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.
May 2018 – February 2019
Dr. Navaneetha Ravichandran, Boston College
Title: Phonon scattering from material boundaries and higher-order anharmonicity
Phonons, which are quantized lattice vibrations, govern the thermal and thermodynamic properties of crystalline solids. Understanding phonon properties is essential to engineer new materials for a wide variety of energy applications such as thermoelectrics, superconductors, energy storage etc., and has been a topic of intense research interest over the past several decades.
In the first part of my talk, I will describe my experimental research at Caltech to answer an important nanoscale phonon transport problem that has remained unsolved for decades: “Do THz-frequency thermal phonons reflect specularly from atomically rough surfaces, thereby preserving their phase? Or do they scatter diffusely and lose it?”. By implementing a novel non-contact optical experiment called the transient grating (TG) on suspended thin silicon (Si) membranes, and by rigorous first-principles analysis of the TG experimental data, I will show that thermal phonons are exquisitely sensitive to the surface roughness of just a few atomic planes on the Si membrane, and that our experimental and computational machinery enables us to obtain the first measurements of the specular phonon reflection probability as a spectral function of phonon wavelength.
In the second part of my talk, I will discuss my computational research at Boston College, where I am developing new first-principles tools to analyze the thermal properties of novel materials, for which the conventional phonon theory fails drastically. I will begin by describing a curious case of thermal transport in boron arsenide (BAs), where the lowest order scattering processes involving three phonons are unusually weak and four-phonon scattering due to higher-order anharmonicity affects the thermal conductivity significantly. Finally I will talk about phonons in sodium chloride (NaCl), where, once again, the conventional phonon theory fails drastically, but for a different reason: the unusually strong anharmonic bonds in NaCl. I will show that the phonons interact so strongly in NaCl that they invalidate the Peierls-Boltzmann description of phonon transport, even below half of the melting temperature. To address this issue, I have developed a new phonon renormalization approach based on many-body theory, which creates new “dressed-up” quasi-particles that interact weakly to admit the Peierls-Boltzmann treatment of heat conduction. I will show that our new phonon renormalization approach along with higher-order four-phonon scattering enables us to get good agreement with several temperature-dependent measurements of phonon dispersions, thermal expansion and thermal conductivity simultaneously.
I am originally from India. I obtained my undergraduate degree from the Indian Institute of Technology, Madras. I obtained my Masters and PhD from Caltech, working with Prof. Austin Minnich. For my PhD, I worked on experimentally investigating phonon boundary scattering in thin silicon membranes using the transient grating experiment. I am currently a postdoctoral fellow at Boston College, where I am working with Prof. David Broido on developing a rigorous predictive first-principles computational tool that simultaneously works for multiple thermal and thermodynamic properties of strongly anharmonic materials.
Abstract: Heat is one of the most fundamental forms of energy, and the ability to control heat plays a critical role in most current and future energy applications. Recently, nanoscale engineering has provided new approaches to manipulate heat transport at the scales of the heat carriers in solids. Despite these advances, we still lack a comprehensive understanding of energy carriers in solids, which would allow us to achieve precise control of energy transport at the nanoscale. My research interests lie in furthering our knowledge of energy carriers, especially electrons and phonons (quantized lattice vibrations). In the first part of my talk, I will give a brief introduction to ultrafast laser spectroscopies that allow us to simultaneously characterize macroscopic thermal properties and microscopic thermal processes of energy carriers in bulk crystals as well as nanometer-thick thin films. In the second part of my talk, I will discuss a generalized Fourier’s law derived from the Boltzmann transport equation that is valid from diffusion to ballistic regimes. This generalized Fourier’s law contains two parts, nonlocality of thermal conductivity, which has been previously hypothesized, and nonlocality of boundary conditions, which has long been ignored in literatures. We apply the derived generalized Fourier’s law to predict the surface temperature responses of an ultrafast laser spectroscopic technique called time-domain thermoreflanctance (TDTR) under various conditions, demonstrating an excellent match between the theoretical predictions and experimental results. Furthermore, by exploiting the generalized Fourier’s law in a synthetic TDTR experiment on a single crystal boron arsenide, we show that in the non-diffusive thermal transport regime, simply interpreting the observation using a Fourier’s law with a modified thermal conductivity, a common practice in the community, would lead to erroneous results. To map the macroscopic observations to intrinsic phonon properties, it is crucial to appropriately take account into the microscopic boundary conditions. Our work shows that in a non-diffusive regime, the two parts of the generalized Fourier’s law are equally important to accurately describing the thermal transport, and we can take advantage of the nonlocal nature of the boundary conditions as an extra knob to manipulate the heat.
The conventional wisdom is that the high-temperature cuprate superconductors are in the clean limit due to their short coherence length. In the clean limit the opening of the superconducting energy gap should be difficult to observe in the optical properties; however, in the cuprates the gapping of the spectrum of excitations with the onset of superconductivity usually has a dramatic optical signature that is suggestive of a system in the dirty limit. To provide guidance, the optical properties of the multiband iron-based superconductor FeTe0.55Se0.45 (Tc~14 K) have been examined. The iron-based material may be described by two bands; one band is coherent, while the other is nearly incoherent, placing this material simultaneously in both the clean and dirty limits . While the optical properties of the cuprates are described by a single band, both it and the superconducting gap are momentum dependent. The similarity of these two classes of materials suggests that the cuprates may also behave as if they are in both the clean and dirty limit at the same time, a view that is supported by the fact that the cuprates and the iron-based superconductors both fall on the same universal scaling line .
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Exact Factorization Approach to Coupled Electron, Ion, and Photon Dynamics
Spin crossover in iron in lower mantle minerals
Department of Applied Physics and Applied Mathematics and
Department of Earth and Environmental Science,
Lamont Doherty Earth Observatory, Columbia University
Pressure and temperature-induced spin state change in iron in lower mantle minerals is an unusual phenomenon with previously unknown consequences. High pressure and high temperature experiments have offered a wealth of new information about this class of materials problems, which includes the insulator to metal transition in Mott systems. I will discuss key experimental data, contrast them with ab initio results and thermodynamic models, show the implications for fundamental phenomena taking place at the atomic scale and their macroscopic manifestations, and discuss potential geophysical consequences of this phenomenon.
Ultrafast terahertz microscopy: from near fields to single atoms
A new experimental frontier has recently emerged with the potential to significantly impact
physics, chemistry, materials science, and biology: the regime of ultrafast time resolution and ultrasmall spatial resolution. This is the domain in which single atoms, molecules, and electronic orbitals move. It also corresponds, on larger length scales, to the territory of low-energy elementary excitations such as plasmons, phonons, and interlevel transitions in excitons. These processes are of particular importance for nanomaterial functionality and typically survive for only femtoseconds to picoseconds after photoexcitation. In this talk, I will show how these diverse dynamics can be studied with new techniques that combine terahertz technology with scanning probe microscopy. First, I will describe how ultrafast near-field microscopy has been employed to perform sub-cycle spectroscopy of single
nanoparticles , reveal hidden structure in correlated electron systems , and resolve transient interface polaritons in van der Waals heterostructures . Then I will discuss the development of a related technique: lightwave-driven terahertz scanning tunneling microscopy [4,5]. In this novel approach, the oscillating electric field of a phase-stable, few-cycle light pulse at an atomically sharp tip can be used to remove a single electron from a single molecular orbital within a time window faster than an oscillation cycle of the terahertz wave. I will show how this technique has been used to take ultrafast snapshot images of the electron density in single molecular orbitals (e.g. Figure 1) and watch the motion of a single molecule for the first time .
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Bright Lights, Big Opportunities for Quantum Materials Research at NSLS-II
Abstract: The realization of quantum materials for energy science and quantum information applications requires an understanding of competing electronic phases spanning multiple scales of energy, time and length. Operating since 2014, the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory has become a nexus for X-ray based studies of the electronic properties to advance knowledge of these key issues. Following an overview of NSLS-II and its beamlines tailored for quantum materials research, this talk will review the capabilities and current status of the soft resonant inelastic X-ray scattering (RIXS) beamline, called SIX. Recent research examples involving transition-metal oxides and 4f Kondo systems will be showcased.
Condensed Matter Physics and Materials Science Division, Brookhaven National Laboratory, Upton, New York 11973
Low energy, laser-based ARPES with variable light polarization, including both linear and circularly polarized, is used to examine the Fe-based superconductor family, FeTe1-xSex. At the center of the Brillouin zone we observe the presence of a Dirac cones with helical spin structure as expected for a topological surface state and as previously reported in the related FeTe0.55Se0.45. These experimental studies are compared with theoretical studies that take account of the disordered local magnetic moments related to the paramagnetism observed in this system. Indeed including the magnetic contributions in the theoretical description is necessary to bring the chemical potential of the calculated electronic band structure into alignment with the experimental observations. In the bulk superconducting state for FeTe0.7Se0.3 the system appears to reflect the presence of some level of orbital selectivity in the pairing even though the system is in the tetragonal phase above and below the transition temperature Tc. At the same time the topological state appears to acquire mass at the superconducting transition, possibly indicative of time reversal symmetry breaking. These observations are discussed in detail.
NEW TWISTS FOR MAGNONS
Matthias Benjamin Jungfleisch
Department of Physics and Astronomy
University of Delaware
In recent years, the exploration of magnons, the quanta of spin waves, as carriers of spin-angular momentum has flourished in spintronics. Magnon spintronics aims at developing novel functional devices that combine magnonic and electronic spin transport phenomena.
In particular, magnetic metamaterials such as artificial spin ice and magnonic crystals offer unique possibilities in magnon spintronics. Here, we present results on high-frequency dynamics in metallic artificial spin-ice lattices by employing broadband ferromagnetic resonance spectroscopy . Furthermore, we explore the possibility to drive and to detect spin dynamics in those systems by dc electrical means using the spin Hall effect [2,3].
Besides magnetic metamaterials, magnetic insulators such as yttrium iron garnet (YIG) are ideal materials for magnonic and spintronic research since they feature long magnon propagation distances and coherence times. Here, we demonstrate the propagation of spin waves in nanometer-thick YIG waveguides  and the electric excitation and detection of spin dynamics via pure spin currents by the spin Hall effect in YIG/Pt micro- and nanostructures [5,6].
This work was supported by the U.S. Department of Energy, Office of Science, Materials Science and Engineering Division.
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