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Register a new userSuperconductivity in Sulfur-Doped Amorphous Carbon Films
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Following our previous investigations on superconductivity in amorphous carbon (aC) based systems; we have prepared thin composite aC-W films using electron-beam induced deposition. The films did not show any sign for superconductivity above 5 K. However, local, non-percolative, superconductivity emerged at Tc = 34.4 K after treatment with sulfur at 250 C for 24 hours. The superconducting features in the magnetization curves were by far sharper compared to our previous results, and the shielding fraction increased by about an order of magnitude. Our data suggest that pairing and localized superconductivity take place in the aC-S regions, whereas phase coherence, assisted by the W inclusions, was enhanced compared to our previous samples, yet still not to the degree of achieving global phase-coherence and percolating superconductivity.
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Magnetization M(T,H) measurements performed on thoroughly characterized commercial amorphous carbon powder doped with sulfur (AC-S), revealed the occurrence of an inhomogeneous superconductivity (SC) below T_c = 38 K. The constructed magnetic field-temperature (H-T) phase diagram resembles that of type-II superconductors. However, AC-S demonstrates a number of anomalies. In particular, we observed (1) a non-monotonic behavior of the lower critical field H_c1(T); (2) a pronounced positive curvature of the upper critical field boundary that we associated with the flux lattice melting line Hm(T); (3) a spontaneous ferromagnetic-like magnetization M0 coexisting with SC. Based on the analysis of experimental results we propose a nonstandard SC state in AC-S.
Traces of superconductivity (SC) at elevated temperatures (up to 65 K) were observed by magnetic measurements in three different inhomogeneous sulfur doped amorphous carbon (a-C) systems: (a) in commercial and (b) synthesized powders and (c) in a-C thin films. (a) Studies performed on commercial (a-C) powder which contains 0.21% of sulfur, revealed traces of non-percolated superconducting phases below Tc = 65 K. The SC volume fraction is enhanced by the sulfur doping. (b) a-C powder obtained by pyrolytic decomposition of sucrose did not show any sign for SC above 5 K. This powder was mixed with sulfur and synthesized at 400 C (a-CS). The inhomogeneous products obtained, show traces of SC phases at TC= 17 and 42 K. (c) Non-superconducting composite a-C-W thin films were grown by electron-beam induced deposition. SC emerged at Tc = 34.4 K only after heat treatment with sulfur. Other parts of the pyrolytic a-CS powder, show unusual magnetic features. (i) Pronounced irreversible peaks around 55-75 K appear in the first zero-field-cooled (ZFC) sweep only. Their origin is not known. (ii) Unexpectedly these peaks are totally suppressed in the second ZFC runs measured a few minutes later. (iii) Around the peak position the field-cooled (FC) curves cross the ZFC plots (ZFC>FC). These peculiar magnetic observations also ascribed to a-CS powder prepared from the commercial a-C powder and are connected to each other. All SC and magnetic phenomena observed are intrinsic properties of the sulfur doped a-C materials. It is proposed that the a-CS systems behave similarly to well known high TC curates and/or pnictides in which SC emerges from magnetic states.
T-carbon has been proposed as a new carbon allotrope in 2011, which was successfully synthesized in recent experiments. Because of its fluffy structure, several kinds of atoms can be intercalated into T-carbon, making it a versatile candidate in various applications such as hydrogen storage, perovskite solar cells, lithium ion batteries, thermoelectrics, photocatalyst, etc. Here we show that superconductivity can appear in Na-doped T-carbon with superconducting transition temperature Tc of 11 K at ambient pressure, and Tc can be enhanced to 19 K under pressure of 14 GPa, which results from an enhancement of the electron-phonon coupling due to the shift of the phonon spectral weight to lower frequencies with the increase of pressure. The calculations on specific heat and electrical and thermal conductivities show that the normal state of the Na-doped T-carbon superconductor reveals a non-Fermi liquid behavior. The prediction of superconductivity in Na-doped T-carbon would spur great interest both experimentally and theoretically to explore novel carbon-based superconductors.
We investigate the possibility of multi-band superconductivity in SrTiO$_{3}$ films and interfaces using a two-dimensional two-band model. In the undoped compound, one of the bands is occupied whereas the other is empty. As the chemical potential shifts due to doping by negative charge carriers or application of an electric field, the second band becomes occupied, giving rise to a strong enhancement of the transition temperature and a sharp feature in the gap functions, which is manifested in the local density of states spectrum. By comparing our results with tunneling experiments in Nb-doped SrTiO$_{3}$, we find that intra-band pairing dominates over inter-band pairing, unlike other known multi-band superconductors. Given the similarities with the value of the transition temperature and with the band structure of LaAlO$_{3}$/SrTiO$_{3}$ heterostructures, we speculate that the superconductivity observed in SrTiO$_{3}$ interfaces may be similar in nature to that of bulk SrTiO$_{3}$, involving multiple bands with distinct electronic occupations.
The recent discovery of high-temperature superconductivity in single-layer iron selenide has generated significant experimental interest for optimizing the superconducting properties of iron-based superconductors through the lattice modification. For simulating the similar effect by changing the chemical composition due to S doping, we investigate the superconducting properties of high-quality single crystals of FeSe$_{1-x}$S$_{x}$ ($x$=0, 0.04, 0.09, and 0.11) using magnetization, resistivity, the London penetration depth, and low temperature specific heat measurements. We show that the introduction of S to FeSe enhances the superconducting transition temperature $T_{c}$, anisotropy, upper critical field $H_{c2}$, and critical current density $J_{c}$. The upper critical field $H_{c2}(T)$ and its anisotropy are strongly temperature dependent, indicating a multiband superconductivity in this system. Through the measurements and analysis of the London penetration depth $lambda _{ab}(T)$ and specific heat, we show clear evidence for strong coupling two-gap $s$-wave superconductivity. The temperature-dependence of $lambda _{ab}(T)$ calculated from the lower critical field and electronic specific heat can be well described by using a two-band model with $s$-wave-like gaps. We find that a $d$-wave and single-gap BCS theory under the weak-coupling approach can not describe our experiments. The change of specific heat induced by the magnetic field can be understood only in terms of multiband superconductivity.
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