Post by williamplayer on Jan 22, 2014 12:30:33 GMT
Physics Today
Graphene Yields evidence of Atomic Collapse
A relativistic phenomenon predicted for highly charged nuclei has been seen in a tabletop experiment
Since the discovery that electronsvin graphene behave as masslessDirac fermions (see PHYSICS TODAY, January 2006, page 21), the single-atom-thick material has become a fertile playground for testing exotic predictions of quantum electrodynamics. Graphene experiments have been used successfully to probe such relativistic phenomena as Klein tunneling—the anomalous, unimpeded passage of electrons through a high potential barrier and the fractional quantum Hall effect (see PHYSICS TODAY, January 2010, page 11). Now add to that list atomic collapse, the spontaneous formation of electrons and positrons in the electrostatic field of a superheavy atomic nucleus. Although the phenomenon has eluded detection in collider experiments, its condensed matter analogue was recently spotted in a collaborative effort headed by Michael Crommie (University of California, Berkeley) and Leonid Levitov (MIT).
Sparking the Vacuum
Atomic collapse is thought to effectively set a ceiling on the periodic table: For any atom whose proton number exceeds some critical value—estimated to be around 170—the Dirac equation predicts that the Coulomb field of the nucleus should be strong enough to create an electron–positron pair from the vacuum. The newly formed electron falls inward and “decharges” the nucleus; its antimatter counterpart is flung outward. To see atomic collapse in an experiment, however, one would first need to create an atom nearly 50% more massive than the current record holder, element 118. Collision experiments designed to fleetingly create the requisite superheavy nuclei have yielded only ambiguous results. Around 2006, Levitov, Antonio Castro Neto (Boston University and the National University of Singapore), and other condensed-matter theorists independently began to explore a two dimensional analogue of the atomic collapse problem. “Our interest at the time was to understand how impurities change graphene’s electrical properties,” recalls Castro Neto. “And we knew that the most important kinds of impurities are atoms that stick to the graphene surface and acquire charge.” In models of atomic collapse, such charge impurities are treated as atomic nuclei. Graphene itself is treated as the vacuum: Its relativistic electrons and holes are analogous to the virtual particles that populate the vacuum in quantum field theories. Crucially, because the effective fine structure constant—which sets the strength of interaction between charges—is large for graphene, theorists predicted that the graphene version of atomic collapse should occur at a critical charge much closer to 1 than to 170. Crommie, an experimentalist, first learned of that prediction when he saw Levitov present theoretical results at a 2009 conference. “I asked him, ‘Is this something we can really see in an experiment?’” As it turned out, not only does the collapse produce an experimentally observable signature, but Crommie’s Berkeley group had already been refining the techniques needed to detect it.
Triggering Collapse
Levitov’s calculations suggested that atomic collapse in graphene should reveal itself as a telltale electronic resonance—a short-lived, quasi-bound electronic state whose energy lies near the Dirac point, the point in energy momentum space where graphene’s conduction and valence bands meet. Crommie’s lab specializes in making precision scanning tunneling microscopy measurements, which can reveal such resonances. In a typical experimental setup, depicted in figure 1, a bias voltage Vis applied between a graphene monolayer and an STM tip. By measuring how the tunneling current I varies with V, the researchers can piece together a picture of how the charge-carrier concentration in the graphene monolayer varies with energy and location. A gate electrode allows the team to manipulate the level of electron or hole doping in the sample.
Read Full Article: www.mit.edu/~levitov/Physics_Today_2013.pdf
Graphene Yields evidence of Atomic Collapse
A relativistic phenomenon predicted for highly charged nuclei has been seen in a tabletop experiment
Since the discovery that electronsvin graphene behave as masslessDirac fermions (see PHYSICS TODAY, January 2006, page 21), the single-atom-thick material has become a fertile playground for testing exotic predictions of quantum electrodynamics. Graphene experiments have been used successfully to probe such relativistic phenomena as Klein tunneling—the anomalous, unimpeded passage of electrons through a high potential barrier and the fractional quantum Hall effect (see PHYSICS TODAY, January 2010, page 11). Now add to that list atomic collapse, the spontaneous formation of electrons and positrons in the electrostatic field of a superheavy atomic nucleus. Although the phenomenon has eluded detection in collider experiments, its condensed matter analogue was recently spotted in a collaborative effort headed by Michael Crommie (University of California, Berkeley) and Leonid Levitov (MIT).
Sparking the Vacuum
Atomic collapse is thought to effectively set a ceiling on the periodic table: For any atom whose proton number exceeds some critical value—estimated to be around 170—the Dirac equation predicts that the Coulomb field of the nucleus should be strong enough to create an electron–positron pair from the vacuum. The newly formed electron falls inward and “decharges” the nucleus; its antimatter counterpart is flung outward. To see atomic collapse in an experiment, however, one would first need to create an atom nearly 50% more massive than the current record holder, element 118. Collision experiments designed to fleetingly create the requisite superheavy nuclei have yielded only ambiguous results. Around 2006, Levitov, Antonio Castro Neto (Boston University and the National University of Singapore), and other condensed-matter theorists independently began to explore a two dimensional analogue of the atomic collapse problem. “Our interest at the time was to understand how impurities change graphene’s electrical properties,” recalls Castro Neto. “And we knew that the most important kinds of impurities are atoms that stick to the graphene surface and acquire charge.” In models of atomic collapse, such charge impurities are treated as atomic nuclei. Graphene itself is treated as the vacuum: Its relativistic electrons and holes are analogous to the virtual particles that populate the vacuum in quantum field theories. Crucially, because the effective fine structure constant—which sets the strength of interaction between charges—is large for graphene, theorists predicted that the graphene version of atomic collapse should occur at a critical charge much closer to 1 than to 170. Crommie, an experimentalist, first learned of that prediction when he saw Levitov present theoretical results at a 2009 conference. “I asked him, ‘Is this something we can really see in an experiment?’” As it turned out, not only does the collapse produce an experimentally observable signature, but Crommie’s Berkeley group had already been refining the techniques needed to detect it.
Triggering Collapse
Levitov’s calculations suggested that atomic collapse in graphene should reveal itself as a telltale electronic resonance—a short-lived, quasi-bound electronic state whose energy lies near the Dirac point, the point in energy momentum space where graphene’s conduction and valence bands meet. Crommie’s lab specializes in making precision scanning tunneling microscopy measurements, which can reveal such resonances. In a typical experimental setup, depicted in figure 1, a bias voltage Vis applied between a graphene monolayer and an STM tip. By measuring how the tunneling current I varies with V, the researchers can piece together a picture of how the charge-carrier concentration in the graphene monolayer varies with energy and location. A gate electrode allows the team to manipulate the level of electron or hole doping in the sample.
Read Full Article: www.mit.edu/~levitov/Physics_Today_2013.pdf