In Kondo lattice systems with mixed valence, such as YbAl3, interactions between localized electrons in a partially filled f shell and delocalized conduction electrons can lead to fluctuations ...between two different valence configurations with changing temperature or pressure. The impact of this change on the momentum-space electronic structure and Fermi surface topology is essential for understanding their emergent properties, but has remained enigmatic due to a lack of appropriate experimental probes. Here by employing a combination of molecular beam epitaxy (MBE) and in situ angle-resolved photoemission spectroscopy (ARPES) we show that valence fluctuations can lead to dramatic changes in the Fermi surface topology, even resulting in a Lifshitz transition. As the temperature is lowered, a small electron pocket in YbAl3 becomes completely unoccupied while the low-energy ytterbium (Yb) 4f states become increasingly itinerant, acquiring additional spectral weight, longer lifetimes, and well-defined dispersions. Our work presents the first unified picture of how local valence fluctuations connect to momentum space concepts including band filling and Fermi surface topology in the longstanding problem of mixed-valence systems.
We measure the Shubnikov-de Haas effect in thin-film Sr\(_2\)RuO\(_4\) grown on an (LaAlO\(_3\))\(_{0.29}\)-(SrAl\(_{1/2}\)Ta\(_{1/2}\)O\(_3\))\(_{0.71}\) (LSAT) substrate. We detect all three known ...Fermi surfaces and extract the Fermi surface volumes, cyclotron effective masses, and quantum lifetimes. We show that the electronic structure is nearly identical to that of single-crystal Sr\(_2\)RuO\(_4\), and that the quasiparticle lifetime is consistent with the Tc of comparably clean, single-crystal Sr\(_2\)RuO\(_4\). Unlike single-crystal Sr\(_2\)RuO\(_4\), where the quantum and transport lifetimes are roughly equal, we find that the transport lifetime is \(1.3\pm0.1\) times longer than the quantum lifetime. This suggests that extended (rather than point) defects may be the dominant source of quasiparticle scattering in these films. To test this idea, we perform cross-sectional STEM and find that out-of-phase boundaries extending the entire thickness of the film occur with a density that is consistent with the quantum mean free path. The long quasiparticle lifetimes make these films ideal for studying the unconventional superconducting state in Sr\(_2\)RuO\(_4\) through the fabrication of devices -- such as planar tunnel junctions and SQUIDs.
We report the growth of thin films of the mixed valence compound YbAl\(_{3}\) on MgO using molecular-beam epitaxy. Employing an aluminum buffer layer, epitaxial (001) films can be grown with sub-nm ...surface roughness. Using x-ray diffraction, in situ low-energy electron diffraction and aberration-corrected scanning transmission electron microscopy we establish that the films are ordered in the bulk as well as at the surface. Our films show a coherence temperature of 37 K, comparable to that reported for bulk single crystals. Photoelectron spectroscopy reveals contributions from both \(\textit{f}^{13}\) and \(\textit{f}^{12}\) final states establishing that YbAl\(_{3}\) is a mixed valence compound and shows the presence of a Kondo Resonance peak near the Fermi-level.
We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow \(\beta\)-Ga\(_2\)O\(_3\) at a growth rate of ~1 \({\mu}\)m/h with control of the silicon doping concentration from 5x10\(^{16}\) ...to 10\(^{19}\) cm\(^{-3}\). In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98\% Ga\(_2\)O, i.e., gallium suboxide, is supplied. Directly supplying Ga2O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of \(\beta\)-Ga\(_2\)O\(_3\) by conventional MBE. As a result, a growth rate of ~1 \({\mu}\)m/h is readily achieved at a relatively low growth temperature (T\(_{sub}\) = 525 \(^\circ\)C), resulting in films with high structural perfection and smooth surfaces (rms roughness of < 2 nm on ~1 \({\mu}\)m thick films). Silicon-containing oxide sources (SiO and SiO\(_2\)) producing an SiO suboxide molecular beam are used to dope the \(\beta\)-Ga\(_2\)O\(_3\) layers. Temperature-dependent Hall effect measurements on a 1 \({\mu}\)m thick film with a mobile carrier concentration of 2.7x10\(^{17}\) cm\(^{-3}\) reveal a room-temperature mobility of 124 cm\(^2\) V\(^{-1}\) s\(^{-1}\) that increases to 627 cm\(^2\) V\(^{-1}\) s\(^{-1}\) at 76 K; the silicon dopants are found to exhibit an activation energy of 27 meV. We also demonstrate working MESFETs made from these silicon-doped \(\beta\)-Ga\(_2\)O\(_3\) films grown by S-MBE at growth rates of ~1 \({\mu}\)m/h.
The sluggish kinetics of the oxygen evolution reaction (OER) is one of the major sources of the overpotentials in many air-breathing electrochemical energy devices such as electrolyzers and ...rechargeable metal-air batteries. To increase the OER kinetics and reduce the overpotentials, it is critical to establish the linkage between the catalyst structure and the OER activity and mechanism. In this contribution, we present our effort in establishing this structure-activity connection, using iridates as model electrocatalysts and strain as a tuning knob for understanding the structure-activity connection. We have grown a series of perovskite iridium oxides with different biaxial strains using Molecular Beam Epitaxy (MBE), which we will use as a model system to elucidate how the OER kinetics can be affected by strain. To further connect strain to the physical properties of the iridates, we subject these MBE-grown oxides to ambient pressure X-ray photoelectron spectroscopy, from which we can determine how strain can affect the oxygen adsorption. We use this information to reveal insights into how strain can serve as a material knob to facilitate the OER and the underlying structure-activity relationship in iridates.
We present our electrochemical studies of epitaxial ruthenate and iridate films grown on single crystal substrates. Iridates and ruthenates are among the most studied oxygen evolution reaction (OER) ...electrocatalysts for water splitting applications. In the past decade, advances in deposition technologies and substrate availabilities have enabled the growth of single-crystal transition-metal oxides with high structural perfection. We use these advances, specifically Molecular-Beam Epitaxy (MBE), to prepare epitaxial ruthenate and iridate films on single crystal substrates. The use of epitaxial, single-crystal films allow us to link the features from our electrochemical characterizations to the physical adsorptions, which we will compare to the first-principle calculations. We further discuss the alloying effect on the surface adsorption process. We use this information to reveal insights into how to tune the surface physicochemical properties on the ruthenates and iridates and discuss the implications on the OER electrocatalysis.
Dielectric capacitors hold a tremendous advantage for energy storage due to their fast charge/discharge times and stability in comparison to batteries and supercapacitors. A key limitation to today's ...dielectric capacitors, however, is the low storage capacity of conventional dielectric materials. To mitigate this issue, antiferroelectric materials have been proposed, but relatively few families of antiferroelectric materials have been identified to date. Here, we propose a new design strategy for the construction of lead-free antiferroelectric materials using interfacial electrostatic engineering. We begin with a ferroelectric material with one of the highest known bulk polarizations, BiFeO3. We show that by confining atomically-precise thin layers of BiFeO3 in a dielectric matrix that we can induce a metastable antiferroelectric structure. Application of an electric field reversibly switches between this new phase and a ferroelectric state, in addition, tuning of the dielectric layer causes coexistence of the ferroelectric and antiferroelectric states. Precise engineering of the structure generates an antiferroelectric phase with energy storage comparable to that of the best lead-based materials. The use of electrostatic confinement provides a new pathway for the design of engineered antiferroelectric materials with large and potentially coupled responses.
In many unconventional superconductors, the presence of a pseudogap - a suppression in the electronic density of states extending above the critical temperature - has been a long-standing mystery. ...Here, we employ combined \textit{in situ} electrical transport and angle-resolved photoemission spectroscopy (ARPES) measurements to reveal an unprecedentedly large pseudogap regime in single-layer FeSe/SrTiO\(_3\), an interfacial superconductor where incoherent Cooper pairs are initially formed above \(T_{\Delta}\) \(\approx\) 60 K, but where a zero resistance state is only achieved below \(T_{0}\) \(<\) 30 K. We show that this behavior is accompanied by distinct transport signatures of two-dimensional phase fluctuating superconductivity, suggesting a mixed vortex state hosting incoherent Cooper pairs which persist well above the maximum clean limit \(T_{c}\) of \(\approx\) 40 K. Our work establishes the critical role of reduced dimensionality in driving the complex interplay between Cooper pairing and phase coherence in two-dimensional high-\(T_c\) superconductors, providing a paradigm for understanding and engineering higher-\(T_{c}\) interfacial superconductors.
We investigate the vibrational properties of VO2, particularly the low temperature M1 phase by first-principles calculations using the density functional theory as well as Raman spectroscopy. We ...perform the structural optimization using SCAN meta-GGA functional and obtain the optimized crystal structures for metallic rutile and insulating M1 phases satisfying all expected features of the experimentally derived structures. Based on the harmonic approximation around the optimized structures at zero temperature, we calculate the phonon properties and compare our results with experiments. We show that our calculated phonon density of states is in excellent agreement with the previous neutron scattering experiment. Moreover, we reproduce the phonon softening in the rutile phase as well as the phonon stiffening in the M1 phase. By comparing with the Raman experiments, we find that the Raman-active vibration modes of the M1 phase is strongly correlated with the V-V dimer distance of the crystal structure. Our combined theoretical and experimental framework demonstrates that Raman spectroscopy could serve as a reliable way to detect the subtle change of V-V dimer in the strained VO\(_2\).
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