Z. Z. Sheng

Tuning the effective properties of metamaterials by changing the substrate properties

Tunable properties of materials are highly desirable in many applications. Metamaterials with negative properties (permittivity, permeability, or both) can have many applications if such properties can be made tunable or can be switched alternatively between positive and negative property behavior. This paper is a numerical study of the effect of substrate properties on the effective properties of a metamaterial slab. We present both simulation results and measurement data for a specific split-ring resonator structure for two different substrate thicknesses and demonstrate very good agreement. Then, using finite element simulation, varying the permittivity of the substrate from 1 to 14 while keeping its physical thickness fixed, we show that the resonance frequency drops from ∼16to∼6GHz. Alternately, when the physical thickness of the substrate is varied from 0.05to2mm, keeping its permittivity fixed, the resonance frequency decreases from ∼13.2to∼9.2GHz. In each case, the effective refractive index is re...

Study on Zn-, Cd-, or Hg-addition into TlBaCuO

Nominal ZnTlBaCuO4.5, CdTlBaCuO4.5, and HgTlBaCuO4.5 samples have been prepared under different conditions and have been studied by resistance and ac‐susceptibility measurements compared to nominal TlBaCuO3.5 samples. In general, the ZnTlBaCuO4.5 samples required less strict heating conditions to reach zero resistance near liquid nitrogen temperature; the CdTlBaCuO4.5 samples had lower Tc; the HgTlBaCuO4.5 samples showed slightly improved Tc at certain conditions. In contrast to La2−xSrxCuO4, Zn‐addition did not significantly depress Tc of TlBaCuO. This observation is worthy to be investigated further. Hardness of the ZnTlBaCuO4.5 and CdTlBaCuO4.5 samples was greatly increased, which could have importance in the practical applications of these materials.

Superconducting TlSr2(Ca,Cr)Cu2O7 thin films with critical current density up to 106 A/cm2

Superconducting TlSr2(Ca,Cr)Cu2O7 thin films with zero resistance temperature Tc up to 102 K and critical current density Jc as high as 106 A/cm2 at 77.7 K have been successfully prepared via laser ablation and thallium diffusion. Prolonged low temperature annealing in air was used for film processing. X‐ray diffraction patterns indicated that the films were highly oriented 1212 phase with c axes normal to the LaAlO3(100) or MgO(100) substrates.

Transport critical current and magnetization measurements of melt‐processed YBa2Cu3O7−x

We report magnetic field dependence of the transport critical current and dc magnetic susceptibility measurements on YBa2Cu3O7−x superconductors formed by melt‐solid reactions at 950u2009°C between Ba‐Cu‐O (or Tb‐Ba‐Cu‐O) and solid nonstoichiometric Y‐Ba‐Cu‐oxide. Four‐probe dc critical current measurements at 77, 64, and 4.2 K show strong depression of the critical current density with increasing magnetic field in agreement with a model of weakly linked superconducting regions. Diamagnetic shielding and Meissner flux expulsion measurements in the temperature range 10–300 K show about one third volume fraction of perfect superconductivity. Both shielding and flux expulsion were observed to be approximately temperature independent below 60 K indicating strong coupling between the grains throughout the entire volume below this temperature.

A new 1223-type high Tc cuprate (Tl, V) Sr2Ca2Cu3O9

A new 1223-type (Tl, V)-based superconducting layered cuprate (Tl1−xVx) Sr2Ca2Cu3O9 withx=0.25 and 0.50 has been successfully synthesized in the nearly-pure form and identified by powder X-ray diffration analyses. An excessive quantity of Tl is necessary for the preparation of the (Tl, V)-based 1223-type compound. Resistance and ac susceptibility measurements showed that the new (Tl, V)-based 1223 cuprate exhibits Tc above 110 K.

Synthesis and superconductivity of a new 1222-type T1-based layered cuprate (T1, Nb) Sr2(Nd, Ce)2Cu2O z

Synthesis and superconductivity of a new 1222-type layered cuprate (Tl1−xNbx) Sr2(Nd1−yCey)2Cu2Oz have been studied. The structure of this cuprate is directly related to that of Nb-1222 NbSr2(Nd, Ce)2Cu2Oz with tetragonal body-center lattice. Partial substitution of Tl for Nb in Nb-1222 phase improves its superconductivity. (Tl1−xNbx) Sr2(Nd0.75Ce0.25)2Cu2Oz samples prepared by the typical procedure exhibit superconductivity withTc of 30–40 K. Effects of Tl and Nb on superconductivity of this cuprate are briefly discussed.

Superconductivity and phases in the MTlSrCaCuO and MTlSrCuO systems with M=Cr, Mo, and W

Cr‐, Mo‐, or W‐substituted TlSrCaCuO and TlSrCuO samples were prepared and investigated by resistance and ac susceptibility measurements and by x‐ray diffraction analyses. The results showed that CrTlSrCaCuO, MoTlSrCaCuO, and WTlSrCaCuO systems form a 1212‐type phase, but with different Tc: 100K, 70 K, and 50 K, respectively. Some MoTlSrCaCuO samples exhibit weak superconductivity at 100 K. The Ca‐free CrTlSrCuO system is superconducting at 40 K and 77 K. The 40 K superconducting phase is a 1201‐type phase (Tl, Cr)Sr2CuO5, whereas the 77 K phase may be anew phase and remains to be identified.

New T1‐based superconductor T1PbSrRCuO without Ca with Tc above 100 K

Ca‐free T1PbSrRCuO samples (R=rare earths) with Tc up to above 100 K were prepared, and studied by resistance and ac susceptibility measurements and by powder x‐ray diffraction analyses. A 1212‐type phase (Tl1−xPbx)Sr2(Sr1−yRy)Cu2Oz is responsible for the observed supercondcutivity. A rare‐earth is required for the formation of the 1212 phase. Pb‐dopping is necessary to increase the Tc of the 1212 phase from 90 K to above 100 K.

Empirical search for new and higher‐temperature perovskite‐type oxide superconductors

Although high Tc oxide superconductors have different chemical compositions, they are all perovskite‐type. In this paper, we define high Tc oxide superconductor‐forming elements, and show distribution of these elements in the periodic table. We present a novel expression from the high Tc perovskite‐type oxide superconductors, which is as simple as a chemical formula, but contains important structural information. Using this expression, we list most of known high Tc superconductors. We found that a perovskite‐type superconductor with certain layers of metallic atoms may reach a highest Tc, and the highest Tc is a function of total metallic atom layers: Tc(highest)=f×L (K) when L≤3×(E‐1), where E is the number of metallic elements, L is the number of metallic atom layers, and f is a coefficient. Therefore, higher Tc perovskite‐type superconductors should be sought in more complex systems.

Direct Comparison of the Magnitude and Phase of Measured S-parameters of Metamaterials with Finite Element Simulation

This paper calculates the S-parameters of the metamaterial structure using finite element simulation. To verify the simulation, a slab composed of arrays of the Omega structures was prepared, and the S-parameters of the metamaterial slab sample were measured using a focused beam free-space microwave measurement setup. The direct comparison of S-parameters shows perfect agreement of the simulation and the measurement. A strong resonance at about 7.5 GHz is observed