Berry curvature, which arises from the asymmetry in a system, is a paramount physical property that results in interesting phenomena such as valley Hall effect. There have been several experimental observations of valley Hall effect in hBN encapsulated bilayer graphene systems as well as hBN/single-layer graphene heterostructures. Although the intent of encapsulating graphene with hBN is to improve the electronic properties of the graphene, the effect of the hBN layer in breaking the layer symmetry and inducing Berry curvature in these systems is not studied explicitly. In this study, we show that for the commensurate state of the heterostructure, the configuration, as well as the orientation of the hBN layer, has an immense effect on the polarity as well as the magnitude of the Berry curvature in bilayer graphene system as they break the layer symmetry. Also, the polarity and the magnitude of the Berry curvature can further be manipulated with the application of an out-of-plane electric field by tuning the layer asymmetry. Whereas, in hBN/single-layer graphene commensurate systems, the presence of an hBN layer breaks the sublattice symmetry and induces Berry curvature, whose magnitude and polarity depend on the configuration and the orientation of the hBN layer. Moreover, although the magnitude can be further tuned with the application of an out-of-plane electric field, the polarity is rather insensitive to the field as the sublattice asymmetry cannot be reversed with the applied field.

Controlling the heat transport and thermal conductivity through a material is of prime importance for thermoelectric applications. Phononic crystals, which are a nanostructured array of specially designed pores, can suppress heat transportation owing to the phonon wave interference, resulting in bandgap formation in their band structure. To control heat phonon propagation in thermoelectric devices, phononic crystals with a bandgap in the THz regime are desirable. In this study, we carried out simulation on snowflake shaped phononic crystal and obtained several phononic bandgaps in the THz regime, with the highest being at ≈2 THz. The phononic bandgap position and the width of the bandgap were found to be tunable by varying the neck-length of the snowflake structure. A unique bandgap map computed by varying the neck-length continuously provides enormous amounts of information as to the size and position of the phononic bandgap for various pore dimensions. We have also carried out transmission spectrum analysis and found good agreement with the band structure calculations. The pressure map visualized at various frequencies validates the effectiveness of snowflake shaped nano-pores in suppressing the phonons partially or completely, depending on the transmission probabilities.

Graphene is widely used in various real-life applications due to its high sensitivity to the change in the carrier concentration. Here, we demonstrate that graphene can be used for implementing a reliable lightning detection network as it shows excellent sensitivity to the electric field of both positive and negative polarities, with a wide range of magnitude. The lowest electric field detected by our graphene sensor is 67 V/m, which is much smaller than the detection limit of previously reported graphene sensors and comparable to that of field mill and MEMS-based sensors. We also present the results of outdoor experiments where the response of the graphene sensor to the atmospheric electric field on a lightning day was tested and found to be in good agreement with the existing field mill sensor.

In this study, we investigate the scope of molybdenum disulfide (MoS₂) as an electric field sensor. We show that MoS₂ sensors can be used to identify the polarity as well as to detect the magnitude of the electric field. The response of the sensor is recorded as the change in the drain current when the electric field is applied. The sensitivity, defined as the percentage change in the drain current, reveals that it has a linear relation with the magnitude of the electric field. Furthermore, the sensitivity is highly dependent on the layer thickness, with the single-layer device being highly sensitive and the sensitivity decreasing with the thickness. We have also compared the electric field sensitivity of MoS₂ devices to that of previously studied graphene devices and found the former to be exceptionally sensitive than the latter for a given electric field magnitude.

Valley current, a stable, dissipationless current, originates due to the emergence of Berry curvature in inversion symmetry broken systems. Several theoretical predictions and experimental observations have explored layer symmetry breaking in AB-stacked bilayer graphene due to long-range Coulomb interactions between the electrons. However, none of the experimental studies conducted so far have observed valley current in unbiased bilayer graphene, which makes it vital to study the Berry curvature in unbiased bilayer graphene. In this study, we observed a non-zero Berry curvature with opposite values at K and K′ valleys, validating the argumentation of the asymmetry persistent in unbiased bilayer graphene. The magnitude, as well as the polarity of the Berry curvature, is tunable with the application of an out-of-plane electric field. These results are especially important because they can lead to the realization of a valley valve, in which the carriers from the K and K′ valleys can be regulated with a gate at the centre of a bilayer graphene nanoribbon.

Valley Hall effect is observed in asymmetric single‐layer and bilayer graphene systems. In single‐layer graphene systems, asymmetry is introduced by aligning graphene with hexagonal boron nitride (hBN) with a near‐zero twist angle, breaking the sub‐lattice symmetry. Although a similar approach is used in bilayer graphene to break the layer symmetry and thereby observe the valley Hall effect, the bilayer graphene is sandwiched with hBN on both sides in those studies. This study looks at a much simpler, non‐encapsulated structure where hBN is present only at the top of graphene. The crystallographic axes of both hBN and bilayer graphene are aligned. A clear signature of the valley Hall effect through non‐local resistance measurement (R_NL) is observed. The observed non‐local resistance can be manipulated by applying a displacement field across the heterostructure. Furthermore, the electronic band structure and Berry curvature calculations validate the experimental observations.