Condensed Matter Physics: Long-standing prediction of quantum physics experimentally proven

In the ground state the magnetic moments are either upward or downward, the spins antiparallel to the external magnetic field (red) are never together (right). By excitation, further spins can align antiparallel and Bethe chains are formed (white spins, left).

In the ground state the magnetic moments are either upward or downward, the spins antiparallel to the external magnetic field (red) are never together (right). By excitation, further spins can align antiparallel and Bethe chains are formed (white spins, left). © HZB

90 years ago, the physicist Hans Bethe postulated that unusual patterns, so-called Bethe strings, appear in certain magnetic solids. Now an international team has succeeded in experimentally detecting such Bethe strings for the first time. They used neutron scattering experiments at various neutron facilities including the unique high-field magnet of BER II* at HZB. The experimental data are in excellent agreement with the theoretical prediction of Bethe and prove once again the power of quantum physics.

The regular arrangement of atoms in a crystal allows complex interactions that can lead to new states of matter. Some crystals have magnetic interactions in only one dimension, i.e. are they magnetically one-dimensional. If, in addition, successive magnetic moments are pointing in opposite directions , then we are dealing with a one-dimensional antiferromagnet. Hans Bethe first described this system theoretically in 1931, predicting also the presence of excitations of strings of two or more consecutive moments pointing in one direction, so called Bethe strings. 

1D-model system to obserbe Bethe strings

However those string states could not be observed under normal experimental conditions because they are unstable and obscured by the other features of the system. The trick used in this paper is to isolate the strings by applying a magnetic field.

Now an international cooperation around the HZB physicist Bella Lake and her colleague Anup Bera was able to experimentally identify and characterise Bethe strings in a real solid for the first time. The team made crystals of SrCo2V2O8, which is a model system one-dimensional antiferromagnnet. Only the cobalt atoms have magnetic moments, they all are aligned along one direction and adjacent moments cancel each other out.

At BER II: External magnetic fields up to 25,9 Tesla

At the Berlin neutron source BER II it was possible to investigate the sample with neutrons under extremely high magnetic fields up to 25.9 Tesla. From the data, the physicists obtained a phase diagram of the sample as a function of the magnetic field, and also further information about the internal magnetic patterns, which could be compared with the idea of Bethe that were quantified by a theoretical group led by Jianda Wu.

Excellent agreement with theory

"The experimental data are in excellent agreement with the theory," says Prof. Bella Lake. "We were able to clearly identify two and even three chains of Bethe strings and determine their energy dependence. These results show us once again how fantastically well quantum physics works."

Nature Physics (2020): Dispersions of Many-Body Bethe Strings Anup Kumar Bera, Jianda Wu, Wang Yang, Robert Bewley, Martin Boehm, Jianhui Xu, Maciej Bartkowiak, Oleksandr Prokhnenko, Bastian Klemke, A. T. M. Nazmul Islam, Joseph Mathew Law, Zhe Wang and Bella Lake

DOI: 10.1038/s41567-020-0835-7

* After 46 years of successful research with neutrons, the operation of the Berlin research reactor BER II ended on 11 December 2019.  The BER II is to be dismantled over the next few years.

arö

You might also be interested in

  • Thermal insulation for quantum technologies
    Science Highlight
    19.05.2022
    Thermal insulation for quantum technologies
    New energy-efficient IT components often only operate stably at extremely low temperatures. Therefore, very good thermal insulation of such elements is crucial, which requires the development of materials with extremely low thermal conductivity. A team at HZB has now used a novel sintering process to produce nanoporous silicon aluminium samples in which pores and nanocrystallites impede the transport of heat and thus drastically reduce thermal conductivity. The researchers have developed a model for predicting the thermal conductivity, which was confirmed using experimental data on the microstructure of the samples and their thermal conductivity. Thus, for the first time, a method is available for the targeted development of complex porous materials with ultra-low thermal conductivity.
  • Magnetic nanoparticles in biological vehicles individually characterised
    Science Highlight
    17.05.2022
    Magnetic nanoparticles in biological vehicles individually characterised
    Magnetic nanostructures are promising tools for medical applications.  Incorporated into biological structures, they can be steered via external magnetic fields inside the body to release drugs or to destroy cancer cells. However, until now, only average information on the magnetic properties of those nanoparticles could be obtained, thus limiting their successful implementations in therapies. Now a team at HZB conceived and tested a new method to assess the characteristic parameters of every single magnetic nanoparticle.
  • Jan Lüning heads HZB Institute for Electronic Structure Dynamics
    News
    09.05.2022
    Jan Lüning heads HZB Institute for Electronic Structure Dynamics
    The HZB Institute for Electronic Structure Dynamics, newly founded on 1 May, develops experimental techniques and infrastructures to investigate the dynamics of elementary microscopic processes in novel material systems. This will help to optimise functional materials for sustainable technologies.