AC transport data query (using a PPMS system)

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Discussion Overview

The discussion centers around the interpretation of AC transport measurements of a semi-conducting sample using a PPMS system, specifically addressing the unexpected negative resistivity values observed in the data. The scope includes technical explanations and potential issues related to data interpretation in experimental physics.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Eve Wildman reports negative resistivity values when measuring a semi-conducting sample in AC transport mode, raising concerns about the validity of the data.
  • Some participants propose that the negative values could be related to the reactive part of impedance, suggesting a need for conversion to the real resistive part.
  • Another participant emphasizes that AC transport measurements should yield an impedance vector, consisting of real and imaginary components, which can be analyzed using equivalent circuit models.
  • Eve clarifies that the PPMS system used does not provide the option to separate the real and imaginary parts of the impedance, complicating the interpretation of the negative resistivity values.
  • A later reply suggests that if the absolute resistivity values appear reasonable, the issue might stem from a software bug.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of the negative resistivity values, with some suggesting it may be an artifact of the measurement system or data processing, while others highlight the importance of impedance analysis. The discussion remains unresolved regarding the cause of the negative values.

Contextual Notes

There are limitations regarding the assumptions made about the measurement system's capabilities, particularly the absence of options for separating impedance components. The discussion also reflects uncertainty about the reliability of the data due to the observed negative resistivity values.

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.