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# Electron–phonon-driven three-dimensional metallicity in an insulating cuprate

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### Significance

Elucidating the role of different degrees of freedom in a phase transition is crucial in the comprehension of complex materials. A phase transformation that attracts significant interest is the insulator-to-metal transition of Mott insulators, in which the electrons are thought to play the dominant role. Here, we use ultrafast laser spectroscopy and theoretical calculations to unveil that the correlated insulator $L a 2 C u O 4$ , precursor to high-temperature superconductivity, is unstable toward metallization when its crystal structure is displaced along the coordinates of specific vibrational modes. This, in turn, supports the involvement of the lattice in this phase transition. Our results pave the way toward the geometrical design of metallic states in Mott insulators, with technological potential for ultrafast switching devices at room temperature.

### Abstract

The role of the crystal lattice for the electronic properties of cuprates and other high-temperature superconductors remains controversial despite decades of theoretical and experimental efforts. While the paradigm of strong electronic correlations suggests a purely electronic mechanism behind the insulator-to-metal transition, recently the mutual enhancement of the electron–electron and the electron–phonon interaction and its relevance to the formation of the ordered phases have also been emphasized. Here, we combine polarization-resolved ultrafast optical spectroscopy and state-of-the-art dynamical mean-field theory to show the importance of the crystal lattice in the breakdown of the correlated insulating state in an archetypal undoped cuprate. We identify signatures of electron–phonon coupling to specific fully symmetric optical modes during the buildup of a three-dimensional (3D) metallic state that follows charge photodoping. Calculations for coherently displaced crystal structures along the relevant phonon coordinates indicate that the insulating state is remarkably unstable toward metallization despite the seemingly large charge-transfer energy scale. This hitherto unobserved insulator-to-metal transition mediated by fully symmetric lattice modes can find extensive application in a plethora of correlated solids.

### Most cited references84

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### Author and article information

###### Journal
Proc Natl Acad Sci U S A
pnas
pnas
PNAS
Proceedings of the National Academy of Sciences of the United States of America
0027-8424
1091-6490
24 March 2020
11 March 2020
11 March 2020
: 117
: 12
: 6409-6416
###### Affiliations
aInstitute of Physics, Laboratory for Ultrafast Microscopy and Electron Scattering, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;
bInstitute of Chemical Sciences and Engineering, Laboratory of Ultrafast Spectroscopy, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;
cMax Planck Institute for the Structure and Dynamics of Matter, D-22761 Hamburg, Germany;
dDepartment of Physics, King’s College London, London WC2R 2LS, United Kingdom;
eWilhelm Ostwald Institut of Physical and Theoretical Chemistry, University of Leipzig, D-04103 Leipzig, Germany;
fDepartment of Physics, University of Fribourg, CH-1700 Fribourg, Switzerland;
gSolid State Chemistry Group, Laboratory for Multiscale Materials Experiments, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland;
hNano-Bio Spectroscopy Group, Departamento de Física de Materiales, Universidad del País Vasco, 20018 San Sebastían, Spain;
iCenter for Computational Quantum Physics, The Flatiron Institute, New York, NY 10010
###### Author notes
1To whom correspondence may be addressed. Email: angel.rubio@ 123456mpsd.mpg.de , cedric.weber@ 123456kcl.ac.uk , or ebaldini@ 123456mit.edu .

Contributed by Angel Rubio, February 11, 2020 (sent for review November 8, 2019; reviewed by Riccardo Comin and Zhi-Xun Shen)

Author contributions: E.B., M.A.S., A.R., and C.W. designed research; E.B., S.A., T.B., E.S., F.L., E.P., M.v.S., A.R., and C.W. performed research; E.B. and F.C. analyzed data; and E.B., M.A.S., C.B., A.R., and C.W. wrote the paper.

Reviewers: R.C., Massachusetts Institute of Technology; and Z.-X.S., Stanford University.

###### Article
201919451
10.1073/pnas.1919451117
7104249
32161128

###### Counts
Pages: 8
###### Categories
Physical Sciences
Physics