Strain Unlocks 100-Fold Increase in Kondo Scattering at Low Temperatures

Researchers investigating heavy-fermion metals have long sought to understand the intricacies of Kondo entanglement and criticality. Soumendra Nath Panja from Experimental Physics VI, Center for Electronic Correlations and Magnetism, University of Augsburg, Jacques G. Pontanel, and Julian Kaiser, working with colleagues at the University of Augsburg and Philipp Gegenwart, present compelling evidence of strain-dependent Kondo scattering in the tetragonal Kondo lattice YbRh₂Si₂. Their study utilises symmetry-decomposed elastoresistance measurements, combining longitudinal and transverse responses under uniaxial strain, to reveal the contribution of Kondo scattering to the strain dependence of magnetic entropy. This innovative approach, probing the and symmetry channels, demonstrates that elastoresistance acts as a sensitive probe of strain-controlled criticality as temperatures approach 2 K, offering new insights into the behaviour of correlated electron systems.

Scientists have uncovered a novel way to probe the exotic behaviour of heavy-fermion metals, materials central to understanding correlated electron systems and the emergence of quantum phenomena. This work details the first symmetry-resolved elastoresistance measurements performed on Ytterbium Rhodium Disilicide (YbRh₂Si₂), a prototypical heavy-fermion metal positioned near a critical point where magnetism and quantum effects intertwine. By meticulously analysing how the material’s electrical resistance changes under precisely applied strain, researchers have revealed fingerprints of strain-dependent Kondo scattering, a key process governing the material’s unusual electronic properties. The study demonstrates that subtle changes in lattice strain directly influence the behaviour of electrons within the material, offering a new avenue for controlling and understanding these complex systems. The research hinges on a technique called elastoresistance, which quantifies the change in electrical resistance in response to applied strain. Crucially, the team decomposed this elastoresistance into its constituent symmetry components, isolating how strain affects different electronic pathways within the material. Measurements along specific crystallographic directions revealed that the response is overwhelmingly dominated by an isotropic channel, indicating that uniform strain effectively tunes the strength of the Kondo effect, the interaction between localized magnetic moments and conduction electrons. YbRh₂Si₂ is particularly interesting because it exists close to a quantum critical point, a state where tiny changes in external parameters can dramatically alter the material’s properties. The team’s experiments, conducted at temperatures down to 2 Kelvin, show that the elastoresistance exhibits characteristic sign changes and reaches remarkably large values at low temperatures. Scaling analysis, combined with comparisons to linear thermal expansion measurements, confirms that the observed elastoresistance directly reflects the contribution of Kondo scattering to the strain dependence of magnetic entropy. This discovery establishes elastoresistance as a powerful tool for investigating the intricate interplay between strain, magnetism, and quantum criticality in heavy-fermion materials. This work not only provides fundamental insights into the behaviour of correlated electron systems but also opens up possibilities for manipulating these materials with unprecedented precision. The ability to control the Kondo effect through strain could pave the way for designing novel electronic devices and exploring new states of matter with tailored properties. Further research will focus on extending these techniques to other heavy-fermion compounds and investigating the potential for inducing and controlling quantum phase transitions via strain engineering. A commercial strain cell from Razorbill Instruments, model CS-100, underpinned the elastoresistance measurements performed on YbRh₂Si₃ (YRS) single crystals. This apparatus facilitated precise control over sample deformation via piezoelectric stacks, allowing for the application of uniaxial strain along specific crystallographic directions. Plate-like YRS crystals, typically measuring 2.0 × 0.2 to 0.6 × 0.05 mm³, were carefully oriented with edges aligned to either the tetragonal in-plane or direction before being bonded to the strain cell using Stycast 2850FT epoxy, establishing an effective strained length of approximately 0.8 to 1mm. The applied strain was continuously monitored using a built-in capacitive displacement sensor, ensuring accurate quantification of sample deformation. Electrical resistance measurements were conducted using a Quantum Design PPMS system, a platform for precise temperature and magnetic field control, employing a modified P450 probe for thermal anchoring. Eight-contact configurations, detailed in supplementary Figure S1, enabled simultaneous measurement of both longitudinal and transverse electrical resistance. Isothermal sweeps of the piezo-voltage were performed, simultaneously recording data from the strain sensor and the electrical resistance, allowing for the precise determination of the strain derivative of the relative resistance change, d(∆R/R₀)/dε. This approach isolated the intrinsic strain dependence of resistivity, separating it from geometric contributions arising from Poisson’s ratio, which typically ranges from 0.25 to 0.35. Symmetry-resolved elastoresistance measurements on tetragonal YbRh₂Si₂ reveal a dominant response in the A1g channel, indicating in-plane strain effectively tunes Kondo hybridization. Longitudinal strain-induced resistance changes, expressed as ∆R/R₀, demonstrate linear behaviour with applied strain up to ±1.3 × 10⁻³ along both the ‘a’ and ‘b’ in-plane crystalline directions. This linearity, observed across all temperatures, confirms fully elastic deformation within the studied strain range. Calculations of the strain derivative of the relative resistance change, d(∆R/R₀)/dε, quantify the material’s sensitivity to mechanical stress. The research establishes that the elastoresistive response is entirely governed by the isotropic A1g symmetry channel, with no detectable contribution from symmetry-breaking channels. This absence of signal in the B1g and B2g channels strongly suggests the lack of nematic fluctuations within the system. Measurements performed at various temperatures reveal a pronounced temperature dependence of the A1g elastoresistance, aligning with theoretical predictions of a sign change in elastoresistance near a maximum in the ambient resistivity curve, and a subsequent decrease to enhanced negative values upon further cooling. Scientists meticulously probing the subtle interplay of electrons in heavy-fermion materials have uncovered a new way to map the critical behaviour of these exotic metals. For years, pinpointing the quantum critical point, where materials transition between distinct states, has been hampered by the difficulty of isolating the relevant signals. The team’s innovation lies in using elastoresistance, measuring how the material’s electrical resistance changes under strain, and then carefully dissecting that response according to its symmetry. By applying stress in different directions, they’ve been able to filter out noise and focus on the contribution from Kondo scattering. The resulting data reveal a strong link between strain, temperature, and the material’s magnetic entropy, suggesting a pathway towards controlling criticality. While the technique offers a powerful new probe, it remains sensitive to the quality of the material and the precision of the strain measurements. Future work will likely focus on applying this method to other heavy-fermion compounds, and exploring whether similar strain-controlled tuning can be used to induce and manipulate novel quantum phases. Ultimately, this research aims to understand fundamental physics and inch closer to materials with tailored electronic properties for future technologies.