arXiv subjects

J. Oppliger

Publications and source records attributed to J. Oppliger.

7 recordsLinked to original sources

arXiv2026

Strain-tunable charge localization coupled to complex magnetic orders in EuAl$_4$

Charge localization is particularly interesting when coupled to antiferromagnetic spin structures. Coupled spin-charge orders are well established in elemental chromium and correlated oxide superconductors, yet the interplay between charge order and more complex magnetic textures -- such as skyrmion lattices and chiral spin structures -- remains largely unexplored. Here we report a comprehensive study of how charge localization couples to the unusually rich sequence of magnetic phases in EuAl$_4$. Using x-ray diffraction under applied magnetic field and uniaxial pressure, we demonstrate a direct coupling between the charge and spin order parameters. In the absence of external stimuli, charge localization is markedly enhanced upon entering the magnetically ordered phases. Strikingly, this effect is highly susceptible to strain: uniaxial pressure applied along the charge-order propagation direction further enhances localization, whereas pressure applied perpendicular to it weakens it. Application of magnetic field reveals both competitive and possible collaborative interactions between spin and charge ordering.This flexible coupling between spin and charge ordering opens a new route to designing symmetry-breaking states. Chiral charge order may for example be patterned from spin structures with that symmetry.

cond-mat.str-el↗

arXiv2026

Autonomous Diffractometry Enabled by Visual Reinforcement Learning

Automation underpins progress across scientific and industrial disciplines. Yet, automating tasks requiring interpretation of abstract visual information remain challenging. For example, crystal alignment strongly relies on humans with the ability to comprehend diffraction patterns. Here we introduce an autonomous system that aligns single crystals without access to crystallography and diffraction theory. Using a model-free reinforcement learning framework, an agent learns to identify and navigate towards high-symmetry orientations directly from Laue diffraction patterns. Despite the absence of human supervision, the agent develops human-like strategies to achieve time-efficient alignment across different crystal symmetry classes. With this, we provide a computational framework for intelligent diffractometers. As such, our approach advances the development of automated experimental workflows in materials science.

cs.LG↗

arXiv2026

Correlated charge order intertwined with time-reversal symmetry-breaking nodal superconductivity in the dual flat band kagome superconductor CeRu$_{3}$Si$_{2}$

Kagome materials provide a powerful platform for exploring how flat electronic bands promote symmetry-breaking quantum states, yet studies have so far focused mainly on kagome-derived $d$-electron flat bands. In this paper, we introduce CeRu$_{3}$Si$_{2}$, a kagome superconductor in which our first-principles calculations show the coexistence of Ru $d$-orbital kagome flat bands and heavy-fermion flat bands derived from Ce$^{4+}$ $4f$-states. X-ray diffraction reveals a dominant 1/2 charge order with a much weaker 1/3 component persisting up to room temperature. Theoretical calculations further highlight the correlated nature of these charge-order states. Deep within the charge-ordered state, magnetoresistance emerges below 80 K and strengthens further below 30 K. Zero-field muon spin-rotation measurements show no time-reversal symmetry (TRS) breaking in the normal state, in contrast to LaRu$_{3}$Si$_{2}$ and YRu$_{3}$Si$_{2}$. However, an applied magnetic field induces weak magnetism. Across the $A$Ru$_{3}$Si$_{2}$ family ($A$ = La, Y, and Ce), the superconducting transition temperature $T_{\rm c}$ scales linearly with the onset temperature of normal-state TRS breaking $T_{\rm {TRSB}}$ and the magnitude of the field-induced magnetic response, revealing a direct positive correlation between normal-state symmetry breaking and superconductivity. Furthermore, we identify that CeRu$_{3}$Si$_{2}$ is the first 132-type kagome compound to host nodal superconductivity together with spontaneous internal magnetic fields, providing clear evidence for intrinsic TRS breaking in the superconducting state. These results establish CeRu$_{3}$Si$_{2}$ as a unique platform where intertwined kagome $d$- and heavy fermion $f$-electron flat bands generate a rich hierarchy of electronic orders.

cond-mat.supr-con↗

arXiv2025

Differentiation of Site-Specific Symmetry Breaking Orders in Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_{6+y}$

Solid matter is classified through symmetry of ordering phenomena. Experimentally, this approach is straightforward, except when distinct orderings occur with identical or almost identical symmetry breaking. Here we show that the cuprate system Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_{6+y}$ hosts two distinct orderings with almost identical translational symmetry breaking. Only when applying site-sensitive resonant elastic x-ray scattering (REXS), charge ordering can be conclusively differentiated from a super-lattice structure. These two orderings occur with almost the same in-plane symmetry but manifest at different atomic sites and display different temperature dependence. Differentiating these orders provides an important clue to the anomalous behavior of PrBa$_2$Cu$_3$O$_7$ within the 123-series of high-temperature superconductors. We conclude that the symmetry breaking at the Pr-site is unfavorable for superconducting pairing.

cond-mat.supr-con↗

arXiv2025

Discovery of High-Temperature Charge Order and Time-Reversal Symmetry-Breaking in the Kagome Superconductor YRu3Si2

The identification of high-temperature unconventional charge order and superconductivity in kagome quantum materials is pivotal for deepening our understanding of geometrically frustrated and correlated electron systems, and for harnessing their exotic properties in future quantum technologies. Here, we report the discovery of a remarkably rich phase diagram in the kagome superconductor YRu$_{3}$Si$_{2}$, uncovered through a unique combination of muon spin rotation (${\mu}$SR), magnetotransport, X-ray diffraction (XRD), and density functional theory (DFT) calculations. Our study reveals the emergence of a charge-ordered state with a propagation vector of (1/2, 0, 0), setting a record onset temperature of 800 K for such an order in a kagome system and for quantum materials more broadly. In addition, we observe time-reversal symmetry (TRS) breaking below $T_{2}^{*}$ ${\simeq}$ 25 K and field-induced magnetism below $T_{1}^{*}$ ${\simeq}$ 90 K, indicating the presence of a hidden magnetic state. These transitions are mirrored in the magnetoresistance data, which show a clear onset at ${\sim}$ $T_{1}^{*}$ and a pronounced increase below ${\sim}$ $T_{2}^{*}$, ultimately reaching a maximum magnetoresistance of 45${\%}$. Band structure calculations identify two van Hove singularities (VHSs) near the Fermi level, one of which resides within a flat band, suggesting a strong interplay between electronic correlations and emergent orders. At low temperatures, we find bulk superconductivity below $T_{\rm c}$ = 3.4 K, characterized by a pairing symmetry with either two isotropic full gaps or an anisotropic nodeless gap. Together, our findings point to a coexistence of high-temperature charge order, tunable magnetism, and multigap superconductivity in YRu$_{3}$Si$_{2}$, positioning it as a compelling platform for exploring correlated kagome physics.

cond-mat.supr-con↗

arXiv2024

Decoupling of Static and Dynamic Charge Correlations revealed by Uniaxial Strain in a Cuprate Superconductor

We use uniaxial strain in combination with ultra-high-resolution Resonant Inelastic X-ray Scattering (RIXS) at the oxygen K- and copper L3-edges to study the excitations stemming from the charge ordering wave vector in La1.875Sr0.125CuO4. By detwinning stripe ordering, we demonstrate that the optical phonon anomalies do not show any stripe anisotropy. The low-energy charge excitations also retain an in-plane four-fold symmetry. As such, we find that both phonon and charge excitations are decoupled entirely from the strength of static charge ordering. The almost isotropic character of charge excitations is indicative of a quantum critical behaviour and remains a possible source for the strange metal properties found in the normal state of cuprate superconductors.

cond-mat.str-el↗

arXiv2024

Discovery of Giant Unit-Cell Super-Structure in the Infinite-Layer Nickelate PrNiO$_2$

Spectacular quantum phenomena such as superconductivity often emerge in flat-band systems where Coulomb interactions overpower electron kinetics. Engineering strategies for flat-band physics is therefore of great importance. Here, using high-energy grazing-incidence x-ray diffraction, we demonstrate how in-situ temperature annealing of the infinite-layer nickelate PrNiO$_2$ induces a giant superlattice structure. The annealing effect has a maximum well above room temperature. By covering a large scattering volume, we show a rare period-six in-plane (bi-axial) symmetry and a period-four symmetry in the out-of-plane direction. This giant unit-cell superstructure likely stems from ordering of diffusive oxygen. The stability of this superlattice structure suggests a connection to an energetically favorable electronic state of matter. As such, our study provides a new pathway - different from Moir\'e structures - to ultra-small Brillouin zone electronics.

cond-mat.supr-con↗