Investigating certainties and uncertainties

Research funded by FAPESP and Texas Tech is studying high energy physics and structural defects in systems that use graphene and attempting to discover new pathways in quantum mechanics

By Heitor Shimizu, in Lubbock  |  Agência FAPESP – A project that is bringing together researchers from the United States and Brazil to study structural defects in graphene-based systems through studies of its chemical properties at the quantum level was presented at FAPESP Week Nebraska-Texas, held September 18-22 in the United States.

Francisco Bolivar Correto Machado, full researcher at the Technological Institute of Aeronautics (ITA), in the State of São Paulo, and Hans Lischka, an adjunct professor in the Department of Chemistry and Biochemistry at TTU, are coordinating the project, selected through a call for proposals under SPRINT – São Paulo Researchers in International Collaboration.

The project is primarily investigating the excited state properties of carbon nanostructures such as graphene and its analogs; the characteristics of structures produced by species chemically absorbed in graphene sheets; and the physical and chemical absorption of transition metals in graphene for applications in spintronics – a field that explores the quantum propensity of electrons to spin as well as applications for the state of their charges.

One of the crystalline forms of carbon – like diamond and graphite –, graphene is considered one of the most promising materials for applications in nanoelectronics. Extremely resistant and an excellent conductor of electricity, graphene is not easily produced. And any defect, no matter how miniscule – in measurements on the order of billionths of a meter –, can render the potential of the new material useless.

“Our project addresses topics such as describing the electronic structure of the defect’s excited state using extended models, chemical reactivity, chemical doping and polycyclic aromatic hydrocarbons as models for interactions on graphene sheets,” said Machado.

The collaboration has already resulted in several publications in high impact journals such as the 2017 paper “How to efficiently tune the biradicaloid nature of acenes by chemical doping with boron and nitrogen” (Physical Chemistry Chemical Physics) and “Single and double carbon vacancies in pyrene as first models for graphene defects: A survey of the chemical reactivity toward hydrogen” (Chemical Physics). Four other articles are in preparation for publication.

Machado and Lischka, in collaboration with other researchers, are organizing the 3rd School of Computational Chemistry – Theory of new materials at atomistic level: Graphene, Graphene Defects and π-Conjugated Polyradical Systems, which will be held in Ribeirão Preto (State of São Paulo) December 11-14, 2017.

Certainty and uncertainty

Bill Poirier, a professor in the Department of Chemistry and Biochemistry at TTU, talked about the project, “Fermi accelerators, inverse fermi accelerators, nonadiabatic dynamics and quantum trajectories: towards a method for electron dynamics,” that he is conducting with Professor Mahir Saleh Hussein, head of the Non-Conventional Astrophysics Group at the Institute of Advanced Studies of the University of São Paulo (IEA-USP).

Selected under a recent SPRINT call for proposal and still in its early stages, the project involves theoretical physics. “Our objective is to solve quantum mechanics problems related to trajectories. We want to find solutions that are both time-dependent and time-independent and have potential applications in electron dynamics,” he said.

“In a previous collaboration with Professor Hussein – who has published more than 340 articles, including 71 in the journals Physical Review Letters and Physical Letters –, we developed a new trajectory-based formulation for quantum mechanics, in which waves have no role,” he said.

Poirier noted that Max Planck preferred “realistic” descriptions of the physical world, as did Albert Einstein. “I like to think that Planck would enjoy our approach, which is a realistic and ontological quantum theory, but as he himself put it, there is no greater obstacle to successfully developing a new hypothesis than overstepping one’s boundaries,” he said.

The approach taken by Poirier and Hussein is based on the classical quantum system of the particle in a box. It is a theoretical problem where a particle in motion inside a box collides with the sides of the box and cannot escape, nor can it lose energy.

In classical mechanics, the solution is that the particle moves in a straight line at a constant speed until it bounces off one of the walls. When it bounces, the speed is altered only in the element perpendicular to the wall, whose sign changes. The unit of speed does not change. One possible solution is a stationary particle with zero speed.

The problem becomes more complex in quantum mechanics and the results are no longer intuitive. The particle can only have certain specific energy levels and zero no longer fits the context. In addition to that, there is no guarantee that the particle can even be detected inside the box. It might be in positions that are impossible to detect. It goes from certainty to uncertainty.

“Based on the particle in the box, we can ask what would happen if one of the walls of the box began to move? The two perspectives: the classical (which involves trajectory) and the quantum (wave) seem to provide radically different answers,” Poirier said.

“The bottom line is that we began to investigate whether nature behaves like a wave or like a particle. That is theory. In practice, the model could prove to be very useful for important processes that are taking place in both nuclear physics and chemical physics,” he said.

High energies

Sergio Ferraz Novaes, full professor at the State University of São Paulo (Unesp) and leader of the São Paulo Research and Analysis Center (SPRACE), talked about the project he is conducting with Nural Akchurin, professor and chair of the Department of Physics and Astronomy at TTU. The project “SPRACE-Unesp and TTU collaboration in high-energy physics in the Compact Muon Solenoid experiment at the Large Hadron collider” was selected under a SPRINT call for proposal.

“The research groups at TTU and SPRACE are active in the CMS Collaboration at CERN [European Organization for Nuclear Research]. Both groups have a participatory and leadership position in activities involving physics analysis other than the standard model. It is hoped that after the discovery of the Higgs boson, the community’s attention will go back to that,” Novaes said.

The standard model of particle physics is a theory that describes the four fundamental forces: gravity, weak, electromagnetic and strong. It also describes the fundamental particles that constitute matter.

“Our common interests include looking for a candidate for dark matter, a new form of matter that would help explain some of the current observations in cosmology. We are also looking for new resonances, predicted in many types of physics models besides the standard model,” Novaes said.

In addition to basic research in high energy physics, Novaes explains that the project also plans to promote innovation, training, education and communication.

“Research and development of new technologies, partnerships with academic institutions and companies, promoting knowledge in cutting-edge fields, training high school teachers and sharing knowledge with society are other project objectives,” he said.