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See detailMapping degenerate vortex states in a kagomé lattice of elongated antidots via scanning Hall probe microscopy
Xue, Cun; Ge, Jun-Yi; He, An et al

in Physical Review. B : Condensed Matter (in press)

We investigate the degeneracy of the superconducting vortex matter ground state by directly visualizing the vortex con gurations in a kagomé lattice of elongated antidots via scanning Hall probe ... [more ▼]

We investigate the degeneracy of the superconducting vortex matter ground state by directly visualizing the vortex con gurations in a kagomé lattice of elongated antidots via scanning Hall probe microscopy (SHPM). The observed vortex patterns, at specifi c applied magnetic fi elds, are in good agreement with the confi gurations obtained using time-dependent Ginzburg-Landau (TDGL) simulation. Both results indicate that the long range interaction in this nano-structured superconductor is unable to lift degeneracy between different vortex states and the pattern formation is mainly ruled by the nearest neighbour interaction. This simpli cation allows the identi cation of a set of simple rules characterizing the vortex con gurations. We demonstrated that these rules can explain both the observed vortex distributions and the magnetic fi eld dependent degree of degeneracy. [less ▲]

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See detailSuperconducting weak links created by electromigration
Silhanek, Alejandro ULg

Conference (2017, June 01)

In this presentation, we explore in-situ controlled electromigration to fabricate superconducting weak links. We show evidence that in Al a transition from thermally assisted phase slips (TAPS) to quantum ... [more ▼]

In this presentation, we explore in-situ controlled electromigration to fabricate superconducting weak links. We show evidence that in Al a transition from thermally assisted phase slips (TAPS) to quantum phase slips may takes place when the effective cross section becomes smaller than ~150 nm^2. In the regime dominated by quantum phase slips the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature. We also discuss the origin of negative magnetoresistance at low magnetic fields in the bow-tie shaped constrictions. Strikingly, the detrimental effect caused by the repeated electromigration can be healed by simply inverting the current direction. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic size constrictions. [less ▲]

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See detailSuperconducting weak links created by electromigration
Silhanek, Alejandro ULg

Conference (2017, June)

In this presentation, we explore in-situ controlled electromigration to fabricate superconducting weak links. We show evidence that in Al a transition from thermally assisted phase slips (TAPS) to quantum ... [more ▼]

In this presentation, we explore in-situ controlled electromigration to fabricate superconducting weak links. We show evidence that in Al a transition from thermally assisted phase slips (TAPS) to quantum phase slips may takes place when the effective cross section becomes smaller than ~150 nm2. In the regime dominated by quantum phase slips the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature. We also discuss the origin of negative magnetoresistance at low magnetic fields in the bow-tie shaped constrictions. Strikingly, the detrimental effect caused by the repeated electromigration can be healed by simply inverting the current direction. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic size constrictions. [less ▲]

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See detailHealing Effect of Controlled Anti-Electromigration on Conventional and High-Tc Superconducting Nanowires
Baumans, Xavier ULg; Lombardo, Joseph ULg; Brisbois, Jérémy ULg et al

in Small : Nano Micro (2017), 1700384

The electromigration process has the potential capability to move atoms one by one when properly controlled. It is therefore an appealing tool to tune the cross section of monoatomic compounds with ... [more ▼]

The electromigration process has the potential capability to move atoms one by one when properly controlled. It is therefore an appealing tool to tune the cross section of monoatomic compounds with ultimate resolution or, in the case of polyatomic compounds, to change the stoichiometry with the same atomic precision. As demonstrated here, a combination of electromigration and anti-electromigration can be used to reversibly displace atoms with a high degree of control. This enables a fine adjustment of the superconducting properties of Al weak links, whereas in Nb the diffusion of atoms leads to a more irreversible process. In a superconductor with a complex unit cell (La 2−x Ce x CuO 4 ), the electromigration process acts selectively on the oxygen atoms with no apparent modification of the structure. This allows to adjust the doping of this compound and switch from a superconducting to an insulating state in a nearly reversible fashion. In addition, the conditions needed to replace feedback controlled electromigration by a simpler technique of electropulsing are discussed. These findings have a direct practical application as a method to explore the dependence of the characteristic parameters on the exact oxygen content and pave the way for a reversible control of local properties of nanowires. [less ▲]

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See detailStatistics of localized phase slips in tunable width planar point contacts
Baumans, Xavier ULg; Zharinov, Vyacheslav; Raymenants, Eline et al

in Scientific Reports (2017), 7

The main dissipation mechanism in superconducting nanowires arises from phase slips. Thus far, most of the studies focus on long nanowires where coexisting events appear randomly along the nanowire. In ... [more ▼]

The main dissipation mechanism in superconducting nanowires arises from phase slips. Thus far, most of the studies focus on long nanowires where coexisting events appear randomly along the nanowire. In the present work we investigate highly confined phase slips at the contact point of two superconducting leads. Profiting from the high current crowding at this spot, we are able to shrink in-situ the nanoconstriction. This procedure allows us to investigate, in the very same sample, thermally activated phase slips and the probability density function of the switching current I sw needed to trigger an avalanche of events. Furthermore, for an applied current larger than I sw , we unveil the existence of two distinct thermal regimes. One corresponding to efficient heat removal where the constriction and bath temperatures remain close to each other, and another one in which the constriction temperature can be substantially larger than the bath temperature leading to the formation of a hot spot. Considering that the switching current distribution depends on the exact thermal properties of the sample, the identification of different thermal regimes is of utmost importance for properly interpreting the dissipation mechanisms in narrow point contacts. [less ▲]

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See detailFlux penetration in a superconducting film partially capped with a conducting layer
Brisbois, Jérémy ULg; Gladilin, V. N.; Tempere, J. et al

in Physical Review B (2017), 95

The influence of a conducting layer on the magnetic flux penetration in a superconducting Nb film is studied by magneto-optical imaging. The metallic layer partially covering the superconductor provides ... [more ▼]

The influence of a conducting layer on the magnetic flux penetration in a superconducting Nb film is studied by magneto-optical imaging. The metallic layer partially covering the superconductor provides an additional velocity-dependent damping mechanism for the flux motion that helps protecting the superconducting state when thermomagnetic instabilities develop. If the flux advances with a velocity slower than w = 2/µ0σt, where σ is the cap layer conductivity and t is its thickness, the flux penetration remains unaffected, whereas for incoming flux moving faster than w, the metallic layer becomes an active screening shield. When the metallic layer is replaced by a perfect conductor, it is expected that the flux braking effect will occur for all flux velocities. We demonstrate this effect by investigating Nb samples with a thickness step. Some of the observed features, namely the deflection and the branching of the flux trajectories at the border of the thick centre, as well as the favoured flux penetration at the indentation, are reproduced by time-dependent Ginzburg-Landau simulations. [less ▲]

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See detailVelocimetry of superconducting vortices based on stroboscopic resonances
Jelic, Zeljko ULg; Milosevic, Milorad; Silhanek, Alejandro ULg

in Scientific Reports (2016), 6

An experimental determination of the mean vortex velocity in superconductors mostly relies on the measurement of flux-flow resistance with magnetic field, temperature, or driving current. In the present ... [more ▼]

An experimental determination of the mean vortex velocity in superconductors mostly relies on the measurement of flux-flow resistance with magnetic field, temperature, or driving current. In the present work we introduce a method combining conventional transport measurements and a frequency-tuned flashing pinning potential to obtain reliable estimates of the vortex velocity. The proposed device is characterized using the time-dependent Ginzburg-Landau formalism, where the velocimetry method exploits the resonances in mean vortex dissipation when temporal commensuration occurs between the vortex crossings and the flashing potential. We discuss the sensitivity of the proposed technique on applied current, temperature and heat diffusion, as well as the vortex core deformations during fast motion. [less ▲]

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See detailUltra-narrow superconducting junctions: electromigration to shed light on quantum point contacts
Baumans, Xavier ULg; Cerbu, Dorin; Adami, Obaïd-Allah ULg et al

Conference (2016, September 06)

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and in quantum computing circuits. In this context, it is of ... [more ▼]

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and in quantum computing circuits. In this context, it is of fundamental importance to better understand the undesired and harmful appearance of thermal and quantum fluctuations of the superconducting order parameter [1]-[3] as a function of the wire width. Although superconductors in the mesoscopic regime (i.e. size comparable to ξ and/or λ) have been explored both experimentally and theoretically in depth, the superconducting nanoworld (i.e. at scales of the fermi wavelength) has received much less attention. The lack of experimental results is in part due to the difficulty of sample fabrication, at dimensions beyond the limit reached by conventional lithographic techniques. A promising direction consists of controlling the local displacement of atom by an electron wind, a process known as electromigration (EM) [4] . This effect relies on the combination of local temperature rise and substantial current crowding at nanoconstrictions. While uncontrolled, EM is responsible for the breakdown of small electronic devices, it can be used in a controllable way to further decrease locally the cross section of the nanowire towards single atomic contacts. In this work, we explore in-situ controlled EM to fabricate nano-constrictions immersed in cryogenic environment. We demonstrate that a transition from thermally assisted phase slips (TAPS) to quantum phase slips (QPS) takes place when the effective cross section becomes smaller than ~ 150 nm 2 . In the regime dominated by QPS the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature [5] . We also demonstrate that the bow-tie shaped constrictions exhibit a negative magnetoresistance at low magnetic fields [5] which can be attributed to the suppression of superconductivity in the contact leads [6] . Strikingly, the detrimental effect caused by the repeated EM can be healed by simply inverting the current direction. These findings reveal the strong potential of the proposed fabrication method to explore various fascinating superconducting phenomena in atomic-size constrictions. [less ▲]

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See detailPhase Slips In Tunable Width Point Contacts Created By Electromigration
Silhanek, Alejandro ULg

Conference (2016, September 05)

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and as interconnects in circuits proposed for quantum computing ... [more ▼]

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and as interconnects in circuits proposed for quantum computing. In this context, it is of fundamental importance to better understand the undesired and harmful appearance of thermal and quantum fluctuations of the superconducting order parameter as a function of the wire width. In this presentation, we explore in-situ controlled electromigration (EM) to fabricate nano-constrictions immersed in cryogenic environment. We demonstrate that a transition from thermally assisted phase slips (TAPS) to quantum phase slips (QPS) takes place when the effective cross section becomes smaller than ~150 nm^2. In the regime dominated by QPS the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature. We also demonstrate that the bow-tie shaped constrictions exhibit a negative magnetoresistance at low magnetic fields which can be attributed to the suppression of superconductivity in the contact leads. Strikingly, the detrimental effect caused by the repeated EM can be healed by simply inverting the current direction. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic size constrictions. [less ▲]

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See detailImpurity scattering effects on the superconducting properties and the tetragonal-to-orthorhombic phase transition in FeSe
Abdel-Hafiez, Mahmoud; Pu, Y. J.; Brisbois, Jérémy ULg et al

in Physical Review B (2016), 93

A comprehensive study of the doping dependence of the phase diagram of FeSe-based superconductors is still required due to the lack of a clean and systematic means of doping control. Here, we report on ... [more ▼]

A comprehensive study of the doping dependence of the phase diagram of FeSe-based superconductors is still required due to the lack of a clean and systematic means of doping control. Here, we report on the magneto-optical imaging, thermodynamic and transport properties, as well as in situ angle-resolved photoemission spectroscopy (ARPES) studies, on the impurity scattering in stoichiometric FeSe single crystals. Co doping at the Fe site is found to decrease the superconducting transition temperature. The upper critical field and specific heat all indicate a possible multiband superconductivity with strong coupling in the Co-doped system. A remarkable feature in FeSe is that its temperature dependent resistivity exhibits a wide hump at high temperatures, a signature of a crossover from a semiconductinglike behavior to metallic behavior. A structural tetragonal-to-orthorhombic phase transition Ts (a consequence of the electronic nematicity) is suppressed by either physical or chemical pressures. Due to the reconstruction of the Fermi surface at Ts, specific heat anomalies at Ts present ΔCp/Ts≈γn, the Sommerfield coefficient at low temperature. This reflects additional electronic instability in the FeSe(1−x)Sx system. ARPES data between 180 and 282 K indicates the existence of a chemical potential shift with increasing thermal excitations, resulting in a change of the Fermi-surface topology and exhibiting a semimetal behavior. We found that the temperature-induced Lifshitz transition is much higher than the temperature for the nematic order. [less ▲]

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See detailImprinting superconducting vortex footsteps in a magnetic layer
Brisbois, Jérémy ULg; Motta, Maycon; Avila Osses, Jonathan ULg et al

in Scientific Reports (2016), 6

Local polarization of a magnetic layer, a well-known method for storing information, has found its place in numerous applications such as the popular magnetic drawing board toy or the widespread credit ... [more ▼]

Local polarization of a magnetic layer, a well-known method for storing information, has found its place in numerous applications such as the popular magnetic drawing board toy or the widespread credit cards and computer hard drives. Here we experimentally show that a similar principle can be applied for imprinting the trajectory of quantum units of flux (vortices), travelling in a superconducting film (Nb), into a soft magnetic layer of permalloy (Py). In full analogy with the magnetic drawing board, vortices act as tiny magnetic scribers leaving a wake of polarized magnetic media in the Py board. The mutual interaction between superconducting vortices and ferromagnetic domains has been investigated by the magneto-optical imaging technique. For thick Py layers, the stripe magnetic domain pattern guides both the smooth magnetic flux penetration as well as the abrupt vortex avalanches in the Nb film. It is however in thin Py layers without stripe domains where superconducting vortices leave the clearest imprints of locally polarized magnetic moment along their paths. In all cases, we observe that the flux is delayed at the border of the magnetic layer. Our findings open the quest for optimizing magnetic recording of superconducting vortex trajectories. [less ▲]

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See detailUltra-narrow superconducting junctions fabricated by controlled electromigration
Silhanek, Alejandro ULg

Conference (2016, April 12)

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and in quantum computing circuits. In this context, it is of ... [more ▼]

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and in quantum computing circuits. In this context, it is of fundamental importance to better understand the undesired and harmful appearance of thermal and quantum fluctuations of the superconducting order parameter as a function of the wire width. Although superconductors in the mesoscopic regime (i.e. size comparable to ξ and/or λ) have been explored both experimentally and theoretically in depth, the superconducting nanoworld (i.e. at scales of the fermi wavelength) has received much less attention. The lack of experimental results is in part due to the difficulty of sample fabrication, at dimensions beyond the limit reached by conventional lithographic techniques. A promising direction consists of controlling the local displacement of atoms by an electron wind, a process known as electromigration (EM). This effect relies on the combination of local temperature rise and substantial current crowding at nanoconstrictions. While uncontrolled, EM is responsible for the breakdown of small electronic devices, it can be used in a controllable way to further decrease locally the cross section of the nanowire towards single atomic contacts. In this work, we explore in-situ controlled EM to fabricate nano-constrictions immersed in cryogenic environment. We demonstrate that a transition from thermally assisted phase slips (TAPS) to quantum phase slips (QPS) takes place when the effective cross section becomes smaller than ~ 150 nm^2. In the regime dominated by QPS the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature. We also demonstrate that the bow-tie shaped constrictions exhibit a negative magnetoresistance at low magnetic fields which can be attributed to the suppression of superconductivity in the contact leads. Strikingly, the detrimental effect caused by the repeated EM can be healed by simply inverting the current direction. These findings reveal the strong potential of the proposed fabrication method to explore various fascinating superconducting phenomena in atomic-size constrictions. [less ▲]

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See detailThermal and quantum depletion of superconductivity in narrow junctions created by controlled electromigration
Silhanek, Alejandro ULg

Conference (2016, April)

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and as interconnects in circuits proposed for quantum computing ... [more ▼]

Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and as interconnects in circuits proposed for quantum computing. In this context, it is of fundamental importance to better understand the undesired and harmful appearance of thermal and quantum fluctuations of the superconducting order parameter as a function of the wire width. In this presentation, we explore in-situ controlled electromigration (EM) to fabricate nano-constrictions immersed in cryogenic environment. We demonstrate that a transition from thermally assisted phase slips (TAPS) to quantum phase slips (QPS) takes place when the effective cross section becomes smaller than ~150 nm^2. In the regime dominated by QPS the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature. We also demonstrate that the bow-tie shaped constrictions exhibit a negative magnetoresistance at low magnetic fields which can be attributed to the suppression of superconductivity in the contact leads. Strikingly, the detrimental effect caused by the repeated EM can be healed by simply inverting the current direction. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic size constrictions. [less ▲]

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See detailMagnetic flux penetration in Nb superconducting films with lithographically defined micro-indentations
Brisbois, Jérémy ULg; Adami, Obaïd-Allah ULg; Avila Osses, Jonathan ULg et al

in Physical Review B (2016), 93(5), 054521

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined border indentations. We demonstrate that discontinuity lines ... [more ▼]

We present a thorough investigation by magneto-optical imaging of the magnetic flux penetration in Nb thin films with lithographically defined border indentations. We demonstrate that discontinuity lines (d-lines), caused by the abrupt bending of current streamlines around the indentations, depart from the expected parabolic trend close to the defect and depend on the shape and size of the indentation as well as on the temperature. These findings are backed up and compared with theoretical results obtained by numerical simulations and analytical calculations highlighting the key role played by demagnetization effects and the creep exponent n. In addition, we show that the presence of nearby indentations and submicrometer random roughness of the sample border can severely modify the flux front topology and dynamics. Strikingly, in contrast to what has been repeatedly predicted in the literature, we do not observe that indentations act as nucleation spots for flux avalanches, but they instead help to release the flux pressure and avoid thermomagnetic instabilities. [less ▲]

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See detailThermal and quantum depletion of superconductivity in narrow junctions created by controlled electromigration
Baumans, Xavier ULg; Cerbu, Dorin; Adami, Obaïd-Allah ULg et al

in Nature Communications (2016), 7

Superconducting nanowires currently attract great interest due to their application in single-photon detectors and quantum-computing circuits. In this context, it is of fundamental importance to ... [more ▼]

Superconducting nanowires currently attract great interest due to their application in single-photon detectors and quantum-computing circuits. In this context, it is of fundamental importance to understand the detrimental fluctuations of the superconducting order parameter as the wire width shrinks. In this paper, we use controlled electromigration to narrow down aluminium nanoconstrictions. We demonstrate that a transition from thermally assisted phase slips to quantum phase slips takes place when the cross section becomes less than 150 nm2 . In the regime dominated by quantum phase slips the nanowire loses its capacity to carry current without dissipation, even at the lowest possible temperature. We also show that the constrictions exhibit a negative magnetoresistance at low-magnetic fields, which can be attributed to the suppression of superconductivity in the contact leads. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic-size contacts. [less ▲]

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See detailControlled electromigration in Al and Au nanoconstrictions - Towards the study of sub-10 nm superconducting nanostructures
Lombardo, Joseph ULg; Baumans, Xavier ULg; Scheerder, Jeroen et al

Poster (2016, January 29)

By applying high current density to a nanowire, it is possible to induce atomic migration resulting from the momentum transfer by the charge carriers. This phenomenon is known as electromigration. In this ... [more ▼]

By applying high current density to a nanowire, it is possible to induce atomic migration resulting from the momentum transfer by the charge carriers. This phenomenon is known as electromigration. In this work, we present successful preliminary results of controlled electromigration in thin Al and Au transport microbridges with predefined structures (constricted nanowires and Dayem bridges). We discuss the conditions necessary to achieve electromigration, the ingredients of the feedback loop control, and present the electrical properties and stability of the nanostructures achieved by this method. This research is a first step towards the use of electromigration for elaborating nanostructures at sub-10 nm scale. The use of superconducting samples would allow us to study some theoretical predictions as the increase of superconducting critical temperature [1] or the appearance of quantum phase slips [2,3]. [less ▲]

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See detailProbing the low-frequency vortex dynamics in a nanostructured superconducting strip
de Souza Silva, Clecio C.; Raes, Bart; Brisbois, Jérémy ULg et al

in Physical Review B (2016)

We investigate by scanning susceptibility microscopy the response of a thin Pb strip, with a square array of submicron antidots, to a low-frequency ac magnetic field applied perpendicularly to the film ... [more ▼]

We investigate by scanning susceptibility microscopy the response of a thin Pb strip, with a square array of submicron antidots, to a low-frequency ac magnetic field applied perpendicularly to the film plane. By mapping the local permeability of the sample within the field range where vortices trapped by the antidots and interstitial vortices coexist, we observed two distinct dynamical regimes occurring at different temperatures. At a temperature just below the superconducting transition, T/Tc = 0.96, the sample response is essentially dominated by the motion of highly mobile interstitial vortices. However, at a slightly lower temperature, T/Tc = 0.93, the interstitial vortices freeze up leading to a strong reduction of the ac screening length. We propose a simple model for the vortex response in this system which fits well to the experimental data. Our analysis suggests that the observed switching to the high mobility regime stems from a resonant effect, where the period of the ac excitation is just large enough to allow interstitial vortices to thermally hop through the weak pinning landscape produced by random material defects. This argument is further supported by the observation of a pronounced enhancement of the out-of-phase response at the crossover between both dynamical regimes. [less ▲]

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See detailOrbitally induced hierarchy of exchange interactions in zigzag antiferromagnetic state of honeycomb silver delafossite Ag3Co2SbO6
Zvereva, E.A.; Stratan, M.I.; Ushakov, A.V. et al

in Dalton Transactions (2016)

We report the revised crystal structure, static and dynamic magnetic properties of quasi-two dimensional honeycomb lattice silver delafossite Ag3Co2SbO6. The magnetic susceptibility and specific heat data ... [more ▼]

We report the revised crystal structure, static and dynamic magnetic properties of quasi-two dimensional honeycomb lattice silver delafossite Ag3Co2SbO6. The magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic long range order at low temperatures with Néel temperature TN ~ 21.2 K. In addition, the magnetization curves revealed a field-induced (spin-flop type) transition below TN in moderate magnetic fields. The GGA+U calculations show the importance of the orbital degrees of freedom, which maintain a hierarchy of exchange interaction in the system. The strongest antiferromagnetic exchange coupling was found in the shortest Co-Co pairs and is due to direct and superexchange interaction between the half-filled xz+yz orbitals pointing directly to each other. The other four out of six nearest neighbor exchanges within the cobalt hexagon are suppressed, since for these bonds active half-filled orbitals turned out to be parallel and do not overlap. The electron spin resonance (ESR) spectra reveal a broad absorption line attributed to Co2+ ion in octahedral coordination with average effective g-factor g=2.40±0.05 at room temperature and show strong divergence of ESR parameters below ~ 150 K, which implies an extended region of short-range correlations. Based on the results of magnetic and thermodynamic studies in applied fields, we propose the magnetic phase diagram for the new honeycomb-lattice delafossite. [less ▲]

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See detailCurrent crowding in nanostructured superconductors
Silhanek, Alejandro ULg

Scientific conference (2015, November 11)

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See detailThe onset, evolution and magnetic braking of vortex lattice instabilities in nanostructured superconducting films
Adami, Obaïd-Allah ULg; Jelic, Zeljko ULg; Xue, Cun et al

in Physical Review. B : Condensed Matter (2015), 92

In 1976 Larkin and Ovchinnikov [Sov. Phys. JETP 41, 960 (1976)] predicted that vortex matter in superconductors driven by an electrical current can undergo an abrupt dynamic transition from a flux-flow ... [more ▼]

In 1976 Larkin and Ovchinnikov [Sov. Phys. JETP 41, 960 (1976)] predicted that vortex matter in superconductors driven by an electrical current can undergo an abrupt dynamic transition from a flux-flow regime to a more dissipative state at suficiently high vortex velocities. Typically this transition manifests itself as a large voltage jump at a particular current density, so-called instability current density J, which is smaller than the depairing current. By tuning the effective pinning strength in Al fi lms, using an artifi cial periodic pinning array of triangular holes, we show that a unique and well defi ned instability current density exists if the pinning is strong, whereas a series of multiple voltage transitions appear in the relatively weaker pinning regime. This behavior is consistent with time-dependent Ginzburg-Landau simulations, where the multiple-step transition can be unambiguously attributed to the progressive development of vortex chains and subsequently phase-slip lines. In addition, we explore experimentally the magnetic braking e ffects, caused by a thick Cu layer deposited on top of the superconductor, on the instabilities and the vortex ratchet effect. [less ▲]

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