New Insight into the Dynamics of Superconductivity

From quantum computers to sustainable energy, high-temperature superconductors are capable of transforming present-day technologies.

Deciphering previously invisible dynamics in superconductors—Higgs spectroscopy could make this possible: Using cuprates, a high-temperature superconductor, as an example, an international team of researchers has been able to demonstrate the potential of the new measurement method. By applying a strong terahertz pulse (frequency ω), they stimulated and continuously maintained Higgs oscillations in the material (2ω). Driving the system resonant to the Eigenfrequency of the Higgs oscillations in turn leads to the generation of characteristic terahertz light with tripled frequency (3ω). Image Credit: HZDR/Juniks.

But in spite of rigorous studies, there is still a lack of fundamental interpretation needed to create these intricate materials for extensive applications. “Higgs spectroscopy” can possibly bring about a watershed as it shows the dynamics of combined electrons present in superconductors.

Now, an international group of researchers from the Max Planck Institute for Solid State Research (MPI-FKF) and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has described the latest measuring technique in the Nature Communications journal.

Fascinatingly, the dynamics also expose the characteristic precursors of superconductivity, even beyond the critical temperature limit at which the analyzed materials achieve superconductivity. Electric current is transported by superconductors without any energy loss. The use of superconductors could drastically cut down people’s energy needs; however, superconductivity demands temperatures of 140 °C and below. Below this point, materials merely “turn on” their superconductivity.

All the recognized superconductors call for extensive cooling techniques, which render them impractical for daily purposes. High-temperature superconductors like cuprates—novel materials based on copper oxide—have demonstrated potential improvement. But in spite of several years of research efforts, their actual mode of operation continues to be vague. It is believed that Higgs spectroscopy may change that.

Higgs Spectroscopy Allows New Insights into High-Temperature Superconductivity

Higgs spectroscopy offers us a whole new ‘magnifying glass’ to examine the physical processes,” reported Dr Jan-Christoph Deinert. At the HZDR Institute of Radiation Physics, the scientist is working on the latest technique along with collaborators from the MPI-FKF, the University of Tokyo, the University of Stuttgart, and other international research institutions.

The researchers mainly wanted to find out how electrons are able to create pairs in high-temperature superconductors.

In the case of superconductivity, the electrons combine together to produce “Cooper pairs,” which allow them to travel via the material in pairs without making any communication with their setting. However, it is still not known what causes the two electrons to bind together when their charge, in fact, makes them repel each other.

There is a physical explanation for traditional superconductors: “The electrons pair up because of crystal lattice vibrations,” described Professor Stefan Kaiser, one of the main authors of the study who is exploring the dynamics in superconductors at the University of Stuttgart and the MPI-FKF.

One electron deforms the crystal lattice, which subsequently pulls the second electron. But in the case of cuprates, it is still not known which kind of mechanism acts in lieu of lattice vibrations.

One hypothesis is that the pairing is due to fluctuating spins, i.e. magnetic interaction. But the key question is: Can their influence on superconductivity and in particular on the properties of the Cooper pairs be measured directly?

Stefan Kaiser, Study Main Author and Professor, Max Planck Institute for Solid State Research

“Higgs oscillations” enter the stage at this point: In high-energy physics, Higgs oscillations explain why elementary particles possess mass. However, they also take place in superconductors, where they can be activated by powerful laser pulses.

Higgs oscillations denote the oscillations of the order parameter—that is, the measure of the superconductive state of a material, which means the Cooper pairs’ density. So much for the concept.

A few years ago, an initial experimental proof turned out to be a success when scientists at the University of Tokyo utilized an ultra-short light pulse to activate Higgs oscillations in traditional superconductors—similar to setting a pendulum in motion.

But with regard to high-temperature superconductors, such a one-off pulse is not sufficient, because the system is dampened excessively by interactions between the non-superconducting and superconducting electrons and the complex symmetry of the ordering parameter.

Terahertz Light Source Keeps the System Oscillating

Now, due to Higgs spectroscopy, the research team centered around HZDR and the MPI-FKF has accomplished the experimental innovation for high-temperature superconductors. The researchers’ solution was to utilize a multi-cyclic and highly intense terahertz pulse that is optimally adjusted to Higgs oscillation and can sustain it in spite of the damping factors—constantly poking at the metaphorical pendulum.

Using the high-performance terahertz light source TELBE at HZDR, the scientists successfully delivered as many as 100,000 such pulses across the samples per second.

Our source is unique in the world due to its high intensity in the terahertz range combined with a very high repetition rate. We can now selectively drive Higgs oscillations and measure them very precisely.

Dr Jan-Christoph Deinert, Institute of Radiation Physics, HZDR

This success can be attributed to the close association between the experimental and theoretical researchers. The concept was hatched at MPI-FKF; the experiment was carried out by the TELBE research team, headed by Dr Jan-Christoph Deinert and Dr Sergey Kovalev at HZDR under the supervisor of the then group leader Professor Michael Gensch, who is currently researching at TU Berlin and the German Aerospace Center.

The experiments are of particular importance for the scientific application of large-scale research facilities in general. They demonstrate that a high-power terahertz source such as TELBE can handle a complex investigation using nonlinear terahertz spectroscopy on a complicated series of samples, such as cuprates,” stated Professor Gensch.

That is the reason why the scientists are expecting to see a huge demand in the days to come.  

Higgs spectroscopy as a methodological approach opens up entirely new potentials. It is the starting point for a series of experiments that will provide new insights into these complex materials. We can now take a very systematic approach.

Dr Hao Chu, Study Main Author and Postdoc, Max Planck-UBC-UTokyo Center for Quantum Materials

Just Above the Critical Temperature: Where Does Superconductivity Start?

When performing many sets of measurements, the scientists initially demonstrated that their technique works for regular cuprates. However, below the critical temperature, the researchers were able to activate Higgs oscillations and also demonstrated that a novel and formerly unseen excitation communicates with the Higgs oscillations of the Cooper pairs.

Additional experiments would be able to expose whether such interactions are indeed magnetic interactions, as strongly argued in expert circles.

The scientists also saw indications that the Cooper pairs are capable of forming above the critical temperature, but without oscillating together. Earlier, other measuring techniques have proposed the possibility of the early formation of the Cooper pairs. Higgs spectroscopy can potentially support this theory and explain how and when the pairs develop, and the factors that cause them to oscillate together in the superconductor.

Journal reference:

Chu, H., et al. (2020) Phase-resolved Higgs response in superconducting cuprates. Nature Communications. doi.org/10.1038/s41467-020-15613-1.

Source: https://www.hzdr.de/

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