The physics of magnetic quantum critical points (QCPs) is currently attracting much interest, especially after the discovery of the appearance at a QCP of unconventional superconductivity and of the causative role of spin and charge fluctuations. In this respect, one of the classes of systems whose electronic and magnetic properties deserve to be more deeply understood is that of strongly correlated 4f electron systems known as Kondo insulators [1]. Among them, SmS is known since the 70's to be a non-magnetic (Sm2+) semiconductor at ambient pressure (black phase). It becomes metallic (golden phase) under the effect of a modest pressure (0.65 GPa at room temperature), at which the Sm ions are in an intermediate valence state. So far it has been considered that, by increasing further the pressure, the golden phase could evolve into a magnetic state where the Sm ions are trivalent (Sm3+). Despite the numerous attempts to find this magnetic phase, to date there has been no clear evidence of it [2].

The availability of an intense X-ray beam which can be focussed to a tiny spot matching the size of a sample in a diamond anvil cell makes it now possible to perform high-pressure nuclear resonant scattering experiments at beamline ID22N. Here we have applied 149Sm nuclear forward scattering to study the electronic properties of SmS across its complicated pressure-temperature phase diagram. Our results give the first direct evidence that a magnetic ground state appears for SmS at a pressure of 2 GPa.


Fig. 2: Pressure dependences of (a) the magnetic hyperfine field (Bhf), (b) the quadrupole splitting (EQ) and (c) the magnetic component fraction at T = 3 K. The dashed lines through the data points are guides to the eye.


Figure 2 shows the pressure dependence of the hyperfine parameters (magnetic hyperfine field Bhf and electric quadrupole splitting EQ) and of the magnetic component fraction at 3 K. The pressure-induced phase transition from a non-magnetic state (where the hyperfine parameters at the 149Sm nuclei are zero) into a magnetically-ordered state at pc ~ 2 GPa is evident. The steep variation of both Bhf and EQ as well as the coexistence of the low and high-pressure phases in the vicinity of pc point towards a first-order transition. The absolute values of Bhf and EQ are consistent with the occurrence of a 8 crystal field ground state for the Sm3+ ions, with a magnetic moment of the order of 0.5 µB. The high-pressure magnetically-ordered phase is stable up to at least 19 GPa, as shown by the very weak pressure dependence of the hyperfine parameters and by the increase with pressure of the ordering temperature Tm (see Figure 3).


Fig. 3: Pressure dependence of the magnetic-ordering temperature Tm of SmS.


In conclusion we have used the microscopic probe of 149Sm nuclear forward scattering to obtain the first clear evidence of the existence at pressures greater than about 2 GPa of a magnetically-ordered ground state in SmS. These results constitute a strong basis for further experimental and theoretical work.

[1] For an overview see P. Wachter, in Handbook on the Physics and Chemistry of Rare Earths, Vol. 19, edited by K. A. Gschneidner Jr., L. Eyring, G.H. Lander and G.R. Choppin, (North-Holland, Amsterdam, 1994), p. 383.
[2] V.N. Antonov, B.N. Harmon and A.N. Yaresko, Phys. Rev. B 66, 165208 (2002).

Principal Publication and Authors
A. Barla (a), J. P. Sanchez (b), Y. Haga (b,c), G. Lapertot (b), B. P. Doyle (a), O. Leupold (a), R. Rüffer (a), M. M. Abd-Elmeguid (d), R. Lengsdorf (d), J. Flouquet (b), accepted by Phys. Rev. Lett.
(a) ESRF
(b) CEA, Grenoble (France)
(c) Japan Atomic Energy Research Institute, Tokai (Japan)
(d) Universität zu Köln (Germany)