88 research outputs found
Intervalley biexcitons and many-body effects in monolayer MoS[subscript 2]
No full textInteractions 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 analog 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 MoS[subscript 2] 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 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.United States. Dept. of Energy (Grant DE-FG02-08ER46521)United States. Dept. of Energy (Grant DE-SC0006423)National Science Foundation (U.S.) (STC Center for Integrated Quantum Materials Grant DMR-1231319)National Science Foundation (U.S.) (Grant DMR-0845358
Spin-Induced Optical Conductivity in the Spin-Liquid Candidate Herbertsmithite
No full textWe report a direct measurement of the low-frequency optical conductivity of large-area single-crystal herbertsmithite, a promising spin-liquid candidate material, by means of terahertz time-domain spectroscopy. In the spectral range below 1.4 THz, we observe a contribution to the real part of the in-plane conductivity σ[subscript ab](ω) from the spin degree of freedom. This spin-induced conductivity exhibits a power-law dependence on frequency σ[subscript ab](ω) ~ ω[superscript β] with β ≈ 1.4. Our observation is consistent with the theoretically predicted low-frequency conductivity arising from an emergent gauge field of a gapless U(1) Dirac spin liquid.United States. Dept. of Energy. Office of Basic Energy Sciences (Division of Materials Sciences and Engineering Grant DE-FG02-08ER46521)United States. Dept. of Energy. Office of Basic Energy Sciences (Division of Materials Sciences and Engineering Grant DE-SC0006423)United States. Dept. of Energy. Office of Basic Energy Sciences (Division of Materials Sciences and Engineering Grant DE-FG02-07ER46134
Webinar: Electronics Technology
No full textThis webinar, provided by the Maricopa Advanced Technology Education Center (MATEC), was presented on September 20 2013. Louis Frenzel, noted author and editor of Electronic Design News, gives an overview of the ultra-current trends and technologies that are driving the industry today. Applications involving wireless, mobile, computers and the green influence on electronic technologies are covered. Frenzel then discusses how colleges are dealing with and educating their students to keep up with these trends in technology. This webinar is accompanied by a corresponding handout and webinar slides.The webinar is one hour and 31 minutes in length
Semiconducting-to-Metallic Photoconductivity Crossover and Temperature-Dependent Drude Weight in Graphene
No full textWe 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.United States. Dept. of Energy. Office of Basic Energy Sciences (Grant DE-SC0006423)National Science Foundation (U.S.) (Grant DMR-1231319)National Science Foundation (U.S.). Graduate Research Fellowship Progra
Trion-Induced Negative Photoconductivity in Monolayer MoS[subscript 2]
No full textOptical 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[subscript 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.United States. Dept. of Energy. Office of Basic Energy Sciences (Grant DE-SC0006423)National Science Foundation (U.S.) (Grant NSF DMR 0845358
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Terahertz Electrodynamics of Dirac Fermions in Graphene
No full textCharge carriers in graphene mimic two-dimensional massless Dirac fermions with linear energy dispersion, resulting in unique optical and electronic properties. They exhibit high mobility and strong interaction with electromagnetic radiation over a broad frequency range. Interband transitions in graphene give rise to pronounced optical absorption in the mid-infrared to visible spectral range, where the optical conductivity is close to a universal value . Free-carrier intraband transitions, on the other hand, cause low-frequency absorption, which varies significantly with charge density and results in strong light extinction at high carrier density. These properties together suggest a rich variety of possible optoelectronic applications for graphene.
In this thesis, we investigate the optoelectronic properties of graphene by measuring transient photoconductivity with 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 are 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. Our measurements also reveal the non-monotonic temperature dependence of the Drude weight in graphene, a unique property of two-dimensional massless Dirac fermions.Physicsgraphene; terahertz; optics; ultrafast; optoelectronics; 2D material
Observation of suppressed terahertz absorption in photoexcited graphene
No full textWhen light is absorbed by a semiconductor, photoexcited charge carriers enhance the absorption of far-infrared radiation due to intraband transitions. We observe the opposite behavior in monolayer graphene, a zero-gap semiconductor with linear dispersion. By using time domain terahertz (THz) spectroscopy in conjunction with optical pump excitation, we observe a reduced absorption of THz radiation in photoexcited graphene. The measured spectral shape of the differential optical conductivity exhibits non-Drude behavior. We discuss several possible mechanisms that contribute to the observed low-frequency non-equilibrium optical response of graphene.United States. Dept. of Energy. Office of Basic Energy Sciences (Grant DE-SC0006423)National Science Foundation (U.S.). Graduate Research Fellowship ProgramUnited States. Air Force Office of Scientific ResearchUnited States. Office of Naval Research. Multidisciplinary University Research Initiative. Graphene Approaches to Terahertz ElectronicsNational Science Foundation (U.S.) (Award DMR-0819762)National Science Foundation (U.S.) (Grant ECS-0335765
Exergetical evaluation of biobased synthesis pathways
No full textThe vast majority of today’s chemical products are based on crude oil. An attractive and sustainable alternative feedstock is biomass. Since crude oil and biomass differ in various properties, new synthesis pathways and processes have to be developed. In order to prioritize limited resources for research and development (R & D), their economic potential must be estimated in the early stages of development. A suitable measure for an estimation of the economic potential is based on exergy balances. Different structures of synthesis pathways characterised by the chemical exergy of the main components are evaluated. Based on a detailed evaluation of the underlying processes, general recommendations for future bio-based synthesis pathways are derived.Values, Technology and InnovationTechnology, Policy and Managemen
Data from: A 3D Analysis of Dendritic Solidification and Mosaicity in Ni-Based Single Crystal Superalloys
No full textAuthor: F. Scholz, M. Cevik, P. Hallensleben, P. Thome, G. Eggeler, J. Frenzel
Affiliation: Ruhr University Bochum
Date: 08/2021
Material: Nickel-base superalloy ERBO/1 (more details: Parsa, A. B., et al. Advanced scale bridging microstructure analysis of single crystal Ni-base superalloys. Adv. Eng. Mater. 2015, 17 (2), 216-230, https://doi.org/10.1002/adem.201400136)
Casting: Bridgman seed technique; Withdrawal rate: 180 mm/h, Thermal gradient 13.3 K/mm (more details: Hallensleben, P., et al. On the evolution of cast microstructures during processing of single crystal Ni-base superalloys using a Bridgman seed technique, Mat. Des. 2017, 128, 98–111, https://doi.org/10.1016/j.matdes.2017.05.001)
Sample: Cross sectional slices extracted perpendicular to the growth direction of a single crystal superalloy cylinder (diameter 12mm, length 120 mm).
Image acquisition: Optical microscope of type Axio (Carl Zeiss GmbH) equipped with a high-resolution CCD-camera of type Leica DFC320 and stepper-motor driven sample stage of type Tango Desktop (Märzhäuser)
Image pre-processing: Preparation of wide-field image collages using the stitching procedures implemented in software package Imagic ims (https://imagic.ch/en/imagic-ims, 07/2021)
Image post-processing: Image registration with CorelDraw X7 (: https://www.coreldraw.com/en/, 07/2021) using a contour reference mask
Quantitate analysis: Dendrite positions were extracted using the software package ImageJ (https://imagej.de.softonic.com/, 07/2021).
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The five optical micrographs cross sections represent image data which were obtained by tomographic characterization of as-cast single crystal nickel-base superalloy prepared by a seeded Bridgman technique. The material has been studied in the frame of the collaborative research center SFB/TR 103. All details on the applied Bridgman technique are described in the literature (Hallensleben, P., et al., Mat. Des. 2017, 128, 98–111, https://doi.org/10.1016/j.matdes.2017.05.001 and Hallensleben, P., et al., Crystals 2019, 9 (3), 149, https://doi.org/10.3390/cryst9030149). The tomographic image slices were prepared by successive electro discharge machining using incremental steps of 1mm. The image series represents the evolution of dendritic microstructures during the early stages of crystal growth from the back melted seed. The five wide-field micrographs were used to retrieve dendrite positions (enclosed as CSV data for each cross section) to evaluate crystal mosaicity on the basis of dendrite growth directions. All information and a detailed interpretation of tomographic are available in (Scholz, F., PhD-thesis, Ruhr University Bochum, https://doi.org/10.13154/294-8079). We hope that our image data will be useful for other types of solidification research. Please provide a notification by personal mail on the re-use of our raw data. Thank you.
All images and dendrite position data were evaluated in the following study concerning dendrite growth behavior, low angle misorientation defects, dendrite arrangements and spacings:
Scholz, F.; Cevik, M.; Hallensleben, P.; Thome, P.; Eggeler, G.; Frenzel, J. A 3D Analysis of Dendritic Solidification and Mosaicity in Ni-based Single Crystal Superalloys, Materials 2021, 14 (17), 4904 (https://doi.org/10.3390/ma14174904)
Poverty, class, and tourism : from seeing poverty to modern philanthropy, social policy, and antipoverty activism
No full textTourism’s relationship to poverty and class is multifaceted and has changed over time. Tourism is a
social phenomenon in which class and poverty often become apparent and are sometimes negotiated.
In this chapter, the author focuses on a certain aspect of the relationship between tourism and class,
namely tourism’s role in enabling and underpinning the politics of class and poverty. Using the context
of Victorian slumming as an empirical backdrop, the chapter discusses three aspects of the politics of
class in tourism: (1) the relationship of knowledge and class in tourism, with tourism producing but
also obscuring knowledge; (2) the relationship of tourism and inequality, interrogating how tourism
enables claims making in contexts of inequality; and (3) the role of free time for political action,
investigating how the availability of free time and space has enabled the formation of class-based
politics. The chapter contributes to better understanding the role of tourism in political and classbased action, a role not widely acknowledged in political history and social movement studies
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