Zhe Qu

Dual evidence of surface Dirac states in thin cylindrical topological insulator Bi2Te3 nanowires

How the surface state (SS) develops and how the spin transport in a curved cylindrical topological insulator nanowire have attracted theoretical attention recently. However, experimental confirmation for the SS in such a real modeling system still remains insufficient. Here we carried out a systematic comparative study on the cylindrical single-crystal Bi2Te3 nanowires of various diameters, and report unambiguously dual evidence for the Dirac SS. Both the predicted anomalous Aharonov-Bohm (AB) quantum oscillations with a period of h/e in H// and the 1/2-shifted Shubnikov-de Haas (SdH) oscillations (i.e., γ = −1/2) in H⊥ were indentified below 1.4 K. In addition, Altshuler-Aronov-Spivak (AAS)-like oscillations with a period of h/2e and ordinary SdH oscillations with γ = 0 were also resolved. These data provide clear evidence of coexistence of the nontrivial topological Dirac state and trivial electron state on the surface of topological insulator nanowire.

Effect of Ru doping in La 0.5 Sr 0.5 Mn O 3 and La 0.45 Sr 0.55 Mn O 3

We investigated the structural, magnetic, and transport properties of polycrystalline doping systems La0.5Sr0.5Mn1−xRuxO3 0 x 0.15 with ferromagnetic matrix and La0.45Sr0.55Mn1−yRuyO3 0 y 0.5 with antiferromagnetic matrix. X-ray photoelectron spectroscopy shows that Ru ions exist mainly in the form of Ru4+ while there also exist a small quantity of Ru5+. For the low doping samples 0.05 x y 0.15 0.2 , the ferromagnetism is enhanced and Curie temperature TC rises with increasing Ru doping level, but TC is much higher than the temperature TIM corresponding to insulating-metallic transition. For the high doping samples 0.3 y 0.5 , oppositely the ferromagnetism is suppressed and the insulating property is enhanced with further increasing Ru doping level. These results show that there is an exchange interaction between Mn3+ and Ru4+ Ru5+ . The electron spin resonance spectra clearly and directly inspect that the interaction between Mn3+ and Ru4+ Ru5+ is ferromagnetic. Furthermore, the electron spin resonance spectra also show that a Ru-Ru antiferromagnetic interaction takes place in the high doping region y=0.4,0.5 . We explained the disagreement between TC and TIM.

Fe-doping–induced superconductivity in the charge-density-wave system 1T-TaS2

We report the interplay between charge-density-wave (CDW) and superconductivity of 1T-FexTa1−xS2 (0≤x≤0.05) single crystals. The CDW order is gradually suppressed by Fe-doping, accompanied by the disappearance of pseudogap/Mott-gap as shown by the density functional theory (DFT) calculations. The superconducting state develops at low temperatures within the CDW state for the samples with the moderate doping levels. The superconductivity strongly depends on x within a narrow range, and the maximum superconducting transition temperature is 2.8 K as x=0.02. For high doping level (x> 0.04), the Anderson localization (AL) state appears, resulting in a large increase of resistivity. We present a complete electronic phase diagram of the 1T-FexTa1−xS2 system that shows a dome-like Tc(x).

Spin Valve Effect and Magnetoresistivity in Single Crystalline Ca3Ru2O7

The laminar perovskite Ca3Ru2O7 naturally forms ferromagnetic double layers of alternating moment directions, as in the spin-valve superlattices. The mechanism of the huge magnetoresistive effect in the material has been controversial due to a lack of clear understanding of various magnetic phases and phase transitions. In this neutron diffraction study in a magnetic field, we identify four different magnetic phases in Ca3Ru2O7 and determine all first-order and second-order phase transitions between them. The spin-valve mechanism then readily explains the dominant magnetoresistive effect in Ca3Ru2O7.

Critical properties of the 3D-Heisenberg ferromagnet \chem{CdCr_{2}Se_{4}}

The critical properties of the ferromagnet CdCr2Se4 around the paramagnetic-ferromagnetic phase transition have been investigated. It is found that the 3D-Heisenberg model is the best one to describe the critical phenomena around the critical point. Critical exponents β=0.337±0.03 and γ=1.296±0.109 at TC=130.48±0.34 are obtained. In addition, the critical exponent δ=4.761±0.129 is determined separately from the isothermal magnetization at TC. These critical exponents fulfill the Widom scaling relation δ=1+γ/β. Based on these critical exponents, the magnetization-field-temperature (M-H-T) data around TC collapses into two curves obeying the single scaling equation M(H, e)=eβf±(H/eβ+γ). Although the 3D-Heisenberg model is the most satisfactory model to describe this system, critical exponents for CdCr2Se4 are slightly smaller than the theoretical exponents (β=0.36, γ=1.39 and δ=4.8). This indicates that the exchange interaction J(r) decays slower than r− 5 in this system, which can be attributed to the spin-lattice coupling.

Giant anisotropic magnetoresistance in bilayered La1.2Sr1.8Mn2O7 single crystals

We report an observation of anomalous anisotropic magnetoresistance (AMR) in bilayered La1.2Sr1.8Mn2O7 single crystals. A giant AMR is found to be 80% under a magnetic field of 1 T near the metal-insulator transition temperature, where AMR is defined as AMR=[ρ(H⊥c)−ρ(H∥c)]/ρ(H⊥c)×100%, and ρ(H⊥c) and ρ(H∥c) are the resistivity with the magnetic field perpendicular and parallel to c-axis, respectively. The AMR effect shows strong temperature and magnetic field dependences, and indicates a close interrelation with the anisotropic field-tuned metal-insulator transition.

Unusual heavy-mass nearly ferromagnetic state with a surprisingly large Wilson ratio in the double layered ruthenates (Sr1−xCax)3Ru2O7

We report an unusual nearly ferromagnetic heavy-mass state with a surprisingly large Wilson ratio Rw e.g., Rw 700 for x = 0.2 in double layered ruthenates Sr1�xCax3Ru2O7 with 0.08x 0.4. This state does not evolve into a long-range ferromagnetically ordered state despite considerably strong ferromagnetic correlations, but it freezes into a cluster-spin glass at low temperatures. In addition, evidence of non-Fermi-liquid behavior is observed as the spin-freezing temperature of the cluster-spin glass approaches zero near x 0.1. We discuss the origin of this unique magnetic state from the Fermi-surface information probed by Hall-effect measurements.

Tricritical behavior of the two-dimensional intrinsically ferromagnetic semiconductor CrGeTe3

CrGeTe3 recently emerges as a new two-dimensional (2D) ferromagnetic semiconductor that is promising for spintronic device applications. Unlike CrSiTe3 whose magnetism can be understood using the 2D-Ising model, CrGeTe3 exhibits a smaller van der Waals gap and larger cleavage energy, which could lead to a transition of magnetic mechanism from 2D to 3D. To confirm this speculation, we investigate the critical behavior CrGeTe3 around the second-order paramagnetic-ferromagnetic phase transition. We obtain the critical exponents estimated by several common experimental techniques including the modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis, which show that the magnetism of CrGeTe3 follows the tricritical mean-field model with the critical exponents \b{eta}, {\gamma}, and {\delta} of 0.240, 1.000, and 5.070, respectively, at the Curie temperature of 67.9 K. We therefore suggest that the magnetic phase transition from 2D to 3D for CrGeTe3 should locate near a tricritical point. Our experiment provides a direct demonstration of the applicability of the tricritical mean-field model to a 2D ferromagnetic semiconductor.

Disorder-induced orbital glass state in FeCr2S4

The effect of disorder on the orbital state in the spinel FeCr2S4 has been investigated with the substitution of Cr by Al, Ga, and Fe, respectively. For polycrystalline FeCr2S4, being related to the orbital-ordering transition around 9 K, the temperature dependence of magnetization shows a step-like transition, and the specific heat displays a well-defined λ-type anomaly correspondingly. However for single-crystal and doped FeCr2S4 samples, the step-like transition in magnetization disappears, and the λ-type anomaly of specific heat is replaced by a broad hump. Moreover, the specific heat obeys a T2-dependence at temperatures below 2 K, suggesting the formation of the orbital glass state in these samples. Consistently with different orbital states, the resistivity at low temperature can be better fitted with a thermally activated model for the polycrystalline FeCr2S4 sample, and better described by Motts variable-range hopping expression for the others. All these results imply that the disorder induces orbital glass state in FeCr2S4.

Critical behavior of the quasi-two-dimensional semiconducting ferromagnet CrSiTe3.

The semiconducting ferromagnet CrSiTe3 is a promising candidate for two-dimensional magnet simply by exfoliating down to single layers. To understand the magnetic behavior in thin-film samples and the possible applications, it is necessary to establish the nature of the magnetism in the bulk. In this work, the critical behavior at the paramagnetic to ferromagnetic phase transition in single-crystalline CrSiTe3 is investigated by bulk magnetization measurements. We have obtained the critical exponents (β = 0.170 ± 0.008, γ = 1.532 ± 0.001, and δ = 9.917 ± 0.008) and the critical temperature TC = 31.0 K using various techniques such as modified Arrott plot, Kouvel-Fisher plot, and critical isotherm analysis. Our analysis suggests that the determined exponents match well with those calculated from the results of renormalization group approach for a two-dimensional Ising model coupled with long-range interaction.