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1. Fingerprints of an orbital-selective Mott phase in the block magnetic state of BaFe2Se3 ladders

   https://doi.org/10.1038/s42005-019-0155-3  

   Patel, N. D. ; Nocera, A. ; Alvarez, G. ; Moreo, A. ; Johnston, S. ; Dagotto, E. ( June 2019 , Communications Physics)

   Resonant Inelastic X-Ray Scattering (RIXS) experiments on the iron-based ladder BaFe₂Se₃unveiled an unexpected two-peak structure associated with local orbital (dd) excitations in a block-type antiferromagnetic phase. A mixed character between correlated band-like and localized excitations was also reported. Here, we use the density matrix renormalization group method to calculate the momentum-resolved charge- and orbital-dynamical response functions of a multi-orbital Hubbard chain. Remarkably, our results qualitatively resemble the BaFe₂Se₃RIXS data, while also capturing the presence of long-range magnetic order as found in neutron scattering, only when the model is in an exotic orbital-selective Mott phase (OSMP). In the calculations, the experimentally observed zero-momentum transfer RIXS peaks correspond to excitations between itinerant and Mott insulating orbitals. We provide experimentally testable predictions for the momentum-resolved charge and orbital dynamical structures, which can provide further insight into the OSMP regime of BaFe₂Se₃.

    

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2. Combining localized orbital scaling correction and Bethe–Salpeter equation for accurate excitation energies

   https://doi.org/10.1063/5.0087498  

   Li, Jiachen ; Jin, Ye ; Su, Neil Qiang ; Yang, Weitao ( April 2022 , The Journal of Chemical Physics)

   We applied localized orbital scaling correction (LOSC) in Bethe–Salpeter equation (BSE) to predict accurate excitation energies for molecules. LOSC systematically eliminates the delocalization error in the density functional approximation and is capable of approximating quasiparticle (QP) energies with accuracy similar to or better than GW Green’s function approach and with much less computational cost. The QP energies from LOSC, instead of commonly used G 0 W 0 and ev GW, are directly used in BSE. We show that the BSE/LOSC approach greatly outperforms the commonly used BSE/ G 0 W 0 approach for predicting excitations with different characters. For the calculations of Truhlar–Gagliardi test set containing valence, charge transfer, and Rydberg excitations, BSE/LOSC with the Tamm–Dancoff approximation provides a comparable accuracy to time-dependent density functional theory (TDDFT) and BSE/ev GW. For the calculations of Stein CT test set and Rydberg excitations of atoms, BSE/LOSC considerably outperforms both BSE/ G 0 W 0 and TDDFT approaches with a reduced starting point dependence. BSE/LOSC is, thus, a promising and efficient approach to calculate excitation energies for molecular systems. 

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3. Dispersionless orbital excitations in (Li,Fe)OHFeSe superconductors

   https://doi.org/10.1038/s41535-022-00492-0  

   Xiao, Qian ; Zhang, Wenliang ; Asmara, Teguh Citra ; Li, Dong ; Li, Qizhi ; Zhang, Shilong ; Tseng, Yi ; Dong, Xiaoli ; Wang, Yao ; Chen, Cheng-Chien ; et al ( December 2022 , npj Quantum Materials)

   Abstract The superconducting critical temperature T c of intercalated iron-selenide superconductor (Li,Fe)OHFeSe (FeSe11111) can be increased to 42 from 8 K of bulk FeSe. It shows remarkably similar electronic properties as the high- T c monolayer FeSe and provides a bulk counterpart to investigate the origin of enhanced superconductivity. Unraveling the nature of excitations is crucial for understanding the pairing mechanism in high- T c iron selenides. Here we use resonant inelastic x-ray scattering (RIXS) to investigate the excitations in FeSe11111. Our high-quality data exhibit several Raman-like excitations, which are dispersionless and isotropic in momentum transfer in both superconducting 28 K and 42 K samples. Using atomic multiplet calculations, we assign the low-energy ~0.3 and 0.7 eV Raman peaks as local e g  −  e g and e g  −  t 2 g orbital excitations. The intensity of these two features decreases with increasing temperature, suggesting a dominating contribution of the orbital fluctuations. Our results highlight the importance of the orbital degree of freedom for high- T c iron selenides. 

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4. Triple electron–electron–proton excitations and second-order approximations in nuclear–electronic orbital coupled cluster methods

   https://doi.org/10.1063/5.0106173  

   Pavošević, Fabijan ; Hammes-Schiffer, Sharon ( August 2022 , The Journal of Chemical Physics)

   The accurate description of nuclear quantum effects, such as zero-point energy, is important for modeling a wide range of chemical and biological processes. Within the nuclear–electronic orbital (NEO) approach, such effects are incorporated in a computationally efficient way by treating electrons and select nuclei, typically protons, quantum mechanically with molecular orbital techniques. Herein, we implement and test a NEO coupled cluster method that explicitly includes the triple electron–electron–proton excitations, where two electrons and one proton are excited simultaneously, using automatic differentiation. Our calculations show that this NEO-CCSDT eep method provides highly accurate proton densities and proton affinities, outperforming any previously studied NEO method. These examples highlight the importance of the triple electron–electron–proton excitations for an accurate description of nuclear quantum effects. Additionally, we also implement and test the second-order approximate coupled cluster with singles and doubles (NEO-CC2) method as well as its scaled-opposite-spin (SOS) versions. The NEO-SOS′-CC2 method, which scales the electron–proton correlation energy as well as the opposite-spin and same-spin components of the electron–electron correlation energy, achieves nearly the same accuracy as the NEO-CCSDT eep method for the properties studied. Because of its low computational cost, this method will enable a wide range of chemical and photochemical applications for large molecular systems. This work sets the stage for a variety of developments and applications within the NEO framework. 

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5. Orbital stability of smooth solitary waves for the Degasperis-Procesi equation

   https://doi.org/10.1090/proc/16087  

   Li, Ji ; Liu, Yue ; Wu, Qiliang ( January 2023 , Proceedings of the American Mathematical Society)

   The Degasperis-Procesi (DP) equation is an integrable Camassa-Holm-type model which is an asymptotic approximation for the unidirectional propagation of shallow water waves. This work establishes the orbital stability of localized smooth solitary waves to the DP equation on the real line, extending our previous work on their spectral stability [J. Math. Pures Appl. (9) 142 (2020), pp. 298–314]. The main difficulty stems from the fact that the natural energy space is a subspace of L 3 L^3 , but the translation symmetry for the DP equation gives rise to a conserved quantity equivalent to the L 2 L^2 -norm, resulting in L 3 L^3 higher-order nonlinear terms in the augmented Hamiltonian. But the usual coercivity estimate is in terms of L 2 L^2 norm for DP equation, which cannot be used to control the L 3 L^3 higher order term directly. The remedy is to observe that, given a sufficiently smooth initial condition satisfying some mild constraint, the L ∞ L^\infty orbital norm of the perturbation is bounded above by a function of its L 2 L^2 orbital norm, yielding the higher order control and the orbital stability in the L 2 ∩ L ∞ L^2\cap L^\infty space. 

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