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1. Anomalous quantum criticality in the electron-doped cuprates

   https://doi.org/10.1073/pnas.1817653116  

   Mandal, P. R. ; Sarkar, Tarapada ; Greene, Richard L. ( March 2019 , Proceedings of the National Academy of Sciences)

   In the physics of condensed matter, quantum critical phenomena and unconventional superconductivity are two major themes. In electron-doped cuprates, the low critical field (H_C2) allows one to study the putative quantum critical point (QCP) at low temperature and to understand its connection to the long-standing problem of the origin of the high-T_Csuperconductivity. Here we present measurements of the low-temperature normal-state thermopower (S) of the electron-doped cuprate superconductor La_2−xCe_xCuO₄(LCCO) fromx= 0.11–0.19. We observe quantum critical$\mathit{S}/\mathit{T}$versus$\mathbf{l}\mathbf{n}\left(\mathbf{1}/\mathit{T}\right)$behavior over an unexpectedly wide doping rangex= 0.15–0.17 above the QCP (x= 0.14), with a slope that scales monotonically with the superconducting transition temperature (T_Cwith H = 0). The presence of quantum criticality over a wide doping range provides a window on the criticality. The thermopower behavior also suggests that the critical fluctuations are linked withT_C. Above the superconductivity dome, atx= 0.19, a conventional Fermi-liquid$\mathit{S}\propto \mathit{T}$behavior is found for$\mathit{T}\le$40 K.
2. Optical spectral weight, phase stiffness, and T _c bounds for trivial and topological flat band superconductors

   https://doi.org/10.1073/pnas.2106744118  

   Verma, Nishchhal ; Hazra, Tamaghna ; Randeria, Mohit ( August 2021 , Proceedings of the National Academy of Sciences)

   We present exact results that give insight into how interactions lead to transport and superconductivity in a flat band where the electrons have no kinetic energy. We obtain bounds for the optical spectral weight for flat-band superconductors that lead to upper bounds for the superfluid stiffness and the two-dimensional (2D)$T_c$. We focus on on-site attraction$\left|U\right|$on the Lieb lattice with trivial flat bands and on the π-flux model with topological flat bands. For trivial flat bands, the low-energy optical spectral weight${\tilde{D}}_{\text{low}}\le \tilde{n}\left|U\right|\mathrm{\Omega }/2$with$\tilde{n}=min\left(n,2-n\right)$, where n is the flat-band density and Ω is the Marzari–Vanderbilt spread of the Wannier functions (WFs). We also obtain a lower bound involving the quantum metric. For topological flat bands, with an obstruction to localized WFs respecting all symmetries, we again obtain an upper bound for${\tilde{D}}_{low}$linear in$\left|U\right|$. We discuss the insights obtained from our bounds by comparing them with mean-field and quantum Monte Carlo results.
3. Precision measurement of the neutral pion lifetime

   https://doi.org/10.1126/science.aay6641  

   Larin, I. ; Zhang, Y. ; Gasparian, A. ; Gan, L. ; Miskimen, R. ; Khandaker, M. ; Dale, D. ; Danagoulian, S. ; Pasyuk, E. ; Gao, H. ; et al ( April 2020 , Science)

   The explicit breaking of the axial symmetry by quantum fluctuations gives rise to the so-called axial anomaly. This phenomenon is solely responsible for the decay of the neutral pion π⁰into two photons (γγ), leading to its unusually short lifetime. We precisely measured the decay width Γ of the${\pi }^0\to \gamma \gamma$process. The differential cross sections for π⁰photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding$\mathrm{\Gamma }\left({\pi }^0\to \gamma \gamma \right)=7.798\pm 0.056\left(\mathrm{stat}\mathrm{.}\right)\pm 0.109\left(\mathrm{syst}\mathrm{.}\right)\mathrm{ eV}$, where stat. denotes the statistical uncertainty and syst. the systematic uncertainty. We combined the results of this and an earlier experiment to generate a weighted average of$\mathrm{\Gamma }\left({\pi }^0\to \gamma \gamma \right)=7.802\pm 0.052\left(\mathrm{stat}\mathrm{.}\right)\pm 0.105\left(\mathrm{syst}\mathrm{.}\right)\mathrm{ eV}$. Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.
4. Stars Crushed by Black Holes. II. A Physical Model of Adiabatic Compression and Shock Formation in Tidal Disruption Events

   https://doi.org/10.3847/1538-4357/ac3fb9  

   Coughlin, Eric R. ; Nixon, C. J. ( February 2022 , The Astrophysical Journal)

   We develop a Newtonian model of a deep tidal disruption event (TDE), for which the pericenter distance of the star,r_p, is well within the tidal radius of the black hole,r_t, i.e., whenβ≡r_t/r_p≫ 1. We find that shocks form forβ≳ 3, but they are weak (with Mach numbers ∼1) for allβ, and that they reach the center of the star prior to the time of maximum adiabatic compression forβ≳ 10. The maximum density and temperature reached during the TDE follow much shallower relations withβthan the previously predicted${\rho }_{\max }\propto {\beta }^3$and$T_{\max }\propto {\beta }^2$scalings. Belowβ≃ 10, this shallower dependence occurs because the pressure gradient is dynamically significant before the pressure is comparable to the ram pressure of the free-falling gas, while aboveβ≃ 10, we find that shocks prematurely halt the compression and yield the scalings${\rho }_{\max }\propto {\beta }^{1.62}$and$T_{\max }\propto {\beta }^{1.12}$. We find excellent agreement between our results and high-resolution simulations. Our results demonstrate that, in the Newtonian limit, the compression experienced by the star is completely independent of the mass of the black hole. We discuss our results in the context of existing (affine) models, polytropic versus non-polytropic stars, and general relativistic effects, which become important when the pericenter ofmore »the star nears the direct capture radius, atβ∼ 12.5 (2.7) for a solar-like star disrupted by a 10⁶M_⊙(10⁷M_⊙) supermassive black hole.

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5. Multiphase magnetism in Yb ₂ Ti ₂ O ₇

   https://doi.org/10.1073/pnas.2008791117  

   Scheie, Allen ; Kindervater, Jonas ; Zhang, Shu ; Changlani, Hitesh J. ; Sala, Gabriele ; Ehlers, Georg ; Heinemann, Andre ; Tucker, Gregory S. ; Koohpayeh, Seyed M. ; Broholm, Collin ( October 2020 , Proceedings of the National Academy of Sciences)

   We use neutron scattering to show that ferromagnetism and antiferromagnetism coexist in the low T state of the pyrochlore quantum magnet${\text{Yb}}_2{\text{Ti}}_2{\mathrm{O}}_7$. While magnetic Bragg peaks evidence long-range static ferromagnetic order, inelastic scattering shows that short-range correlated antiferromagnetism is also present. Small-angle neutron scattering provides direct evidence for mesoscale magnetic structure that we associate with metastable antiferromagnetism. Classical Monte Carlo simulations based on exchange interactions inferred from$⟨111⟩$-oriented high-field spin wave measurements confirm that antiferromagnetism is metastable within the otherwise ferromagnetic ground state. The apparent lack of coherent spin wave excitations and strong sensitivity to quenched disorder characterizing${\text{Yb}}_2{\text{Ti}}_2{\mathrm{O}}_7$is a consequence of this multiphase magnetism.