We have investigated the origin of the large increase in spin-echo decay rates for the $^{77}$Se nuclear spins at temperatures near to $T=30K$ in the organic superconductor (TMTSF)$_2$ClO$_4$. The measured angular dependence of $T_2^{-1}$ demonstrates that the source of the spin-echo decays lies with carrier density fluctuations rather than fluctuations in TMTSF molecular orientation. The very long time scales are directly associated with the dynamics of the anion ordering occurring at $T=25K$, and the inhomogeneously broadened spectra at lower temperatures result from finite domain sizes. Our results are similar to observations of line-broadening effects associated with charge-ordering transitions in quasi-two dimensional organic conductors.

(TaSe4)(2)I has a quasi-one-dimensional electronic structure with charge-density wave (CDW) ground state forming in a phase transition at T-P = 265 K. Here we report on Se-77 nuclear magnetic resonance (NMR) spectrum and spin-spin relaxation time (T-2) measurements in the temperature range of 100 to 380 K. The line positions are shown 10 couple to the CDW order parameter. The spin-spin relaxation rate displays a sharp minimum at T-P; signs of critical slowing down are absent. Together with the fast increase of the order parameter, these findings are consistent with it true gap due to hidden order in the high-temperature phase.

Some mu c-(Si,Ge):H alloys have been grown using low-pressure, reactive ECR plasma deposition with high H dilution and subtle (sub ppm) B-doping. Incorporating these high-quality materials into devices leads to low-gap mu c-(Si,Ge) solar cells with acceptable performance. This justifies a detailed investigation of the electronic transport properties of mu c-(Si,Ge):H alloys by employing the microwave photomixing technique.From the measurements of the electric field dependence of the drift mobility and lifetime, we have found strong evidence for the existence of long-range potential fluctuations in mu c(Si,Ge):H alloys. We determine the depth and range of the potential fluctuations, and subsequently the charged defect density, as a function of the deposition rate. It was found that the film transport properties do not degrade or enhance monotonically with increasing deposition rate; there exists a valley point where the strongest potential fluctuations occur as a result of a significant increase in the charged defect density. Beyond this point, the film quality increases again. The evidence indicates that it is the long-range potential fluctuations that result in the deterioration of the transport properties of mu c-(Si,Ge):H alloys. Specifically, it is the increase in the depth, and a decrease in the length of the potential fluctuations, which lead to a decrease in the mobility, and consequently in the photoconductivity. Our present results demonstrate that aside from the increase of charged scattering centers, compositional disorder in the alloys play an important role with the build-up of the potential fluctuations. (c) 2004 Elsevier B.V. All rights reserved.

Time-resolved photo- and thermoelectric effects (TTE) were used to determine simultaneously trap levels and trap state density differences in amorphous (a-SiGe:H) samples. In particular, the trap state density differences are obtained from the decay of the ambipolar charge distribution (i.e., stage II of the TTE transients). This type of spectroscopy has been applied for the first time to a-SiGe:H samples, and indeed trap states that seem to relate to concentration fluctuations, that is, Si(Ge) and Ge(Si) clusters, are observed.

The temperature/pressure phase diagram for (TMTTF)(2)SbF6 is determined using C-13 NMR spectroscopy. At ambient pressure, a transition to a charge-ordered (CO) state occurs at T-CO=156 K, and antiferromagnetic (AF) order is observed below T-N=8 K. Both are suppressed with pressure: when P>0.5 GPa, there is no evidence for CO, and the low-temperature C-13 NMR spectrum is consistent with a singlet [spin-Peierls (SP)] ground state. At a given pressure, the temperature dependence of the CO order parameter is not monotonic, and provides an opportunity to identify what processes could be controlling the CO amplitude. F-19 NMR spectroscopy provides empirical evidence that electron-counterion coupling is crucial to stabilizing the CO and AF phases.

(TMTTF) 2 AsF 6 and (TMTTF) 2 SbF 6 are both known to undergo a charge ordering phase transition, though their ground states are different. The ground state of the first is Spin-Peierls, and the second is an antiferromagnet. We study the effect of pressure on the ground states and the charge-ordering using 13 C NMR spectroscopy. The experiments demonstrate that the the CO and SP order parameters are repulsive, and consequently the AF state is stabilized when the CO order parameter is large, as it is for (TMTTF) 2 SbF 6 . An extension of the well-known temperature/pressure phase diagram is proposed.

We have improved the quality of our narrow bandgap a-SiGe:H grown by hot-wire chemical vapor deposition (HWCVD) by decreasing our W filament diameter and our substrate temperature. We now grow a-SiGe:H with Tauc bandgaps below 1.5 eV having a photoresponse equal to or better than our plasma enhanced CVD grown alloys. We enhanced the transport properties—as measured by the photoconductivity frequency mixing technique—relative to previous HWCVD results. These improved alloys do not necessarily show an improvement in the degree of structural heterogeneity on the nanometer scale as measured by small-angle X-ray scattering. Decreasing both the filament temperature and substrate temperature produced a film with relatively low structural heterogeneity while photoluminescence showed an order of magnitude increase in defect density for a similar change in the process.

Se-77 Nuclear magnetic resonance spectroscopy and relaxation experiments are used to study the effects of weak counterion disorder in the metaillic phase of (TMTSF)(2)PF6. Inhomogeneous line broadening is observed at low temperatures, very much enhanced relative to clean samples. We show that the effect is the result of an inhomogeneous carrier density developing at low temperatures. The magnitude of the effect is enhanced on approaching the critical pressure for the suppression of the spin-density wave phase.

(TMTTF )2AsF6 and (TMTTF )2SbF6 are both known to undergo a charge ordering phase transition, though their ground states are different. The ground state of the first is Spin-Peierls, and the second is an antiferromagnet. We study the effect of pressure on the ground states and the charge-ordering using C NMR spectroscopy. The experiments demonstrate that the the CO and SP order parameters are repulsive, and consequently the AF state is stabilized when the CO order parameter is large, as it is for (TMTTF )2SbF6. An extension of the well-known temperature/pressure phase diagram is proposed.

We report proton NMR measurements of the effect of very high magnetic fields up to 44.7 T (1.9 GHz) on the spin density wave (SDW) transition of the organic conductor TMTSF2PF6. Up to 1.8 GHz, no effect of critical slowing close to the transition is seen on the proton relaxation rate (1/T1), which is determined by the SDW fluctuations associated with the phase transition at the NMR frequency. Thus, the correlation time for such fluctuations is less than $1O^{-10}$s. A possible explanation for the absence of longer correlation times is that the transition is weakly first order, so that the full critical divergence is never achieved. The measurements also show a dependence of the transition temperature on the orientation of the magnetic field and a quadratic dependence on its magnitude that agrees with earlier transport measurements at lower fields. The UCLA part of this work was supported by NSF Grant DMR-0072524.

(TMTTF)(2)AsF6 undergoes two phase transitions upon cooling from 300 K. At T-CO=103 K a charge ordering (CO) occurs, and at T-SP(B=9 T)=11 K the material undergoes a spin-Peierls transition. Within the intermediate, CO phase, the charge disproportionation ratio is found to be at least 3:1 from C-13 NMR T-1(-1) measurements on spin-labeled samples. Above T-SP up to about 3T(SP) T-1(-1) is independent of temperature, indicative of low-dimensional magnetic correlations. With the application of about 0.15 GPa pressure, T-SP increases substantially, while T-CO is rapidly suppressed, demonstrating that the two orders are competing. The experiments are compared to results obtained from calculations on the one-dimensional extended Peierls-Hubbard model.

We have used a field-penetration method to measure thermodynamic compressibility of a moderately interacting two-dimensional electron system ($r_{s}$ $\approx$ 0.5-3) in a three terminal GaAs/AlGaAs device, fabricated with an epitaxial lift-off technique. We found that the density and temperature dependencies of the compressibility are qualitatively different from that observed in earlier studies of the 2D hole system, where interaction energies are considerably stronger. We show that the observed characteristics can be described by the recently developed formalism for compressibility of the droplet state.

Properties of the charge-ordered (CO) phase for the representative salt (TMTTF) 2 AsF 6 are determined using 13 C and 75 As nuclei as probes. The charge disproportionation ratio is found to be at least 3:1 at temperatures below the transition temperature T CO . At lower temperatures, the material undergoes a second transition to a singlet ground state. Investigations of the relative stability of the two orders under applied pressure reveal that the two orders compete. The phase diagram consists of a region of coexistence and a tetracritical point.

We report extremely high field and frequency NMR measurements of the proton spin-lattice relaxation rate (1/T-1) in (TMTSF)(2)PF6. They probe the spin density wave (SDW) fluctuations in the critical regime near the SDW transition and in the ordered phase at lower temperatures. The highest field measurements were made using the NHMFL hybrid magnet at 44.7 T and a proton NMR frequency of 1.9 GHz. A quadratic increase in the transition temperature is observed that agrees with transport measurements made at lower fields. As the field is increased above 40 T, a change in the SDW critical dynamics is seen. Also, the behavior of 1/T-1 vs. the alignment of the magnetic field suggests a magnetic transition at moderate to high fields with the characteristics of a sharp change in the orientation of the SDW polarization, but with parameters that differ from the spin-flop transition seen at low field.