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Two lock-in amplifiers based $3ω$ technique: a practical guide for thermal conductivity experiments in bulk samples

The accurate determination of thermal conductivity $κ(T)$ in bulk materials at room temperature and above is crucial for evaluating their compatibility for specific applications. The 3$ω$ technique is an established methodology for studying the thermal conductivity of thin films, becoming particularly suitable in the case of bulk specimens for $T\gtrsim$300K, where standard stationary techniques require significant corrections for radiative losses. Although this method has been employed in several works, it remains not widely adopted because its implementation demands considerable sophistication, including experiment design, thin film deposition techniques, and choices of the geometry of the current/heat transducer, electronics, and analytical treatment of the signals. Based on a critical review of the technique's key technical aspects, this work provides practical support for a rapid and user-friendly implementation, from the design phase through to execution and analysis. We release a Python-based graphical user interface that supports a quantitative estimation of the investigated temperature profiles based on the geometrical parameters (width/length) of the deposited transducer (heater/thermometer metal line) before an experiment, guaranteeing an optimal design of the experimental conditions for each given material under scrutiny.