Kevin Ingersent — Research Interests
My research addresses aspects of strongly correlated electron systems: materials in which the motion of each charge carrier is greatly influenced by that of the other carriers. Specific interests include (1) the effects of magnetic impurities in metallic, semi-metallic, and superconductoring hosts, (2) the properties of heavy-fermion materials, and (3) novel physics in magnetic nanostructures.
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Ongoing Projects
Locally critical quantum phase transitions
- Continuous quantum phase transitions (QPTs) play a central role in condensed matter physics. A continuous QPT is a zero-temperature phase transition, driven by some control parameter such as pressure or magnetic field, between two distinct phases of matter. One of the phases is characterized by a nonzero order parameter, which vanishes continuously as the control parameter is tuned to its critical value (and is zero throughout the second phase). In the standard picture, the critical behavior near a QPT is determined by long-wavelength, low-frequency fluctuations of the order parameter.
- In 1999, my collaborators and I proposed a fundamentally new type of QPT, in which the critical degrees of freedom include not only long-wavelength fluctuations, but also spatially localized modes. We believe that such a locally critical QPT occurs in heavy-fermion systems, such as CeCu_{5.9}Au_{0.1} and YbRh_{2}Si_{2}, that exhibit critical local behavior close to a zero-temperature magnetic ordering transition. To find out more, please read this paper (reproduced from Nature) and the accompanying commentary.
- Current investigations in this area aim to extend analysis of the locally critical QPT beyond the perturbative renormalization-group methods employed in the paper cited above. An initial step towards this goal was achieved through the first numerical renormalization-group (NRG) solution of the Bose-Fermi Kondo impurity model, described in this paper (reproduced from Phys. Rev. Lett.). Subsequently, we extended the NRG solution to the Kondo lattice model of heavy fermions, as described in this paper (reproduced from Phys. Rev. Lett.). The numerical calculations required for this study are very demanding. Nonetheless, to within our estimated numerical error, our results support the existence of a locally critical QPT in this model.
Magnetic nanostructures
- With various collaborators, I am studying the properties of (i) small clusters of magnetic adatoms on surfaces of nonmagnetic metals, and (ii) devices consisting of multiple quantum dots. Both types of system can exhibit unusual properties arising from competition (or, in some cases, cooperation) between the Kondo effect, which results in screening of the local degrees of freedom, and ordering (or "quantum entanglement") of those local degrees of freedom.
- One result of these studies has been the identification of a novel type of non-Fermi-liquid (NFL) regime arising from the presence of magnetic frustration in a system of three magnetic adatoms or quantum dots arranged in an equilateral triangle. This regime, which has properties qualitatively distinct from NFL behaviors previously identified in other quantum impurity models, may have been observed in scanning tunneling microscopy experiments on chromium trimers on gold surfaces. For more details, see this paper (reproduced from Phys. Rev. Lett.).
- We have also shown that a double-quantum-dot (DQD) system consisting of two quantum dots—one of which is operating in a Coulomb blockade valley and hence has an unpaired spin, while the other is tuned near to resonance with the leads—can realize the strong-correlation physics of a single magnetic impurity coupled to a conduction band that has nontrivial structure in the density of states. Through variation of gate voltages, DQD systems can be tuned to a quantum-critical point separating two phases. In one phase, the unpaired spin on the interacting dot is quenched via the Kondo effect, while in the other phase the spin remains unscreened. Thus, DQD systems provide a rare example of a controlled setting in which to investigate quantum-critical behavior in a strongly correlated system. For more details, see this paper (reproduced from Phys. Rev. Lett.). Very recently, we have proposed that embedding such a device in a ring that is pierced by a magnetic field adds another degree of control to the double-dot device. Our predictions for this setup are described in this paper (reproduced from Phys. Rev. Lett.).
This material is based upon work supported by the National Science Foundation under Grants DMR-0312939, DMR-0710540, and DMR-1107814. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.