Karen Michaeli

Novel electronic phases at interface structures

It was recently discovered that the interface between LaAlO₃ and SrTiO₃ [1] features a variety of fascinating phenomena, such as superconductivity coexisting with ferromagnetism [2]. This is particularly surprising since by themselves, these materials are both electronically trivial insulators. In my study of these systems, together with Prof. Patrick A. Lee and Andrew C. Potter, we introduced a model for the interface electrons which captures their main physical properties [3]. Most significantly, it explained the origin of ferromagnetism and its coexistence with superconductivity. This is made possible by an unconventional Fulde-Ferrel-Larkin-Ovchinikov (FFLO) pairing between electrons supported by spin-orbit coupling. An additional example of unconventional superconductivity that I have been studying is the odd-parity pairing state. In collaboration with Prof. L. Fu we demonstrated that in materials characterized by chiral symmetry, disorder is not an obstacle to realizing such a superconducting state [4].

Thermal transport in the presence of interactions

To study correlated electron systems via transport measurements, thermal conductivity is an ideal probe due to its high sensitivity to interaction effects. However, for interacting particles the standard theoretical methods for deriving transport properties, such as the Kubo formula, are cumbersome and essentially unfeasible in most situations. During my PhD under the supervision of Prof. Alexander M. Finkel’stein, we developed a new theoretical technique for analyzing thermal and thermoelectric transport based on the quantum kinetic equation, which can be easily applied to study various kinds of interactions of arbitrary strength [5]. This scheme clearly illustrates that in contrast to an electric current, a heat current can be transmitted also by neutral quasiparticles. In particular, energy can be carried by excitations that mediate interactions between other quasiparticles. As a consequence, we showed that the interplay of Coulomb interactions and disorder leads to violations of the Wiedemann-Franz law, suggesting that the thermal conductivity can remain finite across the metal-insulator transition due to long-range interactions.

Transport in the vicinity of the superconducting phase transition

The quantum kinetic approach proved very useful for the analysis of the transverse thermoelectric response, known as the Nernst effect, in disordered superconducting films [6]. We showed that a giant Nernst signal is generated by fluctuations of the superconducting order parameter above the critical temperature or critical magnetic field. Not only does my theory show remarkable agreement with experiments conducted on conventional superconducting films [7]; it has also been used by the group of Prof. L. Taillefer to explain their Nernst measurements on high temperature superconductors [8]. Recently, I have completed the theory of transverse transport in the vicinity of the superconducting phase transition, by analyzing the Hall effect. This work combines theoretical [9] and experimental [10] studies in close collaboration with Prof. A. M. Finkel'stein and the group of Prof. A. Kapitulnik. We demonstrated that the Hall conductivity, similar to the Nernst effect, is a much more sensitive probe of superconducting fluctuations than regular, longitudinal transport.

Metal-Insulator Transition in a System of Tunneling Vortices

Tunneling vortices are at the core of the quantum phase transitions in superconductors. We study the possibility to prevent vortices from tunneling by depositing a metallic gate on top of a superconducting film [11]. Each vortex carries magnetic flux which penetrates the gate and scatters the electrons in a way similar to an Aharonov-Bohm flux tube. A tunneling vortex forces the electrons inside the gate to adjust to the change in its position. The response of the electrons to this sudden change leads to an Orthogonality Catastrophe. Thus, the electrons oppose the tunneling and can even localize the vortices, thus restoring the perfect superconducting properties of the film at low temperatures. We used this example of macroscopic quantum tunneling to explain the experimental observation [12] of a change in the resistance of a superconducting film in a magnetic field, induced by adding a remote gate.

Finite transport coefficients generated by strong correlation in spin liquids

A particularly striking example of extremely atypical quasiparticles is found in quantum spin liquids. These can form in the vicinity of the Mott metal-insulator transition when the charge is gapped while the spin degrees of freedom strongly fluctuate. The effective theory for this Mott insulator consists of spinons, which are neutral fermions with spin 1/2. The spinons exhibit a magnetic interactions that, unlike the Coulomb repulsion, is not screened and leads to non-Fermi liquid behavior. I showed that even in the absence of disorder this strong interaction provides an efficient relaxation mechanism for spin and heat currents, keeping them finite even at the lowest temperatures [13].

References

1. A. Ohtomo, and H. Y. Hwang, Nature 427, 423 (2004).
2. L. Li, C. Richter, J. Mannhart, and R. C. Ashoori, Nat. Phys. 7, 762 (2011).
3. K. Michaeli, A. C. Potter, and P. A. Lee, Phys. Rev. Lett. 108, 117003 (2012).
4. K. Michaeli, L. Fu, arXiv: 1203.1055, accepted to Phys. Rev. Lett.
5. K. Michaeli, and A. M. Finkel’stein, Phys. Rev. B. 80, 115111 (2009).
6. K. Michaeli, and A. M. Finkel’stein, Phys. Rev. B. 80, 214516 (2009).
7. K. Michaeli, and A. M. Finkel’stein, Europhys. Lett. 86, 27007 (2009).
8. J. Chang, N. Doiron-Leyraud, O. Cyr-Choiniere, G. Grissonnanche, F. Laliberte, E. Hassinger, J-Ph. Reid, R. Daou, S. Pyon, T. Takayama, H. Takagi, and L. Taillefer, Nat. Phys. 8, 751 (2012).
9. K. Michaeli, K. S. Tikhonov, A. M. Finkel’stein, Phys. Rev. B. 86, 014515 (2012).
10. N. P. Breznay, K. Michaeli, K. S. Tikhonov, A. M. Finkel’stein, M. Tendulkar, A. Kapitulnik, Phys. Rev. B. 86, 014514 (2012).
11. K. Michaeli, and A. M. Finkel’stein, Phys. Rev. Lett. 97, 117004 (2006).
12. N. Mason, and A. Kapitulnik, Phys. Rev. B. 65, 220505 (2002).
13. K. Michaeli, in preparation.