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)

On-line SPICE-SPIN+X Seminars

On-line Seminar: 11.11.2020 - 15:00 (CET)

Old 2degs with new tricks: Antiferromagnetic order and magnetoelectricity of 2D charge carriers

Ulrich Zuelicke, Victoria University of Wellington

In magnetoelectric media, an electric field can induce a magnetization and a magnetic field can induce an electric polarization, while the system remains in thermal equilibrium.  This effect requires that both space-inversion and time-reversal symmetry are broken.  I will present a comprehensive theory for magnetoelectricity in magnetically ordered quasi-2D systems.  Considering ferromagnetic (FM) zincblende and antiferromagnetic (AFM) diamond structures, quantitative expressions for the magnetoelectric responses due to electric and magnetic fields are obtained that reveal explicitly the inherent duality of these responses required by thermodynamics.  The magnitude of magnetoelectric effects in quasi-2D systems is tunable, and typical values are sizable in quasi-2D hole systems where moderate electric fields can induce a magnetic moment of one Bohr magneton per charge carrier.  For the microscopic understanding of magnetoelectric responses in these systems,  AFM order plays a central role.  We define a Néel operator t that describes AFM order, in the same way a magnetization mreflects FM order.  While m is even under space inversion and odd under time reversal, t describes a toroidal moment that is odd under both symmetries. Thus m and t quantify complementary aspects of magnetic order in solids.  In quasi-2D systems, FM order can be attributed to dipolar equilibrium currents that give rise to a magnetization.  In the same way, AFM order arises from quadrupolar currents that generate the toroidal moment.  The electric-field-induced magnetization can then be attributed to the electric manipulation of the quadrupolar currents.  Our theory provides a broad framework for the manipulation of magnetic order by means of external fields.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 28.10.2020 - 15:00 (CET)

Current-induced gap opening in interacting topological insulator surface states

Mark Spencer Rudner, University of Copenhagen

Nonequilibrium many-body systems may host a variety of internal fields, such as dc currents or ac electric fields, which are not allowed in equilibrium. Through electron-electron interactions, such fields may give rise to intriguing feedback effects that lead to novel types of nonlinear transport phenomena and dynamical phase transitions. In this talk I will show how such feedback is manifested in electronic topological edge states. Two-dimensional topological insulators (TIs) host gapless helical edge states that are predicted to support a quantized two-terminal conductance. Quantization is protected by time-reversal symmetry, which forbids elastic backscattering. Paradoxically, the current-carrying state itself breaks the time-reversal symmetry that protects it. As I will discuss, the combination of electron-electron interactions and momentum-dependent spin polarization in helical edge states gives rise to feedback through which an applied current opens a gap in the edge state dispersion, thereby breaking the protection against elastic backscattering. I will discuss transport signatures of this phenomenon and prospects for its realization in recently discovered large bulk band gap TIs, as well as an analogous current-induced gap opening mechanism for the surface states of three-dimensional TIs.

PDF file of the talk available here