A ten-fold coordinated high-pressure structure in hafnium dihydrogen with increasing superconducting transition temperature induced by enhancive pressure

High pressure is an effective method to induce structural and electronic changes,creating novel high-pressure structures with excellent physical and chemical properties.Herein,we investigate the structural phase transition of hafnium dihydrogen(HfH2)in a pressure range of 0 GPa-500 GPa through the first-principles calculations and the crystal structure analysis by particle swarm optimization(CALYPSO)code.The high-pressure phase transition sequence of HfH2is I4/mmm→Cmma→P-3m1 and the two phase transition pressure points are 220.21 GPa and 359.18 GPa,respectively.A newly trigonal P-3m1 structure with 10-fold coordination first appears as an energy superior structure under high pressure.These three structures are all metallic with the internal ionic bonding of Hf and H atoms.Moreover,the superconducting transition temperature(Tc)values of Cmma at 300 GPa and P-3m1 at 500 GPa are 3.439 K and 19.737 K,respectively.Interestingly,the superconducting transition temperature of the P-3m1 structure presents an upward trend with the pressure rising,which can be attributed to the increase of electron-phonon coupling caused by the enhanced Hf-d electronic density of states at Fermi level under high pressure.

Degradation Mechanism of the Superconducting Transition Temperature in Nb Thin Films

Systemic measurements show that there is no 3D to 2D crossover in the reduction of the superconducting transition temperature Tc in Nb thin films. This result is consistent with all previous measurements while it is contrary to the prevailing understanding based on the interplay between proximity, localization, and lifetime broadening. Our study indicates that the decrease of Tc can be interpreted by the combined effects of electron-phonon coupling parameter λ and the defect scattering rate pw, being uniquely determined by their ratio λ/ρw. Other factors such as film thickness and irradiation do not produce additional effects beyond these two parameters.

Electronic Structure Investigation of 12442 Iron-Based Superconductors Based on Block-Layer Model

Superconducting transition temperature(Tc),as a crucial parameter,exploring its relationship with various macroscopic and microscopic factors helps to understand the mechanism of high-temperature superconductivity from multiple perspectives,aiding in a multidimensional comprehension of high-temperature superconductivity mechanisms.Drawing inspiration from the block-layer structure models of cuprate superconductors,we computationally investigated the interlayer interaction energies in the 12442-type iron-based superconducting materials AkCa_(2)Fe_(4)As_(4)F_(2)(Ak=K,Rb,Cs)systems based on the block-layer model and explored their relationship with Tc.We observed that an increase in interlayer combinative energy leads to a decrease in Tc,while conversely,a decrease in interlayer combination energy results in an increase in Tc.Further,we found that the contribution of the Fe 3d band structure,especially the 3dz2 orbital,to charge transfer is significant.

Fabrication and characterization of Al–Mn superconducting films for applications in TES bolometers

Superconducting transition edge sensor(TES)bolometers require superconducting films to have controllable transition temperatures T_(c)in different practical applications.The value of T_(c)strongly affects thermal conductivity and thermal noise performance of TES detectors.Al films doped with Mn(Al-Mn)of different concentrations can accomplish tunable T_(c)A magnetron sputtering machine is used to deposit the Al-Mn films in this study.Fabrication parameters including sputtering pressure and annealing process are studied and their influences on T_(c)and superconducting transition widthΔT_(c)are optimized.The Al-Mn films withΔT_(c)below 1.0 mK for T_(c)in a range of 520 mK-580 mK are successfully fabricated.

Directional Solidification and Superconductive Transition of Bi_2Sr_2 CaCu_2O_(8+δ)

By laser heated pedestal growth method, Bi_2Sr_2CaCu_2O_(8+δ) as-grown fibers with different morphologies were solidified in non-equilibrium condition at the rates of 0.1～14mm/min.At low rates,a(Sr_(1-x)Ca_x)CuO_2 phase with long and straight strips in shape and a white Cu-rich phase having a morphology of globular shape were found. With the increase of solidification rates,the phase (Sr_(1-x)Ca_x)CuO_2 becomes thinner and the Cu-rich phase becomes smaller and dispersive.When the solidification rates were last.another unstable phase(Sr_(1-x)Ca_x)Cu_2O_3 occurred.In fact,the tran- sition of the semiconductive as-grown fiber to the superconductor is a peritectoid reaction influenced by the annealing temperature and time.

Extraordinary terahertz transmission through subwavelength spindle-like apertures in NbN film

We studied numerically the temperature dependent extraordinary terahertz transmission through niobium nitride(NbN) film perforated with subwavelength spindle-like apertures. Both the resonant frequency and intensity of extraordinary terahertz transmission peaks can be greatly modified by the transition of NbN film from the normal state to the superconducting state. An enhancement of the(±1, 0) NbN/magnesium oxide(MgO) peak intensity as high as 200% is demonstrated due to the combined contribution of both the superconducting transition and the excitation of localized surface plasmons(LSPs) around the apertures. The extraordinary terahertz transmission through spindle-like hole arrays patterned on the NbN film can pave the way for us to explore novel active tuning devices.

Superconducting and structural properties of the phosphorus-rich Nb_(2)P_(5) superconductor under high pressure

Nb_(2)P_(5) is a recently discovered Bardeen,Cooper,Schrieffer theory s-wave superconductor hosting nontrivial topological nodal-line structure,which could serve as an excellent platform for studying the interplay between the superconductivity and nontrivial topological states.We report herein the superconducting and structural properties of Nb2P5studied by measuring the electrical resistivity and structure under high pressure up to ~28 GPa and performing first principles calculations.The superconducting transition temperature TCdecreases with applied pressure,and reaches a minimum of 1.9 K at ~20.0 GPa,followed by a continuous enhancement up to 50.0 GPa without saturation.Furthermore,at pressures above ~8.2 GPa,it is likely that a second superconducting transition appears and coexists with the initial one even up to~28 GPa.Theoretical calculations rule out the correlation between the second superconducting-like transition and structure.Moreover,the Nb2P5showed remarkable axial anisotropic compressibility,in which the b-axis is more compressible than the a-and c-axes.The variation of TCis discussed via considering the evolution of density of states around the Fermi level and the phonon spectrum.

MOSSBAUER EFFECT STUDY OF YBa_(2)Cu_(3-x)Sn_(x)O_(7-y)SYSTEM

Mssbauer spectra measurements reveal that Sn exists in YBa_2Cu_(3_x)Sn_xO_(7_y) system (x=0.05,0.10,0.20,0.30 and 0.40)in form of Sn^(4+) and occupies Cu(1)and Cu(2)sites.The relative rate at which Sn^(4+) occupies Cu(1)or Cu(2)sites varies with x.The superconductivity transition temperature,T_c,depresses slightly due to the substitution of Cu for Sn.

Surface electron doping induced double gap opening in T_(d)-WTe_(2)

By using scanning tunneling microscopy,we investigated the electronic evolution of T_(d)-WTe_(2) via in-situ surface alkali K atoms deposition.The T_(d)-WTe_(2) surface is electron doped upon K deposition,and as the K coverage increases,two gaps are sequentially opened near Fermi energy,which probably indicates that two phase transitions concomitantly occur during electron doping.The two gaps both show a dome-like dependence on the K coverage.While the bigger gap shows no prominent dependence on the magnetic field,the smaller one can be well suppressed and thus possibly corresponds to the superconducting transition.This work indicates that T_(d)-WTe_(2) exhibits rich quantum states closely related to the carrier concentration.

Modulation depth of series SQUIDs modified by Josephson junction area

The superconducting quantum interference device（SQUID） amplifier is widely used in the field of weak signal detection for its low input impedance, low noise, and low power consumption. In this paper, the SQUIDs with identical junctions and the series SQUIDs with different junctions were successfully fabricated. The Nb/Al-AlOx/Nb trilayer and input Nb coils were prepared by asputtering equipment. The SQUID devices were prepared by a sputtering and the lift-off method.Investigations by AFM, OM and SEM revealed the morphology and roughness of the Nb films and Nb/Al-AlOx/Nb trilayer.In addition, the current–voltage characteristics of the SQUID devices with identical junction and different junction areas were measured at 2.5 K in the He^3 refrigerator. The results show that the SQUID modulation depth is obviously affected by the junction area. The modulation depth obviously increases with the increase of the junction area in a certain range. It is found that the series SQUID with identical junction area has a transimpedance gain of 58 Ω approximately.

Review of Moirésuperconductivity and application of the Roeser-Huber formula

Moirésuperconductivity represents a new class of superconducting materials since the discovery of superconductivity in magic‐angle(1.1°)twisted bi‐layer graphene(MATBG),forming a Moirélattice with a much bigger crystal parameter as the original lattice constant of graphene.Hence,experimentally changing the Moirétwist angle,0.93°≤Θ≤1.27,leads to a variation of the superconducting properties and enables a new way of engineering 2D superconducting materials.Details of the robust superconducting state of MATBG as function of charge carrier density,temperature and applied magnetic fields are reviewed.The influence of the top/bottom hexagonal boron nitride layer thickness on the superconducting properties of MATBG was also demonstrated in the literature.In all fabricated MATBG devices,changing of the charge carrier density leads to the appearance of insulating,metallic and even ferromagnetic states,which separate several superconducting domes in the phase diagram(longitudinal resistance,Rxx,as function of temperature T and charge carrier density,n).Further works have considered MATBG combined with WSe2‐layers,twisted bi‐layer WSe2,magic‐angle trilayer graphene(MATTG),and most recently,four‐layer(MAT4G)and five‐layer(MAT5G)stacks.The differences between the layered,cuprate high‐Tc superconductors and the Moirésuperconductors are compiled together.The collected information is then used to apply the Roeser‐Huber formalism to Moiré‐type superconductivity to calculate the superconducting transition temperature,Tc,using only information of the Moirélattice and the electronic configuration.To account for the different charge carrier densities in the experimental data sets and the low charge carrier mass demands that a new parameterηmust be introduced to the Roeser‐Huber formalism to enable the description of several superconducting domes found in the phase diagram for a given Moiréangle.Doing so,the calculated data fit well to the correlation curve defined within the Roeser‐Huber formalism.

Optimization of sintering parameters for fabrication of Ag-sheathed FeSe superconducting wires

Ag-sheathed Fe1.05Se superconducting wires were fabricated with ex-situ powder in tube （PIT） process. Fe and Se powders with molar ratio of 1.05 were firstly mixed and sintered under 600 ℃ for 12 h to form precursor powders. Owing to the complex Fe-Se binary phase diagram, both superconducting tetragonal FeSe and nonsuperconducting hexagonal FeSe could be formed simultaneously during sintering. Aiming at the reduction of hexagonal FeSe phase content and higher superconducting phase volume, the influences of key parameters, including sintering time, cooling rate and heating rate, on the phase composition of sintered wires were systematically studied. Optimal sintering parameters are obtained, and the maximum tetragonal FeSe phase content of ~ 97% is achieved. Meanwhile, the effects of packing density of precursor powders on the phase composition of final wires were also discussed. Owing to the shorter length of diffusion path, more tetragonal FeSe was formed with higher packing density. The superconducting transition signal with critical temperature of ~ 7.5 K was obtained, which proved the effectiveness of our optimal sintering process.

The band structures of some transition metals and their NaCl-type compounds

The band structures of Group IVB (Ti, Zr, Hf), VB (V, Nb, Ta) and VIE (Cr, Mo, W) transition metals and some of their carbides and nitrides (TiN, ZrN, HfN, VC, NbC, TaC, VN, NbN, TaN) with NaCl-type (B1-type) structure have been calculated by using the tight-binding method within the Extended Huckel approximation (EHT). The energy bands, densities of states and crystal orbital overlap populations are given. The relationship between the bonding properties and the superconducting transition temperatures (T-c) of them is discussed. The influences of various kinds of metallic atoms and changes of bond lengths on T-c are also discussed.

Symmetry protected topological Luttinger liquids and the phase transition between them

We show that a doped spin-1/2 ladder with antiferromagnetic intra-chain and ferromagnetic inter-chain coupling is a symmetry protected topologically non-trivial Luttinger liquid.Turning on a large easy-plane spin anisotropy drives the system to a topologically-trivial Luttinger liquid.Both phases have full spin gaps and exhibit power-law superconducting pair correlation.The Cooper pair symmetry is singlet dxy in the non-trivial phase and triplet Sz? 0 in the trivial phase.The topologically non-trivial Luttinger liquid exhibits gapless spin excitations in the presence of a boundary,and it has no non-interacting or mean-field theory analog even when the fluctuating phase in the charge sector is pinned.As a function of the strength of spin anisotropy there is a topological phase transition upon which the spin gap closes.We speculate these Luttinger liquids are relevant to the superconductivity in metalized integer spin ladders or chains.

Nanostructure and thermal-optical properties of vanadium dioxide thin films

A novel nanopolycrystalline structure of vanadium dioxide thin films is deposited on silicon or fused silica substrates by reactive ion sputtering and followed by an annealing. The characteristic analysis＇shows that the films have a columnar nanostructure with an average grain of 8 nm. The resistivities as a function of ambient temperatures tested by four-point probes for as-deposited films present that the transition temperature for nanostructure of vanadium dioxide films is near 35 ℃ which lowers about 33 ℃ in comparison with the transition temperature at 68 ℃ in its microstructure.

Effect of dichroic azo-dye doped on ferroelectric liquid crystals

In order to study the effect of mixing dye in ferroelectric liquid crystal （FLC） materials, the phase transition temperature and electro-optical properties of azo dye doped FLC samples have been investigated. All the properties have been found to be changed drastically. The results have revealed that not only the SmC^＊- SmA^＊ transition temperature decreased markedly by the addition of azo-dye, but also dye-doped FLC had lower threshold voltage and saturation voltage than the pure FLC.

Synthesis and Characterization of Tl-1223 Substituted by Scandium

X-ray powder diffraction, scanning electron microscopy （SEM）, energy dispersive X-ray, electrical resistivity and AC-magnetic susceptibility measurements have been performed for polycrystalline superconducting samples of type TIBa2Ca2_xSCxCU309_δ （0.0 ≤ x 〈 0.6）. The powder X-ray diffractograms indicate that the tetragonal structure of T1-1223 is not affected by Sc-substitution whereas the lattice parameters are changed. The X-ray analysis indicates that the low-contents of scandium （x） enhance the formation of T1-1223 and reduce the secondary phases. The grain-size determined by SEM decreases as x increases. The electrical resistivity measurements show suppression in the superconducting transition temperature, Tc, and an increase in both the residual resistivity and the superconducting transition width as x increases. The suppression in Tc is attributed to the hole-filling mechanism.

Discovery of a novel 112-type iron-pnictide and La-doping induced superconductivity in Eu_(1-x)La_xFeAs_2(x=0–0.15)

We report the discovery and characterization of a novel 112-type iron pnictide EuFeAs2, with La-doping induced superconductivity in a series of Eu1- xLaxFeAs2. The polycrystalline samples were synthesized through solid state reaction method only within a very narrow temperature window around 1073 K. Small single crystals were also grown from a flux method with the size about 100μm. The crystal structure was identified by single crystal X-ray diffraction analysis as a monoclinic structure with space group of P2 1/m. From resistivity and magnetic susceptibility measurements, we found that the parent compound EuFeAs2 shows distinct anomalies probably due to the Fe2＋ related antiferromagnetic/structural phase transition near 110K and the Eu2＋ related antiferromagnetic phase transition near 40K. La-doping suppressed both phase transitions to lower temperatures and induced superconducting transitions with a Tc - 11 K for Eu0.85La0.15FeAs2.