Wenjian Lu

Magnetocaloric effect and Griffiths-like phase in La0.67Sr0.33MnO3 nanoparticles

The effect of grain size on electrical and magnetic properties of La0.67Sr0.33MnO3 nanoparticles with average grain size 32–85 nm has been investigated. The metal-insulator transition temperature TP gradually decreases with decreasing grain size, while the ferromagnetic-paramagnetic transition temperature TC remains almost constant. For the 32 nm sample, the larger effective magnetic moments and the deviation of the inverse susceptibility from the Curie–Weiss law are observed, indicating the possible existence of a Griffiths-like cluster phase. The ferromagnetic transition of the samples is further investigated by measuring magnetocaloric effect (MCE). The presence of short-range magnetic order greatly depresses the magnetic entropy of the paramagnetic phase. Moreover, the analysis of the MCE using Landau theory of phase transition confirms the importance of magnetoelastic coupling and electron interaction in magnetocaloric properties of manganites.

Magnetic properties and magnetocaloric effect of La0.8Ca0.2MnO3 nanoparticles tuned by particle size

La0.8Ca0.2MnO3 particles with the sizes from 17 to 43 nm have been prepared using the sol-gel method and the magnetic properties are systematically studied. The existence of the blocking of the superparamagnetism (SPM), freezing of super-spin-glass, and surface-spin-glass is evidenced. It is found that a core-shell structure can be responsible for the magnetism behavior of the nanoparticles. The phase transition from paramagnetism (PM) to ferromagnetism (FM) is modified from first order to second order as the particle size reduced. The magnetocaloric effect (MCE) thus is modified by the changed magnetism. The observed temperature interval of the magnetic entropy change broadens as the particle size reduced. The magnetic entropy change of superparamagnetic particles has been calculated based on the core-shell model. The relative cooling power (RCP) can be tuned dramatically by particle size due to the change of spontaneous magnetization of the core and the changed ratio of the shell and surface, which show...

Electrically driven reversible insulator-metal phase transition in 1T-TaS2.

In this work, we demonstrate abrupt, reversible switching of resistance in 1T-TaS2 using dc and pulsed sources, corresponding to an insulator-metal transition between the insulating Mott and equilibrium metallic states. This transition occurs at a constant critical resistivity of 7 mohm-cm regardless of temperature or bias conditions and the transition time is significantly smaller than abrupt transitions by avalanche breakdown in other small gap Mott insulating materials. Furthermore, this critical resistivity corresponds to a carrier density of 4.5 × 10(19) cm(-3), which compares well with the critical carrier density for the commensurate to nearly commensurate charge density wave transition. These results suggest that the transition is facilitated by a carrier driven collapse of the Mott gap in 1T-TaS2, which results in fast (3 ns) switching.

Electron-doped phosphorene: A potential monolayer superconductor

We predict by first-principles calculations that the electron-doped phosphorene is a potential BCS-like superconductor. The stretching modes at the Brillouin-zone center are remarkably softened by the electron-doping, which results in the strong electron-phonon coupling. The superconductivity can be introduced by a doped electron density above , and may exist over the liquid-helium temperature when . The superconductivity can be significantly tuned and enhanced by applying tensile strain. The maximum critical temperature of electron-doped phosphorene is predicted to be higher than 10 K. The superconductivity of phosphorene will significantly broaden the applications of this novel material.

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).

Distinct surface and bulk charge density waves in ultrathin 1T-TaS 2

We employ low-frequency Raman spectroscopy to study the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition in 1T-TaS2 ultrathin flakes protected from oxidation. We identify new modes originating from C phase CDW phonons that are distinct from those seen in bulk 1T-TaS2. We attribute these to CDW modes from the surface layers. By monitoring individual modes with temperature, we find that surfaces undergo a separate, low-hysteresis NC-C phase transition that is decoupled from the transition in the bulk layers. This indicates the activation of a secondary phase nucleation process in the limit of weak interlayer interaction, which can be understood from energy considerations.

Microscopic evidence for strong periodic lattice distortion in two-dimensional charge-density wave systems

In quasi-two-dimensional electron systems of layered transition metal dichalcogenides (TMDs) there is still controversy about the nature of the transitions to charge-density wave (CDW) phases, i.e., whether they are described by a Peierls-type mechanism or by a lattice-driven model. By performing scanning tunneling microscopy experiments on canonical TMD-CDW systems, we image the electronic modulation and the lattice distortion separately in 2H-TaS2, TaSe2, and NbSe2. Across the three materials, we found dominant lattice contributions instead of the electronic modulation expected from Peierls transitions, in contrast to the CDW states, which show the hallmark of contrast inversion between filled and empty states. Our results imply that periodic lattice distortion plays a vital role in the formation of CDW phases in TMDs and illustrate the importance of taking into account the more complicated lattice degrees of freedom when studying correlated electron systems.

AlNxMn3 A possible high-temperature soft magnetic material and strongly correlated system

Structural, magnetic, electrical, and thermal transport properties of antiperovskite compounds AlNxMn3 (x=1,1.1,1.2) have been investigated systematically. With increasing x, the lattice constant increases monotonously while the Curie temperature TC decreases. Both the high TC and small coercive fields consistently indicate AlNxMn3 may be a promising high-temperature soft magnetic material. The resistivity displays T2-dependence below 30 K and the Kadowaki–Woods ratio is about 107.7u2002μΩu2009cm/K2, indicating a possible strongly correlated Fermi-liquid behavior in AlNMn3. Further analysis suggests that the electron-type carriers are dominant and the thermal conductivity mainly originates from the lattice contribution.

Low-field magnetoresistance in nanostructured Sr2FeMoO6∕CeO2 composites

Nanosized (Sr2FeMoO6)1−x∕(CeO2)x composites were prepared by Sr2FeMoO6 (SFMO) obtained by sol-gel and nanosized CeO2 powders. The magnetoresistance (MR) of composites is explored as a function of the molybdate/insulator composition and magnetic field. The enhancement of the low-field magnetoresistance is observed with increasing CeO2. The MR ratio at 10K with H=5kOe is 7.9% and 10.2% for the x=0.05 and 0.3 samples, which are 1.1 times and 1.7 times as large as that for pure SFMO, respectively. The enhanced MR was attributed to the spin-dependent tunneling at the interface of grain boundaries.

Scotch tape induced strains for enhancing superconductivity of FeSe0.5Te0.5 single crystals

We investigated the superconducting transition temperature Tc of FeSe0.5Te0.5 single crystals, which can be enhanced up to 14% by attaching onto a commercial Scotch tape. The Scotch tape exhibits a large cooling shrinkage at low temperatures, which is considerably more pronounced than that of the metallic FeSe0.5Te0.5 single crystal, thus providing a compressive strain of 2.4u2009×u200910−3 at 15u2009K. For such strain, we calculated that the lattice parameter of c/a can be increased to ∼0.31%, which corresponds to the enhancement of the superconductivity. The present finding provides a rapid and simple method to examine the microstructure sensitive physical properties of the layered-structure materials by using the Scotch tape as strain generator.