Charge order-driven nematicity in the nickel-pnictide superconductor Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$

Thomas Johnson; Camille Bernal-Choban; Sangjun Lee; Xuefei Guo; Stella Sun; John Collini; Christopher Eckberg; Johnpierre Paglione; Rafael M. Fernandes; Eduardo Fradkin; Peter Abbamonte

arXiv · 2504.13133

Charge order-driven nematicity in the nickel-pnictide superconductor Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$

Thomas Johnson·Camille Bernal-Choban·Sangjun Lee·Xuefei Guo·Stella Sun·John Collini·Christopher Eckberg·Johnpierre Paglione·Rafael M. Fernandes·Eduardo Fradkin·Peter Abbamonte

View original source ↗Read source document ↗Export metadata ↓

Abstract

Nematic order refers to the spontaneous breaking of rotational symmetry while preserving translational symmetry. First identified in classical liquid crystals, nematic order arises from the collective alignment of anisotropic molecules. Its quantum counterpart, electronic nematicity, has been observed in a variety of quantum materials, ranging from unconventional superconductors to kagome metals. Despite its prevalence, there is no universal understanding of the conditions under which nematic order occurs. Electronic nematicity is most firmly established in iron-based superconductors, where it is understood to be a consequence of vestigial spin density wave (SDW) order. However, direct evidence for nematicity arising from other types of order are lacking. Here, we report direct evidence for charge-order-driven electronic nematicity in Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$, a nickel-based analog of the iron pnictides known to exhibit charge density wave (CDW) order. Using x-ray diffraction under applied uniaxial strain, we observe a pronounced symmetry-breaking response-up to $\sim 50 \%$-in the intensity of incommensurate CDW Bragg peaks, even at small strain levels ($\epsilon_{xy} \sim 10^{-3}$). This effect occurs within the same region of the phase diagram where a giant nematic susceptibility is observed in transport measurements. These results provide direct evidence that long-range CDW order can drive nematic behavior in quantum materials.

Explore related subjects

cond-mat.str-el cond-mat.mtrl-sci

Keep this discovery

BibTeX RIS

Thomas Johnson, Camille Bernal-Choban, Sangjun Lee, Xuefei Guo, Stella Sun, John Collini, Christopher Eckberg, Johnpierre Paglione, Rafael M. Fernandes, Eduardo Fradkin, Peter Abbamonte. 2025-04-17. Charge order-driven nematicity in the nickel-pnictide superconductor Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$. https://arxiv.org/abs/2504.13133

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

Competing Interlayer Loop Currents and Superconductivity in the Bilayer $t$-$J_\perp$-$V$ Model

The recent discovery of high-$T_c$ superconductivity in pressurized and thin-film bilayer nickelates, featuring a strong interlayer exchange coupling, and their potential similarities with cuprate superconductors, has made this a very active topic in condensed matter physics. In the present paper we study the strongly correlated one-orbital ($d_{x^2-y^2}$) bilayer $t$-$J_\perp$-$V$ model for nickelates, where $V$ denotes the Coulomb interactions, using a controlled large-$N$ expansion at and beyond the mean-field level. Focusing on the out-of-plane spin exchange interaction ($J_\perp$), we find that it triggers both out-of-plane $s$-wave superconductivity and an out-of-plane bond-order phase ($z$-BOP) instability. The $z$-BOP gives rise to a complex $z$-axis hopping dominated by its imaginary component, which drives out-of-plane currents and induces in-plane ones, spontaneously forming on the vertical plaquettes a loop-current state that breaks time-reversal symmetry. Competition between this loop-current phase and superconductivity yields a dome-shaped superconducting region, with optimal superconductivity occurring near the $z$-BOP quantum critical point. The resulting phase diagram features a pure loop-current region, a low-doping coexistence phase, a pure superconducting state at higher doping, and a correlated metallic state.

cond-mat.str-el↗

arXiv2026

Optically induced metallic state with persistent monoclinic symmetry in NdNiO$_3$

Understanding whether electronic and structural order remain coupled under nonequilibrium conditions is a central challenge in correlated materials. Here, we simultaneously track metallicity and symmetry across the photoinduced insulator-to-metal transition in NdNiO$_3$ using time-resolved optical reflectivity and symmetry-sensitive second-harmonic generation. We find that metallic reflectivity emerges at significantly lower excitation fluence than restoration of the orthorhombic high-temperature symmetry. As a result, optical excitation stabilizes a metastable state that combines the reflectivity of the metallic phase with the monoclinic symmetry of the insulating phase, revealing an optically induced monoclinic metal. Only at substantially higher fluences does the symmetry fully recover to that of the high-temperature phase. These results demonstrate a nonequilibrium decoupling of metallicity and structural symmetry and establish simultaneous multiprobe spectroscopy as a powerful approach for identifying emergent phases in correlated materials.

cond-mat.str-el↗

arXiv2026

Instabilities in self-consistent diagrammatic approaches and how to cure them

While self-consistent diagrammatic approaches are widely used to compute the physical properties of correlated quantum materials, their applicability may get severely hindered precisely in the parameter regions, where the most exciting physics is observed. One of the major issues, referred to as "misleading convergence", is the tendency of iterative schemes to converge to unphysical fixed points for intermediate-to-strong electronic interactions, regardless of numerical accuracy of the computation. Here, we explicitly verify that the origin of this problem in several established self-consistent many-electron approaches, defined in the general diagrammatic framework of the boson-exchange formalism, resides exclusively in the stability condition of the respective iteration schemes, and not in an intrinsic breakdown of their self-consistent diagrammatic description. This insight enables a simple and general remedy, as recently proposed in Phys. Rev. Lett. 137, 016502 (2026): The redefinition of the iterative procedure, by inverting the unstable eigendirections of the Jacobian associated to the fixed point of the self-consistent algorithm. We illustrate the successful outcome of this procedure by means of systematic calculations performed on testbed, exactly solvable, models. Our results demonstrate that the physical fixed point of the diagrammatic schemes we considered can be stabilized, de facto, across the entire parameter range, including the most challenging nonperturbative/strong-coupling regimes.

cond-mat.str-el↗