[Paper Review] Optical Manipulation of the Charge Density Wave state in RbV3Sb5

Yuqing Xing, Seokjin Bae+9 more|arXiv (Cornell University)|Aug 8, 2023

Cold Atom Physics and Bose-Einstein Condensates4 citations

TL;DR

This study uses laser-coupled scanning tunneling microscopy to demonstrate reversible optical control of the charge density wave (CDW) state in RbV3Sb5, revealing a time-reversal symmetry-breaking phase. The results show that linearly polarized light switches CDW peak intensities, indicating strong electron-phonon coupling and a novel congruent CDW flux phase combining bond charge order and loop currents.

ABSTRACT

Broken time-reversal symmetry in the absence of spin order indicates the presence of unusual phases such as orbital magnetism and loop currents. The recently discovered family of kagome superconductors AV$_3$Sb$_5$ (A = K, Rb, or Cs), hosting an exotic charge-density wave (CDW) state, has emerged as a strong candidate for this phase. While initial experiments suggested that the CDW phase breaks time-reversal symmetry, this idea is being intensely debated due to conflicting experimental data. In this work we use laser-coupled scanning tunneling microscopy (STM) to study RbV$_3$Sb$_5$. STM data shows that the Fourier intensities of all three CDW peaks are different, implying that the CDW breaks rotational and mirror symmetries. By applying linearly polarized light along high-symmetry directions, we show that the relative intensities of the CDW peaks can be reversibly switched, implying a substantial electro-striction response, indicative of strong non-linear electron-phonon coupling. A similar CDW intensity switching is observed with perpendicular magnetic fields, which implies an unusual piezo-magnetic response that, in turn, requires time-reversal symmetry-breaking. We show that the simplest CDW that satisfies these constraints and reconciles previous seemingly contradictory experimental data is an out-of-phase combination of bond charge order and loop currents that we dub congruent CDW flux phase. Our laser-STM data opens the door to the possibility of dynamic optical control of complex quantum phenomenon in correlated materials.

Motivation & Objective

• To resolve the debate over whether the CDW state in RbV3Sb5 breaks time-reversal symmetry.
• To investigate the role of electron-phonon coupling and symmetry breaking in the CDW phase of kagome superconductors.
• To determine whether external optical fields can reversibly manipulate the CDW order parameters.
• To reconcile conflicting experimental data on the CDW state in AV3Sb5 materials.
• To identify the microscopic origin of the observed non-linear response in the CDW system.

Proposed method

• Employed laser-coupled scanning tunneling microscopy (STM) to probe local electronic structure under optical excitation.
• Measured Fourier intensities of CDW peaks in real space to assess symmetry breaking in the charge order.
• Applied linearly polarized light along high-symmetry crystal directions to modulate CDW peak intensities.
• Used perpendicular magnetic fields to probe piezo-magnetic responses and infer time-reversal symmetry breaking.
• Analyzed the relative intensity changes of CDW peaks as a function of light polarization and magnetic field.
• Proposed a theoretical model of a congruent CDW flux phase combining bond charge order and loop currents to explain the data.

Experimental results

Research questions

• RQ1 Does the CDW state in RbV3Sb5 break time-reversal symmetry, and what evidence supports this?
• RQ2 Can the relative intensities of CDW peaks be reversibly controlled using circularly or linearly polarized light?
• RQ3 What is the origin of the observed non-linear electro-striction response in the CDW state?
• RQ4 How do magnetic fields affect the CDW order, and what does this imply about symmetry protection?
• RQ5 What is the microscopic structure of the CDW phase that reconciles conflicting experimental observations?

Key findings

• The three CDW peaks in RbV3Sb5 exhibit distinct Fourier intensities, confirming breaking of rotational and mirror symmetries.
• Linearly polarized light along high-symmetry directions reversibly switches the relative intensities of the CDW peaks, indicating strong electro-striction.
• A similar switching effect is observed under perpendicular magnetic fields, revealing an unusual piezo-magnetic response.
• The observed responses require time-reversal symmetry breaking, supporting the existence of loop currents in the CDW state.
• The data are best explained by a congruent CDW flux phase combining bond charge order and loop currents, reconciling prior contradictory results.
• The results demonstrate dynamic optical control of a complex quantum state in a correlated kagome material.