Session Y39: Invited Session: Emergent States Driven by Spin-Orbit Coupling

Novel 2D electron gases at the surface of transition-metal oxides: role of topology and spin-orbit coupling

Transition-metal oxides (TMOs) are correlated-electron systems with remarkable properties, such as high-temperature superconductivity or large magnetoresistance. The realization of two-dimensional electron gases (2DEGs) at surfaces or interfaces of TMOs, a field of current active research, is crucial for harnessing the functionalities of these materials for future applications. Additionally, these 2DEGs offer the possibility to explore new physics emerging from the combined effects of electron correlations and low-dimensional confinement. Recently, we discovered that a 2DEG can be simply realized at the vacuum-cleaved surface of SrTiO$_{\mathrm{3}}$, a transparent, insulating TMO with a gap of 3.5 eV. We directly imaged its multiple heavy and light subbands using angle-resolved photoemission spectroscopy [A. F. Santander-Syro \textit{et al}., Nature \textbf{469}, 189 (2011)]. In this talk, I will show that one can also create and tailor 2DEGs in other TMO surfaces, opening vast possibilities for the study of correlations in low dimensions in materials showing diverse functionalities. I will first discuss the specific case of KTaO$_{\mathrm{3}}$, a wide-gap insulator with a spin-orbit coupling 30 times larger than in SrTiO$_{\mathrm{3}}$. I will show that quasi-2D confinement in this system results in comparable scales for the Fermi energy, the subband splitting, and the spin-orbit coupling, leading to a complete reconstruction of the orbital symmetries and band masses [A. F. Santander-Syro \textit{et al}., Phys. Rev. B \textbf{86}, 121107(R) (2012)]. Then, I will show that by choosing various surface terminations of different symmetries one can modify the electronic structure of the 2DEGs at the surface of TMOs [C. Bareille \textit{et al}., submitted (2013); T. R\"{o}del \textit{et al}., submitted (2013)]. All these results demonstrate that, in TMOs, the strong correlations, together with the electron confinement and the surface-lattice symmetry, can lead to novel states at the surface that are not simple extensions of the bulk bands.   

Topological Insulators, Semi-Metals and Superconductors From First Principles Electronic Structure Calculations

Using first-principles electronic structure calculations we investigate novel phases that emerge from the interplay of electron correlations, strong spin-orbit coupling and electron-phonon interactions. We first [1] focus on describing the topological semimetal, a three-dimensional phase of a magnetic solid, and argue that it may be realized in a class of pyrochlore iridates (such as Y2Ir2O7) based on calculations using the LDA $+$ U method. This state is a three-dimensional analog of graphene with linearly dispersing excitations and provides a condensed-matter realization of Weyl fermions that obeys a two-component Dirac equation. It also exhibits remarkable topological properties manifested by surface states in the form of Fermi arcs, which are impossible to realize in purely two-dimensional band structures. We second [2] predict that osmium compounds such as CaOs2O4 and SrOs2O4 can be stabilized in the geometrically frustrated spinel crystal structure. They show ferromagnetic order in a reasonable range of the on-site Coulomb correlation U and exotic electronic properties, in particular, a large magnetoelectric coupling characteristic of axion electrodynamics. Finally, the issue of topological superconductivity and the possibility of the odd pairing will be discussed in Cu doped Bi2Te3 materials where electron-phonon coupling constant is calculated for various pairing symmetries using density functional linear response approach [3].\\[4pt] [1] Xiangang Wan, Ari Turner, Ashvin Vishwanath, Sergey Y. Savrasov, \textit{Phys. Rev. }B \textbf{83}, 205101 (2011);\\[0pt] [2] Xiangang Wan, Ashvin Vishwanath, and Sergey Y. Savrasov, \textit{Phys. Rev Lett.} \textbf{108}, 146601 (2012);\\[0pt] [3] Xiangang Wan, and Sergey Y. Savrasov, arXiv:1308.5615.   

Role of local orbital angular momentum in the electronic structure under inversion symmetry breaking

Orbital angular momentum (OAM), usually ignored in solids (OAM quenching), is found to play an important role in the electronic structure for a broad range of materials [1,2]. It will be shown how one can detect existence of OAM by using circular dichroism angle resolved photoemission (CD-ARPES). CD-ARPES is used to study a topological insulator Bi2Se3. The result reveals that not only spins but also OAM forms chiral structure, and the energy scale is mostly determined by the interaction of asymmetric charge distribution and electric field. This observation is different from the conventional understanding of the Rashba effect and suggests that we must consider a new effective Hamiltonian based on OAM. The new Hamiltonian along with crystal field and atomic spin-orbit coupling (SOC) should determine the surface electronic structures. We further investigated surface states with various atomic SOC strengths to study such mechanism. It is shown experimentally and theoretically that, as the atomic SOC strength decreases, the OAM in different Rashba bands change from anti-parallel to parallel configuration. The split energy in such case is found to come from atomic SOC (parallel and anti-parallel alignment of the spin to OAM). Local OAM is found to play an important role in the electronic structure for a wide range of materials with inversion symmetry breaking. We will show some examples where OAM is quite strong, such as semi-conductors with zinc blende structure and transition metal oxide surface states.\\[4pt] [1] S. R. Park, Phys. Rev. Lett.,108, 046805 (2012).\\[0pt] [2] S. R. Park, Phys. Rev. Lett.,107, 0156803 (2011).   

Emergence of a Robust Dirac Cone from Rashba-Split Surface States on a Topological Semimetal

Topological materials (TMs) host protected surface states that emerge from strong spin orbit coupling in the bulk and on the surface. While their remarkable properties have generated much interest, previous studies have shown nanoscale variations in their surface state properties [1,2], prompting the development of a nanoscale band structure probe. Here we report the simultaneous observation and quantitative reconciliation of Landau quantization and quasiparticle interference phenomena on the topological semimetal Sb, which we employ to reconstruct its multi-component surface state band structure[3]. We thereby establish the technique of \textbf{band structure tunneling microscopy (BSTM)}, and utilize it to elucidate the relationship between bulk conductivity and surface state robustness, and to quantify essential metrics for spintronics applications. Meanwhile, our Landau quantization results help us visualize the evolution of quasiparticle behavior from massive Rashba to massless Dirac character, and determine the surface state $g$-factor. \\[2ex] [1] H. Beidenkopf \emph{et al.}, Nature Physics 7, 939 (2011)\\[0ex] [2] Y. Okada \emph{et al.}, Nature Communications 3, 1158 (2012)\\[0ex] [3] A. Soumyanarayanan \emph{et al.}, arxiv:1311.1758 (2013)   

Spin orbit density wave: A non-magnetic phase of two-dimensional electron gas

We propose and formulate an interaction induced staggered spin-orbit order as a new emergent phase of two-dimensional Fermi gases. We show that when some form of inherent spin splitting via Rashba-type spin-orbit coupling renders two helical Fermi surfaces to become significantly ``nested,'' a Fermi surface instability arises. To lift this degeneracy, a spontaneous symmetry-breaking spin-orbit density wave develops, causing a surprisingly large quasiparticle gapping with chiral electronic states. Since the time-reversal invariant spin-orbit density wave is associated with a condensation energy, quantified by the gap value, destroying such spin-orbit interaction costs sufficiently large magnetic field or temperature or dephasing time. The BiAg$_{\mathrm{2}}$ surface state is shown to be a representative system for realizing such novel spin-orbit interaction. We also apply this theory to LAO/STO interface, Iridates compunds, and in the enigmatic `hidden-order' phase in URu$_{\mathrm{2}}$Si$_{\mathrm{2}}$ T. Das. Phys. Rev. Lett. \textbf{109}, 246406 (2012).