Thermal excitation of large charge offsets in a single-Cooper-pair transistor

Abstract

Charge offsets and two-level fluctuators are common in single-electron transistors with a typical magnitude of |ΔQ|≪0.1e . We present measurements in a 2e -periodic single-Cooper-pair transistor which exhibited hysteretic charge offsets close to 1e . The real-time capture and escape of individual electrons in metastable trapped states was measured at very low temperatures. This enabled the dynamics of the transitions to be investigated in detail, demonstrating thermal excitation to a hysteretic tunneling transition. We show that, allowing for the hysteresis, the metastable states are in thermal equilibrium with each other. The observed temperature dependence and hysteresis can be explained by the coupling of a two-level fluctuator to a quasiparticle trap.

Full text

Page 1

Thermal excitation of
large charge offsets
in a single-Cooper
pair transistor
Royal Holloway
University of London


Luke Simkins, Dave Rees,
Vladimir Antonov, Phil Glasson, Phil Meeson, Mike Lea.
+ Eddy Collin, Grenoble

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Single Electron Transistors
Second angled evaporation Al
Leads overlap the oxidized island
First angled evaporation Al
Oxidization forms AlO2 layer ~1nm thick
Shadow evaporation technique
Sensitive electrometer theoretical charge sensitivity
∆Q=5 x10-7e√Hz, typical ∆Q=5 x10-4e√Hz
∆Q=5 x10-6e√Hz for rf SET
Consists of an island between source and drain
electrodes separated by a tunnel junctions

Ec≡e2/2C∑ , current will always flow unless KbT<Ec
For T=1K C∑~1x10-15 F , junction size 100nm x 100nm
Charge defects from substrate, in the Al (not 100% pure),
oxygen in the junctions, adsorbed gases and Al grains

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Superconducting SETs
Current flow depends on
superconducting energy gap ∆
as well as charging energy Ec
Ej≡ħIc/2e, Ic is the critical current
Ec and Ej determine the dominate
features seen in the response on
the SET
Main superconducting features
are the Josephson quasiparticle
peak and the Cooper pair peak

Page 4

G. Papageorgiou et al, APL 86, 153106 (2005)
SETs for Electrons on Helium
Guard
Guard
SET
SET
Gate
Gate
Courtesy of Dr. Y. Mukharsky and E. Rousseau at CEA at Saclay, France.
E. Rousseau et al, J. Low Temp. Phys. 148, 193 (2007)
EoH trapped by potential profile of pool above
SET. As gate voltage is swept CBOs are seen on
SET, and potential profile of pool changes. EoH
are lost from the pool, phase jumps seen in CBOs

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Electron array devices (RHUL)
5
SET
SET
Guard
Trap
Trap
Clean system
with large 2e
CBOs