Unusual Superconductivity Spotted in Uranium Compound Could Unlock New Technologies

Scientists investigate the elusive nature of unconventional superconductivity in UTe2, a material thought to host an odd-parity pairing state. Shuqiu Wang, from H. H. Wills Physics Laboratory, University of Bristol, and Clarendon Laboratory, University of Oxford, alongside J. C. Séamus Davis of both the University of Oxford and University College Cork, present compelling evidence for a quasiparticle surface band arising from this unusual superconducting order parameter. Their work visualises this band using scanning tunnelling microscopy, demonstrating characteristics that differentiate between chiral and non-chiral superconducting states and confirming non-chiral superconductivity in UTe2. These findings are significant as they provide direct observation of the order parameter’s topology, offering crucial insights into the mechanisms driving superconductivity in this and potentially other novel materials.

Evidence for a Quasiparticle Surface Band in the Nodal Superconductor UTe2 suggests unconventional superconductivity

Researchers have identified a distinctive characteristic of nodal intrinsic superconductivity in the material UTe2, revealing the presence of a quasiparticle surface band (QSB). This band, appearing only below a critical temperature, signifies a crucial step towards realising topologically protected quantum states essential for advanced quantum devices.
The work visualised a band of Bogoliubov quasiparticles appearing as a characteristic sextet, specifically six interference wavevectors, confirming the QSB dispersions within the superconducting energy gap. Scanning tunneling microscopy, employing a superconducting scan-tip, was used to investigate the electronic structure of UTe2 at the (0-11) crystal termination.

This technique revealed an intense zero-energy Andreev conductance maximum, which subsequently developed into two finite-energy, particle-hole symmetric conductance maxima as the tunnel barrier was reduced. This transformation confirms that superconductivity within UTe2 is non-chiral, a critical aspect of its topological properties.

Quasiparticle interference imaging then focused on the in-gap quasiparticle patterns of the QSB. The research demonstrates that the interference patterns are dominated by the QSB for energies within the superconducting energy gap, allowing exclusive detection of the bulk characteristics of the intrinsic topological superconductivity.

Specifically, the study visualised a band of Bogoliubov quasiparticles appearing as a characteristic sextet of interference wavevectors. These six interference wavevectors demonstrate that QSB dispersions occur only for energies less than or equal to the maximum superconducting energy gap and within a specific range of Fermi momenta projected onto the (0-11) crystal surface.

In combination, these observations strongly support the existence of a bulk superconducting state in UTe2 exhibiting spin triplet, time-reversal conserving, odd-parity, a-axis nodal, B3u symmetry. This finding represents a significant advancement in understanding and harnessing the potential of topologically protected quantum states for future technological applications.

Visualisation of Andreev conductance and quasiparticle interference in UTe2 reveals unconventional superconductivity

Superconducting scan-tip scanning tunneling microscopy (STM) imaging was employed to investigate the quasiparticle surface band (QSB) in UTe2. This technique facilitated the visualisation of Andreev conductance at the (0-11) crystal termination, revealing an intense zero-energy conductance maximum. Development of this peak into two finite-energy, particle-hole symmetric conductance maxima upon reduction of the tunnel barrier signified non-chiral superconductivity.

The experimental setup involved a superconducting scan-tip positioned to measure differential conductance spectra, allowing precise control over the tunneling junction and observation of Andreev bound states. Quasiparticle interference (QPI) imaging was then performed using the same superconducting scan-tip to visualise the in-gap quasiparticle interference patterns of the QSB.

Fourier analysis of the measured data derived scattering interference patterns, a(q, V), which were used to map the energy evolution of QSB states. Specifically, the research visualised a band of Bogoliubov quasiparticles appearing as a characteristic sextet: six interference wavevectors, confirming the QSB dispersions within the superconducting energy gap.

These measurements were conducted at 280 mK in an identical field of view to previous topographic imaging. Energy-resolved QPI was obtained through superconductive-tip a(r, V) measurements at the (0-11) cleave surface, recording data at |V| = 0 μV, 50 μV, 100 μV, 150 μV, 200 μV, and 250 μV. The consequent scattering interference patterns, a(q, V), were derived by Fourier analysis, revealing the energy evolution of scattering interference from the QSB states.

Comparison with theoretical predictions of the density of states, N(q, E), for B2u-QSB and B3u-QSB at the (0-11) surface, confirmed the presence of a B3u-QSB, evidenced by enhanced arc-like scattering intensity and the unique appearance of wavevector q1. The observed QPI features were repeatable in multiple independent experiments, demonstrating the robustness of the findings.

Sextet formation and surface band dominance confirm non-chiral superconductivity in this material

Researchers visualised a band of Bogoliubov quasiparticles appearing as a characteristic sextet, specifically six interference wavevectors, confirming the quasiparticle surface band dispersions within the superconducting energy gap. This observation provides crucial insight into the nature of superconductivity within the material under investigation.

The sextet configuration of interference wavevectors was detected only for energies and within a specific range of Fermi momenta projected onto the crystal surface, further defining the parameters of this unique superconducting state. Development of a zero-energy Andreev conductance peak into two finite-energy particle-hole symmetric conductance maxima as the tunnel barrier is reduced signifies that superconductivity is non-chiral.

Quasiparticle interference imaging revealed that the in-gap quasiparticle interference patterns are dominated by the quasiparticle surface band for energies within the superconducting energy gap. This dominance allows for the exclusive detection of bulk characteristics of the intrinsic superconductivity.

The research demonstrates that the observed phenomena are consistent with a material exhibiting spin triplet, time-reversal conserving, odd-parity symmetry, with a nodal a-axis configuration. These characteristics are fundamental to understanding the topological properties of the superconducting state.

The visualisation of the band of Bogoliubov quasiparticles as a sextet, with six interference wavevectors, definitively confirms the quasiparticle surface band dispersions within the superconducting energy gap. This finding is a key identifier of nodal intrinsic superconductivity and supports the existence of a topological quasiparticle surface band.

Evidence for a spin-triplet superconducting state in UTe2 from quasiparticle surface band imaging and Andreev conductance spectroscopy supports unconventional superconductivity

The visualisation of a quasiparticle surface band (QSB) exhibiting a characteristic sextet of interference wavevectors confirms the presence of specific dispersions within the superconducting energy gap of UTe2. This observation provides crucial insight into the nature of superconductivity within this material, demonstrating a band of Bogoliubov quasiparticles appearing as six interference wavevectors.

These findings establish a clear path toward understanding the topological properties of the superconducting state in UTe2. Further analysis of Andreev conductance measurements, alongside quasiparticle interference imaging, supports a superconducting order parameter with B3u symmetry. The splitting of the zero-energy Andreev conductance peak upon reducing the tunnel barrier between niobium and UTe2, coupled with the unique in-gap quasiparticle interference patterns, strongly suggests a spin-triplet, time-reversal conserving, odd-parity superconducting state.

While the authors acknowledge that features at certain wavevectors are weakly observable in the normal state, the enhancement of these features in the superconducting state provides compelling evidence for the proposed symmetry. These results rule out several alternative order parameters, including those with chiral characteristics or isotropic symmetry, based on discrepancies with the observed experimental data. Future research may focus on exploring the behaviour of this material under varying conditions to further refine the understanding of its topological properties and potential applications in quantum technologies.