Wang Zhe, an alumnus of Grade 2004, and his associates reported on Nature on February 8, 2018, that they had observed Bethe Strings of quantum many-body systems for the first time.
In 1931, German physicist Hans Bethe published his paper “Eigenwerte und Eigenfunktionen der linearen Atomkette” and put forward a theoretical method of precisely studying one-dimensional quantum many-body systems on the basis of the theory by another physicist Werner Heisenberg. Through a method of statistical physics known as Bethe Ansatz today, he could precisely describe the Bethe strings that reflect electronic collective behaviors. The Bethe strings are also known as Complex Bound States Induced by Magnetism. Bethe theoretically predicted the existence of the Bethe strings in his 1931 paper. His theory has been further explored by later generations, but not until recently were proper experimental materials and highly complex technology found to show the existence of the Bethe strings.
The observation began when researchers synthesized SeCo2V2O8 Crystal and accomplished the antiferromagnetic Heisenberg-Ising spin chain. Then they observed the excited state of the spin chain in a strong magnetic field by using high resolution terahertz spectroscopy. Compared with the Bethe Ansatz calculation, the Bethe strings were finally observed in the spectrum. This discovery not only furthers the study of dynamics in quantum magnetic physics, but promotes research in other areas of physics, such as cold atom physics and string theory.
(Picture)
Figure A shows a ground state of SrCo2V2O8 spin chain. Figures B and C respectively show the string state of length 2 and length 3, that is, 2-string and 3-string.
Wang Zhe’s Bio:
2002-2008, bachelor’s degree, Kuang Yaming Honors School, Nanjing University
2008-2010, master’s degree, Augsburg University in Germany, University of Bordeaux in France
2010-2015, doctorate degree, Augsburg University in Germany
2015-2016, post doctorate, Augsburg University in Germany
2017-Present, post doctorate, Helmholtz Research Center in Dresden, Germany
More information:Zhe Wang et al. Experimental observation of Bethe strings, Nature (2018). DOI: 10.1038/nature25466
//m.phys.org/news/2018-02-bethe-experimentally.html
Abstract:Almost a century ago, string states—complex bound states of magnetic excitations—were predicted to exist in one-dimensional quantum magnets. However, despite many theoretical studies, the experimental realization and identification of string states in a condensed-matter system have yet to be achieved. Here we use high-resolution terahertz spectroscopy to resolve string states in the antiferromagnetic Heisenberg–Ising chain SrCo2V2O8 in strong longitudinal magnetic fields. In the field-induced quantum-critical regime, we identify strings and fractional magnetic excitations that are accurately described by the Bethe ansatz. Close to quantum criticality, the string excitations govern the quantum spin dynamics, whereas the fractional excitations, which are dominant at low energies, reflect the antiferromagnetic quantum fluctuations. Today, Bethe's result is important not only in the field of quantum magnetism but also more broadly, including in the study of cold atoms and in string theory; hence, we anticipate that our work will shed light on the study of complex many-body systems in general.
