The pyrochlore iridates RIrOhave emerged as a unique playground for exploring exotic quantum phenomena arising from the intricate interplay of strong spin-orbit coupling, electron correlations, and geometric frustration. While bulk crystals of these materials have revealed a rich landscape of correlated and topological states, recent breakthroughs in epitaxial thin-film synthesis and heterostructure engineering unlocked an entirely new dimension of discovery. This brief Perspective reviews recent advancements highlighting how new tuning knobs such as dimensional confinement, epitaxial strain, and interfacial coupling can be used to manipulate the delicate balance of competing interactions. We discuss several key discoveries enabled by this approach including the realization of the magnetic Weyl semimetal phase in (111) oriented films, strain-engineered magnetic multipolar orders, the emergence of a chiral spin liquid-like state in the quasi-2D limit, and the discovery of novel electronically anisotropic states at interfaces between pyrochlore iridates and other quantum materials, such as spin ice pyrochlores. These findings showcase that low-dimensional pyrochlore iridates provide ample opportunities for both theory and experiment to unravel, control and ultimately design novel quantum states of matter. We conclude by outlining key open questions and future directions ranging from the synthesis of new heterostructures to the application of advanced probes and the exploration of non-equilibrium phenomena.

In magnetic pyrochlore materials, the interplay of spin-orbit coupling, electronic correlations, and geometrical frustration gives rise to exotic quantum phases, including topological semimetals and spin ice. While these phases have been observed in isolation, the interface-driven phenomena emerging from their interaction have never been realized previously. Here, we report on the discovery of interfacial electronic anisotropy and rotational symmetry breaking at a heterostructure consisting of the Weyl semimetal Eu2Ir2O7 and spin ice Dy2Ti2O7. Subjected to magnetic fields, we unveil a sixfold anisotropic transport response that is theoretically accounted by a Kondo-coupled heterointerface, where the spin ice's field-tuned magnetism induces electron scattering in the Weyl semimetal's topological Fermi-arc states. Furthermore, at elevated magnetic fields, we reveal a twofold anisotropic response indicative of the emergence of a symmetry-broken many-body state. This discovery showcases the potential of pyrochlore frustrated magnet/topological semimetal heterostructures in search of emergent interfacial phenomena.

Pyrochlore materials are known for their exotic magnetic and topological phases arising from complex interactions among electron correlations, band topology, and geometric frustration. Interfaces between different pyrochlore crystals characterized by complex many-body ground states hold immense potential for novel interfacial phenomena due to the strong interactions between these phases. However, the realization of such interfaces has been severely hindered by limitations in material synthesis methods. In this study, we discover a robust synthesis method that produces the previously unexplored epitaxial pyrochlore interface between spin ice Dy2Ti2O7 and Weyl semimetal Eu2Ir2O7. The method relies on an ultrahigh supersaturation regime during deposition aided by directional IR-laser-driven thermal gradients, transforming amorphous covalent networks into nearly perfectly ordered, atomically sharp interfaces with a chemically ideal arrangement of ions. The novel pyrochlore interface enables the study of interactions between relativistic Weyl fermions and spin ice magnetic monopoles, opening a path to designing diverse pyrochlore interfaces.

Ultrafast electric-field control of emergent electronic and magnetic states at oxide interfaces offers exciting prospects for the development of the next generation of energy-efficient devices. Here, it is demonstrated that the electronic structure and emergent ferromagnetic interfacial state in epitaxial LaNiO3/CaMnO3 superlattices can be effectively controlled using intense, single-cycle THz electric-field pulses. A suite of advanced X-ray spectroscopic techniques is employed to measure a detailed magneto-optical profile and the thickness of the ferromagnetic interfacial layer. Then, a combination of time-resolved and temperature-dependent optical measurements is used to disentangle several correlated electronic and magnetic processes driven by ultrafast, high-field THz pulses. Sub-picosecond non-equilibrium Joule heating of the electronic system is observed, ultrafast demagnetization of the ferromagnetic interfacial layer, and slower dynamics indicative of a change in the magnetic state of the superlattice due to the transfer of spin-angular momentum to the lattice. These findings suggest a promising avenue for the efficient control of 2D ferromagnetic states at oxide interfaces using ultrafast electric-field pulses.

Emergent magnetic phenomena at interfaces represent a frontier in materials science, pivotal for advancing technologies in spintronics and magnetic storage. In this letter, we utilize a suite of advanced X-ray spectroscopic and scattering techniques to investigate emergent interfacial ferromagnetism in oxide superlattices comprised of antiferromagnetic CaMnO3 and paramagnetic CaRuO3. Our findings challenge prior theoretical models by demonstrating that the ferromagnetism extends beyond the interfacial layer into multiple unit cells of CaMnO3 and exhibits an asymmetric profile. Complementary density functional calculations reveal that the interfacial ferromagnetism is driven by the double exchange mechanism, facilitated by charge transfer from Ru to Mn ions. Additionally, defect chemistry, particularly the presence of oxygen vacancies, likely plays a crucial role in modifying the magnetic moments at the interface, leading to the observed asymmetry between the top and bottom CaMnO3 interfacial magnetic layers. Our findings underscore the potential of manipulating interfacial ferromagnetism through point defect engineering.

The pyrochlore iridates have become ideal platforms to unravel fascinating correlated and topolog?ical phenomena that stem from the intricate interplay among strong spin-orbit coupling, electronic correlations, lattice with geometric frustration, and itinerancy of the 5d electrons. The all-in-all?out antiferromagnetic state, commonly considered as the magnetic ground state, can be dramatically altered in reduced dimensionality, leading to exotic or hidden quantum states inaccessible in bulk. Here, by means of magnetotransport, resonant elastic and inelastic x-ray scattering experiments, we discover an emergent quantum disordered state in (111) Y2Ir2O7 thin films (thickness less than 30 nm) per?sisting down to 5 K, characterized by dispersionless magnetic excitations. The anomalous Hall effect observed below an onset temperature near 135 K corroborates the presence of chiral short-range spin configurations expressed in non-zero scalar spin chirality, breaking the macroscopic time-reversal symmetry. The origin of this chiral state is ascribed to the restoration of magnetic frustration on the pyrochlore lattice in lower dimensionality, where the competing exchange interactions together with enhanced quantum fluctuations suppress any long-range order and trigger spin-liquid-like behavior with degenerate ground-state manifold.

Emergent magnetic phenomena at interfaces represent a frontier in materials science, pivotal for advancing technologies in spintronics and magnetic storage. In this Letter, we utilize a suite of advanced X-ray spectroscopic and scattering techniques to investigate emergent interfacial ferromagnetism in oxide superlattices composed of antiferromagnetic CaMnO3 and paramagnetic CaRuO3. Our findings demonstrate that ferromagnetism exhibits an asymmetric profile and may extend beyond the interfacial layer into multiple unit cells of CaMnO3. Complementary density functional calculations reveal that the interfacial ferromagnetism is driven by the double exchange mechanism, facilitated by charge transfer from Ru to Mn ions. Additionally, defect chemistry, particularly the presence of oxygen vacancies, can play a crucial role in modifying the magnetic moments at the interface, possibly leading to the observed asymmetry between the top and bottom CaMnO3 interfacial magnetic layers. Our findings underscore the potential of manipulating interfacial ferromagnetism through point defect engineering.

The synthesis of stoichiometric and epitaxial pyrochlore iridate thin films presents significant challenges yet is critical for unlocking experimental access to novel topological and magnetic states. Towards this goal, we unveil an in-situ two-stage growth mechanism that facilitates the synthesis of high-quality oriented pyrochlore iridate thin films. The growth starts with the deposition of a pyrochlore titanate as an active iso-structural template, followed by the application of an in-situ solid phase epitaxy technique in the second stage to accomplish the formation of single crystalline, large-area films. This novel protocol ensures the preservation of stoichiometry and structural homogeneity, leading to a marked improvement in surface and interface qualities over previously reported methods. The success of this synthesis approach is attributed to the application of directional laser-heat annealing, which effectively reorganizes the continuous random network of ions into a crystalline structure, as evidenced by our comprehensive analysis of the growth kinetics. This new synthesis approach advances our understanding of pyrochlore iridate film fabrication and opens a new perspective for investigating their unique physical properties.

Interfacial charge transfer in oxide heterostructures gives rise to a rich variety of electronic and magnetic phenomena. Designing heterostructures where one of the thin-film components exhibits a metal-insulator transition opens a promising avenue for controlling such phenomena both statically and dynamically. In this letter, we utilize a combination of depth-resolved soft X-ray standing-wave and hard X-ray photoelectron spectroscopies in conjunction with polarization-dependent X-ray absorption spectroscopy to investigate the effects of the metal-insulator transition in $LaNiO_{3}$ on the electronic and magnetic states at the $LaNiO_{3}/CaMnO_{3}$ interface. We report on a direct observation of the reduced effective valence state of the interfacial Mn cations in the metallic superlattice with an above-critical $LaNiO_{3}$ thickness (6 u.c.) due to the leakage of itinerant Ni 3d $e_{g}$ electrons into the interfacial $CaMnO_{3}$ layer. Conversely, in an insulating superlattice with a below-critical $LaNiO_{3}$ thickness of 2 u.c., a homogeneous effective valence state of Mn is observed throughout the $CaMnO_{3}$ layers due to the blockage of charge transfer across the interface. The ability to switch and tune interfacial charge transfer enables precise control of the emergent ferromagnetic state at the $LaNiO_{3}/CaMnO_{3}$ interface and, thus, has far-reaching consequences on the future strategies for the design of next-generation spintronic devices.

Frustrated rare-earth pyrochlore titanates, Yb$_2$Ti$_2$O$_7$, and Tb$_2$Ti$_2$O$_7$ have been proposed as promising candidates to realize quantum spin ice (QSI). Multiple exotic quantum phases, including Coulombic ferromagnet, quantum valence-bond solid, and quadrupolar ordering, have been predicted to emerge in the QSI state upon application of a (111)-oriented external magnetic field. Here, we report on the primal successful layer-by-layer growth of ultra-thin films of frustrated quantum pyrochlores, R$_2$Ti$_2$O$_7$ (R = Er, Yb, and Tb), along the (111) direction. We confirm their high crystallinity and proper chemical composition by a combination of methods, including in-situ RHEED, x-ray diffraction, reciprocal space mapping, and x-ray photoelectron spectroscopy. The availability of large area (111)-oriented QSI structures with planar geometry offers a new complementary to the bulk platform to explore strain and magnetic field dependent properties in the quasi-2D limit.

In a neutral system such as liquid helium-3, transport of mass, heat, and spin provide information analogous to electrical counterparts in metals, superconductors and topological materials. Of particular interest is transport in strongly confining channels of height approaching the superfluid coherence length, where new quantum states are found and excitations bound to surfaces and edges should be present. Here we report on the thermal conduction of helium-3 in a 1.1~$\mu$m high microfabricated channel. In the normal state we observe a diffusive thermal conductivity that is approximately temperature independent, consistent with recent work on the interference of bulk and boundary scattering. In the superfluid state we measure diffusive thermal transport in the absence of thermal counterflow. An anomalous thermal response is also detected in the superfluid which we suggest may arise from a flux of surface excitations.

We report on the successful fabrication of (111) oriented thin films of pyrochlore iridate Y2Ir2O7 by means of solid phase epitaxy. Essentially, we developed an entirely in situ annealing protocol, which is superior to the conventional ex-situ routine that requires multi-hour annealing to stabilize the proper pyrochlore structure. The morphological quality of the films has been confirmed by x-ray diffraction, reflectivity, and reciprocal space mapping, demonstrating their high crystallinity with a pure pyrochlore phase and an expected epitaxial relation to the substrate. Strikingly, below the magnetic phase transition, the temperature dependence of resistivity shows a power-law behavior with the power exponent of ∼3/2, characteristic of a Weyl semimetal in the presence of impurities and electron–electron correlations. Below 10 K, the magneto-transport measurements reveal the emergence of a non-saturated negative magnetoresistance up to 9 T with a small “valley” around zero field. These findings are further interpreted in relation to the weak anti-localization effect and the non-collinear antiferromagnetic ordering on the Ir sublattice.

AB$_2$O$_4$ normal spinels with a magnetic B site can host a variety of magnetic and orbital frustrations leading to spin-liquid phases and field-induced phase transitions. Here we report the first epitaxial growth of (111)-oriented MgCr$_2$O$_4$ thin films. By characterizing the structural and electronic properties of films grown along (001) and (111) directions, the influence of growth orientation has been studied. Despite distinctly different growth modes observed during deposition, the comprehensive characterization reveals no measurable disorder in the cation distribution nor multivalency issue for Cr ions in either orientation. Contrary to a naive expectation, the (111) stabilized films exhibit a smoother surface and a higher degree of crystallinity than (001)-oriented films. The preference in growth orientation is explained within the framework of heteroepitaxial stabilization in connection to a significantly lower (111) surface energy. These findings open broad opportunities in the fabrication of 2D kagome-triangular heterostructures with emergent magnetic behavior inaccessible in bulk crystals.