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1. Tripling of the superconducting critical current density in BaFe ${}_2$ (As ${}_{1-x}$ P ${}_x$ ) ${}_2$ retained after pressure release

   https://doi.org/10.1088/1361-6668/ae702f  

   Liu, Jiangteng; Lopez, Alex; Slone, Daniel; Ren, Guodong; Liu, Zhaoyu; Idrobo, Juan-Carlos; Chu, Jiun-Haw; Eley, Serena (June 2026, Superconductor Science and Technology)

   Abstract Superconducting performance is tunable not only via chemical modification or defect engineering, but also through external parameters such as pressure, though this method remains less readily accessible. In this work, we study how compression influences vortex dynamics and critical currents in an iron-based superconductor. Specifically, we perform magnetization measurements using an off-the-shelf pressure cell to investigate the effects of hydrostatic pressures up to 1.08 GPa on the magnetic properties of BaFe ${}_2$ (As ${}_{0.62}$ P ${}_{0.38}$ ) ${}_2$ crystals across a range of temperatures $T$ and magnetic fields $H$ . Although these pressures minimally affect the superconducting critical temperature, they produce a clear increase in the critical current density $J_{\mathrm{c}}(T,H)$ , a pronounced reduction in the rate of thermally activated vortex motion $S(T,H)$ , and can change the dominant vortex pinning mechanism. Furthermore, the effects of pressure are irreversible: after pressurization and subsequent release at room temperature, high-density microcracks are observed and the crystals retain their enhanced critical current densities. The second magnetization peak vanishes above 18 K after the pressure cycle, which we attribute to a transition from predominantly $\delta \kappa$ pinning to a mixed mechanism of $\delta T_{\mathrm{c}}$ and surface pinning. Lastly, a threefold increase in $J_{\mathrm{c}}$ , a more than 40% reduction in $S$ at 8 K and 0.5 T, and an expanded elastic-creep region were achieved after $1-2$ pressure cycles. These findings demonstrate the potential utility of pressure cycling for improving $J_{\mathrm{c}}$ , which may offer a simpler alternative compared to approaches such as chemical doping or the introduction of artificial pinning centers. 

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2. Observation of a cross-section enhancement near the $t\overline{t}$ production threshold in $\sqrt{s}=13$  TeV pp collisions with the ATLAS detector

   https://doi.org/10.1088/1361-6633/ae60a0  

   ATLAS (May 2026, Reports on Progress in Physics)

   Abstract A measurement of $t\overline{t}$ production is presented in the invariant-mass region near the pair production threshold, $m_{t\overline{t}}\sim 345$  GeV, in final states with two charged leptons and multiple jets. The measurement is based on $140{\mathrm{fb}}^{-1}$ of proton–proton collision data collected at $\sqrt{s}=13$  TeV with the ATLAS detector at the Large Hadron Collider. The data are compared to two models of $t\overline{t}$ production: a baseline model including only perturbative quantum chromodynamics (pQCD) predictions for the hard process at approximate next-to-next-to leading order accuracy in the strong coupling, and an extended model that, in addition, incorporates non-relativistic QCD simulations that also include the formation of colour-singlet quasi-bound-states near the $t\overline{t}$ threshold. The agreement between the data and the models is quantified via a profile-likelihood fit to the reconstructed $m_{t\overline{t}}$ distributions, in bins of two angular observables sensitive to spin-correlations in the $t\overline{t}$ system. An excess of events is observed over the baseline pQCD prediction, with an observed significance over 8 standard deviations. This excess is consistent with the formation of colour-singlet and spin-singletS-wave quasi-bound $t\overline{t}$ states, as predicted by non-relativistic QCD, and corresponds to an observed cross-section of ${9.3}_{-1.3}^{+1.4}$  pb. 

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3. Enhanced stability and bulk superconducting properties of Ag intercalated Bi _1.6 Pb _0.4 Sr ₂ Ca ₂ Cu ₃ O ${}_{10+\delta }$

   https://doi.org/10.1088/1361-6668/ae5690  

   Gonzalez_Arevalo, D; Do, Huu T; Mark, Alexander C; Rodriguez, David; Zangeneh, Danial; Klie, Robert F; Hemley, Russell J; Man, N K (April 2026, Superconductor Science and Technology)

   Abstract Cuprate high-temperature superconductors (HTSCs) have long stood a promising candidates for various applications due to their high critical temperature ( $T_{\mathrm{c}}$ ) at ambient pressure. Practical limitations such as low bulk critical current densities ( $J_{\mathrm{c}}^{\mathrm{B}}$ ), sample inhomogeneities and chemical instability, however, have hindered their applicability. Increasing ${\mathit{T}}_{\mathrm{c}}$ , $J_{\mathrm{c}}^{\mathrm{B}}$ and the upper critical fields as well as maintaining the chemical stability in cuprates are therefore major goals for these materials science. Here, we characterize the electronic and structural properties of an Ag intercalated ${\mathrm{Bi}}_{1.6}{\mathrm{Pb}}_{0.4}{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\delta }$ sample prepared using a novel growth technique. The sample demonstrates enhanced electrical properties that have remained nearly constant over 8 years, while the non-silver Bi-2223 exhibits much degradation in both high- $T_{\mathrm{c}}$ fractional phase and superconducting properties. We correlate these bulk electronic properties with structural probes to determine the critical role of Ag in these samples. The enhanced properties of these Ag intercalated cuprates suggest novel synthesis pathways will be key in developing more practical superconductors for applications. 

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4. Search for heavy pseudoscalar and scalar bosons decaying to a top quark pair in proton–proton collisions at $\sqrt{s}=13\text{TeV}$

   https://doi.org/10.1088/1361-6633/ae2207  

   Collaboration, The CMS (December 2025, Reports on Progress in Physics)

   Abstract A search for pseudoscalar or scalar bosons decaying to a top quark pair ( $\text{t}\overline{\text{t}}$ ) in final states with one or two charged leptons is presented. The analyzed proton–proton collision data was recorded at $\sqrt{s}=13\text{TeV}$ by the CMS experiment at the CERN LHC and corresponds to an integrated luminosity of 138  ${\text{fb}}^{-1}$ . The invariant mass $m_{\mathrm{t}\overline{\mathrm{t}}}$ of the reconstructed $\text{t}\overline{\text{t}}$ system and variables sensitive to its spin and parity are used to discriminate against the standard model $\text{t}\overline{\text{t}}$ background. Interference between pseudoscalar or scalar boson production and the standard model $\text{t}\overline{\text{t}}$ continuum is included, leading to peak-dip structures in the $m_{\mathrm{t}\overline{\mathrm{t}}}$ distribution. An excess of the data above the background prediction, based on perturbative quantum chromodynamics (QCD) calculations, is observed near the kinematic $\text{t}\overline{\text{t}}$ production threshold, while good agreement is found for high $m_{\mathrm{t}\overline{\mathrm{t}}}$ . The data are consistent with the background prediction if the contribution from a simplified model of a color-singlet ${}^1{\text{S}}_0^{\left[1\right]}$ $\text{t}\overline{\text{t}}$ quasi-bound state ${\eta }_{\mathrm{t}}$ , inspired by nonrelativistic QCD, is added. Upper limits at 95% confidence level are set on the coupling between the pseudoscalar or scalar bosons and the top quark for boson masses in the range 365–1000 GeV, relative widths between 0.5% and 25%, and two background scenarios with or without ${\eta }_{\mathrm{t}}$ contribution. 

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5. Instantons and the large $N=4$ algebra

   https://doi.org/10.1088/1751-8121/ada64d  

   Witten, Edward (January 2025, Journal of Physics A: Mathematical and Theoretical)

   Abstract We investigate the differential geometry of the moduli space of instantons on ${\mathrm{S}}^3\times {\mathrm{S}}^1$ . Extending previous results, we show that a sigma-model with this target space can be expected to possess a large $N=4$ superconformal symmetry, supporting speculations that this sigma-model may be dual to Type IIB superstring theory on ${\mathrm{AdS}}_3\times {\mathrm{S}}^3\times {\mathrm{S}}^3\times {\mathrm{S}}^1$ . The sigma-model is parametrized by three integers—the rank of the gauge group, the instanton number, and a ‘level’ (the integer coefficient of a topologically nontrivialB-field, analogous to a WZW level). These integers are expected to correspond to two five-brane charges and a one-brane charge. The sigma-model is weakly coupled when the level, conjecturally corresponding to one of the five-brane changes, becomes very large, keeping the other parameters fixed. The central charges of the large $N=4$ algebra agree, at least semiclassically, with expectations from the duality. 

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