[Paper Review] Field-induced tuning of the pairing state in a superconductor

Adrien Rosuel, C. Marcenat+13 more|arXiv (Cornell University)|May 9, 2022

Rare-earth and actinide compounds11 citations

TL;DR

This study reports the first complete thermodynamic phase diagram of UTe₂ under magnetic fields along all three crystallographic axes, revealing a field-tuned transition from a low-field spin-triplet to a high-field spin-singlet superconducting state. The key finding is a distinct phase transition within the superconducting phase under high fields along the *b* axis, with strong evidence for field-induced pairing strength enhancement and a reversal of pairing symmetry, a phenomenon unprecedented in superconductors.

ABSTRACT

The recently discovered superconductor UTe$_2$, with a T$_c$ between 1.5~K and 2~K, is attracting much attention due to strong suspicion of spin-triplet and topological superconductivity. Its properties under magnetic field are also remarkable, with field-reinforced and field-induced superconducting phases. Here, we report the first complete thermodynamic determination of the phase diagram for fields applied along the three crystallographic directions. Measurements were performed up to 36~T along the hard magnetisation $b$~axis in order to follow the superconducting transition up to the metamagnetic transition at $H_{m} = 34.75$~T. They reveal the existence of a phase transition line within the superconducting phase, and drastic differences occurring between these two phases. Detailed analysis supports a different spin state between the two phases, implying a low-field spin-triplet to high-field spin-singlet transition, a unique case among superconductors, giving insight on the mechanisms leading to spin-triplet superconductivity.

Motivation & Objective

• To determine the full thermodynamic phase diagram of UTe₂ under magnetic fields applied along the three crystallographic directions (*a*, *b*, *c*).
• To resolve the long-standing ambiguity about whether the field-reinforced superconducting phase in UTe₂ has a different pairing symmetry than the low-field phase.
• To investigate the origin of the upper critical field *H*_c2 enhancement under high fields, particularly along the *b* axis, and its implications for pairing symmetry.
• To establish whether a thermodynamic phase transition separates the low-field and high-field superconducting phases in UTe₂, which would imply distinct pairing mechanisms.
• To test the hypothesis that magnetic field induces a transition from spin-triplet to spin-singlet pairing in this unconventional superconductor.

Proposed method

• Conducted high-precision specific heat measurements on single-crystalline UTe₂ samples up to 36 T to map the superconducting transition temperature *T*_c as a function of field and field orientation.
• Performed angular-dependent specific heat measurements in the (*b*, *c*) plane to probe the field reorientation dependence of the superconducting phase transitions.
• Used Gaussian broadening analysis of the specific heat anomaly to account for a distribution of metamagnetic transition fields *H*_m, with σ = 0.19 T.
• Applied a field-dependent pairing strength model *λ*(*H*/*H*_m) to relate *H*_c2 to *T*_c and extract the sensitivity of *T*_c to *H*_m via ∂*T*_c/∂*H*_m.
• Compared theoretical models of *H*_c2 with and without paramagnetic limitation to assess the impact of pairing symmetry on the observed broadening of the specific heat anomaly.
• Mapped the phase boundary using a Gaussian peak-fitting procedure on *C*/*T* data to extract *T*_c at various field angles and magnitudes.

Experimental results

Research questions

• RQ1 Does the field-reinforced superconducting phase in UTe₂, observed along the *b* axis, represent a distinct thermodynamic phase with different pairing symmetry?
• RQ2 Is there a thermodynamic phase transition within the superconducting state of UTe₂ under high magnetic fields, indicating a change in pairing mechanism?
• RQ3 Can the observed enhancement of *H*_c2 under high fields be explained by a field-tuned transition from spin-triplet to spin-singlet pairing?
• RQ4 How does the field dependence of the pairing strength *λ*(*H*) affect the broadening of the specific heat anomaly, and what does this imply about the underlying pairing symmetry?
• RQ5 What is the role of the distribution of metamagnetic transition fields *H*_m in shaping the observed specific heat features in UTe₂?

Key findings

• For fields along the *a* axis, a strong negative curvature of *H*_c2 near *T*_c reveals a significant suppression of pairing strength at low magnetic fields.
• Along the *b* axis, specific heat measurements up to 36 T confirm a bulk field-reinforced superconducting phase with a critical field *H*_c2 exceeding 30 T.
• A clear phase transition line is observed within the superconducting phase under high fields along the *b* axis, separating a low-field phase from a high-field phase.
• The low-field and high-field superconducting phases exhibit drastically different thermodynamic behaviors, suggesting distinct pairing mechanisms.
• Detailed analysis of the field dependence of *T*_c and *H*_c2 supports a transition from a spin-triplet to a spin-singlet pairing state under high magnetic fields, a unique phenomenon in superconductors.
• The broadening of the specific heat anomaly is significantly larger when paramagnetic limitation is included in the *H*_c2 model, indicating stronger sensitivity to *H*_m distribution under spin-singlet pairing.