Ambient-pressure Superconductor Research Articles

Anisotropic H c2, shubnikov de haas, and angular dependent magnetoresistance in the organic superconductor a-(BEDT-TTF) 2MHg(SCN) 4

We report measurements on the anisotropic critical field, the angular dependent magnetoresistance, and Shubnikov de Haas oscillations in α-(BEDT-TTF) 2NH 4Hg(SCN) 4, the only ambient pressure superconductor (T c = 1.2K) in the α-(BEDT-TTF) 2MHg(SCN) 4 organic conductor family. The anisotropic critical field is well described by a 2-D model for layered supercondcutors where the anistropy ε = H c\\\\/H c⊥ is ∼ 25 at low temperatures, and rises to ∼ 100 near T c. Beating behaviour is observed in the SdH effect in tilted magnetic fields.

Coexistence of superconductivity with CDW (or SDW): ambient-pressure superconductor based on Ni(dmit)2, α-EDT-TTF[Ni(dmit)2

We present experimental evidences showing coexistence of superconductivity with CDW (or SDW) in α-EDT-TTF[Ni(dmit) 2 ], a new ambient-pressure superconductor with donor-acceptor structure. The electrical resistance of this salt exhibits a hump around 10∼14 K and drops at 1.3 K due to the superconducting transition. Based on the electrical anisotropy derived from reflectance spectra and weak-field magnetoresistance, this resistance hump is assigned to the CDW (or SDW) formation within EDT-TTF chain. Above 20 K both EDT-TTF and Ni(dmit) 2 chain contribute to the electrical conduction, while below 10 K electrical conduction is mainly through Ni(dmit) 2 chain. Temperature-pressure phase diagram determined by resistance measurements under pressure is presented.

20 K crystal structure, electrical transport, electronic band structure, scanning tunnelling microscopy and pressure–RF impedance studies on the organic conducting salt κ-(BEDT–TSF)2Cu[N(CN)2]Br

The electrochemical synthesis of κ-(BEDT–TSF)2Cu[N(CN)2]Br, which is virtually isostructural (Pnma) with the 12.5 K ambient pressure superconductor κ-(BEDT–TTF)2Cu[N(CN)2]Br, has been systematically investigated. The electrical resistivity of κ-(BEDT–TSF)2Cu[N(CN)2]Br was measured in the a[ρ300= 2(1)× 10–2Ω cm] and b[ρ300= 1.7(0.7)× 101Ω cm] crystallographic directions. The resistivity decreased monotonically from 300 to 4 K following a T2 relationship, but a superconducting state was never achieved. Tight-binding band calculations showed that the Fermi surface of κ-(BEDT–TSF)2Cu[N(CN)2]Br is indistinguishable from the BEDT–TTF analogue. STM images were obtained of both the radical cation and the polymeric anion layers. The images were in agreement with partial electron density plots. A 20 K X-ray structure of κ-(BEDT–TSF)2Cu[N(CN)2]Br was obtained and is discussed in detail. One of the BEDT–TSF ethylenedithio groups was still disordered at 20 K; this may have been responsible for the failure to observe superconductivity.

Synthesis and characterization of two new organic superconductors, κL- and κH-[bis(ethylenedisulfanyl)tetrathiafulvalene]2Au(CF3)4·(1,1,2-trichloroethane)via microelectrocrystallization

A novel microelectrocrystallization (MEC) procedure (ten-fold decrease in halogenated solvent use, micromolar quantities of electron donor molecule and supporting electrolyte, and Hg free electrodes), necessitated by the very small available quantities of the novel organometallic Au(CF3)4– anion, has been developed to prepare two distinct phases of κ-(ET)2Au(CF3)4·(TCE)[ET = bis(ethylenedisulfanyl)tetrathiafulvalene, TCE = 1,1,2-trichloroethane] which exhibit ambient pressure superconductivity with onset temperatures of 2.1 and 10.5 K.

Transport properties of ET 2Cu[N(CN) 2Cl Cl−x (0 < x < 1) organic superconductors

In plane resistance and thermopower were studied for the ET 2Cu[N(CN) 2]Cl 1−xBr x salts for the compositions that are ambient pressure superconductors (0.5<x<1). Complex relation between compound properties and composition are established.

Optical study of the electronic structure and electron-molecular vibrational coupling in a novel organic superconductor κ-(BEDT-TTF) 2Cu[N(CN) 2]Cl 0.5Br 0.5

Reflectance spectra of a new ambient-pressure superconductor, κ-(BEDT-TTF) 2Cu[N(CN) 2]Cl 0.5Br 0.5 ( T c = 11.3 K), where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, are measured in the range from 750 to 5500 cm −1 and from 9000 to 40 000 cm −1 for different polarizations. The detailed analysis of the infrared conductivity, obtained after a Kramers-Kronig transformation, is performed in the framework of the approach, based on the ‘phase phonons’ theory of M.J. Rice, with the help of special transformation of the experimental data. Parameters of the electronic subsystem and electron-molecular vibration coupling are determined. The determined ‘total’ dimensionless coupling constant (λ) is compared with those of the organic superconductors κ-(BEDT-TTF) 2Cu(NCS) 2 ( T c = 10.4 K) and (BEDT-TTF) 4Hg 2.89Br 8 ( T c = 4.3 K). It is found that a larger λ produces a larger T c in accordance with the known Bardeen-Cooper-Schriffer formula for T c.

First Ambient-pressure Superconductor Based on Ni(dmit)2, α-EDT-TTF[Ni(dmit)2

Abstract The donor-acceptor type molecular conductor, α-(EDT-TTF)[Ni(dmit)2], was found to be a superconductor under ambient pressure below 1.3 K. This is the first ambient-pressure superconductor in a series of M(dmit)2 salts [M = Ni, Pd, Pt]. The sample dependence and magnetic-field dependence on the superconducting transition are described.

Structural and physical properties of two new ambient pressure κ-type BEDT-TTF superconductors and their related salts

The electrooxidation of BEDT-TTF (ET) in the presence of Cu +, N(CN) 2 − and CN − gave three kinds of salts, two of which are ambient pressure superconductors; 1 κ-(ET) 2Cu(CN)[N(CN) 2] + (Tc=11.2K) and 2 κ′-(ET) 2Cu 2(CN) 3 (Tc=3.8K), and the other is a semiconductor; 3 θ-(ET) 2Cu 2(CN)[N(CN) 2] 2. In 1 , CN − bridges Cu + to form a zigzag infinite chain and N(CN) 2 − coordinates to Cu + as a pendant. In 2 , six CN − and six Cu + form a hexagonal ring which constructs honeycomb network. In 1 and 2 the donor dimers fit into the openings of the anion layer. The polymerized anion structure of 3 is unique. Both 1 and 2 show no semiconductive behavior above 100K which has been observed in the 10K ambient pressure superconductors; κ-Cu(NCS) 2 and κ-Cu[N(CN) 2]Br. Tc decreases with pressure ( dTc/dP=−2.47 for 1, −1.25 K/kbar for 2 ). A large inverse isotope effect on Tc is observed in 1 (Tc=12.3K for the deuterated salt, this is the highest Tc at ambient pressure among the ET superconductors). Hc 2 values of 1 are almost comparable to those of κ-Cu(NCS) 2 and κ-Cu[N(CN) 2]Br [Hc 2=21.5T(//), 7.0T(⊥) at 1.4K]. Shubnikov-de Haas oscillations are not observed up to ca.30T in 1 though the calculated Fermi surface indicates the presence of a closed orbit. The thermopower of 1 shows anisotropic temperature dependence which coincides with the calculated Fermi surface qualitatively.

Isotope effect in 13C-substituted (central C=C) κ-phase organic superconductors

The organic electron-donor molecule ET [bis(ethylenedithio)tetrathiafulvalene] was synthesized twice by the identical route: (a) with natural (12C) carbon isotopic distribution; (b) with 95% 13C, in the central C-C double bond position. The superconducting transition temperatures (in magnetic fields of 0.0–0.4 T) of single crystals prepared by both routes of ET synthesis were found, by the use of ac susceptibility measurements, to be identical within the precision of the measurements (∼0.1K or 1%) for the ambient-pressure superconductors κ-(ET)2Cu(NCS)2 κ-(ET)2Cu[N(CN)2]Br. The absence of an effect of such isotopic replacement on Tc indicates that the general C= stretching vibration (a high-energy optical mode at ∼1500 cm−1) is not the dominant contributor to the superconducting pairing mechanism in these systems.

Some new radical-cation salts based on unsymmetrical tetraheterofulvalenes: (MDTTTF) 4Pt(CN) 4, the first κ-phase salt, with a square planar dianion

We report the synthesis of the organic donor radical cations, methylenedithiotetrathiafulvalene (MDTTTF), ethylenedithiotetrathiafulvalene (EDTTTF) and vinylenedithiotetrathiafulvalene (VDTTTF), with the square planar dianions, Pt(CN) 4 and Ni(CN) 4. Among these salts, the κ-(MDTTTF) 4Pt(CN) 4 (CH 2Cl 2) phase constitutes the first κ-phase, produced with a nonlinear dianion. Crystal and electronic structures, electrical and optical properties and ESR measurements have been investigated for these salts. Crystal and electronic structures of κ-(MDTTTF) 4Pt(CN) 4 are similar to those of (MDTTTF) 2AuI 2, which is an ambient pressure superconductor ( T c = 4.5 K). However, d.c. conductivity measurements show a semiconducting behaviour, associated with an ESR linewidth decreasing with temperature in contrast with all other known κ salts. An explanation of the origin of these different behaviours is proposed.

New κ-phase materials, κ-(ET)2Cu[N(CN)2]X.X=Cl, Br and I. The synthesis, structure and superconductivity above 11 K in the Cl (Tc = 12.8 K, 0.3 kbar) and Br(Tc = 11.6 K) salts

The syntheses, structures, selected physical properties, and band electronic structures of three copper (I) dicyanamide halide salts of bis(ethylenedithio)tetrathiafulvalene [κ-(ET)2Cu[N(CN)2]X, where X = Cl, Br, and I] are discussed. X-ray crystallographic studies demonstrate that the three derivatives are isostructural. The bromide salt is an ambient pressure superconductor with an inductive onset at 11.6 K and a resistive onset at 12.5 K. κ-(ET)2Cu[N(CN)2]Cl exhibits the highest reported superconducting transition temperature (Tc = 12.8 K. 0.3 kbar) for an organic superconductor, once a semiconductor-semiconductor transition (42 K) is suppressed. The application of GE varnish or Apiezon N grease to crystals of κ-(ET)2Cu[N(CN)2]Cl provides sufficient stress to induce superconductivity at “ambient pressure”. Crystals of the iodide remain metallic to ∼150 K, where they become weakly semiconductive. No sign of superconductivity was detected at pressures (hydrostatic and shearing) up to 5.2 kbar and at temperatures as low as 1.1 K. The band electronic structures of the three salts are essentially identical. The differences in superconducting properties are explained in terms of differences in lattice softness, which are strongly influenced by short C-H…donor and C-H…anion contacts.

Strain index, lattice softness and superconductivity of organic donor-molecule salts: Crystal and electronic structures of three isostructural salts k-(BEDT- TTF) 2Cu[N(CN) 2]X (X=Cl, Br, I)

The recently discovered k-phase salts k-(BEDT-TTF) 2]Cu[N(CN) 2]X (X=Cl,Br, I) are isostructural but differ in their superconducting properties: the Br-salt is an ambient-pressure superconductor with T c=11.6K, the Cl-salt becomes superconducting ( T c=12.8K) under a slight applied pressure of 0.3 kbar, and the I-salt does not exhibit superconductivity under an applied-pressure of up to 5 kbar. We examine the structural and electronic properties of these three salts on the basis of their crystal structures determined at 127 K by single crystal X-ray diffraction measurements. The band electronic structure of the three salts are virtually the same and do not account for the differences in the conduction properties. The differences in the three salts with regard to superconductivity are examined from the viewpoint of lattice softness by calculating strain indices for short intermolecular contacts. The random potential arising from ethylene group conformational disorder prevents superconductivity in the iodide salt.

From semiconductor-semiconductor transition (42 K) to the highest-Tc organic superconductor, .kappa.-(ET)2Cu[N(CN)2]Cl (Tc = 12.5 K)

The discovery of an organic superconductor with the highest transition temperature (125 K) recorded following the suppression of a semiconductor-semiconductor transition (42 K) is reported. The reported compound is a kappa-phase superconductor, kappa-(ET){sub 2}Cu(N(CN){sub 2})Cl, which is isostructural with the recently prepared ambient-pressure superconductor kappa-(ET){sub 2}Cu(N(CN){sub 2})Br with a transition temperature of 11.5 K that has been previously reported. Band electronic structural calculations show the newly discovered compound to be a two-dimensional metal. 15 refs., 2 figs.

Recent Progress in the Development of Structure-Property Correlations for κ-Phase Organic Superconductors

Abstract An examination of the molecular packing arrangements of the electron-donor molecules in κ-phase organic conductors and superconductors [κ-(BEDT-TTF)2X (X ˭ I3−, Cu(NCS)2−), κ-(MDT-TTF)2AuI2, and κ-(BMDT-TTF)2Au(CN)2] reveal that the main predictors of superconductivity appear to be (i) bond-over-ring intradimer molecular packing and (ii) relatively short (∼ 3.35A) intradimer separations. These structural features in newly discovered κ-phase organic conductors may be requirements for the occurrence of ambient pressure superconductivity.

(MDT-TTF) 2 AuI 2: An ambient pressure organic superconductor (T c = 4.5 K) based on an unsymmetrical electron donor

R f penetration depth measurements show (MDT-TTF) 2AuI 2 to be an ambient pressure superconductor with a T c ∼4.5 K. The effectof hydrostatic pressure on T c was found to be −0.92 K/kbar, which suggests that anions longer than AuI 2 - may puff up the lattice and may yield cation-radical salts of MDT-TTF with even higher superconducting transition temperatures. (MDT-TTF) 2AuI 2 has a room temperature ESR linewidth of ∼ 100 G, which gradually increases as the temperature is lowered. The structure and properties of (MDT-TTF) 2AuI 2 bear a close resemblance to those of the 10 K organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2. The band electronic structure of (MDT-TTF) 2AuI 2 is two-dimensional, with a closed Fermi surface. The MDT-TTF donor molecule prossesses only four CH bonds to form short contacts with anions, as compared to eight for BEDT-TTF. Thus, the translational and/or librational modes of vibration are expected to be substantially different in (MDT-TTF) 2AuI 2 and κ-(BEDT-TTF) 2Cu(NCS) 2.

Static and ESR susceptibilities of the ambient pressure superconductors β- and α t-(BEDT-TTF) 2I 3 and (BEDT-TTF) 2Cu(SCN) 2

Whereas the static spin-susceptibility χ(T) of β- and α t-(BEDT-TTF) 2I 3 at 5.7 tesla increases in a slow monotonic manner with temperature, χ s(T) for the organic superconductor (BEDT-TTF) 2Cu(SCN) 2 displays a markedly structured temperature dependence. This gives evidence for two or more phase transitions in the metallic phase at temperatures near 100 K and 50 K, in addition to the onset of superconductivity at 5.7 tesla below ∼ 8 K. Good agreement is found between the absolute magnitudes of static and ESR susceptibility results, thus confirming that orbital susceptibility contributions are of negligible importance here. Cooling in low magnetic fields reveals that superconductivity in (BEDT-TTF) 2Cu(SCN) 2 sets in at a temperature near 8.5 K, with a 7.6% Meissner effect in 14 G field.

A new ambient pressure organic superconductor (BEDT-TTF) 2Cu(NCS) 2 with Tc above 10 K

An ambient pressure superconductivity in (BEDT-TTF) 2Cu(NCS) 2 is reported. The superconducting critical temperature is the highest among the organic superconductors so far obtained(Tc=10.4 K). The salt prepared using deuterated BEDT-TTF is also an ambient pressure superconductor with a slightly higher Tc (11.0K). The crystal structure analysis and resistivity measurement revealed the highly two-dimensional nature of this salt. The temperature dependence of normal resistivity, superconducting critical field, quantum oscillation of resistivity and so on are reported down to 0.5 K and up to 13.5 T. The superconducting upper critical field shows a peculiar temperature dependence, and the parallel critical field behaviour is ascribed to the dimensional crossover effect. The quantum oscillation is understood as the Shubnikov-de Haas effect, and the possible Fermi surface is presented. The possible superconducting mechanisms are also discussed.

Peculiar Critical Field Behaviour in the Recectly Discovered Ambient Pressure Organic Superconductor (BEDT-TTF)2Cu (NCS)2(Tc=10.4K)

An upper critical field measurement in the recently discovered ambient pressure organic superconductor (BEDT-TTF) 2 Cu(NCS) 2 ( T c =10.4K) is reported. Three- to two-dimensional crossover behaviour is observed in the temperature dependence of the parallel critical field. The anisotropy of the coherence length near T c , ξ c /ξ a * , is about 19 and ξ a * (0) is smaller than the lattice spacing along the a * -axis. An unusual temperature dependence of the perpendicular critical field is observed far below T c . It is confirmed that deuterated salt is also an ambient pressure superconductor with a slightly higher T c (11 K).

A New Ambient Pressure Organic Superconductor Based on BEDT-TTF with TC Higher than 10 K (TC=10.4 K)

Abstract An ambient pressure superconductivity of (BEDT-TTF)2Cu(SCN)2 was observed by d.c. magnetic susceptibility and electrical conductivity measurements. The superconducting critical temperature is the highest (TC=10.4 K) among the organic superconductors so far obtained, even though the anion has a positional disorder in the crystal.

New organic superconductors K- and θ-(BEDT-TTF) 2I 3: Transport property

Transport properties of newly synthesized organic conductors K - and θ-(BEDT-TTF) 2I 3 are investigated. We find that these materials are quasi-two dimensional metals. Both materials exhibit ambient pressure superconductivity with T c around 3.6K. The upper critical magnetic field is determined as a function of temperature and the coherence length is estimated. We find that the coherence lengths for k -type in the two dimensional plane and normal to it are about 360A and 13.5A, respectively.