arXiv subjects

Robert J Birgeneau

Publications and source records attributed to Robert J Birgeneau.

3 recordsLinked to original sources

arXiv2025

Spin Excitations and Flat Electronic Bands in a Cr-based Kagome Superconductor

In the quest for topology- and correlation-driven quantum states, kagome lattice materials have garnered significant interest for their band structures, featuring flat bands (FBs) from the quantum destructive interference of the electronic wavefunction. Tuning an FB to the chemical potential could induce electronic instabilities and emergent orders. Despite extensive studies, direct evidence of FBs tuned to the chemical potential and their role in emergent orders in bulk materials remains lacking. Using angle-resolved photoemission spectroscopy, resonant inelastic X-ray scattering, and density functional theory, we show that the low-energy structure of the Cr-based kagome metal superconductor {\Cr} is dominated by FBs at the Fermi level. We also observe low-energy magnetic excitations evolving across the low-temperature transition, largely consistent with the FB shift. Our results suggest that the low-temperature order contains a magnetic origin and that the kagome FBs may play a role in the emergence of this order.

cond-mat.str-el↗

arXiv2024

Charge Density Fluctuations with Enhanced Superconductivity at the Proposed Nematic Quantum Critical Point

A quantum critical point (QCP) represents a continuous phase transition at absolute zero. At the QCP of an unconventional superconductor, enhanced superconducting transition temperature and magnetic fluctuations strength are often observed together, indicating magnetism-mediated superconductivity. This raises the question of whether quantum fluctuations in other degrees of freedom, such as charge, could similarly boost superconductivity. However, because charge is frequently intertwined with magnetism, isolating and understanding its specific role in Cooper pair formation poses a significant challenge. Here, we report persistent charge density fluctuations (CDF) down to 15 K in the non-magnetic superconductor Sr$_{0.77}$Ba$_{0.23}$Ni$_{2}$As$_{2}$, which lies near a proposed nematic QCP associated with a six-fold enhancement of superconductivity. Our results show that the quasi-elastic CDF does not condense into resolution-limited Bragg peaks and displays non-saturated strength. The CDF completely softens at 25 K, with its critical behavior described by the same mathematical framework as the antiferromagnetic Fermi liquid model, yielding a fitted Curie-Weiss temperature of $θ\approx 0$ K. Additionally, we find that the nematic fluctuations are not lattice-driven, as evidenced by the absence of softening in nematic-coupled in-plane transverse acoustic phonons. Our discovery positions Sr$_{x}$Ba$_{1-x}$Ni$_{2}$As$_{2}$ as a promising candidate for charge-fluctuation-driven nematicity and superconductivity.

cond-mat.supr-con↗

arXiv2021

Charge ordering in superconducting copper oxides

Charge order has recently been identified as a leading competitor of high-temperature superconductivity in moderately doped cuprates. We provide a survey of universal and materials-specific aspects of this phenomenon, with emphasis on results obtained by scattering methods. In particular, we discuss the structure, periodicity, and stability range of the charge-ordered state, its response to various external perturbations, the influence of disorder, the coexistence and competition with superconductivity, as well as collective charge dynamics. In the context of this journal issue which honors Roger Cowley's legacy, we also discuss the connection of charge ordering with lattice vibrations and the central-peak phenomenon. We end the review with an outlook on research opportunities offered by new synthesis methods and experimental platforms, including cuprate thin films and superlattices.

cond-mat.str-el↗