Abstracts

Tuesday, January 11, 2022

09:00-10:00 Karl Landsteiner (Universidad Autónoma de Madrid)Quantum Anomalies (in) Matter.

Abstract: The laws of quantum physics are sometimes incompatible with classical symmetries. One speaks then of an anomaly. The most prominent examples are triangle anomalies leading to a violation of chiral symmetries. While such anomalies are properties of the quantum theory of chiral fermions it is now clear that they are also important in (near) equilibrium states of chiral matter. They lead to new transport phenomena such as the Chiral Magnetic Effect (CME) and Chiral Vortical Effect (CVE). I will review briefly the theory of triangle anomalies and how they induce CME and CVE and then discuss applications of the theory to Weyl semi-metals.

10:00-11:00 Jaakko Nissinen (Aalto University)Torsion and geometric anomalies in (non-relativistic) topological matter.

Abstract: I discuss geometric anomalies in topological matter that extend the definition of relativistic anomalies in terms of gravity to situations where non-relativistic effective geometry and symmetries, in addition to topology, play a key role. Such anomalies can be understood based on chiral gravitational anomalies with torsion and vorticity, the former especially controversial in relativistic field theory, and their suitable generalizations to non-relativistic systems.
As examples I discuss anomalous chiral hydrodynamics and torsional anomalies in chiral Weyl superfluids, superconductors and semimetals. In contrast to usual relativistic anomalies, the discussed geometric anomalies are "unquantized" and feature properties of the momentum space geometry in terms of UV-IR dependencies.

11:00-12:00 Kiril Tuchin (Iowa State University, Ames)Quantum tomography of chiral media.

Abstract: The chiral matter possesses a number of unique properties originating from the quantum phenomenon of the chiral anomaly. These properties can be measured by observing propagation of and radiation by fast charged particles moving through the matter. I show how the chiral anomaly confers distinctive features onto the particle energy loss and its radiation spectrum. I argue then that this makes the quantum tomography a powerful and versatile tool to investigate the properties of chiral systems ranging from the Weyl semimetals to the quark-gluon plasma to the axion stars.

B R E A K

13:00-14:00 Thomas Stegmann (ICF-UNAM)Steering the current flow in twisted bilayer graphene

Abstract: A nanoelectronic device made of twisted bilayer graphene (TBLG) is proposed to steer the direction of the current flow. The ballistic electron current, injected at one edge of the bottom layer, can be guided predominantly to one of the lateral edges of the top layer. The current is steered to the opposite lateral edge, if either the twist angle is reversed or the electrons are injected in the valence band instead of the conduction band, making it possible to control the current flow by electric gates. The steering is explained by the trigonal shape of the energy bands beyond the van Hove singularity due to the Moiré interference pattern. As the shape of the energy bands depends on the valley degree of freedom, the steered current is partially valley polarized. Our findings show how to control and manipulate the current flow in TBLG. Technologically, they are of relevance for applications in twistronics and valleytronics.

14:00-15:00 Alejandro Kunold (UAM-Azcapotzalco)Floquet spectrum of 2D Dirac materials under a periodic drive: Monodromy matrix approach

Abstract: In this work I will comment on the characteristics of the quasienergy spectrum of 2D Dirac materials under the action of a linearly or circularly polarized periodic drive. In the strong-field regime, the spectrum as a function of the momentum components separates into two very distinctive regions. In the first, located in the vicinity of the Dirac points, the quasi spectrum is substantially distorted by the field as the electronic parameters are renormalized by electronic dressing. In the second, far from the Dirac points, all the characteristics of the free carrier spectrum are retained. The separation between these two regions is very abrupt. Linearly polarized light anisotropically deforms the spectrum according to the field polarization direction. In both valleys many Dirac-like points form around the original Dirac point. The quasi spectrum of circularly polarized light, instead, exhibits a gap formation in the Dirac point and has elliptical symmetry. I will describe how the quasienergy spectrum is numerically calculated from the monodromy matrix of the Schrödinger equation via the Floquet theorem for arbitrarily intense electromagnetic fields. The monodromy matrix method is found to be more efficient for obtaining the evolution operator and Floquet spectrum of time-driven systems than the traditional diagonalization using replicas, as this last method requires truncation in both the number of replicas and system size. Deducing a Rabi-like formula in the rotating wave approximation I will prove that, in contrast to the single-photon resonant transitions that characterize the weak-field regime, the strong-field regime is dominated by multiphoton resonances. These processes also manifest themselves in the generation of high harmonics. I will briefly comment on the difficulties of calculating the Berry phase under a time dependent drive.

15:00-15:40 Andrés Gómez (ITP, University of Heidelberg)Effective axial electromagnetic response for general Weyl semimetals.

Abstract: A generalization of the axial contribution to the effective electromagnetic action of Weyl semimetals is presented using thermal field theory methods. The response is described by chiral electrodynamics, where the contributions from the separation of the cones, tilting, anisotropies, chemical potential and chiral chemical potential enter the coupling 𝚯(x) to the unmodified Pontryagin density. This both resolves the problem of the chiral rotation method not being able to obtain electromagnetic responses other than just the separation of the cones and maintains the original form of the chiral anomaly in accordance to previous research.

Wednesday, January 12, 2022

09:00-10:00 Maxim Chernodub (CNRS, Institut Denis Poisson, Tours, France)Transport effects due to scale anomaly

Abstract: Abstract: We overview transport phenomena generated by the scale anomaly in interacting quantum field theories of massless (chiral) particles, making an emphasis on Dirac and Weyl semimetals at the charge neutrality point. We discuss anomaly-induced thermoelectric effects in the background of the magnetic field in the presence of a temperature gradient. We also consider systems with boundaries, where the scale anomaly produces a non-quantized edge current. The magnitude of all these phenomena is proportional to the beta function associated with the renormalization of electric charge.

10:00-11:00 Zeila Zanolli (Utrecht University)2D materials for the quantum age

Abstract: Major advances in human civilization are driven by developments in materials. This is such a remarkable feature that historical eras are named after the material (and the related technology) that dominated that time. Today we live in the silicon era: Silicon technology enables our modern way of life via mobile phones, computers, automation. However, we are reaching the limits of silicon technology, as the related energy demand is not sustainable. It is time to move forward. As a scientist, I work to answer the question: what is the material that will enable the next revolution?
In this talk, I will present my personal perspective to answer this general question. Using the predictive power of first-principles techniques, I will show that a possible way forward is to design new materials, exploiting the quantum effects emerging at reduced dimensionality and at interfaces [1]. By looking at the intersection of topological materials, interfaces, and spin-based electronics, we will see that it is possible to understand and, hence, control key parameters for next-generation devices as spin injection [2], topological (dissipationless) carrier transport [3], spin lifetime [4] and defects signatures [5] in 2D layered materials. Finally, we will discuss to what extent the monolayer physical/chemical properties persist in thin films, up to the mesoscale. [6, 7]
References:[1] N. Wittemeier, M. J. Verstraete, P. Ordejón, Z. Zanolli, Carbon 186, 416 (2022)[2] Z. Zanolli, Sci. Rep. 6 (2016) 31346[3] C. Niu, G. Bihlmayer, Y. Mokrousov, P. Mavropoulos, M. J. Verstraete, S. Blügel, Phys. Rev. B 98 (2018) 155404[4] M. Ersfeld, F. Volmer, P. M. M. C. de Melo, R. de Winter, M. Heithoff, Z. Zanolli, Ch. Stampfer, M. J. Verstraete, B. Beschoten, Nano Lett. 19 (2019) 4083[5] Pedro M. M. C. de Melo, Z. Zanolli, M. J. Verstraete, Adv. Quantum Technol. 3 (2021) 2000118[6] I. Ronneberger, Z. Zanolli, M. Wuttig, R. Mazzarello, Advanced Materials 32, (2020) 2001033[7] A. Dewandre, M. J. Verstraete, N. Grobert, Z. Zanolli, J. Phys. Mater. 2 (2019) 044005

11:00-12:00 Gordon Semenoff (University of British Columbia)Graphene with an Edge

Abstract: An overview of some unusual aspects of gapless relativistic fermions on a two dimensional space with a boundary will be discussed, both from a formal point of view and with some applications to graphene and other Weyl semi-metals in mind.

B R E A K

13:00-14:00 Alfredo Raya (Universidad Michoacana, San Nicolás Hidalgo)Influence of a Chern-Simons term in the dynamical fermion masses in reduced or pseudo QED

Abstract: Mixed-dimensional theories have been used to describe condensed matter systems where fermions are constrained to a plane while the gauge fields they interact with remain four dimensional. Here we investigate dynamical breaking of chiral symmetry in the framework of a mixed-dimensional theory that has been successful in describing graphene and other planar Dirac materials and has been dubbed pseudo or reduced quantum electrodynamics. We explore the interplay between the gauge and fermion sectors when a Chern-Simons term is considered and, by exploring the tensor structure of the fermion self-energy, we show that the radiative corrections may induce both a Dirac and a Haldane mass term. Furthermore, by solving the corresponding Schwinger-Dyson equation in a suitable truncation, we nonperturbatively explore the dynamical generation of fermion masses for different values of the electromagnetic coupling and the Chern-Simons coefficient. We also show that for definite parameter values, the contributions from each chirality cancel out and the chiral symmetry may be restored. Possible implications of this result for physical systems, in particular on what concerns the chiral magnetic effect, are discussed.

14:00-15:00 Omar Jesús Franca (Universität Kassel)Transition radiation near topological insulators.

Abstract: Transition radiation is a radiation phenomenon due to a charged particle that crosses the interface of two dielectric media. It has served to understand deeper the interaction of light and matter and has provided several applications. In this talk, we consider how this phenomenon is modified when it occurs in presence of a plane interface between two three-dimensional topological insulators with different topological parameters, permittivities and permeabilities. To this aim, we employ the Green's function approach to find the electromagnetic field, the angular distribution of the radiation and the frequency distribution for this kind of radiation. We present typical radiation patterns for the topological insulator TlBiSe2 and the magnetoelectric TbPO4. In the ultra-relativistic case, the additional contributions from the magnetoelectric coupling appreciably enhance the global maximum of the angular distribution. We also present an analytic expression for the frequency distribution of the radiation for this case. We find that in the limit where the permittivities are equal there still exists transition radiation of the order of the square of the topological parameter with a pure topological origin. This work offers another indirect measurement of the topological parameter and can be relevant for the current research of dark matter detection.

15:00-15:40 POSTER SESSION

Thursday, January 13, 2022

9:00-10:00 Alberto Cortijo ((Universidad Autónoma de Madrid)Hall viscosity in three-dimensional semimetals

Abstract: In this talk I will discuss how the Hall viscosity, a term in the viscous response of electronic systems related to their geometrical properties paying attention to semimetals. Usually extracted from the coupling of quantum degrees of freedom to curved spacetimes, I will discuss how such response can be obtained within a tight binding approach in crystalline electron systems.

10:00-11:00 César Fosco (Centro Atómico de Bariloche)Chiral anomaly and vacuum polarization in the presence of a defect.

Abstract: We study the vacuum polarization tensor for a massless Dirac field in 1 + 1 and 3 + 1 dimensions, in the presence of a particular kind of defect which, in a special limit, imposes bag boundary conditions. We also show that the chiral anomaly in the presence of such a defect is the same as in its absence, both in 1 + 1 and 3 + 1 dimensions. We comment on the interplay between the two phenomena, and its relevance to the determination of the induced current when an external field is applied.

11:00-12:00 Tatiana Rappoport (Universidade Federal do Rio de Janeiro & Instituto de Telecomunicações - Lisboa)Orbital Hall effect in 2D Materials

Abstract: A renewed interest in orbital magnetism and other orbital effects in solids raises expectations that orbital angular degrees of freedom may be eventually employed to process information in logic and memory devices. This next-generation technology has been coined as orbitronics and utilizes the orbital current as an information carrier.
The orbital Hall effect (OHE), similarly to the spin Hall effect (SHE), refers to the creation of a transverse flow of orbital angular momentum that is induced by a longitudinally applied electric field. I will discuss different aspects of the OHE in 2D materials. In particular, I will show that monolayers and bilayers of transition metal dichalcogenides (TMDs) like MoS2 and WSe2, exhibit OHE in its insulating phase. The orbital Hall conductivity presents a plateau in the semiconductor gap of these materials that can be associated with a topological phase characterized by an orbital Chern number. Our results offer the possibility of using TMDs for orbital current injection and orbital torque transfer that surpass their spin equivalents. I will also explore the connection between the orbital Hall effect and the valley Hall effect in 2D materials.

B R E A K

13:00-14:00 Gerardo Naumis (IF-UNAM)Topological modes of time-driven 2D materials

Abstract: In this talk we will study the emergence of electronic non-trivial topological modes in time-periodically driven 2D materials. Two kinds of Hamiltonians are presented: tight binding and Dirac-like effective equations. Then several non-perturbative methods are presented to solve the system dynamics: 1) an algebraic method to find the effective Hamiltonian, 2) a monodromy matrix approach and 3) Floquet theory leading to Mathieu-Hill equations. Using such methods, the topological phase diagram of the system can be built. The phase diagrams and the topological modes are in excellent agreement with numerical calculations. Finally, we will discuss how the quantum time evolution can be seen as classical particles under oscillating electromagnetic fields. The topological modes are thus associated with trajectories with orbital precession. This allows us to find an alternative, non-perturbative approach to find the topological Berry phases. All these ideas are applied to graphene, borophene and decorated graphene lattices under electromagnetic or time driven deformation fields.

14:00-15:00 Esteban Rodríguez (Universidad de Chile)Spin-polarized tunable photocurrents

Abstract: Circular dichroism, a distinct response to left and right-handed circularly polarized light, is an example of a phenomenon involving light-matter interaction that has been heavily exploited to control valley polarization in two-dimensional materials [1]. In most studies, light-matter interaction enters perturbatively and does not modify the electronic properties. But beyond this weak-coupling regime, Floquet engineering [2-4] has shown that we can use light to change the band-structure of a material [5- 7] and even its topology [2-4, 8], generating a Hall response [9]. Light can also be used to generate directed currents even in the absence of an applied bias voltage, a phenomenon called quantum pumping, and recently it has been shown that by tailoring a selective environment one can take this to the limit of a perfect isolator effect [10], where currents flow in one direction but not the opposite. Here, we go a step further and show how the rich interplay between electron-photon processes (and the additional synthetic dimension), stacking order, spin-orbit coupling, Rashba spin-orbit coupling and the topology of a two-dimensional material can be harnessed to control spin charge, and valley currents in two-dimensional materials, beyond the weak-coupling regime [11].References [1] K. F. Mak, K. He, Jie Shan, and T. F. Heinz, Nature Nanotechnology, 7 (2012) 494. [2] T. Oka, and A. Aoki, Physical Review B 79 (2009) 081406. [3] N. H. Lindner, G. Refael, and V. Galitski, Nature Physics 7 (2011) 490 [4] T. Kitagawa, T. Oka, A. Brataas, L. Fu, and E. Demler, Physical Review B 84 (2011) 235108. [5] H. L. Calvo, H. M. Pastawski, S. Roche, and L. E. F. Foa Torres, Applied Physics Letters 98 (2011) 232103. [6] Y. H. Wang, H. Steinberg, P. Jarillo-Herrero, and N. Gedik, Science 342 (2013) 453. [7] F. Mahmood et al. Nature Physics 12 (2016) 306. [8] L. E. F. Foa Torres, P. M. Perez-Piskunow, C. A. Balseiro, and G. Usaj, Physical Review Letters 113 (2014) 266801. [9] J.W. McIver et al., Nature Physics 16 (2020) 38-41. [10] V. Dal Lago, E. Suárez Morell, and L. E. F. Foa Torres, Physical Review B 96 (2017), 235409. [11] M. Berdakin, E. Rodriguez-Mena, and L. E. F. Foa Torres, Nano Lett. 21 (2021), 3177.

15:00-15:40 POSTER SESSION

Friday, January 14, 2022

09:00-10:00 Jens H. Bardarson (KTH Royal Institute of Technology)Axial anomaly and axial pseudo-electromagnetic fields in magnetic Weyl semimetals.

Abstract: I will begin with a broad introduction to the physics of topological materials with focus on Weyl semimetals. I will then specialise to the realisation of pseudo-electromagnetic fields in magnetic Weyl semimetals, in particular at domain walls, and discuss how they can be experimentally realised.

10:00-11:00 Mariana Mallard (Universidade de Brasília)Less is more: Engineering robust nodal-line semimetals with vacancies

Abstract: Symmetry-enforced nodal-line semimetals are immune to perturbations that preserve the underlying symmetries. This fundamental robustness enables investigations of fundamental phenomena and applications utilizing diverse materials-design techniques. The drawback of symmetry-enforced nodal-line semimetals is that the crossings of energy bands are constrained to symmetry-invariant momenta in the Brillouin zone. On the other end lies accidental nodal-line semimetals whose band crossings, not being enforced by symmetry, are easily destroyed by perturbations. Some accidental nodal-line semimetals have, however, the advantage that their band crossings can occur in generic locations in the Brillouin zone, and thus can be repositioned to tailor material properties. We show that lattice engineering with periodic distributions of vacancies yields a hybrid type of nodal-line semimetals which possess symmetry-enforced nodal lines and accidental nodal lines, with the latter endowed with an enhanced robustness to perturbations. Both types of nodal lines are explained by a symmetry analysis of an effective model which captures the relevant characteristics of the proposed materials and are verified by first-principles calculations of vacancy-engineered borophene and graphene polymorphs. Our findings offer an alternative path to relying on complicated compounds to design robust nodal-line semimetals; one can instead remove atoms from a common monoatomic material.

11:00-12:00 Omar Antolín (IMAT-UNAM)Classifying crystalline interacting topological phases through equivariant cohomology

Abstract: Abstract: I'll describe joint work with Daniel Sheinbaum in which we explain how to compute how many distinct topological phases there are for crystalline interacting gapped systems given their symmetry group. Our answer is given in terms of a tool from algebraic topology called equivariant cohomology. I'll explain in broad strokes how this classification arises from natural assumptions and will compare both the classification and those assumptions with those from other proposals you can find in the literature.

B R E A K

13:00-14:00 Hugo García-Compeán (Cinvestav)A Realization of the Classification of Topological Insulators in terms of KK-theory

Abstract: It is known that topological insulators and topological superconductors (TIs/TSCs) admit a classification in terms of the ten-fold way of K-theory. In 2010 S. Ryu and T. Takayanagi introduced a D-brane realization of TIs/TSCs. This realization consists of a D-brane configuration of Dr-branes intersecting Dd-branes with and without orientifold planes. It is also very well known that the Ramond charge of D-branes is also classified in terms of K-theory, K-homology and KK-theory. In this talk, based in a work in progress, we give the first steps to construct D-brane configurations which determine the classification og TIs/TSCs in terms of KK-theory. In a previous work we find the classifications of Dd-branes in orientifolds in terms of KK-theory. We use some of these results to reproduce the TIs/TSCs classification in terms of KK-theory. We compare our results with other results in the literature.

14:00-15:00 Daniel Sheinbaum (FQ-UNAM)Non-linear analogs of topological matter, Solitons and K-theory

Abstract: There is currently an interest in topological properties of non-linear systems, both in photonics and exciton-polariton systems. In this short talk I present a coarse/partial classification of these systems employing linearized perturbations around a soliton solution. This classification is expressed in terms of K-theory and cohomology, tools from algebraic topology. I will discuss why it is not identical to the linear case and the potential role of the space of Soliton solutions (moduli space).

15:00-15:40 Rafael Flores (Max Planck Institute for the Physics of Complex Systems, Dresden)Quantized Nonlinear transport in Weyl Semimetals

Abstract: We show that under the effect of an external electric field and a gradient of chemical potential, a topological electric current can be induced in Weyl semimetals without inversion and mirror symmetries. We derive analytic expressions for the nonlinear conductivity tensor and show that it is nearly quantized for small tilting when the Fermi levels are close to the Weyl nodes. When the van Hove point is much larger than the largest Fermi level, the band structure is described by two linearly dispersing Weyl fermions with opposite chirality. In this case, the electrochemical response is fully quantized in terms of fundamental constants and the scattering time, and it can be used to measure directly the topological charge of Weyl points. We show that the electrochemical chiral current may be derived from an electromagnetic action similar to axion electrodynamics, where the position-dependent chiral Fermi level plays the role of the axion field. This posits our results as a direct consequence of the chiral anomaly.

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