Session L04: Lars Onsager Prize

Lars Onsager Prize Talk: Quantum phase transitions in quantum matter

I will survey examples of quantum phase transitions, with applications to experiments on a variety of quantum materials. The simplest examples are associated with symmetry-breaking order parameters. More subtle examples involve emergent gauge fields, and their confinement or Higgs transitions, sometimes accompanied by changes in the volume enclosed by the Fermi surface. General aspects of the non-zero temperature dynamics of quantum critical systems will be noted, along with holographic connections to the dynamics of black holes. I will conclude by discussing the phase diagram of the hole-doped cuprate superconductors, and ideas on the transformations near optimal doping.   

LeRoy Apker Award Talk: Laser Printed Flexible Electronics

Organic electronics are at the forefront of the industry for large-area, low-cost flexible electronic applications, and provide an opportunity to incorporate electronics in non-traditional areas such as clothing, electronic paper, bio-integrated applications, and more. While several organic semiconductors have shown excellent electrical properties in single crystal form, reproducing these characteristics using low-complexity manufacturing processes has proven challenging. It is clear that the need to improve the performance of organic semiconductors while preserving their ease of manufacturing is critical for transitioning these devices from the laboratory to industry. To confront this challenge, in this work we explored two alternative methods for device fabrication that offer low-cost processability, compatibility with flexible substrates, and yield good device performance. The first method uses a laser printer for electrophotographic deposition of the organic semiconductor layer. We developed the first electronic device in which the semiconducting layer was laser printed. We performed electrical and structural characterization on the obtained thin-film transistors to relate the performance with the film microstructure and the quality of the semiconductor/dielectric interface. The second method, Graphite Aerosol Spray Lithography, employs laser printing for contact definition and uses a graphite aerosol to spray the electrodes. We show that this method can create contacts without using thermal or electron beam deposition, two common, expensive techniques. Furthermore, it replaces the complex lithography processes with a low-cost alternative based on printer toner. These methods offer insight into environmentally friendly and cost-effective processing for organic devices with an established technology such as the laser printer. This represents a step forward in developing scalable and cost-effective device fabrication techniques.   

Richard L. Greene Dissertation Award Talk: Resonant Ptychographic Tomography of Three Dimensional Magnetic Structures

Three dimensional magnetic systems promise significant opportunities for applications, for example providing higher density data storage devices and new functionality associated with complex topology and greater degrees of freedom. The aim of this work was to address two main challenges concerning the experimental investigation of three dimensional magnetic structures, namely the fabrication of three dimensional tailored architectures, and the characterisation of three dimensional magnetisation vector fields. In particular an artificial magnetic buckyball was fabricated, and a detailed characterisation of the structural and elemental properties performed [1]. For the determination of the magnetic configuration in a three dimensional system, hard X-ray magnetic imaging at the nanoscale was developed [2], and combined with a new iterative reconstruction algorithm to achieve hard X-ray magnetic tomography [3]. In a first demonstration, the three dimensional magnetic nanostructure within the bulk of a soft magnetic micropillar was determined, revealing a complex magnetic configuration consisting of vortices and antivortices that form cross-tie and vortex walls. At the intersections of these magnetic structures, magnetic singularites - Bloch points - are found and their surrounding magnetic configuration determined, allowing for different Bloch point types to be identified [3]. With its high penetration depth, hard X-ray magnetic tomography opens the possibility for the elucidation of complex three dimensional magnetic structures for a range of extended magnetic systems with high spatial resolution.
The nanofabrication and characterisation techniques developed in this thesis provide a basis for future nanoscale investigations of three dimensional magnetic structures in a wide variety of systems.
[1] Donnelly et al, PRL 114, 115501 (2015)
[2] Donnelly et al, PRB 94, 064421 (2016)
[3] Donnelly et al, Nature 547, 328–331 (2017)
  

Richard L. Greene Dissertation Award Talk: Probing Exotic Phases of Interacting Two-dimensional Carriers Using One-dimensional Potential Modulation

Clean two-dimensional electron systems (2DESs) subjected to a perpendicular magnetic field exhibit an array of exotic phases. At and near zero field, the 2D electrons typically form a degenerate Fermi gas and occupy a Fermi sea in their ground-state. At higher fields, however, their ground-state can transform into various phases such as the integer and fractional quantum Hall states, Fermi gas of composite fermions, stripe and bubble phases, Wigner crystal, etc. The evolution into different phases as a function of field, and in some cases temperature, makes the 2DES very exciting. Here we report our recent studies on some of these phases probed by an external, gentle periodic potential modulation. The modulation is achieved through the piezo-electric effect in GaAs as we fabricate a periodic, strain-inducing super-lattice on the surface of our samples. Using this novel technique, we have investigated: (i) the Fermi contour splitting of 2D electrons under strong in-plane magnetic fields, (ii) the spin polarization of composite fermions near the fractional quantum Hall state at filling factor 5/2, (iii) reorientation of the stripe phases at filling factor 7/2, (iv) different behavior of composite fermions near the bilayer fractional quantum Hall states at filling factors 1 and ½, (v) warped Fermi contour of composite fermions at filling factor 1/2, etc. Our results establish the periodic potential modulation as a versatile probe for the different phases of 2DESs.   

Nicholas Metropolis Award Talk: Hydrodynamics of Heat Transport in Crystals

Heat flux in nonmagnetic dielectric crystals is determined by the propagation of lattice vibrations and is often modeled with the kinetic gas theory, where phonons are the gas constituents. However, recent advancements have shown the limitations of this approximation, especially when dealing with crystals at cryogenic temperatures or 2D materials. In these cases, the behavior of the interacting phonon gas departs from conventional expectations and causes high thermal conductivity and hydrodynamic phenomena such as second sound, where heat propagates as a wave, rather than diffusively.
To explain these properties, we introduce a gas of collective phonon excitations called relaxons [1], defined as the eigenvectors of the scattering operator. The exact thermal conductivity from the linearized Boltzmann transport equation is still described by the kinetic gas theory, with the relevant gas consisting not of phonons, but of relaxons. Surface scattering is reinterpreted as a hydrodynamic effect [2] with friction at the materials borders slowing the heat flux, in analogy with the Poiseuille flow of water through a pipe. The dispersion relation of relaxons [3], obtained from the Fourier transform of the linearized Boltzmann equation, describes heat oscillations and provide a new interpretation of second sound. These considerations are applied to a first-principles study of graphene, MoS₂ and silicon, revising the relevant time, velocity and length scales of thermal transport and providing a new viewpoint on semiclassical transport theories.
[1] A. Cepellotti and N. Marzari, Phys. Rev. X 6, 041013 (2016)
[2] A. Cepellotti and N. Marzari, Nano Lett. 17, 4675 (2017)
[3] A. Cepellotti and N. Marzari, Phys. Rev. Mater. 1, 045406 (2017)