High Superconducting Transition Temperature Research Articles

High-temperature superconductivity in iron pnictides and chalcogenides

Superconductivity develops in metals upon the formation of a coherent macroscopic quantum state of electron pairs. Iron pnictides and chalcogenides are materials that have high superconducting transition temperatures. In this Review, we describe the advances in the field that have led to higher superconducting transition temperatures in iron-based superconductors and the wide range of materials that form them. We summarize both the essential aspects of the normal state and the mechanism for superconductivity. We emphasize the degree of electron-electron correlations and their manifestation in properties of the normal state. We examine the nature of magnetism, analyse its role in driving the electronic nematicity, and discuss quantum criticality at the border of magnetism in the phase diagram. Finally, we review the amplitude and structure of the superconducting pairing, and survey the potential settings for optimizing superconductivity.

Anomalous correlation effects and unique phase diagram of electron-doped FeSe revealed by photoemission spectroscopy.

FeSe layer-based superconductors exhibit exotic and distinctive properties. The undoped FeSe shows nematicity and superconductivity, while the heavily electron-doped KxFe2−ySe2 and single-layer FeSe/SrTiO3 possess high superconducting transition temperatures that pose theoretical challenges. However, a comprehensive study on the doping dependence of an FeSe layer-based superconductor is still lacking due to the lack of a clean means of doping control. Through angle-resolved photoemission spectroscopy studies on K-dosed thick FeSe films and FeSe0.93S0.07 bulk crystals, here we reveal the internal connections between these two types of FeSe-based superconductors, and obtain superconductivity below ∼46 K in an FeSe layer under electron doping without interfacial effects. Moreover, we discover an exotic phase diagram of FeSe with electron doping, including a nematic phase, a superconducting dome, a correlation-driven insulating phase and a metallic phase. Such an anomalous phase diagram unveils the remarkable complexity, and highlights the importance of correlations in FeSe layer-based superconductors.

Effects of High-Energy Ball Milling Time on the Sintering Process of FeSe Superconductors

FeSe superconducting bulks were prepared with high energy ball milling (HEBM) aided sintering process, within which process, tetragonal β-FeSe superconducting phase could be formed directly with one step sintering process, and the formation of hexagonal δ-FeSe non-superconducting phase was effectively avoided. The influences of HEBM time on the sintering process of FeSe bulks were systematically investigated. With different HEBM time, the phase composition and morphology of precursor powders changed correspondingly, which thus influenced the final phase composition and superconducting properties of FeSe superconducting bulks. Due to the formation of FeSe bulks with larger tetragonal phase content and higher superconducting transition temperature, HEBM time of 6.0 h was recognized as the optimal parameter. Shorter HEBM time could lead to the insufficient decrease of particle size and low density. While longer HEBM time caused the formation of amorphous hexagonal δ-FeSe, which crystallized during sintering process. Thus no more tetragonal FeSe could be obtained. The FeSe superconducting bulk with the critical temperature Tc(onset) of 8.0 K was obtained with the HEBM time of 6 h, and sintering temperature of 700 oC for 12 h.

Evolution of superconductivity and magnetism in BiS2-based layered compounds

AbstractBiS2-based compounds constitute a new family of layered superconductors. In this article, several experimental findings on the superconductivity and related properties of the BiS2 family are reviewed. In particular, the focus is on the conditions required for the emergence of superconductivity and higher superconducting transition temperature in the BiS2 family: it is shown that optimizing the electron doping and crystal structure are important. Further, a few notable characteristics such as the coexistence of superconductivity and magnetism as well as the relation between the superconductivity and chargedensity- wave instability are introduced in brief.

Superconductivity pairing mechanism from cobalt impurity doping in FeSe: Spin (s±) or orbital (s++) fluctuation

In high-superconducting transition temperature ($T_{\rm c}$) iron-based superconductors, interband sign reversal ($s_{\rm \pm}$) and sign preserving ($s_{\rm ++}$) $s$-wave superconducting states have been primarily discussed as the plausible superconducting mechanism. We study Co impurity scattering effects on the superconductivity in order to achieve an important clue on the pairing mechanism using single crystal Fe$_{1-x}$Co$_x$Se and depict a phase diagram of a FeSe system. Both superconductivity and structural transition / orbital order are suppressed by the Co replacement on the Fe sites and disappear above $x$ = 0.036. These correlated suppressions represent a common background physics behind these physical phenomena in the multiband Fermi surfaces of FeSe. By comparing experimental data and theories so far proposed, the suppression of $T_{\rm c}$ against the residual resistivity is shown to be much weaker than that predicted in the case of a general sign reversal and a full gap $s_{\pm}$ models. The origin of the superconducting paring in FeSe is discussed in terms of its multiband electronic structure.

Superconductivity in Carbide Compounds

The discovery of superconductivity in MgB2 and B-doped diamond has stimulated the search for new superconducting materials in similar systems containing light elements. In the framework of BCS theory, high frequency phonons induced in a network of light elements can yield a higher superconducting transition temperature (Tc). It shows that light element superconductors provide one of the most promising paths to a room-temperature superconductor taking account of the relationship electronic state and bonding state.

BCS-BEC Crossover in Quantum Confined Superconductors

Ultranarrow superconductors are in the strong quantum confinement regime with formation of multiple coherent condensates associated with the many subbands of the electronic structure. Here, we analyze the multiband BCS-BEC crossover induced by the chemical potential tuned close to a subband bottom, in correspondence of a superconducting shape resonance. The evolution of the condensate fraction and of the pair correlation length in the ground state as functions of the chemical potential demonstrates the tunability of the BCS-BEC crossover for the condensate component of the selected subband. The extension of the crossover regime increases when the pairing strength and/or the characteristic energy of the interaction get larger. Our results indicate the coexistence of large and small Cooper pairs in the crossover regime, leading to the optimal parameter configuration for high transition temperature superconductivity.

Fabrication of Nb-sheathed FeSe0.5Te0.5 tape by an ex-situ powder-in-tube method

We report the fabrication of Nb-sheathed FeSe0.5Te0.5 superconducting tapes by the ex-situ powder-in-tube method. In our fabrications, high-quality FeSe0.5Te0.5 polycrystalline material was firstly employed as precursor and different annealing settings were performed on as-rolled FeSe0.5Te0.5 tapes as their last heat-treatment steps. Subsequently, resistance measurements and magnetization measurements were carried out on annealed FeSe0.5Te0.5 tapes. Furthermore, according to our results, the highest superconducting transition temperature and the highest self-field magnetic critical current density of our FeSe0.5Te0.5 tape reached about 15.7 K and 1 × 104 A/cm2 at 5 K, respectively.

Pressure-induced vibrational and superconducting properties of lanthanum hydrides from first principles calculations

Pressure-induced vibrational and superconducting properties of lanthanum hydrides (LaH2 and LaH3) have been studied using first principles calculations. It is found that LaH2 and LaH3 are dynamically stable in the pressure ranges of 0–39 GPa and 4–35 GPa, respectively. The character of phonon dispersion curves for LaH2 and LaH3 is analyzed under pressure. The zone-center phonon mode eigen displacements that represent infrared and Raman activity are also obtained, which are essential to the analysis of spectral experiments. The calculations based on Bardeen–Cooper–Schrieffer theory indicate that LaH2 almost has no superconducting behavior even under pressure, in reasonable agreement with previous theoretical calculations and experiments. Whereas, LaH3 presents a considerable high superconducting transition temperature (Tc) at the onset of the face centered cubic structure, while it decreases exponentially under further compression up to 25 GPa and finally almost approaches zero. Further analysis indicates that the underlying mechanism of these two distinct superconducting behaviors are closely related to the hybridization between the HO-s state and La-d state. The mode Grüneisen parameters of two hydrides are also analyzed under 35 GPa, finding that the hydrogen atoms at octahedral sites are responsible for the superconducting properties of LaH3, and in fact, the unobserved superconducting behavior in LaH2 can be interpreted as the absence of hydrogen at octahedral sites compared with LaH3.

Pressure-induced reappearance of superconductivity in the oC24 phase of lithium

Abstract Elements with low atomic numbers are expected to have high superconducting transition temperatures (Tc). Lithium, the lightest metallic element, has therefore been extensively investigated for superconductivity below 65 GPa. A new metallic phase of Li, oC24, has been recently identified above 95 GPa, but its superconductivity has remained unexplored. We report here an ab initio investigation of the phase׳s structural, electronic, dynamical, and superconducting properties at pressures of 100–200 GPa, and show that this phase is dynamically stable within the considered pressure region. The predicted Tc increases with pressure, and reaches 13.64 K at 200 GPa. This observation is a consequence of the reconstruction of the Fermi surface and the rapidly increased electronic density of states at the Fermi level under compression.

The Ferromagnetic Conditions for Ni3Al and Ni3Ga

We investigated the difference between Ni3Al and Ni3Ga in terms of their ferromagnetism. Using the analogy of our group model for high transition temperature superconductivity (HTSC), we determined the ferromagnetic conditions for Ni3Al and Ni3Ga. The ferromagnetic criteria are represented by formulas that are satisfied by only Ni3Al. The ferromagnetic criteria are not met for Ni3Ga.

Electrochemical Na-intercalation-induced high-temperature superconductivity in FeSe

Iron-chalcogenide-based superconductors have attracted much attention due to their relatively high superconducting transition temperatures (Tc) and their simple layered crystal structures. We have performed electrochemical co-intercalation of Na and propylene carbonate (PC) into FeSe, and successfully synthesized a new superconductor, Nax(PC)yFe2Se2, with Tc = 43 K. The type and amount of intercalated metal, and the electrolyte used in the intercalation affected the superconductivity. Our electrochemical intercalation method should be a useful tool for discovering new superconductors by controlling the intercalation conditions.

Electronic Raman scattering and the electron–phonon interaction in YB6

Electronic Raman scattering in YB6 and in its structural and electronic analog LaB6 has been studied in the temperature range of 10–730 K. The experimental spectra have been compared to those calculated on the basis of ab initio band structures with renormalization owing to the electron–phonon interaction. Good agreement between the calculation and experiment for LaB6 has been obtained throughout the entire temperature range. This allows the determination of the coupling constant λ ep = 0.25. To satisfactorily describe the spectra of electronic light scattering in YB6, it is necessary to introduce an additional electron relaxation channel. In this case, the estimate of the electron–phonon coupling constant λ ep is no more than 0.4; for this reason, a high superconducting transition temperature cannot be explained only by the phonon mechanism.

Epitaxial growth of Ba2YNbO6 films on biaxially-textured NiW substrates as a multifunctional single buffer layer for high Jc epitaxial YBCO film

Abstract

Superconducting Nano Wire Circuits Fabricated using a Focused Helium Beam

Electrical circuits fabricated from high-transition temperature superconductors (HTS) are very difficult to pattern due to the lack of a reliable etching process. Chemical etching can be used for large features, but undercutting limits the feature size to tens of microns. Dry etching is required for smaller features, but there is no anisotropic reactive ion etch for these materials. Therefore dry etching must be done with isotropic argon ion milling. The ion milling process generates excess heat and unfortunately oxide superconductors are sensitive to high temperature. Overheating the material by ion milling causes it to deoxygenate which turns the superconductor into an insulator. Therefore to prevent overheating the critical dimension for argon ion milling is typically limited to a few microns.

Nodal bilayer-splitting controlled by spin-orbit interactions in underdoped high-Tc cuprates.

The highest superconducting transition temperatures in the cuprates are achieved in bilayer and trilayer systems, highlighting the importance of interlayer interactions for high Tc. It has been argued that interlayer hybridization vanishes along the nodal directions by way of a specific pattern of orbital overlap. Recent quantum oscillation measurements in bilayer cuprates have provided evidence for a residual bilayer-splitting at the nodes that is sufficiently small to enable magnetic breakdown tunneling at the nodes. Here we show that several key features of the experimental data can be understood in terms of weak spin-orbit interactions naturally present in bilayer systems, whose primary effect is to cause the magnetic breakdown to be accompanied by a spin flip. These features can now be understood to include the equidistant set of three quantum oscillation frequencies, the asymmetry of the quantum oscillation amplitudes in c-axis transport compared to ab-plane transport, and the anomalous magnetic field angle dependence of the amplitude of the side frequencies suggestive of small effective g-factors. We suggest that spin-orbit interactions in bilayer systems can further affect the structure of the nodal quasiparticle spectrum in the superconducting phase. PACS numbers: 71.45.Lr, 71.20.Ps, 71.18.+y

Fabrication of (200)-Oriented TiN Films on Si (100) Substrates by DC Magnetron Sputtering

TiN films with (200) orientation were fabricated on single-crystal Si (100) substrates by dc magnetron sputtering method. We used HF solution to clean the Si substrates and achieved atomic smooth substrates with single-crystal surface. We deposited TiN films on the substrates with different sputtering conditions and observed that temperature can affect the quality of TiN films dramatically. Both X-ray diffraction and R-T measurement results show that the films deposited at a high temperature have a sharp (200) orientation and very high superconducting transition temperature of 5.4 K.

Development of Research on New High Temperature Superconductors

Since discovered in 1911, the superconductors have evolved from single element, alloy to complex compounds with multiple elements.So far the proved highest superconducting transition temperature is 164 K(under pressure). In the long time of investigation on superconductivity, the understanding on the superconducting mechanism has been promoted significantly. The BCS theory which was greatly successful in describing the conventional superconductivity now is challenged by the new phenomena in some unconventional superconductors.Therefore the investigation of high temperature superconductivity mechanism is also at the dawn of major breakthrough. In this short overview, we will give a survey on the three families of high temperature superconductors, namely cuprates, iron based superconductors.Based on the experience accumulated in past decades, we propose some ideas in exploring high temperature superconductors.

Nano Josephson superconducting tunnel junctions in YBa2Cu3O(7-δ) directly patterned with a focused helium ion beam.

Since the discovery of the high-transition-temperature superconductors (HTSs), researchers have explored many methods to fabricate superconducting tunnel junctions from these materials for basic science purposes and applications. HTS circuits operating at liquid-nitrogen temperatures (∼77 K) would significantly reduce power requirements in comparison with those fabricated from conventional superconductors. The difficulty is that the superconducting coherence length is very short and anisotropic in these materials, typically ∼2 nm in the a-b plane and ∼0.2 nm along the c axis. The electrical properties of Josephson junctions are therefore sensitive to chemical variations and structural defects on atomic length scales. To make multiple uniform HTS junctions, control at the atomic level is required. In this Letter we demonstrate all-HTS Josephson superconducting tunnel junctions created by using a 500-pm-diameter focused beam of helium ions to directly write tunnel barriers into YBa2Cu3O(7-δ) (YBCO) thin films. We demonstrate the ability to control the barrier properties continuously from conducting to insulating by varying the irradiation dose. This technique could provide a reliable and reproducible pathway for scaling up quantum-mechanical circuits operating at liquid-nitrogen temperatures, as well as an avenue to conduct novel planar superconducting tunnelling studies for basic science.

Coexistence of Bulk Superconductivity and Magnetism in CeO1−xFxBiS2

We show the observation of the coexistence of bulk superconductivity and magnetic ordering in CeO1−xFxBiS2 (x = 0–1.0) prepared by annealing under high pressure. In the CeO1−xFxBiS2 system, both superconductivity and two types of ferromagnetic-like ordering with magnetic transition temperatures of 4.5 and 7.5 K are induced upon systematic F substitution. This fact suggests that carriers generated by the substitution of O by F are supplied to not only the BiS2 superconducting layers but also the CeO blocking layers. Furthermore, the highest superconducting transition temperature is observed when the ferromagnetic-like transition temperature increases, which implies that superconductivity and this magnetic ordering are linked to each other in the CeO1−xFxBiS2 system.