Exploration Of New Quantum States In Ce-based Kondo Semiconductors And Frustrated Compounds

In this paper,the transport,magnetic and thermodynamic properties of Ce-based heavy-fermion semiconductors and geometric frustrated compounds are studied at the low temperature.We dicuss possible existence of a"hidden order",topological electronic state and quantum paramagnetism in frustration in CeRu2Al10,CeRu4Sn6 and CePdAl,respectively.At the same time,an exploration of new Ce-based heavy-fermion conpounds was also carried out.（1）Heavy-fermion semiconducting compound CeRu₂Al₁₀0 enters an unkown order phase below T₀=27 K.There is no consistent understanding of the order parameter.And the charge dynamics has a dramatic change at T₀,which has significant influence on the electrical and thermal transport.Most significantly,we have found that the thermal conductivity has an extra contribution in the ordered phase,which can be suppressed by magntic field.This unique feature can be attributed to low-energy magnetic excitation in the unkown phase,which is similar to the"hidden order"phase of URu2Si2.It is a common problem for these two materials that how to generate a great contribution of thermal conductivity through a tiny magnetic moment.（2）The heavy-fermion semiconductor CeRu₄Sn₆ was recently predicted to be the first Weyl semimetal realized by highly renormalized heavy-fermion bands.As the quasiparticle bands in heavy-fermion systems are sensitive to external conditions like pressure and field,extremely abnormal phenomena may emerge with changing external parameters.By performing electrical transport measures in high pressure and down to very low temperature,we found that two energy gaps of CeRu₄Sn₆ are both gradually enhenced with the increase of hydrostatic pressure.This indicates the electron-correlation nature of the hybridized band in this compound.The resistivity platform at the lowest temperature is insensitive to magnetic field and pressure,which semms to be closely related to the topologically nontrivial electronic structure,as is realized in SmB₆.How it evolves at pressures appear to be interesting topics for future investigations.（3）Frustrated Kondo-lattice systems have recently attracted much attention because of the emergence of exciting novel quantum phases.CePdAl is a prototypical heavy-fermion compound with a geometrically frustrated lattice.Reflecting the spin frustration in this compound,it reveals an antiferromagnetically ordered state with only2/3 of the Ce moments involved.The other 1/3 magnetic moments remain to be either disordered down to the lowest temperature,or Kondo screened.Instead of a quantum critical point,we have discovered a field（pressure）-induced quantum paramagnetic state（p phase）between the magnetic order phase and Fermi liquid state.It is believed that the geometrical frustration plays an important role in this phase.Furthermore,we found a sudden jump of the Fermi surface at T=0 K in the vicinity of the quantum paramagnetic state.These results suggest that a Kondo-destroying quantum-critical point can exist in a stoichiometric（non-doping）heavy-fermion metal,near a paramagnetic phase and far away from any kind of magnetic order.