Wenqin Zou

Large low field magnetoresistance in ultrathin nanocrystalline magnetite Fe3O4 films at room temperature

Ultrathin (15nm) Fe3O4 nanocrystalline films with (111) spinel texture have been prepared by rapid annealing of amorphous ion oxide films. Large low field magnetoresistance (LFMR), with the values of about −6.3% at 300K and −10% at 200K under a field of 0.5T, has been observed in the films. The LFMR is mainly attributed to the boundary tunneling of high spin-polarized electrons in Fe3O4 grains of the films and nearly follows a simple relationship between MR and polarization for intergranular tunneling. The fabricating method here seems to be a good approach to prepare high quality Fe3O4 nanocrystalline films with a large LFMR at room temperature.

Influence of the oxidative annealing temperature on the magnetism of (Mn, N)-codoped ZnO thin films

(Mn, N)-codoped ZnO films have been achieved by oxidative annealing of sputtered Zn2N3:Mn films at different temperatures in flowing O2 ambient. X-ray diffraction measurements and x-ray photoelectron spectroscopy studies indicate that both divalent Mn2+ and trivalent N3− ions are incorporated into ZnO lattice. Ferromagnetism with Curie temperature above 300 K was observed in the (Mn, N)-codoped ZnO films and found to be sensitive to the oxidative annealing temperature due to its strong effect on the carrier type and concentration. The strongest ferromagnetism has been found in the (Mn, N)-codoped ZnO films with the highest hole concentration in this study. The results indicate that holes are favorable for ferromagnetic ordering of doped Mn2+ ions in ZnO in agreement with the recent theoretical studies. The mechanism of ferromagnetic coupling in (Mn, N)-codoped ZnO is discussed.

Intrinsic Topological Insulator Bi 1.5 Sb 0.5 Te 3-x Se x Thin Crystals

The quaternary topological insulator (Bi,Sb)2(Te,Se)3 has demonstrated topological surface states with an insulating bulk. Scientists have identified an optimized composition of Bi1.5Sb0.5Te1.7Se1.3 with the highest resistivity reported. But the physics that drive to this composition remains unclear. Here we report the crystal structure and the magneto-transport properties of Bi1.5Sb0.5Te3-xSex (BSTS) series. A correlation between the structure and the physical properties has been revealed. We found out that within the rhombohedral structure, the composition with most Te substituting Se has the highest resistivity. On the other hand, segregation of other composition phases will introduce much higher bulk concentration.

Room-temperature ferromagnetism in p-type (Mn, N)-codoped ZnO thin films achieved by thermal oxidation of sputtered Zn3N2:Mn films

P-type (Mn, N)-codoped ZnO films have been fabricated by oxidative annealing of sputtered Zn2N3 : Mn films on silicon substrates at 550 °C in flowing O2 ambient. X-ray diffraction and x-ray photoelectron spectroscopy studies reveal that the Mn and N ions incorporated into the ZnO lattice are divalent Mn2+ and trivalent N3− ions, respectively. Ferromagnetism with Curie temperature above 300 K was observed in p-type (Mn, N)-codoped ZnO films. The results indicate that holes are favourable for ferromagnetic ordering in doped Mn2+ ions in ZnO, in agreement with recent theoretical studies.

Charge transfer and orbital reconstruction in the (La2/3Sr1/3MnO3)m/(SrRuO3)n superlattices

The structural, electronic, and magnetic properties of (La2/3Sr1/3MnO3)m/(SrRuO3)n superlattices have been investigated based on the first principles calculations. An obvious Jahn-Teller distortion, which depends on m, n, appears in MnO6 octahedron in the superlattices. The stretch along c-axis of MnO6 octahedron at the interface lifts the Mn eg orbital degeneracy, with electrons preferring the lower energy 3z2−r2 to the higher energy x2−y2. Benefitting from the charge transfer at the interface, the still occupied x2−y2 orbital can mediate a robust in-plane double exchange interaction. La2/3Sr1/3MnO3 block is ferromagnetic and metallic, even for the superlattice with m = n = 1.

Observation of Quantum Hall effect in an ultra-thin (Bi0.53Sb0.47)2Te3 film

We report the observation of the Quantum Hall effect from the topological surface states in both the Dirac electron and Dirac hole regions in a 4 quintuple layer (Bi0.53Sb0.47)2Te3 film grown on GaAs (111)B substrates. The Fermi level is sitting within the enlarged bulk band gap due to the quantum confinement of the ultra-thin film and can be tuned through the Dirac point by gate biases. Furthermore, the Hall resistance Rxy shows even denominator plateaus, which could be fractional Quantum Hall states. This may be due to the hybridization between the top and bottom surface states and suggests the possible way to manipulate the interaction of two surfaces for potential spintronic devices.

Magnetoresistance and nonlinear Hall effect in quaternary topological insulator Bi1.5Sb0.5Te1.8Se1.2

The quaternary topological insulator Bi2−xSbxTe3−ySey has exhibited pronounced topological surface states with an insulating bulk. While most transport studies of these compounds are focused on thin flakes peeled from bulk crystals, we carefully analyzed the magneto transport properties of bulk Bi1.5Sb0.5Te1.8Se1.2. We have observed weak anti-localization (WAL) and nonlinear Hall effects at low temperatures. Using a multi-channel model, we decomposed the total conductance into three conducting channels of the bulk, impurity band and surface states at various temperatures. The conductance of the surface states is almost constant in the whole temperature range, and contributes about 19.1% in the total conductance at 1.9 K.

Evidence for Layered Quantized Transport in Dirac Semimetal ZrTe 5

ZrTe5 is an important semiconductor thermoelectric material and a candidate topological insulator. Here we report the observation of Shubnikov-de Hass (SdH) oscillations accompanied by quantized Hall resistance in bulk ZrTe5 crystal, with a mobility of 41,000 cm2V−1s−1. We have found that the quantum oscillations does not originate from the surface states, but from the bulk states. Each single layer ZrTe5 acted like an independent 2D electron system in the quantum Hall regime having the same carrier density and mobilities, while the bulk of the sample exhibits a multilayered quantum Hall effect.

Ultrahigh Hall mobility and suppressed backward scattering in layered semiconductor Bi2O2Se

We report on an ultrahigh Hall mobility exceeding 40 000 cm2/V s and a very long traditional scattering time in a trivial layered semiconductor Bi2O2Se. Shubnikov-de Haas (SdH) oscillations were observed in both the unsaturated longitudinal linear magnetoresistance Rxx and the transverse Hall resistance Rxy. The amplitude ΔRxy of SdH oscillations was phase-shifted approximately 180° with respect to ΔRxx, indicating the strong suppression of electron backward scattering. This was further proved by the evidence of transport lifetime that is 10 times longer than the quantum lifetime. Our results show that the suppressed backward scattering in nontrivial Dirac semimetals can also occur in the trivial semiconductor Bi2O2Se.We report on an ultrahigh Hall mobility exceeding 40 000 cm2/V s and a very long traditional scattering time in a trivial layered semiconductor Bi2O2Se. Shubnikov-de Haas (SdH) oscillations were observed in both the unsaturated longitudinal linear magnetoresistance Rxx and the transverse Hall resistance Rxy. The amplitude ΔRxy of SdH oscillations was phase-shifted approximately 180° with respect to ΔRxx, indicating the strong suppression of electron backward scattering. This was further proved by the evidence of transport lifetime that is 10 times longer than the quantum lifetime. Our results show that the suppressed backward scattering in nontrivial Dirac semimetals can also occur in the trivial semiconductor Bi2O2Se.