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John O | September 2018

Electrons can be tied down by graphene nanoribbons for use in quantum applications

By Josh Perry, Editor


Graphene nanoribbons are being used in a number of applications, including attempts to create next-generation computers, but researchers at the University of California, Berkeley have also discovered that these nanoribbons act as electron traps, which can be utilized in quantum computing applications.


Scanning tunneling microscope image of a topological nanoribbon superlattice.
(University of California, Berkeley)


According to a report from the university, a Berkeley professor theorized last year that two different types of nanoribbons could immobilize a single electron at the junction between the two ribbons.


“In order to accomplish this, however, the electron ‘topology’ of the two nanoribbon pieces must be different,” the article explained. “Topology here refers to the shape that propagating electron states adopt as they move quantum mechanically through a nanoribbon, a subtle property that had been ignored in graphene nanoribbons until [the professor’s] prediction.”


A nanoribbon superlattice was created from ribbons of varying widths and it produced a line of electrons that interact quantum mechanically. “Depending on the strips’ distance apart, the new hybrid nanoribbon is either a metal, a semiconductor or a chain of qubits, the basic elements of a quantum computer,” the article noted.


This theory is based on the concept that nanoribbons are topological insulators, but at the 1-D level. A single electron can get trapped at the ribbon junction and is unable to move, but if another electron is trapped nearby then they can burrow through the ribbon to meet up via the rules of quantum mechanics.


Nanoribbons were built on a gold catalyst inside a vacuum chamber. A scanning tunneling microscope was used to determine the electronic structure of the ribbon and the 50-100 junctions created were each occupied by an individual electron.


“When close together the electrons interact strongly and split into two quantum states (bonding and anti-bonding) whose properties can be controlled, allowing the fabrication of new 1D metals and insulators,” the article added. “When the trapped electrons are slightly more separated, however, they act like small, quantum magnets (spins) that can be entangled and are ideal for quantum computing.”


The research was recently published in Nature. The abstract stated:


“Topological insulators are an emerging class of materials that host highly robust in-gap surface or interface states while maintaining an insulating bulk. Most advances in this field have focused on topological insulators and related topological crystalline insulators in two dimensions and three dimensions, but more recent theoretical work has predicted the existence of one-dimensional symmetry-protected topological phases in graphene nanoribbons (GNRs).


“The topological phase of these laterally confined, semiconducting strips of graphene is determined by their width, edge shape and terminating crystallographic unit cell and is characterized by a Z2Z2 invariant (that is, an index of either 0 or 1, indicating two topological classes—similar to quasi-one-dimensional solitonic systems). Interfaces between topologically distinct GNRs characterized by different values of Z2Z2 are predicted to support half-filled, in-gap localized electronic states that could, in principle, be used as a tool for material engineering.


“Here we present the rational design and experimental realization of a topologically engineered GNR superlattice that hosts a one-dimensional array of such states, thus generating otherwise inaccessible electronic structures. This strategy also enables new end states to be engineered directly into the termini of the one-dimensional GNR superlattice.


“Atomically precise topological GNR superlattices were synthesized from molecular precursors on a gold surface, Au(111), under ultrahigh-vacuum conditions and characterized by low-temperature scanning tunnelling microscopy and spectroscopy. Our experimental results and first-principles calculations reveal that the frontier band structure (the bands bracketing filled and empty states) of these GNR superlattices is defined purely by the coupling between adjacent topological interface states.


“This manifestation of non-trivial one-dimensional topological phases presents a route to band engineering in one-dimensional materials based on precise control of their electronic topology, and is a promising platform for studies of one-dimensional quantum spin physics.”

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