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In recent years, many researchers have struggled to obtain carbon dots (CDs) that possess strong photoluminescence in the red region of light. Success in this area has been limited, although the past few years have brought several promising reports on this topic. The most successful efforts in this area still seem to struggle from a lack of dispersibility/reduced emission in water. This work endeavors to understand the formation process of CDs that do not possess strong performance in an aqueous environment and to improve their capabilities in bioimaging.

Understanding the complexity of the brain ultimately requires insight into the decoding of local microcircuit functionality by noninvasive approaches. Recently, the new field of Spintronics is attracting a lot of attention with its noninvasive abilities to sense the magnetic field of neurons and to modulate their firing with spintronics devices. The two emerging tools are transcranial magnetic stimulation (TMS) and magnetic encephalography (MEG). The proposed nano-TMS device will use magnetic nanowires—the electromagnetic coils’ nanoscale cousins—to generate focused and programmable magnetic fields. Preliminary theoretical calculations show that proposed devices can provide programmable, focused stimulation for noninvasive neuromodulation of neural microcircuits with unprecedented high spatial and temporal resolutions. The nano-MEG is based on a simple version of the magnetometer capable of imaging the neural connections in the brain. The proposed magnetometer will realize the simple quantum limit (SQL) of the ferromagnetic resonance (FMR) of a “YIG” oscillator and/or spin-torque nano-oscillators (STNO) using a phase-locked loop (PLL) synchronized to a quartz clock. This micro-to-nano-metric technology is comparable with silicon integrated circuits and promises a “laboratory on a chip” approach to MEG that permits millions of detectors to be used. The design is aimed at reducing the massive magnetic screening associated with the usual superconducting quantum interference devices (SQUID) or optically pumped magnetometers (OPM).

Since their discovery, carbon dots (CDs) have been a promising nanomaterial in a variety of fields including nanomedicine. Despite their potential in this area, there are many obstacles to overcome for CDs to be approved for biomedical use. One major hindrance to CDs’ approval is related to their poorly defined structure. Herein a structural study of CDs is presented in order to rectify this shortcoming. The properties of three CDs which have significant promise in biomedical applications, black CDs (B-CDs), carbon nitride dots (CNDs), and yellow CDs (Y-CDs), are compared in order to develop a coherent structural model for each nanosystem. Absorption coefficients were measured for each system and this data gave insight on the level of disorder in each system. Furthermore, extensive structural characterization has been performed in order to derive structural information for each system. This data showed that B-CDs and CNDs are functionalized to a greater degree and are also more disordered and amorphous than Y-CDs. These techniques were used to develop a structural model consistent with the obtained data and what is known for carbonic nanostructures. These models can be used to analyze CD emission properties and to better understand the structure-property relationship in CDs.

FeSn is a topological semimetal (TSM) and kagome antiferromagnet (AFM) composed of alternating Fe3Sn kagome planes and honeycomb Sn planes. This unique structure gives rise to exotic features in the band structures such as the coexistence of Dirac cones and flatbands near the Fermi level, fully spin-polarized 2D surface Dirac fermions, and the ability to open a large gap in the Dirac cone by reorienting the N\'eel vector. In this work, we report the synthesis of high quality epitaxial (0001) FeSn films by magnetron sputtering. Using FeSn/Py heterostructures, we show a large exchange bias effect that reaches an exchange field of 220 Oe at 5 K, providing unambiguous evidence of antiferromagnetism and strong interlayer exchange coupling in our films. Field cycling studies show steep initial training effects, highlighting the complex magnetic interactions and anisotropy. Importantly, our work provides a simple, alternative means to fabricate FeSn films and heterostructures, making it easier to explore the topological physics of AFM TSMs and develop FeSn-based spintronics., Comment: accepted by APL

The topological kagome magnet (TKM) Fe3Sn2 exhibits unusual topological properties, flat electronic bands, and chiral spin textures, making it an exquisite materials platform to explore the interplay between topological band structure, strong electron correlations, and magnetism. Here we report the first synthesis of high-quality epitaxial (0001) Fe3Sn2 films with large intrinsic anomalous Hall effect close to that measured in bulk single crystals. In addition, we measured a large, anisotropic anomalous Nernst coefficient Syx of 1.26 {\mu}V/K, roughly 2-5x greater than that of common ferromagnets, suggesting the presence of Berry curvature sources near the Fermi level in this system. Crucially, the realization of high-quality Fe3Sn2 films opens the door to explore emergent interfacial physics and create novel spintronic devices based on TKMs by interfacing Fe3Sn2 with other quantum materials and by nanostructure patterning., Comment: accepted by Physical Review Materials

Topology and strong electron correlations are crucial ingredients in emerging quantum materials, yet their intersection in experimental systems has been relatively limited to date. Strongly correlated Weyl semimetals, particularly when magnetism is incorporated, offer a unique and fertile platform to explore emergent phenomena in novel topological matter and topological spintronics. The antiferromagnetic Weyl semimetal Mn3Sn exhibits many exotic physical properties such as a large spontaneous Hall effect and has recently attracted intense interest. In this work, we report synthesis of epitaxial Mn3+x Sn1-x films with greatly extended compositional range in comparison with that of bulk samples. As Sn atoms are replaced by magnetic Mn atoms, the Kondo effect, which is a celebrated example of strong correlations, emerges, develops coherence, and induces a hybridization energy gap. The magnetic doping and gap opening lead to rich extraordinary properties, as exemplified by the prominent DC Hall effects and resonance-enhanced terahertz Faraday rotation.

The topological kagome metal CoSn hosts orbital-selective Dirac bands and very flat bands near the Fermi energy that lead to a range of exotic phenomena, such as fractional quantum Hall states. In this work, we report the synthesis of high-quality epitaxial (0001) CoSn films by magnetron sputtering. Comprehensive structural characterizations demonstrate high crystalline quality with low disorder, sharp interfaces, and a smooth surface. Complementary magnetic and transport properties show a paramagnetic, metallic ground state as seen in bulk. Our work creates a synthetic foundation to investigate and utilize rich topological physics in CoSn thin films and heterostructures.

The volume charge density for a conducting ellipsoid is expressed in simple geometrical terms, and then used to obtain the known surface charge density as well as the uniform charge per length along any principal axis. Corresponding results are presented for conducting hyperellipsoids in any number of spatial dimensions. The presentation is at a level suitable for use in graduate courses on electrostatics, as a supplement to more traditional material.

Author(s): Karayev, S; Murray, PD; Khadka, D; Thapaliya, TR; Liu, K; Huang, SX | Abstract: Magnetic multilayer thin films with perpendicular magnetic anisotropy (PMA) and interfacial Dzyaloshinskii-Moriya interaction (iDMI) are of intense interest for realizing magnetic skyrmions and modifying topological spin textures. We systematically investigate interlayer exchange coupling (IEC) in Pt/Co/Ru(Ir) superlattices that have PMA and large iDMI. The IEC is greatly tunable by varying Ru(Ir) or Pt thickness and the antiferromagnetic IEC is as large as 1.3mJ/m2 that is on the same order of magnitude as the iDMI. We find unusual magnetic hysteresis loop crossing between field-ascending and -descending magnetization curves. Furthermore, we identify magnetic phase diagrams for antiferromagnetic IEC and hysteresis loop crossing with respect to Ru(Ir) and Pt thickness. Our experimental findings may open a way in the development of synthetic antiferromagnetic spintronics and/or the realization of antiferromagnetic skyrmions.

We report studies of thermal conductivity as functions of magnetic field and temperature in the helimagnetic insulator ${\mathrm{Cu}}_{2}{\mathrm{OSeO}}_{3}$ that reveal novel features of the spin-phase transitions as probed by magnon heat conduction. The tilted conical spiral and low-temperature skyrmion phases, recently identified in small-angle neutron scattering studies, are clearly identified by sharp signatures in the magnon thermal conductivity. Magnon scattering associated with the presence of domain boundaries in the tilted conical phase and regions of skyrmion and conical-phase coexistence are identified.

Magnetic skyrmions have been the focus of intense research due to their potential applications in ultrahigh-density data and logic technologies, as well as for the unique physics arising from their antisymmetric exchange term and topological protections. In this work we prepare a chiral jammed state in chemically disordered (Fe, Co)Si consisting of a combination of randomly oriented magnetic helices, labyrinth domains, rotationally disordered skyrmion lattices, and/or isolated skyrmions. Using small angle neutron scattering, we demonstrate a symmetry-breaking magnetic field sequence which disentangles the jammed state, resulting in an ordered, oriented skyrmion lattice. The same field sequence was performed on a sample of powdered $\mathrm{C}{\mathrm{u}}_{2}\mathrm{OSe}{\mathrm{O}}_{3}$ and again yields an ordered, oriented skyrmion lattice, despite the relatively noninteracting nature of the grains. Micromagnetic simulations confirm the promotion of a preferred skyrmion lattice orientation after field treatment, independent of the initial configuration, suggesting this effect may be universally applicable. Energetics extracted from the simulations suggests that approaching a magnetic hard axis causes the moments to diverge away from the magnetic field, increasing the Dzyaloshinskii-Moriya energy, followed subsequently by a lattice reorientation. The ability to facilitate an emergent ordered magnetic lattice with long-range orientation in a variety of materials despite overwhelming internal disorder enables the study of skyrmions even in imperfect powdered or polycrystalline systems and greatly improves the ability to rapidly screen candidate skyrmion materials.

The advent of topological insulators (TIs), a novel class of materials that harbor a metallic spin-chiral surface state coexisting with band-insulating bulk, opens up new possibilities for spintronics. One promising route is current-induced switching of an adjacent magnetic layer via spin-orbit torque (SOT), arising from the large spin-orbit coupling intrinsically possessed by TIs. The Kondo insulator SmB6 has been recently proposed to be a strongly correlated TI, supported by the observation of a metallic surface state in bulk SmB6, as evidenced by the thickness independence of the low-temperature resistance plateau. We report the synthesis of epitaxial (001) SmB6/Si thin films and a systematic thickness-dependent electrical transport study. Although the low-temperature resistance plateau is observed for all films from 50 to 500 nm in thickness, the resistance is distinctively thickness-dependent and does not support the notion of surface conduction and interior insulation. On the other hand, we demonstrate that SmB6 can generate a large SOT to switch an adjacent ferromagnetic layer, even at room temperature. The effective SOT generated from SmB6 is comparable to that from β-W, one of the strongest SOT materials.

Low-temperature thermal conductivity measurements in the ferrimagnetic insulator Cu${}_{2}$OSeO${}_{3}$ reveal an unprecedentedly large magnonic contribution below T~10K, far exceeding (by nearly two orders of magnitude) that observed previously in any other ferromagnet or ferrimagnet. Features predicted by theory more than 50 years ago are identified, including ballistic behavior with ${\ensuremath{\kappa}}_{m}\ensuremath{\propto}{T}^{2}$, and, possibly, Poiseuille flow, wherein the magnon mean-free path exceeds the specimen dimensions as momentum conserving scattering occurs more frequently than scattering by resistive processes.

Bulk B20 magnets exhibit an exotic skyrmion phase in a narrow phase region at a temperature close to Curie temperature within a magnetic field. The investigation and exploitation of these exotic phases, and especially potential devices, require thin films. In this chapter, we discuss the epitaxial growth, the unusual magnetic and transport properties, with the strong thickness dependence of the B20 films. We have also realized an extended skyrmion phase in thin films.

We compute the exterior Green function for a grounded equi-potential circular ring in two-dimensional electrostatics by treating the system geometrically as a "squashed wormhole" with an image charge located in a novel but obvious position, thereby implementing a method first suggested in 1897 by Sommerfeld. We compare and contrast the strength and location of the image charge in the wormhole picture with that of the conventional point of view where an image charge is located inside the circular ring. While the two viewpoints give mathematically equivalent Green functions, we believe they provide strikingly different physics perspectives. We also comment on earlier Green function results by Hobson in 1900, and by Davis and Reitz in 1971, who applied Sommerfeld's method to analyze a grounded conducting circular disk in three-dimensional electrostatics., Comment: 13 figures

The interfacial Dzyaloshinskii–Moriya interaction (iDMI) in asymmetric magnetic multilayer films has displayed increasingly important roles in the modification of domain walls, stabilization of Skyrmions, and realization of new topological spin textures such as magnetic radial vortices. Unlike magnetization and magnetic anisotropy which can be readily measured, iDMI is difficult to measure. In this work, we measured the iDMI in Pt/Co/Ir and Pt/Co/Ru multilayer films by exploring the spin-orbit torque induced effective field under an in-plane bias magnetic field. Skyrmions have been reported to exist in Pt/Co/Ir multilayers. We found that Pt/Co/Ru multilayers have a similar magnitude of the iDMI for Pt/Co/Ir multilayers, suggesting that Pt/Co/Ru is a good candidate to host Skyrmions.

Driven by the advances in the plasma display panel (PDP) technique and Hg-free fluorescent lamps, the demand on high efficient vacuum ultraviolet (VUV) phosphors has increased dramatically in the past decade. To search for high efficient VUV phosphors in a variety of materials systems using the combinatorial approach, we have set up a scanning multi-inkjet delivery system for the solution-based combinatorial synthesis of libraries and a parallel screening apparatus for VUV photoluminescence based on a home designed cathode gas (Xe/He or Xe/Ne) discharge lamp. We demonstrated the applicability of the combinatorial strategy in high efficient VUV phosphors by fabricating a series of RE doped borophosphate phosphors libraries, characterizing their performance on VUV photoluminescence.

We demonstrate a self-consistent and versatile method to determine the relative spin Hall angle ${\ensuremath{\theta}}_{\text{SH}}$ and spin diffusion length ${\ensuremath{\lambda}}_{\text{SF}}$ of selected $5d$ metals (Pt, Au, Ta, and W) by thermal spin injection from a common ferromagnetic insulator, yttrium iron garnet, into thin metal films of various thicknesses. The values of ${\ensuremath{\theta}}_{\text{SH}}$ for Pt and Au are opposite in sign to those of Ta and W. The inverse spin Hall effect voltage depends on spin Hall resistivity, which is the product of ${\ensuremath{\theta}}_{\text{SH}}$ and electrical resistivity. We also show evidence of magnetic proximity effects including magnetoresistance and the anomalous Hall effect in these $5d$ metals, except for Au.

The B20 chiral magnets with broken inversion symmetry and C4 rotation symmetry have attracted much attention. The broken inversion symmetry leads to the Dzyaloshinskii–Moriya that gives rise to the helical and Skyrmion states.Wereport the unusual magnetoresistance (MR) of B20 chiral magnet Fe0.85Co0.15Si that directly reveals the broken C4 rotation symmetry and shows the anisotropic scattering by Skyrmions with respect to the current directions. The intimacy between unusual MR and broken symmetry is well confirmed by theoretically studying an effective Hamiltonian with spin–orbit coupling. In conclusion, the unusual MR serves as a transport signature for the Skyrmion phase.

The Skyrmion state in epitaxial B20 FeGe(111) thin films, determined by the topological Hall effect, is greatly extended in the phase diagram to cover all temperatures up to the Curie temperature ${T}_{C}\ensuremath{\approx}271\text{ }\text{ }\mathrm{K}$ and over a wide magnetic field range that includes a zero magnetic field. The properties of the Skyrmion phase can be controlled and manipulated by the film thickness, which has a strong effect on the stabilization of Skyrmions.

The pressure effect on the resistivity and superconducting Tc of prestrained thin films of the iron chalcogenide superconductor FeSe0.5Te0.5 is studied. Films with different anion heights above the Fe layer showing different values of ambient pressure Tc's are compressed up to a pressure of 1.7 GPa. All films exhibit a significant increase of Tc with pressure. The results cannot solely be explained by a pressure-induced decrease of the anion height but other parameters have to be considered to explain the data for all films., 4 pages, 3 figures

We present a systematic investigation of the additional resistance $({R}_{E})$, which is an unavoidable consequence of pseudo-four-probe electrical measurements, on the point-contact Andreev reflection (PCAR) spectrum by both modeling and experiments. Instead of considering the total resistance between the two voltage leads across a point contact as a sum of a contact resistance $({R}_{C})$ and a fixed sample resistance $({R}_{S})$, it is essential to treat the total resistance as a sum of the Andreev resistance ${R}_{\text{AR}}$ and the additional resistance ${R}_{E}$, which are, respectively, the resistances affected and unaffected by the Andreev reflection process. We show a detailed formalism of taking ${R}_{E}$ into account in modeling and demonstrate that the PCAR spectrum can be drastically affected by the presence of ${R}_{E}$. Experimentally, we have found that not only ${R}_{E}$ cannot be readily measured or even estimated, it is in fact different for each contact, depending on the contact resistance and whether the contact is near the purely ballistic regime or the purely diffusive regime. A self-consistent process is necessary to analyze the entire PCAR spectrum, properly normalize the conductance, determine ${R}_{E}$, and other parameters including the spin polarization and the superconducting gap for each contact. We determine ${R}_{E}$ for various contacts on specimens with different resistivity and resolve the causes of ${R}_{E}$. For contacts close to the diffusive regime, there are two sources of ${R}_{E}$: a dominant contribution which is linearly proportional to the total resistance and a constant value from the sample resistance. We also address the effects of additional resistance when PCAR is administered in the ballistic limit and in the diffusive limit. With the proper treatment of the additional resistance, we demonstrate that PCAR can quantitatively extract essential information of spin polarization and superconducting gap.

We demonstrate a close relationship between superconductivity and the dimensions of the Fe-Se(Te) tetrahedron in FeSe0.5Te0.5. This is done by exploiting thin film epitaxy, which provides controlled biaxial stress, both compressive and tensile, to distort the tetrahedron. The Se/Te height within the tetrahedron is found to be of crucial importance to superconductivity, in agreement with the scenario that (π, π) spin fluctuations promote superconductivity in Fe superconductors.

Spin-transfer torques (STT) provides a new mechanism to alter the magnetic configurations in magnetic heterostructures, a feat previously only achieved by an external magnetic field. A current flowing perpendicular through a magnetic noncollinear spin structure can induce torques on the magnetization, depending on the polarity of the current. This is because an electron carries angular momentum , or spin, part of which can be transf erred to the magnetic layer as a torque. A spin-polarized current of a substantial current density (e.g., 10 8 A/cm 2 ) is required to observe the effect of the spin transfer torques. Consequently, switching by spin-polarized currents is often realized in small structures with sub-micron cross sections made by nanolithography. Here we demonstrate spin transfer torque effects using point-contact spin injection involving no lithography. In a continuous Co/Cu/Co trila yer, we have observed hysteretic reversal of sub-100 nm magnetic elements by spin injection through a metal tip both at low temperature and at room temperature. A small magnetic domain underneath the tip in the top Co layer can be manipulated to align parallel or anti-parallel to the bottom Co layer with a unique bias voltage. In an exchange-biased single ferromagnetic layer, we have observed a new form of STT effect which is the inverse effect of domain wall magnetore sistance effect rather than gi ant magnetoresistance effect. We further show that in granular solids, the STT effect that can be exploited to in duce a large spin diso rder when combined with a large magnetic field. As a result, we have obtained a spectacular MR effect in ex cess of 400%, the largest ever reported in any metallic systems.

The superconducting gap in FeAs-based superconductor SmFeAs(O1-xFx) (x = 0.15 and 0.30) and the temperature dependence of the sample with x = 0.15 have been measured by Andreev reflection spectroscopy. The intrinsic superconducting gap is independent of contacts while many other "gap-like" features vary appreciably for different contacts. The determined gap value of 2D = 13.34 +/-0.47 meV for SmFeAs(O0.85F0.15) gives 2D/kBTC = 3.68, close to the BCS prediction of 3.53. The superconducting gap decreases with temperature and vanishes at TC, in a manner similar to the BCS behavior but dramatically different from that of the nodal pseudogap behavior in cuprate superconductors., Comment: 13 pages, 9 figures, Special Issue of Physica C on Superconducting Pnictides

Spin-transfer torques (STT) provide a mechanism to alter the magnetic configurations of magnetic heterostructures, a result previously only achieved by an external magnetic field. In granular solids, we demonstrate a new form of STT effect that can be exploited to induce a large spin disorder when combined with a large magnetic field. We have obtained a very large magnetoresistance effect in excess of 400% at 4.2 K in a large magnetic field, the largest ever reported in any metallic systems. The STT characteristics of granular solids differ significantly from those of multilayers, showing no STT effect at low magnetic fields but prominent STT effects at high fields.

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The tunneling magnetoresistance and perpendicular magnetic anisotropy in CoFeB(1.1-1.2 nm)/MgO/CoFeB(1.2-1.7 nm) junctions were found to be very sensitively dependent on annealing time. During annealing at a given temperature, decay of magnetoresistance occurs much earlier compared to junctions with in-plane magnetic anisotropy. Through a rapid thermal annealing study, the decrease of magnetoresistance is found to be associated with the degradation of perpendicular anisotropy, instead of impurity diffusion as observed in common in-plane junctions. The origin of the evolution of perpendicular anisotropy as well as possible means to further enhance tunneling magnetoresistance is discussed.

Point-contact Andreev reflection measurements reveal that amorphous CoxFe80−xB20 (x=20, 40, and 60) alloys possess spin polarization of as much as 65%, much higher than the values of 43%–45% for Co and Fe. This accounts for the high magnetoresistance values in magnetic tunnel junctions incorporating amorphous CoFeB as the ferromagnetic electrodes. The crystallization of the amorphous alloys substantially reduces the spin polarization.

Temperature dependent magnetotransport measurements were performed on polycrystalline and epitaxial chromium dioxide (CrO2) nanowires fabricated using the selective-area growth technique. Polycrystalline nanowires showed a negative temperature coefficient of resistivity at low temperatures because of strong grain boundary scattering. The magnetoresistance (MR) value exhibited a width dependence, reaching a maximum of 20% for a 150nm wide wire. In contrast, the MR response of single crystal CrO2 wires was mainly determined by magnetocrystalline and shape anisotropy.