10:30am  11:00amInvitedMagnetismInduced Massive Dirac Spectra and Topological Spin Textures in the Surface State of Magnetic Topological Insulators
NaiChang YEH^{1}, ChienChang CHEN^{1}, Marcus L. TEAGUE^{1}, Liang HE^{2}, Xufeng KOU^{2}, Murong LANG^{2}, Kang L. WANG^{2}
^{1}California Institute of Technology, United States; ^{2}University of California, Los Angeles, United States
Proximityinduced magnetic effects on the surface Dirac spectra of topological insulators are investigated by scanning tunneling spectroscopic (STS) studies of MBEgrown bilayer structures consisting of undoped Bi_{2}Se_{3} on top of (Bi_{1x}Cr_{x})_{2}Se_{3}.^{[1]} For thickness of the top Bi_{2}Se_{3} layer equal to or smaller than 3 quintuple layers, a spatially inhomogeneous surface spectral gap Δ opens up below a characteristic temperature T_{c}^{2D}, which is much higher than the bulk Curie temperature T_{c}^{3D} determined from the anomalous Hall resistance. The mean value and spatial homogeneity of Δ increase with increasing caxis magnetic field (H) and increasing Cr doping level (x), suggesting that the gap is due to proximityinduced caxis ferromagnetism, whereas the temperature (T) dependence of Δ is nonmonotonic, showing initial increase below T_{c}^{2D} followed by a “dip” and then reaching maximum at T << T_{c}^{3D}. These phenomena may be attributed to proximity magnetism induced by two types of contributions with different temperature dependences: a threedimensional (3D) contribution from the bulk magnetism that dominates at low T, and a twodimensional (2D) contribution associated with the RKKY interactions mediated by surface Dirac fermions, which dominates at T_{c}^{3D} << T < T_{c}^{2D}.^{[1]} In addition, spatially localized sharp resonant spectra are found along the boundaries of gapped and gapless regions.^{[1]} These spectral resonances are longlived at H = 0, with their occurrences being most prominent near T_{c}^{2D} and becoming suppressed under caxis magnetic fields. We attribute the spectral resonances to magnetic impurityinduced topological spin textures of the surface Dirac fermions, which are in stark contrast to the nearly Tindependent nonmagnetic impurity resonances found in undoped Bi_{2}Se_{3}.^{[2]} The longterm stability of these topologically protected twolevel states may find potential applications to quantum information technology.
References:
[1] C.C. Chen et al., New J. Phys. 17, 113042 (2015).
[2] M. L. Teague et al., Solid State Commun. 152, 747 (2012).
11:00am  11:30amInvitedInitializationfree Multilevel States Driven by Spinorbit Torque Switching
ChihHuang LAI, KuoFeng HUANG, HsiuHau LIN
National Tsing Hua University, Taiwan
Among nonvolatile memories, MRAM is one of the most promising candidates for its excellent endurance and speed. In recent developments, the socalled spinorbit torques (SOT) has been proposed as an alternative writing mechanism for the next generation MRAM.^{} Because the writing and reading procedures in the 3terminal design are separated, SOTMRAM possesses better endurance for massive writing cycles and fewer disturbances on the stored bits for reliable reading. However, the 3terminal setup of the SOTMRAM limits its smallest cell size. To reduce the effective cell size, multilevel storage has been developed in various nonvolatile memories. When the reported multilevel states are realized, an extra initialization process is essentially needed, which prolongs write latency and wastes power consumption.
Here we demonstrate how multilevel storage can be achieved in Co/Pt multilayers without extra initialization steps. By stacking Co/Pt repeatedly to N layers, the magnetic property can be tailored with large SOT as in the Co/Pd multilayers. It is appealing that the SOT switching behavior strongly depends on the number of repeated Co/Pt layers. For N=1, the applied electric pulses cause sharp binary switching. When the layer number increases to N=4, multidomain formation becomes energetically favorable and the initial magnetization dissolves into the demagnetized state upon applying electric pulses. For intermediate layer number N=2, the multidomain configurations settle into multilevel states with different net magnetizations, well controlled by modulating the electric pulses and completely independent of its initial states. We perform the read/write test on the N=2 Co/Pt multilayers and demonstrate four wellseparated multilevel states without initialization steps. Moreover, by generalizing the modified LandauLifshitzGilbert equation to the multidomain configurations, the theoretical predictions provide nice agreement with the experimental findings, rendering the SOT switching mechanism in Co/Pt multilayers well understood. The SOT switching mechanism we discovered here paves a promising avenue for further developments.
11:30am  11:45amOralPressure Induced Topological Phase Transition and SpinOrbit Coupling Tuning in Layered Bi2S3
Yongzheng LUO^{1}, Ming YANG^{2,3}, Lei SHEN^{4}, Yuan Ping FENG^{1,3}
^{1}Department of Physics, National University of Singapore, Singapore; ^{2}Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore; ^{3}Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore; ^{4}Engineering Science Programme, National University of Singapore, Singapore
Modulating nontrivially topological states in trivial materials is of both scientific and technological interest. Inspired by the robustness to scattering of conducting edge states in quantum Hall systems, the searching of strong threedimensional (3D) topological insulators (TIs) has successfully given rise to the Bi2Se3 family of compounds, which share a rhombohedral structure containing blocks of quintuple layers. In this study, via hybrid density functional theory based firstprinciples calculations, we first predict that Bi2S3, the lightest bismuthbased chalcogenide binary component, can also be realized in a similar rhombohedral structure, and it becomes a TI by a moderate compressive pressure of about 5.3 GPa with spinorbit coupling (SOC). Interestingly, the inverted band gap is able to be tuned larger than 0.4 eV with an experimentally accessible pressure. Simultaneously, the surface states and Z2 invariants are calculated with maximally localized Wannier functions. The theoretical analyses presented here demonstrate that the topological quantum phase transition between a nontrivial phase and a trivial insulating/metallic phase can be realized by pressure, which also shed light on searching new TIs with large band gap by the tuning of SOC.
11:45am  12:00pmOralStructural Anisotropy and High Energy Product in FePtFe3Pt HardSoft Magnet NanoComposites from ab Initio Calculations
Bheema Lingam CHITTARI^{1}, Vijay KUMAR^{2,3}
^{1}Department of Physics, University of Seoul, Korea; ^{2}Dr. Vijay Kumar Foundation, India; ^{3}Center for Informatics, School of Natural Sciences, Shiv Nadar University, India
We report results of ab initio calculations on FePtFe_{3}Pt hardsoft magnet matrixnanoparticle composites using a supercell approach. It is found that the FePt matrix induces tetragonal anisotropy in L1_{2} Fe_{3}Pt nanoparticles when embedded in L1_{0} FePt matrix. The FePt matrix and the Fe_{3}Pt nanoparticles have the same easy axis. The magnetic moments on Fe and Pt atoms are enhanced in both the matrix as well as the nanoparticles compared with the values in FePt hard magnet even though there is a reduction in Pt contents. The enhanced magnetic moments and reduced volume in these nanocomposites lead to high maximum energy product of 71.8 MGOe which is much higher than the calculated value of 46.8 MGOe for bulk FePt hard magnet. A systematic investigation of the structure and magnetic properties of these nanocomposites reveals charge transfer from Fe to Pt, quantum confinement, and interface effects to play an important role.
12:00pm  12:15pmOralSpin Phonon Coupling and Magnetic Excitations in Spin Orbit Coupled Irridates: A Raman Spectroscopic Study
Dileep Kumar MISRA^{1,2}
^{1}Materials Science Program, Indian Institute of Technology, Kanpur, India; ^{2}Department of Condensed Matter Physics and Materials Science (DCMP and MS), Tata Institute of Fundamental Research, Mumbai, India
In strongly correlated electronic systems, 5d irridates are recent focus of research interest. Presence of strong spinorbit coupling in irridates give rise to novel quantum states of matter termed as topological insulators and quantum spin liquids. The both topological phases have promise applications in spintronics and quantum computing ^{[1]} that led to flurry of intense research investigations for potential applications and fundamental physics point of view. Sr2IrO4 is a spin orbit Coupled Mott Insulator, possesses unconventional electronic and magnetic ground states. Raman Scattering is an extremely sensitive local probe to explore unconventional electronic and magnetic states.
Here, Raman scattering study under extreme temperature condition on the high quality Sr2IrO4 polycrystalline pallet is presented. The low temperature Raman measurements were carried out upto high wave numbers that reveals the existence of two magnon excitations in Sr_{2}IrO_{4} which is dramatically presented well above the T_{N}. Strong Fano assymetry is observed in some of the Raman modes which indicates towards presence of electronic or psuedospin excitation in the paramagnetic state of the system, however the psuedospin excitation are not quenched in magnetic ordered states as observed in single crystalline form of the system ^{[2]}. Spin phonon coupling is also evidenced in magnetic ordered state, the magnetic exchange interaction constant (J) has been calculated using this data.
References:
[1] C. Nayak, A. Stern, M. Freedman, and S. Das Sarma, Rev. Mod. Phys. 80, 1083 (2008).
[2] H. Gretarsson, N. H. Sung, M. Hoeppner, B. J. Kim, B. Keimer, M. Le Tacon , Phys. Rev. Lett. 116, 136401 (2016).
