The thermal Hall effect of spin excitations in a Kagome magnet

At low temperatures, the thermal conductivity of spin excitations in a magnetic insulator can exceed that of phonons. However, because they are charge neutral, the spin waves are not expected to display a thermal Hall effect in a magnetic field. Recently, this semiclassical notion has been upended in quantum magnets in which the spin texture has a finite chirality. In the Kagome lattice, the chiral term generates a Berry curvature. This results in a thermal Hall conductivity $\kappa_{xy}$ that is topological in origin. Here we report observation of a large $\kappa_{xy}$ in the Kagome magnet Cu(1-3, bdc) which orders magnetically at 1.8 K. The observed $\kappa_{xy}$ undergoes a remarkable sign-reversal with changes in temperature or magnetic field, associated with sign alternation of the Chern flux between magnon bands. We show that thermal Hall experiments probe incisively the effect of Berry curvature on heat transport.Comment: 6 pages, 3 figure

Topological Magnon Bands in a Kagome Lattice Ferromagnet

There is great interest in finding materials possessing quasiparticles with topological properties. Such materials may have novel excitations that exist on their boundaries which are protected against disorder. We report experimental evidence that magnons in an insulating kagome ferromagnet can have a topological band structure. Our neutron scattering measurements further reveal that one of the bands is flat due to the unique geometry of the kagome lattice. Spin wave calculations show that the measured band structure follows from a simple Heisenberg Hamiltonian with a Dzyaloshinkii-Moriya interaction. This serves as the first realization of an effectively two-dimensional topological magnon insulator—a new class of magnetic material that should display both a magnon Hall effect and protected chiral edge modes.United States. Dept. of Energy. Office of Basic Energy Sciences (Grant DE-FG02-07ER46134)National Science Foundation (U.S.) (Grant CHE 1041863

A Cu2+ (S = 1/2) Kagom\'e Antiferromagnet: MgxCu4-x(OH)6Cl2

Spin-frustrated systems are one avenue for inducing macroscopic quantum states in materials. However, experimental realization of this goal has been difficult because of the lack of simple materials and, if available, the separation of the unusual magnetic properties arising from exotic magnetic states from behavior associated with chemical disorder, such as site mixing. Here we report the synthesis and magnetic properties of a new series of magnetically frustrated materials, MgxCu4-x(OH)6Cl2. Because of the substantially different ligand-field chemistry of Mg2+ and Cu2+, site disorder within the kagom\'e layers is minimized, as directly measured by X-ray diffraction. Our results reveal that many of the properties of these materials and related systems are not due to disorder of the magnetic lattice but rather reflect an unusual ground state.Comment: Accepted for publication in J. Am. Chem. Soc

Neutron scattering and thermodynamic studies of quantum magnetism on the kagomé lattice

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 191-198).The geometry of the kagome lattice leads to exciting novel magnetic behavior in both ferromagnetic and antiferromagnetic systems. The collective spin dynamics were investigated in a variety of magnetic materials featuring spin-1/2 and spin-1 moments on kagome lattices using neutron scattering and thermodynamic probes. Both ferromagnetic and antiferromagnetic systems were studied. Cu(1,3-bdc) is an organometallic material, where the Cu2+ ions form a ferromagnetic S = 1/2. kagomé system. Synthesis techniques were developed to produce -mg-sized deuterated single crystals, and ~2,000 crystals were partially coaligned to create a sample for neutron scattering measurements. Elastic neutron scattering measurements show the existence of long range magnetic ordering below T = 1.77 K. Integrated Bragg peak intensities were analyzed to determine the structure of ordered magnetic moments. Inelastic neutron scattering measurements show the magnon dispersion spectrum, which consists of a flat high energy band and two dispersive, lower energy bands. The application of a magnetic field perpendicular to the kagome plane opens gaps between these three bands and distorts the flatness of the highest energy band. The system was modelled as a nearest-neighbor Heisenberg ferromagnet with Dzyaloshinskii-Moriya(DM) interaction. The model dispersion and scattering structure factor were calculated and fit to the data to precisely determine the strengths of the nearest-neighbor coupling and DM interaction. The observed manon band structure is a bosonic analog to the band structure of the topological insulator systems. Antiferromagnetic kagome systems can exhibit novel magnetic ground states such as quantum spin liquids and spin nematics. Thermodynamic measurements were performed on the antiferromagnetic kagome materials MgxCu₄-x(OH)₆ Cl₂ , featuring S = 1/2 moments. These measurements reveal magnetic ordering at low values of x that is suppressed with increasing x. At x = 0.75, this ordering is not fully suppressed, but susceptibility and specific heat measurements reveal behavior similar to that of the quantum spin liquid candidate herbertsmithite. Thermodynamic and neutron scattering measurements were performed on the kagome lattice material BaNi₃(OH)₂(VO₄)₂, which features S = 1 moments. These measurements reveal competing interactions, which result in a spin glass ordering transition.by Robin Michael Daub Chisnell.Ph. D