arXiv subjects

Manfred Fiebig

Publications and source records attributed to Manfred Fiebig.

At least 19 recordsLinked to original sources

arXiv2026

Kohn anomaly in a topological phase transition

Topological crystalline insulators extend the concept of topological insulators by hosting surface states protected by crystallographic symmetry. Their topological phase transitions arise from spin-orbit-driven band inversion in the bulk electronic structure, reshaping the low-energy electronic environment and its coupling to lattice excitations. While the electronic aspects of band topology are well established, the corresponding dynamics of lattice and electron-phonon interactions remain largely unexplored. Here, we report a pronounced softening of a low-energy surface phonon mode across the topological phase transition in Pb0.77Sn0.23Se, revealed by temperature-dependent time-domain terahertz spectroscopy. Unlike the well-known phonon softening in ferroelectrics, this effect does not signal a structural instability but instead reflects electronic reconstruction. We attribute the softening to the Kohn anomaly, indicating a strong coupling between lattice vibrations and Dirac-like surface electrons in the topological phase. Consistently, the phonon linewidth deviates from the standard anharmonic temperature dependence, further evidencing enhanced electron-phonon coupling. Our results establish phonon softening as a spectroscopic signature of topological phase transitions and provide a route to distinguish topological and trivial phases.

cond-mat.str-el↗

arXiv2026

Polarizing ultrathin ferroelectric BaTiO3 films through interfacial layer polarization

An important requirement for the integration of ferroelectric thin films into devices is deterministic control of the polarization state in films of only a few unit cells in thickness. Here, we utilize the charged atomic planes of (001)-oriented SmNiO$_3$ (SNO) buffer layers as a polarizing template to stabilize the polarization in ferroelectric BaTiO$_3$ (BTO) model system thin films. We show that an upwards (downwards) oriented polarization is achieved by selection of the [SmO]$^+$ ([NiO$_2$]$^-$) buffer termination. Most importantly, the charged atomic planes of SNO suppress the depolarizing-field-induced critical thickness in BTO, and we record the emergence of a net polarization in our BTO films from the first unit cell deposited. Our experiments, guided by density-functional-theory (DFT) calculations, further highlight the impact of charged defects on the polarizing effectiveness of the SNO buffer. Specifically, oxygen vacancies counteract the polarizing field of the negatively charged, [NiO$_2$]$^-$-terminated surface of the SNO buffer. Our findings provide important insights into the interplay of defect chemistry and polarizing interfaces to stabilize ferroelectric polarization down to the single-unit-cell limit.

cond-mat.mtrl-sci↗

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↗

arXiv2025

Accelerating the standard siren method: Improved constraints on modified gravitational-wave propagation with future data

Gravitational waves (GWs) from compact binary mergers have emerged as one of the most promising probes of cosmology and General Relativity (GR). However, a major challenge in fully exploiting GWs as standard sirens with current and future GW observatories is developing efficient and robust codes capable of analyzing the increasing data volumes that are, and will be, acquired. We present here CHIMERA 2.0, an advanced computational framework for hierarchical Bayesian inference of cosmological, modified gravity, and population hyperparameters using standard sirens and galaxy catalogs. This upgrade introduces novel GPU-accelerated algorithms to estimate the hierarchical likelihood, enabling the analysis of thousands of events - crucial for next-generation experiments - and includes the two-parameter ($\Xi_0-n$) modified GW propagation model. Using CHIMERA 2.0, we forecast cosmological and modified GW propagation constraints for the future LIGO-Virgo-KAGRA O5-like run. We analyze three binary black hole populations of 300 events at SNR>20, each with a different value of $\Xi_0$: 0.6, 1 (corresponding to GR), and 1.8. Multiple analyses were performed each catalog, comprising a population of approximately 5000 events, thanks to CHIMERA 2.0, which is 10-1000 times faster depending on the settings and catalog size. We jointly infer cosmological, modified GW propagation, and population hyperparameters. With spectroscopic galaxy catalogs, the fiducial $\Xi_0$ is recovered with a precision of $22\%$, $7.5\%$, and $10\%$ for $\Xi_0$ = 0.6, 1, and 1.8, respectively; while the precision on $H_0$ is 2-7 times worse than when $\Xi_0$ is not inferred. Finally,in the case of photometric redshifts the constraints degrade on average by 3.5 times in all cases, underscoring the importance of future spectroscopic surveys in maximizing the constraining power of standard sirens.

astro-ph.CO↗

arXiv2025

Magnetoelectric training of multiferroic domains in Mn$_2$GeO$_4$

Magnetoelectric multiferroics promise direct cross-control between coexisting ferroelectric and ferromagnetic orders, which is of interest for applications in magnetism and spintronics. A particularly interesting type of cross-control is found in spin-spiral multiferroic Mn$_2$GeO$_4$, where a ferroelectric multi-domain distribution can be globally inverted by a single magnetic field sweep. In this work we consider the initial domain evolution from zero-field cooling, imaging the evolution of domains under both magnetic and electric fields via optical second harmonic generation. We find that polarization and magnetization domains form independently when entering the multiferroic phase, and a single deterministic initialisation procedure, spanning three quarters of a field cycle, is required to achieve reliable magnetoelectric cross-coupling. This initialisation behaviour originates from a deterministic pathway from metastable to equilibrium domain patterns, in contrast to more common and less reliable domain "training" procedures that require repeated field cycles. Understanding the initial domain evolution thus enables reliable cross-control in magnetoelectric devices with highly interlinked order parameters.

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

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

Discovery of a non-Hermitian phase transition in a bulk condensed-matter system

Phase transitions are fundamental in nature. A small parameter change near a critical point leads to a qualitative change in system properties. Across a regular phase transition, the system remains in thermal equilibrium and, therefore, experiences a change of static properties, like the emergence of a magnetisation upon cooling a ferromagnet below the Curie temperature. When driving a system far from equilibrium, novel, otherwise inaccessible quantum states of matter may arise. Such states are typically non-Hermitian, that is, their dynamics break time-reversal symmetry, a basic law of equilibrium physics. Phase transitions in non-Hermitian systems are of fundamentally new nature in that the dynamical behaviour rather than static properties may undergo a qualitative change at a critical, here called exceptional point. Here we experimentally realize a non-Hermitian phase transition in a bulk condensed-matter system. Optical excitation creates charge carriers in the ferromagnetic semiconductor EuO. In a temperature-dependent interplay with the Hermitian transition to ferromagnetic order, a non-Hermitian change of the relaxation dynamics occurs, manifesting in our time-resolved reflection data as a transition from bi-exponential real to single-exponential complex decay. Our theory models this behavior and predicts non-Hermitian phase transitions for a large class of condensed-matter systems, where they may be exploited to sensitively control bulk-dynamic properties.

cond-mat.str-el↗

arXiv2024

Missing spectral weight in a heavy-fermion system far above N\'eel temperature

The competition between the Kondo spin-screening effect and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in heavy-fermion systems drives the quantum phase transition between the magnetically ordered and the heavy-Fermi-liquid ground states. Despite intensive investigations of heavy quasiparticles on the Kondo-screened side of the quantum phase transition and of their breakdown at the quantum critical point, the magnetically ordering side has not systematically been studied. Using terahertz time-domain spectroscopy, we report a suppression of the Kondo quasiparticle weight in CeCu$_{6-x}$Au$_x$ samples on the antiferromagnetic side of the quantum phase transition at temperatures as much as two orders of magnitude above the N\'{e}el temperature $T_\text{N}$. With our systematic investigations into the high-temperature, paramagnetic region on the antiferromagnetic side of the phase diagram of CeCu$_{6-x}$Au$_x$, i.e., with $x =$ 0.2, 0.3, and 0.5, we show that the suppression results from a quantum frustration effect induced by the temperature-independent RKKY interaction. Hence, our results emphasize that besides critical fluctuations, the RKKY interaction may play an important role in the quantum-critical scenario.

cond-mat.str-el↗

arXiv2024

Magnetoelectric phase control at domain-wall-like epitaxial oxide multilayers

Ferroelectric domain walls are nanoscale objects that can be created, positioned, and erased on demand. They often embody functional properties that are distinct from the surrounding bulk material. Enhanced conductivity, for instance, is observed at charged ferroelectric domain walls. Regrettably, domain walls of this type are scarce because of the energetically unfavorable electrostatics. This hinders the current technological development of domain-wall nanoelectronics. Here we overcome this constraint by creating robust domain-wall-like objects in epitaxial oxide heterostructures. We design charged head-to-head (HH) and tail-to-tail (TT) junctions with two ferroelectric layers (BaTiO$_{3}$ and BiFeO$_{3}$) that have opposing out-of-plane polarization. To test domain-wall-like functionalities, we insert an ultrathin ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ layer into the junctions. The interfacial electron or hole accumulation at the interfaces, set by the HH and TT polarization configurations, respectively, controls the LSMO conductivity and magnetization. We thus propose that trilayers reminiscent of artificial domain walls provide magnetoelectric functionality and may constitute an important building block in the design of oxide-based electronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Magnetoelectric domain engineering from micrometer to Ångstrøm 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↗

arXiv2024

Terahertz crystal electric field transitions in a Kondo-lattice antiferromagnet

Hybridization between the localized f-electrons and the delocalized conduction electrons together with the crystal electric field (CEF) play a determinant role in governing the many-body ground state of a correlated-electron system. Here, we investigate the low-energy CEF states in CeAg_2Ge_2, a prototype Kondo-lattice antiferromagnet where Kondo correlation is found to exist within the antiferromagnetic phase. Using time-domain THz reflection spectroscopy, we show the first direct evidence of two low-energy CEF transitions at 0.6 THz (2.5 meV) and 2.1 THz (8.7 meV). The presence of low-frequency infrared-active phonon modes further manifests as a Fano-modified lineshape of the 2.1 THz CEF conductivity peak. The temporal spectral weights obtained directly from the THz time traces, in addition, corroborate the corresponding CEF temperature scales of the compound.

cond-mat.str-el↗

arXiv2023

Kondo coherence versus superradiance in THz radiation-driven heavy-fermion systems

In strongly correlated systems such as heavy-fermion materials, the coherent superposition of localized and mobile spin states leads to the formation of Kondo resonant states, which on a dense, periodic array of Kondo ions develop lattice coherence. Characteristically, these quantum-coherent superposition states respond to a terahertz (THz) excitation by a delayed THz pulse on the scale of the material's Kondo energy scale and, hence, independent of the pump-light intensity. However, delayed response is also typical for superradiance in an ensemble of excited atoms. In this case, quantum coherence is established by the coupling to an external, electromagnetic mode and, hence, dependent on the pump-light intensity. In the present work, we investigate the physical origin of the delayed pulse, i.e., inherent, correlation-induced versus light-induced coherence, in the prototypical heavy-fermion compound CeCu_5.9Au_0.1. We study the delay, duration and amplitude of the THz pulse at various temperatures in dependence on the electric-field strength of the incident THz excitation, ranging from 0.3 to 15.2 kV/cm. We observe a robust delayed response at approximately 6 ps with an amplitude proportional to the amplitude of the incident THz wave. This is consistent with theoretical expectation for the Kondo-like coherence and thus provides compelling evidence for the dominance of condensed-matter versus optical coherence in the heavy-fermion compound.

cond-mat.str-el↗

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↗

arXiv2023

Current-driven dynamics and ratchet effect of skyrmion bubbles in a ferrimagnetic insulator

Magnetic skyrmions are compact chiral spin textures that exhibit a rich variety of topological phenomena and hold potential for developing high-density memory devices and novel computing schemes driven by spin currents. Here, we demonstrate room temperature interfacial stabilization and current-driven control of skyrmion bubbles in the ferrimagnetic insulator Tm3Fe5O12 (TmIG) coupled to Pt. We track the current-induced motion of individual skyrmion bubbles. The ferrimagnetic order of the crystal together with the interplay of spin-orbit torques and pinning determine the skyrmion dynamics in TmIG and result in a strong skyrmion Hall effect characterized by a negative deflection angle and hopping motion. Further, we show that the velocity and depinning threshold of the skyrmion bubbles can be modified by exchange coupling TmIG to an in-plane magnetized Y3Fe5O12 layer, which distorts the spin texture of the skyrmions and leads to a directional-dependent rectification of their dynamics. This effect, which is equivalent to a magnetic ratchet, is exploited to control the skyrmion flow in a racetrack-like device.

cond-mat.mes-hall↗

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↗