Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Topological Matter in Optical Lattices

30.11.2011
Atoms trapped by laser light have become excellent platforms for simulating solid state systems. These systems are also a playground for exploring quantum matter and even uncovering new phenomena not yet seen in nature.

Researchers at the Joint Quantum Institute* have shown that an optical lattice system exhibits a never-before-seen quantum state called a topological semimetal. The semimetal, which debuts in this week’s Advance Online Publication for the journal Nature Physics (DOI:10.1038/NPHYS2134}, can undergo a new type of phase transition to a topological insulator.

Topological insulators are one of the hottest topics in condensed matter research because of their dual-personality. They are insulators throughout the bulk of the material but are conductors along the edges. Harnessing the underlying phenomena, known as quantum hall physics, is important for developing new types of electronics and quantum information.

Scientists can create this unusual behavior in certain two-dimensional materials--such as a layer of electrons at the interface between two semiconductors-- by employing extremely large magnetic fields. What makes topological insulators special is their ability to exhibit this physics without external magnets.

JQI postdoctoral fellow Kai Sun explains, “Magnetic fields and lattices have nothing to do with topology. If a particular quantum hall state is topological matter, then I should be able to create it in different ways just by constructing the right topology.”

While experiments using these new materials have made great advances, hurdles such as achieving the necessary sample purity, remain. Additionally, real-time control over experimental conditions can be quite difficult and in some cases, not possible.

In the paper, the team proposes an atom-optical lattice system as the ideal test bed. Ultracold gases offer versatility for studying topological matter [see Topology inset] because a single apparatus can be used for repeated experiments. Here, researchers are able to alter the effective material through adjusting laser power.

Kai Sun explains that these advantages motivated the team to “design a system to realize topological state that has not been seen in condensed matter systems.”

An ultracold gas may not sound like a solid, but under certain conditions, this unusual quantum matter behaves just like a crystal made in nature. Neutral atoms trapped by a checkerboard of laser light are analogous to electrons in a crystalline solid. The light intensity determines the mobility of the atom gas around the lattice. If the atoms do not interact with each other, an energy band structure emerges that represents the semimetal. This semimetal has special properties that allow it to transform into topological insulator when the atoms begin to interact.**

What is a semimetal?

Condensed matter physicists classify materials according to their conductivity: insulators and metals. This property is related to the ability for electrons to leave their parent atoms and become mobile in a material.

In a single atom, electrons are restricted to certain energy levels. When many atoms form a solid, the individual energy levels mesh together and a new energy spectrum arises for the sea of electrons in a material. This is called a material’s band structure, where bands and gaps represent allowed and forbidden electron energies, respectively.

A valence band encompasses all the allowed energies if a solid were at absolute zero temperature. Electrons fill the valence band and are relatively localized to their parent atoms in the solid. Above the valence band is a conduction band, a zone that can be occupied but only if the electron somehow gains enough energy to overcome the forbidden gap region, becoming mobile.

So what makes a material an electrical conductor, or metal? Insulators have a gap between this conduction zone and valence band that exceeds the energy of the electrons. A well-known subset of insulators, called semiconductors, has a gap small enough that they can only conduct under certain conditions. Metals have no gap, so they are naturally conductive. This topological semimetal has no gap, but only at a small point in the energy spectrum. [see Figure 1]

Starting with the semimetal, these researchers show that allowing the particles to interact disrupts the system and forces a phase transition. Previously studied topological insulators are the result of single particles interacting with a type of internal magnetic field (called spin-orbit coupling).

W. Vincent Liu, co-author and professor at University Pittsburgh, explains this key feature, "A most exciting aspect of this research is that we seem to have found a first example of optical lattice gases that can reveal a quantum Hall-like topological state without needing an external magnetic field or spin-orbital coupling. Instead it is due to the effects of internal many-body interactions."

The team found that upon adding these interactions the particles spontaneously began to rotate [see Figure 2/animation]. Normally, lasers can generate rotation, either by stirring the sample or engineering effective magnetic fields. The proposal here offers not only novel states, but the spontaneously generated rotation may be a complementary experimental technique.

The proposed experiment uses established atomic physics techniques. Combining these ingredients may prove tricky, but would certainly open new possibilities for studying condensed matter physics.

Note: Hi-res images available upon request. Visit http://jqi.umd.edu/news/289-topological-matter-in-optical-lattices.html to see.

*The JQI researchers are also affiliated with the Condensed Matter Theory Center at UMD. They collaborated with many groups over these three publications. Please see individual publications for all author affiliations.

**The research results described here were presented across three publications:

1. “Topological semimetal in a fermionic optical lattice,” Kai Sun, W. Vincent Liu, Andreas Hemmerich, and S. Das Sarma, Nature Physics, DOI:10.1038/NPHYS2134 (2011)
2. “Fractional quantum Hall effect in the absence of Landau levels,” D.N. Sheng, Zheng-Cheng Gu, Kai Sun, and L. Sheng, Nature Communications, 2, 389, (2011)
3. “Nearly Flatbands with Nontrivial Topology,” Kai Sun, Zheng-Cheng Gu, Hosho Katsura, and S. Das Sarma, Physical Review Letters, 106, 236803, (2011)

[Accompanying Physics Viewpoint]

Media contacts at Joint Quantum Institute:
Emily Edwards, 301-405-2291, eedwards@umd.edu
Phillip F. Schewe, 301-405-0989, pschewe@umd.edu
Research Contacts:
Kai Sun Kai Sun kaisun@umd.edu
Sankar Das Sarma dassarma@umd.edu

Emily Edwards | Newswise Science News
Further information:
http://www.umd.edu

More articles from Physics and Astronomy:

nachricht From rocks in Colorado, evidence of a 'chaotic solar system'
23.02.2017 | University of Wisconsin-Madison

nachricht Prediction: More gas-giants will be found orbiting Sun-like stars
22.02.2017 | Carnegie Institution for Science

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Breakthrough with a chain of gold atoms

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

Im Focus: DNA repair: a new letter in the cell alphabet

Results reveal how discoveries may be hidden in scientific “blind spots”

Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...

Im Focus: Dresdner scientists print tomorrow’s world

The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.

The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...

Im Focus: Mimicking nature's cellular architectures via 3-D printing

Research offers new level of control over the structure of 3-D printed materials

Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...

Im Focus: Three Magnetic States for Each Hole

Nanometer-scale magnetic perforated grids could create new possibilities for computing. Together with international colleagues, scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) have shown how a cobalt grid can be reliably programmed at room temperature. In addition they discovered that for every hole ("antidot") three magnetic states can be configured. The results have been published in the journal "Scientific Reports".

Physicist Dr. Rantej Bali from the HZDR, together with scientists from Singapore and Australia, designed a special grid structure in a thin layer of cobalt in...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Booth and panel discussion – The Lindau Nobel Laureate Meetings at the AAAS 2017 Annual Meeting

13.02.2017 | Event News

Complex Loading versus Hidden Reserves

10.02.2017 | Event News

International Conference on Crystal Growth in Freiburg

09.02.2017 | Event News

 
Latest News

Stingless bees have their nests protected by soldiers

24.02.2017 | Life Sciences

New risk factors for anxiety disorders

24.02.2017 | Life Sciences

MWC 2017: 5G Capital Berlin

24.02.2017 | Trade Fair News

VideoLinks
B2B-VideoLinks
More VideoLinks >>>