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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 quantum entanglement in top quark pair production in proton–proton collisions at $\sqrt{s}=13$  TeV

   https://doi.org/10.1088/1361-6633/ad7e4d  

   CMS_Collaboration, The (October 2024, Reports on Progress in Physics)

   Abstract Entanglement is an intrinsic property of quantum mechanics and is predicted to be exhibited in the particles produced at the Large Hadron Collider. A measurement of the extent of entanglement in top quark-antiquark ( $\mathrm{t}\overline{\mathrm{t}}$) events produced in proton–proton collisions at a center-of-mass energy of 13 TeV is performed with the data recorded by the CMS experiment at the CERN LHC in 2016, and corresponding to an integrated luminosity of 36.3 fb⁻¹. The events are selected based on the presence of two leptons with opposite charges and high transverse momentum. An entanglement-sensitive observableDis derived from the top quark spin-dependent parts of the $\mathrm{t}\overline{\mathrm{t}}$production density matrix and measured in the region of the $\mathrm{t}\overline{\mathrm{t}}$production threshold. Values of $D<-1/3$are evidence of entanglement andDis observed (expected) to be $-{0.480}_{-0.029}^{+0.026}$( $-{0.467}_{-0.029}^{+0.026}$) at the parton level. With an observed significance of 5.1 standard deviations with respect to the non-entangled hypothesis, this provides observation of quantum mechanical entanglement within $\mathrm{t}\overline{\mathrm{t}}$pairs in this phase space. This measurement provides a new probe of quantum mechanics at the highest energies ever produced. 

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3. 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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4. Validation of Elemental and Isotopic Abundances in Late-M Spectral Types with the Benchmark HIP 55507 AB System

   https://doi.org/10.3847/1538-4357/ad1243  

   Xuan, Jerry W; Wang, Jason; Finnerty, Luke; Horstman, Katelyn; Grimm, Simon; Peck, Anne E; Nielsen, Eric; Knutson, Heather A; Mawet, Dimitri; Isaacson, Howard; et al (February 2024, The Astrophysical Journal)

   Abstract M dwarfs are common host stars to exoplanets but often lack atmospheric abundance measurements. Late-M dwarfs are also good analogs to the youngest substellar companions, which share similarT_eff∼ 2300–2800 K. We present atmospheric analyses for the M7.5 companion HIP 55507 B and its K6V primary star with Keck/KPIC high-resolution (R∼ 35,000)K-band spectroscopy. First, by including KPIC relative radial velocities between the primary and secondary in the orbit fit, we improve the dynamical mass precision by 60% and find $M_B={88.0}_{-3.2}^{+3.4}{M}_{\mathrm{Jup}}$, putting HIP 55507 B above the stellar–substellar boundary. We also find that HIP 55507 B orbits its K6V primary star with $a={38}_{-3}^{+4}$au ande= 0.40 ± 0.04. From atmospheric retrievals of HIP 55507 B, we measure [C/H] = 0.24 ± 0.13, [O/H] = 0.15 ± 0.13, and C/O = 0.67 ± 0.04. Moreover, we strongly detect¹³CO (7.8σsignificance) and tentatively detect ${\mathrm{H}}_2^{18}\mathrm{O}$(3.7σsignificance) in the companion’s atmosphere and measure ${}^{12}\mathrm{CO}{/}^{13}\mathrm{CO}={98}_{-22}^{+28}$and ${\mathrm{H}}_2^{16}\mathrm{O}/{\mathrm{H}}_2^{18}\mathrm{O}={240}_{-80}^{+145}$after accounting for systematic errors. From a simplified retrieval analysis of HIP 55507 A, we measure ${}^{12}\mathrm{CO}{/}^{13}\mathrm{CO}={79}_{-16}^{+21}$and ${\mathrm{C}}^{16}\mathrm{O}/{\mathrm{C}}^{18}\mathrm{O}={288}_{-70}^{+125}$for the primary star. These results demonstrate that HIP 55507 A and B have consistent¹²C/¹³C and¹⁶O/¹⁸O to the <1σlevel, as expected for a chemically homogeneous binary system. Given the similar flux ratios and separations between HIP 55507 AB and systems with young substellar companions, our results open the door to systematically measuring¹³CO and ${\mathrm{H}}_2^{18}\mathrm{O}$abundances in the atmospheres of substellar or even planetary-mass companions with similar spectral types. 

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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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