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Molecular beam epitaxy (MBE), a workhorse of the semiconductor industry, has progressed rapidly in the last few decades in the development of novel materials. Recent developments in condensed matter and materials physics have seen the rise of many novel quantum materials that require ultra-clean and high-quality samples for fundamental studies and applications. Novel oxide-based quantum materials synthesized using MBE have advanced the development of the field and materials. In this review, we discuss the recent progress in new MBE techniques that have enabled synthesis of complex oxides that exhibit ‘quantum’ phenomena, including superconductivity and topological electronic states. We show how these techniques have produced breakthroughs in the synthesis of 4d and 5d oxide films and heterostructures that are of particular interest as quantum materials. These new techniques in MBE offer a bright future for the synthesis of ultra-high quality oxide quantum materials.

 

We performed polarized reflection and transmission measurements on the layered conducting oxide PdCoO₂thin films. For theab-plane, an optical peak near Ω ≈ 750 cm⁻¹drives the scattering rate 1/τ(ω) and effective massm^*(ω) of the Drude carrier to increase and decrease respectively forω ≧ Ω. For thec-axis, a longitudinal optical phonon (LO) is present at Ω as evidenced by a peak in the loss function Im[−1/ε_c(ω)]. Further polarized measurements in different light propagation (q) and electric field (E) configurations indicate that the Peak at Ω results from an electron-phonon coupling of theab-plane carrier with thec-LO phonon, which leads to the frequency-dependent 1/τ(ω) andm^*(ω). This unusual interaction was previously reported in high-temperature superconductors (HTSC) between a non-Drude, mid-infrared (IR) band and ac-LO. On the contrary, it is the Drude carrier that couples in PdCoO₂. The coupling between theab-plane Drude carrier andc-LO suggests that thec-LO phonon may play a significant role in the characteristicab-plane electronic properties of PdCoO₂, including the ultra-high dc-conductivity, phonon-drag, and hydrodynamic electron transport.

 

Using molecular beam epitaxy (MBE) to grow multielemental oxides (MEOs) is generally challenging, partly due to difficulty in stoichiometry control. Occasionally, if one of the elements is volatile at the growth temperature, stoichiometry control can be greatly simplified using adsorption-controlled growth mode. Otherwise, stoichiometry control remains one of the main hurdles to achieving high-quality MEO film growths. Here, we report another kind of self-limited growth mode, dubbed diffusion-assisted epitaxy, in which excess species diffuses into the substrate and leads to the desired stoichiometry, in a manner similar to the conventional adsorption-controlled epitaxy. Specifically, we demonstrate that using diffusion-assisted epitaxy, high-quality epitaxial CuCrO2 films can be grown over a wide growth window without precise flux control using MBE.

 

Abstract PdCoO 2 layered delafossite is the most conductive compound among metallic oxides, with a room-temperature resistivity of nearly $$2\,\mu \Omega \,{{{{{\rm{cm}}}}}}$$ 2 μ Ω cm , corresponding to a mean free path of about 600 Å. These values represent a record considering that the charge density of PdCoO 2 is three times lower than copper. Although its notable electronic transport properties, PdCoO 2 collective charge density modes (i.e. surface plasmons) have never been investigated, at least to our knowledge. In this paper, we study surface plasmons in high-quality PdCoO 2 thin films, patterned in the form of micro-ribbon arrays. By changing their width W and period 2 W , we select suitable values of the plasmon wavevector q , experimentally sampling the surface plasmon dispersion in the mid-infrared electromagnetic region. Near the ribbon edge, we observe a strong field enhancement due to the plasmon confinement, indicating PdCoO 2 as a promising infrared plasmonic material. 

Abstract When a three-dimensional material is constructed by stacking different two-dimensional layers into an ordered structure, new and unique physical properties can emerge. An example is the delafossite PdCoO 2 , which consists of alternating layers of metallic Pd and Mott-insulating CoO 2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO 2 , we revealed its layer-resolved electronic structure combining standing-wave X-ray photoemission spectroscopy and ab initio many-body calculations. Experimentally, we have decomposed the measured VB spectrum into contributions from Pd and CoO 2 layers. Computationally, we find that many-body interactions in Pd and CoO 2 layers are highly different. Holes in the CoO 2 layer interact strongly with charge-transfer excitons in the same layer, whereas holes in the Pd layer couple to plasmons in the Pd layer. Interestingly, we find that holes in states hybridized across both layers couple to both types of excitations (charge-transfer excitons or plasmons), with the intensity of photoemission satellites being proportional to the projection of the state onto a given layer. This establishes satellites as a sensitive probe for inter-layer hybridization. These findings pave the way towards a better understanding of complex many-electron interactions in layered quantum materials.