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J. M. Williams

Publications and source records attributed to J. M. Williams.

5 recordsLinked to original sources
arXiv2015

Operation of graphene quantum Hall resistance standard in a cryogen-free table-top system

We demonstrate quantum Hall resistance measurements with metrological accuracy in a small cryogen-free system operating at a temperature of around 3.8K and magnetic fields below 5T. Operating this system requires little experimental knowledge or laboratory infrastructure, thereby greatly advancing the proliferation of primary quantum standards for precision electrical metrology. This significant advance in technology has come about as a result of the unique properties of epitaxial graphene on SiC.

cond-mat.mes-hall↗
arXiv2012

Precision comparison of the quantum Hall effect in graphene and gallium arsenide

The half-integer quantum Hall effect in epitaxial graphene is compared with high precision to the well known integer effect in a GaAs/AlGaAs heterostructure. We find no difference between the quantised resistance values within the relative standard uncertainty of our measurement of $8.7\times 10^{-11}$. The result places new tighter limits on any possible correction terms to the simple relation $R_{\rm K}=h/e^2$, and also demonstrates that epitaxial graphene samples are suitable for application as electrical resistance standards of the highest metrological quality. We discuss the characterisation of the graphene sample used in this experiment and present the details of the cryogenic current comparator bridge and associated uncertainty budget.

cond-mat.mes-hall↗
arXiv2011

Graphene, universality of the quantum Hall effect and redefinition of the SI system

The Système Internationale d'unités (SI system) is about to undergo its biggest change in half a century by redefining the units for mass and current in terms of the fundamental constants h and e, respectively. This change crucially relies on the exactness of the relationships which link these constants to measurable quantities. Here we directly compare the integer quantum Hall effect in epitaxial graphene with that in GaAs/AlGaAs heterostructures. We find no difference of the quantized resistance value within the relative standard uncertainty of our measurement of 8.6\times10-11, being the most stringent test of the universality of the quantum Hall effect in terms of material independence.

cond-mat.mes-hall↗
arXiv1999

Low Temperature Penetration depth of κ-(ET)_2Cu[N(CN)_2]Br and κ-(ET)_2Cu(NCS)_2

We present high precision measurements of the penetration depth λof single crystals of κ-(ET)_2Cu[N(CN)2]Br and κ-(ET)_2Cu(NCS)_2 at temperature down to 0.4 K. We find that, at low temperatures, the in-plane penetration depth varies as a fractional power law, λ\sim T^1.5. W hilst this may be taken as evidence for novel bose excitation processes, we show that the data are also consistent with a quasi-linear variation of the superfluid density, as is expected for a d-wave superconductor with impurities. Our data for the interplane penetration depth show similar features and give a direct measurement of the absolute value, λ(0)=100\pm 20 μm.

cond-mat.supr-con↗