[Paper Review] Is there a hidden chiral density-wave in the iron-based superconductors?

Rafael M. Fernandes, Steven A. Kivelson+1 more|arXiv (Cornell University)|Apr 14, 2015

Iron-based superconductors research3 citations

TL;DR

This paper proposes that in iron-based superconductors, certain magnetically ordered ground states—specifically checkerboard charge-and-spin density waves (CSDW) or spin-vortex crystals (SVC)—can melt in two stages, forming an intermediate phase with vestigial order: a charge density wave (CDW) for CSDW and a novel spin-vorticity density-wave (SVDW) for SVC. The SVDW phase features a vector chiral order parameter that breaks inversion but preserves time-reversal symmetry, hosting Goldstone modes and potentially enhancing superconductivity.

ABSTRACT

Superconductivity in most iron pnictides and iron chalcogenides is found near an instability towards an orthorhombic stripe magnetic state. A remarkable property of the latter is that its melting into the tetragonal paramagnetic phase happens generally in two stages, resulting in a nematic paramagnetic phase that breaks the point-group symmetry. Recent experiments in hole-doped iron arsenides, however, revealed a magnetically ordered ground state that preserves tetragonal symmetry, which is likely either a checkerboard charge-and-spin density wave (CSDW) or a spin-vortex crystal (SVC). Here we show that either of these phases can also melt in two stages, resulting in an intermediate phase with vestigial order, namely: a charge density-wave (CDW) for a CSDW ground state, and a remarkable spin-vorticity density-wave (SVDW) for a SVC ground state. While the CDW has an Ising-like order parameter, the SVDW has a vector chiral order parameter that breaks inversion without breaking time-reversal symmetry, giving rise to Goldstone modes. We propose experimentally detectable signatures of these phases, and point out that their fluctuations can lead to an enhancement of the superconducting transition temperature.

Motivation & Objective

• To investigate the possibility of a two-stage melting transition in iron-based superconductors with tetragonal-symmetric magnetically ordered ground states.
• To explore whether such transitions can lead to intermediate phases with vestigial order, particularly focusing on spin-vorticity density-wave (SVDW) order.
• To identify experimentally detectable signatures of these novel phases, especially the SVDW with vector chiral order parameter.
• To examine how fluctuations in these phases might enhance the superconducting transition temperature.

Proposed method

• Using symmetry analysis and effective field theory to classify possible order parameters in iron-based superconductors with tetragonal symmetry.
• Modeling the two-stage melting of CSDW and SVC states into intermediate phases with vestigial order: CDW and SVDW, respectively.
• Analyzing the order parameter structure: CDW has an Ising-like scalar order parameter, while SVDW features a vector chiral order parameter that breaks inversion but not time-reversal symmetry.
• Identifying the emergence of Goldstone modes in the SVDW phase due to the spontaneous breaking of continuous symmetry associated with the vector chiral order.
• Deriving the effective action and symmetry-allowed couplings to predict observable signatures in neutron scattering, NMR, and ARPES.
• Assessing the impact of SVDW and CDW fluctuations on superconducting pairing via renormalization group or fluctuation exchange approaches.

Experimental results

Research questions

• RQ1 Can a checkerboard charge-and-spin density wave (CSDW) or spin-vortex crystal (SVC) ground state in iron-based superconductors melt in two stages, forming an intermediate phase with vestigial order?
• RQ2 What is the nature of the order parameter in the intermediate phase when the ground state is a spin-vortex crystal (SVC)?
• RQ3 Does the intermediate phase resulting from SVC melting host a vector chiral order parameter that breaks inversion but preserves time-reversal symmetry?
• RQ4 What are the experimentally detectable signatures of the proposed spin-vorticity density-wave (SVDW) phase?
• RQ5 Can fluctuations in the SVDW or CDW phases enhance the superconducting transition temperature in iron-based superconductors?

Key findings

• A two-stage melting transition is possible for both CSDW and SVC ground states in iron-based superconductors, leading to an intermediate phase with vestigial order.
• For a CSDW ground state, the intermediate phase is a charge density wave (CDW) with an Ising-like scalar order parameter.
• For an SVC ground state, the intermediate phase is a spin-vorticity density-wave (SVDW) with a vector chiral order parameter that breaks inversion symmetry without breaking time-reversal symmetry.
• The SVDW phase hosts Goldstone modes due to the spontaneous breaking of continuous symmetry associated with the vector chiral order.
• The SVDW phase is characterized by a distinct chiral texture in spin and orbital degrees of freedom, potentially detectable via circularly polarized neutron scattering or NMR.
• Fluctuations of the SVDW and CDW phases may enhance the superconducting transition temperature by mediating effective pairing interactions.