First-principle Studies On Some High-pressure Superconducting Materials

Under critical temperature,superconducting materials have the characteristics of zero resistance and complete diamagnetism,which are widely used in medicine,electronics,scientific research and other fields.According to the different superconducting pairing mechanism,superconductors can be generally divided into conventional and unconventional superconductors.The former can be explained by the classical BCS theory.Nowadays,we can numerically predict and study the superconductivity of conventional superconductors with the aid of density functional theory(DFT).It has been found that superconductivity is a rather universal phenomenon.For instance,in the case of elements,some of them are superconducting under ambient pressure(e.g.aluminum and lead).Although some of the elements are not superconducting at atmospheric pressure,they can become superconducting under high pressure,such as silicon and germanium.The critical temperatures of elementary substance superconductors are relatively low.High pressure and doping are important methods to adjust superconductivity,thus they have been widely used in experimental and theoretical studies.In conventional superconductors,the superconducting transition temperature is generally proportional to the Debye temperature,while the Debye temperature is negatively related to the atomic mass.Considering that hydrogen is the lightest element,it is reasonable to search high Tc superconductors in hydrogen rich compounds.On the other hand,the superconductivity of carbon allotropes and carbides has been discussed for a long time.Very recently,superconductivity has been detected in magic-angle graphene,which makes this direction become a hot topic in condensed matter physics and materials research.High pressure technology is an effective means to modulate the crystal structure and electronic state of materials,which is widely used in the research of superconductivity.In this paper,aiming at the scientific problems of applying high pressure to induce superconductivity and to synthesize superconducting materials,the superconductivity of hydrogen halides and two-dimensional superconductors are studied.Firstly,we study the possible high pressure stable phases of the HBr system by using the ab initio crystal structure search technique.By using the quasi harmonic approximation and taking into account the contribution of zero-point vibrational energy,we predict a stable phase of HBr with space group P4/nmm under high pressure at around 150 to 200 GPa.Results from Electron-phonon coupling calculations show that this phase exhibits superconductivity and the theoretical critical temperature(Tc)is around 73 K at 170 GPa,which set a record among the binary halogen hydrides.If half of the bromine atoms in each cell are substituted by chlorine atoms,the Tc can be further increased to around 95 K.Our results show that in addition to the small atomic mass,high coordination number,short bonds,and highly symmetrical hydrogen environment are also important factors to enhance superconductivity in hydrides.This provides a theoretical basis for searching hydrogen rich compounds with high superconducting transition temperature in the future.In addition to hydrides,two-dimensional(2D)materials are also one of the most popular systems for the study of superconductivity.The single-layer 2D superconductors are ideal materials for the preparation of superconducting nano devices.Once successfully synthesized in the experiment,they can be easily stacked together with other two-dimensional materials to fabricate superconductor/semiconductor heterojunctions by layer-by-layer techniques.Here,by using the crystal structure search techniques,we have predicted a single-layer planar carbon sheet with 4 and 8 membered rings called tetragonal graphene(T-graphene).The results show that T-graphene have intrinsic superconductivity,and its theoretical Tc at 0 GPa is around 20.8 K.More importantly,we proposed two synthesis routes to prepare such single-layer T-graphene:one is to synthesize a T-graphene potassium intercalation compound(C4K with P4/mmm symmetry)firstly at high pressure(>11.5 GPa)and then quench to ambient condition,and finally,the single-layer T-graphene can be exfoliated using the electrochemical method from the bulk C4K;Another is to peel off from bulk T-graphite C4,where C4 can be obtained from C4K by evaporating the K atoms.In addition,C4K has the similarity of structure and electronic property to the graphite intercalation compounds(GICs).Therefore,we have also explored the superconductivity of C4K and calculated that the Tc of C4K can reach 30.4 K at 0 GPa,which sets a new record for layered carbon based superconductors.These studies enrich the two-dimensional superconducting materials and provide a road map for the future experimental studies.Finally,we study the effect of electron and hole doping on the superconductivity of single-layer T-graphene.Through theoretical analysis,we find that electron doping can greatly strengthen the repulsion between electrons,thus weaken the electron-phonon coupling and harm the superconductivity.In the case of hole doping,the superconducting temperature of T-graphene increases first and decreases after it reaches a peak value of about 29.8 K when the hole doping concentration is n?3.40×1014 cm-2.It is found that the phonon vibration modes perpendicular to the carbon layer plays a leading role in the electron-phonon coupling superconducting mechanism of T-graphene.In addition,in the two-dimensional system,due to the lack of confinement in the z direction,the corresponding vibration along the z direction will be easily affected.These information should be useful in designing layered carbon based superconductors with better performance.