Phase transition from anatase to rutile for the 70nm TiO2 crystallite has been investigated by annealing at different temperatures followed by TDPAC measurement of these TiO2 crystallites adsorbed with 181Hf tracer. The width of the peaks in XRD spectra for TiO2 crystallites annealed at different temperatures indicates the growth of the crystallites with temperature. The samples annealed upto 823K for 4hrs showed no phase transition, except the growth of the crystallites. However, it showed phase transition at the same temperature (823K), when annealed for longer duration. Annealing for shorter duration at 1123K showed phase transition. 181Hf tracer adsorbed on 70 nm anatase TiO2 is found to be in different polymorphs (anatase & rutile) formed during annealing process. Surface to bulk mass-transfer is found to play a significant role in the phase transition process.
Hyperfine interactions, magnetic interactions between the spins of electrons and nuclei, in graphene and related carbon nanostructures are studied. By using a combination of accurate first principles calculations on graphene fragments and statistical analysis, I show that both isotropic and dipolar hyperfine interactions can be accurately described in terms of the local electron spin distribution and atomic structure. A complete set of parameters describing the hyperfine interactions of C-13 and other nuclear spins at substitution impurities and edge terminations is determined.
A theory of electron spin relaxation in semiconducting carbon nanotubes is developed based on the hyperfine interaction with disordered nuclei spins I=1/2 of $^{13}$C isotopes. It is shown that strong radial confinement of electrons enhances the electron-nuclear overlap and subsequently electron spin relaxation (via the hyperfine interaction) in the carbon nanotubes. The analysis also reveals an unusual temperature dependence of longitudinal (spin-flip) and transversal (dephasing) relaxation times: the relaxation becomes weaker with the increasing temperature as a consequence of the particular…
We investigate the optical control possibilities of spin-valley qubit carried by single electrons localized in nanostructures of monolayer TMDs, including small quantum dots formed by lateral heterojunction and charged impurities. The quantum controls are discussed when the confinement induces valley hybridization and when the valley hybridization is absent. We show that the bulk valley and spin optical selection rules can be inherited in different forms in the two scenarios, both of which allow the definition of spin-valley qubit with desired optical controllability. We also investigate nucle…
We present a density functional theory analysis of nitrogen-vacancy (NV) centers in diamond which are located in the vicinity of extended defects, namely intrinsic stacking faults (ISF), extrinsic stacking faults (ESF), and coherent twin boundaries (CTB) on \{111\} planes in diamond crystals. Several sites for NV centers close to the extended defects are energetically preferred with respect to the bulk crystal. This indicates that NV centers may be enriched at extended defects. We report the hyperfine structure (HFS) and zero-field splitting (ZFS) parameters of the NV centers at the extended d…
We implemented a nuclear magnetic resonance (NMR) study on rubrene(C42H28)-Co nano-composites that exhibit an enhanced magnetoresistance (MR) ratio of 80%. The 59Co NMR spin echo experiment enabled clarification of the hyperfine field of Co at the interface between the ferromagnet and the molecules, which has not been investigated for molecular spintronics. An enhanced hyperfine field of the Co was observed in the rubrene-Co nano-composites, which may be related to the enhancement of the MR ratio. This study demonstrates the importance of microscopic investigation of the interface between mole…
We present first-principles calculations of the magnetic hyperfine fields H of 5sp impurities on the (001), (111), and (110) surfaces of Ni. We examine the dependence of H on the coordination number by placing the impurity in the surfaces, on top of them at the adatom positions, and in the bulk. We find a strong coordination dependence of H, different and characteristic for each impurity. The behavior is explained in terms of the on-site s-p hybridization as the symmetry is reduced at the surface. Our results are in agreement with recent experimental findings.
We present results on structural and magnetic properties of highly crystalline alpha-Fe2O3 nanoparticles of average size ~200 nm, synthesized from a novel sol-gel method using metal alkoxide precursor. These particles are multi-domain, showing the weak ferromagnetic-antiferromagnetic (WF-AF) transition (i.e., the Morin transition) at T_M = 256(2) K. Mössbauer measurements revealed a jump in hyperfine parameters at T ~ T_M, which also displays thermal hysteresis upon cooling or heating the sample. The analysis of hyperfine parameters as a function of temperature allowed us to discard temperatur…
Fine and Hyperfine Interactions with Multi-level Spin Relaxation of the purified Giese-Salt in Veterinary Medicine: Prussian Blue Compound Ammonium-Ferric-Hexacyano-Ferrate
University of Veterinary Medicine Hannover Foundation（汉诺威兽医大学基金会）；Leibniz University Hannover（汉诺威莱布尼茨大学）；Technische Universität Berlin（柏林工业大学）；Technische Universität Dresden（德累斯顿工业大学）；TU Bergakademie Freiberg（弗赖贝格矿业技术大学）
A method for the observation of the Knight shift in nanometer-scale region in semiconductors is developed using resistively detected nuclear magnetic resonance (RDNMR) technique in quantum Hall edge channels. Using a gate-induced decoupling of the hyperfine interaction between electron and nuclear spins, we obtain the RDNMR spectra with or without the electron-nuclear spin coupling. By a comparison of these two spectra, the values of the Knight shift can be given for the nuclear spins polarized dynamically in the region between the relevant edge channels in a single two-dimensional electron sy…
We study the hyperfine interaction between the nuclear spins and the electrons in a HgTe quantum well, which is the prime experimentally realized example of a two-dimensional topological insulator. The hyperfine interaction is a naturally present, internal source of broken time-reversal symmetry from the point of view of the electrons. The HgTe quantum well is described by the so-called Bernevig-Hughes-Zhang (BHZ) model. The basis states of the BHZ model are combinations of both S- and P-like symmetry states, which means that three kinds of hyperfine interactions play a role: (i) The Fermi con…
The effects of host isotope mass on the hyperfine interaction of group-V donors in silicon are revealed by pulsed electron nuclear double resonance (ENDOR) spectroscopy of isotopically engineered Si single crystals. Each of the hyperfine-split P-31, As-75, Sb-121, Sb-123, and Bi-209 ENDOR lines splits further into multiple components, whose relative intensities accurately match the statistical likelihood of the nine possible average Si masses in the four nearest-neighbor sites due to random occupation by the three stable isotopes Si-28, Si-29, and Si-30. Further investigation with P-31 donors …
We present a density functional theory analysis of the negatively charged nitrogen-vacancy (NV$^-$) defect complex in diamond located in the vicinity of (111)- or (100)-oriented surfaces with mixed (N,H)-terminations. We assess the stability and electronic properties of the NV$^-$ center and study their dependence on the H:N ratio of the surface termination. The formation energy, the electronic density of states, the hyperfine structure and zero-field splitting parameters of an NV$^-$ center are analyzed as function of its distance and orientation to the surface. We find stable NV$^-$ centers …
We report the synthesis of a bulk nanostructured alloy using the laser-powder bed fusion process. The equiatomic AlCoCrFeMnNi chemically complex alloy forms a nanoscale modulated structure, which is homogeneously distributed in the as-built condition. The nanostructure consisted of Al & Ni-rich ordered (B2) and Cr & Fe-rich disordered (A2) BCC phases. The two phases form an interconnected phase-network with coherent interface boundaries. Atom-probe-tomography and aberration-corrected scanning transmission electron microscopy analysis of the spatial distribution of the modulated structure sugge…
We have studied the role of Ga 3d states in determining the properties of bulk as well as nanoparticles of GaN using PAW potentials. A significant contribution of the Ga d states in the valence band is found to arise from the interaction of Ga 4d states with the dominantly N p states making up the valence band. The errors arising from not treating the Ga 3d states as a part of the valence are found to be similar, ~ 1%, for bulk as well as for nanoclusters of GaN.
Hyperfine driven spin relaxation of charge carriers in metal-halide perovskites
Sorbonne Université, CNRS, Institut des NanoSciences（索邦大学，法国国家科学研究中心，纳米科学研究所）；Instituto de Ciencia de Materiales de Sevilla (ICMS) Consejo Superior de Investigaciones Científicas – Universidad de Sevilla (CSIC-US)（塞维利亚材料科学研究所（ICMS）西班牙国家研究委员会–塞维利亚大学（CSIC-US））；Institute of Advanced Materials (INAM), Universitat Jaume I（先进材料研究所（INAM），卡斯特利翁海梅一世大学）
Ferromagnetic (Fe or Fe20Ni80) and antiferromagnetic (NiO) phases were deformed by high pressure torsion, a severe plastic deformation technique, to manufacture bulk sized nanocomposites and demonstrate an exchange bias, which has been reported predominantly for bilayer thin films. High pressure torsion deformation at elevated temperatures proved to be the key to obtain homogeneous bulk nanocomposites. X-ray diffraction investigations detected nanocrystallinity of the ferromagnetic and antiferromagnetic phases. Furthermore, an additional phase was identified by X-ray diffraction, which formed …
Raman scattering on bulk AsxS1-x glasses showes that vibrational modes of As4S4 monomer first appear near x=0.38, and their concentration increases precipitously with increasing x, suggesting that the stoichiometric glass (x=0.40) is intrinsically phase seperated into small As-rich(As4S4) and large S-righ clusters. Support for the Raman-active vibrational modes of the orpiment-like and realgar-like nanophases is provided by ab-initio density functional theory calculations on appropriate clusters. Nanoscale phase seperation provides a basis for understanding the global maximum in the glass tran…
Gallium arsenide nanowires were synthesized by gallium-assisted molecular beam epitaxy. By varying the growth time, nanowires with diameters ranging from 30 to 160 nm were obtained. Raman spectra of the nanowires ensembles were measured. The small line width of the optical phonon modes agree with an excellent crystalline quality. A surface phonon mode was also revealed, as a shoulder at lower frequencies of the longitudinal optical mode. In agreement with the theory, the surface mode shifts to lower wave numbers when the diameter of the nanowires is decreased or the environment dielectric cons…
Molecular dynamic simulations are used to study the heating of a gold nanorod of 2624 atoms. We show that roughening of surface {110} facets leads to a shape transformation and structural rearrangement of surface and bulk atoms in the rod, in order to reach a more stable configuration. Our results show that the stability of the nanorod is governed by the free energetics of the surface facets.
Using density functional theory plus Hubbard $U$ calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O$_3$ perovskite, a major mineral phase in the Earth's lower mantle, has high-spin ferric iron ($S=5/2$) at both the dodecahedral (A) and octahedral (B) site. As the pressure increases, the B-site iron undergoes a spin-state crossover to the low-spin state ($S=1/2$), while the A-site iron remains in the high-spin state. Our calculation shows that the B-site spin-state crossover in the pressure range of 40-70 GPa is accompanied by a noticeable volume reduction and an increase in quadrup…
Nanoparticles (size 20, 40 and 60 nm) of Pr_0.5Sr_0.5MnO_3 are prepared by sol-gel technique and their magnetic properties are studied using ferromagnetic resonance and magnetization measurements. A comparison with the properties of the bulk material shows that the ferromagnetic transition at 265 K remains unaffected but the anti-ferromagnetic transition at T_N = 150 K disappears in the nanoparticles. Further, the temperature dependence of magnetic anisotropy shows a complex behavior, being higher in the nanoparticles at high temperatures and lower at lower temperatures in comparison with the …
A nanopowder from p-Bi-Sb-Te with particles ~ 10 nm were fabricated by the ball milling using different technological modes. Cold and hot pressing at different conditions and also SPS process were used for consolidation of the powder into a bulk nanostructure and nanocomposites. The main factors allowing slowing-down of the growth of nanograins as a result of recrystallization are the reduction of the temperature and of the duration of the pressing, the increase of the pressure, as well as addition of small value additives (like MoS2, thermally expanded graphite or fullerenes). It was reached …
We studied the effects of high-pressure on the crystalline structure of bulk and nanocrystalline scheelite-type PbMoO4. We found that in both cases the compressibility of the materials is highly non-isotropic, being the c-axis the most compressible one. We also observed that the volume compressibility of nanocrystals becomes higher that the bulk one at 5 GPa. In addition, at 10.7(8) GPa we observed the onset of an structural phase transition in bulk PbMoO4. The high-pressure phase has a monoclinic structure similar to M-fergusonite. The transition is reversible and not volume change is detecte…
The MnAs compound shows a first-order transition at T$_C\approx42$ C, and a second-order transition at T$_t\approx120$ C. The first-order transition, with structural (hexagonal-orthorhombic), magnetic (FM-PM) and electrical conductivity changes, is associated to magnetocaloric, magnetoelastic, and magnetoresistance effects. We report a study in a large temperature range from $-196$ up to $140$ C, using the $γ-γ$ perturbed angular correlations method with the radioactive probe $^{77}$Br$\rightarrow^{77}$Se, produced at the ISOLDE-CERN facility. The electric field gradients and magnetic hyperfin…
Bulk nanocrystalline materials of small and medium ferromagnetic content were produced using severe plastic deformation by high-pressure torsion at roomtemperature. Giant magnetoresistive behavior was found for as deformed materials, which was further improved by adjusting the microstructure with thermal treatments. The adequate range of annealing temperatures was assessed with in-situ synchrotron diffraction measurements. Thermally treated CuCo materials show larger giant magnetoresistance after annealing for 1 h at 300C, while for CuFe this annealing temperature is too high and decreases the…
The ceramic pellets of the nominal compositions Bi$_{0.7}$Ba$_{0.3}$Fe$_{1-x}$Ti$_x$O$_3$ (x = 0.00-0.20) were prepared initially by standard solid state reaction technique. The pellets were then ground into micrometer-sized powders and mixed with isopropanol in an ultrasonic bath to prepare nanoparticles. The X-ray diffraction patterns demonstrate the presence of a significant number of impurity phases in bulk powder materials. Interestingly, these secondary phases were completely removed due to the sonication of these bulk powder materials for 60 minutes. The field and temperature dependent …
We report on an experimental study of the effect of coherent surface-bulk electron scattering on quantum transport in a three-dimensional topological insulator Bi$_2$Te$_3$ nanoplate. The nanoplate is grown via van der Waals epitaxy on a mica substrate and a top-gated Hall-bar device is fabricated from the nanoplate directly on the growth substrate. Top-gate voltage dependent measurements of the sheet resistance of the device reveal that the transport carriers in the nanoplate are of n-type and that, with decreasing top gate voltage, the carrier density in the nanoplate is decreased. However, …
Materials are subjected to energetic particles in a number of radiation environments,and are hence prone to undesirable (radiation) damage.We report here the superiority of the nanocrystalline phase over bulk for radiation tolerance under simultaneous irradiation with high energy (electronic energy loss (Se) dominant) and low energy (nuclear energy loss (Sn) dominant) particles.Nano-crystalline yttria stabilized zirconia is found to exhibit lesser radiation damage (viz.degradation in crystallinity),when compared to its bulklike counterpart,against simultaneous irradiation with high energy 27 M…
Indium (In) doping in topological crystalline insulator SnTe induces superconductivity, making In-doped SnTe a candidate for a topological superconductor. SnTe nanostructures offer well-defined nanoscale morphology and high surface-to-volume ratios to enhance surface effects. Here, we study In-doped SnTe nanoplates, InxSn1-xTe, with x ranging from 0 to 0.1 and show they superconduct. More importantly, we show that In doping reduces the bulk mobility of InxSn1-xTe such that the surface states are revealed in magnetotransport despite the high bulk carrier density. This is manifested by two-dimen…
Understanding the size- and shape-dependent properties of platinum nanoparticles is critical for enabling the design of nanoparticle-based applications with optimal and potentially tunable functionality. Toward this goal, we evaluated nine different empirical potentials with the purpose of accurately modeling faceted platinum nanoparticles using molecular dynamics simulation. First, the potentials were evaluated by computing bulk and surface properties-surface energy, lattice constant, stiffness constants, and the equation of state-and comparing these to prior experimental measurements and qua…
119Sn nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rate (1/T1) in SnO2 nanoparticles were measured as a function of temperature and compared with those of SnO2 bulk sample. A 15% loss of 119Sn NMR signal intensity for the nano sample compared to the bulk sample was observed. This is indicative of ferromagnetism from a small fraction of the sample. Another major finding is that the recovery of the 119Sn longitudinal nuclear magnetization in the nano sample follows a stretched exponential behavior, as opposed to that in bulk which is exponential. Further, the 119Sn 1/T1 a…
Nanoscale Charge Transport in Au@PANI Assemblies: Bulk-like Films and Linear Assemblies
Leibniz Institute of Polymer Research Dresden（德累斯顿莱布尼茨聚合物研究所）；Dresden University of Technology（德累斯顿工业大学）；Helmholtz-Zentrum, Dresden-Rossendorf（德累斯顿-罗斯多夫亥姆霍兹中心）
The Laves phase compound, YCo2, is a well-known exchange-enahnced Pauli paramagnet. We report here that, in the nanocrystalline form, this compound interestingly is an itinerant ferromagnet at room temperature with a low coercive-field. The magnitude of the saturation moment (about 1 Bohr-magneton per formula unit) is large enough to infer that the ferromagnetism is not a surface phenomenon in these nanocrystallites. Since these ferromagnetic nanocrystallines are easy to synthesize with a stable form in air, one can explore applications, particularly where hysteresis is a disadvantage.
A three-dimensional Dirac semimetal has bulk Dirac cones in all three momentum directions and Fermi arc-like surface states, and can be converted into a Weyl semimetal by breaking time-reversal symmetry. However, the highly conductive bulk state usually hides the electronic transport from the surface state in Dirac semimetal. Here, we demonstrate the supercurrent carried by bulk and surface states in Nb-Cd3As2 nanowire-Nb short and long junctions, respectively. For the 1 micrometer long junction, the Fabry-Perot interferences induced oscillations of the critical supercurrent are observed, sugg…
Silver nanowires have great application potential in fields like flexible electronic devices, solar cells and transparent electrodes. It is critical and fundamental to study the thermal and electrical transport properties in a single silver nanowire. In this work, the thermal and electrical transport in an individual silver nanowire is characterized down to 35 K with the steady state electro-thermal technique. The results indicate that, at room temperature, the electrical resistivity increases by around 4 folds compared from that of its bulk counterpart. After fitting the temperature dependent…
