Prediction of time-reversal-symmetry breaking fermionic quadrupling condensate in twisted bilayer graphene

Maccari, Ilaria; Carlström, Johan; Babaev, Egor

doi:10.1103/physrevb.107.064501

Cited by 9 publications

(2 citation statements)

References 36 publications

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“…It has been predicted theoretically that the spin-valley degeneracy of these eight bands can be lifted by strong Coulomb interactions, leading to fully spin-valley polarised insulating states with finite Chern numbers at the integer band fillings [180][181][182]190]. According to theoretical calculations breaking of C 2 and/or T symmetry can gap out Dirac points and produce flat Chern bands with C = ±1 [192][193][194][195][196][197]. In figures 9(a) and (b), we have presented these two distinct ways of symmetry breaking (C 2 and T ) that can open energy gaps at the CNP.…”

Section: Topological Flat Bandsmentioning

confidence: 99%

Emergent phases in graphene flat bands

Bhowmik,

Ghosh,

Chandni

2024

Rep. Prog. Phys.

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Electronic correlations in two-dimensional materials play a crucial role in stabilising emergent phases of matter. The realisation of correlation-driven phenomena in graphene has remained a longstanding goal, primarily due to the absence of strong electron-electron interactions within its low-energy bands. In this context, magic-angle twisted bilayer graphene has recently emerged as a novel platform featuring correlated phases favoured by the low-energy flat bands of the underlying moiré superlattice. Notably, the observation of correlated insulators and superconductivity has garnered significant attention, leading to substantial progress in theoretical and experimental studies aiming to elucidate the origin and interplay between these two phases. A wealth of correlated phases with unprecedented tunability was discovered subsequently, including orbital ferromagnetism, Chern insulators, strange metallicity, density waves, and nematicity. However, a comprehensive understanding of these closely competing phases remains elusive. The ability to controllably twist and stack multiple graphene layers has enabled the creation of a whole new family of moiré superlattices with myriad properties being discovered at a fast pace. Here, we review the progress and development achieved so far, encompassing the rich phase diagrams offered by these graphene-based moiré systems. Additionally, we discuss multiple phases recently observed in non-moiré multilayer graphene systems. Finally, we outline future opportunities and challenges for the exploration of hidden phases in this new generation of moiré materials.

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Section: Topological Flat Bandsmentioning

confidence: 99%

Emergent phases in graphene flat bands

Bhowmik,

Ghosh,

Chandni

2024

Rep. Prog. Phys.

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“…An especially interesting possibility is associated with fermion quadrupling condensates forming above the superconducting phase transition, leading to condensates with principally different properties than superconductors. One such possible state appears when a fermion quadrupling condensate results in a Broken Time-Reversal Symmetry (BTRS) 6 , 11 – 15 . At low temperatures, such a system is a multicomponent superconductor that breaks time-reversal symmetry and can be described by several complex fields Δ i , where i is a component or band index (see e.g., 16 – 20 ).…”

Section: Introductionmentioning

confidence: 99%

Calorimetric evidence for two phase transitions in Ba1−xKxFe2As2 with fermion pairing and quadrupling states

Shipulin,

Stegani,

Maccari

et al. 2023

Nat Commun

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Materials that break multiple symmetries allow the formation of four-fermion condensates above the superconducting critical temperature (Tc). Such states can be stabilized by phase fluctuations. Recently, a fermionic quadrupling condensate that breaks the Z2 time-reversal symmetry was reported in Ba1−xKxFe2As2. A phase transition to the new state of matter should be accompanied by a specific heat anomaly at the critical temperature where Z2 time-reversal symmetry is broken ($${T}_{{{{{{{{\rm{c}}}}}}}}}^{{{{{{{{\rm{Z2}}}}}}}}} \, > \, {T}_{{{{{{{{\rm{c}}}}}}}}}$$ T c Z2 > T c ). Here, we report on detecting two anomalies in the specific heat of Ba1−xKxFe2As2 at zero magnetic field. The anomaly at the higher temperature is accompanied by the appearance of a spontaneous Nernst effect, indicating the breakdown of Z2 symmetry. The second anomaly at the lower temperature coincides with the transition to a zero-resistance state, indicating the onset of superconductivity. Our data provide the first example of the appearance of a specific heat anomaly above the superconducting phase transition associated with the broken time-reversal symmetry due to the formation of the novel fermion order.

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Vestigial singlet pairing in a fluctuating magnetic triplet superconductor and its implications for graphene superlattices

Poduval,

Scheurer

2024

Nat Commun

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Stacking and twisting graphene layers allows to create and control a two-dimensional electron liquid with strong correlations. Experiments indicate that these systems exhibit strong tendencies towards both magnetism and triplet superconductivity. Motivated by this phenomenology, we study a 2D model of fluctuating triplet pairing and spin magnetism. Individually, their respective order parameters, d and N, cannot order at finite temperature. Nonetheless, the model exhibits a variety of vestigial phases, including charge-4e superconductivity and broken time-reversal symmetry. Our main focus is on a phase characterized by finite d ⋅ N, which has the same symmetries as the BCS state, a Meissner effect, and metastable supercurrents, yet rather different spectral properties: most notably, the suppression of the electronic density of states at the Fermi level can resemble that of either a fully gapped or nodal superconductor, depending on parameters. This provides a possible explanation for recent tunneling experiments in the superconducting phase of graphene moiré systems.

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Prediction of time-reversal-symmetry breaking fermionic quadrupling condensate in twisted bilayer graphene

Cited by 9 publications

References 36 publications

Emergent phases in graphene flat bands

Emergent phases in graphene flat bands

Calorimetric evidence for two phase transitions in Ba1−xKxFe2As2 with fermion pairing and quadrupling states

Vestigial singlet pairing in a fluctuating magnetic triplet superconductor and its implications for graphene superlattices

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