All Scheduled Events

Self-organized complex structures in nature, e.g., viral capsids, hierarchical biopolymers, and bacterial flagella, offer efficiency, adaptability, robustness, and multi-functionality. Can we program the self-assembly of three-dimensional (3D) complex structures using simple building blocks, and reach similar or higher level of sophistication in engineered materials? Here we present an analytic theory for the self-assembly of polyhedral nanoparticles (NPs) based on their crystal structures in non-Euclidean space. We show that the unavoidable geometrical frustration of these particle shapes, combined with competing attractive and repulsive interparticle interactions, lead to controllable self-assembly of structures of complex order. Applying this theory to tetrahedral NPs, we find high-yield and enantiopure self-assembly of helicoidal ribbons, exhibiting qualitative agreement with experimental observations. We expect that this theory will offer a general framework for the self-assembly of simple polyhedral building blocks into rich complex morphologies with new material capabilities such as tunable optical activity, essential for multiple emerging technologies.

Topological quantum computing offers a route to lower error rates, enabling fault tolerance and impactful applications. However, it relies on reliably tuning devices into a non-Abelian topological phase. In this talk, I will discuss four lessons that have been learned in the attempt to realize a topological quantum computer.

Exerting controlled forces in a non-contact way is important in biomedical investigations which require holding, moving, or mechanically probing biomedical samples. Optical and acoustic manipulation of microscopic samples both play a prominent role among suitable technologies. The differences in the physical laws and in the typical length scales governing acoustic and optical forces make them complementary: Acoustic forces can levitate large and heavy particles, which optical tweezers could not handle without adverse high-power effects, while optical forces cover subcellular scales. The talk will contrast the two modalities, and identify situations where one or the other is favorable, or when a combination of both is the best choice.

If the Peccei-Quinn symmetry associated to an axion has ever been restored after inflation, axion strings inevitably produce a contribution to the stochastic gravitational wave background. Combining effective field theory analysis with numerical simulations, I will show that the resulting gravitational wave spectrum has logarithmic deviations from a scale invariant form with an amplitude that is significantly enhanced at low frequencies. I will discuss the implications for upcoming experiments and the complementary cosmological constraints.

Once in a while, a discovery comes along that opens up uncharted paths of inquiry. The discovery of atomically thin crystals, starting with graphene and followed up with dozens in the van der Waals family of compounds, has changed the way we think about materials. This new class of designer materials is making it possible to tune and control electronic properties without changing chemical composition, through alternative means such as superposing different layers, misaligning them, inducing strain, etc. In this talk I will describe highlights from this rapidly evolving field, from its serendipitous inception, to the discovery of correlated electron phases in buckled and twisted layers.

1. G. Li et al Observation of Van Hove singularities in twisted graphene layers, Nature Physics 6, 109 (2009)
2. J. Mao et al, Evidence of Flat Bands and Correlated States in Buckled Graphene Superlattices, Nature 584, 215 (2020)
3. S.Wu, Z. Zhang, K. Watanabe, T. Taniguchi, E.Y. Andrei, Chern Insulators and Topological Flat-bands in Magic-angle Twisted Bilayer Graphene , Nature Materials 20, 488 (2021)

While known for a long time, antiferromagnetically ordered systems have previously been considered, as expressed by Louis Néel in his Nobel Prize Lecture, to be “interesting but useless”. However, since antiferromagnets potentially promises faster operation, enhanced stability with respect to interfering magnetic fields and higher integration due to the absence of dipolar coupling, they could potentially become a game changer for new spintronic devices. The zero net moment makes manipulation using conventional magnetic fields challenging. However recently, these materials have received renewed attention due to possible manipulation based on new approaches such as photons [1] or spin-orbit torques [2]. In this talk, we will present an overview of the key features of antiferromagnets to potentially functionalize their unique properties. This includes writing, reading and transporting information using antiferromagnets. We recently realized switching in the metallic antiferromagnet Mn2Au by intrinsic staggered spin-orbit torques [3,4] and characterize the switching properties by direct imaging. While switching by staggered intrinsic spin-orbit torques in metallic AFMs requires special structural asymmetry, interfacial non-staggered spin-orbit torques can switch multilayers of many insulating AFMs capped with heavy metal layers. We probe switching and spin transport in selected collinear insulating antiferromagnets, such as NiO [5-7], CoO [8,9] and hematite [10,11]. In NiO and CoO we find that there are multiple switching mechanisms that result in the reorientation of the Néel vector and additionally effects related to electromigration of the heavy metal layer can obscure the magnetic switching [5,7,9]. For the spin transport, spin currents are generated by heating as resulting from the spin Seebeck effect and by spin pumping measurements and we find in vertical transport short (few nm) spin diffusion lengths [6,8]. For hematite, however, we find in a non-local geometry that spin transport of tens of micrometers is possible [10,11]. We detect a first harmonic signal, related to the spin conductance, that exhibits a maximum at the spin-flop reorientation, while the second harmonic signal, related to the Spin Seebeck conductance, is linear in the amplitude of the applied magnetic field [10]. The first signal is dependent on the direction of the Néel vector and the second one depends on the induced magnetic moment due to the field. We identify the domain structure as the limiting factor for the spin transport [11]. We recently also achieved transport in the easy plane phase [12], which allows us to obtain long distance spin transport in hematite even at room temperature [12]. From the power and distance dependence, we unambiguously distinguish long-distance transport based on diffusion [10,11] from predicted spin superfluidity that can potentially be used for logic [13]. A number of excellent reviews are available for further information on recent developments in the field [14].

References
[1] A. Kimel et al., Nature 429, 850 (2004).
[2] J. Zelezny et al., Phys. Rev. Lett. 113, 157201 (2014); P. Wadley et al., Science 351, 587 (2016).
[3] S. Bodnar et al., Nature Commun. 9, 348 (2018)
[4] S. Bodnar et al., Phys. Rev. B 99, 140409(R) (2019).
[5] L. Baldrati et al., Phys. Rev. Lett. 123, 177201 (2019)
[6] L. Baldrati et al., Phys. Rev. B 98, 024422 (2018); L. Baldrati et al. Phys. Rev. B 98, 014409 (2018)
[7] F. Schreiber et al., Appl. Phys. Lett. 117, 082401 (2020); H. Meer et al., Nano Lett. 21, 114 (2021)
[8] J. Cramer et al., Nature Commun. 9, 1089 (2018)
[9] L. Baldrati et al., Phys. Rev. Lett. 125, 077201 (2020)
[10] R. Lebrun et al., Nature 561, 222 (2018).
[11] A. Ross et al., Nano Lett. 20, 306 (2020).
[12] R. Lebrun et al., Nature Commun. 11, 6332 (2020).
[13] Y. Tserkovnyak et al., Phys. Rev. Lett. 119, 187705 (2017).
[14] Rev. Mod. Phys. 90, 15005 (2018); Nat. Phys. 14, 200-242 (2018); Adv. Mater. 32, 1905603 (2020)

High-precision astrometric data from space observatories, such as the Hubble Space Telescope HST and Gaia, are revolutionizing our ability to study the Local Group. 6D phase space measurements 3-dimensional position and velocity now make it possible to rewind the clock and trace the orbital histories of nearly half of all Local Group satellites to their cosmic origins in the early Universe. These new datasets pave the way for a revised model of the Local Group’s dynamical history and its current dark matter content. In this talk, I will focus on the Magellanic Clouds and M33, the most massive satellite galaxies orbiting around the MW and M31, respectively. These massive satellites are nearly 10% as massive as their host galaxies and LCDM simulations predict that they too host a population of "ultra-faint” satellite galaxies, i.e. a satellites of satellites hierarchy. I will juxtapose these two satellite systems and discuss ongoing efforts to characterize satellites of the Magellanic Clouds and future efforts for finding M33 satellites. Together, these two systems provide a direct test for LCDM predictions at the low mass end and act as a benchmark for next generation studies of analogous galaxies beyond the Local Group in the era of Roman, JWST, and LSST.

The string theory landscape of vacua points to new consistency conditions that a quantum gravitational system must satisfy. There are only a small number of quantum field theories that satisfy these conditions and all the rest belong to the "Swampland," which cannot be consistently coupled to gravity. In this talk I review some of these conditions and their implications for cosmology and particle physics.

TBD