Scientist looks at less to find out more about quantum materials

Physics gets strange when matter gets small. Take electrons orbiting the nucleus of an atom, for example. If they did so with the same dynamics by which planets orbit the sun, classical physics predicts the electrons would spiral toward the nucleus and crash into it in a fraction of a second. But that doesn’t happen.

At very low temperatures, classical physics fails to explain phenomena at tiny scales. This is when quantum mechanics kicks in. Scientists are now chilling materials to near absolute zero – so cold that molecules don’t move enough to shiver – to study the behavior of electrons in the smallest discrete building blocks of matter, such as single atoms or complex molecules. Then they are looking at those materials in reduced dimensions, which confine the flow of electrons, to study novel quantum states.

’’3-D is not very interesting because there are no surprises,’’ says Stanford Professor Aharon Kapitulnik, who conducts research in the departments of Applied Physics and Physics. ’’Usually you get new physics when you impose confinement.’’ On Feb. 15 in Denver, Kapitulnik will speak at the annual meeting of the American Association for the Advancement of Science about superconductors, insulators and other novel quantum states in artificially grown materials. Such materials may someday find application in electronic devices, transportation systems and more.

Surprisingly, Kapitulnik and his colleagues are finding out more by looking at less. As the temperature approaches absolute zero, they’re learning, for example, how electronic materials can change their character, turning from conductors and insulators into their electrical opposites, or even ending up as superconductors. At such low temperatures, these changes are governed by quantum mechanics rather than classical thermodynamics and are thus called ’’quantum phase transitions.’’

’’This is physics that is going to apply to future work – quantum computers and nanotechnology,’’ says Kapitulnik. ’’In general, devices that are going to be used in future technologies are believed to be devices of reduced dimensionality.’’

To study phase transitions of electronic materials at their quantum limits, Kapitulnik says he constrains the materials ’’from the point of view of the electron’’ and cools the system to a very low temperature.

In the three-dimensional world of length, width and height, the rules are well established for a solid in which electrons flow like a liquid: that ’’electron fluid material’’ can be either a superconductor (material with no resistance to electric flow), metal (material with some resistance, but not enough to stop current from flowing) or insulator (material with resistance so high that no current can flow), depending on the nature of the interactions in that material.

But, according to old physics, the same isn’t true in two or fewer dimensions, where only superconductor-to-insulator and insulator-to-superconductor transitions are supposed to be possible. For example, a thin film, where electrons can travel in dimensions of length and width but not height, can be either a superconductor or an insulator. But it can’t be a metal, according to old theory. ’’This is a pure quantum mechanical consequence of the theory of localization and interactions of electronic systems,’’ Kapitulnik explains. The theory says scattering from impurities and strong electron-electron interaction cause the electronic wave function to localize. ’’It has been the paradigm for understanding electronic systems in two dimensions for more than two decades.’’

But Kapitulnik disagrees with this paradigm, suggesting that the theory of how electrons behave in two dimensions – how they interact in the presence of disorder – is not fully understood. By applying a magnetic field to a superconductor, he can force electrons to behave in a way that turns the material into its electronic opposite: though the material partially maintains some of its superconducting nature, it turns into an unusual insulator. And his pioneering experiments over the past 10 years have shown that such systems can exist in a continuum of in-between states as novel metals.

Kapitulnik’s approach is to understand a fundamental question in physics – how do electrons move through materials at their quantum limits, chilled to absolute zero, when they’re in contact with systems that can dissipate energy? ’’This kind of situation is not very well understood,’’ Kapitulnik says.

Getting small

How are devices of reduced dimensionality produced? To make films as thin as a few layers of atoms, Kapitulnik uses evaporation techniques. To pattern the thin films into small structures of 10 or so nanometers, he employs lithography in the Stanford Nanofabrication Facility. The structures can be left as two-dimensional sheets or further confined to one or zero dimensions.

Alternatively, scientists can study superconducting materials found in nature to see which systems are stable, and then create superconducting crystals in the lab for their experiments. The crystals sandwich high-temperature superconductors, such as copper-oxygen compounds, between other materials. By looking at just one superconducting layer in a three-dimensional crystal, Kapitulnik can study superconductors in two dimensions.

Today’s transistors are basically two-dimensional systems, Kapitulnik says, but this is changing rapidly as smaller transistors are used in higher-density integrated systems. Restricting the flow of electrons to even fewer dimensions, carbon nanotubes confine electrons in one dimension, and atoms and molecules confine electrons in zero dimensions.

’’If you would think about making a transistor based on a single molecule, it’s going to be a transistor in the zero-dimensional limit because the electron is confined in all directions,’’ he says. Another zero-dimensional system is the quantum dot – a box that holds a discrete number of electrons. A change in voltage causes electrons to be either released or held. Many technologists believe such devices will be the building blocks of future quantum computers.

Also speaking in the AAAS session are Gabriel Aeppli of NEC Research Institute (quantum phase transition in magnetic systems); Immanuel Bloch of Ludwig-Maximilians-Universität (quantum lattices), Matthew P. A. Fisher of the University of California-Santa Barbara (theory of exotic phases of quantum materials) and Subir Sachdev of Yale (general theory of quantum phase transitions).

CONTACT: Dawn Levy, News Service: 650-725-1944, dawnlevy@stanford.edu

COMMENT: Aharon Kapitulnik, Applied Physics: 650-723-3847, aharonk@stanford.edu