Xingquan Zou

Terahertz Conductivity of Twisted Bilayer Graphene

Using terahertz time-domain spectroscopy, the real part of optical conductivity [σ(1)(ω)] of twisted bilayer graphene was obtained at different temperatures (10-300 K) in the frequency range 0.3-3 THz. On top of a Drude-like response, we see a strong peak in σ(1)(ω) at ~2.7 THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7 THz to an enhanced density of states at that energy, which is caused by the presence of a van Hove singularity arising from a commensurate twisting of the two graphene layers.

Terahertz conductivity of topological surface states in Bi 1.5 Sb 0.5 Te 1.8 Se 1.2

Topological insulators are electronic materials with an insulating bulk and conducting surface. However, due to free carriers in the bulk, the properties of the metallic surface are difficult to detect and characterize in most topological insulator materials. Recently, a new topological insulator Bi1.5Sb0.5Te1.7Se1.3 (BSTS) was found, showing high bulk resistivities of 1–10 Ω.cm and greater contrast between the bulk and surface resistivities compared to other Bi-based topological insulators. Using Terahertz Time-Domain Spectroscopy (THz-TDS), we present complex conductivity of BSTS single crystals, disentangling the surface and bulk contributions. We find that the Drude spectral weight is 1–2 orders of magnitude smaller than in other Bi-based topological insulators, and similar to that of Bi2Se3 thin films, suggesting a significant contribution of the topological surface states to the conductivity of the BSTS sample. Moreover, an impurity band is present about 30 meV below the Fermi level, and the surface and bulk carrier densities agree with those obtained from transport data. Furthermore, from the surface Drude contribution, we obtain a ~98% transmission through one surface layer — this is consistent with the transmission through single-layer or bilayer graphene, which shares a common Dirac-cone feature in the band structure.

Origin of hysteresis in the transfer characteristic of carbon nanotube field effect transistor

Using electrostatic force microscopy, we show direct evidence of charge injection at the carbon nanotube–SiO2 interface leading to the appearance of hysteresis. The dynamic screening effect of the injected charges is revealed step by step. Further temperature dependent tests also demonstrate the effect of SiO2 surface chemistry. Furthermore, we conclude that it is not practical to use such a device for memory application because of data retention and storage density issues.

Ultrafast carrier dynamics in pristine and FeCl3-intercalated bilayer graphene

Ultrafast carrier dynamics of pristine bilayer graphene (BLG) and BLG intercalated with FeCl3 (FeCl3–G), were studied using time-resolved transient differential reflection (ΔR/R). Compared to BLG, the FeCl3–G data showed an opposite sign of ΔR/R, a slower rise time, and a single (instead of double) exponential relaxation. We attribute these differences in dynamics to the down-shifting in the Fermi level in FeCl3–G, as well as the formation of numerous horizontal bands arising from the d-orbitals of Fe. Our work shows that intercalation can dramatically change the electronic structure of graphene, and its associated carrier dynamics.Ultrafast carrier dynamics of pristine bilayer graphene (BLG) and BLG intercalated with FeCl3 (FeCl3–G), were studied using time-resolved transient differential reflection (ΔR/R). Compared to BLG, the FeCl3–G data showed an opposite sign of ΔR/R, a slower rise time, and a single (instead of double) exponential relaxation. We attribute these differences in dynamics to the down-shifting in the Fermi level in FeCl3–G, as well as the formation of numerous horizontal bands arising from the d-orbitals of Fe. Our work shows that intercalation can dramatically change the electronic structure of graphene, and its associated carrier dynamics.

Temperature-dependent terahertz conductivity of tin oxide nanowire films

Temperature-dependent terahertz conductivity of tin oxide (SnO2) nanowire films was measured from 10 to 300K using terahertz time-domain spectroscopy. The optical parameters, including the complex refractive index, optical conductivity and dielectric function, were obtained using a simple effective medium theory. The complex conductivity was fitted with the Drude-Smith model and the plasmon model. The results show that the carrier density (N) and plasmon resonance frequency (ω0) increase while the scattering time decreases with increasing temperature. The reduced carrier mobility compared with bulk SnO2 indicates the presence of carrier localization or trapping in these nanowires. (Some figures may appear in colour only in the online journal)

Ultrafast carrier phonon dynamics in NaOH-reacted graphite oxide film

NaOH-reacted graphite oxide film was prepared by decomposing epoxy groups in graphite oxide into hydroxyl and -ONa groups with NaOH solution. Ultrafast carrier dynamics of the sample were studied by time-resolved transient differential reflection (ΔR/R). The data show two exponential relaxation processes. The slow relaxation process (∼2ps) is ascribed to low energy acoustic phonon mediated scattering. The electron-phonon coupling and first-principles calculation results demonstrate that -OH and -ONa groups in the sample are strongly coupled. Thus, we attribute the fast relaxation process (∼0.17ps) to the coupling of hydroxyl and -ONa groups in the sample.

Understanding Asymmetric Transportation Behavior in Graphene Field-Effect Transistors Using Scanning Kelvin Probe Microscopy

Scanning Kelvin probe microscopy (SKPM) is applied to experimentally understand the asymmetric behaviors in hole and electron transportation regions in graphene field-effect transistors (FETs). With gate modulation, the transition from p-p-p to p-n-p (for a Ag or Pd source/drain junction with graphene) or from n-p-n to n-n-n (for an Al source/drain junction with graphene) is verified by SKPM, which is believed to be responsible for the asymmetric transport. The odd resistance (Rodd) is positive for Ag (or Pd)/single-layer-graphene (SLG) FETs with ΔWFintrinsic >; 0, while Rodd is negative for Al/SLG devices with ΔWFintrinsic <; 0, where ΔWFintrinsic is defined as the work function difference between metal and intrinsic graphene.

Quasiparticle dynamics in overdoped Bi1.4Pb0.7Sr1.9CaCU2O8+δ: Coexistence of superconducting gap and pseudogap below Tc

Photoexcited quasiparticle relaxation dynamics in overdoped Bi

Effect of annealing on the temperature-dependent dielectric properties of LaAlO3 at terahertz frequencies

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Doping dependence of the electron–phonon and electron–spin fluctuation interactions in the high-Tc superconductor Bi2Sr2CaCu2O8+δ

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