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1. Two-loop superstring five-point amplitudes. Part I. Construction via chiral splitting and pure spinors

   https://doi.org/10.1007/JHEP08(2020)135  

   D’Hoker, Eric ; Mafra, Carlos R. ; Pioline, Boris ; Schlotterer, Oliver ( August 2020 , Journal of High Energy Physics)

   null (Ed.)

   A bstract The full two-loop amplitudes for five massless states in Type II and Heterotic superstrings are constructed in terms of convergent integrals over the genus-two moduli space of compact Riemann surfaces and integrals of Green functions and Abelian differentials on the surface. The construction combines elements from the BRST cohomology of the pure spinor formulation and from chiral splitting with the help of loop momenta and homology invariance. The α ′ → 0 limit of the resulting superstring amplitude is shown to be in perfect agreement with the previously known amplitude computed in Type II supergravity. Investigations of the α ′ expansion of the Type II amplitude and comparisons with predictions from S-duality are relegated to a first companion paper. A construction from first principles in the RNS formulation of the genus-two amplitude with five external NS states is relegated to a second companion paper. 

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2. Two-loop superstring five-point amplitudes. Part II. Low energy expansion and S-duality

   https://doi.org/10.1007/JHEP02(2021)139  

   D’Hoker, Eric ; Mafra, Carlos R. ; Pioline, Boris ; Schlotterer, Oliver ( February 2021 , Journal of High Energy Physics)

   null (Ed.)

   A bstract In an earlier paper, we constructed the genus-two amplitudes for five external massless states in Type II and Heterotic string theory, and showed that the α ′ expansion of the Type II amplitude reproduces the corresponding supergravity amplitude to leading order. In this paper, we analyze the effective interactions induced by Type IIB superstrings beyond supergravity, both for U(1) R -preserving amplitudes such as for five gravitons, and for U(1) R -violating amplitudes such as for one dilaton and four gravitons. At each order in α ′, the coefficients of the effective interactions are given by integrals over moduli space of genus-two modular graph functions, generalizing those already encountered for four external massless states. To leading and sub-leading orders, the coefficients of the effective interactions D 2 ℛ 5 and D 4 ℛ 5 are found to match those of D 4 ℛ 4 and D 6 ℛ 4 , respectively, as required by non-linear supersymmetry. To the next order, a D 6 ℛ 5 effective interaction arises, which is independent of the supersymmetric completion of D 8 ℛ 4 , and already arose at genus one. A novel identity on genus-two modular graph functions, which we prove, ensures that up to order D 6 ℛ 5 , the five-point amplitudes require only a single new modular graph function in addition to those needed for the four-point amplitude. We check that the supergravity limit of U(1) R -violating amplitudes is free of UV divergences to this order, consistently with the known structure of divergences in Type IIB supergravity. Our results give strong consistency tests on the full five-point amplitude, and pave the way for understanding S-duality beyond the BPS-protected sector. 

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3. Two loop mass renormalisation in heterotic string theory: NS states

   https://doi.org/10.1007/JHEP11(2023)052  

   Bhattacharya, Ritabrata ( November 2023 , Journal of High Energy Physics)

   In this work computation of the renormalised mass at two loop order for the NS sector of heterotic string theory is attempted. We first implement the vertical integration prescription for choosing a section avoiding the spurious poles due to the presence of a required number of picture changing operators. As a result the relevant amplitude on genus 2 Riemann surface can be written as a boundary term. We then identify the 1PI region of the moduli space having chosen a gluing compatible local coordinates around the external punctures. We also identify the relevant integrands and the relevant region of integration for the modular parameters at the boundary.

    

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4. Observation of strong excitonic magneto-chiral anisotropy in twisted bilayer van der Waals crystals

   https://doi.org/10.1038/s41467-021-22412-9  

   Lan, Shoufeng ; Liu, Xiaoze ; Wang, Siqi ; Zhu, Hanyu ; Liu, Yawen ; Gong, Cheng ; Yang, Sui ; Shi, Jing ; Wang, Yuan ; Zhang, Xiang ( December 2021 , Nature Communications)

   Abstract The interplay between chirality and magnetism generates a distinct physical process, the magneto-chiral effect, which enables one to develop functionalities that cannot be achieved solely by any of the two. Such a process is universal with the breaking of parity-inversion and time-reversal symmetry simultaneously. However, the magneto-chiral effect observed so far is weak when the matter responds to photons, electrons, or phonons. Here we report the first observation of strong magneto-chiral response to excitons in a twisted bilayer tungsten disulfide with the amplitude of excitonic magneto-chiral (ExMCh) anisotropy reaches a value of ~4%. We further found the ExMCh anisotropy features with a spectral splitting of ~7 nm, precisely the full-width at half maximum of the excitonic chirality spectrum. Without an externally applied strong magnetic field, the observed ExMCh effect with a spontaneous magnetic moment from the ferromagnetic substrate of thulium iron garnet at room temperature is favorable for device applications. The unique ExMCh processes provide a new pathway to actively control magneto-chiral applications in photochemical reactions, asymmetric synthesis, and drug delivery. 

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5. Chiral single photons from deterministic quantum emitter arrays via proximity coupling to van der Waals ferromagnets

   https://doi.org/10.1088/2053-1583/ace5bc  

   Liu, Na ; Xiao, Licheng ; Fu, Shichen ; Ma, Yichen ; Liu, Song ; Chen, Siwei ; Hone, James ; Yang, Eui-Hyeok ; Strauf, Stefan ( July 2023 , 2D Materials)

   Chiral single photons are highly sought to enhance encoding capacities or enable propagation-dependent routing in nonreciprocal devices. Unfortunately, most semiconductor quantum emitters (QEs) produce only linear polarized photons unless external magnets are applied. Magnetic proximity coupling utilizing 2D ferromagnets promises to make bulky external fields obsolete. Here we directly grow Fe-doped MoS₂(Fe:MoS₂) via chemical vapor deposition that displays pronounced hard ferromagnetic properties even in monolayer form. This approach with monolayer ferromagnets enables full utilization of the strain from the pillar stressor to form QE in WSe₂deterministically. The Fe:MoS₂/WSe₂heterostructures display strong hysteretic magneto-response and high-purity chiral single photons with a circular polarization degree of 92 ± 1% (74% average) without external magnetic fields. Furthermore, the chiral single photons are robust against uncontrolled twist-angle and external stray-fields. This ability to manipulate quantum states and transform linear polarized photons into high-purity chiral photons on-chip enables nonreciprocal device integration in quantum photonics.

    

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