**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.

## May – October 2019

**Superfluidity of excitons and polaritons in novel two-dimensional materials**

**Abstract:**

This talk reviews the theoretical studies of the Bose-Einstein condensation (BEC) and superfluidity of indirect excitons and microcavity polaritons in quasi-two-dimensional (quasi-2D) van der Waals nanomaterials such as transition metal dichalcogenide (TMDC) heterostructures and phosphorene. Indirect excitons are the Coulomb-bound pairs of electrons and holes confined to different parallel monolayers of a layered planar nanomaterial structure. The high-T superfluidity of the two-component weakly-interacting Bose gas of the *A*-type and *B*-type indirect excitons in the TMDC heterostructures is proposed [1,2]. The critical temperature and superfluid velocity of the indirect excitons in a bilayer phosphorene nanostructure is shown to be anisotropic, dependent strongly on the particular direction of the exciton propagation [3]. The spin Hall effect for polaritons (SHEP) in a TMDC monolayer embedded in a microcavity is predicted [4]. It is demonstrated that two counterpropagating laser beams incident on a TMDC monolayer can deflect a superfluid polariton flow due to the generation the effective gauge vector and scalar potentials [4]. The polaritons cloud is formed due to the coupling of excitons created in a TMDC layer and microcavity photons. It was demonstrated that the polariton flows in the same valley are splitting: the superfluid components of the *A*and *B*polariton flows propagate in opposite directionsalong the counterpropagating beams, while the normal components of the flows slightly deflect in opposite directionsand propagate almost perpendicularly to the beams [4]. The components of polariton conductivity tensor were obtained for polaritons without Bose-Einstein condensation (BEC) and in the presence of BEC and superfluidity [4]. The possible experimental observation of SHEP is discussed. These results open up new avenues for the experimental realization of the exciton and polariton BEC and superfluidity phenomena as well as their practical applications in optoelectronics [5].

[1] O. L. Berman and R. Ya. Kezerashvili, Phys. Rev. B **93**, 245410 (2016)

[2] O. L. Berman and R. Ya. Kezerashvili, Phys. Rev. B **96**, 094502 (2017).

[3] O. L. Berman, G. Gumbs, and R. Ya. Kezerashvili, Phys. Rev. B **96**, 014505 (2017).

[4] O. L. Berman, R. Ya. Kezerashvili, and Yu. E. Lozovik, Phys. Rev. B **99**, 085438 (2019).

[5] D. W. Snoke and J. Keeling, Physics Today **70**, 54 (2017).

**Unconventional thermal transport **

David Broido

Department of Physics

Boston College

According to conventional theories of heat conduction in semiconductors and insulators, only crystals composed of strongly-bonded light elements can have high lattice thermal conductivity, *k*_{L}, and intrinsic thermal resistance comes only from lowest-order anharmonic three-phonon interactions. In this talk, I will discuss aspects of a new paradigm for achieving high *k*_{L} that we proposed, in which the vibrational properties are tailored to reduce the phase space for three-phonon scattering [1]. * *Our* ab initio* calculations predicted that one candidate material, cubic Boron Arsenide (BAs), indeed had ultrahigh three-phonon limited *k*_{L} comparable to that of the best heat conductor, diamond, and significantly higher than any other semiconductor [1]. In BAs, three-phonon scattering can become so weak that four-phonon scattering also plays an important role in limiting *k*_{L} [2, 3]. Such unconventional transport behavior has been confirmed in recent experiments [3-5]. It gives rise to anomalous non-monotonic pressure dependence of *k*_{L} [6]. I will review the challenging material constraints, which must be overcome in order to achieve the unconventional high *k*_{L}. An interesting case is that of group V transition metal carbides, NbC, TaC and VC. These metals have ideal vibrational properties for the desired weak phonon-phonon scattering. But, their nested Fermi surfaces give rise to strong scattering between phonons and electrons, which results in an orders-of-magnitude lower *k*_{L} that is nearly temperature independent, contrary to the typical behavior.

[1] L. Lindsay, D. A. Broido, and T. L. Reinecke, Phys. Rev. Lett. 111, 025901 (2013).

[2] T. Feng, L. Lindsay, and X. Ruan, Phys. Rev. B, 96, 161201 (2017).

[3] F. Tian et al., Science 361, 582 (2018).

[4] J. S. Kang M. Li, H. Wu, H Nguyen, and Y. Hu., Science 361, 575 (2018).

[5] S. Li, Q. Zheng, Y. Lv, X. Liu, X. Wang, P. Y. Huang, D. G. Cahill, and B. Lv, Science 361, 579 (2018).

[6] N. K. Ravichandran and D. Broido, Nature Communications (2019).

[7] C. Li, N. K. Ravichandran, L. Lindsay, and D. Broido,Phys. Rev. Lett. 121, 175901 (2018).

The advent of two-dimensional materials with hexagonal crystal symmetry offers a new electronic degree of freedom, namely valley, the manipulation and detection of which could potentially be exploited to form new many-body ground states as well as new paradigms of electronic applications. In this talk, I will describe our work in creating valley-momentum locked quantum wires, namely quantum valley Hall kink states, along artificial domain walls created by gating in Bernal stacked bilayer graphene. The (quantum valley Hall) kink states can carry current ballistically with a mean free path of several um’s. I will also demonstrate the operations of a topological valley valve and a tunable electron beam splitter, which exploit unique characteristics of the kink states. Because it uses topology, the operation of the valley valve does not require valley-polarized current. The high quality and versatile controls of the system open the door to many exciting possibilities in valleytronics and in pursuing fundamental physics of helical 1D systems.

J. Li, K. Wang, K. J. McFaul, Z. Zern, Y. F. Ren, K. Watanabe, T. Taniguchi, Z. H. Qiao, J. Zhu,** **“Gate-controlled topological conducting channels in bilayer graphene”, *Nature Nanotechnology*, **11**, 1060 (2016)

Jing Li, Rui-Xing Zhang, Zhenxi Yin, Jianxiao Zhang, Kenji Watanabe, Takashi Taniguchi, Chaoxing Liu, Jun Zhu, “A valley valve and electron beam splitter”, *Science ***362**, 1149 (2018)

Jun Zhu - Penn State

Room B-131 Physics, Stony Brook University. Tuesday June 25, 1:30 pm. host: PBA

**Physical properties of (Mn _{0.85 }Fe _{0.15})Si along the critical trajectory**

A.E. Petrova and S.M. Stishov

Institute for High Pressure Physics of RAS, Troitsk, Moscow, Russia

Dirk Menzel

Institut für Physik der Kondensierten Materie, Technische Universität Braunschweig, D-38106 Braunschweig, Germany

The magnetic phase transition temperature in the helical magnet MnSi decreases with pressure and practically reaches zero value at ~15 kbar. However a nature of this transition at zero temperature and high pressure is still a subject of controversial interpretations. Early it was claimed an existence of tricritical point on the phase transition line that might result in a first order phase transition in MnSi at low temperatures, preventing observation a quantum critical point in MnSi. On the other hand some experimental works and the recent Monte-Carlo calculations may indicate a strong influence of inhomogeneous stress arising at high pressures and low temperatures on characteristics of phase transitions that could make any experimental data not entirely conclusive. In this situation it would be appealing to use a different approach to discover a quantum criticality in MnSi, for instance, making use doping as a control parameter. The results of studying the magnetization, specific heat and thermal expansion of a single crystal with nominal composition Mn _{0.85} Fe _{0.15} Si show that the trajectory corresponding to the present composition of (MnFe)Si is a critical one, i.e. approaching quantum critical point at lowering temperature, but some properties may feel the cloud of helical fluctuations bordering the phase transition line.

**Title**: Structure Inference from X-ray Absorption Spectroscopy: Pilot Projects for *Operando* Experimentation

**host**: Phil Allen

**abstract**: X-ray Absorption Near Edge Structure (XANES) is well-adapted for *in situ* and *operando* experiments. It is both atomically specific and it encodes local structure of the surrounding atoms. Due to multiple scattering effects, inferring that structure from the spectra can be complex. In the Center for Functional Nanomaterials, we are pursuing several prototype projects with collaborating groups to explore approaches to solve this inverse problem and pursue nanomaterials research enabled by these methods. We seek to go beyond the empirical fingerprint method, particularly to broaden applicability to structural motifs that emerge in studies of new or nanostructured materials. Our over-arching approach is two-fold: exploit theory for direct computation of XANES spectra for a pertinent database of material structures to support and train data analysis techniques; develop and validate these techniques through comparison to experiments. Following a brief introduction of *operando* experimental techniques and the role of X-ray absorption spectroscopy, I will describe a series of pilot projects [1-3] that illustrate different aspects of structure inference, including the training of artificial neural network models.

Work performed in part at the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

[1] J. Timoshenko, et al., J. Phys. Chem. Lett*.* 8, 5091 (2017).

[2] M. R. Carbone, et al., Phys. Rev. Mater*.* 3, 033604 (2019).

[3] D. Yan, et al., Nano Lett. 19, 3457, (2019).

** Brief Biography:** Mark S. Hybertsen holds a BA in Physics from Reed College in Portland, OR (1980) and a PhD in Physics from The University of California, Berkeley (1986) where his thesis research was directed to many-body perturbation theory and the GW approach. Dr. Hybertsen joined Bell Laboratories in 1986, pursuing a variety of research projects in the theory of the electronic properties of materials. He supervised the Device and Materials Physics Group in the Semiconductor Photonics Research Department for four years. From 2003 to 2006, Dr. Hybertsen was a Senior Research Scientist in the Department of Applied Physics and Applied Mathematics at Columbia University in New York, where he has also been an Adjunct Professor in the Department of Electrical Engineering. In 2006, Dr. Hybertsen joined the new Center for Functional Nanomaterials at Brookhaven National Laboratory. He is a Senior Scientist, leading the Theory and Computation Group. He has also had adjunct research appointments at Columbia University. Dr. Hybertsen is a fellow of the American Physical Society and a member of the IEEE and the American Chemical Society.

**Title: First observations of topological phonons in crystalline materials**

Host: Jen Cano

Condensed matter systems have now become a fertile ground to discover emerging topological quasiparticles with symmetry protected modes. While many studies have focused on fermionic excitations, the same conceptual framework can also be applied to bosons yielding new types of topological states. This idea has, for example, been applied to great effect in macroscopic waveguides. Motivated by the application of these ideas to naturally occurring crystal lattices, we used inelastic x-ray scattering to make the first observation of topological phonons [1]. We demonstrate that new classes of topological crossing can be accessed in this way, such as “Double Weyl” crossings in FeSi and parity-time symmetry protected helical nodal lines in MoB_{2}[2]. Phonon band structures thus provide compelling new playgrounds for exploring topological properties and I will discuss how they differ from the well-studied electronic band structures from this perspective. I will end by speculating how this might be useful in the future.

**References**

[1]H. Miao, T. T. Zhang, L. Wang, D. Meyers, A. H. Said Y. L. Wang, Y. G. Shi, H. M. Weng, Z. Fang, and M. P. M. Dean, Phys. Rev. Lett. 121, 035302 (2018)

[2] T. T. Zhang, H. Miao, Q. Wang, J. Q. Lin, Y. Cao, G. Fabbris, A. H. Said, X. Liu, H. C. Lei, Z. Fang, H. M. Weng, and M. P. M. Dean, submitted (2019)

**Title: Synchrotron Multi-Dimensional & Multi-Modal Study of Functional Materials**

Host — Phil Allen

**Abstract**: Multi-modal and multi-dimensional characterization at synchrotrons can provide unprecedented information for complex, heterogeneous materials system. A multi-modal approach combines multiple synchrotron techniques to gain complementary information. Furthermore, with imaging techniques specifically, multi-dimensional imaging includes techniques such as tomography, spectroscopic microscopy, or *in situ/operando* imaging. These capabilities are particularly powerful when used to study complex structures with morphological and chemical heterogeneity. This talk will address the applications in nano-/meso-porous and bicontinuous metals, energy storage and conversion materials, and molten salts research. Broader impacts regarding cultural heritage and environmentally friendly anti-corrosion surface treatment will also be briefly discussed

host: Jin Wang

**Mechanistic Basis of Spindle Size Control and Scaling**

The size and morphology of intracellular structures such as the nucleus, Golgi apparatus, and mitotic spindle dramatically vary between different cell types, yet the mechanisms that regulate the size of these structures are not understood. Interestingly, the size of most intracellular structures scales with cell size, i.e., larger cells tend to have a larger nucleus and spindle. So far, many models have been proposed to explain such scaling behavior, but rigorous testing of these models inside the cells is challenging, and often not feasible. To overcome this challenge, we combined the statistical framework of quantitative genetics, with cell biology and biophysics to develop a general methodology to quantitatively examine different models of spindle size control and scaling for the first mitotic spindle in C. elegans. We developed a high-throughput microscopy platform to measure the size and dynamics of the spindle for ~200 genotyped recombinant inbred lines, which are created by the random crossing of two genetically distinct C. elegans wild isolates. We observed quantitative variations for all attributes of spindle size and dynamics, as well as cell size, across these lines. We used these variations to discriminate between different models of spindle size regulation and scaling, and we proposed a new model based on the effect of cortical forces on spindle elongation. To further examine our model, we used laser ablation technique to selectively cut different populations of microtubules and compared the results with predictions of the model. The combination of quantitative genetics with cell biology and biophysics provides a systematic and unbiased method to study mechanisms that contribute to size regulation of intracellular structure and also will give us a deeper understanding of the evolution of these structures.

**Odd fluids**

Abstract:

Two-dimensional isotropic fluids can possess an anomalous part of the viscous stress tensor known as odd or Hall viscosity. This peculiar viscosity does not lead to any dissipation in the fluid. Examples of fluids with odd viscosity include rotating superfluids, plasmas in magnetic fields, quantum Hall fluids, and chiral active fluids. I will describe some manifestations of the odd viscosity. In particular, I will focus on surface waves propagating along the boundaries of such fluids. I will also present a variational principle and the corresponding Hamiltonian structure for fluid dynamics with odd viscosity.