Graduate AC transport data query (using a PPMS system)

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The discussion revolves around measuring resistivity in a semi-conducting sample using a PPMS system in AC transport mode, where unexpected negative resistivity values were observed. The user notes that while the data shows a correct trend with frequency, the negative values may arise from the voltage being negative and current positive. A suggestion was made to consider the data as an impedance vector, separating real and imaginary components, to interpret the results accurately. However, the PPMS system used does not support this option, leading to concerns about potential software issues. The user seeks clarification on whether the negative resistivity can be trusted or if it indicates a need for further analysis.
Dr Eve Wildman
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Hello and thanks for looking at this question.

I have a semi-conducting sample which has been run on a PPMS system - measuring it's resistivity as a function of temperature.
I switched to AC transport mode in order to measure the resistivity again while applying frequencies between 1Hz - 1000Hz. The data looks correct when plotted and shows a shift in the insulating transition with increasing frequency at the correct temperature - but it reports negative resistivity values (- ohm.cm) compared to before. I am assuming this isn't right? In the data file - the voltage is negative and current positive therefore giving a negative resistivity value.

To anyone familiar - do you think I can trust this data? Or is the negative value possibly the reactive part of impedance which needs to be converted to the 'real'/resistive part?

Any help from a fellow scientist would be appreciated!

Eve Wildman
 
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When employing AC transport measurements, the data should be provided as an impedance vector as a function of frequency, i.e., a real part (resistance) and an imaginary part (reactance). By this, one can fit the data to some equivalent circuit model in order to interpret the data and to draw conclusions regarding the sample’s behavior. I am not sure, whether the PPMS systems have such an option.
 
Lord Jestocost said:
When employing AC transport measurements, the data should be provided as an impedance vector as a function of frequency, i.e., a real part (resistance) and an imaginary part (reactance). By this, one can fit the data to some equivalent circuit model in order to interpret the data and to draw conclusions regarding the sample’s behavior. I am not sure, whether the PPMS systems have such an option.

Thank you for your reply. I am familiar with impedance spectroscopy where you retrieve the real and imaginary part and can plot it in a complex plane.
The PPMS uses a 4 probe measurement and the measurement I have done does not have this option. The data obtained contains the voltage, current and resistivity of the sample while applying fixed frequencies over a given temperature range. I am concerned as to why the resistivity values are negative! Although the overall trend and values are reasonable.
 
In case the absolute resistivity values look reasonable, there might be a simple software bug.
 

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