We present an infrared magneto-optical study of the highly thermoelectric narrow-gap semiconductor Bi2Se3. Far-infrared and mid-infrared (IR) reflectance and transmission measurements have been performed in magnetic fields oriented both parallel and perpendicular to the trigonal c axis of this layered material, and supplemented with UV-visible ellipsometry to obtain the optical conductivity σ1(ω). With lowering of temperature we observe narrowing of the Drude conductivity due to reduced quasiparticle scattering, as well as the increase in the absorption edge due to direct electronic transitions. Magnetic fields H c dramatically renormalize and asymmetrically broaden the strongest far-IR optical phonon, indicating interaction of the phonon with the continuum freecarrier spectrum and significant magnetoelectric coupling. For the perpendicular field orientation, electronic absorption is enhanced, and the plasma edge is slightly shifted to higher energies. In both cases the direct transition energy is softened in magnetic field. arXiv:0912.2769v2 [cond-mat.str-el]

We investigate the transient photoconductivity of graphene at various gate-tuned carrier densities by optical-pump terahertz-probe spectroscopy. We demonstrate that graphene exhibits semiconducting positive photoconductivity near zero carrier density, which crosses over to metallic negative photoconductivity at high carrier density. These observations can be accounted for by the interplay between photoinduced changes of both the Drude weight and carrier scattering rate. Our findings provide a complete picture to explain the opposite photoconductivity behavior reported in (undoped) graphene grown epitaxially and (doped) graphene grown by chemical vapor deposition. Notably, we observe nonmonotonic fluence dependence of the photoconductivity at low carrier density. This behavior reveals the nonmonotonic temperature dependence of the Drude weight in graphene, a unique property of two-dimensional massless Dirac fermions. DOI: 10.1103/PhysRevLett.113.056602 PACS numbers: 72.80.Vp, 72.40.+w, 73.40.Qv, 78.20.−e Charge carriers in graphene mimic two-dimensional (2D) massless Dirac fermions with linear energy dispersion, resulting in unique optical and electronic properties [1]. They exhibit high mobility and strong interaction with electromagnetic radiation over a broad frequency range [2]. Interband transitions in graphene give rise to pronounced optical absorption in the midinfrared to visible spectral range, where the optical conductivity is close to a universal value σ 0 ¼ πe 2 =2h [3]. Free-carrier intraband transitions, on the other hand, cause lowfrequency absorption, which varies significantly with charge density and results in strong light extinction at high carrier density [4]. In addition to this density dependence, the massless Dirac particles in graphene are predicted to exhibit a distinctive nonmonotonic temperature dependence of the intraband absorption strength, or Drude weight, due to their linear dispersion [5,6]. This behavior contrasts with the temperature-independent Drude weight expected in conventional systems of massive particles with parabolic dispersion [7,8]. Although the unique behavior of the Drude weight in graphene has been considered theoretically, experimental signatures are still lacking.The intrinsic properties of Drude absorption in graphene can be revealed by studying its dynamical response to photoexcitation. In particular, optical-pump terahertz-probe spectroscopy provides access to a wide transient temperature range via pulsed optical excitation, and allows measurement of the ac Drude conductivity by a timedelayed terahertz probe pulse [9]. This technique has been applied to study transient photoconductivity (PC) in graphene, but conflicting results have been reported [9][10][11][12][13][14][15]. Positive PC was observed in epitaxial graphene on SiC (Ref.[15]), while negative PC was seen in graphene grown by chemical vapor deposition (CVD) [11][12][13][14]. It has been argued that the opposite behavior in these samples arises from their different charge densities. Here we study...

Interactions between two excitons can result in the formation of bound quasiparticles, known as biexcitons. Their properties are determined by the constituent excitons, with orbital and spin states resembling those of atoms. Monolayer transition metal dichalcogenides (TMDs) present a unique system where excitons acquire a new degree of freedom, the valley pseudospin, from which a novel intervalley biexciton can be created. These biexcitons comprise two excitons from different valleys, which are distinct from biexcitons in conventional semiconductors and have no direct analogue in atomic and molecular systems. However, their valley properties are not accessible to traditional transport and optical measurements. Here, we report the observation of intervalley biexcitons in the monolayer TMD MoS2 using ultrafast pump-probe spectroscopy. By applying broadband probe pulses with different helicities, we identify two species of intervalley biexcitons with large binding energies of 60 meV and 40 meV. In addition, we also reveal effects beyond biexcitonic pairwise interactions in which the exciton energy redshifts at increasing exciton densities, indicating the presence of many-body interactions among them.