Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L02: Topological Materials -- Optical and other SpectroscopyFocus
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Sponsoring Units: DMP Chair: Kenneth Burch, Boston College Room: BCEC 107A |
Wednesday, March 6, 2019 11:15AM - 11:51AM |
L02.00001: Magneto-infrared Spectroscopy of Topological Materials Invited Speaker: Zhigang Jiang The topological nature of a material is not only reflected on the surfaces or along the edges but also hidden inside the material in the bulk electronic structure. In this talk, I will describe how the bulk-sensitive magneto-infrared spectroscopy technique can be used to probe the electronic structure topology. I will use transition-metal pentatellurides (ZrTe5 and HfTe5, Dirac semimetals) and monopnictides (NbP, Weyl semimetals) as material examples. For the Dirac semimetals, we find that the observed Landau level transitions are similar to that in graphene but with a finite mass and the Zeeman effect opens the inverted band gap due to the large g-factor in the materials. For the Weyl semimetals, we find that the magnetic field opens a sizable gap at the charge neutrality point (Weyl annihilation) due to the finite coupling between the Weyl points and a new optical transition rule appears when the magnetic field breaks the axial symmetry. For both material systems, a semiquantitative agreement between the experiment and the effective Hamiltonian model calculation is achieved. |
Wednesday, March 6, 2019 11:51AM - 12:03PM |
L02.00002: Quantum oscillation evidence of topological semimetal phase in ZrSnTe Jin Hu, Yanglin Zhu, Zhijie Tang, Zhiqiang Mao, David E Graf, Xin Gui, Weiwei Xie The layered WHM - type (W=Zr/Hf/La, H=Si/Ge/Sn/Sb, M=S/Se/Te) materials represent a large family of topological semimetals, which provides an excellent platform to study the evolution of topological semimetal state with the fine tuning of spin-orbit coupling and structural dimensionality for various combinations of W, H and M elements. In this work, through high field de Haas–van Alphen (dHvA) quantum oscillation studies, we have found evidence for the predicted topological non-trivial bands in ZrSnTe. Furthermore, from the angular dependence of quantum oscillation frequency, we have revealed the three-dimensional Fermi surface topologies of this layered material owing to strong interlayer coupling. |
Wednesday, March 6, 2019 12:03PM - 12:15PM |
L02.00003: Identification of candidate species for intercalation doping of ZrTe5 Normand Modine ZrTe5 is an interesting topological quantum material that is predicted to be a Dirac semimetal at the boundary between weak and strong topological insulator phases. In topological materials, it is highly desirable to control the doping of the material in order to adjust the Fermi level to coincide with features in the band structure. Since ZrTe5 is a layered material, one approach to accomplishing this goal is “intercalation doping," in which a dopant diffuses between the layers of the material. We have used van der Waals-corrected density functional theory to investigate the intercalation of a variety of atoms into ZrTe5 with the goal of identifying promising candidates for intercalation doping. We have calculated bulk absorption energies and diffusion barriers, and where bulk interlayer diffusion rates are reasonably rapid, we have also calculated the corresponding surface properties. We conclude that Li and Pd are promising candidates for intercalation doping of ZrTe5. |
Wednesday, March 6, 2019 12:15PM - 12:27PM |
L02.00004: Measurements of out-of-plane magnetoresistance in ZrSiS, ZrSiSe, and HfSiS microstructures Kent Shirer, Kimberly Modic, Tino Zimmerling, Markus Koenig, Leslie Schoop, Andrew Mackenzie Topological nodal-line semimetals with the general formula XSiY (X = Zr, Hf and Y = S, Se, Te) have recently attracted much experimental and theoretical interest due to their properties, particularly their large magnetoresistances and high carrier mobilities. Due to the platelet-like nature of the XSiY crystals and their extremely low residual resistivities, measurements of the resistivity along the [001] direction are extremely challenging. To accomplish such measurements, microstructures of single crystals were prepared using Focused Ion Beam techniques. Microstructures prepared in this manner have very well-defined geometries and maintain their high crystal quality, verified by the quantum oscillations we observed. We will present magnetoresistance and quantum oscillation data for currents applied along both [001] and [100] in ZrSiS, ZrSiSe, and HfSiS and discuss the role microstructuring can play in the study of these materials. |
Wednesday, March 6, 2019 12:27PM - 12:39PM |
L02.00005: Revealing Optical Transitions and Carrier Dynamics within the Bulk Band Structure of Bi2Se3 Giriraj Jnawali, Samuel M Linser, Iraj Abbasian Shojaei, Seyyedesadaf Pournia, Howard E Jackson, Leigh Smith, Ryan Need, Stephen Wilson Bismuth selenide (Bi2Se3) is a prototypical topological insulator which exhibits gapped states within the bulk and topologically protected conducting states on the surface. Here we use mid-infrared pump-probe spectroscopy on Bi2Se3 nanosheets exfoliated from Bridgeman grown single crystals to map the band-edge electronic structure and interrogate carrier relaxation processes over a wide energy range (0.3 to 1.2 eV). We observe direct optical transitions from spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas followed by slow electron-hole recombination within 150 ps at 10 K and 50 ps at 300 K. Knowledge of electronic structure and interaction of electrons and holes within the bulk band structure provides a foundation for understanding coupling of these states with the protected surface states. |
Wednesday, March 6, 2019 12:39PM - 12:51PM |
L02.00006: The thickness dependent optical nonlinearity of graphene/Bi2Te3 heterojunction Jia Chi Lan, Jun Peng Qiao, Wei-Heng Sung, Chun Hu Chen, Cheng Maw Cheng, Chao-Kuei Lee The graphene like surface state of topological insulators (TIs) have been attracted plenty of attentions. However, ultra-low saturation absorption resulting from the intrinsic bulk state of TIs limts its application, such as pulse laser. In this work, thickness dependent optical nonlinearity of graphene/Bi2Te3 heterojunction saturable absorber is investigated. Unlike the low saturation intensity of Bi2Te3 (with order of tens W/cm2), the saturation intensity of graphene/Bi2Te3 heterojunction increases dramitically at least 4 order of magnitude. In addition, the increasing modulation depth of around 60% is characterized and the high quality 1um Q-switched solid state laser based on graphene/Bi2Te3 heterojunction saturable absorber is accordingly performed. Finally, the possible mechanism of carrier dynamics between p-n heterojunction and guideline for material design are proposed and discussed as well. |
Wednesday, March 6, 2019 12:51PM - 1:03PM |
L02.00007: Measuring the Thermal Conductivity of Semiconductors and Semimetals Using Raman Spectroscopy Iraj Abbasian Shojaei, Giriraj Jnawali, Seyyedesadaf Pournia, Samuel M Linser, Howard E Jackson, Leigh Smith, Ryan Need, Stephen Wilson We present a measurement of the thermal conductivity for Bi2Se3 and Te and semimetals at room temperature by using Raman scattering spectroscopy on a clean prepared sample on a SiO2 substrate. Aluminum is deposited on either side of the flake and is used as a heat sink at room temperature. Using a He/Ne laser (633 nm), we measure the Raman shift of the sample as a function of laser power between 0.1mW and 1mW. The linear red shift of Raman modes due to heating of the sample provides a measure of the temperature of the sample around the incident laser area. We calculate the thermal conductivity of the sample by solving a simplified one dimensional heat diffusion equation, ignoring the heat conduction to the ambient air and substrate. Our results for Bi2Se3 suggest a value of 4 W/mK for the thermal conductivity, comparable to the measurements of others; using the same technique we plan make measurements of several Weyl semimetals (eg/ WTe2 and NbIrTe4). |
Wednesday, March 6, 2019 1:03PM - 1:15PM |
L02.00008: Signature of chiral anomaly in TaP through phonon dynamics Thanh Nguyen, Mingda Li, Fei Han, Nina Andrejevic, Songxue Chi, Jaime Fernandez-Baca, Masaaki Matsuda, Ahmet Alatas, Esen Alp Recent theoretical predictions discussed the possibility of detecting chiral anomaly based on phonon spectra as the exotic electronic degrees of freedom in a Weyl semimetal play an important role to influence the phonon structure. I will discuss measurements made of phonon excitations on a high-quality single crystal sample of type-I Weyl semimetal TaP using inelastic neutron and x-ray scattering spectroscopies. These measurements indicate the presence of phonon softening near the Weyl nodes which is driven by the coupling between the chiral Weyl fermions and phonons. We also observe that this coupling differs between the W1 and W2 Weyl nodes. These results provide a method to extract valuable information about the sought-after electron-phonon coupling strength. |
Wednesday, March 6, 2019 1:15PM - 1:27PM |
L02.00009: Band structure of topological insulators via cyclotron resonance Andrei Pimenov In the quasi-classical approximation the cyclotron mass can be directly connected to the band structure. The analysis based on the cyclotron resonance (CR) is especially useful for thin film materials in which capping layers prevent standard approaches like ARPES. Here, using the CR experiments in the terahertz range, we obtain the band structure of topological insulators (TI) based on mercury telluride (HgTe) quantum wells. In three-dimensional HgTe TI the dispersion of surface carriers is close to parabolic and it dominates the cyclotron signal. In addition, several other features are observed that can be attributed the bulk carriers. In two-dimensional HgTe TI the dispersion of electrons is close to linear showing increasingly complicated character in the region of the hole-like carriers. |
Wednesday, March 6, 2019 1:27PM - 1:39PM |
L02.00010: Nonlinear Optical Effect of Impulsive Stimulated Raman Process in type-II Weyl Semimetal Td-WTe2 Elizabeth Drueke, Junjie Yang, Liuyan Zhao Recent studies into Weyl semimetals with noncentrosymmetric crystal structures have revealed a wide range of second order nonlinear phenomena, including the polarization-selective photogalvanic effect and giant second harmonic generation. We here present a study of another second order nonlinear effect, impulsive stimulated Raman scattering, in the type II Weyl semimetal Td-WTe2. Using ultrafast time-resolved optical reflectivity measurements, we demonstrate a large number of coherent phonons excited through this impulsive stimulated Raman scattering process and identify polarization-dependent selection rules for these phonons based on a symmetry analysis of the crystal structure of the material. Further, we investigate the fluence dependence of the phonons and compare this dependence to fluence-dependent carrier dynamics. |
Wednesday, March 6, 2019 1:39PM - 1:51PM |
L02.00011: Fermi Surfaces of Flat-Band Intermetallic APd3 (A = Pb, Sn) Kaya Wei, Kuan-Wen Chen, Jennifer Neu, You Lai, Greta Chappell, Yan Xin, David E Graf, Luis Balicas, Ryan Baumbach, Theo Siegrist DFT calculations for APd3 (A=Pb, Sn) recently revealed an interesting band structure [1] that includes: a dispersionless branch along the Γ-X line, that could result in a large DOS near EF, and Dirac-like surface states. To test these predictions, we synthesized single crystals of PbPd3 and SnPd3 using the Czochralski growth technique and performed magnetoresistance and torque magnetometry measurements in magnetic fields up to 45 T. Through analysis of quantum oscillations, we uncover Fermi surface topographies that suggest the EF is roughly 50 meV above the flat band for PbPd3. This results in the flat bands only having a weak effect on the bulk properties, while the impact of the Dirac cone at the Γ-point is also limited due to it being several hundred meV above EF. We will discuss detailed results of the Fermi surface topology measurements and prospects for tuning the EF to meet either the flat bands or the Dirac cone. |
Wednesday, March 6, 2019 1:51PM - 2:03PM |
L02.00012: Novel plasmons in quantum anomalous Hall systems Jianhui Zhou, Di Xiao, Yugui Yao, Furu Zhang We find that the inverted band structure with the Mexican-hat dispersion could enhance the interband correlation leading to a strong intrinsic plasmon excitation. Its frequency ranges from several meV to tens of meV and can be effectively tuned by the external fields. The electron-hole asymmetric term splits the peak of the plasmon excitation into double peaks. The fate and properties of this plasmon excitation can also act as a probe to characterize the topological phases even in lightly doped systems. We numerically demonstrate the impact of band inversion on plasmon excitations in magnetically doped thin films of three-dimensional strong topological insulators, which support the quantum anomalous Hall states (QAHE). We also study the chiral edge plasmons in QAHE and find many new and remarkable features of chiral edge plasmons. Our work thus sheds some new light on the potential applications of topological materials in plasmonics. |
Wednesday, March 6, 2019 2:03PM - 2:15PM |
L02.00013: Optical circular dichroism in an antiperovskite Dirac semimetal Katsuhisa Taguchi, Takuto Kawakami, Masatoshi Sato We theoretically study selective circular dichroism in an antiperovskite type Dirac semimetal, which hosts Dirac fermions with high total angular momentum J. We find that unconventional circular dichroism depends on J : In J =1/2 Dirac fermion, nonzero circular dichroism is generated in Jz - J'z = ±1, where Jz and J'z are the z-component of J of the conduction band minimum and the valance band maximum around Dirac points, respectively. It is noted that this ±1 reflects on the spin angular momentum of the polarization of the applied light. On the other hand, in J =3/2 Dirac fermion, the nonzero circular dichroism is triggered in Jz - J'z = ±3. Its origin is shown in this presentation. |
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