Breakthrough research by NITheP physicist

Nature journal article highlights research on superfluidity at low

temperatures

Breakthrough findings about superfluidity of particles at very low temperatures have been

published in an article in the influential journal Nature by Dr Fabio Cinti, a member of the

South African National Institute for Theoretical Physics (NITheP), together with co-authors T.

Macrì, W. Lechner, G. Pupillo and T. Pohl.

They point out in the article that more than 40 years ago, Andreev, Lifshitz and Chester

suggested the possible existence of a peculiar solid phase of matter, of which the

microscopic constituents can flow superfluidly, or behave like fluids without resistance in a

way that defies both gravity and surface tension. This has become known as the Andreev–

Lifshitz–Chester (ALC) scenario. Yet, the authors point out that “a physical system where

this theoretical mechanism is unambiguously shown to exist, remains to be found, both

experimentally and theoretically”.

Superfluidity characteristics have been found in liquid helium, and are also thought to exist

inside high-density, low temperature neutron stars. In theoretical physics, work on

superfluidity is one of the avenues of research aimed at developing scientific models capable

of understanding the intrinsic mechanisms of the entire universe. Therefore this is an

example that implies the vast scale of physics, ranging from the smallest to the largest

known matter.

“Travelling in temperature”

Dr Cinti explains that in simple terms their research meant instead of travelling in space, they

“travelled in temperature”. He says “we know more or less how things work and how

particles move at normal temperatures because we are used to them. But when you

decrease temperatures to close to absolute zero, the world is completely different. Particles

start becoming ‘stuck’ in their positions and when this happens, other interesting things also

come up, such as that particles start moving coherently, which is not the case at higher

temperatures.”

He continues: “If one gets very close to zero Kelvin, you will reach a realm which is almost

Alice in Wonderland-like: where things start appearing very different from higher

temperatures, and you will see a macroscopic effect due to the quantum mechanics. For

instance, you can see how the particles start working together at the same lowest energy

and if you move one atom, atoms that are very far away will move in the same way. This

happens because you have basically frozen the thermal fluctuation that will destroy the

coherence.”

Another example is superfluidity, which refers to a liquid without any viscosity. In this sense,

helium is a very important atom for physics, because it is the only real chemical element that

you can find in nature – it is always liquid and you cannot find it as solid without using

pressure.

Together with his co-researchers Dr Cinti reported in the Nature article, on an investigation

of a zero-temperature phase diagram of a special class of particles named “bosons” with

finite-range soft-core interaction. They showed that, for low particle densities, the system of

self-assembled crystals features a solid phase in which zero-point vacancies emerge

spontaneously and give rise to a superfluid flow of particles through the crystal.

Breakthrough

Theirs is therefore a breakthrough finding and the first example of defect-induced,

continuous-space supersolidity consistent with the ALC scenario.

They showed that particles interacting via soft-core singular bosonic potentials (potentials

meaning a field defined in space from which certain physical properties may be derived),

“assemble into a commensurate and insulating solid and that, on removing the singularity,

particles can cluster and, under proper conditions, form an incommensurate crystal with

more particles than lattice sites. Defect delocalisation due to inter-site tunnelling can then

promote a finite superfluid response of the self-assembled crystalline ground state.”

The researchers used exact numerical techniques to determine an underlying zero-

temperature phase diagram, “which reveals the emergence of defect-induced supersolidity in

the vicinity of commensurate solid phases, as conjectured by ALC.”

They conclude that “having identified a physical system that facilitates defect-induced

supersolidity, we hope that this work will provide useful guidance for future experiments and

initiate further theoretical explorations.”

Future research and implications

Is there a practical purpose or implication for their theoretical research? Dr Cinti says that

the purpose of the research is a better understanding of the behaviour of matter at low

temperatures. In a practical sense, “if you are able to control the properties of a material, you

take advantage of it. For instance, if one can control the internal properties of neutral atoms

in future, this has implications for, among others, quantum computing.”

The Nature article is illustrative of the world-class research currently being supported by the

National Institute for Theoretical Physics (NITheP), a national facility leading research

programmes and educational opportunities in the field of theoretical physics in South Africa

and Africa. The NITheP’s aim is to support the country’s endeavours to become an

international player in a truly fundamental field of science.

Asked about the importance of the article in Nature, Prof Frederik Scholtz, the Director of the

NITheP, stresses that as a centre of excellence, the institute among others wants to

establish a very strong publication profile in top journals, since that also “supports our

international visibility and recognition as a place where quality research is being done, and

as an outflow of that we will continue to attract high quality international visitors and long-

term collaborators.”

More about NITheP

NITheP was launched in 2008 in the presence of world-renowned physicists, including physicist Prof Stephen Hawking and Nobel Laureate Prof David Gross, as well as the South African Minister of Science and Technology in attendance. It has its headquarters at the Stellenbosch Institute for Advanced Study (STIAS) at Stellenbosch University. Together with its regional nodes at the University of the Witwatersrand and the University of KwaZulu-Natal, it develops and supports all theoretical physicists in the country. See also the list of the current board members of the NITheP below. Any member of the South African physics community who is affiliated to a South African institute or university could become a NITheP associate by invitation, or by application. Applications should be directed to renekotze@sun.ac.za

More about Dr Fabio Cinti Prior to joining NITheP, Dr Fabio Cinti obtained his PhD in Physics at Università di Modena e

Reggio Emilia (Italy) working on solid state physics (concentrating on quantum magnetism)

and then conducted research at various universities and institutes in Europe and North

America, including the universities of Florence (Italy), Alberta (Canada), and the Max Planck

Institute, Dresden (Germany). Among his research interests are quantum many-body

problems (superconductivity, superfluidity, supersolidity, and Bose-Einstein condensation),

ultra-cold atomic systems, quantum-phase transitions in low dimensions, and computational

physics.

The full article in Nature can be found at

http://www.nature.com/ncomms/2014/140204/ncomms4235/full/ncomms4235.html

Additional information

Current NITheP Board Members

Prof Eugene Cloete (DVC Research Stellenbosch University) http://bit.ly/bbZw6A Prof Robert de Mello Koch (Associate Member of NITheP) (School of Physics, University of the Witwatersrand) http://bit.ly/PDpC1d Prof Roy Maartens (SKA chair at University of the Western Cape, Dept of Physics) http://www.uwcastro.org/ (Also Affiliated to Portsmouth University, UK) http://bit.ly/PDloXn Prof Azwinndini Muronga (Department of Physics, Faculty of Science, University of Johannesburg) http://bit.ly/OVQl4a (Director, UJ Science Centre, Faculty of Science University of Johannesburg, Soweto Campus) http://bit.ly/P95sLB Prof Francesco Petruccione (Deputy-Director of NITheP) (University of KwaZulu-Natal) http://quantum.ukzn.ac.za/ Prof Joao Rodrigues (Deputy-Director of NITheP) (School of Physics, University of the Witwatersrand)http://neo.phys.wits.ac.za/members/joao-rodrigues/ Prof Frederick Scholtz (Director of NITheP) Dr Nthabiseng Taole (Programme Director: Centres of Excellence, Knowledge Fields Development, NRF National Research Foundation)http://www.nrf.ac.za/index.php

Prof Patricia Whitelock (SAAO South African Astronomical Observatory) http://www.saao.ac.za/ (NASSP National Astrophysics and Space Science Programme, University of Cape Town) http://www.star.ac.za/ Dr Daniel Adams (Department of Science and Technology)