arXiv subjects

Zhuozhi Ge

Publications and source records attributed to Zhuozhi Ge.

4 recordsLinked to original sources

arXiv2025

Unraveling Enhanced Superconductivity in Single-layer FeSe through Substrate Surface Terminations

Single-layer FeSe films grown on (001) SrTiO3 substrates have shown a significant increase in superconducting transition temperature compared to bulk FeSe. Several mechanisms have been proposed to explain such enhancement, including electron doping, interfacial electron-phonon coupling, and strong electron correlations. To pinpoint the primary driver, we grew FeSe films on SrTiO3 substrates with coexisting TiO2 and SrO surface terminations. Scanning tunneling spectroscopy revealed a larger superconducting gap of 17 meV for FeSe on TiO2 compared to 11 meV on SrO. Tunneling spectroscopy also showed a larger work function on SrO, leading to reduced charge transfer, as confirmed by angle-resolved photoemission spectroscopy. Scanning transmission electron microscopy revealed distinctive interfacial atomic-scale structures, with the Se-Fe-Se tetrahedral angle changing from 109.9{\deg} on SrO to 105.1{\deg} on TiO2. Compared to dynamical mean field theory calculations, these results suggest optimal electron correlations in FeSe/TiO2 for enhancing high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2019

Evidence for d-wave superconductivity in single layer FeSe/SrTiO3 probed by quasiparticle scattering off step edges

The de Gennes extrapolation length is a direction dependent measure of the spatial evolution of the pairing gap near the boundary of a superconductor, and thus provides a viable means to probe its symmetry. It is expected to be infinite and isotropic for plain s-wave pairing, and finite and anisotropic for d-wave. Here, we synthesize single layer FeSe films on SrTiO3(001) (STO) substrates by molecular beam epitaxy, and measure the de Gennes extrapolation length by scanning tunneling microscopy/spectroscopy. We find a 40% reduction of the superconducting gap near specular [110]Fe edges, yielding an extrapolation length of 8.0 nm. However, near specular [010]Fe edges the extrapolation length is nearly infinite. These findings are consistent with a phase changing pairing with 2-fold symmetry, indicating d-wave superconductivity. This is further supported by the presence of in-gap states near the specular [110]Fe edges, but not the [010]Fe edges. This work provides direct experimental evidence for d-wave superconductivity in single layer FeSe/STO, and demonstrates quasiparticle scattering at boundaries to be a viable phase sensitive probe of pairing symmetry in Fe-based superconductors.

cond-mat.supr-con↗

arXiv2018

Intrinsic interfacial van der Waals monolayers and their effect on the high-temperature superconductor FeSe/SrTiO$_3$

The sensitive dependence of monolayer materials on their environment often gives rise to unexpected properties. It was recently demonstrated that monolayer FeSe on a SrTiO$_3$ substrate exhibits a much higher superconducting critical temperature T$_C$ than the bulk material. Here, we examine the interfacial structure of FeSe / SrTiO$_3$ and the effect of an interfacial Ti$_{1+x}$O$_2$ layer on the increased T$_C$ using a combination of scanning transmission electron microscopy and density functional theory. We find Ti$_{1+x}$O$_2$ forms its own quasi-two-dimensional layer, bonding to both the substrate and the FeSe film by van der Waals interactions. The excess Ti in this layer electron-dopes the FeSe monolayer in agreement with experimental observations. Moreover, the interfacial layer introduces symmetry-breaking distortions in the FeSe film that favor a T$_C$ increase. These results suggest that this common substrate may be functionalized to modify the electronic structure of a variety of thin films and monolayers.

cond-mat.mtrl-sci↗

arXiv2017

Effects of interface oxygen vacancies on electronic bands of FeSe/SrTiO3(001)

Modifications of the electronic bands of thin FeSe films due to oxygen vacancies in the supporting SrTiO 3 (001) substrate - and the interplay with spin-orbit coupling, magnetism, and epitaxy - are investigated by first-principles supercell calculations. Unfolded (k-projected) bands show that the oxygen vacancies both provide electron doping to the interface FeSe layer and also have notable effects on the details of the bands around the Fermi level, including renormalizing the width of the Fe-3d band near the Fermi level by a factor of about 0.6, and causing a splitting of ~40 meV at the M point for the checkerboard antiferromagnetic configuration. For an FeSe bilayer, the modifications to the bands are mainly limited to the interface FeSe layer. While spin-orbit-coupling induced band splittings of ~30 meV at M for the ideal FeSe/SrTiO3 (001) interfaces are comparable to the splitting due to oxygen vacancies, the effects are not simply additive. Calculations and comparison to our scanning tunneling microscopy images of MBE-grown FeSe films on SrTiO3 (001) suggest that a common defect may be Se bound to an oxygen vacancy at the interface

cond-mat.mtrl-sci↗