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A Hubbard exciton fluid in a photo-doped antiferromagnetic Mott insulator

Nature Physics volume 19, pages 1876–1882 (2023)Cite this article

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Abstract

The undoped antiferromagnetic Mott insulator naturally has one charge carrier per lattice site. When it is doped with additional carriers, they are unstable to spin-fluctuation-mediated Cooper pairing as well as other unconventional types of charge, spin and orbital current ordering. Photo-excitation can produce charge carriers in the form of empty (holons) and doubly occupied (doublons) sites that may also exhibit charge instabilities. There is evidence that antiferromagnetic correlations enhance attractive interactions between holons and doublons, which can then form bound pairs known as Hubbard excitons, and that these might self-organize into an insulating Hubbard exciton fluid. However, this out-of-equilibrium phenomenon has not been experimentally detected. Here we report the transient formation of a Hubbard exciton fluid in the antiferromagnetic Mott insulator Sr2IrO4 using ultrafast terahertz conductivity. Following photo-excitation, we observe rapid spectral-weight transfer from a Drude metallic response to an insulating response. The latter is characterized by a finite-energy peak originating from intraexcitonic transitions, whose assignment is corroborated by our numerical simulations of an extended Hubbard model. The lifetime of the peak is short (approximately one picosecond) and scales exponentially with the Mott gap size, implying extremely strong coupling to magnon modes.

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Fig. 1: Electrodynamic properties of Sr2IrO4.
Fig. 2: Photo-doping-induced optical conductivity transients of Sr2IrO4.
Fig. 3: Temperature dependence of HE decay.
Fig. 4: HE spectrum and characteristics obtained from effective model numerics.

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Source data are provided with this paper. All other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

We thank L. Balents, M. Ye, R. D. Averitt, P. A. Lee, M. Mitrano, M. Buzzi, S. K. Cushing, P. Prelovšek, D. Golež and V. Galitski for useful discussions. We thank S. J. Moon for sharing his equilibrium optical conductivity results. This work is supported by ARO MURI grant no. W911NF-16-1-0361. D.H. acknowledges support for instrumentation from the David and Lucile Packard Foundation and from the Institute for Quantum Information and Matter (IQIM), an NSF Physics Frontiers Center (PHY-1733907). S.D.W. acknowledges partial support via NSF award DMR-1729489. X.L. acknowledges support from the Caltech Postdoctoral Prize Fellowship and IQIM. M.B. acknowledges support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy Cluster of Excellence Matter and Light for Quantum Computing (ML4Q) EXC 2004/1-390534769. Z.L. was funded by the Gordon and Betty Moore Foundation’s EPiQS initiative, grant no. GBMF4545, and J1-2463 project and P1-0044 program of the Slovenian Research Agency.

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Author notes

  1. These authors contributed equally: Omar Mehio, Xinwei Li, Honglie Ning.

Authors and Affiliations

  1. Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, CA, USA

    Omar Mehio, Xinwei Li, Honglie Ning, Yuchen Han, Nicholas J. Laurita & David Hsieh

  2. Department of Physics, California Institute of Technology, Pasadena, CA, USA

    Omar Mehio, Xinwei Li, Honglie Ning, Yuchen Han, Nicholas J. Laurita & David Hsieh

  3. Department for Theoretical Physics, Jozef Stefan Institute, Ljubljana, Slovenia

    Zala Lenarčič

  4. Department of Physics, University of California, Berkeley, CA, USA

    Zala Lenarčič

  5. Institute for Theoretical Physics, University of Cologne, Cologne, Germany

    Michael Buchhold

  6. Materials Department, University of California, Santa Barbara, CA, USA

    Zach Porter & Stephen D. Wilson

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Contributions

O.M. and D.H. conceived the experiment. O.M. constructed the time-resolved time-domain THz spectrometer with contributions from X.L. and N.J.L. O.M., X.L., H.N. and Y.H. collected and analysed the data. Z.L. performed the exact diagonalization calculations and interpreted the results with O.M. and M.B. Z.P. and S.W. synthesized and characterized the sample. O.M. and D.H. wrote the paper with input from all authors.

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Correspondence to David Hsieh.

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Extended data

Extended Data Fig. 1 Drude-Lorentz fitting of a typical tr-TDTS spectrum of Sr2IrO4.

a,b Individual components of the Drude-Lorentz fitting of Δσ1(ω) (a) and Δσ2(ω) (b) plotted with the original data at t = 1.5 ps. The pump energy was fixed at 0.6 eV, resonant with the α transition, and set to a fluence of 2 mJ/cm2. The temperature of the sample was 80 K.

Source data

Extended Data Fig. 2 Fluence independence of THz frequency decay dynamics.

a ΔE(tEOS, t) traces taken with tEOS fixed to the time where E(tEOS) is maximal (Methods) plotted as a function of the fluence of the α-resonant (0.6 eV) photo-excitation. Data was collected at a sample temperature of 80 K. b Decay constants extracted from an exponential fitting (Methods) of the traces in panel a. The solid line is a guide to the eye. Error bars are the standard deviation from the least-squares-fitting algorithm.

Source data

Extended Data Fig. 3 Temperature dependence of differential infrared reflectivity.

ΔR/R traces taken on Sr2IrO4 as a function of temperature. The probe energy was fixed at 1.55 eV. The pump energy was fixed at 0.6 eV, resonant with α, and a fluence near 2 mJ/cm2 was used. The black dashed lines are fits to a double exponential function (Methods).

Source data

Extended Data Fig. 4 Simulated dispersion of the HE states.

Eigenenergies of HtJV in the sector of a single HD pair projected onto the kx axis throughout the entire reciprocal space of the 26 site lattice. At zero center-of-mass momentum, \(\overrightarrow{k}=[0,0]\), the four lowest states are colored to indicate which excitonic states are depicted in Fig. 4 of the main text. We define 0 eV to be the energy of the lowest-energy state.

Source data

Supplementary information

Supplementary Information

Supplementary Sections I–V and Figs. 1–10.

Source data

Source Data Fig. 1

Experimental data.

Source Data Fig. 2

Experimental data.

Source Data Fig. 3

Experimental data.

Source Data Fig. 4

Theoretical simulation results.

Source Data Extended Data Fig. 1

Experimental data.

Source Data Extended Data Fig. 2

Experimental data.

Source Data Extended Data Fig. 3

Experimental data.

Source Data Extended Data Fig. 4

Theoretical simulation results.

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Mehio, O., Li, X., Ning, H. et al. A Hubbard exciton fluid in a photo-doped antiferromagnetic Mott insulator. Nat. Phys. 19, 1876–1882 (2023). https://doi.org/10.1038/s41567-023-02204-2

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