Department Calendar

September 2018

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"Unusual superconductivity in FeSe"

"Unusual superconductivity in FeSe"

Date: 
Thu, 09/06/2018 - 2:40pm
Location: 
Pupin Hall 329
Abhay Pasupathy
Columbia University

"Unusual superconductivity in FeSe"

In a bilayer van der Waals material, the application of differential strain between the layers can lead to the formation of strain "solitons". Such solitons are one-dimensional lines where there is a phase slip of one lattice constant between the two layers that comprise the bilayer [1.2]. I will describe scanning tunneling microscopy experiments of such strain solitons in transition-metal dichalcogenides. Using a piezo-driven apparatus, we describe a technique by which uniform as well as differential strain can be applied to single crystal materials with arbitrarily large achievable strains. We apply this to a single crystal of semiconducting MoSe2 which has a single freely-standing monolayer on the surface. We describe the controllable application of strain and the production of strain solitons in this system. We find that strain solitons form along the principal axes of the crystal, and in the case of an arbitrary direction of applied strain the solitons form elongated, quasi-periodic, hexagonal patterns for strain relief. At the vertices of each hexagon, a Y junction is formed, and the vertex of the junction is under triaxial strain. STM spectroscopy measurements show the presence of Landau levels in the Yjunction vertex, with an effective magnetic field of about 300 Tesla.

[1] Alden et al, PNAS 110, 11256 (2013).
[2] Kumar et al, Scientific Reports 6, 21516 (2016).

About the speaker

Abhay Pasupathy works in experimental condensed matter physics. His group uses scanned probe microscopes (scanning tunneling and atomic force), nanofabrication and electron transport measurements to get atomic-resolution structural and electronic information on quantum materials. The research group does a significant amount of microscope building and design, and collaborate with scientists both at Columbia and outside to obtain samples that are of interest. Current projects include studies of graphene and related two-dimensional materials, unconventional superconductors, charge and spin ordered materials.

More details can be found here.

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09/06/2018 - 2:40pm
 
"Tunable strain solitons in transition metal dichalcogenides"

"Tunable strain solitons in transition metal dichalcogenides"

Date: 
Fri, 09/07/2018 - 9:30am
Location: 
Pupin Hall 329
Taka Shibauchi
University of Tokyo

"Tunable strain solitons in transition metal dichalcogenides"

Among iron-based superconductors, FeSe has the simplest crystal structure, but it has several unique features that may be important in understanding the pairing mechanisms [1]. In this lecture I will review the key features of the superconductivity in FeSe including very recent experimental advances. The superconducting order parameter (or the superconducting gap) has a strong momentum dependence indicating the unconventional pairing nature, but surprisingly the detailed structure changes when approaching a twin boundary of nematic domains [2]. The analysis suggests that the time reversal symmetry breaking state is induced near the boundaries, and I will show some experimental evidence for this unusual superconducting state [2,3].
 
Moreover, the gap magnitude is very close to the scale of Fermi energy, placing the system deep inside the BCS-BEC crossover regime [1,4]. The signatures of the BCS-BEC crossover in this system will also be discussed.
 
[1] S. Kasahara et al., Proc. Natl. Acad. Sci. USA 111, 16309-16313 (2014);
[2] T. Watashige et al., Phys. Rev. X 5, 031022 (2015).
[3] T. Hashimoto et al., Nat. Commun. 9, 282 (2018).
[4] S. Kasahara et al., Nat. Commun. 7, 12843 (2016).
 

About the speaker

Takasada Shibauchi (B.S., University of Tokyo 1990; M.S., University of Tokyo 1992; Ph.D., University of Tokyo 1999) is a Professor at the University of Tokyo since 2014. He is a condensed-matter physics experimentalist and his research interests include high-temperature superconductivity, strongly correlated electron systems, low-dimensional electron systems, and quantum magnetism. He received J. Robert Oppenheimer Fellowship from Los Alamos National Laboratory in 2001. He has been an Associate Professor in Kyoto University from 2001 to 2014. In 2014, he was elected as a Highly Cited Researcher by Thomson Reuters. He is an elected fellow of American Physical Society and a member of Physical Society of Japan.

More details can be found here.

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09/07/2018 - 9:30am
 
 
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"The Power of Non-Symmetry"

"The Power of Non-Symmetry"

Date: 
Mon, 09/10/2018 - 2:10pm
Location: 
Pupin Hall Theory Center, 8th Floor
Shu-Heng Shao
Institute for Advanced Study

"The Power of Non-Symmetry"

Global symmetries in two dimensions are implemented by topological defect lines (TDLs).  For continuous global symmetries, the TDLs are nothing but the Noether charges.  However there can be TDLs that are not associated to any global symmetry, and can be thought of as a generalized notion of symmetry.  We study the crossing relations of TDLs, discuss their relation to the 't Hooft anomaly, and use them to constrain renormalization group flows to either conformal critical points or topological quantum field theories (TQFTs). We show that if certain non-symmetry TDLs are preserved along a RG flow, then the vacuum cannot be a non-degenerate gapped state.

About the speaker

Shu-Heng Shao has a wide range of interests in theoretical physics, including supersymmetry and conformal symmetry in diverse dimensions, scattering amplitudes in quantum field theory and string theory, and mathematical physics.

More details can be found here.

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09/10/2018 - 2:10pm
 
"Dipolar Quantum Matter near Absolute Zero Temperature"

"Dipolar Quantum Matter near Absolute Zero Temperature"

Date: 
Mon, 09/10/2018 - 4:15pm
Location: 
428 Pupin Hall
Francesca Ferlaino
Institut für Experimentalphysik, Universität Innsbruck, Innsbruck, Austria and Institute for Quantum Optics and Quantum Information (IQOQI), Innsbruck
"Dipolar Quantum Matter near Absolute Zero Temperature"

Quantum gases of ultracold atoms are a powerful resource to address fundamental questions and realize novel paradigms in few- and many-body quantum physics. The potential of such systems is becoming ever more enabling as scientists acquire an increasingly fine control over optical manipulation (e.g. cooling, trapping, and state preparation) and inter-particle interactions.

Recently, a novel class of atomic species, possessing a large magnetic character and an extraordinary rich atomic spectrum, is entering the stage, offering a new conceptual twist for the field. In our laboratories, we have realized the first dipolar Bose-Einstein condensate and Fermi gas, using a magnetic rare-earth species: Erbium. In the quantum regime, Er atoms possess interparticles interactions of genuinely different nature, in which the ordinary magnetically-tunable contact interaction combines with the long-range and anisotropic magnetic dipolar interaction. The mere existence and competition between these two sources of interactions dictate the physics at play, disclosing a variety of intriguing, yet counter-intuitive, quantum phenomena and phase of matter.

This talk will provide an overview from the Innsbruck prospective of some fascinating dipolar phenomena with dipolar quantum gases (Er) and the newly-achieved heteronuclear dipolar mixtures (Er-Dy).
 

About the speaker

Francesca Ferlaino’s research is dedicated to the experimental study of fundamental few- and many-body phenomena realized with ultracold quantum gases of atoms and molecules. The key advantage of such systems is the high degree of control achievable over their internal and external degrees of freedom. Interactions between particles, trapping environments and quantum states of atoms can be adjusted almost “on demand”, opening enormous possibilities for studying effects belonging to very different branches of physics.  The main focus here is the realization of exotic states of matter such as giant three-body states, quantum dipolar gases of highly magnetic atoms and ultracold polar molecules.

More details can be found here.

  

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09/10/2018 - 4:15pm
 
 
 
"Exploring the physics of spontaneous emission with atomic matter waves"

"Exploring the physics of spontaneous emission with atomic matter waves"

Date: 
Thu, 09/13/2018 - 12:00pm
Location: 
705 Pupin Hall
Dominik Schneble
Stony Brook University

"Exploring the physics of spontaneous emission with atomic matter waves"

The quantitative understanding of spontaneous emission harks back to the early days of QED, when in 1930 Weisskopf and Wigner, using Dirac’s radiation theory, calculated the transition rate of an excited atom undergoing radiative decay. Their model, which describes the emission of a photon through coherent coupling of the atom’s dipole moment to the continuum of vacuum modes, reflects the view that spontaneous emission into free space, driven by vacuum fluctuations, is inherently irreversible. 

In my talk, I will describe recent studies of the Weisskopf-Wigner model in a novel context that allowed us to go beyond the model’s usual assumptions. For this purpose, we created an array of microscopic atom traps in an optical lattice that emit single atoms, rather than single photons, into the surrounding vacuum. Our ultracold system, which provides a tunable matter-wave analog of photon emission in photonic-bandgap materials, revealed behavior beyond standard exponential decay with its associated Lamb shift. It includes partial backflow of radiation into the emitter, and the formation of a long-predicted bound state in which the emitted particle hovers around the emitter in an evanescent wave. My talk will conclude with an outlook on using our new platform for studies of dissipative many-body physics and (non)-Markovian matter-wave quantum optics in optical lattices.

About the speaker

In the Schneble laboratory, they load atoms from BECs into optical lattices, i.e. potentials realized by standing waves of laser light, in order to create and explore ultracold engineered quantum systems. With the help of such optical lattices, it thus becomes possible to address cutting-edge topics ranging from condensed matter physics to quantum information science, through direct quantum simulation. For example, the behavior of atoms in an optical lattice closely mimics that of electrons in a solid, but at a length scale that is three orders of magnitude larger and with exquisite control over all relevant parameters in a naturally defect-free system. Moreover, atoms in optical lattices can act as localized qubits (spins) with controllable interactions, making lattice-based atomic quantum systems a versatile platform for studying the fundamental science driving the development of modern quantum technologies.

More details can be found here.

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09/13/2018 - 12:00pm
 
 
 
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"5d Coulomb branches and 6d Anomaly cancelation"

"5d Coulomb branches and 6d Anomaly cancelation"

Date: 
Mon, 09/17/2018 - 2:10pm
Location: 
Pupin Hall Theory Center, 8th Floor
Monica Jinwoo Kang
Harvard University

"5d Coulomb branches and 6d Anomaly cancelation"

In this talk, I will present Coulomb branches and Chern-Simons terms from the compactification of M-theory on elliptically fibered Calabi-Yau threefolds. I will further check that the uplifted theory in 6d is anomaly-free. I will focus on the theory with a gauge group SU(2)xG2, which plays a major role in the classification of superconformal theories.

About the speaker

Monica Jinwoo Kang is a Ph.D. candidate at Harvard University in the High Energy Theory Group.  Monica's primary interests are M/F-theory, elliptic fibrations, 5/6d supersymmetric gauge theories, and 6d superconformal field theory.  Her advisor is Daniel Jafferis and collaborators incolude Mboyo Esole and Shing-Tung Yau.  Monica was born and raised in Korea where she attended Korea Science Academy and graduated from UC Berkeley in Mathematics and Physics in May 2012.

More details can be found here.

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09/17/2018 - 2:10pm
 
"See the world! with neutrinos: current and future accelerator based neutrino experiments"

"See the world! with neutrinos: current and future accelerator based neutrino experiments"

Date: 
Mon, 09/17/2018 - 4:15pm
Location: 
428 Pupin Hall
Kendall Mahn
Michigan State University

"See the world! with neutrinos: current and future accelerator based neutrino experiments"

One of the most promising investigations of beyond-the-Standard-Model physics has been the study of neutrino oscillation, that is, the conversion of neutrinos from one flavor to another as they propagate.  While neutrino oscillation is studied in a wide variety of experiments, accelerator based experiments, use a muon neutrino or antineutrino beam as a probe, of energies of order 1 GeV.  The most recent analysis of data from the Tokai-to-Kamioka  experiment in Japan hint at differences between neutrino and antineutrino oscillation, indicative of possible CP violation with neutrinos and maximal mixing between tau and muon flavors. This talk will discuss what we aim to learn from current and future experiments, how those experiments operate, and the future challenges of accelerator based programs.

About the speaker

Kendall Mahn joined the MSU Department of Physics and Astronomy in 2014 as a high energy particle (HEP) experimentalist. In 2016, she became the ninth member of the department to receive a prestigious Sloan Research Fellowship since the program's inception in 1955. In the Fall of 2017, she became one of two analysis coordinators for the T2K Experiment.

More details on Kendall's research can be found here.

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09/17/2018 - 4:15pm
 
 
 
 
"Shining light on van der Waals materials"

"Shining light on van der Waals materials"

Date: 
Fri, 09/21/2018 - 5:00pm
Location: 
11M Conference Room – North West Corner
Hyunyong Choi
School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea

"Shining light on van der Waals materials"

In this talk, I will discuss how laser excitation can be used to manipulate the quantum degree of freedom in van der Waals materials. First part of this talk is about inversion-symmetry breaking in multi-layer transition metal dichalcogenides (TMD) and the associated non-local detection of valley-locked spin photocurrent in topological insulators (TI) using the characteristic spin-momentum locking [1]. In the second part, I will discuss the coherent exciton dynamics in group VII TMDs, where the broken in-plane symmetry plays a key role in observing the light-polarization-dependent phenomena [2,3]. Finally, I will show both intrinsic and extrinsic ways of manipulating the interaction dynamics between the surface and bulk states in TIs, where we have observed a strong Fano-like asymmetric response via optical-pump and terahertz (THz) probe spectroscopy. [4,5].

References:

  • [1] Cha, S. et al. Nature Nanotechnology in press (2018).
  • [2] Sim, S. et al. Nature Communications 9, 351 (2018).
  • [3] Sim, S. et al. Nature Communications 7, 13569 (2016).
  • [4] Sim, S. et al. Nature Communications 6, 8814 (2015).
  • [5] In, C. et al. Nano Letters 18, 734 (2018).

 

About the speaker

Professor Hyunyong Choi received his B.S. degree in department of electrical and electronic engineering at Yonsei University in 2002. He did his M.S. and Ph.D. in department of electrical engineering and computer science at the University of Michigan, Ann Arbor (under Prof. Ted Norris) in 2004 and 2007, respectively, and postdocs at the Lawrence Berkeley National Laboratory until 2010. Since then, he has been working at Yonsei University as an Assistant Professor (2011.3-2015.8) and as an Associate Professor (2015.9-present).

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09/21/2018 - 5:00pm
 
 
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"From LIGO-Virgo detections to the measurement of the expansion rate of the Universe"

"From LIGO-Virgo detections to the measurement of the expansion rate of the Universe"

Date: 
Mon, 09/24/2018 - 2:10pm
Location: 
Pupin Hall Theory Center, 8th Floor
Hsin-Yu Chen
Harvard University

"From LIGO-Virgo detections to the measurement of the expansion rate of the Universe"

The discovery of gravitational-wave from the binary mergers and the joint detection of the electromagnetic counterparts have triggered extensive exploration of gravitational-wave science. In this talk, I will discuss the gravitational-wave detection statistics, the electromagnetic follow-up strategies, and focus on one of the many scientific outcomes enabled by the gravitational-wave/electromagnetic-wave multimessenger astronomy--to measure the expansion rate of the Universe. In particular, I will talk about the precision of Hubble constant measured by this method and the expected challenges.

About the speaker

Hsin-Yu Chen's research will focus on gravitational-wave multi-messenger astronomy. Hsin-Yu's research interests include electromagnetic counterpart follow-up of gravitational-wave sources, origin and astrophysics of stellar mass compact binary mergers, and cosmological measurement with gravitational-wave observations.

More details can be found here.

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09/24/2018 - 2:10pm
 
"The Simons Observatory: cosmology from the microwave sky"

"The Simons Observatory: cosmology from the microwave sky"

Date: 
Mon, 09/24/2018 - 4:15pm
Location: 
428 Pupin Hall
Jo Dunkley
Princeton University

"The Simons Observatory: cosmology from the microwave sky"

The Simons Observatory (SO) is a new cosmic microwave background experiment being built in the Atacama Desert in Chile, due to begin observations in the early 2020s. SO will measure the temperature and polarization anisotropy of the cosmic microwave background using 60,000 detectors measuring the sky in six frequency bands. The observatory will have three 0.5-m telescopes and one 6-m telescope. I will describe the scientific goals of the experiment: to characterize the primordial perturbations that were imprinted in the early universe, to measure the mass of neutrino particles and the number of relativistic species, to test for deviations from a cosmological constant, to improve our understanding of galaxy evolution, and to constrain the duration of cosmic reionization.  I will focus in particular on the way in which we will be searching for gravitational waves from the primordial universe, and how we will seek to measure the neutrino mass scale.

About the speaker

Jo Dunkley's research is in cosmology, studying the origins and evolution of the Universe. Her main projects are the Atacama Cosmology Telescope, the Simons Observatory, and the Large Synoptic Survey Telescope.

​Jo has been awarded the Maxwell Medal, the Fowler Prize, the Rosalind Franklin award and the Philip Leverhulme Prize for her work on the Cosmic Microwave Background, and shared the Gruber Prize with the WMAP team. 

More details can be found here.

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09/24/2018 - 4:15pm
 
 
 
"Atomic-scale Engineering of Exotic Superconducting States"

"Atomic-scale Engineering of Exotic Superconducting States"

Date: 
Thu, 09/27/2018 - 12:00pm
Location: 
705 Pupin Hall
Kyle Shen
Cornell University

"Atomic-scale Engineering of Exotic Superconducting States"

Quantum materials host a vast array of emergent electronic phenomena, including high-temperature superconductivity, colossal magnetoresistance, and nanoscale charge / spin ordering. One of the challenges is to be able to precisely and deterministically manipulate the properties of quantum materials. To achieve this control, we employ molecular beam epitaxy (MBE) to synthesize artificial quantum materials with atomic layer precision, combined with angle-resolved photoemission spectroscopy (ARPES) which provides direct insights into the electronic structure. In particular, I will focus on some very recent developments where we have used interfacial engineering and thin film epitaxy to manipulate and control exotic superconductors. The first example is the odd-parity superconductor Sr2RuO4, where we have used epitaxial strain and stabilization to drive a Lifshitz transition in the Fermi surface topology and investigate how epitaxial strain affects the superconducting transition temperature. The second is monolayer FeSe grown on SrTiO3, where the superconducting transition temperature can be enhanced from 8 K in bulk FeSe to approximately 60 K in monolayer thin films grown on SrTiO3 via interfacial engineering.

About the speaker

The Shen research group investigates the emergent properties of quantum materials, with a particular focus on exotic superconductors, artificially engineered materials, and materials in the atomically thin limit. They employ a number of techniques, particularly materials synthesis via molecular beam epitaxy, angle-resolved photoemission spectroscopy, and various x-ray synchrotron-based spectroscopies. They also develop new instrumentation and techniques for synthesizing and investigating materials with atomic precision. 

More details can be found here.

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09/27/2018 - 12:00pm
 
 
 
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