Heavy fermion state of YbNi₂Si₃ without local inversion symmetry

We have succeeded in growing a new heavy fermion compound YbNi₂Si₃ (space group: I4=mmm, #139) by the flux method. The local inversion symmetry at the ytterbium site is broken in tetragonal YbNi₂Si₃, and the site symmetry is 4mm. We have measured the electrical resistivity, magnetization, specific heat, magnetic torque and hard X-ray photoemission spectroscopic measurements. Typical sample whose chemical formula is given by YbNi_1.8Si_3.2 was used in this work. We obtained the H–T phase diagram along the c axis. Antiferromagnetic ordered state is realized below 0.35 K. A large electronic specific heat coefficient of 5 J=mol K² remains at 0.08 K, indicating that heavy fermion state is realized at very low temperatures. The magnetic torque measurement reveals that this material is in the antiferromagnetic ordered state even at ultra-low temperature down to 2 mK. We find that the ytterbium valence at 20 K is estimated as 2.98, and that the Kondo temperature is 2 K. The present work reveals that YbNi₂Si₃ is a heavy fermion compound which exhibits an antiferromagnetic order at 0.35 K.