Hidden order found in a quantum spin liquid

In quantum information processing, data is recorded and manipulated as quantum bits or ‘qubits’, generalizations of the classical ‘0’ and ‘1’ bits which are traditionally represented by the ‘on’ and ‘off’ states of conventional switches. It is widely believed that if large-scale quantum computers can be built, they will be able to solve certain problems, such as code breaking, exponentially faster than classical computers.

Theoretically, the spin of an individual electron is an excellent qubit, but in a real material it interacts with other electrons and its useable quantum properties are rapidly lost. The new research is important because it explicitly demonstrates, using a practical material, that a large number of electron spins can be coupled together to yield a quantum mechanical state with no classical analog. In addition, the team has also established the factors that affect the distance over which the hidden ‘quantum order’ can be maintained.

“We had two objectives,” explains Professor Gabriel Aeppli, Director of the London Centre for Nanotechnology and the paper’s senior author. “The first was to show that we could actually image the quantum order, which is sometimes referred to as phase coherence. The second aim was to manipulate the distance over which it can be maintained.” This distance - and how sensitive it is to changes in temperature or chemical impurities in the material - can be essential in determining whether a material will have real-life applications, where it would be crucial to control and maintain quantum order over predetermined extents in space and time.

The team studied a ceramic material consisting of chains of nickel-centered oxygen octahedra laid end-to-end. The chains are not ordinary magnets such as those used to fix reminders onto refrigerator doors, but an exotic quantum spin liquid in which the electron spins (analogous to tiny bar magnets) point in random directions with no particular order, even at very low temperatures.

To measure the quantum order throughout this classically disordered liquid, the scientists used neutrons to image the magnetic excitations - “flips” or fluctuations of the spins - and the distances over which they could propagate. The experiments were performed at the National Institute of Standards and Technology (NIST) Center for Neutron Research in the US and at the ISIS particle accelerator of the Rutherford Appleton Laboratory in the UK.

The scientists found that despite the apparent classical disorder, magnetic excitations could propagate over long chains of atoms at low temperature - in the otherwise magnetically disordered material.

Other examples of large-scale quantum phase coherence include superconductors and superfluids where quantum physics leads to fascinating properties.

The team also discovered that they could limit the coherence or make it disappear altogether by introducing defects into the material either by adding chemical impurities (doping) or heating. These defects break the chains into independent sub-chains, each with its own, hidden order. This part of the reported research is the first step towards engineered spin-based quantum states in ceramics.

Aeppli and other members of the team note that their work was initially not intended to have direct applications, but that they later realized that what they are learning could be applied in a range of fields from nanotechnology to quantum computing.

Collaborators on this research include: Guangyong Xu, of John Hopkins University and Brookhaven National Laboratory; Collin L. Broholm, Ying Chen, and Michel Kenzelmann of Johns Hopkins University and the NIST Center for Neutron Research; Yeong-Ah Soh of Dartmouth College; Gabriel Aeppli of the London Centre for Nanotechnology and University College London; John. F. DiTusa of Louisiana State University; Christopher D. Frost from the ISIS Facility, Rutherford Appleton Laboratory, U.K.; Toshimitsu Ito and Kunihiko Oka of the National Institute of Advanced Industrial Science and Technology (AIST), Japan; and Hidenori Takagi from AIST and University of Tokyo.

The work was funded by the Office of Basic Energy Sciences within the U.S. Department of Energy’s Office of Science, the National Science Foundation, a Wolfson-Royal Society Research Merit Award (UK), and by the Basic Technologies programme of the UK Research Councils.

Hi-res images are available from the Press Office.

Image 1

Image of quantum order made using neutrons by the LCN (London Centre for Nanotechnology)/UCL team and its collaborators from the US and Japan at the ISIS particle accelerator in the UK. The sharp red peak in the middle of the picture corresponds to nearly perfect quantum coherence or order.

Image 2

Nickel oxide chains which host quantum order in the YBaNiO5 oxide ceramic studied.

About the London Centre for Nanotechnology

The London Centre for Nanotechnology is a joint enterprise between UCL (University College London) and Imperial College London. In bringing together world-class infrastructure and leading nanotechnology research activities, the Centre aims to attain the critical mass to compete with the best facilities abroad. Furthermore by acting as a bridge between the biomedical, physical, chemical and engineering sciences the Centre will cross the 'chip-to-cell interface' - an essential step if the UK is to remain internationally competitive in biotechnology.

Website: www.london-nano.com

About UCL

Founded in 1826, UCL was the first English university established after Oxford and Cambridge, the first to admit students regardless of race, class, religion or gender, and the first to provide systematic teaching of law, architecture and medicine. In the government’s most recent Research Assessment Exercise, 59 UCL departments achieved top ratings of 5* and 5, indicating research quality of international excellence.

UCL is the fourth-ranked UK university in the 2006 league table of the top 500 world universities produced by the Shanghai Jiao Tong University. UCL alumni include Mahatma Gandhi (Laws 1889, Indian political and spiritual leader); Jonathan Dimbleby (Philosophy 1969, writer and television presenter); Junichiro Koizumi (Economics 1969, Prime Minister of Japan); Lord Woolf (Laws 1954, Lord Chief Justice of England & Wales); Alexander Graham Bell (Phonetics 1860s, inventor of the telephone), and members of the band Coldplay.

Website: www.ucl.ac.uk

About Research Councils UK

The seven Research Councils are independent non-departmental public bodies, funded by the Science Budget through the Office of Science and Innovation. They are incorporated by Royal Charter and together manage a research budget of over £2.8 billion a year.

Research Councils UK (RCUK) is the partnership between the UK's seven Research Councils. Through RCUK, the Research Councils work together to champion the research, training and innovation they support.

The seven UK Research Councils are: Arts & Humanities Research Council (AHRC) Biotechnology & Biological Sciences Research Council (BBSRC) Economic & Social Research Council (ESRC) Engineering & Physical Sciences Research Council (EPSRC) Medical Research Council (MRC) Natural Environment Research Council (NERC) Science and Technology Facilities Council (STFC)