Bulletin of the American Physical Society
2018 Annual Meeting of the APS Mid-Atlantic Section
Volume 63, Number 20
Friday–Sunday, November 9–11, 2018; College Park, Maryland
Session F05: Spintronics and Josephson Junctions |
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Chair: Pratibha Dev, Howard University Room: Edward St. John 2208 |
Saturday, November 10, 2018 1:30PM - 2:06PM |
F05.00001: Majorana Fermions in Semiconductor Nanowire Invited Speaker: Sergey M Frolov Majorana fermions are non-trivial quantum excitations that have remarkable topological properties and can be used to protect quantum information against decoherence. Tunneling spectroscopy measurements on one-dimensional superconducting hybrid materials have revealed signatures of Majorana fermions which are the edge states of a bulk topological superconducting phase. We couple strong spin-orbit semiconductor InSb nanowires to conventional superconductors (NbTiN, Al) to obtain additional signatures of Majorana fermions and to explore the topological phase transition. A potent alternative explanation for many of the recent experimental Majorana reports is that a non-topological Andreev state localizes near the end of a nanowire. We compare Andreev and Majorana modes and investigate ways to clearly distinguish the two phenomena. We are also exploring how Andreev states can be chained together along the nanowire to realize the one-dimensional Kitaev model, a discrete way of generating Majorana modes. |
Saturday, November 10, 2018 2:06PM - 2:18PM |
F05.00002: Josephson junctions with weak links of topological crystalline insulator nanowires Christie J Trimble, Pengzi Liu, Judy J Cha, James R Williams Incorporating superconductivity in topological states of matter offers potential routes to novel excitations with properties essential to topological quantum computation. Topological crystalline insulators (TCIs) are topological states of matter protected by crystalline symmetry, in contrast to the more commonly used time-reversal invariant topological insulators. In this talk, we report on the fabrication of Josephson junctions using nanowires of SnTe, a TCI, as the weak link material. We show the divergence of our devices from standard Josephson junction behavior using DC techniques, highlighting intriguing novel behavior in the magnetic diffraction pattern, and discuss the origin of this behavior in terms of pi phase difference across the junction. |
Saturday, November 10, 2018 2:18PM - 2:30PM |
F05.00003: Dynamically-created Josephson Junctions in thin layers of NbSe2 Steven Tran, Albert Davydov, Sergiy Krylyuk, James Williams Niobium Diselenide (NbSe2) is a member of the transition metal dichalcogenides (TMDs) family which displays superconducting properties and is host to charge density waves (CDWs) down to its monolayer limit. These properties as well as its hexagonal structure make it an interesting material to study physics within the two dimensional limit. Here, we report on fabrication and low-temperature transport measurements of atomically-thin layers of NbSe2 in four terminal-configurations. We observe the formation of phase slip lines across our NbSe2 sheets which appear as non-zero resistances in the superconducting state. The formation of the phase slip lines dynamically creates Josephson junctions (JJs) within our devices which we are able to interact with. We interact with these dynamically-created JJs through the application of RF radiation and magnetic fields. Interestingly, we observe a deviation from the conventional behavior of JJs in the presence of RF radiation and a skewed Fraunhofer pattern in the presence of a magnetic field. We attempt to model our observations in the presence of RF by assuming a coupling of the JJ to a CDW. |
Saturday, November 10, 2018 2:30PM - 3:06PM |
F05.00004: Topological spintronics Invited Speaker: Nitin Samarth Tetradymite narrow bandgap semiconductors (Bi2Te3, Bi2Se3, Sb2Te3, and their alloys) are known to support topologically protected, two dimensional (2D) helical Dirac fermion surface states characterized by a spin‐texture in momentum space [1]. The 'spin‐momentum locking' of the 2D surface states in these three dimensional (3D) 'topological insulators' lends itself naturally to 'topological spintronics,' device applications that might exploit efficient spin‐charge interconversion. We present an introductory overview of the emergence of 'topological spintronics' [2] and then focus on recent experiments that probe spin‐charge interconversion at the interface between a 3D topological insulator and an insulating ferrimagnet [3], with a view toward understanding how the spin Hall conductivity in topological insulators varies with chemical potential. An important issue examined in this context is the relative contribution to spin‐charge conversion of the surface and bulk states, both of which have large spin‐orbit coupling in the tetradymites. Finally, we address emerging demonstrations of efficient spin‐orbit torque switching using topological insulator/ferromagnetic metal heterostructures [4].
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Saturday, November 10, 2018 3:06PM - 3:18PM |
F05.00005: AC Josephson effect in a capacitively shunted graphene Josephson junction Fan Yu, Sandesh S Kalantre, James Williams Recent advances in the search of novel sub-gap modes in Josephson junctions have put forth stringent requirements regarding the control of material properties. Thus it has become a common practice to fabricate bias-gate(s) in close proximity to the junction. However, unexpected shunt capacitance might be introduced and its effect on the behaviour of Josephson junction is less well understood, particularly when the junction is under RF radiation. Here we study the AC Josephson effect on a junction made of graphene encapsulated in boron nitride and contacted by electrodes made of a molybdenum-rhenium alloy. The device comes with a back-gate for chemical potential tuning and two top-gates for boundary tuning. In regions where chemical potential is close to the charge neutrality point and the RF drive current is comparable with critical current, this device demonstrated a bi-stability between the first Shapiro steps, indicating the Josephson junction is in a chaotic regime. This observation casts doubts over arguments that AC Josephson effect in the low RF drive amplitude region would offer the opportunity to observe 4-\pi current phase relation in topological Josephson junctions.
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Saturday, November 10, 2018 3:18PM - 3:30PM |
F05.00006: Non-linear dynamical effects and crisis in an RF-driven graphene Josephson junction Sandesh S Kalantre, Fan Yu, James Williams Josephson junctions have been under extensive scientific investigation for the past 50 years with broad applications ranging from magnetometry to quantum computation. The dynamics of the superconducting phase difference across the junction is described by a resistively and capacitively shunted junction (RCSJ) model, which is equivalent to the dynamics of a driven damped simple pendulum. In the overdamped regime, phase-locked solutions exist which correspond to the well-known Shapiro steps. In this work, we present low-temperature voltage measurements on a graphene Josephson junction in the underdamped regime as a function of DC-current bias and RF power. At zero current bias and a certain range of RF power, we observe the presence of two metastable voltage states with intermittent switching between them. The switching timescale is extraordinarily long (~ s) and a power-law scaling for the timescale with RF power is observed. We reconcile our observations with theory by studying the evolution of an underdamped RCSJ model and show the existence of multiple unstable solutions and strange attractors. Our observations can be tentatively explained using the phenomenon of an interior crisis. We outline future measurements to understand non-linear dynamics of underdamped junctions. |
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