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1. Creating superconductivity in WB2 through pressure-induced metastable planar defects

   Lim, J. ; A. C. Hire ; Y. Quan ; J. S. Kim ; S. R. Xie ; S. Sinha ; R. S. Kumar ; D. Popov ; C. Park ; R. J. Hemley ; et al ( January 2022 , Nature communications)

   - (Ed.)

   High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB2 during compression induces superconductivity above 50 GPa with a maximum super-conducting critical temperature, Tc of 17 K at 91 GPa. Upon further compression up to 187 GPa, the Tc gradually decreases. Theoretical calculations show that electron-phonon mediated super-conductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB2 (hP3, space group 191, prototype AlB2). Synchrotron x-ray diffraction measurements up to 145 GPa show that the ambient pressure hP12 structure (space group 194, prototype WB2) continues to persist to this pressure, consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials. 

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2. Pressure-induced superconductivity in a novel germanium allotrope

   https://doi.org/10.1016/j.mtphys.2024.101338  

   Deng, Liangzi ; Zhang, Jianbo ; Sakai, Yuki ; Tang, Zhongjia ; Adnani, Moein ; Dahal, Rabin ; Litvinchuk, Alexander P. ; Chelikowsky, James R. ; Cohen, Marvin L. ; Hemley, Russell J. ; et al ( February 2024 , Materials Today Physics)

   - (Ed.)

   High-pressure studies on elements play an essential role in superconductivity research, with implications for both fundamental science and applications. Here we report the experimental discovery of surprisingly low pressure driving a novel germanium allotrope into a superconducting state in comparison to that for α-Ge. Raman measurements revealed structural phase transitions and possible electronic topological transitions under pressure up to 58 GPa. Based on pressure-dependent resistivity measurements, superconductivity was induced above 2 GPa and the maximum Tc of 6.8 K was observed under 4.6 GPa. Interestingly, a superconductivity enhancement was discovered during decompression, indicating the possibility of maintaining pressure-induced superconductivity at ambient pressure with better superconducting performance. Density functional theory analysis further suggested that the electronic structure of Ge (oP32) is sensitive to its detailed geometry and revealed that disorder in the β-tin structure leads to a higher Tc in comparison to the perfect β-tin Ge. 

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3. Superconductor–insulator transitions in three-dimensional indium-oxide at high pressures

   https://doi.org/10.1088/1361-648X/ac48bf  

   Hen, Bar ; Shelukhin, Victor ; Greenberg, Eran ; Rozenberg, Gregory Kh ; Kapitulnik, Aharon ; Palevski, Alexander ( January 2022 , Journal of Physics: Condensed Matter)

   Experiments investigating magnetic-field-tuned superconductor–insulator transition (HSIT) mostly focus on two-dimensional material systems where the transition and its proximate ground-state phases, often exhibit features that are seemingly at odds with the expected behavior. Here we present a complementary study of a three-dimensional pressure-packed amorphous indium-oxide (InOx) powder where granularity controls the HSIT. Above a low threshold pressure of ∼0.2 GPa, vestiges of superconductivity are detected, although neither a true superconducting transition nor insulating behavior are observed. Instead, a saturation at very high resistivity at low pressure is followed by saturation at very low resistivity at higher pressure. We identify both as different manifestations of anomalous metallic phases dominated by superconducting fluctuations. By analogy with previous identification of the low resistance saturation as a ‘failed superconductor’, our data suggests that the very high resistance saturation is a manifestation of a ‘failed insulator’. Above a threshold pressure of ∼6 GPa, the sample becomes fully packed, and superconductivity is robust, withT_Ctunable with pressure. A quantum critical point atP_C∼ 25 GPa marks the complete suppression of superconductivity. For a finite pressure belowP_C, a magnetic field is shown to induce a HSIT from a true zero-resistance superconducting state to a weakly insulating behavior. Determining the critical field,H_C, we show that similar to the 2D behavior, the insulating-like state maintains a superconducting character, which is quenched at higher field, above which the magnetoresistance decreases to its fermionic normal state value.

    

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4. Tuning magnetic confinement of spin-triplet superconductivity

   https://doi.org/10.1038/s41535-020-00270-w  

   Lin, Wen-Chen ; Campbell, Daniel J. ; Ran, Sheng ; Liu, I-Lin ; Kim, Hyunsoo ; Nevidomskyy, Andriy H. ; Graf, David ; Butch, Nicholas P. ; Paglione, Johnpierre ( December 2020 , npj Quantum Materials)

   null (Ed.)

   Abstract Electrical magnetoresistance and tunnel diode oscillator measurements were performed under external magnetic fields up to 41 T applied along the crystallographic b axis (hard axis) of UTe 2 as a function of temperature and applied pressures up to 18.8 kbar. In this work, we track the field-induced first-order transition between superconducting and magnetic field-polarized phases as a function of applied pressure, showing suppression of the transition with increasing pressure until the demise of superconductivity near 16 kbar and the appearance of a pressure-induced ferromagnetic-like ground state that is distinct from the field-polarized phase and stable at zero field. Together with evidence for the evolution of a second superconducting phase and its upper critical field with pressure, we examine the confinement of superconductivity by two orthogonal magnetic phases and the implications for understanding the boundaries of triplet superconductivity. 

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5. Effect of Fermi surface topology change on the Kagome superconductor CeRu2 under pressure

   https://doi.org/10.1016/j.mtphys.2023.101322  

   Deng, Liangzi ; Gooch, Melissa ; Liu, Hongxiong ; Salke, Nilesh P. ; Bontke, Trevor ; Shao, Sen ; You, Jingyang ; Schulze, Daniel J. ; Kumar, Ravhi ; Yin, Jia-Xin ; et al ( January 2024 , Materials Today Physics)

   - (Ed.)

   The cubic Laves phase compound CeRu2 with a Kagome substructure of Ru has been investigated to understand myriad fascinating phenomena resulting from competition among its various physical and geometric features. Such phenomena include flat bands, van Hove singularities, Dirac cones, reentrant superconductivity, magnetism, the Fulde–Ferrell–Larkin–Ovchinnikov state, valence fluctuations, time-irreversible anisotropic s-state superconductivity, etc. Extensive studies have thus been carried out since 1958 when the highly unusual coexistence of superconductivity and ferromagnetism was proposed for the mixed compounds (Ce,Gd)Ru2. Activity has accelerated in recent years due to increasing interest in topological states in superconductors. However, there has been little investigation of the mutual influence of these fascinating states. Therefore, we systematically investigated the superconductivity and possible Fermi surface topological change in CeRu2 via magnetic, resistivity, and structural measurements under pressure up to ~168 GPa. An unusual phase diagram that suggests an intriguing interplay between the compound’s superconducting order and Fermi surface topological order has been constructed. A resurgence in its superconducting transition temperature was observed above 28 GPa. Our experiments have identified a structural transition above 76 GPa and a few tantalizing phase transitions driven by high pressure. Our high-pressure results further suggest that superconductivity and Fermi surface topology in CeRu2 are strongly intertwined, 

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