On-line SPICE-SPIN+X Seminars

On-line Seminar: 17.03.2021 - 15:00 German Time

Charge and Spin transport physics of organic semiconductors

Henning Sirringhaus, University of Cambridge

Organic semiconductors are characterised by weak intermolecular van der Waals bonding. Many vibrational modes are soft and strongly anharmonic and any electronic processes occur in a strongly fluctuating structural landscape. This gives rise to a unique and interesting transport regime not found in inorganic semiconductors in which electronic excitations are effectively able to surf on the waves of molecular lattice distortion. A key requirement is that the energetic site disorder is sufficiently small that it becomes possible for vibrational modes to couple localized states near the band edges to highly delocalised states within the bands that can then transport excitations over long length scales. In this talk we will provide an overview over how we currently understand the charge and spin transport physics of organic semiconductors in this regime.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 24.02.2021 - 15:00 German Time

Spatio-Temporal Dynamics of Magnon Bose-Einstein Condensates

Sergej O. Demokritov, Münster University

Recent advances in the studies of magnon gases have opened new horizons for the deep understanding of physics of room-temperature macroscopic coherent states including Bose-Einstein condensates of magnons. Although this phenomenon was discovered almost 15 years ago, a lot of important issues associated with the magnon Bose-Einstein condensation still remain unclear. Here I review the recent experimental achievements in the investigations of this phenomenon. I show that magnon condensates are characterized by high degree of temporal and spatial coherency, which enables, for instance, observation of the interference of two condensates in the real space. Discovery of the second sound in magnon condensates is also discussed. I also address the dynamics of the condensate in inhomogeneous time-varying external fields. I show that this approach allows one to implement a magnon laser, which can generate a freely propagating cloud of coherent magnons, as well as to separate in the real space degenerate condensates corresponding to two opposite wavevectors. These studies also allowed us to answer the long-standing question concerning the physical origin of the spatial stability of the condensate. Finally, I examine the applicability of the spin-current paradigm for description of magnon condensates.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 10.03.2021 - 15:00 German Time

All-optical control of magnetism: from fundamentals to brain-inspired computing concepts

Theo Rasing, Radboud University

The ability to switch magnets between two stable bit states is the main principle of digital data storage technologies since the early days of the computer. Due to many new ideas, originating from fundamental research during the last 50 years, this technology has developed in a breath-taking fashion. However, the explosive growth of digital data and its related energy consumption is pushing the need to develop fundamentally new physical principles and materials for faster, smaller and more energy-efficient processing and storage of data.
Since our demonstration of magnetization reversal by a single 40 femtosecond laser pulse, the manipulation of magnetism by ultra-short laser pulses has developed into an alternative and energy efficient approach to magnetic recording. Plasmonic antennas have allowed to push this even down to nanometer length scales while photonic networks allow the development of an opticaly swichable MRAM. However, new ICT technologies, such as Artificial Intelligence push the exponentially increasing energy requirement of data manipulation even more. Therefore, the development of radically new physical principles that combine energy-efficiency with high speeds and high densities is crucial for a sustainable future. One of those is neuromorphic computing, that is inspired by the notion that our brain uses a million times less energy than a supercomputer while, at least for some tasks, it even outperforms the latter.
In this talk, I will discuss the state of the art in ultrafast optical manipulation of magnetic bits and present some first results and the potential of optical control of magnetism to implement brain-inspired computing concepts in magnetic materials.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 17.02.2021 - 15:00 German Time

Seeing or listening: magnetoelastic effects in antiferromagnetic textures

Olena Gomonay, JGU Mainz

Antiferromagnets are considered as prospective materials for spintronic applications as they could be effectively manipulated with the electrical and optical pulses, and also show magnetic dynamics and low susceptibility to the external magnetic field. The mechanisms involved into control and manipulation of antiferromagnetic states were usually related with the current- or laser-induced spin-torques. However, recent experiments demonstrated that heating and heat-induced strains that follow current and laser pulses can produce similar or even stronger effects on the magnetic dynamics. In this presentation we consider behaviour of antiferromagnetic textures in presence of inhomogeneous strain fields of different origin. In particular, we discuss the magnetoelastic mechanism that is responsible for formation of the domain structure, magnetoelastic pinning of the domain walls, and thermo-magnetoelastic mechanism of current-induced switching.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 10.02.2021 - 15:00 German Time

Magnetic Materials and Topology

Claudia Felser, MPI Chemical Physics of Solids, Dresden, Germany

Topology, a mathematical concept, recently became a hot and truly transdisciplinary topic in condensed matter physics, solid state chemistry and materials science. All 200 000 inorganic materials were recently classified into trivial and topological materials: topological insulators, Dirac, Weyl and nodal-line semimetals, and topological metals [1]. Around 20% of all materials host topological bands. Currently, we have focussed also on magnetic materials, a fertile field for new since all crossings in the band structure of ferromagnets are Weyl nodes or nodal lines [2], as for example Co2MnGa and Co3Sn2S2. Beyond a single particle picture and identified antiferromagnetic topological materials [3].

[1] Bradlyn et al., Nature 547 298, (2017), Vergniory, et al., Nature 566 480 (2019)
[2] Belopolski, et al., Science 365, 1278 (2019), Liu, et al. Nature Physics 14, 1125 (2018), Guin, et al. Advanced Materials 31 (2019) 1806622, Liu, et al., Science 365, 1282 (2019), Morali, et al., Science 365, 1286 (2019)
[3] Xu et al. Nature 586 (2020) 702

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 27.01.2021 - 15:00 German Time

The Thermal Chiral Anomaly in ideal field-induced Weyl semimetals

Joseph Heremans, Ohio State University

The Thermal Chiral Anomaly in ideal field-induced Weyl semimetals.
Weyl semimetals (WSMs) are solids with a bulk band structure consisting of pairs of chirally distinct linear Dirac bands that intersect at the Weyl points. In ideal Weyl semimetals, where the electrochemical potential is located at the energy of the Weyl points, the Fermi surface in the bulk amounts to a pair of Weyl points of opposite chirality. Transport in ultra-quantum magnetic fields in an ideal WSM is subject to the chiral anomaly, an additional electrical conductivity that results from the generation of carriers of one chirality and the annihilation of carriers of the other in a magnetic field oriented parallel to the direction of the charge flux. In contrast the chiral anomaly in the electrical conductivity, there is no creation/annihilation of charge in the thermal conductivity, but there is an equivalent effect in the energy at both points, giving an excess thermal conductivity that we put in evidence experimentally. The thermal and electrical anomalies relate to each other by the Wiedemann-Franz law.
The experimental observations are made on the field-induced WSM Bi1-x¬Sbx (8 at%<x 1-to-4 T), transforming the semiconductor into a WSM. The samples display freeze-out upon cooling and show no Shubnikov–de Haas oscillations despite their high mobility (1.9 × 106 cm2V-1s-1 at 10 K), ensuring that the electrochemical potential is at the Weyl points. By construction, no trivial pockets exist in the Fermi surface, making this an ideal WSM. We observe an increase in thermal conductivity zz in longitudinal field Bz that is consistent with the theory and obeys the Wiedemann-Franz law wit the free electron value of the Lorenz ratio.
If time permits, preliminary data on the thermal Hall effect in the same samples will be presented. The thermal Hall effect behaves qualitatively differently from the electrical Hall effect, and may contain contributions from charge carriers on the Fermi arcs.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 03.02.2021 - 15:00 German Time

Chiral magnetism: a geometric perspective

Oleg Tchernyshyov, Johns Hopkins University

Chiral ferromagnets have spatially modulated magnetic order exemplified by helices, spirals, and more complex patterns such as skyrmion crystals. The theoretical understanding of these states is based on a competition of a strong Heisenberg exchange interaction favoring uniform magnetization and a weaker Dzyaloshinskii-Moriya (DM) interaction promoting twists in magnetization. We offer a geometric approach, in which chiral forces are a manifestation of curvature in spin parallel transport [1]. The resulting theory is a gauged version of the Heisenberg model, with the DM vectors serving as background SO(3) gauge fields. This geometrization of chiral magnetism is akin to the treatment of gravity in general relativity, where gravitational interactions are reduced to a curvature of spacetime. The geometric perspective provides a simple way to define a conserved spin current in the presence of spin-orbit interaction. The gauge-dependent nature of the DM term raises questions about its recently proposed linear dependence on the (gauge-independent) spin current [2-3]. We also show that the gauged Heisenberg model in d=2 has a skyrmion-crystal ground state for a magic value of an applied magnetic field.

[1] D. Hill, V. Slastikov, and O. Tchernyshyov, arXiv:2008.08681.
[2] T. Kikuchi, T. Koretsune, R. Arita and G. Tatara, Phys. Rev. Lett. 116, 247201 (2016).
[3] F. Freimuth, S. Blügel, and Y. Mokrousov, Phys. Rev. B 96, 054403 (2017).

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 20.01.2021 - 15:00 German Time

Mòire Samples: The twisted bilayer graphene scenario

Andrei Bernevig, Princeton University

We present a full theory of the interacting insulating phases of twisted bilayer graphene around the first magic angle where the bandwidth of the “active” bands becomes very small. We show that the single particle Hamiltonian is fully anomalous: it contains stable topology for every set of bands. Furthermore, we analyze the Coulomb interaction and obtain exact insulating ground states as well as the full excitation spectrum in certain limits.

PDF file of the talk available here

 

 

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 13.01.2021 - 15:00 German Time

Chirality Induced Spin Selectivity: Open questions and challenges

Bart van Wees, University of Groningen

The phenomenon of chirality induced spin selectivity (CISS) has been known for over two decades [1]. A strong coupling between spin and charge transport has been observed, which depends on the chirality (or helicity) of the molecules. These systems are non-magnetic, so the origin of CISS must lay in manifestations of the spin-orbit interaction between moving electrons and the chirality induced electric fields present in the chiral systems.
In this talk I will give an overview of the manifestations of chirality in nature, as well as the experimental status of CISS in electronic and spintronic transport experiments. In particular I focus on experiments where CISS is observed as a spin-valve magneto resistance (MR) in two-terminal devices, where a ferromagnet is used to polarize or analyze the spins transmitted through the chiral systems. I will discuss our recent theory work, where we emphasize the important symmetry differences between the spin-charge coupling in ferromagnets, which breaks time reversal symmetry, and in (non-magnetic) chiral systems, which preserves time reversal symmetry. This prohibits the observation of CISS induced MR in two-terminal systems (but still allows it in multiterminal devices)[2]. Experiments on CISS also explore the non-linear transport regime, where voltage biases are employed which exceed the thermal energy. In this regime a CISS induced MR is possible. However, its sign depends not only on the chirality, but also on the nature of the transport through the ferromagnet and chiral system[3].
Finally I will discuss a possible connection between the absence/presence of CISS induced MR in electronic transport measurements with experiments which demonstrated magnetization/magnetic field self-assembly of chiral molecules. [4].
[1] K. Ray, S.P. Ananthavel, D.H. Waldeck, R. Naaman, Asymmetric scattering of Polarized electrons by organized organic films made of chiral molecules, Science, 283, 814 (1999).
[2] X. Yang, C.H. van der Wal, and B.J. van Wees, Spin-dependent electron transmission model for chiral molecules in mesoscopic devices, Phys. Rev. B 99, 024418 (2019)
[3] X. Yang, C. H. van der Wal, and B. J. van Wees, Detecting Chirality in Two-Terminal Electronic Nanodevices, Nano Lett. 20, 8, 6148–6154 (2020)
[4] K. Banerjee Kosh et al., Separation of enantiomers by their enantiospecific interaction with achiral magnetic substrates, Science 10 May (2018)

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 02.12.2020 - 15:00 (CET)

Electric-field effects on localized spins

Tomasz Dietl, Polish Academy of Sciences

Since control by voltages is energetically more efficient than by currents, bipolar electronics was replaced, a half-century ago, by CMOS FET technology, in which power dissipation occurs merely only during recharging of FET capacitors. A time ago, some of us realized that gating could also provide a high degree of control over magnetic properties [1].
Winkler and Zülicke have recently presented a comprehensive theory predicting the effects of an electric field on the magnetism of carriers in a semiconductor quantum well under the presence of localized spins breaking time-reversal symmetry [1]. Complementing that theory, I will give an overview of experimental results and quantitative theories demonstrating the influence of the electric field on localized spins in two classes of magnetic semiconductors: (i) dilute magnetic insulator wz-(Ga,Mn)N [3,4], in which ferromagnetic superexchange leads to spin ordering at low temperature; (ii) dilute ferromagnetic semiconductors [5], in which band holes mediate spin-spin coupling. The issue of spin heating by charging currents, as well as preliminary results on gating in antiferromagnetic CuMnAs [6], will also be addressed.

PDF file of the talk available here

[1] A. Haury, A. Wasiela, A. Arnoult, J. Cibert, S. Tatarenko, T. Dietl, Y. Merle d'Aubigné, Phys. Rev. Lett. 79, 511 (1997)
[2] R. Winkler and U. Zülicke, Phys. Rev. Research 2, 043060 (2020)
[3] D. Sztenkiel, K. Gas, M. Foltyn, N. Gonzalez Szwacki, C. Śliwa, J. Domagała, D. Hommel, T. Dietl, and M. Sawicki, JEMS, 7-11 Dec. 2020, and in preparation
[4] D. Sztenkiel, M. Foltyn, G. P. Mazur, R. Adhikari, K. Kosiel, K. Gas, M. Zgirski, R. Kruszka, R. Jakiela, Tian Li, A. Piotrowska, A. Bonanni, M. Sawicki, and T. Dietl, Nat. Commun. 7, 13232 (2016)
[5] T. Dietl and H. Ohno, Rev. Mod. Phys. 86, 187 (2014)
[6] M. J. Grzybowski, P. Wadley, K. W. Edmonds, R. P. Campion, K. Dybko, M. Majewicz, B. L. Gallagher, M. Sawicki, and T. Dietl, AIP Advances 9, 115101 (2019)