arXiv subjects

Massimiliano Stengel

Publications and source records attributed to Massimiliano Stengel.

At least 37 records · Page 2Linked to original sources

arXiv2023

Long-range electrostatic contribution to the electron-phonon couplings and mobilities of two-dimensional and bulk materials

Charge transport plays a crucial role in manifold potential applications of two-dimensional materials, including field effect transistors, solar cells, and transparent conductors. At most operating temperatures, charge transport is hindered by scattering of carriers by lattice vibrations. Assessing the intrinsic phonon-limited carrier mobility is thus of paramount importance to identify promising candidates for next-generation devices. Here we provide a framework to efficiently compute the drift and Hall carrier mobility of two-dimensional materials through the Boltzmann transport equation by relying on a Fourier-Wannier interpolation. Building on a recent formulation of long-range contributions to dynamical matrices and phonon dispersions [Phys. Rev. X 11, 041027 (2021)], we extend the approach to electron-phonon coupling including the effect of dynamical dipoles and quadrupoles. We identify an unprecedented contribution associated with the Berry connection that is crucial to preserve the Wannier-gauge covariance of the theory. This contribution is not specific to 2D crystals, but also concerns the 3D case, as we demonstrate via an application to bulk SrO. We showcase our method on a wide selection of relevant monolayers ranging from SnS2 to MoS2, graphene, BN, InSe, and phosphorene. We also discover a non-trivial temperature evolution of the Hall hole mobility in InSe whereby the mobility increases with temperature above 150 K due to the mexican-hat electronic structure of the InSe valence bands. Overall, we find that dynamical quadrupoles are essential and can impact the carrier mobility in excess of 75%.

cond-mat.mtrl-sci↗

arXiv2023

Accurate prediction of Hall mobilities in two-dimensional materials through gauge-covariant quadrupolar contributions

Despite considerable efforts, accurate computations of electron-phonon and carrier transport properties of low-dimensional materials from first principles have remained elusive. By building on recent advances in the description of long-range electrostatics, we develop a general approach to the calculation of electron-phonon couplings in two-dimensional materials. We show that the nonanalytic behavior of the electron-phonon matrix elements depends on the Wannier gauge, but that a missing Berry connection restores invariance to quadrupolar order. We showcase these contributions in a MoS$_2$ monolayer, calculating intrinsic drift and Hall mobilities with precise Wannier interpolations. We also find that the contributions of dynamical quadrupoles to the scattering potential are essential, and that their neglect leads to errors of 23% and 76% in the room temperature electron and hole Hall mobilities, respectively.

cond-mat.mtrl-sci↗

arXiv2023

Natural optical activity from density-functional perturbation theory

We present an accurate and computationally efficient first-principles methodology to calculate the natural optical activity. Our approach is based on the long-wave density-functional perturbation theory and includes self-consistent field (SCF) terms naturally in the formalism, which are found to be of crucial importance. The final result is expressed exclusively in terms of response functions to uniform field perturbations and avoids troublesome summations over empty states. Our strategy is validated by computing the natural optical activity tensor in representative chiral crystals (trigonal Se, $α$-HgS and $α$-SiO$_2$) and molecules (C$_4$H$_4$O$_2$), finding excellent agreement with experiment and previous theoretical calculations.

cond-mat.mtrl-sci↗

arXiv2023

Tilt-driven antiferroelectricity in PbZrO$_3$

Antiferroelectricity is a state of matter that has so far eluded a clear-cut definition. Even in the best-known material realization, PbZrO$_3$, the physical nature of the driving force towards an antipolar order has not been settled yet. Here, by building a Landau-like continuum Hamiltonian from first-principles via an exact long-wave approach, we reconcile the existing theories in terms of a single physical mechanism. In particular, we find that a formerly overlooked trilinear coupling between tilts, tilt gradients and polarization provides a surprisingly accurate description of the energetics and structure of the antiferroelectric ground state of PbZrO$_3$. We discuss the relevance of our findings to other ferrielectric and incommensurate polar structures that were recently observed in perovskites.

cond-mat.mtrl-sci↗

arXiv2022

Frequency splitting of chiral phonons from broken time reversal symmetry in CrI$_3$

Conventional approaches for lattice dynamics based on static interatomic forces do not fully account for the effects of time-reversal-symmetry breaking in magnetic systems. Recent approaches to rectify this involve incorporating the first-order change in forces with atomic velocities under the assumption of adiabatic separation of electronic and nuclear degrees of freedom. In this work, we develop a first-principles method to calculate this velocity-force coupling in extended solids, and show via the example of ferromagnetic CrI$_3$ that, due to the slow dynamics of the spins in the system, the assumption of adiabatic separation can result in large errors for splittings of zone-center chiral modes. We demonstrate that an accurate description of the lattice dynamics requires treating magnons and phonons on the same footing.

cond-mat.mtrl-sci↗

arXiv2022

Translational covariance of flexoelectricity at ferroelectric domain walls

Macroscopic descriptions of ferroelectrics have an obvious appeal in terms of efficiency and physical intuition. Their predictive power, however, has often been thwarted by the lack of a systematicp rocedure to extract the relevant materials parameters from the microscopics. Here we address this limitation by establishing an unambiguous two-way mapping between spatially inhomogeneous fields and discrete lattice modes. This yields a natural treatment of gradient couplings in the macroscopic regime via a long-wavelength expansion of the crystal Hamiltonian. Our analysis reveals an inherent arbitrariness in both the flexoelectric and polarization gradient coefficients, which we ascribe to a translational freedom in the definition of the polar distortion pattern. Remarkably, such arbitrariness cancels out in all physically measurable properties (relaxed atomic structure and energetics) derived from the model, pointing to a generalized translational covariance in the continuum description of inhomogeneous ferroelectric structures. We demonstrate our claims with extensive numerical tests on 180-degree domain walls in common ferroelectric perovskites, finding excellent agreement between the continuum model and direct first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2022

Convert widespread paraelectric perovskite to ferroelectrics

While nature provides a plethora of perovskite materials, only a few exhibits large ferroelectricity and possibly multiferroicity. The majority of perovskite materials have the non-polar CaTiO$_3$(CTO)structure, limiting the scope of their applications. Based on effective Hamiltonian model as well as first-principles calculations, we propose a general thin-film design method to stabilize the functional BiFeO$_3$(BFO)-type structure, which is a common metastable structure in widespread CaTiO$_3$-type perovskite oxides. It is found that the improper antiferroelectricity in CTO-type perovskite and ferroelectricity in BFO-type perovskite have distinct dependences on mechanical and electric boundary conditions, both of which involve oxygen octahedral rotation and tilt. The above difference can be used to stabilize the highly polar BFO-type structure in many CTO-type perovskite materials.

cond-mat.mtrl-sci↗

arXiv2022

Curved Magnetism in CrI$_3$

Curved magnets attract considerable interest for their unusually rich phase diagram, often encompassing exotic (e.g., topological or chiral) spin states. Micromagnetic simulations are playing a central role in the theoretical understanding of such phenomena; their predictive power, however, rests on the availability of reliable model parameters to describe a given material or nanostructure. Here we demonstrate how non-collinear-spin polarized density-functional theory can be used to determine the flexomagnetic coupling coefficients in real systems. By focusing on monolayer CrI$_3$, we find a crossover as a function of curvature between a magnetization normal to the surface to a cycloidal state, which we rationalize in terms of effective anisotropy and Dzyaloshinskii-Moriya contributions to the magnetic energy. Our results reveal an unexpectedly large impact of spin-orbit interactions on the curvature-induced anisotropy, which we discuss in the context of existing phenomenological models.

cond-mat.mtrl-sci↗

arXiv2022

Nonadiabatic Born effective charges in metals and the Drude weight

In insulators, Born effective charges describe the electrical polarization induced by the displacement of individual atomic sublattices. Such a physical property is at first sight irrelevant for metals and doped semiconductors, where the macroscopic polarization is ill-defined. Here we show that, in clean conductors, going beyond the adiabatic approximation results in nonadiabatic Born effective charges that are well defined in the low-frequency limit. In addition, we find that the sublattice sum of the nonadiabatic Born effective charges does not vanish as it does in the insulating case, but instead is proportional to the Drude weight. We demonstrate these formal results with density functional perturbation theory calculations of Al, and electron-doped SnS$_2$ and SrTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2021

Rotational $g$ factors and Lorentz forces of molecules and solids from density-functional perturbation theory

Applied magnetic fields can couple to atomic displacements via generalized Lorentz forces, which are commonly expressed as gyromagnetic $g$ factors. We develop an efficient first-principles methodology based on density-functional perturbation theory to calculate this effect in both molecules and solids to linear order in the applied field. Our methodology is based on two linear-response quantities: the macroscopic polarization response to an atomic displacement (i.e., Born effective charge tensor), and the antisymmetric part of its first real-space moment (the symmetric part corresponding to the dynamical quadrupole tensor). The latter quantity is calculated via an analytical expansion of the current induced by a long-wavelength phonon perturbation, and compared to numerical derivatives of finite-wavevector calculations. We validate our methodology in finite systems by computing the gyromagnetic $g$ factor of several simple molecules, demonstrating excellent agreement with experiment and previous density-functional theory and quantum chemistry calculations. In addition, we demonstrate the utility of our method in extended systems by computing the energy splitting of the low-frequency transverse-optical phonon mode of cubic SrTiO$_3$ in the presence of a magnetic field.

cond-mat.mtrl-sci↗

arXiv2021

Lattice-mediated bulk flexoelectricity from first principles

We present the derivation and code implementation of a first-principles methodology to calculate the lattice-mediated contributions to the bulk flexoelectric tensor. The approach is based on our recent analytical long-wavelength extension of density-functional perturbation theory [Royo and Stengel, Phys. Rev. X 9, 021050 (2019)], and avoids the cumbersome numerical derivatives with respect to the wave vector that were adopted in previous implementations. To substantiate our results, we revisit and numerically validate the sum rules that relate flexoelectricity and uniform elasticity by generalizing them to regimes where finite forces and stresses are present. We also revisit the definition of the elastic tensor under stress, especially in regards to the existing linear-response implementation. We demonstrate the performance of our method by applying it to representative cubic crystals and to the tetragonal low-temperature polymorph of SrTiO$_3$, obtaining excellent agreement with the available literature data.

cond-mat.mtrl-sci↗

arXiv2021

On the possibility that PbZrO$_3$ not be antiferroelectric

Lead zirconate (PbZrO$_3$) is considered the prototypical antiferroelectric material with an antipolar ground state. Yet, several experimental and theoretical works hint at a partially polar behaviour in this compound, indicating that the polarization may not be completely compensated. In this work we propose a simple ferrielectric structure for lead zirconate. First-principles calculations reveal this state to be more stable than the commonly accepted antiferroelectric phase at low temperatures, possibly up to room temperature, suggesting that PbZrO$_3$ may not be antiferroelectric at ambient conditions. We discuss the implications of our discovery, how it can be reconciled with experimental observations and how the ferrielectric phase could be obtained in practice.

cond-mat.mtrl-sci↗

arXiv2021

Direct and converse flexoelectricity in two-dimensional materials

Building on recent developments in electronic-structure methods, we define and calculate the flexoelectric response of two-dimensional (2D) materials fully from first principles. In particular, we show that the open-circuit voltage response to a flexural deformation is a fundamental linear-response property of the crystal that can be calculated within the primitive unit cell of the flat configuration. Applications to graphene, silicene, phosphorene, BN and transition-metal dichalcogenide monolayers reveal that two distinct contributions exist, respectively of purely electronic and lattice-mediated nature. Within the former, we identify a key $metric$ term, consisting in the quadrupolar moment of the unperturbed charge density. We propose a simple continuum model to connect our findings with the available experimental measurements of the converse flexoelectric effect.

cond-mat.mes-hall↗

arXiv2021

Exact long-range dielectric screening and interatomic force constants in quasi-2D crystals

We develop a fundamental theory of the long-range electrostatic interactions in two-dimensional crystals by performing a rigorous study of the nonanalyticities of the Coulomb kernel. We find that the dielectric functions are best represented by $2\times 2$ matrices, with nonuniform macroscopic potentials that are two-component hyperbolic functions of the out-of-plane coordinate, $z$. We demonstrate our arguments by deriving the long-range interatomic forces in the adiabatic regime, where we identify a formerly overlooked dipolar coupling involving the out-of-plane components of the dynamical charges. The resulting formula is exact up to an arbitrary multipolar order, which we illustrate in practice via the explicit inclusion of dynamical quadrupoles. By performing numerical tests on monolayer BN, SnS$_2$ and BaTiO$_3$ membranes, we show that our method allows for a drastic improvement in the description of the long-range electrostatic interactions, with comparable benefits to the quality of the interpolated phonon band structure.

cond-mat.mes-hall↗

arXiv2020

Controlling ferroelectric hysteresis offsets in PbTiO$_{3}$ based superlattices

Ferroelectric materials are characterized by degenerate ground states with multiple polarization directions. In a ferroelectric capacitor this should manifest as equally favourable up and down polarization states. However, this ideal behavior is rarely observed in ferroelectric thin films and superlattice devices, which generally exhibit a built-in bias which favors one polarization state over the other. Often this polarization asymmetry can be attributed to the electrodes. In this study we examine bias in PbTiO$_3$-based ferroelectric superlattices that is not due to the electrodes, but rather to the nature of the defects that form at the interfaces during growth. Using a combination of experiments and first-principles simulations, we are able to explain the sign of the observed built-in bias and its evolution with composition. Our insights allow us to design devices with zero built-in bias by controlling the composition and periodicity of the superlattices.

cond-mat.mtrl-sci↗

arXiv2020

Phonon-limited electron mobility in Si, GaAs and GaP with exact treatment of dynamical quadrupoles

We describe a new approach to compute the electron-phonon self-energy and carrier mobilities in semiconductors. Our implementation does not require a localized basis set to interpolate the electron-phonon matrix elements, with the advantage that computations can be easily automated. Scattering potentials are interpolated on dense $\mathbf{q}$ meshes using Fourier transforms and ab initio models to describe the long-range potentials generated by dipoles and quadrupoles. To reduce significantly the computational cost, we take advantage of crystal symmetries and employ the linear tetrahedron method and double-grid integration schemes, in conjunction with filtering techniques in the Brillouin zone. We report results for the electron mobility in Si, GaAs, and GaP obtained with this new methodology.

cond-mat.mtrl-sci↗

arXiv2020

Electron-Phonon Beyond Fröhlich: Dynamical Quadrupoles in Polar and Covalent Solids

We include the treatment of quadrupolar fields beyond the Fröhlich interaction in the first-principles electron-phonon vertex in semiconductors. Such quadrupolar fields induce long-range interactions that have to be taken into account for accurate physical results. We apply our formalism to Si (nonpolar), GaAs, and GaP (polar) and demonstrate that electron mobilities show large errors if dynamical quadrupoles are not properly treated.

cond-mat.mtrl-sci↗

arXiv2020

Switching a polar metal via strain gradients

Although rare, spontaneous breakdown of inversion symmetry sometimes occurs in a material which is metallic: these are commonly known as polar metals or ferroelectric metals. Their 'polarization', however, cannot be switched via an electric field, which limits the experimental control over band topology. Here we shall investigate, via first-principles theory, flexoelectricity as a possible way around this obstacle with the well known polar metal LiOsO$_3$. The flexocoupling coefficients are computed for this metal with high accuracy with a completely new approach based on real-space sums of the inter-atomic force constants. A Landau-Ginzburg-Devonshire-type first-principles Hamiltonian is built and a critical bending radius to switch the material is estimated, whose order of magnitude is comparable to that of BaTiO$_3$.

cond-mat.mtrl-sci↗