Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S52: Focus Session: Supercondcutivity, Vortex Matter II |
Hide Abstracts |
Sponsoring Units: DMP Chair: Alex Gurevich, Old Dominion University Room: Mile High Ballroom 1F |
Thursday, March 6, 2014 8:00AM - 8:36AM |
S52.00001: Fractionalizing the vortex lattice in multiband superconductors in the flux flow region Invited Speaker: Shi-Zeng Lin Because of the discovery of $\mathrm{MgB_2}$ and iron-based superconductors, multiband superconductors have attracted considerable attention recently. Multiband superconductors are not always straightforward extensions of the single-band counterpart, and novel features may arise. In multiband superconductors, electrons in different bands form distinct superfluid condensates, which are coupled to the same gauge field. Each condensate thus supports vortex excitation with fractional flux quantum. However the energy of a fractional vortex diverges logarithmically in the thermodynamic limit. In the ground state vortices in different bands are bounded and their normal cores are locked together to form a composite vortex with the standard integer quantum flux. It is interesting to ask whether the vortices in different condensates can decouple under certain conditions. In this talk, I will discuss the dissociation of the composite vortex lattice in the flux flow region when the disparity of superfluid density and coherence length between different bands is large. The fractional vortex lattice in different bands move with different velocities after the dissociation transition, and the dissociation transition shows up as an increase of flux flow resistivity. In the dissociated phase, the Shapiro steps are developed when an ac current is superimposed with a dc current. We also propose to stabilize the fractional vortices by periodic pinning arrays. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S52.00002: Investigation of vortex dynamics in type-II superconductors using a scanning SQUID-on-tip microscope Lior Embon, Yonathan Anahory, Denis Vasyukov, Jo Cuppens, Ella Lachman, Dorri Halbertal, Naren Hoovinakatte, Elad Yaakobi, Aviram Uri, Yuri Myasoedov, Michael Rappaport, Martin Huber, Eli Zeldov A novel scanning microscope based on a nanoSQUID which is fabricated on the apex of a sharp tip has been developed. This SQUID-on-tip (SOT) based system possesses record spin sensitivity, spatial resolution, and operable magnetic fields, combined with a geometry which allows nanoscale sample-probe distance using tuning fork based AFM feedback. Our SQUIDs can operate at liquid 4He temperatures in applied magnetic fields of up to 1T, be made as small as 50 nm and display an extremely low flux noise of 50 $n\Phi_0/\sqrt{Hz}$ which corresponds to a spin sensitivity better than $1\mu _B/\sqrt{Hz}$ [1]. Using these newly acquired capabilities we can now directly image vortices in Pb films over a wide range of fields while running currents through the sample to exert force on the vortices and to controllably drive them from a static state to ``flux creep'' and to a ``flux flow'' regime. \\[4pt] [1] D. Vasyukov, Y. Anahory, L. Embon, D. Halbertal, J. Cuppens, L. Neeman, A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, M. E. Huber, and E. Zeldov, Nature Nanotech. 8, 639 (2013) [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S52.00003: Scanning SQUID-on-tip microscopy of vortex matter Yonathan Anahory, Lior Embon, Denis Vasyukov, Jo Cuppens, Ella Lachman, Dorri Halbertal, Elad Yaakobi, Aviram Uri, Yuri Myasoedov, Michael L. Rappaport, Martin E. Huber, Eli Zeldov We present a scanning nanoSQUID microscope with record spatial resolution, spin sensitivity, and operating magnetic fields for the study of vortex matter. The key element of the microscope is the SQUID-on-tip (SOT) device, which is fabricated by pulling a quartz tube into a sharp pipette, followed by three steps of thermal evaporation of a thin superconducting film onto the sides and the apex of the pipette. The devices operate at 4.2 K in applied fields of up to 1T and can be made with diameters down to 50 nm. The SQUIDs-on-tip display an extremely low flux noise of $\Phi_{\mathrm{n}}$ $=$ 50 n$\Phi_{\mathrm{0}}$/Hz$^{\mathrm{1/2}}$ and corresponding spin sensitivity of better than 1 $\mu_{\mathrm{B}}$/Hz$^{\mathrm{1/2}}$ [1], which is about two orders of magnitude improvement over any previous SQUID. Using this new tool we have investigated static and dynamic behavior of vortices in superconducting Pb films. By driving \textit{ac} and \textit{dc} transport current we can study vortex displacement and the vortex potential landscape with sub-atomic precision. [1] D. Vasyukov, Y. Anahory, L. Embon, D. Halbertal, J. Cuppens, L. Neeman, A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, M. E. Huber, and E. Zeldov, Nature Nanotech. \textbf{8}, 639 (2013) [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S52.00004: Critical Current of Layered Superconductor with Columnar Defects in Tilted Magnetic Field: A Numerical Study Jose Rodriguez The critical current of a layered (a-b plane) superconductor with perpendicular (c-axis) columnar defects in titled external magnetic field is determined by numerical simulations of the corresponding London model for vortex dynamics. Intra-layer vortex dynamics is computed in parallel by central processors units (CPU) assigned to each layer, while inter-layer vortex dynamics is computed by a message passing interface (MPI) between the dedicated CPU's. We find that the critical current versus the angle that the external magnetic field makes with the columnar defects shows a cusp maximum at zero. At fixed tilt angle, we also find that the critical current increases monotonically with increasing electronic mass anisotropy, $m_c/m_{ab}$, in a manner consistent with collective-pinning theory. Comparison with experimental determinations of the critical current in films of high-temperature superconductors with both natural (line dislocation) and artificial (nano-rod) columnar defects is made where possible. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S52.00005: Influence of Domain Width on Vortex Nucleation in Superconductor/Ferromagnet Hybrid Structures S. Moore, J. Fedor, V. Novosad, J. Pearson, S.D. Bader, G. Karapetrov, M. Iavarone We have investigated the effects of spatially inhomogenous magnetic fields on vortex nucleation, domain wall superconductivity and reverse domain superconductivity in magnetically coupled superconductor/ferromagnet hybrid structures. Using low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) we have studied Pb/[Co-Pd] systems with varying magnetic domain widths. Visualization of the underlying magnetic template structure is achieved through field dependent conductance maps. In the case of zero applied fields these maps reveal the absence of vortices below a threshold domain width. In those systems with insufficient domain width to support generation of vortices in zero applied fields, nucleation can be restored through the application of an external magnetic field. We also observe that the domain wall superconductivity is strongly affected by the ferromagnetic domain size. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 10:00AM |
S52.00006: Vortex pinning vs. superconducting wire network in nanostructured superconductors Invited Speaker: Jose L. Vicent Superconducting films with array of ordered defects allow studying effects which are governed by the interplay between lengths of the nanostructured sample and lengths related to physical parameters, as for example coherence length. When the coherence length and the separation between the defects are similar, the sample can mimic a superconducting wire network. In this situation, applied magnetic fields induce Little-Parks oscillations due to fluxoid quantization constraint. These L-P oscillations vanish when the coherence length is smaller than the ``stripe'' superconducting region between the defects. In superconducting films with array of nanodefects periodic oscillations can also be detected in resistance R(H), critical current I$_{c}$(H), magnetization M(H) and \textit{ac-}susceptibility $\chi_{ac}$(H) in a broader temperature range than the temperature interval where L-P oscillations are present. Vortex pinning mechanisms are the origin of these oscillations. These oscillations emerge due to matching effects between two lattices: the vortex lattice and the lattice of defects. These oscillations are detected in a broader temperature interval than the temperature interval where L-P oscillations are present. Worth to note that, due to the coherence length divergence at T$_{c}$, a crossover to wire network behavior is experimentally found always. Interestingly, both mechanisms coexist close to superconducting critical temperatures; i. e. in the temperature region where the sample mimics superconducting wire network. These overlapping effects can be experimentally separated and both origins can be discriminated. We have analyzed and single out, with magnetotransport measurements, both mechanisms: pinning and fluxoid quantization constraint in superconducting films with arrays of non-magnetic and magnetic dots. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S52.00007: Magnetoresistance oscillations in single-crystal NbSe$_2$ nanowires designed for the study of vortex dynamics Shaun Mills, Neal Staley, Jacob Wisser, Chenyi Shen, Zhuan Xu, Ying Liu The dynamic behavior of Abrikosov vortices has been of long-standing interest, both from fundamental and application-based perspectives. While data on static configurations of vortices and the collective motion of a vortex lattice have been accumulating, studies of vortex dynamics in nanoscale samples are rare. We have pursued electrical transport measurements on devices made of single-crystal NbSe$_2$ flakes -- overcoming large challenges in the fabrication of these single-crystal nano-devices -- in order to detect the motion of individual Abrikosov vortices. We also carried out recalculations of vortex configurations in our devices in the London approximation to assist the designing of our experiments. By tuning the strength of an external magnetic field and sample geometry, individual vortices can be confined within our devices. We present our recent progress towards the controlled motion of individual Abrikosov vortices, along with transport measurements on NbSe$_2$ nanowires, the latter of which reveal unexpected magnetoresistance oscillations attributable to vortex related physics. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S52.00008: Vortex Lattice Metastability and Power Law Dynamics in MgB$_2$ Catherine Rastovski, S.J. Kuhn, K. Smith, M.R. Eskildsen, L. DeBeer-Schmitt, C.D. Dewhurst, W.J. Gannon, N.D. Zhigadlo, J. Karpinski Previous small-angle neutron scattering (SANS) studies of the vortex lattice (VL) of MgB$_2$ with H $\parallel$ c found a triangular VL which undergoes a field-driven 30$^\circ$ reorientation transition, forming three distinct ground state phases. A high degree of metastability exists between the VL phases of MgB$_2$ that cannot be attributed to vortex pinning and may be a result of the jamming of VL domains [C.~Rastovski \emph{et al.}, Phys. Rev. Lett. {\bf 111}, 107002 (2013)]. To further investigate the effect of vortex motion on the metastable to ground state VL transition, we applied a small AC magnetic field parallel or perpendicular to the vortices to ``shake" the lattice. The metastable VL volume fraction decreased with a two-step power law dependence on the number of applied AC cycles. The slow and then fast power law decay of the metastable state may indicate first nucleation and then growth of ground state VL domains. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S52.00009: Vortex Transport in Thickness-Modulated Granular Aluminum Films August DeMann, Sara Mueller, Stuart Field, Yaohua Liu, Daniel Reich The nature of superconducting vortices driven in a periodic potential has been the subject of much recent theoretical and experimental interest. We report here the results of transport studies of vortex dynamics in a periodic potential fabricated using a novel technique; this technique yields exceptionally smooth, nearly sinusoidal potentials that are ideal for the investigation of both static and dynamics vortex configurations. Our method starts with a glass substrate into which a periodic square-wave grating is fabricated by electron-beam lithography followed by a subsequent wet etch. The period of the gratings is $\approx 2\ \mu$m. A subsequent annealing step at 650 $^\circ$C smooths the grating into a sinusoidal profile. Finally, a low-pinning granular aluminum film, with $T_c \approx 1.7$ K, is evaporated onto this substrate at an angle with respect to the normal, leading to a superconducting film that also has a sinusoidal modulation in its thickness. We have performed experiments comparing the transport properties of these modulated films to flat films grown at the same time; the modulated films show clear signatures of an increased critical current and effects due to matching and commensurability. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S52.00010: Vortex Flipping in Superconductor-Ferromagnet Spin Valve Structures Edgar J. Patino, Marco Aprili, Mark Blamire, Yoshiteru Maeno We report in plane magnetization measurements on Ni/Nb/Ni/CoO and Co/Nb/Co/CoO spin valve structures with one of the ferromagnetic layers pinned by an antiferromagnetic layer. In samples with Ni, below the superconducting transition Tc, our results show strong evidence of vortex flipping driven by the ferromagnets magnetization. This is a direct consequence of proximity effect that leads to vortex supercurrents leakage into the ferromagnets. Here the polarized electron spins are subject to vortices magnetic field occasioning vortex flipping. Such novel mechanism has been made possible for the first time by fabrication of the F/S/F/AF multilayered spin valves with a thin-enough S layer to barely confine vortices inside as well as thin-enough F layers to align and control the magnetization within the plane. When Co is used there is no observation of vortex flipping effect. This is attributed to Co shorter coherence length. Interestingly instead a reduction in pinning field of about 400 Oe is observed when the Nb layer is in superconducting state. This effect cannot be explained in terms of vortex fields. In view of these facts any explanation must be directly related to proximity effect and thus a remarkable phenomenon that deserves further investigation. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S52.00011: Critical currents in superconducting films with array of magnetic and non-magnetic dots: Interstitial vs. trapped vortices Javier del Valle, Alicia Gomez, Elvira Gonzalez, Jose Vicent Arrays of magnetic Py dots and non-magnetic Cu dots have been embedded in Nb superconducting films by electron beam lithography and sputtering techniques. (I,V) curves have been measured in both systems. The critical current values extracted from the (I,V) curves show periodic maxima which are induced by the well-known matching effect between the vortex lattice and the array of nanodefects. The Nb film with non-magnetic Cu array shows an unexpected larger number of maxima than the Nb film with the magnetic Py array. Furthermore, comparison between critical currents in both samples shows a striking result, a crossover is measured increasing the applied magnetic fields. At low applied magnetic fields critical current values are higher in the magnetic (Py) dot sample than in the non-magnetic (Cu) dot sample, but increasing the applied field the opposite occurs, i. e. the critical current in Nb film with Cu dots is higher than the critical current in Nb film with Py dots. These data are analyzed taking into account the different behavior between interstitial vortices and vortices trapped in the pinning potential wells which are generated by the different arrays. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700