Jiangwei Chen

Suppression of superconductivity in epitaxial NbN ultrathin films

This paper studies the suppression of superconducting transition temperature (T(c)) of ultrathin NbN film. We fabricated epitaxial NbN superconducting thin films of thicknesses ranging from 2.5 to 100 nm on single crystal MgO (100) substrates by dc magnetron sputtering. We performed structure analyses and measured their electric and far infrared properties. The experimental results were compared with several mechanisms of the suppression of superconductivity proposed in the literature, including the weak localization effect, the proximity effect, and quantum size effect (electron wave leakage model). We found that the electron wave leakage model matches best to the experimental data

Low-loss terahertz metamaterial from superconducting niobium nitride films

This paper reports a type of low Ohmic loss terahertz (THz) metamaterials made from low-temperature superconducting niobium nitride (NbN) films. Its resonance properties are studied by THz time domain spectroscopy. Our experiments show that its unloaded quality factor reaches as high as 178 at 8 K with the resonance frequency at around 0.58 THz, which is about 24 times that of gold metamaterial at the same temperature. The unloaded quality factor keeps at a high level, above 90, even when the resonance frequency increases to 1.02 THz, which is close to the gap frequency of NbN film. All these experimental observations fit well into the framework of Bardeen-Copper-Schrieffer theory and equivalent circuit model. These new metamaterials offer an efficient way to the design and implementation of high performance THz electronic devices.

Electronic properties of peapods: effects of fullerene rotation and different types of tube

Effects of encapsulated fullerene rotation on peapod electronic properties are studied by using Slater–Koster tight-binding calculations. We found that the encapsulated fullerenes can rotate freely in the space of a (10, 10) tube at room temperature, and the rotation of fullerenes will affect C60@(10,10) peapod electronic properties significantly; generally, orientational disorder will remove the sharp features of the average density of states (DOS). However, the rotation of fullerenes cannot induce a metal–insulator transition. Electronic properties of peapods formed by fullerenes and different types of single-walled carbon nanotube (SWNT) are studied too. Percentage calculations suggest that, unlike the multicarrier metallic C60@(10,10) peapod, the C60@(17,0) peapod is a semiconductor, and the effects of the encapsulated fullerenes on tube valence bands and conduction bands are asymmetrical.

Left-handed materials made of dilute ferromagnetic wire arrays with gyrotropic tensors

In this article, we theoretically investigate properties of normally incident microwave propagation in dilute metallic ferromagnetic wire array (DMFWA) slabs with gyrotropic tensors. It is found that DMFWA may become left-handed materials (LHM) in a narrow frequency range below the plasma frequency, even the real part of the corresponding inversion permeability element is positive. Comparing with the isotropic materials having same diagonal permeability elements, due to existence of off-diagonal elements of the gyrotropic permeability tensor, the DMFWA becomes a LHM in a higher frequency range depended on its geometric structure. In addition, E, H, and wave vector Re(k) of microwave propagating in the DMFWA form an approximately left-handed triplet of vectors.

Inhomogeneous exciting field dependence of permeability and microwave properties of trilayer ferromagnetic films with in-plane uniaxial anisotropy

Based upon the Landau–Lifshitz equation and Maxwells equations, the permeability and microwave properties of trilayer ferromagnetic films with in-plane uniaxial anisotropy are investigated simultaneously. It is found that, due to coexistence of interlayer exchange coupling and the gyrotropic permeability tensor in the film, the permeability, especially the magnetic parameter 1/ν = μyy−μxy2/μxx, related directly to properties of the normally incident microwave propagation in the film, may be influenced significantly by inhomogeneous exciting fields applied in the layer magnetic moments. In turn, properties of normally incident microwave propagation in the film may be modified strongly; e.g., giant modification of absorption peaks, such as the number, frequency position, shape, and so on, are predicated.

Broadband left-handed materials made of thin soft ferromagnetic films with in-plane uniaxial anisotropy

We study theoretically characteristics of normally incident E-polarized microwave propagation in soft ferromagnetic films with in-plane uniaxial anisotropy, and find that due to the existence of the gyrotropic permeability tensor, the soft ferromagnetic film may become a left-handed material over a broad frequency band. Furthermore, the bandwidth and the frequency positions can be easily modified by the external magnetic field. Finally, the energy loss of the microwave propagating in the film is investigated.

Stripe-vortex transitions in ultrathin magnetic nanostructures

In this work, the magnetic states in ultrathin nanostructures are investigated using Monte Carlo simulation, based on a Heisenberg model involving the short-range exchange coupling, long-range dipole-dipole interaction, and perpendicular anisotropy. An intriguing thermally driven magnetic structural transition from perpendicular stripe domain to flux closure (planar vortex) state, accompanied by an apparent thermal hysteresis effect and typical characteristics of the first-order phase transition, is revealed. Furthermore, it is found that the transition can be remarkably modulated by perpendicular anisotropy. The present work suggests a promising approach to manipulate the spin configurations in nanomagnets by adjusting temperature and perpendicular anisotropy.

Manipulation of magnetic state in nanostructures by perpendicular anisotropy and magnetic field

We investigate the transitions of spin configurations in ultrathin nanostructures by tuning the perpendicular anisotropy (Kz) and out-of-plane magnetic field (H), using the Monte Carlo simulation. It is revealed that enhancing the anisotropy Kz can drive the evolution of in-plane vortex state into intriguing saturated magnetization states under various H, such as the bubble domain state and quadruple-block-domain state etc. The spin configurations of these states exhibit remarkable H-dependence. In addition, the strong effects of geometry and size on the spin configurations of nanostructures are observed. In particular, a series of edged states occur in the circular disk-shaped lattices, and rich intricate saturated magnetization patterns appear in big lattices. It is suggested that the magnetic states can be manipulated by varying the perpendicular anisotropy, magnetic field, and geometry/size of the nanostructures. Furthermore, the stability (retention capacity) of the saturated magnetization states upo...

Tunable magnetic helicity in Mn1−xFexGe: A Monte Carlo simulation

Motivated by recent experiments on B20 compound Mn1−xFexGe, we explore the variations of spin structures and magnetic helicity γm in Mn1−xFexGe as a function of concentration x, using Monte Carlo simulation. We propose a simple spin model including hybrid Dzyaloshinskii-Moriya (DM) interactions with different signs in FeGe and MnGe. The results reveal a series of spin structures with helicity γm varying continuously from negative values to positive values upon decreasing x (0.0 ≤ x ≤ 1.0). The simulated x-dependence of the spin structures is consistent with experimental observations, suggesting the validity of the present hybrid model. This study sheds light on control of magnetic helicity in helimagnets, by mixing crystals with different DM interactions.