Oriented Graphene Nanoribbons Embedded in Hexagonal Boron Nitride Trenches

Graphene nanoribbons (GNRs) are ultra-narrow strips of graphene that have the potential to be used in high-performance graphene-based semiconductor electronics. However, controlled growth of GNRs on dielectric substrates remains a challenge. Here, we report the successful growth of GNRs directly on hexagonal boron nitride substrates with smooth edges and controllable widths using chemical vapour deposition. The approach is based on a type of template growth that allows for the in-plane epitaxy of mono-layered GNRs in nano-trenches on hexagonal boron nitride with edges following a zigzag direction. The embedded GNR channels show excellent electronic properties, even at room temperature. Such in-plane hetero-integration of GNRs, which is compatible with integrated circuit processing, creates a gapped channel with a width of a few benzene rings, enabling the development of digital integrated circuitry based on GNRs.Comment: 32 pages, 4 figures, Supplementary informatio

Irregular universe in the Nieh-Yan modified teleparallel gravity

The Nieh-Yan modified teleparallel gravity is a model which modifies the general relativity equivalent teleparallel gravity by a coupling between the Nieh-Yan density and an axion-like field. This model predicts parity violations in the gravitational waves if the axion-like field has a non-trivial background, and more importantly it is ghost free and avoids the pathologies presented in other parity-violating gravity models. The cosmological dynamics and perturbations of the Nieh-Yan modified teleparallel gravity have been investigated in detail, but all these previous investigations rely on the symmetry requirement that in the background universe both the metric and affine connection are homogeneous and isotropic. In this paper we relax the symmetry constraint on the connection and leave it arbitrary at the beginning, after all the cosmological principle only needs the metric of the background spacetime to meet the symmetry requirement. We find a new flat universe solution for the Nieh-Yan modified teleparallel gravity, for which the background dynamics itself is unchanged but the perturbations around it present a new feature that the scalar and tensor perturbations are coupled together at the linear level. The implications of this peculiar feature in primordial perturbations from inflation are also discussed.Comment: 20 pages, 1 figures, irregular universe, teleparallel gravit

Algebraic Approach to Sensorless Interior Permanent Magnet Synchronous Motor (IPMSM) Drives

This thesis carries out two basic studies for the flux function and the global observability of IPMSMs, leading to, respectively, two lightweight sensorless algorithms. First, we show that the flux function almost always gives a one-to-one correspondence between the rotor angle and flux, implying that the familiar IPMSM flux equations admit a unique position solution. This result eliminates the need for the arctan/atan2 function, leading to a simple flux estimation-based algorithm. Second, we show that the pair (speed, position) of IPMSMs is not globally observable, and the number of all indistinguishable pairs is at most four, an invariant independent of motor parameters and coincides with the number of the solutions of the two fundamental IPMSM equations. This result, which caps the worst-case scenario sensorless IPMSMs can behave, follows as a corollary by characterizing the observability condition involving infinitely many equations as a special limited form of the injectivity nature of finitely many polynomials. This bigger problem is approached through algebraic methodologies. The FOC scheme causes the “collapse” of at most two indistinguishable pairs and joins forces with the knowledge of motor/generator mode to turn the sensorless task “observable”, leading to another simple algorithm.ThesisDoctor of Philosophy (PhD