What did the Scientists Discover?
Chirality – or handedness – is a symmetry property in nature that can be found in many forms and on many length scales, from the spiral-staircase design of DNA to extended spiral galaxies. It simply means that a chiral system is non-superposable on its mirror image. A chiral system we are all very familiar with are our hands: When the thumbs point in the same direction, the palms are opposite. Your hands are mirror images, but not superposable. Each hand is therefore chiral. It has long been debated whether the electrons inside a material responsible for conducting electric current can spontaneously develop chirality, even if the atomic structure of the crystal itself is not chiral. Theory predicts that this type of “spontaneous gyrotropic electronic order” can exist, but experimental evidence has been controversial. Now, a team of researchers lead by Nuh Gedik’s group (MIT) have demonstrated spontaneous chiral symmetry breaking by electrons in a material called 1T-TiSe2, despite its non-chiral crystal structure. They employed a new measurement technique, the “circular photogalvanic effect”: A change in photocurrent is measured when the helicity of light is flipped, which can only occur if the electrons in the material are chiral.
What are the broader impacts of this work?
Electronic symmetry breaking in materials underlies many unconventional materials properties which can be useful in future quantum technologies employed in information technology and novel approaches to data processing.
Why is this important?
Ordinarily, a material would contain equal amounts of the right- and left-handed versions of the charge density waves, and the effects of handedness would cancel out in most measurements. But under the influence of the polarized light, the material mostly prefer one of these chiralities. This similar to the way a magnetic field can induce a magnetic orientation in a material where ordinarily its moments are randomly oriented and thus have no net magnetic effect. In the present research, the researchers have not only observed chirality of conduction electrons but also identified a way to control it.
Why did this research need CHESS?
The gyrotropic phase in TiSe2 emerges from a parent “charge-density-wave” (CDW) ordered state. The CHEXS user program in quantum materials, i.e. the NSF funded QM2 beamline, excels at using x-ray diffraction to identify and understand charge density waves. By comparing the temperature dependence of the circular photogalvanic effect (CPGE, left top) with the temperature dependence of the intensity of CDW peaks measured using x-rays (left bottom), it becomes clear that the CPGE identifies a new, lower-temperature phase transition, distinct from the CDW as predicted by theory.
- Su-Yang Xu, Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA
- Qiong Ma, Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA
- Yang Gao, Department of Physics, Carnegie Mellon University, Pittsburgh, PA, USA
- Anshul Kogar, Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA
- Alfred Zong, Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA
- Andrés M. Mier Valdivia, Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA
- Thao H. Dinh, Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA
- Jacob P. C. Ruff, CHESS, Cornell University, Ithaca, NY, USA
- Kenji Watanabe, National Institute for Materials Science, Tsukuba, Japan
- Takashi Taniguchi, National Institute for Materials Science, Tsukuba, Japan
- Hsin Lin, Institute of Physics, Academia Sinica, Taipei, Taiwan
- Goran Karapetrov, Department of Physics & Department of Materials Science and Engineering, Drexel University, Philadelphia, PA, USA
- Di Xiao, Department of Physics, Carnegie Mellon University, Pittsburgh, PA, USA
- Pablo Jarillo-Herrero, Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA
- Nuh Gedik, Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA
Xu, S., Ma, Q., Gao, Y. et. all, “Spontaneous gyrotropic electronic order in a transition-metal dichalcogenide” Nature 578 , 545–549 (2020). https://doi.org/10.1038/s41586-020-2011-8
How was the work funded?
DoE BES, DMSE; Gordon and Betty Moore Foundation EPiQS Initiative (GBMF4540 and GBMF4541); AFOSR (FA9550-16-1-0382); The Center for the Advancement of Topological Semimetals (an Energy Frontier Research Center funded by DoE BES, through the Ames Laboratory (DE-AC02-07CH11358); MRSEC Shared Experimental Facilities supported by the NSF (DMR-0819762); NSF (ECCS-1711015); CHESS and CHEXS (DMR-1332208 and DMR-1829070); DoE BES (DE-SC0012509); Elemental Strategy Initiative, MEXT, Japan, JSPS KAKENHI (JP18K19136) and CREST (JPMJCR15F3); Young Scholar Fellowship Program in Taiwan, Columbus Program (MOST108-2636-M-006-002); MOST, Taiwan (MOST107-2627-E-006-001 and 105-2112-M-110-014-MY3). Higher Education Sprout Project, Ministry of Education, National Cheng Kung University; Academia Sinica Taiwan, Innovative Materials and Analysis Technology Exploration (AS-iMATE-107-11)