Search query: Detecting photoelectrons from spontaneously formed excitons

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abstract: Excitons, quasiparticles of electrons and holes bound by Coulombic attraction, are created transiently by light and play an important role in optoelectronics, photovoltaics and photosynthesis. They are also predicted to form spontaneously in a small gap semiconductor or a semimetal, leading to a Bose Einstein condensate at low temperature, but there has not been any direct evidence of this effect so far. Here we detect the photoemission signal from spontaneously formed excitons in a debated excitonic insulator candidate, Ta2NiSe5. Our symmetry selective angle resolved photoemission spectroscopy reveals a characteristic excitonic feature above the transition temperature, which provides detailed properties of excitons, such as the anisotropic Bohr radius. The present result provides evidence for so called preformed excitons and guarantees the excitonic insulator nature of Ta2NiSe5 at low temperature. Excitons have been predicted to form spontaneously without external excitation in some materials. Low temperature ARPES measurements on Ta2NiSe5 now provide evidence for such an excitonic insulator and for so called preformed excitons.
author_names: Keisuke Fukutani

Roland Stania

Chang Il Kwon

Ki-jeong Kong

Jaeyoung Kim

Han Woong Yeom
title: Detecting photoelectrons from spontaneously formed excitons
venue: Nature Physics
year: 2021

abstract: We investigate the 3.45 eV luminescence band of spontaneously formed GaN nanowires on Si(111) by photoluminescence and cathodoluminescence spectroscopy. This band is found to be particularly prominent for samples synthesized at comparatively low temperatures. At the same time, these samples exhibit a peculiar morphology, namely, isolated long nanowires are interspersed within a dense matrix of short ones. Cathodoluminescence intensity maps reveal the 3.45 eV band to originate primarily from the long nanowires. Transmission electron microscopy shows that these long nanowires are either Ga polar and are joined by an inversion domain boundary with their short N polar neighbors, or exhibit a Ga polar core surrounded by a N polar shell with a tubular inversion domain boundary at the core/shell interface. For samples grown at high temperatures, which exhibit a uniform nanowire morphology, the 3.45 eV band is also found to originate from particular nanowires in the ensemble and thus presumably from inversion domain boundaries stemming from the coexistence of N and Ga polar nanowires. For several of the investigated samples, the 3.45 eV band splits into a doublet. We demonstrate that the higher energy component of this doublet arises from the recombination of two dimensional excitons free to move in the plane of the inversion domain boundary. In contrast, the lower energy component of the doublet originates from excitons localized in the plane of the inversion domain boundary. We propose that this in plane localization is due to shallow donors in the vicinity of the inversion domain boundaries.
author_names: Carsten Pfuller

Pierre Corfdir

Christian Hauswald

Timur Flissikowski

Xiang Kong

Johannes K Zettler

Sergio Fern'andez-Garrido

Pinar Dougan

Holger T Grahn

Achim Trampert

Lutz Geelhaar

Oliver Brandt
title: Nature of excitons bound to inversion domain boundaries: Origin of the 3.45 eV luminescence lines in spontaneously formed GaN nanowires on Si(111)
year: 2016

abstract: We report on photoluminescence (PL) spectroscopy and electron microscopy studies of an AlGaN quantum well (QW) structure grown by molecular beam epitaxy under metal rich conditions with substrate rotation. Both techniques reveal unintentional formation within the AlGaN barriers of a quasiperiodic structure of thin Ga rich layers, whose period is controlled by both the substrate rotation rate and the AlGaN growth rate. These compositional modulations act as 1 3 monolayer thick QWs emitting below 250 nm with an internal quantum efficiency (IQE) as high as ~30% at room temperature under weak excitation. Variational calculations of the QW exciton properties indicate that the observed high IQE can result from strong three dimensional localization of the excitons confined in the narrow QWs. (c) 2016 WILEY VCH Verlag GmbH Co. KGaA, Weinheim)
author_names: Alexey Toropov

E A Shevchenko

Tatiana V Shubina

Valentin N Jmerik

Dmitrii Nechaev

Galia Pozina

Peder Bergman

Bo Monemar

Sergei Rouvimov

Stefan Ivanov
title: Exciton recombination in spontaneously formed and artificial quantum wells AlxGa1 xN/AlyGa1 yN (x<y~0.8)
year: 2016

abstract: The formation of moire patterns in crystalline solids can be used to manipulate their electronic properties, which are fundamentally influenced by periodic potential landscapes. In two dimensional materials, a moire pattern with a superlattice potential can be formed by vertically stacking two layered materials with a twist and/or a difference in lattice constant. This approach has led to electronic phenomena including the fractal quantum Hall effect1 3, tunable Mott insulators4,5 and unconventional superconductivity6. In addition, theory predicts that notable effects on optical excitations could result from a moire potential in two dimensional valley semiconductors7 9, but these signatures have not been detected experimentally. Here we report experimental evidence of interlayer valley excitons trapped in a moire potential in molybdenum diselenide (MoSe2)/tungsten diselenide (WSe2) heterobilayers. At low temperatures, we observe photoluminescence close to the free interlayer exciton energy but with linewidths over one hundred times narrower (around 100 microelectronvolts) The emitter g factors are homogeneous across the same sample and take only two values, 15.9 and 6.7, in samples with approximate twist angles of 60 degrees and 0 degrees, respectively. The g factors match those of the free interlayer exciton, which is determined by one of two possible valley pairing configurations. At twist angles of approximately 20 degrees the emitters become two orders of magnitude dimmer; however, they possess the same g factor as the heterobilayer at a twist angle of approximately 60 degrees. This is consistent with the umklapp recombination of interlayer excitons near the commensurate 21.8 degree twist angle7. The emitters exhibit strong circular polarization of the same helicity for a given twist angle, which suggests that the trapping potential retains three fold rotational symmetry. Together with a characteristic dependence on power and excitation energy, these results suggest that the origin of the observed effects is interlayer excitons trapped in a smooth moire potential with inherited valley contrasting physics. This work presents opportunities to control two dimensional moire optics through variation of the twist angle.The trapping of interlayer valley excitons in a moire potential formed by a molybdenum diselenide/tungsten diselenide heterobilayer with twist angle control is reported.
author_names: Kyle L Seyler

Pasqual Rivera

Hongyi Yu

N P Wilson

Essance L Ray

David G Mandrus

Jiaqiang Yan

Wang Yao

Xiaodong Xu
title: Signatures of moire trapped valley excitons in MoSe2/WSe2 heterobilayers
venue: Nature
year: 2019

abstract: It is found that the binding energy of the ground state of an exciton formed from an electron and a hole spatially separated from each other (the hole is moving within a quantum dot, and the electron is localized above the spherical (quantum dot) (insulating matrix) interface) in a nanosystem containing insulating Al2O3 quantum dots is substantially increased (by nearly two orders of magnitude) compared to the exciton binding energy in an Al2O3 single crystal. It is established that, in the band gap of an Al2O3 nanoparticle, a band of exciton states (formed from spatially separated electrons and holes) appears. It is shown that there exists the possibility of experimentally detecting the ground and excited exciton states in the band gap of Al2O3 nanoparticles at room temperature from the absorption spectrum of the nanosystem.
author_names: Sergey I Pokutnyi

Yuriy Nikolaevich Kulchin

Vladimir P Dzyuba
title: Binding energy of excitons formed from spatially separated electrons and holes in insulating quantum dots
year: 2015

abstract: Background and Objectives: Recent findings suggest that therapeutic potential of mesenchymal stem cells (MSCs) could be increased through aggregation into three dimensional (3D) bodies, and different culture methods have been employed to obtain 3D spheroids of MSCs. In the current study we report accidentally encountered spontaneous formation of adipose derived stem cell (ASC) bodies in standard ASC culture of a single donor. Methods and Results: Human ASCs from passages 1 to 3, cultured in a medium containing 5% autologous serum (AS) spontaneously clustered and formed floating 3D bodies. After a transfer of floating ASC bodies onto new adherent plastic dish, they attached to the surface and gradual migration of spindle shaped ASCs out of the bodies was detected. A substitution of AS with allogeneic sera did not hinder this ability, but commercial medium containing fetal bovine serum delayed the process. Substantial part of ASCs surrounding transferred ASC bodies showed alkaline phosphatase (AP) activity, while ASC aggregates were AP negative. Similar 3D bodies formed when ASCs were grown on an uncoated glass surface. These ASC aggregates as well as clusters of ASCs, where formation of the 3D bodies is initiated, expressed pluripotency marker NANOG, but the expression of OCT4A was not detected. Conclusions: Obtained results suggest that spontaneously formed ASC aggregates may represent a more primitive cell subpopulation within the individual ASC culture. The ability to form 3D aggregates, the expression of NANOG, and the lack of the AP activity may be used to enrich ASC cultures with potentially more primitive cells serving as an excellent basis for therapeutic applications.
author_names: Ance Bogdanova-Jatniece

Uldis Berzins

Tatjana Kozlovska
title: Growth Properties and Pluripotency Marker Expression of Spontaneously Formed Three dimensional Aggregates of Human Adipose derived Stem Cells
venue: International journal of stem cells
year: 2014

abstract: An unusually large exciton coupling and spontaneous self localization safeguards the long lived excitons of H aggregated perylene bisimide against a notoriously universal excimeric trapping process, and rekindles its potential as a light harvesting material.
author_names: Samaresh Samanta

Subir kumar Ray

Shubham Deolka

Sudipta Saha

Pradeep K R

Rohit Bhowal

Nirmalya Ghosh

Debangshu Chaudhuri
title: Safeguarding long lived excitons from excimer traps in H aggregated dye assemblies+ +Electronic supplementary information (ESI) available: Experimental methods, synthesis and characterization, additional figures and CIF. CCDC 1967792. For ESI and crystallographic data in CIF or other electronic form
venue: Chemical science
year: 2020

abstract: The Pancharatnam Berry phase is a geometric phase acquired over a cycle of parameters in the Hamiltonian governing the evolution of the system. Here, we report on the observation of the Pancharatnam Berry phase in a condensate of indirect excitons (IXs) in a GaAs coupled quantum well structure. The Pancharatnam Berry phase is directly measured by detecting phase shifts of interference fringes in IX interference patterns. Correlations are found between the phase shifts, polarization pattern of IX emission, and onset of IX spontaneous coherence. The evolving Pancharatnam Berry phase is acquired due to coherent spin precession in IX condensate and is observed with no decay over lengths exceeding 10 mm indicating long range coherent spin transport.Geometric phases in matter are of fundamental interest. Here the authors observe the Pancharatnam Berry phase in matter waves by imaging light interference patterns from cold gases of indirect excitons and find long range coherent spin transport in the system.
author_names: J R Leonard

Alexander A High

Aaron Tynes Hammack

Michael M Fogler

L V Butov

Kenneth Lee Campman

Arthur C Gossard
title: Pancharatnam Berry phase in condensate of indirect excitons
venue: 2018 Conference on Lasers and Electro Optics (CLEO)
year: 2018

abstract: Spatiotemporal dynamics of excitons in monolayer and bulk WS2 at room temperature is studied by transient absorption microscopy in the reflection geometry. Excitons are formed from photocarriers injected by a tightly focused 390 nm pump pulse, and monitored by detecting different reflection of a time delayed and spatially scanned 620 nm probe pulse. We obtain exciton lifetimes of 22 1 and 110 10 ps in monolayer and bulk WS2, respectively. Both lifetimes are independent of the exciton density, showing the absence of multi exciton recombination processes. Exciton diffusion coefficients of 60 20 and 3.5 0.5 cm(2) s( 1) are obtained in monolayer and bulk samples, respectively. These results provide a foundation for understanding excitons in this new material and its optoelectronic applications.
author_names: Jiaqi He

Da-wei He

Yongsheng Wang

Qiannan Cui

Frank Ceballos

Hui Zhao
title: Spatiotemporal dynamics of excitons in monolayer and bulk WS2.
venue: Nanoscale
year: 2015

abstract: We calculate the exact time resolved ARPES spectrum of a two band model semiconductor driven out of equilibrium by resonant and nonresonant laser pulses, highlighting the effects of phonon induced decoherence and relaxation. Resonant excitations initially yield a replica of the valence band shifted upward by the energy of the exciton peak in photoabsorption. This phase is eventually destroyed by phonon induced decoherence: the valence band replica lowers in energy by the Stokes shift, locating at the energy of the exciton peak in photoluminescence, and its width grows due to phonon dressing. Nonresonant excitations initially yield a map of the conduction band. Then electrons transfer their excess energy to the lattice and bind with the holes left behind to form excitons. In this relaxed regime a replica of the conduction band appears inside the gap. At fixed momentum the lineshape of the conduction band replica versus the photoelectron energy is proportional to the exciton wavefunction in "energy space" and it is highly asymmetric. Although the two band model represents an oversimplified description of real materials the highlighted features are qualitative in nature; hence they provide useful insights into time resolved ARPES spectra and their physical interpretation.
author_names: Gianluca Stefanucci

Enrico Perfetto
title: From carriers and virtual excitons to exciton populations: Insights into time resolved ARPES spectra from an exactly solvable model
year: 2021