Upt Superconductor Visualizes Half Quantum Vortices and Vortex Sheets Carrying H/2e Flux

Superconductivity, the phenomenon of zero electrical resistance and magnetic field expulsion, typically involves discrete magnetic flux tubes known as vortices penetrating the material, each carrying a single quantum of flux. However, the unconventional superconductor UPt exhibits strikingly different behaviour, as researchers led by P. García-Campos, V. O. Dolocan, and A. D. Huxley, alongside colleagues D. Aoki and K. Hasselbach, have now demonstrated. Using a highly sensitive scanning technique, the team visualised mobile half-vortices, alongside full vortices, and discovered lines of magnetic contrast resembling domain walls when cooling the material under a higher magnetic field. These observations provide compelling evidence supporting theoretical predictions for chiral superconductivity, revealing a complex order-parameter with sheets of half-vortices separating regions of opposite chirality, and fundamentally advancing our understanding of unconventional superconductivity.

UPt3 Superconductivity, Vortex Imaging and Domain Formation

Researchers employed a Scanning SQUID microscope to image magnetic fields in UPt3, an unconventional superconductor, seeking evidence of vortices, domain walls, and potential chiral currents. Observations reveal a complex interplay between vortices and magnetic domains, with circular magnetic structures suggesting domain wall formation, and vortex density influenced by the applied magnetic field. Different field-cooling fields lead to varying domain configurations, demonstrating sensitivity to the sample’s magnetic history. Specific heat measurements confirm a double transition around 0. 535 K, consistent with UPt3’s superconducting state and potentially other ordered phases.

While clear evidence of chiral currents remains elusive, data can be modeled by vortex stray fields, and models including chiral currents do not significantly improve the fit. Analysis addresses the effect of the SQUID’s distance from the sample surface, which can distort observed profiles, demonstrating a reduction in measured peak field when accounting for this distance. The work provides valuable insights into the complex magnetic behavior of this unconventional superconductor and highlights the importance of careful data analysis, even though conclusive evidence of chiral currents remains elusive.

Scanning SQUID Microscopy Reveals Chiral Superconductivity

This study pioneered a scanning SQUID microscopy technique to investigate UPt3, revealing mobile half-vortices and vortex sheets, key indicators of chiral superconductivity. A custom-built scanning force microscope with a quartz tuning fork force detector scanned the UPt3 sample with nanometer precision, enabling highly sensitive detection of magnetic fields at ultra-low temperatures. A high-quality UPt3 crystal was grown using the Czochralski method, achieving a residual resistivity ratio of 580, and optimized through annealing in ultra-high vacuum. Specific heat measurements confirmed a superconducting transition temperature of 0.

514 K with a narrow transition width, demonstrating the crystal’s high quality. The technique involved applying a magnetic field while scanning the SQUID probe, requiring 15 to 30 minutes per image. Researchers employed a monopole model to describe vortex stray fields, allowing for accurate determination of magnetic penetration depth and the distance between the SQUID probe and the sample surface. The model’s validity was demonstrated by characterizing a single vortex, confirming its cylindrical symmetry and achieving a precise fit with established parameters.

Half-Quantum Vortices Observed in UPt3 Superconductor

Scientists directly observed half-quantum vortices and vortex sheets within the B-phase of UPt3 using a scanning SQUID microscope. Following field cooling, the team consistently observed magnetic flux structures corresponding to vortices, with detailed analysis demonstrating a temperature dependence consistent with established parameters between 0. 3 and 0. 5 K. Beyond integer quantum vortices, the research team detected weaker signals from vortices carrying half the maximum field amplitude, confirming the existence of 0.

5Φ0 vortices. Line profiles through both 1Φ0 and 0. 5Φ0 vortices showed isotropic magnetic field distributions with nearly identical parameters for penetration depth and SQUID sample distance. Frequency histograms of field maxima values captured during consecutive cool downs showed distinct peaks at 38 μT (1Φ0 vortices) and 19 μT (half-quantum vortices) in the B-phase. Further experiments revealed dynamic vortex splitting, with initial vortices splitting into half-quantum vortices at higher temperatures, which then disappeared, demonstrating the instability of these structures with increasing temperature. These observations provide strong evidence for time-reversal symmetry breaking in the B-phase of UPt3.

Chiral Superconductivity Visualized Via Vortex Imaging

This research demonstrates the observation of half-quantum vortices and vortex sheets within UPt3, providing new insights into its chiral superconducting state. Using a scanning SQUID microscope, scientists directly imaged these features following controlled cooling in a magnetic field, revealing the formation of distinct superconducting domains separated by these vortex sheets. The observed arrangement supports theoretical predictions for a complex order-parameter with differing chiralities within these domains, offering a compelling explanation for the weak Meissner effect previously noted.