Ashish Chainani

We investigate the Yb valence instabilities in the strongly correlated YbPd compound using resonant x-ray emission spectroscopy as a function of pressure across the charge-order (CO) transition. At a low temperature ( T = 30 K ) in the CO phase, the Yb 4 f valence remains nearly constant up to a pressure P L = 1.5 ± 0.2 GPa , and then increases gradually at higher pressures. In contrast, at room temperature in the normal phase, an anomalous decrease of the Yb 4 f valence is observed, without any accompanying structural phase transition. This behavior is corroborated by a systematic pressure-dependent decrease of the unit-cell volume. Based on a Birch-Murnaghan analysis, the compressibility indicates hardening of the lattice with applied pressure up to a distinct kink seen at P K = 1.6 ± 0.2 GPa . In contrast, for P > P K , the Yb 4 f valence saturates, and the compressibility reveals a counterintuitive pressure-induced softening. The results show a minimum in the compressibility of YbPd (with f 0 – f 1 hole-type mixed valence), reminiscent of the maximum in compressibility seen in the γ – α first-order isostructural phase transition in cerium (with f 0 – f 1 electron-type mixed valence).

Achieving room‐temperature ferromagnetism (RTFM) in diluted magnetic semiconductors (DMSs) has been a long‐standing challenge, with doping transition metals (TM) into oxide semiconductors being one of the most common approaches. However, the underlying physical mechanisms remain poorly understood, particularly for Co‐doped ZnO (Co:ZnO) films, which exhibit high Curie temperatures (T c ) above 300 K. A promising mechanism proposed for high‐T c ferromagnetism is the donor impurity band exchange model, in which donor electrons mediate the coupling between TM spins. Despite its theoretical significance, the nature of the donor band electrons has yet to be experimentally identified. In this work, we use polarization‐dependent, bulk‐sensitive hard x‐ray photoemission spectroscopy (HAXPES) to investigate Co‐doped ZnO epitaxial films. Our results reveal the presence of a weak electron donor band, crossing the Fermi level, and from a polarization dependence analysis, it is unambiguously identify it as having “ s‐ character.” This finding offers new insight into the ferromagnetic mechanism in Co‐doped ZnO, where Zn ¹⁺ 4 s ¹ states mediate the ferromagnetism, contributing to metallic‐like transport and Co ²⁺ spin ordering. These results not only elucidate the complementary role of dopant‐host electronic states but also open avenues for designing novel room‐temperature magnetic semiconductors, particularly in the context of 2D DMSs.

Spin-valley magnetic ordering is restricted to layered van der Waals type transition-metal dichalcogenides with ordering temperatures below 55 K. Recent theoretical studies on non-van der Waals structures have predicted spin-valley polarization induced semiconducting ferromagnetic ground states, but experimental validation is missing. We report high-Curie temperature (T_C ~ 225 K) metallic ferromagnetism with spontaneous spin-valley polarization in monolayer Cr₂Se₃ on graphene. Angle-resolved photoemission spectroscopy (ARPES) reveals systematic temperature-dependent energy shifts and splitting of localized Cr 3 d^↑-t_2g bands, accompanied by occupancy of the itinerant Cr 3d-e_g valleys. The t_2g-e_g spin-valley coupling at the K/K’ points of hexagonal Brillouin zone leads to ferromagnetic ordering. Circular dichroism in ARPES shows clear evidence of spin-valley polarized states. Comparison with bilayer and trilayer Cr₂Se₃ reveals the crucial role of valley carrier density in enhancing T_C and provides a guiding principle to realize 2D ferromagnetism at higher temperatures in non-van der Waals materials.