Journal of Physics D: Applied Physics

Petrov, Eugene P; Schütz, Gerhard J

doi: 10.1088/1361-6463/aa53cbpmid: N/A

Lagerholm, B Christoffer; Andrade, Débora M; Clausen, Mathias P; Eggeling, Christian

doi: 10.1088/1361-6463/aa519epmid: 28458397

Fluorescence correlation spectroscopy (FCS) in combination with the super-resolution imaging method STED (STED-FCS), and single-particle tracking (SPT) are able to directly probe the lateral dynamics of lipids and proteins in the plasma membrane of live cells at spatial scales much below the diffraction limit of conventional microscopy. However, a major disparity in interpretation of data from SPT and STED-FCS remains, namely the proposed existence of a very fast (unhindered) lateral diffusion coefficient, ⩾5 µm2 s−1, in the plasma membrane of live cells at very short length scales, ≈⩽ 100 nm, and time scales, ≈1–10 ms. This fast diffusion coefficient has been advocated in several high-speed SPT studies, for lipids and membrane proteins alike, but the equivalent has not been detected in STED-FCS measurements. Resolving this ambiguity is important because the assessment of membrane dynamics currently relies heavily on SPT for the determination of heterogeneous diffusion. A possible systematic error in this approach would thus have vast implications in this field. To address this, we have re-visited the analysis procedure for SPT data with an emphasis on the measurement errors and the effect that these errors have on the measurement outputs. We subsequently demonstrate that STED-FCS and SPT data, following careful consideration of the experimental errors of the SPT data, converge to a common interpretation which for the case of a diffusing phospholipid analogue in the plasma membrane of live mouse embryo fibroblasts results in an unhindered, intra-compartment, diffusion coefficient of  ≈0.7–1.0 µm2 s−1, and a compartment size of about 100–150 nm.

Chen, Kuan-Chao; Chu, Tung-Wei; Wu, Chong-Rong; Lee, Si-Chen; Lin, Shih-Yen

doi: 10.1088/1361-6463/aa52b6pmid: N/A

Large-area and uniform MoS2 films are fabricated by using sulfurization of pre-deposited molybdenum (Mo) films. One- and three-layer MoS2 films are obtained by sulfurizing 0.5 and 1.0 nm Mo films, respectively. The results have demonstrated the good layer number controllability of this growth technique down to single-layer MoS2. By sequential sulfurization of 0.5 nm W, 0.5 nm Mo and 0.5 nm W under the same condition, three layers of the WS2/MoS2/WS2 hetero-structure are established, which has demonstrated the potential of this growth technique for the establishment of 2D crystal hetero-structures.

Clausen, M P; Colin-York, H; Schneider, F; Eggeling, C; Fritzsche, M

doi: 10.1088/1361-6463/aa52a1pmid: 28458398

Nanoscale spacing between the plasma membrane and the underlying cortical actin cytoskeleton profoundly modulates cellular morphology, mechanics, and function. Measuring this distance has been a key challenge in cell biology. Current methods for dissecting the nanoscale spacing either limit themselves to complex survey design using fixed samples or rely on diffraction-limited fluorescence imaging whose spatial resolution is insufficient to quantify distances on the nanoscale. Using dual-color super-resolution STED (stimulated-emission-depletion) microscopy, we here overcome this challenge and accurately measure the density distribution of the cortical actin cytoskeleton and the distance between the actin cortex and the membrane in live Jurkat T-cells. We found an asymmetric cortical actin density distribution with a mean width of 230 (+105/−125) nm. The spatial distances measured between the maximum density peaks of the cortex and the membrane were bi-modally distributed with mean values of 50  ±  15 nm and 120  ±  40 nm, respectively. Taken together with the finite width of the cortex, our results suggest that in some regions the cortical actin is closer than 10 nm to the membrane and a maximum of 20 nm in others.

Zhang, Le-le; Li, Zhu-bai; Zhang, Xue-feng; Ma, Qiang; Liu, Yan-li; Li, Yong-feng; Zhao, Qian

doi: 10.1088/1361-6463/aa5361pmid: N/A

The element distribution and the magnetic properties were investigated in (Ce,Nd)–Fe–B sintered magnets prepared by mixing Nd13.5Fe80B6.5 and Ce9Nd4.5Fe80B6.5 powders with different mass ratios. Two main phases exist, but element diffusion is evident, and the chemical composition of the main phase is widely different from that of the master alloy. The Ce element tends to be expelled from the Ce-rich Re2Fe14B phase. Compared with the Ce-rich main phase, the Nd-rich Re2Fe14B phase is more stable in structure. Although the microstructure is inhomogeneous and the magnetocrystalline anisotropy is variable, the magnetization reversal is uniform in these dual main-phase magnets, which should ascribe to the existence of the exchange coupling, and magnetization reversal undergoes the nucleation of the reversed domain in irreversible magnetization. It is expected to further improve the coercivity by optimizing the distribution of the Nd-rich main phase in preparing the resource-saving (Ce,Nd)2Fe14B sintered magnets.

Soloviev, S V; Popkov, A F; Knizhnik, A A; Iskandarova, I M

doi: 10.1088/1361-6463/aa52afpmid: N/A

Based on the equation of motion of an antiferromagnetic moment, taking into account a random field of thermal fluctuations, we propose a Monte Carlo (MC) scheme for the numerical simulation of the evolutionary dynamics of an antiferromagnetic particle, corresponding to the Langevin dynamics in the Kramers theory for the two-well potential. Conditions for the selection of the sphere of fluctuations of random deviations of the antiferromagnetic vector at an MC time step are found. A good agreement with the theory of Kramers thermal relaxation is demonstrated for varying temperatures and heights of energy barrier over a wide range of integration time steps in an overdamped regime. Based on the developed scheme, we performed illustrative calculations of the temperature drift of the exchange bias under the fast annealing of a ferromagnet–antiferromagnet structure, taking into account the random variation of anisotropy directions in antiferromagnetic grains and their sizes. The proposed approach offers promise for modeling magnetic sensors and spintronic memory devices containing heterostructures with antiferromagnetic layers.

Bran, C; Gawronski, P; Lucas, I; del Real, R P; Strichovanec, P; Asenjo, A; Vazquez, M; Chubykalo-Fesenko, O

doi: 10.1088/1361-6463/aa4ee3pmid: N/A

(Co(0.4 nm)/Pt(0.7 nm))x(x  =  10, 20, 30) multilayer antidot thin films (films with arrays of nanoholes) have been grown by dc sputtering onto self-assembled pores of anodic alumina membranes with a tailored diameter and a fixed inter-hole distance. The magnetic behavior has been quantified by vibrating sample magnetometry, and the surface magnetization patterns have been imaged by magnetic force microscopy. The magnetization reversal mechanism is characterized by two steps depending on the film thickness and antidot diameter. These steps are ascribed to the nucleation and demagnetization of magnetic stripe domains. Their presence confirms the perpendicular anisotropy of the multilayer antidot films. The coercivity of antidot thin film is larger than that of the continuous films due to additional pinning centers provided by antidots. The width of the stripe domains increases as a function of film thickness. The demagnetization is further investigated through micromagnetic simulations that are in agreement with the measured hysteresis loops and their features. Different reversal mechanisms and an increase of the domain width in antidot thin films are also confirmed as a function of the magnetic anisotropy, antidot diameter and thickness of the thin film. The presence of antidots with a designed geometry is revealed to be successful in tailoring the coercivity of the thin films and magnetic patterns, which is relevant for advances in nanoscale technologies.

Sudakshina, B; Chandrasekhar, K Devi; Yang, H D; Vasundhara, M

doi: 10.1088/1361-6463/aa54depmid: N/A

Crystal structure and magnetic properties of polycrystalline Nd1−xCaxMnO3 (x  =  0.0, 0.2, 0.3, 0.33, 0.4, 0.5, 0.6 and 0.8) manganites were investigated. The fine structural refinement using GSAS was found to undergo a transition from Pnma reflections to Pbnm reflections associated with the Ca substitution at x  =  0.3. The magnetic ordering of these compounds witnessed distinct magnetic phases with variations of Ca substitution. Magnetic ordering of the parent compound, NdMnO3, was found as A-type antiferromagnetic (AFM) in accordance with the earlier reports, which progressively undergoes to canted A-type AFM for x  =  0.2, pseudo CE-type AFM for the intermediate compositions, i.e. x  =  0.3 to x  =  0.5 and CE-type AFM for x  >  0.5. The x  =  0.2 compound exhibited ferromagnetic like (weak AFM) behaviour, and the critical exponent study reinforced the magnetic inhomogeneity of the compound. Hysteresis curves of all the compounds measured at different temperatures implied the presence of metamagnetic like transitions for the intermediate compositions (0.3  ⩽  x  ⩽  0.5). Relative cooling power (RCP) value of Nd0.8Ca0.2MnO3 was observed to be 900 J Kg−1, at the higher magnetic field, making it a promising candidate for magnetic refrigeration applications.

Núñez-Cascajero, A; Valdueza-Felip, S; Monteagudo-Lerma, L; Monroy, E; Taylor-Shaw, E; Martin, R W; González-Herráez, M; Naranjo, F B

doi: 10.1088/1361-6463/aa53d5pmid: N/A

The structural, morphological, electrical and optical properties of In-rich AlxIn1−xN (0  <  x  <  0.39) layers grown by reactive radio-frequency (RF) sputtering on sapphire are investigated as a function of the deposition parameters. The RF power applied to the aluminum target (0 W–150 W) and substrate temperature (300 °C–550 °C) are varied. X-ray diffraction measurements reveal that all samples have a wurtzite crystallographic structure oriented with the c-axis along the growth direction. The aluminum composition is tuned by changing the power applied to the aluminum target while keeping the power applied to the indium target fixed at 40 W. When increasing the Al content from 0 to 0.39, the room-temperature optical band gap is observed to blue-shift from 1.76 eV to 2.0  eV, strongly influenced by the Burstein–Moss effect. Increasing the substrate temperature, results in an evolution of the morphology from closely-packed columnar to compact. For a substrate temperature of 500 °C and RF power for Al of 150 W, compact Al0.39In0.61N films with a smooth surface (root-mean-square surface roughness below 1 nm) are produced.

Liu, Lishu; Mei, Zengxia; Tang, Aihua; Liang, Huili; Du, Xiaolong

doi: 10.1088/1361-6463/50/6/065102pmid: N/A

The degradation of conductivity with increased Mg content for MgxZn1−xO wide bandgap materials has always been a fundamental application-motivated research issue. Herein, the study of self-compensating defects in MgxZn1−xO:F (0  ⩽  x  ⩽  0.29) thin films was performed to reveal their influence on increased resistivity. Our observations solidly evidence that the degradation of conductivity is mainly owing to the increased concentration of Zn vacancy (VZn)-related compensating defects in MgxZn1−xO alloys. The formation enthalpy of intrinsic VZn defects decreases as Mg content (x) increases. Thus, the compensation ratio increases from 0.23 at x  =  0 to 0.47 at x  =  0.29, resulting in deteriorated conductivity in MgxZn1−xO alloys. Cathodoluminescence (CL) spectra further confirm higher VZn concentrations with increased Mg content. The electron transport is demonstrated to be dominated by an ionized scattering mechanism. Formation of – complexes could reduce the concentration of ionized scattering centers and thus increase mobility. These results clarify the reason of increasingly high resistivity in MgxZn1−xO, which is a long-sought-after physics problem in this area, and provide crucial information on controlling the conductivity of MgxZn1−xO alloys.