Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P2: The Kondo Ground State in Graphene |
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Sponsoring Units: DCMP Chair: Piers Coleman, Rutgers University Room: Ballroom A2 |
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P2.00001: Topological Kondo Ground State in Graphene Invited Speaker: Dirac electrons in graphene comprise two-component wavefunctions and quantum symmetries intertwining pseudospin, chirality, and Berry's phase, all ultimately stemming from a node or topological degeneracy in the spectrum known as the Dirac point. Graphene represents one prototype example of a larger class of nodal metals in which a relativistic spectrum causes the density of states to vanish linearly. Based on the unique electronic structure of such systems, a large body of theoretical work has highlighted the propensity for Dirac electrons to condense in strongly correlated ground states when additionally coupled to the real spin degree of freedom. We report the observation of one of these elusive ground states, realized in graphene via unconventional Kondo screening of individual atomic spins by massless Dirac fermions. Low-temperature scanning tunneling microscopy reveals the emergence of a new energy scale and a striking bimodal Kondo resonance localized around magnetic atoms placed on a clean graphene monolayer. Quasiparticle interference maps and concomitant spectroscopy in a high magnetic field demonstrate the spin origin of the associated ground states, and their direct link to local conservation or breaking of effective time-reversal symmetry in the underlying Dirac Hamiltonian. We find these novel spin states to be topologically controlled by Berry phase interference; in the most exotic manifestation, we show experimental evidence for two electron flavors---decoupled in momentum space by a $\pi $ Berry phase shift cancellation---participating in a chiral two-channel Kondo effect. We link these results to a new platform we have developed for the study of topological phases, artificial graphene assembled by atomic manipulation. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P2.00002: Orbitally controlled Kondo effect in graphene Invited Speaker: Graphene differs from usual metals or semiconductors in being a truly two-dimensional material with the charge carriers resembling massless Dirac fermions and the chemical potential being highly tunable by gate voltages. Recently, scanning tunneling spectroscopy experiments opened the exciting possibility to address the interaction of graphene with magnetic adatoms and to investigate the Kondo effect in a material that is simple, of immediate technological importance and offers unprecedented high tunability. Here, we develop a realistic description of the interaction of magnetic adatoms with graphene and explain the role of orbital symmetries: General symmetry arguments show that the Kondo effect in graphene is controlled not only by the spin but also by the orbital degree of freedom and spin-orbit coupling. For the example of Co adatoms, commonly used in experiments, we identify possible scenarios for the Kondo effect based on ab initio calculations. For a Co atom absorbed on top of a carbon atom, the Kondo effect is quenched by spin-orbit coupling below an energy scale of 15K. For Co with spin S=1/2 located in the center of a hexagon, a crossover from SU(4) Kondo physics at higher energies to an SU(2) Kondo effect on the scale of the Co spin-orbit coupling strength is encountered. The interplay of the orbital adatom physics and the peculiar band structure of graphene is directly accessible in Fourier transform tunneling spectroscopy or in the gate-voltage dependence of the Kondo temperature which is predicted to display a very strong, characteristic particle-hole asymmetry. The particular high symmetry situation provided by adatoms on graphene can pave the way for a deeper understanding of Kondo screening in general nanomagnetic structures. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P2.00003: Kondo effect and STM spectroscopy of Dirac electrons in graphene Invited Speaker: We show that graphene, whose low-energy quasiparticles display Dirac like behavior, may exhibit a two-channel Kondo effect in the presence of magnetic impurities. We present a large $N $analysis for a generic spin $S $local moment coupled to Dirac electrons in graphene and demonstrate that the corresponding Kondo temperature can be tuned by an experimentally controllable applied gate voltage. We also study the STM spectra of these Dirac electrons in the presence of such impurities and demonstrate that such spectra depend qualitatively on the position of the impurity atom in the graphene matrix. More specifically, for impurity atoms atop the hexagon center, the zero-bias tunneling conductance, as measured by a STM, shows a peak; for those atop a graphene site, it shows a dip. We provide a qualitative theoretical explanation of this phenomenon and show that this unconventional behavior is a consequence of conservation/breaking of pseudospin symmetry of the Dirac quasiparticles by the impurity. We also predict that tuning the Fermi energy to zero by a gate voltage would not lead to qualitative change in the shape of the conductance spectra when the impurity is atop the hexagon center. A similar tuning of the Fermi energy for the impurity atop a site, however, would lead to a change in the tunneling conductance from a dip to a peak via an antiresonance. We discuss some recent experiments on a doped graphene sample that seem to have qualitative agreement with our theory and suggest further experiments to test our predictions. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P2.00004: Quantum critical Kondo screening in graphene Invited Speaker: Magnetic impurities in neutral graphene provide a realization of the pseudogap Kondo model, which displays a quantum phase transition between phases with screened and unscreened impurity moment. In this talk, I discuss the physics of the pseudogap Kondo model with finite chemical potential $\mu$. While carrier doping restores conventional Kondo screening at lowest energies, properties of the quantum critical fixed point turn out to influence the behavior over a large parameter range. Most importantly, the Kondo temperature $T_K$ shows an extreme asymmetry between electron and hole doping. At criticality, depending on the sign of $\mu$, $T_K$ follows either the scaling prediction $T_K\propto|\mu|$ with a universal prefactor, or $T_K \propto |\mu|^x$ with $x\approx2.6$. This asymmetry between electron and hole doping extends well outside the quantum critical regime and also implies a qualitative difference in the shape of the tunneling spectra for both signs of $\mu$. Finally, the considerations are extended to the two-channel Kondo model where non-Fermi liquid behavior emerges at lowest energies. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 11:00AM |
P2.00005: Gate-Controlled Ionization and Screening of Cobalt Adatoms on a Graphene Surface Invited Speaker: Graphene impurities provide both a source of mobility-limiting disorder as well as a means to alter the graphene electronic structure in a desirable way. While these effects have thus far been primarily studied with spatially averaged techniques, understanding the microscopic physics of such behaviour requires local-probe exploration of the subnanometre-scale electronic and structural properties of impurities on graphene. In this talk I will describe scanning tunnelling microscopy and spectroscopy measurements made on individual Co atoms deposited onto back-gated graphene devices. We observe features in the tunneling local density of states (LDOS) of the Co adatoms related to both atomic resonances and phonon excitations. We also find that the electronic structure of Co adatoms can be tuned by application of the device gate voltage, and that the Co atoms can be reversibly ionized. Large screening clouds are observed to form around Co adatoms ionized in this way, and we observe that some intrinsic graphene defects also show charging behaviour. Our results provide new insight into charged-impurity scattering in graphene, as well as the possibility of using graphene devices as chemical sensors. The relationship between our measurements and recent transport experiments will also be discussed. [Preview Abstract] |
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