Temperature dependent anisotropy and two-band superconductivity revealed by lower critical field in organic superconductor $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br
Resistivity and magnetization have been measured at different temperatures and magnetic fields in organic superconductors $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br. The lower critical field and upper critical field are determined, which allow to depict a complete phase diagram. Through the comparison between the upper critical fields with magnetic field perpendicular and parallel to the conducting ac-planes, and the scaling of the in-plane resistivity with field along different directions, we found that the anisotropy $Γ$ is strongly temperature dependent. It is found that $Γ$ is quite large (above 20) near $T_{c}$, which satisfies the 2D model, but approaches a small value in the low-temperature region. The 2D-Tinkham model can also be used to fit the data at high temperatures. This is explained as a crossover from the orbital depairing mechanism in high-temperature and low-field region to the paramagnetic depairing mechanism in the high-field and low-temperature region. The temperature dependence of lower critical field $H_{c1} (T)$ shows a concave shape in wide temperature region. It is found that neither a single $d$-wave nor a single $s$-wave gap can fit the $H_{c1} (T)$, however a two-gap model containing an $s$-wave and a $d$-wave can fit the data rather well, suggesting two-band superconductivity and an unconventional pairing mechanism in this organic superconductor.
Half-filled two-leg Hubbard ladders have spin-gapped short-range antiferromagnetic correlations while three-leg ladders have power law antoferromagnetic correlations, and both systems have d_{x^2-y^2}-power law pairing correlations when they are doped. Thus these ladders exhibit some of the phenomenology seen in the layered cuprates. Here we report results for half-filled frustrated Hubbard ladders, based upon ladder segments taken from a tight-binding model of kappa-BEDT-TTF. Although these ladders are half-filled, varying the degree of frustration can drive them across an insulator-metal tra…
High-resolution specific-heat measurements of the organic superconductor kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br in the superconducting (B = 0) and normal (B = 14 T) state show a clearly resolvable anomaly at Tc = 11.5 K and an electronic contribution, Ces, which can be reasonably well described by strong-coupling BCS theory. Most importantly, Ces vanishes exponentially in the superconducting state which gives evidence for a fully gapped order parameter.
The first study of thermal conductivity, $κ$, in a quasi-two-dimensional organic superconductor of the $κ$-(BEDT-TTF)$_2$X family reveals features analogous to those already observed in the cuprates. The onset of superconductivity is associated with a sudden increase in $κ$ which can be suppressed by the application of a moderate magnetic field. At low temperatures, a finite linear term - due to a residual electronic contribution- was resolved. The magnitude of this term is close to what is predicted by the theory of transport in unconventional superconductors.
We present magnetic-torque measurements of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2 for in-plane magnetic fields up to 32 T. In this layered two-dimensional compound the superconductivity can persist even in fields above the Pauli limit of about 21 T. There, a pronounced upturn of the upper-critical-field line occurs and the superconducting phase-transition line splits and forms an additional high magnetic field phase. k-(BEDT-TTF)2Cu(NCS)2 is a spin-singlet superconductor, therefore, such a superconducting high-field phase beyond the Pauli limit can originate only from Cooper pairing …
We calculate the lattice phonons and the electron-phonon coupling of the organic superconductor κ-(BEDT-TTF)_2 I_3, reproducing all available experimental data connected to phonon dynamics. Low-frequency intra-molecular vibrations are strongly mixed to lattice phonons. Both acoustic and optical phonons are appreciably coupled to electrons through the modulation of the hopping integrals (e-LP coupling). By comparing the results relevant to superconducting κ- and β-(BEDT-TTF)_2 I_3, we show that electron-phonon coupling is fundamental to the pairing mechanism. Both e-LP and electron-molecular vi…
Angle and temperature dependent torque magnetization measurements are reported for the organic superconductor kappa-(BEDT-TTF)2Cu(NCS)2, at extremely low temperatures (approx. Tc/10^3). Magneto-thermal instabilities are observed in the form of abrupt magnetization (flux) jumps. We carry out an analysis of the temperature and field orientation dependence of these flux jumps based on accepted models for layered type-II superconductors. Using a simple Bean model, we also find a critical current density of 4x10^8 A/m^2 from the remnant magnetization, in agreement with previous measurements.
We report high-resolution specific heat measurements on the quasi-2D organic superconductor $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$, performed by using a most sensitive ac-modulation technique. The main observations are (i) a discontinuity at \tc\ much in excess of what is expected for a weak-coupling BCS superconductor and (ii) a quasiparticle contribution to the specific heat with an exponentially weak temperature dependence at $T \ll T_c$. The latter finding is incompatible with an order parameter that vanishes at certain parts of the Fermi surface. The data for $T \leq T_c$ can be well described by …
The temperature dependence of the linear thermal expansion coefficients (LTEC) of a single crystal of a-(BEDT-TTF)2NH4Hg(SCN)4 where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene was studied by the method of precision capacitive dilatometry in the temperature range 2-250 K along the crystallographic direction b* (perpendicular to the crystal layers). Negative values of LTEC were found below 4 K. Probably it is due to charge fluctuations as the temperature approaches the temperature of transition to the superconducting state Tc. It has been suggested that the bends on temperature dependence…
We present a theoretical study on the superconductivity in β'-(BEDT-TTF)_2ICl_2 at Tc=14.2K under high hydrostatic pressure found by Taniguchi et al., which is the highest record among organic superconductors. Its electronic structure is well expressed by the anisotropic triangular lattice Hubbard model at half filling. In the present work, we study this effective model by using the fluctuation-exchange (FLEX) approximation. In the obtained phase diagram, the superconductivity with d_{x^2-y^2} like symmetry is realized next to the antiferromagnetic (AF) insulating phase, as a result of the 1D-…
To investigate the superconducting (SC) state near a charge instability, we performed ^{13}C NMR experiments on the molecular superconductor beta"-(BEDT-TTF)_{4}[(H_{3}O)Ga(C_{2}O_{4})_{3}]C_{6}H_{5}NO_{2}, which exhibits a charge anomaly at 100 K. The Knight shift which we measured in the SC state down to 1.5 K demonstrates that Cooper pairs are in spin-singlet state. Measurements of the nuclear spin-lattice relaxation time reveal strong electron-electron correlations in the normal state. The resistivity increase observed below 10 K indicates that the enhanced fluctuation has an electric orig…
We compare the theoretical anisotropic upper critical field $H_{C}(Θ,T)$ of a quasi-two-dimensional d-wave superconductor with recent $H_{c2}$ data for the layered organic superconductor $κ-(BEDT-TTF)_2Cu(NCS)_2$. We find agreement both with regard to the angular and the temperature dependence of $H_{C}$. This supports the suggestion that the Fulde-Ferrell-Larkin-Ovchinnikov state (FFLO state) exists in this material for exactly plane-parallel orientation of the magnetic field. Indications of precursor states, occurring for small deviations from the plane-parallel field direction, are also poi…
We have introduced defects into clean samples of the organic superconductor $κ$-(BEDT-TTF)$_2$Cu(SCN)$_2$ in order to determine their effect on the temperature dependence of the conductivity and the critical temperature $T_{\rm c}$. We find a violation of Matthiessen's rule that can be explained by a model of the conductivity involving a defect-assisted interlayer channel which acts in parallel with the band-like conductivity. We observe an unusual dependence of $T_{\rm c}$ on residual resistivity which is not consistent with the generalised Abrikosov-Gor'kov theory for an order parameter with…
The interplane optical spectrum of the organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br was investigated in the frequency range from 40 to 40,000 cm-1. The optical conductivity was obtained by Kramers-Kronig analysis of the reflectance. The absence of a Drude peak at low frequency is consistent with incoherent conductivity but in apparent contradiction to the metallic temperature dependence of the DC resistivity. We set an upper limit to the interplane transfer integral of tb = 0.1 meV. A model of defect-assisted interplane transport can account for this discrepancy. We also assign the pho…
Static susceptibility of kappa-[(BEDT-TTF)1-x(BEDSe-TTF)x]2Cu[N(CN)2]Br alloys with the BEDSe-TTF content near the border-line of ambient pressure superconductivity (x~0.3) has been measured as a function of temperature, magnetic field, and pressure. A non-monotonic pressure dependence is observed for both the superconducting critical temperature and superconducting volume fraction, with both quantities showing growth under pressure in the initial pressure range P < 0.3 kbar. The results are discussed in comparison with the data on the related kappa-phase BEDT-TTF superconductors in which not …
Partial suppression of superconductivity by fast cooling has been observed in the organic superconductor $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br by two means: a marked sample size effect on the magnetic susceptibility and direct imaging of insulating regions by scanning microregion infrared reflectance spectroscopy. Macroscopic insulating regions are found in the vicinity of the crystalline surface after fast cooling, with diameters of 50--100 $μ$m and depths of a few $μ$m. The very large in-plane penetration depth reported to date ($\sim$ 24--100 $μ$m) can be explained by the existence of the insul…
Systematic investigation of the electronic phase separation on macroscopic scale is reported in the organic Mott system $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br. Real space imaging of the phase separation is obtained by means of scanning micro-region infrared spectroscopy using the synchrotron radiation. The phase separation appears near the Mott boundary and changes its metal-insulator fraction with the substitution ratio $x$ in $κ$-[($h$-BEDT-TTF)$_{1-x}$($d$-BEDT-TTF)$_{x}$]$_{2}$Cu[N(CN)$_{2}$]Br, of which band width is controlled by the substitution ratio $x$ between the hydrogenated BEDT-TT…
A muon-spin relaxation (muSR) investigation is presented for the molecular superconductor kappa-(BEDT-TTF)2Cu[N(CN)2Br]. Evidence is found for low-temperature phase-separation, with only a fraction of the sample showing a superconducting signal, even for slow cooling. Rapid cooling reduces the superconducting fraction still further. For the superconducting phase, the in-plane penetration depth is measured to be lambda_{parallel} = 0.47(1) mu m and evidence is seen for a vortex decoupling transition in applied fields above 40 mT. The magnetic fluctuations in the normal state produce Korringa be…
Low temperature scanning tunneling spectroscopy reveals the local density of states of the organic superconductor $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, that was cut in-situ in ultra-high vacuum perpendicular to the superconducting BEDT-TTF layers. The spectra confirm that superconductivity is confined to the conducting BEDT-TTF layers, while the Cu[N(CN)$_2$]Br anion layers are insulating. The density of states comprises a twofold superconducting gap, which is attributed to the two separated bands crossing the Fermi surface.
We study the pressure dependence of the superconducting transition temperature of an organic superconductor $β'$-(BEDT-TTF)$_2$ICl$_2$ by applying the fluctuation exchange method to the Hubbard model on the original two-band lattice at 3/4-filling rather than the single band model in the strong dimerization limit. Our study is motivated by the fact that hopping parameters evaluated from a first-principles study suggest that the dimerization of the BEDT-TTF molecules is not so strong especially at high pressure. Solving the linearized Eliashberg's equation, a d$_{xy}$-wave-like superconducting …
Ultrasonic velocity and attenuation measurements on the quasi-two-dimensional organic conductor $κ$-[BEDT-TTF]$_2$Cu[N(CN)$_2$]Br give the first determination of the coexistence zone of the antiferromagnetic and superconducting phases which extends deep in the metallic part of the pressure-temperature phase diagram of these salts. This zone is identified from a precise control of the ordering in ethylene end groups which acts as an internal pressure. The two phases are found to compete whereas superconducting fluctuations begin to contribute to the attenuation at 15 K, namely at the onset of m…
We report the in-plane penetration depth $λ_{\parallel}$ of single crystals $κ$-(BEDT-TTF)$_2X$ ($X=$ Cu(NCS)$_2$ and Cu[N(CN)$_2$]Br) by means of the reversible magnetization measurements under the control of cooling-rate. In $X$ = Cu(NCS)$_2$, $λ_{\parallel}(0)$ as an extrapolation toward $T$ = 0 K does not change by the cooling-rate within the experimental accuracy, while $T_{\textrm{c}}$ is slightly reduced. On the other hand, in $X$ = Cu[N(CN)$_2$]Br, $λ_{\parallel}(0)$ indicates a distinct increase by cooling faster. The different behavior of $λ_{\parallel}(0)$ on cooling-rate between th…
A cavity perturbation technique is used to study the microwave response of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2. Observation of a Josephson plasma resonance, below Tc (approx. 10 K), enables investigation of the vortex structure within the mixed state of this highly anisotropic, type-II, superconductor. Contrary to previous assumptions, frequency dependent studies (28 - 153 GHz) indicate that the squared plasma frequency depends exponentially on the magnetic field strength. Such behavior has been predicted for a weakly pinned quasi-two-dimensional vortex lattice [Bulaevskii et al. …
We present a first-principles density-functional calculation for the Raman spectra of a neutral BEDT-TTF molecule. Our results are in excellent agreement with experimental results. We show that a planar structure is not a stable state of a neutral BEDT-TTF molecule. We consider three possible conformations and discuss their relation to disorder in these systems.
Spin fluctuation-induced superconductivity in quasi-two dimensional organic compounds, κ-BEDT-TTF salts, is investigated within a fluctuation exchange (FLEX) approximation using a half-filled Hubbard model with a right-angled isosceles triangular lattice (transfer matrices -τ, -τ^\prime), extending a previous work above T_c. An energy gap of A_2 or (x^2-y^2)-type develops with decreasing temperature below T_c more rapidly than in the BCS model. The calculated dynamical susceptibilities enough below T_c show sharp resonance peaks like those in certain cuprates superconductors. The calculated nu…
Utilizing infrared vibrational spectroscopy we have investigated dimerized two-dimensional organic salts in order to search for possible charge redistribution that might cause electronic dipoles and ferroelectricity: the quantum spin liquid κ-(BEDT-TTF)2Cu2(CN)3, the antiferromagnetic Mott insulator κ-(BEDT-TTF)2$Cu[N(CN)2]Cl, and the superconductor κ-(BEDT-TTF)2$Cu[N(CN)2]Br. None of them exhibit any indication of charge disproportionation upon cooling down to low temperatures. No modification in the charge distribution is observed around T = 6 K where a low-temperature anomaly has been repor…
We report detailed measurements of the interlayer magnetoresistance of the layered organic superconductor $κ-(BEDT-TTF)_2Cu(SCN)_2$ for temperatures down to 0.5 K and fields up to 30 tesla. The upper critical field is determined from the resistive transition for a wide range of temperatures and field directions. For magnetic fields parallel to the layers, the upper critical field increases approximately linearly with decreasing temperature. The upper critical field at low temperatures is compared to the Pauli paramagnetic limit, at which singlet superconductivity should be destroyed by the Zee…
We study theoretically the multicritical phenomena of the superconductivity (SC) and antiferromagnetism (AF) in organic conductors $κ$-BEDT salts. The phase diagram and the experimental data on the NMR relaxation rate $1/T_1$ is analysed in terms of the renormalization group method. The bicritical phenomenon observed experimentally indicates the rotational symmetry, i.e., SO(5) symmetry, of the SC and the AF. The critical exponent $x$ for the divergence of $1/T_1$ is well explained by $ x = ν(z - 1 - η)$ with the dynamical exponent $z = 3/2$ for the AF region while $z = ϕ/ν\sim 1.84 $ at the b…
Using a high-sensitivity cavity perturbation technique (40 to 180 GHz), we have probed the angle dependent interlayer magneto-electrodynamic response within the vortex state of the extreme two-dimensional organic superconductor k-(BEDT-TTF)2Cu(NCS)2. A previously reported Josephson plasma resonance [M. Mola et al., Phys. Rev. B 62, 5965 (2000)] exhibits a dramatic re-entrant behavior for fields very close (<1 degree) to alignment with the layers. In this same narrow angle range, a new resonant mode develops which appears to be associated with the non-equilibrium critical state. Fits to the ang…
We report measurements of the de Haas-van Alphen (DHVA) effect and AC susceptibility in the layered organic superconductor $κ$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$. The amplitude of the DHVA oscillations is attenuated over a wide field range above the irreversibility line, B_irr(B), below which a rigid flux lattice is formed. Thus the DHVA effect provides a unique probe of the superconducting fluctuations at high fields in this material. We compare our measurements with other determinations of the superconducting phase diagram of $κ$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$.
Raman and IR spectra of k-(BEDT-TTF)_2Cu[N(CN)_2]Br [BEDT-TTF denotes bis(ethylenedithiolo)tetrathiafulvalene] and its deuterated and partially deuterated analogues were measured at temperatures between 5 and 300 K and cooling rates from 1 to 20 K/min. It was found that, in partially deuterated samples, the interdimer electron-molecular vibration splitting of nu_3 mode in Raman spectra, the magnitude of the resonance enhancement in Raman spectra, and linewidths of some phonon peaks both in Raman and infrared spectra depend on the cooling rate. These observations were explained by disorder-rela…
We investigate the superconductivity in the deuterated BEDT-TTF molecular substitution system $κ$-[(h8-BEDT-TTF)$_{1-x}$(d8-BEDT-TTF)$_x$]$_2$Cu[N(CN)$_2$]Br, where h8 and d8 denote fully hydrogenated and deuterated molecules, respectively. Systematic and wide range ($x$ = 0 -- 1) substitution can control chemical pressure finely near the Mott boundary, which results in the modification of the superconductivity. After cooling slowly, the increase of $T_{\textrm{c}}$ observed up to $x \sim$ 0.1 is evidently caused by the chemical pressure effect. Neither reduction of $T_{\textrm{c}}$ nor suppre…
Electronic phase separation consisting of the metallic and insulating domains with 50 -- 100 $μ$m in diameter is found in the organic Mott system $κ$-[($h$8-BEDT-TTF)$_{1-x}$($d$8-BEDT-TTF)$_{x}$]$_{2}$Cu[N(CN)$_{2}$]Br by means of scanning micro-region infrared spectroscopy using the synchrotron radiation. The phase separation appears below the critical end temperature 35 -- 40 K of the first order Mott transition. The observation of the macroscopic size of the domains indicates a different class of the intrinsic electronic inhomogeneity from the nano-scale one reported in the inorganic Mott …
In the most studied family of organic superconductors kappa-(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 1/2-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 1/2-filled band models find no superconductivity. He…
Infrared reflection measurements of the half-filled two-dimensional organic conductors $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Br$_{x}$Cl$_{1-x}$ were performed as a function of temperature ($5 {\rm K}<T<300$ K) and Br-substitution ($x=0%$, 40%, 73%, 85%, and 90%) in order to study the metal-insulator transition. We can distinguish absorption processes due to itinerant and localized charge carriers. The broad mid-infrared absorption has two contributions: transitions between the two Hubbard bands and intradimer excitations from the charges localized on the (BEDT-TTF)$_2$ dimer. Since the latter coupl…
The polarized optical reflectance spectra of the quasi two dimensional organic correlated electron system $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]$Y$, $Y =$ Br and Cl are measured in the infrared region. The former shows the superconductivity at $T_{\rm c} \simeq$ 11.6 K and the latter does the antiferromagnetic insulator transition at $T_{\rm N} \simeq$ 28 K. Both the specific molecular vibration mode $ν_{3}(a_{g})$ of the BEDT-TTF molecule and the optical conductivity hump in the mid-infrared region change correlatively at $T^{*} \simeq$ 38 K of $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br, although no …
The far-infrared in-plane reflectivity of the organic superconductor $α_t$-(BEDT-TTF)$_{2}$I$_{3}$ is measured down to frequencies of 10 cm$^{-1}$; the optical conductivity is obtained by a Kramers-Kronig procedure. In the superconducting state an energy gap opens at 25 cm$^{-1}$ which corresponds to $2Δ/k_BT_c = 4.4$ in agreement with moderate coupling. Even well below $T_c=8$ K a considerable absorption remains for $\hbarω<2Δ$. We discuss different posibilities to explain this behaviour.
The effect of disorder on the electronic properties near the Mott transition is studied in an organic superconductor $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br, which is systematically irradiated by X-ray. We observe that X-ray irradiation causes Anderson-type electron localization due to molecular disorder. The resistivity at low temperatures demonstrates variable range hopping conduction with Coulomb interaction. The experimental results show clearly that the electron localization by disorder is enhanced by the Coulomb interaction near the Mott transition.
The density of states of the organic superconductor $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, measured by scanning tunneling spectroscopy on \textit{in-situ} cleaved surfaces, reveals a logarithmic suppression near the Fermi edge persisting above the critical temperature $T_\mathrm{c}$. A soft Hubbard gap as predicted by the Anderson-Hubbard model for systems with disorder exactly describes the experimentally observed suppression. The electronic disorder also explains the diminished coherence peaks of the quasiparticle density of states below $T_\mathrm{c}$.
We present direct evidence of a superconducting state existing in the title compound below 300 mK under quasi-hydrostatic pressure. The superconducing transition is observed in the whole pressure range studied, 0 < P < 4 kbar. However, the character of the transition drastically changes with suppressing the charge-density wave state.
The specific heat of the layered organic superconductor $κ$-% (BEDT-TTF)$_2$Cu(NCS)$_2$, where BEDT-TTF is bisethylenedithio-% tetrathiafulvalene, has been studied in magnetic fields up to 28 T applied perpendicular and parallel to the superconducting layers. In parallel fields above 21 T, the superconducting transition becomes first order, which signals that the Pauli-limiting field is reached. Instead of saturating at this field value, the upper critical field increases sharply and a second first-order transition line appears within the superconducting phase. Our results give strong evidence…
The vortex state in a single crystal of the layered organic superconductor κ-(BEDT-TTF2Cu(NCS)2, where BEDT-TTF (or ET) is bis(ethylenedithio)tetrathiafulvalene, was studied by 1H-NMR. Under a low field region around 0.75 T, the vortex glass-liquid transition was demonstrated by a diverging of the longitudinal nuclear spin relaxation rate and peak-broadening in spectra. Under a high field region near the upper critical field Hc2(0)~7 T, the curvature of nuclear spin relaxation curves showed a drastic change at the temperature where the emergence of the quantum vortex slush state was reported. …
We present a theoretical study on the superconductivity of $β'$-(BEDT-TTF)$_2$ICl$_2$ at $T_{\rm c}=$14.2 K under a high hydrostatic pressure recently found, which is the highest among organic superconductors. In the present work, we study an effective model using the fluctuation-exchange (FLEX) approximation based on the results of first-principles calculation. In the obtained phase diagram, the superconductivity with $d_{xy}$-like symmetry is realized next to the antiferromagnetic phase, as a result of the one-dimensional to two-dimensional crossover driven by the pressure.
We propose a mew picture for superconductivity in $κ-(BEDT-TTF)_2X$ salts arguing that small-{\bf q} electron-phonon scattering dominates the pairing. We reproduce the distinct X-shaped d-wave gap reported recently by magnetooptic measurements and we argue that the softness of the momentum structure of the gap and the near degeneracy of s- and d-wave gap states may be at the origin of the experimental controversy about the gap symmetry. We show that a magnetic field applied parallel to the planes may induce extended gapless-regions on the FS accounting for the experimental signatures of a Fuld…
The nature of the low-temperature phase of the quasi-two-dimensional conductors $α-(BEDT-TTF)_2{M}Hg(SCN)_4$[{M}=K,Rb,Tl] is considered. It is argued that the magnetic field dependence of the phase diagram is more consistent with a charge-density wave, rather than a spin-density wave ground state. The phase diagram of a charge-density wave in a magnetic field is discussed using a Ginzburg-Landau free energy derived from microscopic theory. New experiments are proposed to test the charge-density-wave hypothesis and detect an additional high field phase.
The thermal conductivity of organic superconductor kappa-(BEDT-TTF)2Cu(NCS)2 (Tc =10.4 K) has been studied in a magnetic field rotating within the 2D superconducting planes with high alignment precision. At low temperatures (T < 0.5 K), a clear fourfold symmetry in the angular variation, which is characteristic of a d-wave superconducting gap with nodes along the directions rotated 45 degrees relative to the b and c axes of the crystal, was resolved. The determined nodal structure is inconsistent with recent theoretical predictions of superconductivity induced by the antiferromagnetic spin flu…
We propose that the superconducting pairing symmetry of organic superconductors $κ$-(BEDT-TTF)$_{2}$X can be determined by measuring the position in momentum space of the incommensurate peaks of the spin susceptibility. We show that the positions of these peaks are uniquely determined by the symmetry of the superconducting state and the geometry of Fermi surface. Based on the effective Hubbard model on an anisotropic triangular lattice, we demonstrate the different incommensurate patters of spin responses for $d_{x^{2}-y^{2}}$- and $d_{xy}$-like pairing states. In addition, we predict the none…
While the optical properties of the superconducting salt $α$-(BEDT-TTF)$_2$NH$_4$Hg(SCN)$_4$ remain metallic down to 2 K, in the non-superconducting K-analog a pseudogap develops at frequencies of about 200 cm$^{-1}$ for temperatures T < 200 K. Based on exact diagonalisation calculations on an extended Hubbard model at quarter-filling we argue that fluctuations associated with short range charge ordering are responsible for the observed low-frequency feature. The different ground states, including superconductivity, are a consequence of the proximity of these compounds to a quantum phase charg…
We determine the pressure dependence of the electron-phonon coupling constant in $κ$-(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$ by comparison of high pressure Raman scattering and high pressure infrared reflectivity (IR) measurements. We use comparison of the IR reflection spectrum from deuterated and protonated samples to aid the deconvolution of several overlapping phonon modes. These coupled modes are modelled with a Green's function to extract the linear pressure dependence of the individual modes. The Raman active molecular vibrations of the BEDT-TTF dimers stiffen by 0.1-1 %GPa$^{-1}$. In contrast, t…
We report high-field magnetotransport measurements on β''-(BEDT-TTF)_4[(H_3O)M(C_2O_4)_3]Y, where M=Ga, Cr and Fe and Y=C_5H_5N. We observe similar Shubnikov-de Haas oscillations in all compounds, attributable to four quasi-two-dimensional Fermi-surface pockets, the largest of which corresponds to a cross-sectional area ~8.5\% of the Brillouin zone. The cross-sectional areas of the pockets are in agreement with the expectations for a compensated semimetal, and the corresponding effective masses are ~m_e, rather small compared to those of other BEDT-TTF salts. Apart from the case of the smalles…
We present magnetic-torque experiments on the organic superconductor k-(BEDT-TTF)2Cu(NCS)2 for magnetic fields applied parallel to the 2D superconducting layers. The experiments show a crossover from a second-order to a first-order transition when the upper critical field reaches 21 T. Beyond this field, which we interpret as the Pauli limit for superconductivity, the upper critical field line shows a pro-nounced upturn and a phase transition line separates the superconducting state into a low- and a high-field phase. We interpret the data in the framework of a Fulde-Ferrell-Larkin-Ovchinnikov…
The conducting properties of the pressure-induced, layered organic superconductor (BEDT-TTF)_3Cl_2.2H_20 have been studied at 13.5 and 14.0 kbar using low temperatures, high magnetic fields and two-axis rotation. An upper critical field that is significantly larger than that expected from the Pauli paramagnetic limit is observed when the field is applied parallel to the conducting layers. The angle dependent magnetoresistance suggests incoherent transport between the conducting layers at both pressures and the observed negative magnetoresistance at 13.5 kbar can be explained by considering And…
We performed $^{13}$C-NMR experiment and measured spin-lattice relaxation rate divided by temperature $1/T_{1}T$ near the superconducting (SC) transition temperature $T_{c}$ in $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br ($κ$-Br salt), and $κ$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$ ($κ$-NCS salt). We observed the reduction of $1/T_{1}T$ starting at the temperature higher than $T_c$ in $κ$-Br salt. Microscopic observation of quasi-particle density of states in the fluctuating SC state revealed the effects of short-range Cooper pairs induced in the normal state to the quasi-particle density of states. We also …
We show that the anisotropy of the effective spin model for the dimer Mott insulator phase of $κ$-(BEDT-TTF)$_2X$ salts is dramatically different from that of the underlying tight-binding model. Intra-dimer quantum interference results in a model of coupled spin chains, where frustrated interchain interactions suppress long-range magnetic order. Thus, we argue, the "spin liquid" phase observed in some of these materials is a remnant of the Tomonaga-Luttinger physics of a single chain. This is consistent with previous experiments and resolves some outstanding puzzles. An erratum [Phys. Rev. Let…
The superconducting transition in the layered organic compound α-(BEDT-TTF)_2KHg(SCN)_4 has been studied in the two hydrostatic pressure regimes where a charge-density wave is either present or completely suppressed. Within the charge-density-wave state the experimental results reveal a network of weakly coupled superconducting regions. This is especially seen in a strong enhancement of the measured critical field and the corresponding positive curvature of its temperature dependence. Further, it is shown that on lowering the pressure into the density-wave state traces of a superconducting pha…
The electron paramagnetic resonance study for an organic superconductor $β''$-(BEDT-TTF)$_{4}$[(H$_3$O)Ga(C$_2$O$_4$)$_3$]$\cdot$C$_6$H$_5$NO$_2$ reveals that superconductivity coexists uniformly with the charge ordered state in one material. In the charge ordered state, the interplane spin exchange is gapped, while the in-plane conductivity is not significantly modified. This anisotropic behavior is explained by the exotic charge ordered state, in which molecular-site selective carrier localization coexists with conducting carriers on other molecules. Relationship between superconductivity an…
We have studied the interlayer resistivity of the prototypical quasi-two-dimensional organic superconductor $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$ as a function of temperature, current and magnetic field, within the superconducting state. We find a region of non-zero resistivity whose properties are strongly dependent on magnetic field and current density. There is a crossover to non-Ohmic conduction below a temperature that coincides with the 2D vortex solid -- vortex liquid transition. We interpret the behaviour in terms of a model of current- and thermally-driven phase slips caused by the diffusive …
Cyclotron resonance, along with its second and third harmonics, has been observed in the quasi-two-dimensional organic superconductor $β^{\prime\prime}$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$. The harmonic content is richly angle-dependent, and is interpreted in terms of the interlayer warping of the Fermi surface giving rise to the cyclotron resonance. In a departure from Kohn's theorem, but in agreement with recent theoretical predictions, the effective mass deduced from the cyclotron resonance measurements is greater than that determined from magnetic quantum oscillations.
The Mott-insulator κ-(BEDT-TTF)_2Cu_2(CN)_3 is the prime candidate of a quantum spin liquid with puzzling magnetic properties. Our THz and infrared investigations reveal that also the charge dynamics does not follow the expectations for a Mott insulator. The frequency-dependent conductivity exhibits a power-law behavior $σ_1(ω)\propto ω^n$ that grows stronger as the temperature decreases and extends all the way through the far-infrared. With $n\approx 0.8$ to 1.5 we obtain a significantly smaller exponent than predicted by Ng and Lee [Phys. Rev. Lett. {\bf 99}, 156402 (2007)]. We suggest fluct…
The suppression of superconductivity by nonmagnetic disorder is investigated systematically in the organic superconductor $κ$-(BEDT-TTF)$_2$Cu(NCS)$_2$. We introduce a nonmagnetic disorder arising from molecule substitution in part with deuterated BEDT-TTF or BMDT-TTF for BEDT-TTF molecules and molecular defects introduced by X-ray irradiation. A quantitative evaluation of the scattering time $τ_{\rm dHvA}$ is carried out by de Haas-van Alphen (dHvA) effect measurement. A large reduction in $T_{\rm c}$ with a linear dependence on $1/τ_{\rm dHvA}$ is found in the small-disorder region below $1/…
