Strain-induced Evolution of the Electronic Structure in Semiconducting SmS

When pressure is applied to SmS it undergoes a first order, iso-structural transition. At 6.5 kbar, the material switches from black to gold in color and is accompanied by a dramatic drop in resistivity. These changes are caused by the 4f orbitals of the Sm ions hybridizing with the conduction band, transforming them from divalent to mixed valence. Since hydrostatic pressure makes angular resolved photoemission spectroscopy (ARPES) measurements difficult, doping is often used to create chemical pressure and measure the electronic structure of this golden phase. However, previous studies on YxSm1-xS at very light doping values (x<0.05) have shown metallic behavior without any change in color. Clearly, the electrical transport behavior and electronic structure of the black phase depend on whether the pressure is chemical or mechanical. This is to be expected as Y doping introduces effects besides pressure, such as an extra valence electron per atom. However, the previous ARPES results on doped SmS cannot distinguish between these effects. We perform ARPES and valence X-ray photoelectron diffraction (XPD) experiments on locally strained SmS crystals to clarify the evolution of the electronic structure in black phase SmS as a result of purely mechanical strain.