The Experts below are selected from a list of 24375 Experts worldwide ranked by ideXlab platform
Jon A Schuller - One of the best experts on this subject based on the ideXlab platform.
-
even parity self trapped excitons lead to magnetic Dipole Radiation in two dimensional lead halide perovskites
ACS Nano, 2020Co-Authors: Ryan A Decrescent, Naveen R Venkatesan, Clayton J Dahlman, Rhys M Kennard, Michael L Chabinyc, Jon A SchullerAbstract:Recently, unconventional bright magnetic Dipole (MD) Radiation was observed from two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs). According to commonly accepted HOIP band structure calculations, such MD light emission from the ground-state exciton should be strictly symmetry forbidden. These results suggest that MD emission arises in conjunction with an as-yet unidentified symmetry-breaking mechanism. In this paper, we show that MD light emission originates from a self-trapped p-like exciton stabilized at energies below the primary electric Dipole (ED)-emitting 1s exciton. Using suitable combinations of sample and collection geometries, we isolate the distinct temperature-dependent properties of the ED and MD photoluminescence (PL). We show that the ED emission wavelength is nearly constant with temperature, whereas the MD emission wavelength exhibits substantial red shifts with heating. To explain these results, we derive a microscopic model comprising two distinct parity exciton states coupled to lattice distortions. The model explains many experimental observations, including the thermal red shift, the difference in emission wavelengths, and the relative intensities of the ED and MD emission. Thermodynamic analysis of temperature-dependent PL reveals that the MD emission originates from a locally distorted structure. Finally, we demonstrate unusual hysteresis effects of the MD-emitting state near structural phase transitions. We hypothesize that this is another manifestation of the local distortions, indicating that they are insensitive to phase changes in the equilibrium lattice structure.
-
bright magnetic Dipole Radiation from two dimensional lead halide perovskites
Bulletin of the American Physical Society, 2020Co-Authors: Ryan A Decrescent, Naveen R Venkatesan, Clayton J Dahlman, Rhys M Kennard, Xie Zhang, Michael L Chabinyc, Rashid Zia, Jon A SchullerAbstract:Light-matter interactions in semiconductors are uniformly treated within the electric Dipole approximation; multipolar interactions are considered "forbidden." We experimentally demonstrate that this approximation inadequately describes light emission in two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs), solution processable semiconductors with promising optoelectronic properties. By exploiting the highly oriented crystal structure, we use energy-momentum spectroscopies to demonstrate that an exciton-like sideband in 2D HOIPs exhibits a multipolar Radiation pattern with highly directed emission. Electromagnetic and quantum-mechanical analyses indicate that this emission originates from an out-of-plane magnetic Dipole transition arising from the 2D character of electronic states. Symmetry arguments and temperature-dependent measurements suggest a dynamic symmetry-breaking mechanism that is active over a broad temperature range. These results challenge the paradigm of electric Dipole-dominated light-matter interactions in optoelectronic materials, provide new perspectives on the origins of unexpected sideband emission in HOIPs, and tease the possibility of metamaterial-like scattering phenomena at the quantum-mechanical level.
-
bright magnetic Dipole Radiation from two dimensional lead halide perovskites
arXiv: Materials Science, 2019Co-Authors: Ryan A Decrescent, Naveen R Venkatesan, Clayton J Dahlman, Rhys M Kennard, Xie Zhang, Michael L Chabinyc, Rashid Zia, Jon A SchullerAbstract:Light-matter interactions in semiconductor systems are uniformly treated within the electric Dipole (ED) approximation, as multipolar interactions are considered "forbidden". Here, we demonstrate that this approximation inadequately describes light emission in novel two-dimensional hybrid organic-inorganic perovskite materials (2D HOIPs) --- a class of solution processable layered semiconductor with promising optoelectronic properties. Consequently, photoluminescence (PL) spectra become strongly dependent on the experimental geometry, a fact that is often overlooked, though critical for correct optical characterization of materials. Using energy-momentum and time-resolved spectroscopies, we experimentally demonstrate that low-energy sideband emission in 2D HOIPs exhibits a highly unusual, multipolar polarization and angle dependence. Using combined electromagnetic and quantum-mechanical analyses, we attribute this Radiation pattern to an out-of-plane oriented magnetic Dipole transition arising from the 2D character of the excited and ground state orbitals. Symmetry arguments point toward the presence of significant inversion symmetry-breaking mechanisms that are currently under great debate. These results provide a new perspective on the origins of unexpected sideband emission in HOIPs, clarify discrepancies in previous literature, and generally challenge the paradigm of ED-dominated light-matter interactions in novel optoelectronic materials.
Alex Lazarian - One of the best experts on this subject based on the ideXlab platform.
-
electric Dipole Radiation from spinning dust grains
The Astrophysical Journal, 1998Co-Authors: Bruce T. Draine, Alex LazarianAbstract:We discuss the rotational excitation of small interstellar grains and the resulting electric Dipole Radiation from spinning dust. Attention is given to excitation and damping of grain rotation by collisions with neutrals, collisions with ions, "plasma drag," emission of infrared Radiation, emission of electric Dipole Radiation, photoelectric emission, and formation of H2 on the grain surface. Electrostatic "focusing" can substantially enhance the rate of rotational excitation of grains colliding with ions. Under some conditions, "plasma drag"—due to interaction of the electric Dipole moment of the grain with the electric field produced by passing ions—dominates both rotational damping and rotational excitation. Emissivities are estimated for dust in different phases of the interstellar medium, including diffuse H I clouds, warm H I, low-density photoionized gas, and cold molecular gas. Spinning dust grains could explain much, and perhaps all, of the 14-50 GHz background component recently observed by Kogut et al., de Oliveira-Costa et al., and Leitch et al. Future sensitive measurements of angular structure in the microwave sky brightness from the ground and from space should detect this emission from high-latitude H I clouds. It should be possible to detect rotational emission from small grains by ground-based pointed observations of molecular clouds, unless these grains are less abundant there than is currently believed.
-
electric Dipole Radiation from spinning dust grains
arXiv: Astrophysics, 1998Co-Authors: Bruce T. Draine, Alex LazarianAbstract:We discuss the rotational excitation of small interstellar grains and the resulting electric Dipole Radiation from spinning dust. Attention is given to excitation and damping of rotation by: collisions with neutrals; collisions with ions; plasma drag; emission of infrared Radiation; emission of microwave Radiation; photoelectric emission; and formation of H_2 on the grain surface. We introduce dimensionless functions F and G which allow direct comparison of the contributions of different mechanisms to rotational drag and excitation. Emissivities are estimated for dust in different phases of the interstellar medium, including diffuse HI, warm HI, low-density photoionized gas, and cold molecular gas. Spinning dust grains can explain much, and perhaps all, of the 14-50 GHz background component recently observed in CBR studies. It should be possible to detect rotational emission from small grains by ground-based observations of molecular clouds.
Brandon S Hensley - One of the best experts on this subject based on the ideXlab platform.
-
magnetic nanoparticles in the interstellar medium emission spectrum and polarization
The Astrophysical Journal, 2013Co-Authors: B T Draine, Brandon S HensleyAbstract:The presence of ferromagnetic or ferrimagnetic nanoparticles in the interstellar medium would give rise to magnetic Dipole Radiation at microwave and submillimeter frequencies. Such grains may account for the strong millimeter-wavelength emission observed from a number of low-metallicity galaxies, including the Small Magellanic Cloud. We calculate the absorption and scattering cross sections for such grains, with particular attention to metallic Fe, magnetite Fe{sub 3}O{sub 4}, and maghemite {gamma}-Fe{sub 2}O{sub 3}, all potentially present in the interstellar medium. The rate of Davis-Greenstein alignment by magnetic dissipation is also estimated. We determine the temperature of free-flying magnetic grains heated by starlight and calculate the polarization of the magnetic Dipole emission from both free-fliers and inclusions. For inclusions, the magnetic Dipole emission is expected to be polarized orthogonally relative to the normal electric Dipole Radiation. Magnetic Dipole Radiation will contribute significantly to the 20-40 GHz anomalous microwave emission only if a large fraction of the Fe is in metallic Fe iron nanoparticles with extreme elongations. Finally, we present self-consistent dielectric functions for metallic Fe, magnetite Fe{sub 3}O{sub 4}, and maghemite {gamma}-Fe{sub 2}O{sub 3}, enabling calculation of absorption and scattering cross sections from microwave to X-ray wavelengths.
-
magnetic nanoparticles in the interstellar medium emission spectrum and polarization
arXiv: Astrophysics of Galaxies, 2012Co-Authors: B T Draine, Brandon S HensleyAbstract:The presence of ferromagnetic or ferrimagnetic nanoparticles in the interstellar medium would give rise to magnetic Dipole Radiation at microwave and submm frequencies. Such grains may account for the strong mm-wavelength emission observed from a number of low-metallicity galaxies, including the Small Magellanic Cloud. We show how to calculate the absorption and scattering cross sections for such grains, with particular attention to metallic Fe, magnetite Fe3O4, and maghemite gamma-Fe2O3, all potentially present in the interstellar medium. The rate of Davis-Greenstein alignment by magnetic dissipation is also estimated. We determine the temperature of free-flying magnetic grains heated by starlight and we calculate the polarization of the magnetic Dipole emission from both free-fliers and inclusions. For inclusions, the magnetic Dipole emission is expected to be polarized orthogonally relative to the normal electric Dipole Radiation. Finally, we present self-consistent dielectric functions for metallic Fe, magnetite Fe3O4, and maghemite gamma-Fe2O3, enabling calculation of absorption and scattering cross sections from microwave to X-ray wavelengths.
-
the submm and mm excess of the smc magnetic Dipole emission from magnetic nanoparticles
arXiv: Astrophysics of Galaxies, 2012Co-Authors: B T Draine, Brandon S HensleyAbstract:The Small Magellanic Cloud (SMC) has surprisingly strong submm and mm-wavelength emission that is inconsistent with standard dust models, including those with emission from spinning dust. Here we show that the emission from the SMC may be understood if the interstellar dust mixture includes magnetic nanoparticles, emitting magnetic Dipole Radiation resulting from thermal fluctuations in the magnetization. The magnetic grains can be metallic iron, magnetite Fe3O4, or maghemite gamma-Fe2O3. The required mass of iron is consistent with elemental abundance constraints. The magnetic Dipole emission is predicted to be polarized orthogonally to the normal electric Dipole Radiation if the nanoparticles are inclusions in larger grains. We speculate that other low-metallicity galaxies may also have a large fraction of the interstellar Fe in magnetic materials.
Paul A Johnson - One of the best experts on this subject based on the ideXlab platform.
-
effective impedance boundary optimization and its contribution to Dipole Radiation and Radiation pattern control
Nature Communications, 2014Co-Authors: Li Quan, Xu Zhong, Xiufen Gong, Paul A JohnsonAbstract:Controlling sound waves is important to build better acoustic devices, and much can be learnt from optical approaches. To this end, Quan et al. show how periodic arrays of Helmholtz resonators on metal plates can produce Dipole-like Radiation patterns for sound.
-
effective impedance boundary optimization and its contribution to Dipole Radiation and Radiation pattern control
Nature Communications, 2014Co-Authors: Li Quan, Xu Zhong, Xiufen Gong, Xiaozhou Liu, Paul A JohnsonAbstract:Radiation pattern control has generated much interest recently due to its potential applications. Here we report the observation of high-efficiency Dipole-like Radiation of sound with broad bandwidth through a decorated plate with periodical two-dimensional Helmholtz resonators on both sides and a single slit at the centre. The decorated plate was optimally designed to adjust the effective impedance of the boundary, and the underlying mechanism of Radiation pattern control is attributed to wave vector tailoring. The high Radiation efficiency is due to the Fabry-Perot resonances associated with waveguide modes in the centre slit. The method to obtain a collimated beam without any sidelobes is also provided. Our findings should have an impact on acoustic applications.
Ryan A Decrescent - One of the best experts on this subject based on the ideXlab platform.
-
even parity self trapped excitons lead to magnetic Dipole Radiation in two dimensional lead halide perovskites
ACS Nano, 2020Co-Authors: Ryan A Decrescent, Naveen R Venkatesan, Clayton J Dahlman, Rhys M Kennard, Michael L Chabinyc, Jon A SchullerAbstract:Recently, unconventional bright magnetic Dipole (MD) Radiation was observed from two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs). According to commonly accepted HOIP band structure calculations, such MD light emission from the ground-state exciton should be strictly symmetry forbidden. These results suggest that MD emission arises in conjunction with an as-yet unidentified symmetry-breaking mechanism. In this paper, we show that MD light emission originates from a self-trapped p-like exciton stabilized at energies below the primary electric Dipole (ED)-emitting 1s exciton. Using suitable combinations of sample and collection geometries, we isolate the distinct temperature-dependent properties of the ED and MD photoluminescence (PL). We show that the ED emission wavelength is nearly constant with temperature, whereas the MD emission wavelength exhibits substantial red shifts with heating. To explain these results, we derive a microscopic model comprising two distinct parity exciton states coupled to lattice distortions. The model explains many experimental observations, including the thermal red shift, the difference in emission wavelengths, and the relative intensities of the ED and MD emission. Thermodynamic analysis of temperature-dependent PL reveals that the MD emission originates from a locally distorted structure. Finally, we demonstrate unusual hysteresis effects of the MD-emitting state near structural phase transitions. We hypothesize that this is another manifestation of the local distortions, indicating that they are insensitive to phase changes in the equilibrium lattice structure.
-
bright magnetic Dipole Radiation from two dimensional lead halide perovskites
Bulletin of the American Physical Society, 2020Co-Authors: Ryan A Decrescent, Naveen R Venkatesan, Clayton J Dahlman, Rhys M Kennard, Xie Zhang, Michael L Chabinyc, Rashid Zia, Jon A SchullerAbstract:Light-matter interactions in semiconductors are uniformly treated within the electric Dipole approximation; multipolar interactions are considered "forbidden." We experimentally demonstrate that this approximation inadequately describes light emission in two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs), solution processable semiconductors with promising optoelectronic properties. By exploiting the highly oriented crystal structure, we use energy-momentum spectroscopies to demonstrate that an exciton-like sideband in 2D HOIPs exhibits a multipolar Radiation pattern with highly directed emission. Electromagnetic and quantum-mechanical analyses indicate that this emission originates from an out-of-plane magnetic Dipole transition arising from the 2D character of electronic states. Symmetry arguments and temperature-dependent measurements suggest a dynamic symmetry-breaking mechanism that is active over a broad temperature range. These results challenge the paradigm of electric Dipole-dominated light-matter interactions in optoelectronic materials, provide new perspectives on the origins of unexpected sideband emission in HOIPs, and tease the possibility of metamaterial-like scattering phenomena at the quantum-mechanical level.
-
bright magnetic Dipole Radiation from two dimensional lead halide perovskites
arXiv: Materials Science, 2019Co-Authors: Ryan A Decrescent, Naveen R Venkatesan, Clayton J Dahlman, Rhys M Kennard, Xie Zhang, Michael L Chabinyc, Rashid Zia, Jon A SchullerAbstract:Light-matter interactions in semiconductor systems are uniformly treated within the electric Dipole (ED) approximation, as multipolar interactions are considered "forbidden". Here, we demonstrate that this approximation inadequately describes light emission in novel two-dimensional hybrid organic-inorganic perovskite materials (2D HOIPs) --- a class of solution processable layered semiconductor with promising optoelectronic properties. Consequently, photoluminescence (PL) spectra become strongly dependent on the experimental geometry, a fact that is often overlooked, though critical for correct optical characterization of materials. Using energy-momentum and time-resolved spectroscopies, we experimentally demonstrate that low-energy sideband emission in 2D HOIPs exhibits a highly unusual, multipolar polarization and angle dependence. Using combined electromagnetic and quantum-mechanical analyses, we attribute this Radiation pattern to an out-of-plane oriented magnetic Dipole transition arising from the 2D character of the excited and ground state orbitals. Symmetry arguments point toward the presence of significant inversion symmetry-breaking mechanisms that are currently under great debate. These results provide a new perspective on the origins of unexpected sideband emission in HOIPs, clarify discrepancies in previous literature, and generally challenge the paradigm of ED-dominated light-matter interactions in novel optoelectronic materials.