Origin of the Tetragonal-to-Orthorhombic Phase Transition in FeSe: A Combined Thermodynamic and NMR Study of Nematicity

Abstract

The nature of the tetragonal-to-orthorhombic structural transition at Ts ≈ 90 K in single crystalline FeSe is studied using shear-modulus, heat-capacity, magnetization and NMR measurements. The transition is shown to be accompanied by a large shear-modulus softening, which is practically identical to that of underdoped Ba(Fe,Co)2As2, suggesting very similar strength of the electronlattice coupling. On the other hand, a spin-fluctuation contribution to the spin-lattice relaxation rate is only observed below Ts. This indicates that the structural, or "nematic", phase transition in FeSe is not driven by magnetic fluctuations.PACS numbers: 74.70. Xa, 74.25.Bt, 74.25.Ld, 74.25.nj One of the most intriguing questions in the study of iron-based superconductors concerns the relation between structure, magnetism and superconductivity [1][2][3][4][5][6][7][8][9][10]. Stripe-type antiferromagnetic order often occurs at the same or at a slightly lower temperature than the tetragonal-to-orthorhombic structural distortion and the two types of order are closely related by symmetry. They break the four-fold rotational symmetry of the high-temperature phase, which can be associated with a nematic degree of freedom [4,6]. Superconductivity typically is strongest around the point where the structural transition (T s ) and the antiferromagnetic transition (T N ) are suppressed by pressure or chemical substitution. Whether the magnetic or the structural instability is the primary one, is still under intense debate [10], also because of its relevance to the pairing mechanism [5,6]. Recently, scaling relations between the shear modulus related to the structural distortion, C 66 , and the spinlattice relaxation time T 1 as a measure of the strength of spin fluctuations, have been proposed [7,8] in order to address the above question. They were found to be well satisfied in the Ba(Fe,Co) 2 As 2 system [7,8], where T s and T N are in close proximity to each other, suggesting a magnetically-driven structural transition [7]. Clearly, it is of great interest to see if a relation between shear modulus and spin fluctuations is universally observed in other iron-based materials.FeSe is structurally the simplest iron-based superconductor and has attracted a lot of attention because of a nearly four-fold increase of its T c ≈ 8 K under pressure [11]. Moreover, this system is particularly interesting with respect to the relation of structure and magnetism, since it undergoes a tetragonal-to-orthorhombic structural phase transition at T s ∼ 90 K, similar to that found in the 1111-and 122-type parent compounds [2], but does not order magnetically at ambient pressure [12,13]. Spin fluctuations at low temperatures were, however, observed in nuclear magnetic resonance (NMR) measurements [14]. Surprisingly, the orthorhombic distortion of FeSe is not reduced upon entering the superconducting state [9] in strong contrast to underdoped BaFe 2 As 2 [3,15], indicating different couplings between structure and superconductivity. This strongly motivates fu...

Fermi Surface, Pressure-Induced Antiferromagnetic Order, and Superconductivity in FeSe

“…It is recognized that weak spin susceptibility and χ s xy ≪ χ s zx are satisfied within the RPA 26) when the ab initio tight-binding model is adjusted to reproduce ARPES and QO results, and the spinlattice relaxation rate is weak above T s , consistent with experiments. 24,25) From our results, the weakness of the spin fluctuation is confirmed from a self-energy correction.…”

“…The absence of a hole Fermi surface composed of the xy orbital is in agreement with experiments. 27,28) The experimental results of the NMR spin-relaxation rate, 24,25) in which the spin fluctuation is very weak above the structural transition, was verified. We show that the orbital fluctuation enhanced owing to the orbital-polarization interaction.…”

Possible nematic spin liquid in spin-1 antiferromagnetic system on the square lattice: Implications for the nematic paramagnetic state of FeSe

“…Besides, FeSe possesses an electronic nematic order after a tetragonal-to-orthorhombic structural transition at T s 90K [19][20][21][22] . Although the primary origin of this nematic order is still unclear [23][24][25][26][27][28][29][30][31][32][33][34] , neutron scattering measurements indicate the important role of spin degree of freedom 24,25 . These novel properties have triggered wide interests in the magnetic ground state of FeSe [35][36][37][38][39][40][41][42][43][44][45] .…”

Nematic fluctuations and phase transitions in LaFeAsO: A Raman scattering study

“…1(a)), providing a natural mechanism for electronic anisotropy. On the other hand, T S and T AF M are significantly separated for LaFeAsO (LFAO) (T AF M = 140 K and T S = 155 K) [11][12][13], while FeSe does not order magnetically at ambient pressure but still shows a nematic transition at T S = 90 K [14], motivating suggestions that the nematic transition may be driven by charge/orbital degrees of freedom rather than magnetism in the latter [16,17]. However, even for FeSe the magnetic scenario may still apply [18].…”