Yuxiao Zhang

Structure and magnetic properties of Mn-doped ZnO nanoparticles

We report the crystal structure and magnetic properties of the Zn1-xMnxO compounds synthesized by a combustion method. The Zn1-xMnxO compounds with x=0-0.1 crystallize in the wurtzite ZnO structure. The lattice parameters a and c of Zn1-xMnxO increase linearly with the Mn content, indicating that Mn2+ ions substitute for Zn2+ ions. Scanning electron microscopy shows that the average particle radius of Zn0.95Mn0.05O is about 40 nm. From the Curie-Weiss behavior of susceptibility at high temperature, it was found that the Mn-Mn interaction is dominated by antiferromagnetic coupling with effective nearest-neighbor exchange constant J about -32 K and the large negative value of the Curie-Weiss temperature

Observation of Fermi arc and its connection with bulk states in the candidate type-II Weyl semimetal WTe2

Chenlu Wang, Yan Zhang, Jianwei Huang, Simin Nie, Guodong Liu1,∗, Aiji Liang, Yuxiao Zhang, Bing Shen, Jing Liu, Cheng Hu, Ying Ding, Defa Liu, Yong Hu, Shaolong He, Lin Zhao, Li Yu, Jin Hu, Jiang Wei, Zhiqiang Mao, Youguo Shi, Xiaowen Jia, Fengfeng Zhang, Shenjin Zhang, Feng Yang, Zhimin Wang, Qinjun Peng, Hongming Weng, Xi Dai, Zhong Fang, Zuyan Xu, Chuangtian Chen and X. J. Zhou1,5,∗ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. Department of Physics and Engineering Physics, Tulane University, New Orleans, Louisiana 70118, USA Military Transportation University, Tianjin 300161, China. Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Collaborative Innovation Center of Quantum Matter, Beijing 100871, China. These people contributed equally to the present work. ∗Corresponding author: gdliu arpes@iphy.ac.cn, XJZhou@iphy.ac.cn.

Electronic evidence of temperature-induced Lifshitz transition and topological nature in ZrTe 5

The topological materials have attracted much attention for their unique electronic structure and peculiar physical properties. ZrTe5 has host a long-standing puzzle on its anomalous transport properties manifested by its unusual resistivity peak and the reversal of the charge carrier type. It is also predicted that single-layer ZrTe5 is a two-dimensional topological insulator and there is possibly a topological phase transition in bulk ZrTe5. Here we report high-resolution laser-based angle-resolved photoemission measurements on the electronic structure and its detailed temperature evolution of ZrTe5. Our results provide direct electronic evidence on the temperature-induced Lifshitz transition, which gives a natural understanding on underlying origin of the resistivity anomaly in ZrTe5. In addition, we observe one-dimensional-like electronic features from the edges of the cracked ZrTe5 samples. Our observations indicate that ZrTe5 is a weak topological insulator and it exhibits a tendency to become a strong topological insulator when the layer distance is reduced.

ENHANCEMENT OF QUANTUM TUNNELING FOR EXCITED STATES IN FERROMAGNETIC PARTICLES

A formula suitable for a quantitative evaluation of the tunneling effect in a ferromagnetic particle is derived with the help of the instanton method. The tunneling between nth degenerate states of neighboring wells is dominated by a periodic pseudoparticle configuration. The low-lying level-splitting previously obtained with the Lehmann-Symanzik-Zimmermann method in field theory in which the tunneling is viewed as the transition of n bosons induced by the usual (vacuum) instanton is recovered. The observation made with our result is that the tunneling effect increases at excited states. The results should be useful in analyzing results of experimental tests of macroscopic quantum coherence in ferromagnetic particles.

Extraordinary Hall resistance and unconventional magnetoresistance in Pt/LaCoO3 hybrids

We report an investigation of transverse Hall resistance and longitudinal resistance on Pt thin films sputtered on epitaxial LaCoO3 (LCO) ferromagnetic insulator films. The LaCoO3 films were deposited on several single crystalline substrates [LaAlO3, (La, Sr)(Al, Ta)O-3, and SrTiO3] with (001) orientation. The physical properties of LaCoO3 films were characterized by the measurements of magnetic and transport properties. The LaCoO3 films undergo a paramagnetic to ferromagnetic (FM) transition at Curie temperatures ranging from 40 to 85 K, below which the Pt/LCO hybrids exhibit significant extraordinary Hall resistance up to 50 m Omega and unconventional magnetoresistance ratio Delta rho/rho(0) about 1.2 x 10(-4), accompanied by the conventional magnetoresistance. The observed spin transport properties share some common features as well as some unique characteristics when compared with well-studied Y3Fe5O12-based Pt thin films. Our findings call for new theories since the extraordinary Hall resistance and magnetoresistance cannot be consistently explained by the existing theories.

Macroscopic quantum coherence in mesoscopic ferromagnetic systems

Fe/Sb multilayer films with various Fe and Sb layer thicknesses were prepared by vapor deposition. A metastable rhombohedral Fe phase was formed in the Fe/Sb multilayered films, when the Fe layer thickness was of 1.7 nm and the Sb layer thickness was of 10 nm. In an opposite condition, a metastable bcc Sb phase was obtained, when the Fe layer was thicker than 4 nm and the Sb layer was thinner than 2.5 nm. Interestingly, the metastable rhombohedral Fe exhibited ferromagnetic behavior and its average magnetic moment was as high as 1.52 mu(B). The magnetic behavior and electronic and magnetic structures of the rhombohedral Fe were calculated by the first-principles all-electron linearized augmented plane-wave method (LAPW) within the local spin-density functional approximation. The difference between the calculation of the Fe magnetic-moment amplitude (3.32 mu(B)) and the experimental determination (1.52 mu(B)) was discussed in terms of the presence of dead layers at the Fe/Sb interfaces. [S0163-1829(99)02017-2].

Macroscopic quantum coherence in small antiferromagnetic particles and quantum interference effects

Abstract Starting from the Hamiltonian of the uncompensated two-sublattice model of a small antiferromagnetic particle, we derive the effective Lagrangian of a biaxial antiferromagnetic particle in an external magnetic field with the help of spin-coherent-state path integrals. Two unequal level-shifts induced by tunneling through two types of barriers are obtained using the instanton method. The energy spectrum is found from Bloch theory regarding the periodic potential as a superlattice. The external magnetic field indeed removes Kramers degeneracy, however, a new quenching of the energy splitting depending on the applied magnetic field is observed for both integer and half-integer spins due to the quantum interference between transitions through two types of barriers.

CROSSOVER FROM THERMAL HOPPING TO QUANTUM TUNNELING IN MN12AC

The crossover from thermal hopping to quantum tunneling is studied. We show that the decay rate Gamma with dissipation can accurately be determined near the crossover temperature. Besides considering the Wentzel-Kramers-Brillouin (WKB) exponent, we also calculate contribution of the fluctuation modes around the saddle point and give an extended account of a previous study of crossover region. We deal with two dangerous fluctuation modes whose contribution cannot be calculated by the steepest descent method and show that higher-order couplings between the two dangerous modes need to he taken into consideration. At last the crossover from thermal hopping to quantum tunneling in the molecular magnet Mn12Ac is studied. [S0163-1829(99)03409-8].

Evidence of Electron-Hole Imbalance in WTe

WTe2 has attracted a great deal of attention because it exhibits extremely large and nonsaturating magnetoresistance. The underlying origin of such a giant magnetoresistance is still under debate. Utilizing laser-based angle-resolved photoemission spectroscopy with high energy and momentum resolutions, we reveal the complete electronic structure of WTe2. This makes it possible to determine accurately the electron and hole concentrations and their temperature dependence. We find that, with increasing the temperature, the overall electron concentration increases while the total hole concentration decreases. It indicates that the electron-hole compensation, if it exists, can only occur in a narrow temperature range, and in most of the temperature range there is an electron-hole imbalance. Our results are not consistent with the perfect electron-hole compensation picture that is commonly considered to be the cause of the unusual magnetoresistance in WTe2. We identified a flat band near the Brillouin zone center that is close to the Fermi level and exhibits a pronounced temperature dependence. Such a flat band can play an important role in dictating the transport properties of WTe2. Our results provide new insight on understanding the origin of the unusual magnetoresistance in WTe2.

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Abstract We report a successful tuning of the hole doping level over a wide range in high temperature superconductor Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi2212) through successive in situ potassium (K) deposition. By taking high resolution angle-resolved photoemission measurements on the Fermi surface and band structure of an overdoped Bi2212 ( T c = 76xa0K) at different stages of K deposition, we found that the area of the hole-like Fermi surface around the Brillouin zone corner (π,π) shrinks with increasing K deposition. This indicates a continuous hole concentration change from initial ~0.26 to eventual 0.09 after extensive K deposition, a net doping level change of 0.17 that makes it possible to bring Bi2212 from being originally overdoped, to optimally-doped, and eventually becoming heavily underdoped. The electronic behaviors with K deposition are consistent with those of Bi2212 samples with different hole doping levels. These results demonstrate that K deposition is an effective way of in situ controlling the hole concentration in Bi2212. This work opens a good way of studying the doping evolution of electronic structure and establishing the electronic phase diagram in Bi2212 that can be extended to other cuprate superconductors.