Competing Orders in Frustrated Magnets and Nanostructures

University Of Utah, Salt Lake City UT

Investigators

Abstract

TECHNICAL SUMMARY This award supports theoretical research to investigate competition between different ordering tendencies in low-dimensional strongly correlated electron systems such as frustrated quantum magnets and fabricated and self-assembled nanostructures. Both of these areas are at the forefront of modern condensed matter physics. The research has 3 objectives: 1.) Develop a consistent theoretical description of spin excitations in spatially anisotropic quantum antiferromagnets. This description will include both fractionalized high-energy spinons of one-dimensional chains as well as low-energy magnons of the weakly ordered ground state. Particular attention will be paid to a kinematic binding mechanism when a composite spin-1 pair, formed by two spin-1/2 spinons, lowers the energy via delocalization along the direction transverse to the chains. Effects of symmetry-lowering Dzyaloshinskii-Moriya interaction, thermal fluctuations, and external magnetic field will be included. The PI also aims to investigate the singlet sector of multi-spinon continuum which can be probed by resonant inelastic x-ray scattering, phonon-assisted optical absorption, and Raman scattering. 2.) Determine the phase diagram of two families of spatially anisotropic triangular antiferromagnets. Both systems exhibit strong competition between collinear spin fluctuations, dimer ordering tendencies and classical spiral instability, but differ in the role that phonons play. Anisotropic response of this material to the applied magnetic field will be analyzed, and experimental properties of the field-induced ordered phases will be calculated by a combination of the renormalization group, chain mean-field theories and interacting spin waves and large-N Schwinger boson techniques. In addition, a careful analysis of symmetry lowering anisotropic Dzyaloshinskii-Moriya interaction in the quasi-one-dimensional geometry will be carried out. 3.) Analyze spin-orbit induced instabilities of low-dimensional interacting electrons subject to significant structure inversion asymmetry as appropriate for various electrostatically confined and/or surface nanostructures. I will investigate the limit of strongly correlated electron Wigner crystal where spin-orbit effects are bound to dominate over exponentially weak exchange interaction. Graduate students involved in the project will be trained in modern theoretical techniques such as bosonization, renormalization group, quantum field and many-body theories. The project will involve summer research opportunity for undergraduate students. Qualitative discussion of the research on the general physics level will be given during regular undergraduate physics seminars at the University of Utah. In addition, a web site with a graduate student level description of the outlined research topics and their interconnections will be developed in order to communicate results to a broader audience. NON-TECHNICAL SUMMARY This award supports theoretical research and education at a frontier of condensed matter physics. The research is focused on understanding unusual magnetic properties revealed by experiments on recently discovered materials, for example herbertsmithite and volborthite. These materials have the necessary ingredients to become magnets, but do not exhibit magnetism. On the scale of atoms, there is a competition between the interactions that would favor aligning the fundamental building blocks of magnetism and the geometrical arrangements of the atoms. This frustrates the tendency to magnetic order. Experiments continue to deliver more examples, enabling the test of theoretical ideas that new states of matter will arise from failed magnetism. The discovery of new states of matter opens the door to new materials with desirable properties and new materials-related phenomena that may form the basis for future technologies or may solve existing problems. The research will lay the foundations for understanding these materials and for future experiments that will better understand the unusual properties of these frustrated magnets. Graduate students involved in the project will be trained in modern theoretical techniques of condensed matter physics. The project will involve summer research opportunity for undergraduate students. Qualitative discussion of the research on the general physics level will be given during regular undergraduate physics seminars at the University of Utah. In addition, a web site with a graduate student level description of the outlined research topics and their interconnections will be developed in order to communicate results to a broader audience.