Paul Scherrer Institute and EPFL, Switzerland

Abstract：

The core business of condensed matter physics is to find new ground states, new phase transitions and new excitations. In this seminar I review three recent projects in which high magnetic fields applied to quantum magnetic materials induced new physics in all three categories. In SrCu2(BO3)2, which provides a faithful realisation of the Shastry-Sutherland model, the magnetic excitation spectrum contains not only one-triplon excitations but a number of two-triplon bound-state branches. In an applied field, the one- and two-triplon states compete to condense first, and in a recent high-field experiment we have demonstrated that it is a two-triplon state that wins, forming a new type of spin-nematic ground state over the range from 20 to 26 T.

The dimer-based bilayer compound BaCuSi2O6 was proposed to deviate in a different way from the conventional scenario of field-induced triplon condensation due to a frustration-induced reduction of the effective dimensionality, but it was shown later that the interlayer coupling is ferromagnetic and hence free of frustration. We have explained the observed behaviour with by discovering that the system contains three magnetically inequivalent bilayers, with ratios 3:2:1, whose differing interaction parameters create an extra field-temperature scaling regime near the quantum critical point that has a non-trivial but non-universal critical exponent, a result highly relevant to the modulated two-dimensional structures now being created by atomic layering methods.

Despite 50 years as the poster child of frustrated quantum magnetism, the triangular-lattice Heisenberg antiferromagnet remains poorly quantified in most aspects. The new family of Yb-delafossites offers an excellent realisation of the nearest-neighbour model with energy scales such that magnetic saturation is achieved below 10 T. We have exploited this situation to perform detailed measurements of the excitation spectrum at all fields, and have accompanied these by cylinder MPS calculations of the spectrum that provide full quantitative agreement, opening the way for a definitive identification of the analytical framework capturing the physics of this paradigm model.

Brief CV of Prof. Bruce Normand:

Bruce Normand received his Bachelor’s degree from the University of Cambridge and his PhD from MIT, working on high-temperature superconductivity. Has worked extensively in the field of quantum magnetism, in the identification and characterisation of novel phases, phase transitions and excitation spectra, and most recently on nonequilibrium phenomena. His work is always performed close to materials and experiment, with a view to prediction and observation, and often in collaboration with specialists applying advanced numerical methods.  After postdoctoral research in Japan and Europe, he was for some years a Distinguished Professor at Renmin University of China and is now a senior staff scientist at the Paul Scherrer Institute and the Ecole Polytechnique Fédérale Lausanne.

邀  请  人：向    涛    研究员

联  系  人：傅    琦（82649469）