Glass Transition in a Two-Dimensional Electron System in Silicon in a Parallel Magnetic Field

Studies of low-frequency resistance noise show that the glassy freezing of the two-dimensional electron system (2DES) in Si in the vicinity of the metal-insulator transition (MIT) persists in parallel magnetic fields B of up to 9 T. At low B, both the glass transition density $n_g$ and $n_c$, the critical density for the MIT, increase with B such that the width of the metallic glass phase ($n_c<n_s<n_g$) increases with B. At higher B, where the 2DES is spin polarized, $n_c$ and $n_g$ no longer depend on B. Our results demonstrate that charge, as opposed to spin, degrees of freedom are responsible for glassy ordering of the 2DES near the MIT.Comment: 4 pages, 5 figure

Gedenken: over de eerste en de tweede geschiedenis

Commemoration: on the first and second history In this article a basic distinction is made between the first and the second history. The “first history” is taken as an example in which to indicate history as we experience it from within, on the basis of historical interest. The “second history” would then be history as academic professionals construct it on the basis of historical and critical research of the facts. The question that arises is: How does the historical interest that is typical of the first, lived and experienced history, relate to the historical-critical investigation of the facts that have become determinative for the second, i.e. constructed history? In the following sections I first and foremost pay attention to the human ability to commemorate; commemoration appears to be a specific expression of historical interest. Next, I make note of the difference between significant and small historical narratives and I criticise Fukuyama’s view of history. I furthermore analyse the dialogical and critical character of commemoration, describing more precisely this commemoration as an anamnetic experience. I continue by focusing on the romantic misconception of commemoration and on the crucial role of the stranger within the gates. In the following section I contrast the anamnetic and the academic history as an experienced versus a constructive history: is the latter value-free? Finally I come to the conclusion that the second history, as a historical construction, can be seen as an indispensable contribution to commemoration, i.e. to our intimate dealing with the first history

Metal-insulator transition and glassy behavior in two-dimensional electron systems

Studies of low-frequency resistance noise demonstrate that glassy freezing occurs in a two-dimensional electron system in silicon in the vicinity of the metal-insulator transition (MIT). The width of the metallic glass phase, which separates the 2D metal and the (glassy) insulator, depends strongly on disorder, becoming extremely small in high-mobility (low-disorder) samples. The glass transition is manifested by a sudden and dramatic slowing down of the electron dynamics, and by a very abrupt change to the sort of statistics characteristic of complicated multistate systems. In particular, the behavior of the second spectrum, an important fourth-order noise statistic, indicates the presence of long-range correlations between fluctuators in the glassy phase, consistent with the hierarchical picture of glassy dynamics.Comment: Contribution to conference on "Noise as a tool for studying materials" (SPIE), Santa Fe, New Mexico, June 2003; 15 pages, 12 figs. (includes some low-quality figs; send e-mail to get high-quality figs.

Scaling of nano-Schottky-diodes

A generally applicable model is presented to describe the potential barrier shape in ultra small Schottky diodes. It is shown that for diodes smaller than a characteristic length $l_c$ (associated with the semiconductor doping level) the conventional description no longer holds. For such small diodes the Schottky barrier thickness decreases with decreasing diode size. As a consequence, the resistance of the diode is strongly reduced, due to enhanced tunneling. Without the necessity of assuming a reduced (non-bulk) Schottky barrier height, this effect provides an explanation for several experimental observations of enhanced conduction in small Schottky diodes.Comment: 4 pages, 4 figures, accepted for publication in Appl. Phys. Lett., some minor additions and correction

Enhanced tunneling across nanometer-scale metal-semiconductor interfaces

We have measured electrical transport across epitaxial, nanometer-sized metal-semiconductor interfaces by contacting CoSi2-islands grown on Si(111) with an STM-tip. The conductance per unit area was found to increase with decreasing diode area. Indeed, the zero-bias conductance was found to be about 10^4 times larger than expected from downscaling a conventional diode. These observations are explained by a model, which predicts a narrower barrier for small diodes and therefore a greatly increased contribution of tunneling to the electrical transport.Comment: 3 pages, 2 EPS-figures; accepted for publication in Appl. Phys. Let

Interests Informing Dutch Parents' and Teachers' Positioning in their Collaborative Relations

To benefit the schooling and development of a child, the relationship between parents and teachers is considered to be crucial in improving social and academic outcomes. Dutch education policy aims to encourage collaborative parent–teacher relations. However, in practice, this seems to be complex. Evaluations of partnership practices in Dutch education have concluded that more action is needed to develop a collaborative culture. This empirical study aimed to contribute to the existing literature by investigating 1) how conflicting positionings inform parents’ and teachers’ collaborative relations and 2) how the transactional positioning of parents and teachers has an impact on possible conflicts and tensions in interaction. The theoretical framework of ‘transactional positioning’ developed for this research draws upon positioning theory, role theory, and the concepts of agency and transaction. A pragmatic approach was adopted and a mixed methods study designed to provide further insight into the positioning of parents and teachers in their relationship. The sample consisted of 367 parents and 80 teachers sourced from five different urban Dutch schools. A questionnaire and group interviews provided interesting insights into the role conceptions and positioning of parents and teachers. The data revealed how strained relations seem to hinder parent–teacher relations and how parents and teachers seem to be located based on the ambivalent ways in which they position themselves. Individual reflections and assessments guide their positioning rather than mutual coordination of the development of their relationship. The framework of transactional positioning provides a useful tool for further research into positioning practices in educational and other areas. The findings of this research have the potential to inform policy and develop practices in schools. This research contributes to new knowledge on parent–teacher relations by revealing the conflicting interests of parents and teachers and providing insight into the transactional nature of positioning in practice

The controllable pi - SQUID

We have fabricated and studied a new kind of DC SQUID in which the magnitude and sign of the critical current of the individual Josephson junctions can be controlled by additional voltage probes connected to the junctions. We show that the amplitude of the voltage oscillations of the SQUID as a function of the applied magnetic field can be tuned and that the phase of the oscillations can be switched between 0 and $\pi$ in the temperature range of 0.1 - 4.2 K using a suitable control voltage. This is equivalent to the external application of (n+1/2) flux quantum.Comment: 3 Figures, submitted to Applied Physics Letter

Dynamics of spin transport in voltage-biased Josephson junctions

We investigate spin transport in voltage-biased spin-active Josephson junctions. The interplay of spin filtering, spin mixing, and multiple Andreev reflection leads to nonlinear voltage dependence of the dc and ac spin current. We compute the voltage characteristics of the spin current (I_S) for superconductor-ferromagnet-superconductor (SFS) Josephson junctions. The sub-harmonic gap structure of I_S(V) is shown to be sensitive to the degree of spin mixing generated by the ferromagnetic interface, and exhibits a pronounced even-odd effect associated with spin transport during multiple Andreev reflection processes. For strong spin mixing both the magnitude and the direction of the dc spin current can be sensitively controlled by the bias voltage.Comment: 4 pages, 3 figure