Optical and plasmonic properties of twisted bilayer graphene: Impact of interlayer tunneling asymmetry and ground-state charge inhomogeneity

We present a theoretical study of the local optical conductivity, plasmon spectra, and thermoelectric properties of twisted bilayer graphene (TBG) at different filling factors and twist angles $\theta$. Our calculations are based on the electronic band structures obtained from a continuum model that has two tunable parameters, $u_0$ and $u_1$, which parametrize the intra-sublattice inter-layer and inter-sublattice inter-layer tunneling rate, respectively. In this Article we focus on two key aspects: i) we study the dependence of our results on the value of $u_0$, exploring the whole range $0\leq u_0\leq u_1$; ii) we take into account effects arising from the intrinsic charge density inhomogeneity present in TBG, by calculating the band structures within the self-consistent Hartree approximation. At zero filling factor, i.e. at the charge neutrality point, the optical conductivity is quite sensitive to the value of $u_0$ and twist angle, whereas the charge inhomogeneity brings about only modest corrections. On the other hand, away from zero filling, static screening dominates and the optical conductivity is appreciably affected by the charge inhomogeneity, the largest effects being seen on the intra-band contribution to it. These findings are also reflected by the plasmonic spectra. We compare our results with existing ones in the literature, where effects i) and ii) above have not been studied systematically. As natural byproducts of our calculations, we obtain the Drude weight and Seebeck coefficient. The former displays an enhanced particle-hole asymmetry stemming from the inhomogeneous ground-state charge distribution. The latter is shown to display a broad sign-changing feature even at low temperatures ($\approx 5~{\rm K}$) due to the reduced slope of the bands, as compared to those of single-layer graphene.Comment: 28 pages, 16 figures, 6 appendice

Magnetic field induced effects in the high source-drain bias current of weakly coupled vertical quantum dot molecules

We report on the basic properties of recently observed magnetic field resonance, induced time dependent oscillation, and hysteresis effects in the current flowing through two weakly coupled vertical quantum dots at high source-drain bias (up to a few tens of mV). These effects bare some similarity to those reported in the N=2 spin-blockade regime, usually for weak in-plane magnetic field, of quantum dot molecules and attributed to hyperfine coupling, except here the measurements are conducted outside of the spin-blockade regime and the out-of-plane magnetic field is up to ~6 T.Comment: 3 pages, 3 figures, accepted for publication in Physica E in EP2DS 17 conference proceeding

Photo-excited Carrier Dynamics and Impact Excitation Cascade in Graphene

Photo-excitation in solids can trigger a cascade in which multiple particle-hole excitations are generated. We analyze the carrier multiplication cascade of impact excitation processes in graphene and show that the number of pair excitations has a strong dependence on doping, which makes carrier multiplication gate-tunable. We also predict that the number of excited pairs as well as the characteristic time of the cascade scale linearly with photo-excitation energy. These dependences, as well as sharply peaked angular distribution of pair excitations, provide clear experimental signatures of carrier multiplication

A Generic Model for Current Collapse in Spin Blockaded Transport

A decrease in current with increasing voltage, often referred to as negative differential resistance (NDR), has been observed in many electronic devices and can usually be understood within a one-electron picture. However, NDR has recently been reported in nanoscale devices with large single-electron charging energies which require a many-electron picture in Fock space. This paper presents a generic model in this transport regime leading to a simple criterion for the conditions required to observe NDR and shows that this model describes the recent observation of multiple NDR's in Spin Blockaded transport through weakly coupled-double quantum dots quite well. This model shows clearly how a delicate interplay of orbital energy offset, delocalization and Coulomb interaction lead to the observed NDR under the right conditions, and also aids in obtaining a good match with experimentally observed features. We believe the basic model could be useful in understanding other experiments in this transport regime as well.Comment: 10 pages, 10 figures. to appear in Phys Rev

Eliminating interactions between non-neighboring qubits in the preparation of cluster states in quantum molecules

We propose a scheme to eliminate the effect of non-nearest-neighbor qubits in preparing cluster state with double-dot molecules. As the interaction Hamiltonians between qubits are Ising-model and mutually commute, we can get positive and negative effective interactions between qubits to cancel the effect of non-nearest-neighbor qubits by properly changing the electron charge states of each quantum dot molecule. The total time for the present multi-step cluster state preparation scheme is only doubled for one-dimensional qubit chain and tripled for two-dimensional qubit array comparing with the time of previous protocol leaving out the non-nearest-neighbor interactions.Comment: 5 pages, 4 figures, 2 table

Nature inspired solutions Special Interest Group - A UK wide network

This talk wants to give an overview of the newly founded nature inspired solutions special interest group (NIS SIG) which is a UK network of Nature inspired engineering solution providers and industry problem holders that can benefit from Nature Inspired solutions. What is the NIS SIG aiming to do? Like all SIGs run by KTN, the aim is to convene people who wouldn’t normally meet; to learn, connect and explore opportunities. The SIG will initially focus on the application of NIS across transport, infrastructure and energy. Over the next two years, we will build a community (online and offline), organise networking events, showcases and raise awareness by sharing best practice and success stories. A landscape map and a commercial opportunity report will be created to help accelerate growth and market uptake in the relevant sectors. Joining the NIS SIG provides an opportunity to: Discover how nature inspired engineering can help you solve some of your biggest challenges. Seek support from the NIS community if you are a ‘challenge holder’. Showcase your research and demonstrate your nature inspired engineering solutions. Broaden your network and make meaningful connections leading to collaboration opportunities. The talk wants to give an overview of the activities in Nature Inspired Engineering in the UK, inform about the NIS SIG activities and find out where there are synergies with other countries and regions. Here is an example of an event that we ran on 25th April to showcase nature inspired solutions. https://ktn-uk.co.uk/news/can-bees-dragonflies-and-locusts-help-solve-connected-and-autonomous-vehicle-problems Join our LinkedIn Group: Nature Inspired Solutions at KTN- https://www.linkedin.com/groups/13701855/ The talk would be best places in an overview section where various topics are discussed. As it ties in Energy, Infrastructure and Transport, the three conference streams would be best, if this does not exist: •Built Environment: Construction, Architecture & Urban Design •Energy & Environmental Technology •Robotics and Other Applications ----------------------------------------------------------------------------------------------------------------------------------------------------- I am happy to discuss the angle and topic of the talk, just get in touch via email: [email protected] I would also be interested to chair a session if needed, as I have a generalistic overview of various of the mentioned conference themes, best in one of the three areas mentioned above