Bradley J. Siwick

An ab-initio computational methodology for interrogating the phonon contribution to polaron formation in real materials is developed that can be directly compared to experiment. Using LiF as an example, we show that the recent ab-initio theory of Sio et al.¹ makes predictions of the momentum- and branch dependent phonon amplitudes in polaron quasiparticles that are testable using ultrafast electron diffuse scattering (UEDS) and related techniques. The large electron polaron in LiF has UEDS signatures that are qualitatively similar to those expected from a simple isotropic strain field model, but the small hole polaron exhibits a profoundly anisotropic UEDS pattern that is in poor agreement with an isotropic strain field. We also show that these polaron diffuse scattering signatures are directly emblematic of the underlying polaron wavefunction. The combination of new time and momentum resolved experimental probes of nonequilibrium phonons with novel computational methods promises to complement the qualitative results obtained via model Hamiltonians with a first principles, material-specific quantitative understanding of polarons and their properties.

The formation of excited dimer states, so called excimers, is an important phenomenon in many organic molecular semiconductors. In contrast to Frenkel exciton-polaron excited states, an excimer is long-lived and energetically low-lying due to stabilization resulting from a substantial reorganization of the inter-molecular geometry. In this letter, we show that ultrafast electron diffraction can follow the dynamics of solid-state excimer formation in polycrystalline thin films of a molecular semiconductor, revealing both the key reaction modes and the eventual structure of the emitting state. We study the prototypical organic semiconductor zinc-phthalocyanine (ZnPc) in its crystallographic $α$-phase as a model excimeric system. We show, that the excimer forms in a two-step process starting with a fast dimerization ($\lesssim 0.4\, \text{ps}$) followed by a subsequent slow shear-twist motion ($14\, \text{ps}$) leading to an alignment of the $π$-systems of the involved monomers. Furthermore, we show that while the same excimer geomtry is present in partially fluorinated derivatives of ZnPc, the formation kinematics slow down with increasing level of fluorination.

A combined experimental and computational methodology for interrogating the phonon contribution to polaron formation in real materials is developed. Using LiF as an example, we show that the recent ab-initio theory of Sio et. al [PRL 122, 246403 (2019)] makes predictions of the momentum- and branch dependent phonon amplitides in polaron quasiparticles that are testable using ultrafast electron diffuse scattering (UEDS) and related techniques. The large electron polaron in LiF has UEDS signatures that are qualitatively similar to those expected from a simple point-defect model, but the small hole polaron exhibits a profoundly anisotropic UEDS pattern that is in poor agreement with a point-defect model. We also show that these polaron diffuse scattering signatures are directly emblematic of the underlying polaron wavefunction. The combination of new time and momentum resolved experimental probes of nonequilibrium phonons with novel computational methods promises to complement the qualitative results obtained via model Hamiltonians with a first principles, material-specific quantiative understanding of polarons and their properties.

At the 2D limit, hexagonal systems such as monolayer transition metal dichalcogenides (TMDs) and graphene exhibit unique coupled spin and momentum-valley physics (valley pseudospin) owing to broken spatial inversion symmetry and strong spin-orbit coupling. Circularly polarized light provides the means for pseudospin-selective excitation of excitons (or electrons and holes) and can yield momentum-valley polarized populations of carriers that are the subject of proposed valleytronic applications. The chirality of these excited carriers has important consequences for the available relaxation/scattering pathways, which must conserve (pseudo)angular momentum as well as energy. One available relaxation channel that satisfies these constraints is coupling to chiral phonons. Here, we show that chiral carrier-phonon coupling following valley-polarized photoexcitation is expected to lead to a strongly valley-polarized chiral phonon distribution when this relaxation mechanism is dominant. This momentum valley phonon polarization is directly measurable using ultrafast phonon diffuse scattering techniques. Using ab initio calculations, we show how the dynamic phonon occupations and valley anisotropy determined by nonequilibrium observations can provide a new window on the physical processes that drive carrier valley depolarization in monolayer TMDs.

At the 2D limit, hexagonal systems such as monolayer transition metal dichalcogenides (TMDs) and graphene exhibit unique coupled spin and momentum-valley physics (valley pseudospin) owing to broken spatial inversion symmetry and strong spin-orbit coupling. Circularly polarized light provides the means for pseudospin-selective excitation of excitons (or electrons and holes) and can yield momentum-valley polarized populations of carriers that are the subject of proposed valleytronic applications. The chirality of these excited carriers have important consequences for the available relaxation/scattering pathways, which must conserve (pseudo)angular momentum as well as energy. One available relaxation channel that satisfies these constraints is coupling to chiral phonons. Here we show that chiral carrier-phonon coupling following valley-polarized photoexcitation is expected to leads to a strongly valley-polarized chiral phonon distribution that is directly measurable using ultrafast phonon-diffuse scattering techniques. Using ab-initio calculations we show how the dynamic phonon occupations and valley anisotropy determined by nonequilibrium observations can provide a new window on the physical processes that drive carrier valley-depolarization in monolayer TMDs.

Significance: Thermoelectrics can generate electrical power from waste heat and could make an important contribution to sustainable energy production if their efficiency is improved. Engineering efficient thermoelectrics, however, requires a sophisticated understanding of the fundamental interdependencies between electrical and thermal transport, for which improvements in our understanding of how charge carriers are coupled to lattice vibrations (phonons) is needed. Using advanced ultrafast electron-based techniques, we probe the fundamental nature of electron–phonon coupling in SnSe, one of the highest-performing thermoelectric materials, in order to unravel the origins of its thermoelectric efficiency. We show that the charge carriers in SnSe interact strongly with lattice waves to form moving lattice distortions called polarons, which helps to explain some of SnSe’s most unusual properties.

The phonon dynamics and dielectrically screened electron-phonon coupling of atomically thin molybdenum disulfide are directly observed for the first time ever by UEDS diffuse intensity transients in excellent agreement with novel density functional theory calculations.

Despite their fundamental role in determining many important properties of materials, detailed momentum-dependent information on the strength of electron–phonon and phonon–phonon coupling across the entire Brillouin zone has remained elusive. Ultrafast electron diffuse scattering (UEDS) is a recently developed technique that is making a significant contribution to these questions. Here, we describe both the UEDS methodology and the information content of ultrafast, photoinduced changes in phonon-diffuse scattering from single-crystal materials. We present results obtained from Ni, WSe₂, and TiSe₂, materials that are characterized by a complex interplay between electronic (charge, spin) and lattice degrees of freedom. We demonstrate the power of this technique by unraveling carrier–phonon and phonon–phonon interactions in both momentum and time and following nonequilibrium phonon dynamics in detail on ultrafast time scales. By combining ab initio calculations with ultrafast diffuse electron scattering, insights into electronic and magnetic dynamics that impact UEDS indirectly can also be obtained. Graphic