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Register a new userVisualization of local magnetic moments emerging from impurities in the Hunds metal states of FeSe
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Understanding the origin of the magnetism of high temperature superconductors is crucial for establishing their unconventional pairing mechanism. Recently, theory predicts that FeSe is close to a magnetic quantum critical point, and thus weak perturbations such as impurities could induce local magnetic moments. To elucidate such quantum instability, we have employed scanning tunneling microscopy and spectroscopy. In particular, we have grown FeSe film on superconducting Pb(111) using molecular beam epitaxy and investigated magnetic excitation caused by impurities in the proximity-induced superconducting gap of FeSe. Our study provides a deep insight into the origin of the magnetic ordering of FeSe by showing the way local magnetic moments develop in response to impurities near the magnetic quantum critical point.
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We investigate properties of a spin-1 Heisenberg model with extended and biquadratic interactions, which captures crucial aspects of the low energy physics in FeSe. While we show that the model exhibits a rich phase diagram with four different magnetic ordering tendencies, we identify a parameter regime with strong competition between N`eel, staggered dimer, and stripe-like magnetic fluctuations, accounting for the physical properties of FeSe. Through the comparison of numerically evaluated spin and Raman response with experiments, we find evidence for enhanced magnetic frustration between N`eel and co-linear stripe ordering tendencies, which increases with increasing temperature. The explanation of these spectral behaviors with this frustrated spin model supports the idea of local spin interactions in FeSe.
FeSe is classed as a Hunds metal, with a multiplicity of $d$ bands near the Fermi level. Correlations in Hunds metals mostly originate from the exchange parameter emph{J}, which can drive a strong orbital selectivity in the correlations. The Fe-chalcogens are the most strongly correlated of the Fe-based superconductors, with $d_{xy}$ the most correlated orbital. Yet little is understood whether and how such correlations directly affect the superconducting instability in Hunds systems. By applying a recently developed high-fidelity emph{ab initio} theory, we show explicitly the connections between correlations in $d_{xy}$ and the superconducting critical temperature $T_{c}$. Starting from the emph{ab initio} results as a reference, we consider various kinds of excursions in parameter space around the reference to determine what controls $T_{c}$. We show small excursions in $J$ can cause colossal changes in $T_{c}$. Additionally we consider changes in hopping by varying the Fe-Se bond length in bulk, in the free standing monolayer M-FeSe, and M-FeSe on a SrTiO$_{3}$ substrate (M-FeSe/STO). The twin conditions of proximity of the $d_{xy}$ state to the Fermi energy, and the strength of $J$ emerge as the primary criteria for incoherent spectral response and enhanced single- and two-particle scattering that in turn controls $T_{c}$. Using constrained RPA, we show further that FeSe in monolayer form (M-FeSe) provides a natural mechanism to enhance $J$. We explain why M-FeSe/STO has a high $T_{c}$, whereas M-FeSe in isolation should not. Our study opens a paradigm for a unified understanding what controls $T_{c}$ in bulk, layers, and interfaces of Hunds metals by hole pocket and electron screening cloud engineering.
The origin of the pseudogap behavior, found in many high-$T_c$ superconductors, remains one of the greatest puzzles in condensed matter physics. One possible mechanism is fermionic incoherence, which near a quantum critical point allows pair formation but suppresses superconductivity. Employing quantum Monte Carlo simulations of a model of itinerant fermions coupled to ferromagnetic spin fluctuations, represented by a quantum rotor, we report numerical evidence of pseudogap behavior, emerging from pairing fluctuations in a quantum-critical non-Fermi liquid. Specifically, we observe enhanced pairing fluctuations and a partial gap opening in the fermionic spectrum. However, the system remains non-superconducting until reaching a much lower temperature. In the pseudogap regime the system displays a gap-filling rather than gap-closing behavior, consistent with experimental observations. Our results provide the first unambiguous lattice model realization of a pseudogap state in a strongly correlated system, driven by superconducting fluctuations.
We investigate the influence of itinerant carriers on dynamics and fluctuation of local moments in Fe-based superconductors, via linear spin-wave analysis of a spin-fermion model containing both itinerant and local degrees of freedom. Surprisingly against the common lore, instead of enhancing the ($pi$,0) order, itinerant carriers with well nested Fermi surfaces is found to induce significant amount of textit{spatial} and temporal quantum fluctuation that leads to the observed small ordered moment. Interestingly, the underlying mechanism is shown to be intra-pocket nesting-associated long-range coupling, rather than the previously believed ferromagnetic double-exchange effect. This challenges the validity of ferromagnetically compensated first-neighbor coupling reported from short-range fitting to the experimental dispersion, which turns out to result instead from the ferro-orbital order that is also found instrumental in stabilizing the magnetic order.
Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field ($H_{c2}$) of the iron-based superconductor FeSe ($T_c$ = 8.6 K) have been studied by tunnel diode oscillator-based measurements in magnetic fields of up to 55 T and temperatures down to 1.6 K. Several Fourier components enter the SdH oscillations spectrum with frequencies definitely smaller than predicted by band structure calculations indicating band renormalization and reconstruction of the Fermi surface at low temperature, in line with previous ARPES data. The Werthamer-Helfand-Hohenberg model accounts for the temperature dependence of $H_{c2}$ for magnetic field applied both parallel (textbf{H} $|$ $ab$) and perpendicular (textbf{H} $|$ $c$) to the iron conducting plane, suggesting that one band mainly controls the superconducting properties in magnetic fields despite the multiband nature of the Fermi surface. Whereas Pauli pair breaking is negligible for textbf{H} $|$ $c$, a Pauli paramagnetic contribution is evidenced for textbf{H} $|$ $ab$ with Maki parameter $alpha$ = 2.1, corresponding to Pauli field $H_{P}$ = 36.5 T
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