arXiv subjects

Andrew M. Rappe

Publications and source records attributed to Andrew M. Rappe.

At least 37 records · Page 2Linked to original sources

arXiv2024

Ultrafast Vibrational Control of Hybrid Perovskite Devices Reveals the Influence of the Organic Cation on Electronic Dynamics

Vibrational control (VC) of photochemistry through the optical stimulation of structural dynamics is a nascent concept only recently demonstrated for model molecules in solution. Extending VC to state-of-the-art materials may lead to new applications and improved performance for optoelectronic devices. Metal halide perovskites are promising targets for VC due to their mechanical softness and the rich array of vibrational motions of both their inorganic and organic sublattices. Here, we demonstrate the ultrafast VC of FAPbBr3 perovskite solar cells via intramolecular vibrations of the formamidinium cation using spectroscopic techniques based on vibrationally promoted electronic resonance. The observed short (~300 fs) time window of VC highlights the fast dynamics of coupling between the cation and inorganic sublattice. First-principles modelling reveals that this coupling is mediated by hydrogen bonds that modulate both lead halide lattice and electronic states. Cation dynamics modulating this coupling may suppress non-radiative recombination in perovskites, leading to photovoltaics with reduced voltage losses.

physics.app-ph↗

arXiv2024

Ligand Equilibrium Influences Photoluminescence Blinking in CsPbBr3: A Change Point Analysis of Widefield Imaging Data

Photoluminescence intermittency remains one of the biggest challenges to realizing perovskite quantum dots (QDs) as scalable single photon emitters. We compare CsPbBr3 QDs capped with different ligands, lecithin, and a combination of oleic acid and oleylamine, to elucidate the role of surface chemistry on photoluminescence intermittency. We employ widefield photoluminescence microscopy, sampling the blinking behavior of hundreds of QDs. Using change point analysis, we achieve the robust classification of blinking trajectories, and we analyze representative distributions from large numbers of QDs (Nlecithin = 1308, Noleic acid/oleylamine =1317). We find that lecithin suppresses blinking in CsPbBr3 QDs compared to oleic acid/oleylamine. Under common experimental conditions, lecithin-capped QDs are 7.5 times more likely to be non-blinking and spend 2.5 times longer in their most emissive state, despite both QDs having nearly identical solution photoluminescence quantum yields. We measure photoluminescence as a function of dilution and show that the differences between lecithin and oleic acid/oleylamine capping emerge at low concentrations during preparation for single particle experiments. From experiment and first principles calculations, we attribute the differences in lecithin and oleic acid/oleylamine performance to differences in their ligand binding equilibria. Consistent with our experimental data, density functional theory calculations suggest a stronger binding affinity of lecithin to the QD surface compared to oleic acid/oleylamine, implying a reduced likelihood of ligand desorption during dilution. These results suggest that using more tightly binding ligands is a necessity for surface passivation and consequently, blinking reduction in perovskite QDs used for single particle and quantum light experiments.

cond-mat.mtrl-sci↗

arXiv2024

Observation of polarization density waves in SrTiO3

The nature of the "failed" ferroelectric transition in SrTiO3 has been a long-standing puzzle in condensed matter physics. A compelling explanation is the competition between ferroelectricity and an instability with a mesoscopic modulation of the polarization. These polarization density waves, which should become especially strong near the quantum critical point, break local inversion symmetry and are difficult to probe with conventional x-ray scattering methods. Here we combine a femtosecond x-ray free electron laser (XFEL) with THz coherent control methods to probe inversion symmetry breaking at finite momenta and visualize the instability of the polarization on nanometer lengthscales in SrTiO3. We find polar-acoustic collective modes that are soft particularly at the tens of nanometer lengthscale. These precursor collective excitations provide evidence for the conjectured mesoscopic modulated phase in SrTiO3.

cond-mat.mtrl-sci↗

arXiv2023

Automated Discovery of Wurtzite Solid Solutions with Enhanced Piezoelectric Response

While many piezoelectric materials are known, there is still great potential to improve on the figures of merit of existing materials through compositional doping, forming solid solutions. Specifically, it has been shown that doping and alloying wurtzite-structured materials can improve the piezoelectric response; however, a vast compositional space has remained unexplored. In this work, we apply a multi-level screening protocol combining machine learning, chemical intuition, and thermodynamics to systematically discover dopant combinations in wurtzite material space that improve the desired piezoelectric response. Through our protocol, we use computationally inexpensive screening calculations to consider more than 3000 possible ternary wurtzite solid solutions from 9 different wurtzite base systems: AlN, BeO, CdS, CdSe, GaN, ZnO, ZnS, ZnSe, and AgI. Finally, based on thermodynamic analysis and explicit piezoelectric response calculations, we predict 11 materials with improved piezoelectric response, due to the incorporation of electropositive dopants.

cond-mat.mtrl-sci↗

arXiv2023

Li iontronics in single-crystalline T-Nb2O5 thin films with vertical ionic transport channels

The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure that has two-dimensional (2D) layers with very low steric hindrance allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, likely due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film's surface. These vertical 2D channels enable fast Li-ion migration which we show gives rise to a colossal insulator-metal transition where the resistivity drops by eleven orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, that allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way to the exploration of novel thin films with ionic channels and their potential applications.

cond-mat.mtrl-sci↗

arXiv2023

Controllable Topological Insulator Phases in Litharge-phase InBi Monolayer

Despite recent advances of layered square-net topological material models that possess ideal semimetallic electronic structures and promising potential in material applications, the identification of experimentally accessible two-dimensional square-net materials with related topological properties has proven challenging. Due to the highly tunable physical and topological properties of III-V semiconductors, we revisit the class of III-V materials and observe that the litharge-phase InBi is a layered square-net material and can be exfoliated into the InBi monolayer. We present a comprehensive first-principles study of the energy landscape of the InBi monolayer. We identify a paraelastic phase and three ferroelastic phases and study their topological properties. Specifically, we show that the paraelastic InBi monolayer is a trivial insulator due to the orbital-ordering-induced band inversion occurring between states with the same parity. Substituting one Bi atom per cell with another V-group element (N, P, As) or applying an electric field that breaks the inversion symmetry and changes the orbital onsite energy, the paraelastic InBi monolayer can be driven into the topological insulator phase. Furthermore, one of the ferroelastic phases of pure InBi, which can be obtained by gently straining the paraelastic phase, also possesses such topological insulating properties. These results provide several experimentally accessible routes to tune the nontrivial topology in the InBi monolayer, including creating heterostructures with piezoelectric or ferroelectric substrates and applying mechanical strain, making the InBi monolayer an ideal platform to study the interplay of reduced dimensionality, square-net chemical bonding networks, and band topology.

cond-mat.mtrl-sci↗

arXiv2023

Range-Separated Hybrid Functional Pseudopotentials

Consistency between the exchange-correlation (xc) functional used during pseudopotential construction and planewave-based electronic structure calculations is important for an accurate and reliable description of the structure and properties of condensed-phase systems. In this work, we present a general scheme for constructing pseudopotentials with range-separated hybrid (RSH) xc functionals based on the solution of the all-electron radial integro-differential equation for a spherical atomic configuration. As proof-of-principle, we demonstrate pseudopotential construction with the PBE, PBE0, HSE, and sRSH (based on LC-$ω$PBE0) xc functionals for a select set of atoms, and then investigate the importance of pseudopotential consistency when computing band gaps, equilibrium lattice parameters, bulk moduli, and atomization energies of several solid-state systems. In doing so, we find that pseudopotential consistency errors (PSCE) tend to be systematic and can be as large as $1.4\%$ when computing these properties.

cond-mat.mtrl-sci↗

arXiv2023

Discovery of enhanced lattice dynamics in a single-layered hybrid perovskite

Layered hybrid perovskites have attracted much attention in recent years due to their emergent physical properties and exceptional functional performances, but the coexistence of lattice order and structural disorder severely hinders our understanding of these materials. One unsolved problem regards how the lattice dynamics are affected by the dimensional engineering of the inorganic frameworks and the interaction with the molecular moieties. Here, we address this question by using a combination of high-resolution spontaneous Raman scattering, high-field terahertz spectroscopy, and molecular dynamics simulations. This approach enables us to reveal the structural vibrations and disorder in and out of equilibrium and provides surprising observables that differentiate single- and double-layered perovskites. While no distinct vibrational coherence is observed in double-layer perovskites, we discover that an off-resonant terahertz pulse can selectively drive a long-lived coherent phonon mode through a two-photon process in the single-layered system. This difference highlights the dramatic change in the lattice environment as the dimension is reduced. The present findings pave the way for the ultrafast structural engineering of hybrid lattices as well as for developing high-speed optical modulators based on layered perovskites.

cond-mat.mtrl-sci↗

arXiv2022

Domain Wall Enabled Steep Slope Switching in MoS$_2$ Transistors Towards Hysteresis-Free Operation

The device concept of ferroelectric-based negative capacitance (NC) transistors offers a promising route for achieving energy-efficient logic applications that can outperform the conventional semiconductor technology, while viable operation mechanisms remain a central topic of debate. In this work, we report steep slope switching in MoS$_2$ transistors back-gated by single-layer polycrystalline PbZr$_{0.35}$Ti$_{0.65}$O$_3$. The devices exhibit current on/off ratios up to 8$\times$10$^6$ within an ultra-low gate voltage window of $V_g$ = $\pm$0.5 V and subthreshold swing (SS) as low as 9.7 mV decade$^{-1}$ at room temperature, transcending the 60 mV decade$^{-1}$ Boltzmann limit without involving additional dielectric layers. Theoretical modeling reveals the dominant role of the metastable polar states within domain walls in enabling the NC mode, which is corroborated by the relation between SS and domain wall density. Our findings shed light on a hysteresis-free mechanism for NC operation, providing a simple yet effective material strategy for developing low-power 2D nanoelectronics.

physics.app-ph↗

arXiv2022

Ab initio study of proton-exchanged LiNbO3(I): Structural, thermodynamic, dielectric, and optical properties

Using first principles calculations, we study the ground-state structure of bulk proton-exchanged lithium niobate, which is also called hydrogen niobate and is widely used in waveguides. Thermodynamics helps to establish the most favorable nonpolar surface as well as the water-deficient and water-rich phases under different ambient conditions, which we refer to as "dehydrated" and "rehydrated" phases, respectively. We compute the low-frequency dielectric response and the optical refractive indices of hydrogen niobate in different phases. The dielectric constant is greatly enhanced compared to lithium niobate. At shorter wavelengths, the refractive indices vary between each phase and have a sharp contrast to lithium niobate. Our study characterizes the structures and thermal instabilities of this compound and reveals its excellent dielectric and optical properties, which can be important in the future application in waveguides.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic Bulk Photovoltaic Effect: Strong and Weak Field

Shift current and ballistic current have been proposed to explain the bulk photovoltaic effect (BPVE), and there have been experiments designed to separate the two mechanisms. These experiments are based on the assumption that under magnetic field, ballistic current can have a Hall effect while the shift current cannot, which is from some energy-scale arguments and has never been proven. A recent work [Phys. Rev. B 103, 195203 (2021)] using quantum transport formalism achieves a conclusion that shift current indeed has a Hall current, seemingly contradicting the previous assumption and making the situation more confusing. Moreover, the behavior of BPVE under strong magnetic field is still unexplored. In this Letter, using a minimal 2D tight-binding model, we carry out a systematic numerical study of the BPVE under weak and strong magnetic field by treating the field in a non-perturbative way. Our model clearly shows the appearance of the magnetically-induced ballistic current along the transverse direction, which agrees with the previous predictions, and interestingly a sizable longitudinal response of the shift current is also observed, a phenomenon that is not captured by any existing theories where the magnetic field is treated perturbatively. More surprisingly, drastically different shift current is found in the strong-field regime, and the evolution from weak to strong field resembles a phase transition. We hope that our work could resolve the debate over the behavior of BPVE under magnetic field, and the strong-field behavior of shift current is expected to inspire more studies on the relation between nonlinear optics and quantum geometry.

cond-mat.mtrl-sci↗

arXiv2022

Recent Progress in the Theory of Bulk Photovoltaic Effect

The bulk photovoltaic effect (BPVE) occurs in solids with broken inversion symmetry and refers to DC current generation due to uniform illumination, without the need of heterostructures or interfaces, a feature that is distinct from the traditional photovoltaic effect. Its existence has been demonstrated almost 50 years ago, but predictive theories only appeared in the last ten years, allowing for the identification of different mechanisms and the determination of their relative importance in real materials. It is now generally accepted that there is an intrinsic mechanism that is insensitive to scattering, called shift current, where first-principles calculations can now give highly accurate predictions. Another important but more extrinsic mechanism, called ballistic current, is also attracting a lot of attention, but due to the complicated scattering processes, its numerical calculation for real materials is only made possible quite recently. In addition, an intrinsic ballistic current, usually referred to as injection current, will appear under circularly-polarized light and has wide application in experiments. In this article, experiments that are pertinent to the theory development are reviewed, and a significant portion is devoted to discussing the recent progress in the theories of BPVE and their numerical implementations. As a demonstration of the capability of the newly developed theories, a brief review of the materials design strategies enabled by the theory development is given. Finally, remaining questions in the BPVE field and possible future directions are discussed to inspire further investigations.

cond-mat.mtrl-sci↗

arXiv2021

Intrinsic Fermi Surface Contribution to the Bulk Photovoltaic Effect

We study the Fermi surface contribution to the nonlinear DC photocurrent at quadratic order in a spatially uniform optical field in the ultra-clean limit. In addition to shift and injection current, we find that polarized light incident on a metallic system generates an intrinsic contribution to the bulk photovoltaic effect deriving from photoinduced electronic transitions on the Fermi surface. In {velocity} gauge, this contribution originates in both the coherent band off-diagonal and diagonal parts of the density matrix, describing respectively, the coherent wave function evolution and the carrier dynamics of an excited population. We derive a formula for the intrinsic Fermi surface contribution for a time-reversal invariant chiral Weyl semimetal illuminated with circularly-polarized light. At low frequency, this response is proportional to the frequency of the driving field, with its sign determined by the topological charge of the Weyl nodes and with its magnitude being comparable to the recently discovered quantized circular photogalvanic effect. Our work presents a complete derivation for all contributions to nonlinear DC photocurrent and classify them according to the polarization of light in the presence and absence of TRS.

cond-mat.mtrl-sci↗

arXiv2021

Negative piezoelectricity in low--dimensional materials

In this study, we investigate the underlying mechanisms of the negative piezoelectricity in low--dimensional materials by carrying out first--principles calculations. Two--dimensional ferroelectric CuInP$_2$S$_6$ is analyzed in detail as a typical example, but the theory can be applied to all other low--dimensional piezoelectrics. Similar to three--dimensional piezoelectrics with negative piezoelectric responses, the anomalous negative piezoelectricity in CuInP$_2$S$_6$ results from its negative clamped--ion term, which cannot be compensated by the positive internal strain part. Here, we propose a more general rule that having a negative clamped--ion term should be universal among piezoelectric materials, which is attributed to the "lag of Wannier center" effect. The internal--strain term, which is the change in polarization due to structural relaxation in response to strain, is mostly determined by the spatial structure and chemical bonding of the material. In a low--dimensional piezoelectric material as CuInP$_2$S$_6$, the internal--strain term is approximately zero. This is because the internal structure of the molecular layers, which are bonded by the weak van der Waals interaction, responds little to the strain. As a result, the magnitude of the dipole, which depends strongly on the dimension and structure of the molecular layer, also has a small response with respect to strain. An equation bridging the internal strain responses in low--dimensional and three--dimensional piezoelectrics is also derived to analytically express this point. This work aims to deepen our understanding about this anomalous piezoelectric effect, especially in low--dimensional materials, and provide strategies for discovering materials with novel electromechanical properties.

cond-mat.mtrl-sci↗

arXiv2021

First Principles Calculation of Ballistic Current from Electron-Hole Interaction

The bulk photovoltaic effect (BPVE) has attracted an increasing interest due to its potential to overcome the efficiency limit of traditional photovoltaics, and much effort has been devoted to understanding its underlying physics. However, previous work has shown that theoretical models of the shift current and the phonon-assisted ballistic current in real materials do not fully account for the experimental BPVE photocurrent, and so other mechanisms should be investigated in order to obtain a complete picture of BPVE. In this Letter, we demonstrate two approaches that enable the ab initio calculation of the ballistic current originating from the electron-hole interaction in semiconductors. Using BaTiO$_3$ and MoS$_2$ as two examples, we show clearly that for them the asymmetric scattering from electron-hole interaction is less appreciable than that from electron-phonon interaction, indicating more scattering processes need to be included to further improve the BPVE theory. Moreover, our approaches build up a venue for predicting and designing materials with larger ballistic current due to electron-hole interactions.

cond-mat.mtrl-sci↗

arXiv2021

Large Bulk Piezophotovoltaic Effect of Monolayer $2H$-MoS$_2$

The bulk photovoltaic effect in noncentrosymmetric materials is an intriguing physical phenomenon that holds potential for high-efficiency energy harvesting. Here, we study the shift current bulk photovoltaic effect in the transition metal dichalcogenide MoS$_2$. We present a simple automated method to guide materials design and use it to uncover a distortion to monolayer $2H$-MoS$_2$ that dramatically enhances the integrated shift current. Using this distortion, we show that overlap in the Brillouin zone of the distributions of the shift vector (a quantity measuring the net displacement in real space of coherent wave packets during excitation) and the transition intensity is crucial for increasing the shift current. The distortion pattern is related to the material polarization and can be realized through an applied electric field via the converse piezoelectric effect. This finding suggests an additional method to engineer the shift current response of materials to augment previously reported methods using mechanical strain.

cond-mat.mtrl-sci↗

arXiv2020

Phonon-Assisted Ballistic Current From First Principles Calculations

The bulk photovoltaic effect (BPVE) refers to current generation due to illumination by light in a homogeneous bulk material lacking inversion symmetry. In addition to the intensively studied shift current, the ballistic current, which originates from asymmetric carrier generation due to scattering processes, also constitutes an important contribution to the overall kinetic model of the BPVE. In this letter, we use a perturbative approach to derive a formula for the ballistic current resulting from the intrinsic electron-phonon scattering in a form amenable to first-principles calculation. We then implement the theory and calculate the ballistic current of the prototypical BPVE material \ch{BaTiO3} using quantum-mechanical density functional theory. The magnitude of the ballistic current is comparable to that of shift current, and the total spectrum (shift plus ballistic) agrees well with the experimentally measured photocurrents. Furthermore, we show that the ballistic current is sensitive to structural change, which could benefit future photovoltaic materials design.

cond-mat.mtrl-sci↗

arXiv2020

The shift photovoltaic current and magnetically-induced bulk photocurrent in piezoelectric sillenite crystals

Recently, it has been shown how shift and ballistic currents in piezoelectric sillenite crystals Bi$_{12}$GeO$_{20}$ and Bi$_{12}$SiO$_{20}$ can be separated experimentally under the assumption that the shift component of the circular current is small. However, it has been claimed that the shift and ballistic currents cannot be quantified by this method, due to the magneto-photovoltaic effect caused either by the change of the crystal's spatial symmetry in the magnetic field or by the breaking of time-reversal symmetry. Presently, we report observations of photovoltaic currents in Bi$_{12}$SiO$_{20}$, excited by linearly- and circularly-polarized light under weak external magnetic field, as well as measurements of the corresponding photo-Hall signals. We demonstrate that the magneto-photovoltaic current constitutes a significant fraction of the measured current in the Hall direction for Bi$_{12}$SiO$_{20}$ under specific experimental conditions.

cond-mat.mtrl-sci↗