Observation of Magnetoplasmons in Bi₂Se₃ Topological Insulator

Both the collective (plasmon) and the single particle (Drude) excitations of an electron gas can be controlled and modified by an external magnetic field <i>B</i>. At finite <i>B</i>, plasmon gives rise to a magnetoplasmon mode and the Drude term to a cyclotron resonance. These magnetic effects are expected to be extremely strong for Dirac electrons with a linear energy-momentum dispersion, like those present in graphene and topological insulators (TIs). Here, we investigate both the plasmon and the Drude response versus <i>B</i> in Bi₂Se₃ topological insulator. At low <i>B</i>, the cyclotron resonance is still well separated in energy from the magnetoplasmon mode; meanwhile, both excitations asymptotically converge at the same energy for increasing <i>B</i>, consistently with a dynamical mass for Dirac carriers of <i>m</i>_D^* = 0.18 ± 0.01 m_<i>e</i>. In TIs, one then achieves an excellent magnetic control of plasmonic excitations and this could open the way toward plasmon controlled terahertz magneto-optics

DA and receptors blockade increases the number of striatal TH⁺ neurons.

<p>Mice received a single i.p. injection with DA receptor ligands or with a selective nicotinic acetylcholine α4β2 receptor antagonist dihydro-β-erythroidine (DHβE) at PND4 and were killed at PND8. SKF = SKF38393 (10 mg/kg); SCH = SCH23390 (0.1 mg/kg); Q = quinpirole (0.1 mg/kg); RAC = raclopride (1 mg/kg); DHβE = dihydro-β-erythroidine (3.2 mg/kg). (E) PND4 mice subjected to striatal DA depletion by treatment with the TH inhibitor αMpT (150 mg/kg, i.p., twice, with 24 h of interval) were treated with quinpirole (0.1 mg/kg); (n = 6) or SKF38393 (10 mg/kg); (n = 6). Values are means+S.E.M. of 12 (A,C), 17 (B) or 6 (D,E) mice for group. In (A), *p<0.05 (One-way ANOVA+Bonferroni's test) <i>vs.</i> all other values; in (B), *p<0.05 (One-way ANOVA+Bonferroni's test) <i>vs.</i> values obtained in mice treated with saline or quinpirole alone; in (C,D,E), *p<0.05 (Student's t test) <i>vs.</i> values obtained in mice treated with saline.</p

DA depletion increases the number of intrinsic TH⁺ neurons.

<p>DA levels and the number of TH⁺ neurons in the striatum of mice treated with αMpT (150 mg/kg, i.p.; injected twice with 24 h of interval at PND4 and PND5), and killed 24 h (PND6) or 72 h (PND8) later are shown in (D) and (E). Values are means+S.E.M. of 10 mice for group. *p<0.05 (Student's test) <i>vs.</i> saline-treated mice. Correlation analysis between DA levels and the number of TH⁺ neurons in shown in (F) (r² = 0.65; p<0.05). Empty circles = mice treated with saline and killed at PND6; filled circles = mice treated with αMpT and killed at PND6; empty squares = mice treated with saline and killed at PND8; filled squares = mice treated with αMpT and killed at PND8.</p

DA depletion changes the spatial distribution of striatal TH⁺ neurons.

<p>The distribution profile of TH⁺ neurons in the striatum of mice treated with saline or in striatum of mice treated with saline or αMpT at PND4 (2 injections 24 h apart) and killed at PND6 is shown in (A). Representative images of neurons and fibers stained for TH are shown below the graph. The figure shows the triple vectors used for distance determination. Segments connecting the cell body of TH⁺ neurons to the central border and the two peripheral borders of the clusters are indicated). Note that most of the TH⁺ neurons are placed at shorter distance from the clusters of DA fibers in mice treated with αMpT. Double fluorescence immunostaining for TH and GAD, ChAT, Ki-67, and BrdU in mice treated with saline or αMpT as above is shown in (B), (C), (D), and (E), respectively.</p

Phenotypic characterization of intrinsic TH⁺ neurons.

<p>Double fluorescence staining for TH and DAT, or AADC and for TH and BrdU are shown in (A) and in (B), respectively. Co-localization was examined by densitometric analysis of red and green fluorescence in a selected region corresponding to the horizontal line in the right panels. The coincidence of the fluorescence peaks is indicative of a high level of co-localization.</p

Striatal TH⁺ neurons express D2 and D4 receptors.

<p>(A) Double fluorescence staining for TH and D1, D2 or D4 receptors in the striatum of mice at PND4 and PND8 is shown. Co-localization was examined by densitometric analysis of red and green fluorescence in a selected region corresponding to the horizontal line in the right panels. The coincidence of the fluorescence peaks is indicative of a high level of co-localization. (B) Immunoreactivity for D1 and D2 dopamine receptors in striatal sections of adult wild-type and D1 or D2 receptor knockout mice, respectively.</p