
Fermionic Systems I (chair: Volker Meden)

09:00  09:50

JeanMarc Triscone
(Université de Genève)
Structural and electronic coupling at oxide interfaces
Margherita Boselli$^{1}$, Claribel Dominguez$^{1}$, Jennifer Fowlie$^{1}$, Marta Gibert$^{2}$, Celine Lichtensteiger$^{1}$, Hugo Meley$^{1}$, Gernot Scheerer$^{1}$, Adrien Waelchli$^{1}$, Bernat Mundet$^{3}$, Duncan Alexander$^{3}$, Stefano Gariglio$^{1}$, and JeanMarc Triscone$^{1}$
$^{1}$DQMP, University of Geneva, 24 Quai ErnestAnsermet, 1211 Geneva 4, Switzerland
$^{2}$PhysikInstitut, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
$^{3}$EPFL  IPHYS – LSME Station 3, CH1015 Lausanne, Switzerland
In this talk, I will discuss several interfacial couplings that occur in oxide heterostructures. Polar discontinuities can lead to 2dimensional conduction between insulating materials – I will briefly discuss the case of the LaAlO$_3$/SrTiO$_3$ system [1,2]; Structural and electronic coupling at oxide interfaces can also lead to interesting phenomena that we investigated in perovskite nickelates  wellknown for their metal to insulator transition (MIT) and unique antiferromagnetic ground state [35] – there, I will show that NdNiO$_3$/SmNiO$_3$ superlattices display, depending on the thickness of the individual layers, a single or double MIT  a behavior that allows the role of interfacial structural and electronic coupling in heterostructures to be studied. Finally I will present results on vanadatebased heterostructures where we aimed at designing an artificial ferroelectric material [6,7]. In this system, the interface sharpness seems to be controlled by the coupling of distortions and periodicity of the structure.
[1] A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004).
[2] N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.M. Triscone and J. Mannhart, Science 317, 1196 (2007).
[3] M.L. Medarde, Journal of Physics Condensed Matter 9, 1679 (1997).
[4] G. Catalan, Phase Transitions 81, 729 (2008).
[5] S. Catalano, M. Gibert, J. Fowlie, J. Iniguez, J.M. Triscone, J. Kreisel ,Reports on Progress in Physics 81, 046501 (2018).
[6] See, for instance, J. M. Rondinelli and C. J. Fennie, Adv. Mater. 24, 1961 (2012).
[7] H. Meley, Karandeep, L. Oberson, J. de Bruijckere, D. T. L. Alexander, J.M. Triscone, Ph. Ghosez, S. Gariglio, APL Materials 6, 046102 (2018).

09:50  10:15

Laura Classen
(University of Minnesota, Minneapolis)
Spinfermion coupling in itinerant magnets
We study the quantum effects of spinfermion coupling in systems where localized spins and itinerant electrons coexist.
In general, this coupling leads to corrections in the excitation spectrum of both subsystem and affects, e.g., the electronic scattering rate or the spin susceptibility. In particular, it is interesting to analyze the mutual influence when the individual subsystems possess special quantum degrees of freedom. We illustrate this at two complimentary examples. We determine the damping of spinwaves in an antiferromagnet due to interaction with Dirac electrons as relevant for $YbMnBi_2$. Furthermore, we demonstrate the emergence of clear nonFermiliquid behavior in a 3D metal coupled to 1D critical spin chains, a situation realized in $Yb_2Pt_2Pb$.

10:15  10:40

Benjamin Lenz
(École Polytechnique, Palaiseau)
Role of nonlocal correlations for spectral properties of \(Sr_2IrO_4\)
The spinorbit Mott insulator Sr${}_2$IrO${}_4$ has raised tremendous interest recently due to striking similarities to highTc superconducting copper oxides both in the pure and in electrondoped compounds.
Here, we study the evolution of the spectral function of this 5d transition metal system as a function of doping by means of a combined abinitio electronic structure and manybody quantum cluster approach. Thereby, important ingredients like spinorbit coupling and distortions of the oxygen octahedra as well as Hubbard interactions and nonlocal charge fluctuations are taken into account. The spectral function of pure Sr${}_2$IrO${}4$ is analyzed both in its hightemperature paramagnetic and its lowtemperature antiferromagnetic phase and the importance of nonlocal fluctuations is discussed. The spectra compare well with angleresolved photoemission measurements and allow us to study emerging changes under electron and holedoping. For electrondoped Sr${}_2$IrO${}_4$, special emphasis is placed on pseudogap features of the spectral function, which are found to be in good agreement with experiment. Furthermore, we discuss an intriguing kdependence in the composition of the spinorbit entangled $j_{\mathrm{eff}}=1/2$ states.

10:40  11:10

coffee break


Fermionic Systems I (chair: Fakher Assaad)

11:10  11:35

Michael M. Scherer
(Universität zu Köln)
Renormalization group approach to Dirac fermions and (deconfined) critical phenomena
Gapless Dirac fermions appear in various condensedmatter scenarios as for example in graphene and related materials, but also in the dual description of the deconfined quantum critical point between Neel and valence bond solid orders in frustrated quantum magnets. The precise determination of the critical behavior of Dirac fermions defines a prime benchmark for complementary theoretical approaches and moreover will allow us to test conjectures based on duality arguments. In my talk, I present a comprehensive analysis of the GrossNeveu universality classes based on the recently achieved fourloop renormalization group calculations and compare to Monte Carlo and the conformal bootstrap. Further, I will show a threeloop study of deconfined criticality from the dual QED3GrossNeveu model and discuss the implications of our estimates for the critical exponents for nontrivial scaling relations which follow from the emergent SO(5) symmetry implied by the duality conjecture.

11:35  12:00

Cecile Repellin
(Massachusetts Institute of Technology, Cambridge)
Ferromagnetism and quantum anomalous Hall effect in moiré superlattices
Following the remarkable discovery of correlated insulators and superconductivity in magic angle twisted bilayer graphene (TBG), a great deal of attention has been lavished on various ``moire materials". We focus on the variety of moire systems where the conduction and valence bands are separated from each other by energy gaps. This is the case when the TBG is further aligned with a hexagonal Boron Nitride substrate (TBG/hBN), but also in different sets of experiments, such as trilayer graphene aligned with hBN, as well as twisted double bilayer graphene systems (i.e. bilayer graphene twisted relative to another bilayer graphene to a magic angle). We further restrict our analysis to the strong interaction limit i.e. when the Coulomb interaction is much bigger than the bandwidth. We show the emergence of a ferromagnetic ground state at quarter, half and three quarter filling of these nearly flat bands. When the band has a nonzero Chern number, the ferromagnetism is accompanied by a quantized anomalous Hall effect. Our results are based on analytical arguments, as well as exact diagonalization and DMRG results which quantitatively show the stability of the ferromagnetic ground state. They provide further theoretical understanding to the observation of ferromagnetism in TBG/hBN[1], TLG/hBN[2] and twisted double bilayer[3], as well as the observation of quantized anomalous Hall effect in some of these systems[2].
[1] Emergent ferromagnetism near threequarters filling in twisted bilayer graphene, Sharpe et al arXiv:1901.03520
[2] Tunable Correlated Chern Insulator and Ferromagnetism in Trilayer Graphene/Boron Nitride Moire Superlattice, Chen et al arXiv:1905.06535
[3] Spinpolarized Correlated Insulator and Superconductor in Twisted Double Bilayer Graphene, Liu et al arXiv: 1903.08130

12:00  12:25

Matthias Punk
(LudwigMaximiliansUniversität München)
Incommensurate 2kF CDW quantum critical points in metals
Metallic phases which cannot be described in the usual Fermiliquid framework have been observed in a variety of strongly correlated electron systems, such as cuprates, pnictides and heavyfermion materials. A widely discussed scenario for such nonFermi liquid behavior is the presence of a quantum critical point in quasi two dimensional metals, where the strong interaction between electrons and gapless orderparameter fluctuations destroys the electronic quasiparticle coherence.
Here we revisit the problem of twodimensional metals at the onset of incommensurate 2kF charge density wave order. Previous works argued that this transition is potentially first order due to strong fluctuations. Building upon a controlled, perturbative renormalization group approach developed by Dalidovich and Lee [1], which treats electrons and order parameter fluctuations on equal footing, we show that a dynamical nesting of the Fermi surface renders the transition continuous. We discuss properties of this new nonFermi liquid fixed point and highlight experimental signatures.

12:25  13:25

lunch

13:25  14:00

discussion


Fermionic Systems II (chair: Jan von Delft)

14:00  14:25

Inti Sodemann
(MaxPlanckInstitut für Physik komplexer Systeme, Dresden)
The cyclotron resonance as a smoking gun for U(1) spin liquids with gapless fermions
Certain U(1) spin liquids with gapless neutral fermions are expected to have the mindboggling property that their optical conductivity vanishes as a power law of frequency. Thus, they are insulators to DC electric fields but without a "hard" optical gap, allowing them to absorb light at low frequencies. Additionally, they can also develop Landau levels in a magnetic field. In this work, we show that they can also have cyclotron resonance peaks in their optical spectrum analogous to metals, even though they are charge insulators. Interestingly, we have found that in contrast to metals, the principal Kohn harmonic of the cyclotron resonance is missing. The cyclotron resonance could therefore serve as a beautiful smoking gun test for the existence of these states which have been proposed in 2D organic materials and SmB6.

14:25  14:50

Bela Bauer
(Microsoft Station Q, Santa Barbara)
Topologically protected braiding in a single wire using Floquet Majorana modes
Majorana zero modes are a promising platform for topologically protected quantum information processing. Their nonAbelian nature, which is key for performing quantum gates, is most prominently exhibited through braiding. While originally formulated for twodimensional (2d) systems, it has been shown that braiding can also be realized using onedimensional (1d) wires by forming an essentially twodimensional network. Here, we show that in driven systems far from equilibrium, one can do away with the second spatial dimension altogether by instead using quasienergy as the second dimension. To realize this, we use a Floquet topological superconductor which can exhibit Majorana modes at two special eigenvalues of the evolution operator, 0 and pi, and thus can realize four Majorana modes in a single, driven quantum wire. We describe and numerically evaluate a protocol that realizes a topologically protected exchange of two Majorana zero modes in a single wire by adiabatically modulating the Floquet drive and using the pi modes as auxiliary degrees of freedom.

14:50  15:20

coffee break

15:20  15:55

Manuel Weber
(Georgetown University, Washington)
Quantum phase transitions and competing orders in onedimensional fermionboson models
In 1979, Su, Schrieffer, and Heeger (SSH) introduced a simple microscopic model of polyacetylene with a modulation of the electronic hopping as a result of distortions of the carbon bonds. It has since gained a much wider relevance due to its relation to symmetryprotected topological states. While it is known that quantum lattice fluctuations can destroy the Peierls insulating bondorderwave (BOW) ground state and lead to a gapless Luttingerliquid (LL) phase, quantitative results for finite phonon frequencies are rare. Using a recently developed directedloop quantum Monte Carlo method for retarded interactions we calculate the phase diagram of the spinless SSH model at halffilling for the entire range of phonon frequencies. In addition to the known LL and BOW phases, we find an extended chargedensitywave (CDW) phase at high phonon frequencies that has not been found in previous studies. Because of different broken symmetries, BOW and CDW phases are connected by a retardationdriven phase transition. Our results are consistent with the theory of the frustrated XXZ chain, including unconventional powerlaw exponents at criticality, and with an interpretation in terms of deconfined quantum criticality via proliferation of solitons.

15:55  16:20

Richard Schmidt
(MaxPlanckInstitut für Quantenoptik, Garching)
Theory of electronexciton scattering in atomically thin semiconductors
Excitons interacting with charge carriers in vanderWaals materials represent a new venue to study the manybody physics of strongly interacting BoseFermi mixtures. In order to derive an effective lowenergy model for such systems we develop an exact diagonalization approach that predicts the boundstate and scattering properties of electrons, excitons, and trions in twodimensional semiconductors. By solving the quantum mechanical threebody problem of interacting charge carriers we thus obtain binding energies of excitons and trions that are in excellent agreement with quantum Monte Carlo predictions. Importantly, in our approach also excited states are accessible. This allows us to predict exotic excited trion states as well as to calculate the scattering phase shifts of electrons and excitons. From these results we derive an effective lowenergy model of excitonelectron scattering that can serve as an input to advanced manybody techniques. As a demonstration we study the recently observed exciton Fermi polarons, and we demonstrate that effective range corrections predicted by our model have a substantial impact on the optical absorption spectrum of chargedoped transitionmetal dichalcogenides. Our approach can similarly be applied to study a plethora of manybody phenomena realizable in atomically thin semiconductors ranging from exciton lattices and localization to induced superconductivity.

16:20  20:00

poster session  focus on odd poster numbers

20:00  21:00

dinner

21:00  22:00

discussion
