Session A19: Invited Session: New Correlated Electron Physics Using Scanning Tunneling Microscopy and Other Probes

Local Kondo screening and spatial coherence in YbRh$_2$Si$_2$

Heavy fermion metals are often characterized by a variety of relevant energy scales and competing interactions which may result in such fascinating phenomena as quantum criticality and unconventional superconductivity. Therefore, these materials have advanced to suitable model systems by means of which electronic interactions can be studied in detail. This will be discussed for the interplay of localized and itinerant electronic states in Kondo lattice systems in which heavy charge carriers are generated. We investigate the generic Kondo lattice system YbRh$_2$Si$_2$, one of the heaviest heavy fermion metals, by utilizing atomically resolved Scanning Tunneling Spectroscopy (STS) [1]. An analysis of the topography allows for a determination of the terminating surface as well as a comparison to results from chemical analysis. Importantly, the crystal field excitations are unambiguously reflected by our STS measurements, clearly relating STS to bulk properties. The hybridization of conduction and 4$f$ electrons results in a gap-like feature in the tunneling conductance. In addition, a strongly temperature dependent peak in the tunneling conductance is attributed to a resonance resulting from the Kondo lattice. The experimental data are discussed in relation to results obtained within the non-crossing approximation (NCA) and renormalized band structure calculation. In a brief outlook we discuss further investigations by STS, e.g.\ with respect to the quantum critical phenomena observed in YbRh$_2$Si$_2$ [2], to substitutions-induced changes of the relevant energy scales [3], or on heavy fermion superconductors. \\[4pt] [1] S. Ernst {\it et al.}, Nature {\bf 474} (2011) 362.\\[0pt] [2] S. Friedemann {\it et al.}, Proc. Natl. Acad. Sci. USA {\bf 107} (2010) 14547.\\[0pt] [3] S. Friedemann {\it et al.}, Nature Phys. {\bf 5} (2009) 465.   

Correlated Spin Phenomena in Molecular Systems

While a great deal of work has been carried out on molecular magnets, the spatial distribution of the spin wave function and the many body interactions between the delocalized molecular spin and its surrounding electrons can now be obtained with atomic scale resolution with the scanning tunneling microscope (STM). The combination of surface science, self-assembly, and STM enables correlated spin phenomena, such as the Kondo state, to be probed in a wide range of well characterized systems from single molecules to a two-dimensional lattice of interacting spins. Nonlocality, Kondo gap, and the Kondo lattice in correlated electron physics are revealed by the atomic-scale spatial resolution and high energy resolution spectroscopy and imaging with the STM from oxygen to porphyrins and phthalocyanine molecules adsorbed on metal and oxide surfaces.   

Long-range Kondo signature of a single magnetic impurity

Scanning tunneling spectroscopy (STS) has provided an approach to study the Kondo effect - one of the oldest many particle phenomena known in condensed matter physics - in real space. In spite of the high spatial resolution of scanning tunneling spectroscopy, experiments performed on single magnetic atoms on metal surfaces, have shown that the fingerprint of the Kondo effect is only visible if the tip is placed directly above the impurity (e.g. [1]). In the present work we follow a novel route and investigate single isolated Co and Fe impurities not on top but buried below a Cu(100) surface. It has been shown recently [2] that the anisotropy of the copper Fermi surface leads to a strongly directional propagation of quasi particles called electron focusing which gives access to individual bulk impurities in a metal. We have studied the energy-dependent scattering characteristics for single isolated atoms of Ag, Co and Fe buried under a Cu(100) surface using low temperature scanning tunnelling spectroscopy (STS). For the case of a non-magnetic Ag impurity a Friedel oscillation in the local density of states is observed. For both magnetic impurity atoms we observe, in contrast to previous works, a long range Kondo signature which is periodic with the distance to the impurity [3]. The comparison of Co and Fe atoms demonstrates that both impurity species show similar behavior on completely different energy scales, which is determined by the Kondo temperature. We investigate the scattering amplitude as well as the phase. A theoretical interpretation based on a combined approach of band structure and many-body numerical renormalization group calculations is able to describe the rich spatially and spectroscopically resolved experimental data. \\[4pt] [1] M. Ternes et al., Journal of Physics: Condensed Matter 053001 (2009) \\[0pt] [2] A. Weismann et al., Science 323, 1190 (2009) \\[0pt] [3] H. Prueser et al., Nature Physics 7,203 (2011)   

Defects in Heavy-Fermion Materials: Unveiling Strong Correlations in Real Space

Heavy-fermion materials exhibit a plethora of puzzling phenomena which are believed to arise from the competition between Kondo screening and antiferromagnetic ordering. The microscopic origin of these phenomena, such as the non-Fermi-liquid properties observed in the quantum critical region, is still a topic of debate. Recent groundbreaking scanning tunneling spectroscopy (STS) experiments [1-4] have shed new light on this debate by providing important insight into the electronic and magnetic structure of heavy fermion materials. In this talk, I review some recent progress made in our theoretical understanding of the differential conductance, dI/dV and the resulting quasi-particle interference (QPI) patterns [5-7], observed in these experiments. In particular, I will demonstrate that defects in heavy-fermion materials provide an unprecedented opportunity to disentangle electronic correlations arising from Kondo screening, and antiferromagnetic correlations between the magnetic moments by inducing perturbations in the electronic and magnetic structure that exhibit characteristically different spatial patterns. The spatial extent of these perturbations grows with the strength of the magnetic interactions, and thus directly reflects the degree of correlations [5]. In addition, I show that non-magnetic defects (Kondo holes) in heavy fermion materials can give rise to the formation of an impurity bound. Our prediction of spatial hybridization oscillations and the formation of an impurity bound state were recently confirmed by STS experiments [4]. Moreover, I will demonstrate that QPI spectroscopy, utilizing spatial oscillations in the LDOS induced by defects, does not only provide important insight into the electronic structure of heavy fermion materials, but also in the entanglement of electronic and magnetic states [6,7]. Finally, the strongly correlated nature of heavy-fermion materials leads to a highly non-linear quantum interference between defects, and the creation of order from disorder. These results provide unique insight into the spatial complexity of heavy fermion materials. \\[4pt] [1] A.R. Schmidt et al., Nature 465, 570 (2010). \\[0pt] [2] P. Aynajian et al., PNAS 107, 10383 (2010). \\[0pt] [3] S. Ernst et al., Nature 474, 362 (2011). \\[0pt] [4] M. Hamidian et al., PNAS 108, 18233 (2011). \\[0pt] [5] J. Figgins and D.K. Morr, Phys. Rev. Lett. 107, 066401 (2011). \\[0pt] [6] J. Figgins and D.K. Morr, Phys. Rev. Lett. 104, 187202 (2010). \\[0pt] [7] T. Yuan, J. Figgins, and D.K. Morr, arXiv:1101.2636.   

Tunable Kondo Effect in SrTiO$_3$

Correlated, low-dimensional systems offer the possibility of tuning complex physical phenomena. Using strong electric fields applied with an electrolytic gate, we continuously tune through three regimes of transport in SrTiO$_3$: an insulator, a metal and a Kondo metal. Two of these regimes -- the metallic and Kondo metal state -- are investigated as a function of temperature and magnetic field, where clear signatures of each regime are evident and discussed. This diverse system not only displays behavior distinct from conventional two-dimensional electron gases, but also show similarities to the LaAlO$_3$/SrTiO$_3$ interface, elucidating on some of the phenomena observed in this heterostructure of debated origin.