Ke-Xi Xu

Vibration of a double-walled carbon nanotube aroused by nonlinear intertube van der Waals forces

Vibration of a double-walled carbon nanotube aroused by nonlinear interlayer van der Waals (vdW) forces is studied. The interlayer vdW forces as a nonlinear function are described by the interlayer spacing. The inner and outer carbon nanotubes are modeled as two individual elastic beams. Detailed results are demonstrated for double-walled carbon nanotubes (DWCNTs) with an aspect ratios of 10 and 20, based on the simply supported, fixed, or free end conditions, respectively. Harmonic balance method is used to analyze the relation between the amplitudes of deflection and the frequencies of coaxial and noncoaxial free vibrations. Our results indicate that the nonlinear factors of vdW forces have little effect on the coaxial free vibration, and that the deflection amplitudes increase rapidly with the increasing frequency, which are almost the same with those of the linear free vibration. On the other hand, the nonlinear factors of vdW forces have a great effect on noncoaxial free vibration. The relation betwe...

Ferromagnetic resonance investigation in as-prepared NiFe/FeMn/NiFe trilayer

NiFe/FeMn/NiFe trilayer prepared by dc magnetron sputtering was systematically investigated by ferromagnetic resonance technique (FMR) at room temperature. For NiFe/FeMn/NiFe trilayer, there are two distinct resonance peaks both in in-plane and out-of-plane FMR spectra, which are attributed to the two NiFe layers, respectively. The isotropic in-plane resonance field shift is negative for the bottom NiFe layer, while positive for the top NiFe layer. And, such phenomena result from the negative interfacial perpendicular anisotropy at the bottom NiFe/FeMn interface and positive interfacial perpendicular anisotropy at the top FeMn/NiFe interface. The linewidth of the bottom NiFe layer is larger than that of the top NiFe layer, which might be related to the greater exchange coupling at the bottom NiFe/FeMn interface.

A fully superconducting bearing system for flywheel applications

A fully superconducting magnetic suspension structure has been designed and constructed for the purpose of superconducting bearing applications in flywheel energy storage systems. A thrust type bearing and two journal type bearings, those that are composed of melt textured high-Tc superconductor YBCO bulks and Nd-Fe-B permanent magnets, are used in the bearing system. The rotor dynamical behaviors, including critical speeds and rotational loss, are studied. Driven by a variable-frequency three-phase induction motor, the rotor shaft attached with a 25 kg flywheel disc can be speeded up to 15 000 rpm without serious resonance occurring. Although the flywheel system runs stably in the supercritical speeds region, very obvious rotational loss is unavoidable. The loss mechanism has been discussed in terms of eddy current loss and hysteresis loss.

Nonequilibrium photoresponse of YBa2Cu3O7−x granular films to 8 mm microwave radiation

Nonequilibrium photoresponse behavior has been investigated for YBa2Cu3O7−x (YBCO) granular films to 8 mm microwave radiation under various bias currents and magnetic fields. The measurements reveal that the nonequilibrium photoresponse mode occurs only in the tail region of the resistance transition curve R(T) from the normal to the superconducting state, where transportation behavior of the granular superconducting film is found to be characterized by the Kosterlitz–Thouless (KT) phase transition model. Based on the KT model, the photoresponse mechanism has been interpreted in terms of the depinning process of the unbinding vortices, which are generated from the decoupling process of the vortex–antivortex pairs by current, and are held at the intrinsic pinning sites of the granular high-Tc superconducting films at low temperature. Under the co-action of the bias current and the incident microwave photons, these unbinding vortices will be driven out of the pinning center, creating viscous motion in the J...

Abnormal frequency dependence of ac susceptibility in melt texture Gd–Ba–Cu–O single domain

In this paper, the pinning property of the melt texture Gd–Ba–Cu–O single domain has been examined by ac susceptibility measurements. An unexpected frequency dependence behavior of the irreversibility line Birr(T) has been found, in which the power n for Birr(T)∝(1−t)n is affected by frequencies of the ac fields, no longer kept as a constant as in the Yeshurun and Malozemoff theory. We present the abnormal frequency dependent behavior and interpret it in framework of the flux diffusion model.

Microwave detection using Y–Ba–Cu–O granular film bridge device

Abstract High-Tc superconducting microwave video detection is investigated based on the Y–Ba–Cu–O granular film bridge device. By optimizing the device parameters, a detectivity of about 104 V/W is achievable, and the noise equivalent power at T=70 K is about 5.2×10−14 W/Hz1/2. To identify the response time of the device, the response voltage is measured with incident radiation chopped at 500 kHz, and a rise time of less than 3.9×10 −8 s is observed. These results compare favorably with that of high-Tc superconducting bolometers for millimeter wave bands. On the other hand, the dynamic resistance RN of the device is found to increase as the device is illuminated by incident irradiation. This effect cannot be interpreted in terms of the Josephson dissipation mechanism, and a discussion about this is presented.

Optical detector using granular YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// weak link-type junctions

Experimental photoresponse study of granular YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) weak link is reported. The Josephson critical current I/sub c/ is suppressed with light illumination of the junction, produced mainly from optical generation of quasiparticles. At lower temperature (T=64.5 K), the hysteresis in I-V characteristic of the weak link appears, which may be found useful for optical detection purpose. This was supported from preliminary experiments.

Enhancement of trapped field in single grain Y–Ba–Cu–O bulk superconductors by a modified top-seeded melt-textured growth

The modified top-seeded melt-textured growth technique for fabricating single grain Y–Ba–Cu–O (YBCO) bulk superconductors with high field-trapping ability by using modified precursor pellets was reported. The modified precursor pellets are composed of different precursor powders YBa2Cu3O (Y123) + x mol% Y2BaCuO5 (Y211) + 1 wt% CeO2 without any further chemical doping. The modified YBCO bulks up to 25 and 34 mm in diameter were successfully fabricated from the modified precursor pellets. Microstructural observation results showed that the modified YBCO bulk exhibited a homogeneous distribution of Y211 phase particles, which was qualitatively explained by the solute diffusion growth model in combination with the trapping/pushing theory. As a result, it is notable that the peak trapped field values of 0.91 T (maximum 0.96 T) and 1.2 T (maximum 1.28 T) at 77 K were achieved for 25 and 34 mm modified YBCO bulks, respectively. In a word, the results from present work are very helpful to understand the melt growth mechanism and to further improve the properties of YBCO bulk superconductors for practical applications.

Influence of layered precursor pellets on the growth and properties of Y-Ba-Cu-O bulk superconductors by top-seeded melt-textured growth

It is well known that a fine and homogeneous distribution of Y2BaCuO5 (Y211) phase particles in single-grain Y-Ba-Cu-O (YBCO) bulk superconductors is essential for improving field-trapping ability. However, the size and concentration of Y211 phase particles in the fully melt-processed superconducting bulk increase significantly with the distance from the seed, which results in the accumulation of Y211 phase particles and the degradation of superconducting properties. In this paper, we report a new method of fabricating single-grain YBCO using layered precursor pellets. Using the top-seeded melt-textured growth process, single-grain YBCO bulk superconductors of about 22 mm in diameter and 9 mm in thickness were fabricated from layered precursor pellets and standard precursor pellets, respectively. The layered precursor pellets consist of precursor powders with 40 mol% Y211 at the top, 30 mol% Y211 in the middle and 20 mol% Y211 at the bottom of the whole pellets, while standard precursor pellets are prepared from precursor powders with only 40 mol% Y211. The growth morphology, microstructure and magnetic flux properties of the layered samples and standard samples were comparatively studied. The results proved that the layered precursor pellets allow a sufficient growth in the c-growth sector and a more uniform distribution of the Y211 phase in the matrix. The distribution of Y211 phase particles is qualitatively explained by the prevalent trapping/pushing theory. The trapped field at 77 K reaches 0.8 T, nearly 29% higher than the standard sample. The present results are very valuable for further improving the properties of YBCO bulk superconductors.

Decoupling of vortex-antivortex pairs: a possible explanation for nonequilibrium microwave response of YBa2Cu3O7-δ granular films

Abstract A two-dimensional Josephson-junction array has been employed as a model system for YBa 2 Cu 3 O 7− δ (YBCO) granular films. Based on this model, the dissociation process of the vortex–antivortex pairs is discussed in terms of applied current activation, and the unbinding vortices distribution n ( T , I ) is calculated as a function of temperature and applied current. When I / I c ≪1 is satisfied, the value of the n ( T , I ) could only be observed within the temperature region of (2/3) T KT T T KT , this behavior is analogous to that of photoresponse dissipation measured from the granular YBCO films. This similitude implies that the unbinding process of the vortex pairs might be responsible for the nonequilibrium photoresponse dissipation in granular superconducting films.