordering and behaviour of αRuCl_{3 }under a magnetic field. On the basis of exact diagonalization (ED) and DMRG, an antiferromagnetic (AFM) Gamma interaction was found to be essential for stabilizing the KSL under tilted magnetic fields. A subsequent classical study of the KitaevGamma model found a multitude of large unit cell magnetic orders due to the competition between Gamma and the applied field. Furthermore, an infinite tensor product state (iTPS) study found that these classical orders are melted by quantum fluctuations, giving way to two nematic paramagnetic states. The topological nature of these nematic phases and their relation to the KSL remain unknown. Here we present results obtained with ED to address these open questions. 
Tuesday, March 3, 2020 4:06PM  4:42PM 
J47.00009: Vison crystals in an extended Kitaev model on the Honeycomb latttice Invited Speaker: Cristian Batista

Tuesday, March 3, 2020 4:42PM  4:54PM 
J47.00010: Partitioning the phase diagram of pyrochlore and Kitaev magnets using graph theory Ke Liu, Jonas Greitemann, Ludovic DC Jaubert, Han Yan, Nicholas Shannon, Lode Pollet Highly frustrated magnets host rich exotic states of matter such as spin liquids and hidden orders. Those phases can occur in various forms and are notoriously difficult to identify. In this talk, I will show that the combination of a kernel method and graph partitioning theory provides an efficient framework to unravel the complex phase diagram of frustrated magnets. It delimits regimes of both classical spin liquids and broken symmetry phases, including hidden orders, and provides the analytical order parameters and/or characteristic local constraints. The method is demonstrated by examples of pyrochlore and Kitaev magnets but applies to general (semi)classical spin systems. 
Tuesday, March 3, 2020 4:54PM  5:06PM 
J47.00011: Quantum Spin Liquid and Proximate Magnetic Orders in Magnets with SpinOrbit Coupling Animesh Nanda, Kusum Dhochak, Subhro Bhattacharjee Quantum phase transitions out of magnetic orders in quantum spin liquid (QSL) phases have gained much recent attention in the context of several spinorbit coupled magnets such as αRuCl_{3}, Yb_{2}Ti_{2}O_{7} etc. In this talk, I shall report our theoretical calculations about the nature of such unconventional phase transition in a class of experimentally relevant Hamiltonians in the honeycomb magnets. In particular, we shall show how such deconfined quantum phase transitions are naturally captured in terms of the condensation of the fractionalised excitations of the QSL. In addition, we shall show how to think about such transitions in terms of the domain walls of the magnetically ordered phases. 
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