Physics Under Pressure
Stanimir Bonev, an Assistant Professor of physics at Dalhousie University in Halifax, NS has always been the curious type. Growing up, he dreamed about one day working as a physicist and solving life’s basic problems.
“Physics examines basic concepts, such as energy, force and space-time and all that derives from these, such as Earth on which we live and the materials and devices we use in daily life.”
Today, Bonev and his Dalhousie research group—PhD students Isaac Tamblyn and Brian Boates, and research associates Amanuel Teweldeberhan and Vahid Askarpour—are working to discover the very basic electronic, structural and dynamic properties of various materials under diverse conditions.
Bonev and his group are interested in how materials’ properties change under high pressure and temperatures. These discoveries can be somewhat fundamental, but they also have long-term practical applications. In the process of making these discoveries, new types of useful materials can be formed that exhibit and retain new chemical and physical properties. An example of such a material is diamond. Diamonds can be formed from carbon at extremely high pressures where carbon is the lowest energy state, and, therefore, the most stable. Once formed, diamonds are mechanically stable in their new formation, even at atmospheric pressure, where normally the lowest energy state of carbon is graphite, a typically less stable material. Understanding the bonding and thermodynamic properties of carbon has lead to the development of new technologies for the production of significant amounts of diamond at normal conditions.
While more than 90 percent of the matter in the universe exists at pressures over one million atmospheres, most materials’ properties are only known at pressures close to one atmosphere. For example, at the centre of Earth the pressure is 3.6 million atmospheres and the temperature is 7,000 degrees Celsius, while in the centre of the gas giant Jupiter the pressure is 100 million atmospheres and it is 17,000 degrees Celsius. To understand the structure of these planets and how they formed and evolved, we need to know what the state of matter is under diverse pressure and temperature conditions.
“Unfortunately, we can’t probe the centre of Earth or the centre of Jupiter, so to understand their structures we use laboratory experiments or computer simulations,” says Bonev. “Techniques for high-pressure experiments are challenging and often provide only limited information. Computer simulations have thus become an indispensable tool for predicting material properties and explaining experimental findings.”
In order to determine a material’s properties, Bonev and his group need to solve the quantum mechanical equations of the many particles that make up this material; in other words, they need to break down the material at a microscopic or atomic level. These equations can’t be done using a simple scientific calculator and a notepad; rather they are completed through using data-intensive computer simulations.
“They have to be solved numerically, which is very complicated,” says Bonev. “That is why we have to use very large computational resources.”
The simulations of materials’ properties are large-scale calculations beyond the capability of a single computer processor and a feasible amount of time. The calculations are done in parallel over a number of processors using WestGrid’s high performance computing resources.
To determine the properties of a specific material at a given temperature and pressure, a procedure is run called a molecular dynamics simulation at the given temperature and pressure.
“We start by specifying what kind of atoms we have and their positions,” says Bonev. “Then, we find the energy of the system and the forces acting on the atoms by solving the quantum mechanical equation of the electrons and nuclei, identifying the material at a microscopic level. The atoms are then slightly moved by the forces acting on them and we do this by controlling the pressure and temperature of the system. This procedure is repeated thousands of times until we collect sufficient statistical information for the system.”
Additional calculations are then processed to determine various electronic, structural, bonding, vibration and thermodynamic properties. Following the molecular dynamics simulations, there is an exceedingly large amount of data generated. Bonev’s group now has several terabytes of data securely stored within WestGrid storage facilities for later access and examination.
“The information we are discovering is not only challenging to obtain, but it is also quite groundbreaking,” says Bonev. “It must be safely stored and we are pleased with the way WestGrid does this.”