High-Pressure Ground State ofSmB6: Electronic Conduction and Long Range Magnetic Order

Abstract

High-pressure 149Sm nuclear forward scattering of synchrotron radiation and specific heat measurements have been performed on the intermediate valent Kondo insulator SmB6. The results show that at a critical pressure p(c) approximately = 6 GPa, where the charge gap closes, a first order transition occurs to a magnetically ordered state, which shows typical features of trivalent samarium compounds. The similarity with SmS stresses the role of local correlations and gives important insight into the debate on the local or itinerant character of the f electrons in heavy fermion systems.

11B-NMR Study of SmB6under Pressure

“…The fitting errors are 1.9% at 1 bar and 3.7% at 16.5 kbar, which are much smaller than change of 1/T 1 (around 20%) in the temperature range 20-100 K. The value of E g at 1 bar is in good agreement with other NMR experiments [1,10]. However, the decrease of E g with increasing applied pressure is quite large in comparison with other experimental methods [2,9]. One of the possible explanations is that pressure brings about not only the decrease of E g , but also changes in spin correlations.…”

“…One of the possible explanations is that pressure brings about not only the decrease of E g , but also changes in spin correlations. Since the system approaches magnetic order phase with increasing pressure as indicated by previous reports [9,11], low energy spin fluctuations which contribute to the T 1 relaxation may be enhanced in the low temperature region. Another interesting feature is that 1/T 1 tends to decrease compared with the data at ambient pressure, in contrast to the pressure effect seen below 100 K. Similar behavior is observed in some Ce-based heavy fermion systems [12,13], namely 4f local moments become more itinerant with pressure, which seems opposite to the usually expected pressure effect for the Sm-based compounds.…”

“…Below about 5 K the electrical conductivity is constant due to small conductivity channel in the E d band. Experiments under applied pressure show that the insulating gap closes at pressure ≈ 40 kbar and above ≈ 60 kbar long range magnetic order appears [2,9]. * corresponding author; e-mail: gabriel.pristas@saske.sk…”

Pressure-Resistant Intermediate Valence in the Kondo InsulatorSmB6

“…This is remarkable because both the Fermi surface and magnetic ground state are expected to be determined by f-electron fluctuations, as seen in other rare earth compounds. Most striking is that the Sm valence in SmB 6 not only never reaches 3þ, but remains strongly intermediate valent at high pressures, defying predictions [29,30,39,40]. The unusual electronic configuration is surprisingly robust.…”

“…Applied pressure stabilizes a metallic and magnetic ground state over the range 4-10 GPa [28][29][30] and increases the Sm valence [31,32]. By analogy, a pressure-induced trivalent state is anticipated in SmB 6 , but it has not yet been shown spectroscopically, nor is it known how its onset correlates with either metallization or magnetic order [29].…”