Submitted by Pooja Pandey on Thu, 19/10/2023 - 14:58
Scientists in a team led by Prof Malte Grosche from the University of Cambridge have observed how electron-electron interactions can transform a conductor into an insulator. Their experiment, which combined extreme pressures, low temperatures, and high magnetic fields, showed that as the interaction strength increases, the charge carriers slow down, validating ideas that have first been proposed over five decades ago. This phenomenon underlies a wide range of exotic electronic properties in quantum materials, particularly in the realm of unconventional superconductors.
This finding reveals a more nuanced view of the 1970 Brinkman-Rice paradigm and motivates further studies of the metal/insulator boundary region, which is proving a fertile ground for the discovery of novel functional quantum materials. Malte Grosche
At the heart of their research lies the concept of electron-electron interactions in crystalline metals, which can effectively slow down charge carriers, making them appear "heavier." In 1970, Brinkman and Rice proposed that in a class of materials called Mott insulators, the repulsion between electrons can become so dominant that they cannot hop past each other in a lattice, resulting in grid-lock as their effective mass diverges. Many metals near the boundary of this state tend to exhibit exotic electronic properties, such as unconventional high-temperature superconductivity in copper- and nickel-oxide compounds.
While the Brinkman-Rice model and its derivatives have been instrumental in explaining electronic properties of various materials, experimental validation of this fundamental idea of divergent charge carrier mass has eluded scientists for decades. However, this challenge has finally been overcome by a collaboration between researchers from the Quantum Matter group at the Cavendish Laboratory, the University of Bristol, and high magnetic field laboratories in the Netherlands (HFML Nijmegen) and the USA (NHMFL Tallahassee).
Their findings published in PNAS, describe how the project utilised cutting-edge experimental techniques to track the evolution of electronic states as the boundary between insulator and metal was crossed. “With the help of diamond anvil cells, ultra-pure crystals of Mott-insulating compound NiS2 were subjected to high hydrostatic pressure of above 100 000 atmospheres, turning the material into a metallic conductor. Cooling the sample down to sub-Kelvin temperatures, and applying magnetic fields of up to 35 T, the scientists then used the technique of quantum oscillations, probing tiny changes in conductivity at the level of 0.1 parts per million to extract charge carrier velocities and tracking their values as the crystal was tuned towards the insulating regime with pressure,” shares Prof Malte Grosche, Cavendish Laboratory.
The results show that as the material neared the insulating state from the metallic side, the effective mass of electrons initially grew in a divergent manner, seemingly corroborating the long-standing theory. However, just before the charge carrier velocity reached zero, the material abruptly transitioned into an insulator, cutting short the expected gradual mass divergence.
"This finding reveals a more nuanced view of the 1970 Brinkman-Rice paradigm and motivates further studies of the metal/insulator boundary region, which is proving a fertile ground for the discovery of novel functional quantum materials," adds Grosche.
Reference:
Semeniuk, K., et al. 'Truncated Mass Divergence in a Mott Metal', Proc. Natl. Acad. Sci. U.S.A. 2023, 120 (38), e2301456120. DOI: 10.1073/pnas.2301456120.
Image: Dilution refrigerator insert for the group’s 20.4 T cryomagnet | Credit: Prof. Friedrich Malte Grosche