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1. Josephson Detection of Time Reversal Symmetry Breaking s +/- is Superconductivity in SnTe Nanowires

   Trimble, C.; Wei, M. T.; Kalantre, S. S.; Yuan, N. F.; Liu, P.; Cha, J. J.; Fu, L.; Williams, J. R. (January 2019, ArXiv.org)

   Exotic superconductivity, such as high TC, topological, and heavy-fermion superconductors, often rely on phase sensitive measurements to determine the underlying pairing. Here we investigate the proximity-induced superconductivity in nanowires of SnTe, where a s±is′ superconducting state is produced that lacks the time-reversal and valley-exchange symmetry of the parent SnTe. A systematic breakdown of three conventional characteristics of Josephson junctions -- the DC Josephson effect, the AC Josephson effect, and the magnetic diffraction pattern -- fabricated from SnTe nanowire weak links elucidates this novel superconducting state. Further, the AC Josephson effect reveals evidence of a Majorana bound state, tuned by a perpendicular magnetic field. This work represents the definitive phase-sensitive measurement of novel s±is′ superconductivity, providing a new route to the investigation of fractional vortices, topological superconductivity, topological phase transitions, and new types of Josephson-based devices. 

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2. Nature of the spin resonance mode in CeCoIn5

   https://doi.org/10.1038/s42005-020-0365-8  

   Song, Yu; Wang, Weiyi; S. Van Dyke, John; Pouse, Naveen; Ran, Sheng; Yazici, Duygu; Schneidewind, A.; Čermák, Petr; Qiu, Y.; Maple, M. B.; et al (May 2020, Communications Physics)

   Abstract Spin-fluctuation-mediated unconventional superconductivity can emerge at the border of magnetism, featuring a superconducting order parameter that changes sign in momentum space. Detection of such a sign-change is experimentally challenging, since most probes are not phase-sensitive. The observation of a spin resonance mode (SRM) from inelastic neutron scattering is often seen as strong phase-sensitive evidence for a sign-changing superconducting order parameter, by assuming the SRM is a spin-excitonic bound state. Here we show that for the heavy fermion superconductor CeCoIn₅, its SRM defies expectations for a spin-excitonic bound state, and is not a manifestation of sign-changing superconductivity. Instead, the SRM in CeCoIn₅likely arises from a reduction of damping to a magnon-like mode in the superconducting state, due to its proximity to magnetic quantum criticality. Our findings emphasize the need for more stringent tests of whether SRMs are spin-excitonic, when using their presence to evidence sign-changing superconductivity. 

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3. Dual-phase superconductivity in high-pressure high-temperature synthesized TaNbZrHfTi

   https://doi.org/10.1063/5.0214797  

   Sereika, Raimundas; Iwan, Seth; Baker, Paul_A; Bi, Wenli; Vohra, Yogesh_K (June 2024, AIP Advances)

   We report on a novel TaNbZrHfTi-based high entropy alloy (HEA) which demonstrates distinctive dual-phase superconductivity. The HEA was synthesized under high pressures and high temperatures starting from a ball milled mixture of elemental metals in a large-volume Paris–Edinburgh cell with P ≈ 6 GPa and T = 2300 K. The synthesized HEA is a phase mixture of BCC (NbTa)0.45(ZrHfTi)0.55 with Tc1 = 6 K and FCC (NbTa)0.04(ZrHfTi)0.96 with Tc2 = 3.75 K. The measured magnetic field parameters for the HEA are lower critical field, Hc1(0) = 31 mT, and a relatively high upper critical field, Hc2(0) = 4.92 T. This dual-phase system is further characterized by the presence of a second magnetization peak, or the fishtail effect, observed in the virgin magnetization curves. This phenomenon, which does not distort the field-dependent magnetization hysteresis loops, suggests intricate pinning mechanisms that could be potentially tuned for optimized performance. The manifestation of these unique features in HEA superconductivity reinforces phase-dependent superconductivity and opens new avenues in the exploration of novel superconducting materials. 

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4. Unconventional superconducting phase diagram of monolayer ${\mathrm{WTe}}_2$

   https://doi.org/10.1103/PhysRevResearch.7.013224  

   Song, Tiancheng; Jia, Yanyu; Yu, Guo; Tang, Yue; Uzan, Ayelet J; Zheng, Zhaoyi Joy; Guan, Haosen; Onyszczak, Michael; Singha, Ratnadwip; Gui, Xin; et al (February 2025, Physical Review Research)

   The existence of a quantum critical point (QCP) and fluctuations around it are believed to be important for understanding the phase diagram in unconventional superconductors such as cuprates, iron pnictides, and heavy fermion superconductors. However, the QCP is usually buried deep within the superconducting dome and is difficult to investigate. The connection between quantum critical fluctuations and superconductivity remains an outstanding problem in condensed matter. Here combining both electrical transport and Nernst experiments, we explicitly demonstrate the onset of superconductivity at an unconventional QCP in gate-tuned monolayer tungsten ditelluride $\left({\mathrm{WTe}}_2\right)$, with features incompatible with the conventional Bardeen-Cooper-Schrieffer scenario. The results lead to a superconducting phase diagram that is distinguished from other known superconductors. Two distinct gate-tuned quantum phase transitions are observed at the ends of the superconducting dome. We find that quantum fluctuations around the QCP of the underdoped regime are essential for understanding how the monolayer superconductivity is established. The unconventional phase diagram we report here illustrates a previously unknown relation between superconductivity and QCP. Published by the American Physical Society2025 

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5. Beyond the standard model of topological Josephson junctions: From crystalline anisotropy to finite-size and diode effects

   https://doi.org/10.1063/5.0214920  

   Pekerten, Barış; Brandão, David S; Bussiere, Bailey; Monroe, David; Zhou, Tong; Han, Jong E; Shabani, Javad; Matos-Abiague, Alex; Žutić, Igor (June 2024, Applied Physics Letters)

   A planar Josephson junction is a versatile platform to realize topological superconductivity over a large parameter space and host Majorana bound states. With a change in the Zeeman field, this system undergoes a transition from trivial to topological superconductivity accompanied by a jump in the superconducting phase difference between the two superconductors. A standard model of these Josephson junctions, which can be fabricated to have a nearly perfect interfacial transparency, predicts a simple universal behavior. In that model, at the same value of Zeeman field for the topological transition, there is a π phase jump and a minimum in the critical superconducting current, while applying a controllable phase difference yields a diamond-shaped topological region as a function of that phase difference and a Zeeman field. In contrast, even for a perfect interfacial transparency, we find a much richer and nonuniversal behavior as the width of the superconductor is varied or the Dresselhaus spin–orbit coupling is considered. The Zeeman field for the phase jump, not necessarily π, is different from the value for the minimum critical current, while there is a strong deviation from the diamond-like topological region. These Josephson junctions show a striking example of a nonreciprocal transport and superconducting diode effect, revealing the importance of our findings not only for topological superconductivity and fault-tolerant quantum computing but also for superconducting spintronics. 

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