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1. Evidence for a monolayer excitonic insulator

   Jia, Y. ( January 2020 , ArXivorg)

   null (Ed.)

   The interplay between topology and correlations can generate a variety of unusual quantum phases, many of which remain to be explored. Recent advances have identified monolayer WTe2 as a promising material for exploring such interplay in a highly tunable fashion. The ground state of this two-dimensional (2D) crystal can be electrostatically tuned from a quantum spin Hall insulator (QSHI) to a superconductor. However, much remains unknown about the nature of these ground states, including the gap-opening mechanism of the insulating state. Here we report systematic studies of the insulating phase in WTe2 monolayer and uncover evidence supporting that the QSHI is also an excitonic insulator (EI). An EI, arising from the spontaneous formation of electron-hole bound states (excitons), is a largely unexplored quantum phase to date, especially when it is topological. Our experiments on high-quality transport devices reveal the presence of an intrinsic insulating state at the charge neutrality point (CNP) in clean samples. The state exhibits both a strong sensitivity to the electric displacement field and a Hall anomaly that are consistent with the excitonic pairing. We further confirm the correlated nature of this charge-neutral insulator by tunneling spectroscopy. Our results support the existence of an EI phase in the clean limit and rule out alternative scenarios of a band insulator or a localized insulator. These observations lay the foundation for understanding a new class of correlated insulators with nontrivial topology and identify monolayer WTe2 as a promising candidate for exploring quantum phases of ground-state excitons. 

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2. Measurement of the hyperfine coupling constants and absolute energies of the 12s 2S1/2, 13s 2S1/2 , and 11d 2DJ levels in atomic cesium

   https://doi.org/10.1103/PhysRevA.105.022819  

   Quirk, Jonah A ; Damitz, Amy ; Tanner, Carol E ; Elliott, D S ( February 2022 , Physical Review A)

   We report measurements of the absolute energies of the hyperfine components of the $12s \ ^2S_{1/2}$ and $13s \ ^2S_{1/2}$ levels of atomic cesium, $^{133}$Cs. Using the frequency difference between these components, we determine the hyperfine coupling constants for these states, and report these values with a relative uncertainty of $\sim$0.06\%. We also examine the hyperfine structure of the $11d \ ^2D_{J}$ ($J=3/2, 5/2$) states, and resolve the sign ambiguity of the hyperfine coupling constants from previous measurements of these states. We also derive new, high precision values for the state energies of the $12s \ ^2S_{1/2}$, $13s \ ^2S_{1/2}$ and $11d \ ^2D_{J}$ states of cesium. 

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3. Valence transition theory of the pressure-induced dimensionality crossover in superconducting Sr14−xCaxCu24O41

   https://doi.org/10.1103/PhysRevB.108.134510  

   Song, Jeong-Pil ; Clay, R. Torsten ; Mazumdar, Sumit ( October 2023 , Physical Review B)

   Stephen E. Nagler (Ed.)

   One of the strongest justifications for the continued search for superconductivity within the single-band Hubbard Hamiltonian originates from the apparent success of single-band ladder-based theories in predicting the occurrence of superconductivity in the cuprate coupled-ladder compound Sr{14−x}Ca{x}Cu{24}O{41}. Recent theoretical works have, however, shown the complete absence of quasi-long-range superconducting correlations within the hole-doped multiband ladder Hamiltonian including realistic Coulomb repulsion between holes on oxygen sites and oxygen-oxygen hole hopping. Experimentally, superconductivity in Sr{14−x}Ca{x}Cu{24}O{41} occurs only under pressure and is preceded by dramatic transition from one to two dimensions that remains not understood. We show that understanding the dimensional crossover requires adopting a valence transition model within which there occurs transition in Cu-ion ionicity from +2 to +1 , with transfer of holes from Cu to O ions [S. Mazumdar, Phys. Rev. B 98, 205153 (2018)]. The driving force behind the valence transition is the closed-shell electron configuration of Cu^{1+} , a feature shared by cations of all oxides with a negative charge-transfer gap. We make a falsifiable experimental prediction for Sr{14−x}Ca{x}Cu{24}O{41} and discuss the implications of our results for layered two-dimensional cuprates. 

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4. Single-ion properties of the Seff = 12 XY antiferromagnetic pyrochlores NaA′Co2F7 ( A′=Ca2+, Sr2+ )

   https://doi.org/10.1103/PhysRevB.95.144414  

   Ross, K. A. ; Brown, J. M. ; Cava, R. J. ; Krizan, J. W. ; Nagler, S. E. ; Rodriguez-Rivera, J. A. ; Stone, M. B. ( April 2017 , Physical Review B)
5. Angular dependence of spin-orbit torque in monolayer Fe3GeTe2

   https://doi.org/10.1103/PhysRevB.108.144422  

   Xue, Fei ; Stiles, Mark D. ; Haney, Paul M. ( October 2023 , Physical Review B)

   In ferromagnetic systems lacking inversion symmetry, an applied electric field can control the ferromagnetic order parameters through the spin-orbit torque. The prototypical example is a bilayer heterostructure composed of a ferromagnet and a heavy metal that acts as a spin current source. In addition to such bilayers, spin-orbit coupling can mediate spin-orbit torques in ferromagnets that lack bulk inversion symmetry. A recently discovered example is the two-dimensional monolayer ferromagnet Fe3GeTe2. In this paper, we use first-principles calculations to study the spin-orbit torque and ensuing magnetic dynamics in this material. By expanding the torque versus magnetization direction as a series of vector spherical harmonics, we find that higher order terms (up to ℓ=4) are significant and play important roles in the magnetic dynamics. They give rise to deterministic, magnetic field-free electrical switching of perpendicular magnetization. 

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