arXiv subjects

A. D. Caplin

Publications and source records attributed to A. D. Caplin.

17 recordsLinked to original sources

arXiv2009

Evidence for Nodal superconductivity in Sr$_{2}$ScFePO$_{3}$

Point contact Andreev reflection spectra have been taken as a function of temperature and magnetic field on the polycrystalline form of the newly discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak which disappears at the superconducting transition temperature, dominates all of the spectra. Data taken in high magnetic fields show that this feature survives until 7T at 2K and a flattening of the feature is observed in some contacts. Here we inspect whether these observations can be interpreted within a d-wave, or nodal order parameter framework which would be consistent with the recent theoretical model where the height of the P in the Fe-P-Fe plane is key to the symmetry of the superconductivity. However, in polycrystalline samples care must be taken when examining Andreev spectra to eliminate or take into account artefacts associated with the possible effects of Josephson junctions and random alignment of grains.

cond-mat.supr-con↗

arXiv2009

Effect of columnar defects on the pinning properties of NdFeAsO0.85 conglomerate particles

Oxypnictide superconductor NdFeAsO0.85 sample was irradiated with 2 GeV Ta ions at a fluence of 5x10^10 ions/cm2. High resolution transmission electron microscopy study revealed that the irradiation produced columnar-like defects. The effect of these defects on the irreversible magnetisation in polycrystalline randomly oriented fragments was studied as a function of field angle and field sweep rate. We find that the critical current density is enhanced at fields below the matching field (~1 Tesla) but only marginally. The pinning enhancement is anisotropic and maximum along the defect direction at high temperatures but the pinning then becomes more isotropic at low temperatures. The creep rate is suppressed at high temperatures and at fields below the matching field, indicating the columnar defects are efficient pinning sites at these H and T conditions.

cond-mat.supr-con↗

arXiv2008

Mapping the dynamic interactions between vortex species in highly anisotropic superconductors

Here we use highly sensitive magnetisation measurements performed using a Hall probe sensor on single crystals of highly anisotropic high temperature superconductors $Bi_{2}Sr_{2}CaCu_{2}O_{8}$ to study the dynamic interactions between the two species of vortices that exist in such superconductors. We observe a remarkable and clearly delineated high temperature regime that mirrors the underlying vortex phase diagram. Our results map out the parameter space over which these dynamic interaction processes can be used to create vortex ratchets, pumps and other fluxonic devices.

cond-mat.supr-con↗

arXiv2005

Dynamic interactions between pancake vortex stacks and Josephson vortices in Bi2Sr2CaCu2O8 single crystals: relaxation and ratchets

We present a detailed study of the dynamic interactions between Josephson vortices and stacks of pancake vortices in a Bi2Sr2CaCu2O8 single crystal, obtained by measuring the effect of applied inplane magnetic field pulses on the c-axis magnetisation. The predominant interaction is to relax the system towards equilibrium. However, using a highly sensitive AC technique we are able to measure also the forces acting to drive the system away from equilibrium, consistent with the existence of dragging interactions between the Josephson and pancake systems. Such forces were discussed recently as the basis of possible flux ratchet devices.

cond-mat.supr-con↗

arXiv2005

Electron diffusivities in MgB2 from point contact spectroscopy

We demonstrate that the variation of the Andreev reflection with applied magnetic field provides a direct means of comparing the properties of MgB2 with the theory for a dirty two-band superconductor, and we find good agreement between the two. The ratio of electron diffusivities in the s and p bands can be inferred from this experiment. We find that the field dependence of the density of states at the Fermi level in the p band is independent of the field direction, and that the anisotropic upper critical field is determined by the anisotropic diffusivity in the s band.

cond-mat.supr-con↗

arXiv2003

First order magnetic transition in CeFe$_2$ alloys: Phase-coexistence and metastability

First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in doped-CeFe$_2$ alloys is studied with micro-Hall probe technique. Clear visual evidence of magnetic phase-coexistence on micrometer scales and the evolution of this phase-coexistence as a function of temperature, magnetic field and time across the first order FM-AFM transition is presented. Such phase-coexistence and metastability arise as natural consequence of an intrinsic disorder-influenced first order transition. Generality of this phenomena involving other classes of materials is discussed.

cond-mat.mtrl-sci↗

arXiv2003

Superconducting gap structure and pinning in disordered MgB2 films

We have performed a comparative study of two thin films of magnesium diboride (MgB2) grown by different techniques. The critical current density at different temperatures and magnetic fields was evaluated from magnetisation curves, the structure of superconducting order parameter was obtained from point-contact spectroscopy, and the scattering rates were evaluated by fitting the temperature dependent normal-state resistivity to the two-band model. The films have similar critical temperatures close to 39 K, but the upper critical fields were different by a factor of 2 (5.2T and 2.5 T at 20 K). We have found that the film with higher Hc2 also had stronger scattering in the sigma band and smaller value of the superconducting gap in this band. As the scattering in sigma band is primarily due to the defects in boron plane, our results are consistent with the assumption that disordering the boron planes leads to enhanced Hc2 and better pinning properties in magnetic field.

cond-mat.supr-con↗

arXiv2003

The effect of magnetic field on the two superconducting gaps in MgB2

Double-gap superconductivity in an epitaxial MgB2 film has been studied by means of point-contact spectroscopy in magnetic field up to 8 Tesla. The relatively fast disappearance of the feature associated with the pi-band gap at a field around 1 T is caused by the line broadening due to strong pair breaking rather than to a collapse of the double-gap state. This pair breaking was found to increase linearly with field. Field dependences of the order parameters Delta_pi and Delta_sigma in the pi and sigma bands were measured in field applied parallel and perpendicular to the film, at T = 4.2 K. In perpendicular field, both order parameters survive to a common Hc2, which is about 6.5 T for this direction. In parallel field, the decrease of Delta_sigma is much more gradual, consistent with the Hc2 being about 4 times greater in this ori-entation. The difference in Delta_pi measured in the two field orientations is however smaller than the difference of Delta_sigma. We compare these results with the data on tunnelling spectroscopy and specific heat measurements of MgB2 single crystals and find consistency between the different experimental approaches.

cond-mat.supr-con↗

arXiv2003

Effect of mesoscopic inhomogeneities on the critical current of bulk melt-textured YBCO

The downsizing 211-inclusions and an increase of their density leads to rise in mean critical current value in Y-based melt textured material. Very often 211-inclusion are spread in the material volume non-homogeneous, with typical scale 50 - 100 micrometer. Therefore it is difficult to find the real correlation between local critical current and the inclusions distribution. We performed a study of a local critical current using modified magneto-optic technique on a melt-textured YBaCuO ceramic, found the areas with constant current and studied the real structure of the material in the areas, inclusions distribution and their sizes, by scanning electron microscopy and X-ray microanalysis. The estimation of a pinning in these places, by taking into account the amount of inclusions and the length of their boundaries, and comparison with the value of local critical current reveals a strait correlation between the density of inclusions and the current but shows remarkable quantitative disagreement.

cond-mat.supr-con↗

arXiv2002

Critical Fields and Critical Currents in MgB2

We review recent measurements of upper (Hc2) and lower (Hc1) critical fields in clean single crystals of MgB2, and their anisotropies between the two principal crystallographic directions. Such crystals are far into the "clean limit" of Type II superconductivity, and indeed for fields applied in the c-direction, the Ginzburg-Landau parameter k is only about 3, just large enough for Type II behaviour. Because m0Hc2 is so low, about 3 T for fields in the c-direction, MgB2 has to be modified for it to become useful for high-current applications. It should be possible to increase Hc2 by the introduction of strong electron scattering (but because of the electronic structure and the double gap that results, the scatterers will have to be chosen carefully). In addition, pinning defects on a scale of a few nm will have to be engineered in order to enhance the critical current density at high fields.

cond-mat.supr-con↗

arXiv2002

Effective Vortex Pinning in MgB2 thin films

We discuss pinning properties of MgB2 thin films grown by pulsed-laser deposition (PLD) and by electron-beam (EB) evaporation. Two mechanisms are identified that contribute most effectively to the pinning of vortices in randomly oriented films. The EB process produces low defected crystallites with small grain size providing enhanced pinning at grain boundaries without degradation of Tc. The PLD process produces films with structural disorder on a scale less that the coherence length that further improves pinning, but also depresses Tc.

cond-mat.supr-con↗

arXiv2002

Super-conducting critical fields and anisotropy of a MgB2 single crystal

Despite the intense activity in the year since the discovery of superconductivity in MgB2, key parameters, in particular the upper and lower critical fields Hc2 and Hc1 and their anisotropies, are not well-established, largely because of the difficulty of growing MgB2 crystals. Attempts have been made to deduce these parameters from experiments on polycrystalline material, but they have substantial uncertainties. Hc2 is particularly important for applications, as it is the field which quenches bulk super-conductivity. In terms of understanding MgB2, it is now clear that the conventional electron-phonon interaction is strong enough to account for the high transition temperature Tc, but the consequences of the double super-conducting gap for the anisotropy and its dependence on temperature, are uncertain. Here we describe detailed direct measurements of Hc1(T) and Hc2(T) for the two principal crystallographic directions in a clean single crystal of MgB2. For fields in the c-direction, $μ_0 H^c_{c1}(0)$ = $0.28 +- 0.01T$ and $μ_0 H^c_{c2}(0)$ is $3 +- 0.5T$; this ratio of critical fields is rather low and implies that MgB2 is only just a Type II super-conductor. The anisotropies of both critical fields are close to 2.

cond-mat.supr-con↗

arXiv2002

High Critical Current Density and Improved Irreversibility Field in Bulk MgB2 Made By A Scaleable, Nanoparticle Addition Route

Bulk samples of MgB2 were prepared with 5, 10, and 15% wt.% Y2O3 nanoparticles added using a simple solid-state reaction route. Transmission electron microscopy (TEM) showed a fine nanostructure consisting of ~3-5 nm YB4 nanoparticles embedded within MgB2 grains of ~400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density Jc was observed, most notably for 10% doping. At 4.2K, the lower bound Jc value was ~2x105A.cm-2 at 2T. At 20K, the corresponding value was ~8x104A.cm-2. Irreversibility fields were ~11.5 T at 4.2K and 5.5T at 20K, compared to ~4T and ~8T, respectively, for high-pressure synthesized bulk samples.

cond-mat.supr-con↗

arXiv2001

The structure of the superconducting gap in MgB2 from point-contact spectroscopy

We have studied the structure of the superconducting gap in MgB2 thin films by means of point-contact spectroscopy using a gold tip. The films were produced by depositing pure boron on a sapphire substrate, using e-beam evaporation, followed by reaction with magnesium vapour. The films have a Tc of 38.6 +- 0.3 K and resistivity of about 20 microOhm cm at 40 K. The point-contact spectra prove directly the existence of a multi-valued order parameter in MgB2, with two distinct values of the gap, DELTA1=2.3+-0.3 meV and DELTA2=6.2+-0.7 meV at 4.2 K. Analysis of the spectra in terms of the Blonder-Tinkham-Klapwijk model reveals that both gaps close simultaneously at the Tc of the film. Possible mechanisms that can explain the intrinsic co-existence of two values of the gap are discussed.

cond-mat.supr-con↗

arXiv2001

Enhancement of the high-field critical current density of superconducting MgB2 by proton irradiation

A relatively high critical temperature, Tc, approaching 40 K, places the recently-discovered superconductor magnesium diboride (MgB2) intermediate between the families of low- and copper-oxide-based high-temperature superconductors (HTS). Supercurrent flow in MgB2 is unhindered by grain boundaries, unlike the HTS materials. Thus, long polycrystalline MgB2 conductors may be easier to fabricate, and so could fill a potentially important niche of applications in the 20 to 30 K temperature range. However, one disadvantage of MgB2 is that in bulk material the critical current density, Jc, appears to drop more rapidly with increasing magnetic field than it does in the HTS phases. The magnitude and field dependence of Jc are related to the presence of structural defects that can "pin" the quantised magnetic vortices that permeate the material, and prevent them from moving under the action of the Lorentz force. Vortex studies suggest that it is the paucity of suitable defects in MgB2 that causes the rapid decay of Jc with field. Here we show that modest levels of atomic disorder, induced by proton irradiation, enhance the pinning, and so increase Jc significantly at high fields. We anticipate that chemical doping or mechanical processing should be capable of generating similar levels of disorder, and so achieve technologically-attractive performance in MgB2 by economically-viable routes.

cond-mat.supr-con↗

arXiv2001

Microwave Surface Resistance in MgB2

Measurements of the temperature dependence of the surface resistance at 3 GHz of 100 micron size grains of MgB_2 separated powder are presented and discussed. The microwave surface resistance data is compared to experimental results of Nb, Bi_2Sr_2CaCu_2O_{8+δ} (BSCCO) and theoretical predictions of s-wave weak coupling electron-phonon theory (BCS).

cond-mat.supr-con↗

arXiv2001

Critical currents and vortex dynamics in super-conducting MgB2

The recently-discovered MgB2 super-conductor has a transition temperature Tc approaching 40K, placing it intermediate between the families of low and high temperature super-conductors (LTS and HTS). In practical applications, super-conductors are permeated by quantised magnetic flux vortices, and when a current flows there is dissipation unless the vortices are "pinned" in some way, and so inhibited from moving under the influence of the Lorentz force. This vortex motion sets the limiting critical current density Jc in the super-conductor. Vortex behaviour has proved to be more complicated in the HTS than in LTS materials. While this has stimulated extensive theoretical and experimental research, it has impeded applications. Clearly it is important to explore vortex behaviour in MgB2; here we report on Jc, and also on the creep rate S, which is a measure of how fast the "persistent" currents decay. Our results show that naturally-occurring grain boundaries are highly transparent to supercurrent, and suggest that the steep decline in Jc with increasing magnetic field H reflects a weakening of the vortex pinning energy, possibly because this compound forms naturally with a high degree of crystalline perfection.

cond-mat.supr-con↗