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.
Much of current research in frustrated magnetism community is devoted to finding magnetic systems and materialsof a strange "spin-liquid" type whereby atomic spins never freeze in one particular pattern but instead continue theircomplicated quantum-mechanical motion all way to zero temperature. Such a state has been proposed theoretically about 50 years ago but only now we have found promising materials (such as kagome lattice based spin-1/2 antiferromagnets) where this exotic quantum many-body state may indeed be realized. However confirming that a given material is indeed a spin liquid is not a trivial task at all. As a matter of fact theresimply no direct experimental way to check this. My main accomplishment in the past year was to come up witha new way to probe and identify the spin liquid state of matter. This is done with the help of frequently present in real materials deviations of interactions between spins from the simple isotropic exchange form. Particular form of such deviations, known under the name of Dzyaloshinskii-Moriya (DM) interaction, has been studied much in the past but mainly for the usual, magnetically ordered kind of materials. The PI has found that DM interaction plays the role of spin-orbit interaction of Rashba type typical for low-dimensionalconductors. This means that elementary excitations of the spin liquid, known as spinons, respond to DM interaction in the same way as the usual electrons respond to the Rashba spin-orbit interaction. This is very surprising because unlike electrons, spinons have no electric charge! Nonetheless, the discovered similarity allows one to borrow manywell-developed in the field of spintronics tools and apply them to behavior of electrically-neutral spinons. Graduate student Rachel Glenn has carried, under the PI's supervision, an interesting calculation of energy absorption by a system spinons subject to oscillating magnetic field. This calculation predicts peculiar lineshape and temperature dependenceof the signal, as well as its sensitivity to the polarization of the external field. We believe these results are of direct relevance to materials of triangular geometry, such as Cs2CuCl4, and also to those of kagome type, including very actively studied currently herbertsmithite antiferromagnet.
