Scattering studies of density wave orders in unconventional superconductors

Johnson, Thomas Arthur

Permalink

https://hdl.handle.net/2142/122020 Copy

Description

Title

Scattering studies of density wave orders in unconventional superconductors

Author(s)

Johnson, Thomas Arthur

Issue Date

2023-11-28

Director of Research (if dissertation) or Advisor (if thesis)

• Abbamonte, Peter
• MacDougall, Gregory

Doctoral Committee Chair(s)

Fradkin, Eduardo

Committee Member(s)

Kahn, Yonatan F

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Superconductivity, Cuprates, CDW, SDW, nematicity, x-ray scattering, neutronn scattering

Abstract

Unconventional superconductors exhibit multiple electronic phases proximate to superconductivity such as charge density waves (CDW) and spin density waves (SDW). The existence of electronic phases near superconductivity (SC) in a variety of materials, especially high-T$_c$ superconductors, has led to suggestions that these phases are important for superconductivity and may even intertwine to form new states of matter [1]. I studied density wave orders in two different materials: the cuprate La$_{1.8-x}$Eu$_{0.2}$Sr$_x$CuO$_4$ (LESCO), which exhibits CDW and SDW order, and the pnictide superconductor Ba$_{1-x}$Sr$_x$Ni$_2$As$_2$ (BSNA), which exhibits several competing CDW phases [2] and was recently shown to have a 6-fold increase in superconducting T$_c$ near a nematic quantum phase transition [3]. CDW order is ubiquitous among the cuprate materials [4, 5]. In La-based cuprates, the CDW wavevector follows the Yamada relation where the incommensurability increases linearly with doping until saturation above 1/8 hole doping. But in YBa$_2$Cu$_3$O$_{6+\delta}$ (YBCO), the CDW wavevector linearly decreases with doping. One possibility is that the CDW wavevector in La-based cuprates is affected by the static SDW order at low temperatures that is generally absent in other cuprates. To explore this possibility, we grew crystals of LESCO using the traveling solvent floating zone technique and studied the CDW and SDW using x-ray and neutron scattering respectively. We measured the temperature and doping dependence of CDW and SDW wavevectors and showed that the CDW is affected by CDW-SDW coupling and charge compressibility. We also observed a nearly temperature-independent CDW integrated intensity over our measured temperature range, consistent with Ref. [6]. Our results suggest that the CDW in different cuprate materials forms by a common mechanism. BSNA is a charge-ordered pnictide superconductor that exhibits a diverging nematic susceptibility in resistivity measurements under applied uniaxial strain. Recently, we showed that BSNA exhibits a C$_4$-symmetric, incommensurate charge density wave (I-CDW) in the region of the phase diagram where the nematic response is largest [2, 3], which suggests that there may be a relationship between the nematic response and the dynamics of the I-CDW. We performed x-ray measurements of the I-CDW under applied uniaxial strain, to evaluate whether the I-CDW is involved in the nematic response. We found that strain breaks the C$_4$ symmetry of the system by making I-CDW x-ray satellite intensities anisotropic. We measured a nematic susceptibility that diverges near the nematic transition in the same manner as Ref. [3], demonstrating a direct relationship between transport effects and the behavior of the I-CDW. Our results suggest that the I-CDW anisotropy may be the preeminent nematic order parameter in BSNA.

Graduation Semester

2023-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Thomas Johnson

Owning Collections