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Register a new userSingle-gap superconductivity and dome of superfluid density in Nb-doped SrTiO3
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SrTiO$_3$ exhibits a superconducting dome upon doping with Nb, with a maximum critical temperature mbox{$T_mathrm{c} approx 0.4$~K}. Using microwave stripline resonators at frequencies from 2 to 23~GHz and temperatures down to 0.02~K, we probe the low-energy optical response of superconducting SrTiO$_3$ with charge carrier concentration from 0.3 to $2.2times 10^{20}$~cm$^{-3}$, covering the majority of the superconducting dome. We find single-gap electrodynamics even though several electronic bands are superconducting. This is explained by a single energy gap $2Delta$ due to gap homogenization over the Fermi surface consistent with the low level of defect scattering in Nb-doped SrTiO$_3$. Furthermore, we determine $T_mathrm{c}$, $2Delta$, and the superfluid density as a function of charge carrier concentration, and all three quantities exhibit the characteristic dome shape.
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SrTiO$_3$ is a unique example of a system which exhibits both quantum paraelectricity and superconductivity. Thus, it is expected that the superconducting state is closely related to the intrinsic ferroelectric instability. Indeed, recent experiments suggest existence of a coexistent phase of superconductivity and ferroelectricity in Ca-substituted SrTiO$_3$. In this paper, we propose that SrTiO$_3$ can be a platform of the ferroelectric superconductivity, which is characterized by a ferroelectric transition in the superconducting state. By analyzing a multiorbital model for $t_{2g}$ electrons, we show that the ferroelectric superconductivity is stabilized through two different mechanisms which rely on the presence of the spin-orbit coupling. First, the ferroelectric superconducting state is stabilized in the dilute carrier density regime due to a ferroelectricity-induced Lifshitz transition. Second, it is stabilized under a magnetic field independent of the carrier density. The importance of the multiorbital or multiband nature for the ferroelectric superconductivity is clarified. Then, we predict a topological Weyl superconducting state in the ferroelectric superconducting phase of SrTiO$_3$.
The soft ferro-electric phonon in SrTiO3 observed with optical spectroscopy has an extraordinary strong spectral weight which is much stronger than expected in the limit of a perfectly ionic compound. The charged phonon in SrTiO3 is caused by the close-to-covalent character of the Ti-O ionic bond and implies a strong coupling between the soft ferro-electric phonon and the inter band transitions across the 3 eV gap of SrTiO3. We demonstrate that this coupling leads, in addition to the charged phonon effect, to a pairing interaction involving the exchange of two transverse optical phonons. This process owes its relevance to the strong electron-phonon coupling and to the fact that the interaction mediated by a single transverse optical phonon vanishes at low electron density. We use the experimental soft phonon spectral weight to calculate the strength of the bi-phonon mediated pairing interaction in the electron doped material and show that it is of the correct magnitude when compared to the experimental value of the superconducting critical temperature.
We synthesized a series of Nb$_{2}$Pd$_{1-x}$Ru$_{x}$S$_{5}$ polycrystalline samples by a solid-state reaction method and systematically investigated the Ru-doping effect on superconductivity by transport and magnetic measurements. It is found that superconductivity is enhanced with Ru doping and is quite robust upon disorder. Hall coefficient measurements indicate that the charge transport is dominated by hole-type charge carriers similar to the case of Ir doping, suggesting multi-band superconductivity. Upon Ru or Ir doping, emph H$_{c2}$/$emph T_c$ exhibits a significant enhancement, exceeding the Pauli paramagnetic limit value by a factor of approximately 4. A comparison of $T_c$ and the upper critical field ($H_{c2}$) amongst the different doping elements on Pd site, reveals a significant role of spin--orbit coupling.
The search for oxide-based room-temperature ferromagnetism has been one of the holy grails in condensed matter physics. Room-temperature ferromagnetism observed in Nb-doped SrTiO3 single crystals is reported in this Rapid Communication. The ferromagnetism can be eliminated by air annealing (making the samples predominantly diamagnetic) and can be recovered by subsequent vacuum annealing. The temperature dependence of magnetic moment resembles the temperature dependence of carrier density, indicating that the magnetism is closely related to the free carriers. Our results suggest that the ferromagnetism is induced by oxygen vacancies. In addition, hysteretic magnetoresistance was observed for magnetic field parallel to current, indicating that the magnetic moments are in the plane of the samples. The x-ray photoemission spectroscopy, the static time-of-flight and the dynamic secondary ion mass spectroscopy and proton induced x-ray emission measurements were performed to examine magnetic impurities, showing that the observed ferromagnetism is unlikely due to any magnetic contaminant.
The interface between the insulating oxides LaAlO3 and SrTiO3 exhibits a superconducting two-dimensional electron system that can be modulated by a gate voltage. While gating of the conductivity has been probed extensively and gating of the superconducting critical temperature has been demonstrated, the question whether, and if so how, the gate tunes the superfluid density and superconducting order parameter is unanswered. We present local magnetic susceptibility, related to the superfluid density, as a function of temperature, gate voltage and location. We show that the temperature dependence of the superfluid density at different gate voltages collapse to a single curve characteristic of a full superconducting gap. Further, we show that the dipole moments observed in this system are not modulated by the gate voltage.
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