Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R10: Optics in Topological SemimetalsFocus
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Sponsoring Units: DMP Chair: David Tanner, Univ of Florida - Gainesville Room: LACC 301B |
Thursday, March 8, 2018 8:00AM - 8:36AM |
R10.00001: Quantization and enhancement of non-linear responses in topological matter Invited Speaker: Adolfo Grushin In this talk I will discuss three examples of nonlinear responses of topological matter, which can be quantized and/or remarkably large. The first and simplest is a quantized circular dichroism in Chern insulators: it can be probed by heating and it is related to an absorbed power sum rule. It does not rely on translational invariance, and its therefore applicable to fractional quantum Hall states. The second concerns the circular photogalvanic effect, the part of the photocurrent which changes sign when the light's polarization flips. In Weyl semimetals it is quantized in units of e^3/h^2 times the Weyl monopole charge, with no material-dependent parameters. It is predicted to be obsevable in mirror free Weyl semimetals (e.g. SrSi2) and three-dimensional Rashba materials (e.g. doped Te) and its magnitude is relatively large, which enables the direct detection of the monopole charge with current techniques. Lastly, I will discuss the recently observed giant and anisotropic second harmonic generation found in TaAs. In particular I will provide evidence that a class of ferroelectric materials have an upper bound for non-linear effects, and that TaAs is close to saturating it for a particular frequency range. |
Thursday, March 8, 2018 8:36AM - 8:48AM |
R10.00002: Large, Room Temperature Photocurrents in WSM TaAs Gavin Osterhoudt, Laura Katharina-Diebel, Ryan Conrad, Kai Kharpertian, Bing Shen, Ni Ni, Philip Moll, Kenneth Burch Numerous predictions exist for novel photocurrent generation related to the novel topology of inversion breaking Weyl semi-metals. We present here the first observation of room temperature, polarization controlled, photocurrents in mesoscale TaAs devices. From their qualitatively different polarization and frequency dependencies we are able to disentangle these from more mundane thermal effects. The magnitude of the photocurrents is consistent with reports of an intrinsically large second order optical response in TaAs, due to the unique band topology. Furthermore, these results suggest TaAs could be a promising material for optoelectronic and photovoltaic applications. |
Thursday, March 8, 2018 8:48AM - 9:00AM |
R10.00003: Berry Curvature Dipole in Weyl Semimetal Materials Yang Zhang, Yan Sun, Binghai Yan In the band structure of a Weyl semimetal (WSM), the conduction and valence bands cross each linearly through Weyl points that are usually treated as “monopoles” of the Berry curvature. As a second-order response, WSMs were very recently demonstrated to show strong nonlinear optical effects including an exotic nonlinear Hall effect. This is caused by the non-equilibrium distribution of the Berry curvature, described as the “dipole” of the Berry curvature. In this talk, I will talk about our recent ab initio results on nonlinear response for representative WSM materials TaAs and MoTe2. |
Thursday, March 8, 2018 9:00AM - 9:12AM |
R10.00004: Resonance enhanced nonlinear optical conductivity in Weyl semimetal TaAs Shreyas Patankar, Baozhu Lu, Manita Rai, Jason Tran, Liang Wu, Takahiro Morimoto, Adolfo Grushin, Nityan Nair, James Analytis, Joel Moore, Darius Torchinsky, Joseph Orenstein Weyl semimetals are a class of materials with nontrivial band structure geometry, and have recently been shown to host a variety of exotic nonlinear optical properties, including the largest measured second-order optical susceptibility χ(2). Here we report the measurement of the spectrum of nonlinear optical susceptibility χ(2)(ω) in the visible and near-infrared spectral range from photon energies of 0.5eV to 1.5eV. We observe that χ(2)(ω) has a sharp peak in amplitude at 0.7eV, and that the peak value is over four orders of magnitude higher than that of a standard nonlinear optical semiconductor, GaAs. The spectral lineshape of χ(2)(ω) is consistent with the prediction from a theoretical model of optical nonlinearity in 1D Rice-Mele chains and the magnitude of the frequency-integrated response saturates a universal bound that emerges from the model. |
Thursday, March 8, 2018 9:12AM - 9:24AM |
R10.00005: Strong Photothermoelectric Response and Contact Reactivity of the Dirac Semimetal ZrTe5 Francois Leonard, Wenlong Yu, Kimberley Collins, Douglas Medlin, Joshua Sugar, Albert Talin, Wei Pan The family of three-dimensional topological insulators opens new avenues to discover novel photophysics and to develop novel types of photodetectors. ZrTe5 has been shown to be a Dirac semimetal possessing unique topological electronic and optical properties. Here we present spatially-resolved photocurrent measurements on devices made of nanoplatelets of ZrTe5, demonstrating the photothermoelectric origin of the photoresponse. Due to the high electrical conductivity and good Seebeck coefficient, we obtain noise-equivalent powers as low as 42 pW/Hz1/2 at room temperature for visible light illumination at zero bias. We also show that these devices suffer from significant ambient reactivity such as the formation of a Te-rich surface region driven by Zr oxidation, as well as severe reactions with the metal contacts. This reactivity results in significant stresses in the devices, leading to unusual geometries that are useful for gaining insight into the photocurrent mechanisms. Our results indicate that both the large photothermoelectric response and reactivity must be considered when designing or interpreting photocurrent measurements in these systems. |
Thursday, March 8, 2018 9:24AM - 9:36AM |
R10.00006: Chiral pumping at THz frequencies in Weyl semimetal TaAs Antonio Levy, Andrei Sushkov, Fengguang Liu, Howard Drew, Bing Shen, Ni Ni, Gregory Jenkins An optical conductivity enhancement is observed at THz frequencies with polarization parallel to an |
Thursday, March 8, 2018 9:36AM - 9:48AM |
R10.00007: Spatially Dispersive Circular Photogalvanic Effect in Weyl Semimetals Zhurun Ji, Zachariah Addison, Gerui Liu, Wenjing Liu, Peng Yu, Zheng Liu, Andrew Rappe, Charles Kane, Eugene Mele, Ritesh Agarwal We observe a circulating photocurrent in type-II Weyl semimetals MoTe2/Mo0.9W0.1Te2, driven by circularly polarized light at normal incidence. This effect occurs exclusively in the Weyl phase and crucially we find that it is associated with a spatially inhomogeneous beam profile. We assign this response to spatially dispersive contribution to the circular photogalvanic effect (s-CPGE). We systematically analyze the microscopic mechanism of this photocurrent by calculating the quantum density matrix to first order in spatial gradients and second order in the electric field. We find that the observed s-CPGE is dominated by terms in the response function that are non-zero only in the inversion broken Weyl phase. We demonstrate our finding by numerical calculations of its frequency dependence using a minimal model of an inversion broken Weyl semimetal and use the calculation to find low and high frequency scaling behavior of the photocurrent. |
Thursday, March 8, 2018 9:48AM - 10:00AM |
R10.00008: Directly photoexcited Dirac fermions in ZrSiS and NbAs, and their femtosecond dynamics Christopher Weber, Jaehun Kim, Ernest Arushanov, Alex Nateprov, Leslie Schoop, Bettina Lotsch, Stuart S Parkin, Mazhar Ali Despite the strong current interest in Dirac and Weyl semimetals, their ultrafast response is still little studied. Nearly all experiments to date have photoexcited the materials with high energy (few-eV) photons, and relied on relaxation of high-energy carriers to populate the Dirac cone. Here we use pulses of mid-IR photons to photoexcite electrons and holes directly into the Dirac cone, and we follow their subsequent dynamics through the transient reflectivity. We compare the dynamics as a function of the pump and probe photons’ energies. For ZrSiS, the pump always photoexcites electrons within the Dirac line node, where the optical conductivity is flat. This flatness results in a simple response, consisting only in a sub-picosecond increase (spike) in the reflectivity and depending little on pump energy. However, for NbAs the pump wavelengths used straddle 2EF. When pumping with photons below 2EF, only Drude absorption takes place, resulting in a several-picosecond decrease in the reflectivity. However, when the pump energy exceeds 2EF, intraband absorption excites Dirac fermions. Then the reflectivity gains a sub-picosecond spike, as in ZrSiS. We therefore can identify the sub-picosecond spike in reflectivity as the signature of Dirac fermions, in both materials. |
Thursday, March 8, 2018 10:00AM - 10:12AM |
R10.00009: Time-resolved terahertz magneto-optical study of the Weyl semimetal TaAs M. Mehdi Jadidi, Martin Mittendorff, Yigit Aytac, Jacob König-Otto, Bing Shen, Andrei Sushkov, Ni Ni, Stephan Winnerl, Thomas E. Murphy, H. Dennis Drew Weyl semimetals host pairs of singly-degenerate electronic states with linear energy dispersion and opposite chirality that are separated in momentum space. For a typical Weyl semimetal, linear bands and electron pockets with opposite chirality exist only at low energies (<100 meV). This makes the terahertz frequency regime exceptionally attractive for exploring novel optical/magneto-optical effects predicted for Weyl semimetals. |
Thursday, March 8, 2018 10:12AM - 10:24AM |
R10.00010: Probing the interaction between coupled Weyl points via magneto-infrared spectroscopy. Yuxuan Jiang, Zhiling Dun, Seongphill Moon, Haidong Zhou, Dmitry Smirnov, Zhigang Jiang Weyl semimetals (WSMs) are a newly discovered quantum state of matter with topological protection characterized by a pair of Weyl points (WPs) with linear dispersion in all directions. When the bands of two WPs overlap, their interaction can significantly change the energy dispersion, resulting in unique responses in WSM. Here, we present a combined theoretical and experimental study of Niobium Phosphide (NbP), an established WSM. Our magneto-infrared reflectance measurements show that the Landau levels in NbP are strongly modified from an isolated WP picture. By considering the interaction between two WPs, our model can reach a semi-quantitative agreement with the experiment. Our result signifies the crucial role of the interaction effect in understanding the rich physical phenomena in WSM. We also discuss other unique implications of this interaction picture, which could be used to distinguish a WSM from a Dirac semimetal. |
Thursday, March 8, 2018 10:24AM - 10:36AM |
R10.00011: Landau level spectroscopy of NbP in Voigt configuration Seongphill Moon, Yuxuan Jiang, Zhiling Dun, Jennifer Neu, Theo Siegrist, Haidong Zhou, Zhigang Jiang, Dmitry Smirnov Niobium phosphide, NbP, is a member of the recently found noncentrosymmetric Weyl semimetal family with 12 Weyl point (WP) pairs with two different type of Weyl points in the first Brillouin zone. When magnetic field is applied along the c axis, all the Weyl pairs are perpendicular to the magnetic field and split the energy bands of all the pairs in a similar way. Experimentally, this case can be readily addressed using infrared magneto-spectroscopy in Faraday geometry. However, when the field is oriented along the a or b axis, the magnetic field is parallel to half of the Weyl pairs and perpendicular to the other half. Here we present magneto-infrared reflection measurements of NbP in the Voigt geometry with B field parallel to the a or b axis. The observed complex structure of inter Landau level transitions will be compared to the calculated dispersion of Landau levels [1] and the measured data in the Faraday geometry. |
Thursday, March 8, 2018 10:36AM - 10:48AM |
R10.00012: Magneto-polaritons in Weyl semimetals Alexey Belyanin, Zhongqu Long, Yongrui Wang, Maria Erukhimova, Mikhail Tokman Exotic topological and transport properties of Weyl semimetals generated a lot of excitement in the condensed matter community. Here we show that Weyl semimetals in a strong magnetic field are highly unusual optical materials. The hybridization of magnetoplasmons with electromagnetic waves leads to fascinating optical phenomena involving magnetopolaritons: hyperbolic dispersion, the absence of Landau damping for strongly localized excitations, photonic stop bands, coupling-induced transparency, efficient polarization conversion, and pulse compression, to name a few. We show that optical spectroscopic techniques provide a ``clean'' way of detecting properties of low-energy electron states and in particular the chiral anomaly. Moreover, Weyl semimetals show strong promise for future photonic chips enabling a wide array of broadband optoelectronic applications, such as polarizers, modulators, switches, and pulse shapers for mid-infrared through terahertz wavelengths. |
Thursday, March 8, 2018 10:48AM - 11:00AM |
R10.00013: Electrodynamics on Fermi Cyclides in Nodal Line Semimetals Seongjin Ahn, Eugene Mele, Hongki Min We study the frequency-dependent conductivity of nodal line semimetals (NLSMs), focusing on the effects of carrier density and energy dispersion on the nodal line. We find that the low-frequency conductivity has a rich spectral structure which can be understood using scaling rules derived from the geometry of their Dupin cyclide Fermi surfaces. We identify different frequency regimes, find scaling rules for the optical conductivity in each, and demonstrate them with numerical calculations of the inter- and intraband contributions to the optical conductivity using a low-energy model for a generic NLSM. [Phys. Rev. Lett. 119, 147402 (2017)] |
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