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1. 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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2. Creation, stabilization, and investigation at ambient pressure of pressure-induced superconductivity in Bi _0.5 Sb _1.5 Te ₃

   https://doi.org/10.1073/pnas.2423102122  

   Deng, Liangzi; Wang, Busheng; Halbert, Clayton; Schulze, Daniel J; Gooch, Melissa; Bontke, Trevor; Kuo, Ting-Wei; Shi, Xin; Song, Shaowei; Salke, Nilesh; et al (February 2025, Proceedings of the National Academy of Sciences)

   In light of breakthroughs in superconductivity under high pressure, and considering that record critical temperatures (T_cs) across various systems have been achieved under high pressure, the primary challenge for higher T_cshould no longer solely be to increase T_cunder extreme conditions but also to reduce, or ideally eliminate, the need for applied pressure in retaining pressure-induced or -enhanced superconductivity. The topological semiconductor Bi_0.5Sb_1.5Te₃(BST) was chosen to demonstrate our approach to addressing this challenge and exploring its intriguing physics. Under pressures up to ~50 GPa, three superconducting phases (BST-I, -II, and -III) were observed. A superconducting phase in BST-I appears at ~4 GPa, without a structural transition, suggesting the possible topological nature of this phase. Using the pressure-quench protocol (PQP) recently developed by us, we successfully retained this pressure-induced phase at ambient pressure and revealed the bulk nature of the state. Significantly, this demonstrates recovery of a pressure-quenched sample from a diamond anvil cell at room temperature with the pressure-induced phase retained at ambient pressure. Other superconducting phases were retained in BST-II and -III at ambient pressure and subjected to thermal and temporal stability testing. Superconductivity was also found in BST with T_cup to 10.2 K, the record for this compound series. While PQP maintains superconducting phases in BST at ambient pressure, both depressurization and PQP enhance its T_c, possibly due to microstructures formed during these processes, offering an added avenue to raise T_c. These findings are supported by our density-functional theory calculations. 

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3. On the Remarkable Superconductivity of FeSe and Its Close Cousins

   https://doi.org/10.3390/sym12091402  

   Kreisel, Andreas; Hirschfeld, Peter J.; Andersen, Brian M. (September 2020, Symmetry)

   null (Ed.)

   Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity. 

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4. Electronic, vibrational, and electron–phonon coupling properties in SnSe ₂ and SnS ₂ under pressure

   https://doi.org/10.1039/D0TC04356G  

   Kafle, Gyanu Prasad; Heil, Christoph; Paudyal, Hari; Margine, Elena R. (December 2020, Journal of Materials Chemistry C)

   The tin-selenide and tin-sulfide classes of materials undergo multiple structural transitions under high pressure leading to periodic lattice distortions, superconductivity, and topologically non-trivial phases, yet a number of controversies exist regarding the structural transformations in these systems. We perform first-principles calculations within the framework of density functional theory and a careful comparison of our results with available experiments on SnSe 2 reveals that the apparent contradictions among high-pressure results can be attributed to differences in experimental conditions. We further demonstrate that under hydrostatic pressure a superstructure can be stabilized above 20 GPa in SnS 2 via a periodic lattice distortion as found recently in the case of SnSe 2 , and that this pressure-induced phase transition is due to the combined effect of Fermi surface nesting and electron–phonon coupling at a momentum wave vector q = (1/3, 1/3, 0). In addition, we investigate the contribution of nonadiabatic corrections on the calculated phonon frequencies, and show that the quantitative agreement between theory and experiment for the high-energy A 1g phonon mode is improved when these effects are taken into account. Finally, we examine the nature of the superconducting state recently observed in SnSe 2 under nonhydrostatic pressure and predict the emergence of superconductivity with a comparable critical temperature in SnS 2 under similar experimental conditions. Interestingly, in the periodic lattice distorted phases, the critical temperature is found to be reduced by an order of magnitude due to the restructuring of the Fermi surface. 

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5. Superconductivity and phase stability of a high-entropy (NbTa)0.55(HfTiZr)0.45 alloy under high pressures

   https://doi.org/10.1063/5.0265943  

   Chakrabarty, Kallol; Freeman, Thomas; Jose, Greeshma_C; Canales, Francisco_J; Petri, James_L; Thompson, Elizabeth_C; Bi, Wenli; Yadav, Abhinav; Rangari, Vijaya; Vohra, Yogesh_K (June 2025, Journal of Applied Physics)

   High-entropy alloys (HEAs) are a class of multi-element materials that exhibit unique structural and functional properties. This study reports on the synthesis and characterization of a superconducting HEA, (NbTa)0.55(HfTiZr)0.45 fabricated using the vacuum arc melting technique. Scanning electron microscopy and energy-dispersive x-ray spectroscopy were employed to analyze the material's morphology and composition. X-ray diffraction analysis revealed a single-phase body-centered cubic (BCC) structure with a measured nanoindentation hardness of 6.4 GPa and Young's modulus of 132 GPa. This HEA superconductor was investigated by x-ray diffraction at Beamline 13BM-C, Advanced Photon Source, and the BCC phase was stable to the highest pressure of 50 GPa. Superconductivity was characterized by four-probe resistivity measurements in a quantum design physical property measurement system, yielding a superconducting transition temperature (Tc) of 7.2 K at ambient pressure and reaching a maximum of 10.1 K at the highest applied pressure of 23.6 GPa. The combination of high structural stability enhanced superconducting performance under pressure and superior mechanical properties highlights (NbTa)0.55(HfTiZr)0.45 as a promising superconductor under extreme environments. 

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