Ahmet Avsar

Chiral spin textures, largely driven by the Dzyaloshinskii–Moriya interaction, offer significant potential for next‐generation computing technologies due to their chirality and topological stability. Ferroelectric/ferromagnetic van der Waals heterostructures are particularly appealing because they can combine interfacial inversion‐symmetry breaking and spin‐orbit coupling to promote interfacial Dzyaloshinskii–Moriya interaction, while switchable ferroelectric polarization provides a nonvolatile tuning knob. This study investigates interfacial chiral spin textures in few‐layer /‐ heterostructures. Two groups of topological Hall signals are identified just below and above the coercive field, and thickness‐dependent transport reveals a notable reduction in critical temperature with increasing layer thickness. Low‐temperature magnetic force microscopy images reveal two types of magnetic bubbles with opposite magnetic contrasts near the coercive field, each associated with distinct topological Hall signals. Together with atomistic spin‐dynamics simulations and first‐principles calculations, these results support the formation of interfacial DMI‐stabilized chiral spin textures. Switching the ferroelectric polarization of the ‐ layer further enables nonvolatile modulation of both anomalous and topological Hall effects. The resulting ferroelectric and magnetic bistabilities generate four distinguishable Hall resistance states programmable by electric and magnetic fields. These findings highlight the potential of van der Waals interfaces for advanced device applications.

Dilute magnetic semiconductors (DMSs) provide a platform for electrically controlling spin interactions; however, conventional systems face limited gate tunability and structural disorder. Here, we demonstrate gate-switchable magnetism in air-stable, dilute Fe-doped PtSe ₂ , a two-dimensional (2D) DMS that remains structurally homogeneous down to the atomically thin limit. Bulk crystals exhibit ferromagnetism with a Curie temperature of 320 kelvin, and this coupling persists in metallic devices down to seven layers. As thickness and carrier concentration further decrease, the system transitions to antiferromagnetic order, with a gate-tunable Néel temperature reaching 105 kelvin in five-layer semiconducting devices. Our first-principles calculations reveal a carrier-density–dependent crossover from Ruderman-Kittel-Kasuya-Yosida–mediated ferromagnetism to superexchange-driven antiferromagnetism. These findings demonstrate how induced magnetic order evolves from bulk to the 2D limit, providing a pathway to functional spintronic devices with electrically controlled magnetic states.

Graphene can support spin transport over long distances, yet achieving large electrical spin signals remains challenging because spin injection and detection are highly sensitive to disorder at tunnel-barrier interfaces. Here we demonstrate that suppressing such interfacial disorder enables high-fidelity spin injection and detection in graphene. We fabricate van der Waals graphene spin valves by exfoliating and assembling constituent two-dimensional crystals inside an inert glovebox, combined with contamination-suppressing lamination and thorough post-transfer cleaning to realize atomically flat hexagonal boron nitride tunnel barriers. Our four-terminal nonlocal devices exhibit exceptionally large spin polarizations approaching 90 percent and nonlocal spin signals up to 1.6 kΩ. The high tunnel-barrier quality enables robust spin detection down to nanoampere excitation currents and gate-tunable magnetoresistance exceeding 80 percent. Spin precession measurements reveal Elliott–Yafet-type relaxation with nearly isotropic spin dynamics. These results establish interface-controlled van der Waals fabrication as an effective route to high-signal spin transport in graphene.

Ionic-liquid gating enables electrostatic carrier accumulation, allowing transport-based band-edge determination while extending chemical potential into high-density regimes where interaction effects emerge strongly. PtSe₂ exhibits a pronounced thickness-dependent electronic structure, evolving from a gapped semiconductor in the ultrathin limit to a metallic state in thicker crystals. While the metallic regime has been widely studied, quantitative determination of the transport gap and controlled carrier tuning in the ultrathin semiconducting thickness range require electrostatic access beyond conventional dielectric gating. Here, by using ambipolar ionic liquid-gated transistors, we systematically probe transport in ultrathin PtSe₂. We extract electrical bandgaps of ∼1.1 eV (2L), ∼0.8 eV (3L), and ∼0.46 eV (4L), followed by complete gap closure at five layers. In 4L PtSe₂, high carrier densities drive a metal-insulator crossover and reveal distinct temperature-dependent regimes, including Fermi-liquid T² resistivity, extended T-linear behavior, and a low-temperature logarithmic upturn, establishing PtSe₂ as a thickness- and carrier-density-tunable correlated system.

Metal-insulator transitions in low band-gap systems emerge from the interplay of interactions, disorder, and band structure on comparable energy scales. Platinum diselenide, with its thickness-dependent electronic structure, provides an ideal platform to explore this regime, as the five-layer limit lies close to the semimetallic boundary while retaining a small bandgap (~0.1 eV). Here we show that hexagonal boron nitride-encapsulated five-layer PtSe₂ devices with embedded ultra-flat, pre-patterned platinum contacts exhibit high electronic quality, with mobilities up to 1,640 cm²V^-1s^-1 and contact resistances as low as 1.31 kΩ·μm at 1.5 K. These characteristics enable a gate-tunable metal-insulator transition at carrier densities down to 2.75 × 10¹⁰ cm⁻². Transport in this regime is dominated by electron-electron interactions, as supported by finite-temperature scaling. Below 50 K, however, an additional disorder-related energy scale emerges, consistent with percolation. These results demonstrate that disorder remains relevant even in high-mobility devices, challenging a purely interaction-driven transition in two-dimensional systems.

The magnetic proximity effect induces spin splitting in graphene through interfacial exchange coupling, enabling control of spin-resolved band structure, most clearly revealed near charge neutrality where low carrier density enhances spin-dependent transport signatures. Here, we use cobalt contacts to induce magnetic proximity in graphene and probe spin-resolved bands with pure spin currents, observing a gate-tunable inversion of the nonlocal spin signal near the charge neutrality point. Similar inversions occur at satellite neutrality points in graphene-boron nitride aligned superlattices, demonstrating that proximity-induced spin splitting governs spin transport across both primary and reconstructed bands. In a bilayer graphene superlattice device, where a bandgap enhances energy-selective spin filtering, we observe spin polarizations approaching 50% and nonlocal spin resistances exceeding 300 Ω, nearly two orders of magnitude larger than those away from the charge neutrality point. Such electrically controlled spin polarization via proximity interactions at low carrier densities opens opportunities for low-power spintronic devices.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are promising materials for next-generation complementary metal-oxide-semiconductor (CMOS) technologies owing to their atomically thin channels and strong electrostatic control. Achieving the modulation of carrier polarity within a single material system is desirable for CMOS integration but remains challenging. Low-bandgap TMDCs could facilitate effective polarity tuning. Additionally, such materials provide a sensitive medium for probing doping-induced electronic evolution, where small perturbations can strongly shift the Fermi level. Here, we employ five-layer PtSe₂, an air-stable TMDC with a low bandgap of ∼0.1 eV, as a material platform to investigate doping-driven transport modulation. Through dilute (∼2%) incorporation of period-four transition metal dopants, we drive a continuous transition from intrinsic n-type semiconducting behavior (pristine) to p-type semiconducting (V, Mn-doped), through a heavily p-doped regime (Fe-doped), and ultimately to a fully metallic state (Cr-doped). In Cr-PtSe₂, we observe four-terminal (4T) resistivity as low as 200 Ω and achieve a very high hole carrier density of ∼7.8 × 10¹⁴ cm^-2, reflecting the strong dopant-induced Fermi level tuning. This study shows a broad, doping-controlled conduction spectrum within a single TMDC, characterizes dopant-host interactions and electronic structure modulation, and is relevant to CMOS-compatible low-bandgap 2D semiconductors.

Avalanche field-effect transistors (AFETs) based on two-dimensional (2D) materials have attracted growing interest in optoelectronics due to their enhanced performance via carrier multiplication and their potential applications in nanoelectronics. However, most AFETs employing 2D materials face challenges with high breakdown fields and low ionization indexes, which limit their applications in optoelectronics. Here, we report a ReSe₂-based AFET that achieves a breakdown electric field down to 2.55 kVcm^-1 and an ionization index up to 38.79. This performance is attributed to using anisotropic ReSe₂ as the channel material, which reduces unnecessary carrier collisions. Moreover, the incorporation of HfZrO₂ as the dielectric enhances gate modulation, which further mitigates scattering effects. The underlying mechanism is validated through calculations of electron effective masses along both in- and out-of-plane directions. Moreover, scattering probability within ReSe₂ based on simulation model and experimental data further corroborates the proposed mechanism. As a demonstration, ReSe₂ avalanche phototransistors with a high responsivity of 1.71×10⁴AW^-1 and a high gain of 173 are realized based on this platform. By incorporating anisotropic 2D materials and high-k dielectric with less carrier scattering, this AFET design provides a promising pathway for developing high-performance avalanche photodetectors.

The intrinsic magnetic topological insulator MnBi ₂ Te ₄ is a promising platform for exploring quantum phases with nontrivial band topology and for enabling electrical control over coupled magnetic and electronic phase transitions. In‐plane magnetic fields, in particular, offer a distinct means of tuning these properties by strengthening quantized Hall effects, enhancing surface energy gaps, and driving spin reorientation transitions. However, a systematic understanding of how such fields affect magnetotransport is limited. Here, the magnetotransport behavior of few‐layer MnBi ₂ Te ₄ as a function of gate voltage, temperature, and magnetic field angle, with a primary focus on in‐plane field effects, are investigated. A gate‐tunable crossover in magnetoresistance is observed from positive to negative values under in‐plane magnetic fields as the gate voltage is swept below the charge neutrality point at temperatures below the Néel temperature. The in‐plane field drives a transition from the antiferromagnetic ground state to a ferromagnetic configuration with spins aligned in‐plane, while simultaneously altering the electronic structure, as revealed by gate‐dependent transport features. The angle‐dependent measurements reveal strongly gate‐tunable magnetotransport anisotropy. These results establish in‐plane magnetic fields as an effective tuning parameter for modulating spin and charge transport in MnBi ₂ Te ₄ , advancing prospects for reconfigurable spintronic and topological devices.