Quantum simulator prototype replicates structure of graphene

June 7, 2011

(PhysOrg.com) -- A team of researchers from Columbia Engineering, the Italian National Research Council, Princeton University, University of Missouri, and University of Nijmegen (Netherlands) has developed an artificial semiconductor structure that has superimposed a pattern created by advanced fabrication methods that are precise at the nanometer scale. The pattern is similar to the honeycomb lattice that occurs in graphene. The device, called “artificial graphene" (AG), simulates quantum behavior of strongly interacting electrons. The research team sees the AG-device as a first step towards the realization of an innovative class of solid-state quantum simulators to explore fundamental quantum physics.

The research is reported in the June 3rd, 2011, issue of Science. The work is co-authored by Vittorio Pellegrini and Marco Polini of the NEST Laboratory of Istituto Nanoscienze-Cnr and Scuola Normale Superiore of Pisa; and by Aron Pinczuk, Applied Physics Professor at The Fu Foundation School of Engineering and Applied Science and Physics Professor at the School of Arts and Sciences, Columbia University; along with researchers from the Universities of Nijmegen, Missouri, and Princeton.

In order to study quantum phenomena that are difficult to be directly observed, scientists use artificial ad-hoc designed systems — quantum simulators — that can be controlled and manipulated in the laboratory. Researchers have only just begun to develop quantum simulators using different technologies. The AG-device is the first quantum simulator to be based on a semiconductor material that is designed with the goal of uncovering quantum behavior of electrons.

Phenomena such as high-temperature superconductivity, ferromagnetism, and exotic states of matter such as quantum Hall liquids and spin liquids originate from mutual interactions among many electrons. Exact calculations of the behavior of these complex systems are an impossible task even for the more sophisticated and powerful computers. Quantum simulators help bypass the problem by replacing the “uncomputable” quantum system with a controllable artificial one that is able to emulate the dynamics of the original system.

“Quantum simulators based on novel artificial semiconductor structures are at the crossroads of quantum science and innovative technologies,” says Aron Pinczuk, Applied Physics Professor at The Fu Foundation School of Engineering and Applied Science and Physics Professor at the School of Arts and Sciences, Columbia University. “While the frontiers of quantum physics are being explored with giant accelerators, in this branch of condensed matter science we employ advanced methods that expand the state-of-the-art in growth and processing of semiconductors. We could describe our work on quantum simulators as ‘probing quantum weirdness in a nano-nut-shell.’”

The simulator developed by the researchers consists of a realized on the surface of a Gallium Arsenide (GaAs) heterostructure using advanced nanofabrication methods. The artificial honeycomb lattice structure replicates that of , a material in which electrons behave in a peculiar way because of the crystal-lattice geometry. With the ability to modify key parameters such as the lattice constant of the artificial lattice, the researchers are in the position to explore different regimes of electron-electron interactions in graphene-like systems.

Vittorio Pellegrini and Marco Polini from NEST Laboratory of Istituto Nanoscienze-Cnr and Scuola Normale Superiore note that the AG-device has been tested with a “first run” trial that generated an unexpected peculiar quantum state. “The early data we collected are quite promising and show the great potential our device has,” they say. “The next step in this research is a fine-tuning of the AG-device”. The researchers are excited about the potential of creating venues for the uncovering of novel quantum states that could, eventually, lead to new device concepts and eventually to an array of applications, for instance, in advanced information processing or in cryptography.

Pinczuk added that they hope next to achieve new breakthroughs through the creation of smaller nanofabricated structures reaching limits in which individual units in patterns have lengths of five nanometers. “This is a state-of-the-art that should open access to physics and materials science that has not yet been explored!”

Explore further: The 'quantum magnet': Physicists expand prospects for engineering unusual materials

More information: Two-Dimensional Mott-Hubbard Electrons in an Artificial Honeycomb Lattice, Science 3 June 2011: Vol. 332 no. 6034 pp. 1176-1179. DOI: 10.1126/science.1204333

Artificial crystal lattices can be used to tune repulsive Coulomb interactions between electrons. We trapped electrons, confined as a two-dimensional gas in a gallium arsenide quantum well, in a nanofabricated lattice with honeycomb geometry. We probed the excitation spectrum in a magnetic field, identifying collective modes that emerged from the Coulomb interaction in the artificial lattice, as predicted by the Mott-Hubbard model. These observations allow us to determine the Hubbard gap and suggest the existence of a Coulomb-driven ground state.

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