Bulletin of the American Physical Society
Mid-Atlantic Section Fall Meeting 2020
Volume 65, Number 20
Friday–Sunday, December 4–6, 2020; Virtual
Session K04: THz Spectroscopy of QM II |
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Chair: Alexander Boris, MPI Sttuttgart |
Sunday, December 6, 2020 11:30AM - 12:06PM |
K04.00001: Duality and domain wall dynamics in a twisted Kitaev chain Invited Speaker: N. Peter Armitage The Ising chain in transverse field is a paradigmatic model for a host of physical phenomena, including spontaneous symmetry breaking, topological defects, quantum criticality, and duality. Although the quasi-1D ferromagnet CoNb$_2$O$_6$ has been put forward as the best material example of the transverse field Ising model, it exhibits significant deviations from ideality. Through a combination of THz spectroscopy and theory, we show that CoNb$_2$O$_6$ in fact is well described by a different model with strong bond dependent interactions, which we dub the {\it twisted Kitaev chain}, as these interactions share a close resemblance to a one-dimensional version of the intensely studied honeycomb Kitaev model. In this model the ferromagnetic ground state of CoNb$_2$O$_6$ arises from the compromise between two distinct alternating axes rather than a single easy axis. Due to this frustration, even at zero applied field domain-wall excitations have quantum motion that is described by the celebrated Su-Schriefer-Heeger model of polyacetylene. This leads to rich behavior as a function of field. Despite the anomalous domain wall dynamics, close to a critical transverse field the twisted Kitaev chain enters a universal regime in the Ising universality class. This is reflected by the observation that the excitation gap in CoNb$_2$O$_6$ in the ferromagnetic regime closes at a rate precisely twice that of the paramagnet. This originates in the duality between domain walls and spin-flips and the topological conservation of domain wall parity. We measure this universal ratio `2' to high accuracy -- the first direct evidence for the Kramers-Wannier duality in nature. [Preview Abstract] |
Sunday, December 6, 2020 12:06PM - 12:42PM |
K04.00002: Linear and Non linear Terahertz Behavior of Topological Insulators Materials. Invited Speaker: Stefano Lupi The great potential of electrons (Dirac and Weyl) with linear energy/momentum dispersion for integrated photonics has been readily recognized after their discovery in graphene. Dirac carriers are also found in Topological Insulators (TI), quantum systems having an insulating gap in the bulk and intrinsic Dirac metallic states at the surface, while Weyl electrons appear in topological semimetals.In this talk, after a brief panoramic of Topological Quantum Materials properties, I will discuss their applications in linear and non-linear optics and plasmonics. In particular, I will review several experiments proving the exotic optical phenomena appearing in topological matter and its potential applications in quantum devices. [Preview Abstract] |
Sunday, December 6, 2020 12:42PM - 1:18PM |
K04.00003: The chiral anomaly in Weyl semimetals as seen by optical probes Invited Speaker: Artem Pronin The chiral anomaly – a characteristic feature of chiral Weyl bands subjected to parallel electric and magnetic fields – is well known for its manifestations in such dc probes as longitudinal magnetoresistance and planar Hall effect. Unfortunately, disentangling the chiral-anomaly contribution from other contributions in transport measurements is rather nontrivial. Reports on detecting the chiral anomaly by other experimental methods are therefore of paramount importance, but remain rare. Theoretically, it has been shown that low-frequency (terahertz or infrared) optical measurements provide a way to observe the chiral anomaly in Weyl semimetals. In the talk, recent results on optical observations of the chiral anomaly in the Weyl semimetals TaAs, NbAs, and GdPtBi are reported. [Preview Abstract] |
Sunday, December 6, 2020 1:18PM - 1:30PM |
K04.00004: Total angular momentum dichroism of the THz vortex beams at the antiferromagnetic resonances Andrei Sirenko, P. Marsik, C. Bernhard, T. N. Stanislavchuk, S.-W. Cheong Circularly polarized light with spin angular momentum (SAM) is one of the most valuable probes of magnetism. Recently we demonstrated that light beams with orbital angular momentum (OAM), or vortex beams, can also couple to magnetism exhibiting dichroisms in a magnetized medium. Terahertz (THz) vortex beams with various combinations of the orbital angular momentum $l=+$/-1, 2, 3, and 4 and spin angular momentum $\sigma \quad = \quad +$/-1, or conventional circular polarization, were used for studies of the magnon spectra at the antiferromagnetic resonance conditions in TbFe$_{\mathrm{3}}$(BO$_{\mathrm{3}})_{\mathrm{4}}$ and Ni$_{\mathrm{3}}$TeO$_{\mathrm{6}}$ single crystals. In both materials we observed strong vortex beam dichroism for the magnon doublet, which is split in an external magnetic field applied along the spin ordering direction. The absorption conditions at the magnon frequencies depend on the total angular momentum of light $j$ that is determined by the combination of the spin and orbital angular momenta: $j=\sigma +l$. For the higher orders of $l, $the selection rules for AFM resonances dictated by $l$ completely dominate over that for conventional circular polarization. Our results demonstrate the high potential of the vortex beams with OAM as a new spectroscopic probe of magnetism in matter. https://arxiv.org/abs/2008.08670 [Preview Abstract] |
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