Laser-bias ARPES Expands Momentum Space and Enables Full 2 Solid Angle Collection with 6.994 eV Source

Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique for understanding the behaviour of electrons in materials, but conventional instruments typically capture only a limited view of their momentum. Taimin Miao, Yu Xu, Bo Liang, and colleagues at institutions including Wenpei Zhu and Neng Cai, now demonstrate a significant advance in laser-based ARPES by applying a sample bias during measurement. This innovative approach expands the accessible range of electron momentum, enabling complete two-dimensional mapping of the electronic structure, something previously unattainable with this high-resolution technique. By establishing a precise conversion between detector angle and electron momentum, and developing a method for accurate work function determination, the team achieves full solid-angle collection while preserving the exceptional energy and angular resolutions of laser ARPES, opening new avenues for materials research.

Expanded Momentum Coverage in Photoemission Spectroscopy

Scientists have significantly enhanced angle-resolved photoemission spectroscopy (ARPES) by expanding the accessible momentum space and enabling full 2π solid angle photoelectron collection. This advancement directly improves the ability to investigate the electronic structure of materials and probe the behaviour of electrons within them. The research team developed a novel ARPES system incorporating a sample bias voltage, allowing precise manipulation of photoelectron energies and, consequently, a wider mapping of momentum space. Applying a negative sample bias effectively increases the sample’s work function, shifting the photoelectron energy distribution and extending the range of accessible momentum, particularly in specific directions.

This approach overcomes limitations imposed by conventional energy windows and geometrical constraints within ARPES analysis. Full 2π solid angle collection, achieved through a carefully optimised hemispherical analyser, further enhances data acquisition capabilities, allowing for comprehensive mapping of electronic dispersion relations across the entire Brillouin zone and providing a more complete understanding of a material’s electronic properties. The combination of sample biasing and full 2π collection significantly improves signal-to-noise ratio and reduces data acquisition time, enabling efficient and high-resolution measurements. Researchers successfully demonstrated these enhanced capabilities by performing detailed ARPES measurements on benchmark materials, including high-temperature superconductors and topological insulators, revealing previously unobserved features in their electronic band structure and providing valuable insights into their underlying physics.

Expanded Momentum Coverage Via Sample Biasing

Scientists developed a novel bias ARPES technique to overcome limitations in conventional angle-resolved photoemission spectroscopy, which typically captures only a small fraction of the full two-dimensional emission angle. This work elevates laser ARPES by expanding momentum coverage while preserving high resolution, achieved through systematic modification of an existing cryostat and a VUV laser-based ARPES system. The team engineered a method to apply a bias voltage to the sample using a thin sapphire piece inserted between the cryostat’s cold head and the sample holder, serving as an electrical insulator while maintaining efficient thermal conduction and enabling sample temperatures as low as 11. 7 K.

This innovative approach utilises the angle deflection mode of the electron energy analyser to achieve full two-dimensional collection of photoelectrons across a 2π solid angle. Researchers carefully examined the conversion relations between detector angle, emission angle, and electron momentum, establishing that a parallel-plate capacitor model accurately describes the electric field around the sample. The team developed a precise method for determining the sample work function, a critical parameter in accurately converting angles to electron momentum. They demonstrated that applying a bias voltage, while causing minor degradation of energy resolution, still maintains high resolution, better than 5 meV, even with high voltages. Maintaining a small beam size proved crucial for achieving sharp features and high instrumental resolution, and the technique remained effective even when the sample was tilted away from the normal emission direction. By precisely mapping the measured detector angle to the photoelectron emission angle and electron momentum, the team established a robust methodology for determining electron energy and momentum within materials, advancing the capabilities of ARPES for materials science research.

Full Momentum Mapping Via Sample Bias Voltage

This work demonstrates a significant advancement in angle-resolved photoemission spectroscopy (ARPES) through the successful implementation of a bias technique. By applying a voltage to the sample, researchers greatly expanded the accessible momentum space, achieving full two-dimensional collection of photoelectrons, a capability previously limited in laser ARPES measurements. This expansion was achieved with minimal modification to an existing laser ARPES system, inserting only an insulating sapphire piece for electrical isolation. The team carefully established and validated conversion relations to accurately map detector angles to electron momentum in two dimensions, developing a precise method for determining the sample work function, which is critical for accurate angle-momentum conversion.

Investigations revealed a minor degradation in energy resolution with applied bias, but high resolution, better than 5 meV, was maintained even at high voltages. Researchers acknowledged that the electrical field around the sample may deviate from an ideal model, potentially affecting the accuracy of the conversion relations, particularly at high bias voltages, and identified a correction term to account for this deviation, which can be determined from the photoelectron cone boundary. This advancement enables further ARPES studies of complex materials, including investigations into charge density waves, superconductivity, and many-body interactions.