Understanding Of Complex Electronic Materials Advanced By Yale Research of "Spin Dynamic"

Yale physicists have developed a detailed theory for how complex electronic materials, like those used in modern computer disk drives, operate.

“New complex materials are often at the heart of electronic devices that underlie today’s Internet and wireless revolution,” said Subir Sachdev, principal investigator on the study. “Our goal is to understand the electronic properties of these materials by relating it to basic laws of quantum mechanics like the uncertainty principle and Schroedinger”s equation for the wave function of electrons. These laws control the motion of electrons at the microscopic scale.”

Published in the December 24 issue of Science, the study looks at a class of materials called layered transition metal oxides. These materials have complex magnetic properties and some of them are superconducting-allowing electric current to flow through without any measurable resistance. This superconductivity appears below a certain critical temperature and this critical temperature is higher for this class of compounds than any other known materials.

“Our paper focuses on a ‘quantum phase transition’ that can occur in the magnetic properties of these materials,” said Sachdev, professor of physics and applied physics at Yale. “This is a phase transition, like the melting of ice.”

The quantum phase transition is between an ice-like ‘Neel phase’ in which the electron spin axes are arranged in a regular checkerboard pattern, and a water-like ‘spin liquid’ where the spin axes fluctuate in all directions.

In the spin liquid, the magnetic response of a foreign impurity is restricted by the principles of quantum mechanics to be that of free spin with an angular momentum which is precisely 1/2 times Planck’s constant.

“We show that this property holds all the way up to the quantum-critical point to the Neel phase, but at the quantum critical point, the angular momentum is a new irrational number times Planck’s constant,” Sachdev said. “We provide initial estimates for this irrational number-future studies involving numerical computations on supercomputers should determine its value more precisely. Future experiments should also be able to measure this ‘irrational angular momentum’ in the laboratory and so test our theory.”

Sachdev’s research team included graduate student Chiranjeeb Buragohain and postdoctoral fellow Matthias Vojta of Yale.