arXiv subjects

Zhiqiang Li

Publications and source records attributed to Zhiqiang Li.

98 records · Page 6Linked to original sources

arXiv2012

Plasmonics of coupled graphene micro-structures

The optical response of graphene micro-structures, such as micro-ribbons and disks, is dominated by the localized plasmon resonance in the far infrared (IR) spectral range. An ensemble of such structures is usually involved and the effect of the coupling between the individual structures is expected to play an important role. In this paper, the plasmonic coupling of graphene microstructures in different configurations is investigated. While a relatively weak coupling between graphene disks on the same plane is observed, the coupling between vertically stacked graphene disks is strong and a drastic increase of the resonance frequency is demonstrated. The plasmons in a more complex structure can be treated as the hybridization of plasmons from more elementary structures. As an example, the plasmon resonances of graphene micro-rings are presented, in conjunction with their response in a magnetic field. Finally, the coupling of the plasmon and the surface polar phonons of SiO2 substrate is demonstrated by the observation of a new hybrid resonance peak around 500cm-1.

cond-mat.mes-hall↗

arXiv2012

Magnetic field tuning of terahertz Dirac plasmons in graphene

Boundaries and edges of a two dimensional system lower its symmetry and are usually regarded, from the point of view of charge transport, as imperfections. Here we present a first study of the behavior of graphene plasmons in a strong magnetic field that provides a different perspective. We show that the plasmon resonance in micron size graphene disks in a strong magnetic field splits into edge and bulk plasmon modes with opposite dispersion relations, and that the edge plasmons at terahertz frequencies develop increasingly longer lifetimes with increasing magnetic field, in spite of potentially more defects close to the graphene edges. This unintuitive behavior is attributed to increasing quasi-one dimensional field-induced confinement and the resulting suppression of the back-scattering. Due to the linear band structure of graphene, the splitting rate of the edge and bulk modes develops a strong doping dependence, which differs from the behavior of traditional semiconductor two-dimensional electron gas (2DEG) systems. We also observe the appearance of a higher order mode indicating an anharmonic confinement potential even in these well-defined circular disks. Our work not only opens an avenue for studying the physics of graphene edges, but also supports the great potential of graphene for tunable terahertz magneto-optical devices.

cond-mat.mes-hall↗

arXiv2011

Structurally Dependent Fano Resonances in the Infrared Spectra of Phonons in Few-Layer Graphene

The in-plane optical phonons around 200 meV in few-layer graphene are investigated utilizing infrared absorption spectroscopy. The phonon spectra exhibit unusual asymmetric features characteristic of Fano resonances, which depend critically on the layer thickness and stacking order of the sample. The phonon intensities in samples with rhombohedral (ABC) stacking are significantly higher than those with Bernal (AB) stacking. These observations reflect the strong coupling between phonons and interband electronic transitions in these systems and the distinctive variation in the joint density of electronic states in samples of differing thickness and stacking order.

cond-mat.mes-hall↗

arXiv2011

Infrared signatures of high carrier densities induced in semiconducting poly(3-hexylthiophene) by fluorinated organosilane molecules

We report on infrared (IR) absorption and dc electrical measurements of thin films of poly(3-hexylthiophene) (P3HT) that have been modified by a fluoroalkyl trichlorosilane (FTS). Spectra for FTS-treated films were compared to data for electrostatically-doped P3HT in an organic field-effect transistor (OFET). The appearance of a prominent polaron band in mid-IR absorption data for FTS-treated P3HT supports the assertion of hole doping via a charge-transfer process between FTS molecules and P3HT. In highly-doped films with a significantly enhanced polaron band, we find a monotonic Drude-like absorption in the far-IR, signifying delocalized states. Utilizing a simple capacitor model of an OFET, we extracted a carrier density for FTS-treated P3HT from the spectroscopic data. With carrier densities reaching 10$^{14}$ holes/cm$^2$, our results demonstrate that FTS doping provides a unique way to study the metal-insulator transition in polythiophenes.

cond-mat.mtrl-sci↗

arXiv2011

Observation of an electrically tunable band gap in trilayer graphene

A striking feature of bilayer graphene is the induction of a significant band gap in the electronic states by the application of a perpendicular electric field. Thicker graphene layers are also highly attractive materials. The ability to produce a band gap in these systems is of great fundamental and practical interest. Both experimental and theoretical investigations of graphene trilayers with the typical ABA layer stacking have, however, revealed the lack of any appreciable induced gap. Here we contrast this behavior with that exhibited by graphene trilayers with ABC crystallographic stacking. The symmetry of this structure is similar to that of AB stacked graphene bilayers and, as shown by infrared conductivity measurements, permits a large band gap to be formed by an applied electric field. Our results demonstrate the critical and hitherto neglected role of the crystallographic stacking sequence on the induction of a band gap in few-layer graphene.

cond-mat.mes-hall↗

arXiv2011

Distributed Cooperative Spectrum Sensing in Mobile Ad Hoc Networks with Cognitive Radios

In cognitive radio mobile ad hoc networks (CR-MANETs), secondary users can cooperatively sense the spectrum to detect the presence of primary users. In this chapter, we propose a fully distributed and scalable cooperative spectrum sensing scheme based on recent advances in consensus algorithms. In the proposed scheme, the secondary users can maintain coordination based on only local information exchange without a centralized common receiver. We use the consensus of secondary users to make the final decision. The proposed scheme is essentially based on recent advances in consensus algorithms that have taken inspiration from complex natural phenomena including flocking of birds, schooling of fish, swarming of ants and honeybees. Unlike the existing cooperative spectrum sensing schemes, there is no need for a centralized receiver in the proposed schemes, which make them suitable in distributed CR-MANETs. Simulation results show that the proposed consensus schemes can have significant lower missing detection probabilities and false alarm probabilities in CR-MANETs. It is also demonstrated that the proposed scheme not only has proven sensitivity in detecting the primary user's presence, but also has robustness in choosing a desirable decision threshold.

cs.IT↗

arXiv2010

Imaging Stacking Order in Few-Layer Graphene

Few-layer graphene (FLG) has been predicted to exist in various crystallographic stacking sequences, which can strongly influence the electronic properties of FLG. We demonstrate an accurate and efficient method to characterize stacking order in FLG using the distinctive features of the Raman 2D-mode. Raman imaging allows us to visualize directly the spatial distribution of Bernal (ABA) and rhombohedral (ABC) stacking in tri- and tetra-layer graphene. We find that 15% of exfoliated graphene tri- and tetra-layers is comprised of micron-sized domains of rhombohedral stacking, rather than of usual Bernal stacking. These domains are stable and remain unchanged for temperatures exceeding $800^{\circ}$C.

cond-mat.mtrl-sci↗

arXiv2008

A Combinatorial Interpretation for Certain Relatives of the Conolly Sequence

For any integer s >= 0, we derive a combinatorial interpretation for the family of sequences generated by the recursion (parameterized by s) h_s(n) = h_s(n - s - h_s(n - 1)) + h_s(n - 2 - s - h_s(n - 3)), n > s + 3, with the initial conditions h_s(1) = h_s(2) = ... = h_s(s+2) = 1 and h_s(s+3) = 2. We show how these sequences count the number of leaves of a certain infinite tree structure. Using this interpretation we prove that h_s sequences are "slowly growing", that is, h_s sequences are monotone nondecreasing, with successive terms increasing by 0 or 1, so each sequence hits every positive integer. Further, for fixed s the sequence h_s(n) hits every positive integer twice except for powers of 2, all of which are hit s+2 times. Our combinatorial interpretation provides a simple approach for deriving the ordinary generating functions for these sequences.

math.CO↗