Bulletin of the American Physical Society
Mid-Atlantic Section Fall Meeting 2020
Volume 65, Number 20
Friday–Sunday, December 4–6, 2020; Virtual
Session J04: THz Spectroscopy of QM I |
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Chair: Pavel Volkov, Rutgers University |
Sunday, December 6, 2020 9:00AM - 9:36AM |
J04.00001: Infrared spectroscopy of collective excitations in quasi-one-dimensional Ta$_2$NiSe$_5$ and Ta$_2$NiS$_5$ Invited Speaker: Alexander Boris Cooper pairing of fermions is one of the most fundamental and successful concepts of condensed matter physics. In analogy to the superconducting state generated by pairing of electrons in metals, condensation of neutral electron-hole pairs in semimetals or narrow-band semiconductors has been predicted to generate a coherent quantum excitonic insulator state. There has been continued effort to look for an excitonic insulator in real materials. \\ Using spectroscopic ellipsometry, we have studied collective excitation modes in the quasi-one-dimensional chalcogenide Ta$_2$NiSe$_5$, which exhibits a structural phase transition that has been attributed to exciton condensation, and in the isostructural Ta$_2$NiS$_5$ which does not exhibit such a transition [1-3]. We conclude that in Ta$_2$NiS$_5$ the exciton-phonon complexes are sparse and do not develop a long-range order. This local violation of symmetry allows the excitations to have a nonzero energy, which we see in the dielectric function as a soft localized excitation. In Ta$_2$NiSe$_5$, on the other hand, the giant spectral weight of the exciton Fano resonances due to antenna emission of large exciton-phonon complexes implies that they are strongly overlapping and probably span the entire crystal, hence preserving the translational symmetry of the lattice. Due to this symmetry the internal excitations of the exciton-polaron aggregates may support propagating low-energy excitations akin to acoustic phonon modes. The observed difference reflects the different nature of the ground states of these two compounds, where extended and overlapping exciton-phonon complexes in Ta$_2$NiSe$_5$ lead to a lowering of the crystal symmetry, whereas sparse and localized complexes in Ta$_2$NiS$_5$ cause local distortions, which is in good agreement with the excitonic insulator hypothesis in the former, but not in the latter. \\ [1] Y.F. Lu {\it et al.}, Nature Commun. {\bf 8}, 14408 (2017) [2] T.I. Larkin {\it et al.}, Phys. Rev. B {\bf 95}, 195144 (2017) [3] T.I. Larkin {\it et al.}, Phys. Rev. B {\bf 98}, 125113 (2018) [Preview Abstract] |
Sunday, December 6, 2020 9:36AM - 9:48AM |
J04.00002: Dynamically Tunable Single-layer VO2/metasurface-based broadband THz Cross-Polarization Converter Zizwe Chase, Riad Yahiaoui, Chan Kyaw, Thomas Searles, Eric Seabron, Jay Mathews Numerically, we demonstrate a single-layer THz metadevice, comprised of a 2D array of split ring resonators each with a vanadium oxide (VO2) pad integrated into one of the two capacitive gaps of a unit cell, which exhibits cross-polarization transmission. As the conductivity of VO2 increases, the amplitude of the cross-polarization intensity decreases but maintains a wider broadband range than previously reported for single layered hybrid metamaterial (MM) devices as the VO2 transforms from the insulator to metallic phase. The asymmetric transmission, optically modulated by the device, is higher than that of multi-layered MM devices. Due to the materials properties of VO2, our results introduce a promising method that allows for an active sub-cycle dynamic tenability for THz polarization conversion with multiple modalities using optical, electrical or thermal switching. [Preview Abstract] |
Sunday, December 6, 2020 9:48AM - 10:24AM |
J04.00003: Real-time observation of dynamic modulations over a ferro-rotational charge density wave Invited Speaker: Liuyan Zhao Dynamic control over phases of matter with electromagnetic (EM) radiation receives increasing popularity in recent years because of both its ultrafast time scale and its access to thermodynamically unapproachable phenomena. Till now, the realization of dynamic manipulation of unconventional orders, such as a high-rank multipolar order, awaits to be explored largely because the coupling between multipolar orders and EM fields is nonlinear and nontrivial. In this talk, using the commensurate charge density wave (CCDW) in 1T-TaS2 as the archetype, we demonstrate the dynamic control over the ferro-rotational order, the antisymmetric components of the second-rank electric quadrupolar order. We first confirm the ferro-rotational nature of CCDW in 1T-TaS2, broken mirrors but preserved inversion, by performing temperature-dependent rotation anisotropy-second harmonic generation (RA-SHG). We then track in real time the dynamic modulation of this ferro-rotational CCDW order, using time-resolved-RA-SHG (tr-RA-SHG) that adopts the optical-pump, RA-SHG-probe scheme. We find that this ultrafast modulation manifests itself as the breathing and the rotation of RA-SHG patterns at three different frequencies in the neighborhood of the previously reported CCDW amplitude mode frequency, with the mode of the highest (lowest) frequency primarily in the breathing (rotation) channel and the middle one in both channels. We further reveal a sudden shift of these three frequencies and a dramatic increase in the breathing and rotation magnitudes across a critical pump fluence of \textasciitilde 0.5 mJ/cm2, by performing fluence dependent tr-RA-SHG. [Preview Abstract] |
Sunday, December 6, 2020 10:24AM - 11:00AM |
J04.00004: Nonlinear terahertz emission spectroscopy of topological chiral multifold semimetals Invited Speaker: Liang Wu The absence of mirror symmetry, or chirality, is behind striking natural phenomena found in systems as diverse as DNA and crystalline solids. A remarkable example occurs when chiral semimetals with topologically protected band degeneracies are illuminated with circularly polarized light. Under the right conditions, the part of the generated photocurrent that switches sign upon reversal of the light's polarization, known as the circular photogalvanic effect (CPGE), is predicted to depend only on fundamental constants. The conditions to observe quantization are non-universal, and depend on material parameters and the incident frequency. In my talk, I will discuss nonlinear terahertz emission spectroscopy with tunable photon energy from 0.2 eV - 1.1 eV in the chiral topological semimetals CoSi [1,2] and RhSi[3]. Particularly, we identify a large longitudinal photocurrent peaked at 0.4 eV reaching $\sim$ 550 $\mu A/V^{2}$ in CoSi, which is much larger than the photocurrent in any chiral crystal reported in the literature. Using first-principles calculations we establish that the peak originates from topological band crossings, reaching 3.3$\pm$0.3 in units of the quantization constant. Our calculations indicate that the quantized CPGE is within reach in CoSi upon doping and increase of the hot-carrier lifetime. References:[1]Ni, et al. arXiv:2006.09612. [2]Xu, et al. PNAS (2020) https://doi.org/10.1073/pnas.2010752117. [3] Ni, et al. arXiv:2005.13473 npj Quantum Materials (2020) [Preview Abstract] |
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