# During Spring 2021, seminars will be posted on this calendar and take place via Zoom. To join the mailing list and get the link, email Jennifer Cano (first name.lastname@stonybrook.edu)

The CQM Distinguished Lecture series has been established in the Fall of 2015 to bring to Stony Brook University the renowned experts in the physics of quantum matter.

## February – April 2021

**Title: A proposal for determination of polarity orientation in polar metals **

Host: Cyrus Dreyer

In typical ferroelectrics, which are polar insulators, switching of polarity is immediately manifested in a polarization switching current. By contrast, in a polar metal or semimetal, a corresponding experimental response is missing. In this talk, I will discuss that the nonlinear Hall effect (NLHE) can offer a way to detect the polarization orientation and polarization switching in polar metals and semimetals, as well as in narrow bandgap ferroelectric semiconductors, which often show conducting behavior due to the presence of defects and impurities. This effect is particularly enhanced in topological metals or topological semimetals due to the large concentrations in Berry curvature near the Fermi level, which is central to the description of the NLHE [1-2]. However, we find that NLHE can also be realized in topologically trivial materials. The nonlinear Hall response current, which appears as a second-order response to an external electric field, vanishes in the paraelectric phase and reverses its sign upon the polarity reversal in a polar metal [3]. The magnitude of this response current is large enough to be experimentally detected [4].

[1] I. Sodemann and L. Fu, Phys. Rev. Lett. 115, 216806 (2015).

[2] S. Singh, J. Kim, K. M. Rabe, and D. Vanderbilt, Phys. Rev. Lett. 125, 046402 (2020).

[3] R.-C. Xiao, D.-F. Shao, W. Huang, and H. Jiang, Phys. Rev. B 102, 024109 (2020).

[4] Ma et al., Nature 565, 337 (2019).

**Title: Nonlinear terahertz emission spectroscopy of topological chiral multifold semimetals**

The absence of mirror symmetry, or chirality, is behind striking natural phenomena found in systems as diverse as DNA and crystalline solids. A remarkable example occurs when chiral semimetals with topologically protected band degeneracies are illuminated with circularly polarized light. Under the right conditions, the part of the generated photocurrent that switches sign upon reversal of the light’s polarization, known as the circular photogalvanic effect (CPGE), is predicted to depend only on fundamental constants. The conditions to observe quantization are non-universal, and depend on material parameters and the incident frequency. In my talk, I will discuss nonlinear terahertz emission spectroscopy with tunable photon energy from 0.2 eV – 1.1 eV in the chiral topological semimetals CoSi [1,2] and RhSi[3]. Particularly, we identify a large longitudinal photocurrent peaked at 0.4 eV reaching ∼ 550 \mu A/V^2 in CoSi, which is much larger than the photocurrent in any chiral crystal reported in the literature. Using first-principles calculations we establish that the peak originates from topological band crossings, reaching 3.3±0.3 in units of the quantization constant. Our calculations indicate that the quantized CPGE is within reach in CoSi upon doping and increase of the hot-carrier lifetime.

References:

[1]Ni, et al. Nat. Comm. 12, 154 (2021)

[2]Xu, et al. PNAS, 117, 27104 (2020).

[3] Ni, et al. npj Quantum Materials, 5, 96 (2020)

Host: Jen

Tunable chiral symmetry breaking in symmetric Weyl materials

Asymmetric Weyl semimetals, which possess an inherently chiral structure, have different energies and dispersion relations for left- and right-handed fermions. They exhibit certain effects not found in symmetric Weyl semimetals, such as the quantized circular photogalvanic effect and the helical magnetic effect. In this work, we derive the conditions required for breaking chiral symmetry by applying an external field in symmetric Weyl semimetals. We explicitly demonstrate that in certain materials with the Td point group, magnetic fields along low symmetry directions break the symmetry between left- and right-handed fermions; the symmetry breaking can be tuned by changing the direction and magnitude of the magnetic field. In some cases, we find an imbalance between the number of type I left- and right-handed Weyl cones (which is compensated by the number of type II cones of each chirality.)

Ref: Phys. Rev. B 103, 085106 (2021) (ArXiv: 2011.00970)

Host: Dima Kharzeev

**Moiré Materials: A Tunable Platform for Interacting Quantum Phases**

**Thermal susceptibility: the nonlocal temperature response to local heat input**

When a finite sample of a solid absorbs heat from an external source, the temperature response is interesting, especially in nanomaterials. Its understanding is important for heat management of circuit elements. Thermal susceptibility Θ(x-x’,t-t’) was defined by Allen and Perebeinos (2018) as the temperature rise at (x,t) per unit heat insertion at (x’,t’). This linear response function will be discussed for insulating crystals, where heat and temperature are described by phonons. For nanoscale studies, thermal susceptibility is a more useful and appropriate idea than thermal conductivity. It provides a more direct and visualizable understanding of the “ballistic to diffusive crossover”. Two particular issues will be discussed: (1) How can thermal susceptibility of nanoscale systems be studied by Boltzmann theory? (2) Are the results of Boltzmann theory reliable and useful for such systems? Can they help to interpret experiments and molecular dynamics simulations? A phonon Boltzmann theory appropriate for thermal susceptibility was given by Hua and Minnich (2014). The phonon distribution function N(Q) is driven not only by the usual terms, but also by external insertion of heat. This poses several interesting difficulties, which will be discussed. Numerical solutions are difficult. Computations will be discussed.

**Instabilities and the response function in low dimensional materials**

Detecting the presence of charge and spin instabilities in a material is an important step to make predictions about the superconducting transition temperature. Typically, the evidence of sharp peaks in the real part of the static dielectric response function is used as an indication that such instabilities exist. However, there are persistent misconceptions that Fermi surface (FS) nesting guarantees a peak in the response function like in one-dimensional systems, and, in addition, response function matrix elements between empty and occupied states are of secondary importance and set to unity like in the free electron gas case. We explicitly show, through model systems and real materials, within the framework of density functional theory, that predictions about the peaks in the response function, using FS nesting and constant matrix elements yields erroneous results. In all the cases studied, other than the one-dimensional case, we find that the inclusion of matrix elements washes out the structure found with constant matrix elements. Our conclusion is that it is imperative to calculate the full response function, with matrix elements, when making predictions about instabilities in novel materials.

Host: Marivi

**General constraints on metals, with applications to strange metals**

In a solid material, one can consider the physics of the electrons, that move under the influence of a periodic lattice potential and their own mutual electrostatic repulsion. Despite the fact that the same basic microscopic degrees of freedom are present in many different materials, varied and exotic emergent phenomena can occur, and it is an extremely difficult problem to predict the emergent physics for any given material.

Therefore, it is invaluable to develop general theoretical results that constrain the emergent physics, given the properties of the microscopic degrees of freedom. In this talk, I will discuss approaches to obtain such constraints, making contact with field theoretic ideas such as emergent symmetries and anomalies.

I will largely focus on metals. Many metallic materials are successfully described by the so-called “Fermi liquid theory”, but there is also much interest in “non-Fermi liquid metals” that evade such a description. Using the theoretical framework that I introduce, combined with experimental observations, one can derive strong and unexpected conclusions about the nature of a particular kind of non-Fermi liquid metal, the “strange metal” observed in doped cuprates.

**Peak Force Photothermal and Scattering-type Near-field Microscopy for Chemical Nanoscopy**

The combination of atomic force microscope (AFM) with infrared (IR) illuminations opens the route toward label-free spectroscopic imaging well below the diffraction limit. In the presentation, I will present our development of AFM-based infrared microscopy with the peak force tapping mode with two distinctive approaches of photothermal and optical detections. In the photothermal-based peak force infrared (PFIR) microscopy, the tip-enhanced infrared absorption is mechanically probed by the cantilever deflection of AFM through temporal gated detection. The PFIR microscopy allows the collection of both IR imaging and broadband spectroscopy with a quantum cascade laser. We have demonstrated the spatial resolution of the PFIR microscopy to be 6 nm in the air phase and ~10 nm in the liquid phase. The PFIR microscopy is also compatible with simultaneous measurement of mechanical properties and surface potential mapping. In the optical detection-based peak force scattering-type near-field optical microscopy (PF-SNOM), we demonstrated the extension of the scattering-type near-field microscopy to the collection of three-dimensional near-field responses. We observed the momentum localization of hyperbolic phonon polaritons in a boron nitride microdisk that is dependent on the tip-sample distance, as well as the tip-induced relaxation of phonon polaritons in silicon carbide. PF-SNOM also permits multimodal signal collection of mechanical properties and contact current in parallel with near-field imaging. In addition, I will present our work on the development of a compact ultra-broadband laser-driven plasma infrared source for nano-FTIR spectroscopy.

Host: Mengkun

**Revealing the topology of Fermi-surface wave functions from magnetic quantum oscillations**

In the quantum theory of solids, a metal is distinguished from an insulator by having a Fermi surface – a surface (in momentum space) where “the drama of the life of the electron is played out”, leading to all properties unique to metals, e.g., their lustrous appearance, and their ability to conduct heat and electricity. Traditionally, solid-state physicists have focused on experimentally determining the shape of the Fermi surface. Today, emphasis has shifted to determining the quantum geometry of electronic wave functions on the Fermi surface – an electron travelling around the Fermi surface acquires a geometric Berry phase. For some symmetry classes of metals, such Berry phase is nontrivial and unchanging under perturbations of the metal. Such robustness is the hallmark of a new generation of ‘topological metals’, whose recent discovery has revolutionized the field of condensed matter with the promise of new functionalities. As a first step toward such functionalities, material candidates must be grown in laboratories and experimentally verified to be truly topological. For this purpose, I will describe how a time-honored experimental technique (magnetic quantum oscillations) can be refined to unambiguously distinguish a topological metal from a conventional one.

Host: Sasha