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# Optical manipulation of electronic dimensionality in a quantum material

## Abstract

Exotic phenomena can be achieved in quantum materials by confining electronic states into two dimensions. For example, relativistic fermions are realized in a single layer of carbon atoms1, the quantized Hall effect can result from two-dimensional (2D) systems2,3, and the superconducting transition temperature can be considerably increased in a one-atomic-layer material4,5. Ordinarily, a 2D electronic system can be obtained by exfoliating the layered materials, growing monolayer materials on substrates, or establishing interfaces between different materials. Here we use femtosecond infrared laser pulses to invert the periodic lattice distortion sectionally in a three-dimensional (3D) charge density wave material (1T-TiSe2), creating macroscopic domain walls of transient 2D ordered electronic states with unusual properties. The corresponding ultrafast electronic and lattice dynamics are captured by time-resolved and angle-resolved photoemission spectroscopy6 and ultrafast electron diffraction at energies of the order of megaelectronvolts7. Moreover, in the photoinduced 2D domain wall near the surface we identify a phase with enhanced density of states and signatures of potential opening of an energy gap near the Fermi energy. Such optical modulation of atomic motion is an alternative path towards realizing 2D electronic states and will be a useful platform upon which novel phases in quantum materials may be discovered.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Correspondence and requests for materials regarding trARPES experimental data and the simulation should be addressed to W.Z. and regarding UED experimental data should be addressed to D.X.

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## Acknowledgements

W.Z. acknowledges support from the Ministry of Science and Technology of China (2016YFA0300501) and the National Natural Science Foundation of China (11974243), and additional support from a Shanghai talent programme. Y.G. acknowledges support from the National Natural Science Foundation of China (grant no. 11874264). D.Q. acknowledges support from the Ministry of Science and Technology of China (grant no. 2016YFA0301003). D.Q. and W.L. acknowledge support from the National Natural Science Foundation of China (grant nos 12074248 and 11521404). D.X. and J.Z. acknowledge support from the National Natural Science Foundation of China (grant nos 11925505, 11504232 and 11721091) and from the office of Science and Technology, Shanghai Municipal Government (nos 16DZ2260200 and 18JC1410700). First-principles computations were performed at the Center for High Performance Computing of Shanghai Jiao Tong University.

## Author information

Authors

### Contributions

W.Z. and D.X. proposed and designed the research. S.D., Y.Y., C.H., T.T. and W.Z. contributed to the development and maintenance of the trARPES system. S.D., Y.Y. and C.H. collected the trARPES data. Y.C. and F.Q. took the UED measurements. W.X. and Y.G. prepared the single-crystal sample. S.D., Y.C., D.X. and W.Z. performed the phenomenological simulation. C.X. and W.L. performed the DFT calculation. W.Z. wrote the paper with S.D., Y.C., W.L., D.Q., D.X. and J.Z. All authors discussed the results and commented on the manuscript.

### Corresponding authors

Correspondence to Dao Xiang or Wentao Zhang.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Claude Monney and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Simulated and experimental electron diffraction patterns in TiSe2.

a, b, Simulated and experimental electron diffraction patterns based on the kinetical theory with the sample thickness of 60 nm. c, Experimental diffraction patterns at 12 ps with a tilted angle. Data were taken with the same pump fluence and equilibrium temperature as in Fig. 1d. The intensity units on the colour scale are arbitrary.

### Extended Data Fig. 2 Ultrafast electronic dynamics.

a, Time-dependent photoemission spectroscopy intensity at different pump fluences. The intensity is the integration of non-equilibrium electrons between 0 and 0.03 eV above the Fermi energy. b, The decay rates of nonequilibrium electrons as a function of pump fluence. The three different regions separated by the two critical pump fluences are indicated by different colours in the background.

### Extended Data Fig. 3 Additional measurements on two other samples (samples II and III).

a, Spectral intensity as a function of the pump fluence integrated from −0.1 eV (Se 4px,y band top) to the Fermi level for samples II and III at a delay time of 12 ps. b, Photoemission spectra difference between fluences I′ and I (I(I) − I(I′)) from 3 to 40 ps for sample II. The colour scale is in arbitrary units, and ‘–’ (green) indicates <0 and ‘+’ (red) indicates >0. c, EDCs at the momentum of the dashed line shown in the inset of Fig. 4b for pump fluences at I′ and I for samples II and III. EDCs are normalized to the same height. d, Spectral intensity as a function of the pump fluence integrated from −0.1 eV (Se 4px,y band top) to the Fermi level for sample II. e, Original EDCs without normalization between 3 and 40 ps for sample II. f, Normalized EDCs from e.

### Extended Data Fig. 4 DFT calculations and experimental electronic structures of the bulk and 2D domain wall.

a, Charge density isosurface plot of the domain-wall bands at the Γ point. The Ti and Se atoms are shown in blue and green spheres, and the outer and inner surfaces of the density state isosurface on the domain wall are shown in purple and blue. b, Band structure of the periodic eight-layer supercell with domain wall. The px and py orbitals are projected to the three layers at the domain wall (shown in red circles); the symbol size denotes the relative weight of the orbitals. The reference bands are drawn in orange lines. c, Energy shifts of the domain-wall bands at different strengths of CDW displacements. The label ‘CDW’ means a single CDW phase, and ‘domain’ means the presence of sharp domain wall. d, Time-resolved photoemission spectra at 12 ps for pump fluences at I′ and I, as indicated in the inset to e. e, EDCs at the momentum of −0.13 Å−1 for pump fluence I′ and I, as indicated by the solid line cuts (the same colour as the corresponding EDC) in d. Inset shows the same spectra at 12 ps for sample II as that shown in Extended Data Fig. 3a. EDCs are normalized to the same height.

## Supplementary information

### Supplementary Information

This Supplementary Information file contains sections (I) Numerical solution of the motion equation; and (II) Pump probe fluence resolution; including Supplementary figure 1 and 2.

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Duan, S., Cheng, Y., Xia, W. et al. Optical manipulation of electronic dimensionality in a quantum material. Nature 595, 239–244 (2021). https://doi.org/10.1038/s41586-021-03643-8

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