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Tuesday, November 22, 2011

New Material Quantum State From Simple Research

Optical lattice in the solid state has an important role to develop a technology. The recent implementation of orbital degrees of freedom in chequerboard and hexagonal optical lattices opens up a new avenue towards discovering novel quantum states of matter that have no prior analogues in solid-state electronic materials.

Band structure for the momenta along the contour from Γ to M to X and back to Γ
Band structure for the momenta along the contour from Γ to M to X and back to Γ (nature.com)

A research conducted by the University of Pittsburgh (W. Vincent Liu, associate professor of physics in Pitt’s Department of Physics and Astronomy) and his colleagues from the University of Maryland and the University of Hamburg in England, has been studying topological states in order to advance quantum computing, a method that harnesses the power of atoms and molecules for computational tasks. Through his research, with more than $1 million in funding from two consecutive four-year grants from the U.S. Army Research Office and a five-year shared grant from the DARPA Optical Lattice Emulator Program, Liu and his team have been studying orbital degrees of freedom and nano-Kelvin cold atoms in optical lattices (a set of standing wave lasers) to better understand new quantum states of matter.

They assume an exotic topological semimetal emerges as a parity-protected gapless state in the orbital bands of a two-dimensional fermionic optical lattice. Assumption they are characterized by a parabolic band-degeneracy point with Berry flux 2 π , in sharp contrast to the π flux of Dirac points as in graphene. They also show  that the appearance of this topological liquid is universal for all lattices with D 4 point-group symmetry, as long as orbitals with opposite parities hybridize strongly with each other and the band degeneracy is protected by odd parity.

“We never expected a result like this based on previous studies,” said Liu. “We were surprised to find that such a simple system could reveal itself as a new type of topological state—an insulator that shares the same properties as a quantum Hall state in solid materials.”

“This new quantum state is very reminiscent of quantum Hall edge states,” said Liu. “It shares the same surface appearance, but the mechanism is entirely different: This Hall-like state is driven by interaction, not by an applied magnetic field.”

Liu and his collaborators have come up with a specific experimental design of optical lattices and tested the topological semimetal state by loading very cold atoms onto this “checkerboard” lattice. Generally, these tests result in two or more domains with opposite orbital currents; therefore the angular momentum remains at zero. However, in Liu’s study, the atoms formed global rotations, which broke time-reversal symmetry: The momentum was higher, and the currents were not opposite.

“By studying these orbital degrees of freedom, we were able to discover liquid matter that had no origins within solid-state electronic materials,” said Liu.

Liu says this liquid matter could potentially lead toward topological quantum computers and new quantum devices for topological quantum telecommunication. Next, he and his team plan to measure quantities for a cold-atom system to check these predicted quantum-like properties.

Source: http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2134.html#/author-information and http://www.news.pitt.edu/Nature_QuantumPhysics

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