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The interface between two-dimensional topological Dirac states and an s-wave superconductor is expected to support Majorana bound states that can be used for quantum computing applications. Realizing these novel states of matter and their applications requires control over superconductivity and spin-orbit coupling to achieve spin-momentum locked topological surface states which are simultaneously superconducting. While signatures of Majorana bound states have been observed in the magnetic vortex cores of bulk FeTe0.55Se0.45, inhomogeneity and disorder from doping makes these signatures unclear and inconsistent between vortices. Here we report superconductivity in monolayer FeTe1-ySey (Fe(Te,Se)) grown on Bi2Te3 by molecular beam epitaxy. Spin and angle resolved photoemission spectroscopy directly resolve the interfacial spin and electronic structure of Fe(Te,Se)/Bi2Te3 heterostructures. We find that for y = 0.25 the Fe(Te,Se) electronic structure overlaps with the topological Bi2Te3 interfacial states which disrupts the desired spin-momentum locking. In contrast, for y = 0.1 a smaller Fe(Te,Se) Fermi surface allows for clear spin-momentum locking observed in the topological states. Hence, we demonstrate the Fe(Te,Se)/Bi2Te3 system is a highly tunable platform for realizing Majorana bound states where reduced doping can improve characteristics important for Majorana interrogation and potential applications.

Epitaxial Fe(Te,Se) thin films have been grown on various substrates but never been realized on magnetic layers. Here we report the epitaxial growth of four-fold Fe(Te,Se) film on a six-fold antiferromagnetic insulator, MnTe. The Fe(Te,Se)/MnTe heterostructure shows a clear superconducting transition at around 11 K and the critical magnetic field measurement suggests the origin of the superconductivity to be bulk-like. Structural characterizations suggest that the uniaxial lattice match between Fe(Te,Se) and MnTe allows a hybrid symmetry epitaxy mode, which was recently discovered between Fe(Te,Se) and Bi2Te3. Furthermore, Te/Fe flux ratio during deposition of the Fe(Te,Se) layer is found to be critical for its superconductivity. Now that superconducting Fe(Te,Se) can be grown on two related hexagonal platforms, Bi2Te3 and MnTe, this result opens a new possibility of combining topological superconductivity of Fe(Te,Se) with the rich physics in the intrinsic magnetic topological materials (MnTe)n(Bi2Te3)m family., 19 pages, 5 figures, accepted by Nano Letters

We report the crystal growth and structural and electronic properties of superconducting, van der Waals layered PtTe. Easily cleavable crystals with a plate-like morphology consistent with the layered structure were grown from a platinum rich flux. A consistent determination of $T_c = 0.57$ K is made from the onset of diamagnetism, the zero of resistivity, and the midpoint of the heat capacity jump. The observed behavior is consistent with type-II superconductivity, with upper critical field at $T=0$ estimated using the Werthamer-Helfand-Hohenberg theory to be 143 and 65 Oe for fields out of and in the plane, respectively. The heat capacity discontinuity is close to the weak coupling BCS value. Density functional theory calculations and analysis of the electronic structure finds that PtTe is a topological semimetal with numerous surface states, but suggests the superconducting state itself may be topologically trivial. Angle resolved photoemission spectroscopy reveals a normal-state Fermi surface in remarkable agreement with theory, and confirms the overall topological nature of the material by experimental identification of the surface bands. Together, these findings identify PtTe as an interesting example of a cleavable, topological, and superconducting material.

In this work, the high entropy oxide A2CuO4 Ruddlesden-Popper (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 is explored by charge doping with Ce+4 and Sr+2 at concentrations known to induce superconductivity in the simple parent compounds, Nd2CuO4 and La2CuO4. Electron doped (La0.185Pr0.185Nd0.185Sm0.185Eu0.185Ce0.075)2CuO4 and hole doped (La0.18Pr0.18Nd0.18Sm0.18Eu0.18Sr0.1)2CuO4 are synthesized and shown to be single crystal, epitaxially strained, and highly uniform. Transport measurements demonstrate that all as-grown films are insulating regardless of doping. Annealing studies show that resistivity can be tuned by modifying oxygen stoichiometry and inducing metallicity but without superconductivity. These results in turn are connected to extended x-ray absorption fine structure (EXAFS) results indicating that the lack of superconductivity in the high entropy cuprates likely originates from a large distortion within the Cu-O plane ({\sigma}2>0.015 {\AA}2) due to A-site cation size variance, which drives localization of charge carriers. These findings describe new opportunities for controlling charge- and orbital-mediated functional responses in Ruddlesden-Popper crystal structures, driven by balancing of cation size and charge variances that may be exploited for functionally important behaviors such as superconductivity, antiferromagnetism, and metal-insulator transitions, while opening less understood phase spaces hosting doped Mott insulators, strange metals, quantum criticality, pseudogaps, and ordered charge density waves.

An extremely large linear magnetoresistance (LMR) is a ubiquitous phenomenon emerging from topological Dirac and Weyl semimetals. However, the connection between an LMR and a nontrivial topology is under extensive debate. In this paper, by precisely controlling the thickness of ${\mathrm{SrNbO}}_{3}$ thin films grown on ${\mathrm{SrTiO}}_{3}$ substrates, we observe an LMR over a large carrier density range with a magnetoresistance as high as $150\phantom{\rule{0.16em}{0ex}}000%$ at a carrier density $n\ensuremath{\sim}{10}^{21}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, far away from the quantum-limit regime. The temperature-, magnetic-field-, and carrier-density-dependent LMR in ${\mathrm{SrNbO}}_{3}/{\mathrm{SrTiO}}_{3}$ heterostructures provides compelling evidence of a mobility-driven LMR in coherent electronic systems. Our results uncover the general principle of an LMR and shed light on proper categorization of transport properties in topological and correlated materials.

The influence of hole doping on magnetic properties is mapped for the compositionally complex high-entropy oxide ${AB\mathrm{O}}_{3}$ perovskite ${\mathrm{La}}_{1\text{\ensuremath{-}}x}{\mathrm{Sr}}_{x}({\mathrm{Cr}}_{0.2}{\mathrm{Mn}}_{0.2}{\mathrm{Fe}}_{0.2}{\mathrm{Co}}_{0.2}{\mathrm{Ni}}_{0.2}){\mathrm{O}}_{3}$ $(0\ensuremath{\le}x\ensuremath{\le}0.5)$. It is found that aliovalent A-site substitution is a viable means to manipulate the magnetically active B-site sublattice. A series of single-crystal films are synthesized and show a general trend toward stronger ferromagnetic response and a shift in magnetic anisotropy as the Sr concentration increases. Magnetometry demonstrates complex and nonuniform responses similar to rigid and uncoupled composites at intermediate dopings. This behavior points to the presence of locally inhomogeneous magnetic phase competition, where ferromagnetic and antiferromagnetic magnetic contributions create a frustrated matrix containing uncompensated spins at the boundaries between these regions. The observations are discussed in the context of known responses to hole doping in the less complex ternary $\mathrm{La}{T}_{M}{\mathrm{O}}_{3}$ $({T}_{M}=\mathrm{Cr},\mathrm{Mn},\mathrm{Fe},\mathrm{Co},\mathrm{Ni})$ oxides, and they are found to be different from a simple sum of the doped parents. The results are summarized in a preliminary magnetic phase diagram.

Proposed approaches to topological quantum computation based on Majorana bound states may enable new paths to fault-tolerant quantum computing. Several recent experiments have suggested that the vortex cores of topological superconductors, such as iron-based superconductors, may host Majorana bound states at zero energy. To facilitate quantum computation with these zero-energy vortex bound states, a precise and fast manipulation of individual vortices is crucial. However, handling individual vortices remains a challenge, and a theoretical framework for describing individually controlled vortex motion is still critically needed. We propose a scheme for the use of local heating based on scanning optical microscopy to manipulate Majorana bound states emergent in the vortex cores of topological superconductors. Specifically, we derive the conditions required for transporting a single vortex between two stationary defects in the superconducting material by trapping it with a "hot spot" generated by local optical heating. Using these critical conditions for the vortex motion, we then establish the ideal material properties for the implementation of our manipulation scheme, which paves the way toward the controllable handling of zero-energy vortex bound states.

The combination of two-dimensional Dirac surface states with s-wave superconductivity is expected to generate localized topological Majorana zero modes in vortex cores. Putative experimental signatures of these modes have been reported for heterostructures of proximitized topological insulators, iron-based superconductors or certain transition metal dichalcogenides. Despite these efforts, the Majorana nature of the observed excitation is still under debate. We propose to identify the presence of Majorana vortex modes using a nonlocal transport measurement protocol originally employed for one-dimensional settings. In the case of an isolated subgap state, the protocol provides a spatial map of the ratio of local charge- and probability-density which offers a clear distinction between Majorana and ordinary fermionic modes. We show that these distinctive features survive in the experimentally relevant case of hybridizing vortex core modes., 12 pages, 6 Figures

The availability of native substrates is a cornerstone in the development of microelectronic technologies relying on epitaxial films. If native substrates are not available, virtual substrates - crystalline buffer layers epitaxially grown on a structurally dissimilar substrate - offer a solution. Realizing commercially viable virtual substrates requires the growth of high-quality films at high growth rates for large-scale production. We report the stoichiometric growth of SrTiO3 exceeding 600 nm hr−1. This tenfold increase in growth rate compared to SrTiO3 grown on silicon by conventional methods is enabled by a self-regulated growth window accessible in hybrid molecular beam epitaxy. Overcoming the materials integration challenge for complex oxides on silicon using virtual substrates opens a path to develop new electronic devices in the More than Moore era and silicon integrated quantum computation hardware., A scalable method for the growth of perovskite oxides thin films on silicon is desirable for integration of buffer layers in devices. Here the authors demonstrate the stoichiometric growth of thin SrTiO3 layers on silicon at high growth rates by hybrid molecular beam epitaxy.

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 PdCoO2, which consists of alternating layers of metallic Pd and Mott-insulating CoO2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO2, 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 CoO2 layers. Computationally, we find that many-body interactions in Pd and CoO2 layers are highly different. Holes in the CoO2 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. PdCoO2 belongs to a class of materials where both weakly and strongly correlated electrons co-exist side-by-side in different layers of the crystal structure. Here, the authors investigate PdCoO2 using standing wave photoemission spectroscopy and many-body calculations reporting layer-specific details about the electronic structure.

Relaxor ferrolectrics are important in technological applications due to a strong electromechanical response, energy storage capacity, electrocaloric effect, and pyroelectric energy conversion properties. Current efforts to discover and design new materials in this class generally rely on substitutional doping of known ferroelectrics, as slight changes to local compositional order can significantly affect the Curie temperature, morphotropic phase boundary, and electromechanical responses. In this work, we demonstrate that moving to the strong limit of compositional complexity in an ABO3 perovskite allows stabilization of novel relaxor responses that do not rely on a single narrow phase transition region. Entropy-assisted synthesis approaches are used to create single crystal Ba(Ti0.2Sn0.2Zr0.2Hf0.2Nb0.2)O3 [Ba(5B)O] films. The high levels of configurational disorder present in this system is found to influence dielectric relaxation, phase transitions, nano-polar domain formation, and Curie temperature. Temperature-dependent dielectric, Raman spectroscopy and second-harmonic generation measurements reveal multiple phase transitions, a high Curie temperature of 570 K, and the relaxor ferroelectric nature of Ba(5B)O films. The first principles theory calculations are used to predict possible combinations of cations to quantify the relative feasibility of formation of highly disordered single-phase perovskite systems. The ability to stabilize single-phase perovskites with such a large number of different cations on the B-sites offers new possibilities for designing high-performance materials for piezoelectric, pyroelectric and tunable dielectric applications.

Motivated by the possibility of an intermediate U(1) quantum spin liquid phase in out-of-plane magnetic fields and enhanced magnetic fluctuations in exfoliated {\alpha}-RuCl3 flakes, we study magneto-Raman spectra of exfoliated multilayer {\alpha}-RuCl3 in out-of-plane magnetic fields of -6 T to 6 T at temperatures of 670 mK - 4 K. While the literature currently suggests that bulk {\alpha}-RuCl3 is in an antiferromagnetic zigzag phase with R3bar symmetry at low temperature, we do not observe R3bar symmetry in exfoliated {\alpha}-RuCl3 at low temperatures. While we saw no magnetic field driven transitions, the Raman modes exhibit unexpected stochastic shifts in response to applied magnetic field that are above the uncertainties inferred from Bayesian analysis. These stochastic shifts are consistent with the emergence of magnetostrictive interactions in exfoliated {\alpha}-RuCl3., Comment: 7 pages, 5 figures

${\mathrm{PdCoO}}_{2}$, belonging to a family of triangular oxides called delafossite, is one of the most conducting oxides. Its in-plane conductivity is comparable to those of the best metals and exhibits hydrodynamic electronic transport with an extremely long mean free path at cryogenic temperatures. Nonetheless, it is nonmagnetic despite the presence of the cobalt ion. Here, we show that a mild hydrogenation process reduces ${\mathrm{PdCoO}}_{2}$ thin films to an atomically mixed alloy of PdCo with strong out-of-plane ferromagnetism and sign-tunable anomalous Hall effect. Considering that many other compounds remain little affected under a similar hydrogenation condition, this discovery may provide a route to creating novel spintronic heterostructures combining strong ferromagnetism, involving oxides and other functional materials.

Disorder can have a dominating influence on correlated and quantum materials leading to novel behaviors which have no clean limit counterparts. In magnetic systems, spin and exchange disorder can provide access to quantum criticality, frustration, and spin dynamics, but broad tunability of these responses and a deeper understanding of strong limit disorder is lacking. In this work, we demonstrate that high entropy oxides present an unexplored route to designing quantum materials in which the presence of strong local compositional disorder hosted on a positionally ordered lattice can be used to generate highly tunable emergent magnetic behavior--from macroscopically ordered states to frustration-driven dynamic spin interactions. Single crystal La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 films are used as a structurally uniform model system hosting a magnetic sublattice with massive microstate disorder in the form of site-to-site spin and exchange type inhomogeneity. A classical Heisenberg model is found to be sufficient to describe how compositionally disordered systems can paradoxically host long-range magnetic uniformity and demonstrates that balancing the populating elements based on their discrete quantum parameters can be used to give continuous control over ordering types and critical temperatures. Theory-guided experiments show that composite exchange values derived from the complex mix of microstate interactions can be used to design the required compositional parameters for a desired response. These predicted materials are synthesized and found to possess an incipient quantum critical point when magnetic ordering types are designed to be in direct competition; this leads to highly controllable exchange bias sensitivity in the monolithic single crystal films previously accessible only in intentionally designed bilayer heterojunctions.

Magnetic insulators are important materials for a range of next-generation memory and spintronic applications. Structural constraints in this class of devices generally require a clean heterointerface that allows effective magnetic coupling between the insulating layer and the conducting layer. However, there are relatively few examples of magnetic insulators that can be synthesized with surface qualities that would allow these smooth interfaces and precisely tuned interfacial magnetic exchange coupling, which might be applicable at room temperature. In this work, we demonstrate an example of how the configurational complexity in the magnetic insulator layer can be used to realize these properties. The entropy-assisted synthesis is used to create single-crystal (Mg

Two-dimensional (2D) van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temperatures that are well below room temperature, making it challenging to incorporate them into device applications. In this work, we synthesized a room-temperature vdW magnetic crystal Fe$_5$GeTe$_2$ with a Curie temperature T$_c = 332$ K, and studied its magnetic properties by vibrating sample magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy. The experiments were performed with external magnetic fields applied along the c-axis (H$\parallel$c) and the ab-plane (H$\parallel$ab), with temperatures ranging from 300 K to 10 K. We have found a sizable Land\'e g-factor difference between the H$\parallel$c and H$\parallel$ab cases. In both cases, the Land\'e g-factor values deviated from g = 2. This indicates contribution of orbital angular momentum to the magnetic moment. The FMR measurements reveal that Fe$_5$GeTe$_2$ has a damping constant comparable to Permalloy. With reducing temperature, the linewidth was broadened. Together with the VSM data, our measurements indicate that Fe$_5$GeTe$_2$ transitions from ferromagnetic to ferrimagnetic at lower temperatures. Our experiments highlight key information regarding the magnetic state and spin scattering processes in Fe$_5$GeTe$_2$, which promote the understanding of magnetism in Fe$_5$GeTe$_2$, leading to implementations of Fe$_5$GeTe$_2$ based room-temperature spintronic devices.

Controlling electronic band structure is an important step toward harnessing quantum materials for practical uses. This is exemplified in the spin-polarized bands near the Fermi level of a ferromagnet where small variation to the Berry curvature can be used to control the intrinsic anomalous Hall effect (AHE). Since these pathways are highly sensitive to crystal structure, iterative post-synthesis manipulation of the underlying lattice can act as a fundamental probe and provide new opportunities for functionalization. In this work, depth-dependent magnetic inhomogeneity in a $4d$ itinerant ferromagnet ${\mathrm{SrRuO}}_{3}$ film is controlled by applying low energy He ion irradiation. Combined magnetometry, polarized neutron reflectometry, and magnetotransport experiments demonstrate that the Berry phase and associated AHE can be continuously and iteratively modified post-synthesis. The findings of this work should be widely applicable to other quantum materials where crystal distortions and symmetry are tightly bound to band structure and resulting Berry phase effects.

It is challenging to grow an epitaxial four-fold compound superconductor (SC) on six-fold topological insulator (TI) platform due to stringent lattice-matching requirement. Here, we demonstrate that Fe(Te,Se) can grow epitaxially on a TI (Bi2Te3) layer due to accidental, uniaxial lattice match, which is dubbed as "hybrid symmetry epitaxy". This new growth mode is critical to stabilizing robust superconductivity with TC as high as 13 K. Furthermore, the superconductivity in this FeTe1-xSex/Bi2Te3 system survives in Te-rich phase with Se content as low as x = 0.03 but vanishes at Se content above x = 0.56, exhibiting a phase diagram that is quite different from that of the conventional Fe(Te,Se) systems. This unique heterostructure platform that can be formed in both TI-on-SC and SC-on-TI sequences opens a route to unprecedented topological heterostructures., Comment: 41 pages, 14 figures, accepted by Nano Letters

Understanding band alignment and charge transfer at complex oxide interfaces is critical to tailoring and utilizing their diverse functionality. Towards this goal, we design and experimentally validate both Ohmic- and Schottky-like charge transfers at oxide/oxide semiconductor/metal interfaces. We utilize a method for predicting band alignment and charge transfer in ABO3 perovskites, where previously established rules for simple semiconductors fail. The prototypical systems chosen are the rare class of oxide metals, SrBO3 with B=V-Ta, when interfaced with the multifaceted semiconducting oxide, SrTiO3. For B=Nb and Ta, we confirm that a large accumulation of charge occurs in SrTiO3 due to higher energy Nb and Ta d-states relative to Ti; this gives rise to a high mobility metallic interface, which is an ideal epitaxial oxide/oxide Ohmic contact. On the other hand, for B=V, there is no charge transfer into the SrTiO3 interface, which serves as a highly conductive epitaxial gate metal. Going beyond these specific cases, this work opens the door to integrating the vast phenomena of ABO3 perovskites into a wide range of practical devices., Comment: Accepted in Small Science

We report the growth of the intrinsic magnetic topological system $\mathrm{MnTe}{({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})}_{n}$ by molecular beam epitaxy. By mapping the temperature and the Bi:Mn flux ratio, it is shown that there is a narrow growth window for the $n=1$ phase $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ with $2.0l\mathrm{Bi}:\mathrm{Mn}l2.6$ at 225 \ifmmode^\circ\else\textdegree\fi{}C. Here the films are stoichiometric and excess Bi and Te is not incorporated. At higher flux ratios (Bi:Mn\ensuremath{\ge}4.5) it is found that the $n=2\mathrm{Mn}{\mathrm{Bi}}_{4}{\mathrm{Te}}_{7}$ phase is stabilized. Transport measurements indicate that the $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ and $\mathrm{Mn}{\mathrm{Bi}}_{4}{\mathrm{Te}}_{7}$ undergo magnetic transitions around 25 and 10 K, respectively, consistent with antiferromagnetic phases found in the bulk. Further, for Mn-rich conditions (Bi:Mn2), ferromagnetism emerges that exhibits a clear hysteretic state in the Hall effect, which likely indicates Mn-doped $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$. Understanding how to grow ternary chalcogenide phases is the key to synthesizing new materials and to interface magnetism and topology, which together are routes to realize and control exotic quantum phenomena.

Recent optical conductivity measurements reveal the presence of Hubbard excitons in certain Mott insulators. In light of these results, it is important to revisit the dynamics of these materials to account for excitonic correlations. We investigate time-resolved excitation and relaxation dynamics as a function of temperature in perovskite-type ${\mathrm{LaVO}}_{3}$ thin films using ultrafast optical pump-probe spectroscopy. ${\mathrm{LaVO}}_{3}$ undergoes a series of phase transitions at roughly the same critical temperature ${T}_{C}\ensuremath{\cong}140$ K, including a second-order magnetic phase transition ($\mathrm{PM}\stackrel{}{\ensuremath{\rightarrow}}\mathrm{AFM}$) and a first-order structural phase transition, accompanied by $C$-type spin order and $G$-type orbital order. Ultrafast optical pump-probe spectroscopy at 1.6 eV monitors changes in the spectral weight of the Hubbard exciton resonance which serves as a sensitive reporter of spin and orbital fluctuation dynamics. We observe dramatic slowing down of the spin, and orbital dynamics in the vicinity of ${T}_{C}\ensuremath{\cong}140$ K, reminiscent of a second-order phase transition, despite the (weakly) first-order nature of the transition. We emphasize that since it is spectral weight changes that are probed, the measured dynamics are not reflective of conventional exciton generation and recombination, but are related to the dynamics of Hubbard exciton formation in the presence of a fluctuating many-body environment.

Chiral interactions in magnetic systems can give rise to rich physics manifested, for example, as nontrivial spin textures. The foremost interaction responsible for chiral magnetism is the Dzyaloshinskii-Moriya interaction (DMI), resulting from inversion symmetry breaking in the presence of strong spin-orbit coupling. However, the atomistic origin of DMIs and their relationship to emergent electrodynamic phenomena, such as topological Hall effect (THE), remain unclear. Here, we investigate the role of interfacial DMIs in 3d-5d transition metal-oxide-based LaMnO3/SrIrO3 superlattices on THE from a chiral spin texture. By additively engineering the interfacial inversion symmetry with atomic-scale precision, we directly link the competition between interfacial collinear ferromagnetic interactions and DMIs to an enhanced THE. The ability to control the DMI and resulting THE points to a pathway for harnessing interfacial structures to maximize the density of chiral spin textures useful for developing high-density information storage and quantum magnets for quantum information science.

The 2D layered Ruddlesden-Popper crystal structure can host a broad range of functionally important behaviors. Here we establish extraordinary configurational disorder in a two dimensional layered Ruddlesden-Popper (RP) structure using entropy stabilization assisted synthesis. A protype A2CuO4 RP cuprate oxide with five components (La, Pr, Nd, Sm, Eu) on the A-site sublattice is designed and fabricated into epitaxial single crystal films using pulsed laser deposition. By comparing (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 crystals grown under identical conditions but different substrates, it is found that heteroepitaxial strain plays an important role in crystal phase formation. When grown on a near lattice matched substrate, the high entropy oxide film features a T'-type RP structure with uniform A-site cation mixing and square-planar CuO4 units, however, growing under strong compressive strain results in a single crystal non-RP cubic phase consistent with a CuX2O4 spinel structure. These observations are made with a range of combined characterizations using X-ray diffraction, atomic-resolution scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray absorption spectroscopy measurements. Designing configurational complexity and moving between 2D layered RP and 3D cubic crystal structures in this class of cuprate materials opens many opportunities for new design strategies related to magnetoresistance, unconventional superconductivity, ferroelectricity, catalysis, and ion transport.

Alternate stacking of a highly conducting metallic layer with a magnetic triangular layer found in delafossite PdCrO2 provides an excellent platform for discovering intriguing correlated quantum phenomena. Thin film growth of delafossites may enable not only the tuning of the basic physical properties beyond what bulk materials can exhibit, but also the development of novel hybrid materials by interfacing with dissimilar materials, yet this has proven to be extremely challenging. Here, we report the epitaxial growth of metallic delafossite PdCrO2 films by pulsed laser epitaxy (PLE). The fundamental role of the PLE growth conditions, epitaxial strain, and chemical and structural characteristics of the substrate is investigated by growing under various growth conditions and on various types of substrates. While strain plays a large role in improving the crystallinity, the direct growth of epitaxial PdCrO2 films without impurity phases was not successful. We attribute this difficulty to both the chemical and structural dissimilarities with the substrate and volatile nature of the PdO sublayer, which make nucleation of the right phase difficult. This difficulty was overcome by growing CuCrO2 buffer layers before PdCrO2 films were grown. Unlike PdCrO2, CuCrO2 films were readily grown with a relatively wide growth window. Only a monolayer thick buffer layer was sufficient to grow the correct PdCrO2 phase. This result indicates that the epitaxy of Pd-based delafossites is extremely sensitive to the chemistry and structure of the interface, necessitating near perfect substrate materials. The resulting films are commensurately strained and show an antiferromagnetic transition at 40 K that persists down to as thin as 3.6 nm in thickness. This work provides key insights into advancing the epitaxial growth of the broader class of metallic delafossites for both studying the basic physical properties and developing new spintronic and computing devices.

Configurational disorder can have a dominating role in the formation of macroscopic functional responses in strongly correlated materials. Here, we use entropy-stabilization synthesis to create single-crystal epitaxial $\mathrm{AB}{\mathrm{O}}_{3}$ perovskite thin films with equal atomic concentration of 3d transition-metal cations on the B-site sublattice. X-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy of $\mathrm{La}(\mathrm{C}{\mathrm{r}}_{0.2}\mathrm{M}{\mathrm{n}}_{0.2}\mathrm{F}{\mathrm{e}}_{0.2}\mathrm{C}{\mathrm{o}}_{0.2}\mathrm{N}{\mathrm{i}}_{0.2}){\mathrm{O}}_{3}$ (L5BO) films demonstrate excellent crystallinity, smooth film surfaces, and uniform mixing of the 3d transition-metal cations throughout the B-site sublattice. The magnetic properties are strongly dependent on substrate-induced lattice anisotropy and suggest the presence of long-range magnetic order in these exceptionally disordered materials. The ability to populate multiple elements onto a single sublattice in complex crystal structures opens new possibilities to design functionality in correlated systems and enable novel fundamental studies seeking to understand how diverse local bonding environments can work to generate macroscopic responses, such as those driven by electron-phonon channels and complex exchange interaction pathways.

Hybrid oxide molecular beam epitaxy (hMBE), a thin-film deposition technique in which transition metal cations are delivered using a metal-organic precursor, has emerged as the state-of-the-art approach to the synthesis of electronic-grade complex oxide films with a stoichiometric growth window. However, numerous questions remain regarding the chemical mechanisms of the growth process and the surface properties of the resulting films. To examine these properties, thin film SrTiO$_{3}$ (STO) was prepared by hMBE using a titanium tetraisopropoxide (TTIP) precursor for Ti delivery and an elemental Sr source on annealed STO and Nb-doped STO substrates with varying TTIP:Sr flux ratios to examine the conditions for the reported stoichiometric growth window. The films were transferred in vacuo to an x-ray photoelectron spectroscopy system to study the surface elemental composition. Samples were examined using x-ray diffraction to compare our surface sensitive results with previously reported measurements of the bulk of the films in the literature. Ex situ studies by atomic force microscopy, scanning tunneling microscopy and low energy electron microscopy confirmed the presence of surface reconstructions and an Ehrlich-Schwoebel barrier consistent with an A-site SrO termination. We find that a surface exhibiting a mixture of SrO and TiO$_{2}$ termination, or a full SrO termination is necessary to obtain stoichiometric adsorption-controlled growth. These results indicate that surface Sr is necessary to maintain chemical equilibrium for stoichiometric growth during the hMBE process, which is important for the design of future interfacial systems using this technique., Comment: 37 pages, 6 figures, 10 supplemental figures