Kohei Matsuura

Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe

The coexistence and competition between superconductivity and electronic orders, such as spin or charge density waves, have been a central issue in high transition-temperature (Tc) superconductors. Unlike other iron-based superconductors, FeSe exhibits nematic ordering without magnetism whose relationship with its superconductivity remains unclear. Moreover, a pressure-induced fourfold increase of Tc has been reported, which poses a profound mystery. Here we report high-pressure magnetotransport measurements in FeSe up to ∼15 GPa, which uncover the dome shape of magnetic phase superseding the nematic order. Above ∼6 GPa the sudden enhancement of superconductivity (Tc≤38.3 K) accompanies a suppression of magnetic order, demonstrating their competing nature with very similar energy scales. Above the magnetic dome, we find anomalous transport properties suggesting a possible pseudogap formation, whereas linear-in-temperature resistivity is observed in the normal states of the high-Tc phase above 6 GPa. The obtained phase diagram highlights unique features of FeSe among iron-based superconductors, but bears some resemblance to that of high-Tc cuprates.

Nematic quantum critical point without magnetism in FeSe1−xSx superconductors

Significance The electronic nematic order that spontaneously breaks rotational symmetry of the system is perhaps one of the most surprising states of matter. A key issue is the relationship between the fluctuations of such nematic order and high-temperature superconductivity in cuprates and iron pnictides. However, because of coexisting antiferromagnetic or charge density wave orders, it is difficult to pinpoint the impact of nematic fluctuations on superconductivity. Here we report a quantum critical point (QCP) of pure nematic order without accompanying other orders in FeSe1−xSx superconductors. We find that the nematic fluctuations are divergently enhanced at the nematic QCP. This discovery opens up a new avenue to study the unconventional superconductivity mediated by exotic mechanisms different from the well-studied spin fluctuations. In most unconventional superconductors, the importance of antiferromagnetic fluctuations is widely acknowledged. In addition, cuprate and iron-pnictide high-temperature superconductors often exhibit unidirectional (nematic) electronic correlations, including stripe and orbital orders, whose fluctuations may also play a key role for electron pairing. In these materials, however, such nematic correlations are intertwined with antiferromagnetic or charge orders, preventing the identification of the essential role of nematic fluctuations. This calls for new materials having only nematicity without competing or coexisting orders. Here we report systematic elastoresistance measurements in FeSe1−xSx superconductors, which, unlike other iron-based families, exhibit an electronic nematic order without accompanying antiferromagnetic order. We find that the nematic transition temperature decreases with sulfur content x; whereas, the nematic fluctuations are strongly enhanced. Near x≈0.17, the nematic susceptibility diverges toward absolute zero, revealing a nematic quantum critical point. The obtained phase diagram for the nematic and superconducting states highlights FeSe1−xSx as a unique nonmagnetic system suitable for studying the impact of nematicity on superconductivity.

Maximizing T c by tuning nematicity and magnetism in FeSe 1−x S x superconductors

A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature (Tc). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving non-magnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains perplexing. Here we construct the three-dimensional electronic phase diagram, temperature (T) against pressure (P) and isovalent S-substitution (x), for FeSe1−xSx. By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended non-magnetic tetragonal phase emerges, where Tc shows a striking enhancement. The completed phase diagram uncovers that high-Tc superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas Tc remains low near the nematic critical point.The overlap between different phases has hindered the understanding of how each phase affects superconductivity in FeSe. Here, Matsuura et al. achieve a complete separation of non-magnetic nematic and antiferromagnetic phases for FeSe1-xSx, observing a tetragonal phase in between with a strikingly enhanced Tc.

High- Tc Superconductivity in FeSe at High Pressure: Dominant Hole Carriers and Enhanced Spin Fluctuations

The importance of electron-hole interband interactions is widely acknowledged for iron-pnictide superconductors with high transition temperatures (T_{c}). However, the absence of hole pockets near the Fermi level of the iron-selenide (FeSe) derived high-T_{c} superconductors raises a fundamental question of whether iron pnictides and chalcogenides have different pairing mechanisms. Here, we study the properties of electronic structure in the high-T_{c} phase induced by pressure in bulk FeSe from magnetotransport measurements and first-principles calculations. With increasing pressure, the low-T_{c} superconducting phase transforms into the high-T_{c} phase, where we find the normal-state Hall resistivity changes sign from negative to positive, demonstrating dominant hole carriers in contrast to other FeSe-derived high-T_{c} systems. Moreover, the Hall coefficient is enlarged and the magnetoresistance exhibits anomalous scaling behaviors, evidencing strongly enhanced interband spin fluctuations in the high-T_{c} phase. These results in FeSe highlight similarities with high-T_{c} phases of iron pnictides, constituting a step toward a unified understanding of iron-based superconductivity.

Weakening of the diamagnetic shielding in FeSe1−xSx at high pressures

The superconducting transition of FeSe

Magnetic fluctuations under pressure on S-doped FeSe studied via 77Se NMR

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Two different superconducting states and possible antiferromagnetic quantum critical points in S-doped FeSe under pressure

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Anomalous magnetization of the chiral antiferromagnet Mn 3 Sn

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High-pressure electronic phase diagrams in FeSe

with three distinct sulphur concentrations

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