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 GaTa_{4}Se_{8}, 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, GaTa_{4}Se_{8} (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 GaTa_{4}Se_{8}

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).