3,875 research outputs found
Sub-micron thin film intrinsic Josephson junctions
No full textWe have fabricated sub-micron intrinsic Josephson junctions in thin films of Tl-Ba-Ca-Cu-O using two differing techniques suited to different applications. By using lateral focussed ion-beam milling we have created arrays of intrinsic junctions in c-axis oriented films. Such arrays, with areas as low as 0.25 mum(2), display large hysteresis comparable to that observed in single-crystal intrinsic junctions. By using normal focussed ion-beam milling we have created arrays in mis-aligned films grown on vicinal substrates. In arrays of area less than 0.4 mum(2) we observe Josephson phase diffusion and a suppressed critical current, showing that charging effects may be significant in these junctions
Intrinsic Josephson pi-junctions for novel devices
No full textThe properties of YBCO grain boundary Josephson junctions have been studied experimentally. Due to the d-wave symmetry of the order parameter of YBCO it is possible to have junctions with a 0-π phase discontinuity along the barrier profile. Such 0-π-junctions can have a ground state where spontaneous flux is generated at the discontinuity point. This work has been focused on understanding if it is possible to fabricate biepitaxial grain boundary 0-π-junctions that, despite the unavoidable faceting along the grain boundary line, can have a ground state with spontaneously generated flux. The interest in the biepitaxial technique lies in its flexibility for circuit design. The microstructure and transport properties of the grain boundary junctions have been studied in details. Scanning SQUID Microscope imaging has revealed the presence of spontaneous flux in our junctions. Finally the dynamics of 0-π-junctions compared to conventional 0-junctions have been studied and compared to simulation
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Novel applications of the Josephson Effect: Ferroelectric Characterisation and Capacitively Shunted Grain Boundary Junctions
No full textThis thesis describes applications of the ac Josephson effect. Firstly, results are presented
from bicrystal grain boundary YBa2Cu3O7-d junctions shunted with a YBa2Cu3O7-d/SrTiO3/Au
multilayer external capacitor, to make a junction with a hysteretic current voltage characteristic
operating at high temperatures. A hysteretic junction with a McCumber parameter of 1.01 at
72.3K, with a critical current of 451mA and a resistance of 0.56W was achieved for a junction
shunted with a 150mm2 external capacitor with a 50nm SrTiO3 dielectric. The measured
capacitance was less than that expected from a calculation of the parallel plate shunt
capacitance. The explanation was thermal noise suppression of the hysteresis and the junction
saw the shunt capacitor as a distributed impedance rather than a lumped circuit element.
It was found during these investigations that the influence of the SrTiO3 substrate on the
intrinsic junction capacitance was poorly understood. The permittivity of SrTiO3 is 24000 at
4.2K. A series of YBa2Cu3O7-d Josephson junctions of lengths from 2mm to 20mm was
patterned on a SrTiO3 bicrystal and the Fiske resonance dispersion relation was measured. The
dispersion relation consisted of two branches, one at low frequencies with a high resonator
capacitance per unit length and a high frequency branch with a low resonator capacitance per
unit length. This was due to the frequency dependence of the permittivity of bulk SrTiO3,
which drops above the soft optic phonon frequency. From the dispersion relation, the
permittivity of bulk SrTiO3 was 750 and the soft optic phonon frequency was 145GHz.
The ac Josephson effect was exploited to measure the permittivity of thin films of
SrTiO3 at microwave frequencies using Josephson junctions coupled to external resonators.
The permittivity of 50nm, 100nm and 200nm SrTiO3 films was frequency independent
between 100GHz and 900GHz and to decrease with film thickness. The permittivity of the
50nm film was 35 and that of the 200nm film was 187 at 4.2K. The permittivity of the 200nm
film was tunable with a dc voltage bias between 245 and 112 at 30K and 116GHz.
The grain boundary capacitance was used to probe grain boundary current transport. The
capacitance per unit area scaled inversely with resistance area product and increased linearly
with critical current density, for undoped and Ca doped YBa2Cu3O7-d grain boundaries on 24°
bicrystals. This behaviour could not be explained by tunneling models of grain boundary
current transport, and requires current flow over a fraction of the area of the grain boundary
Evidence for the Josephson Effect in Quantum Hall Bilayers
No full textThis thesis presents tunneling measurements on bilayer two-dimensional (2D) electrons systems in GaAs/AlGaAs double quantum wells. 2D-2D tunneling is applied here as a probe of the inter-layer correlated quantum Hall state at total Landau level filling factor νT = 1. This bilayer state is theoretically expected to be an excitonic superfluid with an associated dissipationless current and Josephson effect.
In addition to the conventional signatures of the quantum Hall effect ? a pronounced minimum in Rxx and associated quantization of Rxy - the strong inter-layer correlations lead to a step-like discontinuity in the tunneling I ? V. Although reminiscent of the DC Josephson effect, the tunneling discontinuity has a finite extent even at the lowest temperatures (the peak in conductance, dI/dV, is strongly temperature dependent even below 15 mK. The correlations develop when the inter- and intra-layer Coulomb interactions become comparable. The relative importance of which is determined by the ratio of layer separation to average electron spacing. Although this state is theoretically expected to be an excitonic superfluid, the degree to which intra-layer tunneling is Josephson-like is controversial. At a critical layer separation the zero-bias tunneling feature is lost, which we interpret as signaling the quantum phase transition to the uncorrelated state. We study the dependence of the phase transition on electron density and relative density imbalance. In the presence of a parallel magnetic field tunneling probes the response of the spectral function at finite wave vector. These tunneling spectra directly detect the expected linearly dispersing Goldstone mode; our measurement of this mode is in good agreement with theoretical expectations. There remains deep theoretical and experimental interest in this state, which represents a unprecedented convergence in the physics of quantum Hall effects and superconductivity.</p
Quantum dot Josephson junctions in the Kondo regime
No full textIn this thesis, we study quantum dots (QD) fabricated from single-wall carbon nanotubes (SWCNT). The SWCNT are connected to source (S) and drain (D) leads by tunnelling contacts whose transparency is characterized by their respective coupling parameters, S and D. The leads consist of a Ti/Al double layer and become superconducting below the critical temperatureTC ~ 1K. By application of an external magnetic field B, we can suppress superconductivity and drive the leads into the normal state.
We measure the electrical transport through these QD systems at temperatures well below 1K. The differential conductivity G in dependency of a bias voltage VSD and a backgate voltage VBG reflects the successive filling of electron states on the QD, revealing whether the occupation is odd or even. For an odd occupation and normal metal leads, the Kondo effect will dominate transport when the coupling of the QD to the leads, = S + D, is large enough. The associated energy scale is parameterized by the Kondo temperature TK. In the presence of a BCS density of states (DOS) in the leads, a competition sets in between superconductivity and the Kondo effect. Both processes rely on the formation of opposing spin singlet states involving electrons in the leads. For TC > TK, the Kondo effect is suppressed, whereas it persists in the opposite regime, TC < TK. Both regimes have previously been studied in similar samples by electrical transport spectroscopy (PRL 89, 256801 (2002); NJP 9, 124 (2007)).
In chapter III, we report on the discovery of a third regime by studying a QD with highly asymmetrical coupling, S / D ~ 50. With superconducting leads, this asymmetry gives rise to a Kondo effect pinned to one lead only, illustrating the fact that TK dominates over TC at one contact, whereas TC > TK at the other. As a result, we observe a significant enhancement of a particular feature in non-equilibrium transport, the Andreev reflection, in states with this kind of asymmetrical Kondo effect.
In chapter IV, we expand our scope of investigation by comparing Kondo physics to the supercurrent IC that is transported through the QD. We extract the full supercurrent by following a fitting procedure based on the `extended resistively and capacitively shunted junction model' for overdamped Josephson junctions (Nature 439, 953 (2006); Nano Lett. 7, 2441 (2007)). Using VBG to tune T within one odd charge state, we drive the QD from
one regime (TK > TC) to the other (TC > TK). Since IC is very sensitive to the Kondo effect, it acts as a probe for the transition that occurs at the crossing between the two regimes (Nature Nanotech. 1, 53 (2006)). In a second odd charge state with larger , TK is always larger than TC, and consequently no transition is observed.
Chapters V and VI document our efforts to gain more direct control over the Kondo effect by means of topgates (TG). Ideally, such TG would allow us to tune S
and D individually, thus greatly increasing the possibilities for comprehension and manipulation of these complex systems. In chapter VI, we report on a TG dependent Kondo effect in the presence of an effective ferromagnetic field (Science 306, 86 (2004); Nature Physics 4, 373 (2008)). Albeit not based on direct control over S,D, this result has caught our interest and might stimulate further research in the same direction
Teaching Community Organizing and the Practice of Democracy
No full textThis paper calls on political science to develop a thicker and deeper idea of civic engagement, linked to building full participation with community partners who take the lead in designing student civic engagement projects and activities. Based on a current curricular experiment with a two-course sequence at an urban, highly diverse public university, the paper argues for treating community partners as leaders and full partners in the design and implementation of community engaged learning. The courses combine democratic theory, community organizing and leadership development, and partnership between students and a community organization in carrying out research and action projects on education, public safety, and affordable housing.Paper delivered at the American Political Science Association (APSA) Teaching and Learning Conference, Long Beach, California, February 10-12, 2017
Democracy and Higher Education
No full textHigher education has a democracy problem, a democratic deficit. This is not a problem of tone deaf administrators, or of coddled students, or of over-privileged faculty. It is not just a problem of “access”, though that is a problem that must be addressed. It is a problem of underfunding and misplaced priorities and treating higher education as a zero sum game rather than a public good and a civic and political and economic resource for the polity. This is a deep institutional problem, and a deep political problem. The time is ripe to re-think through how to make higher education more democratic. Fortunately, we have many resources in the history of higher education in the US, and the history of American political thought, to bring to bear on how to address the deep institutional democratic deficit in higher education
Language Purism and Gender: Icelandic Trans* Activists and the Icelandic Linguistic Gender Binary
No full textIn this essay, the authors offer the case of Iceland as a “language frontier” for the trans* community, given the nationalistic linguistic context and the deeply gendered nature of the Icelandic language.We begin by briefly outlining the legal situation with respect to trans* identification and the ability to transition. We then outline the conflict over terminology in the context of the Icelandic language and Icelandic national identity. Using empirical interview data, we discuss the difficulty Icelandic poses as a language for trans* identified people, given the deeply gendered nature of the language. We see no easy solution to this complex problem of language, nationalism, and identity.Peer reviewe
Higher Education and the Public Interest
No full textPaper presented at the annual meeting of the Northeastern Political Science Association, Boston, Mass., November 10-12, 2016
Gender Equality and Trans Issues in Iceland
No full textThis paper was prepared for the Institute for Research on Women, Rutgers University, Seminar on Trans Studies, 2012-2013
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