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1. Modified supersymmetric indices in AdS3/CFT2

   https://doi.org/10.1007/JHEP01(2024)099  

   Ardehali, Arash Arabi; Krishna, Hare (January 2024, Journal of High Energy Physics)

   A<sc>bstract</sc> We consider the 𝒩 = (2, 2) AdS₃/CFT₂dualities proposed by Eberhardt, where the bulk geometry is AdS₃× (S³×T⁴)/ℤ_k, and the CFT is a deformation of the symmetric orbifold of the supersymmetric sigma modelT⁴/ℤ_k(withk= 2, 3, 4, 6). The elliptic genera of the two sides vanish due to fermionic zero modes, so for microstate counting applications one must consider modified supersymmetric indices. In an analysis similar to that of Maldacena, Moore, and Strominger for the standard 𝒩 = (4, 4) case ofT⁴, we study the appropriate helicity-trace index of the boundary CFTs. We encounter a strange phenomenon where a saddle-point analysis of our indices reproduces only a fraction (respectively$$ \frac{1}{2} $$ $\frac{1}{2}$,$$ \frac{2}{3} $$ $\frac{2}{3}$,$$ \frac{3}{4} $$ $\frac{3}{4}$,$$ \frac{5}{6} $$ $\frac{5}{6}$) of the Bekenstein-Hawking entropy of the associated macroscopic black branes. 

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2. Instanton contributions to the ABJM free energy from quantum M2 branes

   https://doi.org/10.1007/JHEP10(2023)029  

   Beccaria, M; Giombi, S; Tseytlin, A A (October 2023, Journal of High Energy Physics)

   A<sc>bstract</sc> We present a quantum M2 brane computation of the instanton prefactor in the leading non-perturbative contribution to the ABJM 3-sphere free energy at largeNand fixed levelk. Using supersymmetric localization, such instanton contribution was found earlier to take the form$$ {F}^{inst}\left(N,k\right)=-{\left({\sin}^2\frac{2\pi }{k}\right)}^{-1}\exp \left(-2\pi \sqrt{\frac{2N}{k}}\right)+.\dots $$ $F^{\text{inst}}\left(N,k\right)=-{\left({sin}^2\frac{2\pi }{k}\right)}^{-1}exp\left(-2\pi \sqrt{\frac{2N}{k}}\right)+.\dots$The exponent comes from the action of an M2 brane instanton wrapped onS³/ℤ_k, which represents the M-theory uplift of the ℂP¹instanton in type IIA string theory on AdS₄× ℂP³. The IIA string computation of the leading largekterm in the instanton prefactor was recently performed in arXiv:2304.12340. Here we find that the exact value of the prefactor$$ {\left({\sin}^2\frac{2\pi }{k}\right)}^{-1} $$ ${\left({sin}^2\frac{2\pi }{k}\right)}^{-1}$is reproduced by the 1-loop term in the M2 brane partition function expanded near theS³/ℤ_kinstanton configuration. As in the Wilson loop example in arXiv:2303.15207, the quantum M2 brane computation is well defined and produces a finite result in exact agreement with localization. 

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3. Softening of a flat phonon mode in the kagome ScV6Sn6

   https://doi.org/10.1038/s41467-023-42186-6  

   Korshunov, A; Hu, H; Subires, D; Jiang, Y; Călugăru, D; Feng, X; Rajapitamahuni, A; Yi, C; Roychowdhury, S; Vergniory, M G; et al (December 2023, Nature Communications)

   Abstract Geometrically frustrated kagome lattices are raising as novel platforms to engineer correlated topological electron flat bands that are prominent to electronic instabilities. Here, we demonstrate a phonon softening at thek_z = πplane in ScV₆Sn₆. The low energy longitudinal phonon collapses at ~98 K andq = $$\frac{1}{3}\frac{1}{3}\frac{1}{2}$$ $\frac{1}{3}\frac{1}{3}\frac{1}{2}$due to the electron-phonon interaction, without the emergence of long-range charge order which sets in at a different propagation vectorq_CDW = $$\frac{1}{3}\frac{1}{3}\frac{1}{3}$$ $\frac{1}{3}\frac{1}{3}\frac{1}{3}$. Theoretical calculations corroborate the experimental finding to indicate that the leading instability is located at$$\frac{1}{3}\frac{1}{3}\frac{1}{2}$$ $\frac{1}{3}\frac{1}{3}\frac{1}{2}$of a rather flat mode. We relate the phonon renormalization to the orbital-resolved susceptibility of the trigonal Sn atoms and explain the approximately flat phonon dispersion. Our data report the first example of the collapse of a kagome bosonic mode and promote the 166 compounds of kagomes as primary candidates to explore correlated flat phonon-topological flat electron physics. 

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4. Species scale in diverse dimensions

   https://doi.org/10.1007/JHEP05(2024)112  

   van_de_Heisteeg, Damian; Vafa, Cumrun; Wiesner, Max; Wu, David H (May 2024, Journal of High Energy Physics)

   A<sc>bstract</sc> In a quantum theory of gravity, the species scale Λ_scan be defined as the scale at which corrections to the Einstein action become important or alternatively as codifying the “number of light degrees of freedom”, due to the fact that$$ {\Lambda}_s^{-1} $$ ${\Lambda }_s^{-1}$is the smallest size black hole described by the EFT involving only the Einstein term. In this paper, we check the validity of this picture in diverse dimensions and with different amounts of supersymmetry and verify the expected behavior of the species scale at the boundary of moduli space. This also leads to the evaluation of the species scale in the interior of the moduli space as well as to the computation of the diameter of the moduli space. We also find evidence that the species scale satisfies the bound$$ {\frac{\left|\nabla {\Lambda}_s\right|}{\Lambda_s}}^2\le \frac{1}{d-2} $$ ${\frac{\left(\nabla {\Lambda }_s\right)}{{\Lambda }_s}}^2\le \frac{1}{d-2}$all over moduli space including the interior. 

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5. The $$ {D}_3^{(2)} $$ spin chain and its finite-size spectrum

   https://doi.org/10.1007/JHEP11(2023)095  

   Frahm, Holger; Gehrmann, Sascha; Nepomechie, Rafael I.; Retore, Ana L. (November 2023, Journal of High Energy Physics)

   A<sc>bstract</sc> Using the analytic Bethe ansatz, we initiate a study of the scaling limit of the quasi-periodic$$ {D}_3^{(2)} $$ $D_3^{\left(2\right)}$spin chain. Supported by a detailed symmetry analysis, we determine the effective scaling dimensions of a large class of states in the parameter regimeγ∈ (0,$$ \frac{\pi }{4} $$ $\frac{\pi }{4}$). Besides two compact degrees of freedom, we identify two independent continuous components in the finite-size spectrum. The influence of large twist angles on the latter reveals also the presence of discrete states. This allows for a conjecture on the central charge of the conformal field theory describing the scaling limit of the lattice model. 

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