Abstract

The formation of long range order in a material undergoing a phase transition is accompanied by the breakdown of a corresponding symmetry. Magnetic order for example, breaks spin rotational symmetry, while superconductivity breaks a global phase symmetry, and density waves break translational symmetry. Apart from these direct effects on the symmetry of the material, concomitant changes in the interactions of the lattice, orbital, spin and charge degrees of freedom may lead to the ‘accidental’ breakdown of additional symmetries. These additional broken symmetries are key in our understanding of many material properties.

In this seminar, I will describe how the onset of a specific charge ordered state may be accompanied by the breakdown of inversion symmetry, giving rise to a polar or even a chiral phase. Unlike the case of magnetic materials, in which this type of broken inversion symmetry is common, owing to the ease of forming helical arrangements of spin vectors, the scalar charge density is not so readily coerced into a chiral arrangement. I will indicate how the interplay between charge and orbital degrees of freedom is essential in the formation of this novel state of matter.

The chiral or polar charge and orbital order can be observed using a variety of experimental probes. I will discuss recent X-ray and transport measurements performed at Argonne’s Materials Science Division, which provide the first experimental evidence for the presence of a hierarchy of transitions involved in the onset of chiral charge order in TiSe2, in close agreement with theoretical predictions.

References: 1. J. Ishioka et al., Phys. Rev. Lett., 105, 176401 (2010). 2. J. van Wezel, Europhys. Lett., 96, 67011 (2011). 3. J. van Wezel, Phys. Rev. B, 85, 35131 (2012). 4. J.-P. Castellan et al., arxiv/1204.1374