Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G55: Invited Session: Proximity Induced Ferromagnetism in Topological Insulators |
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Sponsoring Units: DCMP Chair: Ilya Eremin, Ruhr University Bochum Room: Four Seasons Ballroom 1 |
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G55.00001: Broken lattice-symmetry influence on electronic anisotropy and interface exchange-coupling-induced ferromagnetic state in TI thin films and heterostructures Invited Speaker: Jagadeesh Moodera Inducing an exchange gap locally on the Dirac surface states of a topological insulator (TI) is ideal for observing the predicted unique features such as the quantized topological magnetoelectric effect, half-integer quantized Hall effect, as well as to con?ne Majorana fermions. Our work experimentally demonstrated the proximity-induced interface ferromagnetism in a heterostructure combining a ferromagnetic insulator EuS layer with Bi2Se3, without introducing defects [1]. An exchange gap was observed to be induced on the surface of the TI. Extensive magnetic and magneto-transport (magnetoresistance and anomalous Hall effect) investigation of the heterostructures, including synchrotron interfacial (XAS and XMCD measurements) studies have shown the emergence of a ferromagnetic phase in TI, which is a step forward to unveiling the above exotic properties. Also, to understand the intrinsic properties of TI it is necessary to correlate structure with the exotic electronic properties as well as interaction with other materials. Molecular beam epitaxy (MBE) ideally allows us to engineer the system whereas using synchrotron and electron diffraction based experimental techniques helps us to investigate with atomic resolution. We will elucidate our studies on well-defined TI films and heterostructure, and the role of imperfections on the symmetry of the material that leads to internal atomic ordering by the decoration of the defects. Charge transport and mobility are seen to relate with film growth strain and relaxation as well as display strong directional dependence on the defect geometry. Work done in collaboration with Peng Wei, Ferhat Katmis and others. \\[4pt] [1] P. Wei, F. Katmis, B. A. Assaf, H. Steinberg, P. Jarillo-Herrero, D. Heiman, and J. S. Moodera, PRL, 110, 186807 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:27PM |
G55.00002: Emerging weak localization effects on a topological insulator--insulating ferromagnet (Bi$_{2}$Se$_{3}$-EuS) interface Invited Speaker: Qi Yang A topological insulator (TI) has a full energy gap in the bulk, and possesses gapless Dirac-like surface states. Because of time reversal symmetry, the surface states cannot be back-scattered by non-magnetic impurities [1]. When a thin magnetic layer is applied on the surface, a full insulating gap is opened, and an electric charge close to the surface is predicted to induce an image magnetic monopole [2]. To further elucidate the uniqueness of transport in the surface state of TI materials, and to investigate such predicted interplay with magnetic materials, we studied the interface between a thin film TI (Bi$_{2}$Se$_{3})$ and an insulating ferromagnet (EuS). While above the Curie temperature ($T_{C})$ of the EuS we observed positive magnetoresistance (MR), which is obtainted ubiquitously in similar TI thin films and interpreted as weak antilocalization (WAL) effects [3], below $T_{C}$ the MR becomes negative near zero field, clearly indicating a proximity effect between the TI and the IF [4]. This phenomenon is consistent with recent theories that predict weak localization (WL) effects in TIs resulted from gap-opening at their surface state Dirac point [5,6].\\[4pt] [1] X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. \textbf{83}, 1057 (2011).\\[0pt] [2] J. E. Moore, Nature \textbf{464}, 194 (2010). \\[0pt] [3] H.-T. He, G. Wang, T. Zhang, I.-K. Sou, G. K. L. Wong, J.-N. Wang, H.-Z. Lu, S.-Q. Shen, and F.-C. Zhang, Phys. Rev. Lett. \textbf{106}, 166805 (2011).\\[0pt] [4] Q. I. Yang, M. Dolev, L. Zhang, J. Zhao, A. D. Fried, E. Schemm, M. Liu, A. Palevski, A. F. Marshall, S. H. Risbud, and A. Kapitulnik, Phys. Rev. B \textbf{88}, 081407 (2013).\\[0pt] [5] I. Garate and L. Glazman, Phys. Rev. B \textbf{86}, 035422 (2012).\\[0pt] [6] H.-Z. Lu, J. Shi, and S.-Q. Shen, Phys. Rev. Lett. \textbf{107}, 076801 (2011). [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 1:03PM |
G55.00003: Weak localization and antilocalization in topological insulator thin films with coherent bulk-surface coupling Invited Speaker: Ion Garate This talk will review the theory and experiments concerning quantum corrections to conductivity in thin films of three dimensional topological insulators. In particular, I will discuss how the magnitude and sign of the low-field magnetoresistance are influenced by bulk-surface coupling, gate voltage, size quantization effects and magnetic order. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:39PM |
G55.00004: Proximity-induced magnetization dynamics, interaction effects, and phase transitions on a topological surface Invited Speaker: Flavio Nogueira When a ferromagnetic or antiferromagnetic insulating layer is grown on the surface of a three-dimensional topological insulator, ferromagnetic order is induced on its surface. In the case of an out-of-plane magnetization, the Dirac fermion surface states become gapped. Quantum fluctuations of the Dirac fermions significantly affect the dynamics of the magnetization at the interface between the topological insulator and the magnetic material. In this talk we will discuss different aspects of the proximity-induced effective action on the topological surface and show that the Coulomb interaction between the Dirac fermions play a crucial role. In the case of a ferromagnet inducing an out-of-plane magnetization, the magnetization dynamics is modified due to a fluctuation-induced Chern-Simons term in the effective action. Such a topological term leads to a topological magnetoelectric torque in the Landau-Lifshitz-Gilbert equation, which is coupled to a non-local Poisson equation for the fluctuating electric field associated to the Coulomb interation. At finite temperature this leads to a downwards shift of the Curie temperature on the topological surface relative to the Curie temperature of the ferromagnet in the absence of the topological insulator. We also analyze the influence of the chemical potential on the magnetization dynamics and phase transitions at finite temperature and show that a thermoelectric screening takes place depending on the magnitude of the fermionic gap. For the case of an in-plane magnetization, a quantum phase transition occurs as the strength of the Coulomb interaction is varied, leading in this way to a semimetal-insulator transition on the topological surface. A Chern-Simons term is generated only when the Coulomb interaction is large enough. In this case parity and time-reversal symmetries are spontaneously broken. A semimetal insulator transition also happens when the magnetic layer is antiferromagnetic. In this case there is quantum criticality with unconventional critical exponents, and the magnetic susceptibility features a large anomalous dimension. \\[4pt] [1] F. S. Nogueira and I. Eremin, Phys. Rev. Lett. {\bf 109}, 237293 (2012).\\[0pt] [2] F. S. Nogueira and I. Eremin, Phys. Rev. B {\bf 88}, 085126 (2013).\\[0pt] [3] F. S. Nogueira and I. Eremin, arXiv:1309.3451. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 2:15PM |
G55.00005: Thin-Film Magnetization Dynamics on the Surface of a Topological Insulator Invited Speaker: Yaroslav Tserkovnyak We theoretically study the magnetization dynamics of a thin ferromagnetic film exchange coupled with a surface of a strong three-dimensional topological insulator. We focus on the role of electronic zero modes imprinted by domain walls (DWs) or other topological textures in the magnetic film. Thermodynamically reciprocal hydrodynamic equations of motion are derived for the DW responding to electronic spin torques, on the one hand, and fictitious electromotive forces in the electronic chiral mode fomented by the DW, on the other. An experimental realization illustrating this physics is proposed based on a ferromagnetic strip, which cuts the topological insulator surface into two gapless regions. In the presence of a ferromagnetic DW, a chiral mode transverse to the magnetic strip acts as a dissipative interconnect, which is itself a dynamic object that controls (and, inversely, responds to) the magnetization dynamics. [Preview Abstract] |
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