Coherent Spin-Phonon Coupling in the Layered Ferrimagnet Mn3Si2Te6

We utilize ultrafast photoexcitation to drive coherent lattice oscillations in the layered ferrimagnetic crystal Mn3Si2Te6, which significantly stiffen below the magnetic ordering temperature. We suggest that this is due to an exchange-mediated contraction of the lattice, stemming from strong magneto-structural coupling in this material. Additionally, simulations of the transient incoherent dynamics reveal the importance of spin relaxation channels mediated by optical and acoustic phonon scattering. Our findings highlight the importance of spin-lattice coupling in van der Waals magnets and a promising route for their dynamic optical control through their intertwined electronic, lattice, and spin degrees of freedom

Surface Effects on Anisotropic Photoluminescence in One-Dimensional Organic Metal Halide Hybrids

One-dimensional (1D) organic metal halide hybrids exhibit strongly anisotropic optical properties, highly efficient light emission, and large Stokes shift, holding promises for novel photodetection and lighting applications. However, the fundamental mechanisms governing their unique optical properties and in particular the impacts of surface effects are not understood. Here, we investigate 1D C4N2H14PbBr4 by polarization-dependent time-averaged and time-resolved photoluminescence (TRPL) spectroscopy, as a function of photoexcitation energy. Surprisingly, we find that the emission under photoexcitation polarized parallel to the 1D metal halide chains can be either stronger or weaker than that under perpendicular polarization, depending on the excitation energy. We attribute the excitation-energy-dependent anisotropic emission to fast surface recombination, supported by first-principles calculations of optical absorption in this material. The fast surface recombination is directly confirmed by TRPL measurements, when the excitation is polarized parallel to the chains. Our comprehensive studies provide a more complete picture for a deeper understanding of the optical anisotropy in 1D organic metal halide hybrids

Correlated Excitonic Signatures in a Nanoscale van der Waals Antiferromagnet

Composite quasi-particles with emergent functionalities in spintronic and quantum information science can be realized in correlated materials due to entangled charge, spin, orbital, and lattice degrees of freedom. Here we show that by reducing the lateral dimension of correlated antiferromagnet NiPS3 flakes to tens of nanometers, we can switch-off the bulk spin-orbit entangled exciton in the near-infrared (1.47 eV) and activate visible-range (1.8 to 2.2 eV) transitions with charge-transfer character. These ultra-sharp lines (<120 ueV at 4.2 K) share the spin-correlated nature of the bulk exciton by displaying a Neel temperature dependent linear polarization. Furthermore, exciton photoluminescence lineshape analysis reveals a polaronic character via coupling with at-least 3 phonon modes and a comb-like Stark effect through discretization of charges in each layer. These findings augment the knowledge on the many-body nature of excitonic quasi-particles in correlated antiferromagnets and also establish the nanoscale platform as promising for maturing integrated magneto-optic devices

Sulfurization Engineering of One-Step Low-Temperature MoS2 and WS2 Thin Films for Memristor Device Applications

2D materials have been of considerable interest as new materials for device applications. Non-volatile resistive switching applications of MoS2 and WS2 have been previously demonstrated; however, these applications are dramatically limited by high temperatures and extended times needed for the large-area synthesis of 2D materials on crystalline substrates. The experimental results demonstrate a one-step sulfurization method to synthesize MoS2 and WS2 at 550 °C in 15 min on sapphire wafers. Furthermore, a large area transfer of the synthesized thin films to SiO2/Si substrates is achieved. Following this, MoS2 and WS2 memristors are fabricated that exhibit stable non-volatile switching and a satisfactory large on/off current ratio (103–105) with good uniformity. Tuning the sulfurization parameters (temperature and metal precursor thickness) is found to be a straightforward and effective strategy to improve the performance of the memristors. The demonstration of large-scale MoS2 and WS2 memristors with a one-step low-temperature sulfurization method with simple strategy to tuning can lead to potential applications such as flexible memory and neuromorphic computing.This research was primarily supported by the National Science Foundation through the Center for Dynamics and Control of Materials: an NSF MRSEC under Cooperative Agreement No. DMR-1720595. The work was partly done at the Texas Nanofabrication Facility supported by NSF grant NNCI-2025227. This work was performed in part at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is managed by Triad National Security, LLC for the U.S. Department of Energy’s NNSA, under contract 89233218CNA000001.Center for Dynamics and Control of Material

Thermal Properties of Graphene, Carbon Nanotubes and Nanostructured Carbon Materials

Recent years witnessed a rapid growth of interest of scientific and engineering communities to thermal properties of materials. Carbon allotropes and derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range - of over five orders of magnitude - from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. I review thermal and thermoelectric properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. A special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe prospects of applications of graphene and carbon materials for thermal management of electronics.Comment: Review Paper; 37 manuscript pages; 4 figures and 2 boxe

Anomalous Heat Conduction and Anomalous Diffusion in Low Dimensional Nanoscale Systems

Thermal transport is an important energy transfer process in nature. Phonon is the major energy carrier for heat in semiconductor and dielectric materials. In analogy to Ohm's law for electrical conductivity, Fourier's law is a fundamental rule of heat transfer in solids. It states that the thermal conductivity is independent of sample scale and geometry. Although Fourier's law has received great success in describing macroscopic thermal transport in the past two hundreds years, its validity in low dimensional systems is still an open question. Here we give a brief review of the recent developments in experimental, theoretical and numerical studies of heat transport in low dimensional systems, include lattice models, nanowires, nanotubes and graphenes. We will demonstrate that the phonon transports in low dimensional systems super-diffusively, which leads to a size dependent thermal conductivity. In other words, Fourier's law is breakdown in low dimensional structures

Giant Mechano-Optoelectronic Effect in an Atomically Thin Semiconductor.

Transition metal dichalcogenides (TMDs) are particularly sensitive to mechanical strain because they are capable of experiencing high atomic displacements without nucleating defects to release excess energy. Being promising for photonic applications, it has been shown that as certain phases of layered TMDs MX2 (M = Mo or W; X = S, Se, or Te) are scaled to a thickness of one monolayer, the photoluminescence response is dramatically enhanced due to the emergence of a direct electronic band gap compared with their multilayer or bulk counterparts, which typically exhibit indirect band gaps. Recently, mechanical strain has also been predicted to enable direct excitonic recombination in these materials, in which large changes in the photoluminescence response will occur during an indirect-to-direct band gap transition brought on by elastic tensile strain. Here, we demonstrate an enhancement of 2 orders of magnitude in the photoluminescence emission intensity in uniaxially strained single crystalline WSe2 bilayers. Through a theoretical model that includes experimentally relevant system conditions, we determine this amplification to arise from a significant increase in direct excitonic recombination. Adding confidence to the high levels of elastic strain achieved in this report, we observe strain-independent, mode-dependent Grüneisen parameters over the entire range of tensile strain (1-3.59%), which were obtained as 1.149 ± 0.027, 0.307 ± 0.061, and 0.357 ± 0.103 for the E2g, A1g, and A21g optical phonon modes, respectively. These results can inform the predictive strain-engineered design of other atomically thin indirect semiconductors, in which a decrease in out-of-plane bonding strength may lead to an increase in the strength of strain-coupled optoelectronic effects

Locally defined quantum emission from epitaxial few-layer tungsten diselenide

Recently, single photons have been observed emanating from point defects in two-dimensional (2D) materials including WSe2, WS2, hexagonal-BN, and GaSe, with their energy residing in the direct electronic bandgap. Here, we report single photon emission from a nominal weakly emitting indirect bandgap 2D material through deterministic strain induced localization. A method is demonstrated to create highly spatially localized and spectrally well-separated defect emission sites in the 750-800nm regime in a continuous epitaxial film of few-layer WSe2 synthesized by a multistep diffusion-mediated gas source chemical vapor deposition technique. To separate the effects of mechanical strain from the substrate or dielectric-environment induced changes in the electronic structure, we created arrays of large isotropically etched ultrasharp silicon dioxide tips with spatial dimensions on the order of 10 mu m. We use bending based on the small radius of these tips-on the order of 4nm-to impart electronic localization effects through morphology alone, as the WSe2 film experiences a uniform SiO2 dielectric environment in the device geometry chosen for this investigation. When the continuous WSe2 film was transferred onto an array of SiO2 tips, an similar to 87% yield of localized emission sites on the tips was observed. The outcomes of this report provide fundamental guidelines for the integration of beyond-lab-scale quantum materials into photonic device architectures for all-optical quantum information applications.U. S. National Science Foundation [CAREER-1553987, REU-1560098]; FEI Company Graduate Fellowship [2018AU0058]; Laboratory Directed Research and Development Program of Los Alamos National Laboratory [20190516ECR]; Los Alamos National Laboratory; U. S. Department of Energy's NNSA [89233218CNA000001, 2DCC-MIP]; NSF [DMR-1539916]; Air Force Office of Scientific Research [FA9550-15RYCOR159]Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]