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1. Quantum oscillations in the hole-doped cuprates and the confinement of spinons

   https://doi.org/10.1073/pnas.2418633121  

   Bonetti, Pietro_M; Christos, Maine; Sachdev, Subir (December 2024, Proceedings of the National Academy of Sciences)

   A long-standing problem in the study of the under-hole-doped cuprates has been the description of the Fermi surfaces underlying the high magnetic field quantum oscillations, and their connection to the higher temperature pseudogap metal. Harrison and Sebastian [Phys. Rev. Lett.106, 226402 (2011)] proposed that the pseudogap “Fermi arcs” are reconstructed into an electron pocket by field-induced charge density wave order. But computations on such a model [Zhang and Mei,Europhys. Lett.114, 47008 (2016)] show an unobserved additional oscillation frequency from a Fermi surface arising from the backsides of the hole pockets completing the Fermi arcs. We describe a transition from a fractionalized Fermi liquid (FL*) model of the pseudogap metal, to a metal with bidirectional charge density wave order without fractionalization. We show that the confinement of the fermionic spinon excitations of the FL* across this transition can eliminate the unobserved oscillation frequency. 

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2. Thermodynamic behavior of correlated electron-hole fluids in van der Waals heterostructures

   https://doi.org/10.1038/s41467-023-43799-7  

   Qi, Ruishi; Joe, Andrew Y.; Zhang, Zuocheng; Zeng, Yongxin; Zheng, Tiancheng; Feng, Qixin; Xie, Jingxu; Regan, Emma; Lu, Zheyu; Taniguchi, Takashi; et al (December 2023, Nature Communications)

   Abstract Coupled two-dimensional electron-hole bilayers provide a unique platform to study strongly correlated Bose-Fermi mixtures in condensed matter. Electrons and holes in spatially separated layers can bind to form interlayer excitons, composite Bosons expected to support high-temperature exciton condensates. The interlayer excitons can also interact strongly with excess charge carriers when electron and hole densities are unequal. Here, we use optical spectroscopy to quantitatively probe the local thermodynamic properties of strongly correlated electron-hole fluids in MoSe₂/hBN/WSe₂heterostructures. We observe a discontinuity in the electron and hole chemical potentials at matched electron and hole densities, a definitive signature of an excitonic insulator ground state. The excitonic insulator is stable up to a Mott density of ~0.8 × 10¹²cm⁻²and has a thermal ionization temperature of ~70 K. The density dependence of the electron, hole, and exciton chemical potentials reveals strong correlation effects across the phase diagram. Compared with a non-interacting uniform charge distribution, the correlation effects lead to significant attractive exciton-exciton and exciton-charge interactions in the electron-hole fluid. Our work highlights the unique quantum behavior that can emerge in strongly correlated electron-hole systems. 

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3. Emergence of nodal Bogoliubov quasiparticles across the transition from the pseudogap metal to the d-wave superconductor

   https://doi.org/10.1038/s41535-023-00608-0  

   Christos, Maine; Sachdev, Subir (January 2024, npj Quantum Materials)

   Abstract We model the pseudogap state of the hole- and electron-doped cuprates as a metal with hole and/or electron pocket Fermi surfaces. In the absence of long-range antiferromagnetism, such Fermi surfaces violate the Luttinger requirement of enclosing the same area as free electrons at the same density. Using the Ancilla theory of such a pseudogap state, we describe the onset of conventionald-wave superconductivity by the condensation of a chargeeHiggs boson transforming as a fundamental under the emergent SU(2) gauge symmetry of a backgroundπ-flux spin liquid. In all cases, we find that thed-wave superconductor has gapless Bogoliubov quasiparticles at 4 nodal points on the Brillouin zone diagonals with significant velocity anisotropy, just as in the BCS state. This includes the case of the electron-doped pseudogap metal with only electron pockets centered at wavevectors (π, 0), (0, π), and an electronic gap along the zone diagonals. Remarkably, in this case, too, gapless nodal Bogoliubov quasiparticles emerge within the gap at 4 points along the zone diagonals upon the onset of superconductivity. 

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4. Realistic Electron Diffusion Rates and Lifetimes Due to Scattering by Electron Holes

   https://doi.org/10.1029/2021JA029380  

   Shen, Yangyang; Vasko, Ivan Y.; Artemyev, Anton; Malaspina, David M.; Chu, Xiangning; Angelopoulos, Vassilis; Zhang, Xiao‐Jia (August 2021, Journal of Geophysical Research: Space Physics)

   Abstract Plasma sheet electron precipitation into the diffuse aurora is critical for magnetosphere‐ionosphere coupling. Recent studies have shown that electron phase space holes can pitch‐angle scatter electrons and may produce plasma sheet electron precipitation. These studies have assumed identical electron hole parameters to estimate electron scattering rates (Vasko et al., 2018,https://doi.org/10.1063/1.5039687). In this study, we have re‐evaluated the efficiency of this scattering by incorporating realistic electron hole properties from direct spacecraft observations into computing electron diffusion rates and lifetimes. The most important electron hole properties in this evaluation are their distributions in velocity and spatial scale and electric field root‐mean‐square intensity (). Using direct measurements of electron holes during a plasma injection event observed by the Van Allen Probe at, we find that when4 mV/m electron lifetimes can drop below 1 h and are mostly within strong diffusion limits at energies below10 keV. During an injection observed by the THEMIS spacecraft at, electron holes with even typical intensities (1 mV/m) can deplete low‐energy (a few keV) plasma sheet electrons within tens of minutes following injections and convection from the tail. Our results confirm that electron holes are a significant contributor to plasma sheet electron precipitation during injections. 

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5. Ultrafast transverse and longitudinal response of laser-excited quantum wires

   https://doi.org/10.1364/OE.448934  

   Gulley, Jeremy_R; Huang, Danhong (March 2022, Optics Express)

   We couple 1D pulse propagation simulations with laser-solid dynamics in a GaAs quantum wire, solving for the electron and hole populations and the interband and intraband coherences between states. We thus model not only the dynamical dipole contributions to the optical polarization (interband bound-charge response) but also the photo-generation and back-action effects of the net free-charge density (intraband free-charge response). These results show that solving for the dynamic electron and hole intraband coherences leads to plasma oscillations at THz frequencies, even in a 1D solid where plasma screening is small. We then calculate the transverse and longitudinal response of the quantum wire and characterize the dispersion relation for thee-hplasma. This approach allows one to predict the optoelectronic response of 1D semiconductor devices during and after exposure to resonant ultrashort pulses. 

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