In-Plane ESR Microwave Conductivity Measurements and Electronic

Paul G. Nixon, Rolf W. Winter, and Gary L. Gard. Department of Chemistry, Portland State UniVersity, Portland, Oregon 97207-0751. ReceiVed: April 19, ...
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J. Phys. Chem. B 1999, 103, 5493-5499

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In-Plane ESR Microwave Conductivity Measurements and Electronic Band Structure Studies of the Organic Superconductor β′′-(BEDT-TTF)2SF5CH2CF2SO3 H. Hau Wang,* Michael L. VanZile, John A. Schlueter, Urs Geiser, Aravinda M. Kini, and Paul P. Sche Chemistry and Materials Science DiVisions, Argonne National Laboratory, Argonne, Illinois 60439

H.-J. Koo and M.-H. Whangbo Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204

Paul G. Nixon, Rolf W. Winter, and Gary L. Gard Department of Chemistry, Portland State UniVersity, Portland, Oregon 97207-0751 ReceiVed: April 19, 1999

The electronic structure of the organic superconductor β′′-(BEDT-TTF)2SF5CH2CF2SO3 (BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene) was characterized with the use of electron spin resonance (ESR) spectroscopy and electronic band structure calculations. The room-temperature ESR line width is 24-27 G in the plane of a donor molecule layer (i.e., in the ab-plane) and ∼32 G along the normal to this plane (i.e., along the c*-direction). The ab-plane anisotropy of the microwave conductivity was extracted for the first time from the ESR Dysonian line shape analysis. The in-plane conductivity varies sinusoidally, is maximal along the interstack direction (b-axis), and is minimal along the donor stack direction (a-axis). The Fermi surfaces of the title compound consist of a 2D hole pocket and a pair of 1D wavy lines. The directions for the in-plane conductivity maximum and minimum are in excellent agreement with the electronic band structure calculated for β′′-(BEDT-TTF)2SF5CH2CF2SO3, and the origin of the in-plane conductivity anisotropy lies in the one-dimensional part of the Fermi surface. This is the first time that an organic conductor shows Dysonian ESR line shape due to its 2D and strongly metallic nature, yet the 1D character is revealed simultaneously through the in-plane conductivity anisotropy.

1. Introduction All BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene or ET] radical cation based organic superconductors consist of twodimensional highly conductive donor layers alternating with charge-compensating and charge-insulating anion layers. Information concerning in-plane conductivity anisotropy that bears direct correlation to the theoretical Fermi surface is not easily accessible. Conventional four-probe measurements provide conductive information in the plane but only along one direction because of the fact that direct contact is required for the method. In an attempt to obtain in-plane angular-dependent conductivity, we have carried out noncontact electron spin resonance (ESR) measurements on the newly reported organic superconductor β′′-(ET)2SF5CH2CF2SO3. The in-plane microwave conductivities were extracted for the first time for this class of materials. In this article, we demonstrate that the resulting microwave conductivity anisotropy correlates well with the calculated Fermi surface by extended Hu¨ckel theory. Organic conductors and superconductors based on organic donor molecules exhibit novel physical properties such as superconductivity, field-induced spin density wave, quantum Hall effect, etc.1,2 The first organic superconductor, (TMTSF)2PF6 salt, where TMTSF denotes tetramethyltetraselenafulvalene, was realized in 1980 with a superconducting transition temperature, * Author for correspondence.

Tc of 0.9 K under 12 kbar pressure.3 The organic superconductors with the highest Tc’s are found in the family of κ-(ET)2Cu[N(CN)2]X, X ) Br (Tc is 11.6 K at ambient pressure) and X ) Cl (12.8 K under 0.3 kbar).4,5 The ET molecules in the oxidized form are nearly flat and tend to form a layer of parallel stacked donor molecules (β-phase) or a layer of orthogonally packed dimers (κ-phase). The crystal structures of typical ET salts consist of such two-dimensional donor layers alternating with anion layers. As a result, they are highly conducting within the layer due to S‚‚‚S orbital overlap and poorly conducting normal to the layer. The β-phase organic superconductors consist of four isostructural members, β-(ET)2X (X ) I3, IBr2, AuI2, and β*-I3),6-9 while about 30 κ-phase superconductors are known.2,10 Recently, we reported a new ambient pressure organic superconductor β′′-(ET)2SF5CH2CF2SO3, with Tc at 5.2 K.11 In the donor-molecule layers of the β′′-phase, the plane of the ET molecule is tilted away from the donor stacking axis by about 60°, and the nearest-neighbor ET molecules between stacks are almost coplanar. There are three other β′′-like superconductors, i.e., (ET)3Cl2‚2H2O (Tc ) 2K under 16 kbar)12 and (ET)4(H2O)M(C2O4)3‚C6H5CN (Tc ) 7.0 K for M ) Fe and Tc ) 6.0 K for M ) Cr).13,14 These β′′-like salts are not isostructural but have common structural features in the donor layers. Herein, we present the normal-state ESR properties, extracted in-plane microwave conductivities, and electronic band structure of the new superconductive β′′-(ET)2SF5CH2CF2SO3 salt.

10.1021/jp991268j CCC: $18.00 © 1999 American Chemical Society Published on Web 06/11/1999

5494 J. Phys. Chem. B, Vol. 103, No. 26, 1999

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2. Experimental Section Crystals of the β′′-(ET)2SF5CH2CF2SO3 salt were prepared using electrocrystallization techniques as previously described.11 Both 1,1,2-trichloroethane (TCE) and tetrahydrofuran (THF) were used as solvents for crystal growth. Crystals from both solvents showed superconductivity during ac susceptibility measurements, and the samples with the highest Tc (5.2 K) were obtained from TCE solvent. The ESR measurements were carried out using an IBM ER200 X-band spectrometer with a TE102 rectangular cavity. A single-crystal sample of β′′-(ET)2SF5CH2CF2SO3 was mounted to a quartz rod with a thin layer of Dow Corning silicone grease. The rotation of the sample was performed with a single-axis ESR goniometer with 1° resolution. Low temperatures (4-300 K) were achieved with an Oxford 900 flow-through cryostat, and high temperatures (300-420 K) with a 4111VT temperature controller. A strong pitch (g ) 2.0028) was used for g-value calibration. Data acquisition was accomplished using the LabView software (National Instruments Corporation) and a locally developed user interface program. Typically, 400 data points were taken for each spectrum. Spectrum fits were carried out using the program KaleidaGraph (Synergy Software) on a Macintosh Power PC computer. 3. Results and Discussion ESR measurements of the organic conductors at ambient temperature provide angle-resolved data for g-values, peak-topeak line widths (∆H), and spin susceptibilities (χ) that are related to a type of crystal packing.2 In addition, anisotropic microwave conductivities can be deduced from the Dysonian line shape analysis. A. Orientation Dependence of g-Factor and Line Width at Room Temperature. (1) Rotation around the b-Axis. The donor layers of the β′′-(ET)2SF5CH2CF2SO3 salt lie in the abplane.11 A rectangular platelike crystal of β′′-(ET)2SF5CH2CF2SO3 was mounted with the long crystal axis (i.e., the b-axis) vertical in the ESR cavity. The crystal was rotated around the b-axis with 0° corresponding to the static magnetic field parallel to the crystal (ab-) plane or nearly along the a-axis, and 90° corresponding to exactly along the c*-axis. The spectra were taken with 10° steps and were fitted with a single Lorentzian derivative curve.15 All fits were better than 0.9999 in Pearson’s R value. The observed peak-to-peak line widths (∆Hobs) and g-value squares (gobs2) were fitted with the following equations: 15,16

∆Hobs ) (∆H|) sin2 θ + (∆Hx) sin 2θ + (∆H⊥) cos2 θ (1) gobs2 ) g|2 sin2 θ + gx2 sin 2θ + g⊥2 cos2 θ

(2)

The ∆H| is the line width value at 0°, ∆H⊥ is that at 90°, and ∆Hx is a nonzero term, which allows the maximum and minimum line widths to be fitted away from 90 and 180°. For samples of the triclinic crystal system (space group P1h in this case), the ∆Hx term (∼4.3 G) is allowed to be nonzero. The corresponding g-values have the same meanings. The angular dependence of the peak-to-peak line widths (triangles) and g-values (circles) and the least-squares-fit curves using eqs 1 and 2 shown as thin lines are plotted in Figure 1. The resultant line widths and g-values range from 23 to 34 G and from 2.004 to 2.012, respectively. The line widths and the g-values are in phase, which is typical for β-phase salts.17,18 The maximum values (both ∆H and g) occur near 65°, which is 25° away from the c*-axis (at 90°). There are two independent ET molecules

Figure 1. Line widths in G (triangles) and g-values (circles) of β′′(ET)2SF5CH2CF2SO3 as a function of rotation around the crystallographic b-axis. The inset shows a typical Lorentzian derivative ESR spectrum.

in the unit cell (see Figure 3), and the central CdC bonds of these molecules form an angle of 27.2° and 27.5°, respectively, with the c-axis. Since the c- and c*-axes differ within ∼5°, the maximum ∆H and g values correspond to the central CdC double bond direction, i.e., the long molecular axis direction. The minimum values for both ∆H and g occur at 155°, which are 90° from the maximum values. This result is consistent with that of the other β-phase salts; i.e., the minimum g-value is found along the normal to the molecular plane.17 Similar orientation studies for a β′′-(ET)2SF5CH2CF2SO3 crystal grown from THF reveal the same g-value range (2.004-2.012) with a slightly broader line width from 23.3 to 34.5 G, indicating slightly inferior crystal quality. (2) Rotation in the ab-Plane. The same β′′-(ET)2SF5CH2CF2SO3 rectangular crystal was oriented horizontally in the ESR cavity. The crystal was rotated around the c*-axis with 0° indicating that the static magnetic field was perpendicular to the long crystal axis (i.e., along the b-axis). Under these conditions, the microwave electric field was always parallel to the highly conductive ab-plane, and the ESR spectra were Dysonian in line shape. The spectra were taken in 10° steps, and all spectra were fitted with a single Dysonian derivative curve with eq 3 below (see Appendix).19

Y′(H) )

-I [2y - A (y2 - 1)] + C0 (1 + y2)2 y)

(3)

H - H0

x3 ∆H 2

where I is the intensity of the ESR absorption, H0 is the center of the absorption, ∆H is the ESR line width, A is the asymmetry factor (i.e., a measure of the admixture of the dispersive component), and C0 is a constant to compensate for the vertical spectra displacement. The five parameters, I, H0, ∆H, A, and C0, were optimized in the Dysonian line shape analysis. Fits for all spectra were better than 0.9997 in Pearson’s R value. The angle dependence of the line widths and g-values obtained in this fashion were again fitted to eqs 1 and 2. The line widths (triangles), g-values (circles), and the best-fit curves are plotted in Figure 2. As shown in Figure 2, the line widths and g-values range from 24 G (2.004) to 27 G (2.008). Since the crystal was rotated along the c*-axis, the g-value probes the plane containing the short molecular axis and the normal to the molecular plane. The maximum g-value (2.008) occurs at 50°, which is the direction along the short molecular axis. The minimum g-value

β′′-(BEDT-TTF)2SF5CH2CF2SO3

Figure 2. Line widths in G (triangles) and g-values (circles) of β′′(ET)2SF5CH2CF2SO3 as a function of rotation around the crystallographic c*-axis. The inset shows a typical Dysonian derivative ESR spectrum.

J. Phys. Chem. B, Vol. 103, No. 26, 1999 5495

Figure 4. Asymmetry factors A and relative microwave conductivities (σ/σmin) in the ab-plane as a function of the rotation around the c*axis.

B. Conductivity Anisotropy in the ab-plane. Microwave conductivities of highly conductive samples can be extracted from the Dysonian derivative line shapes. The Dysonian absorption (W) is described as a linear combination of the absorption (χ′′) and dispersion (χ′) of the sample as follows:19

W ) D(χ′′ + Aχ′)

(4)

where D is a constant and A is an asymmetry factor that can be determined experimentally from the Dysonian line shape analysis (see eq 3 and Appendix). The admixture of a dispersion component to the absorption signal is due to induced eddy currents in the electrically conducting sample. The microwave conductivity is extracted according to the equations

σ)

c2 c2 ) ) Kp2 2πωδ2 2πω 2t 2 p

()

(5)

and

( )

p σ ) σmin pmin Figure 3. Perspective view of a donor molecule layer of β′′-(ET)2SF5CH2CF2SO3 salt showing the in-plane g-values and the approximate directions of the minimum and the maximum microwave conductivities (structure redrawn from ref 11).

(2.004) occurs around 140°, which is along the normal to the ET molecular plane (Figure 3). In summary, the three g-values extracted from the orientation studies are 2.012 along the long molecular axis, 2.008 along the short molecular axis, and 2.004 normal to the molecular plane. These data are similar to those determined for β-(ET)2I3 (i.e., 2.0111, 2.0065, and 2.0025, respectively).17 The line width of the title compound is 32 G along the direction perpendicular to the ab-plane and 24-27 G in the ab-plane. Other β′′-like salts gave similar line widths, i.e., 22-34 G for (ET)2ClO4(TCE)0.5,20 ∼40 G for β′′-(ET)2AuBr2,21 ∼37 G for (ET)3Cl2‚ 2H2O,22 and 23-35 G for (ET)4(H2O)Fe(C2O4)3‚C6H5CN.13 The line widths for the β′′-type salts are generally larger than that of the β-phase salts (15-23 G).2 The similar g-values and line widths from many β′′-type salts with different anions show that contributions to the conduction electron ESR signal come mainly from the ET molecules and their structural packing motifs, and contributions from the anion are minimal. In β′′-(ET)2AuBr2, the line width is slightly broader because spin-orbit coupling is stronger because of the heavier element bromine.

2

(6)

where c is the speed of light, δ is the skin depth, 2t is the sample thickness, p is the ratio between the sample thickness and the skin depth (2t/δ), and K is a constant. From the Dysonian derivative spectra associated with the crystal rotation around the c*-axis, the asymmetry factors (A) were extracted. On the basis of the correlation between the p and asymmetry factor, which has been derived analytically for flat plate samples,19 the relative microwave conductivities (σ/σmin) were calculated from eq 6. The resulting asymmetry factors (circles), relative microwave conductivities (diamonds), and the best-fit sinusoidal curves are plotted in Figure 4 as a function of the ab-plane rotation angle. As shown in Figure 4, the in-plane relative microwave conductivities were extracted for the first time, and the minimum and maximum microwave conductivities in the ab-plane occur at 20° and 110°, respectively. The 20° direction is along the ET stacking axis, and the 110° direction is along the b-axis (see Figure 3). Thus, Figures 3 and 4 show that the microwave conductivity in the ab-plane is minimum along the stacking direction, is maximum along the interstack direction, and varies sinusoidally between these values as a function of the rotation angle θ. To rule out that the observed microwave conductivity anisotropy is due to a slightly uneven microwave electric field in the rectangular cavity, we carried out similar measurements

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Figure 5. Dispersion relations of the four highest occupied bands calculated for β′′-(ET)2SF5CH2CF2SO3, where the dashed line refers to the Fermi level, Γ ) (0, 0), X ) (a*/2, 0), Y ) (0, b*/2), and M ) (a*/2, b*/2).

on a crystal sample of another organic superconductor, κ-(ET)2Cu(NCS)2, which is known to be highly isotropic in the ab-plane.2 The crystal was oriented horizontally in the same cavity and rotated around the normal to the donor layer plane. Although Dysonian line shape was observed throughout the measurement, no systematic trend for the asymmetry factor was observed. Therefore, the observed sinusoidal in-plane microwave conductivity is a unique characteristic of the β′′-phase organic conductors. The origin of the ab-plane conductivity anisotropy in the title salt is discussed in the next section. C. Electronic Band Structure and ab-Plane Conductivity. To understand the reason for the angular dependence of the abplane conductivity, the electronic band structure of β′′-(ET)2SF5CH2CF2SO3 was calculated using the extended Hu¨ckel tight binding (EHTB) method.23,24 Figure 5 shows dispersion relations of the highest four occupied bands, which are primarily made up of the HOMOs of ET molecules. The two highest-lying bands are partially filled and determine the anisotropy of the electrical conductivity of β′′-(ET)2SF5CH2CF2SO3. The partially filled bands are substantially more dispersive along M f X and Γ f Y than along Γ f X and M f Y. This means that the HOMO-HOMO interactions between nearest-neighbor ET molecules are much stronger along the interstack direction than along the stacking direction. Therefore, the energies of interaction Hij ) 〈ψi|Heff|ψj〉, where Heff is an effective Hamiltonian and ψi and ψj are the HOMOs of ET molecules i and j, respectively, should be much larger in magnitude along the interstack direction. This is indeed the case as shown in Figure 6. The Fermi surfaces associated with the two partially filled bands are presented in Figure 7 in an extended zone scheme. Within the first primitive zone (FPZ), the Fermi surfaces consist of a hole pocket centered at X and a pair of wavy lines straddling along M f Y. Thus, β′′-(ET)2SF5CH2CF2SO3 has both onedimensional (1D) and two-dimensional (2D) Fermi surfaces. The 2D hole pocket has the shape of a rounded rectangle, and its size is about 14.8% of the FPZ. This is largely consistent with the results of a recent magnetoresistance study of β′′-(ET)2SF5CH2CF2SO3, which showed the presence of a noncircular closed pocket with 5% of the FPZ in size.25 To a first approximation, the 2D hole pocket predicts an isotropic conductivity in the abplane. A system with a 1D surface composed of parallel lines has a minimum (maximum) conductivity along the direction parallel (perpendicular) to the lines. Thus, according to the 1D Fermi surface of Figure 7, the maximum conductivity is

Figure 6. HOMO-HOMO interaction energies βij (in meV) between nearest-neighbor ET molecules in a donor molecule layer of β′′(ET)2SF5CH2CF2SO3. The calculated values are based on structure determined at 123 K.

Figure 7. Fermi surfaces calculated for β′′-(ET)2SF5CH2CF2SO3. The closed pockets located at X and its equivalent points are hole surfaces, and the wavy lines straddling along the Y-M direction are electron surfaces.

expected along the b-direction (i.e., perpendicular to M f Y) and the minimum conductivity along Γ f X (i.e., along the stacking direction). This is in agreement with the observed anisotropy in the ab-plane microwave conductivity (Figure 3), and the origin of the anisotropy is from the 1D part of the Fermi surfaces. D. Line Width and Susceptibility at Low Temperature. The β′′-(ET)2SF5CH2CF2SO3 crystal was mounted vertically in the cavity for low-temperature ESR studies. The b-axis was kept vertical, and the static magnetic field was parallel to the c*axis. The relative spin susceptibility (diamonds) and the peakto-peak line width (triangles) are plotted against temperatures in Figure 8. Both spin susceptibility and line width are temperature-dependent. The spin susceptibility decreased by 35% from room temperature to 140 K. For this temperature region, a very small activation energy of 8.9 meV is extracted from the slope of a ln[χ(T)/χ(295)] versus 1/T plot. A similar activated behavior in the ESR spin susceptibility was previously reported in the temperature range 60-300 K for the pressureinduced organic superconductor β′′-like (ET)3Cl2‚2H2O. This was attributed to Peierls fluctuations, which reduce the number of carriers with temperature.21 It is reasonable to propose the same explanation for β′′-(ET)2SF5CH2CF2SO3 because the partially nested 1D Fermi surface can provide such an electronic

β′′-(BEDT-TTF)2SF5CH2CF2SO3

J. Phys. Chem. B, Vol. 103, No. 26, 1999 5497

Figure 8. Line widths in G (triangles) and relative spin susceptibilities (diamonds) of β′′-(ET)2SF5CH2CF2SO3 as a function of temperature in the low-temperature region (5-300 K).

instability. Below 140 K, the spin susceptibility of β′′-(ET)2SF5CH2CF2SO3 is nearly constant to 20 K. This behavior is indicative of a metallic property. Below 10 K, the spin susceptibility decreases precipitously, which is consistent with the superconducting onset temperature near 5 K. The line width decreases with decreasing temperature. There is a notable slope change at 150 K (0.035 G/K above 150 K, and 0.155 G/K below 150 K). Below 20 K, the slope decreased again and the ESR signal turns into a very narrow residual line width of 0.26 G at 5 K. The four-probe conductivity measurements also show a similar behavior, i.e., semiconducting between room temperature and ∼120 K, metallic between 120 and 6 K, and superconducting below 6 K (Figure 9). The activation energy extracted from the initial semiconducting region was ∼150 meV, which is considerably larger than the corresponding value determined from ESR measurements (i.e., 8.9 meV). This large difference could be caused by several reasons. The microwave measurements (X-band ESR) are carried out at ∼9.5 GHz frequency, while four-probe measurements are performed with the use of a dc or low-frequency ac current. The ESR method has no direct contacts and is based on the bulk of the sample, while the fourprobe method requires direct contacts and might be sensitive to surface conditions and microcrackings in the specimen.26 For the temperature-dependent line width caused by spinorbit coupling, the relationship between line width and resistivity is described by eq 7:15,18,27

∆H ∝

[

]

(∆g)2 ne2(∆g)2 F ) τ m

(7)

where τ is the relaxation time, n is the number of carriers, m is the effective mass, and F is the resistivity. The κL-(ET)2Cu(CF3)4(TCE) salt follows this relationship closely.15 The κL(ET)2Cu(CF3)4(TCE) salt differs from the β′′-(ET)2SF5CH2CF2SO3 salt in that the spin susceptibility of the κL-phase is nearly temperature-independent while that of the β′′-phase decreases with decreasing temperature. A decrease in the susceptibility means a decrease in the number of carriers n. Equation 7 shows that the line width ∆H becomes broader with increasing F but becomes narrower with decreasing n. For the β′′-(ET)2SF5CH2CF2SO3 salt, the net result of these competing effects is that the line width decreases with decreasing temperatures but with a slope change, as evidenced by the change at ∼150 K in the line width-versus-temperature plot (Figure 8). Finally, we comment on the residual line width of the β′′(ET)2SF5CH2CF2SO3 salt. This salt shows a very sharp residual line width of 0.26 G at 5 K, which is comparable to two other

Figure 9. Relative electrical resistivity of β′′-(ET)2SF5CH2CF2SO3 in the 4-270 K region measured by the four-probe method. The inset shows a close-up of the 4-10 K region showing the onset of superconductivity below ∼6 K.

Figure 10. Line widths (G) and relative spin susceptibilities of β′′(ET)2SF5CH2CF2SO3 as a function of temperature in the hightemperature region (300-410 K).

superconducting β-(ET)2X salts (0.2 G at 5 K for X ) IBr2 and 0.6 G at 2 K for X ) AuI2) but not to β-(ET)2I3 (2 G at 5 K), β-(ET)2I2Br (2.5 G at 5 K), and (ET)2ClO4(TCE)0.5 (5 G at 3 K).20,28 The β-(ET)2I3 salt has disorder in the ethylene group conformations, while the β-(ET)2I2Br and (ET)2ClO4(TCE)0.5 salts have disorder in the anion and solvent arrangements.29-31 Such a disorder gives rise to broader residual ESR line widths. The sharp line width observed for the β′′-(ET)2SF5CH2CF2SO3 salt is consistent with the fact that the ethylene groups and the anion are nicely ordered in the 123 K crystal structure and mostly likely remain ordered at 5 K.11 E. Line Width and Susceptibility at High Temperature. A crystal of β′′-(ET)2SF5CH2CF2SO3 was oriented vertically in a capillary tube and was wedged with cotton to avoid sample movement during heating. The sample was heated from 300 to 410 K at a 10 K increment. The line width (triangles) and relative spin susceptibility (squares) are plotted in Figure 10. The line width shows a small increase with increasing temperature. The slope of the line width change is comparable to that of the line width change in the 300-150 K region. The spin susceptibility also increases with increasing temperature, which indicates a semiconducting state. The activation energy is estimated to be 19.8 meV. The high-temperature studies of β′′(ET)2SF5CH2CF2SO3 revealed no phase transition in the 300410 K region and a semiconducting behavior similar to that in the 150-300 K region. No experiments were carried out above 410 K because ET salts decompose beyond that temperature.

5498 J. Phys. Chem. B, Vol. 103, No. 26, 1999 4. Concluding Remarks The normal-state ESR properties of the β′′-(ET)2SF5CH2CF2SO3 salt were examined. The line width range is 24-27 G in the highly conductive ab-plane and 32 G along the c*-direction. This salt is semiconducting in the temperature range between 410 and 150 K, metallic between 150 and 10 K, and superconducting below 5 K. The microwave conductivity in the ab-plane is extracted for the first time from the Dysonian line shape analysis. It varies sinusoidally, is minimal along the ET stacking direction, and maximal along the interstack direction, and the σmax/σmin ratio is ∼1.35. The in-plane anisotropy in conductivity originates from the 1D Fermi surface, which provides conductivity mainly along the interstack direction. The expected maximum and minimum conductivities in the ab-plane from calculations are in excellent agreement with ESR measurements. Preliminary measurements on two isostructural β′′-(ET)2X salts (X ) SF5CHFCF2SO3 and SF5CHFSO3) indicate similar inplane microwave conductivity anisotropy. Additional measurements on β-(ET)2X organic superconductors, which do not have a 1D Fermi surface, are in progress. Acknowledgment. Work at Argonne National Laboratory is sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences, under Contract W-31-109-ENG-38. M.L.V. and P.P.S. are student undergraduate research participants, sponsored by the Argonne Division of Educational Programs, from Michigan Technological University, Houghton, MI and the University of Connecticut, Storrs, CT, respectively. Work at North Carolina State University was supported by the U.S. Department of Energy, Office of Basic Sciences, Division of Materials Sciences, under Grant DE-FG05-86ER45259. Work at Portland State University is supported by the National Science Foundation (CHE-9632815) and the Petroleum Research Fund (ACS-PRF Grant 31099AC1). Appendix The Dysonian absorption function can be described as a linear combination of absorption (χ′′) and dispersion (χ′). A moderate metallic Dysonian ESR resonance derivative curve is described by19

Y′(H) )

d [χ′′ + Aχ′] dy

where

( ) ( )

χ′′ ) I

1 1 + y2

χ′ ) I

-y 1 + y2

I is a measure of the ESR signal intensity, and A is an asymmetry factor, i.e., a measure of the admixture of the dispersive component χ′. The equation is rewritten as

Y′(H) )

-I [2y - A(y2 - 1)] + C0 (1 + y2)2 y)

H - H0

x3 ∆H 2

Wang et al. where H0 is the center of the ESR resonance curve and ∆H is the ESR line width. Five parameters, I, H0, ∆H, A, and C0 (for vertical spectra displacement), are used for the Dysonian line shape analysis. References and Notes (1) The Physics and Chemistry of Organic Superconductors; Saito, G., Kagoshima, S., Eds.; Springer-Verlag: Berlin, Heidelberg, 1990. (2) Williams, J. M.; Ferraro, J. R.; Thorn, R. J.; Carlson, K. D.; Geiser, U.; Wang, H. H.; Kini, A. M.; Whangbo, M.-H. Organic Superconductors (Including Fullerenes): Synthesis, Structure, Properties and Theory; Prentice Hall: Englewood Cliffs, NJ, 1992. (3) Je´rome, D.; Mazaud, A.; Ribault, M.; Bechgaard, K. J. Phys. Lett. (Orsay) 1980, 41, L95-L98. (4) Kini, A. M.; Geiser, U.; Wang, H. H.; Carlson, K. D.; Williams, J. M.; Kwok, W. K.; Vandervoort, K. G.; Thompson, J. E.; Stupka, D. L.; Jung, D.; Whangbo, M.-H. Inorg. Chem. 1990, 29, 2555-2557. (5) Williams, J. M.; Kini, A. M.; Wang, H. H.; Carlson, K. D.; Geiser, U.; Montgomery, L. K.; Pyrka, G. J.; Watkins, D. M.; Kommers, J. M.; Boryschuk, S. J.; Strieby Crouch, A. V.; Kwok, W. K.; Schirber, J. E.; Overmyer, D. L.; Jung, D.; Whangbo, M.-H. Inorg. Chem. 1990, 29, 32723274. (6) Yagubskii, EÄ . B.; Shchegolev, I. F.; Laukhin, V. N.; Kononovich, P. A.; Kartsovnik, M. V.; Zvarykina, A. V.; Buravov, L. I. Pis’ma Zh. Eksp. Teor. Fiz. 1984, 39, 12-15 (English translation: JETP Lett. 1984, 39, 12). (7) Williams, J. M.; Wang, H. H.; Beno, M. A.; Emge, T. J.; Sowa, L. M.; Copps, P. T.; Behroozi, F.; Hall, L. N.; Carlson, K. D.; Crabtree, G. W. Inorg. Chem. 1984, 23, 3839-3841. (8) Wang, H. H.; Beno, M. A.; Geiser, U.; Firestone, M. A.; Webb, K. S.; Nun˜ez, L.; Crabtree, G. W.; Carlson, K. D.; Williams, J. M.; Azevedo, L. J.; Kwak, J. F.; Schirber, J. E. Inorg. Chem. 1985, 24, 2465-2466. (9) Laukhin, V. N.; Kostyuchenko, E. EÄ .; Sushko, Y. V.; Shchegolev, I. F.; Yagubskii, EÄ . B. Pis’ma Zh. Eksp. Teor. Fiz. 1985, 41, 68-70 (English translation: JETP Lett. 1985, 41, 81). (10) Schlueter, J. A.; Williams, J. M.; Geiser, U.; Dudek, J. D.; Kelly, M. E.; Sirchio, S. A.; Carlson, K. D.; Naumann, D.; Roy, T.; Campana, C. F. AdV. Mater. 1995, 7, 634-639. (11) Geiser, U.; Schlueter, J. A.; Wang, H. H.; Kini, A. M.; Williams, J. M.; Sche, P. P.; Zakowicz, H. I.; Vanzile, M. L.; Dudek, J. D.; Nixon, P. G.; Winter, R. W.; Gard, G. L.; Ren, J.; Whangbo, M.-H. J. Am. Chem. Soc. 1996, 118, 9996-9997. (12) Mori, T.; Inokuchi, H. Solid State Commun. 1987, 64, 335-337. (13) Kurmoo, M.; Graham, A. W.; Day, P.; Coles, S. J.; Hursthouse, M. B.; Caulfield, J. L.; Singleton, J.; Pratt, F. L.; Hayes, W.; Ducasse, L.; Guionneau, P. J. Am. Chem. Soc. 1995, 117, 12209-12217. (14) Martin, L.; Turner, S. S.; Day, P.; Mabbs, F. E.; McInnes, E. J. L. Chem. Commun. 1997, 1367-1368. (15) Wang, H. H.; Schlueter, J. A.; Geiser, U.; Williams, J. M.; Naumann, D.; Roy, T. Inorg. Chem. 1995, 34, 5552-5557. (16) Drago, R. S. Physical Methods in Chemistry; W. B. Saunders Company: Philadelphia, 1977. (17) Sugano, T.; Saito, G.; Kinoshita, M. Phys. ReV. B: Condens. Matter 1986, 34, 117-125. (18) Sugano, T.; Saito, G.; Kinoshita, M. Phys. ReV. B: Condens. Matter 1987, 35, 6554-6559. (19) Chapman, A. C.; Rhodes, P.; Seymour, E. F. W. Proc. Phys. Soc. 1957, LXX (4-B), 345. (20) Enoki, T.; Imaeda, K.; Kobayashi, M.; Inokuchi, H.; Saito, G. Phys. ReV. B: Condens. Matter 1986, 33, 1553-1558. (21) Kurmoo, M.; Talham, D. R.; Day, P.; Parker, I. D.; Friend, R. H.; Stringer, A. M.; Howard, J. A. K. Solid State Commun. 1987, 61, 459464. (22) Rosseinsky, M. J.; Kurmoo, M.; Talham, D. R.; Day, P.; Chasseau, D.; Watkin, D. J. Chem. Soc., Chem. Commun. 1988, 88-90. (23) Whangbo, M.-H.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 6093. (24) Whangbo, M.-H.; Williams, J. M.; Leung, P. C. W.; Beno, M. A.; Emge, T. J.; Wang, H. H.; Carlson, K. D.; Crabtree, G. W. J. Am. Chem. Soc. 1985, 107, 5815-5816. (25) Beckmann, D.; Wanka, S.; Wosnitza, J.; Schlueter, J. A.; Williams, J. M.; Nixon, P. G.; Winter, R. W.; Gard, G. L.; Ren, J.; Whangbo, M.-H. Eur. Phys. J. B 1998, 1, 295-300. (26) A very recent paper reported a monotonic decrease in resistance for the same material from room temperature down to the superconducting onset temperature.25 Similar behavior, i.e., metallic versus semiconductive property between 150 and 300 K, has also been observed in a few κ-phase superconductors. The reason for the variation is still being studied.

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