KIT - Campus South - Wolfgang-Gaede-Str. 1
Seminar Room 10.01, Bldg. 30.23 (Physikhochhaus)

11.01.2010

Dr. Salvatore R. Manmana
Ecole Polytechnique Federale de Lausanne, Switzerland

14:00

Abstract: In addition to the usual Heisenberg type of exchange interactions, real quantum magnetic materials possess anisotropic interactions due to the presence of spin-orbit coupling. This leads to small contributions of Dzyaloshinskii-Moriya (DM) type which, in spite of their smallness, may alter the physics of the system significantly or introduce new, interesting aspects. In this talk, I will discuss the implications of DM interactions on two-leg ladders and Shastry-Sutherland tubes in strong magnetic fields and the kagome lattice with non-magnetic impurities in weak magnetic fields. The common starting point of the considerations for these very different systems will be a single dimer with DM interaction in a magnetic field. For the ladder systems in the strong rung limit we find pronounced peak anomalies at magnetization plateaux which can be understood in terms of the single dimer picture and which we propose to use for the precise determination of critical values of the fields in ongoing magnetic torque measurements, e.g., on SrCu₂(BO₃)₂, a system which is modeled by the Heisenberg model on a 2D Shastry-Sutherland lattice. This picture is supported by results for Shastry-Sutherland spin tubes, which represent a quasi-1D version of the Shastry-Sutherland lattice. For the kagome system in weak magnetic fields, we find that the basic picture provided by single dimers can be recognized in the very close vicinity of the impurity for weak DM interactions due to the existing frustration relief. While the bulk system shows a phase transition to the semiclassical 120◦ state at D/J ∼ 10%, surprisingly, the dimers next to the vacancy remain strong up to D/J ∼ 1 − 3, i.e., well above the critical point. Implications for the interpretation of recent NMR experiments on Herbertsmithite ZnCu₃(OH)₆Cl₂ will be discussed.