Nematic quantum critical point without magnetism in FeSe _{1− x} S _x superconductors

Abstract

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 FeSe 1−x S x 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 FeSe 1−x S x as a unique nonmagnetic system suitable for studying the impact of nematicity on superconductivity.electronic nematicity | iron-based superconductors | nematic susceptibility | unconventional superconductivity | quantum critical point T he prime candidate for the unconventional mechanism of superconductivity in many strongly correlated electron systems including cuprate, iron-based, and heavy-fermion superconductors is based on magnetic fluctuations (1-4). In these materials, domeshaped superconducting phases appear in the vicinity of end point of the antiferromagnetic (AFM) order, where spin fluctuations are strongly enhanced. Recently, however, other competing or coexisting orders that break rotational symmetry of the system have been frequently found in these materials (5-8), and the importance of fluctuations of these orders on superconducting pairing has been suggested theoretically (9-12).In underdoped cuprate superconductors, unidirectional electronic correlations (stripe correlations) appear in the pseudogap state, whose relation with superconductivity is a center of debate. It has become more complicated after the charge density wave (CDW) order has been observed in a portion of this pseudogap region of the phase diagram (5). In iron pnictides, the tetragonal-toorthorhombic structural transition always precedes or coincides with the AFM transition (3). Below the structural transition temperature T s , electronic nematicity that represents a large electronic anisotropy breaking the C 4 rotational symmetry, is observed (7), which may have a similar aspect with the stripe correlations in underdoped cuprates. In both cases, however, the nematicity is largely coexisting and intertwined with other CDW and AFM orders. Large nematic fluctuations have been experimentally observed in BaFe 2 As 2 systems above T s , and these nematic fluctuati...

Nematic Quantum Critical Fluctuations inBaFe2−xNixAs2Liu1, Gu2, Zhang3 et al. 2016Phys. Rev. Lett.476751View full textAdd to dashboardCite

“…3 is that ζ (100) becomes maximum right at the nematic QCP, which clearly suggests that it is directly associated with quantum critical fluctuations. In a recent elastoresistance study on the FeSe 1−x S x system [9], the nematic signal along the (110) direction is found to be largest at the nematic QCP, which is different from the Co-doped Ba-122 system [3] and our results. Since an obvious difference between FeSe 1−x S x and Ba-122 systems is the lack of AF order in the former, the large thermal critical fluctuations present in the underdoped Ba-122 samples may be attributed to the fluctuations of striped AF order [28][29][30][31], which gives the same rotational symmetry breaking as the nematic order.…”

“…Xa, The normal-state electronic states in many iron-based superconductors show strong in-plane anisotropic properties that break the fourfold rotational symmetry of the lattice due to the presence of nematic order [1][2][3][4][5][6][7][8][9]. The nematic order is typically accompanied by a structural transition at T s following an antiferromagnetic (AF) transition at T N ≤ T s , except for FeSe where no AF order is found [10,11].…”

“…It is shown that the nematic order may go through a zero-temperature order-to-disorder quantum phase transition as shown by elastoresistance [3,9], elastic constants [4,5], and Raman scattering [7,8]. The nematic susceptibility shows divergent behavior with the form of (1/T ) γ around optimal doping [3,5,[7][8][9]20].…”

Comparative Review on Thin Film Growth of Iron-Based Superconductors

“…The S-substitution for Se introduces chemical pressure in FeSe, and T s gradually decreases with increasing S content x [219]. The structural/nematic transition is completely suppressed at x = 0.17, where the nematic fluctuations were found to diverge toward T → 0 K from the measurements of nematic susceptibility [219]. This result shows that the nematic quantum critical point (QCP) exists around x = 0.17 [219].…”

“…In bulk single crystals, the detailed phase diagram of FeSe under hydrostatic pressures [216][217][218] and FeSe 1−x S x [219,220] have been reported because the high-quality single crystals of FeSe 1−x S x and FeSe can be available. The S-substitution for Se introduces chemical pressure in FeSe, and T s gradually decreases with increasing S content x [219]. The structural/nematic transition is completely suppressed at x = 0.17, where the nematic fluctuations were found to diverge toward T → 0 K from the measurements of nematic susceptibility [219].…”