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

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.

The lectures in this series will attract a broad audience of physicists from SBU and BNL,
and SBU graduate students.

February – April 2021

Sobhit Singh (Rutgers)
Feb 19 @ 1:30 pm – 2:30 pm

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).

Liang Wu (University of Pennsylvania)
Feb 26 @ 1:30 pm – 2:30 pm

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.


[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

Sahal Kaushik (SBU)
Mar 5 @ 1:30 pm – 2:30 pm

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

SPECIAL SEMINAR: Eslam Khalaf (Harvard)
Mar 12 @ 10:00 am – 11:00 am
Moiré Materials: A Tunable Platform for Interacting Quantum Phases
Understanding the nature of strong electronic correlations is one of the central problems of condensed matter physics. Despite intense efforts, several aspects of the strong correlation problem remain unsolved due to the theoretical difficulty, structural complexity, and limited tunability of most strongly correlated materials. In this talk, I will discuss an exciting new platform that overcomes many of these limitations: Moiré materials.


Moiré materials are simple, highly tunable strongly correlated systems that display a wide array of exotic phases: correlated insulators, unconventional superconductors, orbital magnetism, and topological phases. The simplest Moiré material is twisted bilayer graphene (TBG) consisting of two graphene sheets twisted by a small angle relative to each other. Despite TBG being such a strongly interacting system, I will show how we can understand its basic features using a simple toy model that relates it to a pair of time-reversed multilayer quantum Hall systems. This model explains the appearance of correlated insulators as seen in recent experiments and predicts the nature of their collective excitations. The model also suggests a new topological mechanism for strong-coupling unconventional superconductivity that is distinct from conventional weak coupling mechanisms as well as strong coupling mechanisms proposed in other unconventional superconductors such as cuprates. I will show how this mechanism provides insights into the essential ingredients of superconductivity in TBG and explains why superconductivity is not observed in other related Moiré systems. Furthermore, I will show how insights gained from this model have led to the discovery of a new Moiré superconductor, twisted trilayer graphene, and how they can be used in the future to predict new platforms for unconventional superconductivity and other exotic phases. At the end, I will discuss how insights from Moiré materials can help develop a new understanding of the effects of band topology in strongly interacting materials in general.


Host: Sasha Abanov
Practice March meeting talk: Phil Allen
Mar 12 @ 1:30 pm – 2:30 pm

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.

No seminar — March meeting
Mar 19 all-day
Simon Divilov (Autonomous University of Madrid)
Mar 26 @ 1:30 pm – 2:30 pm

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

Mar 29 @ 10:00 am – 11:00 am

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.

Xiaoji Xu (Lehigh)
Apr 2 @ 1:30 pm – 2:30 pm

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

SPECIAL SEMINAR: Aris Alexandradinata (UIUC)
Apr 7 @ 10:00 am – 11:00 am

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