Abstract
Oxygen vacancy is known to play an important role for the physical properties in SrTiO3(STO)-based systems. On the surface, rich structural reconstructions had been reported owing to the oxygen vacancies, giving rise to metallic surface states and unusual surface phonon modes. More recently, an intriguing phenomenon of a huge superconducting transition temperature enhancement was discovered in a monolayer FeSe on STO substrate, where the surface reconstructed STO (SR-STO) may play a role. In this work, SR-STO substrates were prepared via thermal annealing in ultra-high vacuum followed by low energy electron diffraction analyses on surface structures. Thin Nb films with different thicknesses (d) were then deposited on the SR-STO. The detailed studies of the magnetotransport and superconducting property in the Al(1 nm)/Nb(d)/SR-STO samples revealed a large positive magnetoresistance and a pronounced resistance peak near the onset of the resistive superconducting transition in the presence of an in-plane field. Remarkably, the amplitude of the resistance peak increases with increasing fields, reaching a value of nearly 57% of the normal state resistance at 9 T. Such resistance peaks were absent in the control samples of Al(1 nm)/Nb(d)/STO and Al(1 nm)/Nb(d)/SiO2. Combining with DFT calculations for SR-STO, we attribute the resistance peak to the interface resistance from the proximity coupling of the superconducting niobium to the field-enhanced long-range magnetic order in SR-STO that arises from the spin-polarized in-gap states due to oxygen vacancies.
Highlights
One intriguing factor governing the physical property in an oxide system is the oxygen content
Oxygen vacancy and charge transfer from STO substrate are potential factors[14], and more experimental efforts are keenly required. In light of this quest, we study the influence of the superconducting property in thin niobium (Nb) films deposited on surface reconstructed STO (SR-STO) substrates
The low energy electron diffraction (LEED) pattern transformed from a simple cubic structure of major diffraction spots (Fig. 1a) into a much more complex pattern with additional secondary diffraction spots that come from the surface structural reconstruction effect
Summary
One intriguing factor governing the physical property in an oxide system is the oxygen content. It is known that oxygen vacancies in SrTiO3 (STO) serve as electron donors[4], which turns the STO from a wide band-gap insulator into an unusual dilute superconductor[5]. As it turns out, the influence of the oxygen vacancies to STO can be multifold according to some recent investigations[6,7,8]. Oxygen vacancies locally break the d-orbital symmetry of titanium ions, giving rise to unusual ingap states and possible magnetism Such in-gap states in oxygendeficient STO have been observed in angle resolved photoemission spectroscopy and scanning tunneling spectroscopy measurements[9,10].
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