Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E22: Nematicity and the Valley Degree of FreedomInvited
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Sponsoring Units: DCMP Chair: Steve Kivelson, Stanford University Room: New Orleans Theater A |
Tuesday, March 14, 2017 8:00AM - 8:36AM |
E22.00001: Quantum Hall Electron Nematics Invited Speaker: Allan MacDonald In 2D electron systems hosted by crystals with hexagonal symmetry, electron nematic phases with spontaneously broken C3 symmetry are expected to occur in the quantum Hall regime when triplets of Landau levels associated with three different Fermi surface pockets are partially filled. The broken symmetry state is driven by intravalley Coulombic exchange interactions that favor spontaneously polarized valley occupations. I will discuss three different examples of 2D electron systems in which this type of broken symmetry state is expected to occur: i) the SnTe (111) surface \footnote{Xiao Li, Fan Zhang, and A.H. MacDonald, Phys. Rev. Lett. {\textbf 116}, 026803 (2016).}, ii) the Bi (111) surface \footnote{Benjamin E. Feldman \textit{et al.}, Science, \textbf{354}, 316-321 (2016).}. and iii) unbalanced bilayer\footnote{Xiao Li and A.H. MacDonald, to be published (2017).} graphene. This type of quantum Hall electron nematic state has so far been confirmed only in the Bi (111) case, in which the anisotropic quasiparticle wavefunctions of the broken symmetry state were directly imaged. In the SnTe case the nematic state phase boundary is controlled by a competition between intravalley Coulomb interactions and intervalley scattering processes that increase in relative strength with magnetic field. An in-plane Zeeman field alters the phase diagram by lifting the three-fold Landau level degeneracy, yielding a ground state energy with 2$\pi$/3 periodicity as a function of Zeeman-field orientation angle. I will comment on the possibility of observing similar states in the absence of a magnetic field. [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 9:12AM |
E22.00002: Visualizing Landau Orbits and a Nematic Quantum Hall Liquid with the Scanning Tunneling Microscope Invited Speaker: Ali Yazdani Nematic quantum fluids with wavefunctions that break the underlying crystalline symmetry can form in interacting electronic systems. We examine the quantum Hall states that arise in high magnetic fields from anisotropic hole pockets on the Bi(111) surface. Spectroscopy performed with a scanning tunneling microscope shows that a combination of local strain and many-body Coulomb interactions lift the six-fold Landau level (LL) degeneracy to form three valley-polarized quantum Hall states. We image the resulting anisotropic LL wavefunctions and show that they have a different orientation for each broken-symmetry state. The wavefunctions correspond precisely to those expected from pairs of hole valleys and provide a direct spatial signature of a nematic electronic phase. Reference: Benjamin E. Feldman, Mallika T. Randeria, Andras Gyenis, Fengcheng Wu, Huiwen Ji, R. J. Cava, Allan H. MacDonald, Ali Yazdani, Science 354, 6310 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:48AM |
E22.00003: Quantum Hall Valley Nematics: From Field Theories to Microscopic Models Invited Speaker: Siddharth Parameswaran The interplay between quantum Hall ordering and spontaneously broken ``internal'' symmetries in two-dimensional electron systems with spin or pseudospin degrees of freedom gives rise to a variety of interesting phenomena, including novel phases, phase transitions, and topological excitations. I will discuss a theory of broken-symmetry quantum Hall states, applicable to a class of multivalley systems, where the symmetry at issue is a point-group element that combines a spatial rotation with a permutation of valley indices. I will explore its ramifications for the phase diagram of a variety of experimental systems, such as AlAs and Si quantum wells and the surface states of bismuth. I will also discuss unconventional transport phenomena in these phases in the presence of quenched randomness, and the possible mechanisms of selection between degenerate broken-symmetry phases in clean systems. \newline\newline References: \newline [1] D.A. Abanin, S.A. Parameswaran, S.A. Kivelson and S.L. Sondhi, Phys. Rev. B {\bf 82}, 035428 (2010). \newline [2] A. Kumar, S.A. Parameswaran and S.L. Sondhi, Phys. Rev. B {\bf 88}, 045133 (2013). \newline [3] A. Kumar, S.A. Parameswaran and S.L. Sondhi, Phys. Rev. B. {\bf 93}, 014442 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:24AM |
E22.00004: Valleytronics and Nematicity in bulk bismuth Invited Speaker: Zengwei Zhu In bismuth, a low-carrier-density and high-mobility semi-metal, the three rotationally equivalent electron pockets (valleys) have a Dirac-like dispersion and can be confined to their lowest Landau level with a relatively small magnetic field. A magnetic field rotating in the binary-bisectrix plays the role of a valley valve tuning the contribution of each valley to the total conductivity along trigonal axis [1]. An extensive study of angle-dependent transverse magnetoresistance quantifies the mobility tensor of both electrons and holes [2]. The temperature dependence of mobility indicates that inelastic resistivity is dominated by carrier-carrier scattering. The C3 symmetry of the underlying lattice is suddenly lost at low temperature and high magnetic field [1, 2]. This may be caused by a valley-nematic phase transition [3] driven by the large anisotropy in the effective mass of electrons. By extending the measurements to still higher magnetic fields (far beyond the quantum limit), we found that one (when the field is oriented along the bisectrix axis) or two (when it is along the binary axis) valleys become totally empty. Drying up a Fermi sea leads to a dramatic enhancement in electric conductance. We attribute this enhancement to transfer of carriers between valleys with highly anisotropic mobilites. The non-interacting picture can explain most of the data. However, Coulomb interaction may play a role in shaping the fine details [4]. 1. Z. Zhu et al. Nature Phys 8, 89 (2012) 2. A. Collaudin et al. Phys. Rev. X 5, 021022 (2015) 3. D. A. Abanin et al. , Phys. Rev. B 82, 035428 (2010) 4. Z. Zhu et al. arXiv:1608.06199 (2016) [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 11:00AM |
E22.00005: Suppressed magnetic circular dichroism and valley-polarized magnetoabsorption due to the mass anisotopy in Bi Invited Speaker: Alexey B. Kuzmenko We measure broadband far-infrared magneto-optical conductivity spectra of pure bismuth separately for left- and right-handed circular polarizations in magnetic fields up to 7 T that allows us to obtain the magnetic circular dichroism (MCD). Thanks to a high spectral resolution we distinguish the Landau level (LL) transitions in the Dirac-like electron and the parabolic hole bands. The hole transitions exhibit a full (100\%) MCD as is indeed expected for a circular cyclotron orbit. However, the MCD for electron-pocket transitions is reduced to only 13$\pm$1\%. This strong suppression can be attributed to the huge effective-mass anisotropy ($\sim$ 200) in the electron pockets and can be generally interpreted as a signature of the mismatch between the spatial metric experienced by the photons and the electrons. An important consequence of this observation is that the magneto-absorption in bismuth is highly valley sensitive, which paves the way to future valleytronic applications in materials with a strong effective-mass anisotropy. [1] P.J. de Visser, J. Levallois, M.K. Tran, J.-M. Poumirol, I.O. Nedoliuk, J. Teyssier, C. Uher, D. van der Marel, and A. B. Kuzmenko, Phys. Rev. Lett. \textbf{117}, 017402 (2016). [Preview Abstract] |
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