High Superconducting Transition Temperature Research Articles

Fragile Topology and Flat-Band Superconductivity in the Strong-Coupling Regime.

In flat bands, superconductivity can lead to surprising transport effects. The superfluid "mobility", in the form of the superfluid weight D_{s}, does not draw from the curvature of the band but has a purely band-geometric origin. In a mean-field description, a nonzero Chern number or fragile topology sets a lower bound for D_{s}, which, via the Berezinskii-Kosterlitz-Thouless mechanism, might explain the relatively high superconducting transition temperature measured in magic-angle twisted bilayer graphene (MATBG). For fragile topology, relevant for the bilayer system, the fate of this bound for finite temperature and beyond the mean-field approximation remained, however, unclear. Here, we numerically use exact MonteCarlo simulations to study an attractive Hubbard model in flat bands with topological properties akin to those of MATBG. We find a superconducting phase transition with a critical temperature that scales linearly with the interaction strength. Then, we investigate the robustness of the superconducting state to the addition of trivial bands that may or may not trivialize the fragile topology. Our results substantiate the validity of the topological bound beyond the mean-field regime and further stress the importance of fragile topology for flat-band superconductivity.

<i>In situ</i> resistance analysis of MgB<sub>2</sub> formation process from Mg(BH<sub>4</sub>)<sub>2</sub>

Mg(BH<sub>4</sub>)<sub>2</sub> was previously studied as a promising hydrogen storage material, because of its high gravimetric storage capacities for hydrogen and suitable thermodynamic properties. Mg(BH<sub>4</sub>)<sub>2</sub> began to decompose at about 300 ℃, and formed MgB<sub>2</sub> at the end of hydrogen desorption process with the weight content of 14.9% of hydrogen lost. Aside from the prominent hydrogen storage property, the decomposition process from Mg(BH<sub>4</sub>)<sub>2</sub> to MgB<sub>2</sub> can be a potential method for fabricating superconducting MgB<sub>2</sub> at a low sintering temperature. In this paper, MgB<sub>2</sub> bulk was prepared by an <i>in-situ</i> reaction, using the Mg(BH<sub>4</sub>)<sub>2</sub> pressed block as a precursor. The resistance change of the sample was monitored during the Mg(BH<sub>4</sub>)<sub>2</sub> decomposition process and the resistance-temperature (<i>R</i>-<i>T</i>) curve of this process was recorded. Phase of MgH<sub>2</sub>, Mg and B were formed as the block slowly release its hydrogen before MgB<sub>2</sub> occurred. According to the <i>R</i>-<i>T</i> curve, the phase formation of MgB<sub>2</sub> started in a relatively low temperature of 410 ℃. Because MgB<sub>2</sub> was critically formed by Mg and B derived from Mg(BH<sub>4</sub>)<sub>2</sub>, we can compare our formation temperature with previous study on MgB<sub>2</sub> prepared by Mg and B in different particle size. The fitting result indicated that the particle size of Mg and B harvest from Mg(BH<sub>4</sub>)<sub>2</sub> decomposition was only 3.4 nm on average. The nearly atomic level mixture of Mg and B resulted in a high chemical reactivity, which was the main reason for low sintering temperature. X-ray diffraction results showed that the purity of MgB<sub>2</sub> was 95.2%, and the size of MgB<sub>2</sub> grains was 10–18 nm. SEM images showed that the MgB<sub>2</sub> bulk had a porous structure and poor connectivity, which was caused by large amount the hydrogen release during the decomposition. MgB<sub>2</sub> nanofibers can also be observed inside the bulk. In the superconductivity test, the superconducting transition temperature of the bulk was 35 K. After all, such <i>in situ</i> method to fabricate MgB<sub>2</sub> showed a great advantage in some aspects, as its low-cost precursors, low sintering temperature, small grain-size and high superconducting transition temperature in the formed MgB<sub>2</sub>, which have the potential in industrial scale fabrication of MgB<sub>2</sub> bulks and wires.

Theoretical study on the Y-Ba-H hydrides at high pressure

It has been a hot issue to search for hydrides at high pressure with high superconducting transition temperature (Tc). In this work, we studied YBaHn (n = 2∼12) at 50 - 200 GPa and found that most of YBaH2−12 are thermodynamically stable with respect to the elementary YBa and H systems at this pressure range. The most stable phases of YBaH4, YBaH5 and YBaH8 from genetic algorithm (GA) search have C2/m, P3m1 and P4/mmm symmetries at 50 GPa, and P6/mmm, R3m and P4/mmm at 150 GPa, respectively. The P4/mmm-YBaH8 was suggested to be formed by BaH2 + YH6 → YBaH8. In addition, the electric properties, phonon spectra and the superconductivity of the YBaH8 and BaH2 were calculated and discussed. The Tc of P4/mmm-YBaH8 was predicted to be 49.6 K at 50 GPa and 89.4 K at 150 GPa.

Recent development of iron-based superconducting films

<p indent="0mm">The discovery of F-doped LaFeAsO in 2008 brought a worldwide research surge of iron-based superconductors. As a new type of high-temperature superconductors, iron-based superconductors have attracted much attention due to their unique properties and significant differences from copper oxide and traditional superconductors. Iron-based superconductors have small anisotropy, high superconducting transition temperature and upper critical field, which are preferred in film preparation and applications. At the same time, the features of iron-based superconductor films, such as higher superconducting transition temperature in epitaxial films and the ability to stabilize metastable equivalents, are interesting to the researchers. The studies on films are very useful for the basic research on superconducting physics and the search of new superconducting materials. The iron-based superconductors discovered so far can be divided into four systems: (1) 1111 system: LnFeAs (O, F) (Ln= lanthanides); (2) 122 system: AEFe₂As₂ (AE = alkaline earth metals); (3) 111 system: LiFeAs and NaFeAs; (4) 11 system: FeCh (Ch=chalcogenides). 1111 system is the first reported iron-based superconductor. It has a high critical transition temperature up to <sc>58 K</sc> as well as a higher upper critical field. The disadvantage of 1111 superconductors is the lager anisotropy and the broadened superconducting transition under the magnetic field. The critical transition temperature of 122 system is lower than 1111 system, but it can still reach <sc>38 K,</sc> and its anisotropy and elemental stability is better than 1111 system. The structure of 11 system is the simplest in iron-based superconductor family. It contains no toxic element As, but the transition temperature is low <sc>(8 K).</sc> 111 system is unstable in the air, limiting its application prospects. In addition, the critical current density is very important for the application of superconductors, which has been obtained in 122-system superconducting tapes. Meanwhile, the 11-system superconducting film stands out because of its easy growth, simple structure and high transport current density, which can exceed <sc>10⁵ A/cm²</sc> even at <sc>30 T.</sc> During the past years, researchers around the world have made great progress in the fabrication and synthesis of iron-based superconducting single crystals, bulk materials, tapes and thin films. Among them, the development of thin films cannot be ignored. The fabrication of high-quality thin films has three purposes: Basic research, tape application and electronic application. Basic and application-oriented research is developing quickly. Increasing the critical current density is an important strategy for realizing commercialized and cost-effective high-temperature superconductor technology, which can be improved by microstructure engineering. Naturally grown pinning centers and artificial pinning centers are able to pin on magnetic vortices in magnetic fields, which can effectively prevent the dissipation of energy and raise critical current density. Based on the results achieved in 1111 system, doped 122 system, and 11 system, we analyzed the possible connections between the growth technique, microstructure and superconducting properties of iron-based superconducting films.

Hydrogen Pentagraphenelike Structure Stabilized by Hafnium: A High-Temperature Conventional Superconductor.

The recent discovery of H_{3}S and LaH_{10} superconductors with record high superconducting transition temperatures T_{c} at high pressure has fueled the search for room-temperature superconductivity in the compressed superhydrides. Here we introduce a new class of high T_{c} hydrides with a novel structure and unusual properties. We predict the existence of an unprecedented hexagonal HfH_{10}, with remarkably high value of T_{c} (around 213-234K) at 250GPa. As concerns the novel structure, the H ions in HfH_{10} are arranged in clusters to form a planar "pentagraphenelike" sublattice. The layered arrangement of these planar units is entirely different from the covalent sixfold cubic structure in H_{3}S and clathratelike structure in LaH_{10}. The Hf atom acts as a precompressor and electron donor to the hydrogen sublattice. This pentagraphenelike H_{10} structure is also found in ZrH_{10}, ScH_{10}, and LuH_{10} at high pressure, each material showing a high T_{c} ranging from 134 to 220K. Our study of dense superhydrides with pentagraphenelike layered structures opens the door to the exploration of a new class of high T_{c} superconductors.

Coexistence of Two Components in Magnetic Excitations of La2−xSrxCuO4 (x = 0.10 and 0.16)

To elucidate the spin dynamics of La$_{2-x}$Sr$_x$CuO$_4$, which couples with the charge degree of freedom, the spin excitations spanning the characteristic energy ($E_{\rm cross}$ $\sim$35-40 meV) are investigated for underdoped $x$ = 0.10 and optimally doped (OP) $x$ = 0.16 through inelastic neutron scattering measurements. Analysis based on a two-component picture of high-quality data clarified the possible coexistence of uprightly standing incommensurate (IC) excitations with gapless commensurate (C) excitations in the energy-momentum space. The IC component weakens the intensity toward $E_{\rm cross}$ with increasing energy transfer ($\hbar\omega$), whereas the C component strengthens at high-$\hbar\omega$ regions. The analysis results imply that the superposition of two components with a particular intensity balance forms an hourglass-shaped excitation in appearance. Furthermore, the temperature dependence of the intensity of each component exhibits different behaviors; the IC component disappears near the pseudo-gap temperature ($T$*) upon warming, whereas the C component is robust against temperature. These results suggest the itinerant electron spin nature and the localized magnetism of the parent La$_2$CuO$_4$ in IC and C excitations, respectively, similar to the spin excitations in the OP YBa$_2$Cu$_3$O$_{6+\delta}$ and HgBa$_2$CuO$_{4+\delta}$ with higher superconducting-transition temperatures.

Electrical transport measurements for superconducting sulfur hydrides using boron-doped diamond electrodes on beveled diamond anvil

A diamond anvil cell (DAC) has become an effective tool for investigating physical phenomena that occur at extremely high pressure, such as high-transition temperature superconductivity. Electrical transport measurements, which are used to characterize one of the most important properties of superconducting materials, are difficult to perform using conventional DACs. The available sample space in conventional DACs is very small and there is an added risk of electrode deformation under extreme operating conditions. To overcome these limitations, we herein report the fabrication of a boron-doped diamond microelectrode and undoped diamond insulation on a beveled culet surface of a diamond anvil. Using the newly developed DAC, we have performed in-situ electrical transport measurements on sulfur hydride H2S, which is a well-known precursor of the pressure-induced, high-transition temperature superconducting sulfur hydride, H3S. These measurements conducted under high pressures up to 192 GPa, indicated the presence of a multi-step superconducting transition, which we have attributed to elemental sulfur and possibly HS2.

Route to high-T_{c} superconductivity of hbox {BC}_{{7}} via strong bonding of boron\u2013carbon compound at high pressure

We have analyzed the compositions of boron–carbon system, in which the hbox {BC}_{{7}} compound is identified as structural stability at high pressure. The first-principles calculation is used to identify the phase diagram, electronic structure, and superconductivity of hbox {BC}_{{7}}. Our results have demonstrated that the hbox {BC}_{{7}} is thermodynamically stable in the diamond-like P{bar{4}}m2 structure at a pressure above 244 GPa, and under temperature also. Feature of chemical bonds between B and C atoms is presented using the electron localization function. The strong chemical bonds in diamond-like P{bar{4}}m2 structure are covalent bonds, and it exhibits the s–p hybridization under the pressure compression. The Fermi surface shape displays the large sheet, indicating that the diamond-like P{bar{4}}m2 phase can achieve a high superconducting transition temperature (hbox {T}_{{c}}). The outstanding property of hbox {BC}_{{7}} at 250 GPa has manifested very high-hbox {T}_{{c}} of superconductivity as 164 K, indicating that the carbon-rich system can induce the high-hbox {T}_{{c}} value as well.

Refinement of the Low-Temperature Phase with Nano SnO Doping in Ba-Ca-Cu-O Ceramics

The SnO nano dopant effects on to the low-temperature phase have studied by the selected composition of Ba2Ca3Cu6-xSnxOy (x = 0.0; 0.5; 1.0; 1.5; 2.0). The samples produced by Melt-Growth Method (MGM). As well-known, the BaCaCuO superconductor has two phases; one of them occurs in the high superconducting transition temperature at 126 K, but this phase is unstable and requires high pressure. Because of these unfavorable conditions, Ba-Ca-Cu-O superconductors out of practically be an application to technology. The other is the low-temperature phase, in which superconducting transition temperature is nearly at 90-95 K. In this work, we focused on investigating the low transition temperature phase. The phase has a Tetragonal crystal structure with a P4 / mmm symmetry and lattice constants a = 6.517 Å and c = 8.410 Å. The effects investigated by the differential scanning calorimetry, energy dispersive X-ray, scanning electron microscope, and X-ray powder diffraction method results. Structural analysis has shown that the matrix has infiltrated by the use of the SnO as a dopant. Namely, the SnO doping has resulted as the low-temperature phase has a growing percentage of 73.875% within the volume fraction of the matrix.

Superconductivity of Lanthanum Superhydride Investigated Using the Standard Four-Probe Configuration under High Pressures*Supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB33000000 and XDB25000000), the Beijing Natural Science Foundation (Grant No. Z190008), the National Natural Science Foundation of China (Grant Nos. 11575288, 11921004, 11888101, 11904391, 11834016 and 11874400), the National Key R&D Program of

Recently, the theoretically predicted lanthanum superhydride, LaH10 ± δ, with a clathrate-like structure was successfully synthesized and found to exhibit a record high superconducting transition temperature Tc ≈ 250 K at ∼ 170 GPa, opening a new route for room-temperature superconductivity. However, since in situ experiments at megabar pressures are very challenging, few groups have reported the ∼ 250 K superconducting transition in LaH10 ± δ. Here, we establish a simpler sample-loading procedure that allows a relatively large sample size for synthesis and a standard four-probe configuration for resistance measurements. Following this procedure, we successfully synthesized LaH10 ± δ with dimensions up to 10 × 20 μm2 by laser heating a thin La flake and ammonia borane at ∼ 1700 K in a symmetric diamond anvil cell under the pressure of 165 GPa. The superconducting transition at Tc ≈ 250 K was confirmed through resistance measurements under various magnetic fields. Our method will facilitate explorations of near-room-temperature superconductors among metal superhydrides.

The higher superconducting transition temperature T c and the functional derivative of T c with α 2 F(ω) for electron–phonon superconductors

This work presents an analysis of the functional derivative of the superconducting transition temperature T c with respect to the electron-phonon coupling function α 2 F(ω) [δT c/δα 2 F(ω)] and α 2 F(ω) spectrum of H3S (), in the pressure range where the high-T c was measured (155–225 GPa). The calculations are done in the framework of the Migdal–Eliashberg theory. We find for this electron–phonon superconductor, a correlation between the maximums of δT c/δα 2 F(ω) and α 2 F(ω) with its higher T c. We corroborate this behavior in other electron–phonon superconductors by analyzing data available in the literature, which suggests its validity in this type of superconductors. The correlation observed could be considered as a theoretical tool that in an electron–phonon superconductor, allows describing qualitatively the proximity to its highest T c, and determining the optimal physical conditions (pressure, alloying or doping concentration) that lead to the superconductor reaching its highest T c possible.

Discovery of an insulating parent phase in single-layer FeSe/SrTiO3 films

The single-layer FeSe/SrTiO3 (FeSe/STO) films have attracted much attention because of their simple crystal structure, distinct electronic structure and record high superconducting transition temperature (Tc). The origin of the dramatic Tc enhancement in single-layer FeSe/STO films and the dichotomy of superconductivity between single-layer and multiple-layer FeSe/STO films are still under debate. Here we report a comprehensive high resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy measurements on the electronic structure evolution with doping in single-layer and multiple-layer FeSe/STO films. We find that the single-layer FeSe/STO films have a distinct parent phase and a route of doping evolution to superconductivity that are fundamentally different from multiple-layer FeSe/STO films. The parent phase of the single-layer FeSe/STO films is insulating, and its doping evolution is very similar to that of doping a Mott insulator in cuprate superconductors. In multiple-layer FeSe/STO films, high-Tc superconductivity occurs by suppressing the nematic order in the parent compound with electron doping. The single-layer FeSe/STO films represent the first clear case in the iron-based superconductors that the parent compound is an insulator. Our observations of the unique parent state and doping evolution in the single-layer FeSe/STO films provide key insight in understanding its record high-Tc superconductivity. They also provide a new route of realizing superconductivity in iron-based superconductors that is common in high temperature cuprate superconductors.

Electronic structures and electron–phonon superconductivity of Nb2C-based MXenes

Motivated by the recent discovery of superconductivity in Nb2C MXene, we perform theoretical studies on both pristine and functionalized Nb2C using density functionals theory. First, the possible absorbing sites and structures for various functionalized groups are determined by calculating the binding energy. Second, electronic structures of Nb2C as well as all the functionalized systems are obtained which indicate that they are metallic. The band structures of both Nb2C and Nb2CS2 exhibit Dirac points near the Fermi level. Last but most importantly, the lattice dynamics and electron phonon coupling (EPC) are studied for pristine and functionalized Nb2C. Though the EPC in Nb2C is weak, the functional-groups significantly enhance the EPC and thus obtain much high superconducting transition temperature (Tc). Considering the fact that MXenes easily absorb the oxygen atoms in experiments, we suggest that the estimated Tc∼ 12 K for Nb2CO2 may explain the experimentally observed Tc∼ 12.5 K for Nb2C.

Strain-induced structural transition in DyBa2Cu3O7−x films grown by atomic layer-by-layer molecular beam epitaxy

We have used atomic layer-by-layer molecular beam epitaxy to synthesize coherently lattice-matched thin films of the high-temperature superconductor DyBa2Cu3O7−x with minimal defect density. A systematic set of x-ray reciprocal-space maps reveals tetragonal and orthorhombic structures with different twinning patterns and elucidates their evolution with the thickness, the oxygenation state, and the epitaxial relationship with the substrate. We also show that films with more pronounced orthorhombicity exhibit lower normal-state resistivities and higher superconducting transition temperatures. These findings provide guidance for the synthesis of optimized superconducting heterostructures and devices.

A picture of pseudogap phase related to charge fluxes

Recently, charge density fluctuations or charge fluxes attract strong interests in understanding the unconventional superconductivity. In this paper, a new emergent configuration in cuprates is identified by density functional theory simulations, called the charge pseudoplane, which exhibits the property of confining the dynamic charge fluxes for higher superconducting transition temperatures. It further redefines the fundamental collective excitation in cuprates as pQon with the momentum-dependent and ultrafast localization-delocalization duality. It is shown that both pseudogap and superconducting phases can be born from and intertwined through the charge flux confinement property of the charge pseudoplane region. Our experimental simulations based on the new picture provide good agreements with previous angle resolved photoemission spectroscopy and scanning tunneling microscopy results. Our work thus opens a new perspective into the origin of the pseudogap phase and other related phases in cuprates, and further provides a critical descriptor to search and design higher temperature superconductors.

Micrometer Scale Y–Ba–Cu–O SQUID Arrays Fabricated With a Focused Helium Ion Beam

We present the fabrication and testing of microarrays of superconducting quantum interference devices (SQUIDs) fabricated from the high-transition temperature superconductor YBa $_2$ Cu $_3$ O $_{7-\delta }$ (YBCO). The arrays consisted of 84 SQUIDs, each with loop area 25 $\mu \text{m}^{2}$ , connected electrically in series with a meander line structure. The entire array was contained in an area of just $\text{100}\,\mu \text{m}\times \text{100}\,\mu \text{m}$ . The Josephson junctions in the arrays were directly written into YBCO thin films with ion irradiation from a focused helium ion beam. We found that the electrical properties can be controlled by changing the diameter of the ion beam through selection of either a 20 or 5 $\mu \text{m}$ beam aperture. In particular, larger aperture or beam size results in a weaker link junction with higher operating temperature and smaller critical voltage. The voltages across the dc-biased arrays were measured as a function of magnetic field. The data exhibited maximum voltage modulations of 300 $\mu \text{V}$ at 65 K and 1 mV at 52 K for the 20 and 5 $\mu \text{m}$ beam aperture, respectively. The magnetic field periodicity for both arrays was approximately 30 $\mu$ T per flux quantum.

Demonstration of electric double layer gating under high pressure by the development of field-effect diamond anvil cell

We have developed an approach to control the carrier density in various materials under high pressure by the combination of an electric double layer transistor (EDLT) and a diamond anvil cell (DAC). In this study, this “EDLT-DAC” was applied to a Bi thin film, and here, we report the field effect under high pressure in the material. Our EDLT-DAC is a promising device for exploring unknown physical phenomena such as high transition-temperature superconductivity.

T−J model on the effective brick-wall lattice for the recently discovered high-temperature superconductor Ba2CuO3+δ

Layered copper oxides have highest superconducting transition temperatures at ambient pressure. Its mechanism remains a grand challenge in condensed matter physics. The essential physics lying in 2-dimensional copper-oxygen layers is well described by a single band Hubbard model or its strong coupling limit t-J model in 2-dimensional square lattice. Recently discovered high temperature superconductor Ba$_2$CuO$_{3+\delta}$ with $\delta \sim 0.2$ has different crystal structure with large portion of in-plane oxygen vacancies. We observe that an oxygen vacancy breaks the bond of its two neighboring copper atoms, and propose ordered vacancies in Ba$_2$CuO$_{3+\delta}$ lead to extended t-J model on an effective brick-wall lattice. For the nearest neighbor hopping, the brick-wall model can be mapped onto t-J model on honeycomb lattice. Our theory explains the superconductivity of Ba$_2$CuO$_{3+\delta}$ at high charge carrier density, and predict a time reversal symmetry broken pairing state.

Portable Solid Nitrogen Cooling System for High Transition Temperature Superconductive Electronics

In this article, we present a miniature solid nitrogen cooling system designed for high-transition temperature (high-T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ) superconductor Josephson devices operating in the temperature range of 40-55 K. The system utilizes a small fiberglass Dewar and an external pump to solidify nitrogen in thermal contact with a circuit board insert. Our cooling tests of two different Dewars with 100 mL and 200 mL capacity reveal that the systems can hold at their lowest base temperature of 42 K for 1.2 hours and 45 K for 4.7 hours for the 100 mL and 200 mL Dewars, respectively. This can provide a low power, low weight cooling solution for ultra-portable superconductive electronics and other cryogenic sensors.

Multiband nature of room-temperature superconductivity in LaH10 at high pressure

Recently, the discovery of room-temperature superconductivity (SC) was experimentally realized in the fcc phase of LaH$_{10}$ under megabar pressures. This SC of compressed LaH$_{10}$ has been explained in terms of strong electron-phonon coupling (EPC), but the mechanism of how the large EPC constant and high superconducting transition temperature $T_{\rm c}$ are attained has not yet been clearly identified. Based on the density-functional theory and the Migdal-Eliashberg formalism, we reveal the presence of two nodeless, anisotropic superconducting gaps on the Fermi surface (FS). Here, the small gap is mostly associated with the hybridized states of H $s$ and La $f$ orbitals on the three outer FS sheets, while the large gap arises mainly from the hybridized state of neighboring H $s$ or $p$ orbitals on the one inner FS sheet. Further, we find that the EPC constant of compressed YH$_{10}$ with the same sodalite-like clathrate structure is enhanced due to the two additional FS sheets, leading to a higher $T_{\rm c}$ than LaH$_{10}$. It is thus demonstrated that the multiband pairing of hybridized electronic states is responsible for the large EPC constant and room-temperature SC in compressed hydrides LaH$_{10}$ and YH$_{10}$.