A coincidence of errors: Critical exponent for the quantum Hall transition

One of the most studied problems in the last twenty years is the integer quantum Hall effect, which describes the response of electrons confined in two-dimensions to a strong perpendicular magnetic field: The electrons move in circular orbits that fill bands, called Landau levels, and the Hall conductance ( i.e., the conductance perpendicular to an induced current ) of the electrons is quantized.

Like any phase transition, the change of the Hall conductance between quantized values ( as the electrons are moved between Landau levels by an external voltage or magnetic field ) can be described by a critical exponent, ν. Both numerical calculations and experiment agree fairly well about the value of ν.

But as Keith Slevin at Osaka University and Tomi Ohtsuki at Sophia University, both in Japan, argue in a Rapid Communication appearing in Physical Review B, this may be because of a coincidence of errors. Namely, numerical calculations have so far ignored Coulomb interactions between electrons, which should in fact be important, while the experimentally measured value of ν is based on approximations.

Escuela Franco-Venezolana de Nanotecnología 2009 ( ENANO2009 ) del 2 al 6 de noviembre de 2009 en Caracas y Choroní, Venezuela

Contacto

http://www.redvnano.org/congreso/contactanos.html

La Red Venezolana de Nanotecnología está ayudando a organizar la Escuela Franco-Venezolana de Nanotecnología 2009 ( ENANO2009 ) del 2 al 6 de noviembre de 2009 en Caracas y Choroní ( Venezuela ), bajo el patrocinio de:

• FONACIT
• Programa de Cooperación de Postgraduados ( PCP )
• Embajada de Francia en Caracas
• Fundación IDEA
• Academia de Ciencias Físicas, Matemáticas y Naturales
• Universidad Central de Venezuela

Para el primer día del evento ( 2/11/09 ) se ha programado una jornada en la Fundación IDEA ( Caracas, Venezuela ) en la cual varios investigadores presentarán los desarrollos obtenidos fruto de la cooperación franco-venezolana en nanociencia y nanotecnología, gracias al apoyo de las herramientas de cooperación científica entre estos dos países.

El Comité Organizador de la ENANO2009 también ha programado sesiones de cursos, charlas y carteles del 3 al 6 de noviembre de 2009. Esta segunda parte de la Escuela tendrá lugar en Choroní, Edo. Aragua. Las sesiones van dirigidas a estudiantes e investigadores sensibles al desarrollo de estos conocimientos en Venezuela.

Se programarán además mesas de trabajo sobre aspectos asociados al programa de doctorado interinstitucional de nanotecnología que la Red se encuentra promoviendo, la cooperación nacional e internacional en actividades de I+D+i dirigidas a atender los problemas de interés nacional, la definición de mecanismos que faciliten la comunicación entre el sector científico, productivo y la sociedad en general.

Formalismo de Keldysh (Keldysh Technique)

This technique provides the correct evaluation of the quantum mechanical average of an observable. It means that there is not any assumption about the final system state whenever we perform the above mentioned average. As a consequence, it is a suitable tool to study small systems in contact with macroscopic systems which are in "different" thermodynamic equilibrium states.

See the original Keldysh's papers:

• L. V. Keldysh, ZhEFT 47, 1515 (1964)
• L. V. Keldysh, Soviet Physics JEPT 20, 1018 (1965)

A Mesoscopic Mystery

Researchers continue to push rival interpretations of a vexing problem in mesoscopic physics, the size scale where quantum and classical worlds co-exist. The conductance across a narrow bridge between two reservoirs of electrons--a so-called quantum point contact--was expected to follow simple quantum rules, but doesn't. A group publishing in the 9 December print issue of PRL proposes a model of automatic electron spin alignment to explain the conductance anomaly. But in the 4 November print issue, another group's theory supports the claim that physics concepts borrowed from the world of material impurities is key. Both teams hope they're approaching a resolution to a problem that has turned mesoscopic physics on its head.

MUBs and the 0.7 Anomaly in QPC conductance

Thank Nanoscale for bringing to my attention a long standing puzzle in mesocopic scale condensed matter physics, the “0.7 anomaly”. The problem is the behavior of the conductance (inverse of resistance) for a quantum point contact (QPC). Two experimental papers showing the effect are cond-mat/0706.0792 and cond-mat/0005082, which see. Shot noise decreases at the 0.7 anomaly just as it does at the integer conductance points, see cond-mat/0311435. A recent perturbation analysis of the situation is given by cond-mat/0707.1989. This blog post gives a non perturbational calculation.