The theory of the "0.7 anomaly" in quantum point contacts

The phenomenology of the "0.7 anomaly" in quantum point contacts is fully explained in terms of a quasi-localized state, which forms as the point contact opens up. Detailed numerical calculations within spin-density functional theory indeed con rm the emergence of such a state. Quantitative calculations of the conductance and the noise are obtained using a model based on these observations, and are in excellent agreement with existing experimental observations.Comment: A summary paper, to be included in JPCM Special Issue on 0.7 Featur

Ultrasmall volume Plasmons - yet with complete retardation effects

Nano particle-plasmons are attributed to quasi-static oscillation with no wave propagation due to their subwavelength size. However, when located within a band-gap medium (even in air if the particle is small enough), the particle interfaces are acting as wave-mirrors, incurring small negative retardation. The latter when compensated by a respective (short) propagation within the particle substantiates a full-fledged resonator based on constructive interference. This unusual wave interference in the deep subwavelength regime (modal-volume<0.001lambda^3) significantly enhances the Q-factor, e.g. 50 compared to the quasi-static limit of 5.5.Comment: 16 pages, 6 figure

Evidence for localization and 0.7 anomaly in hole quantum point contacts

Quantum point contacts implemented in p-type GaAs/AlGaAs heterostructures are investigated by low-temperature electrical conductance spectroscopy measurements. Besides one-dimensional conductance quantization in units of $2e^{2}/h$ a pronounced extra plateau is found at about $0.7(2e^{2}/h)$ which possesses the characteristic properties of the so-called "0.7 anomaly" known from experiments with n-type samples. The evolution of the 0.7 plateau in high perpendicular magnetic field reveals the existence of a quasi-localized state and supports the explanation of the 0.7 anomaly based on self-consistent charge localization. These observations are robust when lateral electrical fields are applied which shift the relative position of the electron wavefunction in the quantum point contact, testifying to the intrinsic nature of the underlying physics.Comment: 4.2 pages, 3 figure

Israel's ethical code for public libraries: liberal democratic values for a non-western multicultural society?

In 1996 a code of ethics for public libraries was formulated by an ad hoc Committee of the Israeli Council of Public Libraries and endorsed by the Israeli Librarians’ Association. As stated by the chair of the committee, the main motivation was to improve the status of librarianship in Israel, for if it was to be taken seriously, it needed a code of ethics just like the medical or legal professions. The process of formulating the code involved little input from practicing public or school librarians, parents, educators or readers, and relied mainly on the analysis of existing codes in other countries. The specific values held by different segments of Israeli society - notably orthodox Jews, Muslims or immigrants from the former USSR – were largely ignored. The code is loosely based on the American Libraries Association’s Code of Ethics, emphasizing commitment to professional standards, freedom from censorship, equality and confidentiality of users’ records. Since its official introduction, the code has made practically no impact on the Israeli library scene. Not all of the academic training programs for librarians in Israel teach about information or library ethics, and those that do devote little if any attention to it. Awareness among readers of their rights as users is low, and the code is rarely mentioned in public debate about the type or quality of services that should be provided by public libraries in Israel, or about the level of public funding. There are no standing committees that can be consulted by readers or librarians about its implementation; and it hasn’t been updated at a time of rapid change in library service. Why has the code failed to become integrated into everyday life? Its critics consider it an attempt to impose western liberal democratic values on a non-western multicultural society. A survey of school librarians and principals showed that censorship is considered positively by many of them, mainly in religious schools. In Israel ethics is all too often considered a luxury rather than a necessity. A survey of library directors in Israel showed that technical competence is more highly valued than awareness of ethical issues when recruiting librarians. Perhaps the main questions that should be addressed are: Do we need an ethical code? What do we expect to achieve with it? Can having an ethical code prevent us in certain cases from dealing with the main issues hampering the quality of library services? What are the lessons to be learned about the process of introducing an ethical code for information services? Can we make everyone a stakeholder by soliciting broad participation in the deliberations leading up to the formulation of a code

The transformative potential of using participatory community sport initiatives to promote social cohesion in divided community contexts

Sports are popularly believed to have positive integrative functions and are thought, therefore, to be able to galvanise different, and sometimes divided communities through a shared sporting interest. UK government and policy rhetoric over the last two decades has consistently emphasised the positive role sport can play in building more cohesive, empowered and active communities. These positive impacts are particularly important for communities with high numbers of young people from disadvantaged backgrounds. The purpose of this article is to reflect on the challenges associated with co-producing a participatory community sport initiative with 28 young people, the aim of which was to work towards greater social cohesion in an ethnically segregated borough in north-west England. Although a great deal was learned from working towards this, the initiative was ultimately unsuccessful because, for a variety of reasons, the young people removed themselves from the process. A major contribution of this article is its consideration of how we reflect on the realities of project failure and how future community sport initiatives might have greater success. In particular, we argue that for sport to make a difference, participants must be enabled to develop a sense of civic engagement and critical awareness which go beyond either sport or community development, emphasising wider sociopolitical development instead

Origins of conductance anomalies in a p-type GaAs quantum point contact

Low temperature transport measurements on a p-GaAs quantum point contact are presented which reveal the presence of a conductance anomaly that is markedly different from the conventional `0.7 anomaly'. A lateral shift by asymmetric gating of the conducting channel is utilized to identify and separate different conductance anomalies of local and generic origins experimentally. While the more generic 0.7 anomaly is not directly affected by changing the gate configuration, a model is proposed which attributes the additional conductance features to a gate-dependent coupling of the propagating states to localized states emerging due to a nearby potential imperfection. Finite bias conductivity measurements reveal the interplay between the two anomalies consistently with a two-impurity Kondo model

Kondo effect in complex mesoscopic structures

We study the Kondo effect of a quantum dot placed in a complex mesoscopic structure. Assuming that electronic interactions are taking place solely on the dot, and focusing on the infinite Hubbard interaction limit, we use a decoupling scheme to obtain an explicit analytic approximate expression for the dot Green function, which fulfills certain Fermi-liquid relations at zero temperature. The details of the complex structure enter into this expression only via the self-energy for the non-interacting case. The effectiveness of the expression is demonstrated for the single impurity Anderson model and for the T-shaped network.Comment: 12 pages 6 figure

Orbital Magnetism and Current Distribution of Two-Dimensional Electrons under Confining Potential

The spatial distribution of electric current under magnetic field and the resultant orbital magnetism have been studied for two-dimensional electrons under a harmonic confining potential V(\vecvar{r})=m \omega_0^2 r^2/2 in various regimes of temperature and magnetic field, and the microscopic conditions for the validity of Landau diamagnetism are clarified. Under a weak magnetic field (\omega_c\lsim\omega_0, \omega_c being a cyclotron frequency) and at low temperature (T\lsim\hbar\omega_0), where the orbital magnetic moment fluctuates as a function of the field, the currents are irregularly distributed paramagnetically or diamagnetically inside the bulk region. As the temperature is raised under such a weak field, however, the currents in the bulk region are immediately reduced and finally there only remains the diamagnetic current flowing along the edge. At the same time, the usual Landau diamagnetism results for the total magnetic moment. The origin of this dramatic temperature dependence is seen to be in the multiple reflection of electron waves by the boundary confining potential, which becomes important once the coherence length of electrons gets longer than the system length. Under a stronger field (\omega_c\gsim\omega_0), on the other hand, the currents in the bulk region cause de Haas-van Alphen effect at low temperature as T\lsim\hbar\omega_c. As the temperature gets higher (T\gsim\hbar\omega_c) under such a strong field, the bulk currents are reduced and the Landau diamagnetism by the edge current is recovered.Comment: 15 pages, 11 figure

Effect of Quantum Confinement on Electron Tunneling through a Quantum Dot

Employing the Anderson impurity model, we study tunneling properties through an ideal quantum dot near the conductance minima. Considering the Coulomb blockade and the quantum confinement on an equal footing, we have obtained current contributions from various types of tunneling processes; inelastic cotunneling, elastic cotunneling, and resonant tunneling of thermally activated electrons. We have found that the inelastic cotunneling is suppressed in the quantum confinement limit, and thus the conductance near its minima is determined by the elastic cotunneling at low temperature ($k_BT \ll \Gamma$, $\Gamma$: dot-reservoir coupling constant), or by the resonant tunneling of single electrons at high temperature ($k_BT \gg \Gamma$).Comment: 11 pages Revtex, 2 Postscript figures, To appear in Phys.Rev.

1/f noise in a dilute GaAs two-dimensional hole system in the insulating phase

We have measured the resistance and the 1/f resistance noise of a two-dimensional low density hole system in a high mobility GaAs quantum well at low temperature. At densities lower than the metal-insulator transition one, the temperature dependence of the resistance is either power-like or simply activated. The noise decreases when the temperature or the density increase. These results contradict the standard description of independent particles in the strong localization regime. On the contrary, they agree with the percolation picture suggested by higher density results. The physical nature of the system could be a mixture of a conducting and an insulating phase. We compare our results with those of composite thin films.Comment: 4 pages, 3 figures; to appear in Physica E (EP2DS-16 proceedings