Flat bands, electronic states with nearly dispersionless energy-momentum structure, provide fertile ground for unconventional quantum phases. Recent observations of flat bands at the Fermi level in kagome metals open the possibility of unifying topology and correlation-driven heavy-fermion physics. Here we show that topology and heavy-fermion correlations coexist in the layered kagome metal YbCr6Ge6. At high temperatures, an intrinsic kagome flat band-arising from frustrated hopping on the kagome lattice-dominates the Fermi level. Upon cooling, localized Yb 4f-states hybridize with the topological kagome flat bands, transforming this state into momentum-independent Kondo resonance states across the entire Brillouin zone. Topological analysis of the hybridization gaps reveals filling-tunable weak and strong topological Kondo-insulating regimes, and identifies a topological Dirac-Kondo semimetal. Taken together, these results identify YbCr6Ge6 as a prototype of a topological heavy-fermion system and a platform where geometric frustration, strong correlations, and topology converge, with broad implications for correlated quantum matter.

High temperature superconductivity is typically associated with strong coupling and a large superconducting gap, yet these characteristics have not been demonstrated in the nickelates. Here, we provide experimental evidence that Eu substitution in the spacer layer of Nd1-xEuxNiO2 (NENO) thin films enhances the superconducting gap, driving the system toward a strong-coupling regime. This is accompanied by a magnetic-exchange-driven magnetic-field-enhanced superconductivity. We investigate the upper critical magnetic field, Hc2, and the superconducting gap of superconducting NENO thin films with x = 0.2 to 0.35. Magnetoresistance measurements reveal magnetic-field-enhanced superconductivity in NENO films. We interpret this phenomenon as a result of an interaction between magnetic Eu ions and superconducting states in the Ni dx2-y2 orbital. The upper critical magnetic field strongly violates the weak-coupling Pauli limit. Infrared spectroscopy confirms a large gap-to-Tc ratio 2 Δ / k B T c ≃ 5 - 6 , indicating a stronger coupling pairing mechanism in NENO relative to the Sr-doped NdNiO2. The substitution of Eu in the rare-earth layer causes pronounced modifications of the superconducting gap and magnetic interactions in Nd-based nickelates, opening new pathways to engineer high-Tc superconductivity in infinite-layer nickelates.

High-frequency mechanical oscillators with long coherence times are essential to realizing a variety of high-fidelity quantum sensors, transducers, and memories. However, the unprecedented coherence times needed for quantum applications require exquisitely sensitive new techniques to probe the material origins of phonon decoherence and new strategies to mitigate decoherence in mechanical oscillators. Here, we combine non-invasive laser spectroscopy techniques with materials analysis to identify key sources of phonon decoherence in crystalline media. Using micro-fabricated high-overtone bulk acoustic-wave resonators (μHBARs) as an experimental testbed, we identify phonon-surface interactions as the dominant source of phonon decoherence in crystalline quartz; lattice distortion, subsurface damage, and high concentration of elemental impurities near the crystal surface are identified as the likely causes. Removal of this compromised surface layer using an optimized polishing process is seen to greatly enhance coherence times, enabling μHBARs with Q-factors of > 240 million at 12 GHz frequencies, corresponding to > 6 ms phonon coherence times and record-level f-Q products. Complementary phonon linewidth and time-domain ringdown measurements, performed using a new Brillouin-based pump-probe spectroscopy technique, reveal negligible dephasing within these oscillators. Building on these results, we identify a path to > 100 ms coherence times as the basis for high-frequency quantum memories. These findings clearly demonstrate that, with enhanced control over surfaces, dissipation and noise can be significantly reduced in a wide range of quantum systems.

A two-dimensional electron gas (2DEG) forms at the interface of complex oxides like SrTiO_3 (STO) and LaTiO_3 (LTO), despite each material having a low native conductivity, as a band and a Mott insulator, respectively. The interface 2DEG hosts charge carriers with moderate charge carrier density and mobility that raised interest as a material system for applications like field-effect transistors or detectors. Of particular interest is the integration of these oxide systems in silicon technology. To this end we study the carrier dynamics in a STO/LTO/STO heterostructure epitaxially grown on Si(001) both experimentally and theoretically. Linear THz spectroscopy was performed to analyze the temperature dependent charge carrier density and mobility, which was found to be in the range of 10^12 cm^2 and 1000 cm^2V^-1s^-1, respectively. Pump-probe measurements revealed a very minor optical nonlinearity caused by hot carriers with a relaxation time of several 10 ps, even at low temperature. Density functional theory calculations with a Hubbard U term on ultrathin STO-capped LTO films on STO(001) show an effective mass of 0.64-0.68 m_e.

Dimensionality and electronic correlations are crucial elements of many quantum material properties. An example is the change of the electronic structure accompanied by the loss of quasiparticles when a metal is reduced from three dimensions to a lower dimension, where the Coulomb interaction between carriers becomes poorly screened. Here, using angle-resolved photoemission spectroscopy, we report an orbital-selective decoherence of spectral density in the perovskite nickelate LaNiO_{3} toward the monolayer limit. The spectral weight of the d_{z2} band vanishes much faster than that of the d_{x2−y2} band as the thickness of the LaNiO_{3} layer is decreased to a single unit cell, indicating a stronger correlation effect for the former upon dimensional confinement. Dynamical mean-field theory calculations show an orbital-selective Mott transition largely due to the localization of d_{z2} electrons along the c axis in the monolayer limit. This orbital-selective correlation effect underpins many macroscopic properties of nickelates, such as metal-to-insulator transition and superconductivity, where most theories are built upon a d_{x2−y2}–d_{z2} two-band model.

Angle-resolved photoemission spectroscopy (ARPES) has been a widely adopted technique in the studies of quantum materials. The surface sensitivity of photoelectric effect also makes it a powerful tool to investigate surface and shallow interface phenomena. While an overwhelming majority of its use focuses on extracting the eigenenergy of the electron Bloch states in momentum space, attempts to extract information of the wave function via ARPES has been limited to molecular systems. In this perspective, it is proposed and advocated use ARPES to investigate and unravel wave function properties, as opposed to only the electron energy-momentum dispersion relation, in crystalline solids and their interfaces. This can help enhance the rapidly growing development of material properties based on the spatial and geometric properties of the electronic wave functions. Angle-resolved photoemission spectroscopy (ARPES) has been a widely adopted technique to investigate surface and shallow interface phenomena. An overwhelming majority of its use focuses on extracting the energy-momentum dispersion (band structure) in momentum space. A new way of using ARPES is proposed to investigate wave function properties in crystalline solids and surfaces. image

Rare-earth ion-doped dielectric crystals are a promising materials platform for quantum device applications due to their stable and highly coherent optical transitions. Recently, REIs in thin film form have become attractive because of their enhanced control of stoichiometry, lattice structure, and dimensionality. This flexibility provides a versatile host crystal environment. Control of surface and interface structures of host crystals at the atomic scale offers an avenue to further improve the optical properties of the system by mitigating defects, which can otherwise compromise the coherence time of quantum devices. In this work, we have investigated the impact of thermal annealing on the surface morphology of a promising host crystal, CaWO4. Our findings reveal that crystal miscut plays a significant role in determining the surface step-terrace structure at the atomic level. Additionally, by iterating an annealing-wet etch cycle, we have achieved atomically flat surfaces with a roughness of less than 0.5 Å rms over a 1 × 1 μm2 area. Homoepitaxial thin film growth using molecular beam epitaxy on an atomically flat surface of CaWO4 results in high-quality thin films. Our study establishes guiding principles to realize a novel quantum optical system based on REI-doped CaWO4 thin films.

Two-dimensional electron gas (2DEG) states at oxide interfaces between two ferroic materials have been fertile ground to realize controllable multiferroicity. Here, we investigate the 2DEG states at the interface of ferroelectric BaTiO 3 and a magnetic layer of iron using angle -resolved photoemission spectroscopy. Orbitalselective charge transfer occurs on the surprisingly robust 2DEG. Based on first -principles calculations, we show how the interfacial hybridization can give rise to the unexpected charge transfer in the magnetic 2DEG. Our study reveals a close interplay on a 2DEG between magnetic and ferroelectric interfaces, which sheds light on future design principles of multiferroic 2DEG states.

Interlayer excitons in solid‐state systems have emerged as candidates for realizing novel platforms ranging from excitonic transistors and optical qubits to exciton condensates. Interlayer excitons have been discovered in 2D transition metal dichalcogenides, with large exciton binding energies and the ability to form various van der Waals heterostructures. Here, an oxide system consisting of a single unit cell of Mg 2 TiO 4 on MgO (001) is proposed as a platform for hosting interlayer excitons. Using a combination of density functional theory (DFT) calculations, molecular beam epitaxy growth, and in situ crystal truncation rod measurements, it is shown that the Mg 2 TiO 4 ‐MgO interface can be precisely controlled to yield an internal electric field suitable for hosting interlayer excitons. The atoms in the polar Mg 2 TiO 4 layers are observed to be displaced to reduce polarity at the interface with the non‐polar MgO (001) surface. Such polarity‐driven atomic displacements strongly affect electrostatics of the film and the interface, resulting in localization of filled and empty band‐edge states in different layers of the Mg 2 TiO 4 film. The DFT calculations suggest that the electronic structure is favorable for localization of photoexcited electrons in the bottom layer and holes in the top layer, which may bind to form interlayer exciton states.

We report on superconductivity in Nd 1− x Eu x NiO 2 using Eu as a 4f dopant of the parent NdNiO 2 infinite-layer compound. We use an all–in situ molecular beam epitaxy reduction process to achieve the superconducting phase, providing an alternate method to the ex situ CaH 2 reduction process to induce superconductivity in the infinite-layer nickelates. The Nd 1− x Eu x NiO 2 samples exhibit a step-terrace structure on their surfaces, have a T c onset of 21 K at x = 0.25, and have a large upper critical field that may be related to Eu 4f doping.

We report on superconductivity in Nd1-xEuxNiO2 using Eu as a 4f dopant of the parent NdNiO2 infinite-layer compound. We use an all-in situ molecular beam epitaxy reduction process to achieve the superconducting phase, providing an alternate method to the ex situ CaH2 reduction process to induce superconductivity in the infinite-layer nickelates. The Nd1-xEuxNiO2 samples exhibit a step-terrace structure on their surfaces, have a T-c onset of 21 K at x = 0.25, and have a large upper critical field that may be related to Eu 4f doping.

Interfaces between dissimilar correlated oxides can offer devices with versatile functionalities, and great efforts have been made to manipulate interfacial electronic phases. However, realizing such phases is often hampered by the inability to directly access the electronic structure information; most correlated interfacial phenomena appear within a few atomic layers from the interface. Here, atomic-scale epitaxy and photoemission spectroscopy are utilized to realize the interface control of correlated electronic phases in atomic-scale ruthenate-titanate heterostructures. While bulk SrRuO3 is a ferromagnetic metal, the heterointerfaces exclusively generate three distinct correlated phases in the single-atomic-layer limit. The theoretical analysis reveals that atomic-scale structural proximity effects yield Fermi liquid, Hund metal, and Mott insulator phases in the quantum-confined SrRuO3 . These results highlight the extensive interfacial tunability of electronic phases, hitherto hidden in the atomically thin correlated heterostructure. Moreover, this experimental platform suggests a way to control interfacial electronic phases of various correlated materials.

The square-planar nickelates are a class of superconductors analogous to the cuprates and promise to provide insight into the pairing mechanism in high-temperature superconducting oxides. The parent phase of doped superconducting films is NdNiO2, which is prepared by reducing 3(+) Ni in NdNiO3 films. In this paper, we develop an ultrahigh vacuum reduction method using aluminum deposited on top of the 3(+) nickelates and monitor the reduction process in real time through in situ crystal truncation rod measurements and diffraction x-ray absorption near edge spectroscopy measurements across the Ni K edge. We establish a relation between Ni valence and the lattice constant of NdNiO3-x and show that the process can precisely control the oxygen content in the films. Finally, we extend the reduction process to quintuple square-planar nickelates.

In research on functional materials, the focus of attention has increasingly shifted from ferroelectrics, with electric‐field‐driven switching between two or more symmetry‐related insulating polar states, to materials with functional behavior based on the competition of antipolar and polar states, or two or more symmetry inequivalent polar states, with switching driven by applied electric fields and/or stresses. When the competing states are not symmetry related, they can have quite distinct magnetic, optical, transport, and topological properties, with functionality derived from modulating these properties or even turning them on and off. In this perspective, the authors set out joint experimental–theoretical strategies for the discovery and development of new functional materials in this class.

Atomically engineered heterostructures of ferroelectric oxides are promising avenues to study rich interface physics and to design nonvolatile functionalities for future energy-efficient nanoelectronics. Towards designing such ferroelectric-based devices, not only achieving precise control of domains and domain walls, but also understanding interfacial charge-lattice coupling phenomena are essential [1]. In situ transmission electron microscopy allows to observe real-time domain switching and to probe atomistic lattice responses at ferroelectric/correlated oxide heterostructures. In this presentation, two in situ TEM experiments performed on ferroelectric-oxide-based heterostructures are discussed. Rich correlated-electron physics in nickelates, including metal-insulator transition, charge and magnetic ordering, is due to intimate competition among electronic bandwidth, on-site Coulomb repulsion, charge transfer energy, and a wide range of structural variants [2]. Pursuing understanding and controlling the interplay between structural distortions and electronic correlations in nickelates, we investigated ferroelectric PbZr 0.2 Ti 0.8 O 3 (PZT)/LaNiO 3 (LNO, 5 unit cell) heterostructures grown on (001) plane of SrTiO 3 substrate using in situ electrical biasing and atomic resolution imaging [3]. Two distinct polarization states at the PZT/LNO interface were induced by in situ biasing (Fig. 1(a)). The presence of metallic LNO layer ensures uniform bidirectional bias

Rare-earth ions (REIs) doped into solid-state crystal hosts offer an attractive platform for realizing quantum interconnects that can function as quantum memories and quantum repeaters. The 4f valence electrons of REIs are shielded by 5s and 5p electrons and undergo highly coherent transitions even when embedded in host crystals. In particular, Er3+ has an optical transition in the telecom band that is suitable for low-loss communication. Recently, REIs in thin film systems have gained interest due to potential advantages in providing a flexible host crystal environment, enabling scalable on-chip integration with other quantum devices. Here, we investigate the structural and optical properties of Er-doped anatase TiO2 thin films on LaAlO3 (001) substrates. By choosing a system with minimal lattice mismatch and adjusting Er-dopant concentration, we achieve optical inhomogeneous linewidths of 5 GHz at 4.5 K. We show that 9 nm-thick buffer and capping layers can reduce the linewidth by more than 40%, suggesting a pathway to further narrowing linewidths in this system. We also identify that Er3+ ions mainly incorporate into substitutional Ti4+ sites with non-polar D2d symmetry, which makes Er dopants insensitive to the first order to local electric fields from impurities and is desirable for coherence properties of Er3+ spins.

We develop a resonant scattering technique to measure cation stoichiometry of lanthanum-doped BaSnO3 (BSO) thin films on a DyScO3 substrate. Samples are grown by a hybrid molecular beam epitaxy method and display high room-temperature carrier mobilities. The measured thin films are grown with widely differing cation arrival rates, with Ba being evaporated from an elemental source and Sn from a SnO2 source. Differences in mobilities in these films may arise from differences in Ba/Sn cation stoichiometry. Owing to the similar scattering strength of the Ba and Sn cations, odd-order Bragg peaks of BSO are particularly sensitive to the material's cation stoichiometry, i.e., the Ba/Sn ratio. Sensitivity to cation stoichiometry is further enhanced using the technique of resonant x-ray scattering, which changes the scattering strength of a single element across the Ba L absorption edges. We determine that the Ba/Sn cation stoichiometry varies from unity by less than 1% for films of mobility ranging from 84.8 to 144 cm(2)/(V s) and conclude that the mobility of these films scales with film thickness and growth rate rather than stoichiometry.

Ferroelectric interfacial devices consist of materials systems whose interfacial electronic properties (such as a 2D electron gas or an interfacial magnetic spin configuration) are modulated by a ferroelectric layer set in its immediate vicinity. While the prototypical example of such a system is the ferroelectric field effect transistor first proposed in the 1950s, only with the recent advances in the controlled growth of epitaxial thin films and heterostructures, and the recent physical understanding down to the atomic scale of screening processes at ferroelectric-semiconducting and -metallic interfaces made possible by first principles calculations, have the conditions been met for a full development of the field. In this review, we discuss the recent advances in ferroelectric interfacial systems with emphasis on the ferroelectric control of the electronic properties of interfacial devices with well ordered (epitaxial) interfaces. In particular, we consider the cases of ferroelectric interfacial systems aimed at controlling the correlated state, including superconductivity, Mott metallic-insulator transition, magnetism, charge, and orbital order, and charge and spin transport across ferroelectric tunnel junctions. The focus is on the basic physical mechanisms underlying the emergence of interfacial effects, the nature of the ferroelectric control of the electronic state, and the role of extreme electric field gradients at the interface in giving rise to new physical phenomena. Such understanding is key to the development of ferroelectric interfacial systems with characteristics suitable for next generation electronic devices based on controlling the correlated state of matter.

BaTiO3 exhibits several functional properties, such as high dielectric constant, large Pockels coefficient, and strong ferroelectricity/piezoelectricity. These properties can be used for a variety of applications, such as ferroelectric tunnel junctions in non-volatile memory devices. To achieve large-scale integration of BaTiO3, however, one requires the synthesis of high-quality BaTiO3 films at low temperatures in order to be compatible with the thermal budget of electronic processes in use today. Here, we describe the synthesis of BaTiO3 thin films by molecular beam epitaxy and find that coherently strained and ferroelectric BaTiO3 can be grown at temperatures as low as 310 °C. Using reflection high energy diffraction, we demonstrate a surface mobility of BaO and TiO2 adatoms that is high enough to promote ferroelectric crystal growth at low temperatures. A clear ferroelectric polarization switching is observed using piezoresponse force microscopy. Our results pave the way toward large-scale integration of ferroelectric BaTiO3 with mainstream electronics platforms.