# 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.

## March – November 2018

**Matt Reuter**, Stony Brook Applied Math and IACS

B131 Physics, Stony Brook University

**Abstract**

Exploiting the structure of a quantum mechanical Hamiltonian often leads to fast algorithms for computational simulations involving the system. For instance, sparsity of the Hamiltonian can lead to efficient algorithms for obtaining the Green’s function. But can this structure also provide physical insights? In this talk we will discuss the types of structure that can hide in a Hamiltonian by examining the Hamiltonian’s information content. We then apply this idea to two systems. First, we will investigate the complex band structure of an almost-crystalline system, showing that complex band structure is the minimal, intrinsic material information for describing the system [1]. Second, we will tie this hidden Hamiltonian structure to complete destructive interference effects in electron transport through molecules [2, 3].

[1] M. G. Reuter. J. Phys.: Condens. Matter 29, 053001 (2017).

[2] M. G. Reuter, T. Hansen. J. Chem. Phys. 141, 181103 (2014).

[3] P. Sam-ang, M. G. Reuter. New J. Phys. 19, 053002 (2017).

Speaker: **Jiadong Zang**, Univ. of New Hampshire

**Topological Spin Textures in Chiral Magnets**

B-131 Physics, Stony Brook University

**Abstract**

Chiral magnets are a series of magnets with broken inversion symmetry. A new type of spin interaction therein, the Dzyaloshinskii-Moriya interaction, stimulates the formation of many novel topological spin textures. One typical example is the emergence of magnetic skyrmion, whose nontrivial topology enables unique dynamical property and thermal stability and gives out promise on future magnetic memory devise. Inspired by skyrmions, in this talk, I will give a comprehensive introduction of skyrmions and their behavior in confined geometries. I will also present three other relevant spin textures in chiral magnets. One is the target skyrmion we recently observed, both theoretically and experimentally, in ultra-small nanodisks of chiral magnets. Zero-field target skyrmions and their polarization switch will be discussed. Putting in heterostructures, we also found a new type of topological configuration dubbed the Hopfion therein. Finally, I will discuss emergent topology driven by thermal fluctuations.

An Inside View of Physical Review Family of Journals

The Physical Review journals of the APS have a long tradition of publishing important physics papers, and serving as the bedrock of physics research. In the past six years, the APS has launched four new journals to broaden this collection: Physical Review X, a highly selective Open Access journal; Physical Review Applied, dedicated to publishinghigh-quality papers that bridge the gap between engineering and physics, and between current and future technologies; Physical Review Fluids, an journal publishing innovative research that will significantly advance the fundamental understanding of fluid dynamics; and Physical Review Materials: a new broad-scope journal serving the multidisciplinary community working on materials research.

In this talk, I will give a brief overview of our journal family and the peer review process, and offer some guidelines on how to communicate effectively with editors and referees during the review process. I will also discuss our current scope and standard of published papers on condensed matter, AMO physics, and materials science. I will be available to answer questions, hear ideas, and discuss comments about the journals.

Yiming Xu received his B.Sc. from Nanjing University in China and his Ph.D. from Boston College, both in experimental condensed matter physics. Prior to joining PRX in 2014, he was a postdoctoral fellow in the Materials Sciences Division at Lawrence Berkeley National Laboratory. His research focus was on the electronic properties of strongly correlated materials.

Dr. Navaneetha Ravichandran, Boston College

Title: Phonon scattering from material boundaries and higher-order anharmonicity

Abstract:

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.

bio:

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.

**Abstract**

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 FeTe_{0.55}Se_{0.45} (T_{c}~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 [1]. 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 [2].

[1] C. C. Homes *et al.*, Phys. Rev. B **91**, 144503 (2015).

[2] C. C. Homes *et al.*, Nature **430**, 539 (2004).

**Exact Factorization Approach to Coupled Electron, Ion, and Photon Dynamics**

**abstract**

**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.

** **

**Abstract**: Scattering scanning near-field optical microscopy (s-SNOM) has emerged as a powerful imaging and spectroscopic tool for investigating nanoscale heterogeneities in biology, quantum matter, and electronic and photonic devices. However, many materials are defined by a wide range of fundamental molecular and quantum states at far-infrared (FIR) resonant frequencies currently not accessible by s-SNOM. Here we show ultrabroadband FIR s-SNOM nano-imaging and spectroscopy by combining synchrotron infrared radiation with a novel fast and low-noise copper-doped germanium (Ge:Cu) photoconductive detector [1]. This approach of FIR synchrotron infrared nanospectroscopy (SINS) extends the wavelength range of s-SNOM to 33µm (330 cm −1 , 10 THz), exceeding conventional limits [2] by an octave toward lower energies. We demonstrate this new nano-spectroscopic window by measuring elementary excitations of exemplary functional materials, including surface phonon-polariton waves and optical phonons in oxides and layered ultrathin van der Waals materials, skeletal and conformational vibrations in molecular systems, and the highly tunable plasmonic response of graphene.

**References**

**5**(7), 2773–2779(2018).

**111**(20), 7191–7196 (2014).

**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 [1], reveal hidden structure in correlated electron systems [2], and resolve transient interface polaritons in van der Waals heterostructures [3]. 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 [5].

**References:**

[1] M. Eisele et al., Nature Photon. 8. 841 (2014).

[2] M. A. Huber et al., Nano Lett. 16, 1421 (2016).

[3] M. A. Huber et al., Nature Nanotech. 12, 207 (2017).

[4] T. L. Cocker et al., Nature Photon. 7, 620 (2013).

[5] T. L. Cocker et al., Nature 539, 263 (2016).