Browsing by Author "Wu, Chien-Te"

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Collective mode contributions to the Meissner effect: Fulde-Ferrell and pair-density wave superfluids
(2017-06-02)
Boyack, Rufus
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Wu, Chien-Te
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Anderson, Brandon M.
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Levin, K.
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電子物理學系
In this paper we demonstrate the necessity of including the generally omitted collective-mode contributions in calculations of the Meissner effect for nonuniform superconductors. We consider superconducting pairing with nonzero center-of-mass momentum, as is possibly relevant to high transition temperature cuprates, cold atoms, and color superconductors in quantum chromodynamics. For the concrete example of the Fulde-Ferrell phase we present a quantitative calculation of the superfluid density, showing not only that the collective-mode contributions are appreciable but also that they derive from the amplitude mode of the order parameter. This latter mode is generally viewed as being invisible in conventional superconductors. However, our analysis shows that it is extremely important in pair-density-wave-type superconductors, where it destroys stable superfluidity well before the mean-field order parameter vanishes.
Full proximity treatment of topological superconductors in Josephson-junction architectures
(2019-05-21)
Setiawan, F.
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Wu, Chien-Te
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Levin, K.
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電子物理學系
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Department of Electrophysics
Experiments on planar Josephson-junction architectures have recently been shown to provide an alternative way of creating topological superconductors hosting accessible Majorana modes. These zero-energy modes can be found at the ends of a one-dimensional channel in the junction of a two-dimensional electron gas (2DEG) proximitized by two spatially separated superconductors. The channel, which is below the break between the superconductors, is not in direct contact with the superconducting leads, so that proximity coupling is expected to be weaker and less well controlled than in the simple nanowire configuration widely discussed in the literature. This provides a strong incentive for this paper which investigates the nature of proximitization in these Josephson-junction architectures. At a microscopic level we demonstrate how and when it can lead to topological phases. We do so by going beyond simple tunneling models through solving self-consistently the Bogoliubov-de Gennes equations of a heterostructure multicomponent system involving two spatially separated s-wave superconductors in contact with a normal Rashba spin-orbit-coupled 2DEG. Importantly, within our self-consistent theory we present ways of maximizing the proximity-induced superconducting gap by studying the effect of the Rashba spin-orbit coupling, chemical potential mismatch between the superconductor and 2DEG, and sample geometry on the gap. Finally, we note (as in experiment) a Fulde-Ferrell-Larkin-Ovchinnikov phase is also found to appear in the 2DEG channel, albeit under circumstances which are not ideal for the topological superconducting phase.
How Do Acquired Political Identities Influence Our Neural Processing toward Others within the Context of a Trust Game?
(2018-02-02)
Wu, Chien-Te
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Fan, Yang-Teng
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Du, Ye-Rong
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Yang, Tien-Tun
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Liu, Ho-Ling
Trust is essential for mutually beneficial human interactions in economic exchange and politics and people's social identities notably have dramatic effects on trust behaviors toward others. Previous literature concerning social identities generally suggests that people tend to show in-group favoritism toward members who share the same identity. However, how our brains process signals of identity while facing uncertain situations in interpersonal interactions remains largely unclear. To address this issue, we performed an fMRI experiment with 54 healthy adults who belonged to two identity groups of opposing political orientations. The identity information of participants was extracted from a large-scale social survey on the 2012 Taiwan presidential election. Accordingly, participants were categorized as either the Kuomintang (KMT) or the Democratic Progressive Party (DPP) supporters. During the experiment, participants played trust games with computer agents with labels of the same or the opposing political identity. Interestingly, our results suggest that the behaviors of the two groups cannot be equally attributed to in-group favoritism. Behaviorally, only the DPP supporter group showed a significant trust preference toward in-group members, which did not hold for the KMT supporter group. Consistently, neurophysiological findings further revealed that only the DPP supporter group showed neuronal responses to both unexpected negative feedback from in-group members in anterior insula, temporoparietal junction, and dorsal lateral prefrontal cortex, as well as to unexpected rewards from out-group members in caudate. These findings therefore suggest that acquired identities play a more complex role in modulating people's social expectation in interpersonal trust behaviors under identity-relevant contexts.
Majorana zero modes in spintronics devices
(2017-01-24)
Wu, Chien-Te
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Anderson, Brandon M.
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Hsiao, Wei-Han
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Levin, K.
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電子物理學系
We show that topological phases should be realizable in readily available and well-studied heterostructures. In particular we identify a new class of topological materials which are well known in spintronics: helical ferromagnet-superconducting junctions. We note that almost all previous work on topological heterostructures has focused on creating Majorana modes at the proximity interface in effectively two-dimensional or one-dimensional systems. The particular heterostructures we address exhibit finite-range proximity effects leading to nodal superconductors with Majorana modes localized well away from this interface. To show this, we implement a Bogoliubov-de Gennes (BdG) proximity numerical scheme, which importantly involves two finite dimensions in a three-dimensional junction. Incorporating this level of numerical complexity serves to distinguish ours from alternative numerical BdG approaches which are limited by generally assuming translational invariance or periodic boundary conditions along multiple directions. With this access to the edges, we are then able to illustrate in a concrete fashion the wave functions of Majorana zero modes and, moreover, address finite-size effects. In the process we establish consistency with a simple analytical model.
Quantum phase transitions in proximitized Josephson junctions
(2018-08-13)
Wu, Chien-Te
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Setiawan, F.
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Anderson, Brandon M.
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Hsiao, Wei-Han
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Levin, K.
We study fermion-parity-changing quantum phase transitions (QPTs) in platform Josephson junctions. These QPTs, associated with zero-energy bound states, are rather widely observed experimentally. They emerge from numerical calculations frequently without detailed microscopic insight. Importantly, they may incorrectly lend support to claims for the observations of Majorana zero modes. In this paper, we present a fully consistent solution of the Bogoliubov-de Gennes equations for a multicomponent Josephson junction. This provides insights into the origin of the QPTs. It also makes it possible to assess the standard self-energy approximations, which are widely used to understand proximity coupling in topological systems. The junctions we consider are complex and chosen to mirror experiments. Our full proximity calculations associate the mechanism behind the QPT as deriving from a spatially extended, proximity-induced magnetic "defect." This defect arises because of the insulating region, which effects a local reorganization of the bulk magnetization in the proximitized superconductor. Our results suggest more generally that QPTs in Josephson junctions generally do not require the existence of spin-orbit coupling and should not be confused with, nor are they indicators of, Majorana physics.
Spin transport in half-metallic ferromagnet-superconductor junctions
(2018-08-30)
Wu, Chien-Te
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Halterman, Klaus
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電子物理學系
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Department of Electrophysics
We investigate the charge and spin transport in half-metallic ferromagnet (F) and superconductor (S) nanojunctions. We utilize a self-consistent microscopic method that can accommodate the broad range of energy scales present, and ensures that proximity effects that account for the interactions at the interfaces are accurately determined. Two experimentally relevant half-metallic junction types are considered: The first is an F1F2S structure, where a half-metallic ferromagnet F-1 adjoins a weaker conventional ferromagnet F-2. The current is injected through the F-1 layer by means of an applied bias voltage. The second configuration involves an SF1F2F3S Josephson junction whereby a phase difference Delta phi between the two superconducting electrodes generates the supercurrent flow. In this case, the central half-metallic F-2 layer is surrounded by two weak ferromagnets F-1 and F-3. By placing a ferromagnet with a weak exchange field adjacent to an S layer, we are able to optimize the conversion process in which opposite-spin triplet pairs are converted into equal-spin triplet pairs that propagate deep into the half-metallic regions in both junction types. For the tunnel junctions, we study the bias-induced local magnetization, spin currents, and spin-transfer torques for various orientations of the relative magnetization angle theta in the F layers. We find that the bias-induced equal-spin triplet pairs are maximized in the half metal for theta approximate to 90 degrees and, as part of the conversion process, are anticorrelated with the opposite-spin pairs. We show that the charge current density is maximized, corresponding to the occurrence of a large amplitude of equal-spin triplet pairs, when the exchange interaction of the weak ferromagnet is about 0.1E(F). For the half-metallic Josephson junctions we often find that the spin current flowing in the half metal is equivalent to the charge supercurrent flowing throughout the junction. This is indicative that the current consists of spin-polarized triplet pairs. The conversion process of the opposite-spin triplet pairs to the equal-spin triplet pairs in the weaker magnets is clearly demonstrated. This is exemplified by the fact that the supercurrent in the half metal was found to be relatively insensitive to its thickness.