Gejian Zhao

Above 400-K robust perpendicular ferromagnetic phase in a topological insulator

Topological surface states reveal proximity-induced ferromagnetism with perpendicular anisotropy persisting above 400 K. The quantum anomalous Hall effect (QAHE) that emerges under broken time-reversal symmetry in topological insulators (TIs) exhibits many fascinating physical properties for potential applications in nanoelectronics and spintronics. However, in transition metal–doped TIs, the only experimentally demonstrated QAHE system to date, the QAHE is lost at practically relevant temperatures. This constraint is imposed by the relatively low Curie temperature (Tc) and inherent spin disorder associated with the random magnetic dopants. We demonstrate drastically enhanced Tc by exchange coupling TIs to Tm3Fe5O12, a high-Tc magnetic insulator with perpendicular magnetic anisotropy. Signatures showing that the TI surface states acquire robust ferromagnetism are revealed by distinct squared anomalous Hall hysteresis loops at 400 K. Point-contact Andreev reflection spectroscopy confirms that the TI surface is spin-polarized. The greatly enhanced Tc, absence of spin disorder, and perpendicular anisotropy are all essential to the occurrence of the QAHE at high temperatures.

Erbium concentration control and optimization in erbium yttrium chloride silicate single crystal nanowires as a high gain material

Increasing erbium concentration while minimizing the reduction of photoluminescence is an important task for achieving erbium-based high-gain materials for integrated photonics applications. Here, we demonstrate a strategy of controlled variation of Erbium density in the growth of erbium yttrium chloride silicate (EYCS) ((ErxY1−x)3(SiO4)2Cl)) single crystal nanowires by systematically varying x between 0 and 1. We show that, as a trade-off between high Er density and suppressed upconversion, Er composition x = 0.3 provides the best compromise with the strongest photoluminescence. This optimized Er-composition corresponds to an Er density of 5×1021 cm−3, five times larger than the optimized Er density demonstrated previously for other thin film materials. We estimate that this optimized EYCS is promising in achieving optical gain exceeding 100 dB/cm.

Half-metallicity in highly L21-ordered CoFeCrAl thin films

The structural, magnetic, and electron-transport properties of Heusler-ordered CoFeCrAl thin films are investigated experimentally and theoretically. The films, sputtered onto MgO and having thicknesses of about 100 nm, exhibit virtually perfect single-crystalline epitaxy and a high degree of L21 chemical order. X-ray diffraction and transmission-electron microscopy show that the structure of the films is essentially of the L21 Heusler type. The films are ferrimagnetic, with a Curie temperature of about 390 K, and a net moment of 2 μB per formula unit. The room temperature resistivity is 175 μΩ cm; the carrier concentration and mobility determined from the low temperature (5 K) measurement are 1.2 × 1018 cm−3 and 33 cm2/V s, respectively. In contrast to the well-investigated Heusler alloys such as Co2(Cr1−xFex)Al, the CoFeCrAl system exhibits two main types of weak residual A2 disorder, namely, Co-Cr disorder and Fe-Cr disorder, the latter conserving half-metallicity. Point-contact Andreev reflection yiel...

Demonstration of Net Gain in an Erbium Chloride Silicate Single Nanowire Waveguide

Absorption coefficient of erbium chloride silicate nanowire is studied by monitoring the decay of upconversion emission from a single nanowire waveguide. 30 dB/cm low bound for the net gain is demonstrated for erbium chloride silicate.

Continuous control of spin polarization using a magnetic field

The giant magnetoresistance (GMR) of a point contact between a Co/Cu multilayer and a superconductor tip varies for different bias voltage. Direct measurement of spin polarization by Andreev reflection spectroscopy reveals that the GMR change is due to a change in spin polarization. This work demonstrates that the GMR structure can be utilized as a spin source and that the spin polarization can be continuously controlled by using an external magnetic field.

High spin polarization in epitaxial Fe4N thin films using Cr and Ag as buffer layers

Fe4N thin films with (001) texture were prepared by reactive sputtering on MgO substrates, utilizing either a Cr or Ag buffer layer to facilitate the epitaxial growth. X-ray diffraction, atomic force microscopy, and vibrating sample magnetometry measurements show that the Fe4N thin film grown on the Ag buffer layer is superior to that grown on the Cr buffer layer. The point contact Andreev reflection measurement was then conducted, and the spin polarizations were determined to be 61.1% and 81.3% for Fe4N thin films with Cr and Ag buffer layers, respectively. The 81.3% spin polarization is significantly higher than the ratio reported previously for Fe4N and is comparable with that of state-of-the-art Heusler alloys. This result is in agreement with the theoretical prediction on the discrepancy between the two differently defined spin polarizations for Fe4N. Moreover, our study indicates that an optimized growth process for Fe4N thin films is crucial for achieving a high spin polarization and that true half-metallicity could potentially be realized with Fe4N. The high spin polarization of Fe4N combined with its low fabrication temperature and simple composition makes Fe4N a competitive candidate to be a half-metallic ferromagnet in spintronic devices.Fe4N thin films with (001) texture were prepared by reactive sputtering on MgO substrates, utilizing either a Cr or Ag buffer layer to facilitate the epitaxial growth. X-ray diffraction, atomic force microscopy, and vibrating sample magnetometry measurements show that the Fe4N thin film grown on the Ag buffer layer is superior to that grown on the Cr buffer layer. The point contact Andreev reflection measurement was then conducted, and the spin polarizations were determined to be 61.1% and 81.3% for Fe4N thin films with Cr and Ag buffer layers, respectively. The 81.3% spin polarization is significantly higher than the ratio reported previously for Fe4N and is comparable with that of state-of-the-art Heusler alloys. This result is in agreement with the theoretical prediction on the discrepancy between the two differently defined spin polarizations for Fe4N. Moreover, our study indicates that an optimized growth process for Fe4N thin films is crucial for achieving a high spin polarization and that true half...

Zero bias anomaly in Andreev reflection spectroscopy

Zero bias anomalies have been observed in point contact Andreev reflection spectroscopy using conventional superconductors, and they cannot be suppressed by a highly spin-polarized current. By systematically varying the size of the point contact, the anomalies are found to be evolved from conductance dips outside the superconducting gap. These anomalies, which depend on the size of the interface, are not intrinsic to the superconductors but due to a large resistance change when the contact region is driven to normal by an Oersted field.

Singlet superconductivity in SmO 0.82 F 0.18 FeAs

In this paper, the Andreev reflection method which employs magnetic materials with various spin polarization has been used to study the superconductivity of SmO0.82F0.18FeAs. It is shown that this method can be utilized to measure the spin of Cooper pairs if a highly spin polarized current is used.

Control of thermal gradient using thermoelectric coolers for study of thermal effects

Thermoelectric coolers based on the Peltier effect have been utilized to control temperature gradient to study thermal effects in both bulk and thin film samples. The temperature gradient is controlled by two coolers and the polarity of the thermal gradient can be reversed by reversing an electric driven voltage. With appropriate controlled thermal gradient using this technique, the Nernst and the Seebeck effects can be measured in both bulk and thin film samples free of spurious contributions. In an arbitrary direction of thermal gradient, the Seebeck and the Nernst components can be decomposed from the measured signal based on the symmetry of the effects in a magnetic field.