晶场劈裂序参量导致稀土-硼十二化合物中磁性和比热反常的理论研究

Mechanism of the Magnetic and Specific-heat Anomalies in Rare-earth Dodecaborides RB_(12)(R=Tb-Tm):an Effect of Crystal-field-splitting Order Parameter

在低温条件(TC≈3~22 K)下,除了4f电子满壳层的LuB12之外,其他稀土-硼十二化合物(简称RB12,R为稀土元素Tb,Dy,Ho,Er,Tm)存在磁性和比热的反常现象,该反常现象的机制至今仍存争议。尽管磁化率曲线上的隆起与R3+离子的反铁磁序类似,但是中子衍射谱和莫斯堡尔谱的实验均没有明确给出稀土离子的磁结构。这些化合物磁化率、熵的反常行为和低温中子谱的结果都与RB12具有幅度调制的磁结构一致。本课题组采用模型和数值计算相结合的方法来探究该反常行为的物理机制。研究表明:由于稀土离子meV量级的晶场劈裂易受热涨落扰动,晶场劈裂能不再是一个量子力学参量,而是一个热力学序参量。物理图像中轨道角动量逐渐淬灭很好地解释了磁性和比热的反常现象。结合该类晶体低温下在(111)晶向的二聚化行为和非弹性散射谱的结果,对中子散射谱中的双峰结构也给予了一个可能的解释。

Rare-earth dodecaborides RB_(12)(R=Tb,Dy,Ho,Er,Tm)are all characterized by a controversial and unresolved magnetic and specific-heat anomaly at low temperature(T_(C)≈3–22 K)except LuB12 with closed f-shell.Although the bump feature in magnetic susceptibilities resembles that of antiferromagnetically ordered R3+ions,no definite magnetic structure was identified by either neutron scattering or Mössbauer spectra.The anomalies in susceptibilities,entropy,and low temperature neutron diffraction pattern all pointed to the amplitude-modulated nature of complex magnetic structure.In view of the rather small crystal-field-splitting energies of the order of meV,we propose to treat crystal-field-splitting(CFS)energy as an order parameter rather than a quantum mechanical quantity.In this way,we found that not only the magnetic and specific-heat anomalies can be explained properly by a gradual quenching of R orbital moments,but also the low temperature satellite peaks of neutron scattering spectra can be understood in terms of the spontaneously(111)dimerised structure and inelastically absorbed rattling phonon modes.