arXiv subjects

Thomas Lottermoser

Publications and source records attributed to Thomas Lottermoser.

8 recordsLinked to original sources

arXiv2025

Emergent domain topology in the multiferroic hexagonal manganites

Emergent topological phenomena in multiferroic materials arise from the intricate coupling between structural, electric, and magnetic order parameters. Hexagonal manganites provide a paradigmatic platform for such studies. These compounds exhibit a strongly coupled distortive-improper ferroelectric order, arising from trimerizing lattice distortions, and a 120{\deg} noncollinear antiferromagnetic spin structure. While their two-dimensional domain topology has been extensively studied, the full three-dimensional multiferroic domain architecture has remained largely unexplored, mainly due to the experimental challenges of probing bulk structures beyond surfaces. Here, we employ a Landau free-energy framework combined with large-scale phase-field simulations to reveal the intricate three-dimensional multiferroic domain network of hexagonal manganites. We demonstrate that the coupling between the structural and antiferromagnetic order parameters gives rise to a rich variety of three-dimensional topological features. In particular, these features give rise to an attraction between different types of domain walls. Moreover, we identify bifurcations of vortex-like lines at domain-wall intersections, a phenomenon that can exist only in three dimensions and fundamentally alters the topology of the domain network. Our results provide a comprehensive theoretical basis for understanding three-dimensional domain interactions in multiferroics and highlight the essential role of dimensionality in coupling improper ferroelectricity, magnetism, and topology in hexagonal manganites.

cond-mat.mtrl-sci↗

arXiv2025

Coherent phonon control beyond amplitude saturation in a sliding ferroelectric

The breakdown of Hooke's law marks the onset of nonlinear behaviour: when displacements become large, restoring forces weaken and conventional proportionality fails. In quantum materials, intense optical excitation can drive the crystal lattice into a similar regime, where established linear relations between light, electrons, and phonons no longer hold. Sliding ferroelectrics are particularly susceptible, as controlling their polarization requires large interlayer shifts. Displacive excitation of coherent phonons, the principal mechanism for launching structural motion, typically assumes that lattice-driving forces scale linearly with the photo-excited carrier density. Whether this linearity survives at high excitation, however, remains largely unexplored, and its breakdown can fundamentally limit accessible lattice displacements. Here we show that such nonlinear limitations can be surpassed in a sliding ferroelectric by timing, rather than strengthening the optical drive. Time-resolved second-harmonic generation reveals that the interlayer sliding phonon governing ferroelectricity saturates and even diminishes under single-pulse excitation. First-principles calculations attribute this nonlinearity to band-specific electron-phonon coupling that induces competing forces on the lattice. By splitting the optical energy into two well-timed pulses that avoid populating counteracting states, we achieve markedly larger phonon amplitudes at fixed total fluence. The resulting enhanced sliding motion exposes a regime of anharmonic phonon coupling that emerges only far from equilibrium. Our findings show that nonlinear limits in driven solids can be overcome, opening new pathways for steering lattice motion in quantum materials.

physics.optics↗

arXiv2025

Distribution of antiferromagnetic rare-earth domains in multiferroic Dy$_{0.7}$Tb$_{0.3}$FeO$_3$

In many multiferroics, rare-earth and transition-metal orders exist side by side. For analyzing their interaction and its consequences for the multiferroic state, the associated domain patterns and their spatial correlation can give valuable insight. Unfortunately, this is often hampered by the lack of access to the domains of the rare-earth order. Here, we uncover such a domain pattern for the antiferromagnetic and multiferroic Dy$_{0.7}$Tb$_{0.3}$FeO$_3$. Optical second harmonic generation reveals the formation of column-like Dy/Tb domains. Interestingly, the columns form perpendicular to the magnetically induced electric polarization. Hence, the antiferromagnetic rare-earth order forces the ferroelectric domains to form nominally charged head-to-head and tail-to-tail domain walls, thus playing a leading role in the domain formation within the multiferroic phase. In turn, to reduce energy cost, the ferroelectric order causes a reduced rare-earth domain-wall density along the direction of the electric polarization. This interplay highlights the multiferroic character of the Dy$_{0.7}$Tb$_{0.3}$FeO$_3$ domain pattern. We position Dy$_{0.7}$Tb$_{0.3}$FeO$_3$ within the broader landscape of rare-earth multiferroics and identify three distinct scenarios for the role of rare-earth order in these.

cond-mat.mtrl-sci↗

arXiv2024

Dynamic control of ferroic domain patterns by thermal quenching

Controlling the domain structure of ferroic materials is key to manipulating their functionality. Typically, quasi-static electric, magnetic, or strain fields are exploited to transform or pole ferroic domains. In contrast, metallurgy makes use of fast thermal quenches across phase transitions to create new functional states and domain structures. This approach employs the rapid temporal evolution of systems far from equilibrium to overcome the constraints imposed by comparably slow interactions. However, guiding the nonequilibrium evolution of domains towards otherwise inaccessible configurations remains largely unexplored in ferroics. Here, we harness thermal quenches to exert control over a ferroic domain pattern. Cooling at variable speed triggers transitions between two ferroic phases in a rare-earth orthoferrite, with transient domain evolution enabling the selection of the final domain pattern. Specifically, by tuning the quench rate, we can either generate the intrinsic domain structure of the low-temperature phase or transfer the original pattern of the high-temperature phase - creating a hidden metastable domain configuration inaccessible at thermal equilibrium. Real-time imaging during rapid quenching reveals two distinct time scales governing domain evolution: a fast fragmentation phase, followed by a slower relaxation towards a new pattern or back to the original one. This dynamic control of domain configurations, alongside external fields, strain engineering, and all-optical switching, offers a novel approach for actively manipulating ferroic order.

cond-mat.mtrl-sci↗

arXiv2024

Magnetoelectric domain engineering from micrometer to {\AA}ngstr{\o}m scales

The functionality of magnetoelectric multiferroics depends on the formation, size, and coupling of their magnetic and electric domains. Knowing the parameters guiding these criteria is a key effort in the emerging field of magnetoelectric domain engineering. Here we show, using a combination of piezoresponse-force microscopy, non-linear optics, and x-ray scattering, that the correlation length setting the size of the ferroelectric domains in the multiferroic hexagonal manganites can be engineered from the micron range down to a few unit cells under the substitution of Mn$^{3+}$ ions with Al$^{3+}$ ions. The magnetoelectric coupling mechanism between the antiferromagnetic Mn$^{3+}$ order and the distortive-ferroelectric order remains intact even at substantial replacement of Mn$^{3+}$ by Al$^{3+}$. Hence, chemical substitution proves to be an effective tool for domain-size engineering in one of the most studied classes of multiferroics.

cond-mat.mtrl-sci↗

arXiv2023

Effect of the depolarizing field on the domain structure of an improper ferroelectric

We show that, contrary to common belief, the depolarizing electric field generated by bound charges at thin-film surfaces can have a substantial impact on the domain structure of an improper ferroelectric with topological defects. In hexagonal-manganite thin films, we observe in phase-field simulations that through the action of the depolarizing field, (i) the average magnitude of the polarization decreases, (ii) the local magnitude of the polarization decreases with increasing distance from the domain walls, and (iii) there is a significant alteration of the domain-size distribution and average domain size, which is visualized with the pair-correlation function. We conclude that, in general, it is not appropriate to ignore the effects of the depolarizing field for thin film ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2020

Local control of improper ferroelectric domains in YMnO$_3$

Improper ferroelectrics are described by two order parameters: a primary one, driving a transition to long-range distortive, magnetic or otherwise non-electric order, and the electric polarization, which is induced by the primary order parameter as a secondary, complementary effect. Using low-temperature scanning probe microscopy, we show that improper ferroelectric domains in YMnO$_3$ can be locally switched by electric field poling. However, subsequent temperature changes restore the as-grown domain structure as determined by the primary lattice distortion. The backswitching is explained by uncompensated bound charges occuring at the newly written domain walls due to the lack of mobile screening charges at low temperature. Thus, the polarization of improper ferroelectrics is in many ways subject to the same electrostatics as in their proper counterparts, yet complemented by additional functionalities arising from the primary order parameter. Tailoring the complex interplay between primary order parameter, polarization, and electrostatics is therefore likely to result in novel functionalities specific to improper ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2012

First-order multi-k phase transitions and magnetoelectric effects in multiferroic Co3TeO6

A theoretical description of the sequence of magnetic phases in Co3TeO6 is presented. The strongly first-order character of the transition to the commensurate multiferroic ground state, induced by coupled order parameters corresponding to different wavevectors, is related to a large magnetoelastic effect with an exchange energy critically sensitive to the interatomic spacing. The monoclinic magnetic symmetry C2' of the multiferroic phase permits spontaneous polarization and magnetization as well as the linear magnetoelectric effect. The existence of weakly ferromagnetic domains is verified experimentally by second harmonic generation measurements.

cond-mat.str-el↗