Skip to content

Physics 599: Condensed Matter Physics Seminars

Spring 2021 Speakers

(Updated throughout the semester)


Date Speaker Title

February 1

Wonhee Ko
Oak Ridge National Laboratory Center for Nanophase Materials Sciences

Probing Quasiparticles in Topological Quantum Materials by Advanced Scanning Tunneling Microscopy

February 8

Xiaojian Bai
Oak Ridge National Laboratory

FeI2: a Quasi-particle Wonderland

February 15

Hao Zhang
UT

Semiclassical Expansion Based on SU(3) Coherent States

February 22

Seung Sae Hong
UC Davis

Designing Low-dimensional Quantum Materials

March 1

Casey Eichstaedt
UT

An Optimized Disentanglement Procedure for Constructing a Wannier Basis for Correlated Electron Materials

March 8

Luis E. F. Foa Torres
Universidad de Chile

TBA

March 15

No Seminar
APS March Meeting Week

 

March 22

Pooya Ronagh
University of Waterloo

TBA

March 29

Matthew Brahlek
Oak Ridge National Laboratory

TBA

April 5

Seunghwan Do
Oak Ridge National Laboratory

TBA

April 12

Jinu Thomas, Rudra Bista
UT

TBA

April 19

Gustavo Adolfo Alvarez-Suchini
UT

TBA


Abstracts
February 1

Wonhee Ko (ORNL Center for Nanophase Materials Sciences)

Probing Quasiparticles in Topological Quantum Materials by Advanced Scanning Tunneling Microscopy

Topological quantum materials are gaining great interest because they host exotic quasiparticles that can be used as basic elements for fault-tolerant quantum computation. In this talk, I will present the recent work in CNMS using advanced scanning tunneling microscopy (STM) to probe the quasiparticles in topological quantum materials. Especially, we focus on Bi-based topological insulators that host 2D Dirac fermions on the surface. The extreme stability of low-temperature STM enables the visualization of quasiparticle interference that reveals the non-trivial topology of the band structure. Multiprobe STM exhibits the transport of quasiparticles, especially their spin states by spin-polarized measurements. The interplay between magnetism and topology can be further investigated by incorporating magnetic elements and applying a magnetic field together with STM. By incorporating the atom manipulation capability of STM, the research will pave the way for precise control of the quasiparticles to perform quantum information processes.

This research was performed at the Center for Nanophase Materials Sciences which is a DOE Office of Science User Facility.


February 8

Xiaojian Bai (Oak Ridge National Laboratory)

FeI2: a Quasi-particle Wonderland

Searching for novel quasi-particles is one of the basic drives in the study of quantum materials. Transverse single-magnons (SM) with |\Delta S|=1 are ubiquitous excitations in the low-energy spectrum of a fully-ordered magnet. The two-magnon single-ion bound state (SIBS), as a categorically different type of quasi-particles with |\Delta S|=2, has long been theorized to exist in the large spin (S>1/2) systems and believed to be evasive to conventional probes. Yet, it was subsequently observed in the uniaxial triangular-lattice S=1 compound FeI2 using various common spectroscopy techniques since the 1970s. Not until recently, its observation was fully understood as a hybridization effect with SM via anisotropic, spin non-conserving exchange interactions using an SU(3) generalized spin-wave theory [1]. In this talk, I will go a step further and demonstrate that, by the same token, different flavors of quasi-particles in FeI2 can interact with each other and produce intrinsic magnon damping effect in magnetic fields. Moreover, I will show that such magnon decay phenomenon can be suppressed as in response to the formation of emergent 4-magnon bound states. Our series of works on FeI2 highlight the importance of anisotropic interactions in large-spin systems and opens up avenues for discovering quantum many-body phenomena remarkably similar to high-energy particle physics.

[1] Bai, Xiaojian, et al. "Hybridized quadrupolar excitations in the spin-anisotropic frustrated magnet FeI2."Nature Physics (2021): 1-6.


February 15

Hao Zhang (University of Tennessee)

Semiclassical Expansion Based on SU(3) Coherent States

The classical mechanics of a point particle can be recovered by taking the ℏ → 0 of the quantum mechanics. However, this classical limit is not well defined for spins, which are intrinsically quantum mechanical objects. Coherent states [1], which originated from the early paper of Schrödinger [2] were always thought of as providing a possible link between quantum and classical mechanics. In this seminar I will first elucidate that the limit ℏ → 0 for spins must be taken while simultaneously sending the dimension of the Hilbert space 𝑑! to infinity. Here the Hilbert space is the representation space of any compact Lie group [3]. In particular, I will construct a classical limit of S=1 spins by introducing SU(3) coherent states of degenerate representations. By treating 1/𝑑! as a small parameter, I will introduce a systematic semiclassical expansion for spin one Hamiltonians, that generalizes the well-known spin wave approximation or 1/S-expansion. Interestingly, I will demonstrate this SU(3) spin wave approximation is more appropriate to describe the low and high temperature dynamics of a large class of spin one systems [4, 5]. Finally, I will use this approach to explain different aspects of the inelastic neutron scattering spectrum of Ba2FeSi2O7 [5]. In particular, by including one-loop corrections to the linear SU(3) spin wave result, we can reproduce the decay and renormalization of the longitudinal mode that is revealed by the inelastic neutron scattering experiment.

[1] Klauder J R and Skagerstam B-S (eds). 1985 Coherent States, Applications in Physics and Mathematical Physics (Singapore: World Scientific)
[2] Schrödinger E 1926. Naturwiss. 14 664
[3] Sven Gnutzmann and Marek Kus. 1988 J. Phys. A: Math. Gen. 31 9871.
[4] X. Bai, S-S. Zhang, Z. Dun, H. Zhang, Q. Huang, H. Zhou, M. B. Stone, A. I. Kolesnikov, F. Ye, C. D. Batista, and M. Mourigal. Nat. Phys. (2021).
[5] S-H. Do, H. Zhang, T. J. Williams, T. Hong, V. O. Garlea, T-H. Jang, S-W. Cheong, J-H. Park, C. D. Batista and A. D. Christianson. arXiv:2012.05445 [cond-mat.str-el].


February 22

Seung Sae Hong (UC Davis)

Designing Low-dimensional Quantum Materials

Materials in the reduced dimension offer many intriguing platforms in condensed matter physics, including mesoscopic electronics, monolayer crystals, and heterointerfaces between dissimilar materials. The talk will introduce several examples of quantum materials design in nanoscale in order to manipulate and create new electromagnetic ground states. The first part of the talk will focus on topological insulators, where the in-situ synthesis control of nanostructures enables 1D confinement of the spin-textured surface states. The second part will cover the recent effort to manipulate complex oxides in a 2D freestanding platform, aiming to understand electron correlation effects by accessing previously unexplored strain states beyond epitaxy.


March 1

Casey Eichstaedt (UTK)

An Optimized Disentanglement Procedure for Constructing a Wannier Basis for Correlated Electron Materials

In 1963, John Hubbard introduced the narrow band Hamiltonian to describe ferromagnetism observed in Nickel. Since then, this model has been the locus of studying electronic and magnetic properties for correlated electron materials (CEM). It is well-established that the need for multi-orbital Hamiltonians for certain CEM, thus the number of adjustable parameters grows very quickly. To estimate these parameters, Wannier functions are constructed via ab initio band structure. It is often the case that this task is very difficult to construct a minimal size Wannier basis due to the ‘entangled’ nature of the band structure. Entanglement is defined as ‘unwanted’ bands intruding in the desired energy range which one wants to define. To remedy this, either more Wannier functions must be added, or a disentanglement procedure must be invoked. Currently the disentanglement techniques have been developed by Souza, Marzari, and Vanderbilt Phys. Rev. B. 65, 035109 (2001). While this procedure has its merit, it still has difficulty faithfully reproducing the low-energy band structure. Here I will discuss work that has been done in our research group to rigorously disentangle a Hilbert space spanned by Wannier functions which exactly reproduces the band structure. In addition, this procedure allows systematic control of the number of Wannier functions in the Hilbert space. I will show results for this procedure applied to one-dimensional and two-dimensional Cuprates.


The flagship campus of the University of Tennessee System and partner in the Tennessee Transfer Pathway.