Quasi-2D superconductivity in FeTe_0.55Se_0.45 ultrathin film

Abstract

Iron-chalcogenide FeTe 0.55 Se 0.45 was found to be a promising topological superconducting candidate recently, which may host Majorana bound state in the vortex core and thus attracts intensive research interests in this material. In this report, mechanically exfoliated FeTe 0.55 Se 0.45 superconducting thin films close to the two-dimensional (2D) limit, i.e. sample thickness is on the order of coherence length, were studied systematically by means of electrical transport and point-contact Andreev-reflection spectroscopy (PCARS) measurements. The quasi-2D nature of FeTe 0.55 Se 0.45 thin films is evidenced by the observation of Berezinskii-Kosterlitz-Thouless (BKT) transition and anisotropic upper critical fields in the vicinity of superconducting transition. Compared to bulk samples, we found that the superconducting transition temperature is only slightly suppressed even for films down to 5 nm. The superconducting gap symmetry remains unchanged and the gap size is weakly affected by tailoring thickness. Our findings suggest that the superconductivity of FeTe 0.55 Se 0.45 thin films is rather robust against reduced dimensions. It provides a novel platform for device applications for quantum computations in combination with possible realization of Majorana modes in this material.

Berezinskii–Kosterlitz–Thouless Transition in the Type-I Weyl Semimetal PtBi2

“…Among van der Waals superconductors, materials with a non-trivial band structure are intriguing because they may host interesting unconventional superconducting states when the electron spin degeneracy of Bloch states is lifted, such as mixed singlet and triplet superconductivity − or Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) finite momentum pairing. − In particular, superconducting Weyl semimetals constitute promising systems to study topological superconductivity, , and the spontaneous emergence in nanostructures of vortices induced by a BKT transition , could further allow for the investigation of Majorana bound states in the absence of an external magnetic field, as recently proposed for iron-based superconductors. , Despite this great interest, experimental evidence of BKT transitions are scarce and generally limited to some high-quality films as a result of the sensitivity of the ordered phase to any structural disorder, , and no BKT transition has been observed in any type-I Weyl semimetal thus far. The need for high-quality crystal growth and ultraclean fabrication techniques as a result of this sensitivity to disorder constitutes one of the main challenges for studying superconductivity at low dimensions.…”

Observation of two-dimensional superconductivity in an ultrathin iron–arsenic superconductor

“…For the cuprates, mechanical exfoliation method was adopted to create the ultrathin Bi 2 Sr 2 CaCu 2 O 8+x (Bi2212) superconductor down to half unit-cell in recent years, where a rather high-T c very close to the bulk compound and the absence of dimensionality effect were revealed [18,19]. As for the iron-based superconductors, investigations focus on the iron-selenic (FeSe, including FeSe 1−x Te x ) materials by the method of the precisely controlled molecular beamepitaxy (MBE) growth [13] or mechanical exfoliation [20]. During this process, high temperature superconductivity was observed in the single-layer FeSe film grown on SrTiO 3 substrates [13].…”

“…A rather high-T c very close to the bulk compound [19][20][21][22] and the absence of dimensionality effect [21] were revealed in this system. As for the iron-based superconductors, investigations focus on the iron-selenic (FeSe, including FeSe 1 − x Te x ) materials by the method of the precisely controlled molecular beamepitaxy (MBE) growth [13] or mechanical exfoliation [23,24]. During this process, high temperature superconductivity was observed in the singlelayer FeSe film grown on SrTiO 3 substrates [13].…”

Ambipolar Superconductivity with Strong Pairing Interaction in Monolayer 1T′-MoTe2

“…Figure A schematically shows the setup (method details in Section 12, SI). The point contact is a clean van der Waals-type contact formed by placing the monolayer on top of a Au electrode in a glovebox without any additional measures such as pressure. − Figure B displays several traces of the differential conductance G contact = I ac / V ac as a function of the dc voltage V dc recorded on the Au/1T′-MoTe 2 junction #1 of monolayer device D5 for an electron density of 1.38 × 10 13 cm –2 . Temperature serves as an additional parameter.…”

“…We employ the extended single-band isotropic s -wave Blonder–Tinkham–Klapwijk (BTK) model to fit the point contact data − in Figure D and F to obtain the temperature-dependent gap Δ­( T ). Included in these graphs is also the BCS model for the gap.…”