All That Matters

Ultracold atoms might no longer be the only hot game in the town of cold condensates. A few weeks ago I highlighted the analogies between the science of ultracold atoms and other areas of physics, down to lasers even. Now meet the new kids on the block: the polaritons. Even though they sound more like the name of a 1960s rock’n’roll band, polaritons are the basis of some of the hottest research not only in condensed-matter physics but also in photonics.

Polaritons form when light couples to electronic excitations in a material. A widely studied type of polaritons, which I mentioned previously in the context of enhanced solar cells, is surface plasmon polaritons. Surface-plasmon polaritons are successfully used in photonics because they enable a versatile, highly local control of light by nanoscale structures. Applications range from sensing and the guiding of light to solar cells and other optical devices.

Schematic of the acoustic waves applied to a polariton condensate. The polaritons are shown in red. DBR are the mirror layers between which the polaritons are confined. (c) 2010 American Physical Society

A perhaps lesser known variety of polaritons are exciton-polaritons, which are quickly turning into a hot research area themselves because they enable the study of fundamental quantum physics phenomena directly in a semiconductor.

Exciton-polaritons form in semiconductors such as GaAs, which have a band structure where the lower energy band, the valence band, is occupied by electrons, and the higher energy band, the conduction band, is empty. If an electron is excited to the conduction band, an empty unoccupied space, a hole, remains in the valence band. The hole left behind in the sea of valence band electrons has a positive charge. The electrostatic interaction between the positive hole in its low-energy state and the negative electron in the high-energy state leads to the formation of a combined entity known as an excition. And these excitons can interact with light to form exciton-polaritons. Usually, they are simply referred to as polaritons.

The high materials quality that can be achieved in semiconductors such as GaAs means that polaritons exist long enough to do experiments with them. Even though the interaction between polaritons is much different than that between ultracold atoms, there are also similarities. Bose-Einstein condensation for example, known from ultracold atom systems, has been observed in polariton systems.

Maurice Skolnick from the University of Sheffield in the UK and his colleagues have now shown in a paper published in Physical Review Letters that polariton condensates can be dynamically controlled by sound waves applied to the semiconductor. “The dynamic modulation allows for the first time a tunable periodic potential to be applied to the polariton condensate,” says Skolnick. This, so Benoit Deveaud-Plédran, a physicist from École Polytechnique Fédérale de Lausanne in Switzerland who also works on polaritons, offers the opportunity to study the reactions of polaritons to these changes in environment: “I really like the idea that is proposed here, playing with condensates with a surface acoustic wave is indeed great.”

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This week two interesting papers have been published that I did not get around to highlight here. In terms of topic they could not be more different, one about a possible new data storage material, and the other one about string theory!

The next big thing in computing could be silicon!

A new memory element made from silicon oxide (SiOx) between two silicon contacts. It promises a silicon-compatible non-volatile memory device. Reprinted with permission from Nano Letters DOI: 10.1021/nl102255r. Copyright 2010 American Chemical Society.

It is not often that advances in condensed matter physics are highlighted on the front page of the New York Times, but the possibility of a new silicon memory technology was compelling enough it seems. The New York Times story is a bit short on details, but one of the authors of the paper published in Nano Letters, Douglas Natelson from Rice University in Houston, describes details of the work on his blog.

What Doug and his colleagues have discovered is that by applying a sufficiently high voltage to an insulating silicon oxide the material suddenly becomes conducting. Increase the voltage further and the devices becomes insulating again. The effect is very pronounced and fully reversible for at least 10,000 cycles.  What happens during the initial switching is that electrochemical processes lead to the formation of electrically conducting silicon nanocrystals within the oxide. The nanocrystals are close together so that electrons can easily hop from one to the other so that the device becomes conducting. At even higher voltages the device becomes insulating again as heating effects mean that probably some of the silicon oxidizes again.

The channels between the conducting silicon nanocrystals are only 5 nanometers wide. This suggests that memory devices could be fabricated that are much smaller than present memory devices with sizes of a few tens of nanometers. The device design is really simple. Switching occurs through the application of voltage pulses, which requires only two wires: one input and one output. Moreover, the fabrication of silicon oxide is fully compatible with present silicon technology. And in comparison to the silicon chips in your computer this memory device is non-volatile, it doesn’t lose the information when the computer is turned off. Rather, it behaves more like the flash memory used in solid-state hard drives.

Obviously, a lot of work is required to bring this to the market. At the same time, conceptually this memory effect is somewhat related to that memristor devices. The same New York Times article mentions how Hewlett-Packard is in the process of developing memristor memory chips, so prospects seem realistic.

Reference:
Yao, J., Sun, Z., Zhong, L., Natelson, D., & Tour, J. M. (2010). Resistive Switches and Memories from Silicon Oxide Nano Letters DOI: 10.1021/nl102255r

Could string theory be proven experimentally?

One of the geometrical objects studied in string theory. The image shows a three-dimensional projection of this six-dimensional object. Image by Lunch via Wikimedia Commons.

String theory promises nothing less than to unify the most fundamental concepts in physics, quantum physics and general relativity. In string theory fundamental particles like electrons are not billiard ball-like particles but vibrating strings that can be imagined as little rubber bands.

Unfortunately, string theory is not nearly as straightforward as this simple picture may suggest. The mathematical description of these strings is very challenging and requires more than the four dimensions we know from our universe (space, time). Indeed, most string theories are based on ten-dimensional spaces. To explain the fact that we observe only four of them, one has to imagine the remaining dimensions to be reduced so that they cannot be observed. It is as if you compress a cubic object in one direction until it almost looks like a flat sheet — effectively you see only two dimensions.

One of the biggest problems of string theory is, however, that it cannot be proven experimentally. Or so we thought. In a Physical Review Letters paper Michael Duff and colleagues from Imperial College London claim to have found an experimental proof. They realized that an identical mathematical formulation describes so-called quantum entanglement experiments with single particles of light, photons, as well as black holes. According to the Imperial College press release Duff had this idea at a conference when listening to a talk on entanglement. “I suddenly recognised his formulae as similar to some I had developed a few years earlier while using string theory to describe black holes. When I returned to the UK I checked my notebooks and confirmed that the maths from these very different areas was indeed identical.”

I cannot claim to understand much about Duff’s paper, but it seems that experiments on the entanglement of four photons should be able to replicate certain properties of black holes — if string theory is correct. String theory is controversial, and I am sure this won’t be the last word on the topic. But I am sure somewhere someone is already planning such entanglement experiments. It is too tempting.

Reference:
L. Borsten, D. Dahanayake, M. J. Duff, A. Marrani, & W. Rubens (2010). Four-qubit entanglement from string theory Phys.Rev.Lett.105:100507,2010 arXiv: 1005.4915v2

In the absence of GPS, a compass is the best option to find your way around. However, although the earth’s magnetic field is a great way to find your own position, doing the reverse, measuring magnetic fields with a high accuracy — on an atomic scale — remains a challenge. Sure, there are electron microscopes, which are great instruments that can image single atoms and other physical objects. However, when it comes to measuring magnetic fields, the achievable resolution is much worse than the size of an atom.

The schematic of a Lorentz microscope. Differently oriented magnetic domains deflect an electron beam in opposite directions, creating an image contrast at the border between magnetic domains.

The problem is even bigger in the fourth dimension, time, when we like to know how magnetic fields evolve over time on a microscopic scale. Then, the best resolution that can be achieved is about 500 times worse than what is possible in the imaging of atoms using state-of-the-art electron microscopes. The group of Ahmed Zewail at Caltech in Pasadena, California has now developed a technique that could significantly enhance the resolution of time-resolved measurements of magnetic fields.

There are of course a number of methods to measure a magnetic field with high spatial resolution. One is magnetic force microscopy, where essentially the tip of a magnetic needle gets moved across the surface of a magnetic material. The force between tip and sample is then a measure of the magnetic field. If the tip is atomically sharp, resolutions of a couple of tens of nanometers can be achieved.

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