We report on the optical properties of the hole-doped manganites Nd 0.7 Sr 0.3 MnO 3 , La 0.7 Ca 0.3 MnO 3 , and La 0.7 Sr 0.3 MnO 3 . The low-energy optical conductivity in the paramagnetic-insulating state of these materials is characterized by a broad maximum near 1 eV. This feature shifts to lower energy and grows in optical oscillator strength as the temperature is lowered into the ferromagnetic state. It remains identifiable well below T c and transforms eventually into a Drude-like response. This optical behavior and the activated transport in the paramagnetic state of these materials are consistent with a Jahn-Teller small polaron. The optical spectra and oscillator strength changes compare well with models that include both double exchange and the dynamic Jahn-Teller effect in the description of the electronic structure.

Quantum light sources in solid-state systems are of major interest as a basic ingredient for integrated quantum photonic technologies. The ability to tailor quantum emitters via site-selective defect engineering is essential for realizing scalable architectures. However, a major difficulty is that defects need to be controllably positioned within the material. Here, we overcome this challenge by controllably irradiating monolayer MoS 2 using a sub-nm focused helium ion beam to deterministically create defects. Subsequent encapsulation of the ion exposed MoS 2 flake with high-quality hBN reveals spectrally narrow emission lines that produce photons in the visible spectral range. Based on ab-initio calculations we interpret these emission lines as stemming from the recombination of highly localized electron–hole complexes at defect states generated by the local helium ion exposure. Our approach to deterministically write optically active defect states in a single transition metal dichalcogenide layer provides a platform for realizing exotic many-body systems, including coupled single-photon sources and interacting exciton lattices that may allow the exploration of Hubbard physics.

We report the THz response of thin films of the topological insulator Bi2Se3. At low frequencies, transport is essentially thickness independent showing the dominant contribution of the surface electrons. Despite their extended exposure to ambient conditions, these surfaces exhibit robust properties including narrow, almost thickness-independent Drude peaks, and an unprecedentedly large polarization rotation of linearly polarized light reflected in an applied magnetic field. This Kerr rotation can be as large as 65• and can be explained by a cyclotron resonance effect of the surface states.Ordered states of matter are typically categorized by their broken symmetries. With the ordering of spins in a ferromagnet or the freezing of a liquid into a solid, the loss of symmetry distinguishes the ordered state from the disordered one. In contrast, topological states are distinguished by specific topological properties that are encoded in their quantum mechanical wavefunctions [1]. Frequently, a consequence of these properties is that there are robust "topologically protected" states on the sample's boundaries. The edge states of the quantum Hall effect (QHE) are the classic example [2]. In the last few years, it was realized that another class of such topological matter may exist in 3D band insulators with large spin-orbit interaction [3][4][5][6]. These so-called topological insulators are predicted to host robust surface states, which exhibit a number of interesting properties including spin helicity, immunity to back-scattering, and weak anti -localization. There are predictions of a number of unusual phenomena associated with these surface states, including a proximity-effect-induced exotic superconducting state with Majorana fermions bound to a vortex [7,8] and an axion electromagnetic response [9,10], and proposals for applications, such as their use in terahertz (THz) devices [11].Most of the signatures of topological behavior in these materials thus far have come from surface probes such as angle resolved photoemission (ARPES) and scanning tunneling spectroscopy [12][13][14][15][16][17]. These experiments have revealed that the surface states indeed show signatures of the predicted topological properties, such as a Diraclike dispersion, chiral spin textures, and the absence of backscattering. Direct observation of the topological behavior in transport has been hampered by the lack of a true bulk insulating state. Only recently have transport experiments started to distinguish the surface contribution from the bulk [18][19][20][21].As opposed to the case of the quantum Hall effect, in topological insulators, the quantization of the offdiagonal conductivity is not a requirement for the existence of the topological state. This, along with the problem of bulk conduction, has made finding a unique signature of this state difficult. It has been proposed that topological insulators may be characterized by their electrodynamic properties [9] due to the existence of an axionic term in the action ∆L = αθ dxdtE · B, where...

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By monitoring changes in excitonic photoluminescence that are induced by far-infrared (FIR) radiation, we observed resonant FIR absorption by magnetoexcitons in GaAs͞AlGaAs quantum wells. The dominant resonance is assigned to the 1s ! 2p 1 transition of the heavy-hole exciton, and agrees well with theory. At low FIR and interband excitation intensities, the 1s ! 2p 1 absorption feature is very narrow and broadens as either of these intensities is increased. The 1s ! 2p 1 absorption feature persists even when the FIR electric field is comparable to the electric field which binds the exciton. [S0031-9007 (96) Correlated electron-hole pairs form excitons in semiconductor heterostructures. Excitons in GaAs are hydrogenlike systems with Bohr radii of order 100 Å, and binding energies of order 10 meV. The importance and much of the rich structure of excitons have been revealed by extensive studies using one-and two-photon interband spectroscopies (0.75-1.5 eV in GaAs) [1]. However, very limited research has succeeded in directly exploring the internal dynamics of excitons [2][3][4]. In such studies, near-infrared (NIR) photons create excitons, and then far-infrared (FIR) radiation (of order 10 meV, 2.4 THz) manipulates them. At low FIR intensities, one expects to observe directly transitions between even-and odd-parity states of the exciton, which are not observable with linear interband spectroscopy. Such transitions provide new, sensitive tests for the theory of excitons, which is fundamental in the physics of semiconductors. At higher FIR intensities, it is possible to reach a nonperturbative regime in which the energy associated with the FIR electric field coupling to the exciton is comparable to both the binding energy and the FIR photon energy.Undoped direct (type I) quantum wells (QWs) are especially interesting since they are so commonly used and provide a simple model system for theoretical analysis. However, the short lifetime of excitons in type I QWs makes it difficult to achieve the large population of cold excitons required for FIR absorption studies. Recent experimental progress has been made in QWs using photoinduced absorption in staggered (type II) QWs [3] and time-resolved terahertz spectroscopy in type I QWs [4].

Temperature induced delocalization of charge carriers and metallic phase in Co0.6Sn0.4Fe2O4 nanoparticles J. Appl. Phys. 112, 063718 (2012) Determining the activation volumes in ZnO J. Appl. Phys. 112, 013504 (2012) Capacitance-voltage analysis of high-carrier-density SrTiO3/GdTiO3 heterostructures Appl. Phys. Lett. 100, 232106 (2012) Pauli spin blockade in undoped Si/SiGe two-electron double quantum dots Appl. Phys. Lett. 99, 063109 (2011) Transport properties of free carriers in semiconductors studied by terahertz time-domain magneto-optical ellipsometry Appl. Phys. Lett. 98, 212108 (2011) Additional information on Appl. Phys. Lett.

Rapid developments in material research of metallic ferromagnetic (III,Mn)V semiconductors over the past few years have brought a much better understanding of these complex materials. We review here some of the main developments and current understanding of the bulk properties of these systems within the metallic regime, focusing principally on the magneto-transport and magneto-optical properties. Although several theoretical approaches are reviewed, the bulk of the review uses the effective Hamiltonian approach, which has proven useful in describing many of these properties namely in (Ga,Mn)As and (In,Mn)As. The model assumes a ferromagnetic coupling between Mn d-shell local moments mediated by holes in the semiconductor valence band.

We report the observation of the generation and routing of single plasmons generated by localized excitons in a WSe monolayer flake exfoliated onto lithographically defined Au-plasmonic waveguides. Statistical analysis of the position of different quantum emitters shows that they are (3.3 ± 0.7) times more likely to form close to the edges of the plasmonic waveguides. By characterizing individual emitters, we confirm their single-photon character via the observation of antibunching in the signal ( g(0) = 0.42) and demonstrate that specific emitters couple to modes of the proximal plasmonic waveguide. Time-resolved measurements performed on emitters close to and far away from the plasmonic nanostructures indicate that Purcell factors up to 15 ± 3 occur, depending on the precise location of the quantum emitter relative to the tightly confined plasmonic mode. Measurement of the point spread function of five quantum emitters relative to the waveguide with <50 nm precision is compared with numerical simulations to demonstrate the potential for greater increases in the coupling efficiency for ideally positioned emitters. The integration of such strain-induced quantum emitters with deterministic plasmonic routing is a step toward deep-subwavelength on-chip single quantum light sources.