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 tbd
Abstract tbd

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

Emergent Phenomena Seminar 290S/QM Seminar Speaker Marc Vila Tusell (UC Berkeley) Wednesday, January 26 at 2:00 pm

Time/Venue Wednesday, January 26 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)

Posted in QM Seminar | Comments Off on Emergent Phenomena Seminar 290S/QM Seminar Speaker Marc Vila Tusell (UC Berkeley) Wednesday, January 26 at 2:00 pm

Emergent Phenomena Seminar 290S/QM Seminar Speaker Igor Markovic (U of British Columbia) Wednesday, December 15 at 2:00 pm

Time/Venue Wednesday, December 15 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 James Analytis
Title Electronically driven spin-reorientation transition of the correlated polar metal Ca3Ru2O7
Abstract Layered perovskite ruthenates are known to host a wide variety of exotic phenomena, stemming from the interplay of their structure and the different interactions in the electronic sector of these materials. Bilayer calcium ruthenate, Ca3Ru2O7, is a polar metal which undergoes two magnetic transitions [1]: a Néel ordering at 56 K and a spin-reorientation at 48 K which has yet to be completely understood. We use angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to study the low-energy electronic structure of Ca3Ru2O7 across a wide temperature range [2], with a focus on the spin-reorientation transition. At low temperatures we find good general agreement with previous ARPES [3] and de Haas-van Alphen [4] experiments. Our measurements across the 48 K transition, however, reveal dramatic changes in the low-energy electronic structure, where upon cooling through the transition we observe the destruction of a large Fermi surface, accompanied by a sudden onset of quasiparticle coherence. Through a series of DFT calculations we demonstrate how the Fermi surface is gapped away by a Rashba-type spin-orbit coupling term, enabled by the bulk structural distortions and unlocked when the spins reorient perpendicular to the local symmetry-breaking potential at the Ru sites. We argue that the electronic energy gain associated with the band hybridisation is actually the key driver for the spin-reorientation phase transition, reflecting a delicate interplay between spin-orbit coupling, structural distortions, and electronic correlations in this system. This mechanism represents a new form of magnetocrystalline anisotropy, where the magnetic moment orientation is ultimately driven by Fermi surface transitions.
[1] D. A. Sokolov et al., Nat. Phys. 15, 671 (2019)
[2] I. Marković et al., PNAS 117, 15524 (2020)
[3] F. Baumberger et al., Phys. Rev. Lett. 96 107601 (2006)
[4] N. Kikugawa et al., J. Phys. Soc. Jpn. 79 024704 (2010)

Posted in QM Seminar | Comments Off on Emergent Phenomena Seminar 290S/QM Seminar Speaker Igor Markovic (U of British Columbia) Wednesday, December 15 at 2:00 pm

Emergent Phenomena Seminar 290S/QM Seminar Speaker Thomas Scaffidi (U Toronto) Wednesday, December 8 at 2:00pm

Time/Venue Wednesday, December 8 at 2 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 
Evidence for deconfined U(1) gauge theory at the transition between toric code and double semion
Abstract Building on quantum Monte Carlo simulations, we study the phase diagram of a one-parameter Hamiltonian interpolating between trivial and topological Ising paramagnets in two dimensions, which are dual to the toric code and the double semion. We discover an intermediate phase with stripe order which spontaneously breaks the protecting Ising symmetry. Remarkably, we find evidence that this intervening phase is gapless due to the incommensurability of the stripe pattern and that it is dual to a U(1) gauge theory exhibiting Cantor deconfinement.

Posted in QM Seminar | Comments Off on Emergent Phenomena Seminar 290S/QM Seminar Speaker Thomas Scaffidi (U Toronto) Wednesday, December 8 at 2:00pm

Special Emergent Phenomena Seminar 290S/QM Seminar Speaker Erez Berg (Weizmann Institute) Tuesday, December 7 at 2:00 pm in 402 Physics South

Time/Venue Tuesday, December 7 at 2:00 pm in 402 Physics South and via Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 , Meeting ID: 995 2349 9113 Passcode: 600704
Host 
Ehud Altman
Title: Handle with care: Gapping fragile topological bands by interactions

Abstract
Twisted bilayer graphene (TBG) has emerged as a new, fascinating platform to study the interplay of band topology and electron-electron interactions. In particular, the narrow bands of TBG are characterized by a subtle form of symmetry-protected structure known as fragile topology. Without electron-electron interactions, this topological structure enforces a band touching between the two bands, that cannot be lifted without breaking the protecting space-time symmetry. In contrast, we show that when the fragile bands are half filled, interactions can open a gap in the many-body spectrum without breaking any symmetry or mixing degrees of freedom from remote bands. Furthermore, the resulting ground state is not topologically ordered. Thus, a fragile topological band structure does not present a fundamental obstruction to forming a “featureless insulator” ground state. We will then turn to discuss sign problem free quantum Monte Carlo simulations of a realistic model of TBG at charge neutrality, showing signatures of a transition from a Kramers Inter-Valley Coherent (KIVC) state to a semi-metal

Posted in QM Seminar | Comments Off on Special Emergent Phenomena Seminar 290S/QM Seminar Speaker Erez Berg (Weizmann Institute) Tuesday, December 7 at 2:00 pm in 402 Physics South

Emergent Phenomena Seminar 290S/QM Seminar Speaker Peter Czajka (Princeton) Wednesday, December 1 at 2:00 pm

Time/Venue Wednesday, December 1 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 James Analytis
Title Exotic Thermal Transport in a Kitaev Magnet
Abstract In the class of materials called spin liquids, a magnetically ordered state cannot be attained even at milliKelvin temperatures because of conflicting constraints on each spin — for example, from geometric or exchange frustration. The resulting quantum spin-liquid (QSL) state is currently of intense interest because it exhibits unusual excitations as well as wave-function entanglement. The layered insulator RuCl3 orders as a zigzag antiferromagnet at low temperature in zero magnetic field. The zigzag order is destroyed when a magnetic field is applied parallel to the zigzag axis. At moderate magnetic field, there is growing evidence that a QSL state exists. In this talk, we present measurements of the system’s field-dependent thermal transport tensor. We observe oscillations in the longitudinal thermal conductivity that are reminiscent of Shubnikov de Haas oscillations in metals as well as a strongly temperature-dependent thermal Hall conductivity. We argue that the latter is consistent with a boson-based signal rather than the Majorana one reported by some groups.

Posted in QM Seminar | Comments Off on Emergent Phenomena Seminar 290S/QM Seminar Speaker Peter Czajka (Princeton) Wednesday, December 1 at 2:00 pm

Emergent Phenomena Seminar 290S/QM Seminar Speaker Kai Sun (U of Michigan) Wednesday, November 24 at 2:00 pm

Time/Venue Wednesday, November 24 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 Mike Zaletel
Title Integer and fractional topological states in TMD moire lattices
Abstract In this talk, we consider non-interacting massive spinless (spin-polarized) fermions moving in a moire potential, which can be achieved in a heterobilayer of transition metal dichalcogenides, and explore minimum/essential ingredients needed for the formation of a topological band. We show analytically that as long as the fermion band carriers some nonzero Berry flux, a topologically nontrivial moire band can always be achieved, if proper periodic potential is applied. We demonstrate this mechanism using MoTe2/WSe2 heterobilayer as an example, and show that a narrow topological phase emerges when a vertical electric field is applied. In the second part of the talk, we further explore topological flat-bands in TMD moire lattices, as well as numerical evidences of fractional topological states.

Posted in QM Seminar | Comments Off on Emergent Phenomena Seminar 290S/QM Seminar Speaker Kai Sun (U of Michigan) Wednesday, November 24 at 2:00 pm

Emergent Phenomena Seminar 290S/QM Seminar Speaker Zhehao Dai (UC Berkeley) Wednesday, November 3 at 2:00pm

Time/Venue Wednesday, November 3 at 2 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
 Photo-induced superconductivity at integer filling = activated discrete time crystal?
Abstract Floquet engineering, the manipulation of matter by periodic drive has become a powerful tool to probe quantum materials and to reveal the rich physics beyond thermal equilibrium. A particularly interesting phenomena is photo-induced superconductivity: a transient superconducting response after a laser pump, which is found in cuprates, fullerides, and organic molecular materials at temperatures much higher than their equilibrium transition temperatures. In this talk, I discuss an effective theory of nonequilibrium superconducting steady states induced by periodic drive and a surprising connection with activated discrete time crystal.

Posted in QM Seminar | Comments Off on Emergent Phenomena Seminar 290S/QM Seminar Speaker Zhehao Dai (UC Berkeley) Wednesday, November 3 at 2:00pm

Emergent Phenomena Seminar 290S/QM Seminar Speaker Binghai Yan (Weizmann Institute) Wednesday, October 27 at 2:00 pm

Time/Venue Wednesday, October 27 at 2 pm in 402 Physics South and Zoom: https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09 Meeting ID: 995 2349 9113 Passcode: 600704
Host Ehud Altman
Title
 Band structure topology and nonlinear effects in quantum materials
Abstract
 Topological quantum materials provide fascinating platforms to investigate band structure topology and explore its impacts on material properties. They also triggered renewed interest in the bulk photovoltaic effect (BPVE), where a homogenous material (instead of a p-n junction) rectifies light into a dc current. For example, nonmagnetic Weyl semimetals (e.g., TaAs and WTe2) are found to exhibit giant photocurrent and second harmonic generation. Recently, we proposed that magnetism can lead to unexpected, even stronger nonlinear effects [1]. For example, linearly polarized light can generate an injection current in a magnet [2], which is forbidden by symmetry in ordinary materials. Surprisingly, the magnetic indued photocurrent does not vanish even in the subgap regime (hν < Eg) [3]. In the dc electric field limit, related nonlinear transport remains and results in the unidirectional magnetoresistance, i.e., a diode controlled by magnetism or magnetic field. The diode rectification can be significantly enhanced by flat bands for example in magic angle twisted bilayer graphene [4,5].
References: [1] Phys. Rev. Research 2, 033100 (2020). [2] Nature Commun. 10,3783 (2019). [3] Phys. Rev. Lett. 125, 227401(2020). [4] The Innovation 2 (1), 100085(2021). [5] arXiv:2101.07539

Posted in QM Seminar | Comments Off on Emergent Phenomena Seminar 290S/QM Seminar Speaker Binghai Yan (Weizmann Institute) Wednesday, October 27 at 2:00 pm

Special Seminar Speaker Haim Beidenkopf (Weizmann Institute) Monday, October 25 at 3:00pm

Time/Venue Monday, October 25 at 3 pm in 375 Physics North 
Host Ehud Altman
Title
 Correlations and topology in a transition metal dichalcogenide compound
Abstract A small change in the crystal structure of a material can completely modify its electronic properties. This is the case in the transition metal dichalcogenide TaS2 where a slight rotation of the sulphur atoms from the trigonal prismatic 1H structure to the octahedral 1T varies it from a superconductor to a Mott insulator possibly hosting a spin liquid state. We have studied 4Hb-TaS2 that interleaves the 1T and 1H polytypes. This has allowed us to investigate using scanning tunneling spectroscopy the fate of a Mott insulator when coupled to a metal and the superconducting phase that results from sandwiching it with Mott insulating layers. In the former we find a quantum phase transition that can be induced by the STM tip, electric field and temperature which possibly signifies the onset of correlations among Anderson impurities (or of a Kondo lattice). In the latter we identify a topological nodal superconducting state which we attribute to interaction-promoted inter-orbital pairing.

Posted in QM Seminar | Comments Off on Special Seminar Speaker Haim Beidenkopf (Weizmann Institute) Monday, October 25 at 3:00pm