How do chiral superconductors break time-reversal symmetry? – Kerr spectroscopy study

Informations projet

Kerr

N° de convention de subvention: 885413

DOI

10.3030/885413

Date de signature de la CE 28 Avril 2020

Date de début 1 Juillet 2021

Date de fin 30 Juin 2027

Financé au titre de

• EXCELLENT SCIENCE - European Research Council (ERC)

Coût total

€ 2 489 976,00

Contribution de l’UE

€ 2 489 976,00

Coordonné par

KEEMILISE JA BIOLOOGILISE FUUSIKA INSTITUUT
Estonia

Periodic Reporting for period 3 - Kerr (How do chiral superconductors break time-reversal symmetry? – Kerr spectroscopy study)

Période du rapport: 2024-07-01 au 2025-12-31

Résumé du contexte et des objectifs généraux du projet

Spontaneous symmetry-breaking – the phenomenon where an infinitesimal perturbation can cause the system to break the underlying symmetry – is a cornerstone concept in the understanding and manipulation of quantum orders.

Unconventional superconductivity is extensively sought for in contemporary research. Of particular interest are chiral superconductors which possess non-trivial topological properties resulting in superconducting orders that may break time-reversal symmetry. The possibility of applications to topological quantum computation have placed such materials at the forefront of condensed matter research. The polar Kerr effect, in which a rotation of polarization is detected for a beam of light reflected from the surface of a superconductor, have emerged as a key experimental probe of the time-reversal symmetry breaking (spontaneous development of magnetic moment).

The newly developed THz range polar Kerr effect spectroscopy enables to study broken time-reversal symmetry, the origin of unconventional pairing, the in-gap collective modes, and the structures of the superconducting order parameters. The objective of the project is to measure the polar Kerr effect at sub-THz frequencies and with milli-radian angular resolution from a variety of unconventional superconductors cooled to 100 mK temperature, deep into the superconducting state. The aim is to understand the basic mechanisms leading to unconventional superconductivity in these systems to answer the fundamental questions, such as:
- What is the structure of the superconducting gap in Sr2RuO4, URu2Si2, and UPt3 compounds?
- Is the time-reversal symmetry broken in (a) the ‘Hidden Order’ state and in (b) the superconducting state of URu2Si2? Which symmetries are broken at the transition from the ‘Hidden Order’ state into the unconventional superconducting state? –
and to elucidate the microscopic origin of superconductivity in the new families of unconventional superconductors.

The same technique will be also applied for the studies of THz-range anomalous Hall effect in variety of quantum materials and for the studies of the integer and fractional quantum Hall states in ultra-clean two-dimensional electron gas systems.

In a broader view, the project will keep Estonian physics on the forefront of science through new scientific contacts and will promote physics education by engaging students and postdocs in the research.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

The team has developed one-of-a-kind Kerr-spectroscopy apparatus based on modified Martin-Puplett interferometer, having the unique capability of measuring small changes in light polarization upon reflection from a sample, using low-power sub-THz frequency continues wave (cw) radiation at sub-Kelvin temperatures. The instrument brings to light opportunities to study optical Hall conductivity at ac frequencies and at sub-Kelvin temperatures, essential for the studies of the origin of pairing in unconventional superconductors. A patent application “Techniques for Precise Determination of the Magneto-Optic Kerr Effect (MOKE) In Materials in Controlled Environments” has been filed for this novel technology.

The PI and collaborators published 13 papers describing novel states in systems with strong electron interactions and/or strong spin-orbit coupling.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

Unconventional superconductivity which possesses non-trivial topological properties resulting in a state with broken time-reversal symmetry is extensively sought for in the modern field of quantum materials. Theoretically, spontaneous TRS-breaking arises naturally from fundamental symmetry considerations. Potential applications to topological quantum computation have placed such materials at the forefront of experimentation: search for such phases of matter remains a major direction in the contemporary study of superconductivity.

Although recent polar Kerr effect studies have identified spontaneous time-reversal symmetry breaking in a number of unconventional superconductors, difficulties with interpretation rose because all the measurements were restricted to a single near-visible radiation frequency, which is more than three orders of magnitude higher than relevant to unconventional superconductivity energy scale, typically at 0.1THz. Consequently, the results brought considerable interpretation debates: e.g. is this a bulk property of the superconductor, or does the signal being influenced by inhomogeneities and is not representative of the sample as a whole? More importantly, the microscopic origin and quantitative understanding of the time-reversal-symmetry-broken state remains topic of ongoing disputations that can only be resolved by ultra-low-frequency spectroscopic measurements of Kerr angle rotation as function of frequency in the window relevant to superconductivity and magnetism that originates the time-reversal-symmetry-breaking effect (development of a spontaneous magnetic moment). Thus, new spectroscopic methods as probes of strong electron correlations are required.

The developed in this project one-of-a-kind apparatus for the magneto-optical polar Kerr angle spectroscopy at sub-THz frequency – the new diagnostics method with wide-ranging transformative impact – brings to light the opportunities to study optical Hall conductivity at ac frequencies, the broken time-reversal symmetry in unconventional superconductors, the origin of unconventional pairing, the in-gap collective modes, and the structures of the superconducting order parameters. These studies will remain in the focus till the end of this project.

MOKE THz spectrometer based on modified Martin-Puplett interferometer coupled to dilution fridge