Emerging Leaders 2023

The identification of electronic processes at the charge-selective contact buried interface is very important for photovoltaic research. The main loss of perovskite solar cell (PeSCs) is generally bound up with its charge transfer layer. Especially, the current record for the highest power conversion efficiency of quasi-two-dimensional (quasi-2D) PeSCs is achieved by inverted device configurations, compared with the efficiency of upright structures. This study investigated, the carrier recombination and charge extraction in quasi-2D PeSCs by leveraging scanning probe microscope technology, steady-state photoluminescence (PL) measurements, and time-resolved PL spectroscopy. The built-in potential in quasi-2D bulk perovskite can be regarded as a budget to hinder energy loss in inverted device configurations. Interface photogenerated recombination in quasi-2D PeSCs can be fully comprehended only when the complete device is under consideration. Our work underlines the significance of considering restructuring loss from the perspective of the complete device instead of individual layers or interfaces in quasi-2D PeSCs.

Interfacial rheology studies were conducted to establish a connection between the rheological characteristics of particle-laden interfaces and the stability of Pickering foams. The behavior of foams stabilized with fumed and spherical colloidal silica particles was investigated, focusing on foam properties such as bubble microstructure and liquid content. Compared to a sodium dodecyl sulfate-stabilized foam, Pickering foams exhibited a notable reduction in bubble coarsening. Drop shape tensiometry measurements on particle-coated interfaces indicated that the Gibbs stability criterion was satisfied for both particle types at various surface coverages, supporting the observed arrested bubble coarsening in particle-stabilized foams. However, although the overall foam height was similar for both particle types, foams stabilized with fumed silica particles demonstrated a higher resistance to liquid drainage. This difference was attributed to the higher yield strain of interfacial networks formed by fumed silica particles, as compared to those formed by spherical colloidal particles at similar surface pressures. Our findings highlight that while both particles can generate long-lasting foams, the resulting Pickering foams may exhibit variations in microstructure, liquid content, and resistance to destabilization mechanisms, stemming from the respective interfacial rheological properties in each case.

Control of polarization states of light is crucial for any photonic system. However, conventional polarization-controlling elements are typically static and bulky. Metasurfaces open a new paradigm to realize flat optical components by engineering meta-atoms at sub-wavelength scale. Tunable metasurfaces can provide enormous degrees-of-freedom to tailor electromagnetic properties of light and thus have the potential to realize dynamic polarization control in nanoscale. In this study, we propose a novel electro-tunable metasurface to enable dynamic control of polarization states of reflected light. The proposed metasurface comprises a two-dimensional array of elliptical Ag-nanopillars deposited on indium-tin-oxide (ITO)–Al₂O₃–Ag stack. In unbiased condition, excitation of gap-plasmon resonance in the metasurface leads to rotation of x-polarized incident light to orthogonally polarized reflected light (i.e., y-polarized) at 1.55 μm. On the other hand, by applying bias-voltage, we can alter the amplitude and phase of the electric field components of the reflected light. With 2 V applied bias, we achieved a linearly polarized reflected light with a polarization angle of −45°. Furthermore, we can tune the epsilon-near-zero wavelength of ITO at the vicinity of 1.55 μm wavelength by increasing the bias to 5 V, which reduces y-component of the electric field to a negligible amplitude, thus, resulting in an x-polarized reflected light. Thus, with an x-polarized incident wave, we can dynamically switch among the three linear polarization states of the reflected wave, allowing a tri-state polarization switching (viz. y-polarization at 0 V, −45° linear polarization at 2 V, and x-polarization at 5 V). The Stokes parameters are also calculated to show a real-time control over light polarization. Thus, the proposed device paves the way toward the realization of dynamic polarization switching in nanophotonic applications.

Many-body localised (MBL) phases of disordered, interacting quantum systems allow for exotic localisation protected quantum order in eigenstates at arbitrarily high energy densities. In this work, we analyse the manifestation of such order on the Hilbert-space anatomy of eigenstates. Quantified in terms of non-local Hilbert-spatial correlations of eigenstate amplitudes, we find that the spread of the eigenstates on the Hilbert-space graph is directly related to the order parameters which characterise the localisation protected order, and hence these correlations, in turn, characterise the order or lack thereof. Higher-point eigenstate correlations also characterise the different entanglement structures in the many-body localised phases, with and without order, as well as in the ergodic phase. The results pave the way for characterising the transitions between MBL phases and the ergodic phase in terms of scaling of emergent correlation lengthscales on the Hilbert-space graph.

We leverage the Born–Oppenheimer approximation to present a general description of topological defects dynamics in p-atic materials on curved surfaces. Focusing on the case of an active nematic, we find that activity induces a geometric contribution to the motility of the $+1/2$ defect. Moreover, in the case of a cone, the simplest example of a geometry with curvature singularity, we find that the motility depends on the deficit angle of the cone and changes sign when the deficit angle is greater than π, leading to the change in active behavior from contractile (extensile) to extensile (contractile) behavior. Using our analytical framework, we then identify for positively charged defects the basin of attraction to the cone apex and present closed-form predictions for defect trajectories near the apex. The analytical results are quantitatively corroborated against full numerical simulations, with excellent agreement when the capture radius is small compared to the cone size.

Artificial microswimmers, i.e. colloidal scale objects capable of self-propulsion, have garnered significant attention due to their central role as models for out of equilibrium systems. Moreover, their potential applications in diverse fields such as biomedicine, environmental remediation, and materials science have long been hypothesized, often in conjunction with their ability to deliver cargoes to overcome mass transport limitations. A very efficient way to load molecular cargoes is to disperse them in a liquid compartment, however, fabricating microswimmers with multiple liquid compartments remains a significant challenge. To address this challenge, we present a modular fabrication platform that combines microfluidic synthesis and sequential capillarity-assisted particle assembly (sCAPA) for microswimmers with various liquid compartments. We demonstrate the synthesis of monodisperse, small polymer-based microcapsules (Ø = 3–6 μm) with different liquid cargoes using a flow-focusing microfluidic device. By employing the sCAPA technique, we assemble multiple microcapsules into microswimmers with high precision, resulting in versatile microswimmers with multiple liquid compartments and programmable functionalities. Our work provides a flexible approach for the fabrication of modular microswimmers, which could potentially actively transport cargoes and release them on demand in the future.

In this paper we present a first-principles study of the high-pressure superconducting phase diagram of calcium alanates (Ca–Al–H), based on ab-initio crystal structure prediction and anisotropic Migdal–Eliashberg Theory. Calcium alanates have been intensively studied at ambient pressure for their hydrogen-storage properties, but their high-pressure behavior is largely unknown. By performing a full scan of the ternary convex hull at several pressures between 0 and 300 GPa, we identify several new structural motifs, characterized by a high Al–H coordination, where Al d orbitals participate in the bonding. Among all new phases thus identified, we focus in particular on a phase with CaAlH₇ composition, which lies on the convex hull at 300 GPa, and remains dynamically stable down to 50 GPa, with a predicted superconducting T_c of 82 K, which likely represents a new promising template to achieve increase chemical precompression in ternary hydrides. Our findings reveal important insights into the structure-property relationships of calcium alanates under high pressure, and highlight a possible strategy to achieve conventional superconductivity at low pressures.

A major shortcoming of ultrawide-bandgap (UWBG) semiconductors is unipolar doping, in which either n-type or p-type conductivity is typically possible, but not both within the same material. For UWBG oxides, the issue is usually the p-type conductivity, which is inhibited by a strong tendency to form self-trapped holes (small polarons) in the material. Recently, rutile germanium oxide (r-GeO₂), with a band gap near 4.7 eV, was identified as a material that might break this paradigm. However, the predicted acceptor ionization energies are still relatively high (∼0.4 eV), limiting p-type conductivity. To assess whether r-GeO₂ is an outlier due to its crystal structure, the properties of a set of rutile oxides are calculated and compared. Hybrid density functional calculations indicate that rutile TiO₂ and SnO₂ strongly trap holes at acceptor impurities, consistent with previous work. Self-trapped holes are found to be unstable in r-SiO₂, a metastable polymorph that has a band gap near 8.5 eV. Group-III acceptor ionization energies are also found to be lowest among the rutile oxides and approach those of GaN. Acceptor impurities have sufficiently low formation energies to not be compensated by donors such as oxygen vacancies, at least under O-rich limit conditions. Based on the results, it appears that r-SiO₂ has the potential to exhibit the most efficient p-type conductivity when compared to other UWBG oxides.

We report the pressure (P) effect on the superconducting transition temperature T_c and the upper critical field μ₀H_c2 of infinite-layer Nd_0.8Sr_0.2NiO₂ thin films by measuring the electrical transport properties under various hydrostatic pressures to 4.6 GPa. At ambient pressure, it shows the clear superconducting transition with T_c ∼ 10 K. Based on the evolution of resistance R(T), we found that the T_c is monotonically enhanced to ∼14 K upon increasing pressure to 2.9 GPa. The constructed temperature–pressure phase diagram indicates that the calculated slope dT_c/dP is about 1.14 K GPa⁻¹ and the superconducting T_c shows no signatures of saturation with pressure. It thus gives the possibility to further enhance T_c by employing higher pressures or heterostructure engineering. In addition, the normalized slope of upper critical field μ₀H_c2(0) implies that the electron correlations are gradually decreasing with pressure, which exhibits an opposite evolution with superconducting T_c. Our work further confirms the positive pressure effects in nickelate superconductors and gives more insight to further enhance its superconducting transition temperature.

Single-molecule junctions (SMJs) may bring exotic physical effects. In this work, a significant thermal rectification effect is observed in a cross-dimensional system, comprising a diamond, a single-molecule junction, and a carbon nanotube (CNT). The molecular dynamics simulations indicate that the interfacial thermal resistance varies with the direction of heat flow, the orientation of the crystal planes of the diamond, and the length of the CNT. We find that the thermal rectification ratio escalates with the length of the CNT, achieving a peak value of 730% with the CNT length of 200 nm. A detailed analysis of phonon vibrations suggests that the primary cause of thermal rectification is the mismatched vibrations between the biphenyl and carbonyl groups. This discovery may offer theoretical insights for both the experimental exploration and practical application of SMJs in efficient thermal management strategy for high power and highly integrated chips.

Identifying phase boundaries of interacting systems is one of the key steps to understanding quantum many-body models. The development of various numerical and analytical methods has allowed exploring the phase diagrams of many Hermitian interacting systems. However, numerical challenges and scarcity of analytical solutions hinder obtaining phase boundaries in non-Hermitian many-body models. Recent machine learning methods have emerged as a potential strategy to learn phase boundaries from various observables without having access to the full many-body wavefunction. Here, we show that a machine learning methodology trained solely on Hermitian correlation functions allows identifying phase boundaries of non-Hermitian interacting models. These results demonstrate that Hermitian machine learning algorithms can be redeployed to non-Hermitian models without requiring further training to reveal non-Hermitian phase diagrams. Our findings establish transfer learning as a versatile strategy to leverage Hermitian physics to machine learning non-Hermitian phenomena.

Bringing advances in machine learning to chemical science is leading to a revolutionary change in the way of accelerating materials discovery and atomic-scale simulations. Currently, most successful machine learning schemes can be largely traced to the use of localized atomic environments in the structural representation of materials and molecules. However, this may undermine the reliability of machine learning models for mapping complex systems and describing long-range physical effects because of the lack of non-local correlations between atoms. To overcome such limitations, here we report a graph attention neural network as a unified framework to map materials and molecules into a generalizable and interpretable representation that combines local and non-local information of atomic environments from multiple scales. As an exemplary study, our model is applied to predict the electronic structure properties of metal-organic frameworks (MOFs) which have notable diversity in compositions and structures. The results show that our model achieves the state-of-the-art performance. The clustering analysis further demonstrates that our model enables high-level identification of MOFs with spatial and chemical resolution, which would facilitate the rational design of promising reticular materials. Furthermore, the application of our model in predicting the heat capacity of complex nanoporous materials, a critical property in a carbon capture process, showcases its versatility and accuracy in handling diverse physical properties beyond electronic structures.

The discovery of new superconductors based on topological insulators always captures special attention due to their unique structural and electronic properties. High pressure is an effective way to regulate the lattice as well as electronic states in the topological insulators, thus altering their electronic properties. Herein, we report the structural and electrical transport properties of the topological insulator GeBi₂Te₄ by using high-pressure techniques. The synchrotron x-ray diffraction revealed that GeBi₂Te₄ underwent two structural phase transitions from R-3m (phase I) to C2/m (phase II) and then into Im-3m (phase III). Superconductivity was observed at 6.6 GPa to be associated with the first structural phase transition. The superconducting transition temperature T_c reached a maximum value of 8.4 K, accompanied by the R_H sign changing from negative to positive at 14.6 GPa, then gradually decreased with increasing pressure in phase III, showing a dome-shaped phase diagram. The present results provide a platform for understanding the interplay between the crystal structure and superconductivity by the regulation of pressure in the topological insulator materials.

Floquet engineering has recently emerged as a technique for controlling material properties with light. Floquet phases can be probed with time- and angle-resolved photoelectron spectroscopy (Tr-ARPES), providing direct access to the laser-dressed electronic bands. Applications of Tr-ARPES to date focused on observing the Floquet-Bloch bands themselves, and their build-up and dephasing on sub-laser-cycle timescales. However, momentum and energy resolved sub-laser-cycle dynamics between Floquet bands have not been analyzed. Given that Floquet theory strictly applies in time-periodic conditions, the notion of resolving sub-laser-cycle dynamics between Floquet states seems contradictory—it requires probe pulse durations below a laser cycle that inherently cannot discern the time-periodic nature of the light-matter system. Here we propose to employ attosecond pulse train probes with the same temporal periodicity as the Floquet-dressing pump pulse, allowing both attosecond sub-laser-cycle resolution and a proper projection of Tr-ARPES spectra on the Floquet–Bloch bands. We formulate and employ this approach in ab-initio calculations in light-driven graphene. Our calculations predict significant sub-laser-cycle dynamics occurring within the Floquet phase with the majority of electrons moving within and in-between Floquet bands, and a small portion residing and moving outside of them in what we denote as 'non-Floquet' bands. We establish that non-Floquet bands arise from the pump laser envelope that induces non-adiabatic electronic excitations during the pulse turn-on and turn-off. By performing calculations in systems with poly-chromatic pumps we also show that Floquet states are not formed on a sub-laser-cycle level. This work indicates that the Floquet-Bloch states are generally not a complete basis set for sub-laser-cycle dynamics in steady-state phases of matter.

Perpendicular magnetic anisotropy (PMA) of magnets is paramount for electrically controlled spintronics due to their intrinsic potentials for higher memory density, scalability, thermal stability and endurance, surpassing an in-plane magnetic anisotropy (IMA). Nickel film is a long-lived fundamental element ferromagnet, yet its electrical transport behavior associated with magnetism has not been comprehensively studied, hindering corresponding spintronic applications exploiting nickel-based compounds. Here, we systematically investigate the highly versatile magnetism and corresponding transport behavior of nickel films. As the thickness reduces within the general thickness regime of a magnet layer for a memory device, the hardness of nickel films' ferromagnetic loop of anomalous Hall effect increases and then decreases, reflecting the magnetic transitions from IMA to PMA and back to IMA. Additionally, the square ferromagnetic loop changes from a hard to a soft one at rising temperatures, indicating a shift from PMA to IMA. Furthermore, we observe a butterfly magnetoresistance resulting from the anisotropic magnetoresistance effect, which evolves in conjunction with the thickness and temperature-dependent magnetic transformations as a complementary support. Our findings unveil the rich magnetic dynamics and most importantly settle down the most useful guiding information for current-driven spintronic applications based on nickel film: The hysteresis loop is squarest for the ∼8 nm-thick nickel film, of highest hardness with R_xy^r/R_xy^s ∼ 1 and minimum H_s−H_c, up to 125 K; otherwise, extra care should be taken for a different thickness or at a higher temperature.

We show that tailoring the geometric curvature profile of magnets can be used for bespoke design of an effective non-relativistic spin–orbit coupling, which may be used to control proximity effects if the magnet is coupled to a superconductor. We consider proximity-coupled one-dimensional magnetic wires with variable curvatures, specifically three distinct shapes classified as J-, C-, and S-type. We demonstrate a chirality-dependent spin polarization of the superconducting correlations, and show the role of curvature in determining the ground state of mixed-chirality junctions. We speculate on how this may be implemented in novel device design, and include analysis of its usage in a spin-triplet SQUID.

Lindbladian formalism, as tuned to dissipative and open systems, has been all-pervasive to interpret non-equilibrium steady states of quantum many-body systems. We study the fate of free fermionic and superconducting phases in a dissipative one-dimensional Kitaev model—where the bath acts both as a source and a sink of fermionic particles with different coupling rates. As a function of these two couplings, we investigate the steady state, its entanglement content, and its approach from varying initial states. Interestingly, we find that the steady state phase diagram retains decipherable signatures of ground state critical physics. We also show that early-time fidelity is a useful marker to find a subclass of phase transitions in such situations. Moreover, we show that the survival of critical signatures at late-times, strongly depend on the thermal nature of the steady state. This connection hints at a correspondence between quantum observables and classical magnetism in the steady state of such systems. Our work uncovers interesting connections between dissipative quantum many-body systems, thermalization of a classical spin and many-body quantum critical phenomena.

The chiral Majorana fermion is an exotic particle that is its own antiparticle. It can arise in a one-dimensional edge of topological materials, and especially that in a topological superconductor can be exploited in non-Abelian quantum computation. While the chiral Majorana mode (CMM) remains elusive, a promising situation is realized when superconductivity coexists with a topologically non-trivial surface state. Here, we perform fully non-empirical calculation for the CMM considering superconductivity and surface relaxation, and show that hexagonal close-packed thallium (Tl) has an ideal electronic state that harbors the CMM. The $k_z = 0$ plane of Tl is a mirror plane, realizing a full-gap band inversion corresponding to a topological crystalline insulating phase. Its surface and hinge are stable and easy to make various structures. Another notable feature is that the surface Dirac point is very close to the Fermi level, so that a small Zeeman field can induce a topological transition. Our calculation indicates that Tl will provide a new platform of the Majorana fermion.