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1. Theory of Josephson scanning microscopy with $s$ -wave tip on unconventional superconducting surface: Application to ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$

   https://doi.org/10.1103/PhysRevB.110.184519  

   Choubey, Peayush; Hirschfeld, P J (November 2024, Physical Review B)

   Josephson scanning tunneling microscopy (JSTM) is a powerful probe of the local superconducting order parameter, but studies have been largely limited to cases where the superconducting sample and superconducting tip both have the same gap symmetry—either s-wave or d-wave. It has been generally assumed that, in an ideal s-to-d JSTM experiment, the critical current would vanish everywhere, as expected for ideal c-axis planar junctions. We show here that this is not the case. Employing first-principlesWannier functions for Bi2Sr2CaCu2O8+δ , we develop a scheme to compute the Josephson critical current (Ic) and quasiparticle tunneling current measured by JSTM with subangstrom resolution. We demonstrate that the critical current for tunneling between an s-wave tip and a superconducting cuprate sample has the largest magnitude above O sites and it vanishes above Cu sites. Ic changes sign under π/2 rotation and its average over a unit cell vanishes, as a direct consequence of the d-wave gap symmetry in cuprates. Further, we show that Ic is strongly suppressed in the close vicinity of a Zn-like impurity owing to suppression of the superconducting order parameter. More interestingly, Ic acquires nonvanishing values above the Cu sites near the impurity. The critical current modulations produced by the impurity occur at characteristic wave vectors distinct from the quasiparticle interference (QPI) analog. Furthermore, the quasiparticle tunneling spectra in the JSTM setup shows coherence peaks and impurity-induced resonances shifted by the s-wave tip gap. We discuss the similarities and differences in JSTM observables and conventional STM observables, making specific predictions that can be tested in future JSTM experiments. 

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2. Spontaneous time-reversal symmetry breaking by disorder in superconductors

   https://doi.org/10.3389/fphy.2024.1353425  

   Andersen, Brian M; Kreisel, Andreas; Hirschfeld, P J (May 2024, Frontiers in Physics)

   A growing number of superconducting materials display evidence for spontaneous time-reversal symmetry breaking (TRSB) below their critical transition temperatures. Precisely what this implies for the nature of the superconducting ground state of such materials, however, is often not straightforward to infer. We review the experimental status and survey different theoretical mechanisms for the generation of TRSB in superconductors. In cases where a TRSB complex combination of two superconducting order parameter components is realized, defects, dislocations and sample edges may generate superflow patterns that can be picked up by magnetic probes. However, even single-component condensates that do not break time-reversal symmetry in their pure bulk phases can also support signatures of magnetism inside the superconducting state. This includes, for example, the generation of localized orbital current patterns or spin-polarization near atomic-scale impurities, twin boundaries and other defects. Signals of TRSB may also arise from a superconductivity-enhanced Ruderman-Kittel-Kasuya-Yosida exchange coupling between magnetic impurity moments present in the normal state. We discuss the relevance of these different mechanisms for TRSB in light of recent experiments on superconducting materials of current interest. 

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3. Studying the interaction between charm and light-flavor mesons

   https://doi.org/10.1103/PhysRevD.110.032004  

   Acharya, S; Adamová, D; Aglieri_Rinella, G; Aglietta, L; Agnello, M; Agrawal, N; Ahammed, Z; Ahmad, S; Ahn, S U; Ahuja, I; et al (August 2024, Physical Review D)

   The two-particle momentum correlation functions between charm mesons ( ${\mathrm{D}}^{*\pm }$and ${\mathrm{D}}^{\pm }$) and charged light-flavor mesons ( ${\pi }^{\pm }$and ${\mathrm{K}}^{\pm }$) in all charge combinations are measured for the first time by the ALICE Collaboration in high-multiplicity proton–proton collisions at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$. For DK and ${\mathrm{D}}^{*}\mathrm{K}$pairs, the experimental results are in agreement with theoretical predictions of the residual strong interaction based on quantum chromodynamics calculations on the lattice and chiral effective field theory. In the case of $\mathrm{D}\pi$and ${\mathrm{D}}^{*}\pi$pairs, tension between the calculations including strong interactions and the measurement is observed. For all particle pairs, the data can be adequately described by Coulomb interaction only, indicating a shallow interaction between charm and light-flavor mesons. Finally, the scattering lengths governing the residual strong interaction of the $\mathrm{D}\pi$and ${\mathrm{D}}^{*}\pi$systems are determined by fitting the experimental correlation functions with a model that employs a Gaussian potential. The extracted values are small and compatible with zero. © 2024 CERN, for the ALICE Collaboration2024CERN 

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4. Electronic stripe patterns near the fermi level of tetragonal Fe(Se,S)

   https://doi.org/10.1038/s41535-023-00592-5  

   Walker, M.; Scott, K.; Boyle, T. J.; Byland, J. K.; Bötzel, S.; Zhao, Z.; Day, R. P.; Zhdanovich, S.; Gorovikov, S.; Pedersen, T. M.; et al (October 2023, npj Quantum Materials)

   Abstract FeSe_1−xS_xremains one of the most enigmatic systems of Fe-based superconductors. While much is known about the orthorhombic parent compound, FeSe, the tetragonal samples, FeSe_1−xS_xwithx > 0.17, remain relatively unexplored. Here, we provide an in-depth investigation of the electronic states of tetragonal FeSe_0.81S_0.19, using scanning tunneling microscopy and spectroscopy (STM/S) measurements, supported by angle-resolved photoemission spectroscopy (ARPES) and theoretical modeling. We analyze modulations of the local density of states (LDOS) near and away from Fe vacancy defects separately and identify quasiparticle interference (QPI) signals originating from multiple regions of the Brillouin zone, including the bands at the zone corners. We also observe that QPI signals coexist with a much stronger LDOS modulation for states near the Fermi level whose period is independent of energy. Our measurements further reveal that this strong pattern appears in the STS measurements as short range stripe patterns that are locally two-fold symmetric. Since these stripe patterns coexist with four-fold symmetric QPI around Fe-vacancies, the origin of their local two-fold symmetry must be distinct from that of nematic states in orthorhombic samples. We explore several aspects related to the stripes, such as the role of S and Fe-vacancy defects, and whether they can be explained by QPI. We consider the possibility that the observed stripe patterns may represent incipient charge order correlations, similar to those observed in the cuprates. 

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5. Stabilization of three-dimensional charge order through interplanar orbital hybridization in PrxY1−xBa2Cu3O6+δ

   https://doi.org/10.1038/s41467-022-33607-z  

   Ruiz, Alejandro; Gunn, Brandon; Lu, Yi; Sasmal, Kalyan; Moir, Camilla M.; Basak, Rourav; Huang, Hai; Lee, Jun-Sik; Rodolakis, Fanny; Boyle, Timothy J.; et al (October 2022, Nature Communications)

   Abstract The shape of 3d-orbitals often governs the electronic and magnetic properties of correlated transition metal oxides. In the superconducting cuprates, the planar confinement of the$${d}_{{x}^{2}-{y}^{2}}$$ $d_{x^2-y^2}$orbital dictates the two-dimensional nature of the unconventional superconductivity and a competing charge order. Achieving orbital-specific control of the electronic structure to allow coupling pathways across adjacent planes would enable direct assessment of the role of dimensionality in the intertwined orders. Using CuL₃and PrM₅resonant x-ray scattering and first-principles calculations, we report a highly correlated three-dimensional charge order in Pr-substituted YBa₂Cu₃O₇, where the Prf-electrons create a direct orbital bridge between CuO₂planes. With this we demonstrate that interplanar orbital engineering can be used to surgically control electronic phases in correlated oxides and other layered materials. 

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