Superconductivity in the vicinity of a ferroelectric quantum phase transition (original) (raw)
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Low-density superconductivity in SrTiO$_3$ bounded by the adiabatic criterion
arXiv (Cornell University), 2021
SrTiO 3 exhibits superconductivity for carrier densities 10 19 − 10 21 cm −3 .Across this range, the Fermi level traverses a number of vibrational modes in the system, making it ideal for studying dilute superconductivity. We use high-resolution planar-tunneling spectroscopy to probe chemically-doped SrTiO 3 across the superconducting dome. The over-doped superconducting boundary aligns, with surprising precision, to the Fermi energy crossing the Debye energy. Superconductivity emerges with decreasing density, maintaining throughout the Bardeen-Cooper-Schrieffer (BCS) gap to transition-temperature ratio, despite being in the anti-adiabatic regime. At lowest superconducting densities, the lone remaining adiabatic phonon van Hove singularity is the soft transverse-optic mode, associated with the ferroelectric instability. We suggest a scenario for pairing mediated by this mode in the presence of spin-orbit coupling, which naturally accounts for the superconducting dome and BCS ratio.
Superconductivity mediated by polar modes in ferroelectric metals
Nature Communications, 2020
The occurrence of superconductivity in doped SrTiO 3 at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades. We report experimental results showing the pressure dependence of the super-conducting transition temperature, T c , near to optimal doping that sheds light on the nature of this interaction. We find that T c increases dramatically when the energy gap of the ferro-electric critical modes is suppressed, i.e., as the ferroelectric quantum critical point is approached in a way reminiscent to behaviour observed in magnetic counterparts. However, in contrast to the latter, the coupling of the carriers to the critical modes in ferroelectrics is predicted to be small. We present a quantitative model involving the dynamical screening of the Coulomb interaction and show that an enhancement of T c near to a ferroelectric quantum critical point can arise due to the virtual exchange of longitudinal hybrid-polar-modes, even in the absence of a strong coupling to the transverse critical modes.
Quantum Critical Origin of the Superconducting Dome in SrTiO_{3}
Physical review letters, 2015
We expand the well-known notion that quantum criticality can induce superconductivity by proposing a concrete mechanism for superconductivity due to quantum ferroelectric fluctuations. To this end, we investigate the origin of superconductivity in doped SrTiO_{3} using a combination of density functional and strong coupling theories within the framework of quantum criticality. Our density functional calculations of the ferroelectric soft mode frequency as a function of doping reveal a crossover related to quantum paraelectricity at a doping level coincident with the experimentally observed top of the superconducting dome. Thus, we suggest a model in which the soft mode fluctuations provide the pairing interaction for superconductivity carriers. Within our model, the low doping limit of the superconducting dome is explained by the emergence of the Fermi surface, and the high doping limit by departure from the quantum critical regime. We predict that the highest critical temperature w...
High TcT_cTc superconductivity at the FeSe/SrTiO$_3$ Interface
2012
In a recent experiment the superconducting gap of a single unit cell thick FeSe film on SrTiO$_3$ substrate is observed by scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. The value of the superconducting gap is much larger than that of the bulk FeSe under ambient pressure. In this paper we study the effects of screening due to the ferroelectric phonons on Cooper pairing. We conclude it can significantly enhance the energy scale of Cooper pairing and even change the pairing symmetry. Our results also raise some concerns on whether phonons can be completely ignored for bulk iron-based superconductors.
Strain and ferroelectric soft-mode induced superconductivity in strontium titanate
Physical Review B, 2018
We investigate the effects of strain on superconductivity with particular reference to SrTiO3. Assuming that a ferroelectric mode that softens under tensile strain is responsible for the coupling, an increase in the critical temperature and range of carrier densities for superconductivity is predicted, while the peak of the superconducting dome shifts towards lower carrier densities. Using a Ginzburg-Landau approach in 2D, we find a linear dependence of the critical temperature on strain: if the couplings between the order parameter and strains in different directions differ while their sum is fixed, different behaviours under uniaxial and biaxial strain can be understood.
Ferroelectricity, Superconductivity, and SrTiO3—Passions of K.A. Müller
Condensed Matter
SrTiO3 is an insulating material which, using chemical doping, pressure, strain or isotope substitution, can be turned into a ferroelectric material or into a superconductor. The material itself, and the two aforementioned phenomena, have been subjects of intensive research of Karl Alex Müller and have been a source of inspiration, among other things, for his Nobel prize-winning research on high temperature superconductivity. An intriguing outstanding question is whether the occurrence of ferroelectricity and superconductivity in the same material is just a coincidence, or whether a deeper connection exists. In addition there is the empirical question of how these two phenomena interact with each other. Here we show that it is possible to induce superconductivity in a two-dimensional layer at the interface of SrTiO3 and LaAlO3 when we make the SrTiO3 ferroelectric by means of 18O substitution. Our experiments indicate that the ferroelectricity is perfectly compatible with having a s...
2012
To gain insight into the high-T c mechanism, we need experiments which identify the parameters that determine T c and link them to the interaction(s) that establish the superconducting state. Here we show that for pnictides, cuprates, and bismuthates T c depends systematically on the primary electron energy relaxation rate 1/τ 1 . We find that 1/τ 1 is a direct experimental measure of the strength of the electron-phonon interaction (EPI) and correlates with structural parameters, in particular the length of the crystallographic a-axis. T c (1/τ 1 ) is a non-monotonic function with the maximum at intermediate relaxation rates (~16 ps -1 ), suggesting that EPI provides the attractive interaction for the high-T c pairing mechanism, where the highest T c occurs in the crossover region between weak and strong EPI.
Role of phonons in the mechanism of high-temperature superconductivity
Physica C: Superconductivity, 2004
While phonons are generally considered to be irrelevant to the mechanism of high-temperature superconductivity, a large number of experimental results indicate otherwise. We argue that the electron-phonon coupling in strongly correlated electron systems is unconventional, and results in a strong spin-charge-phonon coupling. It could contribute significantly to the superconductivity of the cuprates through the synergetic spin-phonon mechanism.
Materials' specific theory expounding high-temperature superconductivity in monolayer FeSe/SrTiO$_3$
arXiv: Superconductivity, 2017
Superconductivity in monolayer-thick FeSe on SrTiO$_3$ reaches amazingly high transition temperatures of typically T$_{\rm c}$=50-70 K [1-6] and up to 100 K [7], much higher than the 8 K value of bulk FeSe[8]. A coupling between SrTiO$_3$ phonons and FeSe electrons occurs at the FeSe/SrTiO$_3$ interface, which manifests itself as electron replica bands [6]. This coupling is commonly believed to enhance T$_{\rm c}$ moderately but not be enough to explain it [6,9,10]. Here, we examine the impact of interfacial phonons on the superconducting state of FeSe/SrTiO$_3$ developing full bandwidth, multiband Eliashberg theory. Our selfconsistent calculations prove that the interfacial electron-phonon interaction which is hidden behind a seemingly weak coupling constant, lambdam\lambda_mlambdam=0.4 [6,10], fully explains the high-T$_{\rm c}$ as well as puzzling experimental observations like the s-wave symmetry and replica bands. We find that deep Fermi-sea Cooper pairing has a T$_{\rm c}$ enhancing effec...