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Precision Measurement of the quantized anomalous Hall resistance

Categories:
  • Fundamentals of Metrology
28.11.2017

A recently discovered novel solid state material, so called 'topological insulators' (TI), is electrically conductive only at its surface. Thin layers of ferromagnetic TI exhibit quantization of the anomalous Hall resistance without external magnetic field. For the first time, this has now been measured with an uncertainty in the sub-ppm range.

 

 

 

Topological insulator crystals such as (BIxSB1-x)2Te3 become ferromagnetic by admixture of, e.g., vanadium. The anomalous Hall effect in such a material and its theoretically predicted quantization without an external magnetic field were already demonstrated some time ago, but only with a relative measurement uncertainty of about 1 part in 104.


For electrical quantum metrology, this effect is of special interest as it could in future lead to devices which combine, on a single chip, quantized Hall resistance standards working without magnetic field with superconducting voltage standards which do not tolerate a magnetic field.


Before that happens, however, some challenges must be solved. One the one hand, the material fabrication by molecular beam epitaxy is difficult and mastered by only few groups worldwide. Further, the quality of the crystals is currently still so limited that (contrary to theoretical estimate) extremely low temperatures in the millikelvin range and very small measuring currents of a few nanoamps allow precise measurements. This is the main reason for the up to now rather high measurement uncertainty.


In cooperation with a group of the University of Würzburg, worldwide one of the leading in the field and the only one in Europe which can fabricate such material, the measurement uncertainty of the quantization was now improved to a level of 3 parts in 107. This underpins the expectation that the effect, though still not understood in many details, can in future be exploited for precision quantum metrology.


The results have been submitted for publication and are available on a preprint server [1].

 

 

Hall-bar structure produces a Hall resistance

Figure 1: A Hall-bar structure (rectangle marked "Topgate" in the inserted image) produces a Hall resistance which agrees, at current levels up to 5 nA, with the expected value of the von-Klitzing-constant RK = h e2  within 3 part in 107

 

 

 

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