Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T52: Invited Session: Dirac Physics in Bulk Semimetals |
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Sponsoring Units: DCMP Chair: Kamran Behnia, ESPCI ParisTech Room: Grand Ballroom C2 |
Thursday, March 5, 2015 11:15AM - 11:51AM |
T52.00001: Spin Hall effect and Landau spectrum of Dirac electrons in bismuth Invited Speaker: Yuki Fuseya Bismuth has played an important role in solid-state physics. Many key phenomena were first discovered in bismuth, such as diamagnetism, Seebeck, Nernst, Shubnikov-de Haas, and de Haas-van Alphen effects. These phenomena result from particular electronic states of bismuth. The strong spin-orbit interaction ($\sim$ 1.5eV) causes strong spin-dependent interband couplings resulting in an anomalous spin magnetic moment. We investigate the spin Hall effect and the angular dependent Landau spectrum of bismuth paying special attention to the effect of the anomalous spin magnetic moment. It is shown that the spin Hall insulator is possible and there is a fundamental relationship between the spin Hall conductivity and orbital diamagnetism in the insulating state of the Dirac electrons. Based on this theoretical finding, the magnitude of spin Hall conductivity is estimated for bismuth by that of orbital susceptibility. The magnitude of spin Hall conductivity turns out to be as large as 10$^{4} \Omega^{-1}$cm$^{-1}$, which is about 100 times larger than that of Pt. It is also shown that the ratio of the Zeeman splitting to the cyclotron energy, which reflects the effect of crystalline spin-orbit interaction, for holes at the $T$-point can be larger than 1.0 (the maximum of previous theories) and exhibit strong angular dependence, which gives a possible solution to the long-standing mystery of holes at the $T$-point. \\[4pt] In collaboration with Masao Ogata, Hidetoshi Fukuyama, Zengwei Zhu, Beno\^it Fauqu\'{e}, Woun Kang, and Kamran Behnia. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:27PM |
T52.00002: Thermodynamic evidence for a valley-dependent density of states in bulk bismuth Invited Speaker: Lucia Steinke A moderate magnetic field confines both hole-like and electron-like carriers of semi-metallic bismuth to their lowest Landau levels. In contrast to holes, which are ordinary quasi-particles, electrons in bismuth are described by the Dirac Hamiltonian with a band mass becoming a thousandth of the bare electron mass along one crystalline axis. These Dirac electrons can occupy each of the three rotationally equivalent elongated ellipsoids of the Fermi surface. The valley degeneracy offers electrons an additional degree of freedom, a subject of recent attention. Here, we employ magnetostriction to map the angle-resolved Landau spectrum and quantify the number of electrons in each valley for a magnetic field slightly tilted off the trigonal axis. Unlike transport measurements, magnetostriction provides a thermodynamic probe that directly couples to the density of states: resonances in the magnetostriction coefficient can be linked to the evacuation of Landau levels with increasing field, and electron and hole spectra are distinguished by tracing the angle dependence of the magnetostriction peaks. We find that while the electron valleys remain identical in their spectrum, they substantially differ in their density of states at the Fermi level, with an $\approx$ 20\% difference in the peak height for two different electron valleys. This experimental observation establishes that, even in the absence of internal strain, the electron fluid does not keep the rotational symmetry of the lattice, at low temperature and high magnetic field. The valley imbalance is found to be restricted to electrons in the immediate vicinity of the Fermi level. This effect, reminiscent of Coulomb pseudo-gap in localized electronic state, emerges as the most striking departure from the non-interacting picture of electrons in bulk bismuth in the vicinity of the quantum limit. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 1:03PM |
T52.00003: Ultrahigh mobility and giant magnetoresistance in the Dirac Semimetals Cd$_{3}$As$_{2}$ and Na$_{3}$Bi Invited Speaker: N. Phuan Ong Dirac semimetals and Weyl semimetals are 3D analogs of graphene in which crystalline symmetry protects the nodes against gap formation. Na$_{3}$Bi and Cd$_{3}$As$_{2}$ were predicted to be Dirac semimetals, and recently confirmed to be so by photoemission. Several novel transport properties in a magnetic field H have been proposed for Dirac semimetals. Here we report an interesting property in Cd$_{3}$As$_{2}$ that was unpredicted, namely a remarkable protection mechanism that strongly suppresses back-scattering in zero H. In single crystals, the protection results in a very high mobility, 10$^{7}$ cm$^{2}$/Vs at 5 K. Suppression of backscattering results in a transport lifetime 10$^{4} $longer than the quantum lifetime. The lifting of this protection by H leads to very large magnetoresistance with a striking H-linear profile. I will also report transport results on Na$_{3}$Bi and compare them with results in Cd$_{3}$As$_{2}$. I discuss how this may relate to changes to the Fermi surface induced by H.\\[4pt] Coauthors: Tian Liang, Jun Xiong, Quinn Gibson, Minhao Liu, Satya Kushwaha, Jason Krizan Maz Ali, and R. J. Cava [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:39PM |
T52.00004: Valley-Polarized Interlayer Conduction of Anisotropic Dirac Fermions in SrMnBi$_{2}$ Invited Speaker: Jun Sung Kim We report anisotropic Dirac fermions in a Bi square net of SrMnBi$_{2}$ and their valley-selective interlayer conduction under in-plane magnetic fields. In contrast to the commonly-observed isotropic Dirac Fermi surfaces, the Dirac Fermi surface in SrMnBi$_{2}$ is highly anisotropic with strong momentum-dependence of Fermi velocity as well as interlayer coupling. The resulting $c$-axis resistivity exhibits clear angular magnetoresistance oscillations indicating coherent interlayer conduction. Strong fourfold variation of the coherent peak in the $c$-axis resistivity reveals that the contribution of each Dirac valley is significantly modulated by the in-plane field orientation. Furthermore, we found a signature of broken valley symmetry at high magnetic fields. These findings demonstrate that a quasi-two-dimensional anisotropic Dirac system can host a valley-polarized interlayer current through magnetic valley control. [Preview Abstract] |
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