Session B05: Iron Chalcogenites (Mostly Te Based)

Superconductivity Mediated by Nematic Fluctuations in Tetragonal FeSe1-xSx.

Electronic nematic exist in several families of unconventional superconductors. Although superconductivity mediated by nematic fluctuations is well established theoretically [1-4], it has yet to be unambiguously identified experimentally [5, 6]. A major challenge is that nematicity is often intertwined with other degrees of freedom, such as magnetism and charge order. The FeSe_1−xS_x family of iron-based superconductors provides a unique opportunity to explore this concept, as it features an isolated nematic phase that can be suppressed by sulfur substitution at a quantum critical point (QCP) near x_c = 0.17, where nematic fluctuations are the largest [7, 8]. We performed scanning tunneling spectroscopy measurements to visualize Boguliubov quasiparticle interference patterns, from which we determined the momentum structure of the superconducting gap near the Brillouin zone Γ point of FeSe_0.81S_0.19. The results reveal an anisotropic, near nodal gap with minima that are 45° rotated with respect to the Fe-Fe direction, characteristic of a nematic pairing interaction. In contrast, the gap maxima in pristine FeSe are aligned with the Fe-Fe direction. Thus, the measurements reveal a gap structure that not only is at odds with general expectations from the traditional spin-fluctuation scenario, but which also displays the form expected for superconductivity mediated by nematic fluctuations. While the qualitative analysis of the data already supports the scenario of superconductivity due to nematic fluctuations, we also made a quantitative comparison with a theoretical model, further substantiating our conclusions.

 

[1] S. Lederer, Y. Schattner, E. Berg, S. A. Kivelson. PRL 114, 097001 (2015).

[2] M. A. Metlitski, D. F. Mross, S. Sachdev, T. Senthil. PRB 91, 115111 (2015).

[3] A. Klein, A. Chubukov. PRB 98, 220501 (2018).

[4] J. Kang, R. M. Fernandes. PRL 117, 217003 (2016).

[5] R. M. Fernandes, A. I. Coldea, H. Ding, I. R. Fisher, P. J. Hirschfeld, G. Kotliar. Nature 601, 35 (2022).

[6] A. E. Bohmer, J.-H. Chu, S. Lederer, M. Yi. Nature Physics 18, 1412–1419 (2022).

[7] A. I. Coldea. Frontiers in Physics 8, 528 (2021).

[8] S. Hosoi, et al. PNAS 113, 8139–8143 (2016).   

Progress in Searching for Majorana Bound State by Shapiro-step measurement in FeTe0.55Se0.45 Josephson Junction

 Topological superconductors have a Majorana bound state (MBS) at their boundaries or vortices on the surface [1]. Such MBS follow non-Abelian exchange statistics, so they can be used as a fault-tolerant topological quantum computer [2], FeTe_0.55Se_0.45 (FTS), an iron-based superconductor, is one of the candidates for topological superconductors [3]. FTS is expected to have a topological surface state and zero-energy vortex bound state (ZVBS) under the external magnetic field [4]. To prove the topological superconductivity of FTS, we fabricated full-van der Waals vertical Josephson junction through low-temperature micro-cleaving technique. We will discuss our investigation about the Shapiro steps upon irradiating microwave as a function of the strength of the external magnetic field.

[1] Rev. Mod. Phys.. 82, 4. (2010)

[2] Phys. –Usp. 44, 131 (2001)

[3] PRL. 117, 047001 (2016)

[4] Nature Materials. 18, 811-815 (2019)   

How Does the Interface-Induced Superconductivity Emerge in (Bi,Sb)2Te3/FeTe Heterostructures?

Recently, the Dirac-fermion-assisted interfacial superconductivity has been discovered in (Bi,Sb)₂Te₃/FeTe heterostructures. In this work, we used molecular beam epitaxy (MBE) to synthesize a series of one quintuple layer (QL) (Bi,Sb)₂Te₃/FeTe heterostructures with different Bi/Sb ratio and performed low-temperature scanning tunneling microscopy and spectroscopy (STM/S) measurements. On the top surface of 1QL (Bi,Sb)₂Te₃, we observed a U-shaped superconducting gap concurrent with the appearance of a periodic modulation in atomic-resolution STM images. This periodic modulation varies in response to changes in the Bi/Sb ratio. We found that an out-of-plane magnetic field can induce a set of in-gap states on 1QL Bi₂Te₃/FeTe, whereas the superconducting gap on 1QL Sb₂Te₃/FeTe remains unchanged under an out-of-plane magnetic field up to 11T. These different behaviors appear to be linked to the influence of crystal and electronic structures as a consequence of altering the Bi/Sb ratio. Our observation strongly suggests that the appearance of the interfacial superconductivity in (Bi,Sb)₂Te₃/FeTe heterostructures is a result of the formation of the periodic modulation across their interface.   

Coexistence of Superconductivity and Ferromagnetism in CrTe2/FeTe Heterostructures

In this work, we employed molecular beam epitaxy (MBE) to synthesize the 1T-CrTe₂/FeTe heterostructures. 1T-CrTe₂ is a two-dimensional van der Waals ferromagnet with Curie temperature up to room temperature, while FeTe is a non-superconducting antiferromagnetic iron chalcogenide. Through electrical transport measurements, we observed a sharp superconducting phase transition with a zero-resistance T_c ~11 K. The ferromagnetism property of the CrTe₂ layer is confirmed by the appearance of the non-zero anomalous Hall traces at temperatures up to 150 K. These observations strongly indicate the coexistence of superconductivity and ferromagnetism in our MBE-grown CrTe₂/FeTe heterostructures. We systematically investigated the CrTe₂ and FeTe thickness dependence of the induced superconductivity and ferromagnetism. We found the superconductivity persists even as the CrTe₂ and FeTe layers are scaled down to a few atomic layers. The successful synthesis of CrTe₂/FeTe heterostructures with atomically sharp interfaces provides a novel platform for studying the interplay between superconductivity and magnetism.   

Coexistence of Superconductivity and Antiferromagnetism in Epitaxial Topological Magnet MnBi2Te4 Films

The interface of two materials has been demonstrated to give rise to completely unexpected emergent phenomena, such as interface-induced superconductivity. Moreover, when one of the two materials has a strong spin-orbit coupling, this interface-induced superconductivity may host the topological superconducting (TSC) phase. In this work, we employ molecular beam epitaxy (MBE) to grow a series of heterostructures formed by stacking together two non-superconducting antiferromagnetic materials, an intrinsic antiferromagnetic topological insulator MnBi₂Te₄ and an antiferromagnetic iron chalcogenide (FeTe). Our electrical transport measurements reveal emergent interface-induced superconductivity in these heterostructures. By performing scanning tunneling microscopy and spectroscopy measurements, we observe a proximity-induced superconducting gap on the top surface of the MnBi₂Te₄ layer, confirming the interaction between superconductivity and antiferromagnetism in the MnBi₂Te₄ layer. Our findings will advance the fundamental inquiries into the TSC in hybrid devices and provide a promising platform for the exploration of chiral Majorana physics in MnBi₂Te₄-based heterostructures.   

Scanning tunneling spectroscopy of vortex core states in Fe(Se,Te) using superconducting STM tip

In iron-based superconductors with topological surface states, the proximity effect of the bulk superconductivity to the surface states induces topological superconductivity at the surface, giving rise to the emergence of the Majorana quasiparticle at a vortex core. Although the recent scanning tunneling microscopy (STM) experiments have revealed the zero-energy vortex bound states (ZVBS) suggestive of the Majorana zero mode (MZM) at the vortex cores of various iron-based superconductors [1,2], it has been still elusive whether the observed ZVBSs are the MZM or not. In this study, we focus on the predicted spin polarization of the MZM, which is a peculiar nature of MZM and have conducted the spin-polarized spectroscopy around vortex cores of FeSe_0.4Te_0.6 at a magnetic field of 1.5 T, using superconducting Nb STM tip with Zeeman-split coherence peaks. The spectra along a path through a vortex core indicate spatially non-dispersive two pairs of peaks at the energies of Zeeman-split coherence peaks of the STM tip, assuring that these spectra capture the ZVBS. We will discuss the spin-polarization of the observed ZVBS in the presentation. 

References

[1] D. Wang et al., Science 362, 333 (2018).

[2] T. Machida et al., Nat. Mater. 18, 811 (2019).   

Combined ARPES and DFT+eDMFT investigation on the effects of strain & doping in Fe(Se,Te) systems.

Recently, the effect of doping in the Fe(Se,Te) systems has been a point of interest in the literature as Fe(Se,Te) system can potentially exhibit topological superconductivity at relatively high temperatures. We use a combination of molecular beam epitaxial (MBE) growth, time and angle-resolved-photoemission-spectroscopy (trARPES), and ab initio embedded dynamical mean-field theory (eDMFT) to understand the correlated electronic and vibrational properties of Fe(Se,Te) systems under the applied in-plane strain. Using eDMFT, we compute structural parameters for various FeSe_xTe_{1-x} systems including the chalcogen heights and important phonon frequencies. eDMFT computed orbital-resolved spectral functions are found to be in excellent agreement with the ARPES data.   

Microwave complex conductivity measurement of superconducting very thin FeSe1-xTex films (x = 0 - 0.5)

Studies on FeSe_1-xTe_x films have attracted significant attention. Although the nematic transition temperature of the FeSe_1-xTe_x films decreased after Te substition, the superconducting transition temperature, T_c, was largely enhanced following the disappearance of nematic order, which is contrary to the FeSe_1-xS_x films and bulk FeSe_1-xTe_x. To investigate the origin of this increase in T_c, we studied complex conductivity using microwaves. In particular, complex conductivity measurements spanning the entire temperature range, including the vicinity of T_c, were conducted on systematically varied FeSe_1-xTe_x (x = 0 - 0.5) thin films. To accomplish this, we developed a novel technique for measuring complex conductivity in very thin films with a thickness significantly smaller than the penetration depth using a microwave electric field.

By applying this novel technique to FeSe_1-xTe_x films, we observed a definite change in the temperature dependences of superfluid density and quasi-particle scattering rate at the nematic boundary. These changes at the nematic boundary suggest different superconducting gap structures between samples in the nematic and non-nematic phases. Furthermore, superconducting fluctuation is visible up to 1.2 T_c, irrespective with or without the nematic order. Our experimental results offer insights into the electronic states of FeSe_1-xTe_x thin films.   

Two-fold symmetric anisotropy of the superconducting phase of Fe Te0.5 Se0.5/Bi2 Te3 thin film

We report the epitaxial MBE-growth of FeTe0.5Se0.5 (a topological superconductor)/Bi2Te3 (a topological insulator) thin film on sapphire substrate. From the electrical transport measurements, we show that the critical current and the longitudinal resistance (measured at close to the critical temperature) of the system, are two-fold symmetric, in presence of a parallel magnetic field. Moreover, the anisotropy completely disappears in the normal state, at 15K, which clearly indicates that the anisotropy is the intrinsic property of the superconducting phase. Thus, our result shows an unconventional superconductivity in a heterostructure of an Iron based SC and a topological insulator, making it a potential candidate for topological quantum computation.   

Disordered Superconductivity in Fe(Te,Se)

The interplay of topology, superconductivity, and magnetism has made iron chalcogenide compounds an exciting playground for exploring exotic quantum states; however, the role of disorder has been largely neglected to date. Using time domain terahertz spectroscopy, we find anomalous low-energy absorption in the superconducting state of Fe(Te,Se) thin films and heterostructures. These features are accurately modeled according to theories of disordered superconductivity originally developed for cuprates and the superconductor-insulator transition. These results suggest that disorder plays an overlooked role in the behavior of Fe(Te,Se), which may have wide-ranging consequences for the interpretation of the various exotic phenomena observed in these exciting compounds.   

Electronic structure and transport properties of partially Ni-substituted superconducting iron chalcogenide FeSe0.45Te0.55

Iron chalcogenides bring together superconductivity, non-trivial electronic topology, and magnetism in a single material, making it a potential platform for hosting Majorana fermions which are the building blocks for topological quantum computing. We investigated the change of electronic structure and transport properties in a series of superconducting iron-nickel chalcogenides (FeNi)Se_0.45Te_0.55 by partial Ni substitution for Fe. Ni substitution increases the electron count. Ni 3d spin-orbit energy is ~ 50% higher than that of an Fe atom, leading to enhanced topological properties in this class of materials. From combined ARPES and transport measurements, we found that Ni substitution reduces superconducting transition temperature T_c and superconducting gap. Temperature dependent normal state resistivity changes from a weak metallic behavior to an insulating-like behavior with Ni-substitution level above 2% (nominal composition). A low level of Ni-substitution was found to enhance the critical current density in iron chalcogenide superconductors that peaks around 2% substitution level.   

Realizing topological Kondo effect in Fe(Te,Se) based simple mesoscopic device.

Kondo effect (KE) occurs when conduction electrons couple to local quantum degree of freedom with spin degenerate energy levels. However, symmetry breaking perturbations can destroy this effect. Interestingly a robust topological Kondo effect (TKE) [1, 2] can arise due to coupling between conduction electrons and topologically degenerate Majorana fermions (MF). Here Majorana bound states (MBS) play the role of the quantum impurity spin in KE. The TKE can further result into unique non-Fermi liquid transport signatures. Hence, if such transport signature is realized in experiment, it can contribute to the exploration of MF. One way to realize such TKE is by employing mesoscopic superconducting device that can host MBS. It is known that topological superconductor (TSC) is promising candidate to host MF. Motivated by these, In this work we propose a simple mesoscopic setup made of iron based TSC FeSe_0.45Te_0.55 [3] containing magnetic domain wall, having a solid potential to realize TKE. As it is easy to grow high quality single crystals and thin films of Fe(Te,Se), our proposed device is a lot easier to fabricate as compared to s-wave superconductors based heterostructure devices.  Additionally it will be advantageous, as FeSe_0.45Te_0.55 is intrinsic TSC with a relatively high T_c.

References:

1. B. Beri et.al., PRL 109, 156803 (2012)

2. A. Altland et.al., PRL 113, 076401 (2014)

3. P. Zhang et. al., Science 360, 6385 (2018)