Theoretical Design And Physical Properties Of Superconducting Hydride And Boride

Superconductors have potential applications and significant academic value in many fields,and the related research has been one of the focuses in condensed matter physics field.Recently,it has been found that the superconducting transition temperature of H3 S is 203 K,while that of LaH10 is 250-260 K.These important discoveries of high-temperature superconductors provide an important opportunity for the design of conventional superconductors,lead to the research on the superconductivity of hydrides,and publish a lot of theoretical and experimental papers.According to the traditional superconducting micro theory,the superconducting transition temperature of the material is directly proportional to its Debye temperature,while the Debye temperature is inversely proportional to the mass of the material.Therefore,the ultra-high transition temperature can be obtained by studying some light elements such as hydrogen.Different from other typical alkali metal elements,hydrogen element forms molecular phase at atmospheric pressure,and hydrogen molecular crystal is insulator.However,the high-density metal phase structure formed by shortening the hydrogen atom spacing under high-pressure and other external conditions is a strong candidate structure for high-temperature superconductors.However,the experiment of metal hydrogen is very challenging,thus the research of light element compounds has attracted a lot of scientific workers' attention.This is because the metallization pressure of the system is significantly reduced in the state of pre-compression under extremely high pressure.Moreover,the high-density phase formed by this kind of compounds has strong phonon vibration frequency and a large number of charges occupy the Fermi level the coupling between electron and phonon is enhanced significantly,which is the necessary factor for the formation of conventional superconductors,and supports the academic view of finding superconductors in this kind of compounds.In this paper,three typical light element compound systems,Ta-H,Th-H and BO,are selected,and the CALYPSO crystal structure method and software independently developed by the research group are used for a series of demonstration studies,and the following innovative research results are obtained:Firstly,the high-pressure crystal structure of the Ta-H system was systematically explored at 100-400 GPa.New metallic monoclinic TaH5 compounds were found at100 GPa,and more hydrogen-rich TaH10,TaH0 and TaH16 compounds were found to be stable at higher pressures.The discovery of these metallic hydrogen-rich compounds indicates that the Ta-H system may have a high superconducting transition temperature under high pressure.Based on this,the electron-phonon coupling calculations based on the density functional perturbation theory are carried out to estimate the superconducting temperature of TaH5 at 100 GPa.The results show that the superconducting temperature of TaH5 is 23 K.Further,the electron-phonon coupling calculations for TaH10,TaH14 and TaH16 are carried out at 300 – 400 GPa,and we find that the superconducting transition temperature of C2/m-TaH10 is 91 K,and the superconducting transition temperature of C2/m-TaH14 is 70 K.The superconducting transition temperature of I4/mmm-TaH16 reaches 195 K at 400 GPa,which is close to the superconducting transition temperature of H3 S.These results indicate that under extreme pressures,TaH16 with higher superconducting transition temperature can be stable in Ta-H system,which provides theoretical guidance for the design of high-temperature superconducting materials.Secondly,we studied the high-pressure structural and superconducting properties of the Th-H system.As early as the 1970 s,experimental scientists began to study the superconductivity of binary metal hydride.It was found that the hydrogen rich compound Th4h15 contained the most hydrogen at atmospheric pressure,and the superconducting transition temperature was 9 K.A key scientific question is whether it has high superconductivity of other Th-H compounds? Therefore,Th-H system is studied in this paper.The results of structure prediction show that the new Th H3 and Th2h7 are thermodynamically stable at atmospheric pressure.The calculated results show that the superconducting temperatures of the new Th H3 and Th2H7 are 6 K and0.4 K respectively.High pressure is an effective way to modulate the electronic energy band and material structure.The results show that the hydrogen rich high-pressure phase Thh18 can exist stably under high pressure.The structure has a peculiar H36 cage structure,which shows that the structure can have a high superconducting transition temperature.Therefore,further calculation of electro-acoustic coupling is carried out.The results show that superconducting temperature of the structure can reach 221 K.Further analysis shows that the unique cage structure H36 is the key to the high conductivity of the structure.At last,we designed a two-dimensional superconducting borophene-like material B₂O,and investigated the influence of tensile strain on its electronic,mechanical,and superconducting properties.Two-dimensional superconducting materials have important potential applications in superconducting micro-nano devices,such as smaller portable magnetic resonance imagers and single-spin detection and control on high-precision miniature magnetic field detectors.At present,two-dimensional superconducting materials have become a frontier area of interest due to their unique physical properties and potential applications.Borophene is the only metallic twodimensional material with monoatomic layer.It is theoretically predicted that its superconducting temperature may reach 10-20 K.However,since borophene is not stable in air or without substrate,it is necessary to design a stable borophene-like material to explore its application in nanoelectronics and superconducting micro-nano devices.Based on first-principles calculations,we designed a metallic borophene-like two-dimensional material B₂O,and found that this material can be prepared from van der Waals layered B₂O bulk materials by mechanical exfoliation.Under normal circumstances,single-layer B₂O has excellent mechanical properties,its Young's modulus is comparable to graphene,and it is superior to single-layer borophene materials in both x and y directions,and it is also superior to multilayer borophenes in the y direction.Its Poisson's ratio is comparable to single-layer borophene but larger than that of multi-layer borophene.B₂O monolayer also has a high specific modulus and is a potential mechanical reinforcement material.Electron-phonon interaction calculations show that the single layer B₂O has superconductivity,and its superconducting transition temperature is 4.62 K,which is similar to borophene.After considering the tensile strain in the x direction,the Poisson's ratio of single-layer B₂O changes from positive to negative,which enhances its fracture resistance and improves its mechanical properties,which is expected to enhance its applicability under complex stress conditions.More importantly,the superconductivity of single-layer B₂O has also been improved due to the enhancement of electron-phonon coupling,and the superconducting transition temperature has increased to 5.89 K.The discovery of single-layer B₂O provides an example for designing stable two-dimensional superconducting materials based on boron and exploring its application in nanoelectronics and superconducting micro-nano devices.