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Elastic and inelastic scattering effects in conductance measurements at the nanoscale: A theoretical treatise
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. (Department of materials theory)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Elastic and inelastic interactions are studied in tunnel junctions of a superconducting nanoelectromechanical setup and in response to resent experimental superconducting scanning tunneling microscope findings on a paramagnetic molecule. In addition, the electron density of molecular graphene is modeled by a scattering theory approach in very good agreement with experiment. All studies where conducted through the use of model Hamiltonians and a Green function formalism. The nanoelectromechanical system comprise two fixed superconducting leads in-between which a cantilever suspended superconducting island oscillates in an asymmetric fashion with respect to both fixed leads. The Josephson current is found to modulate the island motion which in turn affects the current, such that parameter regions of periodic, quasi periodic and chaotic behavior arise. Our modeled STM setup reproduces the experimentally obtained spin excitations of the paramagnetic molecule and we show a probable cause for the increased uniaxial anisotropy observed when closing the gap distance of tip and substrate. A wider parameter space is also investigated including effects of external magnetic fields, temperature and transverse anisotropy. Molecular graphene turns out to be well described by our adopted scattering theory, producing results that are in good agreement with experiment. Several point like scattering centers are therefore well suited to describe a continuously decaying potential and effects of impurities are easily calculated.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 87 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1282
Keyword [en]
Scattering theory, Scanning tunneling microscopy, tunnel junctions, molecular graphene, paramagnetic molecules, spin interaction, nano electromechanical system, Josephson junction, superconductivity, chaos
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
URN: urn:nbn:se:uu:diva-261609ISBN: 978-91-554-9321-9OAI: diva2:850716
Public defence
2015-10-16, Häggsalen, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Swedish Research Council
Available from: 2015-09-25 Created: 2015-09-02 Last updated: 2015-10-01
List of papers
1. Stability and chaos of a driven nanoelectromechanical Josephson junction
Open this publication in new window or tab >>Stability and chaos of a driven nanoelectromechanical Josephson junction
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 19, 195439- p.Article in journal (Refereed) Published
Abstract [en]

We consider the motion of and Josephson current through a mechanically oscillating superconducting island asymmetrically embedded in a Josephson junction. The electromechanical coupling is provided by distance-dependent tunneling rates between the electrodes and the island. The system asymmetry, resulting from the geometrical configuration, leads, for weak coupling, to an equation of the mechanical motion that reduces to the well-known Duffing equation. At zero bias voltage the island motion is determined by the homogenous Duffing equation that opens up two separate regions of solutions depending on the superconducting phases. The island either moves under influence of an anharmonic single-well potential or is governed by a double-well potential that allows for off-center oscillations. Under applied bias voltage the island equation of motion turns into a modified Duffing equation, with time-dependent coefficients, that demonstrate both quasiperiodic and chaotic behavior.

National Category
Physical Sciences
urn:nbn:se:uu:diva-175613 (URN)10.1103/PhysRevB.85.195439 (DOI)000304173400005 ()
Available from: 2012-06-12 Created: 2012-06-11 Last updated: 2015-10-01Bibliographically approved
2. Spin inelastic electron tunneling spectroscopy on local magnetic moment embedded in Josephson junction
Open this publication in new window or tab >>Spin inelastic electron tunneling spectroscopy on local magnetic moment embedded in Josephson junction
2014 (English)In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 108, no 6, 67009Article in journal (Refereed) Published
Abstract [en]

Recent experimental conductance measurements performed on paramagnetic molecular adsorbates on a superconducting surface, using superconducting scanning tunneling microscopy techniques, are theoretically investigated. For low temperatures, we demonstrate that tunneling current assisted excitations of the local magnetic moment cannot occur for voltage biases smaller than the superconducting gap of the scanning tunneling microscope. The magnetic moment is only excited for voltages corresponding to the sum of the superconducting gap and the spin excitation energies. In excellent agreement with experiment, we show that pumping into higher excitations gives additional current signatures by accumulation of density in the lower ones. Using external magnetic fields, we Zeeman-split the possible degeneracy and thereby resolve all excitations comprised in the magnetic moment.

National Category
Physical Sciences
urn:nbn:se:uu:diva-245544 (URN)10.1209/0295-5075/108/67009 (DOI)000348204400021 ()
Available from: 2015-03-09 Created: 2015-02-26 Last updated: 2015-10-01Bibliographically approved
3. Theory of spin inelastic tunneling spectroscopy for superconductor-superconductor and superconductor-metal junctions
Open this publication in new window or tab >>Theory of spin inelastic tunneling spectroscopy for superconductor-superconductor and superconductor-metal junctions
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 20, 205438Article in journal (Refereed) Published
Abstract [en]

We address the tunneling conductance and spin inelastic tunneling spectroscopy of localized paramagnetic moments in a superconducting environment, pertaining to recent measurements on Fe-octaethylporphyrin-chloride using superconducting scanning tunneling microscopy. We demonstrate that the Cooper pair correlations in the tip and substrate generate a finite uniaxial anisotropy field acting on the local spin moment, and we argue that this field may be a source for the observed changes in the conductance spectrum for decreasing distance between the scanning tunneling tip and the local magnetic moment. We make a side-by-side comparison between the superconductor-superconductor junction and normal-metal-superconductor junction, and find qualitative agreement between the two setups while quantitative differences become explicit. When simulating the effects of electron pumping, we obtain additional peaks in the conductance spectrum that can be attributed to excitations between higher-energy spin states. The transverse anisotropy field couples basis states of the local spin which opens for transitions between spin states that are otherwise forbidden by conservation of angular momentum. Finally, we explore the influences of temperature, which tend to enable in-gap transitions, and an external magnetic field, which enables deeper studies of the spin excitation spectrum. We especially notice the appearance of a low and high excitation peak on each side of the main coherence peak as an imprint of transitions between the Zeeman split ground states.

National Category
Physical Sciences
urn:nbn:se:uu:diva-256836 (URN)10.1103/PhysRevB.91.205438 (DOI)000355091600020 ()
Available from: 2015-06-26 Created: 2015-06-26 Last updated: 2015-10-01Bibliographically approved
4. Molecular graphene under the eye of scattering theory
Open this publication in new window or tab >>Molecular graphene under the eye of scattering theory
2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 24, 245418- p.Article in journal (Refereed) Published
Abstract [en]

The recent experimental observations of designer Dirac fermions and topological phases in molecular graphene are addressed theoretically. Using scattering theory, we calculate the electronic structure of finite lattices of scattering centers dual to the honeycomb lattice. In good agreement with experimental observations, we obtain a V-shaped electron density of states around the Fermi energy. By varying the lattice parameter we simulate electron and hole doping of the structure, and by adding and removing scattering centers we simulate, respectively, vacancy and impurity defects. Specifically, for the vacancy defect we verify the emergence of a sharp resonance near the Fermi energy for increasing strength of the scattering potential.

National Category
Medical and Health Sciences
urn:nbn:se:uu:diva-215913 (URN)10.1103/PhysRevB.88.245418 (DOI)000328681900001 ()
Available from: 2014-01-20 Created: 2014-01-17 Last updated: 2015-10-01Bibliographically approved

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