Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session X66: Superconducting Electronics and Cryogenic MemoryInvited
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Sponsoring Units: DCMP Chair: Nathan Satchell, Univ of Leeds Room: Four Seasons 1 |
Friday, March 6, 2020 11:15AM - 11:51AM |
X66.00001: Demonstration of JMRAM arrays Invited Speaker: Donald Miller
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Friday, March 6, 2020 11:51AM - 12:27PM |
X66.00002: Superconducting vortex-based memory cells Invited Speaker: Vladimir Krasnov Non-volatile quantized states are ideal for the realization of classical Boolean logics. Abrikosov vortex represents the most compact magnetic object in superconductors with the size determined by the London penetration depth ~100 nm. Therefore, it can be utilized for creation of high-density digital cryoelectronics. In this talk we will describe operation of memory cells, in which a single vortex is used as an information bit [1]. The vortex is pinned at a nano-scale trap and is read-out by a nearby Josephson junction [2,3]. Unlike SQUID-based memory cells, such cells have non-degenerate 0 and 1 states, which greatly simplify the device architecture. Furthermore, SQUID-based devices have a problem with increasing write current upon decreasing the SQUID loop size, preventing a straightforward miniaturization. To the contrary, write current for a vortex is determined by the depinning current density and, therefore, scales with the size. All together this allows simple miniaturization down to sub-micron sizes. We demonstrate that vortex memory cells have a high-endurance operation, are characterized by an infinite magnetoresistance, do not require external magnetic field, have a short access time, and a low write energy. Non-volatility and perfect reproducibility are inherent for such devices due to the quantized nature of the vortex. We argue that vortex-based memory can be used in superconducting digital supercomputers. |
Friday, March 6, 2020 12:27PM - 1:03PM |
X66.00003: Superconducting-nanowire-based memories Invited Speaker: Karl Berggren A persistent current in a superconductor provides a nearly perfect physical token for data storage. It is nonvolatile (so long as the loop remains cold), and can be read out precisely, thus enabling digital (and perhaps even multi-level) electronic memory. However, the quantisation of flux in units of Φo, means that relatively large inductors (on the order of 10s of μm's long) are typically required for such systems, somewhat limiting their potential. The solution until recently was to use Josephson junctions as a compact source of inductance, but even this approach resulted in memory-cell dimensions at the many-μm2 level. Recently, however, high-quality thin superconducting films with long penetration depths (and thus high kinetic inductance) have become available. These materials enable much smaller persistent-current loops without requiring the use of a Josephson junction. Novel nanowire-based electronic devices, also not using Josephson junctions, have also emerged that can be used to read out the nanowire state. With this, working single-cell memories with negligible bit-error rates have been realized. I will describe these developments. I will also present a new family of novel superconducting electronic elements that can be used with these memories to enhance their capabilities and function. |
Friday, March 6, 2020 1:03PM - 1:39PM |
X66.00004: Controlling supercurrents and their spatial distribution in ferromagnets Invited Speaker: Jan Aarts Spin-triplet Cooper pairs induced in ferromagnets form the centerpiece of the emerging field of superconducting spintronics. Control over the magnetization of the stacked ferromagnetic layers that generate the triplets allows, for instance, making controllable pi-junctions. In such stacks, crucial for triplet generation is the presence of different uniform magnetization directions, The mechanism at work is that the spin-dependent scattering of a singlet in one layer generates an mS = 0 component of the triplet state, which, in the next ferromagnet with a different magnetization direction, manifest itself as an mS = 1, or ‘equal-spin’ triplet. There are other configurations, however, which give rise to controllable triplet currents. Using more intricate magnetic texture even permits control over the spatial distribution of supercurrent. Here we discuss two types of experiments, both based on a disk-shaped ferromagnetic layer as supercurrent carrier. The in-plane magnetization forms a vortex, with in the center a core where the magnetic flux is forced out. In the first experiment, two superconductor / ferromagnet contacts on top of the disk allow us to generate a triplet supercurrent in the disk, and by using superconducting quantum interferometry, we show the existence of two channels. Moreover, we show how the supercurrent can be controlled by moving the vortex with an in-plane magnetic field. Micromagnetic simulations are used to make the connection between the behavior of the supercurrent and the magnetic texture of the disk. In the second experiment, we show that a triplet current can even be generated by placing the superconducting contacts directly on top of the disk, without the presence of the second ferromagnet. Again, the supercurrent is sensitive to the position of the magnetic vortex core. The rotating magnetization, combined with the edges of the structure, yields the correct kind of inhomogeneity. This is not entirely trivial as will be discussed. |
Friday, March 6, 2020 1:39PM - 2:15PM |
X66.00005: Electromagnetic long ranged proximity effect in superconductor-ferromagnet structures Invited Speaker: Alexandre Buzdin The spread of the Cooper pairs into the ferromagnet in proximity-coupled superconductor – ferromagnet (SF) structures is shown to cause a strong inverse electrodynamic phenomenon, namely, the long-range transfer of the magnetic field from the ferromagnet to the superconductor. Contrary to the previously investigated inverse proximity effect resulting from the spin polarization of superconducting surface layer, we found a very generic orbital mechanism of the magnetic moment transfer from a ferromagnet to a superconductor, which is unavoidable in S/F hybrids. It is related with the fact that the common superconducting wave function in S and F (near the interface) does not permit to exclude the vector-potential of the magnetization by gauge transformation. From the experimental point of view, this phenomenon reminds the Aharonov-Bohm effect since the current inside the attached superconductor is induced by the ferromagnetic layer, which cannot create the magnetic field in the outside in the absence of such superconducting environment. At the same time, the true physical key point is that the wave function penetrating the ferromagnet is responsible for this effect. Let us stress that the characteristic length of the proposed inverse electrodynamic effect is of the order of the London penetration depth. |
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