All That Matters

The visible spectrum of neon and its characteristic emission lines. By Jan Homann via Wikimedia Commons

In 1814 the German physicist Joseph von Fraunhofer observed narrow dark lines in the otherwise continuous spectrum of light emitted by the sun. Hundreds of them. As Gustav Kirchhoff and Robert Bunsen later showed, these lines correspond to the absorption of light by various chemical elements in the sun. Each element has its own unique set of lines that correspond to energetic transition between the electronic states of these atoms. This discovery has laid the foundation to the field of spectroscopy, where the interaction of matter and light is probed.

A study published in Nature Physics this week by Hartmut Abele and colleagues from the University of Vienna in Austria now reports how gravity can be used instead to probe quantum states. And they’re not using atoms either, but neutrons, which are the electrically neutral particles in the atom’s core.

These neutrons are produced in nuclear research reactors, for example at the Institute Laue-Langevin (ILL) in Grenoble, which I visited last year. In fact, the experiment by Abele and colleagues was done at ILL because there ultracold neutrons are available for research – “still the only source of ultracold neutrons for users in the world,” says Peter Geltenbort from the ILL, who took part in the experiments. […]

The past year has been a great year for science with major advances in several areas. Too many exciting results to mention here. Instead, to reflect about the past year I have chosen a representative paper for each month of the year that I hope can serve as an example of the great science going on in a number of research fields. Of course, this is a highly subjective and personal collection, and indeed there might be others worth mentioning. But the aim was also to provide a balanced overview of the year that covers a variety of topics.

Of course, if you have an exciting paper to add, please feel free to use the comments section below to let us know!

Anyway, enough said, here are some of my highlights from the past year:

Simulations of electronic excitations in an iron-based superconductor. Image by Oak Ridge National Laboratory via flickr.

JANUARY – iron-based superconductors

Since they were discovered in 2008, iron-based superconductors, the pnictides, have been one of the hottest topics in condensed matter physics. Part of their appeal stems from the fact that they are based on iron, which is a magnetic element. Normally, magnets and superconductivity exclude each other.

The iron-based compounds have a similar crystal structure as the so-called cuprates, which are the materials with the highest superconducting temperatures known. The mechanism for these high-temperature superconductors is unknown, and studying the iron-based superconductors may also be relevant to the understanding of the cuprates.

This paper published in Science shows for the first time that the electrons in the iron-based superconductors show a periodic arrangement that is different to the periodicity of the atoms in the crystal. Similar observations have been made in the cuprates, and their understanding is considered important to the mechanism of high-temperature superconductivity.

Chuang, T., Allan, M., Lee, J., Xie, Y., Ni, N., Bud’ko, S., Boebinger, G., Canfield, P., & Davis, J. (2010). Nematic Electronic Structure in the “Parent” State of the Iron-Based Superconductor Ca(Fe1-xCox)2As2 Science, 327 (5962), 181-184 DOI: 10.1126/science.1181083

[…]

Last week I blogged about the potential of using the magnetic properties of an electron, its spin, for novel electronics. And already this week we have come a step further towards spin electronics through the demonstration of a spin-based transistor device!

In spin electronics, it is the spin of the electron and not its electrical charge that could be used for computing. Indeed, this could be done entirely without electrical currents, and would be more energy efficient as it is easier to switch a spin than carry an electric current. In such devices, the spin can assume two orientations, which can be used to represent the 1 and 0 of computer bits.

The fundamental unit of a computer is the transistor. So what about the equivalent for spin electronics, the spin transistor? Well, the concept of a transistor that only switches an electron’s spin instead of charge was proposed 20 years ago by Supriyo Datta and Biswajit Das, but was never realized. The problem has been to control the spin of an electron in a clear and efficient way by electrical voltages while it is in transit through a nanoscale device.

The Spin Hall effect transistor. (c) Science 330, 1801 (2010)

Work by Jörg Wunderlich from the Hitachi Laboratory in Cambridge, Tomas Jungwirth from the Institute of Physics in Prague and the University of Nottingham in the UK, and their colleagues now published in Science comes the closest yet to the Datta-Das spin transistor: they present a spin Hall effect transistor.

Unlike the Datta-Das transistor, which basically is the concept of the conventional transistor transferred to spin electronics, the spin Hall effect transistor is a little more elaborate. The researchers excite electrons with a predefined spin (yellow cylinder in the figure). As the electrons travel from there to the other end of the device they scatter and get diverted either to the left or the right, depending on the direction their spin is pointing at. If the electrons all have spins pointing in the same direction, as in the experiment, they all get deflected in the same direction. This creates a Hall voltage along a crossbar (R_H in the figure), even though no electric current flows in this device.

[…]