Superconductor’s Unusual Electronic Structure Revealed by Laser Measurements at 133 Kelvin

Researchers have investigated the electronic structure and superconducting properties of the complex material HgBa₂Ca₂Cu₃O₈₊δ, offering new insights into high-temperature superconductivity. Taimin Miao from the Beijing National Laboratory for Condensed Matter Physics, alongside Wenshan Hong and Qinghong Wang, led a study employing laser-based angle-resolved photoemission spectroscopy to map the material’s electronic behaviour at 133 K. Their work reveals the presence of distinct regions on the material’s surface exhibiting single and double Fermi surfaces, and demonstrates a superconducting gap whose momentum dependence diverges from conventional d-wave symmetry. These findings suggest a closer resemblance between HgBa₂Ca₂Cu₃O₈₊δ and underdoped cuprates, potentially reshaping our understanding of charge carrier behaviour and superconductivity within this family of materials.

Spatially resolved electronic structure reveals distinct Fermi surface regions in Hg1223 superconductors

Scientists have achieved spatially-resolved measurements of the electronic structure within the HgBa₂Ca₂Cu₃O₈₊δ (Hg1223) superconductor, revealing distinct regions on its cleaved surface. Performing laser-based angle-resolved photoemission spectroscopy (ARPES) on optimally-doped Hg1223 with a critical temperature of 133 K, researchers identified a single Fermi surface region and a double Fermi surface region originating from the inner and outer CuO₂ planes.
These two regions exhibit differing electronic characteristics, providing new insight into the behaviour of this high-temperature superconductor. The study meticulously maps the momentum dependence of the superconducting gap and band structures within each region, demonstrating that electronic states are primarily concentrated near the nodal region of the Brillouin zone.

This work overcomes significant challenges in preparing a clean surface for ARPES analysis of Hg1223, a material known for its high superconducting transition temperature but lacking a natural cleavage plane. By employing spatially-resolved ARPES, the research team successfully distinguished two surface terminations arising from cleavage between the HgOδ-BaO layers.

Detailed analysis of the Fermi surface topology in the single Fermi surface region confirms the presence of a well-defined band near the nodal region, while the double Fermi surface region exhibits two distinct sheets corresponding to the inner and outer CuO₂ planes. The observed band dispersion and superconducting gap opening are consistent with a superconducting state.

Furthermore, the momentum dependence of the superconducting gap deviates from the standard d-wave form, suggesting that the surface electronic structure of Hg1223 more closely resembles that of underdoped cuprates. This finding is crucial for understanding the complex interplay between doping, electronic structure, and superconductivity in this material.

The ability to resolve these subtle differences in electronic behaviour opens avenues for tailoring the properties of Hg1223 and potentially other high-temperature superconductors. These results provide a foundation for future investigations into the mechanisms driving high-temperature superconductivity and could contribute to the development of advanced materials with enhanced superconducting properties.

Optimised Hg1223 single crystal growth and photoemission spectroscopy setup

A bias laser-ARPES system equipped with a 6.994 eV vacuum-ultra-violet laser and a DA30L hemispherical electron energy analyzer was central to the investigation of HgBa₂Ca₂Cu₃O₈₊δ (Hg1223). High-quality single crystals were grown via the self-flux method and subsequently annealed at 530°C under an oxygen pressure of approximately 12 atmospheres for seven days, resulting in optimally doped samples exhibiting a superconducting transition temperature (T onset c) of 133 K with a narrow transition width of around 2.5 K.

ARPES measurements utilized a copper sample holder electrically connected to a Keithley 2450 source meter, enabling controlled sample biasing to expand the accessible momentum space. The laser beam was focused to a spot size of approximately 10μm on the sample surface to facilitate high spatial resolution.

Overall energy resolution reached ∼8.5 meV with a 10 eV pass energy and a 0.3mm slit at -90V sample bias, and improved to ∼4.5 meV using a 5 eV pass energy and a 0.3mm slit at -30V sample bias. Angular resolution was better than 0.2° with zero sample bias, ensuring precise momentum mapping. Spatially resolved ARPES measurements involved scanning the sample position to acquire photocurrent maps or band structure maps, revealing variations across the cleaved surface.

Samples were cleaved in situ at low temperature and measured under ultrahigh vacuum conditions, maintaining a base pressure below 2 × 10⁻¹¹ mbar. The Fermi level was carefully referenced using a clean polycrystalline gold standard. Investigations focused on identifying two distinct regions on the cleaved surface: a single Fermi surface region and a double Fermi surface region originating from the inner and outer CuO₂ planes, allowing for comparative analysis of electronic structure and superconducting gap behaviour in each area. This spatially-resolved approach proved crucial for understanding the complex electronic properties of Hg1223 and its relationship to other cuprate superconductors.

Fermi surface topology and superconducting gap anisotropy in optimally-doped Hg1223

Spatially-resolved laser-based angle resolved photoemission spectroscopy measurements were performed on optimally-doped HgBa₂Ca₂Cu₃O₈₊δ, designated Hg1223, at 133 K. Two distinct regions were identified on the cleaved surface: a single Fermi surface region and a double Fermi surface region originating from both the inner and outer CuO₂ planes.

Electronic structure and superconducting gap measurements were obtained from both regions, revealing that observed electronic states are primarily concentrated near the nodal region. The momentum dependence of the superconducting gap deviates from the standard d-wave form, suggesting the surface electronic structure of Hg1223 more closely resembles that of underdoped cuprates.

High-quality Hg1223 single crystals were grown via the self-flux method and post-annealed at 530°C under an oxygen pressure of approximately 12 atmospheres for seven days. This process yielded optimally doped samples exhibiting a superconducting transition temperature of 133 K with a narrow transition width of 2.5 K.

ARPES measurements utilized a 6.994 eV vacuum-ultra-violet laser and a DA30L hemispherical electron energy analyzer, achieving an overall energy resolution of 8.5 meV with a 10 eV pass energy and 0.3mm slit, or 4.5 meV with a 5 eV pass energy and 0.3mm slit. Angular resolution was better than 0.2° with zero sample bias.

Spatially resolved ARPES measurements, performed by scanning the sample surface, revealed spatial inhomogeneity in the photocurrent map. The majority of the cleaved surface corresponded to Region 1, exhibiting a single Fermi surface, while a smaller fraction comprised Region 2, displaying two resolved Fermi surface sheets.

Detailed Fermi surface mapping and band structure analysis in Region 1, with the single Fermi surface, showed a well-defined band near the nodal region. As momentum cuts moved towards the antinodal region, the lower binding energy portion of the band became less distinct, while the high binding energy portion remained observable, indicating the opening of the superconducting gap.

Surface electronic reconstruction in optimally-doped mercury barium calcium copper oxide

Spatially-resolved angle-resolved photoemission spectroscopy measurements performed on optimally-doped mercury barium calcium copper oxide reveal distinct electronic properties across the material’s cleaved surface. Two regions were identified: one exhibiting a single Fermi surface and another displaying two Fermi surface sheets originating from both the inner and outer copper-oxide planes.

Detailed analysis of both regions demonstrates that electronic states are concentrated near the nodal region of the superconducting gap. The observed momentum dependence of the superconducting gap deviates from the expected d-wave form, suggesting a surface electronic structure more characteristic of underdoped cuprates despite the bulk material being optimally doped.

This unexpected behaviour may be influenced by the polar nature of the cleavage surface, variations in local termination, or minor oxygen loss during sample preparation, all of which can alter the near-surface doping environment. The authors acknowledge the need for further measurements conducted under more rigorously controlled surface conditions to fully elucidate these effects and to investigate the doping evolution of the electronic structure and superconducting gap within this material system. Future research should focus on controlling the doping level to better understand the observed discrepancies and refine the understanding of high-temperature superconductivity in mercury barium calcium copper oxide.