Electrons in a solid usually either run freely (metal) or are locked in place (insulator). But sometimes strong mutual repulsion locks them into a halfway state: charges stand still, while spins, tiny magnetic arrows, continue to live their own life. This is called a Mott insulator.

The authors of the new work built a one-dimensional chain of atoms of two types — A and B. On A sites sits a fixed electron that strongly repels neighbors. On B sites, electrons are mobile. After a mathematical projection, a lattice of only B sites emerged, but with an unusual interaction: electrons feel each other at a distance set by a special momentum scale k*. Adding antiferromagnetic interaction J between neighboring spins, the scientists discovered a phase of Mott semimetal.

Numerical calculations using exact diagonalization and renormalization group showed that the ground state has central charge c=3—a number characterizing the types of collective motions. Upon symmetry breaking, the particle-hole semimetal smoothly turns into an insulator, with polarization jumping by exactly 1/2. Such a jump is a clear signature of a topological transition.

The work provides the first reliable example of a Mott semimetal in the ground state. It is a step toward understanding how exotic phases with charge-spin separation emerge from simple ingredients — repulsion, exchange interaction, and symmetries.