arXiv subjects

Marta D. Rossell

Publications and source records attributed to Marta D. Rossell.

12 recordsLinked to original sources

arXiv2026

THz-induced phonon mode mixing and collective dynamics in a polar nanolattice

Manipulating phonons through symmetry is a fundamental approach to alter the dynamic responses of materials. Most often new phases are sought through control of the unit-cell (e.g. via strain, doping, light, etc.). By comparison, there is vast potential to look beyond the unit-cell into higher-order architectures to control wave scattering and interference effects that remains less explored. We describe the THz-induced dynamics of an SrTiO$_{3}$ thin film with a nanoscale ordered interfacial dislocation network probed with 2- and 3-dimensional time-resolved x-ray diffraction and classical molecular dynamics simulations. We find that symmetry breaking at all scales is an effective approach to create a dynamical electric polarization and to control phonon mixing that generates previously unreported collective modes in the THz regime with circular vortex-like displacements. This work opens a new pathway to explore dynamical functional properties that can be extended to magnetic, electric, and ferroelectric systems by controlling the real-space topology via epitaxy.

cond-mat.str-el↗

arXiv2026

Contrastive Image-Metadata Pre-Training for Materials Transmission Electron Microscopy

The transmission electron microscope facilitates the highest-resolution imaging of any instrument ever created, and its limiting factor is no longer spatial resolution but dose efficiency. Low electron doses avoid sample damage but produce noisy images for which, unlike in classical computer vision, there is no ground truth. Autonomous materials experimentation poses a related problem, since closed-loop instruments need representations grounded in the microscope state at acquisition. Both demand representations grounded in how an image was acquired. We release 7,330 paired high-angle annular dark-field scanning-TEM (HAADF-STEM) images and their seven-dimensional acquisition metadata, and propose Contrastive Image-Metadata Pre-training (CIMP), a CLIP-style encoder that aligns the two modalities and reaches 84.4% Top-1 cross-modal retrieval on a held-out split. All seven parameters are individually recoverable from the frozen visual embedding through a linear probe, and we use the embedding to condition a metadata-conditioned style-transfer model that re-renders experimental images under different acquisition parameters. Virtually scaling dwell time and beam current of low-dose images turns this model into a physics-informed denoiser; in a blind user study, experimental microscopists prefer it over the current state-of-the-art denoiser for STEM imagery on 70.2% of trials.

cs.LG↗

arXiv2025

Control of polarization and polar chiral textures in BiFeO$_3$ by epitaxial strain and interfacial chemistry

The balance between interfacial chemistry, electrostatics, and epitaxial strain plays a crucial role in stabilizing polarization in ferroelectric thin films. Here, we bring these contributions into competition in BiFeO$_3$ (BFO) thin films grown on the charged-surface-terminated La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO)-buffered NdGaO$_3$ (001) substrates. The large anisotropic compressive strain from the substrate promotes the formation of ferroelectric domains despite the expected stabilization of a uniform out-of-plane polarization by the (La,Sr)O$^{0.7+}$ termination of the metallic buffer. Piezoresponse force microscopy and scanning transmission electron microscopy reveal that the resulting nanoscale domain architecture is stabilized by the deterministic formation of a fluorite-like Bi$_2$O$_2$ surface layer on regions polarized oppositely to the LSMO-imposed polarization orientation. Leveraging this polarization compensation mechanism, we stabilize a uniform out-of-plane polarization in our highly strained BFO films by inserting a Bi$_2$O$_2$-terminated Aurivillius film as a buffer layer. Additionally, we reveal signatures of homochiral polarization textures in our BFO films on the level of domain configurations using local polarization switching experiments. Our work thus brings new strategies for controlling polarization direction and chiral textures in oxide ferroelectrics, opening pathways for functional domain-wall and domain-based electronics.

cond-mat.mtrl-sci↗

arXiv2025

Nanoscale electrostatic control in ferroelectric thin films through lattice chemistry

Nanoscale electrostatic control of oxide interfaces enables physical phenomena and exotic functionalities beyond the realm of the bulk material. In technologically-relevant ferroelectric thin films, the interface-mediated polarization control is usually exerted by engineering the depolarizing field. Here, in contrast, we introduce polarizing surfaces and lattice chemistry engineering as an alternative strategy. Specifically, we engineer the electric-dipole ordering in ferroelectric oxide heterostructures by exploiting the charged sheets of the layered Aurivillius model system. By tracking in-situ the formation of the Aurivillius charged Bi$_{2}$O$_{2}$ sheets, we reveal their polarizing effect leading to the characteristic Aurivillius out-of-plane antipolar ordering. Next, we use the polarizing Bi$_{2}$O$_{2}$ stacking as a versatile electrostatic environment to create new electric dipole configurations. We insert multiferroic BiFeO$_3$ into the Aurivillius framework to stabilize a ferrielectric-like non-collinear electric-dipole order in the final heterostructure while maintaining the antiferromagnetic order of BiFeO$_3$. We thus demonstrate that engineering the lattice chemistry stabilizes unconventional ferroic orderings at the nanoscale, a strategy that may be expanded beyond the realm of electrically ordered materials.

cond-mat.mtrl-sci↗

arXiv2024

Magnetostructural Coupling at the Néel point in YNiO3 Single Crystals

The recent discovery of superconductivity in infinite layer thin films and bulk Ruddlesden-Popper nickelates has stimulated the investigation of other predicted properties of these materials. Among them, the existence of magnetism-driven ferroelectricity in the parent compounds RNiO3 (R = 4f lanthanide and Y) at the onset of the Néel order, TN, has remained particularly elusive. Using diffraction techniques, we reveal here the existence of magnetostriction at TN in bulk YNiO3 single crystals. Interestingly, the associated lattice anomalies are much more pronounced along the b crystal axis, which coincides with the electric polarization direction expected from symmetry arguments. This axis undergoes an abrupt contraction below TN that reaches deltab/b ~ - 0.01 %, a value comparable to those found in some magnetoresistive manganites and much larger than those reported for magnetism-driven multiferroics. This observation suggests a strong spin-lattice coupling in these materials, consistent with theoretical predictions. Using the symmetry-adapted distortion mode formalism we identify the main ionic displacements contributing to the lattice anomalies and discuss the most likely polar displacements below TN. Furthermore, our data support symmetric superexchange as the most likely mechanism responsible for the magnetoelastic coupling. These results, that may be common to the full RNiO3 family, provide new experimental evidence supporting the predicted existence of magnetism-driven ferroelectricity in RNiO3 perovskites

cond-mat.str-el↗

arXiv2024

Engineering the Magnetic Transition Temperatures and the Rare Earth Exchange Interaction in Oxide Heterostructures

The properties of functional oxide heterostructures are strongly influenced by the physics governing their interfaces. Modern deposition techniques allow us to accurately engineer the interface physics through the growth of atomically precise heterostructures. This enables minute control over the electronic, magnetic, and structural characteristics. Here, we investigate the magnetic properties of tailor-made superlattices employing the ferromagnetic and insulating double perovskites RE$_2$NiMnO$_6$ (RE = La, Nd), featuring distinct Curie temperatures. Adjusting the superlattice periodicity at the unit cell level allows us to engineer their magnetic phase diagram. Large periodicity superlattices conserve the individual para- to ferromagnetic transitions of the La$_2$NiMnO$_6$ and Nd$_2$NiMnO$_6$ parent compounds. As the superlattice periodicity is reduced, the Curie temperatures of the superlattice constituents converge and, finally, collapse into one single transition for the lowest period samples. This is a consequence of the magnetic order parameter propagating across the superlattice interfaces, as supported by a minimal Landau theory model. Further, we find that the Nd-Ni/Mn exchange interaction can be enhanced by the superlattice interfaces. This leads to a field-induced reversal of the Nd magnetic moments, as confirmed by synchrotron X-ray magnetic circular dichroism measurements and supported by first-principles calculations. Our work demonstrates how superlattice engineering can be employed to fine-tune the magnetic properties in oxide heterostructures and broadens our understanding of magnetic interfacial effects.

cond-mat.mtrl-sci↗

arXiv2023

Combined theoretical and experimental study of the Moiré dislocation network at the SrTiO$_3$-(La,Sr)(Al,Ta)O$_3$ interface

Recently a highly ordered Moiré dislocation lattice was identified at the interface between a \ce{SrTiO3} (STO) thin film and the (LaAlO$_3$)$_{0.3}$(Sr$_2$TaAlO$_6$)$_{0.7}$ (LSAT) substrate. A fundamental understanding of the local ionic and electronic structure around the dislocation cores is crucial to further engineer the properties of these complex multifunctional heterostructures. Here we combine experimental characterization via analytical scanning transmission electron microscopy with results of molecular dynamics and density functional theory calculations to gain insights into the structure and defect chemistry of these dislocation arrays. Our results show that these dislocations lead to undercoordinated Ta/Al cations at the dislocation core, where oxygen vacancies can easily be formed, further facilitated by the presence of cation vacancies. The reduced Ti$^{3+}$ observed experimentally at the dislocations by electron energy-loss spectroscopy are a consequence of both the structure of the dislocation itself, as well as of the electron-doping due to oxygen vacancy formation. Finally, the experimentally observed Ti diffusion into LSAT around the dislocation core occurs only together with cation-vacancy formation in LSAT or Ta diffusion into STO.

cond-mat.mtrl-sci↗

arXiv2022

Defeating depolarizing fields with artificial flux closure in ultrathin ferroelectrics

Material surfaces encompass structural and chemical discontinuities that often lead to the loss of the property of interest in the so-called dead layers. It is notably problematic in nanoscale oxide electronics, where the integration of strongly correlated materials into devices is obstructed by the thickness threshold required for the emergence of their functionality. Here, we report the stabilization of ultrathin out-of-plane ferroelectricity in oxide heterostructures through the design of an artificial flux-closure architecture. Inserting an in-plane polarized ferroelectric epitaxial buffer provides continuity of polarization at the interface, and despite its insulating nature we observe the emergence of polarization in our out-of-plane-polarized model ferroelectric BaTiO$_{3}$ from the very first unit cell. In BiFeO$_{3}$, the flux-closure approach stabilizes a conceptually novel 251$^{\circ}$ domain wall. Its unusual chirality is likely associated with the ferroelectric analog to the Dzyaloshinskii-Moriya interaction. We thus see that in an adaptively engineered geometry, the depolarizing-field-screening properties of an insulator can even surpass those of a metal and be a source of new functionalities. This should be a useful insight on the road towards the next generation of ferroelectric-based oxide electronics.

cond-mat.mtrl-sci↗

arXiv2022

Combined electrostatic and strain engineering of BiFeO$_3$ thin films at the morphotropic phase boundary

Multiferroic BiFeO$_3$ (BFO) possesses a rich phase diagram that allows strain tuning of its properties in thin-film form. In particular, at large compressive strain, a supertetragonal (T) phase with giant polarization is stabilized over the more common rhombohedral (R) structure. To utilize the functionality of such metastable BFO phases in device applications, it is essential to understand the ferroelectric phase evolution upon insertion in nanoscale heterostructures. Here, we explore the emergence of ferroelectric phases close to the morphotropic phase boundary in compressively strained BFO during thin-film growth using in-situ optical second harmonic generation. We find that the epitaxial films form at the growth temperature in the ideal T phase without critical thickness for the polarization. Signatures of T-like and R-like monoclinically distorted phases only appear upon sample cooling. We furthermore demonstrate a robustness of single-domain polarization in the high-temperature T phase during the growth of capacitor-like metal|ferroelectric|metal heterostructures. Here, a reduction in tetragonality of the T phase, rather than domain formation, lowers the electrostatic energy. At this lower tetragonality, density-functional calculations and scanning transmission electron microscopy point to the stabilization of a new metastable R-like monoclinic structure upon cooling the heterostructure down to room temperature. Our results thus show that the combination of strain and electrostatic phase stabilization in BFO heterostructures yields a prominent platform for exploring ferroelectric phases and realizing ultrathin ferroelectric devices.

cond-mat.mtrl-sci↗

arXiv2018

Domain Wall Motion and Interfacial Dzyaloshinskii-Moriya Interactions in Pt/Co/Ir$(t_\mathrm{Ir})$/Ta Multilayers

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is important for chiral domain walls (DWs) and for stabilizing magnetic skyrmions. We study the effects of introducing increasing thicknesses of Ir, from zero to 2 nm, into a Pt/Co/Ta multilayer between the Co and Ta. We observe a marked increase in magnetic moment, due to the suppression of the dead layer at the interface with Ta, but the perpendicular anisotropy is hardly affected. All samples show a universal scaling of the field-driven domain wall velocity across the creep and depinning regimes. Asymmetric bubble expansion shows that DWs in all of the samples have the left-handed Néel form. The value of in-plane field at which the creep velocity shows a minimum drops markedly on the introduction of Ir, as does the frequency shift of the Stokes and anti-Stokes peaks in Brillouin light scattering measurements. Despite this qualitative similarity, there are quantitative differences in the DMI strength given by the two measurements, with BLS often returning higher values. Many features in bubble expansion velocity curves do not fit simple models commonly used to date, namely a lack of symmetry about the velocity minimum and no difference in velocities at high in-plane field. These features are explained by the use of a model in which the depinning field is allowed to vary with in-plane field in a way determined from micromagnetic simulations. This theory shows that velocity minimum underestimates the DMI field, consistent with BLS returning higher values. Our results suggest that the DMI at an Ir/Co interface has the same sign as the DMI at a Pt/Co interface.

cond-mat.mes-hall↗

arXiv2017

Domain wall architecture in tetragonal ferroelectric thin films

Domain walls in ferroelectrics exhibit a plethora of phases and functionalities not found in the bulk. The interplay of electrostatic, chemical, topological, and distortive inhomogeneities at the walls can be so complex, however, that this obstructs their technological performance. In tetragonal ferroelectrics like PbZrxTi1-xO3, for example, the desired functional 180° domain walls within out-of-plane-polarized c-domains are interspersed by in-plane-polarized a-domains and the associated network of domain walls remains challenging to analyze. Here we use a combination of STEM and optical second harmonic generation (SHG) to determine the relation between strain, film thickness, local electric fields and the resulting domain and domain-wall structures across the entire thickness of a set of PZT films. We quantify the distribution of a-domains in the c-domain matrix of the films. Using locally applied electric fields we control the a/c distribution and induce the technologically preferable 180° domain walls. We find that these voltage induced walls are tilted and exhibit a mixed Ising-Néel type transverse rotation of polarization across the wall with a specific nonlinear optical response.

cond-mat.mes-hall↗

arXiv2016

Heavy hole states in Germanium hut wires

Hole spins have gained considerable interest in the past few years due to their potential for fast electrically controlled qubits. Here, we study holes confined in Ge hut wires, a so far unexplored type of nanostructure. Low temperature magnetotransport measurements reveal a large anisotropy between the in-plane and out-of-plane g-factors of up to 18. Numerical simulations verify that this large anisotropy originates from a confined wave function which is of heavy hole character. A light hole admixture of less than 1% is estimated for the states of lowest energy, leading to a surprisingly large reduction of the out-of-plane g-factors. However, this tiny light hole contribution does not influence the spin lifetimes, which are expected to be very long, even in non isotopically purified samples.

cond-mat.mes-hall↗