arXiv subjects

John M. Lupton

Publications and source records attributed to John M. Lupton.

At least 19 recordsLinked to original sources

arXiv2024

Electrically tunable layer-hybridized trions in doped WSe$_2$ bilayers

Doped van der Waals heterostructures host layer-hybridized trions, i.e. charged excitons with layer-delocalized constituents holding promise for highly controllable optoelectronics. Combining a microscopic theory with photoluminescence (PL) experiments, we demonstrate the electrical tunability of the trion energy landscape in naturally stacked WSe$_2$ bilayers. We show that an out-of-plane electric field modifies the energetic ordering of the lowest lying trion states, which consist of layer-hybridized $Λ$-point electrons and layer-localized K-point holes. At small fields, intralayer-like trions yield distinct PL signatures in opposite doping regimes characterized by weak Stark shifts in both cases. Above a doping-asymmetric critical field, interlayer-like species are energetically favored and produce PL peaks with a pronounced Stark red-shift and a counter-intuitively large intensity arising from efficient phonon-assisted recombination. Our work presents an important step forward in the microscopic understanding of layer-hybridized trions in van der Waals heterostructures and paves the way towards optoelectronic applications based on electrically controllable atomically-thin semiconductors.

cond-mat.mes-hall↗

arXiv2022

Supercurrent diode effect and magnetochiral anisotropy in few-layer NbSe$_2$

Nonreciprocal transport refers to charge transfer processes that are sensitive to the bias polarity. Until recently, nonreciprocal transport was studied only in dissipative systems, where the nonreciprocal quantity is the resistance. Recent experiments have, however, demonstrated nonreciprocal supercurrent leading to the observation of a supercurrent diode effect in Rashba superconductors, opening the vision of dissipationless electronics. Here we report on a supercurrent diode effect in NbSe$_2$ constrictions obtained by patterning NbSe$_2$ flakes with both even and odd layer number. The observed rectification is driven by valley-Zeeman spin-orbit interaction. We demonstrate a rectification efficiency as large as 60%, considerably larger than the efficiency of devices based on Rashba superconductors. In agreement with recent theory for superconducting transition metal dichalcogenides, we show that the effect is driven by an out-of-plane magnetic field component. Remarkably, we find that the effect becomes field-asymmetric in the presence of an additional in-plane field component transverse to the current direction. Supercurrent diodes offer a further degree of freedom in designing superconducting quantum electronics with the high degree of integrability offered by van der Waals materials.

cond-mat.supr-con↗

arXiv2022

Robust excitons across the phase transition of two-dimensional hybrid perovskites

Two-dimensional halide perovskites are among intensely studied materials platforms profiting from solution based growth and chemical flexibility. They feature exceptionally strong interactions among electronic, optical as well as vibrational excitations and hold a great potential for future optoelectronic applications. A key feature for these materials is the occurrence of structural phase transitions that can impact their functional properties, including the electronic band gap and optical response dominated by excitons. However, to what extent the phase-transitions in two-dimensional perovskites alter the fundamental exciton properties remains barely explored so far. Here, we study the influence of the phase transition on both exciton binding energy and exciton diffusion, demonstrating their robust nature across the phase transition. These findings are unexpected in view of the associated substantial changes of the free carrier masses, strongly contrast broadly considered effective mass and drift-diffusion transport mechanisms, highlighting the unusual nature of excitons in two-dimensional perovskites.

cond-mat.mes-hall↗

arXiv2021

Twist-angle engineering of excitonic quantum interference and optical nonlinearities in stacked 2D semiconductors

Twist-engineering of the electronic structure of van-der-Waals layered materials relies predominantly on band hybridization between layers. Band-edge states in transition-metal-dichalcogenide semiconductors are localized around the metal atoms at the center of the three-atom layer and are therefore not particularly susceptible to twisting. Here, we report that high-lying excitons in bilayer WSe2 can be tuned over 235 meV by twisting, with a twist-angle susceptibility of 8.1 meV/°, an order of magnitude larger than that of the band-edge A-exciton. This tunability arises because the electronic states associated with upper conduction bands delocalize into the chalcogenide atoms. The effect gives control over excitonic quantum interference, revealed in selective activation and deactivation of electromagnetically induced transparency (EIT) in second-harmonic generation. Such a degree of freedom does not exist in conventional dilute atomic-gas systems, where EIT was originally established, and allows us to shape the frequency dependence, i.e. the dispersion, of the optical nonlinearity.

cond-mat.mes-hall↗

arXiv2020

Large-scale mapping of moiré superlattices by Raman imaging of interlayer breathing mode and moiré phonons

Moiré superlattices can induce correlated-electronic phases in twisted van-der-Waals materials. Strongly correlated quantum phenomena emerge, such as superconductivity and the Mott-insulating state. However, moiré superlattices produced through artificial stacking can be quite inhomogeneous, which hampers the development of a clear correlation between the moiré period and the emerging electrical and optical properties. Here we demonstrate in twisted-bilayer transition-metal dichalcogenides that low-frequency Raman scattering can be utilized not only to detect atomic reconstruction, but also to map out the inhomogeneity of the moiré lattice over large areas. The method is established based on the finding that both the interlayer-breathing mode and moiré phonons are highly susceptible to the moiré period and provide characteristic fingerprints. We visualize microscopic domains with an effective twist-angle resolution of ~0.1°. This ambient non-invasive methodology can be conveniently implemented to characterize and preselect high-quality areas of samples for subsequent device fabrication, and for transport and optical experiments.

cond-mat.mes-hall↗

arXiv2020

Complete polarization of electronic spins in OLEDs

At low temperatures and high magnetic fields, electron and hole spins in an organic light-emitting diode (OLED) become polarized so that recombination preferentially forms molecular triplet excited-state species. For low device currents, magnetoelectroluminescence (MEL) perfectly follows Boltzmann activation, implying a virtually complete polarization outcome. As the current increases, the MEL effect is reduced because spin polarization is suppressed by the reduction in carrier residence time within the device. Under these conditions, an additional field-dependent process affecting the spin-dependent recombination emerges, which appears to relate to the build-up of triplet excitons and the interaction with free charge carriers. Suppression of the EL at high fields on its own does not, strictly, prove electronic spin polarization. We therefore probe changes in the spin statistics of recombination directly in a dual singlettriplet emitting OLED, which shows a concomitant rise in phosphorescence intensity as fluorescence is suppressed. Finite spin-orbit coupling in these materials gives rise to a microscopic distribution in effective g factors of electrons and holes, $Δg$, i.e. a distribution in Larmor frequencies, leading to singlet-triplet mixing within the electronhole pair as a function of applied field. This $Δg$ effect in the pair further suppresses singlet-exciton formation in addition to thermal spin polarization of the individual carriers. Since the $Δg$ process involves weakly bound carrier pairs rather than free electrons and holes, in contrast to thermal spin polarization, the effect does not depend significantly on current or temperature.

cond-mat.mes-hall↗

arXiv2020

Picosecond time-resolved antibunching measures nanoscale exciton motion, annihilation, and true number of chromophores

The particle-like nature of light becomes evident in the photon statistics of fluorescence of single quantum systems as photon antibunching. In multichromophoric systems, exciton diffusion and subsequent annihilation occurs. These processes also yield photon antibunching but cannot be interpreted reliably. Here, we develop picosecond time-resolved antibunching (psTRAB) to identify and decode such processes. We use psTRAB to measure the true number of chromophores on well-defined multichromophoric DNA-origami structures, and precisely determine the distance-dependent rates of annihilation between excitons. Further, psTRAB allows us to measure exciton diffusion in mesoscopic H- and J-type conjugated-polymer aggregates. We distinguish between one-dimensional intra-chain and three-dimensional inter-chain exciton diffusion at different times after excitation and determine the disorder-dependent diffusion lengths. Our method provides a new lens through which excitons can be studied at the single-particle level, enabling the rational design of improved excitonic probes such as ultra-bright fluorescent nanoparticles, and materials for optoelectronic devices.

physics.chem-ph↗

arXiv2020

Bright excitons with negative-mass electrons

Bound electron-hole excitonic states are generally not expected to form with charges of negative effective mass. We identify such excitons in a single layer of the semiconductor WSe2, where they give rise to narrow-band upconverted photoluminescence in the UV, at an energy of 1.66 eV above the first band-edge excitonic transition. Negative band curvature and strong electron-phonon coupling result in a cascaded phonon progression with equidistant peaks in the photoluminescence spectrum, resolvable to ninth order. Ab initio GW-BSE calculations with full electron-hole correlations unmask and explain the admixture of upper conduction-band states to this complex many-body excitation: an optically bright, bound exciton in resonance with the semiconductor continuum. This exciton is responsible for atomic-like quantum-interference phenomena such as electromagnetically induced transparency. Since band curvature can be tuned by pressure or strain, synthesis of exotic quasiparticles such as flat-band excitons with infinite reduced mass becomes feasible.

cond-mat.mes-hall↗

arXiv2020

Hybridized intervalley moiré excitons and flat bands in twisted WSe$_2$ bilayers

The large surface-to-volume ratio in atomically thin 2D materials allows to efficiently tune their properties through modifications of their environment. Artificial stacking of two monolayers into a bilayer leads to an overlap of layer-localized wave functions giving rise to a twist angle-dependent hybridization of excitonic states. In this joint theory-experiment study, we demonstrate the impact of interlayer hybridization on bright and momentum-dark excitons in twisted WSe$_2$ bilayers. In particular, we show that the strong hybridization of electrons at the $Λ$ point leads to a drastic redshift of the momentum-dark K-$Λ$ exciton, accompanied by the emergence of flat moiré exciton bands at small twist angles. We directly compare theoretically predicted and experimentally measured optical spectra allowing us to identify photoluminescence signals stemming from phonon-assisted recombination of layer-hybridized dark excitons. Moreover, we predict the emergence of additional spectral features resulting from the moiré potential of the twisted bilayer lattice.

cond-mat.mes-hall↗

arXiv2019

Ultrafast transition between exciton phases in van der Waals heterostructures

Heterostructures of atomically thin van der Waals bonded monolayers have opened a unique platform to engineer Coulomb correlations, shaping excitonic, Mott insulating, or superconducting phases. In transition metal dichalcogenide heterostructures, electrons and holes residing in different monolayers can bind into spatially indirect excitons with a strong potential for optoelectronics, valleytronics, Bose condensation, superfluidity, and moiré-induced nanodot lattices. Yet these ideas require a microscopic understanding of the formation, dissociation, and thermalization dynamics of correlations including ultrafast phase transitions. Here we introduce a direct ultrafast access to Coulomb correlations between monolayers; phase-locked mid-infrared pulses allow us to measure the binding energy of interlayer excitons in WSe2/WS2 hetero-bilayers by revealing a novel 1s-2p resonance, explained by a fully quantum mechanical model. Furthermore, we trace, with subcycle time resolution, the transformation of an exciton gas photogenerated in the WSe2 layer directly into interlayer excitons. Depending on the stacking angle, intra- and interlayer species coexist on picosecond scales and the 1s-2p resonance becomes renormalized. Our work provides a direct measurement of the binding energy of interlayer excitons and opens the possibility to trace and control correlations in novel artificial materials.

cond-mat.mtrl-sci↗

arXiv2019

Perdeuteration of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (d-MEHPPV): control of microscopic charge-carrier spin-spin coupling and of magnetic-field effects in optoelectronic devices

Control of the effective local hyperfine fields in a conjugated polymer, poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV), by isotopic engineering is reported. These fields, evident as a frequency-independent line broadening mechanism in electrically detected magnetic resonance spectroscopy (EDMR), originate from the unresolved hyperfine coupling between the electronic spin of charge carrier pairs and the nuclear spins of surrounding hydrogen isotopes. The room temperature study of effects caused by complete deuteration of this polymer through magnetoresistance, magnetoelectroluminescence, coherent pulsed and multi-frequency EDMR, as well as inverse spin-Hall effect measurements, confirm the weak hyperfine broadening of charge carrier magnetic resonance lines. As a consequence, we can resolve coherent charge-carrier spin-beating, allowing for direct measurements of the magnitude of electronic spin-spin interactions. In addition, the weak hyperfine coupling allows us to resolve substantial spin-orbit coupling effects in EDMR spectra, even at low magnetic field strengths. These results illustrate the dramatic influence of hyperfine fields on the spin physics of organic light-emitting diode (OLED) materials at room temperature, and point to routes to reaching exotic ultra-strong resonant-drive regimes needed for the study of light-matter interactions.

cond-mat.mtrl-sci↗

arXiv2019

OLEDs as models for bird magnetoception: detecting electron spin resonance in geomagnetic fields

Certain species of living creatures are known to orientate themselves in the geomagnetic field. Given the small magnitude of approximately 48 μT, the underlying quantum mechanical phenomena are expected to exhibit coherence times approaching the millisecond regime. In this contribution, we show sensitivity of organic light-emitting diodes (OLEDs) to magnetic fields far below Earth's magnetic field, suggesting that coherence times of the spins of charge-carrier pairs in these devices can be similarly long. By electron paramagnetic resonance (EPR) experiments, a lower bound for the coherence time can be assessed directly. Moreover, this technique offers the possibility to determine the distribution of hyperfine fields within the organic semiconductor layer. We extend this technique to a material system exhibiting both fluorescence and phosphorescence, demonstrating stable anticorrelation between optically detected magnetic resonance (ODMR) spectra in the singlet (fluorescence) and triplet (phosphorescence) channel. The experiments demonstrate the extreme sensitivity of OLEDs to both static as well as dynamic magnetic fields and suggest that coherent spin precession processes of Coulombically bound electron spin pairs may play a crucial role in the magnetoreceptive ability of living creatures.

cond-mat.mes-hall↗

arXiv2018

Quantum interference in second-harmonic generation from monolayer WSe2

A hallmark of wave-matter duality is the emergence of quantum-interference phenomena when an electronic transition follows different trajectories. Such interference results in asymmetric absorption lines such as Fano resonances, and gives rise to secondary effects like electromagnetically induced transparency (EIT) when multiple optical transitions are pumped. Few solid-state systems show quantum interference and EIT, with quantum-well intersubband transitions in the IR offering the most promising avenue to date to devices exploiting optical gain without inversion. Quantum interference is usually hampered by inhomogeneous broadening of electronic transitions, making it challenging to achieve in solids at visible wavelengths and elevated temperatures. However, disorder effects can be mitigated by raising the oscillator strength of atom-like electronic transitions - excitons - which arise in monolayers of transition-metal dichalcogenides (TMDCs). Quantum interference, probed by second-harmonic generation (SHG), emerges in monolayer WSe2, without a cavity, splitting the SHG spectrum. The splitting exhibits spectral anticrossing behaviour, and is related to the number of Rabi flops the strongly driven system undergoes. The SHG power-law exponent deviates strongly from the canonical value of 2, showing a Fano-like wavelength dependence which is retained at room temperature. The work opens opportunities in solid-state quantum-nonlinear optics for optical mixing, gain without inversion and quantum-information processing.

cond-mat.mes-hall↗

arXiv2018

Role of Triplet-State Shelving in Organic Photovoltaics: Single-Chain Aggregates of Poly(3-hexylthiophene) versus Mesoscopic Multichain Aggregates

Triplet excitons have been the focus of considerable attention with regards to the functioning of polymer solar cells, because these species are long-lived and quench subsequently generated singlet excitons in their vicinity. The role of triplets in poly(3-hexylthiophene) (P3HT) has been investigated extensively with contrary conclusions regarding their importance. We probe the various roles triplets can play in P3HT by analyzing the photoluminescence (PL) from isolated single-chain aggregates and multi-chain mesoscopic aggregates. Solvent vapor annealing allows deterministic growth of P3HT aggregates consisting of ~20 chains, which exhibit red-shifted and broadened PL compared to single-chain aggregates. The multi-chain aggregates exhibit a decrease of photon antibunching contrast compared to single-chain aggregates, implying rather weak interchain excitonic coupling and energy transfer. Nevertheless, the influence of triplet-quenching oxygen on PL and a photon correlation analysis of aggregate PL reveal that triplets are quenched by intermolecular interactions in the bulk state.

physics.chem-ph↗

arXiv2018

Determining the True Optical Gap in a High-Performance Organic Photovoltaic Polymer Using Single-Molecule Spectroscopy

Low-gap conjugated polymers have enabled an impressive increase in the efficiencies of organic solar cells, primarily due to their red absorption which allows harvesting of that part of the solar spectrum. Here, we report that the true optical gap of one prototypical material, PTB7, is in fact at significantly higher energy than has previously been reported, indicating that the red absorption utilized in these materials in solar cells is entirely due to chain aggregation. Using single-molecule spectroscopy we find that PL from isolated nanoscale aggregates consists of multiple independently emitting chromophores. At the single-molecule level, however, straight single chains with a high degree of emission polarization are observed. The PL is found to be ~0.4 eV higher in energy, with a longer lifetime than the red aggregates, and is attributed to single chromophores. Our findings indicate that the impressive light-harvesting abilities of PTB7 in the red spectral region arises solely from chain aggregation.

physics.chem-ph↗

arXiv2018

Light emission from gold nanoparticles under ultrafast near-infrared excitation: thermal emission, inelastic light scattering or multiphoton luminescence?

Gold nanoparticles emit broad-band upconverted luminescence upon irradiation with pulsed infrared laser radiation. Although the phenomenon is widely observed, considerable disagreement still exists concerning the underlying physics - most notably over the applicability of concepts such as multiphoton absorption, inelastic scattering, and interband and intraband electronic transitions. Here, we study single particles and small clusters of particles by employing a spectrally resolved power-law analysis of the irradiation-dependent emission as a sensitive probe of these physical models. Two regimes of emission are identified: at low irradiance levels of kW/cm2, the emission follows a well-defined integer-exponent power law suggestive of a multiphoton process. However, at higher irradiance levels of several kW/cm2, the nonlinearity exponent itself depends on the photon energy detected, a tell-tale signature of a radiating heated electron gas. We show that in this regime, the experiments are incompatible with both interband transitions and inelastic light scattering as the cause the luminescence, while they are compatible with the notion of luminescence linked to intraband transitions.

cond-mat.mtrl-sci↗

arXiv2017

Characterization of highly crystalline lead iodide nanosheets prepared by room-temperature solution processing

Two-dimensional semiconducting materials are particularly appealing for many applications. Although theory predicts a large number of two-dimensional materials, experimentally only a few of these materials have been identified and characterized comprehensively in the ultrathin limit. Lead iodide, which belongs to the transition metal halides family and has a direct bandgap in the visible spectrum, has been known for a long time and has been well characterized in its bulk form. Nevertheless, studies of this material in the nanometer thickness regime are rather scarce. In this article we demonstrate an easy way to synthesize ultrathin, highly crystalline flakes of PbI2 by precipitation from a solution in water. We thoroughly characterize the produced thin flakes with different techniques ranging from optical and Raman spectros-copy to temperature-dependent photoluminescence and electron microscopy. We compare the results to ab initio calculations of the band structure of the material. Finally, we fabricate photodetectors based on PbI2 and study their optoelectronic properties.

cond-mat.mes-hall↗

arXiv2017

Valley-Polarized Exciton Dynamics in Exfoliated Monolayer WSe$_2$

Semiconducting transition metal dichalcogenide monolayers have emerged as promising candidates for future valleytronics-based quantum information technologies. Two distinct momentum-states of tightly-bound electron-hole pairs in these materials can be deterministically initialized via irradiation with circularly polarized light. Here, we investigate the ultrafast dynamics of such a valley polarization in monolayer tungsten diselenide by means of time-resolved Kerr reflectometry. The observed Kerr signal in our sample stems exclusively from charge-neutral excitons. Our findings support the picture of a fast decay of the valley polarization of bright excitons due to radiative recombination, intra-conduction-band spin-flip transitions, intervalley-scattering processes, and the formation of long-lived valley-polarized dark states.

cond-mat.mes-hall↗