National High Magnetic Field Laboratory
Los Alamos National Laboratory
With advancements in the way very high magnetic fields are produced in the laboratory, a new class of physical phenomena has attracted attention into an area of condensed matter physics seldom visited: the physics of magnetic elemental excitations that are created and live in quantum paramagnets. These excitations, known as triplons, behave in ways that can be described with identical formalism to that used for neutral atoms that form condensates or electron-pairs that superconduct. While the thermodynamic phase transition that leads into these states has been studied in excruciating detail in liquid He (including gravity-less experiments carried out in the NASA Space Shuttle), the quantum phase transition had to wait for longer since it is hard to change the chemical potential continuously in an atomic gas. In the case of triplons, instead, high magnetic fields can be used to induce a quantum phase transition by chemical potential, i.e. controlling the concentration of particles in a reversible way at temperature near the absolute zero. In these conditions, when the host material is highly symmetric, the magnetic phase transition into a XY-AFM state is mathematically indistinguishable from a Bose-Einstein condensation. On the other hand, if the underlying crystal symmetry is complex the resulting magnetic frustration has dramatic effects and creates ideal conditions for the observation of dimensional reduction such as first observed in the ancient Chinese pigment BaCuSi2O6, [1-3] and magnetic texture with similar origin to that of Quantum Hall Effect in the Shastry-Sutherland compound SrCu2(BO3)2.  In my seminar I will discuss recent experiments conducted at the National High Magnetic Field Laboratory in the above and other compounds , as well as some theoretical efforts to model our results.
*Work done in collaboration with Neil Harrison, Suchitra Sebastian, Vivien Zapf, and Cristian Batista, among several others.
References: 1. M. Jaime et al., Phys. Rev. Lett. 93, 087203 (2004). 2. S. Sebastian et al., Phys. Rev. B 72, 100404 (2005). 3. S. Sebastian et al., Nature 441, 617 (2006). 4. S. Sebastian et al., arXiv:0707.2075v1 [cond-mat.str-el] 5. V.S. Zapf et al., Phys. Rev. Lett. 96, 077204 (2006).