The esr of single crystal organic metals is studied at the SJSU esr laboratory to ascertain the nature of the phase transitions these undergo (Mol. Cryst. Liq. Cryst., 284, 411-417 (1996)). This is an example of how chemists can learn about phase transitions using all the power of magnetic resonance :

The structure shown in Fig. 1:

was determined by S.S.P. Parkin. It consists of alternating anion Ta2F11- layers and (BEDT-TTF)3+ cation layers. The high conductivity along the c-axis is due to intermolecular S-S overlap in the cation trimers .

The esr spectra are recorded in a Bruker 300 spectrometer equipped with an Oxford 900 cryostat. One measures the derivative of the power P absorbed versus the change in external field Bz, i.e., dP/dBz and this can be integrated numerically in the instrument. The signal is very weak at room temperature. Fig.2:

shows that the line shape changes from Lorentzian to Gaussian below T00=150 K indicating a different relaxation mechanism above and below T00. A spin-spin interaction term is ascertained from the ratio of the second moment to the width at half height of the Gaussian line shape to be:

Jspin-spin = v2/v1/2=15 mT.

The dynamics of the disappearance of the doublet state D is determined from the temperature T dependence of the line widths in Fig. 3:

shows that the natural line width ~1/T2 is increased by the first order rate constant for the chemical disappearance of D:

k1(D) = A0exp((S-H/T)/R)

the logarithmic dependence of the line width ln(Hz,ms) vs 1/T obtains:

{H>/R, A0exp((S>/R)}T> 150 K= 111 K, 3.5 E+7 s-1

{H</R, A0exp((S</R)}T< 150 K = 11 K, 7 E+6 s-1

The change in activation enthalpy and entropy from the high to the low T regimes is:

(H/R)= -100 K and (S/R) = -2~ -3 ln(2)

This means that:

the relaxation barrier for the D states is reduced by 100 K below 150 K because there is an easier path for disappearance, most probably the formation of T* states, and

the entropy barrier is reduced by ~ 3 ln(2) most probably because triplet paring appears at this T and there are three possible ways to pair the D states.

The complete esr spectrum is extremely rich. Fig. 4a:

shows that anti-ferromagnetic, AF resonance is also detected but that a triplet T* half field absorption indicates a condensation of the doublet states near 85 K as the AF resonance broadens because of a shorter lifetime of the AF domains as T decreases in Fig. 4b:

The AF resonance field allowed transition in the experimental orientation (H1 normal to Bz) is fitted to the relation:

Bz,AF = hv/gAF

where fit to the data obtains an exchange field BE = 150 mT and an axial anisotropy field BA =220 mT, parallel to the b-axis and The fit to the orientation dependence of the field Bz,AF =BAz = BA cos(b^Bz) at 145 K is shown in Fig. 4c:

The data indicates that the anisotropy field is along the b-axis (Ta-F-Ta bond direction in Fig. 1). The AF resonance is strongest at 145 K, i.e., BE >> Jspin-spin at 150 K and therefore it prevails above the formation of the T* states.

As the temperature is lowered below 150 K the AF resonance broadens until it disappears, indicating a relaxation time shorter than 10-10 s near 44 K (Fig. 4b). At this temperature the triplet half field esr T* is identified by the half field resonance S=0.

The half field esr relaxation time is determined by saturation measurements:

when r is the ratio of amplitudes of the absorption derivatives for different attenuation of H1 (x, y dB) in Fig. 5:

 

The transition to superconductivity is then identified by the appearance of an energy loss at exactly H=0 and the increase in the T* relaxation time T1 as shown in Fig.6:

The phase transition can also be induced optically as shown in Fig. 7:

The broadening of the D esr transition in 21 minutes of irradiation 400 m wavelength pulsed light is explained as a phase transition, whilst two pairs of S=1 T* transitions (sharpen) indicates that the D states have a faster relaxation time in the new phase while the opposite is true for the T* states which sharpen. The effect of light is important in order to understand the superconducting state. Fig. 8:

indicates that when the external field is parallel to the b-axis the phase transition is displaced to superconducting, however when it is parallel to the c-axis (highest conductivity axis) the transition reverts to the high temperature phase (non-superconducting) with sharp D esr. The cycle can be repeated any number of times as long as the light is incident on the single crystal. When the light is turned off, whichever phase was stable then remains at any orientation in the external field.