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Creators/Authors contains: "Sankar, Raman"

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  1. Abstract

    Novel anti‐ambipolar transistors (AATs) are gate tunable rectifiers with a marked potential for multi‐valued logic circuits. In this work, the optoelectronic applications of AATs in cryogenic conditions are studied, of which the AAT devices consist of vertically stackedp‐SnS andn‐MoSe2nanoflakes to form a type‐II staggered band alignment. An electrostatically tunable p‐SnS/n‐MoSe2cryo‐phototransistor is presented with unique anti‐ambipolar characteristics and cryogenic‐enhanced optoelectronic performance. The cryo‐phototransistor exhibits a sharp and highly symmetric anti‐ambipolar transfer curve at 77 K with the peak‐to‐valley ratio of 103operating under a low bias voltage of 1 V. The high cooling‐enhanced charge mobilities in the cryo‐phototransistor grant this AAT device remarkable photodetection capabilities. At 77 K, thep‐SnS/n‐MoSe2cryo‐phototransistor, holding a broad photoresponse in the spectral range of 250−900 nm, demonstrates its high responsivity of 2 × 104 A W−1and detectivity of 7.5 × 1013 Jones with the excitation at 532 nm. The high‐performancep‐SnS/n‐MoSe2low‐dimensional phototransistor with low operating voltages at 77−150 K is eligible for optoelectronic applications in cryogenic environments. Furthermore, the cryo‐characteristics of this heterostructure can be further extended to design the mul‐tivalued logic circuits operated in cryogenic conditions.

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  2. Using polarized optical and magneto-optical spectroscopy, we have demonstrated universal aspects of electrodynamics associated with Dirac nodal lines that are found in several classes of unconventional intermetallic compounds. We investigated anisotropic electrodynamics ofNbAs2where the spin-orbit coupling (SOC) triggers energy gaps along the nodal lines. These gaps manifest as sharp steps in the optical conductivity spectraσ1(ω). This behavior is followed by the linear power-law scaling ofσ1(ω)at higher frequencies, consistent with our theoretical analysis for dispersive Dirac nodal lines. Magneto-optics data affirm the dominant role of nodal lines in the electrodynamics ofNbAs2.

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  3. Abstract

    A long‐standing pursuit in materials science is to identify suitable magnetic semiconductors for integrated information storage, processing, and transfer. Van der Waals magnets have brought forth new material candidates for this purpose. Recently, sharp exciton resonances in antiferromagnet NiPS3have been reported to correlate with magnetic order, that is, the exciton photoluminescence intensity diminishes above the Néel temperature. Here, it is found that the polarization of maximal exciton emission rotates locally, revealing three possible spin chain directions. This discovery establishes a new understanding of the antiferromagnet order hidden in previous neutron scattering and optical experiments. Furthermore, defect‐bound states are suggested as an alternative exciton formation mechanism that has yet to be explored in NiPS3. The supporting evidence includes chemical analysis, excitation power, and thickness dependent photoluminescence and first‐principles calculations. This mechanism for exciton formation is also consistent with the presence of strong phonon side bands. This study shows that anisotropic exciton photoluminescence can be used to read out local spin chain directions in antiferromagnets and realize multi‐functional devices via spin‐photon transduction.

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  4. Abstract

    Optical response of crystalline solids is to a large extent driven by excitations that promote electrons among individual bands. This allows one to apply optical and magneto‐optical methods to determine experimentally the energy band gap —a fundamental property crucial to our understanding of any solid—with a great precision. Here it is shown that such conventional methods, applied with great success to many materials in the past, do not work in topological Dirac semimetals with a dispersive nodal line. There, the optically deduced band gap depends on how the magnetic field is oriented with respect to the crystal axes. Such highly unusual behavior is explained in terms of band‐gap renormalization driven by Lorentz boosts which results from the Lorentz‐covariant form of the Dirac Hamiltonian relevant for the nodal line at low energies.

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