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1. High-Temperature Superconductivity in Hydrides: Experimental Evidence and Details

   https://doi.org/10.1007/s10948-022-06148-1  

   Eremets, M. I. ; Minkov, V. S. ; Drozdov, A. P. ; Kong, P. P. ; Ksenofontov, V. ; Shylin, S. I. ; Bud’ko, S. L. ; Prozorov, R. ; Balakirev, F. F. ; Sun, Dan ; et al ( March 2022 , Journal of Superconductivity and Novel Magnetism)

   Since the discovery of superconductivity at ~ 200 K in H₃S [1], similar or higher transition temperatures,T_cs, have been reported for various hydrogen-rich compounds under ultra-high pressures [2]. Superconductivity was experimentally proved by different methods, including electrical resistance, magnetic susceptibility, optical infrared, and nuclear resonant scattering measurements. The crystal structures of superconducting phases were determined by X-ray diffraction. Numerous electrical transport measurements demonstrate the typical behavior of a conventional phonon-mediated superconductor: zero resistance belowT_c, shift ofT_cto lower temperatures under external magnetic fields, and pronounced isotope effect. Remarkably, the results are in good agreement with the theoretical predictions, which describe superconductivity in hydrides within the framework of the conventional BCS theory. However, despite this acknowledgement, experimental evidences for the superconducting state in these compounds have recently been treated with criticism [3–7], which apparently stems from misunderstanding and misinterpretation of complicated experiments performed under very high pressures. Here, we describe in greater detail the experiments revealing high-temperature superconductivity in hydrides under high pressures. We show that the arguments against superconductivity [3–7] can be either refuted or explained. The experiments on the high-temperature superconductivity in hydrides clearly contradict the theory of hole superconductivity [8] and eliminate it [3].

    

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2. Superconducting phase diagram of H3S under high magnetic fields

   https://doi.org/10.1038/s41467-019-10552-y  

   Mozaffari, Shirin ; Sun, Dan ; Minkov, Vasily S. ; Drozdov, Alexander P. ; Knyazev, Dmitry ; Betts, Jonathan B. ; Einaga, Mari ; Shimizu, Katsuya ; Eremets, Mikhail I. ; Balicas, Luis ; et al ( June 2019 , Nature Communications)

   The discovery of superconductivity at 260 K in hydrogen-rich compounds like LaH₁₀re-invigorated the quest for room temperature superconductivity. Here, we report the temperature dependence of the upper critical fieldsμ₀H_c2(T) of superconducting H₃S under a record-high combination of applied pressures up to 160 GPa and fields up to 65 T. We find thatH_c2(T) displays a linear dependence on temperature over an extended range as found in multigap or in strongly-coupled superconductors, thus deviating from conventional Werthamer, Helfand, and Hohenberg (WHH) formalism. The best fit ofH_c2(T) to the WHH formalism yields negligible values for the Maki parameterαand the spin–orbit scattering constantλ_SO. However,H_c2(T) is well-described by a model based on strong coupling superconductivity with a coupling constantλ~ 2. We conclude that H₃S behaves as a strong-coupled orbital-limited superconductor over the entire range of temperatures and fields used for our measurements.

    

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3. High-temperature superconductivity on the verge of a structural instability in lanthanum superhydride

   https://doi.org/10.1038/s41467-021-26706-w  

   Sun, Dan ; Minkov, Vasily S. ; Mozaffari, Shirin ; Sun, Ying ; Ma, Yanming ; Chariton, Stella ; Prakapenka, Vitali B. ; Eremets, Mikhail I. ; Balicas, Luis ; Balakirev, Fedor F. ( November 2021 , Nature Communications)

   The possibility of high, room-temperature superconductivity was predicted for metallic hydrogen in the 1960s. However, metallization and superconductivity of hydrogen are yet to be unambiguously demonstrated and may require pressures as high as 5 million atmospheres. Rare earth based “superhydrides”, such as LaH₁₀, can be considered as a close approximation of metallic hydrogen even though they form at moderately lower pressures. In superhydrides the predominance of H-H metallic bonds and high superconducting transition temperatures bear the hallmarks of metallic hydrogen. Still, experimental studies revealing the key factors controlling their superconductivity are scarce. Here, we report the pressure and magnetic field dependence of the superconducting order observed in LaH₁₀. We determine that the high-symmetry high-temperature superconductingFm-3mphase of LaH₁₀can be stabilized at substantially lower pressures than previously thought. We find a remarkable correlation between superconductivity and a structural instability indicating that lattice vibrations, responsible for the monoclinic structural distortions in LaH₁₀, strongly affect the superconducting coupling.

    

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4. Anomalous quantum criticality in the electron-doped cuprates

   https://doi.org/10.1073/pnas.1817653116  

   Mandal, P. R. ; Sarkar, Tarapada ; Greene, Richard L. ( March 2019 , Proceedings of the National Academy of Sciences)

   In the physics of condensed matter, quantum critical phenomena and unconventional superconductivity are two major themes. In electron-doped cuprates, the low critical field (H_C2) allows one to study the putative quantum critical point (QCP) at low temperature and to understand its connection to the long-standing problem of the origin of the high-T_Csuperconductivity. Here we present measurements of the low-temperature normal-state thermopower (S) of the electron-doped cuprate superconductor La_2−xCe_xCuO₄(LCCO) fromx= 0.11–0.19. We observe quantum critical$\mathit{S}/\mathit{T}$versus$\mathbf{l}\mathbf{n}\left(\mathbf{1}/\mathit{T}\right)$behavior over an unexpectedly wide doping rangex= 0.15–0.17 above the QCP (x= 0.14), with a slope that scales monotonically with the superconducting transition temperature (T_Cwith H = 0). The presence of quantum criticality over a wide doping range provides a window on the criticality. The thermopower behavior also suggests that the critical fluctuations are linked withT_C. Above the superconductivity dome, atx= 0.19, a conventional Fermi-liquid$\mathit{S}\propto \mathit{T}$behavior is found for$\mathit{T}\le$40 K.

    

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5. Correlation between scale-invariant normal-state resistivity and superconductivity in an electron-doped cuprate

   https://doi.org/10.1126/sciadv.aav6753  

   Sarkar, Tarapada ; Mandal, P. R. ; Poniatowski, N. R. ; Chan, M. K. ; Greene, Richard L. ( May 2019 , Science Advances)

   An understanding of the normal state in the high-temperature superconducting cuprates is crucial to the ultimate understanding of the long-standing problem of the origin of the superconductivity itself. This so-called “strange metal” state is thought to be associated with a quantum critical point (QCP) hidden beneath the superconductivity. In electron-doped cuprates—in contrast to hole-doped cuprates—it is possible to access the normal state at very low temperatures and low magnetic fields to study this putative QCP and to probe the T ➔ 0 K state of these materials. We report measurements of the low-temperature normal-state magnetoresistance (MR) of the n-type cuprate system La 2− x Ce x CuO 4 and find that it is characterized by a linear-in-field behavior, which follows a scaling relation with applied field and temperature, for doping ( x ) above the putative QCP ( x = 0.14). The magnitude of the unconventional linear MR decreases as T c decreases and goes to zero at the end of the superconducting dome ( x ~ 0.175) above which a conventional quadratic MR is found. These results show that there is a strong correlation between the quantum critical excitations of the strange metal state and the high- T c superconductivity. 

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