Making the strange metal state in high temperature superconductors even stranger

Date:
October 27, 2021
Source:
Chalmers University of Technology
Summary:
Researchers have uncovered a striking new behavior of the 'strange
metal' state of high temperature superconductors. The discovery
represents an important piece of the puzzle for understanding
these materials.



FULL STORY ========================================================================== Researchers from Chalmers University of Technology, Sweden, have uncovered
a striking new behavior of the 'strange metal' state of high temperature superconductors. The discovery represents an important piece of the
puzzle for understanding these materials, and the findings have been
published in the journal Science.


========================================================================== Superconductivity, where an electric current is transported without any
losses, holds enormous potential for green technologies. For example, if
it could be made to work at high enough temperatures, it could allow for lossless transport of renewable energy over great distances. Investigating
this phenomenon is the aim of the research field of high temperature superconductivity. The current record stands at ?130 degrees celsius,
which might not seem like a high temperature, but it is when compared to standard superconductors which only work below ?230 degrees celsius. While standard superconductivity is well understood, several aspects of high temperature superconductivity are still a puzzle to be solved. The
newly published research focusses on the least understood property --
the so called 'strange metal' state, appearing at temperatures higher
than those that allow for superconductivity.

"This 'strange metal' state is aptly named. The materials really
behave in a very unusual way, and it is something of a mystery
among researchers. Our work now offers a new understanding of the
phenomenon. Through novel experiments, we have learned crucial new
information about how the strange metal state works" says Floriana
Lombardi, Professor at the Quantum Device Physics Laboratory at the
Department of Microtechnology and Nanoscience at Chalmers.

Believed to be based on quantum entanglement The strange metal state
got its name because its behavior when conducting electricity is, on
the face of it, far too simple. In an ordinary metal, lots of different processes affect the electrical resistance -- electrons can collide with
the atomic lattice, with impurities, or with themselves, and each process
has a different temperature dependence. This means that the resulting
total resistance becomes a complicated function of the temperature. In
sharp contrast, the resistance for strange metals is a linear function
of temperature -- meaning a straight line from the lowest attainable temperatures up to where the material melts.

"Such a simple behavior begs for a simple explanation based on a powerful principle, and for this type of quantum materials the principle is
believed to be quantum entanglement." says Ulf Gran, Professor at the
Division of Subatomic, High-Energy and Plasma Physics at the Department
of Physics at Chalmers.

"Quantum entanglement is what Einstein called 'spooky action at a
distance' and represents a way for electrons to interact which has
no counterpart in classical physics. To explain the counterintuitive
properties of the strange metal state, all particles need to be entangled
with each other, leading to a soup of electrons in which individual
particles cannot be discerned, and which constitutes a radically novel
form of matter." Exploring the connection with charge density waves
The key finding of the paper is that the authors discovered what kills
the strange metal state. In high temperature superconductors, charge
density waves (CDW), which are ripples of electric charge generated by
patterns of electrons in the material lattice, occur when the strange
metal phase breaks down. To explore this connection, nanoscale samples
of the superconducting metal yttrium barium copper oxide were put under
strain to suppress the charge density waves.

This then led to the re-emergence of the strange metal state. By
straining the metal, the researchers were able to thereby expand the
strange metal state into the region previously dominated by CDW --
making the 'strange metal' even stranger.

"The highest temperatures for the superconducting transition have been
observed when the strange metal phase is more pronounced. Understanding
this new phase of matter is therefore of utmost importance for being
able to construct new materials that exhibit superconductivity at even
higher temperatures," explains Floriana Lombardi.

The researchers' work indicates a close connection between the emergence
of charge density waves and the breaking of the strange metal state --
a potentially vital clue to understand the latter phenomenon, and which
might represent one of the most striking evidence of quantum mechanical principles at the macro scale. The results also suggest a promising new
avenue of research, using strain control to manipulate quantum materials.

========================================================================== Story Source: Materials provided by
Chalmers_University_of_Technology. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Eric Wahlberg, Riccardo Arpaia, Go"tz Seibold, Matteo Rossi, Roberto
Fumagalli, Edoardo Trabaldo, Nicholas B. Brookes, Lucio Braicovich,
Sergio Caprara, Ulf Gran, Giacomo Ghiringhelli, Thilo Bauch,
Floriana Lombardi. Restored strange metal phase through suppression
of charge density waves in underdoped YBa 2 Cu 3 O 7-d. Science,
2021; 373 (6562): 1506 DOI: 10.1126/science.abc8372 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211027085412.htm

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