Study of the energy gap structure in iron-based superconductors using London penetration depth and controlled artificial disorder

A combination of London penetration depth and artificial disorder was used to probe the energy gap structure and symmetry of a few members of the iron-based superconductor systems. Information regarding the gap structure and symmetry is an important clue which helps uncover the mechanism behind the unconventional, non-BCS-type superconductors. We used the tunnel-diode resonator method to do London penetration depth measurements with high precision down to 50~mK base temperature. The disorder is introduced by electron irradiation, which was performed at the SIRIUS facility in Ecole Polytechnique (Palaiseau France) to produce point-like disorder in the materials of study. Non-magnetic defects induced by the irradiation influence each material differently depending on its underlying susceptibility to impurity scattering. The response to irradiation provides another key clue about the gap structure and symmetry of iron-based superconductors. This dissertation describes the details of the experimental work on 16 samples from the Ba$_{1-x}$K$_x$Fe$_2$As$_2$ system across the superconducting dome, with the results can be explained coherently with $s_{\pm}$-pairing symmetry. The same gap symmetry was also found in the CaKFe$_4$As$_4$ system. We found that this series is remarkably similar to the Ba$_{1-x}$K$_x$Fe$_2$As$_2$ system in many ways, consistent with other reports in literature. London penetration depth measurements and electron irradiation were also performed on FeSe, which is a unique system in the iron-based superconductor family. Surprisingly, $T_c$ in FeSe was \textit{enhanced} by irradiation which paints a different picture of superconductivity compared to Ba$_{1-x}$K$_x$Fe$_2$As$_2$ and CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$. However, the FeSe experimental data could still be explained within the (extended) $s_{\pm}$ paradigm. In conclusion, we found a strong evidence supporting the $s_{\pm}$ pairing symmetry which manifested into different gap structures in several representative systems in the iron-based superconductors family

Superconductivity of Amorphous and Crystalline Re–Lu Films

We report on superconducting properties of a novel material: rhenium-lutetium films. Different compositions of RexLu binary are explored from x ≈ 3.8 to close to pure Re stoichiometry. The highest critical temperature, up to 7 K, is obtained for x ≈ 10.5 in accordance with electron dispersive spectroscopy results. Depending on the deposition conditions, polycrystalline or amorphous films are obtainable, both of which are interesting for practical use. Crystalline structure of polycrystalline phase is identified as a non-centrosymmetric superconductor using grazing incidence x-ray diffractometry. Superconducting properties were characterized both resistively and magnetically. Magnetoresistivity and AC/DC susceptibility measurements allowed us to determine and of these films, as well as estimate coherence length and magnetic penetration depth . We also provide information on surface morphology of these films. Demonstration of superconductivity in this material justifies the point of view that Lu plays a role of group 3 transition metal in period 6 of the Periodic table of elements. Then, in analogy with Re–Nb, Re–Ti, Re–Hf and Re–Zr, one can expect that crystalline Re–Lu also breaks time-reversal symmetry. If that is proven by future experiments, in combination with noncentrosymmetric feature, these films could be used for forming nonreciprocal current devices, such as superconducting diodes, without involvement of external magnetic fields

Superconducting Polycrystalline Rhenium Films Deposited at Room Temperature

We report on magnetron deposition of thin superconducting rhenium films on sapphire substrates. During the deposition, substrates were held at ambient temperature. Critical temperature of the films is Tc~3.6 K. Films have polycrystalline structure, and grazing incidence X-ray diffractometry indicates that crystalline lattice parameters are somewhat larger compared to the bulk ones. Magnetoresistive and AC/DC susceptibilities allowed us to determine $H_{c1}$ and $H_{c2}$ of these films, as well as estimate coherence length $\xi$(0) and magnetic penetration depth $\lambda_L$(0). We also provide information on surface morphology of these films.Comment: arXiv admin note: text overlap with arXiv:2307.1631

Flux-Quanta Injection for Nonreciprocal Current Control in a Two-Dimensional Noncentrosymmetric Superconducting Structure

We designed and experimentally demonstrated a four-terminal superconducting device, a “quadristor,” that can function as a nonlatching (reversible) superconducting switch from the diode regime to the resistive state by application of a control current much smaller than the main transport current. The device uses a vortex-based superconducting-diode mechanism that is switched back and forth via the injection of flux quanta through auxiliary current leads. Our finding opens a new research area in the field of superconducting electronics

Quadristor: a novel device for superconducting electronics

We designed and experimentally demonstrated a four-terminal superconducting device which can function as a non-latching (reversible) superconducting switch from the diode regime to the resistive state by applying a control current much smaller than the main transport current. The device utilizes a vortex-based superconducting diode mechanism which is switched back and forth via the injection of flux quanta through auxiliary current leads. Various applications in superconducting electronics can be foreseen

Raman Spectroscopy as a Tool for Rapid Feedback of Perovskite Growth Crystallinity and Composition

Perovskites are ubiquitous in science and technology, from solar cells to high-temperature and exotic superconductivity. Exciting representatives of the perovskite family, such as metal-doped oxy-chalcogen calcium ruthenates, recently attracted the attention of researchers. Rapid modification of their properties is a key for research competitiveness. Crystalline structure plays an important role in the electronic properties of these materials, in particular superconductivity. Flux-assisted growth may deliver an advanced performance of these perovskites due to a higher level of crystallinity. In this article, we demonstrate that Raman spectroscopy enables a fast evaluation of improvement of the crystalline quality of Ca2RuO4 by using chlorine as a flux. In particular, ~0.6 wt.% of chlorine significantly modifies the properties of polycrystalline material. While standard energy dispersive X-ray analysis (EDX) diagnostics could not resolve the presence of chlorine and distinguish between samples with and without it, Raman spectroscopy resolved the chlorine-related features with very high accuracy at initial synthesis. Moreover, after specially performed high-temperature treatment, which sublimated chlorine from the specimen, Raman analysis confirmed the absence of chlorine and presence of required crystalline phase. The feedback received in Raman analysis is very informative, and faster than provided by any other method

Diode effect in superconducting Nb3_3Sn micro-bridges at high frequencies

The superconducting diode effect has been recently reported in a variety of systems. Various symmetry breaking mechanisms have been examined. However, frequency ranges of these potentially important devices are still obscure. We investigated superconducting micro-bridges of Nb$_{3}$Sn with a relatively large diode efficiency of $\sim$ 5% using an out-of-plane magnetic field as small as 2.5 mT; optimum magnetic fields (up to 10 mT) generate higher efficiency (up to 10%) while higher fields, $\sim$ 15-20 mT, quench the effect. The diode changes its polarity with the magnetic field. Interestingly, the bridge resistance at diode operation reaches a value with a factor of 2 times smaller than in its normal state, which is compatible with the vortex-caused mechanism of resistivity. Further confirmation of this mechanism was revealed by scanning electron microscopy: dissimilar edges of the superconductor strip can be responsible for the inversion symmetry breaking. Superconductive diode rectification was observed at frequencies up to 100 kHz, the highest reported as of today. Estimates are in favor of much higher, GHz, range of frequencies

High-Frequency Diode Effect in Superconducting Nb\u3csub\u3e3\u3c/sub\u3eSn Microbridges

The superconducting diode effect has recently been reported in a variety of systems and different symmetry-breaking mechanisms have been examined. However, the frequency range of these potentially important devices still remains obscure. We investigated superconducting microbridges of Nb3Sn in out-of-plane magnetic fields; optimum magnetic fields of ∼10 mT generate ∼10% diode efficiency, while higher fields of ∼15–20 mT quench the effect. The diode changes its polarity with magnetic field reversal. We documented superconductive diode rectification at frequencies up to 100 kHz, the highest reported as of today. Interestingly, the bridge resistance during diode operation reaches a value that is a factor of two smaller than in its normal state, which is compatible with the vortex-caused mechanism of resistivity. This is confirmed by finite-element modeling based on time-dependent Ginzburg-Landau equations. To explain experimental findings, no assumption of lattice thermal inequilibrium has been required. Dissimilar edges of the superconductor strip can be responsible for the inversion symmetry breaking by the vortex penetration barrier; visual evidence of this opportunity was revealed by scanning electron microscopy. Estimates are in favor of a much higher (GHz) range of frequencies for this type of diode

Novel results obtained by modeling of dynamic processes in superconductors: phase-slip centers as cooling engines

Based on a time-dependent Ginzburg-Landau system of equations and finite element modeling, we present novel results related with the physics of phase-slippage in superconducting wires surrounded by a non-superconductive environment. These results are obtained within our previously reported approach related to superconducting rings and superconductive gravitational wave detector transducers. It is shown that the phase-slip centers (PSCs) can be effective in originating not only positive but also negative thermal fluxes. With an appropriate design utilizing thermal diodes, PSCs can serve as cryocooling engines. Operating at $T\sim 1$ K cryostat cold-finger, they can achieve sub-Kelvin temperatures without using $^3$He

Violation of Magnetic Flux Conservation by Superconducting Nanorings

The behavior of magnetic flux in the ring-shaped finite-gap superconductors is explored from the view-point of the flux-conservation theorem which states that under the variation of external magnetic field the magnetic flux through the ring remains constant (see, e.g., [L.D. Landau and E.M. Lifshitz, Electrodynamics of Continuos Media, vol. 8 (New York, Pergamon Press, 1960), Section 42]). Our results, based on the time-dependent Ginzburg-Landau equations and COMSOL modeling, made it clear that in the general case, this theorem is incorrect. While for rings of macroscopic sizes the corrections are small, for micro and nanorings they become rather substantial. The physical reasons behind the effect are discussed. The dependence of flux deviation on ring sizes, bias temperature, and the speed of external flux evolution are explored. The detailed structure of flux distribution inside of the ring opening, as well as the electric field distribution inside the ring\u27s wire cross section are revealed. Our results and the developed finite element modeling approach can assist in elucidating various fundamental topics in superconducting nanophysics and in the advancement of nanosize superconducting circuits prior to time-consuming and costly experiments