Showing 1–24 of 24 results for author: Haber, J B

Exciton valley depolarization in monolayer MoS2: non-Markovian quantum dynamics, intervalley scattering, and the breakdown of the Dyakonov-Perel mechanism

Authors: Yang-hao Chan, Jonah B. Haber, Mit H. Naik, Felipe H. da Jornada, Diana Y. Qiu

Abstract: In monolayer transition metal dichalcogenides, exciton valley relaxation is typically attributed to the Dyakonov-Perel (DP) mechanism, where frequent intravalley scattering with phonons or defects suppresses the intervalley exchange-induced precession and the role of intervalley scattering is considered secondary. Employing a first-principles nonequilibrium exciton Green's function (NEGF) approach… ▽ More In monolayer transition metal dichalcogenides, exciton valley relaxation is typically attributed to the Dyakonov-Perel (DP) mechanism, where frequent intravalley scattering with phonons or defects suppresses the intervalley exchange-induced precession and the role of intervalley scattering is considered secondary. Employing a first-principles nonequilibrium exciton Green's function (NEGF) approach with the generalized Kadanoff-Baym ansatz (GKBA), which treats exchange-driven precession and exciton-phonon scattering on an equal footing across the full Brillouin zone, we demonstrate that even in the small momentum regime most favorable to DP physics, realistic exciton-phonon scattering is too weak to induce the motional narrowing that defines the DP regime. Upon accounting for excitons across the entire Brillouin zone, large momentum intervalley scattering becomes the dominant pathway for valley relaxation, shortening the depolarization by a factor of 3-4 relative to intravalley-only models. We find that valley depolarization and decoherence time are approximately 50 fs at 300 K and lengthen to 130 fs at 10 K. By comparing our results with a Lindblad-type collision framework, we explicitly demonstrate the role of non-Markovian effects and reveal that the Markovian Lindblad framework is highly basis dependent. In the valley-pseudospin basis underlying prior analyses, the Lindblad approach artificially amplifies scattering-induced equilibration, biasing the dynamics toward a DP interpretation. Our study provides a comprehensive picture of valley relaxation, and establishes the exciton density matrix approach derived from NEGF+GKBA as a powerful tool for investigating ultrafast exciton dynamics. △ Less

Submitted 9 July, 2026; originally announced July 2026.

Comments: 17 pages, 8 figures

Tailoring phonon-driven responses in α-MoO3 through isotopic enrichment

Authors: Thiago S. Arnaud, Ryan W. Spangler, Johnathan D. Georgaras, Jonah B. Haber, Daniel Hirt, Maximilian Obst, Gonzalo Álvarez-Pérez, Mackey Long III, Felix G. Kaps, Jakob Wetzel, Courtney Ragle, John E. Buchner, Youngji Kim, Aditha S. Senarath, Richarda Niemann, Mingze He, Giulia Carini, Unai Arregui-Leon, Akash C. Behera, Ramachandra Bangari, Nihar Sahoo, Niels C. Brumby, J. Michael Klopf, Martin Wolf, Lukas M. Eng , et al. (7 additional authors not shown)

Abstract: The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material is α-MoO3, which as an orthorhombic crystal boasts three distinct phonon dispersions, providing three polaritonic dispersions of hyperbolic phonon polaritons (HPhPs) across the mid-infrared (MIR). Here, the tunability of bo… ▽ More The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material is α-MoO3, which as an orthorhombic crystal boasts three distinct phonon dispersions, providing three polaritonic dispersions of hyperbolic phonon polaritons (HPhPs) across the mid-infrared (MIR). Here, the tunability of both optical and thermal responses in isotopically enriched α-MoO3 (98MoO3, Mo18O3 and 98Mo18O3) are explored. A uniform ~5 % spectral redshift from 18O enrichment is observed in both Raman- and IR-active TO phonons. Both the in- and out-of-plane thermal conductivities for the isotopic variations are reported. Ab initio calculations both replicate experimental findings and analyze the select-mode three-phonon scattering contributions. The HPhPs from each isotopic variation are probed with s-SNOM and their Q- factors are reported. A Q-factor maxima increase of ~50 % along the [100] in the RB2 and ~100 % along the [001] in the RB3 are reported for HPhPs supported in 98Mo18O3. Observations in both real and Fourier space of higher-order HPhP modes propagating in single slabs of isotopically enriched α-MoO3 without the use of a subdiffractional surface scatterer are presented here. This work illustrates the tunability of α-MoO3 for thermal and nanophotonic applications. △ Less

Submitted 5 January, 2026; originally announced January 2026.

Real-Space Quantification of Exciton Localization in Acene Crystals Using Wannier Function Decomposition

Authors: Zui Tao, Jonah B. Haber, Jeffrey B. Neaton

Abstract: We introduce the Wannier function decomposition of excitons (WFDX) method to quantify exciton localization in solids within the ab initio Bethe-Salpeter equation framework. By decomposing each Bloch exciton wavefunction into products of single-particle electron and hole maximally localized Wannier functions, this real-space approach provides well-defined orbital- and spatial- resolved measures of… ▽ More We introduce the Wannier function decomposition of excitons (WFDX) method to quantify exciton localization in solids within the ab initio Bethe-Salpeter equation framework. By decomposing each Bloch exciton wavefunction into products of single-particle electron and hole maximally localized Wannier functions, this real-space approach provides well-defined orbital- and spatial- resolved measures of both Frenkel and charge-transfer excitons at low computational cost. We apply WFDX to excitons in acene crystals, quantifying how the number of rings, the exciton spin state, and the center-of-mass momntum affect spatial localization. Additionally, we show how this real-space representation reflects structural nonsymmorphic symmetries that are hidden in standard reciprocal-space descriptions. We demonstrate how the WFDX framework can be used to efficiently interpolate exciton expansion coefficients in reciprocal-space and outline how it may facilitate evaluation of observables involving position operators, highlighting its potential as a general tool for both analyzing and computing excitonic properties in solids. △ Less

Submitted 7 October, 2025; originally announced October 2025.

Tunable electronic energy level alignment and exciton diversity in organic-inorganic van der Waals heterostructures

Authors: Aurélie Champagne, Olugbenga Adeniran, Jonah B. Haber, Antonios M. Alvertis, Zhen-Fei Liu, Jeffrey B. Neaton

Abstract: van der Waals stacking of two-dimensional (2D) materials offers a powerful platform for engineering material interfaces with tailored electronic and optical properties. While most van der Waals multilayers have featured inorganic monolayers, incorporating molecular monolayers introduces new degrees of tunability and functionality. Here, we investigate hybrid bilayers composed of atomically thin pe… ▽ More van der Waals stacking of two-dimensional (2D) materials offers a powerful platform for engineering material interfaces with tailored electronic and optical properties. While most van der Waals multilayers have featured inorganic monolayers, incorporating molecular monolayers introduces new degrees of tunability and functionality. Here, we investigate hybrid bilayers composed of atomically thin perylene-based molecular crystals interfaced with monolayer transition metal dichalcogenides (TMDs), specifically MoS2 and WS2. Using ab initio many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation approach, we predict emergent properties beyond those of the isolated constituent systems. Notably, we find substantial renormalization of monolayer molecular crystal band gap due to TMD-induced polarization. Furthermore, by varying the TMD monolayer, we demonstrate tuning of the energy level alignment of the bilayer and subsequent control over a diversity of lowest-energy excitons, which include strongly bound hybrid excitons and long-lived charge-transfer excitons. These findings establish organic-inorganic van der Waals heterostructures as a promising class of materials for tunable optoelectronic devices and quantum excitonic phenomena, expanding the design space for low-dimensional systems. △ Less

Submitted 6 October, 2025; originally announced October 2025.

Comments: 23 pages, 5 figures

Terahertz field-induced giant symmetry modulations in a van der Waals antiferromagnet

Authors: Sheikh Rubaiat Ul Haque, Martin J. Cross, Sangeeta Rajpurohit, Jonah B. Haber, Christopher J. Ciccarino, Alexandra C. Zimmerman, Isabelle J. Sealey, Vadym Kulichenko, Monique Tie, Huaiyu Wang, Sharon S. Philip, Choongwon Seo, Jake D. Koralek, Luis Balicas, Mykhaylo Ozerov, Dmitry Smirnov, Liang Z. Tan, Felipe H. da Jornada, Tadashi Ogitsu, Matthias C. Hoffmann, Tony F. Heinz, Aaron M. Lindenberg

Abstract: Strong-field terahertz (THz) excitations enable dynamic control over electronic, lattice and symmetry degrees of freedom in quantum materials. Here, we uncover pronounced terahertz-induced symmetry modulations and coherent phonon dynamics in the van der Waals antiferromagnet MnPS3, in which inversion symmetry is broken by its antiferromagnetic spin configuration. Time-resolved second harmonic gene… ▽ More Strong-field terahertz (THz) excitations enable dynamic control over electronic, lattice and symmetry degrees of freedom in quantum materials. Here, we uncover pronounced terahertz-induced symmetry modulations and coherent phonon dynamics in the van der Waals antiferromagnet MnPS3, in which inversion symmetry is broken by its antiferromagnetic spin configuration. Time-resolved second harmonic generation measurements reveal long-lived giant oscillations in the antiferromagnetic phase, with amplitudes comparable to the equilibrium signal, driven by phonons involving percent-level atomic displacements relative to the equilibrium bond lengths. The temporal evolution of the rotational anisotropy patterns indicate a dynamic breaking of mirror symmetry, modulated by two vibrational modes at 1.7 THz and 4.5 THz, with the former corresponding to a hidden mode not observed in equilibrium spectroscopy. We show that these effects arise in part from a field-induced charge rearrangement mechanism that lowers the local crystal symmetry, and couples to the phonon modes. A long-lived field-driven response was uncovered with a complex THz polarization dependence which, in comparison to theory, indicates evidence for an antiferromagnetic-to-ferrimagnetic transition. Our results establish an effective field-tunable pathway for driving excitations otherwise weak in equilibrium, and for manipulating magnetism in low-dimensional materials via dynamical modulation of symmetry. △ Less

Submitted 1 October, 2025; originally announced October 2025.

Exciton-defect interaction and optical properties from a first-principles T-matrix approach

Authors: Yang-hao Chan, Jonah B. Haber, Mit H. Naik, Diana Y. Qiu, Felipe H. da Jornada

Abstract: Understanding exciton-defect interactions is critical for optimizing optoelectronic and quantum information applications in many materials. However, ab initio simulations of material properties with defects are often limited to high defect density. Here, we study effects of exciton-defect interactions on optical absorption and photoluminescence spectra in monolayer MoS2 using a first-principles T-… ▽ More Understanding exciton-defect interactions is critical for optimizing optoelectronic and quantum information applications in many materials. However, ab initio simulations of material properties with defects are often limited to high defect density. Here, we study effects of exciton-defect interactions on optical absorption and photoluminescence spectra in monolayer MoS2 using a first-principles T-matrix approach. We demonstrate that exciton-defect bound states can be captured by the disorder-averaged Green's function with the T-matrix approximation and further analyze their optical properties. Our approach yields photoluminescence spectra in good agreement with experiments and provides a new, computationally efficient framework for simulating optical properties of disordered 2D materials from first-principles. △ Less

Submitted 21 May, 2025; originally announced May 2025.

Comments: 7 pages, 4 figures

Ultrafast Spontaneous Exciton Dissociation via Phonon Emission in BiVO$_4$

Authors: Stephen E. Gant, Antonios M. Alvertis, Christopher J. N. Coveney, Jonah B. Haber, Marina R. Filip, Jeffrey B. Neaton

Abstract: Monoclinic bismuth vanadate (m-BiVO$_4$) is a promising indirect band gap semiconductor for photoelectrochemical water splitting, yet the characteristics of its low-lying photoexcitations, or excitons, remain poorly understood. Here, we use an ab initio Bethe-Salpeter equation approach that incorporates phonon screening to compute the nature and lifetimes of the low-lying excitons of m-BiVO$_4$. O… ▽ More Monoclinic bismuth vanadate (m-BiVO$_4$) is a promising indirect band gap semiconductor for photoelectrochemical water splitting, yet the characteristics of its low-lying photoexcitations, or excitons, remain poorly understood. Here, we use an ab initio Bethe-Salpeter equation approach that incorporates phonon screening to compute the nature and lifetimes of the low-lying excitons of m-BiVO$_4$. Our calculations indicate that at 0 K, the lowest-lying exciton energy exceeds the indirect band gap, enabling spontaneous dissociation into free carriers via phonon emission within picoseconds. At 300 K, both phonon emission and absorption effects reduce this timescale to only a few femtoseconds. Phonon screening also greatly reduces the binding energy of the lowest-lying exciton, leading to an optical absorption spectrum that better reproduces experimental measurements. Overall, our findings establish the general conditions under which phonon emission-driven exciton dissociation can occur in indirect gap semiconductors, and they emphasize the critical role phonon screening can play in predictive calculations of photophysical properties of complex materials. △ Less

Submitted 29 July, 2025; v1 submitted 31 March, 2025; originally announced April 2025.

Exciton-polaritons and exciton localization from a first-principles interacting Green's function formalism

Authors: Zachary N. Mauri, Christopher J. Ciccarino, Jonah B. Haber, Diana Y. Qiu, Felipe H. da Jornada

Abstract: Exciton-polaritons -- hybrid states of photons and excitons -- offer unique avenues for controlling electronic, optical, and chemical properties of materials. However, their modeling is mostly limited to formalisms that wash out atomistic details and many-body physics critical to describing real systems. Here, we present an ab initio Green's function formalism based on the Bethe-Salpeter equation… ▽ More Exciton-polaritons -- hybrid states of photons and excitons -- offer unique avenues for controlling electronic, optical, and chemical properties of materials. However, their modeling is mostly limited to formalisms that wash out atomistic details and many-body physics critical to describing real systems. Here, we present an ab initio Green's function formalism based on the Bethe-Salpeter equation (BSE) wherein exciton-polaritons naturally emerge through an attractive, dynamical electron-hole exchange interaction. In MgO and crystalline pentacene, this attractive interaction dramatically reduces exciton Bohr radii and increases, by one order of magnitude, transition dipole moments of exciton-polaritons. Our calculations are in good agreement with experimental polariton dispersions in wurtzite CdS, and allow one to capture how electronic and collective excitations in materials are qualitatively modified through polaritonic effects. △ Less

Submitted 17 March, 2025; originally announced March 2025.

Comments: 5 figures, 8 pages

Exciton thermalization dynamics in monolayer MoS2: a first-principles Boltzmann equation study

Authors: Yang-hao Chan, Jonah B. Haber, Mit H. Naik, Steven G. Louie, Jeffrey B. Neaton, Felipe H. da Jornada, Diana Y. Qiu

Abstract: Understanding exciton thermalization is critical for optimizing optoelectronic and photocatalytic processes in many materials. However, it is hard to access the dynamics of such processes experimentally, especially on systems such as monolayer transition metal dichalcogenides, where various low-energy excitations pathways can compete for exciton thermalization. Here, we study exciton dynamics due… ▽ More Understanding exciton thermalization is critical for optimizing optoelectronic and photocatalytic processes in many materials. However, it is hard to access the dynamics of such processes experimentally, especially on systems such as monolayer transition metal dichalcogenides, where various low-energy excitations pathways can compete for exciton thermalization. Here, we study exciton dynamics due to exciton-phonon scattering in monolayer MoS2 from a first-principles, interacting Green's function approach, to obtain the relaxation and thermalization of low-energy excitons following different initial excitations at different temperatures. We find that the thermalization occurs on a picosecond timescale at 300 K but can increase by an order of magnitude at 100 K. The long total thermalization time, owing to the nature of its excitonic band structure, is dominated by slow spin-flip scattering processes in monolayer MoS2. In contrast, thermalization of excitons in individual spin-aligned and spin-anti-aligned channels can be achieved within a few hundred fs when exciting higher-energy excitons. We further simulate the intensity spectrum of time-resolved angle-resolved photoemission spectroscopy (TR-ARPES) experiments and anticipate that such calculations may serve as a map to correlate spectroscopic signatures with microscopic exciton dynamics. △ Less

Submitted 5 December, 2024; originally announced December 2024.

Comments: 19 pages, 8 figures

Electronic structure and optical properties of halide double perovskites from a Wannier-localized optimally-tuned screened range-separated hybrid functional

Authors: Francisca Sagredo, Stephen E. Gant, Guy Ohad, Jonah B. Haber, Marina R. Filip, Leeor Kronik, Jeffrey B. Neaton

Abstract: Halide double perovskites are a chemically-diverse and growing class of compound semiconductors that are promising for optoelectronic applications. However, the prediction of their fundamental gaps and optical properties with density functional theory (DFT) and {\it ab initio} many-body perturbation theory has been a significant challenge. Recently, a nonempirical Wannier-localized optimally-tuned… ▽ More Halide double perovskites are a chemically-diverse and growing class of compound semiconductors that are promising for optoelectronic applications. However, the prediction of their fundamental gaps and optical properties with density functional theory (DFT) and {\it ab initio} many-body perturbation theory has been a significant challenge. Recently, a nonempirical Wannier-localized optimally-tuned screened range-separated hybrid (WOT-SRSH) functional has been shown to accurately produce the fundamental band gaps of a wide set of semiconductors and insulators, including lead halide perovskites. Here we apply the WOT-SRSH functional to five halide double perovskites, and compare the results with those obtained from other known functionals and previous $GW$ calculations. We also use the approach as a starting point for $GW$ calculations and we compute the band structures and optical absorption spectrum for Cs\textsubscript{2}Ag{Bi}Br\textsubscript{6}, using both time-dependent DFT and the $GW$-Bethe-Salpeter equation approach. We show that the WOT-SRSH functional leads to accurate fundamental and optical band gaps, as well as optical absorption spectra, consistent with spectroscopic measurements, thereby establishing WOT-SRSH as a viable method for the accurate prediction of optoelectronic properties of halide double perovskites. △ Less

Submitted 7 August, 2024; originally announced August 2024.

Rearrangement collision theory of phonon-driven exciton dissociation

Authors: Christopher J. N. Coveney, Jonah B. Haber, Antonios M. Alvertis, Jeffrey B. Neaton, Marina R. Filip

Abstract: Understanding the processes governing the dissociation of excitons to free charge carriers in semiconductors and insulators is of central importance for photovoltaic applications. Dyson's $\mathcal{S}$-matrix formalism provides a framework for computing scattering rates between quasiparticle states derived from the same underlying Hamiltonian, often reducing to familiar Fermi's golden rule like ex… ▽ More Understanding the processes governing the dissociation of excitons to free charge carriers in semiconductors and insulators is of central importance for photovoltaic applications. Dyson's $\mathcal{S}$-matrix formalism provides a framework for computing scattering rates between quasiparticle states derived from the same underlying Hamiltonian, often reducing to familiar Fermi's golden rule like expressions at first order. By presenting a rigorous formalism for multi-channel scattering, we extend this approach to describe scattering between composite quasiparticles and in particular, the process of exciton dissociation mediated by the electron-phonon interaction. Subsequently, we derive rigorous expressions for the exciton dissociation rate, a key quantity of interest in optoelectronic materials, which enforce correct energy conservation and may be readily used in ab initio calculations. We apply our formalism to a three-dimensional model system to compare temperature-dependent exciton rates obtained for different scattering channels. △ Less

Submitted 22 May, 2024; originally announced May 2024.

Exciton-phonon coupling induces new pathway for ultrafast intralayer-to-interlayer exciton transition and interlayer charge transfer in WS2-MoS2 heterostructure: a first-principles study

Authors: Yang-hao Chan, Mit H. Naik, Jonah B. Haber, Jeffrey B. Neaton, Steven G. Louie, Diana Y. Qiu, Felipe H. da Jornada

Abstract: Despite the weak, van-der-Waals interlayer coupling, photoinduced charge transfer vertically across atomically thin interfaces can occur within surprisingly fast, sub-50fs timescales. Early theoretical understanding of the charge transfer is based on a noninteracting picture, neglecting excitonic effects that dominate the optical properties of such materials. Here, we employ an ab initio many-body… ▽ More Despite the weak, van-der-Waals interlayer coupling, photoinduced charge transfer vertically across atomically thin interfaces can occur within surprisingly fast, sub-50fs timescales. Early theoretical understanding of the charge transfer is based on a noninteracting picture, neglecting excitonic effects that dominate the optical properties of such materials. Here, we employ an ab initio many-body perturbation theory approach which explicitly accounts for the excitons and phonons in the heterostructure. Our large-scale first-principles calculations directly probe the role of exciton-phonon coupling in the charge dynamics of the WS$_2$/MoS$_2$ heterobilayer. We find that the exciton-phonon interaction induced relaxation time of photo-excited excitons at the $K$ valley of MoS$_2$ and WS$_2$ is 67 fs and 15 fs at 300 K, respectively, which sets a lower bound to the intralayer-to-interlayer exciton transfer time and is consistent with experiment reports. We further show that electron-hole correlations facilitate novel transfer pathways which are otherwise inaccessible to non-interacting electrons and holes. △ Less

Submitted 31 January, 2024; originally announced January 2024.

Comments: 15 pages, 4 figures

Phonon screening and dissociation of excitons at finite temperatures from first principles

Authors: Antonios M. Alvertis, Jonah B. Haber, Zhenglu Li, Christopher J. N. Coveney, Steven G. Louie, Marina R. Filip, Jeffrey B. Neaton

Abstract: The properties of excitons, or correlated electron-hole pairs, are of paramount importance to optoelectronic applications of materials. A central component of exciton physics is the electron-hole interaction, which is commonly treated as screened solely by electrons within a material. However, nuclear motion can screen this Coulomb interaction as well, with several recent studies developing model… ▽ More The properties of excitons, or correlated electron-hole pairs, are of paramount importance to optoelectronic applications of materials. A central component of exciton physics is the electron-hole interaction, which is commonly treated as screened solely by electrons within a material. However, nuclear motion can screen this Coulomb interaction as well, with several recent studies developing model approaches for approximating the phonon screening to the properties of excitons. While these model approaches tend to improve agreement with experiment for exciton properties, they rely on several approximations that restrict their applicability to a wide range of materials, and thus far they have neglected the effect of finite temperatures. Here, we develop a fully first-principles, parameter-free approach to compute the temperature-dependent effects of phonon screening within the ab initio GW-Bethe Salpeter equation framework. We recover previously proposed models of phonon screening as well-defined limits of our general framework, and discuss their validity by comparing them against our first-principles results. We develop an efficient computational workflow and apply it to a diverse set of semiconductors, specifically AlN, CdS, GaN, MgO and SrTiO3. We demonstrate under different physical scenarios how excitons may be screened by multiple polar optical or acoustic phonons, how their binding energies can exhibit strong temperature dependence, and the ultrafast timescales on which they dissociate into free electron-hole pairs. △ Less

Submitted 6 December, 2023; originally announced December 2023.

Optical absorption spectra of metal oxides from time-dependent density functional theory and many-body perturbation theory based on optimally-tuned hybrid functionals

Authors: Guy Ohad, Stephen E. Gant, Dahvyd Wing, Jonah B. Haber, María Camarasa-Gómez, Francisca Sagredo, Marina R. Filip, Jeffrey B. Neaton, Leeor Kronik

Abstract: Using both time-dependent density functional theory (TDDFT) and the ``single-shot" $GW$ plus Bethe-Salpeter equation ($GW$-BSE) approach, we compute optical band gaps and optical absorption spectra from first principles for eight common binary and ternary closed-shell metal oxides (MgO, Al$_2$O$_3$, CaO, TiO$_2$, Cu$_2$O, ZnO, BaSnO$_3$, and BiVO$_4$), based on the non-empirical Wannier-localized… ▽ More Using both time-dependent density functional theory (TDDFT) and the ``single-shot" $GW$ plus Bethe-Salpeter equation ($GW$-BSE) approach, we compute optical band gaps and optical absorption spectra from first principles for eight common binary and ternary closed-shell metal oxides (MgO, Al$_2$O$_3$, CaO, TiO$_2$, Cu$_2$O, ZnO, BaSnO$_3$, and BiVO$_4$), based on the non-empirical Wannier-localized optimally-tuned screened range-separated hybrid functional. Overall, we find excellent agreement between our TDDFT and $GW$-BSE results and experiment, with a mean absolute error less than 0.4 eV, including for Cu$_2$O and ZnO, traditionally considered to be challenging for both methods. △ Less

Submitted 5 September, 2023; originally announced September 2023.

Maximally-Localized Exciton Wannier Functions for Solids

Authors: Jonah B. Haber, Diana Y. Qiu, Felipe H. da Jornada, Jeffrey B. Neaton

Abstract: We introduce a maximally-localized Wannier function representation of Bloch excitons, two-particle correlated electron-hole excitations, in crystalline solids, where the excitons are maximally-localized with respect to an average electron-hole coordinate in real space. As a proof-of-concept, we illustrate this representation in the case of low-energy spin-singlet and triplet excitons in LiF, compu… ▽ More We introduce a maximally-localized Wannier function representation of Bloch excitons, two-particle correlated electron-hole excitations, in crystalline solids, where the excitons are maximally-localized with respect to an average electron-hole coordinate in real space. As a proof-of-concept, we illustrate this representation in the case of low-energy spin-singlet and triplet excitons in LiF, computed using the ab initio Bethe-Salpeter equation approach. We visualize the resulting maximally-localized exciton Wannier functions (MLXWFs) in real space, detail the convergence of the exciton Wannier spreads, and demonstrate how Wannier-Fourier interpolation can be leveraged to obtain exciton energies and states at arbitrary exciton crystal momenta in the Brillouin zone. We further introduce an approach to treat the long-range dipolar coupling between singlet MLXWFs and discuss it in depth. The MLXWF representation sheds light on the fundamental nature of excitons and paves the way towards Wannier-based post-processing of excitonic properties, enabling the construction of ab initio exciton tight-binding models, efficient interpolation of the exciton-phonon vertex, the computation of Berry curvature associated with exciton bands, and beyond. △ Less

Submitted 5 August, 2023; originally announced August 2023.

Phonon-driven femtosecond dynamics of excitons in crystalline pentacene from first principles

Authors: Galit Cohen, Jonah B. Haber, Jeffrey B. Neaton, Diana Y. Qiu, Sivan Refaely-Abramson

Abstract: Non-radiative exciton relaxation processes are critical for energy transduction efficiencies in optoelectronic materials, but how these processes are connected to the underlying crystal structure and its associated electron, exciton, and phonon band structures is poorly understood. Here, we present a first-principles approach to explore exciton relaxation pathways in pentacene, a paradigmatic mole… ▽ More Non-radiative exciton relaxation processes are critical for energy transduction efficiencies in optoelectronic materials, but how these processes are connected to the underlying crystal structure and its associated electron, exciton, and phonon band structures is poorly understood. Here, we present a first-principles approach to explore exciton relaxation pathways in pentacene, a paradigmatic molecular crystal and optoelectronic semiconductor. We compute the momentum- and band-resolved exciton-phonon interactions, and use them to analyse key scattering channels. We find that exciton intraband transitions on femtosecond timescales leading to dark-state occupation is a dominant nonradiative relaxation channel in pentacene. We further show how the nature of real-time propagation of the exciton wavepacket is connected with the longitudinal-transverse exciton splitting, stemming from crystal anisotropy, and concomitant anisotropic exciton and phonon dispersions. Our results provide a framework for understanding time-resolved exciton propagation and energy transfer in molecular crystals and beyond. △ Less

Submitted 7 May, 2023; originally announced May 2023.

Quasiparticle and Optical Properties of Carrier-Doped Monolayer MoTe$_2$ from First Principles

Authors: Aurelie Champagne, Jonah B. Haber, Supavit Pokawanvit, Diana Y. Qiu, Souvik Biswas, Harry A. Atwater, Felipe H. da Jornada, Jeffrey B. Neaton

Abstract: The intrinsic weak and highly non-local dielectric screening of two-dimensional materials is well known to lead to high sensitivity of their optoelectronic properties to environment. Less studied theoretically is the role of free carriers on those properties. Here, we use ab initio GW and Bethe-Salpeter equation calculations, with a rigorous treatment of dynamical screening and local-field effects… ▽ More The intrinsic weak and highly non-local dielectric screening of two-dimensional materials is well known to lead to high sensitivity of their optoelectronic properties to environment. Less studied theoretically is the role of free carriers on those properties. Here, we use ab initio GW and Bethe-Salpeter equation calculations, with a rigorous treatment of dynamical screening and local-field effects, to study the doping-dependence of the quasiparticle and optical properties of a monolayer transition metal dichalcogenide, 2H MoTe$_2$. We predict a quasiparticle band gap renormalization of several hundreds meV for experimentally-achievable carrier densities, and a similarly sizable decrease in the exciton binding energy. This results in an almost constant excitation energy for the lowest-energy exciton resonance with increasing doping density. Using a newly-developed and generally-applicable quasi-2D plasmon-pole model and a self-consistent solution of the Bethe-Salpeter equation, we reveal the importance of accurately capturing both dynamical and local-field effects to understand detailed photoluminescence measurements. △ Less

Submitted 21 March, 2023; originally announced March 2023.

Comments: 22 pages, 5 figures, under review in Nano Letters

Rydberg Excitons and Trions in Monolayer MoTe$_2$

Authors: Souvik Biswas, Aurélie Champagne, Jonah B. Haber, Supavit Pokawanvit, Joeson Wong, Hamidreza Akbari, Sergiy Krylyuk, Kenji Watanabe, Takashi Taniguchi, Albert V. Davydov, Zakaria Y. Al Balushi, Diana Y. Qiu, Felipe H. da Jornada, Jeffrey B. Neaton, Harry A. Atwater

Abstract: Monolayer transition metal dichalcogenide (TMDC) semiconductors exhibit strong excitonic optical resonances which serve as a microscopic, non-invasive probe into their fundamental properties. Like the hydrogen atom, such excitons can exhibit an entire Rydberg series of resonances. Excitons have been extensively studied in most TMDCs (MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$), but detailed exploration… ▽ More Monolayer transition metal dichalcogenide (TMDC) semiconductors exhibit strong excitonic optical resonances which serve as a microscopic, non-invasive probe into their fundamental properties. Like the hydrogen atom, such excitons can exhibit an entire Rydberg series of resonances. Excitons have been extensively studied in most TMDCs (MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$), but detailed exploration of excitonic phenomena has been lacking in the important TMDC material molybdenum ditelluride (MoTe$_2$). Here, we report an experimental investigation of excitonic luminescence properties of monolayer MoTe$_2$ to understand the excitonic Rydberg series, up to 3s. We report significant modification of emission energies with temperature (4K to 300K), quantifying the exciton-phonon coupling. Furthermore, we observe a strongly gate-tunable exciton-trion interplay for all the Rydberg states governed mainly by free-carrier screening, Pauli blocking, and band-gap renormalization in agreement with the results of first-principles GW plus Bethe-Salpeter equation approach calculations. Our results help bring monolayer MoTe$_2$ closer to its potential applications in near-infrared optoelectronics and photonic devices. △ Less

Submitted 7 February, 2023; originally announced February 2023.

Comments: 5 figures (main text)

Phonon-induced localization of excitons in molecular crystals from first principles

Authors: Antonios M. Alvertis, Jonah B. Haber, Edgar A. Engel, Sahar Sharifzadeh, Jeffrey B. Neaton

Abstract: The spatial extent of excitons in molecular systems underpins their photophysics and utility for optoelectronic applications. Phonons are reported to lead to both exciton localization and delocalization. However, a microscopic understanding of phonon-induced (de)localization is lacking, in particular how localized states form, the role of specific vibrations, and the relative importance of quantum… ▽ More The spatial extent of excitons in molecular systems underpins their photophysics and utility for optoelectronic applications. Phonons are reported to lead to both exciton localization and delocalization. However, a microscopic understanding of phonon-induced (de)localization is lacking, in particular how localized states form, the role of specific vibrations, and the relative importance of quantum and thermal nuclear fluctuations. Here we present a first-principles study of these phenomena in solid pentacene, a prototypical molecular crystal, capturing the formation of bound excitons, exciton-phonon coupling to all orders, and phonon anharmonicity, using density functional theory, the \emph{ab initio} $GW$-Bethe-Salpeter equation approach, finite difference, and path integral techniques. We find that for pentacene zero-point nuclear motion causes uniformly strong localization, with thermal motion providing additional localization only for Wannier-Mott-like excitons. Anharmonic effects drive temperature-dependent localization, and while such effects prevent the emergence of highly delocalized excitons, we explore the conditions under which these might be realized. △ Less

Submitted 27 January, 2023; originally announced January 2023.

Journal ref: Phys. Rev. Lett. 130, 086401 (2023)

Exciton lifetime and optical linewidth profile via exciton-phonon interactions: Theory and first-principles calculations for monolayer MoS$_2$

Authors: Y. -H. Chan, Jonah B. Haber, Mit H. Naik, J. B. Neaton, Diana Y. Qiu, Felipe H. da Jornada, Steven G. Louie

Abstract: Exciton dynamics dictate the evolution of photoexcited carriers in photovoltaic and optoelectronic devices. However, interpreting their experimental signatures is a challenging theoretical problem due to the presence of both electron-phonon and many-electron interactions. We develop and apply here a first-principles approach to exciton dynamics resulting from exciton-phonon coupling in monolayer M… ▽ More Exciton dynamics dictate the evolution of photoexcited carriers in photovoltaic and optoelectronic devices. However, interpreting their experimental signatures is a challenging theoretical problem due to the presence of both electron-phonon and many-electron interactions. We develop and apply here a first-principles approach to exciton dynamics resulting from exciton-phonon coupling in monolayer MoS2 and reveal the highly selective nature of exciton-phonon coupling due to the internal spin structure of excitons, which leads to a surprisingly long lifetime of the lowest energy bright A exciton. Moreover, we show that optical absorption processes rigorously require a second-order perturbation theory approach, with photon and phonon treated on an equal footing, as proposed by Toyozawa and Hopfield. Such a treatment, thus far neglected in first-principles studies, gives rise to off-diagonal exciton-phonon coupling matrix elements, which are critical for the description of dephasing mechanisms, and yields exciton linewidths in excellent agreement with experiment. △ Less

Submitted 16 December, 2022; originally announced December 2022.

Comments: 11 pages, 4 figures

An Optimally-Tuned Starting Point for Single-Shot $GW$ Calculations of Solids

Authors: Stephen E. Gant, Jonah B. Haber, Marina R. Filip, Francisca Sagredo, Dahvyd Wing, Guy Ohad, Leeor Kronik, Jeffrey B. Neaton

Abstract: The dependence of ab initio many-body perturbation theory within the $GW$ approximation on the eigensystem used in calculating quasiparticle corrections limits this method's predictive power. Here, we investigate the accuracy of the recently developed Wannier-localized optimally tuned screened range-separated hybrid (WOT-SRSH) functional as a generalized Kohn-Sham starting point for single-shot… ▽ More The dependence of ab initio many-body perturbation theory within the $GW$ approximation on the eigensystem used in calculating quasiparticle corrections limits this method's predictive power. Here, we investigate the accuracy of the recently developed Wannier-localized optimally tuned screened range-separated hybrid (WOT-SRSH) functional as a generalized Kohn-Sham starting point for single-shot $GW$ ($G_0W_0$) calculations for a range of semiconductors and insulators. Comparison to calculations based on well-established functionals, namely PBE, PBE0, and HSE, as well as to self-consistent $GW$ schemes and to experiment, shows that band gaps computed via $G_0W_0$@WOT-SRSH have a level of precision and accuracy that is comparable to that of more advanced methods such as quasiparticle self-consistent $GW$ (QS$GW$) and eigenvalue self-consistent $GW$ (ev$GW$). We also find that $G_0W_0$@WOT-SRSH improves the description of states deeper in the valence band manifold. Finally, we show that $G_0W_0$@WOT-SRSH significantly reduces the sensitivity of computed band gaps to ambiguities in the underlying WOT-SRSH tuning procedure. △ Less

Submitted 24 May, 2022; v1 submitted 1 February, 2022; originally announced February 2022.

Journal ref: Phys. Rev. Materials 6, 053802 (2022)

Phonon Screening of Excitons in Semiconductors: Halide Perovskites and Beyond

Authors: Marina R. Filip, Jonah B. Haber, Jeffrey B. Neaton

Abstract: The ab initio Bethe-Salpeter equation (BSE) approach, an established method for the study of excitons in materials, is typically solved in a limit where only static screening from electrons is captured. Here, we generalize this framework to also include dynamical screening from phonons at lowest order in the electron-phonon interaction. We apply this generalized BSE approach to a series of inorgan… ▽ More The ab initio Bethe-Salpeter equation (BSE) approach, an established method for the study of excitons in materials, is typically solved in a limit where only static screening from electrons is captured. Here, we generalize this framework to also include dynamical screening from phonons at lowest order in the electron-phonon interaction. We apply this generalized BSE approach to a series of inorganic lead halide perovskites, CsPbX3, with X = Cl, Br, and I. We find that inclusion of screening from phonons significantly reduces the computed exciton binding energies of these systems. By deriving a simple expression for phonon screening effects, we reveal general trends for the importance of phonon screening effects in semiconductors and insulators, based on a hydrogenic exciton model. We demonstrate that the magnitude of the phonon screening correction in isotropic materials can be reliably predicted using four material specific parameters: the reduced effective mass, the static and optical dielectric constants, and the phonon frequency of the most strongly coupled LO phonon mode. This framework helps to elucidate the importance of phonon screening and its relation to excitonic properties in a broad class of semiconductors. △ Less

Submitted 16 June, 2021; originally announced June 2021.

Journal ref: Phys. Rev. Lett. 127, 067401, 2021

Band gaps of crystalline solids from Wannier-localization based optimal tuning of a screened range-separated hybrid functional

Authors: Dahvyd Wing, Guy Ohad, Jonah B. Haber, Marina R. Filip, Stephen E. Gant, Jeffrey B. Neaton, Leeor Kronik

Abstract: Accurate prediction of fundamental band gaps of crystalline solid state systems entirely within density functional theory is a long standing challenge. Here, we present a simple and inexpensive method that achieves this by means of non-empirical optimal tuning of the parameters of a screened range-separated hybrid functional. The tuning involves the enforcement of an ansatz that generalizes the io… ▽ More Accurate prediction of fundamental band gaps of crystalline solid state systems entirely within density functional theory is a long standing challenge. Here, we present a simple and inexpensive method that achieves this by means of non-empirical optimal tuning of the parameters of a screened range-separated hybrid functional. The tuning involves the enforcement of an ansatz that generalizes the ionization potential theorem to the removal of an electron in an occupied state described by a localized Wannier function in a modestly sized supercell calculation. The method is benchmarked against experiment for a set of systems ranging from narrow band gap semiconductors to large band gap insulators, spanning a range of fundamental band gaps from 0.2 to 14.2 eV and is found to yield quantitative accuracy across the board, with a mean absolute error of $\sim$0.1 eV and a maximal error of $\sim$0.2 eV. △ Less

Submitted 11 October, 2021; v1 submitted 6 December, 2020; originally announced December 2020.

Comments: 10 pages, 2 figures

Vibronic response of a spin-1/2 state from a carbon impurity in two-dimensional WS$_2$

Authors: Katherine A. Cochrane, Jun-Ho Lee, Christoph Kastl, Jonah B. Haber, Tianyi Zhang, Azimkhan Kozhakhmetov, Joshua A. Robinson, Mauricio Terrones, Jascha Repp, Jeffrey B. Neaton, Alexander Weber-Bargioni, Bruno Schuler

Abstract: We demonstrate the creation of a spin-1/2 state via the atomically controlled generation of magnetic carbon radical ions (CRIs) in synthetic two-dimensional transition metal dichalcogenides (TMDs). Hydrogenated carbon impurities located at chalcogen sites introduced by chemical doping can be activated with atomic precision by hydrogen depassivation using a scanning probe tip. In its anionic state,… ▽ More We demonstrate the creation of a spin-1/2 state via the atomically controlled generation of magnetic carbon radical ions (CRIs) in synthetic two-dimensional transition metal dichalcogenides (TMDs). Hydrogenated carbon impurities located at chalcogen sites introduced by chemical doping can be activated with atomic precision by hydrogen depassivation using a scanning probe tip. In its anionic state, the carbon impurity exhibits a magnetic moment of 1 $μ_\text{B}$ resulting from an unpaired electron populating a spin-polarized in-gap orbital of C$^{\bullet -}_\text{S}$. Fermi level control by the underlying graphene substrate can charge and decharge the defect, thereby activating or quenching the defect magnetic moment. By inelastic tunneling spectroscopy and density functional theory calculations we show that the CRI defect states couple to a small number of vibrational modes, including a local, breathing-type mode. Interestingly, the electron-phonon coupling strength critically depends on the spin state and differs for monolayer and bilayer WS$_2$. These carbon radical ions in TMDs comprise a new class of surface-bound, single-atom spin-qubits that can be selectively introduced, are spatially precise, feature a well-understood vibronic spectrum, and are charge state controlled. △ Less

Submitted 27 August, 2020; originally announced August 2020.

Journal ref: Nat. Commun. 12, 7287 (2021)