Muhammad Avicenna Naradipa

The surfaces of strongly correlated materials can exhibit exotic properties that differ substantially from the bulk. Investigation of these surface effects requires both high quality surfaces, and an experimental technique capable of disentangling surface and underneath layers or bulk. Herein, we overcome these two challenges, using newly developed grazing-incident-angle resonant soft x-ray and magnetic scattering techniques to study ultrathin films of Spin(S) = 1/2 La₂CuO₄, the parent compound of high-critical-temperature (high-T_c) superconductor. We observe emergent surface magnetic ordering and surface-like electronic dipole layers at room temperature. As a function of temperature, intrinsic holes from Cu²⁺ and O^2- ions are mitigated from the surface into subsurface layers and back into the surface. The intrinsic hole and oxygen ion mitigation have different rates, yielding strong temperature hysteresis. Supported with theoretical calculations, these surface orderings consist of a mixture of Cu²⁺ 3d⁹ (S = 1/2) and Cu¹⁺ 3d¹⁰-like (S = 0) with strong magneto-electronic coupling through the many-body upper Hubbard band. Our results highlight intrinsic holes and oxygen ion mitigations, yielding surface-like electronic dipole and magnetic orderings and the potential of grazing-incident-angle resonant soft X-ray and magnetic scattering to elucidate, layer-by-layer, magnetic and electronic structures."

Interactions of chiral light with chiral matter, such as Raman optical activity (ROA) and, independently, spin-polarized materials have attracted a lot of interest for both fundamental science and applications. The ROA, on the one hand, provides information on chiral phonons of molecules. However, the short-lifetime ROA signal in general is extremely weak and requires long exposure times, making it not accessible for many important systems with short lifetime. Materials exhibiting high spin polarization in d or f band, on the other hand, remain very limited even at very low temperature. There has been no report on materials exhibiting spin polarization in s band. Herewith, we report a room temperature, full spin polarization in unconventional, correlated s band of highly oriented single-crystalline gold quantum-dots (HOSG-QDs). Intriguingly, the HOSG-QDs produce a chiral correlated-plasmons enhanced Raman optical activity (CP-ROA) with anomalous ROA enhancement and strong spin-dependent chiral coupling. We then address a fundamental problem in crystal violet. Using spin-polarized HOSG-QDs chips, we observe strong CP-ROA signal, revealing chiral properties. The chiral correlated-plasmons of HOSG-QDs interact with the spin, electronic, and lattice structures of crystal violet, revealing chiral phonons and chiral electronic Raman excitations of crystal violet. Such a strong CP-ROA spectrum is obtained within a minute of measurement and a simple preparation without patterning. Our result shows that the CP-ROA based on a spin-polarized HOSG-QDs is extremely sensitive to the chiral property of phonon and spin and electronic structures and a fast, label-free chiral spectroscopic-based detection.

Ferromagnetic insulators and plasmons have attracted a lot of interest due to their rich fundamental science and applications. Recent research efforts have been made to find dopant-free ferromagnetic insulators and unconventional plasmons independently both in strongly correlated electron systems. However, our understanding of them is still lacking. Existing dopant-free ferromagnetic insulator materials are mostly limited to complex d- or f-systems with extremely low Curie temperature, low-symmetry structure, and strict growth conditions on specific substrates, limiting their compatibility with industrial applications. Unconventional plasmon is, on the other hand, a quasiparticle that originates from the collective excitation of correlated-charges, yet they are rarely explored, particularly in ferromagnetic insulator materials. Herewith, we present a novel, room temperature dopant-free ferromagnetic Mott-like insulator with a high-symmetry structure in unconventional strongly correlated s band of low-dimensional highly oriented single-crystal gold quantum dots (HOSG-QDs) on MgO(001). Interestingly, HOSG-QDs show new high-energy correlated-plasmons with low-plasmonics-loss. With a series of state-of-the-art experimental techniques, we find that the Mott-insulating state is tunable with surprisingly strong spin-splitting and spin polarization accompanied by strong s–s transitions, disappearance of Drude response, and generating new Mott-like gap. Supported with a series of theoretical calculations, the interplay of quantum confinement, many-body electronic correlations, and hybridizations tunes electron–electron correlations in s band and determines the ferromagnetism, Mott-like insulator, and high-energy correlated-plasmons. Our result shows a new class of room temperature dopant-free ferromagnetic Mott-like insulator and high-energy correlated-plasmons with low-loss in strongly correlated s band and opens unexplored applications of low-dimensional gold in spin field-effect transistors and plasmonics.

Low-dimensional systems host many exotic physical properties. Using extended resonant soft x-ray scattering supported with theoretical calculations, we observe concomitantly a Wigner-like crystal in a stripe-like QHKL = (001) superlattice with surprisingly long coherence lengths and new soft x-ray correlated plasmons in WS2. A new electronic transition is found at 125 K upon cooling, which is a transition from two-dimensional ordering to three-dimensional ordering, a result of p–d hybridization anisotropy and electronic correlations. Our result shows the importance of the interplay of long-range electron–electron correlation and short-range electronic screening in determining electronics and optical properties of transition metal dichalcogenides.

Metallic contaminants in complex heterostructures are important topics due to their significant roles in determining physical properties as well as devices' performance. Particularly, heterostructures of polyimide via on Al pad and Cu redistribution layer (RDL) on polyimide have shown exotic properties and are important for advanced semiconductor packaging systems. One main problem is significant leakage current variations, which affect the performance of the devices, yet its origin is far from understood. Furthermore, metal contaminations, if any, would occur at the buried interfaces, and it is particularly challenging to probe them. Until now, the electronic and optical properties of complex polyimide heterostructures and the roles of metallic contaminants have not been studied extensively. Herewith, using spectroscopic ellipsometry in broad deep ultraviolet (DUV) range supported with finite-difference time-domain calculations, we determine optical properties with various concentration of contaminants and their influence on device performance of under-bump vias and RDL architectures, especially at the metal–bump interface and surface between RDL. The complex dielectric function reveals varying contamination levels and different metals responsible for chip performance. Metallic contaminants are embedded within ∼50 nm in the polyimide, and different metals are distinguishable with varying concentrations (1.3%–30% relative volume fraction), in agreement with contact measurements in highly complex structures. Our result shows the potency of spectroscopic ellipsometry in the DUV and paves the way for nondestructive, advanced quality control and metrology applications in integrated advanced electronics packaging systems.

Two-dimensional (2D) perovskite oxide interfaces are ideal systems to uncover diverse emergent properties, such as the arising polaronic properties from short-range charge–lattice interactions. Thus, a technique to detect this quasiparticle phenomenon at the buried interface is highly coveted. Here, we report the observation of 2D small-polarons at the LaAlO3/SrTiO3 conducting interface using high-resolution spectroscopic ellipsometry. First-principles investigations show that interfacial electron–lattice coupling mediated by the longitudinal phonon mode facilitates the formation of these polarons. This study resolves the long-standing question by attributing the formation of interfacial 2D small polarons to the significant mismatch between experimentally measured interfacial carrier density and theoretical values. Our study sheds light on the complexity of broken periodic lattice-induced quasi-particle effects and its relationship with exotic phenomena at complex oxide interfaces. Meanwhile, this work establishes spectroscopic ellipsometry as a useful technique to detect and locate optical evidence of polaronic states and other emerging quantum properties at the buried interface.

A many-body large polaron, which is important for both fundamental physics and technological applications, has been predicted to occur in bismuth vanadate (BiVO₄). Herein, using a combination of high-resolution spectroscopic ellipsometry, X-ray absorption spectroscopy at the V L_3,2- and O K-edges, and high-resolution X-ray diffraction supported by theoretical calculations, we reveal a new many-body large-hole polaron in W-doped BiVO₄ films and the interplay of the large-hole polaron and indirect bandgap when determining the photocatalytic activity. With various W doping concentrations and temperatures, anomalous spectral weight transfers in the complex dielectric function are observed, revealing electronic correlations, particularly the on-site Coulomb interactions of O p (U_pp) and V d (U_dd), and screening in BiVO₄. Due to the distortion of BiO₈ dodecahedra and U_dd, Bi 6 s is lifted to the top of the valance band, which results in the formation of an indirect bandgap and a large-hole polaron. The large-hole polaron is found to form as a localized midgap state, consisting of O p hybridized with the V d and Bi sp orbitals, and this is important when determining the high photocatalytic activity of BiVO_4. Our results show the importance of the interplay among the charge, orbital, and lattice degrees of freedom in forming the many-body large-hole polaron, which improves the conductivity and results in a transition metal oxide with high photocatalytic activity.