Organic Superconductor Research Articles

Elucidation of local structure deformation in κ−(BEDT−TTF)2Cu[N(CN)2]Br by x-ray fluorescence holography

$\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{Br} (\ensuremath{\kappa}\text{\ensuremath{-}}\mathrm{Br})$ is an organic superconductor that changes to an Anderson-type insulator due to the random defects introduced by x-ray irradiation. In this study, we directly investigated the effects of irradiation on the local structures in the anion layer of $\ensuremath{\kappa}\text{\ensuremath{-}}\mathrm{Br}$ using x-ray fluorescence holography. The local structures around the Cu atoms in the anion layer were directly reconstructed from holograms. Prior to radiation damage, the reconstruction clearly shows the atomic images of the Br and N atoms that are directly coordinated to Cu. In the irradiated samples, the N images in the reconstructions were strongly suppressed as a result of the positional fluctuations of N introduced by the defects induced by irradiation. Using the previously proposed ``bond-shifted'' model and the thermal vibration amplitudes calculated from molecular dynamics simulations, we simulated the holograms and the atomic reconstructions of the anion layer. The simulations show that the ``bond-shifted'' model, along with thermal vibrations at 100 K, suppresses the N atomic images, as was observed in the experimental reconstructions.

Merohedral disorder and impurity impacts on superconductivity of fullerenes

Local quasiparticle states around impurities provide essential insight into the mechanism of unconventional superconductivity, especially when the candidate materials are proximate to an antiferromagnetic Mott-insulating phase. While such states have been reported in atom-based cuprates and iron-based compounds, they are unexplored in organic superconductors which feature tunable molecular orientation. Here we employ scanning tunneling microscopy and spectroscopy to reveal multiple forms of robustness of an exotic s-wave superconductivity in epitaxial Rb3C60 films against merohedral disorder, non-magnetic single impurities and step edges at the atomic scale. Yu-Shiba-Rusinov (YSR) states, induced by deliberately incurred Fe adatoms that act as magnetic scatterers, have also been observed. The YSR bound states show abrupt spatial decay and vary in energy with the Fe adatom registry. These results and a doping-dependent study of superconductivity point towards local electron pairing in which the multiorbital electronic correlations and intramolecular phonons together drive the high-temperature superconductivity of doped fullerenes.

Thermodynamic evidence for the formation of a Fulde-Ferrell-Larkin-Ovchinnikov phase in the organic superconductor λ−(BETS)2GaCl4

In this work, the thermodynamic properties of the organic superconductor $\lambda$-(BETS)$_2$GaCl$_4$ are investigated to study a high-field superconducting state known as the putative Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. We observed a small thermodynamic anomaly in the field $H_{\rm FFLO}$ $\sim$ 10~T, which corresponds to the Pauli limiting field $H_{\rm P}$. This anomaly probably originates from a transition from a uniform superconducting state to the FFLO state. $H_{\rm FFLO}$ does not show a strong field-angular dependence due to a quasi-isotropic paramagnetic effect in $\lambda$-(BETS)$_2$GaCl$_4$. The thermodynamic anomaly at $H_{\rm FFLO}$ is smeared out and low-temperature upper critical field $H_{\rm c2}$ changes significantly if fields are not parallel to the conducting plane even for a deviation of $\sim$0.5$^{\circ}$. This behavior indicates that the high-field state is very unstable, as it is influenced by the strongly anisotropic orbital effect. Our results are consistent with the theoretical predictions on the FFLO state, and show that the high-field superconductivity is probably an FFLO state in $\lambda$-(BETS)$_2$GaCl$_4$ from a thermodynamic point of view.

Interplay between Vortex Dynamics and Superconducting Gap Structure in Layered Organic Superconductors

Layered organic superconductors motivate intense investigations because they provide various unexpected issues associated with their low dimensionality and the strong electron correlation. Since layered organic superconductors possess simple Fermi surface geometry and they often share similarities to the high temperature oxide superconductors and heavy fermion compounds, research on layered organic superconductors is suitable for understanding the essence and nature of strongly correlated electron systems. In strongly correlated electron systems, one of the central problems concerning the superconducting (SC) state is the symmetry of the SC gap, which is closely related to the paring mechanism. Thus, experimental determination of the SC gap structure is of essential importance. In this review, we present the experimental results for the in-plane angular variation of the flux-flow resistance in layered organic superconductors k-(ET)2Cu(NCS)2, β″-(ET)2SF5CH2CF2SO3, and λ-(BETS)2GaCl4. The interplay between the vortex dynamics and nodal structures is discussed for these superconductors.

Absence of Superconductivity in the Hubbard Dimer Model for κ-(BEDT-TTF)2X

In the most studied family of organic superconductors κ-(BEDT-TTF)2X, the BEDT-TTF molecules that make up the conducting planes are coupled as dimers. For some anions X, an antiferromagnetic insulator is found at low temperatures adjacent to superconductivity. With an average of one hole carrier per dimer, the BEDT-TTF band is effectively 12-filled. Numerous theories have suggested that fluctuations of the magnetic order can drive superconducting pairing in these models, even as direct calculations of superconducting pairing in monomer 12-filled band models find no superconductivity. Here, we present accurate zero-temperature Density Matrix Renormalization Group (DMRG) calculations of a dimerized lattice with one hole per dimer. While we do find an antiferromagnetic state in our results, we find no evidence for superconducting pairing. This further demonstrates that magnetic fluctuations in the effective 12-filled band approach do not drive superconductivity in these and related materials.

Prediction of pseudogap formation due to d-wave bond-order in organic superconductor κ−(BEDT-TTF)2X

Rich hidden unconventional orders with pseudogap formation, such as the inter-site bond-order (BO), attract increasing attention in condensed matter physics. Here, we investigate the hidden order formation in organic unconventional superconductor $\kappa$-(BEDT-TTF)$_2$X. We predict the formation of $d$-wave BO at wavelength $q=Q_B=(\delta,\delta)$ ($\delta=0.38\pi$) for the first time, based on both the functional renormalization group (fRG) and the density-wave equation theories. The origin of the BO is the quantum interference among antiferromagnetic spin fluctuations. This prediction leads to distinct pseudogap-like reduction in the NMR $1/T_1$ relaxation rate and in the density-of-states, consistently with essential experimental reports. The present theory would be applicable for other strongly correlated metals with pseudogap formation.

Fate of the Hebel-Slichter peak in superconductors with strong antiferromagnetic fluctuations

We show that magnetic fluctuations can destroy the Hebel-Slichter peak in conventional superconductors. The Hebel-Slichter peak has previously been expected to survive even in the presence of strong electronic interactions. However, we show that antiferromagnetic fluctuations suppress the peak at $\bf{q}=0$ in the imaginary part of the magnetic susceptibility, $\chi_{+-}''\left(\bf{q},\omega\right)$, which causes the Hebel-Slichter peak. This is of general interest as in many materials superconductivity is found near a magnetically ordered phase, and the absence of a Hebel-Slichter peak is taken as evidence of unconventional superconductivity in these systems. For example, no Hebel-Slichter peak is observed in the $\kappa$-(BEDT-TTF)$_2X$ organic superconductors but heat capacity measurements have been taken to indicate $s$-wave superconductivity. If antiferromagnetic fluctuations destroy the putative Hebel-Slichter peak in organic superconductors then the peak should be restored by applying a pressure, which is known to suppress antiferromagnetic correlations in these materials.

Symmetry change of d -wave superconductivity in κ -type organic superconductors

Magnetic-field-angle-resolved heat capacity of dimer-Mott organic superconductors $\kappa$-(BEDT-TTF)$_2$$X$ is reported. Temperature and field dependence of heat capacity indicates that the superconductivity has line nodes on the Fermi surface, which is a typical feature of $d$-wave superconductivity. In-plane field-angle dependence exhibits fourfold oscillations as well as twofold oscillations due to anisotropy of superconducting gap functions. From analyses of the fourfold term, the gap symmetry is determined as $d_{x^{2}-y^{2}}$+$s_{\pm}$ for $X$=Cu[N(CN)$_2$]Br and $d_{xy}$ for $X$=Ag(CN)$_2$H$_2$O. We suggest that the symmetry difference comes from the competition of $d_{xy}$ and $d_{x^{2}-y^{2}}$ antiferromagnetic fluctuations depending on lattice geometry.

Experimental evidence for Zeeman spin\u2013orbit coupling in layered antiferromagnetic conductors

Most of solid-state spin physics arising from spin–orbit coupling, from fundamental phenomena to industrial applications, relies on symmetry-protected degeneracies. So does the Zeeman spin–orbit coupling, expected to manifest itself in a wide range of antiferromagnetic conductors. Yet, experimental proof of this phenomenon has been lacking. Here we demonstrate that the Néel state of the layered organic superconductor κ-(BETS)2FeBr4 shows no spin modulation of the Shubnikov–de Haas oscillations, contrary to its paramagnetic state. This is unambiguous evidence for the spin degeneracy of Landau levels, a direct manifestation of the Zeeman spin–orbit coupling. Likewise, we show that spin modulation is absent in electron-doped Nd1.85Ce0.15CuO4, which evidences the presence of Néel order in this cuprate superconductor even at optimal doping. Obtained on two very different materials, our results demonstrate the generic character of the Zeeman spin–orbit coupling.

Evidence for metastable photo-induced superconductivity in K3C60

Excitation of high-Tc cuprates and certain organic superconductors with intense far-infrared optical pulses has been shown to create non-equilibrium states with optical properties that are consistent with transient high-temperature superconductivity. These non-equilibrium phases have been generated using femtosecond drives, and have been observed to disappear immediately after excitation, which is evidence of states that lack intrinsic rigidity. Here we make use of a new optical device to drive metallic K3C60 with mid-infrared pulses of tunable duration, ranging between one picosecond and one nanosecond. The same superconducting-like optical properties observed over short time windows for femtosecond excitation are shown here to become metastable under sustained optical driving, with lifetimes in excess of ten nanoseconds. Direct electrical probing, which becomes possible at these timescales, yields a vanishingly small resistance with the same relaxation time as that estimated by terahertz conductivity. We provide a theoretical description of the dynamics after excitation, and justify the observed slow relaxation by considering randomization of the order-parameter phase as the rate-limiting process that determines the decay of the light-induced superconductor.

Anisotropic zero-resistance onset in organic superconductors

We study the coexistence of superconductivity (SC) and density-wave state and reconcile various puzzling experimental data in organic superconductors (TMTSF)$_{2}$PF$_{6}$ and (TMTSF)$_{2}$ClO$_{4}$. The anisotropic resistance drop above $T_c$ is qualitatively described by nascent isolated SC islands within a bulk analytical model. However, the observed anisotropic SC onset is explained only when the finite size and flat needle shape of samples is considered. Our results pave a way to estimate the volume fraction and the typical size of SC islands in far from the sample surface, and apply to many inhomogeneous superconductors, including high-$T_c$ cuprate or Fe-based ones.

Evolution of Shape and Volume Fraction of Superconducting Domains with Temperature and Anion Disorder in (TMTSF)2ClO4

In highly anisotropic organic superconductor (TMTSF)2ClO4, superconducting (SC) phase coexists with metallic and spin-density wave phases in the form of domains. Using the Maxwell-Garnett approximation (MGA), we calculate the volume ratio and estimate the shape of these embedded SC domains from resistivity data at various temperature and anion disorder, controlled by the cooling rate or annealing time of (TMTSF)2ClO4 samples. We found that the variation of cooling rate and of annealing time affect differently the shape of SC domains. In all cases the SC domains have oblate shape, being the shortest along the interlayer z-axis. This contradicts the widely assumed filamentary superconductivity along the z-axis, used to explain the anisotropic superconductivity onset. We show that anisotropic resistivity drop at the SC onset can be described by the analytical MGA theory with anisotropic background resistance, while the anisotropic Tc can be explained by considering a finite size and flat shape of the samples. Due to a flat/needle sample shape, the probability of percolation via SC domains is the highest along the shortest sample dimension (z-axis), and the lowest along the sample length (x-axis). Our theory can be applied to other heterogeneous superconductors, where the size d of SC domains is much larger than the SC coherence length ξ, e.g., cuprates, iron-based or organic superconductors. It is also applicable when the spin/charge-density wave domains are embedded inside a metallic background, or vice versa.

Enhancement of electron correlations and spin density wave fluctuations of the organic superconductor λ−(BETS)2GaCl4 under pressure proved by C13 NMR

We performed $^{13}\mathrm{C}$ NMR measurements on an organic superconductor $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{B}\mathrm{E}\mathrm{T}\mathrm{S})}_{2}\mathrm{Ga}{\mathrm{Cl}}_{4}$ [BETS: bis(ethylenedithio)tetraselenafulvalene] to investigate the pressure effect on the spin density wave (SDW) fluctuations at low temperatures, which were observed at ambient pressure. We found that the spin-lattice relaxation rate divided by temperature $(1/{T}_{1}T)$ is significantly enhanced at low temperatures at 3 kbar, implying enhancement of the SDW fluctuation. When further pressure is applied, increase in $1/{T}_{1}T$ at low temperatures is suppressed and $1/{T}_{1}T$ becomes temperature independent, indicating the Fermi liquid state. Moreover, we revealed that the Korringa factor under pressure becomes double compared to its value at ambient pressure, suggesting a significant change in electron correlation. We discuss the effect of the SDW fluctuation and the electron correlation on the unconventional superconductivity.

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Spin fluctuations and uncoventional superconducting pairing

High-&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; cuprates, iron-based superconductors, heavy-fermion superconductors and &lt;i&gt;κ&lt;/i&gt;-type layered organic superconductors share some common features − the proximity of the superconducting state to the magnetic ordered state and the non-&lt;i&gt;s&lt;/i&gt;-wave superconducting pairing function. It is generally believed that the Cooper pairings in these unconventional superconductors are mediated by spin fluctuations. In this paper, we present a brief overview on the spin dynamics and unconventional pairing, focusing on high-&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; cuprates and iron-based superconductors. In particular, we will overview the properties of the neutron spin resonance and its possible origin, the pairing mechanism in Hubbard model within the weak-coupling framework and its application to the aforesaid unconventional superconductors. We point out that the interplay between magnetism and superconductivity is still an area of active research.

Selective observation of spin and charge dynamics in an organic superconductor λ−(BETS)2GaCl4 using Ga69,71 NMR measurements

For the unconventional organic superconductor $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ [BETS = bis(ethylenedithio) tetraselenafulvalene], the importance of both spin and charge degrees of freedom has been discussed based on the broadening of the NMR linewidth due to charge disproportionation and the occurrence of a spin-density-wave phase near the superconducting phase. NMR with the nuclear spin of $1/2$, previously used for studying organic conductors, is an effective method for revealing electronic states microscopically; however, it cannot distinguish between charge and spin anomalies. To resolve this problem, in this paper, we performed $^{69,71}\mathrm{Ga}$-NMR measurements, which enabled us to study both charge and spin dynamics using different gyromagnetic ratios and quadrupole moments between two isotopes. The spin-lattice relaxation rate is dominated by electric-field gradient fluctuations originating from molecular dynamics above 150 K and, below this temperature, it is dominated by spin fluctuations derived from the $\ensuremath{\pi}$ electrons of BETS layers. This change in the relaxation mechanism is considered to be due to the development of interactions between ${\mathrm{GaCl}}_{4}$ ions and BETS layers upon freezing of the molecular motion by cooling. Below 150 K, the contribution of spin fluctuations monotonically increases, and no increase in the charge fluctuation was observed, suggesting that the spin degree of freedom plays a major role in low-temperature physical properties. Our findings will aid theoretical studies on superconducting properties.

Microscopic Observation of π Spin Polarization by d Localized Spin in λ Type BETS Based Organic Superconductors

In this study, 77Se NMR measurements were carried out to detect the π spin polarization of the organic BETS (BETS = Bis(ethylenedithio)tetraselenafulvalene) molecule of the field induced superconductor, λ-(BETS)2Fe1−xGaxCl4, which shows a superconducting transition at relatively low magnetic field compared to the non-doped λ-(BETS)2FeCl4. From the analysis of the NMR spectrum at low temperature, it was clarified that the exchange interaction between π and 3d spins in the Ga doping system is smaller than that in the Fe salt. It is also clarified that the conduction π spins feel the “averaged” exchange field from the localized 3d spins at the dilute Fe sites.

Anisotropic Fully Gapped Superconductivity Possibly Mediated by Charge Fluctuations in a Nondimeric Organic Complex.

We investigate low-temperature electronic properties of the nondimeric organic superconductor β^{''}-(ET)_{4}[(H_{3}O)Ga(C_{2}O_{4})_{3}]PhNO_{2}. By examining ultrasonic properties, charge disproportionation (CD) without magnetic field dependence is detected below T_{CD}∼8 K just above the superconducting critical temperature T_{c}∼6 K. From quantum oscillations in high fields, we find variation in the Fermi surface and mass enhancement induced by the CD. Heat capacity studies elucidate that the superconducting gap function is fully gapped in the Fermi surface, but anisotropic with fourfold symmetry. We point out that the pairing mechanism of the superconductivity is possibly dominated by charge fluctuations.

Superfluid density versus transition temperature in a layered organic superconductor κ−(BEDT−TTF)2Cu[N(CN)2]Br under pressure

This work studies the pressure dependence of the superfluid density through the magnetization measurements in a layered organic superconductor and finds that the superfluid density is pressure-independent while the transition temperature substantially decreases.

Unconventional superconductivity in λ -(BETS) 2GaCl4 probed by C13 NMR

We performed $^{13}\mathrm{C}$-NMR measurements on an organic superconductor $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ to investigate the superconducting (SC) gap symmetry, where BETS stands for bis(ethylenedithio)tetraselenafulvalene. As the temperature approaches 0 K, the Knight shift in the SC state decreases; the nuclear spin-lattice relaxation rate $1/{T}_{1}$ shows cubic temperature dependence below the SC transition temperature ${T}_{c}$ without a coherence peak. These results can be understood in terms of $d$-wave superconductivity. Moreover, an increase in $1/{T}_{1}T$ at lower temperatures was observed just above ${T}_{c}$; this behavior can be interpreted as an effect of spin-density-wave (SDW) fluctuation. We suggest that the $d$-wave superconductivity of $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ originates from SDW fluctuation.