Vortices In high-Tc Superconductors Research Articles

Domain structure and magnetic pinning in ferromagnetic/superconducting hybrids

Magnetic patterns in three iron-garnet films with different magnetic properties covered with 100-nm superconducting Nb film are studied using a magneto-optical imaging technique. In all samples the strong coupling between ferromagnetic domains and vortices noticeably modifies the magnetization process. However, depending on the type and width of the magnetic domains, we observe different flux dynamics in the Nb film. Wide domains give rise to a combined domain structure and type I-like superconducting response resulting in enhanced attenuation of the flux motion due to cooperative pinning of vortices and magnetic domain walls. These combined domains strongly shrink in the ac fields due to a dynamic instability triggered by oscillating domain walls. Combined domains formed on narrow magnetic domains do not shrink but they guide the motion of vortices and, in turn, align in the vortex motion direction. This introduces superconducting current anisotropy due to strong pinning on the magnetic domain walls. Irregular magnetic domain structures with immobilized domain walls stochastically modify the vortex entry patterns. The presence of magnetic domains essentially arrests thermomagnetic avalanches in the superconducting layer. The studied magnetic pinning has a good potential for slowing down vortices in high-Tc superconductors.

Erratum: Phase diagram of vortices in high-Tcsuperconductors from lattice defect model with pinning [Phys. Rev. B75, 144513 (2007)

Erratum: Phase diagram of vortices in high-Tcsuperconductors from lattice defect model with pinning [Phys. Rev. B75, 144513 (2007)

Interaction between the Ferromagnetic Dots and Vortices: Numerical Calculation and Experimental Results

Enhancing the pinning force in high-Tc superconductors can be achieved by externally introduced periodic magnetic dots. We numerically calculate the interaction between ferromagnetic dots and vortices in high-Tc superconductors. The London equation is used to generate two-dimensional vortex lattice. In the matching condition, we calculate the attraction force between magnetic dots and vortices. It is found that in an ideal condition, the pinning force of the magnetic dot reaches 2.5×10−11 N that is more than one order magnitude stronger than the intrinsic pinning force in YBa2Cu3O7 thin films. In the experimental side, we use a novel nano-technique to deposit periodic submicron Ni dots on YBa2Cu3O7 thin films. The current versus voltage characteristics of an YBa2Cu3O7 thin film strip with uniform Ni dots are measured at various temperatures and magnetic fields. They are compared with the current versus voltage characteristics of a bare YBa2Cu3O7 thin film strip without magnetic dots. It is found the critical current value of the strip with Ni dots reduces with a much slower pace as the magnetic field strength increases in comparison with the value of the bare sample.

Phase diagram of interlayer Josephson vortices in underdoped YBa2Cu3Oy

In order to study the phase diagram of interlayer vortices in high-Tc superconductors, we have prepared untwinned single crystals of YBa2Cu3Oy with the various oxygen content. We also measured the c-axis resistivity ρc(T) in magnetic fields H parallel to the ab plane. In YBa2Cu3Oy (Tc∼60K), the vortex liquid phase freezes into the Josephson vortex glass through two-stage processes with decreasing temperature in the high-H region above 5T. Since the two phase transition lines consist of the first-order transition line TL(H) and the second-order transition line Tg(H), the intermediated phase in the region of Tg(H)<T<TL(H) is considered to be the vortex slush phase. We find that the oscillatory field dependence of the TL(H) line and ρc(T) under high current densities corresponds to the commensurability between the vortex spacing and the period of the CuO2 planes.

Oscillations of Josephson-vortex flow resistance in high-Tc superconductors

Using the coupled sine-Gordon equations，we investigate the motion of Josephson vortex in high-Tc superconductors. It is found that the oscillation behavior in the average voltage and resistance of Josephson vortex flow appear with the increasing of magnetic field in a fixed bias current. The period of the oscillation corresponds to the field, which has the magnitude of adding one Josephson vortex quantum per one intrinsic Josephson junction.

Dependence of maximum static friction on waiting time using dynamics of vortices in superconductors

Vortices in high-Tc superconductors were investigated in terms of the microscopic friction. The I-V characteristics and the transient response of driven vortices were measured in La2-xSrxCuO4 thin films with different Sr concentrations and samples with columnar defects. We found the remarkable dependence of the maximum static friction force on the waiting time at low temperatures. The result suggests that the competition between the flux creep by the thermal fluctuation and the pinning yields a characteristic time scale to stabilize vortices. This relaxation phenomenon is very similar to the so-called boundary lubrication, which occurs under the existence of a thin lubricant film between interfaces. Such a strong dependence changed into a weak logarithmic dependence at higher temperatures. These results imply that the dynamics of vortices can be used for the model not only of the dry friction but also of the lubricated friction. Also, the dependence of the kinetic friction force on the sliding velocity showed the non-Amontons-Coulomb-like behavior even for the lubricated friction. Thus, we confirmed that the Amontons-Coulomb's law does not realize when the thermal fluctuation is sufficiently large.

Study of friction using driven vortices of superconductor as a model system

Dynamics of driven vortices in high-Tc superconductors was investigated in terms of the elementary process of the motion and the microscopic friction. The I-V characteristics and the transient response of driven vortices were measured in La2-xSrxCuO4 thin films and in Bi2Sr2CaCu2Oy bulk crystals. First, with an aid of a recently proposed scaling relation between the driving force and the velocity, we found that the non-Arrhenius process did exist for interacting vortices at low temperatures. With increasing magnetic field, Arrhenius process revived suddenly at the vortex-glass vs Bragg-glass transition, which shows that this approach is suitable as a new method to investigate the equilibrium phase diagram of vortices. After constructing an explicit formula connecting the motion of driven vortices to the physics of friction, the dependence of the kinetic friction force on the sliding velocity was investigated. Based on the experimental results, we propose that the non-Amontons-Coulomb behavior is regarded as the consequence of the broadened dynamic phase transition. We also found the remarkable dependence of the maximum static friction force on the waiting time at low temperatures. Such a strong dependence changed into a weak logarithmic dependence at higher temperatures. The strong time dependence suggests that a characteristic time scale to stabilize vortices exists at low temperatures. This relaxation phenomenon is very similar to the so-called boundary lubrication. These results imply that the dynamics of vortices can be used for a model not only of the dry friction but also of the lubricated friction.

Noise study in condensed matter physics—Towards extension to surrounding fields

I briefly review noise studies in condensed matter physics, such as the shot noise measurement in metals, the dynamic-coherent-volume investigation in charge-density waves, the macroscopic quantum tunneling in superconductors, and the experimental investigation of dynamic phase diagram of driven vortices in high-Tc superconductors. With these examples, one finds that the noise studies have played many crucial roles in condensed matter physics. I also discuss a recent theoretical suggestion that noise measurements in Josephson junction may clarify the origin of the dark energy in the universe.

AC Losses in High-TC Superconductors with Finite Thickness

AbstractThe structure and dynamics of magnetic vortices in high-Tc superconductors are studied. The model we use is applicable to highly anisotropic high-Tc superconductors, such as Bi and Th compounds, where the Josephson coupling is neglected between the superconducting layers. In this paper, we studied the ac losses occurring in a finite number of superconducting layers due to an applied ac current. It is found that the ac losses increase with the frequency and the amplitude of the applied current. By introducing random pinning centers in the layers, the effect of pinning on the reduction of ac losses is also studied.

1-MV Field Emission Electron Microscope and Its Applications

With our newly developed 1-MV field emission electron microscope, which has the brightest electron beam and the highest lattice resolution, we obtained new results about unconventional behaviors of quantized vortices in high-Tc superconductors. Although vortices have hitherto been observed only at the superconductor surfaces by conventional methods such as Bitter method, vortices inside high-Tc superconductors became observable by Lorentz microscopy using our 1-MV microscope. Making full use of this technique, we found various behaviors of vortices peculiar to high-Tc superconductors. [DOI: 10.1380/ejssnt.2004.17]

Numerical Calculation of Interaction Between the Ferromagnetic Dots and Vortices

Enhancing the pinning force in high-Tc superconductors can be achieved by externally introduced periodic magnetic dots. We have numerically calculated the interaction between the magnetic dot and the vortex in high-Tc superconductors. The London equation is used to generate two-dimensional vortex lattice. In the matching condition, we calculate the attraction force between the magnetic dots and vortices at various conditions. It is found that the magnitude of the pinning from the dots is compatible with the intrinsic pinning force of YBCO thin film materials.

Direct observation of vortices in high-Tc superconductors

The structures of vortices inside high-Tc superconductors are now observable using Lorentz microscopy with our newly developed 1-MV field-emission transmission electron microscope, which has the brightest electron beam with the narrowest monochromaticity yet reported. Using this microscope, we identified previously unseen unconventional behaviors of vortices in anisotropic high-Tc superconductors: we distinguished different structures of vortex lines inside high-Tc superconductors, such as vortex lines perpendicular to the layer plane and tilted ones trapped by columnar defects, as different images. We found to our surprise that when the sample temperature decreased below 14 K, the vortices trapped and strongly pinned along tilted columnar defects became perpendicular to the film plane, as if the defects had disappeared. We were also able to clarify the mechanisms of unconventional arrangements, such as the chain state of vortices in YBCO and the chain-lattice state of vortices in Bi-2212, in an inclined magnetic field and also the mechanism of the unexpected chain-vortices image disappearance in Bi-2212.

Dynamic phase diagram of vortices in high-Tc superconductors determined by experimental studies

Dynamic phase of driven vortices of high- T c cuprate, Bi 2 Sr 2 CaCu 2 O y , was investigated by noise measurement and AC–DC interference measurement. We obtained the magnetic field ( H )–temperature ( T )–driving force ( F ) diagram of the dynamic phase. The obtained phase diagram was rather different from that of conventional superconductors and the theoretically expected diagram. We consider that the differences are characteristic of high- T c cuprates.

Lorentz microscopy observation of unconventional behaviors of vortices in High-Tc superconductors

Vortices in high-T c superconductors have been predicted to behave in unconventional manners that reflect the layered structure of the materials. Our newly developed 1-MV field-emission electron microscope has opened the way to enable individual vortices to be observed inside high-T c superconducting thin films. This makes it possible to investigate the inner structures of vortices, such as those trapped along tilted columnar defects and those of chain vortices that are formed when a magnetic field is obliquely applied with respect to the normal of the layer plane of the materials.

Charging Effects of Vortex Core in High Temperature Superconductors Probed by Nuclear Quadrupole Interaction

From high resolution measurements of the nuclear quadrupole frequencies we obtain experimental evidence that a vortex in high Tc superconductors (HTSC) traps a finite electric charge. In slightly overdoped YBa2Cu3O7 the vortex is negatively charged by trapping electrons, while in underdoped YBa2Cu4O8 it is positively charged by expelling electrons. The sign of the trapped charge is opposite to the sign predicted by the conventional BCS theory. Moreover, in both materials the deviation of the magnitude of the charge from the theory is significant. These features can be attributed to the novel electronic structure of the vortex in HTSC

Direct observations of vortices in Bi2212 single crystals by scanning SQUID microscopy

In order to investigate the behaviour of vortices in high-Tc superconductors under low magnetic fields, we performed scanning SQUID microscopy (SSM) on the ab surfaces of Bi2.1Sr1.8Ca(Cu0.98Co0.02)2Oy (Bi2212) single crystals at different temperatures and under different magnetic fields up to 0.8 G. The observed magnetic images clearly demonstrated vortex assembly structures for Bi2212. Vortices trapped inside Bi2212 single crystals tended to be arranged in a one-dimensional manner.

Direct observations of vortices in a high-Tc superconductor (La1-xSrx)2CuO4 by scanning SQUID microscopy

In order to investigate the behavior of vortices in high-Tc superconductors under low magnetic fields, we have performed scanning SQUID microscopy (SSM) on the ab-surfaces of (La1-xSrx)2CuO4 (LSCO) single crystals at low temperatures. The observed magnetic images clearly demonstrated vortices in units of Φ0 (=hc/2e) below Tc. Vortices trapped inside the overdoped LSCO tended to be arranged in a one-dimensional manner, while those in optimally doped crystals were randomly distributed. From the variable-temperature SSM measurements, it was directly visualized that the normal state with regular magnetic-field distribution gradually changed into the inhomogeneous Meissner state, and finally vortices evolved, with decreasing temperature.

Electronic States and Mixing of Different Symmetry or Antiferromagnetism in Vortex Cores in dx2-y2-wave Superconductivity

The microscopic structure of vortices in high-Tc superconductors is studied using the two-dimensional t-J model. The spatial dependences of order parameters are determined self-consistently using the Gutzwiller approximation, in which the effect of correlation is into account. In the high-doping region, the pair potential has d x2-y2-wave nature and the local density of states in the vicinity of the core shows a zero-energy peak. However, in the low-doping region, a spatially oscillating (extended) s-wave-type order parameter is locally induced around the vortex core. As a result, the local density of states near the core shows a splitting of the zero-energy peak. This gives a possible explanation for the experimental data of scanning tunneling spectroscopy for YBCO. The possibility of the antiferromagnetic vortex core is also studied by modifying the Gutzwiller approximation. It is found that, close to the boundary to the antiferromagnetically ordered state, the antiferromagnetic correlation develops inside the vortex core, even if the bulk state is the pure d-wave state. In this case the local density of states near the core does not have a zero-energy peak but instead shows a gap-like feature, which can be observed experimentally. Finally the sign change of the charge of the vortex core as a function of the doping rate is found.

Microwave surface resistance and vortices in high-Tc superconductors: Observation of flux pinning and flux creep.

History dependence and logarithmic time dependence of the microwave surface resistance have been observed in polycrystalline ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ in a magnetic field. We interpret our data in terms of granularity and use the model of Gittleman and Rosenblum to relate the surface resistance to the number of vortices in the sample. At a given field, we find that the surface resistance of a zero-field-cooled sample is higher than that of a field-cooled sample, suggesting that the former contains more intergranular vortices. It is also observed that once the external dc field is removed, flux creep out of the sample results in a logarithmic time decay of the surface resistance, from which we estimate a thermal activation energy at 77 K of about 0.1 eV.