Condensed Matter/Biophysics Seminars
Summer 2020

Condensed Matter/Biophysics Seminars are via Zoom until further notice
on Mondays @ 4:05 pm to 4:55 pm

Contact: Yasu Takano or Dmitrii Maslov

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May 18
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May 25 (No seminar - Memorial Day)
	Speaker	NA
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June 1, 10:00 am (Note special time)
	Speaker	Kamran Behnia (CNRS/ESPCI, PSL Research Univ., Paris)
	Title	Charge and entropy transport in strontium titanate
	Abstract	The ferroelectric instability in pristine strontium titanate is aborted by quantum fluctuations. Therefore, the static electric permittivity saturates to an extremely large value at low temperature and the effective Bohr radius approaches a micron. In this context, removing a tiny fraction of oxygen atoms turns the system to a dilute metal with a sharp Fermi surface and a superconducting instability. The focus of this talk will be charge and entropy transport by electrons in this dilute metal and their broad implications. The temperature dependence of the resistivity of this dilute metal at low temperature is quadratic, even though Umklapp scattering is absent and there is a single Fermi pocket at the zone center. The prefactor of this T-square resistivity corresponds to what is expected in an extended Kadowaki-Woods scaling. In all known Fermi liquids, knowing the Fermi energy allows one to predict the rough magnitude of the T-square resistivity. The origin of this empirical rule, valid over 4 orders of magnitude, is not known. At high temperature, on the other hand, the quasi-particle picture of transport in dilute metallic strontium titanate breaks up. The magnitude of resistivity leads to a mean-free-path too short to be plausible. The temperature dependence of the Seebeck coefficient implies that non-degenerate electrons are becoming heavier with warming. This points to a new route towards mass amplification through entropy accumulation. These observations appear to reside beyond available polaronic theories.
	Host	Dmitrii Maslov

June 8
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June 15, 3:00 pm (Note special time)
	Speaker	Haruki Watanabe (Univ. Tokyo)
	Title	Generalized f-sum rules and Kohn formulas on non-linear conductivities
	Abstract	The f-sum rule and the Kohn formula are well established general constraints on the electric conductivity in quantum many-body systems. We present their generalization to non-linear conductivities at all orders of the response in a unified manner, by considering two limiting quantum time-evolution processes: a quench process and an adiabatic process. Our generalized formulas are valid in any stationary state, including the ground state and finite temperature Gibbs states, regardless of the details of the system such as the specific form of the kinetic term, the strength of the many-body interactions, or the presence of disorders. Refs: HW, M. Oshikawa, arXiv:2003.10390; HW, Y. Liu, M. Oshikawa, arXiv:2004.04561.
.	Host	Yasu Takano

June 22
	Speaker	Philip Phillips (Univ. Illinois Urbana-Champaign)
	Title	Beyond BCS: An exact model for superconductivity and Mottness
	Abstract	Because the cuprate superconductors are doped Mott insulators, it would be advantageous to solve even a toy model that exhibits both Mottness and superconductivity. We consider the Hatsugai-Kohmoto model, an exactly solvable system that is a prototypical Mott insulator above a critical interaction strength at half filling. Upon doping or reducing the interaction strength, our exact calculations show that the system becomes a non-Fermi liquid metal with a superconducting instability. In the presence of a weak pairing interaction, the instability produces a thermal transition to a superconducting phase, which is distinct from the BCS state, as evidenced by a gap-to-transition temperature ratio exceeding the universal BCS limit. The elementary excitations of this superconductor are not Bogoliubov quasiparticles but rather superpositions of doublons and holons, composite excitations signaling that the superconducting ground state of the doped Mott insulator inherits the non-Fermi liquid character of the normal state. An unexpected feature of this model is that it exhibits a superconductivity-induced transfer of spectral weight from high to low energies as seen in the cuprates.
	Host	Peter Hirschfeld

June 29, 10:00 am (Note special time)
	Speaker	Christian Balz (ISIS Neutron and Muon Source, UK)
	Title	Neutron scattering in a Kitaev honeycomb compound
	Abstract	The Kitaev model on a honeycomb lattice predicts a quantum spin liquid (QSL) ground state with unique magnetic excitations resembling Majorana fermions and gauge flux excitations. These dynamic features are exciting prospects to both fundamental physics and applications towards technology for quantum computation. Neutron scattering has proven to be an ideal tool to measure static and dynamic properties of magnetic materials. In this talk, I will describe our range of experiments studying the magnetic Mott insulator α-RuCl₃ built of weakly coupled honeycomb layers. Despite a long-range ordered ground state, neutron scattering reveals a continuum of excitations resembling predictions from the Kitaev model, confirming that the material is proximate to a QSL. Further, an external magnetic field has been shown to drive the material through several phase transitions which have been controversially discussed in literature. Neutron scattering in combination with complimentary measurements probing magnetic bulk properties is used to understand the T-B phase diagram of α-RuCl₃.
	Host	Yasu Takano

July 6
	Speaker	Lei Wang (Nanjing Univ.)
	Title	Correlated electronic phases in twisted bilayer transition metal dichalcogenides
	Abstract	Emergent quantum phases driven by electronic interactions can manifest in materials with narrowly dispersing, i.e. “flat", energy bands. And the advent of van der Waals heterostructures [1] has opened up new avenues to band engineering simply by placing one monolayer on top of another. Recently, flat bands have been realized in a variety of graphene-based heterostructures using the tuning parameters of twist angle, layer stacking and pressure, and resulting in correlated insulator and superconducting states. In this talk, I am going to present our recent experimental observation [2] of correlated phases in twisted bilayer tungsten diselenide (tWSe₂), a semiconducting transition metal dichalcogenide (TMD). Unlike twisted bilayer graphene where the flat band appears only within a narrow range around a “magic angle", we observe correlated states over a continuum of angles, spanning 4° to 5.1°. Hall measurements supported by ab initio calculations suggest that the strength of the insulator is driven by the density of states at half filling, consistent with a 2D Hubbard model in a regime of moderate interactions. At 5.1° twist, we observe evidence of superconductivity upon doping away from half filling, reaching zero resistivity around 3 K. Our results establish twisted bilayer TMDs as a model system to study interaction-driven phenomena in flat bands with dynamically tunable interactions. [1] L. Wang et al, Science, 614-617 (2013). [2] L. Wang, et al. Nature Materials (2020) https://doi.org/10.1038/s41563-020-0708-6.
	Host	Xiao-Xiao Zhang

July 13
	Speaker	Leenoy Meshulam (MIT)
	Title	Coarse graining and hints of scaling in large populations of neurons
	Abstract	Faced with a system that has many degrees of freedom, it is natural to search for a reduced description. Thus, we describe the flow of fluids not by tracking positions and velocities of all the constituent molecules, but by coarse grained density, velocity, and pressure fields that reflect averages over large numbers of molecules. The evident complexity of biological systems has led many people to wonder if similar, systematic coarse graining could be effective in these cases as well. The rapid development of experimental methods to measure, simultaneously, the activity of large numbers of neurons (1000+) has made this question more urgent. We focus on the neural activity underlying the behavior of mice running in a virtual reality environment. To write down minimal models for the collective behavior of these large populations of cells, we seek theoretical approaches that will help us simplify the rich dynamics. First, I will show that we can reliably build maximum entropy models for different subsets of neurons out of the whole population. These models, which are equivalent to Ising models with competing interactions, make surprisingly accurate predictions for the activity of individual neurons given the state of the rest of the network. Next, we use different coarse graining methods, in the spirit of the renormalization group, to uncover macroscopic features of the large network. We see hints of scaling and of behavior that is controlled by a non-trivial fixed point. Perhaps, then, these hints of emergent simplicity in this very complex system, can help us understand what is special about networks of real neurons.
	Host	Purushottam Dixit

July 20
	Speaker	Kazuhiro Fujita (Brookhaven National Lab.)
	Title	Visualizing the cuprate pair density wave state
	Abstract	The defining characteristic [1,2] of Cooper pairs with finite center-of-mass momentum is a spatially modulating superconducting energy gap D(r). Recently, this concept has been generalized to the pair density wave (PDW) state predicted to exist in cuprates [3,4]. Although the signature of a cuprate PDW has been detected in Cooper-pair tunneling [5] and electronic structures of an underdoped cuprates can be explained very well by a coexisting d-wave superconductivity and PDW model [6], the distinctive signature in single-electron tunneling of a periodic D(r) modulation has never been observed. Here, using a new approach, we discover strong D(r) modulations in Bi₂Sr₂CaCu₂O_8+δ that have eight-unit-cell periodicity or wavevectors Q~2p/a₀(1/8,0); 2p/a₀(0,1/8). Simultaneous imaging of the local-density-of-states N(r,E) reveals electronic modulations with wavevectors Q and 2Q, as anticipated when the PDW coexists with superconductivity. Finally, by visualizing the topological defects in these N(r,E) density waves at 2Q, we discover them to be concentrated in areas where the PDW spatial phase changes by p, as predicted by the theory of half-vortices in a PDW state [7,8]. Overall, this is a compelling demonstration, from multiple single-electron signatures, of a PDW state coexisting with superconductivity in the canonical cuprate Bi₂Sr₂CaCu₂O_8+δ. In this talk, I will present the recent development of the cuprate PDW studies as summarized above and discuss a possible role of the PDW in the cuprate. This work was done in collaboration with Zengyi Du, Hui Li Sanghyun Joo, Elizabeth P. Donoway, Jinho Lee, J. C. Séamus Davis, Genda D. Gu, Peter D. Johnson, and has been published in Nature 580 65-70 (2020). [1] Flude, P. and Ferrel, R. A., Phys. Rev. 135, A550 (1965). [2] Larkin, A. I. and Ovchinnikov, Yu. N., Sov. Phys. JETP 20, 762 (1965). [3] Fradkin, E., Kivelson, S. A., Tranquada, J. M., Rev. Mod. Phys. 87, 457 (2015). [4] Agterberg, D. F. et al., Annu. Rev. Condens. Matter Phys. 11 231-270 (2020). [5] Hamidian, M. H. et al., Nature 532, 343-347 (2016). [6] Choubey, P. et al., Proc. Natl Acad. Sci. 117, 14805–14811 (2020). [7] Agterberg, D. F. and Tsunetsugu, H., Nat. Phys. 4, 639-642 (2008). [8] Berg, E., Fradkin, E., Kivelson, S. A., Nat. Phys. 5, 830-833 (2009).
	Host	Yasu Takano

July 27
	Speaker	Xueda Wen (MIT)
	Title	TBA
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	Host	Yuxuan Wang

August 3
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August 10
	Speaker	Saurabh Maiti (Concordia Univ., Montreal)
	Title	TBA
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	Host	Dmitrii Maslov

Condensed Matter/Biophysics Seminars
Fall 2020

Condensed Matter/Biophysics Seminars are via Zoom until further notiice
on Mondays @ 4:05 pm to 4:55 pm

Contact: Yasu Takano or Dmitrii Maslov

August 31
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September 7 (No seminar – Labor Day)
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September 14
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September 21
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September 28
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October 5
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October 12
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October 19
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October 26
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November 2
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November 9
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November 16
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November 23
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November 30
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December 7
	Speaker	Suchitra Sebastian (Univ. of Cambridge)
	Title	TBA
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	Host	Yasu Takano

December 14
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