Stanford Advisors

All Publications

  • Strong electron-phonon coupling driven pseudogap modulation and density-wave fluctuations in a correlated polar metal. Nature communications Wang, H. H., Xiong, Y., Padma, H., Wang, Y., Wang, Z., Claes, R., Brunin, G., Min, L., Zu, R., Wetherington, M. T., Wang, Y., Mao, Z., Hautier, G., Chen, L., Dabo, I., Gopalan, V. 2023; 14 (1): 5769


    There is tremendous interest in employing collective excitations of thelattice, spin, charge, and orbitals to tune strongly correlated electronic phenomena. We report such an effect in a ruthenate, Ca3Ru2O7, where two phonons with strong electron-phonon coupling modulate the electronic pseudogap as well as mediate charge and spin density wave fluctuations. Combining temperature-dependent Raman spectroscopy with density functional theory reveals two phonons, B2P and B2M, that are strongly coupled to electrons and whose scattering intensities respectively dominate in the pseudogap versus the metallic phases. The B2P squeezes the octahedra along the out of plane c-axis, while the B2M elongates it, thus modulating the Ru 4d orbital splitting and the bandwidth of the in-plane electron hopping; Thus, B2P opens the pseudogap, while B2M closes it. Moreover, the B2 phonons mediate incoherent charge and spin density wave fluctuations, as evidenced by changes in the background electronic Raman scattering that exhibit unique symmetry signatures. The polar order breaks inversion symmetry, enabling infrared activity of these phonons, paving the way for coherent light-driven control of electronic transport.

    View details for DOI 10.1038/s41467-023-41460-x

    View details for PubMedID 37723139

  • Bipolaronic Nature of the Pseudogap in Quasi-One-Dimensional (TaSe4)2I Revealed via Weak Photoexcitation. Nano letters Zhang, Y., Murthy, C., Kafle, T. R., You, W., Shi, X., Min, L., Wang, H. H., Li, N., Gopalan, V., Mao, Z., Rossnagel, K., Yang, L., Kapteyn, H., Nandkishore, R., Murnane, M. 2023


    The origin of the pseudogap in many strongly correlated materials has been a longstanding puzzle. Here, we present experimental evidence that many-body interactions among small Holstein polarons, i.e., the formation of bipolarons, are primarily responsible for the pseudogap in (TaSe4)2I. After weak photoexcitation of the material, we observe the appearance of both dispersive (single-particle bare band) and flat bands (single-polaron sub-bands) in the gap by using time- and angle-resolved photoemission spectroscopy. Based on Monte Carlo simulations of the Holstein model, we propose that the melting of pseudogap and emergence of new bands originate from a bipolaron to single-polaron crossover. We also observe dramatically different relaxation times for the excited in-gap states in (TaSe4)2I (∼600 fs) compared with another 1D material Rb0.3MoO3 (∼60 fs), which provides a new method for distinguishing between pseudogaps induced by polaronic or Luttinger-liquid many-body interactions.

    View details for DOI 10.1021/acs.nanolett.3c01078

    View details for PubMedID 37682637