Forum für Wissenschaft, Industrie und Wirtschaft

Hauptsponsoren:     3M 
Datenbankrecherche:

 

An Ideal Material: Solving a Mystery Leads to the Discovery of a True Topological Insulator

04.02.2013
An old material gets a new name, and with it, topological insulators have another chance to shine.

Samarium hexaboride (SmB6) has been around since the late 1960s--but understanding its low temperature behavior has remained a mystery until recently. Experimentalists* have recently confirmed that this material is the first true 3D topological insulator—as originally predicted by JQI/CMTC☨ theorists in 2010.


JQI E. Edwards
Experimentalists have recently confirmed that SmB6 is the first true 3D topological insulator—as originally predicted by JQI/CMTC☨ theorists in 2010.

Topological insulators have been discussed widely as a new area of material science, with the potential to study quantum Hall physics and exotic states such as Majorana fermions. While this finding provides a conclusion to one mystery, it is also the beginning of a new chapter that will certainly lead to a clearer understanding of this strange physics and even new quantum devices.

As insulators are cooled to absolute zero, their ability to insulate effectively becomes infinite. About 40 years ago, scientists observed that, under these conditions, some insulators atypically retain a tiny bit of conductivity. These materials, termed Kondo insulators, were not well-understood, until recently.

In Kondo insulators two ingredients combine to create what are called “heavy fermions.” In materials such as SmB6, some of the electrons are effectively pinned, only having a spin degree of freedom. This is in contrast to the speedy conduction electrons, which can also move in the crystal, endowing it with metallic character. The energy-momentum relationship (band structure) is flat for the pinned electrons. The conduction electrons would normally have a quadratic ("U" shaped) energy-momentum relationship, but at low temperatures, they strongly interact with the effectively stationary electrons. The band structure reorganizes to take on more of the flattened character of the stationary electrons. This hybridization gives rise to electrons that act as if they are sluggish and is the origin of the term ‘heavy fermions.’ The transition from metal to insulator, where the electrons behave, in effect, as if they are 1000 times heavier, starts to occur as the system is cooled below 50 K (see illustration). But then something strange happens a few degrees above absolute zero.

Maxim Dzero is an expert in heavy fermion materials: “Decades ago, people went through the periodic table making all sorts of combinations of elements. With this material, the major mystery in 1970s was that it was an insulator that at low temperatures, but still retained some small residual conductivity.”

The missing piece of the puzzle lay with theory that wouldn’t been developed until recent years. Above 4 K, SmB6 appears to be an insulator; looking below 4 K, it is a metal with high resistivity. This seemed confusing until recently when condensed matter theorists who study topology claim that this is exactly what you should see in a topological insulator.

A perfect topological insulator would be insulating in the bulk, but, in 3-dimensions, allow current to pass over the surface. Predicting these materials is tricky and while scientists have some hints as to the ingredients, finding them is somewhat serendipitous. Even cold atoms interacting with lasers have been proposed as a candidate for realizing this kind of physics. In recent years, researchers have studied bismuth compounds as a topological material. Unfortunately, interactions and defects tend to destroy their bulk insulating behavior, making it difficult to study the existence of conducting surface states.

In 2010**, JQI and PFC-supported scientists at the CMTC made were able to show that the mystery surrounding Kondo insulators could be explained using topological theory. It turns out that the strong interactions create a situation where the surface conducting states are truly independent and isolated from the bulk. Recently, experimental groups have verified that SmB6 is indeed a true 3D topological insulator, and in fact, is the first compound to be classified as such.

An experiment at University of Michigan involves attaching 8 electrical contacts to a thin sample in a novel way so as to unambiguously distinguish between bulk and surface conduction. Independently, a group from the University of California at Irvine has made voltage measurements, probing the Hall effect. They observe that the Hall resistance is independent of sample thickness, which is consistent with SmB6 supporting surface conduction. If the bulk was conducting, then the resistance would increase as the sample thickness was decreased.

In a related, third experiment at the University of Maryland’s Center for Nanophysics and Advanced Materials, researchers have reported careful measurements of the Kondo-insulating behavior of SmB6 at different temperatures, which supports the presence of underlying topological physics.

The surface conducting states of a topological insulator are expected to be quite impervious to disorder. Indeed, the experiments indicate this. Dzero explains, “The transport properties are quite good in spite of the crude things that are done to the sample. It is remarkable that the surface conductivity does not change.”

Victor Galitski, in response to the experimental work that is posted currently on the open-access arXiv discusses the strength of the measurements, “There is one major qualitative prediction to distinguish the topological insulator: surface conduction. This is the first material in the world that does this. There is no other conceivable theory that will explain it besides topological insulators.”

☨Affiliations [The theory work and prediction of SmB6 as a topological insulator was done primarily in Victor Galitski's group at JQI/CMTC, with support from the NSF PFC @JQI]:

Victor Galitski is a JQI fellow and a professor at the University of Maryland Physics Department/Condensed Matter Theory Center (CMTC).

Maxim Dzero was a postdoctoral researcher at JQI and is now a professor at Kent State University

Kai Sun was a postdoctoral researcher at JQI and is now a professor at the University of Michigan--Sun is an author on the previous theoretical works, as well as the current experimental results from U. Michigan

Piers Coleman is a professor at Rutgers University

**Theory papers:

Theory of topological Kondo insulators, Maxim Dzero, Kai Sun, Piers Coleman, and Victor Galitski, Physical Review B (2012)

Topological Kondo Insulators, Maxim Dzero, Kai Sun, Piers Coleman, and Victor Galitski, Physical Review Letters, (2010)

*Experimental papers:

Discovery of the First Topological Kondo Insulator: Samarium Hexaboride, Steven Wolgast, Çaðlýyan Kurdak, Kai Sun, J. W. Allen, Dae-Jeong Kim, Zachary Fisk, arXiv:1211.5104v2

Robust Surface Hall Effect and Nonlocal Transport in SmB6: Indication for an Ideal Topological Insulator, J. Botimer, D.J. Kim, S. Thomas, T. Grant, Z. Fisk and Jing Xia, arXiv:1211.6769

Hybridization, Correlation, and In-Gap States in the Kondo Insulator SmB6 Xiaohang Zhang, N. P. Butch, P. Syers, S. Ziemak, Richard L. Greene, and J. Paglione, arXiv:1211.5532

Victor Galitski | Newswise
Further information:
http://www.physics.umd.edu

More articles from Materials Sciences:

nachricht Combining the elements palladium and ruthenium for industry
22.09.2016 | National Institute for Materials Science

nachricht Defects at the spinterface disrupt transmission
21.09.2016 | Eberhard Karls Universität Tübingen

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: Synthese-chemischer Meilenstein: Neues Ferrocenium-Molekül entdeckt

Wissenschaftler der Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) haben zusammen mit Kollegen der Freien Universität Berlin ein neues Molekül entdeckt: Die Eisenverbindung in der seltenen Oxidationsstufe +4 gehört zu den Ferrocenen und ist äußerst schwierig zu synthetisieren.

Metallocene werden umgangssprachlich auch als Sandwichverbindungen bezeichnet. Sie bestehen aus zwei organischen ringförmigen Verbindungen, den...

Im Focus: Neue Entwicklungen in der Asphären-Messtechnik

Kompetenzzentrum Ultrapräzise Oberflächenbearbeitung (CC UPOB) lädt zum Expertentreffen im März 2017 ein

Ob in Weltraumteleskopen, deren Optiken trotz großer Abmessungen nanometergenau gefertigt sein müssen, in Handykameras oder in Endoskopen − Asphären kommen in...

Im Focus: Mit OLED Mikrodisplays in Datenbrillen zur verbesserten Mensch-Maschine-Interaktion

Das Fraunhofer-Institut für Organische Elektronik, Elektronenstrahl- und Plasmatechnik FEP arbeitet seit Jahren an verschiedenen Entwicklungen zu OLED-Mikrodisplays, die auf organischen Halbleitern basieren. Durch die Integration einer Bildsensorfunktion direkt im Mikrodisplay, lässt sich u.a. die Augenbewegung in Datenbrillen aufnehmen und zur Steuerung von Display-Inhalten nutzen. Das verbesserte Konzept wird erstmals auf der Augmented World Expo Europe (AWE), vom 18. – 19. Oktober 2016, in Berlin, Stand B25 vorgestellt.

„Augmented Reality“ (erweiterte Realität) und „Wearable Displays“ (tragbare Displays) sind Schlagworte, denen man mittlerweile fast täglich begegnet. Beide...

Im Focus: OLED microdisplays in data glasses for improved human-machine interaction

The Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP has been developing various applications for OLED microdisplays based on organic semiconductors. By integrating the capabilities of an image sensor directly into the microdisplay, eye movements can be recorded by the smart glasses and utilized for guidance and control functions, as one example. The new design will be debuted at Augmented World Expo Europe (AWE) in Berlin at Booth B25, October 18th – 19th.

“Augmented-reality” and “wearables” have become terms we encounter almost daily. Both can make daily life a little simpler and provide valuable assistance for...

Im Focus: Künstliche Intelligenz ermöglicht die Entdeckung neuer Materialien

Mit Methoden der künstlichen Intelligenz haben Chemiker der Universität Basel die Eigenschaften von rund 2 Millionen Kristallen berechnet, die aus vier verschiedenen chemischen Elementen zusammengesetzt sind. Dabei konnten die Forscher 90 bisher unbekannte Kristalle identifizieren, die thermodynamisch stabil sind und als neuartige Werkstoffe in Betracht kommen. Das berichten sie in der Fachzeitschrift «Physical Review Letters».

Elpasolith ist ein glasiges, transparentes, glänzendes und weiches Mineral mit kubischer Kristallstruktur. Erstmals entdeckt im El Paso County (USA), kann man...

Alle Focus-News des Innovations-reports >>>

Anzeige

Anzeige

IHR
JOB & KARRIERE
SERVICE
im innovations-report
in Kooperation mit academics
Veranstaltungen

Einsteins Geburtsstadt wird für eine Woche Hauptstadt der Physik

23.09.2016 | Veranstaltungen

Industrie und Wissenschaft diskutieren künftigen Mobilfunk-Standard 5G auf Tagung in Kassel

23.09.2016 | Veranstaltungen

Fachgespräch Feste Biomasse diskutiert Fragen zum Thema "Qualitätshackschnitzel"

23.09.2016 | Veranstaltungen

 
B2B-VideoLinks
Weitere VideoLinks >>>
Aktuelle Beiträge

Das Korallenthermometer muss neu justiert werden

23.09.2016 | Biowissenschaften Chemie

Doppel-Infektion macht Erreger aggressiver

23.09.2016 | Biowissenschaften Chemie

Synthese-chemischer Meilenstein: Neues Ferrocenium-Molekül entdeckt

23.09.2016 | Biowissenschaften Chemie