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.

The lectures in this series will attract a broad audience of physicists from SBU and BNL,
and SBU graduate students.
Jun Zhu, Penn State University: Topological Valleytronic in Bilayer Graphene
May 17 @ 1:30 pm – 2:30 pm

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 Nanotechnology11, 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
May 17 @ 1:30 pm – 2:30 pm

Jun Zhu - Penn State

Sergei Stishov (Troitsk)
Jun 25 @ 1:30 pm – 2:30 pm

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.



Mark S. Hybertsen (Center for Functional Nanomaterials, BNL)
Sep 6 @ 1:30 pm – 2:30 pm

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.

Mark P. M. Dean (BNL)
Sep 13 @ 1:30 pm – 2:30 pm

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 MoB2[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.


[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)

Yu-chen Karen Chen-Wiegart (USB – Materials Science & BNL-NSLSII)
Sep 20 @ 1:30 pm – 2:30 pm

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

Reza Farhadifar (Flatiron Institute)
Sep 27 @ 1:30 pm – 2:30 pm

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.

Sasha Abanov (Stony Brook)
Oct 4 @ 1:30 pm – 2:30 pm

Odd fluids

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.

Riley Hanus (Northwestern U and ORNL)
Oct 11 @ 1:30 pm – 2:30 pm

host: Phil

Title: Heat conduction in defective and complex crystals:  phonon scattering and beyond

Riley Hanus*

* Material Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

* Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA.

The flow of heat through materials is a topic of scientific interest and technological importance in fields of microelectronics, power generation, heat management, and thermoelectrics. For example, advancement in microelectronic technologies (e.g. microprocessors, and high-power electronics) demands ever more efficient removal of the heat generated in these devices. In contrast, technologies such as thermal barrier coatings and thermoelectric materials are designed to stop the flow of heat. In simple, defect-free crystals the thermal conductivity is generally well understood. However, in materials containing defects and/or in those with very complex crystal structures there is a lack of basic understanding which inhibits technological progress.

In this presentation, I will highlight several experimental and theoretical results which aim to establish a fundamental understanding of heat transport in defective materials. First, I will discuss several studies related to heat conduction across interfaces. Secondly, I will demonstrate in several material systems how defects can soften a materials lattice which reduces the phonon group velocity and thus decreases thermal conductivity. Lastly, the transition from crystalline-like to amorphous-like thermal conduction is investigated by studying the lattice dynamics of crystals with very complex crystal structures both computationally and experimentally. Through this analysis emerges a description of phonon transport which is divided between two channels. One is the standard phonon-gas transport mechanism and the other we term the diffuson-channel since it is mathematically the same mechanism in which ‘diffusons’ were defined.

Mehdi Jadidi (Columbia)
Oct 18 @ 1:30 pm – 2:30 pm

Title:  Optical Control of Chiral Charge Pumping in a Topological Weyl Semimetal

host: Dima Kharzeev

Abstract: Solids with topologically robust electronic states exhibit unusual electronic and optical transport properties that do not exist in other materials. A particularly interesting example is chiral charge pumping, the so-called chiral anomaly, in recently discovered topological Weyl semimetals, where simultaneous application of parallel DC electric and magnetic fields creates an imbalance in the number of carriers of opposite topological charge (chirality). Here, using time-resolved terahertz measurements on the Weyl semimetal TaAs in a magnetic field, we optically interrogate the chiral anomaly by dynamically pumping the chiral charges and monitoring their subsequent relaxation. Theory based on Boltzmann transport shows that the observed effects originate from an optical nonlinearity in the chiral charge pumping process. Our measurements reveal that the chiral population relaxation time is much greater than 1 ns. The observation of terahertz-controlled chiral carriers with long coherence times and topological protection suggests the application of Weyl semimetals for quantum optoelectronic technology.