Abstract-A Compact Broadband Terahertz Range Quarter-Wave Plate

Liang Wu, A. Farid, N. J. Laurita, T. Mueller, N. P. Armitage

https://link.springer.com/article/10.1007/s10762-020-00686-2

We detail the design and characterization of a terahertz range achromatic quarter-wave plate based on a stack of aligned variable thickness birefringent sapphire discs. The disc thicknesses and relative rotations of the discs are determined through a basin-hopping Monte Carlo thermal annealing routine. The basin-hopping scheme allows an improved refinement of the required thicknesses and rotations to give a predicted phase error from the ideal π/2 of only 0.5%, which is a factor of approximately 6 better than previous efforts. Moreover, the large contrast between extraordinary and ordinary axes of sapphire allows us to greatly decrease the overall optical path length of our wave plate design by approximately a factor of 10 over similar designs based on quartz discs. However, this very same contrast requires very precise tolerances in the milled thicknesses of the discs and their assembly. We detail a method to compensate for differences in the thickness from their calculated ideal values. We have constructed one of our designs and found it similar in performance to other configurations, but with our much more compact geometry.

Abstract-Room-temperature terahertz anomalous Hall effect in Weyl antiferromagnet Mn3Sn thin films

• Takuya Matsuda, 
• Natsuki Kanda, 
• Tomoya Higo, 
• N. P. Armitage, 
• Satoru Nakatsuji & 
• Ryusuke Matsunaga 

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   Crystal and magnetic structures of Mn3Sn and the THz longetudinal conductivity spectra.

https://www.nature.com/articles/s41467-020-14690-6

Antiferromagnetic spin motion at terahertz (THz) frequencies attracts growing interests for fast spintronics, however, their smaller responses to external field inhibit device application. Recently the noncollinear antiferromagnet Mn3Sn, a Weyl semimetal candidate, was reported to show large anomalous Hall effect (AHE) at room temperature comparable to ferromagnets. Dynamical aspect of such large responses is an important issue to be clarified for future THz data processing. Here the THz anomalous Hall conductivity in Mn3Sn thin films is investigated by polarization-resolved spectroscopy. Large anomalous Hall conductivity Reσxy(ω)∼20Ω−1cm−1$\mathrm{R}\mathrm{e}{\sigma }_{xy}\left(\omega \right)\sim 20{\mathrm{\Omega }}^{-1}{\mathrm{c}\mathrm{m}}^{-1}$ at THz frequencies is clearly observed as polarization rotation. A peculiar temperature dependence corresponding to the breaking/recovery of symmetry in the spin texture is also discussed. Observation of the THz AHE at room temperature demonstrates the ultrafast readout for the antiferromagnetic spintronics using Mn3Sn, and will also open new avenue for studying nonequilibrium dynamics in Weyl antiferromagnets.

Abstract-Electric field modulated topological magnetoelectric effect in Bi 2 Se 3

Mintu Mondal, Dipanjan Chaudhuri, Maryam Salehi, Cheng Wan, N. J. Laurita, Bing Cheng, Andreas V. Stier, Michael A. Quintero, Jisoo Moon, Deepti Jain, Pavel P. Shibayev, James R. Neilson, Seongshik Oh, and N. P. Armitage

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.121106

Topological insulators have been predicted to exhibit a variety of interesting phenomena including a quantized magnetoelectric response and novel spintronics effects due to spin textures on their surfaces. However, experimental observation of these phenomena has proved difficult due to the finite bulk carrier density which may overwhelm the intrinsic topological responses that are expressed at the surface. Here, we demonstrate an ionic gel gating technique to tune the chemical potential of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$thin films while simultaneously performing THz spectroscopy. We can tune the carrier concentration by an order of magnitude and shift the Fermi energy $E_F$ to as low as $\simeq 10$ meV above the Dirac point. At high-bias voltages and magnetic fields, we observe a quantized Faraday angle consistent with the topological magnetoelectric effect that can be tuned by ionic gel gating through a number of plateau states.

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Abstract-Nonvolatile Solid-State Charged-Polymer Gating of Topological Insulators into the Topological Insulating Regime

R. M. Ireland, Liang Wu, M. Salehi, S. Oh, N. P. Armitage, and H. E. Katz

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.9.044003

We demonstrate the ability to reduce the carrier concentration of thin films of the topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by utilizing a nonvolatile electrostatic gating via corona charging of electret polymers. Sufficient electric field can be imparted to a polymer-TI bilayer to result in significant electron density depletion, even without the continuous connection of a gate electrode or the chemical modification of the TI. We show that the Fermi level of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is shifted toward the Dirac point with this method. Using terahertz spectroscopy, we find that the surface chemical potential is lowered into the bulk band gap (approximately 50 meV above the Dirac point and 170 meV below the conduction-band minimum), and it is stabilized in the intrinsic regime while enhancing electron mobility. The mobility of surface state electrons is enhanced to a value as high as approximately $1600{\mathrm{cm}}^2/Vs$ at 5 K.

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Abstract-A compact broadband terahertz range quarter-wave plate

Liang Wu, A. Farid, N. J. Laurita, T. Mueller, N. P. Armitage

https://arxiv.org/abs/1712.08229

We detail the design and characterization of a terahertz range achromatic quarter-wave plate based on a stack of aligned variable thickness birefringent sapphire discs. The disc thicknesses and relative rotations of the discs are determined through a basin-hopping Monte Carlo thermal annealing routine. The basin-hopping scheme allows an improved refinement of the required thicknesses and rotations to give a predicted phase error from the ideal π/2 of only 0.5%, which is a factor of approximately 6 better than previous efforts. Moreover, the large contrast between extraordinary and ordinary axes of sapphire allow us to greatly decrease the overall optical path length of our wave plate design by approximately a factor of 10 over similar designs based on quartz discs. However, this very same contrast requires very precise tolerances in the milled thicknesses of the discs and their assembly. We detail a method to compensate for differences in the thickness from their calculated ideal values. We have constructed one of our designs and found it similar in performance to other configurations, but with our much more compact geometry.

Abstract- Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator

 Liang Wu, M. Salehi, N. Koirala, J. Moon, S. Oh, N. P. Armitage

http://esciencenews.com/sources/science.now/2016/12/02/report.quantized.faraday.and.kerr.rotation.and.axion.electrodynamics.a.3d.topological.insulator

Topological insulators have been proposed to be best characterized as bulk magnetoelectric materials that show response functions quantized in terms of fundamental physical constants. Here, we lower the chemical potential of three-dimensional (3D) Bi2Se3 films to ~30 meV above the Dirac point and probe their low-energy electrodynamic response in the presence of magnetic fields with high-precision time-domain terahertz polarimetry. For fields higher than 5 tesla, we observed quantized Faraday and Kerr rotations, whereas the dc transport is still semiclassical. A nontrivial Berry’s phase offset to these values gives evidence for axion electrodynamics and the topological magnetoelectric effect. The time structure used in these measurements allows a direct measure of the fine-structure constant based on a topological invariant of a solid-state system

Abstract-Anomalous gap-edge dissipation in disordered superconductors on the brink of localization

Bing Cheng, Liang Wu, N. J. Laurita, Harkirat Singh, Madhavi Chand, Pratap Raychaudhuri, and N. P. Armitage

https://journals.aps.org/prb/accepted/6307cY98Z8e1bc50801465e805c29452f5d55bb8f

Superconductivity in disordered systems close to an incipient localization transition has been an area of investigation for many years, but many fundamentally important aspects are still not understood. It has been noted that in such highly disordered superconductors, anomalous spectral weight develops in their conductivity near and below the superconducting gap energy. In this work we investigate the low frequency conductivity in disordered superconducting NbN thin films close to the localization transition with time-domain terahertz spectroscopy. In the normal state, strong deviations from the Drude form due to incipient localization are found. In the superconducting state we find substantial spectral weight at frequencies well below the superconducting gap scale derived from tunneling. We analyze this spectral weight in the context of a model of disorder induced broadening of the quasiparticle density of states. We find that aspects of the optical and tunneling data can be consistently modeled in terms of this effect of mesoscopic disorder, showing that in this disorder and frequency range, quasiparticle effects and not collective modes are the source of low energy absorption. Interestingly, we also find that as a function of frequency the optical conductivity recovers to the normal state value much faster than any model predicts. This points to the non-trivial interplay of superconductivity and disorder close to localization.

Abstract-Low-energy electrodynamics of novel spin excitations in the quantum spin ice Yb2Ti2O7

• • 
    
• LiDong Pan,
• Se Kwon Kim,
• A. Ghosh,
• Christopher M. Morris,
• Kate A. Ross,
• Edwin Kermarrec,
• Bruce D. Gaulin,
• S. M. Koohpayeh,
• Oleg Tchernyshyov
• & N. P. Armitage

• Affiliations
• Contributions
• Corresponding author

Nature Communications

 

5,

 

Article number:

 

4970

 

doi:10.1038/ncomms5970

Received

 

04 April 2014 

Accepted

 

12 August 2014 

Published

 

18 September 2014

http://www.nature.com/ncomms/2014/140918/ncomms5970/full/ncomms5970.html

In condensed matter systems, formation of long-range order (LRO) is often accompanied by new excitations. However, in many geometrically frustrated magnetic systems, conventional LRO is suppressed, while non-trivial spin correlations are still observed. A natural question to ask is then what is the nature of the excitations in this highly correlated state without broken symmetry? Frequently, applying a symmetry breaking field stabilizes excitations whose properties reflect certain aspects of the anomalous state without LRO. Here we report a THz spectroscopy study of novel excitations in quantum spin ice Yb2Ti2O7 under a <001> directed magnetic field. At large positive fields, both right- and left-handed magnon and two-magnon-like excitations are observed. The g-factors of these excitations are dramatically enhanced in the low-field limit, showing a crossover of these states into features consistent with the quantum string-like excitations proposed to exist in quantum spin ice in small <001> fields.

Abstract-Polarization modulation time-domain terahertz polarimetry

http://arxiv.org/abs/1203.5019

C. M. Morris, R. Valdés Aguilar, A. V. Stier, N. P. Armitage

(Submitted on 22 Mar 2012)

We present high precision measurements of polarization rotations in the frequency range from 0.1 to 2.5 THz using a polarization modulation technique. A motorized stage rotates a polarizer at ~80 Hz, and the resulting modulation of the polarization is measured by a lock-in technique. We achieve an accuracy of 0.05{\deg} (900 {\mu}rad) and a precision of 0.02{\deg} (350 {\mu}rad) for small rotation angles. A detailed mathematical description of the technique is presented, showing its ability to fully characterize elliptical polarizations from 0.1 to 2.5 THz.

Comments:	15 pages, 3 figures, submitted to Optics Express
Subjects:	Optics (physics.optics); Materials Science (cond-mat.mtrl-sci); Strongly Correlated Electrons (cond-mat.str-el); Instrumentation and Detectors (physics.ins-det)
Cite as:	arXiv:1203.5019v1 [physics.optics]