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Reinforcement learning (RL) has recently shown promise in predicting Alzheimer's disease (AD) progression due to its unique ability to model domain knowledge. However, it is not clear which RL algorithms are well-suited for this task. Furthermore, these methods are not inherently explainable, limiting their applicability in real-world clinical scenarios. Our work addresses these two important questions. Using a causal, interpretable model of AD, we first compare the performance of four contemporary RL algorithms in predicting brain cognition over 10 years using only baseline (year 0) data. We then apply SHAP (SHapley Additive exPlanations) to explain the decisions made by each algorithm in the model. Our approach combines interpretability with explainability to provide insights into the key factors influencing AD progression, offering both global and individual, patient-level analysis. Our findings show that only one of the RL methods is able to satisfactorily model disease progression, but the post-hoc explanations indicate that all methods fail to properly capture the importance of amyloid accumulation, one of the pathological hallmarks of Alzheimer's disease. Our work aims to merge predictive accuracy with transparency, assisting clinicians and researchers in enhancing disease progression modeling for informed healthcare decisions. Code is available at https://github.com/rfali/xrlad., Comment: Previous versions accepted to NeurIPS 2023's XAIA and AAAI 2024's XAI4DRL workshops

Single-photon emitters serve as building blocks for many emerging concepts in quantum photonics. The recent identification of bright, tunable, and stable emitters in hexagonal boron nitride (hBN) has opened the door to quantum platforms operating across the infrared to ultraviolet spectrum. While it is widely acknowledged that defects are responsible for single-photon emitters in hBN, crucial details regarding their origin, electronic levels, and orbital involvement remain unknown. Here, we employ a combination of resonant inelastic X-ray scattering and photoluminescence spectroscopy in defective hBN unveiling an elementary excitation at 285 meV that gives rise to a plethora of harmonics correlated with single-photon emitters. We discuss the importance of N $\pi^*$ antibonding orbitals in shaping the electronic states of the emitters. The discovery of the elementary excitations of hBN provides new fundamental insights into quantum emission in low-dimensional materials, paving the way for future investigations in other platforms.

The growing field of quantum information technology requires propagation of information over long distances with efficient readout mechanisms. Excitonic quantum fluids have emerged as a powerful platform for this task due to their straightforward electro-optical conversion. In two-dimensional transition metal dichalcogenides, the coupling between spin and valley provides exciting opportunities for harnessing, manipulating and storing bits of information. However, the large inhomogeneity of single layers cannot be overcome by the properties of bright excitons, hindering spin-valley transport. Nonetheless, the rich band structure supports dark excitonic states with strong binding energy and longer lifetime, ideally suited for long-range transport. Here we show that dark excitons can diffuse over several micrometers and prove that this repulsion-driven propagation is robust across non-uniform samples. The long-range propagation of dark states with an optical readout mediated by chiral phonons provides a new concept of excitonic devices for applications in both classical and quantum information technology.

Atomic-scale control of light-matter interactions represent the ultimate frontier for many applications in photonics and quantum technology. Two-dimensional semiconductors, including transition metal dichalcogenides, are a promising platform to achieve such control due to the combination of an atomically thin geometry and convenient photophysical properties. Here, we demonstrate that a variety of durable polymorphic structures can be combined to generate additional optical resonances beyond the standard excitons. We theoretically predict and experimentally show that atomic-sized patches of 1T phase within the 1H matrix form unique electronic bands that give rise to new and robust optical resonances with strong absorption, circularly polarized emission and long radiative lifetime. The atomic manipulation of two-dimensional semiconductors opens unexplored scenarios for light harvesting devices and exciton-based photonics.

Transition metal dichalcogenides (TMDs) are layered materials that have a semiconducting phase with many advantageous optoelectronic properties, including tightly bound excitons and spin-valley locking. In Tungsten-based TMDs, spin and momentum forbidden transitions give rise to dark excitons that typically are optically inaccessible but represent the lowest excitonic states of the system. Dark excitons can deeply affect transport, dynamics and coherence of bright excitons, hampering device performance. Therefore, it is crucial to create conditions in which these excitonic states can be visualized and controlled. Here, we show that compressive strain in WS2 enables phonon scattering of photoexcited electrons between momentum valleys, enhancing the formation of dark intervalley excitons. We show that the emission and spectral properties of momentum-forbidden excitons are accessible and strongly depend on the local strain environment that modifies the band alignment. This mechanism is further exploited for strain sensing in two-dimensional semiconductors revealing a gauge factor exceeding 10^4.

The Weyl semimetal WTe$_{2}$ has shown several correlated electronic behaviors, such as the quantum spin Hall effect, superconductivity, ferroelectricity, and a possible exciton insulator state, all of which can be tuned by various physical and chemical approaches. Here, we discover a new electronic phase in WTe$_{2}$ induced by lithium intercalation. The new phase exhibits an increasing resistivity with decreasing temperature and its carrier density is almost two orders of magnitude lower than the carrier density of the semi-metallic T$_{d}$ phase, probed by in situ Hall measurements as a function of lithium intercalation. Our theoretical calculations predict the new lithiated phase to be a charge density wave (CDW) phase with a bandgap of ~ 0.14 eV, in good agreement with the in situ transport data. The new phase is structurally distinct from the initial T$_{d}$ phase, characterized by polarization angle-dependent Raman spectroscopy, and large lattice distortions close to 6 % are predicted in the new phase. Thus, we report the first experimental evidence of CDW in T$_{d}$-WTe$_{2}$, projecting WTe$_{2}$ as a new playground for studying the interplay between CDW and superconductivity. Our finding of a new gapped phase in a two-dimensional (2D) semi-metal also demonstrates electrochemical intercalation as a powerful tuning knob for modulating electron density and phase stability in 2D materials., Comment: 6 figures

Phase transitions of two-dimensional materials and their heterostructures enable many applications including electrochemical energy storage, catalysis, and memory; however, the nucleation pathways by which these transitions proceed remain underexplored, prohibiting engineering control for these applications. Here, we demonstrate that the lithium intercalation-induced 2H-1T' phase transition in MoS2 proceeds via nucleation of the 1T' phase at a heterointerface by monitoring the phase transition of MoS2/graphene and MoS2/hexagonal boron nitride (hBN) heterostructures with Raman spectroscopy in situ during intercalation. We observe that graphene-MoS2 heterointerfaces require an increase of 0.8 V in applied electrochemical potential to nucleate the 1T' phase in MoS2 compared to hBN-MoS2 heterointerfaces. The increased nucleation barrier at graphene-MoS2 heterointerfaces is due to the reduced charge transfer from lithium to MoS2 at the heterointerface as lithium also dopes graphene based on ab initio calculations. Further, we show that the growth of the 1T' domain propagates along the heterointerface, rather than through the interior of MoS2. Our results provide the first experimental observations of the heterogeneous nucleation and growth of intercalation-induced phase transitions in two-dimensional materials and heterointerface effects on their phase transition., Comment: 5 figures

We investigate the electronic structure of tungsten ditelluride (WTe$_2$) flakes with different thicknesses in magneto-transport studies. The temperature-dependent resistance and magnetoresistance (MR) measurements both confirm the breaking of carrier balance induced by thickness reduction, which suppresses the `turn-on' behavior and large positive MR. The Shubnikov-de-Haas oscillation studies further confirm the thickness-dependent change of electronic structure of WTe$_2$ and reveal a possible temperature-sensitive electronic structure change. Finally, we report the thickness-dependent anisotropy of Fermi surface, which reveals that multi-layer WTe$_2$ is an electronic 3D material and the anisotropy decreases as thickness decreases.

The intercalation-induced phase transition of MoS2 from the semiconducting 2H to the semimetallic 1T' phase has been studied in detail for nearly a decade; however, the effects of a heterointerface between MoS2 and other two-dimensional (2D) crystals on the phase transition have largely been overlooked. Here, ab initio calculations show that intercalating Li at a MoS2-hexagonal boron nitride (hBN) interface stabilizes the 1T phase over the 2H phase of MoS2 by ~ 100 mJ m-2, suggesting that encapsulating MoS2 with hBN may lower the electrochemical energy needed for the intercalation-induced phase transition. However, in situ Raman spectroscopy of hBN-MoS2-hBN heterostructures during electrochemical intercalation of Li+ shows that the phase transition occurs at the same applied voltage for the heterostructure as for bare MoS2. We hypothesize that the predicted thermodynamic stabilization of the 1T'-MoS2-hBN interface is counteracted by an energy barrier to the phase transition imposed by the steric hindrance of the heterointerface. The phase transition occurs at lower applied voltages upon heating the heterostructure, which supports our hypothesis. Our study highlights that interfacial effects of 2D heterostructures can go beyond modulating electrical properties and can modify electrochemical and phase transition behaviors., Comment: 4 figures and 1 table

Owing to the small energy differences between its polymorphs, MoTe2 can access a full spectrum of electronic states, from the 2H semiconducting state to the 1T semimetallic state, and from the Td Weyl semimetallic state to the superconducting state in the 1T and Td phase at low temperature. Thus, it is a model system for phase transformation studies as well as quantum phenomena such as the quantum spin Hall effect and topological superconductivity. Careful studies of MoTe2 and its potential applications require large area MoTe2 thin films with high crystallinity and thickness control. Here, we present cm2 scale synthesis of 2H MoTe2 thin films with layer control and large grains that span several microns. Layer control is achieved by controlling the initial thickness of the precursor MoOx thin films, which are deposited on sapphire substrates by atomic layer deposition and subsequently tellurized. Despite the van der Waals epitaxy, the precursor-substrate interface is found to critically determine the uniformity in thickness and grain size of the resulting MoTe2 films: MoTe2 grown on sapphire show uniform films while MoTe2 grown on amorphous SiO2 substrates form islands. This synthesis strategy decouples the layer control from the variabilities of growth conditions for robust growth results, and is applicable to grow other transition metal dichalcogenides with layer control., Comment: 6 figure

We report electron transport studies in an encapsulated few-layer WTe$_2$ at low temperatures and high magnetic fields. The magnetoconductance reveals a temperature-induced crossover between weak antilocalization (WAL) and weak localization (WL) in quantum diffusive regime. We show that the crossover clearly manifests coexistence and competition among several characteristic lengths, including the dephasing length, the spin-flip length, and the mean free path. In addition, low temperature conductance increases logarithmically with the increase of temperature indicating an interplay of electron-electron interaction (EEI) and spin-orbit coupling (SOC). We demonstrate the existences and quantify the strengths of EEI and SOC which are considered to be responsible for gap opening in the quantum spin hall state in WTe2 at the monolayer limit.

For topological insulators and topological crystalline insulators (TCIs), their exotic surface states are promising for fundamental condensed matter physics research as well as future electronics such as low-dissipation electronics and spintronics. However, the high bulk carrier density that often dominates the transport property is the major materials challenge, critically hindering our ability to study and manipulate the surface states. In this manuscript, we demonstrate an alloying strategy, SnSexTe1-x, to effectively reduce the bulk carrier density. As long as SnSexTe1-x remains in the cubic crystal structure, it is predicted to be a TCI. We show systematic decrease of the bulk carrier density with the increasing Se concentration, demonstrating that the alloying principle works. In addition, we map out the phase diagram of the cubic to the orthorhombic structural transition as a function of the Se concentration. This was made possible by studying alloy nanoplates which remain single-crystalline and is either in the cubic or the orthorhombic phase, in contrast to bulk alloys that would exhibit polycrystalline grains. Lastly, we investigate systematically the ferroelectric transition associated with the structural transition from the cubic to the rhombohedral phase for SnSexTe1-x. This is the first ferroelectric transition study of the alloy system SnSexTe1-x., Comment: 25 pages, 4 figures