News & Highlights
Ferroelectrics shine with sparkling discoveries!
A little over a decade ago, materials scientists stumbled upon a fascinating object: magnetic skyrmions, tiny whirlpools of the magnetic order within materials that act like nanoscale particles. These objects elicit new physical phenomena, and have also been proposed for unconventional computing schemes (like reservoir computing or stochastic computing). More recently, the scientific community has also predicted ans observed their electric counterparts, electric skyrmions, which hold the promise of more energy-efficient and faster applications. However, many fundamental questions about these intriguing entities remain unanswered.
Our recent research delves into the world of ferroelectric bubbles, peculiar formations that emerge in ferroelectric/paraelectric superlattices when subject to an electric bias. What's truly exciting is that these ferroelectric bubbles exhibit particle-like behaviour. They attract each other when separated by long distances, but exhibit repulsion when they get extremely close. Perhaps most interestingly, these bubbles can undergo Brownian motion at room temperature, a behaviour previously observed and harnessed in their magnetic counterparts.
Our findings shed light on the captivating world of electric bubbles and suggest that they could open up exciting possibilities for new computational techniques like those proposed (and partially tested) for magnetic skyrmions. Imagine harnessing these tiny, unpredictable entities to power the future of computation and technology. The world of ferroelectrics is indeed sparkling with potential!
For more details see:
Brownian electric bubble quasiparticles, H. Aramberri and J. Íñiguez-González, Physical Review Letters 132, 136801 (2024). (Open access).
Weaving ferroelectrics at the nanoscale
Our colleagues at the Universidad Complutense de Madrid (UCM), in collaboration with the Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC) managed to exploit a new degree of freedom of oxide crystals not present in nature: a controlled rotation between atomic-scale crystalline layers – a strategy termed 'twistronics'.
In this work, ultrathin layers of single crystals of a ferroelectric perovskite (BaTiO3) were rotated with respect to each other to create structural interaction motifs. When viewed through the lens of an electronic transmission microscope, the interface reveals a gorgeous moiré pattern showing dipole vortices and antivortices in a periodic array.
Our calculations helped to understand the hidden mechanisms at play, linking the nanoscale strains at the interface to the formation of polar vortex patterns. It turns out that the local strains induced at the interface vary rather strongly, promotes the vortex texture on the dipole structure via a flexoelectric coupling.
These nanovortices are not just a spectacle for the eyes. Such nano-vortices may eventually become the building block for future memory technologies, allowing for very high storage densities. And that's not all – this work opens new avenues for exploring and harnessing novel effects and properties in crystalline oxides with ferroic orders or other collective states.
For more details see:
A 2D ferroelectric vortex pattern in twisted BaTiO3 freestanding layers, G. Sánchez-Santolino, V. Rouco et al., Nature 626, 529 (2024). (Open access)
A fresh perspective on hafnia
Hafnia (HfO2) has been subject of keen scientific interest ever since its ferroelectric properties were discovered more than a decade ago. The reason is not minor: this material is compatible with silicon-based semiconductors, a rarity among traditional ferroelectric materials like perovskites. However, understanding the various phases oh hafnia has proven to be a persistent challenge.
Traditionally, hafnia's polymorphism has been described using tetragonal or cubic reference structures, which poses difficulties in explaining crucial experimental findings and developing simple models for this compound.
Recently, in collaboration with Jorge Íñiguez, we proposed a novel approach to look at hafnia. By adopting a specific orthorhombic phase (where the active oxygen atoms remain in the same planes as the hafnium atoms), we revealed that the low-energy phases of hafnia could be understood as relatively simple condensations of its structural instabilities. While this doesn't resolve all the open questions about hafnia, such as its switching, we believe it offers a more promising framework for discussing the fundamental Physics at play.
For more details see:
Theoretical approach to ferroelectricity in hafnia and related materials, H. Aramberri and J. Íñiguez, Communications Materials 4, 95 (2023). (Open access).
Ferroelectric/paraelectric superlattices for energy storage
When surrounding a ferroelectric by dielectric layers, a competition between the polar distortion and the depolarizing field arises, often resulting in a rupture of the ferroelectric into domains with opposite polarization. Such system can be considered antipolar and can switch into a polar state under an electric field, and hence responds as an antiferroelectric.
In this work we study the antiferroelectric-like response of PbTiO3/SrTiO3 superlattices by means of high-throughput second-principles calculations. Check out the parallel coordinates plot showing the resulting database. The plot is interactive: you can change the column ordering and highlight a range of values in each column. This type of plots help to quickly identify correlations among the superlattice parameters and optimal energy-storing performance at a given applied field (see article for detailed description of each column and even more of these interactive plots).
For more details see:
Ferroelectric/paraelectric superlattices for energy storage, H. Aramberri, N. Fedorova and J. Íñiguez, Science Advances 8, eabn4880 (2022). (Open Access).
See also this interview by Charles Q. Choi about our work.
Is the prototypical antiferroelectric PbZrO3 really antiferroelectric?
Lead zirconate (PbZrO3) was the first material identified as antiferroelectric, shortly after the concept of antiferroelectricity was proposed in the 1950s. Not only that, it is considered the prototypical antiferroelectric perovskite. Yet, many experimental works on this compound hint at uncompensated dipoles in this compound, which would correspond to a ferrielectric phase. We build one of the simplest ferrielectric phases one can imagine for PbZrO3, and, to our surprise, first-principles calculations reveal this phase to be more stable than the usual antiferroelectric phase! By studying the phase stability as a function of temperature, we find that the ferrielectric phase could even be the most stable at ambient conditions. There might even be other phases with more complex (uncompensated) dipole arrangements that show an even greater stability.
For more details see:
On the possibility that PbZrO3 not be antiferroelectric, H. Aramberri, C. Cazorla, M. Stengel and J. Íñiguez, npj Computational Materials 7, 196 (2021). (Open Access).
Antiferroelectrics in disguise!
Antiferroelectrics can be very useful for energy storage, but we know relatively few materials of this type. We studied many oxides looking for antiferroelectric candidates, and we found at least three based on vanadium. Surprisingly, these are simple ternary oxides known since the 1950s. The origin of antiferroelectricity in these vanadates is linked to their crystal structure, which is essentially equivalent to that of one of the most abundant mineral families on Earth: pyroxenes. Are there many antiferroelectrics out there flying under the radar?
For more details see:
Antiferroelectricity in a family of pyroxene-like oxides with rich polymorphism, H. Aramberri and J. Íñiguez. Commun Mater 1, 52 (2020) (Open Access)
See also related Behind the Paper contribution in the Device & Materials Engineering community from Nature Research.
Topological Homojunctions
In a Bi2Se3 crystal, you can obtain topological interface states by applying tensile strain to certain layers only (read more about this effect in Strain & Topological Interface States). In such a system the bulk-to-boundary correspondence (which dictates that a topological interface state must arise at the junction between two materials with different topological invariants) can be probed with the smallest possible number of different elements, since both subsystems at the interface will have the same crystal symmetries and chemical composition (and can even have the same electronic gap!). Thus, this topological homojunction can righteously be called the 'Hydrogen atom' of topological states of matter, in the sense that it is the simplest possible scenario for topological states to develop.
In this work we propose this idea and explore the effects of strain gradient and other strain profiles in Bi2Se3 superlattices and thin films. In the latter, we additionally investigate the interaction between topological surface states and topological interface states, finding a close resemblance to the surface-surface interaction in unstrained topological insulators.
For more details see
Strain-driven tunable topological states in Bi2Se3, H. Aramberri and M.C. Muñoz, J. Phys. Mater. 1 015009 (2018). (Open Access)
Electric control of the heat flux
Manipulating phonons is a complicated task due to their lack of mass or electric charge. Nevertheless, some vibrational modes in polar crystals can be strongly affected by external electric fields. In this work we exploit this idea to propose a thermal transistor based on ferroelectric PbTiO3. We explore how electric fields with different orientations with respect to the ferroelectric polarization affect the phonon spectrum and thus the lattice thermal conductivity (klat) of this compound. We find that a reduction of as much as 50% on the klat can be achieved at room temperature with experimentally accessible electric fields.
For more details see:
Electric control of the heat flux through electrophononic effects, J.A. Seijas-Bellido, H. Aramberri, J.Íñiguez, R. Rurali. Phys. Rev. B 97 184306 (2018) (Check also in the supplementary information available online the hysteretic behaviour we found for klat!). Preprint available in arXiv 1805.08213
High pressure silica
Silicon dioxide is the most abundant compound on Earth. We have investigated the thermal properties of SiO2 as it undergoes a pressure-induced phase transition from the stishovite phase to the CaCl2-type phase. We found an astonishingly large peak of the in-plane thermal conductivity close to the critical pressure due to an increase in the population of acoustic modes. Since these two high pressure phases of SiO2 naturally occur inside the Earth's mantle, we also computed the lattice thermal conductivity of silica along the geotherm.
For more details see:
Thermal conductivity changes across a second-order structural phase transition: the case of high pressure silica, H. Aramberri, R. Rurali, J. Íñiguez. Phys. Rev. B 96, 195201 (2017). Preprint available in arXiv 1707.05321
Strain & Topological Interface States
In this paper we show how strain turns the newly discovered topological insulators into normal insulators and disclose a path towards strained-based topological electronics. We calculated the effect of strain on the electronic band structure of the bismuth dichalcogenide family, which hosts several of the best-known topological insulators, and found that strain changes the band gap of the materials and may suppress their topological conducting surface states. We derived a universal phase diagram for these compounds, that describes the strain at which the transition to the normal insulating o metallic state occurs. Base on our findings, we propose an electronic device with switchable topological conducting states. The conducting states are formed at the interface of two different bismuth dichalcogenides, have a precise value of their spin and are topological protected, i.e.: immune to defects in the samples, as well as safe from ambient impurities, as opposed to topological states at surfaces. Additionally, these states can be switched on or off, while preserving their topological nature.
For more details see:
Strain effects in topological insulators: Topological order and the emergence of switchable topological interface states in Sb2Te3/Bi2Te3 heterojunctions, H. Aramberri and M.C. Muñoz, Phys. Rev. B 95, 205422 (2017). Preprint available in arXiv, 1603.01268.
Topological PN junction
Modern day electronic technology is widely based on semiconductor pn junctions. In this article we propose and calculate a lateral topological pn junction based on three dimensional topological insulator Bi2Se3. In a previous work, we showed how twin boundaries (a kind of inversion symmetry breaking planar defects) induce a spontaneous polarisation in Bi2Se3 which in turn generate a self doping of the topological surface states. In this work we exploit this feature along with surface Green's function matching techniques to calculate the electronic and transport properties of a lateral topological pn junction of Bi2Se3 with no extrinsic dopants.
For more details see
A realistic topological p–n junction at the Bi2Se3 (0001) surface based on planar twin boundary defects, H. Aramberri, M.C. Muñoz, and J.I. Cerdá, Nano Research. pp 1-10 (2017).