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3 Dec 2007

Volume 91, Issue 23, Articles (23xxxx)

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Appl. Phys. Lett. 91, 232101 (2007); http://dx.doi.org/10.1063/1.2818712 (3 pages)

T. Kita, D. Chiba, Y. Ohno, and H. Ohno
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Microstructure of NbN epitaxial ultrathin films grown on A-, M-, and R-plane sapphire

R. Espiau de Lamaëstre, Ph. Odier, and J.-C. Villégier

Appl. Phys. Lett. 91, 232501 (2007); http://dx.doi.org/10.1063/1.2820607 (3 pages) | Cited 10 times

Online Publication Date: 3 December 2007

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We compared ultrathin NbN films (2.5–10 nm) simultaneously grown by dc reactive sputtering at 600 °C on A, M, and R orientations of sapphire substrates. Film structures and superconducting properties were characterized. We show that actual NbN device films on R-Al2O3 are (135) oriented and suffer from detrimental disoriented twin domains. On the contrary, NbN on M-Al2O3 is shown to be untwined, leading to a lower resistivity, an increased critical current density Jc (>4 MA cm−2 at 4.2 K), and a higher critical temperature Tc (11.3 K for 4.4 nm). These results offer promising alternative for better performances of superconducting detectors and mixers.
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74.78.Na Mesoscopic and nanoscale systems
74.25.Sv Critical currents
74.25.F- Transport properties
81.15.Cd Deposition by sputtering
61.72.Mm Grain and twin boundaries

High-contrast readout of superconducting qubits beyond the single-shot resolution limit

J. Lisenfeld, A. Lukashenko, and A. V. Ustinov

Appl. Phys. Lett. 91, 232502 (2007); http://dx.doi.org/10.1063/1.2818693 (3 pages) | Cited 1 time

Online Publication Date: 4 December 2007

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The states of superconducting quantum bits, as the Josephson phase qubits and flux qubits, are distinguished by magnetic flux generated by a persistent current in a superconducting loop. As the readout circuit contributes decoherence, its coupling to the qubit should be made as weak as possible. We demonstrate a method to maximize the measurement contrast of the two qubit states, thus allowing to significantly decrease the coupling to the readout. This technique is promising for multiqubit circuits since it allows to read out the states of several qubits by using a single readout circuit.
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74.50.+r Tunneling phenomena; Josephson effects
85.25.-j Superconducting devices
84.30.-r Electronic circuits

Tuning of the in-plane strain in cuprate thin films using a solid-solution compound as a buffer layer

M. Hanawa and I. Tsukada

Appl. Phys. Lett. 91, 232503 (2007); http://dx.doi.org/10.1063/1.2822435 (3 pages) | Cited 1 time

Online Publication Date: 5 December 2007

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In order to tune the epitaxial strain between a superconducting cuprate thin film and a substrate, we introduce a buffer layer of LaSrAl1−zGazO4 which forms solid solutions for the entire range from z = 0–1. Thin films of La1.875Ba0.125CuO4/LaSrAl1−zGazO4 grown by pulsed laser deposition have been characterized by x-ray diffraction and resistivity measurements. The c-axis length of La1.875Ba0.125CuO4 on the buffer layer reveals a systematic change with z indicating that the epitaxial strain is reasonably controlled. The films within the composition range of z = 0.2–0.6 show superconductivity with systematic change of the critical temperature, while the sample with z = 0.8 is insulating below 100 K.
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74.72.-h Cuprate superconductors
74.78.-w Superconducting films and low-dimensional structures
81.15.Fg Pulsed laser ablation deposition
74.25.F- Transport properties
74.25.Ld Mechanical and acoustical properties, elasticity, and ultrasonic attenuation

Very low 1/f noise at room temperature in fully epitaxial Fe/MgO/Fe magnetic tunnel junctions

F. G. Aliev, R. Guerrero, D. Herranz, R. Villar, F. Greullet, C. Tiusan, and M. Hehn

Appl. Phys. Lett. 91, 232504 (2007); http://dx.doi.org/10.1063/1.2822812 (3 pages) | Cited 15 times

Online Publication Date: 6 December 2007

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We report on room temperature 1/f noise in fully epitaxial Fe (45 nm)/MgO (2.6 nm)/Fe (10 nm) magnetic tunnel junctions (MTJs) with and without carbon doping of the Fe/MgO bottom interface. We have found that the normalized noise (Hooge factor) asymmetry between parallel and antiparallel states may strongly depend on the applied bias and its polarity. Both types of MTJs exhibit record of low Hooge factors being at least one order of magnitude smaller than previously reported.
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72.70.+m Noise processes and phenomena
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Longitudinal magneto-optic Kerr effect detection of latching vortex magnetization chirality in individual mesoscale rings

S. R. Bowden, K. K. L. Ahmed, and U. J. Gibson

Appl. Phys. Lett. 91, 232505 (2007); http://dx.doi.org/10.1063/1.2822814 (3 pages) | Cited 5 times

Online Publication Date: 6 December 2007

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We report on a method for breaking the symmetry of the optical signal arising from vortex magnetization in individual micron scale rings. A dielectric coating enhances the longitudinal magneto-optical Kerr effect signal from one half of the ring, while leaving the magnetic interactions unperturbed. The chirality of a single ring can be determined using this method, even with a beam waist much larger than the ring diameter. We observed switchable chirality in clipped 5 μm diameter Permalloy (Ni80Fe20) rings using a longitudinal measurement field and demonstrated that the rotation sense could be reversed repeatedly with a momentary transverse field of ±150 Oe.
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78.20.Ls Magneto-optical effects
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Radiation field screening in photoconductive antennae studied via pulsed terahertz emission spectroscopy

Gabriel C. Loata, Mark D. Thomson, Torsten Löffler, and Hartmut G. Roskos

Appl. Phys. Lett. 91, 232506 (2007); http://dx.doi.org/10.1063/1.2823590 (3 pages) | Cited 4 times

Online Publication Date: 6 December 2007

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We report terahertz emission experiments on low-temperature-grown GaAs photoconductive antennae. Two field-screening effects determine the device response: space-charge screening on a long time scale and radiation field screening of the local electric field. This latter effect is the principal cause for saturation of terahertz emission observed when the emitters are driven hard with high-repetition-rate femtosecond laser pulses. We present an equivalent-circuit model consisting of three elements: a resistor with time-dependent conductance (photoswitch), a time-dependent voltage source (space-charge screening), and the antenna impedance (terahertz emission and radiation field screening). The simulations with this voltage divider reproduce the measured data well.
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73.61.Ey III-V semiconductors
81.05.Ea III-V semiconductors
72.40.+w Photoconduction and photovoltaic effects
85.60.-q Optoelectronic devices
84.40.Ba Antennas: theory, components and accessories

Micromagnetic modeling of laser-induced magnetization dynamics using the Landau-Lifshitz-Bloch equation

U. Atxitia, O. Chubykalo-Fesenko, N. Kazantseva, D. Hinzke, U. Nowak, and R. W. Chantrell

Appl. Phys. Lett. 91, 232507 (2007); http://dx.doi.org/10.1063/1.2822807 (3 pages) | Cited 14 times

Online Publication Date: 7 December 2007

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We present a dynamic approach to micromagnetics based on the Landau-Lifshitz-Bloch equation and Langevin dynamics. This type of modeling will be necessary at high temperatures when the magnetization length is not conserved, especially close to the Curie temperature. We model the laser-induced magnetization dynamics with various laser pulse fluences and show that the results are consistent with both experiments and atomistic modeling. Our results show different recovery rates depending on the final demagnetized state.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.40.Mg Numerical simulation studies
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.10.Hk Classical spin models
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