Condensed Matter Seminars This Term

Topological superconductors provide a promising route to fault-tolerant quantum computing; however, it proved hard to find or engineer them. Recently, topological superconductivity was predicted to arise at the interface between quantum Hall and conventional superconducting states. Since both ingredients are readily available in the lab, topological superconductivity seemed to be within the reach. The predictions, however, focus on the idealized “clean” case, whereas only strongly disordered superconductors are compatible with high magnetic fields needed for the quantum Hall effect. Can topological superconductivity survive the presence of disorder?

We develop a theory of two counter-propagating quantum Hall edge states coupled via a narrow disordered superconductor. We show that, in contrast to the clean-case predictions, the edge states do not turn into a topological superconductor. Instead, the disorder tunes them to the critical point between the trivial insulating phase and the topological phase. We determine the manifestations of this criticality in the charge transport, finding that the critical conductance is a random, sample-specific quantity with a zero average and unusual bias dependence. The developed theory of disordered superconductor-quantum Hall interfaces offers an interpretation of recent experiments.

TBA

The desire for multifunctional devices has driven significant research toward exploring multiferroics, where the coupling between electric, magnetic, optical, and structural order parameters can provide new functionality. While BiFeO3 is a well-studied multiferroic, recent research has shown that the addition of BaTiO3 can improve material properties.1 In this talk we focus on coherent phonon (CP) generation in BaTiO3-BiFeO3 (BTO-BFO) layered structure as well as nanorod arrays. Usually, CPs are used to provide detailed spectroscopy information and for characterizing surfaces and buried interfaces. However, the ability to generate strain via ultrafast optics offers the intriguing possibility of dynamically manipulating the strain with ultrashort optical pulses and opens the possibility of creating a new class of devices, where the strain is manipulated in time to control the properties and operation of a device. As shown in Fig. 1 for our nanorod arrays of BTO-BFO, we observed a clear sinusoidal modulation in the transient reflectivity, a characteristic of CPs. In these nanorod arrays, we demonstrated several coherent modes, with possible signatures of the coexistence of CPs and magnons.2While magnons, in general, are hard to manipulate and control, a strong magneto-elastic interaction between phonons and magnons can be important for a variety of reasons: (i) Coherent Acoustic Phonons generated with ultrafast optical pulses can propagate long distances from the surface, into the sample. With strong magneto-elastic coupling, they can carry the spin information along with them into the sample, perhaps between different regions of a chip. (ii) Strong interactions with phonons can enhance the excitation, manipulation, and detection of the magnons for possible applications in memory devices. In this talk, I will present our observations in several BTO-BFO films and nanorod arrays with different interfaces to demonstrate the tunability of CPs and discuss the possibility of the co-existence of CPs and magnons. I will also discuss the possibility of controlling these coherent states using external magnetic fields which have been demonstrated to increase the sensitivity of the CPs’ detection in other systems.3 References: [1] S.-C. Yang, A. Kumar, V. Petkov, and S. Priya, J. of Appl. Phys., 113, 144101 (2013). [2] R. R. H. H. Mudiyanselage, B. A. Magill, J. Burton, M. Miller, J. Spencer, K. McMillan, G. A. Khodaparast, H.-B. Kang, M.-G. Kang, D. Maurya, S. Priya, J. Holleman, S. McGill, and C. J. Stanton, J. Mater. Chem. C, 7, 14212 (2019). [3] B. A. Magill, S. Thapa, J. Holleman, S. McGill, H. Munekata, C. J. Stanton, and G. A. Khodaparast, Phys. Rev. B 102, 045306 (2020). Acknowledgment: This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-17-1-0341 and DURIP funding (FA9550-16-1-0358). A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida.

 I will discuss a mechanism of microwave absorption in conventional and unconventional  superconductors which is similar to the Debye absorption mechanism in molecular gases. The contribution of this mechanism to AC conductivity is proportional to the inelastic quasiparticle relaxation time rather than the elastic one and therefore it can be much larger than the conventional one. The Debye contribution to the linear conductivity arises only in the presence of a DC supercurrent in the system and its magnitude depends strongly on the orientation of the microwave field relative to the supercurrent. The Debye contribution to the non-linear conductivity exists even in the absence of the supercurrent. It provides an anomalously low non-linear threshold.

 I will also discuss a closely related problems of resistance of superconductor-normal metal-superconductor junctions, and the resistance of superconductors in the magnetic flux flow regime.