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 When AI and optoelectronics meet: Researchers take control of light properties
20.11.2018 | Institut national de la recherche scientifique - INRS

nachricht How to melt gold at room temperature
20.11.2018 | Chalmers University of Technology

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: Nonstop Tranport of Cargo in Nanomachines

Max Planck researchers revel the nano-structure of molecular trains and the reason for smooth transport in cellular antennas.

Moving around, sensing the extracellular environment, and signaling to other cells are important for a cell to function properly. Responsible for those tasks...

Im Focus: UNH scientists help provide first-ever views of elusive energy explosion

Researchers at the University of New Hampshire have captured a difficult-to-view singular event involving "magnetic reconnection"--the process by which sparse particles and energy around Earth collide producing a quick but mighty explosion--in the Earth's magnetotail, the magnetic environment that trails behind the planet.

Magnetic reconnection has remained a bit of a mystery to scientists. They know it exists and have documented the effects that the energy explosions can...

Im Focus: A Chip with Blood Vessels

Biochips have been developed at TU Wien (Vienna), on which tissue can be produced and examined. This allows supplying the tissue with different substances in a very controlled way.

Cultivating human cells in the Petri dish is not a big challenge today. Producing artificial tissue, however, permeated by fine blood vessels, is a much more...

Im Focus: A Leap Into Quantum Technology

Faster and secure data communication: This is the goal of a new joint project involving physicists from the University of Würzburg. The German Federal Ministry of Education and Research funds the project with 14.8 million euro.

In our digital world data security and secure communication are becoming more and more important. Quantum communication is a promising approach to achieve...

Im Focus: Research icebreaker Polarstern begins the Antarctic season

What does it look like below the ice shelf of the calved massive iceberg A68?

On Saturday, 10 November 2018, the research icebreaker Polarstern will leave its homeport of Bremerhaven, bound for Cape Town, South Africa.

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Optical Coherence Tomography: German-Japanese Research Alliance hosted Medical Imaging Conference

19.11.2018 | Event News

“3rd Conference on Laser Polishing – LaP 2018” Attracts International Experts and Users

09.11.2018 | Event News

On the brain’s ability to find the right direction

06.11.2018 | Event News

 
Latest News

Nonstop Tranport of Cargo in Nanomachines

20.11.2018 | Life Sciences

Researchers find social cultures in chimpanzees

20.11.2018 | Life Sciences

When AI and optoelectronics meet: Researchers take control of light properties

20.11.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>