arXiv subjects

Evgueni F. Talantsev

Publications and source records attributed to Evgueni F. Talantsev.

4 recordsLinked to original sources

arXiv2019

Classifying superconductivity in Moiré graphene superlattices

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 10^(12) 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 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.

cond-mat.supr-con↗

arXiv2019

Classifying hydrogen-rich superconductors

The era of near-room-temperature superconductivity started after experimental discovery by Drozdov et al (2015 Nature 525 73) who found that compressed H3S exhibits superconducting transition at Tc = 203 K. To date, the record near-room-temperature superconductivity stands with another hydrogen-rich highly compressed compound, LaH10 (Somayazulu et al 2019 Phys. Rev. Lett. 122 027001), which has critical temperature of Tc>240 K. In this paper, we analyse available upper critical field, Bc2(T), data for LaH10 (Drozdov et al 2019 Nature 569 528) and report that this compound in all considered scenarios has the ratio of Tc to the Fermi temperature, Tf, 0.009 < Tc/Tf < 0.038, which is typical range for unconventional superconductors. In attempt to extend our finding, we examined experimental Bc2(T) data for superconductors in the palladium-hydrogen (PdHx) and thorium-hydrogen-deiterium (ThH-ThD) systems and surprisingly find that superconductors in both these systems also fall into unconventional superconductors band. Taking in account that H3S has the ratio of 0.012 < Tc/Tf < 0.039 (Talantsev 2019 Mod. Phys. Lett. B 33 1950195) we come to conclusion that in the Uemura plot all discovered to date hydrogen-rich superconductors, i.e., Th4H15-Th4D15, PdHx, H3S and LaH10 (in this list we do not include NbTiHx, PtHx, SiH4, and PH3 for which experimental data beyond Tc are unknown), lie in same band as all unconventional superconductors, particularly heavy fermions, fullerenes, pnictides, and cuprates, and former should be classified as a new class of unconventional superconductors.

cond-mat.supr-con↗

arXiv2019

Classifying induced superconductivity in atomically thin Dirac-cone materials

Recently, Kayyalha et al. (2019 Phys. Rev. Lett. 122 047003) reported on anomalous enhancement of the self-field critical currents, Ic(sf,T), at low temperatures in Nb/BiSbTeSe2-nanoribbon/Nb Josephson junctions. The enhancement was attributed to the low-energy Andreev bound states arising from winding of the electronic wave function around the circumference of the topological insulator BiSbTeSe2 nanoribbon. In this paper, we show that identical enhancement in Ic(sf,T) and in the upper critical field, Bc2(T), at approximately same reduced temperatures, were reported by several research groups in atomically thin junctions based on a variety of Dirac-cone materials (DCM) earlier. Our analysis shows that in all these S/DCM/S systems the enhancement is due to a new superconducting band opening. Taking in account that several intrinsic superconductors also exhibit the effect of new superconducting band(s) opening when sample thickness becomes thinner than the ground state out-of-plane coherence length, we strength our previous proposal that there is a new phenomenon of additional superconducting band(s) opening in atomically thin films.

cond-mat.supr-con↗

arXiv2017

On the fundamental definition of critical current in superconductors

Transport critical current, Ic, is usually defined in terms of a threshold electric field criterion, Ec, with the convention Ec = 1 microVolt/cm, chosen somewhat arbitrarily to provide "reasonably small" electric power dissipation in practical devices. Thus Ic is not fundamentally determined. However, recently it was shown, that the self-field critical current of thin-film superconductors is indeed a fundamental property governed only by the London penetration depth. Here we reconsider the definition of critical current and resolve the apparent contradiction. We measure the field distribution across the width of 2G high-Tc superconducting tapes as the transport current is increased to Ic. We identify a threshold current, Ic_surfB, at which two physical events occur simultaneously: (i) an abrupt crossover from non-linear to linear dependence of the local surface magnetic flux density, Bsurf, as a function of transport current measured at any point on the superconductor surface. This effect was not reported previously. (ii) the appearance of a non-zero electric field, just above of the sensitivity of measuring system. In the present examples Ic_surfB is 10-15% lower than Ic_E determined by the Ec criterion. We propose the transition of Bsurf(I) from non-linear to linear as the most reliable and more fundamental technique for measuring transport critical currents.

cond-mat.supr-con↗