Nanostructured Superconductors Research Articles

19aPS-128 ナノ構造超伝導体の統合的理論へ向けて : Eliashberg理論の拡張

19aPS-128 ナノ構造超伝導体の統合的理論へ向けて : Eliashberg理論の拡張

Extended Molecular Dynamics Methods for Vortex Dynamics in Nano-structured Superconductors

Using improved molecular dynamics simulation method, we study vortex dynamics in nano-scaled superconductors. Heat generations during vortex motion, heat transfer in superconductors, and entropy forces to vortices are incorporated. Also quasi-particle relaxations after vortex motion, and their attractive “retarded” forces to other vortices are incorporated using the condensation-energy field. We show the time development of formation of vortex channel flow in a superconducting Corbino-disk.

Dumping topological charges on neighbors: ice manifolds for colloids and vortices

We investigate the recently reported analogies between pinned vortices in nano-structured superconductors or colloids in optical traps, and spin ice materials. It has been found experimentally and numerically that both colloids and vortices exhibit ice or quasi-ice manifolds. However, the frustration of colloids and vortices differs essentially from spin ice at the vertex level. We show that the effective vertex energetics of the colloidal/vortex systems is made identical to that of spin ice materials by the contribution of an emergent field associated to the topological charge of the vertex. The similarity extends to the local low-energy dynamics of the ice manifold, where the effect of geometric hard constraints can be subsumed into the spatial modulation of the emergent field, which mediates an entropic interaction between topological charges. There, as in spin ice materials, genuine ice manifolds enter a Coulomb phase, whereas quasi-ice manifolds posses a well defined screening length, provided by a plasma of embedded topological charges. We also show that such similarities break down in lattices of mixed coordination because of topological charge transfer between sub-latices. This opens interesting perspective for extensions beyond physics, to social and economical networks.

Flux penetrations into two- and three-dimensional nanostructured superconductors

We have fabricated two- and three-dimensional nanostructured superconductors, and observed vortex penetrations by magneto-optical imaging. In the case of two-dimensional superconducting networks with square holes on a square lattice, anomalous diagonal penetrations are widely observed. Two kinds of diagonal vortex penetrations at high and low temperatures have been interpreted as originating from the repulsive interaction of vortices and sharp fan-shaped vortex penetration from the corners of the square holes, respectively. In the case of three-dimensional stack of superconducting strip arrays with double and triple layers, vortex avalanches have been observed in a wide temperature and dimension ranges due to enhanced demagnetization effect. While spotlike avalanches are observed when the overlap of strips is small, anomalous linear avalanches traversing many strips in different layers are observed when the overlap is large. In triple-layer strip arrays, in addition to the spotlike and linear avalanches, vortex penetrations along the line of strips are also observed. Origins of the anomalous diagonal penetration and vortex avalanches are discussed.

Quantum rotor in nanostructured superconductors

Despite its apparent simplicity, the idealized model of a particle constrained to move on a circle has intriguing dynamic properties and immediate experimental relevance. While a rotor is rather easy to set up classically, the quantum regime is harder to realize and investigate. Here we demonstrate that the quantum dynamics of quasiparticles in certain classes of nanostructured superconductors can be mapped onto a quantum rotor. Furthermore, we provide a straightforward experimental procedure to convert this nanoscale superconducting rotor into a regular or inverted quantum pendulum with tunable gravitational field, inertia, and drive. We detail how these novel states can be detected via scanning tunneling spectroscopy. The proposed experiments will provide insights into quantum dynamics and quantum chaos.

Charge and spin transport in mesoscopic superconductors.

Background: Non-equilibrium charge transport in superconductors has been investigated intensely in the 1970s and 1980s, mostly in the vicinity of the critical temperature. Much less attention has been paid to low temperatures and the role of the quasiparticle spin.Results: We report here on nonlocal transport in superconductor hybrid structures at very low temperatures. By comparing the nonlocal conductance obtained by using ferromagnetic and normal-metal detectors, we discriminate charge and spin degrees of freedom. We observe spin injection and long-range transport of pure, chargeless spin currents in the regime of large Zeeman splitting. We elucidate charge and spin transport by comparison to theoretical models.Conclusion: The observed long-range chargeless spin transport opens a new path to manipulate and utilize the quasiparticle spin in superconductor nanostructures.

Frustrated colloidal ordering and fully packed loops in arrays of optical traps

We propose that a system of colloidal particles interacting with a honeycomb array of optical traps that each contain three wells can be used to realize a fully packed loop model. One of the phases in this system can be mapped to Baxter's three-coloring problem, offering an easily accessible physical realization of this problem. As a function of temperature and interaction strength, we find a series of phases, including long range ordered loop or stripe states, stripes with sliding symmetries, random packed loop states, and disordered states in which the loops break apart. Our geometry could be constructed using ion trap arrays, BEC vortices in optical traps, or magnetic vortices in nanostructured superconductors.

Dynamics of Vortices in Nano-Structured Superconductors with Periodic Arrays of Various Antidots

Stable vortex configurations and its dynamics in superconductors with various types of antidotes are examined, using the molecular dynamics simulation. A pinning potential function, which parameterizes spatial shapes and pinning potential structure of the antidot, is introduced. Dependence of saturation number, which is a maximum number of vortices pinned in a single antidot, on these spatial shape and potential structure are investigated.

Nonadiabatic ratchet effect in superconducting films with a tilted cosine pinning potential

The influence of an ac current of arbitrary amplitude and frequency on the mixed-state dc-voltage-ac-drive ratchet response of a superconducting film with a dc current-tilted uniaxial cosine pinning potential at finite temperature is theoretically investigated. The results are obtained in the single-vortex approximation, i.e., for non-interacting vortices, within the frame of an exact solution of the appropriate Langevin equation in terms of a matrix continued fraction. The dc voltage ratchet response is discussed as a function of ac current amplitude and frequency for various dc biases, in a wide range of corresponding dimensionless parameters. Our theoretical results are discussed in comparison with recent experimental work on the high-frequency ratchet response in nanostructured superconducting films.

Crossover behaviors in magnetoresistance oscillations for Nb thin film with rectangular arrays of antidots

Superconducting Nb thin films with rectangular arrays of submicron antidots (holes) have been systemically investigated by transport measurements. A series of crossover behaviors is found in magnetoresistance oscillations, corresponding to three different superconducting states: the wire network-like state, the interstitial vortex state and the single-loop–like state. These states are identified by the field intervals and hysteretic effect. The crossover fields between them are found to be both temperature and geometry dependent. Furthermore, in dense arrays, the saturation number is distinctly larger than the theoretical calculation for a single insulating inclusion. Our results indicate that in the process of magnetic field penetration into nanostructured superconductors, the order parameters are strongly modulated and finally localized near the edges, resulting in changes of oscillation modes.

Direct observation of vortices by scanning SQUID microscope on small superconducting Mo80Ge20 circular disks

The amorphous superconducting films, which tend to have weakened pinning centers, are remarkably useful as model substance in studying nanostructured superconductors. The amorphous Mo80Ge20 films have been deposited by a sputtering apparatus using a Mo80Ge20 target and the small Mo80Ge20 plates were fabricated with the aid of photolithography. The vortex distribution in a Mo80Ge20 disk has been investigated by means of a scanning SQUID microscope. We found that vortices form different configurations as well as shell structures, which evolves with the increase in applied magnetic fields. The observed results are in good agreement with theoretical studies predicted for vortices in mesoscopic circular disks. We also applied a novel numerical method to improve the spatial resolution of the scanning SQUID microscope.

Vortex configuration and vortex–vortex interaction in nano-structured superconductors

We study the vortex structures and quasi-particle structures in nano-structured superconductors. We used the Bogoliubov–de Gennes equation and the finite element method and obtained stable magnetic flux structures and the quasi-particle states. We found the vortex configurations are affected by the interference of the quasi-particle bound states around the vortices. In order to clarify the interference between the quasi-particle wave-functions around two vortices we have developed a numerical method using the elliptic coordinates and the Mathieu functions. We apply this method to two singly quantized vortex state in a conventional s-wave superconductor and a pair of half-quantum vortices in a chiral p-wave superconductor.

Frequency-dependent ratchet effect in superconducting films with a tilted washboard pinning potential

The influence of an ac current of arbitrary amplitude and frequency on the mixed-state dc-voltage-ac-drive ratchet response of a superconducting film with a dc current-tilted uniaxial cosine pinning potential at finite temperature is theoretically investigated. The results are obtained in the single-vortex approximation, i.e., for noninteracting vortices, within the frame of an exact solution of the appropriate Langevin equation in terms of a matrix continued fraction. Formulas for the dc voltage ratchet response and absorbed power in ac response are discussed as functions of ac current amplitude and frequency as well as dc current induced tilt in a wide range of corresponding dimensionless parameters. Special attention is paid to the physical interpretation of the obtained results in adiabatic and high-frequency ratchet responses taking into account both running and localized states of the (ac+dc)-driven vortex motion in a washboard pinning potential. Our theoretical results are discussed in comparison with recent experimental work on the high-frequency ratchet response in nanostructured superconducting films [B. B. Jin et al., Phys. Rev. B 81, 174505 (2010)].

Development and realization methods for the study of local magnetic and transport characteristics of modern nanostructured superconducting materials

It is well known that the Hall probe magnetic imaging represents a powerful technique for the investigation of the magnetic field distribution in nanostructured superconducting materials of many different types and geometries. We present a new method of the evaluation of current distributions in thin nanostructured superconducting tapes and films from the data of two-dimensional magnetic field profiles measured over the sample surface. We have analyzed the different conditions of magnetic flux trap of external magnetic field and the magnetic field distribution of a superconductor at the flow of transport current. Based on the standard algorithm was developed original method of non-contact investigation of local transport properties of modern nanostructured superconducting materials.

Scanning laser microscope for imaging nanostructured superconductors

The nanofabrication of superconductors yields various interesting features in superconducting properties. A variety of different imaging techniques have been developed for probing the local superconducting profiles. A scanning pulsed laser microscope has been developed by the combination of the XYZ piezo-driven stages and an optical fiber with an aspheric focusing lens. The scanning laser microscope is used to understand the position-dependent properties of a superconducting MgB 2 stripline of length 100 μm and width of 3 μm under constant bias current. Our results show that the superconducting stripline can clearly be seen in the contour image of the scanning laser microscope on the signal voltage. It is suggested from the observed image that the inhomogeneity is relevant in specifying the operating conditions such as detection efficiency of the sensor.

Jamming and diode effects for vortices in nanostructured superconductors

We examine jamming and ratchet effects for vortex matter in superconductors with asymmetric funnel geometries. We show that the vortex–vortex interactions can induce a clogging or jamming effect where it becomes increasingly difficult for the vortices to move through the system. We also find that commensurability effects can arise when certain vortex configurations form highly symmetrical structures in the funnel plaquettes. Due to the asymmetry, the critical currents are different for driving in different directions, leading to a diode effect. We also discuss other possible geometries and approaches that could be used to explore jamming in vortex matter, such as an analog to a granular hopper and a single driven vortex probe moving through an array of other vortices.

Levitation force at different temperatures and superconducting properties of nano-structured MgB2 superconductors

The levitation and pinning force properties of nanocrystalline MgB2 superconductors prepared by a mechanical alloying technique with varying milling duration have been investigated. Levitation force measurements under zero-field-cooled regime were carried out at different temperatures (16, 25 and 30 K) as a function of the gap between bottom surface of the superconductor and the top surface of a cylindrical Nd–Fe–B permanent magnet whose diameter and height are 30 mm. An increase in the levitation force was observed due to the decrease in particle size caused by increasing the milling time. It was determined the levitation force of the sample milled for 72 h was higher than that milled for 24 h at the same measurement temperature. This is indicative of an enhancing flux pinning capability at the grain boundaries of the MgB2. Also, it was observed that the pinning properties of the superconductor shifted from surface pinning to normal point pinning above Bmax when the milling time was increased from 24 to 72 h.

Superconducting nanowires: quantum confinement and spatially dependent Hartree–Fockpotential

It is well known that, in bulk, the solution of the Bogoliubov–de Gennes equations is thesame whether or not the Hartree–Fock term is included. Here the Hartree–Fock potential isposition independent and so gives the same contribution to both the single-electronenergies and the Fermi level (the chemical potential). Thus, the single-electron energiesmeasured from the Fermi level (they control the solution) stay the same. This is not thecase for nanostructured superconductors, where quantum confinement breaks thetranslational symmetry and results in a position-dependent Hartree–Fock potential. In thiscase its contribution to the single-electron energies depends on the relevant quantumnumbers. We numerically solved the Bogoliubov–de Gennes equations with theHartree–Fock term for a clean superconducting nanocylinder and found a shift of the curverepresenting the thickness-dependent oscillations of the critical superconductingtemperature to larger diameters.

Creating Artificial Ice States Using Vortices in Nanostructured Superconductors

We demonstrate that it is possible to realize vortex ice states that are analogous to square and kagome ice. With numerical simulations, we show that the system can be brought into a state that obeys either global or local ice rules by applying an external current according to an annealing protocol. We explore the breakdown of the ice rules due to disorder in the nanostructure array and show that in square ice, topological defects appear along grain boundaries, while in kagome ice, individual defects appear. We argue that the vortex system offers significant advantages over other artificial ice systems.

Rocking ratchets in nanostructured superconducting–magnetic hybrids

Two rectification mechanisms in vortex lattice dynamics in Nb films have been studied.These two effects are based on ratchet effects, that is, an ac driving force inducesa net dc vortex flow. In our case, an input ac current applied to the Nb films,grown on top of arrays of Ni nanotriangles, yields an output dc voltage. These tworectification effects occur when the vortex lattice moves in periodic asymmetricpotentials. These pinning potentials are induced by the array of Ni triangles. In oneconfiguration (longitudinal effect) the driven force is applied perpendicular to the trianglereflection symmetry axis; in the second one (transverse effect) the input current isinjected parallel to the triangle reflection symmetry axis. In the framework ofthe rocking ratchet mechanism, the appropriate Langevin equation allows us tomodel the experimental data, taking into account the vortex–vortex interaction.