[Paper Review] Light-Enhanced Spin Fluctuations and d-Wave Superconductivity at a Phase Boundary

Yao Wang, Cheng-Chien Chen+2 more|arXiv (Cornell University)|Sep 26, 2017

Cold Atom Physics and Bose-Einstein Condensates4 citations

TL;DR

This study demonstrates that light-induced enhancement of d-wave superconductivity emerges near a quantum phase boundary between charge and spin density waves in a Mott-Peierls system. Using time-resolved exact diagonalization, it shows that photoexcitation enhances d-wave pairing by amplifying spin fluctuations—particularly d-symmetry projected spin fluctuations—while suppressing competing orders, enabling transient superconductivity even in a non-superconducting equilibrium phase.

ABSTRACT

Time-domain techniques have shown the potential of photo-manipulating existing orders and inducing new states of matter in strongly correlated materials. Using time-resolved exact diagonalization, we perform numerical studies of pump dynamics in a Mott-Peierls system with competing charge and spin density waves. A light-enhanced $d$-wave superconductivity is observed when the system resides near a quantum phase boundary. By examining the evolution of spin, charge and superconducting susceptibilities, we show that a sub-dominant state in equilibrium can be stabilized by photomanipulating charge order to allow superconductivity to appear and dominate. This work provides an interpretation of light-induced superconductivity from the perspective of order competition, and offers a promising approach for designing novel emergent states out of equilibrium.

Motivation & Objective

• To investigate nonequilibrium superconducting instabilities in strongly correlated systems near a quantum phase boundary.
• To determine whether light-induced superconductivity arises from kinetic effects or from enhanced effective interactions.
• To disentangle the roles of spin, charge, and quasiparticle dynamics in driving transient d-wave superconductivity.
• To identify the microscopic mechanism behind photoenhanced superconductivity in systems with competing orders.

Proposed method

• Numerical simulation of a two-dimensional Peierls-Hubbard model with electron-electron and electron-phonon interactions.
• Application of time-resolved exact diagonalization to study pump-induced dynamics in nonequilibrium conditions.
• Analysis of spin, charge, and superconducting susceptibilities to track the evolution of competing orders during the pump.
• Projection of spin fluctuations onto d-wave symmetry to assess their role as pairing glue.
• Comparison of density of states (DOS) evolution and quasiparticle weight changes under varying pump strengths.
• Use of a pulsed optical field to transiently perturb the system and probe the response near the CDW-SDW phase boundary.

Experimental results

Research questions

• RQ1 Can light induce significant d-wave superconductivity in a system that is not superconducting in equilibrium?
• RQ2 What is the dominant microscopic mechanism—spin fluctuations, quasiparticle weight, or bandwidth engineering—behind photoenhanced superconductivity?
• RQ3 Why is the enhancement of d-wave pairing more pronounced near the quantum phase boundary than in the bulk phases?
• RQ4 How do competing charge and spin density wave orders influence the stability of light-induced superconducting states?
• RQ5 What role do many-body effects such as gap-like structures in the DOS play in sustaining transient superconducting instabilities?

Key findings

• Photoenhanced d-wave superconductivity is observed only when the system is near the quantum phase boundary between charge and spin density waves, not deep in either phase.
• The enhancement of pairing correlations is strongly correlated with increased d-symmetry projected spin fluctuations, indicating their role as pairing glue.
• In the Peierls phase, the maximum pairing enhancement occurs toward the end of the pump, while in the Mott phase, it occurs only at the beginning, indicating different dynamical pathways.
• The transient survival of enhanced pairing is attributed to a many-body effect: a gap-like structure in the density of states that suppresses recovery of spin fluctuations after the pump.
• Quasiparticle weight enhancement alone does not drive superconductivity; instead, the interplay between suppressed charge fluctuations and enhanced spin excitations is critical.
• The results show that effective interactions, particularly spin fluctuations, are more important than kinetic or quasiparticle effects in driving the observed nonequilibrium superconducting instability.