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1. Observation of the polaronic character of excitons in a two-dimensional semiconducting magnet CrI3

   https://doi.org/10.1038/s41467-020-18627-x  

   Jin, Wencan; Kim, Hyun Ho; Ye, Zhipeng; Ye, Gaihua; Rojas, Laura; Luo, Xiangpeng; Yang, Bowen; Yin, Fangzhou; Horng, Jason Shih An; Tian, Shangjie; et al (September 2020, Nature Communications)

   Abstract Exciton dynamics can be strongly affected by lattice vibrations through electron-phonon coupling. This is rarely explored in two-dimensional magnetic semiconductors. Focusing on bilayer CrI₃, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th order in Raman spectra, attributed to the polaronic character of excitons. We establish that this polaronic character is dominated by the coupling between the charge-transfer exciton at 1.96 eV and a longitudinal optical phonon at 120.6 cm⁻¹. We further show that the emergence of long-range magnetic order enhances the electron-phonon coupling strength by ~50% and that the transition from layered antiferromagnetic to ferromagnetic order tunes the spectral intensity of the periodic broad modes, suggesting a strong coupling among the lattice, charge and spin in two-dimensional CrI₃. Our study opens opportunities for tailoring light-matter interactions in two-dimensional magnetic semiconductors. 

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2. Deconfined quantum criticality of nodal $d$ -wave superconductivity, Néel order, and charge order on the square lattice at half-filling

   https://doi.org/10.1103/PhysRevResearch.6.033018  

   Christos, Maine; Shackleton, Henry; Sachdev, Subir; Luo, Zhu-Xi (July 2024, Physical Review Research)

   We consider a SU(2) lattice gauge theory on the square lattice, with a single fundamental complex fermion and a single fundamental complex boson on each lattice site. Projective symmetries of the gauge-charged fermions are chosen so that they match with those of the spinons of the $\pi$-flux spin liquid. Global symmetries of all gauge-invariant observables are chosen to match with those of the particle-hole symmetric electronic Hubbard model at half-filling. Consequently, both the fundamental fermion and fundamental boson move in an average background $\pi$-flux, their gauge-invariant composite is the physical electron, and eliminating gauge fields in a strong gauge-coupling expansion yields an effective extended Hubbard model for the electrons. The SU(2) gauge theory displays several confining/Higgs phases: a nodal $d$-wave superconductor, and states with Néel, valence-bond solid, charge, or staggered current orders. There are also a number of quantum phase transitions between these phases that are very likely described by $(2+1)$-dimensional deconfined conformal gauge theories, and we present large flavor expansions for such theories. These include the phenomenologically attractive case of a transition between a conventional insulator with a charge gap and Néel order, and a conventional $d$-wave superconductor with gapless Bogoliubov quasiparticles at four nodal points in the Brillouin zone. We also apply our approach to the honeycomb lattice, where we find a bicritical point at the junction of Néel, valence bond solid (Kekulé), and Dirac semimetal phases. Published by the American Physical Society2024 

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3. Spin-orbital Jahn-Teller bipolarons

   https://doi.org/10.1038/s41467-024-46621-0  

   Celiberti, Lorenzo; Fiore_Mosca, Dario; Allodi, Giuseppe; Pourovskii, Leonid V; Tassetti, Anna; Forino, Paola Caterina; Cong, Rong; Garcia, Erick; Tran, Phuong M; De_Renzi, Roberto; et al (December 2024, Nature Communications)

   Abstract Polarons and spin-orbit (SO) coupling are distinct quantum effects that play a critical role in charge transport and spin-orbitronics. Polarons originate from strong electron-phonon interaction and are ubiquitous in polarizable materials featuring electron localization, in particular 3d transition metal oxides (TMOs). On the other hand, the relativistic coupling between the spin and orbital angular momentum is notable in lattices with heavy atoms and develops in 5d TMOs, where electrons are spatially delocalized. Here we combine ab initio calculations and magnetic measurements to show that these two seemingly mutually exclusive interactions are entangled in the electron-doped SO-coupled Mott insulator Ba₂Na_1−xCa_xOsO₆(0 < x < 1), unveiling the formation ofspin-orbital bipolarons. Polaron charge trapping, favoured by the Jahn-Teller lattice activity, converts the Os 5d¹spin-orbital J_eff = 3/2 levels, characteristic of the parent compound Ba₂NaOsO₆(BNOO), into a bipolaron 5d²J_eff = 2 manifold, leading to the coexistence of different J-effective states in a single-phase material. The gradual increase of bipolarons with increasing doping creates robust in-gap states that prevents the transition to a metal phase even at ultrahigh doping, thus preserving the Mott gap across the entire doping range from d¹BNOO to d²Ba₂CaOsO₆(BCOO). 

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4. Lieb-Schultz-Mattis, Luttinger, and 't Hooft - anomaly matching in lattice systems

   https://doi.org/10.21468/SciPostPhys.15.2.051  

   Cheng, Meng; Seiberg, Nathan (January 2023, SciPost Physics)

   We analyze lattice Hamiltonian systems whose global symmetries have ’t Hooft anomalies. As is common in the study of anomalies, they are probed by coupling the system to classical background gauge fields. For flat fields (vanishing field strength), the nonzero spatial components of the gauge fields can be thought of as twisted boundary conditions, or equivalently, as topological defects. The symmetries of the twisted Hilbert space and their representations capture the anomalies. We demonstrate this approach with a number of examples. In some of them, the anomalous symmetries are internal symmetries of the lattice system, but they do not act on-site. (We clarify the notion of “on-site action.”) In other cases, the anomalous symmetries involve lattice translations. Using this approach we frame many known and new results in a unified fashion. In this work, we limit ourselves to 1+1d systems with a spatial lattice. In particular, we present a lattice system that flows to the c=1 compact boson system with any radius (no BKT transition) with the full internal symmetry of the continuum theory, with its anomalies and its T-duality. As another application, we analyze various spin chain models and phrase their Lieb-Shultz-Mattis theorem as an ’t Hooft anomaly matching condition. We also show in what sense filling constraints like Luttinger theorem can and cannot be viewed as reflecting an anomaly. As a by-product, our understanding allows us to use information from the continuum theory to derive some exact results in lattice model of interest, such as the lattice momenta of the low-energy states. 

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5. Reversible non-volatile electronic switching in a near-room-temperature van der Waals ferromagnet

   https://doi.org/10.1038/s41467-024-46862-z  

   Wu, Han; Chen, Lei; Malinowski, Paul; Jang, Bo Gyu; Deng, Qinwen; Scott, Kirsty; Huang, Jianwei; Ruff, Jacob_P C; He, Yu; Chen, Xiang; et al (December 2024, Nature Communications)

   Abstract Non-volatile phase-change memory devices utilize local heating to toggle between crystalline and amorphous states with distinct electrical properties. Expanding on this kind of switching to two topologically distinct phases requires controlled non-volatile switching between two crystalline phases with distinct symmetries. Here, we report the observation of reversible and non-volatile switching between two stable and closely related crystal structures, with remarkably distinct electronic structures, in the near-room-temperature van der Waals ferromagnet Fe_5−δGeTe₂. We show that the switching is enabled by the ordering and disordering of Fe site vacancies that results in distinct crystalline symmetries of the two phases, which can be controlled by a thermal annealing and quenching method. The two phases are distinguished by the presence of topological nodal lines due to the preserved global inversion symmetry in the site-disordered phase, flat bands resulting from quantum destructive interference on a bipartite lattice, and broken inversion symmetry in the site-ordered phase. 

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