Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Cool Calculations for Cold Atoms

03.09.2014

New theory of universal three-body encounters

Chemical reactions drive the mechanisms of life as well as a million other natural processes on earth. These reactions occur at a wide spectrum of temperatures, from those prevailing at the chilly polar icecaps to those at work churning near the earth’s core.

At nanokelvin temperatures, by contrast, nothing was supposed to happen. Chemistry was expected to freeze up. Experiments and theoretical work have now show that this is not true. Even at conditions close to absolute zero atoms can interact and manage to form chemical bonds.

Within this science of ultracold chemistry, there is a sub-field that deals with “Efimov states,” named for Russian physicist Vitaly Efimov. In 1970 he predicted that under some conditions all two-particle bound states would be unstable while (paradoxically) some three-particle states could exist. Such states were eventually seen experimentally in 2006, among cesium atoms (see Related JQI Article below).

Two scientists at the Joint Quantum Institute have now formulated a universal theory to describe the properties of these Efimov states, a theory that, for the first time, does not need extra adjustable unknown parameters . This should allow physicists to predict the rates of chemical processes involving three atoms---or even more---using only a knowledge of the interaction forces at work.

The JQI authors, Yujun Wang and Paul Julienne, publish their results in the journal Nature Physics (see Reference Publication below).

PICO-ELECTRON VOLTS

Efimov states are fragile. They depend for their existence on quantum effects and on the subtle interplay of two phenomena: Feshbach resonance and van der Waal forces. Quantum effects are necessarily at work at ultracold temperatures in the nano-kelvin regime. Here atoms should be viewed not as hard balls, typically a few tenths of nanometers across, but as wave packets, blobs extending over hundreds of nm.

It is common, when talking about colliding particles, to see them as cars speeding toward each other, perhaps meeting head on or glancing off at a relative angle. It is more unusual to visualize the collision if the “particles” are so large as to overlap each other at relatively great distances. More strange still if three such particles are involved in an interaction whose result will be a loosely-bound confederation.

In the study of Efimov states, the primary force at work among the atoms is the van der Waals force, named for Dutch physicist Johannes Diderik van der Waals. This long-range force among atoms or molecules arises from the temporary appearance of electric dipole moments in the particles.

Even for a neutral atom, a momentary imbalance of charge---more of the atomic electrons’ negative charge might appear to the left, say, leaving a positive preponderance on the right---will constitute an electric dipole, which in turn can attract an atom with a complementary dipole orientation. This induced-dipole force varies at the inverse sixth power of the distance between the two particles.

Another way of controlling inter-particle collisions at ultracold temperatures is to turn on an external magnetic field. For certain ranges of field strength, two particles can be coaxed to form semi-stable objects called Feshbach resonances, named for US physicist Herman Feshbach. Feshbach resonances are commonly used in cold-physics to control interactions, and this is especially true in the study of Efimov states.

Often Feshbach resonances are described in terms of a parameter, a, called the scattering length, denoting the effective distance over which the interaction takes place If a is positive and large (much larger than the nominal range of the force between the atoms), weak binding of atoms can happen. If a is negative, a slight attraction of two atoms can occur but not binding. If, however, a is large and three atoms are present, then the Efimov state can appear. Indeed an infinite number of such states can occur.

In general since it allows interactions over large distances, the Feshbach effect is more important than the van der Waals force. But the JQI research has shown how the van der Waals force can be decisive in forming Efimov states, especially when the scattering length is short. Many scientists had believed that making consistent predictions of triplet-forming interactions would be difficult to make. Instead, the Wang-Julienne model successfully incorporates this short-distance regime.

Thus there should be a series of Efimov states, with various binding energies. But unlike atoms, where the quantum energy levels (denoting how much energy is needed to liberate the electron from its atomic binding) are in the electron volt (eV) range, Efimof states are typified by quantum energies of billionths of an eV or less.

THE NEW JQI THEORY

Wang and Julienne build their theory of 3-body van der Waals physics around the Schrödinger equation, the equation introduced by Erwin Schrödinger in the 1920s to treat particles as waves. Only here it is three particles---viewed as three sets of waves, or rather as a complex of waves representing the three particles---carefully studied in pairwise fashion to simulate an effective composite force field in which the three particles operate.

The result is a theoretical tool that can predict the important Efimov properties, namely the energies of the Efimov states, the widths of those states (essentially the fuzziness of our knowledge of the precise energy value), and the rates at which the three-particle states will form inside a gas of ultracold atoms.

“Our theory works for a full range of scattering lengths,” said Yujun Wang describing the JQI work, “whereas the previous theories could only apply to large scattering lengths. We don't need adjustable parameters. The only inputs in our theory are the known two-body Feshbach parameters and our calculations using the Schrodinger equation. So our theory does not rely on any of the unknown three-body inputs that have been used in previous theories to fit the experimental data. In these two aspects our theory is more comprehensive and powerful. We can make quantitative predictions without relying on the unknowns, so that our results can be directly compared to experiments.”


Figure 1. Three panels illustrate the condition of Efimov states (3-atom stable states). The upper, bell-shaped surface represents the probability density for each of the three geometries, while the gray surface represents the strength of the van der Waals force for that geometry. Left panel: the three interacting atoms lie in an equilateral triangle formation. Middle panel: two of the atoms are rather closer together than to the third atom. Right panel: the geometry of the middle panel but with the atoms lying farther apart from each other. The dimples in the probability density surface reflect the more complicated interaction when two of the atoms are close together.Credit JQI/Yujun

Research Contact
Paul Julienne
Yujun Wang
Media Contact
Phillip F. Schewe|
(301) 403-0989

Phillip F. Schewe | Eurek Alert!

More articles from Physics and Astronomy:

nachricht Physicists precisely measure interaction between atoms and carbon surfaces
29.05.2015 | University of Washington

nachricht How comets were assembled
29.05.2015 | Universität Bern

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: Lasers are the key to mastering challenges in lightweight construction

Many joining and cutting processes are possible only with lasers. New technologies make it possible to manufacture metal components with hollow structures that are significantly lighter and yet just as stable as solid components. In addition, lasers can be used to combine various lightweight construction materials and steels with each other. The Fraunhofer Institute for Laser Technology ILT in Aachen is presenting a range of such solutions at the LASER World of Photonics trade fair from June 22 to 25, 2015 in Munich, Germany, (Hall A3, Stand 121).

Lightweight construction materials are popular: aluminum is used in the bodywork of cars, for example, and aircraft fuselages already consist in large part of...

Im Focus: Solid-state photonics goes extreme ultraviolet

Using ultrashort laser pulses, scientists in Max Planck Institute of Quantum Optics have demonstrated the emission of extreme ultraviolet radiation from thin dielectric films and have investigated the underlying mechanisms.

In 1961, only shortly after the invention of the first laser, scientists exposed silicon dioxide crystals (also known as quartz) to an intense ruby laser to...

Im Focus: Advance in regenerative medicine

The only professorship in Germany to date, one master's programme, one laboratory with worldwide unique equipment and the corresponding research results: The University of Würzburg is leading in the field of biofabrication.

Paul Dalton is presently the only professor of biofabrication in Germany. About a year ago, the Australian researcher relocated to the Würzburg department for...

Im Focus: Basel Physicists Develop Efficient Method of Signal Transmission from Nanocomponents

Physicists have developed an innovative method that could enable the efficient use of nanocomponents in electronic circuits. To achieve this, they have developed a layout in which a nanocomponent is connected to two electrical conductors, which uncouple the electrical signal in a highly efficient manner. The scientists at the Department of Physics and the Swiss Nanoscience Institute at the University of Basel have published their results in the scientific journal “Nature Communications” together with their colleagues from ETH Zurich.

Electronic components are becoming smaller and smaller. Components measuring just a few nanometers – the size of around ten atoms – are already being produced...

Im Focus: IoT-based Advanced Automobile Parking Navigation System

Development and implementation of an advanced automobile parking navigation platform for parking services

To fulfill the requirements of the industry, PolyU researchers developed the Advanced Automobile Parking Navigation Platform, which includes smart devices,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

International symposium: trends in spatial analysis and modelling for a more sustainable land use

20.05.2015 | Event News

15th conference of the International Association of Colloid and Interface Scientists

18.05.2015 | Event News

EHFG 2015: Securing health in Europe. Balancing priorities, sharing responsibilities

12.05.2015 | Event News

 
Latest News

Physicists precisely measure interaction between atoms and carbon surfaces

29.05.2015 | Physics and Astronomy

Godwits are flexible...when they get the chance

29.05.2015 | Life Sciences

Project start: New active substance targeting dreaded hospital pathogens

29.05.2015 | Health and Medicine

VideoLinks
B2B-VideoLinks
More VideoLinks >>>