Hard x-ray photoelectron spectroscopy (HAXPES) is establishing itself as an essential technique for the characterisation of materials. The number of specialised photoelectron spectroscopy techniques making use of hard x-rays is steadily increasing and ever more complex experimental designs enable truly transformative insights into the chemical, electronic, magnetic, and structural nature of materials. This paper begins with a short historic perspective of HAXPES and spans from developments in the early days of photoelectron spectroscopy to provide an understanding of the origin and initial development of the technique to state-of-the-art instrumentation and experimental capabilities. The main motivation for and focus of this paper is to provide a picture of the technique in 2020, including a detailed overview of available experimental systems worldwide and insights into a range of specific measurement modi and approaches. We also aim to provide a glimpse into the future of the technique including possible developments and opportunities.

We use polarization- and temperature-dependent x-ray absorption spectroscopy, in combination with photoelectron microscopy, x-ray diffraction, and electronic transport measurements, to study the driving force behind the insulator-metal transition in ${\mathrm{VO}}_2$. We show that both the collapse of the insulating gap and the concomitant change in crystal symmetry in homogeneously strained single-crystalline ${\mathrm{VO}}_2$ films are preceded by the purely electronic softening of Coulomb correlations within V-V singlet dimers. This process starts 7 K ($\pm 0.3\mathrm{K}$) below the transition temperature, as conventionally defined by electronic transport and x-ray diffraction measurements, and sets the energy scale for driving the near-room-temperature insulator-metal transition in this technologically promising material.

We demonstrate that the interfacial dipole associated with bonding across the $\mathrm{SrTi}{\mathrm{O}}_3/\mathrm{Si}$ heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into $\mathrm{SrTi}{\mathrm{O}}_3$. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type II to III. The transferred charge, accompanying built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Modulating the interface dipole to enable electrostatic altering of band alignment opens additional pathways to realize functional behavior in semiconducting hybrid heterojunctions.

Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO₂) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO₂—especially without inducing structural changes—has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO₂-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard X-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5 × 10²¹ cm⁻³ without inducing any measurable structural changes. We find that the MIT temperature (T_MIT) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e⁻/vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions.

Rational design of low-dimensional electronic phenomena at oxide interfaces is currently considered to be one of the most promising schemes for realizing new energy-efficient logic and memory devices. An atomically abrupt interface between paramagnetic $\mathrm{LaNi}{\mathrm{O}}_3$ and antiferromagnetic $\mathrm{CaMn}{\mathrm{O}}_3$ exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in $\mathrm{LaNi}{\mathrm{O}}_3$. Once fully understood, such emergent functionality could turn this archetypal Mott-interface system into a key building block for the above-mentioned future devices. Here, we use depth-resolved standing-wave photoemission spectroscopy in conjunction with scanning transmission electron microscopy and x-ray absorption spectroscopy, to demonstrate a depth-dependent charge reconstruction at the $\mathrm{LaNi}{\mathrm{O}}_3/\mathrm{CaMn}{\mathrm{O}}_3$ interface. Our measurements reveal an increased concentration of $\mathrm{M}{\mathrm{n}}^{3+}$ and $\mathrm{N}{\mathrm{i}}^{2+}$ cations at the interface, which create an electronic environment favorable for the emergence of interfacial ferromagnetism mediated via the $\mathrm{M}{\mathrm{n}}^{4+}-\mathrm{M}{\mathrm{n}}^{3+}$ ferromagnetic double exchange and $\mathrm{N}{\mathrm{i}}^{2+}-\mathrm{O}-\mathrm{M}{\mathrm{n}}^{4+}$ superexchange mechanisms. Our findings suggest a strategy for designing functional Mott oxide heterostructures by tuning the interfacial cation characteristics via controlled manipulation of thickness, strain, and ionic defect states.

Vanadium dioxide is of broad interest as a spin-$\frac{1}{2}$ electron system that realizes a metal-insulator transition near room temperature, due to a combination of strongly correlated and itinerant electron physics. Here, resonant inelastic x-ray scattering is used to measure the excitation spectrum of charge and spin degrees of freedom at the vanadium $L$ edge under different polarization and temperature conditions, revealing excitations that differ greatly from those seen in optical measurements. These spectra encode the evolution of short-range energetics across the metal-insulator transition, including the low-temperature appearance of a strong candidate for the singlet-triplet excitation of a vanadium dimer.