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Register a new userImpurity scattering effect on the zero-energy peak of the local density of states in a multi-quantum vortex core
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We theoretically study a non-magnetic impurity effect on the vortex bound states of a multi-quantum vortex. The zero-energy peak of the local density of states is investigated for vortex cores with the winding numbers 2 and 4 within the framework of the quasiclassical theory of superconductivity. We find that the zero-energy peaks, which appear away from the vortex center in the clean limit, move towards the vortex center with increasing the impurity scattering rate, resolving a contradiction between an experimental result and previous theoretical predictions.
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We numerically study the effect of non-magnetic impurities on the vortex bound states in noncentrosymmetric systems. The local density of states (LDOS) around a vortex is calculated by means of the quasiclassical Greens function method. We find that the zero energy peak of the LDOS splits off with increasing the impurity scattering rate.
We numerically investigate the electronic structures around a vortex core in a bilayer superconducting system, with s-wave pairing, Rashba spin-orbit coupling and Zeeman magnetic field, with use of the quasiclassical Greens function method. The Bardeen-Cooper-Schrieffer (BCS) phase and the so-called pair-density wave (PDW) phase appear in the temperature-magnetic-field phase diagram in a bulk uniform system [Phys. Rev. B 86, 134514 (2012)]. In the low magnetic field perpendicular to the layers, the zero-energy vortex bound states in the BCS phase are split by the Zeeman magnetic field. On the other hand, the PDW state appears in the high magnetic field, and sign of the order parameter is opposite between the layers. We find that the vortex core suddenly shrinks and the zero-energy bound states appear by increasing the magnetic field through the BCS-PDW transition. We discuss the origin of the change in vortex core structure between the BCS and PDW states by clarifying the relation between the vortex bound states and the bulk energy spectra. In the high magnetic field region, the PDW state and vortex bound states are protected by the spin-orbit coupling. These characteristic behaviors in the PDW state can be observed by scanning tunneling microscopy/spectroscopy.
The search for the Majorana fermions in condensed matter physics has attracted much attention, partially because they may be used for the fault-tolerant quantum computation. It has been predicted that the Majorana zero mode may exist in the vortex core of topological superconductors. Recently, many iron-based superconductors are claimed to exhibit a topologically nontrivial surface state, including Fe(Te,Se). Some previous experiments through scanning tunneling microscopy (STM) have found zero-bias conductance peaks (ZBCP) within the vortex cores of Fe(Te,Se). However, our early experimental results have revealed the Caroli-de Gennes-Matricon (CdGM) discrete quantum levels in about 20% vortices. In many other vortices, we observed a dominant peak locating near zero bias. Here we show further study on the vortex core state of many more vortices in FeTe$_{0.55}$Se$_{0.45}$. In some vortices, if we take a certain criterion of bias voltage window near zero energy, we indeed see a zero energy mode. Some vortices exhibit zero energy bound state peaks with relatively symmetric background, which cannot be interpreted as the CdGM discrete states. The probability for vortices showing the ZBCP lowers down with the increase of magnetic field. Meanwhile it seems that the presence and absence of the ZBCP has no clear relationship with the Te/Se ratio on the surface. Temperature dependence of the spectra reveals that the ZBCP becomes weakened with increasing temperature and disappears at about 4 K. Our results provide a confirmed supplementary to the early claimed zero energy modes within the vortex cores of Fe(Te,Se). Detailed characterization of these zero energy modes versus magnetic field, temperature and spatial distribution of Te/Se will help to clarify its origin.
We theoretically investigate a non-magnetic impurity effect on the temperature dependence of the vortex core shrinkage (Kramer-Pesch effect) in a single-band s-wave superconductor. The Born limit and the unitary limit scattering are compared within the framework of the quasiclassical theory of superconductivity. We find that the impurity effect inside a vortex core in the unitary limit is weaker than in the Born one when a system is in the moderately clean regime, which results in a stronger core shrinkage in the unitary limit than in the Born one.
The magnetic field (B) dependence of the electronic specific heat for a simple BCS type-II superconductor has been determined from measurements on pure niobium (Nb). Contrary to expectations, the electronic specific heat coefficient gamma(T,B) is observed to be a sublinear function of B at fields above the lower critical field H_{c1}. This behavior is attributed to the delocalization of quasiparticles bound to the vortex cores. The results underscore the ambiguity of interpretation that arises in specific heat studies of this kind on newly discovered type-II superconductors, and also emphasize the need to such measurements under field-cooled conditions.
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