133 research outputs found
Charge Transport and Quantum Phase Transitions in Singlet Superconductor - Ferromagnet - Singlet Superconductor Junctions
We study the Josephson current, I_J, in a junction consisting of two s-wave
superconductors that are separated by a ferromagnetic barrier possessing a
magnetic and non-magnetic scattering potential, g and Z, respectively. We
discuss the general dependence of I_J on g, Z, and the phase difference \phi
between the two superconductors. Moreover, we compute the critical current, I_c
for given g and Z, and show that it possesses two lines of non-analyticity in
the (g, Z)-plane. We identify those regions in the (g, Z)-plane where the
Josephson current changes sign with increasing temperature without a change in
the relative phase between the two superconductors, i.e., without a transition
between a 0 and \pi state of the junction. Finally, we show that by changing
the relative phase \phi, it is possible to tune the junction through a
first-order quantum phase transition in which the spin polarization of the two
superconductors' combined ground state changes from =0 to =1/2.Comment: final version, published in Phys. Rev.
Diffusion based degradation mechanisms in giant magnetoresistive spin valves
Spin valve systems based on the giant magnetoresistive (GMR) effect as used
for example in hard disks and automotive applications consist of several
functional metallic thin film layers. We have identified by secondary ion mass
spectrometry (SIMS) two main degradation mechanisms: One is related to oxygen
diffusion through a protective cap layer, and the other one is interdiffusion
directly at the functional layers of the GMR stack. By choosing a suitable
material as cap layer (TaN), the oxidation effect can be suppressed.Comment: 3 pages, 3 figures. to be published in Appl. Phys. Let
Controlled oxygen vacancy induced p-type conductivity in HfO{2-x} thin films
We have synthesized highly oxygen deficient HfO thin films by
controlled oxygen engineering using reactive molecular beam epitaxy. Above a
threshold value of oxygen vacancies, p-type conductivity sets in with up to 6
times 10^{21} charge carriers per cm3. At the same time, the band-gap is
reduced continuously by more than 1 eV. We suggest an oxygen vacancy induced
p-type defect band as origin of the observed behavior.Comment: 4 pages, 3 figure
{001}-textured Pb(Zr, Ti)O₃ thin films on stainless steel by pulsed laser deposition
In this work, we report nearly single oriented {001}-textured ferroelectric PbZr0.52Ti0.48O3 thin films grown by pulsed laser deposition onto AISI 304 stainless steel substrates. Pt, Al2O3, and LaNiO3 buffer layers promote the PbZr0.52Ti0.48O3 {001} texture and protect the substrate against oxidation during deposition. The dominant {001} texture of the PbZr0.52Ti0.48O3 layer was confirmed using x-ray and electron backscatter diffraction. Before poling, the films exhibit a permittivity of about 350 at 1 kHz and a dielectric loss below 5%. The films display a remanent polarization of about 16.5 μC cm⁻² and a high coercive field of up to Ec ¼ 135.9 kV cm⁻¹. The properties of these PbZr0.52Ti0.48O3 thin films on stainless steel are promising for various MEMS applications such as transducers or energy harvesters
The importance of self-consistency in determining interface properties of S-I-N and D-I-N structures
We develop a method to solve the Bogoliubov de Gennes equation for
superconductors self-consistently, using the recursion method. The method
allows the pairing interaction to be either local or non-local corresponding to
and --wave superconductivity, respectively. Using this method we examine
the properties of various and interfaces. In particular we
self-consistently calculate the spatially varying density of states and the
superconducting order parameter. We see that changing the strength of the
insulating barrier, at the interface, does not, in the case of an --wave
superconductor, dramatically, change the low energy local density of states, in
the superconducting region near the interface. This is in stark contrast to
what we see in the case of a interface where the local particle density
of states is changed dramatically. Hence we deduce that in calculating such
properties as the conductance of and structures it is far more
important to carry out a self-consistent calculations in the --wave case.Comment: 14 pages, 8 figures, submitted to special issue of "Superlattices and
Microstructures
Optical Properties of Highly Conductive SrMoO₃ Oxide Thin Films in the THz Band and Beyond
Strontium molybdate (SrMoO₃) thin films are grown epitaxially by pulsed laser deposition onto gadolinium scandate (GdScO₃) substrates and characterized in the terahertz (THz) and visible part of the electromagnetic spectrum. X-ray diffraction measurements prove a high crystallinity and phase-pure growth of the thin films. The high-quality SrMoO₃ thin films feature a room temperature DC conductivity of around 3 1/μΩm. SrMoO₃ is characterized in the THz frequency range by time domain spectroscopy. The resulting AC conductivity is in excellent agreement with the DC value. A Lorentz-Drude oscillator approach models the THz and visible conductivity of SrMoO₃ very well. We compare the results of the SrMoO₃ thin films to a standard, sputtered gold film, with a resulting THz conductivity of 8 1/μΩm. The comparison demonstrates that oxide thin film–based devices can play an important role in future THz technology
On the origin of incoherent magnetic exchange coupling in MnBi/FeCo bilayer system
In this study we investigate the exchange coupling between the hard magnetic
compound MnBi and the soft magnetic alloy FeCo including the interface
structure between the two phases. Exchange spring MnBi-FeCo (x =
0.65 and 0.35) bilayers with various thicknesses of the soft magnetic layer
were deposited onto quartz glass substrates in a DC magnetron sputtering unit
from alloy targets. Magnetic measurements and density functional theory (DFT)
calculations reveal that a Co-rich FeCo layer leads to more coherent exchange
coupling. The optimum soft layer thickness is about 1 nm. In order to take into
account the effect of incoherent interfaces with finite roughness, we have
combined a cross-sectional High Resolution Transmission Electron Microscopy
(HR-TEM) analysis with DFT calculations and micromagnetic simulations. The
experimental results can be consistently described by modeling assuming a
polycrystalline FeCo layer consisting of crystalline (110) and amorphous grains
as confirmed by HR-TEM. The micromagnetic simulations show in general how the
thickness of the FeCo layer and the interface roughness between the hard and
soft magnetic phases both control the effectiveness of exchange coupling in an
exchange spring system
Gradual reset and set characteristics in yttrium oxide based resistive random access memory
This paper addresses the resistive switching behavior in yttrium oxide based resistive random access memory (RRAM) (TiN/yttrium oxide/Pt) devices. We report the coexistence of bipolar and unipolar resistive switching within a single device stack. For bipolar DC operation, the devices show gradual set and reset behavior with resistance ratio up to two orders of magnitude. By using nanosecond regime pulses (20 to 100 ns pulse width) of constant voltage amplitude, this gradual switching behavior could be utilized in tuning the resistance during set and reset spanning up to two orders of magnitude. This demonstrates that yttrium oxide based RRAM devices are alternative candidates for multibit operations and neuromorphic applications
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Geometric conductive filament confinement by nanotips for resistive switching of HfO2-RRAM devices with high performance
Filament-type HfO2-based RRAM has been considered as one of the most promising candidates for future non-volatile memories. Further improvement of the stability, particularly at the “OFF” state, of such devices is mainly hindered by resistance variation induced by the uncontrolled oxygen vacancies distribution and filament growth in HfO2 films. We report highly stable endurance of TiN/Ti/HfO2/Si-tip RRAM devices using a CMOS compatible nanotip method. Simulations indicate that the nanotip bottom electrode provides a local confinement for the electrical field and ionic current density; thus a nano-confinement for the oxygen vacancy distribution and nano-filament location is created by this approach. Conductive atomic force microscopy measurements confirm that the filaments form only on the nanotip region. Resistance switching by using pulses shows highly stable endurance for both ON and OFF modes, thanks to the geometric confinement of the conductive path and filament only above the nanotip. This nano-engineering approach opens a new pathway to realize forming-free RRAM devices with improved stability and reliability
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