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.

Colloids have provided versatile model systems to investigate rudimentary characteristics of material phases and have been employed as elementary units for spontaneous and directed self-assembly, mimicking atoms. Their size observable in the microscopy is feasible to track individual positions and orientations, so many phenomena including melting and nucleation have been studied at the single-particle level for better understanding atomic scale dynamics. In the same manner, we used Janus colloids to imitate spins in the observable scale under the microscope. We proved the phase transition of Janus spheres’ spin (orientation) order in Janus colloidal crystal, where the spin interaction is controlled by the external electric field. The spin configuration evolves from a random pattern to vortex and zigzag patterns, as increased the electric field. During this process, we measured the spin arrangement is changed from the short-range order to the quasi-long-range order through the power law decay of spatial correlation functions. Furthermore, we found the density of topological defects, which are vortex and anti-vortex, is correlated with the spin phase transition that is corroborated with the susceptibility of the spin order parameter. To describe the spin phase transition, we suggested the 2D Heisenberg model. Therefore, we expect that the designed system provides a platform to help understand spin behaviors in the single-particle level as well as plenty of phenomena induced by orientational interaction in atomic and molecular materials.

A recurring theme in classical and quantum dynamics is finding examples of systems which fail to thermalize.  We introduce a class of 1D translationally invariant classical and quantum dynamics that have an unusual approach to equilibrium.  For certain examples, expectation values of local operators relax in a time which is exponentially large in system size, implying an unusual kind of hydrodynamics.  In other examples, thermalization only occurs when the system is connected to a bath which is at least exponentially large in system size, a phenomenon that we call fragile fragmentation.  A field of mathematics called geometric group theory plays an important role in constructing these examples, and we discuss extensions of these results to 2D.

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 to characterize 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. In our nanorod arrays, we demonstrated several coherent modes, with possible signatures of the coexistence of CPs and magnons.² While 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.

Metal-insulator transitions are among the most fascinating and least understood phenomena in condensed matter physics. Metal-insulator transitions lead to significant changes in the electronic conductivity and optical properties, and are generally accompanied by structural and magnetic transformations due to a complex interplay between charge, spin, orbital, and lattice degrees of freedom. The thermally-driven metal-insulator transition (MIT) in bulk vanadium dioxide (VO₂) is accompanied by a structural distortion that leads to pairing of all the vanadium atoms in the insulating phase. This V-V pairing has long been thought critical to the emergence of insulating behavior. We shall present our latest experiments on ultrathin VO₂ films grown on TiO₂ substrates. We demonstrate that the MIT in ultrathin VO₂ films occurs without the V-V structural distortion. Our results establish a route to a purely electronic MIT that is driven by electron-electron interactions. We shall also present our recent experiments and results on infrared nano-imaging and nano-spectroscopy of VO₂ films. The development of table-top, broadband infrared light sources in my lab has enabled nano-spectroscopy experiments on VO₂ and other materials.

Quantum materials are a collection of atoms with interacting electrons and nuclei displaying emergent behaviour and topological properties—properties robust to local defects. The past two decades has witnessed an explosion in the field of topological materials: from weak interacting electrons to strongly correlated ones, topological materials represent one of the most exciting discoveries bot hat fundamental and application leve. High performance electronics, quantum information or ultrafast spintronics are just a few of the possible technologies that can be developed based on these materials. In this talk I will discuss the route to go from pure mathematical prediction of topological properties, through high through-put materials search to experimental realization. I will discuss both topological insulators, in non magnetic and magnetic phases as well as topological (chiral) semimetals using the the modern theory of topological band structure—Topological Quantum Chemistry — built upon symmetry-based considerations and complemented with chemical theories of bonding, ionization, and covalence. Consequently, it describes the universal global properties of all possible band structures and materials. Going beyond the single-particle perspective, I will introduce our formalism grounded in Green’s functions. This approach is designed to uncover topologically correlated phases in materials exhibiting electronic entanglement, such as Mott phases. Additionally, I will discuss recent results centered on Green’s function zeros, which are aimed at diagnosing topology in correlated materials.

Superconductivity is an almost ubiquitous feature in the low temperature phase diagram of multilayer graphene allotropes – moire or crystalline. While the microscopic electronic structures of these systems differ, supporting devices with monolayer WSe2 has been shown to increase superconductivity along many axes of the phase space like density, magnetic field and temperature. Here, we study two superconducting domes (SC1 and SC2) in Bernal Bilayer graphene on WSe2 in light of their resilience to in-plane magnetic fields. While SC1 appears in a symmetry unbroken phase, quantum oscillation measurements show that the normal state of SC2 is nematic, breaking C3 symmetry. Despite this difference, both superconductors violate the Pauli limit consistent with spin singlet pairing between opposite valleys protected from de-pairing by Ising SOC. Our results suggest that the induced SOC is central to the observed enhancement of superconductivity in many graphene multilayer systems - favoring pairing between time reversal symmetric partners.