Stress Boosts Superconductivity in Novel Material

Researchers are increasingly focused on understanding how kagome lattice structures influence quantum ground states. P. Král, V. Sazgari, and Yongheng Ge, working with colleagues from the Institute of Physics of the Czech Academy of Sciences, the Max Planck Institute for Chemical Physics of Solids, and the University of Tokyo, have investigated the kagome superconductor LaRu₃Si₂ using in-plane uniaxial stress as a key tuning parameter. This collaborative effort, also involving researchers at the Helmholtz-Zentrum Dresden-Rossendorf, the London Centre for Nanotechnology, Vilnius University, and Zhejiang University, reveals a pronounced anisotropy in both the upper critical field and normal-state magnetoresistance, demonstrating strong electronic anisotropy within this three-dimensional crystal structure. Significantly, the team observed an increase in the superconducting transition temperature and a substantial enhancement of magnetoresistance under applied stress, suggesting a positive correlation between superconductivity and the material’s normal-state electronic and magnetic properties and offering new insights into the complex interplay of these phenomena in kagome superconductors.

Can applying pressure to certain materials boost their ability to conduct electricity with zero resistance. New work demonstrates that squeezing a kagome lattice material alters its electronic behaviour and modestly increases the temperature at which it becomes superconducting. This tuning reveals a link between superconductivity and unusual magnetic properties within the material’s structure.

Scientists are increasingly focused on understanding the complex behaviour of kagome materials, a class of compounds possessing a unique two-dimensional lattice structure resembling traditional Japanese weaving patterns. These materials exhibit strong potential for hosting exotic quantum states, including unconventional superconductivity and correlated electron phenomena.

Recent research has concentrated on LaRu₃Si₂, a kagome superconductor displaying both superconductivity below approximately 7 Kelvin and charge ordering at much higher temperatures, around 400 Kelvin. This unusual combination presents an opportunity to investigate the interaction between these competing states. Manipulating these materials to optimise their properties has proven difficult.

Previous high-pressure studies revealed a dome-shaped superconducting phase diagram for LaRu₃Si₂, suggesting a delicate balance between superconductivity, charge order, and the material’s normal-state electronic behaviour. Researchers have now turned to applying in-plane uniaxial stress as a means of tuning the electronic structure of LaRu₃Si₂ and probing its impact on both superconducting and normal-state characteristics.

By carefully controlling the stress applied to the material, they aimed to modify the arrangement of atoms within the kagome lattice and, as a result, its electronic properties. At the core of this investigation lies the kagome lattice itself, a network of corner-sharing triangles that promotes geometric frustration and can lead to unusual electronic behaviour.

Detailed magnetotransport measurements, combined with first-principles calculations, have revealed a pronounced anisotropy in both the upper critical field and the normal-state magnetoresistance of LaRu₃Si₂, indicating a strong directional dependence of its electronic properties. Applying in-plane stress resulted in a modest increase in the superconducting transition temperature, reaching approximately 0.3 Kelvin at 0.6 GPa, alongside a substantial enhancement of the normal-state magnetoresistance.

Magnetotransport characterisation via controlled uniaxial stress application to LaRu3Si2 crystals

A force-controlled uniaxial stress cell, the FC100 model from Razorbill Instruments, underpinned the magnetotransport experiments performed on single-crystalline LaRu₃Si₂ samples. This apparatus, compatible with a Physical Property Measurement System (PPMS) manufactured by Quantum Design, allowed for the precise application and measurement of force onto the sample, thereby inducing controlled in-plane uniaxial stress.

The LaRu₃Si₂ crystal was carefully mounted between two piezoelectrically actuated plates using Stycast epoxy, ensuring stable contact and efficient stress transmission. Standard four-probe measurements were undertaken to characterise magnetotransport properties under varying stress conditions, with the current perpendicular to both the applied magnetic field and the crystallographic c-axis, while maintaining the stress direction parallel to the electrical current.

Knowing the sample dimensions, 0.9mm width and 0.4mm thickness, the corresponding stress was calculated from the measured force, with a maximum force of 214 N applied, equating to a stress of 0.60 GPa. This precise control over stress is vital for isolating the effects of electronic structure modification. First-principles calculations were completed using the Vienna Ab initio Simulation Package (VASP), a software framework based on density functional theory (DFT).

The generalised gradient approximation (GGA) with the Perdew, Burke, and Ernzerhof (PBE) parameterisation governed the treatment of the exchange-correlation functional. A plane-wave cutoff energy of 380 eV and a 9 × 3 × 7 Monkhorst-Pack k-point mesh were employed to ensure accurate and well-converged calculations of the electronic band structure. To further refine the understanding of transport phenomena, a tight-binding Hamiltonian was constructed, utilising maximally localised Wannier functions (MLWFs) generated via the Wannier90 package.

This Hamiltonian, based on the room-temperature crystal structure, enabled the calculation of magnetoresistance, incorporating quasiparticle mass renormalisation arising from the flat band present in the kagome lattice. The WannierTools package was then used to perform these magnetoresistance calculations, providing a detailed theoretical framework to complement the experimental findings.

Anisotropic superconductivity and stress-induced modifications in LaRu3Si2

At zero stress, the superconducting transition temperature of LaRu3Si2 is approximately 7 K, as determined by resistivity measurements. Detailed magnetotransport measurements reveal a pronounced anisotropy in the upper critical field, μ0Hc2, varying between 4.5 T at 2 K for configurations where the magnetic field lies in the kagome plane, and exceeding 9 T at 2 K when aligned along the crystallographic c-axis.

This difference indicates that superconductivity is dominated by the kagome planes and is primarily limited by orbital effects rather than Pauli limiting. Normal-state magnetoresistance measurements show an onset around 80 K, coinciding with the reported charge-ordering temperature, followed by enhancement below 35 K, suggesting a link to the emergence of a weak magnetic state.

Applying in-plane uniaxial stress up to 0.6 GPa induces measurable changes in the material’s properties. The superconducting transition temperature experiences a modest increase, reaching approximately 0.3 K at 0.6 GPa. Simultaneously, the absolute magnetoresistance exhibits a substantial increase, rising from about 22% at zero stress to 35% at 0.6 GPa.

These concurrent enhancements of both superconductivity and normal-state electronic responses suggest a positive correlation between these phenomena within LaRu3Si2. For the i ⊥μ0H ⊥c configuration, the magnetoresistance is strongly enhanced, reaching nearly 23%, exceeding the previously reported maximum under hydrostatic pressure. Detailed calculations demonstrate that the stress-induced changes in Tc arise from the combined evolution of the total density of states and the flat band present in the kagome lattice.

The large enhancement in magnetoresistance is primarily driven by a downward shift of the Ru kagome flat band under stress. The upper critical field μ0Hc2 is governed by Cooper-pair breaking mechanisms, with orbital pair breaking dominating due to the layered structure and large in-plane coherence lengths. For the i ∥c, μ0H ⊥c and i ∥μ0H ∥c configurations, a moderate magnetoresistance of approximately 8% is observed at 10 K and 9 T, while the i ⊥c, μ0H ∥c configuration shows a slightly higher value of around 10%. These results confirm a strong connection between superconductivity and the normal-state electronic response, highlighting the electronic origin of these intertwined phases.

Applied stress reveals connections between electronic behaviour and superconductivity in lanthanum ruthenium silicate

Researchers applying pressure to a peculiar material have uncovered a subtle but telling link between its superconducting properties and its unusual electronic behaviour. For years, the quest to understand and control superconductivity, the lossless flow of electricity, has been hampered by the complexity of the materials exhibiting this phenomenon. This work, focusing on lanthanum ruthenium silicate, doesn’t deliver a room-temperature superconductor, but it does offer a rare glimpse into the interaction of electronic structure and superconductivity, a connection that has remained frustratingly opaque in many systems.

The significance extends beyond simply adding another data point to the field. Unlike previous investigations, this study demonstrates a clear correlation between applied stress, the enhancement of superconductivity, and changes in the material’s normal-state electronic characteristics. Understanding how these properties are linked is a step towards designing materials with improved superconducting performance.

The observed increase in the superconducting transition temperature is modest, and the precise mechanisms driving the observed changes require further investigation. The challenge lies in bridging the gap between these controlled laboratory experiments and the creation of practical, scalable devices. While applying uniform pressure is achievable in a research setting, replicating this on a larger scale presents considerable engineering hurdles.

This work highlights the potential of tuning materials through external stimuli within the broader effort to unlock high-temperature superconductivity. Beyond lanthanum ruthenium silicate, the techniques employed here could be applied to other complex materials, potentially revealing similar connections and opening new avenues for materials discovery. It reinforces the idea that superconductivity isn’t just about finding the right elements, but about carefully sculpting their electronic environment.