Magnetics Business & Technology - Fall 2012 - (Page 14)

RESEARCH & DEVELOPMENT First Observation of the Hall Effect in a Bose-Einstein Condensate National Institute of Standards and Technology (NIST) researchers have observed for the first time the Hall effect in a gas of ultracold atoms. The Hall effect is an important interaction of magnetic fields and electric current more commonly associated with metals and semiconductors. Variations on the Hall effect are used throughout engineering and physics with applications ranging from automobile ignition systems to fundamental measures of electricity. The new discovery could help scientists learn more about the physics of quantum phenomena such as superfluidity and the quantum Hall effect. Discovered in 1879 by Edwin Hall, the Hall effect is easiest to visualize in a rectangular conductor like a copper plate when a current is flowing along its length. A magnetic field applied at a right angle to the electric current (down into the plate) deflects the path of the charge carriers in the current (electrons, for example) by inducing a force in the third direction at right angles to both the magnetic field and the current flow. This pushes the charge carriers toward one side of the plate and induces an electrical potential, or “Hall voltage.” The Hall voltage can be used to measure the hidden internal properties of electrical systems, such as the concentration of the current carriers and the sign of their charge. “Cold atom systems are a great platform for studying complicated physics because they are nearly free of obscuring impurities, the atoms move much more slowly than electrons in solids, and the systems are much simpler,” said NIST researcher Lindsay LeBlanc. “The trick is creating the conditions that will get the atoms to behave the right way.” Measuring the Hall effect in a BoseEinstein condensate builds upon previous NIST work generating synthetic electric and magnetic fields. First, the group uses lasers to tie the atoms’ energy to their momentum, putting two internal states into a relationship called a superposition. This causes the electrically neutral atoms to act as if they are charged particles. With the cloud of about 20,000 atoms gathered into a loose ball, the researchers then cyclically vary the trapping force, pushing the atoms in the cloud together and pulling them apart, to simulate the movement of charge carriers in an alternating current. In response, the atoms begin to move in a manner that is mathematically identical to how charged particles experiencing the Hall effect would move, i.e., at right angles to both the direction of the “current” flow and the artificial magnetic field. According to LeBlanc, measuring the Hall effect offers another tool for study- Starting with a cloud of about 20,000 atoms, the researchers varied the trapping force, pushing the atoms together and pulling them apart, to simulate the movement of charge carriers in an alternating current. In response, the atoms begin to move in a manner mathematically identical to how charged particles experiencing the Hall effect would move--at right angles to both the direction of the “current” flow and the artificial magnetic field. This causes the tilting motion. Credit: NIST ing the physics of superfluidity, a lowtemperature quantum-based condition where liquids flow without friction, as well as the so-called quantum Hall effect, where the ratio of the Hall voltage and the current through the material is quantized, allowing for the determination of fundamental constants. 'Tuning' Graphene Drums Might Turn Conductors to Semiconductors Tightening or relaxing the tension on a drumhead will change the way the drum sounds. The same goes for drumheads made from graphene, only instead of changing the sound, stretching graphene dramatically alters the material’s electrical properties. Researchers working at the National Institute of Standards and Technology (NIST) and the University of Maryland have shown that subjecting graphene to mechanical strain can mimic the effects of magnetic fields and create a quantum dot, an exotic type of semiconductor with a wide range of potential uses in electronic devices. Graphene is a single layer of carbon atoms arranged in a honeycomb lattice. Pure graphene is a phenomenal conductor, transmitting electricity with little resistance at room temperature. But microelectronic devices depend on semiconductors that can be turned "on" and “off” because they have an energetic threshold beneath which they won't conduct electricity. This new work demonstrates that mechanical strain can be used to make tiny regions of graphite act like a classic semiconductor. The research team suspended a sheet of graphene over shallow holes in a substrate of silicon dioxide, essentially making a set of graphene drumheads. In probing the drumheads with a scanning probe microscope, they found that the graphene rose up to meet the tip of the microscope, a result of the van der Waals force, a weak electrical force that creates attraction between objects that are very close to each other. Calculations by the Univerwww.MagneticsMagazine.com 14 Magnetics Business & Technology • Fall 2012 http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Fall 2012

Magnetics Business & Technology - Fall 2012
Contents
Editor's Choice
Permanent Magnet Motors in Application
Three-Axis Magnetic Field Measurement: From Nanoteslas to 14 Tesla
Characterizing Permanent Magnet Materials with a Vibrating Sample Magnetometer
Research & Development
Magnets • Materials • Measurement
Application • Component Developments
Industry News
Marketplace
Advertising Index
Spontaneous Thoughts: And the Wisdom to Know the Difference

Magnetics Business & Technology - Fall 2012