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Quantum effects in nanoscale Josephson junction circuits
KTH, School of Engineering Sciences (SCI), Physics.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis presents the results of an experimental study on single-charge effects in nanoscale Josephson junctions and Cooper pair transistors (CPTs).

In nanoscale Josephson junctions the charging energy EC becomes significant at sub-Kelvin temperatures and single-charge effects, such as the Coulomb blockade of Cooper pair tunneling, influence the transport properties. In order to observe charging effects in a single Josephson junction, the impedance of the electromagnetic environment surrounding the junction has to be larger than the quantum resistance (RQ=h/4e2≈6.45kΩ).

In this work the high impedance environment is obtained by biasing the sample under test (single Josephson junction or CPT) with four one-dimensional Josephson junction arrays having SQUID geometry. The advantage of this configuration is the possibility of tuning in situ the effective impedance of the electromagnetic environment. By applying a magnetic field perpendicular to the SQUID loops, the Josephson energy EJ of the SQUIDs is suppressed, resulting in an increase of the measured zero bias resistance of the arrays of several orders of magnitude (104< R0 (Ω) <109). This bias method enables the measurement of the same sample in environments with different impedance.

As the impedance of the environment is increased, the current-voltage characteristics (IVCs) of the single Josephson junction and of the CPT show a well defined Coulomb blockade feature with a region of negative differential resistance, signature of the coherent tunneling of single Cooper pairs.

The measured IVCs of a single Josephson junction with SQUID geometry in the high impedance environment show a qualitative agreement with the Bloch band theory as the EJ/EC ratio of the junction is tuned with the magnetic field. We also studied a single nontunable Josephson junction with strong coupling (EJ/EC > 1), where the exact dual of the overdamped Josephson effect is realized, resulting in a dual shape of the IVC, where the roles of current and voltage are exchanged. Here, we make for the first time a detailed quantitative comparison with a theory which includes the effect of fluctuations due to the finite temperature of the environment.

The measurements on CPTs in the high impedance environment showed that the Coulomb blockade voltage is modulated periodically by the gate-induced charge. The gate-voltage dependence of the CPT changes from e-periodic to 2e-periodic as the impedance of the environment is increased. The high impedance environment reduces quasiparticle tunneling rates, thereby restoring the even parity of the CPT island. This behavior suggests that high impedance leads can be used to effectively suppress quasiparticle poisoning.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , 103 p.
Series
Trita-FYS, ISSN 0280-316X ; 2006:31
Keyword [en]
Josephson junctions, Josephson junction arrays, electromagnetic environment, Cooper pair transistor, parity effects
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-4004ISBN: 91-7178-353-9 (print)OAI: oai:DiVA.org:kth-4004DiVA: diva2:10345
Public defence
2006-06-09, FA32, Albanova University Centrum, Roslagstullsbacken 21, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20100928 Available from: 2006-05-30 Created: 2006-05-30 Last updated: 2010-09-28Bibliographically approved
List of papers
1. Cooper pair transistor in a tunable environment
Open this publication in new window or tab >>Cooper pair transistor in a tunable environment
2005 (English)In: Quantum Computation in Solid State Systems: proceedings of the conference Macroscopic QuantumCoherence and Computing / [ed] B. Ruggiero, P. Delsing, C. Granata, Y. Pashkin, and P. Silvestrini, 2005Conference paper, Published paper (Refereed)
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5832 (URN)
Conference
Macroscopic Quantum Coherence and Computing, Naples 2004
Note

QC 20100928

Available from: 2006-05-30 Created: 2006-05-30 Last updated: 2012-09-26Bibliographically approved
2. Coulomb blockade of Cooper pair tunneling and parity effects in the Cooper pair transistor
Open this publication in new window or tab >>Coulomb blockade of Cooper pair tunneling and parity effects in the Cooper pair transistor
2006 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 22Article in journal (Refereed) Published
Abstract [en]

We have measured the Cooper pair transistor (CPT) in a tunable electromagnetic environment consisting of four one-dimensional superconducting quantum interference device arrays. The transport properties of the CPT in the high impedance limit, Z(env)> R-Q similar or equal to 6.45 k Omega, are studied for different ratios of the Josephson coupling energy to the charging energy. As the impedance of the environment is increased, the current-voltage characteristic (IVC) of the CPT develops a Coulomb blockade of Cooper pair tunneling and the measured IVCs agree qualitatively with a theory based on quasicharge dynamics for a CPT. Increasing the impedance of the environment induces a transition in the modulation of the IVC with the gate charge from e-periodic to 2e-periodic.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-24648 (URN)10.1103/PhysRevB.74.224505 (DOI)000243195600090 ()2-s2.0-33845499080 (Scopus ID)
Note
QC 20100921Available from: 2010-09-21 Created: 2010-09-21 Last updated: 2017-12-12Bibliographically approved
3. Phase-Charge duality of a Josephson junction in a fluctuating electromagnetic environment
Open this publication in new window or tab >>Phase-Charge duality of a Josephson junction in a fluctuating electromagnetic environment
2006 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 97, no 9Article in journal (Refereed) Published
Abstract [en]

We have measured the current-voltage characteristics of a single Josephson junction placed in a high impedance environment. The transfer of Cooper pairs through the junction is governed by overdamped quasicharge dynamics, leading to Coulomb blockade and Bloch oscillations. Exact duality exists to the standard overdamped phase dynamics of a Josephson junction, resulting in a dual shape of the current-voltage characteristic, with current and voltage changing roles. We demonstrate this duality with experiments which allow for a quantitative comparison with a theory that includes the effect of fluctuations due to the finite temperature of the electromagnetic environment.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-5830 (URN)10.1103/PhysRevLett.97.096802 (DOI)000240239000046 ()2-s2.0-33748645370 (Scopus ID)
Note
QC 20100920Available from: 2006-05-30 Created: 2006-05-30 Last updated: 2017-12-14Bibliographically approved
4. Fast Switching Current Detection at low Critical Currents
Open this publication in new window or tab >>Fast Switching Current Detection at low Critical Currents
2005 (English)In: Realizing Controllable Quantum States - MESOSCOPIC SUPERCONDUCTIVITY AND SPINTRONICS, 2005, 255-262 p.Conference paper, Published paper (Refereed)
Abstract [en]

A pulse-and-hold technique is used to measure the switching of small critical current Josephson junctions. This technique allows one to achieve a good binary detection and therefore measure switching probabilities. The technique overcomes limitations on simple square pulses and allows for the measurement of junctions with critical currents of the order of 10nA with bias pulses of the order of 100ns. A correlation analysis of the switching events is performed to show how the switching probability depends on the wait time between repeated bias pulses.

Keyword
tunnel-junctions, circuit, state
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-6278 (URN)000234363700039 ()981-256-468-3 (ISBN)
Conference
3rd International Symposium on Mesoscopic Superconductivity and Spintronics Atsugi City, JAPAN, MAR 01-04, 2004
Note
QC 20100924Available from: 2006-10-23 Created: 2006-10-23 Last updated: 2010-09-28Bibliographically approved

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