Coupling of structure to magnetic and superconducting orders in quasi-one-dimensional K2Cr3As3

Abstract

Quasi-one-dimensional A2Cr3As3 (with A = K, Cs, Rb) is an intriguing new family of superconductors which exhibit many similar features to the cuprate and iron-based unconventional superconductor families. Yet in contrast to these systems, no charge or magnetic ordering has been observed which could provide the electronic correlations presumed necessary for an unconventional superconducting pairing mechanism -an absence which defies predictions of first principles models. We report the results of neutron scattering experiments on polycrystalline K2Cr3As3 (Tc ∼ 7K) which probed the low temperature dynamics near Tc. Neutron diffraction data evidence a strong response of the nuclear lattice to the onset of superconductivity while inelastic scattering reveals a highly dispersive column of intensity at the commensurate wavevector q = (00 1 2 ) which loses intensity beneath Tc -indicative of short-range magnetic fluctuations. Using linear spin-wave theory we model the observed scattering and suggest a possible structure to the short-range magnetic order. These observations suggest that K2Cr3As3 is in close proximity to a magnetic instability and that the incipient magnetic order both couples strongly to the lattice and competes with superconductivity -in direct analogy with the iron-based superconductors.PACS numbers: 74.25. Dw, 74.62.Dh, 74.70.Xa, 61.05.fm Understanding how superconductivity arises from myriad competing ground states and exotic phenomena such as quantum criticality has been an overarching theme in the study of unconventional superconductors (UNSC) -particularly in the well-studied quasi-two-dimensional (Q2D) cuprate and iron-based (FBS) families 1 . Recently, a new family of superconducting quasi-one-dimensional (Q1D) A 2 Cr 3 As 3 (233) materials (with A = K, Cs, Rb) have proven fertile grounds for applying this general narrative to another system which has further lowereddimensionality 2,3 .The 233 family orders in non-centrosymmetric hexagonal P 6m2 space group symmetry with a structural motif of double-walled sub-nano tubes (DWS) coaxial to the caxis and of [(Cr 3 As 3 )−2 ] ∞ stoichiometry (see Fig. 1(d)) while the A-site ion acting as a spacer/charge reservoir 'layer' 2,4,5 . DFT calculations predict a Fermi surface built of complex mixtures of the Cr 3d shells with strong Q1D character 3,6,7 . Consequently, predictions of Tomonaga-Luttinger liquid (TLL)/general non-Fermiliquid (nFL) physics, Peierls distortions, ferromagnetic (FM) fluctuations/magnetic ordering and spin-triplet superconductivity have arisen creating a sea of possible ground states and interactions out of which superconductivity stabilizes.3,5,7-10 .Experimentally a similarly complex picture has emerged. nFL behaviors are observed in transport, nuclear magnetic resonance (NMR) and muon spectroscopy (µSR) measurements, which indicate significant electron correlations and strong magnetic fluctuations [11][12][13] . More exotically, measurements of the penetration depth find nodes in the superconducting gap while those of the u...

Evidence for triplet superconductivity near an antiferromagnetic instability in CrAs

“…Moreover, there is evidence for spin fluctuations in the normal states of both compounds. In the case of K 2 Cr 3 As 3 both nuclear magnetic resonance and neutron scattering measurements indicate that there are short range antiferromagnetic spin fluctuations 23,24 . Meanwhile the presence of spin fluctuations in CrAs is inferred from the low temperature increase of 1/T 1 T from NQR measurements, where the suppression of both 1/T 1 T and T c with increasing pressure suggests that an important role is played by spin fluctuations in the formation of superconductivity 12,13 .…”

Lifshitz transition enhanced triplet $p_z$-wave superconductivity in hydrogen doped KCr$_3$As$_3$

“…In recent years, the quasi-one-dimensional (1D) superconductors family A 2 TM 3 As 3 (A = Na, K, Rb, and Cs; TM = Cr and Mo) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] with highest superconducting T c above 10 K [17] have attracted tremendous research interests. These compounds consist of alkali-metal-atomsseparated [(TM 3 As 3 ) 2− ] ∞ double-walled subnanotubes with the low-energy degrees of freedom dominated by the TM-3d orbitals [19,20], which are proposed to be stronglycorrelated [5,8,14,[21][22][23][24][25][26], implying an electron-interactiondriven pairing mechanism. Various experiments [1-6, 8, 9, 12, 13, 18] have revealed unconventional pairing feature of this superconductors family, with evidences suggesting the existence of line gap nodes [2,4] and possible spin triplet pairing states [1-3, 8, 12, 18, 27, 28].…”

Tuning interchain ferromagnetic instability in A2Cr3As3 ternary arsenides by chemical pressure and uniaxial strain

“…In this context, one of the most relevant examples is represented by the recently discovered Cr-based arsenide family A 2 Cr 3 As 3 (A = Na, K, Rb, and Cs) [1][2][3][4]. This series has been shown to represent an ideal platform for the study of the interplay between magnetism and structural instabilities [5][6][7], as well as of other relevant features such as noncentrosymmetry and quasi-one-dimensional properties [8,9].…”