Hidden Frustration In CaFeTi₂O₆ And Uniaxial Pressure Tuning Of Iron-based Superconductors

Quantum spin liquid（QSL）describes the ground state with a large number of multibody entanglements of spin system.Superconductivity,on the other hand,is an ordered condensed state formed by electron pairing in materials at low temperatures.Surprisingly,these two completely different states are deeply related in understanding high-temperature superconductivity.P.W.Anderson pointed out that superconductiv-ity can be achieved by doping QSLs,which provided a new angle for understanding high-temperature superconductivity.The main content of this paper includes two parts,which are related to QSL and superconductivity,respectively.The first part is a study on the magnetism of Ca Fe Ti₂O₆（CFTO）,a frustrated magnet in which QSL might be achieved.The second part is a study of the tuning of superconductivity in iron-based su-perconductors with uniaxial pressure techniques.The following is a brief introduction to these two parts:1.Frustrated magnets are common platforms for realizing QSLs.There are usually two sources of magnetic frustration:geometric frustration caused by lattice symmetry and exchange frustration of non-Heisenberg interactions caused by spin-orbit coupling.We studied the magnetic properties of CFTO using high magnetic field,specific heat and electron spin resonance,and found that CFTO is a new type of frustrated magnet in which QSL might be achieved.Although the magnetic susceptibility data gives a Curie-Weiss temperature of almost zero,the magnetic moment is still not fully polarized in a magnetic field up to 60 T,indicating that the energy scale of spin exchange interaction is large.However,the system does not have long range magnetic order but exhibits spin-glass behavior below 5.5 K,indicating strong frustration in this system.In addition,we found that magnetic field suppresses the spin glass state and makes the system enter a possible QSL state,whose temperature dependence of the specific heat is^1.6.More importantly,this system cannot be described by conventional frustration mechanisms since our analysis shows that there is no geometric frustration and exchange frustration is unlikely,too.Combined with theoretical analysis,we believe that the modulation effect of orbits on exchange interactions might be the source of hidden frustration in CFTO.A complete description of this mechanism may open up a new road to QSLs.2.In the phase diagram of iron-based superconductors,superconductivity is often adjacent to antiferromagnetic phase and nematic phase.There is a complex competi-tion or coexistence relationship between them.Understanding the relationship between these different states in the phase diagram and superconductivity is helpful to under-stand the origin of iron-based superconductivity.There is a strong coupling between electronic states and lattice in iron-based superconductors.Therefore,uniaxial pres-sure can be used to tune iron-based superconductors to explore the relationship between superconductivity and other ordered phases.In terms of uniaxial pressure tuning ma-terials,the uniaxial pressure devices based on piezoelectric materials have been widely used because they can provide in situ and continuously tunable uniaxial pressure under low temperature and magnetic field environments.However,existing uniaxial pressure devices of this type often have some shortcomings.We developed a focused-ion-beam assisted large uniaxial pressure technique,which can improve the tuning range of our group’s home-made uniaxial pressure device by one or two orders of magnitude.With this method,we measured the uniaxial pressure control of Ba Fe_1.83Ni_0.17As₂samples,and gave a_cchange of⁶ K,showing the great potential of this method in tuning the physical properties of materials.In addition,we carried out the measurement of the_ccontrol of the optimally doping Ba(Fe_0.985Cr_0.015)₂(As_0.64P_0.36)₂samples using a uniaxial strain device based on piezo stacks.We found that_cis rapidly suppressed by strain along the[110]direction of the tetragonal phase with a B₂₂)strain dominant behavior.The sensitive response of_cto B₂₂)strain implies the promotion of nematic quantum critical fluctuations to superconductivity.