"Transport on an interacting helical edge with resonant impurities"Youjian Chen , University of Virginia  Department of Physics [Host: Prof. Dima Pesin]
ABSTRACT:
The quantum spin Hall insulator, also known as a twodimensional (2D) topological insulator, is a topological state of matter supporting the helical edge states, which are counterpropagating, spinmomentum locked 1D modes protected by time reversal symmetry. It exhibits special magnetotransport properties under external magnetic field. In my talk, I will construct modified Anderson impurity models to study the magnetoconductance of the quantum spin hall insulator. Firstly, I will solve the transmission through single impurity on helical Luttinger liquid in the presence of magnetic field using LippmannSchwinger equation. I will show the analytical expression for trans mission and reflection coefficient in terms of the difference between energy of particle and the impurity level, the hybridization coefficient and the magnetic field. Then, I will show the effect of Coulomb interaction on helical Luttinger liquid at HartreeFock level. Using renormalization group, I will derive the temperature dependence of conductance. Lastly, I will show the coherent transport of transmission through many impurities with different energy and hybridization coefficient in the presence of magnetic field. I will compare my theoretical result with experiment. 
Condensed Matter Seminar Thursday, April 21, 2022 3:30 PM Clark Hall, Room G004 Note special room. 
"Probing correlations in fermionic triangular Hubbard systems"Jirayu Mongkolkiattichai , University of Virginia  Department of Physics [Host: Prof. Peter Schauss]
ABSTRACT:
Quantum gas microscopes have expanded the understanding of manyparticle physics with their unique ability of single atom resolved imaging. Quantum gas microscopes provide microscopic information of quantum manybody states through spatial correlation functions. Relying on the unique tunability of ultracold atoms in atomic interactions via Feshbach resonances, density, and spinimbalance, we study a wide parameter range in the phase diagram. Interestingly, a triangular lattice is the simplest example of geometric frustration because three spins with antiferromagnetic interactions cannot be antiparallel, leading to large degeneracies in the manybody ground state [1]. In this talk, I present a Mott insulator of lithium6 on a symmetric triangular lattice with a lattice spacing of 1003 nm. The lattice is imaged via a Raman sideband cooling technique with imaging fidelity of 98% [2]. We calibrated tunneling by extracting lattice depth from band excitation and the interaction is determined using doublon formation. We can access singlespecies singles components with the use of doublon hiding [3] and spin removal techniques [4] to detect spinspin correlations. We compare the results to Determinantal Quantum Monte Carlo calculations, plan to investigate 120° Neel ordering in Heisenberg antiferromagnets, and search for quantum spin liquids in the triangular lattice Hubbard system. [1] L. Balents, Nature 464, 7286 (2010). [2] J. Yang, et al., PRX Quantum 2, 020344 (2021). [3] P. T. Brown, et al., Science 357, 6358 (2017). [4] M. F. Parsons, et al., Science 353, 1253 (2016). 
Condensed Matter Seminar Thursday, April 14, 2022 4:00 PM Clark Hall, Room G004 Note special time. Note special room. 
"Driven Majorana Zero Modes: A Route to Synthetic px+ipy Superconductivity"Lingyu Yang , University of Virginia  Department of Physics [Host: Prof. GiaWei Chern]
ABSTRACT:
In the Kitaev's toy model with a constant chemical potential, the Majorana zero modes (MZMs) can exist but stay localized at the edges of a 1D spinless chain. In this talk, I will introduce the Kitaev's toy model with a sitedependent chemical potential. In this case, one is able to create segments of topological and normal superconducting phases. The MZMs exist at the domain walls between the two phases, but not necessarily at the edges of the whole chain. By tuning the chemical potential such that the domain walls can change in space, the MZMs are able to move in space as well. We call this motion of MZMs the Majorana pump and argue that it leads to px+ipy superconductivity. I will present how the py pairing emerges, and how to realize this model in experiments. 
Condensed Matter Seminar Thursday, April 7, 2022 2:30 PM Clark Hall, Room G004 Note special time. Note special room. Join Zoom Meeting:https://virginia.zoom.us/j/94358162934?pwd=aVZ3UWp2UzltUnlWREV1azdjcUFqUT09Meeting ID: 943 5816 2934 Passcode: 414834 
"Seas of Spin liquids: a glimpse from Kitaev Model"Professor G. Baskaran , The Institute of Mathematical Sciences and IIT Madras, India; Perimeter Institute for Theoretical Physics, Waterloo, Canada [Host: Prof. Bellave Shivaram]
ABSTRACT:
P.W. Anderson envisaged a novel situation of quantum paramagnetic (quantum spin liquid) phase of low spin Mott insulators, back in 1973 and described it using Pauling's resonating valence bonds states. RVB idea got a resurgence, with the discovery of high Tc superconductivity by Bednorz and Muller in 1986. Low dimensionality and frustrations enhance quantum fluctuations in low spin systems, resulting in a variety of spin liquids and many ideas  emergent gauge fields, Majorana Fermi sea, to topological phases etc. I will discuss a delightful and exactly solvable model by Kitaev, which realized dreams of RVB theorists and more, in an exact fashion. This fertile model is experimentally realized now, thanks to Khaliullin and Jackeli's prediction. There are continuing surprises, including very recent discovery of anomalous nonlinear susceptibility by Shivaram and collaborators. 
Condensed Matter Seminar Thursday, February 24, 2022 3:30 PM Online, Room via Zoom Note special room. Join Zoom Meeting:https://virginia.zoom.us/j/91531198174Meeting ID: 915 3119 8174 Passcode: 486564 
" New Direct Electron Imaging Techniques for Quantum Materials"Dr. Kayla Nguyen , University of Illinois UrbanaChampaign [Host: Utpal Chatterjee]
ABSTRACT:
Electron microscopy is transforming the physical sciences. Aided by a new generation of direct imaging detectors, cryoelectron microscopy won the 2017 Nobel Prize in Chemistry for advancements in visualization of biomolecules. To go beyond traditional electron microscopy, new detectors must also be developed for the diffraction imaging; here, the scattered electron beam encodes a wealth of information about the structure, chemistry, electrical, optical, and magnetic properties of matter. During my PhD, I coinvented the electron microscopy pixel array detector (EMPAD), a fast, highly efficient detector designed to capture the full scattered electron information. The EMPAD has been licensed to Thermo Fisher Scientific and sold around the world. In my talk, I will highlight how the EMPAD enables new characterization techniques for imaging topological magnetic and ferroelectric structures. These approaches can be used to uncover polarization fields, orbital angular momentum and chirality of polar and magnetic textures. By developing new characterization methods in combination with theoretical predictions, new physics in emerging quantum materials can be revealed with electron microscopy at atomic resolution.

Condensed Matter Seminar Thursday, December 16, 2021 3:30 PM Online, Room via Zoom Note special room. Join via Zoom: https://virginia.zoom.us/j/97711165035?pwd=aHc3VHYxNkZUL29oaEZHMExjMUJ1UT09 
"Tlinear resistivity down to Tâ0 at the pseudogap critical point of holeddoped cuprates and Planckian dissipation"Dr. Anaelle Legros , John Hopkins University [Host: Utpal Chatterjee]
ABSTRACT:
: The perfectly linear temperature dependence of the resistivity observed as T→0 in a variety of metals close to a quantum critical point is a major puzzle of condensed matter physics. In cuprates, this phenomenon is observed in the vicinity of the pseudogap critical point p*. Using high magnetic fields to suppress superconductivity, one can access the normal state properties down to T→0 close to this critical point. I will present highfield magnetotransport measurements of two holedoped cuprates, near their respective p*, supporting that Tlinear resistivity as T→0 is a generic property of cuprates, associated with a universal scattering rate. We measured the lowT resistivity of Bi2Sr2CaCu2O8+δ just above p* [1] and found that it exhibits a Tlinear dependence, quantitatively similar to other very different cuprates. We also observed, using the Drude formula, that in various cuprates showing this lowT phenomenon the slope of this Tlinear resistivity is given by a universal relation implying a specific scattering rate for charge carriers: 1/�� = αh/2πk_{B}T (corresponding to what is called the Planckian limit [2]), where h is Planck’s constant, k_{B} is the Boltzmann constant and α a constant of order unity. Finally, we directly measured the scattering rate in La_{1.6}_{−}_{x}Nd_{0.4}Sr_{x}CuO_{4}, just above p* and in the lowT limit, using angledependent magnetoresistance measurements [4]: these experiments reveal an inelastic scattering rate which is isotropic and linear in temperature, and whose magnitude is consistent with Planckian dissipation. 
Condensed Matter Seminar Thursday, December 9, 2021 3:30 PM Online, Room via Zoom Note special room. Join Zoom Meetinghttps://virginia.zoom.us/j/96936622285Meeting ID: 969 3662 2285 Passcode: 792554 
"Crossover functions for entanglement entropy in manybody energy eigenstates and universality"Thomas Barthel , Duke University [Host: Israel Klich ]
ABSTRACT:
We consider the entanglement entropies of energy eigenstates in quantum manybody systems. For the typical models that allow for a fieldtheoretical description of the longrange physics, we find that the entanglement entropy of (almost) all eigenstates is described by a single crossover function. The eigenstate thermalization hypothesis (ETH) implies that such crossover functions can be deduced from subsystem entropies of thermal ensembles and that they assume universal scaling forms in quantumcritical regimes. They describe the full crossover from the groundstate entanglement scaling for low energies and small subsystem size (area or logarea law) to the extensive volumelaw regime for high energies or large subsystem size. For critical 1d systems, the scaling function follows from conformal field theory (CFT). We use it to also deduce the scaling function for Fermi liquids in d>1 dimensions. These analytical results are complemented by numerics for large noninteracting systems of fermions in d=1,2,3 and the harmonic lattice model (free scalar field theory) in d=1,2. Lastly, we demonstrate ETH for entanglement entropies and the validity of the scaling arguments in integrable and nonintegrable interacting spin chains. References: PRL 127, 040603 (2021); PRA 104, 022414 (2021); arXiv:2010.07265. 
Condensed Matter Seminar Thursday, December 2, 2021 3:30 PM Physics Building, Room 204 Note special room. 
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