arXiv subjects

Karin Cedergren

Publications and source records attributed to Karin Cedergren.

3 recordsLinked to original sources

arXiv2021

Exceptionally accurate large graphene quantum Hall arrays for the new SI

The quantum Hall effect (QHE) is a cornerstone in the new International System of Units (SI), wherein the base units are derived from seven fundamental constants such as Planck's constant h and elementary charge e. Graphene has revolutionized practical resistance metrology by enabling the realization of quantized resistance h/2e^2 = 12.9... kOhm under relaxed experimental conditions. Looking ahead, graphene also has the potential to improve realizations of the electronic kilogram using the Kibble balance, and the quantum Ampere in wide current ranges. However, these prospects require different resistance values than practically achievable in single QHE devices, while also imposing stringent demands on energy dissipation in single QHE devices, ultimately requiring currents almost two orders of magnitude higher than the typical QHE breakdown currents IC ~ 100 uA achievable in graphene. Here we present unprecedented accuracy in the quantization of a record sized quantum Hall array (QHA), demonstrating RK/236 ~ 109 Ohm with 0.2 part-per-billion (nOhm/Ohm) accuracy with IC over 5 mA (~ 1 nOhm/Ohm accuracy for IC = 8.5 mA), using epitaxial graphene on silicon carbide (epigraphene). The array quantization accuracy, comparable to the most precise universality tests of QHE in single Hall bar devices, together with the scalability and reliability of this approach pave the road for superior realizations of three key units in the modern SI: the ohm, the ampere, and the kilogram.

cond-mat.mes-hall↗

arXiv2018

Magnetic-field-induced parity effect in insulating Josephson junction chains

We report the experimental manifestation of even-odd parity effects in the transport characteristics of insulating Josephson junction chains which occur as the superconducting gap is suppressed by applied magnetic fields at millikelvin temperatures. The primary signature is a non-monotonic dependence of the critical voltage, $V_c$, for the onset of charge transport through the chain, with the parity crossover indicated by a maximum of $V_c$ at the parity field $B^*$. We also observe a distinctive change in the transport characteristics across the parity transition, indicative of Cooper-pair dominated transport below $B^*$, giving way to single-electron dominated transport above $B^*$. For fields applied in the plane of the superconducting aluminum films, the parity effect is found to occur at the field, $B^*_{||}$, such that the superconducting gap equals the single-electron charging energy, $Δ(B^*_{||})=E_C$. On the contrary, the parity effect for perpendicularly applied fields can occur at relatively lower fields, $B^*_\perp\simeq 2Φ_0/A_I$, depending only on island area, $A_I$. Our results suggest a novel explanation for the insulating peak observed in disordered superconducting films and one-dimensional strips patterned from such films.

cond-mat.mes-hall↗

arXiv2017

Insulating Josephson-junction chains as pinned Luttinger liquids

Quantum physics in one spatial dimension is remarkably rich, yet even with strong interactions and disorder, surprisingly tractable. This is due to the fact that the low-energy physics of nearly all one-dimensional systems can be cast in terms of the Luttinger liquid, a key concept that parallels that of the Fermi liquid in higher dimensions. Although there have been many theoretical proposals to use linear chains and ladders of Josephson junctions to create novel quantum phases and devices, only modest progress has been made experimentally. One major roadblock has been understanding the role of disorder in such systems. We present experimental results that establish the insulating state of linear chains of sub-micron Josephson junctions as Luttinger liquids pinned by random offset charges, providing a one-dimensional implementation of the Bose glass, strongly validating the quantum many-body theory of one-dimensional disordered systems. The ubiquity of such an electronic glass in Josephson-junction chains has important implications for their proposed use as a fundamental current standard, which is based on synchronisation of coherent tunnelling of flux quanta (quantum phase slips).

cond-mat.supr-con↗