arXiv subjects

Youngki Yeo

Publications and source records attributed to Youngki Yeo.

5 recordsLinked to original sources

arXiv2026

Polytype-Dependent Upconversion Photoluminescence in 3R-MoS2

Ferroelectric van der Waals materials offer switchable polarization states, yet optical readout of their stacking configurations remains challenging. Here, building on the resonant exciton-exciton annihilation (EEA) mechanism recently identified in 2H-phase TMDs, we report the first observation of upconversion photoluminescence (UPL) in rhombohedral MoS$_2$ and demonstrate that this many-body process is strongly polytype-dependent. Using low-temperature spectroscopy, we observe anti-Stokes emission with superlinear power dependence characteristic of EEA. Beyond serving as an accurate layer-number sensor due to discrete bandgap variations, UPL provides a sensitive probe of stacking order across thicknesses. The two neutral trilayer polytypes, which remain indistinguishable by surface potential measurements and second harmonic generation, exhibit markedly different UPL intensities. This sensitivity persists in thicker samples where multiple configurations coexist. First-principles calculations suggest that the intensity contrast originates primarily from the layer confinement of the annihilating excitons, while energy matching to the $Γ$ final-state manifold provides additional intensity selectivity. Power-dependent spectroscopy further disentangles two distinct annihilation channels originating from different dark exciton valleys, identified through their contrasting intensity scaling and opposite density-induced energy shifts. Crucially, the annihilation process doubles the energy separation of nearly degenerate dark excitons while converting their weak emission into bright signal, providing experimental access to valley-specific responses that are obscured in direct dark-exciton spectroscopy. Our findings demonstrate that ferroelectric configurations provide a new degree of freedom for controlling nonlinear optical processes.

cond-mat.mtrl-sci↗

arXiv2026

A Sliding Ferroelectric Resonant Tunnel Junction

Ferroelectric tunnel junctions (FTJs) leverage polarization-dependent tunneling through ultrathin barriers to enable two-terminal, non-volatile memory and logic. Although conceptually appealing, the practical implementation of conventional FTJs has been hindered by high coercive voltages, low readout currents, limited cycling endurance, and significant device-to-device variability. Here, we overcome these bottlenecks by introducing the sliding ferroelectric resonant tunnel (SFeRT) junction, integrating three cooperative mechanisms: (i) spontaneous interfacial polarization of atomically thin, depolarization-resilient barriers; (ii) superlubric sliding of shear-solitons, enabling ultra-low-friction, wear-free switching; and (iii) momentum-conserving, elastic resonant tunneling between lattice-aligned graphitic electrodes, providing sensitive readouts at both positive and negative biases. We demonstrate nanometer-scale SFeRT junctions using polar polytypes of hexagonal boron nitride (hBN) or transition metal dichalcogenides (TMDs) as barriers, achieving configurable writing voltages below $0.5$ V and tunable reading biases under $0.1$ V. These devices yield current densities exceeding $50$ nA $μ$m$^{-2}$, with a robust room-temperature ON/OFF ratio $> 7$. The crystalline and polarization integrity of sliding van der Waals (vdW) polytypes, down to the atomically thin limit, ensures exceptional device uniformity and performance that remains scalable down to sub-$0.1$ $μ$m$^{2}$ footprints. Furthermore, we provide a predictive model for SFeRT performance across diverse doping levels, temperatures, electrodes, and polytype configurations. Integrated within a Superlubric Array of Polytypes (SLAP) architecture, SFeRT junctions enable switching energies below $1$ fJ, establishing a scalable and durable foundation for low-energy ``slidetronic'' logic and memory.

cond-mat.other↗

arXiv2025

Switching Graphitic Polytypes in Elastically Coupled Islands

Van der Waals polytypes are commensurate configurations of two-dimensional layers with discrete crystalline symmetries and distinct stacking-dependent properties. In graphitic polytypes, the different stacking arrangements of graphene sheets exhibit rich electronic phases, such as intrinsic electric polarizations, orbital magnetizations, superconductivity, and anomalous fractional Hall states. Switching between these metastable periodic configurations by controlling interlayer shifts unlocks intriguing multiferroic responses. Here, we report super-lubricant arrays of polytypes (SLAP) devices, with nanometer-scale islands of Bernal polytypes that switch into Rhombohedral crystals and vice versa under a shear force as low as 6 nano-Newtons. We assemble these four-layer SLAP structures by aligning a pair of graphene bilayers above and under circular cavities in a misaligned spacer layer. Using local current measurements, we detect the shifts between the active bilayers and reveal long-range elastic relaxations outside the cavities that enable efficient nucleation and spontaneous sliding of stacking dislocation inside the islands. We demonstrate configurable, deterministic, and robust polytype switching by confining these boundary strips in narrow cavity channels that connect the islands. Such controlled switching between elastically-coupled single-crystalline islands is appealing for novel multiferroic SlideTronic applications.

cond-mat.mes-hall↗

arXiv2024

Switchable Crystalline Islands in Super Lubricant Arrays

Expanding the performance of field effect devices is a key challenge of the ever-growing chip industry at the core of current technologies. A highly desired nonvolatile response in tiny multiferroic transistors is expected by electric field control of atomic movements rather than the typical electronic redistribution. Recently, such field effect control of structural transitions was established in commensurate stacking configurations of honeycomb van der Waals (vdW) polytypes by sliding narrow boundary dislocations between oppositely polarized domains. The interfacial ferroelectric response, however, relied on preexisting boundary strips between relatively large micron-scale domains, severely limiting practical implementations. Here, we report the robust switching of single-domain polytypes in nm-scale islands embedded in super lubricant vdW arrays. We etch cavities into a thin layered spacer and then encapsulate it with parallel functional flakes. The incommensurate flakes above and under the spacer sag and touch at each cavity to form uniform crystalline islands free from interlayer deformations. By imaging the polytypes' ferroelectric response, we observe reversible nucleation and annihilation of boundary strips and geometry-adaptable hysteresis loops. Using mechanical stress, we accurately position the boundary strip, modify the interlayer twist angle, and nucleate intermediate polar domain patterns. By precisely designing the size, shape, symmetry, and distribution of the islands in these Super Lubricant Arrays of Polytype (SLAP), we envision numerous device functionalities and SlideTronics applications. These range from ultra-sensitive detectors of atomic-scale shifts to nonvolatile multi-ferroic tunneling transistors with tunable coercive switching fields, and even elastically-coupled memory cells for neuromorphic architectures.

cond-mat.mtrl-sci↗

arXiv2022

Reversibly controlled ternary polar states and ferroelectric bias promoted by boosting square-tensile-strain

Interaction between dipoles often emerges intriguing physical phenomena, such as exchange bias in the magnetic heterostructures and magnetoelectric effect in multiferroics, which lead to advances in multifunctional heterostructures. However, the defect-dipole tends to be considered the undesired to deteriorate the electronic functionality. Here, we report deterministic switching between the ferroelectric and the pinched states by exploiting a new substrate of cubic perovskite, BaZrO$_{3}$, which boosts square-tensile-strain to BaTiO$_{3}$ and promotes four-variants in-plane spontaneous polarization with oxygen vacancy creation. First-principles calculations propose a complex of an oxygen vacancy and two Ti$^{3+}$ ions coins a charge-neutral defect-dipole. Cooperative control of the defect-dipole and the spontaneous polarization reveals ternary in-plane polar states characterized by biased/pinched hysteresis loops. Furthermore, we experimentally demonstrate that three electrically controlled polar-ordering states lead to switchable and non-volatile dielectric states for application of non-destructive electro-dielectric memory. This discovery opens a new route to develop functional materials via manipulating defect-dipoles and offers a novel platform to advance heteroepitaxy beyond the prevalent perovskite substrates.

cond-mat.mtrl-sci↗