Bandwidth Tuning Triggers Interplay of Charge Order and Superconductivity in Two-Dimensional Organic Materials

Abstract

We observe charge-order fluctuations in the quasi-two-dimensional organic superconductor β ′′ -(BEDT-TTF)2SF5CH2CF2SO3 both by means of vibrational spectroscopy, locally probing the fluctuating charge order, and investigating the in-plane dynamical response by infrared reflectance specctroscopy. The decrease of effective electronic interaction in an isostructural metal suppresses both charge-order fluctuations and superconductivity, pointing on their interplay. We compare the results of our experiments with calculations on the extended Hubbard model. PACS numbers: 74.70.Kn, 74.25.Gz, 71.30.+h, 71.10.Hf From a naive point of view, superconducting and ordered insulating states are incompatible. This is true, for example, for the competition of charge-density wave and superconductivity [1], or stripes in cuprates [2,3]. However, experimental and theoretical studies on materials with strong electronic correlations suggest that fluctuations of an ordered state may mediate superconductivity. Prime candidates for this mechanism are magnetic order in heavy fermions [4,5] and high-temperature superconductors [6], incommensurate charge-density waves in dichalcogenides [7], or fluctuating charge order in quasitwo-dimensional organic conductors [8][9][10][11][12][13][14].In BEDT-TTF-based 1/4-filled conductors the ground state can be tuned by modifying effective electronic correlations via changing the bandwidth [15,16] (Fig. 1a): A charge-ordered insulating state is observed when the effective Coulomb repulsion is large enough [16][17][18][19][20], while compounds with weaker effective electronic correlations are metallic [21]. In the metallic state close to the metalinsulator phase boundary, charge fluctuations are observed [9,20,22], while the response of coherent carriers is still present. These experimental results are in agreement to calculations on the extended Hubbard model. It is the minimum model that can describe a metalinsulator transition in quasi-two-dimensional molecular conductors with 1/4-filled conduction band [7]. It takes into account the effective on-site U/t and inter-site V /t Coulomb repulsion, where t is the hopping integral related to the bandwidth. This model predicts that fluctuations of checker-board charge order (CO) can act as an attractive interaction of quasiparticles forming Cooper pairs and lead to a superconducting state [8,23]. In this Letter we present an experimental evidence for a bandwidth tuned CO fluctuations in the normal state of β ′′ family of quasi-2D organic conductors. We see an unambiguous relation between the presence of CO fluctuations and superconductivity and discuss its origin. The studied materials are layered compounds, where a slightly anisotropic quasi-two-dimensional conducting electronic system of the (ab) plane is created by the overlap of the neighboring BEDT-TTF (bis-(ethylenedithio)tetrathiafulvalene) molecules [24] (Fig. 1b). The bandwidth is tuned by changing the size of the anion, by so called 'chemical pressure'. β ′′ -(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 (...

Infrared and Raman Studies of Charge Ordering in Organic Conductors, BEDT-TTF Salts with Quarter-Filled Bands

“…SC paring mediated by CO fluctuation was proposed by Merino and McKenzie [22]. Spectroscopic studies which may support the theoretical prediction for the SC mechanism have been reported [29][30][31][32][33][34]. It was also predicted that spin fluctuation mediates the SC pairing in a CO system [35].…”

“…Bangura et al, attributed the splitting of ν 2 in metallic β″-(BEDT-TTF) 4 [(H 3 O)Ga(C 2 O 4 )]·C 6 H 5 NO 2 to the signature of charge order [150]. Recently, Kaiser et al, reported the splitting of ν 27 in metallic β″-(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 , and they interpreted the growth of splitting as a fluctuation of CO [31]. All of these compounds involve non-equivalent sites in the unit cell: three sites in α-I 3 and two sites in the latter two compounds.…”

Charge-order fluctuations and superconductivity in two-dimensional organic metals

“…24 In the present case the mode is difficult to identify also in IR, since it overlaps with low-energy electronic transitions. 15,25 The three bands seen in the low-temperature spectrum of β ′′ -SC clearly correspond to the a ν (in the order of decreasing wavenumber). The latter is rather insensitive to the charge (Fig.…”

Dimer-Mott and charge-ordered insulating states in the quasi-one-dimensional organic conductors δP′ - and δC′−(BPDT-TTF)Kobayashi1, Hashimoto2, Yoneyama3 et al. 2017Phys. Rev. B3010View full textAdd to dashboardCite

“…Figure 5(d) shows the temperature dependence of the center of gravity of σ 1 (ω) defined by ω = 6000 1000 σ 1 (ω)ωdω/ 6000 1000 σ 1 (ω)dω, which shifts to lower frequencies with decreasing temperature. Generally, in a CO state, a transition between Hubbard-like bands induced by the off-site Coulomb repulsion V , i.e., a site-to-site transition within the CO pattern, gives rise to a broad band in the midinfrared region of the order of V [35][36][37][38], which is almost temperature independent. In the present system, however, the center of the spectral weight shifts to much lower frequencies with decreasing temperature [see Fig.…”