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III–V compound nanowires have electrical and optical properties suitable for a wide range of applications, including photovoltaics and photodetectors. Furthermore, their elastic nature allows the use of strain engineering to enhance their performance. Here we have investigated the effect of mechanical strain on the photocurrent and the electrical properties of single GaAs nanowires with radial p-i-n junctions, using a nanoprobing setup. A uniaxial tensile strain of 3% resulted in an increase in photocurrent by more than a factor of 4 during NIR illumination. This effect is attributed to a decrease of 0.2 eV in nanowire bandgap energy, revealed by analysis of the current–voltage characteristics as a function of strain. This analysis also shows how other properties are affected by the strain, including the nanowire resistance. Furthermore, electron-beam-induced current maps show that the charge collection efficiency within the nanowire is unaffected by strain measured up to 0.9%.

Two-dimensional nanomaterials, as one of the most widely used substrates for energy storage devices, have achieved great success in terms of the overall capacity. Despite the extensive research effort dedicated to this field, there are still major challenges concerning capacitance modulation and stability of the 2D materials that need to be overcome. Doping of the crystal structures, pillaring methods and 3D structuring of electrodes have been proposed to improve the material properties. However, these strategies are usually accompanied by a significant increase in the cost of the entire material preparation process and also a lack of the versatility for modification of the various types of the chemical structures. Hence in this work, versatile, cheap, and environmentally friendly method for the enhancement of the electrochemical parameter of various MXene-based supercapacitors (Ti

Semiconductor–superconductor hybrids are widely used to realize complex quantum phenomena, such as topological superconductivity and spins coupled to Cooper pairs. Accessing new, exotic regimes at high magnetic fields and increasing operating temperatures beyond the state-of-the-art requires new, epitaxially matched semiconductor–superconductor materials. One challenge is the generation of favourable conditions for heterostructural formation between materials with the desired properties. Here we harness an increased knowledge of metal-on-semiconductor growth to develop InAs nanowires with epitaxially matched, single-crystal, atomically flat Pb films with no axial grain boundaries. These highly ordered heterostructures have a critical temperature of 7 K and a superconducting gap of 1.25 meV, which remains hard at 8.5 T, and therefore they offer a parameter space more than twice as large as those of alternative semiconductor–superconductor hybrids. Additionally, InAs/Pb island devices exhibit magnetic field-driven transitions from a Cooper pair to single-electron charging, a prerequisite for use in topological quantum computation. Semiconductor–Pb hybrids potentially enable access to entirely new regimes for a number of different quantum systems. Semiconductor–superconductor hybrids are used for realizing complex quantum phenomena but are limited in the accessible magnetic field and temperature range. Now, hybrid devices made from InAs nanowires and epitaxially matched, single-crystal, atomically flat Pb films present superior characteristics, doubling the available parameter space.

Strain engineering provides an effective way of tailoring the electronic and optoelectronic properties of semiconductor nanomaterials and nanodevices, giving rise to novel functionalities. Here, we present direct experimental evidence of strain-induced modifications of hole mobility in individual gallium arsenide (GaAs) nanowires, using in situ transmission electron microscopy (TEM). The conductivity of the nanowires varied with applied uniaxial tensile stress, showing an initial decrease of ∼5–20% up to a stress of 1–2 GPa, subsequently increasing up to the elastic limit of the nanowires. This is attributed to a hole mobility variation due to changes in the valence band structure caused by stress and strain. The corresponding lattice strain in the nanowires was quantified by in situ four dimensional scanning TEM and showed a complex spatial distribution at all stress levels. Meanwhile, a significant red shift of the band gap induced by the stress and strain was unveiled by monochromated electron energy loss spectroscopy.

The attractive electronic and magnetic properties together with their biocompatibility make iron-oxide nanoparticles appear as functional materials. In Fe-oxide nanoparticle (IONP) ensembles, it is crucial to enhance their performance thanks to controlled size, shape, and stoichiometry ensembles. In light of this, we conduct a comprehensive investigation in an ensemble of ca. 28 nm cuboid-shaped IONPs in which all the analyses concur with the coexistence of magnetite/maghemite phases in their cores. Here, we are disclosing the Verwey transition by temperature dependent (4?210 K) Raman spectroscopy. This work was supported by EU FP7 604448 (NanoMag) and MAT2017-83631-C3-R. Dr Norbert Schell is acknowledged for the time at the HEMS beamline, and J. F. thanks MUDP (MST- 141-01415).

Inversion-symmetry breaking for second-harmonic generation in centrosymmetric Si and Ge is artificially realized with a pair of asymmetric-coupled SiGe quantum wells. Laser spectroscopy experiments demonstrate a nonlinear susceptibility four orders of magnitude larger than that of natural nonlinear materials in the visible range.

We show results of nanorheological studies of different concentrations of xanthan (non-Newtonian fluid) in water using magnetic nanoparticles (MNPs) together with the AC susceptibility (ACS) vs frequency method. For comparison we also show the ACS response for different concentrations of glycerol in water (Newtonian fluid). The ACS response is measured, and the data is modelled using dynamic magnetic models and different viscoelastic models. We study the ACS response (in-phase and out-of-phase ACS components) at different concentrations of xanthan in water (up to 1 wt% xanthan) and with a constant concentration of MNPs. We use MNP systems that show Brownian relaxation (sensitive to changes in the environmental properties around the MNPs). ACS measurements are performed using the DynoMag system. The Brownian relaxation of the MNP system peak is shifting down in frequency and the ACS response is broadening and decreases due to changes in the viscoelastic properties around the MNPs in the xanthan solution. The viscosity and the storage moduli are determined at each excitation frequency and compared with traditional macroscopic small amplitude oscillatory shear rheological measurements. The results from the traditional rheological and nanorheological measurements correlate well at higher xanthan concentration.

Scientific interest in iron-oxides and in particular magnetite has been renewed due to the broad scope of their fascinating properties, which are finding applications in electronics and biomedicine. Specifically, iron oxide nanoparticles (IONPs) are gathering attraction in biomedicine. Their cores are usually constituted by a mixture of maghemite and magnetite phases. In view of this, to fine-tune the properties of an ensemble of IONPs towards their applications, it is essential to enhance mass fabrication processes towards the production of monodisperse IONPs with controlled size, shape, and stoichiometry. We exploit the vacancy sensitivity of the Verwey transition to detect the presence of magnetite. Here we provide direct evidence for the Verwey transition in an ensemble of IONPs through neutron diffraction. This transition is observed as a variation in the Fe magnetic moment at octahedral sites and, in turn, gives rise to a change of the net magnetic moment. Finally, we show this variation as the microscopic ingredient driving the characteristic kink that hallmarks the Verwey transition in thermal variation of magnetization., This work has received funding from EU FP7 604448 (NanoMag) and MAT2017-83631-C3-R. The Institute Laue-Langevin is acknowledged for provision of beamtime on the D1B instrument.

Semiconductor-superconductor hybrids are widely used to realize complex quantum phenomena, such as topological superconductivity and spins coupled to Cooper pairs. Accessing new, exotic regimes at high magnetic fields and increasing operating temperatures beyond the state-of-the-art requires new, epitaxially matched semiconductor-superconductor materials. One challenge is the generation of favourable conditions for heterostructural formation between materials with the desired properties. Here we harness an increased knowledge of metal-on-semiconductor growth to develop InAs nanowires with epitaxially matched, single-crystal, atomically flat Pb films with no axial grain boundaries. These highly ordered heterostructures have a critical temperature of 7 K and a superconducting gap of 1.25 meV, which remains hard at 8.5 T, and therefore they offer a parameter space more than twice as large as those of alternative semiconductor-superconductor hybrids. Additionally, InAs/Pb island devices exhibit magnetic field-driven transitions from a Cooper pair to single-electron charging, a prerequisite for use in topological quantum computation. Semiconductor-Pb hybrids potentially enable access to entirely new regimes for a number of different quantum systems.

Vertical arrays of semiconductor nanowires show great potential for material-efficient and high-performance solar cells. The characterization and correlation between material structure and properties of the individual nanowires are crucial for the continued performance improvement of such devices. In this work, we developed a method with a scanning tunneling microscope (STM) probe inside a scanning electron microscope (SEM) to enable the studies of single photovoltaic nanowires. The STM probe is used to contact individual nanowires in ensembles. We combine the STM-SEM with an in situ light emitting diode (LED) illumination source to study both the electrical and photovoltaic properties of vertical GaAs nanowires with radial p-i-n junctions. We also illustrate that the local charge separation ability within the nanowires can be studied by electron beam induced current (EBIC) measurements. The in situ SEM setup allows the correlation between properties and nanowire structure. The data show that the quality of the electrical contact to the semiconductor nanowire is crucial to be able to investigate the inherent properties of the nanowires. We have established a procedure to make high-quality ohmic contacts to the nanowires with the STM probe. We also show that the effect of mechanical strain on the electrical properties can be investigated by the STM-SEM setup.

Free-standing semiconductor nanowires constitute an ideal material system for the direct manipulation of electrical and optical properties by strain engineering. In this study, we present a direct quantitative correlation between electrical conductivity and nanoscale lattice strain of individual InAs nanowires passivated with a thin epitaxial In0.6Ga0.4As shell. With an in situ electron microscopy electromechanical testing technique, we show that the piezoresistive response of the nanowires is greatly enhanced compared to bulk InAs, and that uniaxial elastic strain leads to increased conductivity, which can be explained by a strain-induced reduction in the band gap. In addition, we observe inhomogeneity in strain distribution, which could have a reverse effect on the conductivity by increasing the scattering of charge carriers. These results provide a direct correlation of nanoscale mechanical strain and electrical transport properties in free-standing nanostructures.

We present the theoretical investigation and the experimental demonstration of second harmonic generation in the mid-infrared by hole-doped Ge/SiGe asymmetric quantum wells.

The widespread use of magnetic nanoparticles in the biotechnical sector puts new demands on fast and quantitative characterization techniques for nanoparticle dispersions. In this work, we report the use of asymmetric flow field-flow fractionation (AF4) and ferromagnetic resonance (FMR) to study the properties of a commercial magnetic nanoparticle dispersion. We demonstrate the effectiveness of both techniques when subjected to a dispersion with a bimodal size/magnetic property distribution: i.e., a small superparamagnetic fraction, and a larger blocked fraction of strongly coupled colloidal nanoclusters. We show that the oriented attachment of primary nanocrystals into colloidal nanoclusters drastically alters their static, dynamic, and magnetic resonance properties. Finally, we show how the FMR spectra are influenced by dynamical effects; agglomeration of the superparamagnetic fraction leads to reversible line-broadening; rotational alignment of the suspended nanoclusters results in shape-dependent resonance shifts. The AF4 and FMR measurements described herein are fast and simple, and therefore suitable for quality control procedures in commercial production of magnetic nanoparticles.

We present experimental intrinsic loss power (ILP) values, measured at an excitation frequency of 1 MHz and at relatively low field amplitudes of 3.4–9.9 kA/m, as a function of the mean core diameter, for selected magnetic nanoparticles (MNPs). The mean core sizes ranged from ca. 8 nm to 31 nm. Transmission electron microscopy indicated that those with smaller core sizes (less than ca. 22 nm) were single-core MNPs, while those with larger core sizes (ca. 29 nm to 31 nm) were multi-core MNPs. The ILP data showed a peak at core sizes of ca. 20 nm. We show here that this behaviour correlates well with the predicted ILP values obtained using either a non-interacting Debye model, or via dynamic Monte-Carlo simulations, the latter including core-core magnetic interactions for the multi-core particles. This alignment of the models is a consequence of the low field amplitudes used. We also present interesting results showing that the core-core interactions affect the ILP value differently depending on the mean core size., The work in this study was financed by the EU FP7 NMP project NanoMag, grant agreement no: 604448

The crystal and electronic structure of semiconductor nanowire systems have shown sensitive response to mechanical strain, enabling novel and improved electrical, and optoelectrical properties in nanowires by strain engineering. Here, the response of current–voltage (I–V) characteristics and band structure of individual p-doped GaAs nanowires to bending deformation is studied by in situ electron microscopy combined with theoretical simulations. The I–V characteristics of the nanowire change from linear to nonlinear as bending deformation is applied. The nonlinearity increases with strain. As opposed to the case of uniaxial strain in GaAs, the bending deformation does not give rise to a change in the band gap of GaAs nanowire according to in situ electron energy loss spectroscopy (EELS) measurements. Instead, the response to bending deformation can be explained by strain induced valence band shift, which results in an energy barrier for charge carrier transport along the nanowire. Moreover, the electron effective mass decreases as the strain changes from compressive to tensile across the GaAs nanowire in the bent region. Results from this study shed light on the complex interplay between lattice strain, band structure, and charge transport in semiconductor nanomaterials.

Clustering of magnetic nanoparticles can drastically change their collective magnetic properties, which in turn may influence their performance in technological or biomedical applications. Here, we investigate a commercial colloidal dispersion (FeraSpin (TM) R), which contains dense clusters of iron oxide cores (mean size around 9 nm according to neutron diffraction) with varying cluster size (about 18-56 nm according to small angle x-ray diffraction), and its individual size fractions (FeraSpin (TM) XS, S, M, L, XL, XXL). The magnetic properties of the colloids were characterized by isothermal magnetization, as well as frequency-dependent optomagnetic and AC susceptibility measurements. From these measurements we derive the underlying moment and relaxation frequency distributions, respectively. Analysis of the distributions shows that the clustering of the initially superparamagnetic cores leads to remanent magnetic moments within the large clusters. At frequencies below 10(5) rad s(-1), the relaxation of the clusters is dominated by Brownian (rotation) relaxation. At higher frequencies, where Brownian relaxation is inhibited due to viscous friction, the clusters still show an appreciable magnetic relaxation due to internal moment relaxation within the clusters. As a result of the internal moment relaxation, the colloids with the large clusters (FSL, XL, XXL) excel in magnetic hyperthermia experiments. We acknowledge the Institut Laue Langevin for provision of beamtime at the instrument D2B. This project (NanoMag) has received funding from the European Commission Framework Programme 7 under grant agreement no 604448.

The effects of grain size on the dynamic deformation behavior of rolled AZ31B alloy at high strain rates were investigated. Rolled AZ31B alloy samples with grain sizes of 6,18 and 37 pm, were subjected to shock loading tests using Split Hopkinson Pressure Bar at room temperature and at a strain rate of 1100 s(-1) It was found that a double-peak basal texture formed in the shock loaded samples. The strength and ductility of the alloy under the high strain-rate compressive loading increased with decreasing grain size. However, twinning fraction and strain hardening rate were found to decrease with decreasing grain size. In addition, orientation imaging microscopy showed a higher contribution of double and contraction twins in the deformation process of the coarse-grained samples. Using transmission electron microscopy, pyramidal dislocations were detected in the shock loaded sample, proving the activation of pyramidal slip system under dynamic impact loading.

Magnesium alloys are widely used in automotive and aerospace industries, where they can be exposed to high strain rate conditions such as car crash and ballistic impact. Grain size is an important factor that can affect the mechanical behavior of magnesium alloys at high strain rates. Therefore, it is very important to evaluate the effects of grain size on the dynamic mechanical response of magnesium alloys under shock-loading conditions. In this research, texture evolution, microstructural changes and dynamic mechanical behavior of rolled Mg-4Y-3RE alloy samples, with grain sizes of 8, 25 and 46 mu m, deformed under compressive shock-loading are investigated. Dynamic shock loading tests were conducted using Split Hopkinson Pressure Bar at room temperature at a strain rate of 1200 s(-1). Texture measurements indicate development of a double-peak (00.2) basal texture in all the samples during shock loading. However, slightly higher intensities were observed for coarse-grained samples. Both strength and ductility were found to decrease with increasing grain size, while twining fraction and strain hardening rate increase with increasing grain size. The activity of double and contraction twins increased with increase in grain size. Furthermore, activation of pyramidal (c + a) slip system during the shock loading of the Mg-4Y-3RE alloy was confirmed using the 'g.b' analysis method.

The response of magnetic nanoparticles (MNPs) to an oscillating magnetic field outside the linear response region is important for several applications including magnetic hyperthermia, magnetic resonance imaging and biodetection. The size and magnetic moment are two critical parameters for the performance of a colloidal MNP dispersion. We present and demonstrate the use of optomagnetic (OM) and AC susceptibility (ACS) measurements vs. frequency and magnetic field strength to obtain the size and magnetic moment distributions including the correlation between the distributions. The correlation between the size and the magnetic moment contains information on the morphology and intrinsic structure of the particle. In OM measurements, the variation of the second harmonic light transmission through a dispersion of MNPs is measured in response to an oscillating magnetic field. We solve the Fokker-Planck equations for MNPs with a permanent magnetic moment, and develop analytical approximations to the ACS and the OM signals that also account for the change in the curve shapes with increasing field strength. Further, we describe the influence of induced magnetic moments on the signals, by solving the Fokker-Planck equation for particles, which apart from the permanent magnetic moment may also have an induced magnetic moment and shape anisotropy. Using the results from the Fokker-Planck calculations we fit ACS and OM measurements on two multi-core particle systems. The obtained fit parameters also describe the correlations between the magnetic moment and size of the particles. From such an analysis on a commercially available polydisperse multicore particle system with an average particle size of 80 nm, we find that the MNP magnetic moment is proportional to the square root of the hydrodynamic size.

We investigated, in depth, the interrelations among structure, magnetic properties, relaxation dynamics and magnetic hyperthermia performance of magnetic nanoflowers. The nanoflowers are about 39 nm in size, and consist of densely packed iron oxide cores. They display a remanent magnetization, which we explain by the exchange coupling between the cores, but we observe indications for internal spin disorder. By polarized small-angle neutron scattering, we unambiguously confirm that, on average, the nanoflowers are preferentially magnetized along one direction. The extracted discrete relaxation time distribution of the colloidally dispersed particles indicates the presence of three distinct relaxation contributions. We can explain the two slower processes by Brownian and classical Néel relaxation, respectively. The additionally observed very fast relaxation contributions are attributed by us to the relaxation of disordered spins within the nanoflowers. Finally, we show that the intrinsic loss power (ILP, magnetic hyperthermia performance) of the nanoflowers measured in colloidal dispersion at high frequency is comparatively large and independent of the viscosity of the surrounding medium. This concurs with our assumption that the observed relaxation in the high frequency range is primarily a result of internal spin relaxation, and possibly connected to the disordered spins within the individual nanoflowers. The authors thank the Institute Laue-Langevin for provision of neutron beamtime at instrument D33. This project has received funding from the European Commission Framework Programme 7 under Grant Agreement No. 604448 (NanoMag).

In this research, effect of yttrium on the texture formation, microstructural evolution and mechanical response of AE42 and AE44 cast magnesium alloys were investigated under ballistic impact. The selected strain rates were 800 and 1100 s(-1) and the tests were conducted using Split Hopkinson Pressure Bar. It was inferred that after high velocity impact, a weaker basal texture developed in the samples with lower content of yttrium. Experimental results also showed that by increasing the concentration of yttrium in the cast AE alloys, strength, ductility and dislocation density of the impacted alloys increased but, the fraction of twinning decreased, which indicate the effective influence of yttrium on the nucleation and growth of twins. Moreover, it was inferred that accumulation of dislocations at intersections of the twins led to the cracking and fracture of the samples under shock loading conditions.

Single-core iron-oxide nanoparticles with nominal core diameters of 14 nm and 19 nm were analyzed with a variety of non-magnetic and magnetic analysis techniques, including transmission electron microscopy (TEM), dynamic light scattering (DLS), static magnetization vs. magnetic field (M-H) measurements, ac susceptibility (ACS) and magnetorelaxometry (MRX). From the experimental data, distributions of core and hydrodynamic sizes are derived. Except for TEM where a number-weighted distribution is directly obtained, models have to be applied in order to determine size distributions from the measurand. It was found that the mean core diameters determined from TEM, M-H, ACS and MRX measurements agree well although they are based on different models (Langevin function, Brownian and Néel relaxation times). Especially for the sample with large cores, particle interaction effects come into play, causing agglomerates which were detected in DLS, ACS and MRX measurements. We observed that the number and size of agglomerates can be minimized by sufficiently strong diluting the suspension., This work was supported by the European Commission Framework Programme 7 under the NanoMag project (grant agreement no: 604448).

The structural and magnetic properties of magnetic multi-core particles were determined by numerical inversion of small angle scattering and isothermal magnetisation data. The investigated particles consist of iron oxide nanoparticle cores (9 nm) embedded in poly(styrene) spheres (160 nm). A thorough physical characterisation of the particles included transmission electron microscopy, X-ray diffraction and asymmetrical flow field-flow fractionation. Their structure was ultimately disclosed by an indirect Fourier transform of static light scattering, small angle X-ray scattering and small angle neutron scattering data of the colloidal dispersion. The extracted pair distance distribution functions clearly indicated that the cores were mostly accumulated in the outer surface layers of the poly(styrene) spheres. To investigate the magnetic properties, the isothermal magnetisation curves of the multi-core particles (immobilised and dispersed in water) were analysed. The study stands out by applying the same numerical approach to extract the apparent moment distributions of the particles as for the indirect Fourier transform. It could be shown that the main peak of the apparent moment distributions correlated to the expected intrinsic moment distribution of the cores. Additional peaks were observed which signaled deviations of the isothermal magnetisation behavior from the non-interacting case, indicating weak dipolar interactions. This project has received funding from the European Commission Framework Programme 7 under grant agreement no 604448. ISIS-STFC is thanked for granting SANS beam time on SANS2D and Diego A. Venero (ISIS-STFC) for valuable discussions. José I. Espeso (Universidad de Cantabria) is acknowledged for helping with the XRD analysis.

Magnesium alloys components are potentially used under shock loading conditions. Considering the fact that deformation behavior is completely different under high strain rate conditions compared to quasi-static conditions, it is very important to evaluate the dynamic mechanical response and deformation mechanisms of magnesium alloys under impact loading. In this research, texture formation, microstructural changes and dynamic deformation behavior of rolled AZ31B in the tempered H24 stress-relieved conditions, shock-loaded under compressive dynamic loading, were investigated. Texture measurements showed that the as-received AZ31B alloy had a strong (00.2) basal texture. Four groups of cylindrical samples were cut in the rolling direction (RD), in 45 degrees to the RD, in the transverse direction (TD) and in the direction perpendicular to the RD-TD plane. Dynamic shock loading of the test samples were conducted using Split Hopkinson Pressure Bar at room temperature at strain rates ranging from 600 to 1100 s(-1), while loading direction was parallel to the longitudinal axis of the cylindrical samples. After high strain rate deformation, although the loading direction was different, a strong (00.2) basal texture was observed in all samples. It was inferred that increasing the strain rate led to an increase in strength and ductility, but to a decrease in twinning fraction, indicating the possible activation of non-basal slip systems. Besides, a high degree of mechanical anisotropy was found for all strain rates used such that the lowest strength was registered for the samples cut along the direction parallel to the rolling direction. A viscoplastic self-consistent model with a tangent approach was used to corroborate the experimental textures and possible deformation mechanisms by simulation.

Gavilán, Helena et al., The assembly of magnetic cores into regular structures may notably influence the properties displayed by a magnetic colloid. Here, key synthesis parameters driving the self-assembly process capable of organizing colloidal magnetic cores into highly regular and reproducible multi-core nanoparticles are determined. In addition, a self-consistent picture that explains the collective magnetic properties exhibited by these complex assemblies is achieved through structural, colloidal, and magnetic means. For this purpose, different strategies to obtain flower-shaped iron oxide assemblies in the size range 25–100 nm are examined. The routes are based on the partial oxidation of Fe(OH)2, polyol-mediated synthesis or the reduction of iron acetylacetonate. The nanoparticles are functionalized either with dextran, citric acid, or alternatively embedded in polystyrene and their long-term stability is assessed. The core size is measured, calculated, and modeled using both structural and magnetic means, while the Debye model and multi-core extended model are used to study interparticle interactions. This is the first step toward standardized protocols of synthesis and characterization of flower-shaped nanoparticles., This work was partially supported by the European Commission Framework Program 7 (NanoMag project, NO 604448) and by the Spanish Ministry of Economy and Competitiveness (Mago project, No. MAT2014-52069-R).

Semiconducting nanowires grown by quasi-van-der-Waals epitaxy on graphite flakes are a new class of hybrid materials that hold promise for scalable nanostructured devices within opto-electronics. Here we report on high aspect ratio and stacking fault free Ag-seeded InAs nanowires grown on exfoliated graphite flakes by molecular beam epitaxy. Ag catalyzes the InAs nanowire growth selectively on the graphite flakes and not on the underlying InAs substrates. This allows for easy transfer of the flexible graphite flakes with as-grown nanowire ensembles to arbitrary substrates by a micro-needle manipulator. Besides the possibilities for fabricating novel nanostructure device designs, we show how this method is used to study the parasitic growth and bicrystal match between the graphite flake and the nanowires by transmission electron microscopy.

Al/AlOx/Al Josephson junctions are the building blocks of a wide range of superconducting quantum devices that are key elements for quantum computers, extremely sensitive magnetometers and radiation detectors. The properties of the junctions and the superconducting quantum devices are determined by the atomic structure of the tunnel barrier. The nanoscale dimension and disordered nature of the barrier oxide have been challenges for the direct experimental investigation of the atomic structure of the tunnel barrier. Here we show that the miniaturized dimension of the barrier and the interfacial interaction between crystalline Al and amorphous AlOx give rise to oxygen deficiency at the metal/oxide interfaces. In the interior of the barrier, the oxide resembles the atomic structure of bulk aluminium oxide. Atomic defects such as oxygen vacancies at the interfaces can be the origin of the two-level systems and contribute to decoherence and noise in superconducting quantum circuits.

The structural features of the “5M” martensitic phase in Ni2FeGa alloys have been determined by electron diffraction using the multi-slice least-squares (MSLS) method. The results demonstrate that the “5M” phase contains an evident cooperative effect of monoclinic distortion and sinusoidal modulation along the [110]c direction. Theoretical simulations based on our refined data suggest that the “5M” martensitic phase observed in Ni–Fe–Ga and Ni–Mn–Ga has visible common behaviors in both stacking sequence and local structural distortion. Considering the cooperative effect of monoclinic distortion and sinusoidal modulation, we demonstrate that the “7M” martensitic phase could adopt two equivalent structural phases corresponding with the stacking sequences of ( 4 3 − ) 2 and ( 5 2 − ) 2 , respectively.

Transmission electron microscopy (TEM) investigations on Nd2/3−xLi3xTiO3 with x=0.14 reveal a rich variety of structural features in the samples prepared under different conditions, such as superstructures, anti-phase domains, and nano-chessboard structures. Our careful analysis shows that these structural phenomena can be fundamentally understood as the combination of structural effects of (Nd, Li)-ordering along the 〈001〉p direction and the spinodal decomposition along the 〈100〉p/〈010〉p direction. The coexistence of phase separation and cation ordering can lead to visible nano-structural complexities in many crystals, as the typical results, the regular lamella structure, nano-chessboard structures and anti-phase boundaries have been extensively studied.

Nonlinear transport behaviors are crucial for applications in electronic technology. At the nonlinear critical turning point, the nonequilibrium states cause rich physics responses to environment. The corresponding study in this field is crucial for physics and industry application. Here nonlinear conductivity in charge-ordered (CO) LuFe(2)O(4 )has been demonstrated. Remarkable resistivity switching behavior was observed and the gas-sensing property can be reversibly tuned by a small alternation of partial pressure and/or chemical components of the environment. These facts allow us to use LuFe(2)O(4) materials as a sensitive chemical gas sensor in technological applications. Careful analysis of the gas sensing process in LuFe(2)O(4) suggests a novel sensing mechanism in sharp contrast with that discussed for the conventional gas sensors which depend fundamentally on surface chemical reactions.

Structural properties and hole distribution in the spin-ladder compound Sr(14-x)Ca(x)Cu(24)O(41) have been extensively investigated by transmission electron microscopy (TEM). The complex electron diffraction patterns and high-resolution TEM observations reveal a clear incommensurate structural modulation along the c-axis direction attributable to two mismatched sublattices; this modulation strongly depends on hole distribution in the compounds. The fine structures of the O K and Cu L(2,3) ionization edges for the Sr(14-x)Ca(x)Cu(24)O(41) compounds recorded under different conditions indicate that more doped holes reside in the chains than the ladders, and that substituting Ca for Sr atoms results in a charge redistribution between the chains and ladders. Based on the experimental findings, the theoretical results, including the partial density of states and optical conductivity spectra calculated by the density functional theory, are also discussed.

The nature of the charge ordering (CO) in ${\text{Fe}}_{2}{\text{BO}}_{4}$ was carefully examined by in situ TEM observations from 360 K down to 20 K. An incommensurate structural modulation with the wave vector $q=(1/2,0,\ensuremath{\epsilon})$ becomes plainly visible in association with the CO transition at ${T}_{\text{CO}}\ensuremath{\approx}340\text{ }\text{K}$. Careful structural analysis suggests that this incommensurate structure originates from a well-defined ${\text{Fe}}^{2+}/{\text{Fe}}^{3+}$ ionic order along the ${\text{FeO}}_{6}$ octahedral ribbons (in the $\mathbit{a}$-axis direction) and from a discommensurate antiphase stripe order along the $\mathbit{c}$-axis direction. The average periodicity of antiphase stripes changes notably with lowering temperature and, via certain intermediate states, toward a superstructure phase with modulation wave vector of $q=(1/2,0,0)$ at $\ensuremath{\sim}150\text{ }\text{K}$ in association with a magnetic phase transition.

We report in detail our ab initio local density approximation (LDA)+Gutzwiller method, in which the Gutzwiller variational approach is naturally incorporated with the density-functional theory through the "Gutzwiller density-functional theory" (which is a generalization of original Kohn-Sham formalism). This method can be used for ground-state determination of electron systems ranging from weakly correlated metal to strongly correlated insulators with long-range ordering. We will show that its quality for ground state is as high as that by dynamic mean-field theory, and yet it is computationally much cheaper. In addition, the method is fully variational, the charge-density self-consistency can be naturally achieved, and the quantities, such as total energy and linear response, can be accurately obtained similarly as with LDA-type calculations. Applications on several typical systems are presented, and the characteristic aspects of this method are clarified. The obtained results using LDA+Gutzwiller are in better agreement with existing experiments, suggesting significant improvements over LDA or LDA+U.

The structural changes associated with the charge ordering (CO) transition in layered La(0.5)Sr(1.5)MnO(4) have been determined by electron diffraction using the multislice least-squares method. The results demonstrate that the structural deformations of the Mn(3+)O(6) octahedra can be well characterized by a transverse modulation wave along the direction. Based on these structural data, we have performed a theoretical calculation of the electronic structure by density functional theory plus the on-site Coulomb interaction U. The resultant density of states exhibits a clear band gap at the Fermi level for this low-temperature CO insulating state. It is also noted that the essential Jahn-Teller effects in this layered system can evidently affect the electronic structures and orbital hybridization appearing with the CO transition. Charge disproportionation and orbital ordering in La(0.5)Sr(1.5)MnO(4) have been extensively discussed in comparison with the data obtained for the cubic perovskite La(0.5)Ca(0.5)MnO(3).

We have directly measured the thickness distribution of the tunnel barriers in state-of-the-art Al/AlOx/Al tunnel junctions. From the distribution we can conclude that less than 10% of the junction area dominates the electron tunnelling. The barriers have been studied by transmission electron microscopy, specifically using atomic resolution annular dark field (ADF) scanning transmission electron microscopy (STEM) imaging. The direct observation of the local barrier thickness shows a Gaussian distribution of the barrier thickness variation along the junction, from ~1 to ~2nm. We have investigated how the thickness distribution varies with oxygen pressure (Po) and oxidation time (to) and we find, in agreement with resistance measurements, that an increased to has a larger impact on barrier thickness and its uniformity compared to an increased Po.

The interface between the Al bottom contact layer and Si substrates in Al based Josephson junctions is believed to have a significant effect on the noise observed in Al based superconducting devices. We have studied the atomic structure of it by transmission electron microscopy. An amorphous layer with a thickness of ∼5 nm was found between the bottom Al electrode and HF-treated Si substrate. It results from intermixing between Al, Si, and O. We also studied the chemical bonding states among the different species using energy loss near edge structure. The observations are of importance for the understanding of the origin of decoherence mechanisms in qubits based on these junctions.

An interaction layer is found at the Al/SiO2 interface in Al/AlOx/Al tunnel junctions grown on SiO2 substrates. The amorphous intermixing layer has an average thickness of about 5 nm. We present the detailed structure of this interfacial layer as determined by transmission electron microscopy. The layer contains alumina with aluminum being octahedrally coordinated according to electron energy loss spectroscopy analysis rather than tetrahedrally coordinated, where the latter coordination is the most common type in amorphous alumina. Depth profiles of the Al-O and Si-O bonding characteristics were also investigated using energy loss near edge structure.

Changes in bottom electrode morphology and adhesion layer composition upon deposition of BaxSr1−xTiO3 (BSTO) at elevated temperatures have been found, which have a negative impact on acoustic wave resonator device performance. The difference between nominal and actual adhesion layer composition are explained by grain boundary diffusion of Ti or W and their oxidation by in-diffusing oxygen, which leads to an increased interface roughness between the Pt bottom electrode and the BSTO. It is shown, that room-temperature deposited TiO2 diffusion barriers fail to protect against Ti oxidation and diffusion. Also W adhesion layers are prone to this phenomenon, which limits their ability to act as high temperature resistant adhesion layers for bottom electrodes for ferroelectric thin films.

The nonlinear current-voltage (I-V) relation of charge ordered (CO) LuFe2O4 has been studied in the temperature range from 220 to 520 K. Strong nonlinear effects are identified in the CO state and disappear in high-temperature charge disordered state. Alternation of the CO state under applied electric field is directly revealed by in situ transmission electron microscopy observations. Careful investigations of time-dependent resistance under the applied electric field near the threshold value (Eth) often show three distinct steps for the nonlinear transitions. The CO collapse likely at a critical conductivity yields a sharp drop in resistance.

Polycrystalline samples of Ba(1-x)Sr(x)Fe(2)As(2) (0≤x≤1) and Ba(1-x)Sr(x)Fe(1.8)Co(0.2)As(2) (0≤x≤1) have been synthesized by a solid state reaction method. Structural analysis by means of x-ray diffraction shows that the lattice parameters and unit cell volume decrease monotonically with the increase of x for Ba(1-x)Sr(x)Fe(2)As(2). The measurements of transport properties demonstrate that the average size of the Ba(Sr)-site cations could evidently influence the spin density wave (SDW) behavior in Ba(1-x)Sr(x)Fe(2)As(2) and superconductivity in Ba(1-x)Sr(x)Fe(1.8)Co(0.2)As(2) as well. The critical temperature for SDW (T(SDW)) increases with the Sr substitution for Ba in Ba(1-x)Sr(x)Fe(2)As(2) and, on the other hand, the superconducting T(c) decreases with the increase of Sr content in Ba(1-x)Sr(x)Fe(1.8)Co(0.2)As(2). The inhomogeneous distributions of Ba/Sr ions and structural distortions in Ba(0.5)Sr(0.5)Fe(2)As(2) have been investigated by transmission-electron microscopy (TEM) observations.

The ferroelectricity, magnetic properties, and charge-ordering modulations in the layered Lu2Fe2Fe1−xMnxO7 have been investigated for x ranging from 0 to 0.86. Experimental measurements reveal that substitution of Mn for Fe could evidently influence the low temperature magnetic transition. Moreover, ferroelectric analysis suggests that introduction of Mn ions in this layered system can efficiently suppress the conducting leakage and result in clear hysteresis loops in the polarization–electric field switching curves. The charge ordering states in Lu2Fe2Fe1−xMnxO7 characterized by two incommensurate modulations has been observed and discussed in association with the local spontaneous polarizations.

The charge/orbital ordering (COO) behaviors in the double-layered Pr(SrxCa2-x)Mn2O7 have been investigated by in situ transmission electron microscopy (TEM) observations. The experimental results reveal that two charge ordering (CO) phase transitions, so-called Tco1 and Tco2, commonly exist in the orthorhombic P r(SrxCa2-x)Mn2O7 (0≤x≤0.45) samples. Moreover, a "checkerboard" orbital-stripe phase characterized by two orthogonal COO modulations, qa and qb, is observed between Tco1 and Tco2 in all samples. Detailed TEM investigations on the x=0.2 sample, in which a visible electric polarization was found (Tokunaga Y. et al., Nat. Mater., 5 (2006) 937) demonstrate that the qb modulation only survives between Tco1 and Tco2, and the "checkerboard" phase transforms into the conventional one-dimensional COO stripe phase below Tco2. The reversal of in-plane optical anisotropy across the Tco2 transition (Tokunaga Y. et al., Nat. Mater., 5 (2006) 937) is also explained based on TEM result.

Electric transport measurements of the charge frustrated LuFe2O4, in which the charge ordering (CO) and electronic ferroelectricity are found, reveal strong nonlinear electric conduction upon application of electrical field in both single crystalline and polycrystalline samples. The threshold electric fields (Et) in single crystalline LuFe2O4 are estimated respectively to be about 60V/cm and 10V/cm with E parallel and perpendicular to the c-axis direction. Experimental measurements also demonstrate that the I-V nonlinearity increases quickly with lowering temperature. Furthermore, our in-situ TEM investigations evidently reveal that the nonlinear I-V behavior is intrinsically in correlation with a current driven charge ordering insulator-metal transition, and the applied electrical field triggers a visible CO collapse recognizable as the fading of satellite spots of the CO modulations.

The structural properties associated with the orbitally induced Peierls transition in MgTi(2)O(4) are characterized by in situ cooling TEM observations. A distinctive structural modulation with the wavevector of q(1) = 1/4 (0, 0, 4) has been well demonstrated below a critical temperature of about 260 K for MgTi(2)O(4). Systematic analysis demonstrates that this structural modulation can be well explained by an orbital order existing in the low-temperature insulating phase. It is also noted that the nonstoichiometric feature commonly appearing in the present system could yield visible changes in both physical and structural properties. A low-temperature study of the microstructure of Mg[Ti(1.9)Mg(0.1)]O(4) reveals that a little substitution of Mg(2+) for Ti(3+) ions on the octahedral sites can disrupt the long-range order of the t(2g) orbitals, resulting in complex tweed structures in the superstructure phase.