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"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  two-dimensional  (2D) topological  insulator,  is  a  topological  state  of  matter  supporting  the  helical edge states, which are counter-propagating, spin-momentum locked 1D modes protected  by  time  reversal  symmetry. It  exhibits  special  magneto-transport properties under external magnetic field. In my talk, I will construct modified Anderson impurity models to study the magneto-conductance 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 Lippmann-Schwinger 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 Hartree-Fock 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
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"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 many-particle physics with their unique ability of single atom resolved imaging. Quantum gas microscopes provide microscopic information of quantum many-body states through spatial correlation functions. Relying on the unique tunability of ultracold atoms in atomic interactions via Feshbach resonances, density, and spin-imbalance, 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 many-body ground state [1]. In this talk, I present a Mott insulator of lithium-6 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 single-species singles components with the use of doublon hiding [3] and spin removal techniques [4] to detect spin-spin 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
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"Driven Majorana Zero Modes: A Route to Synthetic px+ipy Superconductivity"


Lingyu Yang , University of Virginia - Department of Physics
[Host: Prof. Gia-Wei 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 site-dependent 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=aVZ3UWp2UzltUnlWREV1azdjcUFqUT09
Meeting ID: 943 5816 2934  Passcode: 414834


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"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 non-linear susceptibility by Shivaram and collaborators.

Condensed Matter Seminar
Thursday, February 24, 2022
3:30 PM
Online, Room via Zoom
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Join Zoom Meeting:  
https://virginia.zoom.us/j/91531198174
Meeting ID: 915 3119 8174  Passcode: 486564


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" New Direct Electron Imaging Techniques for Quantum Materials"


Dr. Kayla Nguyen , University of Illinois Urbana-Champaign
[Host: Utpal Chatterjee]
ABSTRACT:

Electron microscopy is transforming the physical sciences. Aided by a new generation of direct imaging detectors, cryo-electron 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 co-invented 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


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ABSTRACT:

: The perfectly linear temperature dependence of the resistivity observed as T0 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 T0 close to this critical point. I will present high-field magneto-transport measurements of two hole-doped cuprates, near their respective p*, supporting that T-linear resistivity as T0 is a generic property of cuprates, associated with a universal scattering rate. We measured the low-T resistivity of Bi2Sr2CaCu2O8+δ just above p* [1] and found that it exhibits a T-linear dependence, quantitatively similar to other very different cuprates. We also observed, using the Drude formula, that in various cuprates showing this low-T phenomenon the slope of this T-linear resistivity is given by a universal relation implying a specific scattering rate for charge carriers: 1/�� = αh/2πkBT (corresponding to what is called the Planckian limit [2]), where h is Planck’s constant, kB is the Boltzmann constant and α a constant of order unity. Finally, we directly measured the scattering rate in La1.6xNd0.4SrxCuO4, just above p* and in the low-T limit, using angle-dependent magneto-resistance measurements [4]: these experiments reveal an inelastic scattering rate which is isotropic and linear in temperature, and whose magnitude is consistent with Planckian dissipation.
[1] Legros et al., Nat. Phys. 15, 142 (2019)
[2] Zaanen, SciPost Phys. 6, 061 (2019)
[3] Grissonnanche et al., Nature 595, 667 (2021)

Condensed Matter Seminar
Thursday, December 9, 2021
3:30 PM
Online, Room via Zoom
Note special room.

Join Zoom Meeting
https://virginia.zoom.us/j/96936622285
Meeting ID: 969 3662 2285  Passcode: 792554


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ABSTRACT:

We consider the entanglement entropies of energy eigenstates in quantum many-body systems. For the typical models that allow for a field-theoretical description of the long-range 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 quantum-critical regimes. They describe the full crossover from the groundstate entanglement scaling for low energies and small subsystem size (area or log-area law) to the extensive volume-law 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 non-interacting 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 non-integrable 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
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