Yilmaz Simsek

Interferometer measurements of terahertz waves from Bi2Sr2CaCu2O8+d mesas

We fabricated rectangular mesa structures of superconducting Bi2Sr2CaCu2O8+d (Bi2212) using e-beam lithography and Ar ion beam etching techniques for terahertz (THz) emission. c-axis resistance versus temperature (R?T), current?voltage (I?V) characteristics and bolometric THz power measurements were performed to characterize Bi2212 mesas. The emission frequency of mesas was determined using a Michelson interferometer setup which also demonstrates polarized emission. Interference patterns of THz radiation from Bi2212 mesas were detected by various detectors such as a liquid helium cooled silicon composite bolometer, a Golay cell and a pyroelectric detector. An emitted power as high as 0.06?mW was detected from Bi2212 mesas. For the first time, most of the pumped power was extracted as THz emission from a Bi2212 mesa. The radiation at 0.54?THz was detected using the Michelson interferometric setup.

Emission of the THz waves from large area mesas of superconducting Bi2Sr2CaCu2O8+δ by the injection of spin polarized current

The effect of electromagnetic wave irradiation on the phase dynamics of intrinsic Josephson junctions in high temperature superconductors is investigated. We predict three novel effects by variation of the radiation amplitude and frequency: changing of the longitudinal plasma wavelength at parametric resonance; double resonance of the Josephson oscillations with radiation and longitudinal plasma wave; charging of superconducting layers in the current interval corresponding to the Shapiro step. The ”bump” structure in IVC recently observed experimentally is demonstrated. We also observe ragged Shapiro steps at double resonance.

Tuning superconductivity by carrier injection

We have found that by extensive current injection along the c-axis, the superconducting properties of Bi2Sr2CaCu2O8+δ can be changed effectively. We show that critical temperature, c-axis resistivity, and critical current of intrinsic Josephson junctions can be tuned in a large range from underdoping to extreme overdoping. This effect is reversible and persistent. Our results can be explained by trapping charges in the insulating layers, which induce a change of carrier concentration in superconducting planes. This floating gate concept can be a general property of layered materials where the insulating charge reservoir layers are separated from the conducting planes.

Superconductivity induced by carrier injection into non-superconducting Bi2Sr2CaCu2O8 + δ

Doping by carrier injection was investigated in strongly oxygen-depleted Bi2Sr2CaCu2O8 + δ. In order to perform four point c-axis measurements in non-superconducting samples, a new fabrication technique was developed. We were able to show that non-superconducting Bi2Sr2CaCu2O8 + δ can be converted into a superconducting, even overdoped state only by applying a large bias current along the c-axis. A critical temperature of 91 K and a c-axis critical current density of 2000 A cm−2 were achieved. Moreover, the doping effect is persistent and reversible resembling the resistive-memory switching effect observed earlier in various oxides. The mutual influence of oxygen doping and change of the charge carrier concentration by carrier injection is discussed.

Adjustable tunneling barrier in bi-based high-T c cross-whisker junctions

We present a study of cross-whisker junctions with electronically modified tunneling barriers. Cross-whisker junctions were successfully prepared by annealing two crossed Bi-based whisker bars at temperatures of around 840 °C. In addition to the artificially produced junction at the interface, intrinsic Josephson junctions in the respective bars of the cross were observed. The artificial junction exhibited reproducible and almost ideal junction characteristics. The tunneling barrier/interface properties were controlled by carrier injection in c-axis direction. Using this process, we were able to reduce the tunneling resistance from 408 Ω to 30 Ω. At the same time, the critical current did rise by a factor of 4. Subsequently, the critical current was doubled while the tunneling resistance stayed constant. We interpret this observation in terms of the counterplay between transparency of the barrier and the carrier concentration of the electrodes. In this sense, we can consider the current injection procedure as an electronic gluing process of the two cross-junction whisker bars.

Interlayer tunneling spectroscopy of mixed-phase BSCCO superconducting whiskers

In this work, we present a study on the interlayer tunneling spectroscopy (ITS) of mixed-phase BiSrCaCuO (BSCCO) superconducting whiskers. The tunneling experiments were carried out on the artificial cross-whisker (twist angle of 90?) junctions. A multiple superconducting energy gap in the cross-whisker junctions was observed, which is attributed to the presence of different doping levels of two Bi2Sr2CaCu2O8+? phases (Bi-2212), rather than two different phases, in the BSCCO whiskers, namely Bi2Sr2CaCu2O8+? and Bi2Sr2Ca2Cu3O8+? (Bi-2212 and Bi-2223). The temperature dependence of the energy gaps was discussed in the framework of the BCS T-dependence. On the other hand, the carrier concentration of the cross-whisker junction was changed by the carrier injection process. The effects of the carrier injection on the critical current, I c, and the ITS of intrinsic Josephson junctions were investigated in details.

Terahertz wave emission from layered superconductors: Interferometer measurements

Rectangular Bi2Sr2CaCu2O8+δ (Bi2212) mesa structures were fabricated on as-grown Bi2212 single crystal superconductors using standard photolithography and Ar ion beam etching techniques. We have performed c-axis resistance versus temperature (R-T), current-voltage (I-V) characteristics and bolometer measurements. Furthermore, in contrast to previous studies, the emission frequency was determined using interferometer set up instead of FTIR. The interference patterns were detected outside the cryostat after traveling long way through ambient space. The emission frequency calculated by Fourier transform of interference data is consistent with Josephson frequency-voltage relation.

Spin polarized tunneling in large area mesas of superconducting Bi 2 Sr 2 CaCu 2 O 8+δ for the generation of the THz waves

Rectangular intrinsic Josephson junction mesa structures of superconducting Bi2Sr2CaCu2O8+δ (Bi2212) can be used as a source of continuous, coherent and polarized terahertz (THz) radiation. THz emitting mesas are below a certain underdoped level of Bi2212. They have small Josephson critical current in contrast to optimally doped and overdoped Bi2212. We deposited Au/Co/Au multilayer top of mesa and obtained small critical current from mesas fabricated by as-grown Bi2212 single crystals due to injection of spin polarized current. The spin injection eliminates adjustment of doping level for successful THz emission.