Magnetism And Electron Correlation In Geometrically Frustrated Kondo System PrPdAl And Intermediate Valence Compound YbPd

Rare-earth based Kondo lattice compounds often show rich quantum states such as heavy fermion,unconventional superconductivity,non-Fermi liquid,and quantum critical point,owing to the competition between various interactions such as the RKKY interaction and Kondo effect.Though research on those materials has been at the focus for nearly half a century,new issues are still emerging constantly,and attracting growing interest of researchers in the field of condensced matter physics.Among them,how the geometrical frustration of local spins affects the quantum critical states is a new issue in this field.This question has been studied by our group by focusing on Ce Pd Al,where an extended quantum critical phase was found at zero temperature.In this thesis,an iso-structural compound PrPdAl with non Kramers ion will be investigated.Actually,previous neutron scattering result on polycrystalline sample has revealed a partial order of PrPdAl,i.e.,two-thirds Pr ions possess 3.1that is closed to the full magnetic moment of free Pr³⁺and one-third Pr magnetic moments is reduced to only 1.9due to strong quantum spin fluctuations at kagome lattice.This partial order is considerably different to that of Ce Pd Al,where the one-third magnetic moments is fully Kondo screened.A careful comparison of the low temperature properties of both compounds may has great implication for understanding the quantum critical states of Kramers and non Kramers magnetic ions.Moreover,rare-earth based intermediate valence compounds are another important focus of this thesis.Though typical intermediate valence compounds have non-magnetic ground states because of the strong c-f interaction,cubic compound YbPd reveals a variety of intriguing features such as intermediate valence,magnetic order,heavy fermion and structure phase transitions.Aiming to explore new quantum states in rare-earth based Kondo systems,we have performed detailed investigations on PrPdAl and YbPd by low temperature transport,magnetic and thermodynamic probes.The main research results are as follows:1:We studied the magnetism,resistivity,seebeck coefficient and thermodynamic properties such as specific heat and thermal expansion of single crystalline PrPdAl at low temperature for the first time and found two successive phase transitions at T_N1=4 K and T_N2≈1.3 K.A large electronic specific-heat coefficientγ≈940 m J/mol²at near zero temperature was found and the Kadowaki-Woods（KW）ratio of PrPdAl at low temperature is reduced to 1/25 comapared with the typical Ce-based heavy fermion compounds.This indicates that this material has anomalous quantum fluctuations in its ground state.In comparison,Ce Pd Al has only one antiferromagnetic phase transition and has normal KW ratio as observed in many heavy fermion compounds.The similar and the different quantum properties of PrPdAl and Ce Pd Al partly indicate the difference of the Kondo effect between non Kramers Pr³⁺and Kramers Ce³⁺ions.Among them,the 4f² feature of the fromer usually induces two channel Kondo effect leading to the deviating of Fermi liquid behavior at the ground state.Moreover,two step-like metamagnetic transitions are observed by isothermal magnetization measured with B//a indicating that the magnetic moments reside in the ab plane as the same as the result of neutron diffraction experiment.However,magnetic susceptibility measurements indicate c axis to be the magnetically easy axis.This strange phenomenon may hint at strong frustration and enhanced spin fluctuations in the ab planes,i.e.,the frustration in the ab planes force the magnetic moments to point along c more easily.The thermodynamic Gruneisen parameter obtained by measuring the thermal expansion reveals enhanced values around T_N2 indicating an extreme sensitivity of the lower-temperature ordering against pressure comparing to the antiferromagnetic phase transition at T_N1.We have also prepared and studied Pr Ni Al to confirm the pressure effect of PrPdAl.Indeed,Pr Ni Al,which is a chemically pressurized analog of PrPdAl,shows only one higher antiferromagnetic transition at 6.9 K and no another phase transition at lower temperature can be observed.To some extent,this result verifys the pressure sensitivity at the T_N2 phase transition,which this behavior has clear difference to the evolution of Ce（Pd,Ni）Al under chemical pressure.Above studies demonstrate that PrPdAl and related Pr-based homologues are suitable cases for clarifying the different roles of competing interactions such as RKKY interaction and Kondo effect in frustrated lattices with non-Kramers magnetic ions.2:We have grown the single crystalline of intermediate valence compound YbPd and measured its electronic and thermal transport,magnetism and thermodynamic properties.These results have confirmed the coexisting features of intermediate valence states,heavy fermion,magnetic order and structural phase transition.Sharp peaks caused by two structure phase transitions can be observed at T₁ and T₂ through specific heat measurement.However,the magnetic susceptibility measurements show no abnormal at these structure phase transitions,indicating that the valence change is not the reason for these structure phase transitions.More interestingly,we have found drastic softening of transverse and longitudinal elastic moduli and even bulk modulus when the temperature decreases from room temperature to T₁.This behavior is very different form the structure change caused by quadruploe order and charge order.Noticeablely,softening of specific transverse modulus above the temperature of structure phase transition usually means the fluctuation of order parameter,howerer,the softening of bulk modulus means the strong coupling between electron and phonons.This result supports the inference of band Jahn-Teller phase transition at T₁.Besides,we have found opposite signs of the thermal expansion at T₁ and T₂,which are indicative of the opposite effects of pressure of the two phase transitons.However,the two phase transitons can both be suppressed by pressure,indicating that the two phase transitions are strongly independent to each other.A suppressed lattice thermal conductivity and an enhanced thermopower above T₁ are also observed,which again are in line with the band Jahn-Teller scenario for the structure phase transiton at T₁.Marked,these results indicate a new idea to design thermoelectric materials,i.e.,the band Jahn-Teller effect can be used to improve ZT values through reducing the thermal conductivity and increasing the seebeck coefficient.