Effect of magnetic field on the electronic transport in trilayer graphene

Spintronics in Two-Dimensional Materials

Spintronics, exploiting the spin degree of electrons as the information vector, is an attractive field for implementing the beyond Complemetary metal-oxide-semiconductor (CMOS) devices. Recently, two-dimensional (2D) materials have been drawing tremendous attention in spintronics owing to their distinctive spin-dependent properties, such as the ultra-long spin relaxation time of graphene and the spin-valley locking of transition metal dichalcogenides. Moreover, the related heterostructures provide an unprecedented probability of combining the different characteristics via proximity effect, which could remedy the limitation of individual 2D materials. Hence, the proximity engineering has been growing extremely fast and has made significant achievements in the spin injection and manipulation. Nevertheless, there are still challenges toward practical application; for example, the mechanism of spin relaxation in 2D materials is unclear, and the high-efficiency spin gating is not yet achieved. In this review, we focus on 2D materials and related heterostructures to systematically summarize the progress of the spin injection, transport, manipulation, and application for information storage and processing. We also highlight the current challenges and future perspectives on the studies of spintronic devices based on 2D materials.

Direct Observation of the Linear Dichroism Transition in Two-Dimensional Palladium Diselenide

The linear dichroism (LD) transition within anisotropic photonic materials displays promising prospects for applications in polarization-wavelength-selective detectors, optical switching, and optical communication. In conventional two-dimensional (2D) anisotropic materials, the LD is predominantly uniaxial over a broad spectrum of wavelengths and arises principally from the reduced symmetry of the materials. However, the LD transition behavior in crystalline 2D materials remains elusive. Here, we demonstrate the observation of a unique LD conversion phenomenon at a wavelength of 472 nm in palladium diselenide (PdSe₂) using polarization-resolved absorption spectroscopy. This material exhibits prominent anisotropic responses and a high absorption ratio of α_y/α_x ≈ 1.11 at 364 nm, 1.15 at 532 nm, and 0.84 at 633 nm. We propose that this abnormal LD conversion behavior originates from the forceful localization rules at different parallel energy bands that exist within this material. Furthermore, the robust periodicity of A_g and B_1g modes in polarization-resolved Raman spectroscopy is in good agreement with the theoretical structure symmetry analysis. This indicates the strong intrinsic LD effect in the anisotropic nature of PdSe₂, which offers a macrolevel determination of crystal orientations. Such unique LD conversion features, in combination with strong LD effects, enable the air-stable PdSe₂ to be a potential candidate for technological innovations in multispectral imaging, sensing, and polarization-sensitive and wavelength-controllable photoelectronic applications.

Quantum parity Hall effect in Bernal-stacked trilayer graphene

The quantum Hall effect has recently been generalized from transport of conserved charges to include transport of other approximately conserved-state variables, including spin and valley, via spin- or valley-polarized boundary states with different chiralities. Here, we report a class of quantum Hall effect in Bernal- or ABA-stacked trilayer graphene (TLG), the quantum parity Hall (QPH) effect, in which boundary channels are distinguished by even or odd parity under the system's mirror reflection symmetry. At the charge neutrality point, the longitudinal conductance [Formula: see text] is first quantized to [Formula: see text] at a small perpendicular magnetic field [Formula: see text], establishing the presence of four edge channels. As [Formula: see text] increases, [Formula: see text] first decreases to [Formula: see text], indicating spin-polarized counterpropagating edge states, and then, to approximately zero. These behaviors arise from level crossings between even- and odd-parity bulk Landau levels driven by exchange interactions with the underlying Fermi sea, which favor an ordinary insulator ground state in the strong [Formula: see text] limit and a spin-polarized state at intermediate fields. The transitions between spin-polarized and -unpolarized states can be tuned by varying Zeeman energy. Our findings demonstrate a topological phase that is protected by a gate-controllable symmetry and sensitive to Coulomb interactions.

Recent Progress in the Fabrication, Properties, and Devices of Heterostructures Based on 2D Materials

With a large number of researches being conducted on two-dimensional (2D) materials, their unique properties in optics, electrics, mechanics, and magnetics have attracted increasing attention. Accordingly, the idea of combining distinct functional 2D materials into heterostructures naturally emerged that provides unprecedented platforms for exploring new physics that are not accessible in a single 2D material or 3D heterostructures. Along with the rapid development of controllable, scalable, and programmed synthesis techniques of high-quality 2D heterostructures, various heterostructure devices with extraordinary performance have been designed and fabricated, including tunneling transistors, photodetectors, and spintronic devices. In this review, we present a summary of the latest progresses in fabrications, properties, and applications of different types of 2D heterostructures, followed by the discussions on present challenges and perspectives of further investigations.

Stretchable thin-film transistors with molybdenum disulfide channels and graphene electrodes

Two-dimensional (2D) materials including graphene and transition metal dichalcogenides (TMDCs) have attracted great interest as new electronic materials, given their superior properties such as optical transparency, mechanical flexibility, and stretchability, especially for application in next-generation displays. In particular, the integration of graphene and TMDCs enables the implementation of 2D materials-based thin-film transistors (TFTs) in stretchable displays, given that TFTs are the fundamental element of various modern devices. In the present study, we demonstrate chemical-vapor-deposited molybdenum disulfide and graphene-based TFTs on a polymer substrate and investigate the electrical characteristics of TFTs under mechanical deformation to determine the stretchability of our devices. Furthermore, the mechanisms leading to TFT performance degradation are investigated, as they relate to the change in the contact resistance that is closely associated with the relative deformation of 2D materials under mechanical stress. Therefore, the synergetic integration of 2D materials with versatile electrical properties provides an important strategy for creating 2D materials-based stretchable TFTs, thus extending the excellent potential of 2D materials as innovative materials for stretchable active-matrix displays.

Observation of Anomalous Resistance Behavior in Bilayer Graphene

Our measurement results have shown that bilayer graphene exhibits an unexpected sharp transition of the resistance value in the temperature region 200~250 K. We argue that this behavior originates from the interlayer ripple scattering effect between the top and bottom ripple graphene layer. The inter-scattering can mimic the Coulomb scattering but is strongly dependent on temperature. The observed behavior is consistent with the theoretical prediction that charged impurities are the dominant scatters in bilayer graphene. The resistance increase with increasing perpendicular magnetic field strongly supports the postulate that magnetic field induces an excitonic gap in bilayer graphene. Our results reveal that the relative change of resistance induced by magnetic field in the bilayer graphene shows an anomalous thermally activated property.

Tunable Symmetries of Integer and Fractional Quantum Hall Phases in Heterostructures with Multiple Dirac Bands

The copresence of multiple Dirac bands in few-layer graphene leads to a rich phase diagram in the quantum Hall regime. Using transport measurements, we map the phase diagram of BN-encapsulated ABA-stacked trilayer graphene as a function charge density n, magnetic field B, and interlayer displacement field D, and observe transitions among states with different spin, valley, orbital, and parity polarizations. Such a rich pattern arises from crossings between Landau levels from different subbands, which reflect the evolving symmetries that are tunable in situ. At D=0, we observe fractional quantum Hall (FQH) states at filling factors 2/3 and -11/3. Unlike those in bilayer graphene, these FQH states are destabilized by a small interlayer potential that hybridizes the different Dirac bands.

Rich magneto-absorption spectra of AAB-stacked trilayer graphene

A generalized tight-binding model is developed to investigate the feature-rich magneto-optical properties of AAB-stacked trilayer graphene. Three intragroup and six intergroup inter-Landau-level (inter-LL) optical excitations largely enrich magneto-absorption peaks. In general, the former are much higher than the latter, depending on the phases and amplitudes of LL wavefunctions. The absorption spectra exhibit single- or twin-peak structures which are determined by quantum modes, LL energy spectra and Fermion distribution. The splitting LLs, with different localization centers (2/6 and 4/6 positions in a unit cell), can generate very distinct absorption spectra. There exist extra single peaks because of LL anti-crossings. AAB, AAA, ABA, and ABC stackings considerably differ from one another in terms of the inter-LL category, frequency, intensity, and structure of absorption peaks. The main characteristics of LL wavefunctions and energy spectra and the Fermi-Dirac function are responsible for the configuration-enriched magneto-optical spectra.

Temperature dependence of the electrical transport properties in few-layer graphene interconnects

We report a systematic investigation of the temperature dependence of electrical resistance behaviours in tri- and four-layer graphene interconnects. Nonlinear current-voltage characteristics were observed at different temperatures, which are attributed to the heating effect. With the resistance curve derivative analysis method, our experimental results suggest that Coulomb interactions play an essential role in our devices. The room temperature measurements further indicate that the graphene layers exhibit the characteristics of semiconductors mainly due to the Coulomb scattering effects. By combining the Coulomb and short-range scattering theory, we derive an analytical model to explain the temperature dependence of the resistance, which agrees well with the experimental results.

Observation of oscillatory resistance behavior in coupled Bernal and rhombohedral stacking graphene

We report on the first observation of an anomalous temperature-dependent resistance behavior in coupled Bernal and rhombohedral stacking graphene. At low-temperature regime (<50 K) the temperature-dependent resistance exhibits a drop while at high-temperature regions (>250 K), the resistance increases. In the transition region (50-250 K) an oscillatory resistance behavior was observed. This property is not present in any layered graphene structures other than five-layer. We propose that the temperature-dependent resistance behavior is governed by the interplay of the Coulomb and short-range scatterings. The origin of the oscillatory resistance behavior is the ABCAB and ABABA stacking configurations, which induces tunable bandgap in the five-layer graphene. The obtained results also indicate that a perpendicular magnetic field opens an excitonic gap because of the Coulomb interaction-driven electronic instabilities, and the bandgap of the five-layer graphene is thermally activated. Potentially, the observed phenomenon provides important transport information to the design of few-layer graphene transistors that can be manipulated by a magnetic field.