Yi Wu

Dimensionality tuning is an important method to modify the electronic states of quantum materials. However, the mechanism of such tuning in heavy fermion systems and its connection with transport properties remain largely unexplored. Here by combining molecular beam epitaxy, angle-resolved photoemission spectroscopy and transport measurements, we study the electronic states of the heavy-fermion compound CeSi 2 as a function of film thickness. In three-dimensional thick films, our measurements reveal a dispersive Kondo peak at the Fermi level ( E F ) and satellite peaks originating from crystal electric field (CEF) excitations, characteristic of heavy-fermion systems. For two-dimensional ultrathin films, the CEF satellites are largely suppressed while the ground-state Kondo peak at E F remains strong, although it develops at lower temperatures. Simultaneously, the maximum temperature T max of the magnetic resistivity, ρ m ( T ) , changes from ≈ 100 K in thick films to ≈ 35 K in ultrathin films. This can be attributed to the dimensionality-driven reduction of CEF excitations during the Kondo process, in good agreement with spectroscopic results. Our work provides direct insight to understand the quantum confinement effects on strongly correlated 4 f -electron systems and opens up new opportunities to explore emergent phenomena in two-dimensional heavy-fermion materials.

We report evidence for superconductivity with onset temperatures up to 11 K in thin films of the infinite-layer nickelate parent compound NdNiO 2 . A combination of oxide molecular beam epitaxy and atomic hydrogen reduction yields samples with high crystallinity and low residual resistivities, a substantial fraction of which exhibit superconducting transitions. We survey a large series of samples with a variety of techniques, including electrical transport, scanning transmission electron microscopy, x-ray absorption spectroscopy, and resonant inelastic x-ray scattering, to investigate the possible origins of superconductivity. We propose that superconductivity could be intrinsic to the undoped infinite-layer nickelates but suppressed by disorder due to a possibly sign-changing order parameter, a finding which would necessitate a reconsideration of the nickelate phase diagram. Another possible hypothesis is that the parent materials can be hole doped from randomly dispersed apical oxygen atoms, which would suggest an alternative pathway for achieving superconductivity.

We present an integrated procedure for the synthesis of infinite-layer nickelates using molecular-beam epitaxy with gas-phase reduction by atomic hydrogen. We first discuss challenges in the growth and characterization of perovskite NdNiO3/SrTiO3, arising from post growth crack formation in stoichiometric films. We then detail a procedure for fully reducing NdNiO3 films to the infinite-layer phase, NdNiO2, using atomic hydrogen; the resulting films display excellent structural quality, smooth surfaces, and lower residual resistivities than films reduced by other methods. We utilize the in situ nature of this technique to investigate the role that SrTiO3 capping layers play in the reduction process, illustrating their importance in preventing the formation of secondary phases at the exposed nickelate surface. A comparative bulk- and surface-sensitive study indicates that the formation of a polycrystalline crust on the film surface serves to limit the reduction process.

The phase stability, elastic and electronic properties of binary Hf–Rh compounds have been studied using first-principles calculations based on density functional theory. The equilibrium lattice constants, formation enthalpies, elastic constants, and elastic moduli are presented. Among the binary Hf–Rh compounds, Hf₃Rh₅ is the most stable with the lowest formation enthalpy. For the equiatomic HfRh phase, it tends to crystallize in the ZrIr-type structure, followed by L1₀, and then B2 at the ground state based on the analysis of formation enthalpies. Therefore, the crystal structure of the lower temperature HfRh phase is suggested to be the ZrIr-type. This conclusion is in agreement with the experimental reports in the literature. Besides, Hf₃Rh₄ are proposed to be the Pu₃Pd₄-type for the first time. Furthermore, our calculated elastic constants for Hf₂Rh, ZrIr-HfRh, L1₀-HfRh, B2-HfRh, Hf₃Rh₄, Hf₃Rh₅ and HfRh₃ can all satisfy the Born criteria, indicating their mechanical stabilities. When ZrIr-HfRh is adopted, the bulk modulus (B) increases linearly with the growing Rh atomic concentration. Meanwhile, Young's modulus linearly increases with growing shear modulus, and the compound with a higher Poisson's ratio owns a higher B/G ratio simultaneously. Overall, the results also indicate that all the considered Hf–Rh compounds should be ductile. Finally, the electronic structure is analyzed to understand the essence of structural stability of the binary compound.

MnSi _1.73 and MnSi samples were grown by spark plasma sintering (SPS) from different Si / Mn ratio powders, at different sintering temperatures, and for different sintering times. X-ray diffraction (XRD) measurements showed samples containing MnSi and MnSi _1.73 and Si phases, depending on the initial stoichiometries. Measurements of the Seebeck coefficient revealed p-type conductance for all samples. The Seebeck coefficients of the samples with MnSi pure phase were very low (about 10 μ V/K) and changed little at the temperature range measured. The Seebeck coefficients of the samples with MnSi _1.73, MnSi and Si phases were similar to that of the sample with near-pure MnSi _1.73 phase, which were larger than those of the samples with MnSi _1.73 and MnSi phases, but a little smaller than those of the samples with MnSi _1.73 and Si phases. It seems that, for the samples with the same phases, larger ratio of the strongest intensity peak of MnSi _1.73 to [Formula: see text], MnSi _1.73 to [Formula: see text] or both lead to larger Seebeck coeffiecients.