Vortex Penetration Research Articles

Magnesium diboride coated bulk niobium: a new approach to higher acceleration gradient.

Bulk niobium Superconducting Radio-Frequency cavities are a leading accelerator technology. Their performance is limited by the cavity loss and maximum acceleration gradient, which are negatively affected by vortex penetration into the superconductor when the peak magnetic field at the cavity wall surface exceeds the vortex penetration field (Hvp). It has been proposed that coating the inner wall of an SRF cavity with superconducting thin films increases Hvp. In this work, we utilized Nb ellipsoid to simulate an inverse SRF cavity and investigate the effect of coating it with magnesium diboride layer on the vortex penetration field. A significant enhancement of Hvp was observed. At 2.8 K, Hvp increased from 2100 Oe for an uncoated Nb ellipsoid to 2700 Oe for a Nb ellipsoid coated with ~200 nm thick MgB2 thin film. This finding creates a new route towards achieving higher acceleration gradient in SRF cavity accelerator beyond the theoretical limit of bulk Nb.

Magnetic field imaging of a tungsten carbide film by scanning nano-SQUID microscope

We present the results of magnetic field imaging by scanning nano-superconducting quantum interference device (SQUID) microscopy on a tungsten carbide (W-C) film fabricated using focused-ion-beam chemical vapor deposition. We have investigated magnetic field change by a W-C film in an external magnetic field using a scanning nano-SQUID microscope system. We have found that the reduction of the magnetic field above the W-C film was 0.9%, indicating the penetration of vortices in the W-C at an external magnetic field of 0.171 mT.

A water-model experimental study of vortex characteristics due to nozzle clogging in slab caster mould

Vortex formation in the continuous slab casting mould is a troublesome phenomenon that can pull mould powder/slag deep down into the liquid core and potentially damage the product quality. Submerged entry nozzle (SEN) clogging is one of the prime causes of vortex formation. This work is designed to investigate and quantify the inter-related effects of nozzle clogging, casting speed (CS) and SEN submergence depth on vortex frequency and penetration depth through water-model experiments. It is observed that the increase in degree of clogging and CS has an inter-related effect on flow asymmetry and impacted the vortex characteristics. At the combination of highest values of degree of clogging and CS, vortices formed are found to have high values of top-diameter, rotational speed, penetration depth, frequency and life. On the contrary, the SEN submergence depth has showed limited effect on these aspects except the penetration depth. With the increase in SEN submergence depth values, it is found that the number of vortices having depth greater than 80 mm is increased.

Pinning of superconducting vortices in MoGe/Au Thin nano–squares

In this work, we report a scanning tunneling spectroscopy study of vortex patterns in mesoscopic superconducting squares and explore the impact of defects and corrugations inherently present in nanofabricated structures. We find that a hillock at the edge can function as an attractive or repulsive pinning center for vortices deforming the, theoretically predicted, symmetry-induced vortex configurations. In addition, we exploit the inherently present imperfections, creating metastable states, to visualize the dynamics of vortex penetration during magnetic field sweeps.

Unidimensional thermal gradients Tn in a nanoscopic superconductor

In general, few studies of the Abrikosov state in mesoscopic superconductors have been carried out with the presence of temperature gradients. We know that there can be situations where a sample near a heater source leads to a variable temperature system. Thus, in this work we studied the influence of several thermal gradients on the vortex configuration and the thermodynamics properties of a low critical temperature nanoscopic superconducting long square prism of size d × d. For this purposes, we simulated a hypothetical physical situation where the temperature varies into the sample as T=T1+(x/d)n(T2−T1), with T=T1(x=0),T=T2(x=d) and n=1,2,3,4,5,6,T1 < T2. The field for the first vortex penetration H1 presents a slow dependence on n and d.

Superconducting NbTiN thin films for superconducting radio frequency accelerator cavity applications

Current superconducting radio frequency technology, used in various particle accelerator facilities across the world, is reliant upon bulk niobium superconducting cavities. Due to technological advancements in the processing of bulk Nb cavities, the facilities have reached accelerating fields very close to a material-dependent limit, which is close to 50 MV/m for bulk Nb. One possible solution to improve upon this fundamental limitation was proposed a few years ago by Gurevich [Appl. Phys. Lett. 88, 012511 (2006)], consisting of the deposition of alternating thin layers of superconducting and insulating materials on the interior surface of the cavities. The use of type-II superconductors with Tc &amp;gt; TcNb and Hc &amp;gt; HcNb, (e.g., Nb3Sn, NbN, or NbTiN) could potentially greatly reduce the surface resistance (Rs) and enhance the accelerating field, if the onset of vortex penetration is increased above HcNb, thus enabling higher field gradients. Although Nb3Sn may prove superior, it is not clear that it can be grown as a suitable thin film for the proposed multilayer approach, since very high temperature is typically required for its growth, hindering achieving smooth interfaces and/or surfaces. On the other hand, since NbTiN has a smaller lower critical field (Hc1) and higher critical temperature (Tc) than Nb and increased conductivity compared to NbN, it is a promising candidate material for this new scheme. Here, the authors present experimental results correlating film microstructure with superconducting properties on NbTiN thin film coupon samples while also comparing films grown with targets of different stoichiometry. It is worth mentioning that the authors have achieved thin films with bulk-like lattice parameter and transition temperature while also achieving Hc1 values larger than bulk for films thinner than their London penetration depths.

Vortex Penetrations in Parallel-connected two Stacks of Intrinsic Josephson Junctions

In mesoscopic stacks of intrinsic Josephson junctions (IJJs) in Bi2Sr2CaCu2O8+y (Bi2212), the penetrations of individual vortices are detectable by the measurements of the transport properties, i.e., c-axis resistance or critical current. We have measured the c-axis resistance as a function of magnetic field in samples with two stacks of IJJs connected in parallel by Bi2212 itself to study any interaction of individual vortex penetrations into them. Since the superconducting loop containing two stacks of IJJs is the same geometry as that of superconducting quantum interference device (SQUID), we might expect a periodic resistance (or current) modulation as a function of magnetic field, whose period corresponds to the area in the loop. However, the results were just simple mixing of the resistive changes by the individual vortex penetrations into each of the stacks; behavior like SQUID has not been observed in present samples.

Considerations for Measuring Compressor Aerodynamic Excitations Including Rotor Wakes and Tip Leakage Flows

The unsteady flow field generated by the rotor provides unsteady aerodynamic excitations to the downstream stator, which can result in vibrations such as forced response. In this paper, measurements of the rotor wake and rotor tip leakage flow from an embedded rotor in a multistage axial compressor are presented. A unique feature of this work is the pitchwise traverse of the flow field used to highlight the changes in the rotor exit flow field with respect to the position of the surrounding vane rows. Results acquired at midspan focus on characterizing an average rotor wake, including the effects on the frequency spectrum, from a forced response perspective. While many analyses use an average rotor wake to characterize the aerodynamic forcing function to the downstream stator, this study explores the factors that influence changes in the rotor wake shape and the resulting impact on the spectrum. Additionally, this paper investigates the flow near the endwall where the tip leakage vortex is an important contributor to the aerodynamic excitations for the downstream vane. For the first time, experimental data are presented at the rotor exit, which show the modulation in size and radial penetration of the tip leakage vortex as the rotor passes through the upstream vane wake. As computational models become more advanced, the ability to incorporate these aerodynamic excitation effects should be considered to provide better predictions for vane vibratory response.

Single-vortex pinning and penetration depth in superconductingNdFeAsO1−xFx

We use a magnetic force microscope (MFM) to investigate single vortex pinning and penetration depth in NdFeAsO$_{1-x}$F$_x$, one of the highest-$T_c$ iron-based superconductors. In fields up to 20 Gauss, we observe a disordered vortex arrangement, implying that the pinning forces are stronger than the vortex-vortex interactions. We measure the typical force to depin a single vortex, $F_{\mathrm{depin}} \simeq 4.5$ pN, corresponding to a critical current up to $J_c \simeq 7 \times 10^5$ A/cm$^2$. Furthermore, our MFM measurements allow the first local and absolute determination of the superconducting in-plane penetration depth in NdFeAsO$_{1-x}$F$_x$, $\lambda_{ab}=320 \pm 60$ nm, which is larger than previous bulk measurements.

Profile and Crowding of Currents in Mesoscopic Superconductors With an Array of Antidots

Studies with mesoscopic superconducting materials have made significant advances on the last decades. One of the applications of such systems is in devices for single photon and single electron detectors. However, depending on the geometry of these systems, crowding current effects take place and, as a consequence, the total critical current could decrease which facilitates the penetration of vortices. This effect could also be responsible for a variety of penetration morphologies of flux avalanches in macroscopic samples. Thus, in this work we used the time-dependent Ginzburg-Landau theory to study the crowding current effects in mesoscopic superconducting systems with an array of antidots. It is demonstrated that the profile of the currents is influenced by the antidots, i.e., in the vertices of the antidots the intensity of the currents increases. On the other hand, the profile of the currents in between the antidots is more affected in the smaller system which presents closer antidots.

Layer number dependence of flux avalanches in superconducting shifted strip array

We have fabricated multi-layer superconducting shifted strip arrays (SSAs) of Nb up to 4 layers and systematically studied the vortex penetrations into these structures. We observed the vortex penetration as a function of the number of layers and the ratio of overlap between neighboring layers by using magneto-optical (MO) imaging. In the case of 2- and 3-layer SSAs, spot-like avalanches occur when the overlap is small, while linear avalanches occur when the overlap is large, consistent with our previous reports. In the 4-layer SSAs, the smallest limit of the overlap between the neighboring layers for the linear avalanche is lower. Flux penetrations parallel to the strip which were observed in the 3-layer SSA were also observed in the 4-layer SSAs with smaller ratio of overlap. Larger demagnetization effects in the middle two layers in 4-layer SSA help to make avalanches larger and more extended.

Enhancement of lower critical field by reducing the thickness of epitaxial and polycrystalline MgB2 thin films

For potential applications in superconducting RF cavities, we have investigated the properties of polycrystalline MgB2 films, including the thickness dependence of the lower critical field Hc1. MgB2 thin films were fabricated by hybrid physical-chemical vapor deposition on (0001) SiC substrate either directly (for epitaxial films) or with a MgO buffer layer (for polycrystalline films). When the film thickness decreased from 300 nm to 100 nm, Hc1 at 5 K increased from around 600 Oe to 1880 Oe in epitaxial films and to 1520 Oe in polycrystalline films. The result is promising for using MgB2/MgO multilayers to enhance the vortex penetration field.

Ferromagnetic/superconducting interface in a hybrid nanoscopic disc

The oscillations exhibited by the magnetization of superconducting nano and mesoscopics structures as a function of the applied magnetic field has attracted attention in the last few years. Nano/Mesoscopic hybrid systems in contact with ferromagnets exhibit interesting transport properties related to the influence of the exchange field on the density of states of clean ferromagnetic structures in contact with superconductor. In this work we study the vortex configurations in a superconducting disk with one central defect inside. The sample is surrounded by a ferromagnetic medium. We calculate the spatial distribution of the Cooper pairs and the phase of the superconducting order parameter and curves of magnetization and vorticity as a function of the magnetic applied field. We found that the first vortex penetration field decrease when a ferromagnetic/superconducting interface is used.

Size Dependence of Individual Vortex Penetration into Intrinsic Josephson Junction Stacks of Bi2Sr2CaCu2O8+y

By c-axis transport measurements in an intrinsic Josephson junctions (IJJs) stack of Bi2Sr2CaCu2O8+y in magnetic fields parallel to c-axis, a penetration of individual vortices can be detected as a sudden jump or drop of c-axis resistance or critical current, which has already been reported in a sample of small in-plane area (<2μm2). We explored the individual vortex penetrations in IJJs stacks with much larger in-plane areas and successfully observed the same phenomenon even in a sample as large as 100μm2. Behavior of the c-axis resistance and critical current caused by the individual vortex penetrations are investigated through the sample-size dependence in various temperatures and magnetic fields.

Maximum screening fields of superconducting multilayer structures

It is shown that a multilayer comprised of alternating thin superconducting and insulating layers on a thick substrate can fully screen the applied magnetic field exceeding the superheating fields Hs of both the superconducting layers and the substrate, the maximum Meissner field is achieved at an optimum multilayer thickness. For instance, a dirty layer of thickness ∼0.1 μm at the Nb surface could increase Hs ≃ 240 mT of a clean Nb up to Hs ≃ 290 mT. Optimized multilayers of Nb3Sn, NbN, some of the iron pnictides, or alloyed Nb deposited onto the surface of the Nb resonator cavities could potentially double the rf breakdown field, pushing the peak accelerating electric fields above 100 MV/m while protecting the cavity from dendritic thermomagnetic avalanches caused by local penetration of vortices.

Distinct magnetic signatures of fractional vortex configurations in multiband superconductors

Vortices carrying fractions of a flux quantum are predicted to exist in multiband superconductors, where vortex core can split between multiple band-specific components of the superconducting condensate. Using the two-component Ginzburg-Landau model, we examine such vortex configurations in a two-band superconducting slab in parallel magnetic field. The fractional vortices appear due to the band-selective vortex penetration caused by different thresholds for vortex entry within each band-condensate, and stabilize near the edges of the sample. We show that the resulting fractional vortex configurations leave distinct fingerprints in the static measurements of the magnetization, as well as in ac dynamic measurements of the magnetic susceptibility, both of which can be readily used for the detection of these fascinating vortex states in several existing multiband superconductors.

Effect of a superconducting defect on the Cooper pairs of a mesoscopic sample

We investigate the vortex state in a very long prism of square cross section with a central square defect in the presence of an external perpendicular magnetic field. We considered that the inner defect edge is in contact with a thin superconducting layer at higher critical temperature and/or with a dielectric material, while the outer edge of the sample is in contact with the vacuum. We have evaluated the superconducting order parameter, magnetization and vorticity as a function of the size of the defect at the first vortex penetration field. Therefore we conclude and we are able to show that circular geometry of the vortices near to the defect is mildly modified by the enhanced superconductivity at the edge of the hole.

Vortex Doping Into Superconducting ${\rm Mo}_{80}{\rm Ge}_{20}$ Square Network

We have fabricated the finite-sized amorphous Mo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">80</sub> Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> superconducting networks with 10×10 square holes and directly observed the vortex structures by means of using the scanning SQUID microscope. On the increase of the field in a stepwise manner, the vortex penetration into the network is likely to follow the Bean critical model, wherein vortices first enter the network from the edges and penetrate deeper toward the center of the network as the field increases. We observed that the evolution of vorticity mainly occurred along the diagonal direction of the network, which has a narrow wire compared with the hole size. The vortex pattern was strongly modified as the system was cooled down below T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . A diagonal vortex array appeared at intermediate temperatures, and is transformed into the isolated vortices at lower temperatures due to the strong repulsive interaction between vortices.

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.

Determination of the lower critical field Hc1(T) in FeSe single crystals by magnetization measurements

In a recent work, Abdel-Hafiez et al. [1] we have determined the temperature dependence of the lower critical field Hc1(T) of a FeSe single crystal under static magnetic fields H parallel to the crystallographic c axis. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding Hc1(T) was deduced by taking into account demagnetization factors. In general, the first vortex penetration field may not reflect the true Hc1(T) due to the presence of surface barriers. In this work we show that magnetic hysteresis loops are very symmetric close to the critical temperature Tc = 9K evidencing the absence of surface barriers and thus validating the previously reported determination of Hc1(T) and the main observations that the superconducting energy gap in FeSe is nodeless.