Optical manipulation of electronic dimensionality in a quantum material

Exotic phenomenon can be achieved in quantum materials by confining electronic states into two dimensions. For example, quantized Hall effect can be resulted in a unit cell of a periodic 2D system (Nobel prize in 1988), relativistic fermions are realized in a single layer of carbon atoms arranged in a two-dimensional (2D) honeycomb lattice while such electronic state is absent in the bulk graphite (Nobel prize in 2010), superconducting transition temperature can be enhanced by confining materials into 2D, and so on. Ordinarily, the 2D electronic system can be artificially created by exfoliating the layered materials, growing on substrates via molecular beam epitaxy, or building interfaces between two different materials. Searching for new methods to confine electronic states into 2D is important in condensed matter physics.

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In our study, we proposed a new way to generate 2D electronic states by optical manipulation. The method is based on the physics that ultrafast laser excitation can induce macroscopic periodic lattice distortion sectionally in material, which is a result of photoexcited coherent electron-phonon coupling, and long-range-ordered 2D electronic layers are established between the lattice distorted layers and original lattice layers.

We experimentally demonstrate that the electronic dimensionality in a three-dimensional (3D) charge-density-wave material can be manipulated with a laser pulse, and signature of light-induced superconductivity emerges when the 3D electronic structure turns into 2D. Specifically, we use infrared ultrashort laser pulse to pump the sample, and monitor the electronic structure and lattice dynamics by high resolution time- and angle-resolved photoemission (trARPES) and MeV ultrafast electron diffraction (UED) respectively. With improved time, energy, and pump fluence resolution, trARPES experiments evidence 2D electronic states on the surface due to the ultrafast phase inversion induced macroscopic domain wall, which is confirmed by the temporal lattice distortion from high-resolution UED experiments and consistent with the phenomenological theory based on a spatially- and temporally-dependent double wall Ginzburg-Landau potential. Interestingly, a gaped electronic state, which is possibly the signature of superconductivity, is discovered in the 2D electronic structure on the macroscopic domain wall.

The optical induced macroscopic domain wall exhibited the behavior of a 2D electronic system, and thus, it is a platform for realizing novel phases, for example, phases with superconductivity. Our work presents a novel approach for manipulating quantum materials using ultrafast laser pumping, and open a new window in ultrafast science with profound implications for next-generation devices with new functionalities. However, further studies are necessary to clarify the precise mechanism for producing such macroscopic domain walls, to determine if such methods are universal and applicable for other CDW materials or even other ordered solids, and most interestingly, to identify if the observed energy gap was a result of photoinduced superconductivity.


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