Emergent Phenomena 290S/QM Speaker Ruihua Fan (Harvard) Wednesday, April 6 at 2:00 pm


Time/Venue Wednesday, April 6 at 2:00 pm Pacific Time in Physics South 402 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host
Ehud Altman
Title
Fun with geometric flow in conformal field theories: from Floquet dynamics to t’ Hooft anomalies
Abstract Real-time evolution in many-body systems is usually hard to describe. It gets simplified in one-dimensional conformal field theory, where certain evolution can be reduced to geometric flow. We will start with reviewing the quasi-particle picture that was introduced in the study of entanglement in CFT, the intuition built on which will suggest some dynamical problems to study. One is a Floquet problem, where we can design the Floquet driving to have both non-heating and various heating phases. The heating phase is not thermalizing but, paradoxically, can be used for cooling. The other problem is to extract some ’t Hooft anomalies, or equivalently, some edge properties from a wavefunction.

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Special Seminar: Emergent Phenomena 290S/QM Speaker Jörg Schmalian (Karlsruhe Institute of Technology) Wednesday, March 30 at 3:15 pm

Time/Venue Wednesday, March 30 at 3:15 pm Pacific Time in Physics South 325 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title Superconductivity without quasiparticles: Quantum critical Eliashberg theory and its holographic dual
Abstract Superconductivity is abundant near quantum-critical points, where fluctuations suppress the formation of Fermi liquid quasiparticles and the Bardeen-Cooper-Schrieffer theory no longer applies. Two very distinct approaches have been developed to address this issue: quantum-critical Eliashberg theory and holographic superconductivity. The former includes a strongly retarded pairing interaction of ill-defined fermions, the latter is rooted in the duality of quantum field theory and gravity theory. We demonstrate that both are different perspectives of the same theory. We derive holographic superconductivity in form of a gravity theory with emergent space-time from a quantum many-body Hamiltonian – the Yukawa SYK model and finite-dimensional generalizations thereof – where the Eliashberg formalism is exact. Exploiting the power of holography, we then determine the dynamic pairing susceptibility of the model. Our holographic map comes with the potential to use quantum gravity corrections to go beyond the Eliashberg regime.

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Emergent Phenomena Seminar 290S/QM Seminar Speaker Andy Mackenzie (Max Planck Institute for Chemical Physics of Solids and University of St. Andrews) Wednesday, March 30 at 2:00 pm

Time/Venue Wednesday, March 30 at 2:00 pm in 402 Physics South and Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09

Meeting ID: 995 2349 9113 Passcode: 600704 
Host Joseph Orenstein
Title Uniaxial pressure and the superconducting phase diagram of Sr2RuO
Abstract In this talk I will discuss the use of controlled uniaxial pressure as a method of tuning the properties of quantum materials [1,2].  To demonstrate the potential of the new techniques that are being developed, I will describe the results of a series of experiments on the ultra-high purity unconventional superconductor Sr2RuO4.  I will give a snapshot of the ongoing (and winding!) quest to understand its superconducting order parameter [3-7], and report on recent progress in establishing its phase diagram.  The latter was achieved using a novel technique, the a.c. elastocaloric effect [8], that has enormous potential to establishing the thermodynamics not just of Sr2RuO4, but of any quantum material with a significant coupling of the electronic properties to the lattice.
1.  C.W. Hicks, M.E. Barber, S.D. Edkins, D.O. Brodsky and A.P. Mackenzie, Rev. Sci. Inst. 85, 65003 (2014)
2.  M.E. Barber, A. Steppke, A.P. Mackenzie and C.W. Hicks, Rev. Sci. Inst. 90, 023904 (2019)
3.  C.W. Hicks, D.O. Brodsky, E.A. Yelland, A.S. Gibbs, J.A.N. Bruin, M.E. Barber, S.D. Edkins, K. Nishimura, S. Yonezawa, Y. Maeno and A.P. Mackenzie, Science 344, 283 (2014)
4.  A. Steppke, L. Zhao, M.E. Barber, T. Scaffidi, F. Jerzembeck, H. Rosner, A.S. Gibbs, Y. Maeno, S.H. Simon, A.P. Mackenzie and C.W. Hicks, Science 355, aaf9398 (2017)
5.  A. Pustogow, Yongkang Luo, A. Chronister, Y.-S. Su, D.A. Sokolov, F. Jerzembeck,  A. P. Mackenzie, C. W. Hicks, N. Kikugawa, S. Raghu, E. D. Bauer, and S. E. Brown, Nature 574, 72 (2019)
6.  V. Grinenko, S. Ghosh, R. Sarkar, J.-C. Orain, A. Nikitin, M. Elender, D. Das, Z. Guguchia, F. Brückner, M. E. Barber, J. Park, N. Kikugawa, D.A. Sokolov, J.S. Bobowski, T. Miyoshi, Y. Maeno, A.P. Mackenzie, H. Luetkens, C.W. Hicks and H.-H. Klauss, arXiv:2001.08152, Nature Physics 17, 748 (2021)
7.  Y.-S. Li, M. Garst, J. Schmalian, N. Kikugawa, D.A. Sokolov, C.W. Hicks, F. Jerzembeck, M.S. Ikeda, A.W. Rost, M. Nicklas & A.P. Mackenzie, arXiv:2201.04147
8.  M.S. Ikeda, J.A.W. Straquadine, A.T. Hristov, T. Worasaran, J.C. Palmstrom, M. Sorensen, P. Walmsley, and I.R. Fisher, Rev. Sci. Inst. 90, 083902 (2019)

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Emergent Phenomena 290S/QM Seminar Speaker Chris Wächtler (Max Planck Institute for the Physics of Complex Systems) Wednesday, March 23 at 2:00 pm

Time/Venue Wednesday, March 23 at 2:00 pm Pacific Time in Physics South 402 and via Zoom https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title Thermodynamics, synchronization and topology: Various aspects of electron shuttling 
Abstract With rapidly growing abilities to fabricate systems with submicron dimensions it is possible to explore the interplay of electronic and mechanical properties at the nanoscale. These so called nanoelectromechanical systems allow us to study fundamental phenomena like phonon-assisted electron transport or negative differential resistance, and have many applications including signal processing and high-precision measurements of force and electric charge. One particularly intriguing effect is the shuttling of electrons, where the mechanical oscillation of a grain is achieved by sequential electron tunneling between the grain and two connecting leads. The coupled system of mechanical and electronic degrees of freedom has drawn considerable theoretical and experimental attention since its original proposal. While the dynamical description is well understood, the electron shuttle has not been thoroughly investigated from a thermodynamic perspective. In our work, we provide the fundamentals for such a thermodynamic analysis of the electron shuttle and its onset of self-oscillation based on the rigorous formalism of stochastic thermodynamics. As a practical application, we propose an autonomous nano-engine utilizing the shuttling mechanism, which does not require any active regulation from the outside. We then go one step further and explore collective dynamics in a chain of interacting electron shuttles. Through mechanical coupling of adjacent systems we observe synchronization of both ends of the chain, which is topologically protected against local disorder. A thermodynamic analysis of the collective system shows that these synchronized boundary modes directly affect the thermodynamic properties of the chain.

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Special Emergent Phenomena 290S/QM Seminar Speaker Johannes Mitcherling, (Max Planck Institute for Solid State Research) Wednesday, March 23 at 11:00 am

Time/Venue Wednesday, March 23 at 11:00 1m Pacific Time in Physics South 325 and via Zoom https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title Quantum metric, resistivity bound, and flat bands – Electrical conductivity in multiband systems 
Abstract Geometric concepts provide a very fruitful language for quantum (interband) contributions to the dc electrical conductivity of multiband systems. A well-known example is the intrinsic anomalous Hall conductivity, which is based on the Berry curvature. The quantum metric is a second central quantity of band theory but has so far not been related to many response coefficients due to its nonclassical origin. In this talk, I show that the electrical conductivity yields an interband contribution based on the quantum metric. I discuss the implications of this observation in several examples, which range from spiral magnetism in the context of cuprate superconductors to flat-band models. In the former case the interband contribution due to the quantum metric is crucial for a consistent theoretical description of the Hall number close to the onset of order. In the latter case interband effects due to the quantum metric can be significantly enhanced and even dominate the conductivity. This is true in particular for topological flat-band materials of nonzero Chern number, where an upper bound exists for the resistivity due to the common geometrical origin of quantum metric and Berry curvature. I close by proposing a conductivity minimum due to the quantum metric in low-mobility rhombohedral trilayer graphene.

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Emergent Phenomena 290S/QM Seminar Speaker Vir Bulchandani (Princeton, formerly UCB) Wednesday, March 2 at 2:00 pm

Time/Venue Wednesday, March 2 at 2:00 pm in Physics South 402 and via Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title Onset of many-body quantum chaos due to breaking integrability
Abstract Generic perturbations of an integrable system lead to chaos. How they lead to chaos in the classical setting has been understood for decades, and is captured by the Kolmogorov-Arnold-Moser theorem. No such result exists for quantum systems; moreover, the scaling of the threshold to chaos with system size is not well understood even classically. We propose some answers to these questions for quantum many-body systems, arguing that the onset of chaos can be understood as a Fock-space delocalization process. We corroborate our theoretical predictions against large-scale numerical simulations in a variety of model systems.

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Emergent Phenomena 290S/QM Seminar Speaker Rahul Nandkishore (U of Colorado) Wednesday, February 16 at 2:00 pm

Time/Venue Wednesday, February 16 at 2:00 pm in Physics North 402 and via Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title Insights from multidimensional coherent spectroscopy
Abstract I explain how multi-pulse coherent spectroscopy can reveal new information about solid state systems that is difficult or impossible to extract through conventional means. I illustrate the possibilities with a discussion of two specific applications – extracting qubit lifetimes in disordered silicon, and revealing spectroscopic fingerprints of gapped spin liquids.
Nature Physics volume 17, pages627–631 (2021)
Phys. Rev. Research 3, 013254 (2021)

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Emergent Phenomena Seminar 290S/QM Seminar Speaker Zhaodong Chu (University of Pennsylvania) Wednesday, February 9 at 2:00 pm

Time/Venue Wednesday, February 9 at 2:00 pm in 402 Physics South and Zoom: 
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Professor Eric Ma
Title Imaging Photo-Carrier Dynamics and Correlated states in 2D Semiconductors
Abstract Semiconducting transition-metal dichalcogenides (TMDs) exhibit remarkable electrical and optical properties, such as strong light-matter interaction and high photoresponsivity. Moreover, moiré superlattices composed of TMD offer a fantastic platform to manipulate the correlated quantum states of matter. Understanding charge dynamics and electronic correlation at nanoscale is critical for both fundamental science and practical applications. In this presentation, I will first talk about the simultaneous spatial and temporal photoconductivity imaging in TMDs monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and carrier lifetime were directly measured. Time-resolved experiments indicate that the critical process for photo-carriers is the escape of holes from defect-induced trap states, which prolong the apparent lifetime of mobile electrons in the conduction band. In the second part, I will present the nanoscale conductivity imaging of correlated electronic states in TMDmoiré superlattices. The noncontact microwave probe allows us to observe the Mott insulating state at both hole-doped and electron-doped sides that persists for temperatures up to 150 K. Appreciable inhomogeneity of the correlated states is directly visualized in the hetero-bilayer region, indicative of local disorders in the moiré superlattice potential or electrostatic doping. These works provide essential insights into TMD-based optoelectronics and 2D correlated physics down to the microscopic level.

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Emergent Phenomena Seminar 290S/QM Seminar Speaker Marc Vila Tusell (UC Berkeley) Wednesday, February 2 at 2:00 pm

Time/Venue Wednesday, February 2 at 2:00 pm in 402 Physics South and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title Spin dynamics and topological phases in two-dimensional quantum materials
Abstract Nonlocal transport in mesoscopic devices provides unique fingerprints of the underlying processes taking place. Quantization plateaus of quantum spin Hall phases, detection of the spin Hall effect or measurement of spin relaxation in nonmagnetic materials are only a few examples where nonlocal transport schemes are employed. In this talk, I will show how quantum transport within the Landauer-Büttiker formalism can be used to study spin transport in nonlocal multi-terminal devices as well as to detect a novel valley-polarized quantum anomalous Hall effect in bilayer graphene.

In regards of spin transport, I will show how nonlocal spin valves, a device typically employed in experiments to study spin relaxation, is simulated with quantum transport methodologies. This allows us to go beyond the diffusive, semiclassical picture of spin transport and simulate regimes of current interest, such as the (quasi)ballistic regime of spin transport in ultraclean graphene [1]. Furthermore, I will illustrate how this method allows us to simulate simultaneously the spin relaxation and the spin Hall effect in strong spin-orbit coupling (SOC) materials such as MoTe2 or WTe2 (in their T’ or Td phases), and how both the spin relaxation length and the spin Hall angle relate with the form of the SOC [2]. Also, I will briefly describe how the low symmetry of such materials affects the spin texture and generates a canted quantum spin Hall phase [3].

Finally, I will explain our recent prediction that bilayer graphene, encapsulated between a magnetic insulator and a strong SOC material, becomes a quantum anomalous Hall insulator that is valley polarized [4]. Only one Dirac cone develops a topological gap with well-defined quantized Hall resistance, as inferred by our quantum transport methods.

[1] M. Vila et al. Phys. Rev. Let. 124, 196602 (2020)
[2] M. Vila et al. Phys. Rev. Research. 3, 043230 (2021)
[3] J.H. Garcia, M. Vila et al. Phys. Rev. Lett. 125, 256603 (2020)
[4] M. Vila et al. Phys. Rev. B 104, L161113 (2021)

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Special Emergent Phenomena Seminar 290S/QM Speaker Shubhayu Chatterjee (UC Berkeley) Wednesday, January 26 at 3:30 pm

Time/Venue Wednesday, January 26 at 3:30 pm only via Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Mike Zaletel
Title Skyrmion pairing: a topological route to superconductivity
Abstract Atomically thin Van der Waals materials have emerged as a highly versatile platform to advance our understanding of quantum matter driven by strong electron correlations. Recent experimental breakthroughs in stabilizing few-layered graphene structures with a “magic” relative twist between layers has led to the discovery of a wide variety of correlated states ranging from magnetism to superconductivity. Despite compelling experimental evidence for unconventional superconductivity, the glue which binds electrons into Cooper pairs remains a mystery. In this talk I will propose a novel resolution: the Cooper pairs are composed of electrically charged topological spin textures called “skyrmions,” rather than electrons. First proposed by Tony Skyrme to model baryons in particle physics, I will explain how their topological properties can give rise to superconductivity in an electronic model with purely repulsive interactions, and without recourse to phonons which are conventionally responsible for pairing

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