Superconductor-insulator quantum phase transition in disordered FeSe thin films

Signatures of a Quantum Griffiths Phase Close to an Electronic Nematic Quantum Phase Transition

In the vicinity of a quantum critical point, quenched disorder can lead to a quantum Griffiths phase, accompanied by an exotic power-law scaling with a continuously varying dynamical exponent that diverges in the zero-temperature limit. Here, we investigate a nematic quantum critical point in the iron-based superconductor FeSe_{0.89}S_{0.11} using applied hydrostatic pressure. We report an unusual crossing of the magnetoresistivity isotherms in the nonsuperconducting normal state that features a continuously varying dynamical exponent over a large temperature range. We interpret our results in terms of a quantum Griffiths phase caused by nematic islands that result from the local distribution of Se and S atoms. At low temperatures, the Griffiths phase is masked by the emergence of a Fermi liquid phase due to a strong nematoelastic coupling and a Lifshitz transition that changes the topology of the Fermi surface.

Chemical Composition Control at the Substrate Interface as the Key for FeSe Thin-Film Growth

The strong fascination exerted by the binary compound of FeSe demands reliable engineering protocols and more effective approaches toward inducing superconductivity in FeSe thin films. Our study addresses the peculiarities in pulsed laser deposition that determine FeSe thin-film growth and focuses on the film/substrate interface, which has only been considered hypothetically in the past literature. The FeSe/MgO interface has been assumed (1) to be clean and (2) to obey lattice-matching epitaxy. Our studies reveal that both assumptions are misleading and demonstrate the tendency for domain-matching epitaxial growth, which accompanies the problem of chemical heterogeneity. We propose that homogenization of the film/substrate interface by an Fe buffer can improve the control of stoichiometry and nanostrain in a way that favors superconductivity even in ultrathin FeSe films. We will also show that on a chemically homogenized FeSe/Fe interface, the control of film texture with preparation conditions is still possible.

Induced Superconducting Transition in Ultra-Thin Iron-Selenide Films by a Mg-Coating Process

Binary Iron selenide (FeSe) thin films have been widely studied for years to unveil the high temperature superconductivity in iron-based superconductors. However, the origin of superconducting transition in this unconventional system is still under debate and worth deep investigations. In the present work, the transition from insulator to superconductor was achieved in non-superconducting FeSe ultrathin films (~8 nm) grown on calcium fluoride substrates via a simple in-situ Mg-coating by a pulsed laser deposition technique. The Mg-coated FeSe film with an optimized amount of Mg exhibited a superconducting critical temperature as 9.7 K and an upper critical field as 30.9 T. Through systematic characterizations on phase identification, carrier transport behavior and high-resolution microstructural features, the revival of superconductivity in FeSe ultrathin films is mostly attributed to the highly crystallized FeSe and extra electron doping received from external Mg-coating process. Although the top few FeSe layers are incorporated with Mg, most FeSe layers are intact and protected by a stable magnesium oxide layer. This work provides a new strategy to induce superconductivity in FeSe films with non-superconducting behavior, which might contribute to a more comprehensive understanding of iron-based superconductivity and the benefit to downstream applications such as magnetic resonance imaging, high-field magnets and electrical cables.

Challenges for Pulsed Laser Deposition of FeSe Thin Films

Anti-PbO-type FeSe shows an advantageous dependence of its superconducting properties with mechanical strain, which could be utilized as future sensor functionality. Although superconducting FeSe thin films can be grown by various methods, ultrathin films needed in potential sensor applications were only achieved on a few occasions. In pulsed laser deposition, the main challenges can be attributed to such factors as controlling film stoichiometry (i.e., volatile elements during the growth), nucleation, and bonding to the substrate (i.e., film/substrate interface control) and preventing the deterioration of superconducting properties (i.e., by surface oxidization). In the present study, we address various technical issues in thin film growth of FeSe by pulsed laser deposition, which pose constraints in engineering and reduce the application potential for FeSe thin films in sensor devices. The results indicate the need for sophisticated engineering protocols that include interface control and surface protection from chemical deterioration. This work provides important actual limitations for pulsed laser deposition (PLD) of FeSe thin films with the thicknesses below 30 nm.

Film-thickness-driven superconductor to insulator transition in cuprate superconductors

The superconductor-insulator transition induced by film thickness control is investigated for the optimally doped cuprate superconductor La_1.85Sr_0.15CuO₄. Epitaxial thin films are grown on an almost exactly matched substrate LaAlO₃ (001). Despite the wide thickness range of 6 nm to 300 nm, all films are grown coherently without significant relaxation of the misfit strain. Electronic transport measurement exhibits systematic suppression of the superconducting phase by reducing the film thickness, thereby inducing a superconductor-insulator transition at a critical thickness of ~10 nm. The emergence of a resistance peak preceding the superconducting transition is discussed based on the weak localization. X-ray photoelectron spectroscopy results show the possibility that oxygen vacancies are present near the interface.

Double quantum criticality in superconducting tin arrays-graphene hybrid

Two magnetic-field-induced quantum critical behaviors were recently discovered in two dimensional electron gas (2DEG) at LaTiO₃/SrTiO₃ interface and interpreted by disordered superconducting puddles coupled through 2DEG. In this scenario, the 2DEG is proposed to undergo a spontaneous phase separation and breaks up into locally superconducting puddles in a metallic matrix. However, as the inhomogeneous superconducting 2DEG is only illative, this proposal still lacks the direct experimental demonstration. Here, we artificially construct superconducting puddles-2DEG hybrid system by depositing tin nanoislands array on single crystalline monolayer graphene, where the two quantum critical behaviors are reproduced. Through the finite-size scaling analysis on magnetoresistivity, we show that the two quantum critical behaviors result from the intra-island and inter-island phase coherence, respectively, which are further illustrated by the phase diagram. This work provides a platform to study superconducting quantum phase transitions in a 2D system and helps to integrate superconducting devices into semiconductor technology.

Two quantum critical behaviors appear in a two dimensional electron gas (2DEG) but its origin remains to be attested. Here, Sun et al. construct superconducting puddles-2DEG hybrid system by depositing tin nano-islands array on monolayer graphene where the two quantum critical behaviors are reproduced, suggesting the formation of inhomogeneous superconducting 2DEG.

Tunable critical temperature for superconductivity in FeSe thin films by pulsed laser deposition

Stabilized FeSe thin films in ambient pressure with tunable superconducting critical temperature would be a promising candidate for superconducting electronic devices. By carefully controlling the depositions on twelve kinds of substrates using a pulsed laser deposition technique single crystalline FeSe thin films were fabricated. The high quality of the thin films was confirmed by X-ray diffraction with a full width at half maximum of 0.515° in the rocking curve and clear four-fold symmetry in φ-scan. The films have a maximum T_c ~ 15 K on the CaF₂ substrate and were stable in ambient conditions air for more than half a year. Slightly tuning the stoichiometry of the FeSe targets, the superconducting critical temperature becomes adjustable below 15 K with quite narrow transition width less than 2 K. These FeSe thin films deposited on different substrates are optimized respectively. The T_c of these optimized films show a relation with the out-of-plane (c-axis) lattice parameter of the FeSe films.

The Interface Structure of FeSe Thin Film on CaF2 Substrate and its Influence on the Superconducting Performance

The investigations into the interfaces in iron selenide (FeSe) thin films on various substrates have manifested the great potential of showing high-temperature-superconductivity in this unique system. In present work, we obtain FeSe thin films with a series of thicknesses on calcium fluoride (CaF₂) (100) substrates and glean the detailed information from the FeSe/CaF₂ interface by using scanning transmission electron microscopy (STEM). Intriguingly, we have found the universal existence of a calcium selenide (CaSe) interlayer with a thickness of approximate 3 nm between FeSe and CaF₂ in all the samples, which is irrelevant to the thickness of FeSe layers. A slight Se deficiency occurs in the FeSe layer due to the formation of CaSe interlayer. This Se deficiency is generally negligible except for the case of the ultrathin FeSe film (8 nm in thickness), in which the stoichiometric deviation from FeSe is big enough to suppress the superconductivity. Meanwhile, in the overly thick FeSe layer (160 nm in thickness), vast precipitates are found and recognized as Fe-rich phases, which brings about degradation in superconductivity. Consequently, the thickness dependence of superconducting transition temperature (T_c) of FeSe thin films is investigated and one of our atmosphere-stable FeSe thin film (127 nm) possesses the highest T_c^onset/T_c^zero as 15.1 K/13.4 K on record to date in the class of FeSe thin film with practical thickness. Our results provide a new perspective for exploring the mechanism of superconductivity in FeSe thin film via high-resolution STEM. Moreover, approaches that might improve the quality of FeSe/CaF₂ interfaces are also proposed for further enhancing the superconducting performance in this system.

Scaling analysis of field-tuned superconductor-insulator transition in two-dimensional tantalum thin films

The superconductor–insulator (SI) transition in two-dimensional Ta thin films is investigated by controlling both film thickness and magnetic field. An intriguing metallic phase appears between a superconducting and an insulating phase within a range of film thickness and magnetic field. The temperature and electric field scaling analyses are performed to investigate the nature of the SI transition in the thickness-tuned metallic and superconducting samples. The critical exponents product of νz obtained from the temperature scaling analysis is found to be approximately 0.67 in the entire range of film thickness. On the other hand, an apparent discrepancy is measured in the product of ν(z + 1) by the electric filed analysis. The product values are found to be about 1.37 for the superconducting films and about 1.86 for the metallic films respectively. We find that the discrepancy is the direct consequence of electron heating that introduces additional dissipation channels in the metallic Ta films.

Electronic structure and superconductivity of FeSe-related superconductors

FeSe superconductors and their related systems have attracted much attention in the study of iron-based superconductors owing to their simple crystal structure and peculiar electronic and physical properties. The bulk FeSe superconductor has a superconducting transition temperature (Tc) of ~8 K and it can be dramatically enhanced to 37 K at high pressure. On the other hand, its cousin system, FeTe, possesses a unique antiferromagnetic ground state but is non-superconducting. Substitution of Se with Te in the FeSe superconductor results in an enhancement of Tc up to 14.5 K and superconductivity can persist over a large composition range in the Fe(Se,Te) system. Intercalation of the FeSe superconductor leads to the discovery of the AxFe2-ySe2 (A = K, Cs and Tl) system that exhibits a Tc higher than 30 K and a unique electronic structure of the superconducting phase. A recent report of possible high temperature superconductivity in single-layer FeSe/SrTiO3 films with a Tc above 65 K has generated much excitement in the community. This pioneering work opens a door for interface superconductivity to explore for high Tc superconductors. The distinct electronic structure and superconducting gap, layer-dependent behavior and insulator-superconductor transition of the FeSe/SrTiO3 films provide critical information in understanding the superconductivity mechanism of iron-based superconductors. In this paper, we present a brief review of the investigation of the electronic structure and superconductivity of the FeSe superconductor and related systems, with a particular focus on the FeSe films.

Excess conductivity and Berezinskii-Kosterlitz-Thouless transition in superconducting FeSe thin films

Temperature-dependent electronic transport in the vicinity of the superconducting transition is reported for quasi-two-dimensional textured FeSe thin films. The conspicuous rounding of the resistive transitions and large transition widths are indications of excess conductivity due to thermal Cooper-pair fluctuations, which can be well-described by the two-dimensional Aslamazov-Larkin theory. The Halperin-Nelson form of the sheet resistance between the phase-ordering temperature TBKT and the mean-field temperature TMF, and the power-law behaviour of the voltage-current characteristics, with a distinctive jump of the exponent at TBKT, are indicative of a Berezinskii-Kosterlitz-Thouless transition. The complementary results suggest a two-dimensional character of superconductivity in the FeSe films and allow a quantitative estimate of the Ginzburg number Gi.

Thin film growth of Fe-based superconductors: from fundamental properties to functional devices. A comparative review

Fe-based superconductors bridge a gap between MgB2 and the cuprate high temperature superconductors as they exhibit multiband character and transition temperatures up to around 55 K. Investigating Fe-based superconductors thus promises answers to fundamental questions concerning the Cooper pairing mechanism, competition between magnetic and superconducting phases, and a wide variety of electronic correlation effects. The question addressed in this review is, however, is this new class of superconductors also a promising candidate for technical applications? Superconducting film-based technologies range from high-current and high-field applications for energy production and storage to sensor development for communication and security issues and have to meet relevant needs of today’s society and that of the future. In this review we will highlight and discuss selected key issues for Fe-based superconducting thin film applications. We initially focus our discussion on the understanding of physical properties and actual problems in film fabrication based on a comparison of different observations made in the last few years. Subsequently we address the potential for technological applications according to the current situation.