Alex Frenzel

Infrared spectroscopy and nano-imaging of the insulator-to-metal transition in vanadium dioxide

We present a detailed infrared study of the insulator-to-metal transition IMT in vanadium dioxide VO2 thin films. Conventional infrared spectroscopy was employed to investigate the IMT in the far field. Scanning near-field infrared microscopy directly revealed the percolative IMT with increasing temperature. We confirmed that the phase transition is also percolative with cooling across the IMT. We present extensive near-field infrared images of phase coexistence in the IMT regime in VO2. We find that the coexisting insulating and metallic regions at a fixed temperature are static on the time scale of our measurements. A distinctive approach for analyzing the far-field and near-field infrared data within the Bruggeman effective medium theory was employed to extract the optical constants of the incipient metallic puddles at the onset of the IMT. We found divergent effective carrier mass in the metallic puddles that demonstrates the importance of electronic correlations to the IMT in VO2. We employ the extended dipole model for a quantitative analysis of the observed near-field infrared amplitude contrast and compare the results with those obtained with the basic dipole model.

Trion-Induced Negative Photoconductivity in Monolayer MoS 2

Optical excitation typically enhances electrical conduction and low-frequency radiation absorption in semiconductors. We, however, observe a pronounced transient decrease of conductivity in doped monolayer molybdenum disulfide (MoS(2)), a two-dimensional (2D) semiconductor, using ultrafast optical-pump terahertz-probe spectroscopy. In particular, the conductivity is reduced to only 30% of its equilibrium value at high pump fluence. This anomalous phenomenon arises from the strong many-body interactions in the 2D system, where photoexcited electron-hole pairs join the doping-induced charges to form trions, bound states of two electrons and one hole. The resultant increase of the carrier effective mass substantially diminishes the conductivity.

Optical characterization of Bi2Se3 in a magnetic field: Infrared evidence for magnetoelectric coupling in a topological insulator material

We present an infrared magneto-optical study of the highly thermoelectric narrow-gap semiconductor Bi

Nanoscale imaging and control of resistance switching in VO2 at room temperature

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Intervalley biexcitons and many-body effects in monolayer MoS 2

Se

Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene

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Observation of suppressed terahertz absorption in photoexcited graphene

. Far-infrared and mid-infrared (IR) reflectance and transmission measurements have been performed in magnetic fields oriented both parallel and perpendicular to the trigonal

Spin-induced optical conductivity in the spin-liquid candidate herbertsmithite.

c

Ferromagnetism and infrared electrodynamics of Ga1-xMnxAs

axis of this layered material, and supplemented with UV-visible ellipsometry to obtain the optical conductivity

Infrared probe of the insulator-to-metal transition in Ga1−xMnxAs and Ga1−xBexAs

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