New connections among titanate, cuprate, and iron-pnictide superconductors

High-temperature superconductivity remains one of the biggest current challenges in condensed matter physics, even while the prospect of room-temperature superconductivity holds enormous potential as a transformative breakthrough in energy technology. In recent years, it has been found that unusual electronic states that break subtle symmetries of the crystal lattice are often found in close proximity to the superconducting state in high-temperature cuprate and iron-pnictide superconductors, but the exact relationship between superconductivity and these special electronic phases is not yet clear.

Very recently, research in the Billinge group has led to the discovery of a similar broken-symmetry state in an entirely new family of superconductors with the chemical formula BaTi2(As,Sb)2O (see Nature Communications article here). These titanium-based materials are not high-temperature superconductors, but they nevertheless share structural and chemical similarities with the cuprate and iron-pnictide superconductors, and it was suspected that they possessed some type of low-temperature electronic phase close to the superconducting state. Neutron scattering experiments at Los Alamos National Lab spearheaded by Emil and Ben showed that a long-range structural transition takes place at low temperature, with the result of lowering the lattice symmetry from tetragonal to orthorhombic. The rotational symmetry is likewise broken, changing from C4 in the tetragonal phase to C2 in the orthorhombic phase.

Complementary  electron diffraction measurements were carried out by collaborators of the Billinge group at Brookhaven. High-sensitivity measurements revealed that no additional superlattice Bragg peaks appear in the electron diffraction pattern, which proves that the low-symmetry phase does not break the lattice translational symmetry. This is significant because many broken-symmetry electronic states, such as conventional charge density waves, have a finite modulation length and therefore cause discrete superlattice peaks. On the other hand, the type of distortion discovered by these neutron and electron diffraction measurements breaks rotational symmetry while preserving translational symmetry. This is a special type of distortion called a nematic state, and similar nematic states have recently been found in the cuprates and iron-pnictides.

Using energy- and symmetry-based arguments, we showed that this nematic distortion can be naturally explained by a transfer of charge between neighboring Ti sites within the unit cell. This causes increased electrostatic repulsion along one of the crystallographic axes, thereby elongating one of the axes and distorting the C4-symmetric tetragonal structure to a C2-symmetric orthorhombic structure. Having discovered this low-temperature nematic phase and identified its origin, the next step is to study its relationship to the neighboring superconducting state. The discovery of nematic order in this additional group of superconductors will provide researchers with an entirely new material system for learning about unconventional superconductivity and its connection to nematicity, promising to yield deeper insights into the ongoing challenge of high-temperature superconductivity.

For more information, see the article published in Nature Communications: