Spin-orbit coupling and phase-coherence in InAs nanowires

We investigated the magnetotransport of InAs nanowires grown by selective area metal-organic vapor phase epitaxy. In the temperature range between 0.5 and 30 K reproducible fluctuations in the conductance upon variation of the magnetic field or the back-gate voltage are observed, which are attributed to electron interference effects in small disordered conductors. From the correlation field of the magnetoconductance fluctuations the phase-coherence length l_phi is determined. At the lowest temperatures l_phi is found to be at least 300 nm, while for temperatures exceeding 2 K a monotonous decrease of l_phi with temperature is observed. A direct observation of the weak antilocalization effect indicating the presence of spin-orbit coupling is masked by the strong magnetoconductance fluctuations. However, by averaging the magnetoconductance over a range of gate voltages a clear peak in the magnetoconductance due to the weak antilocalization effect was resolved. By comparison of the experimental data to simulations based on a recursive two-dimensional Green's function approach a spin-orbit scattering length of approximately 70 nm was extracted, indicating the presence of strong spin-orbit coupling.Comment: 8 pages, 7 figure

The distance between P680 and QA in Photosystem II determined by ESEEM spectroscopy

AbstractLight induced spin-polarized P680+Q−A radical pairs were studied by two pulse electron spin echo envelope modulation (ESEEM) spectroscopy in the cyanide-treated and Zn-substituted Photosystem II core complexes and in the isolated D1-D2-cyt b559 reaction center complexes reconstituted with dibromoisopropyl-p-benzoquinone. The observed strong out-of phase ESEEM signals were identified as those of the P680+Q−A radical pairs based on the time variation of the transient CW EPR spectra. The shapes of ESEEM spectra were attributed to dipolar D and spin exchange J interactions in the radical pairs. The values of D and J were derived from a sine Fourier transformation and the center-to-center distance between P680 and QA was determined to be 27.2±1.0Å for all three preparations

Low-field magnetoresistance in GaAs 2D holes

We report low-field magnetotransport data in two-dimensional hole systems in GaAs/AlGaAs heterostructures and quantum wells, in a large density range, $2.5 \times 10^{10} \leq p \leq 4.0 \times 10^{11}$ cm$^{-2}$, with primary focus on samples grown on (311)A GaAs substrates. At high densities, $p \gtrsim 1 \times 10^{11}$ cm$^{-2}$, we observe a remarkably strong positive magnetoresistance. It appears in samples with an anisotropic in-plane mobility and predominantly along the low-mobility direction, and is strongly dependent on the perpendicular electric field and the resulting spin-orbit interaction induced spin-subband population difference. A careful examination of the data reveals that the magnetoresistance must result from a combination of factors including the presence of two spin-subbands, a corrugated quantum well interface which leads to the mobility anisotropy, and possibly weak anti-localization. None of these factors can alone account for the observed positive magnetoresistance. We also present the evolution of the data with density: the magnitude of the positive magnetoresistance decreases with decreasing density until, at the lowest density studied ($p = 2.5 \times 10^{10}$ cm$^{-2}$), it vanishes and is replaced by a weak negative magnetoresistance.Comment: 8 pages, 8 figure

Tensile Fracture of Welded Polymer Interfaces: Miscibility, Entanglements and Crazing

Large-scale molecular simulations are performed to investigate tensile failure of polymer interfaces as a function of welding time $t$. Changes in the tensile stress, mode of failure and interfacial fracture energy $G_I$ are correlated to changes in the interfacial entanglements as determined from Primitive Path Analysis. Bulk polymers fail through craze formation, followed by craze breakdown through chain scission. At small $t$ welded interfaces are not strong enough to support craze formation and fail at small strains through chain pullout at the interface. Once chains have formed an average of about one entanglement across the interface, a stable craze is formed throughout the sample. The failure stress of the craze rises with welding time and the mode of craze breakdown changes from chain pullout to chain scission as the interface approaches bulk strength. The interfacial fracture energy $G_I$ is calculated by coupling the simulation results to a continuum fracture mechanics model. As in experiment, $G_I$ increases as $t^{1/2}$ before saturating at the average bulk fracture energy $G_b$. As in previous simulations of shear strength, saturation coincides with the recovery of the bulk entanglement density. Before saturation, $G_I$ is proportional to the areal density of interfacial entanglements. Immiscibiltiy limits interdiffusion and thus suppresses entanglements at the interface. Even small degrees of immisciblity reduce interfacial entanglements enough that failure occurs by chain pullout and $G_I \ll G_b$

Spin polarized transports through a narrow-gap semiconductor wire with ferromagnetic contacts formed on InAlAs step-graded buffer layers

We investigated the transport properties of ferromagnetic/semiconductor hybrid structures utilizing an InAs/In_Al_As modulation-doped heterostructures formed on a GaAs (001) substrate with In_xAl_As step-graded buffer layers. We used NiFe as ferromagnetic electrodes for injection/detection of spin-polarized electrons, which were formed on side walls of the semiconductor mesa to contact electron channel directly. We measured magneto-transport properties of the samples with current flow between the ferromagnetic electrodes at low temperatures. Under vertical magnetic fields, magneto-resistance oscillations were clearly observed, thus the ferromagnetic electrodes worked as ohmic contacts. In addition, we successfully found spin-valve properties under parallel magnetic fields. Furthermore, we observed the enhancement of spin-valve properties by squeezing the channel width

Spin-splitting analysis of a two-dimensional electron gas in an almost strain-free In0.89Ga0.11Sb/In0.88Al0.12Sb by magneto-resistance measurements

We investigated the spin-splitting in an almost strain-free In0.89Ga0.11Sb/In0.88Al0.12Sb two-dimensional electron gas (2DEG) by magnetoresi stance measurements at 1.5 K. A large effective gyromagnetic factor (g factor) vertical bar g*vertical bar=33-34 was obtained by means of the coincidence method, which assumes an effective mass m*=0.021m(0) at the Fermi energy. In spite of the large g factor and the high mobility (mu=9.8 x 10(4) cm(2)/V s), a vanishing spin-splitting was also found around B similar to 0.8 T by analyzing the second derivative of the magnetoresistance. This effect originates from the interplay between the Rashba and Dresselhaus spin-orbit interactions, and we theoretically confirmed the fact that the Dresselhaus spin-splitting energy Delta E-0D=3.5 meV was more than twice as large as the Rashba spin-splitting energy Delta E-0R=1.5 meV. Moreover, we demonstrated that the theoretical curves of the normalized spin splitting, including the g factor and the spin-orbit interactions, were well fitted to the experimental points with the Dresselhaus spin-orbit interaction. Therefore, we concluded that the Dresselhaus spin-orbit interaction is dominant in our 2DEG in spite of its narrow band gap