Session B38: Heineman/Faculty/Greene Prize Session

The Hofstadter's butterfly: from playing with numbers to studying quantum materials (Dannie Heineman Prize for Mathematical Physics talk)

We discuss some results on the Harper's operator - the model behind the Hofstadter's butterfly: metal-insulator transitions of two kinds, the Ten Martini problem, the proof of Thouless' conjecture from the early 80s: that Hausdorff dimension of the spectrum is bounded by 1/2 for all irrational fluxes, as well as the discovery of self-similar (reflective-)hierarchical structure of eigenfunctions throughout the localization regime.
Partially based on joint works with S. Becker and R. Han, I. Krasovsky, and W. Liu .   

A common tool for uncommon measurements: Adventures in atomic force microscopy

Many people are familiar with the atomic force microscope, but fewer have experience using its capabilities to do more than image the topography of a sample, usually in air. In this talk, I’ll present three different subfields of research that all involve the atomic force microscope. The “force curve” enables the measurement of mechanical properties, including Young’s modulus and adhesion. Normally, the tip is brought into contact with a surface, and the force required to push into the sample is measured. We have used the AFM to image natively adhered bacteria to surfaces in fluid and perform force curves on individual bacteria. We have also grown the biofilm on a cantilever and brought it into contact with different surfaces, to study the early stages of adhesion. Magnetic force microscopy (MFM) may be a familiar technique that can reveal the magnetic state of nanostructures. In rings of the correct size, there is a vortex state in which all moments lie circumferentially in plane, creating a closed-flux state that does not have any MFM contrast in a perfectly symmetric ring. It is challenging to experimentally control the clockwise or counterclockwise circulation of this vortex state with a uniform external applied field. Instead, we pass a current through a solid platinum AFM tip in order to create a local circular Oersted field to probe the magnetic behavior of rings and discs. Finally, I’ll present our current efforts using Kelvin probe force microscopy (KPFM) to image real-time charge motion in organic semiconductors. All of this work was completed in my lab at Mount Holyoke College, where I have mentored 50 undergraduate women over the past thirteen years.   

Richard L. Greene Dissertation Award Talk: Probing Hidden Symmetries with Raman Scattering

The properties of quantum materials are dominated by electronic correlations, which often lead to novel emergent phenomena and spontaneous symmetry breaking at low temperatures or material interfaces. The underlying correlations are encoded in the collective excitations out of the ground state, which are sometimes unfortunately hidden from most experimental techniques. One example is the collective modes that transform as a pseudovector, e.g. the A_2g representation of the D_4h and D_3d groups. Here, we use polarization resolved Raman scattering to directly probe the collective modes with A_2g symmetry in two examples. In the heavy fermion metal, URu₂Si₂, the A_2g collective mode uniquely couples to the reflection symmetry breaking ground state in the low temperature phase -- a chirality density wave that was hidden from other spectroscopic tools in the past [1,2]. Whereas on the surface of the 3D topological insulator, due to the strong Rashba spin orbit coupling, the electrons acquire chiral spin textures that results in novel collective modes and composite particles. In the example of Bi₂Se₃, we observed an A_2g mode as the collective spin excitation from the surface states [3], and circularly polarized photoluminescence from chiral excitons [4].
Main References:
[1] H.-H. Kung, R. E. Baumbach, E. D. Bauer, V. K. Thorsmølle, W.-L. Zhang, K. Haule, J. A. Mydosh and G. Blumberg, Science 347, 1339 (2015).
[2] H.-H. Kung, S. Ran, N. Kanchanavatee, V. Krapivin, A. Lee, J. A. Mydosh, K. Haule, M. B. Maple, G. Blumberg, Phys. Rev. Lett. 117, 227601 (2016).
[3] H.-H. Kung, S. Maiti, X. Wang, S.-W. Cheong, D.L. Maslov and G. Blumberg, Phys. Rev. Lett. 119, 136802 (2017).
[4] H.-H. Kung, A.P. Goyal, D.L. Maslov, X. Wang, A. Lee, A.F. Kemper, S.-W. Cheong, and G. Blumberg, Proc. Natl. Acad. Sci. 116, 4006 (2019).   

Delafossite oxides: natural, ultra-pure metal-insulator heterostructures

Delafossite oxides are layered compounds, which can be thought of as natural heterostructures of triangularly coordinated metallic sheets and transition metal oxide blocks. A fascinating range of electronic states can be found both in their bulk and on their surfaces, including extremely high conductivity in PtCoO₂ and PdCoO₂ [1], maximal Rashba-like spin-splitting on the transition metal terminated surfaces of PtCoO₂, PdCoO₂ and PdRhO₂ [2], Stoner ferromagnetism on the Pd-terminated surface of PdCoO₂ [3] and, perhaps most remarkably, an intertwined spin-charge response due to a Kondo coupling between metallic and Mott insulating layers in PdCrO₂ [4]. Our group has investigated these states experimentally with angle resolved photoemission, and theoretically with first principles calculations and model Hamiltonians, where applicable.
Concentrating on the metal/Mott insulator heterostructure and the Rashba-like surface states, I will show how the simplicity of the materials allows us to pinpoint to the underlying cause for the remarkable electronic behaviour, and in turn to use delafossites as model systems to understand complex phenomena.

[1] Kushwaha, P., Sunko, V. et al., 2015, Science Advances 1, e1500692.
[2] Sunko, V. et al., 2017, Nature 549, 492
[3] Mazzola, F., Sunko, V. et al., 2017., PNAS 51, 12956
[4] Sunko, V. et al., 2018, arXiv:1809.08972   

Hybridizing Spin, Charge and Photon on a Quantum Chip

As we witness the transition of quantum information science from the singular objective of quantum supremacy to the eclectic world of NISQ algorithms, the metrics for success have become rapidly more complex. The simultaneous need for long coherence times, fast gates, accurate readout and high connectivity has both highlighted the respective advantages of various quantum hardwares and accentuated their shortcomings. Hybrid device developments, where technologies across different qubit incarnations are combined to compensate for their inherent physical limitations, have enjoyed much success in recent years. In this lecture, I will review one such example where the technique of circuit quantum electrodynamics (cQED) was successfully grafted from superconducting qubits to semiconductor quantum-dot spin qubits. We will cover the physics and technical developments that have led to the first observations of strong-coupling between a single electron charge in gate-defined quantum dots and a microwave photon [1] and strong-coupling between a single electron spin and a single photon [2]. As time permits, applications of the hybrid device to valley physics in silicon will also be covered [3, 4].

[1] X. Mi et al., Science 355, 156 (2017).
[2] X. Mi et al., Nature 555, 599 (2018).
[3] X. Mi et al., PRL 119, 176803 (2017).
[4] X. Mi et al., PRB 98, 161404(R) (2018).