Visualization of electron nematicity and unidirectional antiferroic fluctuations at high temperatures in NaFeAs

Abstract

Abstract:The driving forces behind electronic nematicity in the iron pnictides remain hotly debated. We use atomic-resolution variable-temperature scanning tunneling spectroscopy to provide the first direct visual evidence that local electronic nematicity and unidirectional antiferroic (stripe) fluctuations persist to temperatures almost twice the nominal structural ordering temperature in the parent pnictide NaFeAs. Low-temperature spectroscopic imaging of nematically-ordered NaFeAs shows anisotropic electronic features that are not observed for isostructural, non-nematic LiFeAs. The local electronic features are shown to arise from scattering interference around crystalline defects in NaFeAs, and their spatial anisotropy is a direct consequence of the structural and stripe-magnetic order present at low temperature. We show that the anisotropic features persist up to high temperatures in the nominally tetragonal phase of the crystal. The spatial distribution and energy dependence of the anisotropy at high temperatures is explained by the persistence of large amplitude, short-range, unidirectional, antiferroic (stripe) fluctuations, indicating that strong density wave fluctuations exist and couple to near-Fermi surface electrons even far from the structural and density wave phase boundaries. Main Text:The nature of the normal state from which superconductivity emerges in unconventional superconductors remains a mystery. It is suspected that electronic interactions present in the normal state play a key role in the formation of the superconducting state 1 . In both the cuprates and the pnictide phase diagrams, magnetically ordered states exist in proximity to the superconducting state, and the pnictides additionally exhibit orbital ordering 2,3 . A crucial additional feature of the pnictides is the appearance of a "nematic'' phase in which the tetragonal rotational symmetry of the ideal pnictide lattice is spontaneously broken below a temperature T S . Recent bulk transport and scattering measurements have suggested that the nematic phase is driven by electronic, rather than lattice, degrees of freedom [4][5][6][7][8] and is observed in all electronic channels -charge 9,10 , orbital 4 , and spin 7,11 . Spin order and spin fluctuations [12][13][14][15][16] (which couple quadratically to nematicity) as well as orbital order 17,18 and orbital fluctuations 19 (which can couple linearly) have been invoked to explain the nematicity.However, the dominant interaction responsible for the nematic ordering and fluctuations remains 2 unknown and identifying it is a key experimental goal. In this paper, we use variable temperature scanning tunneling spectroscopy to provide new insights into this issue by showing that our spectroscopic signals reveal that nematicity occurs in conjuction with strong antiferroic fluctuations and that both phenomena persist up to temperatures much greater than the temperatures at which longrange order is established.The arsenide superconductors consist of one or more iron-arsenide layers with the...

Magnetic interactions in iron superconductors: A review

“…In addition, the magnitude of the anisotropy is largest when the structural distortion and the magnetism are weaker [44][45][46][47][48]. Signatures of anisotropy have also been found in other experiments [49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65]. The degeneracy of yz and zx is broken in both the magnetic and non magnetically ordered phases below the structural transition as revealed by ARPES [66][67][68] and X-ray experiments [69].…”

“…The opposite resistivity anisotropy has been found in a holedoped material [63]. Anisotropy has also been measured in optical conductivity [59-62, 64, 223], elastic shear modulus [49,50], Raman [51], scanning tunnelling microscopy [52][53][54], magnetic torque [55] and inelastic neutron scattering experiments [56][57][58]65]. Signatures of orbital-dependent reconstruction of electronic structure in the magnetic and non-magnetic states are found in ARPES experiments [66-68, 224, 225] and in X-ray absortion spectroscopy [69].…”

Stripe and Short Range Order in the Charge Density Wave of 1T−CuxTiSe2

“…A CDW is a spatial modulation of the electron density associated with local lattice distortions. CDWs are found in a number of quasitwo-dimensional superconductors, including transition metal dichalcogenides [1], intercalated graphite [2], cuprates [3-5] and pnictides [6]. Of particular interest, largely driven by the puzzle of high temperature superconductivity, is whether charge order is competing, cooperating, or simply coexisting with superconductivity [7].…”

Persistence of slow fluctuations in the overdoped regime of Ba(Fe_1-xRh_x)₂As₂ superconductors

“…While evidence for spin nematic and orbital nematic fluctuations, even well above the ordering temperature, have been reported in the underdoped regime of iron-based superconductors [38,[54][55][56], no clear evidence for the persistence of slow fluctuations driven by nematicity has been presented for the overdoped iron-based superconductors. It is interesting to notice that the vanishing of the spin fluctuations probed by 1/T 2 is accompanied by a decrease in the amplitude of charge fluctuations of nematic character probed by inelastic Raman scattering [57], as well as by a decrease of the orbital anisotropy [58].…”

“…Thus, we can write the field gradient probed by the nuclei as where γ is the nuclear gyromagnetic ratio. From the equation above, by taking a linewidth of 30 kHz [6], and 2a equal to few lattice steps [38,39], the internal field gradient results ∇B ∼ 10 8 G/cm. Therefore two As nuclei separated by 0.6 nm experience a Larmor frequency difference of about 4 kHz, which is much larger than the dipolar interaction, estimated from lattice sums to be ∼200 Hz.…”