Comparison of hexagonal boron nitride and MgO tunnel barriers in Fe,Co magnetic tunnel junctions

Magnetic tunnel junctions (MTJ) with MgO/Fe based interfaces and out-of-plane spin direction form the basis of present-day spin-transfer-torque magnetic random-access memory (STT-MRAM) devices. They are a leading type of nonvolatile memory due to their very long endurance times and lack of reliability problems. Many semiconductor devices, such as the field effect transistor or nonvolatile memories, have undergone fundamental changes in materials design as dimensional scaling has progressed. Here, we consider how the future scaling of the MTJ dimensions might affect materials choices and compare the performance of different tunnel barriers, such as 2D materials like h-BN with the existing MgO tunnel barriers. We first summarize key features of MgO-based designs of STT-MRAM. We then describe general aspects of the deposition of 2D materials and h-BN on metals. We compare the band structures of MgO and h-BN with their band gaps corrected for the GGA band error. The different absorption sites of h-BN on Fe or Co are compared in terms of physisorbtive or chemisorbtive bonding sites and how this affects their spin-polarized bands and the transmission magneto-resistance (TMR). The transmission magneto-resistance is found to be highest for the physisorptive sites. We look at how these changes would affect the overall TMR and how scaling might progress.</jats:p

Instantons in the Higgs Phase

When instantons are put into the Higgs phase, vortices are attached to instantons. We construct such composite solitons as 1/4 BPS states in five-dimensional supersymmetric U(Nc) gauge theory with Nf(>=Nc) fundamental hypermultiplets. We solve the hypermultiplet BPS equation and show that all 1/4 BPS solutions are generated by an Nc x Nf matrix which is holomorphic in two complex variables, assuming the vector multiplet BPS equation does not give additional moduli. We determine the total moduli space formed by topological sectors patched together and work out the multi-instanton solution inside a single vortex with complete moduli. Small instanton singularities are interpreted as small sigma-model lump singularities inside the vortex. The relation between monopoles and instantons in the Higgs phase is also clarified as limits of calorons in the Higgs phase. Another type of instantons stuck at an intersection of two vortices and dyonic instantons in the Higgs phase are also discussed.Comment: 32 pages, 6 figures, typos corrected, comments and references adde

High-spectral-resolution pulsed photoluminescence study of molecular-beam-epitaxy-grown GaAs/AlxGa1−xAs multi-quantum-well structures using a very-low-power tunable pulsed dye laser

Ultralow-power, high-resolution, pulsed-laser photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies were carried out in molecular-beam-epitaxial GaAs/AlxGa1−xAs multi-quantum-well structures at 5 K. Fine structures were observed for the first time in the PLE spectra, both in the heavy-hole and light-hole excitonic regions. Most of the fine structures are considered to arise from monolayer fluctuations in the thicknesses of the GaAs wells. Dramatic changes in the line shapes and the peak positions of the PL and PLE spectra were observed by applying selective PL detection and excitation spectroscopic techniques

Homodyne detection for measuring internal quantum correlations of optical pulses

A new method is described for determining the quantum correlations at different times in optical pulses by using balanced homodyne detection. The signal pulse and sequences of ultrashort test pulses are superimposed, where for chosen distances between the test pulses their relative phases and intensities are varied from measurement to measurement. The correlation statistics of the signal pulse is obtained from the time-integrated difference photocurrents measured.Comment: 7 pages, A4.sty include

Quantitative characterization of the Antarctic ozone hole

The long-term evolution of the Antarctic ozone hole is studied based on the TOMS data and the JMA data-set of stratospheric temperature in relation with the possible role of polar stratospheric clouds (PSC's). The effective mass of depleted ozone in the ozone hole at its annual mature stage reached a historical maximum of 55 Mt in 1991, 4.3 times larger than in 1981. The ozone depletion rate during 30 days before the mature ozone hole does not show any appreciable long-term trend but the interannual fluctuations do, ranging from 0.169 to 0.689 Mt/day with the average of 0.419 Mt/day for the period of 1979 - 1991. The depleted ozone mass has the highest correlation with the region below 195 K on the 30 mb surface in June, whereas the ozone depletion rate correlates most strongly with that in August. The present result strongly suggests that the long-term evolution of the mature ozone hole is caused both by the interannual change of the latitudinal coverage of the early PSC's, which may control the latitude and date of initiation of ozone decrease, and by that of the spatial coverage of the mature PSC's which may control the ozone depletion rate in the Antarctic spring

Intersecting Solitons, Amoeba and Tropical Geometry

We study generic intersection (or web) of vortices with instantons inside, which is a 1/4 BPS state in the Higgs phase of five-dimensional N=1 supersymmetric U(Nc) gauge theory on R_t \times (C^\ast)^2 \simeq R^{2,1} \times T^2 with Nf=Nc Higgs scalars in the fundamental representation. In the case of the Abelian-Higgs model (Nf=Nc=1), the intersecting vortex sheets can be beautifully understood in a mathematical framework of amoeba and tropical geometry, and we propose a dictionary relating solitons and gauge theory to amoeba and tropical geometry. A projective shape of vortex sheets is described by the amoeba. Vortex charge density is uniformly distributed among vortex sheets, and negative contribution to instanton charge density is understood as the complex Monge-Ampere measure with respect to a plurisubharmonic function on (C^\ast)^2. The Wilson loops in T^2 are related with derivatives of the Ronkin function. The general form of the Kahler potential and the asymptotic metric of the moduli space of a vortex loop are obtained as a by-product. Our discussion works generally in non-Abelian gauge theories, which suggests a non-Abelian generalization of the amoeba and tropical geometry.Comment: 39 pages, 11 figure

Domain Wall Junction in N=2 Supersymmetric QED in four dimensions

An exact solution of domain wall junction is obtained in N=2 supersymmetric (SUSY) QED with three massive hypermultiplets. The junction preserves two out of eight SUSY. Both a (magnetic) Fayet-Iliopoulos (FI) term and complex masses for hypermultiplets are needed to obtain the junction solution. There are zero modes corresponding to spontaneously broken translation, SUSY, and U(1). All broken and unbroken SUSY charges are explicitly worked out in the Wess-Zumino gauge in N=1 superfields as well as in components. The relation to models in five dimensions is also clarified.Comment: 27 pages, 6 figures, comments on zero modes added, a few references adde

Spin torque resonant vortex core expulsion for an efficient radio-frequency detection scheme

Spin-polarised radio-frequency currents, whose frequency is equal to that of the gyrotropic mode, will cause an excitation of the core of a magnetic vortex confined in a magnetic tunnel junction. When the excitation radius of the vortex core is greater than that of the junction radius, vortex core expulsion is observed, leading to a large change in resistance, as the layer enters a predominantly uniform magnetisation state. Unlike the conventional spin-torque diode effect, this highly tunable resonant effect will generate a voltage which does not decrease as a function of rf power, and has the potential to form the basis of a new generation of tunable nanoscale radio-frequency detectors