arXiv subjects

Justin C. Johnson

Publications and source records attributed to Justin C. Johnson.

7 recordsLinked to original sources

arXiv2026

Localized Excitons and Exciton-Phonon Coupling in Antiferromagnetic AgCrP2S6

AgCrP2S6 combines a low-symmetry thiophosphate framework with an antiferromagnetic Cr sublattice and nonmagnetic Ag sites, providing a setting in which covalency and magnetic localization compete in the low-energy optical response. We combine single-crystal x-ray diffraction, Raman spectroscopy, lattice-dynamical calculations, and self-consistent vertex corrected Feynman diagrammatic many-body approaches to determine the structural, electronic, and excitonic properties of bulk AgCrP2S6. The material remains monoclinic between 100 and 300 K, with additional Ag-site disorder resolved at low temperature, and the optimized structure is dynamically stable. The many-body calculations yield a reduced quasiparticle gap relative to Cr trihalides, but the lowest excitons remain predominantly local: the calculated low-energy (~1.4 eV) excitation cluster is best described as a weakly bright, strongly anisotropic Frenkel exciton with dominant onsite d-d character and substantial ligand-assisted d-p admixture. Symmetry analysis in the C2h setting identifies this state as predominantly Bu and constrains its leading exciton-phonon coupling channels to Ag diagonal renormalization and Bg-mediated bright-dark mixing. These results place AgCrP2S6 in a localized excitonic regime in which the stronger p-d hybridization, relative to the more ionic Cr trihalides, narrows the quasiparticle gap, while the antiferromagnetic exchange and Ag-site dilution disfavor the intersite coherence associated with strongly delocalized low-energy excitons.

cond-mat.mtrl-sci↗

arXiv2026

Bright and Dark Excitons in CrSBr: Local Ligand-Field Character and Band-Coherent Optical Selection Rules

Magnetic van der Waals semiconductors such as CrSBr host an intricate exciton landscape whose physical interpretation has converged only recently. A many-body Feynman diagrammatic approach based on quasiparticle self-consistent GW with electron-hole ladder vertex corrections to the screened Coulomb interaction has established the electronic band gap, excitonic orbital character, real-space extent, binding energies, and bosonic-coupling signatures of the bright XA exciton near 1.34 eV and the higher XB manifold near 1.8 eV. These results agree well with ARPES and magneto-optical experiments and supersede the early Rydberg-like assignment of the excitons. What has remained unresolved is why these intense bright excitons coexist, within a few tens of meV, with companion states that are several orders of magnitude darker despite drawing from essentially the same single-particle transition manifold. Here we show that brightness is a band-coherent property of the excitonic eigenfunctions: bright and dark partners are sublattice-symmetric and sublattice-antisymmetric superpositions of the same ligand-field-like Bloch transitions across the two Cr atoms of the orthorhombic primitive cell. The commonly used Frenkel and Wannier-Mott labels describe what an exciton is made of, but brightness requires a symmetry-adapted interference rule between transition dipoles. Disentangling this bare excitonic structure is a prerequisite for interpreting the optical response of CrSBr once magnon, phonon, and photon couplings are included.

cond-mat.mtrl-sci↗

arXiv2025

Spin-quenching in molecule-transition-metal-dichalcogenide heterostructure through inverse proximity effect

A functional heterostructure is central to integrated circuitry in quantum photonics, optoelectronics, neuromorphic computing, spintronics, and straintronics. Recently, heterostructures combining 2D magnets and nonmagnetic transition metal dichalcogenides (TMDs) have been explored. In these, electron and hole wavefunctions are localized in 2D magnets but delocalized in TMDs. When combined, a proximity induced magnetic inter layer exciton can emerge, with energy differing by 20 to 30 meV from intra layer excitons and being two orders of magnitude darker, making it hard to detect and functionalize. Using a high fidelity ab initio many body diagrammatic approach, we show that functionality can be significantly enhanced in a transition metal molecule TMD interface. The molecular exciton exhibits charge transfer character and is extended, unlike the localized Frenkel excitation in 2D magnets. Moreover, the degree of localization and magnetic moment can be tuned by varying the molecular orientation relative to the TMD. This changes the proximity to the magnetic ion, altering screening and enabling a pathway to quench the ion's spin moment. This inverse proximity effect tunes the energies, spin states, and brightness of molecular and inter layer magnetic excitons, a mechanism absent in 2D magnet TMD systems. We also identify conditions under which the interlayer exciton becomes well separated and brighter than intra layer excitons, making it promising for protocols that probe and manipulate magnetic excitonic states.

cond-mat.str-el↗

arXiv2024

Modulating Endothermic Singlet Fission by Controlling Radiative Rates in Perylene Dimers

Endothermic singlet fission (SF), an exciton multiplication process that produces a pair of high-energy triplet excitons (T1T1), is appealing for photovoltaic or photoelectrochemical applications, as it allows the conversion of entropy into electronic or chemical energy. The mechanistic aspects of this process are not entirely known, and strategies for improving the yield of triplets via endothermic SF have not been developed. In this work we provide experimental evidence that in photoexcited dimers of perylene, S1 is initially in equilibrium with 1(T1T1), and that the lifetime of this equilibrium can be controlled through strategic changes in the radiative rate. Through careful molecular design we fine-tune both the degree of endothermicity and excited state lifetimes in four perylene dimers. Using transient absorption and time resolved fluorescence, we reveal that the dimer with the slowest radiative rate constant produces the most prolonged 1(T1T1). However, in the dimers, the annihilation of the 1(T1T1) state results in a single long-lived triplet rather than a pair, and increasing the free triplet yield above 100% would require additional chromophores.

physics.chem-ph↗

arXiv2023

Modulating spin-valley relaxation in WSe$_2$ with variable thickness VOPc layers

Combining the synthetic tunability of molecular compounds with the optical selection rules of transition metal dichalcogenides (TMDC) that derive from spin-valley coupling could provide interesting opportunities for the readout of quantum information. However, little is known about the electronic and spin interactions at such interfaces and the influence on spin-valley relaxation. In this work, vanadyl phthalocyanine (VOPc) molecular layers are thermally evaporated on WSe$_2$ to explore the effect of molecular layer thickness on excited-state spin-valley polarization. The thinnest molecular layer supports an interfacial state which destroys the spin-valley polarization almost instantaneously, whereas a thicker molecular layer results in longer-lived spin-valley polarization than the WSe$_2$ monolayer alone. The mechanism appears to involve a tightly-bound species at the molecule/TMDC interface that strengthens exchange interactions and is largely avoided in thicker VOPc layers that isolate electrons from WSe$_2$ holes.

cond-mat.mes-hall↗

arXiv2022

Optical readout of singlet fission biexcitons with photoluminescence detected magnetic resonance

Molecular spin systems based on photoexcited triplet pairs formed via singlet fission (SF) are attractive as carriers of quantum information because of their potentially pure and controllable spin polarization, but developing systems that offer optical routes to readout as well as initialization is challenging. Herein, we characterize the electron spin magnetic resonance change in photoluminescence intensity for a tailored organic molecular crystal while sweeping a microwave drive up to 10 GHz in a broadband loop structure. We observe resonant transitions for both triplet and quintet spin sublevel populations showing their optical sensitivity, and revealing zero-field parameters for each. We map the evolution of these spectra in both microwave frequency and magnetic field, producing a pattern of optically-detected magnetic resonance (ODMR) peaks. Fits to this data using a suitable model suggest significant spin polarization in this system with orientation selectivity. Unusual excitation intensity dependence is also observed, which inverts the sign of the ODMR signal for triplet features, but not for quintet. These observations demonstrate optical detection of the spin sublevel population dictated by SF and intermolecular geometry, and provide unique insight into the dynamics of triplet pairs.

cond-mat.mes-hall↗

arXiv2022

Triplet-Pair Spin Signatures from Macroscopically Aligned Heteroacenes in an Oriented Single Crystal

The photo-driven process of singlet fission generates coupled triplet pairs (TT) with fundamentally intriguing and potentially useful properties. The quintet 5TT0 sublevel is particularly interesting for quantum information because it is highly entangled, addressable with microwave pulses, and could be detected using optical techniques. Previous theoretical work on a model Hamiltonian and nonadiabatic transition theory, called the JDE model, has determined that this sublevel can be selectively populated if certain conditions are met. Among the most challenging, the molecules within the dimer undergoing singlet fission must have their principal magnetic axes parallel to one another and to an applied Zeeman field B0. Here, we present time-resolved paramagnetic resonance (TR-EPR) spectroscopy of a single crystal sample of a novel tetracenethiophene compound featuring arrays of dimers aligned in this manner, mounted so that the orientation of the field relative to the molecular axes could be controlled. The observed spin sublevel populations in the paired TT and unpaired (T+T) triplets are consistent with predictions from the JDE model, including preferential 5TT0 formation at z||B0, with one caveat - two 5TT spin sublevels have little to no population. This may be due to crossings between the 5TT and 3TT manifolds in the field range investigated by TR-EPR, consistent with the inter-triplet exchange energy determined by monitoring photoluminescence at varying magnetic fields.

physics.chem-ph↗