High Pressure Studies On The Kagome Superconductors And Antiferromagnetic Topological Materials

In condensed matter physics,the macroscopic properties of crystals are determined by the electrons inside,and the properties of electrons are inseparably related to the lattice structure.For example,the antiferromagnetic triangular lattice has spin frustration,which is promising to realize quantum spin liquid.The honeycomb lattice can realize the Dirac band with linear dispersion.In the Dirac band,the effective mass of electrons is close to zero,the motion speed of electrons is very fast,and the electronic density of states is very low,which is considered to be related to many exotic physical properties,including(?)Landau level,tunable Dirac gaps,Chen gaps and quantum anomalous Hall effect,etc.In the Lieb lattice,due to the destructive interference of adjacent lattice transitions,a flat band without dispersion will appear.In the flat band,the effective mass of electrons is infinite,and the density of electron states is very large,which is thought to lead to many exotic physical effects,such as high-temperature superconductivity,ferromagnetism,Wigner crystals,and high-temperature fractional quantum Hall effects;the kagome lattice consists of triangles that share corners,because of its unique lattice structure,it has the above three characteristics,so the material system with kagome lattice has always been one of the hot research objects of condensed matter physics.Recently,a new class of kagome superconductors AV3Sb5(A=K/Rb/Cs)has been discovered.Its rich physical properties provide a good platform for everyone to study the correlation and competition of SC,CDW and non-trivial topologies in kagome lattices.Pressure is an independent dimension for studying the physical properties of materials.We can not only discover many novel physical phenomena and properties,but also understand the physical principles deeply by pressure.High pressure technology is one of the important means of condensed matter physics research.Based on this,the main content of this paper is researching the physical properties under pressure of the kagome lattice superconductor CsV3Sb5 and two other materials Pd3Pb2Se2,EuIn2As2.The dissertation is divided into seven chapters as follows:1.IntroductionIn this chapter,we mainly give a brief introduction to the research background and research content.We first introduce the geometric frustration,band structure of kagome lattices and several kagome lattice materials;Then we briefly introduce the basic physical properties of the kagome lattice superconductors AV3Sb5(A=K/Rb/Cs).Finally,two commonly used high-pressure experimental devices are introduced,namely diamond anvil cell(DAC)and piston-cylinder pressure cell(PCC).2.Concurrence of anomalous Hall effect and charge density wave in a superconducting topological kagome metalIn this chapter,we carried out electrical transport and magnetic susceptibility measurements on the kagome superconductor CsV3Sb5 at ambient-pressure,and observed a giant AHE.The anomalous Hall conductivity reaches up to 2.1 × 104Ω-1cm-1 which is larger than those observed in most of the ferromagnetic metals.Strikingly,the emergence of AHE exactly follows the higher-temperature charge-density-wave(CDW)transition with TCDW～94 K,indicating a strong correlation between the CDW state and AHE.The origin for AHE is attributed to enhanced skew scattering in the CDW state and large Berry curvature arising from the kagome lattice.These discoveries make CsV3Sb5 as an ideal platform to study the interplay among nontrivial band topology,CDW,and unconventional superconductivity.3.Physical Properties in a compressed topological kagome metal CsV3Sb5In this chapter,we conducted high-pressure electrical transport and magnetic susceptibility measurements to study kagome lattice superconductor CsV3Sb5.While the CDW transition is monotonically suppressed by pressure,superconductivity is enhanced with increasing pressure up to P1≈0.7 GPa,then an unexpected suppression on superconductivity happens until pressure around 1.1 GPa,after that,Tc is enhanced with increasing pressure again.The CDW is completely suppressed at a critical pressure P2 ≈ 2 GPa together with a maximum Tc of about 8 K.In contrast to a common dome-like behavior,the pressure-dependent Tc shows an unexpected double-peak behavior.The unusual suppression of Tc at P1 is concomitant with the rapidly damping of quantum oscillations,sudden enhancement of the residual resistivity and rapid decrease of magnetoresistance.Our discoveries indicate an unusual competition between superconductivity and CDW state in pressurized kagome latticeIn addition to the pressure phase diagrams of CDW and SC,we also systematically investigated the pressure effect of the AHE in CsV3Sb5.Our results confirm the concurrence of AHE and CDW in the compressed CsV3Sb5.Remarkably,distinct from the negative AHE at ambient pressure,a positive anomalous Hall resistivity sets in below 35 K with pressure around 0.75 GPa,which can be attributed to the Fermi surface reconstruction and/or Fermi energy shift in the new CDW phase under pressure.Our work indicates that the anomalous Hall effect in CsV3Sb5 is tunable and highly related to the band structure.4.Mechanism study of new SC phase in vanadium-based kagome materials under high pressureIn this chapter,we carried out the high-pressure electrical measurements and the high-pressure X-ray diffraction measurements on vanadium-based kagome materials CsV3Sb5 and AV6Sb6(A=K/Rb/Cs).We found the new superconducting phase appears in all four materials around 15 GPa,and this new superconducting phase was very robust.By first-principles calculations,the appearance of superconducting SC-Ⅱ at high pressure in CsV3Sb5 can be attributed to the formation of interlayer Sb2-Sb2 bonds,which enhances the three-dimensionality of the system,and pushes the pz orbital of Sb2 upward across the Fermi level,resulting in enhanced density of states and increase of Tc.In addition,in CsV3Sb5 we also found a pressure-induced topological phase transition at 20 GPa.We also found pressure-induced superconductivity in bilayer vanadium-based kagome materials for the first time,which originates from pressure-induced structural phase transitions.And this behavior is the general behavior of the AV6Sb6 family.Discovery of superconductivity in AV6Sb6 under pressure,SC-Ⅰ phase and SC-Ⅱ phase in AV3Sb5(A=K/Rb/Cs)will stimulate a general mechanistic study of superconductivity in layered vanadium-based kagome lattice materials.5.Pressure-induced superconductivity in a shandite compound Pd3Pb2Se2 with the Kagome latticeIn this chapter,high pressure electric transport and synchrotron X-ray diffraction(XRD)measurements together with the first-principles calculations are performed on a shandite compound Pd3Pb2Se2 which contains the Kagome lattice of the transition metal Pd.A pressure-induced superconducting transition is observed above 25 GPa,for the first time in the shandite compounds,although the crystal structure of the compound seems to be very robust and persists up to the highest pressure in the XRD study(76.3 GPa).The superconducting transition temperature is about 2.2 K and almost does not change with pressure.The carrier density suddenly increases around 20 GPa possibly due to the emergence of two electron pockets at the Γ point.Our work indicates that the superconductivity in Pd3Pb2Se2 is strongly correlated to its electronic structure.6.Elevating the magnetic exchange coupling in the compressed antiferromagnetic topological materials EuIn2As2In this chapter,by means of high pressure,we have significantly increased the magnetic transition temperature in an antiferromagnetic axion insulator candidate EuIn2As2.Both crystal and magnetic structures remain the same with pressure up to 17 GPa.The Neel temperature can be monotonously increased from 16 K(ambient pressure)to 65 K(14.7 GPa).This is mainly attributed to the enhancement of intralayer ferromagnetic exchange coupling by pressure.With increasing pressure up to 17 GPa,a crystalline to amorphous phase transition occurs,which impedes further enhancement of the Neel temperature.Our results show that high pressure is an effective pathway to greatly enhance the magnetic transition temperature in topological materials.It is helpful for the realization of novel quantum states at elevated temperatures.7.Summary and OutlookFinally,a summary and outlook are given for the full dissertation.