Suchergebnisse

The mechanism of charge carrier interaction in twisted bilayer graphene (TBG) remains an unresolved problem, where some researchers proposed the dominance of the electron–phonon interaction, while the others showed evidence for electron–electron or electron–magnon interactions. Here we propose to resolve this problem by generalizing the Bloch–Grüneisen equation and using it for the analysis of the temperature dependent resistivity in TBG. It is a well-established theoretical result that the Bloch–Grüneisen equation power-law exponent, p, exhibits exact integer values for certain mechanisms. For instance, p = 5 implies the electron–phonon interaction, p = 3 is associated with the electron–magnon interaction and p = 2 applies to the electron–electron interaction. Here we interpret the linear temperature-dependent resistance, widely observed in TBG, as p → 1, which implies the quasielastic charge interaction with acoustic phonons. Thus, we fitted TBG resistance curves to the Bloch–Grüneisen equation, where we propose that p is a free-fitting parameter. We found that TBGs have a smoothly varied p-value (ranging from 1.4 to 4.4) depending on the Moiré superlattice constant, λ, or the charge carrier concentration, n. This implies that different mechanisms of the charge carrier interaction in TBG superlattices smoothly transition from one mechanism to another depending on, at least, λ and n. The proposed generalized Bloch–Grüneisen equation is applicable to a wide range of disciplines, including superconductivity and geology. © 2021 by the author. Licensee MDPI, Basel, Switzerland. ; This research was funded by the Ministry of Science and Higher Education of Russia (theme "Pressure" No. AAAA-A18-118020190104-3) and by Act 211 Government of the Russian Federation, contract No. 02.A03.21.0006.

The primary mechanism governing the emergence of near-room-temperature superconductivity (NRTS) in superhydrides is widely accepted to be the electron-phonon interaction. If so, the temperature-dependent resistance, R(T), in these materials should obey the Bloch-Grüneisen (BG) equation, where the power-law exponent, p, should be equal to the exact integer value of p= 5. However, there is a well-established theoretical result showing that the pure electron-magnon interaction should be manifested by p= 3, and p= 2 is the value for pure electron-electron interaction. Here we aimed to reveal the type of charge carrier interaction in the layered transition metal dichalcogenides PdTe2, high-entropy alloy (ScZrNb)0.65[RhPd]0.35 and highly-compressed elemental boron and superhydrides H3S, LaH x, PrH9 and BaH12 by fitting the temperature-dependent resistance of these materials to the BG equation, where the power-law exponent, p, is a free-fitting parameter. The results showed that the high-entropy alloy (ScZrNb)0.65[RhPd]0.35 exhibited pure electron-phonon mediated superconductivity with p = 4.9 0.4. Unexpectedly, we revealed that all studied superhydrides exhibit 1.8 < p < 3.2. This implies that it is unlikely that the electron-phonon interaction is the primary mechanism for the Cooper pairs formation in highly-compressed superhydrides, and alternative pairing mechanisms, for instance, the electron-magnon, the electron-polaron, the electron-electron and other pairing mechanisms should be considered as the origin for the emergence of NRTS in these compounds. © 2021 IOP Publishing Ltd. ; The author is grateful for financial support provided by the Ministry of Science and Higher Education of Russia (theme 'Pressure' No. AAAA-A18-118020190104-3) and by Act 211 Government of the Russian Federation, Contract No. 02.A03.21.0006.

Since the pivotal experimental discovery of near-room-temperature superconductivity (NRTS) in highly compressed sulphur hydride by Drozdov et al. (Nature 2015, 525, 73–76), more than a dozen binary and ternary hydrogen-rich phases exhibiting superconducting transitions above 100 K have been discovered to date. There is a widely accepted theoretical point of view that the primary mechanism governing the emergence of superconductivity in hydrogen-rich phases is the electron–phonon pairing. However, the recent analysis of experimental temperature-dependent resistance, R(T), in H3S, LaHx, PrH9 and BaH12 (Talantsev, Supercond. Sci. Technol. 2021, 34, accepted) showed that these compounds exhibit the dominance of non-electron–phonon charge carrier interactions and, thus, it is unlikely that the electron–phonon pairing is the primary mechanism for the emergence of superconductivity in these materials. Here, we use the same approach to reveal the charge carrier interaction in highly compressed lithium, black phosphorous, sulfur, and silane. We found that all these superconductors exhibit the dominance of non-electron–phonon charge carrier interaction. This explains the failure to demonstrate the high-Tc values that are predicted for these materials by first-principles calculations which utilize the electron–phonon pairing as the mechanism for the emergence of their superconductivity. Our result implies that alternative pairing mechanisms (primarily the electron–electron retraction) should be tested within the first-principles calculations approach as possible mechanisms for the emergence of superconductivity in highly compressed lithium, black phosphorous, sulfur, and silane. © 2021 by the author. Licensee MDPI, Basel, Switzerland. ; Funding: This research was funded by the Ministry of Science, Higher Education of Russia (theme "Pressure" No. AAAA-A18-118020190104-3), and by Government Act 211 of the Russian Federation, contract No. 02.A03.21.0006.

The experimental discovery of near-room-temperature (NRT) superconductivity in highlycompressed H3S, LaH10 and YH6 has restored fundamental interest in the electron-phonon pairing mechanism in superconductors. One of the prerequisites of phonon-mediated NRT superconductivity in highly compressed hydrides is strong electron-phonon interactions, which can be quantified by dimensionless ratios of the Bardeen-Cooper-Schrieffer (BCS) theory vs variable, where Tc is the critical temperature and ωln is the logarithmic phonon frequency (Mitrovic et al 1984 Phys. Rev. B 29 184). However, all known strong-coupling correction functions for the BCS ratios are applicable for < 0.20, which is not a high enough range for NRT superconductors, because the latter exhibit variable values of 0.13 < < 0.32. In this paper, we reanalyze the full experimental dataset (including data for highly compressed H3S) and find that the strong-coupling correction functions for the gap-to-critical-temperature ratio and for the specific-heat-jump ratio are double-valued nearly linear functions of . © 2020 IOP Publishing Ltd. ; The author acknowledges financial support provided by the Ministry of Education and Science of the Russian Federation (theme 'Pressure' No. AAAA-A18-118020190104-3) and by Act 211 Government of the Russian Federation, contract No. 02.A03.21.0006.

High-temperature superconductivity in iron-based layered compounds discovered by Hosono group (Kamihara et al. 2006 J. Am. Chem. Soc. 128 10012) is fascinating physical phenomenon which still has many unanswered questions. One of these questions is the superconducting gap symmetry in iron-based superconductors (IBS), for which the most agreed concept is multiple-band s-wave symmetry. Recently, an alternative concept of single-band p-wave symmetry has been proposed. To disprove/reaffirm the latter concept, in this paper we analyse temperature dependent in-plane coherence length, ξab(T), in FeSe, FeSe1-xTex, Ba(Fe1-x(Co/Ni)x)2As2 and Ca10(Pt4As8)((Fe1−xPtx)2As2)5 in order to extract the gap-to-critical-temperature ratio, 2Δ(0)/kBTc, and the specific-heat-jump ratio, ΔC/C, in these compounds. In the result, we report that deduced ratios are in a good agreement with the concept of single-band p-wave superconductivity in iron-based superconductors. © 2020 The Author ; AAAA-A18-118020190104-3 ; Author thanks financial support provided by the state assignment of Minobrnauki of Russia (theme "Pressure" No. AAAA-A18-118020190104-3) and by Act 211 Government of the Russian Federation, contract No. 02.A03.21.0006.

Recently Li et al. (2019 Nature 572 624) discovered a new type of oxide superconductor Nd0.8Sr0.2NiO2 with Tc = 14 K. To classify superconductivity in this infinite-layer nickelate experimental upper critical field, Bc2(T), and the self-field critical current densities, Jc(sf,T), reported by Li et al. (2019 Nature 572 624), are analysed in assumption of s-, d-, and p-wave pairing symmetries and single- and multiple-band superconductivity. Based on deduced the ground-state superconducting energy gap, Δ(0), the London penetration depth, λ(0), the relative jump in electronic specific heat at Tc, ΔC/C, and the ratio of 2Δ(0)/kBTc, we conclude that Nd0.8Sr0.2NiO2 is type-II high-κ weak-coupled single-band s-wave superconductor. © 2020 The Author ; AAAA-A18-118020190104-3 ; Author thanks Dr. W. P. Crump (Aalto University) for invaluable help. Author also thanks financial support provided by the state assignment of Minobrnauki of Russia (theme "Pressure" No. AAAA-A18-118020190104-3) and by Act 211 Government of the Russian Federation, contract No. 02.A03.21.0006.

Recently, several research groups have reported on anomalous enhancement of the self-field critical currents, Ic(sf,T), at low temperatures in superconductor/Dirac-cone material/superconductor (S/DCM/S) junctions. Some papers attributed the enhancement to the low-energy Andreev bound states arising from winding of the electronic wave function around DCM. In this paper, Ic(sf,T) in S/DCM/S junctions have been analyzed by two approaches: Modified Ambegaokar-Baratoff and ballistic Titov-Beenakker models. It is shown that the ballistic model is an inadequate tool to analyze experimental data from S/DCM/S junctions. The primary mechanism for limiting superconducting current in S/DCM/S junctions is different from the conventional view that the latter is the maximum value within the order parameter phase variation. Thus, there is a need to develop a new model for self-field critical currents in S/DCM/S systems. © 2019 by the authors. Licensee MDPI, Basel, Switzerland. ; 02.A03.21.0006 ; AAAA-A18-118020190104-3 ; Funding: This research was funded by the State Assignment of Minobrnauki of Russia, theme "Pressure" No. AAAA-A18-118020190104-3, and by Act 211 Government of the Russian Federation, contract No. 02.A03.21.0006.

Dissipative-free electric current flow is one of the most fascinating and practically important properties of superconductors. Theoretical consideration of the charge carriers flow in infinitely long rectangular slab of superconductor in the absence of external magnetic field (so called, self-field) is based on an assumption that the charge carriers have rectilinear trajectories in the direction of the current flow whereas the current density and magnetic flux density are decaying towards superconducting slab with London penetration depth as characteristic length. Here, we calculate charge particle trajectories (as single electron/hole, as Cooper pair) at self-field conditions and find that charge carriers do not follow intuitive rectilinear trajectories along the slab surface, but instead ones have meander shape trajectories cross the whole thickness of the slab. Moreover, if the particle velocity is below some value, the charge moves in opposite direction to nominal current flow. This disturbance of the canonical magnetic flux density distribution and backward movement of Cooper pairs can be entire mechanism for power dissipation in superconductors. © 2021 World Scientific Publishing Company. ; EFT thanks financial support provided by the state assignment of Minobrnauki of Russia (theme "Pressure" No. AAAA-A18-118020190104-3) and by Act 211 Government of the Russian Federation, contract No. 02.A03.21.0006.

Several research groups have reported on the observation of superconductivity in bilayer graphene structures where single atomic layers of graphene are stacked and then twisted at angles θ forming Moiré superlattices. The characterization of the superconducting state in these 2D materials is an ongoing task. Here we investigate the pairing symmetry of bilayer graphene Moiré superlattices twisted at θ = 1.05°, 1.10° and 1.16° for carrier doping states varied in the range of n = (0.5 − 1.5) · 1012 cm−2 (where superconductivity can be realized) by analyzing the temperature dependence of the upper critical field Bc2(T) and the self-field critical current Jc(sf,T) within currently available models – all of which start from phonon-mediated BCS theory – for single- and two-band s−, d−, p− and d + id-wave gap symmetries. Extracted superconducting parameters show that only s-wave and a specific kind of p-wave symmetries are likely to be dominant in bilayer graphene Moiré superlattices. More experimental data is required to distinguish between the s- and remaining p-wave symmetries as well as the suspected two-band superconductivity in these 2D superlattices. © 2020, The Author(s). ; Authors thank Mr. Yuan Cao and Prof. P. Jarillo-Herrero (Massachusetts Institute of Technology) for providing raw experimental data and comments on the manuscript and Prof. J.L. Tallon (Victoria University of Wellington) for valued impact and help during manuscript preparation. EFT thanks financial support provided by the state assignment of Minobrnauki of Russia (theme "Pressure" No. АААА-А18-118020190104-3) and by Act 211 Government of the Russian Federation, Contract No. 02.A03.21.0006.

The electrical transport properties of (Li,Fe)OHFeSe films have been investigated in detail. The sharply textured films, prepared by matrix-assisted hydrothermal epitaxy (MHE) on LaAlO3, show a zero-resistance critical temperature T c of ∼42 K, J c values well above 1 MA cm-2 at low temperatures, and a maximum pinning force density F P of ∼100 GN m-3 at 4 K. The activation energy U 0 for thermal depinning of flux lines has been resolved for low magnetic fields, it agrees well with literature data. The coherence lengths and penetration depth were estimated via upper critical field B c2 and self-field J c, respectively, to be ξ ab ∼ 2.7 nm, ξ c = 0.24 nm, and λ ab ∼ 160-200 nm. The layered crystal structure leads to highly anisotropic and two-dimensional electrical properties, including trapping and lock-in of vortices. © 2020 The Author(s). Published by IOP Publishing Ltd. ; Original content from this work may be used under the terms of the . Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. National Key Research and Development Program of China 2017YFA0303003 2016YFA0300300 Strategic Priority Research Program and Key Research Program of Frontier Sciences of the Chi-nese Academy of Sciences QYZDY-SSW-SLH001 XDB25000000 state assignment of Minobrnauki of Russia Act 211 Government of the Russian Federation 02.A03.21.0006 National Natural Science Foundation of China 11888101 11834016.

The possibility of p-wave pairing in superconductors has been proposed more than five decades ago, but has not yet been convincingly demonstrated. One difficulty is that some p-wave states are thermodynamically indistinguishable from s-wave, while others are very similar to d-wave states. Here we studied the self-field critical current of NdFeAs(O,F) thin films in order to extract absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the superconducting transition temperature, and find that all the deduced physical parameters strongly indicate that NdFeAs(O,F) is a bulk p-wave superconductor. Further investigation revealed that single atomic layer FeSe also shows p-wave pairing. In an attempt to generalize these findings, we re-examined the whole inventory of superfluid density measurements in iron-based superconductors and show quite generally that single-band weak-coupling p-wave superconductivity is exhibited in iron-based superconductors. © 2019, The Author(s). ; Japan Science and Technology Corporation, JST: JPMJCR18J4 ; Government Council on Grants, Russian Federation ; Japan Society for the Promotion of Science, JSPS: 16H04646 ; АААА-А18-118020190104-3 ; The authors thank Prof. Jeffery L. Tallon (Victoria University of Wellington, New Zealand) and Prof. Christian Bernhard (University of Fribourg, Switzerland) for helpful discussions, and also for reading and commenting on the manuscript. EFT is grateful for financial support provided by the state assignment of Minobrnauki of Russia (theme "Pressure" No. АААА-А18-118020190104-3) and by Act 211 of the Government of the Russian Federation, contract No. 02.A03.21.0006. KI and HI acknowledge support by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (B) Grant Number 16H04646, as well as JST CREST Grant Number JPMJCR18J4. The Article Processing Charge for this publication was provided by Ural Federal University, Russia.

Superconductivity in the vicinity of a competing electronic order often manifests itself with a superconducting dome, centered at a presumed quantum critical point in the phase diagram. This common feature, found in many unconventional superconductors, has supported a prevalent scenario in which fluctuations or partial melting of a parent order are essential for inducing or enhancing superconductivity. Here we present a contrary example, found in IrTe2 nanoflakes of which the superconducting dome is identified well inside the parent stripe charge ordering phase in the thickness-dependent phase diagram. The coexisting stripe charge order in IrTe2 nanoflakes significantly increases the out-of-plane coherence length and the coupling strength of superconductivity, in contrast to the doped bulk IrTe2. These findings clarify that the inherent instabilities of the parent stripe phase are sufficient to induce superconductivity in IrTe2 without its complete or partial melting. Our study highlights the thickness control as an effective means to unveil intrinsic phase diagrams of correlated van der Waals materials. © 2021, The Author(s). ; The authors thank K.T. Ko, G.Y. Jo, Y.K. Bang for fruitful discussion. We also thank H. G. Kim in Pohang Accelerator Laboratory (PAL) for the technical support. This work was supported by the Institute for Basic Science (IBS) through the Center for Artificial Low Dimensional Electronic Systems (no. IBS-R014-D1), the National Research Foundation of Korea (NRF) through SRC (Grant No. NRF-2018R1A5A6075964), and the Max Planck-POSTECH Center for Complex Phase Materials (Grant No. NRF-2016K1A4A4A01922028). J.K., M.J.K. acknowledge the support from the NRF of Korea grant (Grant No. NRF-2017R1C1B2012729 and NRF-2020R1A4A1018935). K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, Grant Number JPMXP0112101001, JSPS KAKENHI Grant Numbers JP20H00354 and the CREST (JPMJCR15F3), JST. E. F. T. thanks financial support provided by the state assignment of Minobrnauki of Russia (theme Pressure No. AAAA-A18-118020190104-3) and by Act 211 Government of the Russian Federation, contract no. 02.A03.21.0006. S.K. acknowledges the support from the NRF of Korea grant (Grant No. NRF-2019R1F1A1052026) and KISTI supercomputing center (Grant No. KSC-2019-CRE-0172). K.K. acknowledges the support from the NRF of Korea grant (Grant No. 2016R1D1A1B02008461) and Internal R&D programme at KAERI (Grant No. 5244460-21). SWC was partially supported by the NSF under Grant No. DMR-1629059.