Unfortunately, seminars are cancelled for the remainder of the semester until further notice, due to COVID-19.
NEWS: The CQM Distinguished Lecture series has been established in the Fall of 2015 to bring to Stony Brook University the renown experts in the physics of quantum matter.
October 2019 – January 2020
Title: Heat conduction in defective and complex crystals: phonon scattering and beyond
* Material Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
* Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA.
The flow of heat through materials is a topic of scientific interest and technological importance in fields of microelectronics, power generation, heat management, and thermoelectrics. For example, advancement in microelectronic technologies (e.g. microprocessors, and high-power electronics) demands ever more efficient removal of the heat generated in these devices. In contrast, technologies such as thermal barrier coatings and thermoelectric materials are designed to stop the flow of heat. In simple, defect-free crystals the thermal conductivity is generally well understood. However, in materials containing defects and/or in those with very complex crystal structures there is a lack of basic understanding which inhibits technological progress.
In this presentation, I will highlight several experimental and theoretical results which aim to establish a fundamental understanding of heat transport in defective materials. First, I will discuss several studies related to heat conduction across interfaces. Secondly, I will demonstrate in several material systems how defects can soften a materials lattice which reduces the phonon group velocity and thus decreases thermal conductivity. Lastly, the transition from crystalline-like to amorphous-like thermal conduction is investigated by studying the lattice dynamics of crystals with very complex crystal structures both computationally and experimentally. Through this analysis emerges a description of phonon transport which is divided between two channels. One is the standard phonon-gas transport mechanism and the other we term the diffuson-channel since it is mathematically the same mechanism in which ‘diffusons’ were defined.
Title: Optical Control of Chiral Charge Pumping in a Topological Weyl Semimetal
host: Dima Kharzeev
host: Laszlo Mihaly
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host: Phil Allen
title: Carrier lifetime effects on thermoelectric efficiency
Recent developments in electronic structure algorithms based on the Wannier function interpolation of electronic wave functions have enabled accurate first-principles calculations of electron-phonon interactions and intrinsic carrier lifetimes in the relaxation time approximation. This has supplied the final missing piece of the puzzle for predicting the thermoelectric figure of merit zT=s S2 T/k, where the conductivity s, the Seebeck coefficient S, and the total thermal conductivity k now can all be obtained from the density-functional theory (DFT). This opens up exciting possibilities for theoretically understanding and reliably predicting new materials with high values of zT. We will review several examples from our recent work, including a Li-intercalated analogue of lead telluride (Li2TlBi), an intermetallic compound with unexpectedly high value of S (CoSi), and a theoretically predicted full Heusler compound with ultrahigh zT (Ba2BiAu). General factors for high thermoelectric power factors in these compounds include energy dependence of carrier lifetimes for high S, high degeneracy of carrier pockets at the Fermi level, weak electron-phonon scattering for high mobility, and concomitantly low Lorentz numbers for low electronic thermal conductivity.
Title: Emergent Phenomena at the Interface of Complex Oxides
Host: Cyrus Dreyer
Progress in epitaxial growth of complex oxides have led to heterostructures with exquisite physical phenomena, such as the formation of a high-density two-dimensional electron gas (2DEG) at the interface between two normally insulating materials—e.g. SrTiO3/LaAlO3. Superconductivity and magnetic ordering have been demonstrated in these systems, sparking the interest in novel device applications. The formation of a 2DEG at the interface between SrTiO3 and Mott insulators, such as GdTiO3, has also been demonstrated, with electron densities that are over an order of magnitude higher than those realized with conventional semiconductors. Charge transport in these systems exhibit intriguing behavior, varying drastically from metal to insulator depending on the thickness of the building-block layers. Intensive research efforts in the last decade have raised questions regarding the origin of the excess charge, the mechanisms that determine the density of the 2DEG, and fundamental properties of the 2DEG. In this presentation, we will discuss how computer simulations can provide insights into the origin and nature of the 2DEG. Based on results of first-principles calculations we will discuss electron correlation effects and how the electronic structure of these heterostructures can be drastically altered, turning from metallic into insulating, through charge localization in ultrathin layers. Finally, we will address the interplay between orbital, charge, and spin in the manipulation of the magnetic ordering observed in some of these heterostructures.
title: Manganese Cyanide Tinkertoys
This is a special seminar (Wednesday instead of Friday) held in SCGP (Simons Center) room 102. It is jointly hosted by AMO, Condensed Matter, and the SCGP.
speaker: Immanuel Bloch (Max Planck Institute for Quantum Optics, Garching)
title: Quantum Matter under the Microscope
abstract: More than 30 years ago, Richard Feynman outlined his vision of a quantum simulator for carrying out complex calculations on physical problems. Today, his dream is a reality in laboratories around the world. This has become possible by using complex experimental setups of thousands of optical elements, which allow atoms to be cooled to nanokelvin temperatures, where they almost come to rest. Recent experiments with quantum gas microscopes allow for an unprecedented view and control of such artificial quantum matter in new parameter regimes and with new probes. In our fermionic quantum gas microscope, we can detect both charge and spin degrees of freedom simultaneously, thereby gaining maximum information on the intricate interplay between the two in the paradigmatic Hubbard model. In my talk, I will show how we can reveal hidden magnetic order, directly image individual magnetic polarons or probe the fractionalisation of spin and charge in dynamical experiments. For the first time we thereby have access to directly probe non-local ‘hidden’ correlation properties of quantum matter and to explore its real space resolved dynamical features also far from equilibrium. Furthermore, I will show how quantum gas microscopy can open new avenues for the field of quantum chemistry when probing and controlling the formation of huge Rydberg macrodimers in optical lattices.
Title: Incommensurate transitions and twist disorder in microscopic models of twisted bilayer graphene
Abstract: Recent experiments in twisted bilayer graphene have set off a flurry of work due to the observation of purportedly correlated phases at the so-called “magic-angle.” However, the current models in the literature for magic-angle graphene suffer from two flaws: they assume commensurate twist angles and have great difficulty modeling the exact experimental setup where patches of different twist angles appear (“twist disorder”). We introduce and study a family of microscopic models that begins to address these concerns. In these models, the twist angle enters as a free parameter in real space. We can use this to simulate both incommensurate effects and disorder effects. We find that incommensuration leads to an Anderson-like delocalization transition in momentum space. The result is a small metallic phase at the “magic-angle” (that we speculate is unstable to correlated phases). We further study twist-disorder effects and find that while the minibandwidth is renormalized substantially, the Fermi velocity is not significantly altered.
Host: Jen Cano
Title: Microscopic view of heat conduction in solids
Microscopic quantum mechanical interactions among heat carriers called phonons govern the macroscopic thermal properties of semiconducting and electrically insulating crystalline solids, which find applications in thermal management of electronics, thermal barrier coatings and thermoelectric modules. In this talk, I will describe my recent work on how our newly developed first-principles computational framework to predict these microscopic interactions among phonons unveils a new paradigm for heat conduction in several of these materials. As an example, I will describe a curious case of heat conduction in boron arsenide (BAs), where the lowest order interactions involving three phonons are unusually weak and higher-order scattering among four phonons affects the thermal conductivity significantly, in stark contrast with several other semiconductors such as silicon and diamond . I will show that this competition between three and four phonon scattering can be exquisitely tuned with the application of hydrostatic pressure, resulting in an unusual non-monotonic pressure dependence of the thermal conductivity in BAs unlike in most other materials . I will also briefly describe my prior experimental effort to probe the scattering of phonons at atomically rough surfaces of a nanoscale silicon film, where they showed extreme sensitivity to the changes in surface roughness of just a few atomic planes .
 Fei Tian, Bai Song, Xi Chen, Navaneetha K. Ravichandran et al., Science 361 (6402), 582-585, 2018
 Navaneetha K. Ravichandran & David Broido, Nature Communications 10 (827), 2019
 Navaneetha K. Ravichandran, Hang Zhang & Austin Minnich, Physical Review X 8 (4), 041004, 2018
Intertwined Orders in Cuprate Superconductors
Condensed Matter Physics & Materials Science Division
Brookhaven National Laboratory, Upton, NY 11973-5000
While the nature of cuprate superconductors remains controversial, the concept of intertwined orders provides a consistent way to understand multiple types of superconductivity in these hole-doped antiferromagnets . Neutron and x-ray scattering experiments have demonstrated the tendency for the doped holes to segregate into stripes that are separated by locally-antiferromagnetic regions. As originally proposed by Emery, Kivelson, and Zachar , the hole stripes can develop pairing correlations, but superconducting order is limited by the ability to establish phase order by Josephson coupling through the intervening magnetic regions. When those intervening spin correlations can be gapped, spatially-uniform superconductivity can develop, where the coherent gap is limited by the spin gap . When the spin-stripe correlations are strong, an alternative superconducting state involves a spatially-modulated pair wave function (pair density wave) intertwined with spin stripe order. A sufficiently strong magnetic field destroys the superconducting order without disrupting the pair correlations within the stripes .
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