arXiv subjects

M. S. Senn

Publications and source records attributed to M. S. Senn.

6 recordsLinked to original sources

arXiv2026

Hidden Magnetic Octupolar Order driven by Spin-Orbit coupling in KFeF$_3$

Phase transitions are typically driven by symmetry-breaking structural distortions that lift electronic degeneracies, yet in some cases, these transitions may be driven by a hidden order without conventional structural signatures. Here, we demonstrate that the 3$d$ perovskite, KFeF$_3$, hosts such a hidden instability. Using a combination of high-resolution powder diffraction, magnetometry, symmetry-based analysis and first principles calculations, we reveal that while the 90 K cubic to rhombohedral transition arises from conventional magnetostriction accompanying antiferromagnetic order, a second transition at 40 K lowers the symmetry to monoclinic without any detectable Jahn-Teller distortion or translational symmetry-breaking. Symmetry-based Landau analysis supports a hidden zone centered magnetic octupole order parameter, whose improper coupling accounts for the weak ferromagnetism and pronounced symmetry-breaking strain. Density functional calculations show that spin-orbit coupling suppresses the competing Jahn-Teller instability, and thereby favors higher rank magnetic multipolar degrees of freedom. These findings establish KFeF$_3$ as a model system in which the competing energy scales between spin-orbit coupling and orbital degeneracy result in the emergence of hidden, multipolar order. We show that the control of such magnetic, multipolar order could, in principle, provide a route to induce altermagnetism.

cond-mat.str-el↗

arXiv2025

Chiral lone-pair helices with handedness coupling to electric-strain fields

Ferrochiral materials with an achiral-to-chiral phase transition and switchable chirality have unique application opportunities, enabling control of the angular momentum of circularly polarized lattice vibrations (chiral phonons) and chirality-related electronic phenomena. Materials that fall into this class are, however, extremely rare, and often accompanied by other types of ferroic order that interfere with the ferrochiral responses. In this work, we demonstrate ferrochirality in two tetragonal tungsten bronzes, K4Bi2Nb10O30 and Rb4Bi2Nb10O30. Using high-resolution X-ray powder diffraction combined with transmission electron microscopy, we solve the incommensurately modulated and chiral structures. Temperature dependent X-ray powder diffraction reveals that both materials undergo an achiral-to-chiral phase transition from P4/mbm to P4212(00γ)q00. The chirality originates from a cooperative helical displacement of Bi3+ atoms perpendicular to the c direction and represents the primary order parameter. As a secondary effect of the ferrochiral order, a spatially varying piezoelectric response is observed, consistent with the polycrystalline nature of the investigated materials. Through invariant analysis, an external electric-strain-field coupling with the piezoelectricity is proposed as a conjugate field for switching chirality, establishing tetragonal tungsten bronzes as a versatile playground for the emergent field of ferrochirality.

cond-mat.mtrl-sci↗

arXiv2025

Goldstone-mediated polar instability in hexagonal barium titanate

We discover a rare structural manifestation of the Goldstone paradigm in a hexagonal polytype of the archetypal ferroelectric BaTiO3. First-principles calculations confirm the Goldstone character of the order parameter, and high-resolution diffraction measurements link this to a quasi-continuous domain texture in the vicinity of the low-temperature phase transitions. Our findings highlight how changes in structural topology may be exploited to realize rich polar topologies in bulk ferroelectric perovskites.

cond-mat.mtrl-sci↗

arXiv2021

Recovery of harmonic-like behaviour of the polar mode in BaTiO$_3$ at high pressure

The local structure of high pressure BaTiO$_3$ has been interrogated by neutron total scattering methods, up to pressures of 4.18 GPa at ambient temperature. Competitive refinements of cubic, tetragonal and rhombohedral distortion modes against pair distribution functions indicate contrasting local structure behaviour of temperature- and pressure-induced cubic BaTiO$_3$. Suppression of the mode amplitude, isotropy of the order parameter direction and loss of sensitivity to correlated Ti displacements at high pressure all suggest a high-pressure local structure that is more consistent with the harmonic approximation, rather than an order-disorder model which better describes high-temperature cubic BaTiO$_3$ in the vicinity of the tetragonal phase transition.

cond-mat.mtrl-sci↗

arXiv2015

Emergence of long-range order in BaTiO3 from local symmetry-breaking distortions

By using a symmetry motivated basis to evaluate local distortions against pair distribution function data (PDF), we show without prior bias, that the off-centre Ti displacements in the archetypal ferroelectric BaTiO3 are zone centred and rhombohedral-like in nature across its known ferroelectric and paraelectric phases. With our newly-gained insight we construct a simple Monte Carlo (MC) model which captures our main experimental findings and demonstrate how the rich crystallographic phase diagram of BaTiO3 emerges from correlations of local symmetry-breaking distortions alone. Our results strongly support the order-disorder picture for these phase transitions, but can also be reconciled with the soft-mode theory of BaTiO3 that is supported by some spectroscopic techniques.

cond-mat.mtrl-sci↗

arXiv2014

Charge density wave fluctuations in La2-xSrxCuO4 and their competition with superconductivity

We report hard (14 keV) x-ray diffraction measurements on three compositions (x=0.11,0.12,0.13) of the high-temperature superconductor La2-xSrxCuO4. All samples show charge-density-wave (CDW) order with onset temperatures in the range 51-80 K and ordering wavevectors close to (0.23,0,0.5). The CDW is strongest with the longest in-plane correlation length near 1/8 doping. On entering the superconducting state the CDW is suppressed, demonstrating the strong competition between the charge order and superconductivity. CDW order coexists with incommensurate magnetic order and wavevectors of the two modulations have the simple relationship $δ_{charge}= 2δ_{spin}$. The intensity of the CDW Bragg peak tracks the intensity of the low-energy (quasi-elastic) spin fluctuations. We present a phase diagram of La2-xSrxCuO4 including the pseudogap phase, CDW and magnetic order.

cond-mat.supr-con↗