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Strong coupling between the spin and orbital degrees of freedom is crucial in generating the exotic band structure of topological insulators. The combination of spin-orbit coupling with electronic correlations could lead to exotic effects; however, these two types of interactions are rarely found to be strong in the same material. Gotlieb et al. used spin- and angle-resolved photoemission spectroscopy to map out the spin texture in the cuprate Bi2212. Surprisingly, they found signatures of spin-momentum locking, not unlike that seen in topological insulators. Thus, in addition to strong electronic correlations, this cuprate also has considerable spin-orbit coupling. Science , this issue p. 1271 Spin- and angle-resolved photoemission spectroscopy reveals a rich spin texture in the cuprate Bi2212. Cuprate superconductors have long been thought of as having strong electronic correlations but negligible spin-orbit coupling. Using spin- and angle-resolved photoemission spectroscopy, we discovered that one of the most studied cuprate superconductors, Bi2212, has a nontrivial spin texture with a spin-momentum locking that circles the Brillouin zone center and a spin-layer locking that allows states of opposite spin to be localized in different parts of the unit cell. Our findings pose challenges for the vast majority of models of cuprates, such as the Hubbard model and its variants, where spin-orbit interaction has been mostly neglected, and open the intriguing question of how the high-temperature superconducting state emerges in the presence of this nontrivial spin texture.

In high-temperature superconductivity, the process that leads to the formation of Cooper pairs, the fundamental charge carriers in any superconductor, remains mysterious. We used a femtosecond laser pump pulse to perturb superconducting Bi₂Sr₂CaCu₂08+δ and studied subsequent dynamics using time and angle-resolved photoemission and infrared reflectivity probes. Gap and quasiparticle population dynamics revealed marked dependencies on both excitation density and crystal momentum. Close to the d-wave nodes, the superconducting gap was sensitive to the pump intensity, and Cooper pairs recombined slowly. Far from the nodes, pumping affected the gap only weakly, and recombination processes were faster. These results demonstrate a new window into the dynamical processes that govern quasiparticle recombination and gap formation in cuprates.

Ultrafast spectroscopy is an emerging technique with great promise in the study of quantum materials, as it makes it possible to track similarities and correlations that are not evident near equilibrium. Thus far, however, the way in which these processes modify the electron self-energy—a fundamental quantity describing many-body interactions in a material—has been little discussed. Here we use time- and angle-resolved photoemission to directly measure the ultrafast response of self-energy to near-infrared photoexcitation in high-temperature cuprate superconductor. Below the critical temperature of the superconductor, ultrafast excitations trigger a synchronous decrease of electron self-energy and superconducting gap, culminating in a saturation in the weakening of electron–boson coupling when the superconducting gap is fully quenched. In contrast, electron–boson coupling is unresponsive to ultrafast excitations above the critical temperature of the superconductor and in the metallic state of a related material. These findings open a new pathway for studying transient self-energy and correlation effects in solids. Superconductivity is the result of many-body interactions between excitations in a solid. Zhang et al. use time- and angle-resolved photoemission to compare photo-induced changes in the electron self-energy of a unconventional superconductor to those in a related material in the metallic state.

The concept of stimulated emission of bosons has played an important role in modern science and technology and constitutes the working principle for lasers. In a stimulated emission process, an incoming photon enhances the probability that an excited atomic state will transition to a lower energy state and generate a second photon of the same energy. It is expected, but not experimentally shown, that stimulated emission contributes significantly to the zero resistance current in a superconductor by enhancing the probability that scattered Cooper pairs will return to the macroscopically occupied condensate instead of entering any other state. Here, we use time- and angle-resolved photoemission spectroscopy to study the initial rise of the non-equilibrium quasiparticle population in a Bi 2 Sr 2 CaCu 2 O 8+δ cuprate superconductor induced by an ultrashort laser pulse. Our finding reveals significantly slower buildup of quasiparticles in the superconducting state than in the normal state. The slower buildup only occurs when the pump pulse is too weak to deplete the superconducting condensate and for cuts inside the Fermi arc region. We propose this is a manifestation of stimulated recombination of broken Cooper pairs and signals an important momentum space dichotomy in the formation of Cooper pairs inside and outside the Fermi arc region.

The COVID-19 pandemic imposed profound changes on the way we think about undergraduate physics education. Online courses became mainstream. Exam formats were reimagined. Digital whiteboards replaced face-to-face discussions. Laboratory classes were outfitted with home-delivered supply kits. And all of us developed a more intimate knowledge of Greek letters and symbols (delta, omicron, etc.) than we might have comfortably liked to admit. Having weathered these transformations from the point of view of both an undergraduate student (S.L.J.) and classroom instructors (A.H. and C.L.S.), we have found that among the most challenging aspects of the in-person learning experience to replicate in an online environment have been the relational ones. To highlight some of the ways in which these issues can be mitigated, we report here on the activities of the San Jose State University (SJSU) Physics and Astronomy Student Reading Society (PhASRS), which was an online reading group at SJSU founded by ourselves and others running from the summer of 2020 until the end of the fall 2020 semester. Elements of the reading group's structure and guiding principles are described, as well as student and faculty reflections on what worked well and what did not. The manuscript underlines the power of astronomy- and physics-themed journal clubs as vehicles for learning and more generally emphasizes the importance of community-building initiatives in the discipline. Our hope is that this summary of activities will inspire faculty members and students at colleges and perhaps high schools to imagine new possibilities for developing communities of people in science that might not otherwise be able to exist.

We report on the construction and characterization of a low-cost Mach-Zehnder optical interferometer in which quadrature signal detection is achieved by means of polarization control. The device incorporates a generic green laser pointer, home-built photodetectors, 3D-printed optical mounts, a circular polarizer extracted from a pair of 3D movie glasses, and a Python-enabled microcontroller for analog-to-digital data acquisition. Components fit inside of a 12\"x6\" space and can be assembled on a budget of less than US\\$500. The device has the potential to make quadrature interferometry accessible and affordable for instructors, students, and enthusiasts alike.

We characterize a high-density sample of negatively charged silicon-vacancy (SiV\\(^-\\)) centers in diamond using collinear optical multidimensional coherent spectroscopy. By comparing the results of complementary signal detection schemes, we identify a hidden population of \\ce{SiV^-} centers that is not typically observed in photoluminescence, and which exhibits significant spectral inhomogeneity and extended electronic \\(T_2\\) times. The phenomenon is likely caused by strain, indicating a potential mechanism for controlling electric coherence in color-center-based quantum devices.

We report coherent interactions within an ensemble of silicon-vacancy color centers in diamond. The interactions are ascribed to resonant dipole-dipole coupling. Further, we demonstrate control over resonant center pairs by using a driving optical pulse to induce collective, interaction-enabled Rabi-oscillations in the ensemble. Non-resonant center pairs do not undergo collective oscillations.

An ensemble of silicon vacancy centers in diamond (\\ce{SiV-}) is probed using two coherent spectroscopy techniques. Two main distinct families of \\ce{SiV-} centers are identified using multidimensional coherent spectroscopy, and these families are paired with two orientation groups by comparing spectra from different linear polarizations of the incident laser. By tracking the peak centers in the measured spectra, the full diamond strain tensor is calculated local to the laser spot. Such measurements are made at multiple points on the sample surface and variations in the strain tensor are observed.