arXiv subjects

Philippe Ghosez

Publications and source records attributed to Philippe Ghosez.

At least 55 records · Page 3Linked to original sources

arXiv2021

Band-Edge Orbital Engineering of Perovskite Semiconductors for Optoelectronic Applications

Lead (Pb) halide perovskites have achieved great success in recent years due to their excellent optoelectronic properties, which is largely attributed to the lone-pair s orbital-derived antibonding states at the valence band edge. Guided by the key band-edge orbital character, a series of ns2-containing (i.e., Sn2+, Sb3+, Bi3+) Pb-free perovskite alternatives have been explored as potential photovoltaic candidates. On the other hand, based on the band-edge orbital components (i.e., M2+ s and p/X- p orbitals), a series of strategies have been proposed to optimize their optoelectronic properties by modifying the atomic orbitals and orbital interactions. Therefore, understanding the band-edge electronic features from the recently reported halide perovskites is essential for future material design and device optimization. Here, this Perspective first attempts to establish the band-edge orbital-property relationship using a chemically intuitive approach, and then rationalizes their superior properties and understands the trends in electronic properties. We hope that this Perspective will provide atomic-level guidance and insights toward the rational design of perovskite semiconductors with outstanding optoelectronic properties.

cond-mat.mtrl-sci↗

arXiv2020

Ferroelectricity and multiferroicity in anti-Ruddlesden-Popper structures

Combining ferroelectricity with other properties such as visible light absorption or long-range magnetic order requires the discovery of new families of ferroelectric materials. Here, through the analysis of a high-throughput database of phonon band structures, we identify a new structural family of anti-Ruddlesden-Popper phases A$_4$X$_2$O (A=Ca, Sr, Ba, Eu, X=Sb, P, As, Bi) showing ferroelectric and anti-ferroelectric behaviors. The discovered ferroelectrics belong to the new class of hyperferroelectrics which polarize even under open-circuit boundary conditions. The polar distortion involves the movement of O anions against apical A cations and is driven by geometric effects resulting from internal chemical strains. Within this new structural family, we show that Eu$_4$Sb$_2$O combines coupled ferromagnetic and ferroelectric order at the same atomic site, a very rare occurrence in materials physics.

cond-mat.mtrl-sci↗

arXiv2020

Orbital-Energy Splitting in Anion Ordered Ruddlesden-Popper Halide Perovskites for Tunable Optoelectronic Applications

The electronic orbital characteristics at the band edges plays an important role in determining the electrical, optical and defect properties of perovskite photovoltaic materials. It is highly desirable to establish the relationship between the underlying atomic orbitals and the optoelectronic properties as a guide to maximize the photovoltaic performance. Here, using first-principles calculations and taking anion ordered Ruddlesden-Popper (RP) phase halide perovskites Cs$_{n+1}$Ge$_n$I$_{n+1}$Cl$_{2n}$ as an example, we demonstrate how to rationally optimize the optoelectronic properties (e.g., band gap, transition dipole matrix elements, carrier effective masses, band width) through a simple band structure parameter. Our results show that reducing the splitting energy $|Δc|$ of p orbitals of B-site atom can effectively reduce the band gap and carrier effective masses while greatly improving the optical absorption in the visible region. Thereby, the orbital-property relationship with $Δc$ is well established through biaxial compressive strain. Finally, it is shown that this approach can be reasonably extended to several other non-cubic halide perovskites with similar p orbitals characteristics at the conduction band edges. Therefore, we believe that our proposed orbital engineering approach provides atomic-level guidance for understanding and optimizing the device performance of layered perovskite solar cells.

cond-mat.mtrl-sci↗

arXiv2020

Polarity-field driven conductivity in SrTiO$_3$/LaAlO$_3$: a hybrid functional study

The origin of the 2-dimensional electron system (2DES) appearing at the (001) interface of band insulators $\rm SrTiO_3$ and $\rm LaAlO_3$ has been rationalized in the framework of a polar catastrophe scenario. This implies the existence of a critical thickness of polar $\rm LaAlO_3$ overlayer ($4~\rm u.c.$) for the appearance of the 2DES: polar catastrophe for thick $\rm LaAlO_3$ overlayer is avoided either through a Zener breakdown or a stabilization of donor defects at the $\rm LaAlO_3$ surface, both providing electrons to dope the substrate. The observation of a critical thickness is observed in experiments, supporting these hypotheses. Yet, there remains an open debate about which of these possible mechanisms actually occurs first. Using hybrid functional Density Functional Theory, we re-examine these mechanisms at the same level of approximation. Particularly, we clarify the role of donor defects in these heterostructures, and argue that, under usual growth conditions, electric-field driven stabilization of oxygen vacancies and hydrogen adsorbates at the LAO surface occur at a smaller LAO thickness than required for Zener breakdown.

cond-mat.mtrl-sci↗

arXiv2020

Electron-Lattice Interplays in LaMnO3 from Canonical Jahn-Teller Distortion Notations

LaMnO$_3$ is considered as a prototypical Jahn-Teller perovskite compound, exhibiting a metal to insulator transition at $T_{JT} = 750K$ related to the joint appearance of an electronic orbital ordering and a large lattice Jahn-Teller distortion. From first-principles, we revisit the behavior of LaMnO$_3$ and show that it is not only prone to orbital ordering but also to charge ordering. Both charge and orbital orderings appear to be enabled by rotations of the oxygen octahedra and the subtle competition between them is monitored by a large tetragonal compressive strain, that is itself a Jahn-Teller active distortion. Equally, the competition of ferromagnetic and antiferromagnetic orders is slave of the same tetragonal strain. Our results further indicate that the metal to insulator transition can be thought as a Peierls transition that is enabled by spin symmetry breaking. Therefore, dynamical spin fluctuations in the paramagnetic state stabilize the insulating phase by the instantaneous symmetry breaking they produce and which is properly captured from static DFT calculations. As a basis to our discussion, we introduce canonical notations for lattice distortions in perovskites that distort the oxygen octhedra and are connected to charge and orbital orderings.

cond-mat.str-el↗

arXiv2020

Optical spectra of rare-earth nickelates

Based on the electronic band structure obtained from first principles DFT calculations, the opticalspectra of yttrium and neodymium nickelates are computed. We show that the results are in fairagreement with available experimental data. We clarify the electronic transitions at the origin of thefirst two peaks, highlighting the important role of transitions from t2g states neglected in previousmodels. We discuss the evolution of the optical spectra from small to large rare-earth cations andrelate the changes to the electronic band structure.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic instabilities in doped Fe$_2YZ$ full-Heusler thermoelectric compounds

Thermoelectricity is a promising avenue for harvesting energy but large-scale applications are still hampered by the lack of highly-efficient low-cost materials. Recently, Fe$_2YZ$ Heusler compounds were predicted theoretically to be interesting candidates with large thermoelectric power factor. Here, we show that under doping conditions compatible with thermoelectric applications, these materials are prone to an unexpected magnetic instability detrimental to their thermoelectric performance. We rationalize the physics at the origin of this instability, provide guidelines for avoiding it and discuss its impact on the thermoelectric power factor. Doing so, we also point out the shortcomings of the rigid band approximation commonly used in high-throughput theoretical searches of new thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2019

Length-scales of interfacial coupling between metal-insulator phases in oxides

Controlling phase transitions in transition metal oxides remains a central feature of both technological and fundamental scientific relevance. A well-known example is the metal-insulator transition which has been shown to be highly controllable while a less well understood aspect of this phenomenon is the length scale over which the phases can be established. To gain further insight into this issue, we have atomically engineered an artificially phase separated system through fabricating epitaxial superlattices consisting of SmNiO$_{3}$ and NdNiO$_{3}$, two materials undergoing a metal-to-insulator transition at different temperatures. By combining advanced experimental techniques and theoretical modeling, we demonstrate that the length scale of the metal-insulator transition is controlled by the balance of the energy cost of the domain wall between a metal and insulator and the bulk energetics. Notably, we show that the length scale of this effect exceeds that of the physical coupling of structural motifs, introducing a new paradigm for interface-engineering properties that are not available in bulk

cond-mat.str-el↗

arXiv2019

Structural and electronic phenomena at oxyfluoride KTaO$_3$/K$M$F$_3$ ($M$ = Zn and Ni) superlattices: Rashba splitting and 2DEG

Here, we present the theoretical analysis of the structural and electronic degrees of freedom of two different oxide/fluoride perovskite superlattices, KTaO$_3$/KZnF$_3$ and KTaO$_3$/KNiF$_3$. Using first-principles calculations, we found the appearance of a two-dimensional electron, 2DEG, and hole, 2DHG, gases as a function of the number of layers of the different pristine materials. We demonstrate that the phonon-dynamics at the KTO/K$M$F superlattices play a crucial role in the appearance of these effects. Additionally, our results reveal a rather sizeable Rashba-type spin-splitting at these interfaces in comparison to another oxide/oxide counterparts.

cond-mat.mtrl-sci↗

arXiv2019

Lattice dynamics and Raman spectrum of BaZrO3 single crystals

BaZrO3 is a perovskite that remains in the simple cubic phase at all temperatures, hence with no first-order Raman-active phonon mode allowed by symmetry. Yet, it exhibits an intense Raman spectrum with sharp and well-defined features. Here, we report the evolution of the Raman spectrum of BaZrO3 single crystals in a broad temperature range (4--1200~K) and discuss its origin with the support of detailed first-principle calculations of the lattice dynamics. Phonon calculations are performed not only for the cubic phase of BaZrO3, but also for the low-symmetry phases with octahedra tilts that have been suspected to exist at the nanoscale. We show that the Raman spectrum shows no direct evidence for these nanodomains, but can instead be explained by classical second-order Raman scattering. We provide an assignment of the dominant features to phonon modes combinations. In particular, we show that the high frequency range of the spectrum is dominated by overtones and shows an image of the phonon density of states corresponding to the stretching modes of the oxygen octahedra.

cond-mat.mtrl-sci↗

arXiv2019

Artificial quantum confinement in LAO3/STO heterostructure

Heterostructures of transition metal oxides (TMO) perovskites represent an ideal platform to explore exotic phenomena involving the complex interplay between the spin, charge, orbital and lattice degrees of freedom available in these compounds. At the interface between such materials, this interplay can lead to phenomena that are present in none of the original constituents such as the formation of the interfacial 2D electron system (2DES) discovered at the LAO3/STO3 (LAO/STO) interface. In samples prepared by growing a LAO layer onto a STO substrate, the 2DES is confined in a band bending potential well, whose width is set by the interface charge density and the STO dielectric properties, and determines the electronic band structure. Growing LAO (2 nm) /STO (x nm)/LAO (2 nm) heterostructures on STO substrates allows us to control the extension of the confining potential of the top 2DES via the thickness of the STO layer. In such samples, we explore the dependence of the electronic structure on the width of the confining potential using soft X-ray ARPES combined with ab-initio calculations. The results indicate that varying the thickness of the STO film modifies the quantization of the 3d t2g bands and, interestingly, redistributes the charge between the dxy and dxz/dyz bands.

cond-mat.str-el↗

arXiv2019

Rashba spin-splitting in ferroelectric oxides: from rationalizing to engineering

Ferroelectric Rashba semiconductors (FERSC), in which Rashba spin-splitting can be controlled and reversed by an electric field, have recently emerged as a new class of functional materials useful for spintronic applications. The development of concrete devices based on such materials is, however, still hampered by the lack of robust FERSC compounds. Here, we show that the coexistence of large spontaneous polarisation and sizeable spin-orbit coupling is not sufficient to have strong Rashba effects and clarify why simple ferroelectric oxide perovskites with transition metal at the B-site are typically not suitable FERSC candidates. By rationalizing how this limitation can be by-passed through band engineering of the electronic structure in layered perovskites, we identify the Bi$_2$WO$_6$ Aurivillius crystal as the first robust ferroelectric with large and reversible Rashba spin-splitting, that can even be substantially doped without losing its ferroelectric properties. Importantly, we highlight that a unidirectional spin-orbit field arises in layered Bi$_2$WO$_6$, resulting in a protection against spin-decoherence.We highlight moreover that a unidirectional spin-orbit field arises in Bi$_2$WO$_6$, in which the spin-texture is so protected against spin-decoherence.

cond-mat.mtrl-sci↗

arXiv2019

Magnetism tailored by mechanical strain engineering in PrVO$_3$ thin films

Transition-metal oxides with an ABO$_3$ perovskite structure exhibit strongly entangled structural and electronic degrees of freedom and thus, one expects to unveil exotic phases and properties by acting on the lattice through various external stimuli. Using the Jahn-Teller active praseodymium vanadate Pr$^{3+}$V$^{3+}$O$_3$ compound as a model system, we show that PrVO$_3$ Néel temperature T$_N$ can be raised by 40 K with respect to the bulk when grown as thin films. Using advanced experimental techniques, this enhancement is unambiguously ascribed to a tetragonality resulting from the epitaxial compressive strain experienced by the films. First-principles simulations not only confirm experimental results, but they also reveal that the strain promotes an unprecedented orbital-ordering of the V$^{3+}$ d electrons, strongly favouring antiferromagnetic interactions. These results show that an accurate control of structural aspects is the key for unveiling unexpected phases in oxides.

cond-mat.mtrl-sci↗

arXiv2018

A Polar large polaron in WO3

This version withdrawn by arXiv administrators because the author did not have the right to agree to our license at the time of submission

cond-mat.mtrl-sci↗

arXiv2018

Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces

Polar discontinuities occurring at interfaces between two different materials constitute both a challenge and an opportunity in the study and application of a variety of devices. In order to cure the large electric field occurring in such structures, a reconfiguration of the charge landscape sets in at the interface via chemical modifications, adsorbates or charge transfer. In the latter case, one may expect a local electronic doping of one material: one sparkling example is the two-dimensional electron liquid (2DEL) appearing in SrTiO$_3$ once covered by a polar LaAlO$_3$ layer. Here we show that tuning the formal polarisation of a (La,Al)$_{1-x}$(Sr,Ti)$_x$O$_3$ (LASTO:$x$) overlayer through chemical composition modifies the quantum confinement of the 2DEL in SrTiO$_3$ and its electronic band structure. The analysis of the behaviour in magnetic field of superconducting field-effect devices reveals, in agreement with $ab\ initio$ calculations and self-consistent Poisson-Schrödinger modelling, that quantum confinement and energy splitting between electronic bands of different symmetries strongly depend on interface charge densities. These results not only strongly support the polar discontinuity mechanisms with a full charge transfer to explain the origin of the 2DEL at the celebrated LaAlO$_3$/SrTiO$_3$ interface, but also demonstrate an effective tool for tailoring the electronic structure at oxide interfaces.

cond-mat.mtrl-sci↗

arXiv2018

First-principles study of (Ba,Ca)TiO$_3$ and Ba(Ti,Zr)O$_3$ solid solutions

(Ba,Ca)TiO$_3$ and Ba(Ti,Zr)O$_3$ solid solutions are the building blocks of lead-free piezoelectric materials that attract a renewed interest. We investigate the properties of these systems by means of first-principles calculations, with a focus on the lattice dynamics and the competition between different ferroelectric phases. We first analyze the four parent compounds in order to compare their properties and their different tendency towards ferroelectricity. The core of our study is systematic characterization of the binary systems (Ba,Ca)TiO$_3$ and Ba(Ti,Zr)O$_3$ within both the Virtual Crystal Approximation and direct supercell calculations. In the case of Ca doping, we find a gradual transformation from $A$-site to $B$-site ferroelectricity due to steric effects that largely determines the behavior of the system. In the case of Zr doping, in contrast, the behavior is eventually dominated by cooperative Zr-Ti motions and the local electrostatics. In addition, our comparative study reveals that the specific microscopic physics of these solid sets severe limits to the applicability of the Virtual Crystal Approximation for these systems.

cond-mat.mtrl-sci↗

arXiv2017

Origin of the orbital and spin orderings in rare-earth titanates

Rare-earth titanates RTiO$_3$ are Mott insulators displaying a rich physical behavior, featuring most notably orbital and spin orders in their ground state. The origin of their ferromagnetic to antiferromagnetic transition as a function of the size of the rare-earth however remains debated. Here we show on the basis of symmetry analysis and first-principles calculations that although rare-earth titanates are nominally Jahn-Teller active, the Jahn-Teller distortion is negligible and irrelevant for the description of the ground state properties. At the same time, we demonstrate that the combination of two antipolar motions produces an effective Jahn-Teller-like motion which is the key of the varying spin-orbital orders appearing in titanates. Thus, titanates are prototypical examples illustrating how a subtle interplay between several lattice distortions commonly appearing in perovskites can produce orbital orderings and insulating phases irrespective of proper Jahn-Teller motions.

cond-mat.mtrl-sci↗

arXiv2017

Structurally Triggered Metal-Insulator Transition in Rare-Earth Nickelates

Rare-earth nickelates form an intriguing series of correlated perovskite oxides. Apart from LaNiO3, they exhibit on cooling a sharp metal-insulator electronic phase transition, a concurrent structural phase transition and a magnetic phase transition toward an unusual antiferromagnetic spin order. Appealing for various applications, full exploitation of these compounds is still hampered by the lack of global understanding of the interplay between their electronic, structural and magnetic properties. Here, we show from first-principles calculations that the metal-insulator transition of nickelates arises from the softening of an oxygen breathing distortion, structurally triggered by oxygen-octahedra rotation motions. The origin of such a rare triggered mechanism is traced back in their electronic and magnetic properties, providing a united picture. We further develop a Landau model accounting for the evolution of the metal-insulator transition in terms of the $R cations and rationalising how to tune this transition by acting on oxygen rotation motions.

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