Quantum Hall Interferometry in Triangular Domains of Marginally Twisted Bilayer Graphene.

Quantum Hall (QH) interferometry provides an archetypal platform for the experimental realization of braiding statistics of fractional QH states. However, the complexity of observing fractional statistics requires phase coherence over the length of the interferometer, as well as suppression of Coulomb charging energy. Here, we demonstrate a new type of QH interferometer based on marginally twisted bilayer graphene (mtBLG), with a twist angle θ ≈ 0.16°. With the device operating in the QH regime, we observe distinct signatures of electronic Fabry-Pérot and Aharonov-Bohm oscillations of the magneto-thermopower in the density-magnetic field phase space, at Landau level filling factors ν = 4, 8. We find that QH interference effects are intrinsic to the triangular AB/BA domains in mtBLG that show diminished Coulomb charging effects. Our results demonstrate phase-coherent interference of QH edge modes without any additional gate-defined complex architecture, which may be beneficial in experimental realizations of non-Abelian braiding statistics.

Breakdown of semiclassical description of thermoelectricity in near-magic angle twisted bilayer graphene.

The planar assembly of twisted bilayer graphene (tBLG) hosts multitude of interaction-driven phases when the relative rotation is close to the magic angle (θm = 1.1∘). This includes correlation-induced ground states that reveal spontaneous symmetry breaking at low temperature, as well as possibility of non-Fermi liquid (NFL) excitations. However, experimentally, manifestation of NFL effects in transport properties of twisted bilayer graphene remains ambiguous. Here we report simultaneous measurements of electrical resistivity (ρ) and thermoelectric power (S) in tBLG for several twist angles between θ ~ 1.0 - 1.7∘. We observe an emergent violation of the semiclassical Mott relation in the form of excess S close to half-filling for θ ~ 1.6∘ that vanishes for θ ≳ 2∘. The excess S (≈2 µV/K at low temperatures T ~ 10 K at θ ≈ 1.6∘) persists upto ≈40 K, and is accompanied by metallic T-linear ρ with transport scattering rate (τ-1) of near-Planckian magnitude τ-1 ~ kBT/ℏ. Closer to θm, the excess S was also observed for fractional band filling (ν ≈ 0.5). The combination of non-trivial electrical transport and violation of Mott relation provides compelling evidence of NFL physics intrinsic to tBLG.

Misorientation-Controlled Cross-Plane Thermoelectricity in Twisted Bilayer Graphene.

The introduction of "twist" or relative rotation between two atomically thin van der Waals membranes gives rise to periodic moiré potential, leading to a substantial alteration of the band structure of the planar assembly. While most of the recent experiments primarily focus on the electronic-band hybridization by probing in-plane transport properties, here we report out-of-plane thermoelectric measurements across the van der Waals gap in twisted bilayer graphene, which exhibits an interplay of twist-dependent interlayer electronic and phononic hybridization. We show that at large twist angles, the thermopower is entirely driven by a novel phonon-drag effect at subnanometer scale, while the electronic component of the thermopower is recovered only when the misorientation between the layers is reduced to <6°. Our experiment shows that cross-plane thermoelectricity at low angles is exceptionally sensitive to the nature of band dispersion and may provide fundamental insights into the coherence of electronic states in twisted bilayer graphene.

Tailoring the transfer characteristics and hysteresis in MoS2 transistors using substrate engineering.

We demonstrate a novel form of transfer characteristics in substrate engineered MoS2 field effect transistors. Robust hysteresis with stable threshold voltages and a large gate voltage window is observed, which is suppressed at low temperatures. We analyse the dependence of the device characteristics on gate voltage range, gate stressing and sweep rates. We infer that the hysteresis originates from artificially created charged traps near the MoS2-SiO2 interface. These charge traps act as long range Coulomb scatterers and are screened at high carrier densities. The hysteresis is strongly suppressed in measurements on wafers devoid of the substrate treatment, providing a new extrinsic route to carefully tune the transfer characteristics.

Giant UV Photoresponse of GaN-Based Photodetectors by Surface Modification Using Phenol-Functionalized Porphyrin Organic Molecules.

Organic molecular monolayers (MoLs) have been used for improving the performance of various electronic device structures. In this work, the concept of organic molecular surface modification is applied for improving the performance of GaN-based metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors (PDs). Organic molecules of phenol-functionalized metallated porphyrin (hydroxyl-phenyl-zinc-tetra-phenyl-porphyrin (Zn-TPPOH)) were adsorbed on GaN, and Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni PD structures were fabricated. This process was beneficial in two ways: first, the reverse-bias dark current was reduced by 1000 times, and second, the photocurrent was enhanced by ∼100 times, in comparison to the dark and photocurrent values obtained for Ni/GaN/Ni MSM PDs, at high voltages of ±10 V. The responsivity of the devices was increased from 0.22 to 4.14 kA/W at 5 µW/cm2 optical power density at -10 V bias and at other voltages also. In addition to this, other PD parameters such as photo-to-dark current ratio and UV-to-visible rejection ratio were also enhanced. The spectral selectivity of the PDs was improved, which means that the molecularly modified devices became more responsive to UV spectral region and less responsive to visible spectral region, in comparison to bare GaN-based devices. Photoluminescence measurements, power-dependent photocurrent characteristics, and time-resolved photocurrent measurements revealed that the MoL was passivating the defect-related states on GaN. In addition, Kelvin probe force microscopy showed that the MoL was also playing with the surface charge (due to surface states) on GaN, leading to increased Schottky barrier height in dark conditions. Resultant to both these phenomena, the reverse-bias dark current was reduced for metal/MoL/GaN/MoL/metal PD structures. Further, the unusual photoconductive gain in the molecularly modified devices has been attributed to Schottky barrier lowering for UV-illuminated conditions, leading to enhanced photocurrent.

Understanding of MoS2/GaN Heterojunction Diode and its Photodetection Properties.

Fabrication of heterojunction between 2D molybdenum disulfide (MoS2) and gallium nitride (GaN) and its photodetection properties have been reported in the present work. Surface potential mapping at the MoS2/GaN heterojunction is done using Kelvin Probe Force Microscopy to measure the conduction band offset. Current-voltage measurements show a diode like behavior of the heterojunction. The origin of diode like behavior is attributed to unique type II band alignment of the heterojunction. The photocurrent, photoresponsivity and detectivity of the heterojunction are found to be dependent on power density of the light. Photoresponse investigations reveal that the heterojunction is highly sensitive to 405 nm laser with very high responsivity up to 105 A/W. The heterojunction also shows very high detectivity of the order of 1014 Jones. Moreover, the device shows photoresponse in UV region also. These observations suggest that MoS2/GaN heterojunction can have great potential for photodetection applications.