Charge, lattice and magnetism across the valence crossover in EuIr₂Si₂ single crystals

Abstract

We present a detailed study of the temperature evolution of the crystal structure, specific heat, magnetic susceptibility and resistivity of single crystals of the paradigmatic valence-fluctuating compound EuIr 2 Si 2 . A comparison to stable-valent isostructural compounds EuCo 2 Si 2 (with Eu 3+ ), and EuRh 2 Si 2 , (with Eu 2+ ) reveals an anomalously large thermal expansion indicative of the lattice softening associated to valence fluctuations. A marked broad peak at temperatures around 65-75 K is observed in specific heat, susceptibility and the derivative of resistivity, as thermal energy becomes large enough to excite Eu into a divalent state, which localizes one f electron and increases scattering of conduction electrons. In addition, the intermediate valence at low temperatures manifests in a moderately renormalized electron mass, with enhanced values of the Sommerfeld coefficient in the specific heat and a Fermi-liquid-like dependence of resistivity at low temperatures. The high residual magnetic susceptibility is mainly ascribed to a Van Vleck contribution. Although the intermediate/fluctuating valence duality is to some extent represented in the interconfiguration fluctuation model commonly used to analyze data on valence-fluctuating systems, we show that this model cannot describe the different physical properties of EuIr 2 Si 2 with a single set of parameters.

Emerging 2D-ferromagnetism and strong spin-orbit coupling at the surface of valence-fluctuating EuIr2Si2

“…1 shows the temperature dependence of the Eu valence derived from bulk-sensitive X-ray absorption spectroscopy (XAS) measurements. 11 They imply that the Eu valence ν is equal to 2.5 at 200 K and gradually increases up to about 2.83 at 7 K as indicated by red stars. Undoubtedly, the bulk-sensitive XAS results are in contradiction with our surface-sensitive PE measurements.…”

“…The temperature dependence of the valence as obtained by XAS is given as inset. 11 Red stars mark the valence of EuIr 2 Si 2 in the bulk at 7 K (ν = 2.83) and 200 K (ν = 2.5) at which the PE spectra were acquired given in Supplementary Fig. S1.…”

“…Thus we conclude that the experimental observations presented above fully confirm the mixed valence of EuIr 2 Si 2 in the bulk in general, and the observed valence changes from XAS in particular. 11 Spin-orbit coupling and exchange magnetism in SISE Now we turn to the main subject of our story by focusing on the surface-related properties of EuIr 2 Si 2 which can be derived from a study of the M surface state. In Fig.…”

“…Here, we explore EuIr 2 Si 2 , which in the bulk reveals a fluctuating valence of the Eu ions continuously changing from about 2.35 at room temperature to about 2.83 at low temperature. [9][10][11] Remarkably, in the surface region we discovered very different properties. When EuIr 2 Si 2 is terminated by Si, relaxation of the topmost Si-Ir-Si block tends to create more space for the Eu ions in the fourth layer, and thus stabilizes the magnetic 4f 7 configuration, which has a slightly larger ionic radius.…”

“…The temperature-dependent ARPES experiments from 200 K down to 7 K were performed at the I05 beamline of the Diamond Light Source (DLS). 33 High quality single crystalline samples of EuIr 2 Si 2 11 were cleaved in-situ under ultra-high vacuum conditions.…”