Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J3: Kondo Screening and Quantum Criticality from the Spatial Limit: From Single Spins to Droplets to Lattices |
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Sponsoring Units: DCMP Chair: James Davis, Cornell University Room: Morial Convention Center RO2 - RO3 |
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J3.00001: How does a Kondo impurity respond to its local environment? Invited Speaker: The interplay between localized electrons on a magnetic atom and the conducting electrons in a metal can lead to intriguing many-body ground states such as the Kondo effect. When a spin is Kondo screened by conduction electrons the entire spin system performs a complicated dance that results in the formation of a spin singlet at sufficiently low temperature. For simplicity, most theoretical considerations of Kondo screening focus on magnetic impurities with the lowest possible spin S~=~1/2. Such systems can be studied experimentally in exquisite detail and with great control using quantum dots in semiconductor heterostructures or carbon nanotubes. However, in Kondo systems consisting of localized magnetic atoms, the spin is often larger, making the Kondo effect richer and more complex. Here we use the imaging and spectroscopy capabilities of a scanning tunnelling microscope to study how the Kondo screening of a known high-spin atom is determined by its local environment. Co and Ti atoms were deposited on a thin insulating layer (Cu$_{2}$N) on a copper substrate. We study the influence of external magnetic fields, crystalline magnetic anisotropy, as well as spin-coupling to surrounding atomic spins on the Kondo effect that forms on the Co or Ti atoms. We find that the anisotropy of the crystalline field quenches the high-spin system of Co (S~=~3/2) into an effective S~=~1/2 Kramers doublet. Surprisingly, much of the impact of these environmental factors on the complex many-body ground state can be understood simply through their effects on the energy levels of the unscreened spin. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:27PM |
J3.00002: Geometric Manipulation of Quantum Phase and Correlations in Nanoassembled Spin Systems Invited Speaker: The single-impurity Kondo problem, in which an isolated magnetic impurity in a non-magnetic metallic host has its spin screened by spins of conduction electrons, has been extensively studied both theoretically and through bulk experiments. Recently new methods have allowed detailed experimental probing of the prototype single-impurity Kondo effect of individual magnetic atoms. In addition, spin interactions with confined electron states were used to materialize ``quantum mirages'' consisting of a nonlocal single-impurity Kondo effect. When many spin and spatial states are present in a bulk conductor or on its surface, the interactions between them may engender novel collective effects. Using a scanning tunneling microscope we assembled and studied atomically precise arrangements of (magnetic) Co atoms and (non-magnetic) CO molecules on the Cu(111) surface. The spin degeneracy of single magnetic atoms and the conduction electrons alone provide necessary ingredients for Kondo physics. When combined with quantum resonators, Kondo phase shifts can be measured by using the nanostructures as quantum interferometers. We study the effects of adding to the degeneracy of the system in two controlled ways: engineering degeneracies in the spatial states of confined electrons coupled to the spins, and engineering lattices of many spins coherently coupled through electrons. The first type of experiment has enabled a novel method to read out and geometrically manipulate the quantum phase associated with state superpositions. The second class of experiments has enabled investigation of the finite size spin physics of Kondo droplets. In these periodic structures we observe signs of quantum interference and spin correlation effects when the geometries are suitable tuned relative to the Fermi wavelength of the host electron systems. These new quantum materials act as model systems for understanding complementary physics in complex matter. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J3.00003: Kondo Physics at the Nanoscale Invited Speaker: Recent experimental advances have made it possible to study the Kondo effect in nanoscale structures. These achievements are of great importance because they do not only allow us to manipulate the Kondo screening of single magnetic impurities, but also provide us with the unique opportunity to study how Kondo screening and coherence evolve on different lengthscales from a single Kondo impurity to the Kondo lattice. In this talk, I present two examples of novel Kondo physics emerging in nanostructures. First, I demonstrate that the presence of electronic eigenmodes in a nanostructure gives rise to unconventional properties of a Kondo screened magnetic impurity [1]. In particular, the Kondo temperature, $T_K$, of a magnetic impurity located inside the nanostructure varies with the impurity's location and is determined by the eigenmodes' spatial structure. Moreover, the modes' frequency dependence leads to a linear relation between $T_K$ and the local density of states, in stark contrast to the conventional Kondo effect. Second, I discuss Kondo screening and the onset of coherence in finite size Kondo lattices, so-called {\it Kondo droplets}. I show that in such Kondo nanostructures, the hybridization and the coherent coupling of the Kondo resonances can be resonantly enhanced or suppressed via changes in the droplet's geometry and lattice constant. Moreover, I demonstrate how these properties of the Kondo droplet evolve with increasing droplet size. Finally, I discuss how the ability to manipulate the properties of Kondo droplets might provide novel insight into the origin of quantum criticality, which is a central point in understanding the unconventional non-Fermi liquid properties of Kondo lattice systems, such as the heavy-fermion materials. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:39PM |
J3.00004: Bi-layer $^{3}$He: a simple two dimensional heavy fermion system with quantum criticality Invited Speaker: Two dimensional helium films provide simple model systems for the investigation of quantum phase transitions in two dimensions. Monolayer $^{3}$He absorbed on graphite, with various pre-platings, behaves as a two dimensional Mott-Hubbard system, complete with a density driven ``metal-insulator'' transition [1, 2] into what appears to be a gapless spin-liquid. In two dimensions the corrections to the temperature dependence of the fluid heat capacity, beyond the term linear in $T$, are anomalous and attributed to quasi-1D scattering [3]. On the other hand, bi-layer $^{3}$He films adsorbed on the surface of graphite show evidence of two-band heavy-fermion behavior and quantum criticality [4, 5]. The relevant control parameter is the total density of the $^{3}$He film. The $^{3}$He bilayer system can be driven toward a quantum critical point (QCP) at which the effective mass appears to diverge, the effective inter-band hybridization vanishes, and a local moment state appears. A theoretical model in terms of a ``Kondo breakdown selective Mott transition'' has recently been suggested [6]. * In collaboration with: A Casey, M Neumann, J Nyeki, B Cowan. [1] Evidence for a Mott-Hubbard Transition in a Two-Dimensional $^{3}$He Fluid Monolayer, A. Casey, H. Patel, J. Ny\'{e}ki, B. P. Cowan, and J. Saunders Phys. Rev. Lett. \textbf{90}, 115301 (2003) [2] D Tsuji et al. J. Low Temp. Phys. 134, 31 (2004) [3] A V Chubukov et al. Phys. Rev. \textbf{B71}, 205112 (2005) [4] Bilayer $^{3}$He; a simple two dimensional heavy fermion system with quantum criticality, Michael Neumann, Jan Nyeki, Brian Cowan, John Saunders. Science \textbf{317}, 1356 (2007) [5] Heavy fermions in the original Fermi liquid. Christopher A Hooley and Andrew P Mackenzie. Science \textbf{317}, 1332 (2007) [6] C Pepin, Phys. Rev. Lett. \textbf{98}, 206401 (2007) and A Benlagra and C Pepin, arXiv: 0709.0354 [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 2:15PM |
J3.00005: Quantum Criticality in 3He bi-layers Invited Speaker: |
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