arXiv subjects

V. V. Moshchalkov

Publications and source records attributed to V. V. Moshchalkov.

At least 91 records · Page 5Linked to original sources

arXiv2004

Nucleation of superconductivity in a mesoscopic loop of varying width

We study the evolution of the superconducting state in a perforated disk by varying the size of the hole. The superconducting properties are investigated by means of transport measurements around the superconducting/normal phase boundary Tc(H). A transition from a one-dimensional to a two-dimensional regime is seen when increasing the magnetic field for disks with small holes. A good agreement is found between the measured Tc(H) line and the calculations performed in the framework of the linearized Ginzburg-Landau theory. The effect of breaking the axial symmetry of the structure by moving the hole away from the center of the disk is also studied. An enhanced critical field is found for the asymmetric structures, possibly due to the recovery of the singly connected state.

cond-mat.supr-con↗

arXiv2004

Experimental determination of the number of flux lines trapped by micro-holes in superconducting samples

The influence of a periodic landscape of pinning sites on the vortex dynamics in Pb thin films is explored by ac-susceptibility measurements. For different amplitudes h of the ac-drive, the ac-susceptibility x=x'+ix'' exhibits a complex field dependence associated with different dynamic regimes. At very low ac-drives where both, multiquanta vortices trapped by the antidots and interstitial vortices oscillate inside the pinning potential (intra-valley motion), a small kink in x'(H) together with a very low dissipation is observed. At intermediate ac-excitations such that vortices in the antidots remain pinned whereas interstices are driven out of the pinning well, a more pronounced kink in the screening coinciding with the onset of dissipation (x''(H) ~ 0) indicates the entrance of interstitial vortices. Eventually, at high enough amplitudes all vortices are set in motion and the penetration of interstitial vortices appears as a sudden reduction of the shielding power. We show that these distinctive features allow us to clearly identify the maximum number of flux quanta per hole regardless the vortex dynamic regime.

cond-mat.supr-con↗

arXiv2004

Paramagnetic reentrance of the ac-screening: Evidence of vortex avalanches in Pb thin films

We have studied the influence of a square array of pinning centers on the dynamics of vortex avalanches in Pb thin films by means of ac- and dc- magnetization measurements. Close to the superconducting transition Tc the commensurability between the vortex lattice and the pinning array leads to the well known local increments of the critical current. As temperature T decreases, matching features progressively fade out and eventually disappear. Further down in temperature vortex avalanches develop and dominate the magnetic response. These avalanches manifest themselves as jumps in the dc-magnetization and produce a lower ac-shielding giving rise to a paramagnetic reentrance in the ac-screening x'(T). Within the flux jump regime two subregimes can be identified. Close to the boundary where vortex avalanches develope, the field separation between consecutive jumps follow the periodicity of the pinning array and a field and temperature dependent screening is observed. In this regime, the response also depends on frequency f in agreement with theoretical models for magnetothermal instabilities. At low enough temperatures and fields, the screening saturates to a constant value independent of T, H, and f, where jumps are randomly distributed. We have also found that vortex instabilities occupy a larger portion of the H-T diagram in patterned samples than in films without nanoengineered pinning sites. Finally, we discuss the possible origin of the vortex avalanches and compare our results with previous experimental and theoretical studies.

cond-mat.supr-con↗

arXiv2004

Influence of the sample geometry on the vortex matter in superconducting microstructures

The dependence of the vortex penetration and expulsion on the geometry of mesoscopic superconductors is reported. Hall magnetometry measurements were performed on a superconducting Al square and triangle. The stability of the vortex patterns imposed by the sample geometry is discussed. The field-temperature $H-T$ diagram has been reconstructed showing the transitions between states with different vorticity. We have found that the vortex penetration is only weakly affected by the vortex configuration inside the sample while the expulsion is strongly controlled by the stability of the vortex patterns. A qualitative explanation for this observation is given.

cond-mat.supr-con↗

arXiv2004

Andreev reflection observation in Nb/FeSi/Nb and Nb/FeSi/Si/Nb Josephson junctions

Electrical properties of Josephson junctions Nb/FeSi/Nb with superconductor/ferromagnet (S/F)interfaces are presented. Due to Andreev reflection the nearly exact quadruple enhancement of the tunnel junction differential conductance compared with that of the normal state was achieved. The transparency of the S/F interfaces in our junctions was estimated to be close to unity. This almost ideal value is obtained due to the use of a very smooth amorphous magnetic FeSi alloy for the barrier preparation. The real structure of the Nb/FeSi/Nb tunnel junction is described as a S/F/I/F/S junction. Also Nb/FeSi/Si/Nb Josephson junctions were investigated and the results found on these junctions confirm the effects observed in Nb/FeSi/Nb.

cond-mat.supr-con↗

arXiv2004

Vortex rectification effects in plain superconducting films

Superconducting films in perpendicular magnetic field are found to rectify alternating currents. The effect has been observed both in plain and nanostructured superconducting films (niobium and lead). The rectified voltage appears both along and transverse to alternating current and strongly depends on the magnetic field. Rectification phenomena is based on the property of superconductors to have permanent circulating currents in magnetic field (vortices) and to develop electric fields at high enough currents. In contrast to previous findings, at higher current frequencies no asymmetric pinning sites are needed to produce rectification and related guided flux motion.

cond-mat.supr-con↗

arXiv2004

Pulsed field studies of the magnetization reversal in molecular nanomagnets

We report experimental studies of crystals of Mn12 molecular magnetic clusters in pulsed magnetic fields with sweep rates up to 4x10^3 T/s. The steps in the magnetization curve are observed at fields that are shifted with respect to the resonant field values. The shift systematically increases as the rate of the field sweep goes up. These data are consistent with the theory of the collective dipolar relaxation in molecular magnets.

cond-mat.other↗

arXiv2004

Flux pinning properties of superconductors with an array of blind holes

We performed ac-susceptibility measurements to explore the vortex dynamics and the flux pinning properties of superconducting Pb films with an array of micro-holes (antidots) and non-fully perforated holes (blind holes). A lower ac-shielding together with a smaller extension of the linear regime for the lattice of blind holes indicates that these centers provide a weaker pinning potential than antidots. Moreover, we found that the maximum number of flux quanta trapped by a pinning site, i.e. the saturation number ns, is lower for the blind hole array.

cond-mat.supr-con↗

arXiv2004

Magnetic phase shifter for superconducting qubit

We have designed and investigated a contactless magnetic phase shifter for flux-based superconducting qubits. The phase shifter is realized by placing a perpendicularly magnetized dot at the centre of a superconducting loop. The flux generated by this magnetic dot gives rise to an additional shielding current in the loop, which, in turn, induces a phase shift. By modifying the parameters of the dot an arbitrary phase shift can be generated in the loop. This magnetic phase shifter can, therefore, be used as an external current source in superconducting circuits, as well as a suitable tool to study fractional Josephson vortices.

cond-mat.supr-con↗

arXiv2004

Dynamic Regimes in Films with a Periodic Array of Antidots

We have studied the dynamic response of Pb thin films with a square array of antidots by means of ac susceptibility chi(T,H) measurements. At low enough ac drive amplitudes h, vortices moving inside the pinning potential give rise to a frequency- and h-independent response together with a scarce dissipation. For higher amplitudes, the average distance travelled by vortices surpasses the pinning range and a critical state develops. We found that the boundary h*(H,T) between these regimes smoothly decreases as T increases whereas a step-like behavior is observed as a function of field. We demonstrate that these steps in h*(H) arise from sharp changes in the pinning strength corresponding to different vortex configurations. For a wide set of data at several fields and temperatures in the critical state regime, we show that the scaling laws based on the simple Bean model are satisfied.

cond-mat.supr-con↗

arXiv2003

Novel pinning phenomena in a superconducting film with a square lattice of artificial pinning centers

We study the transport properties of a superconducting Nb film with a square lattice of artificial pinning centers (APCs) as a function of dc current, at a temperature close to the superconducting transition temperature of the film. We find that, at low dc currents, the differential resistance of the film shows the standard matching field anomaly, that is, the differential resistance has a local minimum at magnetic fields corresponding to an integer number of flux lines per APC. However, at higher dc currents, the differential resistance at each matching field turns to a local maximum, which is exactly opposite to the low current behavior. This novel effect might indicate that the flux lines in the APC system change their flow mode as the dc current is increased.

cond-mat.mes-hall↗

arXiv2003

Influence of the stray field of magnetic dot on the nucleation of superconductivity in a disk

We have investigated the nucleation of superconductivity in an Al mesoscopic disk, with a magnetic dot on the top. The dot is magnetized perpendicularly, and its magnetic field is inhomogeneous. The Al disk and magnetic dot are separated by an insulating layer, which ensures that there is only magnetic interaction between them. This hybrid superconductor/ferromagnet structure exhibits the maximum critical temperature for a finite value of the perpendicular applied magnetic field, which is parallel to the magnetization of magnetic dot. We have found a good agreement between the experimental data for the superconductor/normal metal phase boundary and the theoretical predictions based on the Ginzburg-Landau theory.

cond-mat.supr-con↗

arXiv2003

Phase diagram of a superconductor / ferromagnet bilayer

The magnetic field (H) - temperature (T) phase diagram of a superconductor is significantly altered when domains are present in an underlying ferromagnet with perpendicular magnetic anisotropy. When the domains have a band-like shape, the critical temperature Tc of the superconductor in zero field is strongly reduced, and the slope of the upper critical field as a function of T is increased by a factor of 2.4 due to the inhomogeneous stray fields of the domains. Field compensation effects can cause an asymmetric phase boundary with respect to H when the ferromagnet contains bubble domains. For a very inhomogeneous domain structure, Tc~H^2 for low H and Tc~H for higher fields, indicating a dimensional crossover from a one-dimensional network-like to a two-dimensional behavior in the nucleation of superconductivity.

cond-mat.supr-con↗

arXiv2003

Little-Parks effect in a superconducting loop with magnetic dot

We have studied the nucleation of superconductivity in a mesoscopic Al loop, enclosing magnetic dot with perpendicular magnetization. The superconducting phase boundary Tc(B), determined from transport measurements, is asymmetric with respect to the polarity of an applied magnetic field. The maximum critical temperature has been found for a finite applied magnetic field, which is antiparallel to the magnetization of the dot. Theoretical phase boundary shows a good agreement with the experimental data.

cond-mat.supr-con↗

arXiv2003

'Giant' normal state magnetoresistances of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$

Magnetoresistance (MR) of Bi-2212 single crystals with T$_{c}$ $\approx 87-92 K$ is studied in pulsed magnetic fields up to 50T along the c-axis in a wide temperature range. The negative out-of-plane and the positive in-plane MRs are measured in the normal state. Both MRs have similar magnitudes, exceeding any orbital contribution by two orders in magnitude. These are explained as a result of the magnetic pair-breaking of preformed pairs. Resistive upper critical fields H$_{c2}$(T) determined from the in-- and out-of-plane MRs are about the same. They show non-BCS temperature dependences compatible with the Bose-Einstein condensation field of preformed charged bosons.

cond-mat.supr-con↗

arXiv2003

High field magnetoresistivity of epitaxial La2-xSrxCuO4 thin films

A large positive magnetoresistivity (up to tens of percents) is observed in both underdoped and overdoped superconducting La2-xSrxCuO4 epitaxial thin films at temperatures far above the superconducting critical temperature Tc. For the underdoped samples, this magnetoresistance far above Tc cannot be described by the Kohler rule and we believe it is to be attributed to the influence of superconducting fluctuations. In the underdoped regime, the large magnetoresistance is only present when at low temperatures superconductivity occurs. The strong magnetoresistivity, which persists even at temperatures far above Tc, can be related to the pairs forming eventually the superconducting state below Tc. Our observations support the idea of a close relation between the pseudogap and the superconducting gap and provide new indications for the presence of pairs above Tc.

cond-mat.supr-con↗

arXiv2003

Nucleation of superconductivity in Al mesoscopic disk with magnetic dot

We have studied the nucleation of superconductivity in a mesoscopic Al disk wit a Co/Pd magnetic dot placed on the top by measuring the normal superconducting phase boundary Tc(B). The measurements have revealed a pronounced asymmetry with respect to the direction of the applied magnetic field, indicating an enhancement of the critical field when an applied magnetic field is oriented parallel to the magnetization of the magnetic dot. The theoretical Tc(B) curve is in a good agreement with the experimental data.

cond-mat.supr-con↗

arXiv2003

Nanoengineered magnetic-field-induced superconductivity

The perpendicular critical fields of a superconducting film have been strongly enhanced by using a nanoengineered lattice of magnetic dots (dipoles) on top of the film. Magnetic-field-induced superconductivity is observed in these hybrid superconductor / ferromagnet systems due to the compensation of the applied field between the dots by the stray field of the dipole array. By switching between different magnetic states of the nanoengineered field compensator, the critical parameters of the superconductor can be effectively controlled.

cond-mat.supr-con↗