Various device-based experiments have indicated that electron transfer in certain chiral molecules may be spin-dependent, a phenomenon known as the Chiral Induced Spin Selectivity (CISS) effect. However, due to the complexity of these devices and a lack of theoretical understanding, it is not always clear to what extent the chiral character of the molecules actually contributes to the magnetic-field-dependent signals in these experiments. To address this issue, we report here an electron transmission model that evaluates the role of the CISS effect in two-terminal and multiterminal linear-regime electron transport experiments. Our model reveals that for the CISS effect, the chirality-dependent spin transmission is accompanied by a spin-flip electron reflection process. Furthermore, we show that more than two terminals are required in order to probe the CISS effect in the linear regime. In addition, we propose two types of multi-terminal nonlocal transport measurements that can distinguish the CISS effect from other magnetic-field-dependent signals. Our model provides an effective tool to review and design CISS-related transport experiments, and to enlighten the mechanism of the CISS effect itself. arXiv:1810.02662v2 [cond-mat.mes-hall]
Central
to spintronics is the interconversion between electronic
charge and spin currents, and this can arise from the chirality-induced
spin selectivity (CISS) effect. CISS is often studied as magnetoresistance
(MR) in two-terminal (2T) electronic nanodevices containing a chiral
(molecular) component and a ferromagnet. However, fundamental understanding
of when and how this MR can occur is lacking. Here, we uncover an
elementary mechanism that generates such an MR for nonlinear response.
It requires energy-dependent transport and energy relaxation within
the device. The sign of the MR depends on chirality, charge carrier
type, and bias direction. Additionally, we reveal how CISS can be
detected in the linear response regime in magnet-free 2T nanodevices,
either by forming a chirality-based spin-valve using two or more chiral
components or by Hanle spin precession in devices with a single chiral
component. Our results provide operation principles and design guidelines
for chirality-based spintronic nanodevices and technologies.
We succeeded in observing the two-photon absorption of germanium for both the direct and the indirect gaps, using the Vanderbilt free-electron laser. The two-photon absorption for the indirect gap was found to be 3 orders of magnitude smaller than the direct gap. The indirect-gap threshold position indicates that the two-photon absorption is assisted by an LO phonon, providing a long-delayed positive test of the Bassani-Hassan predictions.
RF/microwave magnetic measurement systems can be categorized into broadband or narrowband systems. Narrowband systems utilize high Q cavities for magnetic material characterizations such as electron paramagnetic resonance or electron spin resonance. Example broadband characterization systems are permeameters or pulsed inductive magnetometers. Narrowband measurement systems have high sensitivity but limited bandwidth whereas broadband systems suffer from lower sensitivity. In particular, it has been challenging to precisely measure weak magnetization, high anisotropy and high ferromagnetic resonance linewidth across a broad bandwidth. In this work, we report a new highly sensitive broadband resonance measurement system with lock-in detection utilizing a broadband transmission line. The frequency of the RF source is swept while the magnetic field is fixed. The resulting curve of dP/dH versus f is similar to the dP/dH versus H curve from conventional narrowband magnetic measurement system. The imaginary component of magnetic susceptibility χ" is calculated by the equation of χ"(f)=C∫(dP/dH*dH/df)df, in which C is a constant and dH/df is determined from the Kittel equation for magnetic films. Broadband measurements of sputtered low-moment lossy NiCr (4πMs=1100 G) as well as NiFe (4πMs=11000 G) were taken and compared with conventional broadband measurement using a network analyzer.
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