Bani(as,p) Superconductivity Reveals Orbital Ordering and Symmetry Breaking in Charge Density Waves

The complex behaviour of electrons in materials can give rise to exotic states of matter, and recent research focuses on understanding these interactions in the superconductor barium nickel arsenide phosphide. Tom Lacmann, Robert Eder, and Igor Vinograd, along with their colleagues, investigate how the arrangement of electron orbitals relates to the formation of charge density waves within this material. Their work demonstrates that both common forms of these charge density waves strongly respond to resonant X-ray scattering, revealing a surprising link to the ordering of nickel’s electron orbitals. This discovery provides crucial insight into the fundamental mechanisms driving the material’s properties and sheds light on the subtle symmetry changes that occur as it approaches superconductivity, potentially guiding the development of higher-temperature superconductors.

Scientists investigate the interplay between orbital order and charge density waves (CDWs) in the superconductor BaNi₂(As₁₋ₓPₓ)₂ using resonant X-ray scattering. Both the incommensurate and commensurate CDWs in this material exhibit strong resonant enhancement, indicating a strong connection to the arrangement of electrons in specific orbitals. This enhancement varies with energy and polarization, confirming the presence of orbital ordering that influences the formation of these charge density waves. The research aims to understand how the arrangement of electrons in specific orbitals influences the formation and behaviour of charge density waves within this superconducting material, potentially revealing new insights into the mechanisms governing superconductivity and related phenomena.

Charge Density Waves in Nickel Arsenides Investigated

This study details research on charge density waves (CDWs) in nickel-based materials, specifically barium nickel arsenide and related compounds. Researchers employed resonant inelastic X-ray scattering and X-ray absorption spectroscopy, carefully analyzing the polarization of the scattered X-rays to determine the symmetry of the CDW order and whether it breaks rotational symmetry, potentially leading to a nematic phase. Analysis of the data reveals the tensorial properties of the CDW susceptibility, which describes the material’s response to external fields, and uses mathematical tools to describe the CDW order. The research suggests the possibility of an electronic nematic liquid phase in barium nickel arsenide.

Orbital Order Drives Charge Density Wave Formation

Resonant X-ray scattering experiments have revealed crucial details about the interplay between orbital order and charge density waves (CDWs) in the superconductor BaNi₂(As₁₋ₓPₓ)₂. Scientists discovered that both the incommensurate and commensurate CDWs exhibit strong resonant enhancement, indicating a direct link to the arrangement of electron orbitals within the material. Detailed analysis of the scattering patterns demonstrates a lowering of the local symmetry around the nickel atoms, suggesting a distortion from a simple tetragonal structure. Importantly, the observed CDW signatures are dominated by contributions from specific nickel orbitals, the dxz and dyz, and share a similar orbital character despite differing in their spatial arrangement, supporting a shared, orbitally-driven mechanism for the formation of both CDW phases.

Further investigations reveal that the symmetry lowering at the nickel sites represents a local distortion of the electronic environment, and the incommensurate CDW’s atomic form factor remains robust across a range of phosphorus doping levels. Remarkably, the similarities in resonant behavior between the incommensurate and commensurate CDWs, despite differences in their average structures and responses to pressure, suggest a common underlying orbital mechanism. The research identifies the Ni dxz,yz orbitals as primary contributors to the CDW states, and confirms that the observed CDW peaks arise from a genuine modulation of electronic charge density associated with orbital degrees of freedom. Despite differences in their crystallographic structures and responses to external factors like pressure, both CDWs share a similar orbital signature, providing new insight into the symmetry breaking and orbital fluctuations occurring in the high-temperature regime of the superconductor. Differences in their underlying lattice dynamics exist, as evidenced by the identification of a driving phonon mode for only one of the CDW transitions.