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A solid state physics analogue of the water-vapor transition

The electrical properties of a solid may show a transition between a "fluid" conductive state and an insulating state where the electrons suddenly become localized at their sites. Theorists predicted this transition to be the analogue of the familiar water-vapor transition. An actual realization has been sought for many decades.

Mott predicted in the 40s that, as a function of the electronic density, a solid state system may
undergo a dramatic transition between a metallic and an insulator state. This follows
from the competition between the potential energy, due to the Coulomb repulsion between
electrons, and their kinetic energy. Eventually, this metal-insulator transition (MIT)
became known as the Mott transition and it is now a paradigmatic example of strongly
correlated electron physics. In the 90s, theoretical work on the Hubbard model predicted
that the Mott transition in magnetically disorder systems should occur along a first
order line and end in a finite-temperature second-order critical point, just like in the
familiar liquid-gas transition of water.

Figure 1 : Strongly interacting electronic structure of GaTa4Se8, obtained by LDA+DMFT calculations (color intensity plot). With respect to the LDA bands(solid lines), one observes that the Coulomb repulsion opens a Mott gap at the Fermi energy, and turns the system into a strongly correlated Mott insulator. In the actual systems the gap can be collapsed by external pressure.


Since then, condensed matter physicists have been searching for an actual realization of the Mott transition.
A team of researchers from the LPS, the IMN (Nantes) and Argentina have recently reported in the
Physical Review Letters the finding of such a system, GaTa4Se8 (GTS).

The relative strength of the Coulomb and kinetic energy in GTS can be controlled in practice by
applying high pressure. In fact, pressures of the order of GPa (about the pressure
due to the weight of an elephant standing on a high-heel shoe) turn our to be necessary to
discover the first order MIT in this compound. In addition, it was necessary to perform
the experiment at temperatures well below -200 °C.

Figure 2 : Resistivity as a function of temperature at 3.8 GPa. The data was obtained upon cooling and heating. They show a dramatic first-order jump. Inset : The metal-insulator transition curves for various pressures show more than nine orders of
magnitude change in the resistivity at low temperature, while they are almost identical above 100 K.


The experiments and theoretical results published in this work demonstrated that the GTS phase diagram is
in excellent agreement with the theoretical expectations for the Mott transition. In particular,
the work reported the finding of the long-sought hallmark of the first-order metal-insulator
transition, namely a strong hysteresis effect in the resistivity as a function of
temperature and pressure.


Contact :
Marcelo Rozenberg

Reference :
First-Order Insulator-to-Metal Mott Transition in the Paramagnetic 3D System GaTa4Se8
A. Camjayi, C. Acha, R. Weht, M.G. Rodriguez, B. Corraze, E. Janod, L. Cario, and M.J. Rozenberg
Physical Review Letters 113, 086404 (2014).