Phase transitions
Can a laser flash change a metal's Fermi surface topology?
Open access · cc by · source: Europe PMC
A femtosecond infrared pulse briefly pushes an empty electron pocket in MoTe2 below the Fermi level, changing its Fermi surface topology for under a picosecond.
Study at a glance
- Design
- Other — Pump-probe time-resolved multidimensional photoemission on a cooled crystal, combined with time-dependent DFT+U simulations
- N
- No sample N; bulk Td-MoTe2 crystals cleaved and measured at 30 K across pump-probe delays
- Population
- Bulk crystals of the type-II Weyl semimetal Td-MoTe2
- Outcome
- Time evolution of the Fermi surface and energy position of the γ electron pocket after infrared excitation
Structured fields used in claim comparison tables when every cited study has a complete layer.
Key findings
After the pump, the γ pockets appeared on the Fermi surface, the signature of a Lifshitz transition, and vanished within about a picosecond. The pocket bottom shifted down by about 70 meV, crossing the equilibrium Fermi level by 17 ± 7 meV, and the shift lagged the pump, ruling out Floquet states. The pump fluence was below the structural transition threshold and heated the lattice only to about 71 K, far below 250 K. Simulations reproduced the transition only when both photoexcited populations and a drop in Hubbard U were included, and the needed drop in U was much smaller than equilibrium calculations would require.
Methodology
The researchers pumped Td-MoTe2 crystals at 30 K with 1030 nm infrared pulses and probed them with 21.7 eV extreme-ultraviolet pulses, recording photoelectrons in energy, both in-plane momenta and time with a momentum microscope. They tracked the electron pockets near the Y point and tested four explanations: simple filling of empty states, light-dressed Floquet states, a structural phase change, or reduced electron-electron repulsion (Hubbard U). Time-dependent DFT+U simulations with a frozen lattice modelled the electronic response.
Limitations
The simulations assume a frozen lattice and omit electron-phonon coupling, so they cannot reproduce the recovery and are only qualitative. Time resolution was limited to about 140 fs by the long pump pulse. The structural-transition scenario is excluded partly by comparison with a separate optical study and a heating estimate rather than a direct structural probe in this experiment. Only one material was studied, so generality to other correlated materials is argued, not shown.
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
Light can trigger a transient Lifshitz transition without heating the crystal through a structural change.
Time-resolved photoemission showed new electron pockets appearing on the Fermi surface after a light pulse and vanishing within about a picosecond, with the pocket bottom crossing the Fermi level by 17 ± 7 meV, while the lattice stayed far below its structural transition temperature.
Evidence for the claim as stated.
Light can trigger a transient Lifshitz transition without heating the crystal through a structural change.
Time-resolved photoemission showed new electron pockets appearing on the Fermi surface after a light pulse and vanishing within about a picosecond, with the pocket bottom crossing the Fermi level by 17 ± 7 meV, while the lattice stayed far below its structural transition temperature.
Scope note — One material; simulations are frozen-lattice and qualitative.
Limits the claim's scope: a different population, assay, or outcome.
The two Lifshitz studies differ in drive (steady magnetic field at millikelvin vs femtosecond light pulse), material and probe (surface STM vs photoemission), so they illustrate the same kind of transition but cannot be directly compared quantitatively.
Evidence for the claim as stated.
Open questions
Tensions this paper is part of
From concept pages' “where studies disagree.” Disagreement means the same question; scope means different assays, populations, or outcomes.
The two Lifshitz studies differ in drive (steady magnetic field at millikelvin vs femtosecond light pulse), material and probe (surface STM vs photoemission), so they illustrate the same kind of transition but cannot be directly compared quantitatively.
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