Session D05: Systems: Thin Films and Monolayer

Superconductivity in FeSe enhanced and modulated by quantum geometetry

While the dispersion relation in the band structure is a fundamental property of electron systems in solids, quantum geometry in the band structure is essential for various many-body states and unconventional responses in quantum materials. In particular, the real part of the quantum geometric tensor, namely the Fubini-Study quantum metric, is recently attracting attention, although the imaginary part is well known as the Berry curvature.

While research of quantum geometry in superconductors was triggered by the studies of flat band systems [1], quantum geometry is essential in a broader class of superconductors. We show that Fe-based superconductors are playgrounds of quantum geometry in superconductors.

We formulate the superfluid weight in unconventional superconductors based on the geometric properties of Bloch electrons. We apply the formula to a model of monolayer FeSe obtained by first-principles calculation. Our numerical calculations point to a significant enhancement of the Berezinskii-Kosterlitz-Thouless transition temperature due to the geometric contribution to the superfluid weight [2], which is not included in the Fermi liquid theory. This means that superconductivity in monolayer FeSe is significantly enhanced by quantum geometry. The quantum geometric effect on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state acquiring a finite center of mass momentum in Cooper pairs is also studied [3]. The quantum metric gives a negative contribution to the superfluid weight in some unconventional superconducting states and stabilizes the FFLO state.

These results reveal that the geometric properties of Bloch electrons play an essential role in superconductors and pave the way for clarifying hidden aspects of superconductivity from the viewpoint of quantum geometry.

 

[1] S. Peotta and P. Törmä, Nat. Commun. 6, 8944 (2015).

[2] T. Kitamura, T. Yamashita, J. Ishizuka, A. Daido, Y. Yanase, Phys. Rev. Research 4, 023232 (2022).

[3] T. Kitamura, A. Daido, Y. Yanase, Phys. Rev. B 106, 184507 (2022).   

Effects of transition metal doping on the superconducting and magnetic ordering of thin film FeSe/SrTiO3

Single unit-cell FeSe films grown on (001) SrTiO₃ have attracted extensive theoretical and experimental attention because they have a significantly enhanced superconducting critical temperature (T_c) compared to that observed in bulk FeSe crystals. This is due in part to finely tuned levels of electron doping at the interface. Here we describe the growth of Co_xFe_1-xSe thin films by molecular beam epitaxy on (001) SrTiO₃. We investigate the superconducting properties of these films over the complete concentration range (0 < x < 1) and study the evolution of their electronic structure using laser-based angle resolved photoemission spectroscopy. By increasing the Co concentration, we demonstrate a suppression in the superconducting critical temperature compared to pure FeSe films, along with the emergence of a concomitant, and possibly competing, ferromagnetic order with perpendicular anisotropy. Additionally, we inspect the distribution of substituted Co atoms using scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy.   

Spin-resolved photoemission spectroscopy of FeSe thin films

The iron-based superconductor FeSe hosts rich intertwined behavior including superconductivity, nematic order, and strong magnetic fluctuations. Angle resolved photoemission has proven to be an invaluable tool to probe these intertwined states, revealing the effects of nematic order on the electronic structure and probing the superconducting gap structure. Here we report spin and angle resolved photoemission spectroscopy (SR-ARPES) measurements of FeSe thin films grown via molecular beam epitaxy. In spite of the fact that bulk FeSe preserves both time reversal and inversion symmetry which ensures fully spin degenerate bands, we detect observable spin asymmetry in the emitted photoelectrons. This effect is present even in thicker samples (~10 unit cells) where interfacial effects and associated inversion symmetry breaking from the film-substrate interface are presumed to be negligible. The spin texture of the photoelectrons is fully in the 2D FeSe plane and evolves nontrivially as a function of energy and momentum, with the measured spin switching sign across the Γ point and exhibiting spin-momentum locking. In this talk I will outline the experimental phenomenology of this effect and discuss its implications and possible origins.   

Importance of interfacial layers of FeSe/SrTiO3 superconductors

The electron-phonon coupling at the interfaces between two distinct materials can lead to emergent physical properties and an unexpectedly high superconducting temperature. A notable example is the FeSe monolayer grown on doped SrTiO₃, which was found to exhibit a transition temperature (T_c) over 100 K, much higher than its bulk value of 8 K. This has inspired extensive research efforts to investigate the mechanism behind such extraordinary enhancement. Yet, the role of the oxide substrate remains unclear to date. Here, we discuss how interfacial phonon bands and electron-phonon interactions are influenced by complex reconstructions and segregation at the FeSe/STO interface. A notable discovery is that the formation of an unusual interfacial structure plays an important role in the enhancement of T_c. Our research provides a layer-by-layer breakdown of the electron and phonon states’ contributions to the superconductivity.   

Engineering superconductivity in single-layer FeSe on SrTiO3 (001) with TiO2- and SrO- terminations

The discovery of high-temperature superconductivity in single-layer FeSe grown on a SrTiO₃(001) (STO) substrate raises a fundamental question about the role of the interface in superconductivity. Extensive research so far suggests that the origin of the enhanced T_C is two-fold: electron doping from the substrate and interfacial coupling. Interestingly, most studies have been conducted on films grown on the TiO₂-terminated STO substrate. In this work, we synthesized single-layer FeSe on both TiO₂- and SrO-terminated STO substrates by molecular beam epitaxy and compared their superconducting properties by scanning tunneling microscopy/spectroscopy measurements. Nb-doped STO was first annealed under an oxygen partial pressure of 1 × 10^-5 Torr at 900 ° C for 30 min, resulting in a surface with mixed TiO₂ and SrO terminations. After further annealing under Se flux at 350 °C for 60 min, dI/dz spectroscopy revealed a higher work function for the SrO termination than the TiO₂ termination. For single-layer FeSe films grown on such STO substrates, a superconducting gap of ~15 meV was observed in the FeSe/TiO₂ regions by dI/dV tunneling spectroscopy, which agrees well with previous reports. In contrast, a smaller gap ~10 meV is observed on the FeSe/SrO. By comparing these experimental observations with DMFT calculations, our findings indicate the interfacial spacing and Se-Fe-Se angle of the FeSe tetrahedra are optimal at the FeSe-TiO₂ interface for enhancing superconducting transition temperature in single-layer FeSe/STO.   

Probing phonons at FeSe/SrTiO3 interfaces by monochromted STEM-EELS

Monolayer FeSe films grown on SrTiO₃ substrate show over ten-fold increase in superconducting Tc compared to bulk FeSe. The interfacial electron-phonon coupling has been thought as one key factor for the Tc enhancement. So far, the main experimental evidence for electron-phonon coupling is the ~100 meV replica bands observed in angle resolved photoemission spectroscopy. In order to provide an atomic scale understanding of the electron-phonon interaction, here we study the phonons at FeSe/SrTiO₃ interface by Å-sized electron beam. We study FeSe/SrTiO₃ samples by high resolution electron energy loss spectroscopy in a monochromated scanning transmission electron microscope. We will present atomically resolved phonon spectral images of the FeSe/SrTiO₃ interface. Together with the interface atomic structure and chemistry, we will also discuss possible origin of the phonon modes with strong EPC.   

Transport properties of PLD-grown ultrathin films of FeSe under magnetic field

Iron chalcogenide superconductor, FeSe, has attracted much attention because monolayer films on SrTiO₃ (STO) grown by molecular beam epitaxy (MBE) exhibit interface-enhanced superconductivity with superconducting transition temperature (T_c) of 40-65 K, which is much higher than that of 9 K in bulk form [1,2]. The ultrathin films of FeSe had been grown only by the MBE technique, however, we reported the successful realization of interface-enhanced superconductivity in films thicker than 7 nm using the PLD technique. In the early stage, a thinner film with thickness d of 5 nm did not show superconductivity probably due to degradation by air exposure [3].

Here, we report the successful growth of superconducting ultrathin films of FeSe/STO with d ≦ 4 nm with a capping layer of amorphous Si. The ultrathin film with d = 4 nm showed T_c^onset of 30 K and zero resistivity below 10 K. Negative hall coefficient, which is in contrast to thick FeSe films, was observed at low temperatures, suggesting the electron transfer from STO. The film showed large anisotropy of the upper critical fields as high as 14.8 and the GL coherence length along the c-axis ξ_c of 0.2 nm, which is smaller than the c-axis lattice parameter of FeSe. Similar short coherence length was also observed in thicker film (d  = 19 nm), suggesting a confinement of superconductivity at FeSe/STO interface [4]. Furthermore, we will discuss the vortex state and superconducting fluctuation using Nernst effect measurement.

Reference

[1]. Q. Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012).

[2]. S. He et al., Nat. Mater. 12, 605 (2013).

[3]. T. Kobayashi et al., Supercond. Sci. Technol. 35, 07LT01 (2022).

[4]. T. Kobayashi et al., J. Phys.: Condens. Matter 35, 41LT01 (2023).   

Unconventional superconductivity in iron-based van der Waals heterostructures

High-temperature superconductivity is a holy grail of condensed matter physics. It shifts the conventional BCS paradigm and opens an appealing future for application. In this context, iron-based superconductors (FeSC) play a vital role in understanding unconventional superconductivity in multiorbital systems. Moreover, the interplay between strong electronic correlations, magnetism, nematicity, and band topology gives rise to universal sign-changed superconducting pairing orders accompanied by a rich phase diagram that depends on the exact compound composition. For these reasons, making a unified theoretical description of superconductivity in this system is still challenging. Previous measurements of FeSC were mainly focused on macroscopic-size samples, while potentially significant quantum effects in finite-size samples have yet to be explored. In this talk, we discuss fabricated van der Waals structures based on exfoliated thin flakes of FeSC. Using scanning tunneling microscopy and spectroscopy, we discovered modulation of superconductivity, in huge contrast with its bulk crystal counterpart. We further discussed several possible mechanisms for the origin of the exotic behaviors with the help of a microscopic model. Our results reveal new insights into the unconventional nature of FeSC and provide a novel method to study other unconventional superconductors.   

0-pi Juxtaposed Interference Patterns in FeSe/FeSe van der Waals Josephson Junctions

Recently, van der Waals (vdW) Josephson junctions (JJs) made of two thin flakes of Fe(Te,Se) have been fabricated to reveal the spontaneous time-reversal symmetry breaking in Fe(Te,Se) from a new angle [1]. Here, we investigated the superconducting state of FeSe by fabricating vdW JJ made of FeSe thin flakes, where there is nematicity and fewer impurities compared to Fe(Te,Se). Instead of a typical JJ Fraunhofer interference pattern that is symmetric with respect to DC current bias, we observed 0-junction behavior with positive current bias and pi-junction behavior with negative current bias. Accompanying the juxtaposed Fraunhofer pattern is a superconducting diode effect with maximum efficiency at zero field and symmetric field dependence with multiple sign changes. The implications of our results on spontaneous time-reversal symmetry breaking and superconducting pairing are discussed.

[1] G. Qiu*, H.-Y. Yang* (co-first author), L. Hu, H. Zhang, CY. Chen, Y. Lyu, C. Eckberg, P. Deng, S. Krylyuk, A.V. Davydov, R. Zhang, and K. L. Wang, "Emergent ferromagnetism with superconductivity in Fe(Te,Se) van der Waals Josephson junctions", Nat. Commun. 14, 6691 (2023)   

Terahertz Time-Domain Spectroscopy in FeSe thin films

FeSe is one of the simplest crystal structures of iron-based superconductors, and exhibits remarkable control of its critical temperature, as well as displays competition of magnetic and superconducting order in its ground state. In addition to possible applications, the system offers great potential for experiments in the area of strongly correlated systems and understanding of unconventional superconductivity. 

Previous studies have revealed a dependence of Tc on pressure, with compressive strain increasing Tc up to 37 K from the bulk value of 8 K. In addition, tensile strain has been shown to decrease Tc in this compound, and completely suppress it down to Tc = 200 mK.

In this study, we use broadband terahertz time-domain spectroscopy to probe the superconductivity of FeSe thin films grown by pulsed laser deposition on MgO, and characterized by X-ray diffraction and photoelectron spectroscopy with depth profiling. In our measurements, we evaluate the electrical conductivity down to T = 200 mK. Our work may provide insight into the viability of this and similar compounds for optical study in low temperature environments.   

Photoinduced stress, electron-phonon coupling, and nematicity dynamics in FeSe0.8Te0.2

The transient nematicity, coherent phonon, and quasiparticle dynamics were explored for FeSe_0.8Te_0.2 superconducting epitaxial films. A nematicity in coherent phonon dynamics was resolved by applying an ultrafast pump-probe polarization optical technique. This dynamic is inherent for FeSe_0.8Te_0.2 near and below Tc, and disappears as soon as the material switches to a normal state. The superconducting state persists during electron-electron thermalization within several hundreds of femtoseconds and further decays into a normal state during ~1 ps via electron-phonon scattering.  By applying a two-temperature model, we modeled processes of electron-electron and electron-phonon scattering, and calculated photoinduced stress in the film. Derived coherent phonon dynamics show a dependence of electron-phonon coupling on the stress in the film. To resolve structural changes in optical signal, we applied the 3D angle-resolved light scattering method, including the diffraction conoscopy technique. Here we retrieved surface autocorrelation, surface roughness spectral power density, and lattice distortion in the region of superconducting transition. These results show good agreement with observed photoinduced transient dynamics at corresponding temperatures. Analysis is performed with additional DFT calculations of the phonon and electron density of states. Our findings hint at novel underlying mechanisms governing the photoinduced relaxation dynamics in FeSe_1-xTe_x superconductors.