'Science:' Novel quantum effect directly observed and explained

February 13, 2009

An international research team has succeeded in gaining an in-depth insight into an unusual phenomenon, as reported in the current edition of the high-impact journal "Science". The researchers succeeded for the first time in directly measuring the spin of electrons in a material that exhibits the quantum spin Hall effect, which was theoretically predicted in 2004 and first observed in 2007. Astonishingly, the spin currents flow without any external stimulus as a result of the internal structure of the material. The flow of information is loss-free, even for slight irregularities. This paves the way towards fault-tolerant quantum computers and towards a source of spin currents.

The spin is a quantum-mechanical property of elementary particles and as a rule it occurs in two variations. This is what makes it suitable for use as a binary information carrier. In hard disk drives, for example, spins are already being used to store digital information.

In 2007, physicists from Germany and the USA observed a new phenomenon that could make it possible to transport and electrically manipulate information in future storage media almost loss-free - the quantum spin Hall effect. The discovery was hailed by the high-impact journal "Science" as one of the ten most important scientific breakthroughs of 2007.

The first study that succeeded in directly observing the spin of flowing particles was published this week in "Science" by an international research team, which included Dr. Gustav Bihlmayer from Forschungszentrum Jülich, member of the Helmholtz Association. Until now, the quantum spin Hall effect could only be indirectly proven.

"We were able to show for the first time that two spin currents flow in opposite directions in the edge region of an alloy of bismuth and antimony. An external energy supply is not required; losses cannot occur," explained Dr. Gustav Bihlmayer from the Jülich Institute of Solid State Research. The causes of this astonishing phenomenon are interactions within the material. Of particular interest to materials scientists is the fact that imperfections in the material do not impair the spin currents. "This means that materials known as topological materials have spin currents that can be manipulated electrically and are therefore suitable for use as spin sources. They could even pave the way towards fault-tolerant quantum computers," said Bihlmayer. "Our process will make it possible to test the suitability of materials for this purpose in the future."

The current study makes use of theoretical calculations and photoelectron spectroscopy. The photons in a synchrotron beam cause electrons to be emitted from the material surface. The energy and momentum distribution, as well as the spin of the particles, can be used to derive concrete information on the occurrence of the quantum spin Hall effect. Previous methods were based on measurements of the conductivity in the materials at variable voltages.

Spins for data processing

Spins are a hot topic in research. Physicists and nanoelectricians have high hopes for what is known as spin electronics. Spin electronics does not just exploit the electric charge of electrons and nuclei but also their spin, and should therefore lead to the development of new approaches for the processing and coding of information in information processing. Faster, smaller and more energy-efficient computers could thus become a reality, as could completely new components capable of performing a number of different functions such as storage, logic and communication. One of the most prominent ideas is that of the quantum computer. For spin-electronic concepts, scientists conducting basic research are desperately searching for new materials and phenomena that will make it possible to control both spin orientation and spin flow.

Helmholtz Association of German Research Centres

Mapping of Parent Hamiltonians: From Abelian and non-Abelian Quantum Hall States to Exact Models of Critical Spin Chains (Springer Tracts in Modern Physics)
by Martin Greiter (Author)

This monograph introduces an exact model for a critical spin chain with arbitrary spin S, which includes the Haldane--Shastry model as the special case S=1/2. While spinons in the Haldane-Shastry model obey abelian half-fermi statistics, the spinons in the general model introduced here obey non-abelian statistics. This manifests itself through topological choices for the fractional momentum spacings. The general model is derived by mapping exact models of quantized Hall states onto spin chains. The book begins with pedagogical review of all the relevant models including the non-abelian statistics in the Pfaffian Hall state, and is understandable to every student with a graduate course in quantum mechanics.

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Quantum Principles and Particles
by Walter Wilcox (Author)

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—R. Feynman The core of modern physics, quantum theory is counter-intuitive and challenging for those new to the field. Quantum Principles and Particles presents the fundamental quantum principles in a particularly visual manner and applies them to aspects of particle interactions. Inspired by the author’s work with Nobel laureate Julian Schwinger, it introduces the primary principles of the microscopic world through an analysis of the simplest possible quantum mechanical system—spin 1/2. A Visual Approach to Quantum Mechanics This two-semester introductory undergraduate textbook balances...

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by Oliver Gywat (Author), Hubert J. Krenner (Author), Jesse Berezovsky (Author)

Filling a gap in the literature, this up-to-date introduction to the field provides an overview of current experimental techniques, basic theoretical concepts, and sample fabrication methods.
Following an introduction, this monograph deals with optically active quantum dots and their integration into electro-optical devices, before looking at the theory of quantum confined states and quantum dots interacting with the radiation field. Final chapters cover spin-spin interaction in quantum dots as well as spin and charge states, showing how to use single spins for break-through quantum computation. A conclusion and outlook round off the volume.
The result is a primer providing the essential basic knowledge necessary for young researchers entering the field, as well as semiconductor...

Handbook of Spin Transport and Magnetism
by Evgeny Y. Tsymbal (Editor), Igor Zutic (Editor)

In the past several decades, the research on spin transport and magnetism has led to remarkable scientific and technological breakthroughs, including Albert Fert and Peter Grünberg’s Nobel Prize-winning discovery of giant magnetoresistance (GMR) in magnetic metallic multilayers. Handbook of Spin Transport and Magnetism provides a comprehensive, balanced account of the state of the art in the field known as spin electronics or spintronics. It reveals how key phenomena first discovered in one class of materials, such as spin injection in metals, have been revisited decades later in other materials systems, including silicon, organic semiconductors, carbon nanotubes, graphene, and carefully engineered nanostructures. The first section of the book offers a historical and personal...

An Invitation to Quantum Field Theory (Lecture Notes in Physics, Vol. 839)
by Luis Alvarez-Gaumé (Author), Miguel A. Vázquez-Mozo (Author)

This book provides an introduction to Quantum Field Theory (QFT) at an elementary level—with only special relativity, electromagnetism and quantum mechanics as prerequisites. For this fresh approach to teaching QFT, based on numerous lectures and courses given by the authors, a representative sample of topics has been selected containing some of the more innovative, challenging or subtle concepts. They are presented with a minimum of technical details, the discussion of the main ideas being more important than the presentation of the typically very technical mathematical details necessary to obtain the final results. Special attention is given to the realization of symmetries in particle physics: global and local symmetries, explicit, spontaneously broken, and anomalous continuous...

Quantum Mechanics: A Modern and Concise Introductory Course (Advances Texts in Physics)
by Daniel Bes (Author)

Starting from basic principles, the book systematically covers both Heisenberg and Schrödinger realizations of quantum mechanics (in this order). It provides excellent didactic introduction to the essential principles and treats recent concepts such as entanglement and decoherence. The book gives the background needed to understand quantum cryptography, teleportation and computation, and it is especially suitable for introducing the spin. This second edition includes a more friendly presentation to Hilbert spaces, and more practical applications e.g. scanning tunneling microscope (potential barrier).

Spin Electronics
by David D. Awschalom (Editor), Robert A. Buhrman (Editor), James M. Daughton (Editor), Stephan von Molnár (Editor), Michael L. Roukes (Editor)

This book is a comparative review of spin electronics ("spintronics") research and development activities in the United States, Japan, and Western Europe conducted by a panel of leading U.S. experts in the field. It covers materials, fabrication and characterization of magnetic nanostructures, magnetism and spin control in magnetic nanostructures, magneto-optical properties of semiconductors, and magneto electronics and devices. Spin Electronics is unique in that it combines tutorial and introductory information on the significance of spin electronics research and applications development in this rapidly developing field. It reviews profound technical and policy implications of spin-based devices for the future of semiconductor electronics, non-volatile computer memory, nanotechnology,...

Spin-Statistics Connection and Commutation Relations: Experimental Tests and Theoretical Implications, Anacapri, Capri Island, Italy 31 May-3 June 2000 (AIP Conference Proceedings)
by Robert C. Hilborn (Editor), Guglielmo M. Tino (Editor)

The conference focused on the issues of the behavior of systems of identical particles in quantum mechanics. These questions are important in all areas of physics, in much of chemistry (via the Pauli Exclusion Principle) and mathematics, and in the philosophy of science. The conference brought together experimentalists, theorists, and philosophers to survey work done during the past ten years that challenges the traditional view of these issues. Several papers explore connections with rapidly developing fields such as fundamental symmetries, supersymmetry, quantum gravity, and quantum computation.