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Zero- and longitudinal-field muon-spin relaxation μ+SR measurements have been performed on La_(2-x)Sr_xCuO_4 alloys in both single-crystal and sintered powder forms above and below their magnetic transition temperatures, T_f. The μ+ precession frequency v depends only weakly on x and T_f, an observation which together with resistivity data implies classical freezing of magnetic moments in the regime where the carriers are localized. For x=0.05, critical dynamics are observed near T_f. The μ^+SR technique is shown to be very sensitive to ferromagnetically aligned pairs of Cu^(2+) moments; the population of such pairs increases greatly with x.

Neutron scattering and muon spin relaxation studies of some materials related to high-T c superconductors are described. For La 2− x Sr x Cu O 4 with x as large as 0.05, our data indicate spin glass greezing with a frozen local moment of the same size as in the nominally pure(x=0) antiferromagnet. Magnetic fluctuations similar to those for La 2 Cu O 4 are found for La 2 Ni O 4 , which thus far has not been shown to be the parent compound of a superconductor.

We report both linear and nonlinear magnetoconductance measurements on two different density samples of similar stoichiometry Hg_(1-x)Cd_xTe for 0.01

A contribution to the magnetoresistance is observed at temperatures below 100 mK in bulk metallic Si: P that is unanticipated within theoretical analyses of localization. This contribution is positive, approximately independent of sample orientation, and varying roughly as the square root of the applied field. An analysis of Coulomb interactions including spin splitting is presented which, when combined with localization, describes the magnetoresistance.

The magnetoresistance of single-crystal ErRh_4B_4 has been measured from 0.05 to 10 K with magnetic fields from 0 to 5 T applied along the magnetically easy a direction. The spin disorder resistivity in the normal state is interpreted with a crystal field model which also fits the magnetic anisotropy and specific heat. The data show that substantial spin disorder remains in ErRh_4B_4 even at T = 0 K in the ferromagnetic state.

We describe the controlled exfoliation of single crystalgraphite flakes intercalated to stage eleven with SbCl_5. The resulting exposed surface remains well oriented in the basal plane. Our best sample to date has an internal surface area of 6 Torr cc and an in‐plane mosaic of 2.3° HWHM (half‐width at half‐maximum). High resolution x‐ray scattering from adsorbed krypton reveals an average step‐free surface size of at least 2000 Å. Experiments which probe orientational information in the layer planes should now be possible.

We find that ESR linewidth measurements of amorphous FeMn alloys indicate the ferromagnet-spin glass transition temperature without a scaling analysis. Comparison with neutron scattering data shows the importance of studying the long-wavelength limit.

We have carefully characterized the low-temperature magnetotransport of a series of samples of the metallic glass (Fe_(1-x)Mn_x)_(75)P_(16)B_6Al_3. We find that both localization and electron interactions are important, but the variation of the magnetoresistance with x indicates that this amorphous metal will only satisfy scaling theories which include spin fluctuations.

We have propagated transverse sound in a magnetic field in the basal plane of UPt_3, with the polarization vector oriented both in the basal plane and perpendicular to it. We observe a strong anisotropy in the magnetic field dependence of the attenuation for the two polarizations. Using a simple phenomenological model, we can understand the low-temperature field dependence as a natural consequence of the anisotropy with temperature in zero field reported earlier [Phys. Rev. Lett. 56, 1078 (1986)]. However, for increasing temperatures there are significant deviations from this model. In no case do we find evidence for new superconducting phases in a magnetic field.

We have measured the magnetic susceptibility, χ’+iχ’’, of the dilute dipolar-coupled Ising magnet LiHo_(0.045)Y_(0.955)F_4 over six decades of frequency from 0.02 Hz to 20 kHz. The system behaves as an ideal relaxational glass with Arrhenius behavior in temperature of the peak in χ’’. Scaling data from T=100 mK to T=300 mK by the peak in χ’’ shows an enhanced low-frequency response at high temperatures, in contrast to expectations for spin-glasses and random-field magnets.

We report measurements of the nonlinear current-voltage characteristics on low-carrier-concentration n-type samples of Hg_(0.76)Cd_(0.24)Te at high magnetic fields and low temperatures. We find the onset of nonlinear conductivity occurs at applied electric fields of less than 1 mV/cm. The effect can be easily distinguished from sample heating and is consistent with collective transport produced by the sliding of a Wigner crystal pinned by disorder. We estimate a disorder-induced correlation length of tens of micrometers.

We present measurements of the electrical conductivity at low temperatures of bulk samples of Si:P under uniaxial stress controlled to bring the samples within 0.1% of the metal-insulator transition. As the metal approaches the critical point, we find that the power law of the temperature correction to the conductivity predicted for weak Coulomb interactions continues to fit, but that its sign, size, and range of validity change. Its size defines a diffusion temperature which tends towards zero at the critical density, at which point the power law itself appears to change.

Topological ideas can be used to predict a variety of textures and defects in condensed matter systems under certain conditions. Liquid crystals exhibit a rich variety of such textures, and provide an accessible system on which to perform tabletop experiments. One means of creating and observing various liquid crystal textures is described, and results of these observations are presented.

Detailed measurements are discussed of the far-infrared absorption coefficient and dc conductivity of phosphorous donors in uncompensated silicon at very low temperature throughout the metal-insulator transition. The results obtained support an Anderson like model and formally agree with a general scaling description of the transition.

We measure the low-temperature longitudinal and Hall conductivities in a series of Ge:Sb samples at the approach to the metal-insulator transition. Both conductivities critically vanish with the same exponent of 1, in contradiction to the ratio of 2 predicted by the scaling theory of localization.

We have measured the ac magnetic susceptibility of the model dilute dipolar-coupled Ising system LiHo_xY_(1−x)F_4. The x=0.46 material displays an ordinary ferromagnetic transition, while the x=0.045 and 0.167 samples are two very different magnetic glasses. Thermal relaxation times are more than five times longer for x=0.167 than for x=0.045. In addition, the more concentrated glass shows history dependence and metastability upon field cooling.

We report the first measurements of the attenuation of ultrasound in the basal plane of superconducting UPt_3. Transverse sound propagating along the b axis shows a marked anisotropy in its temperature dependence when the polarization is rotated in and out of the basal plane. For polarization in the basal plane the attenuation varies linearly with temperature down to 35 mK and the slope scales as the square of the frequency. Our results appear to indicate the presence of an additional attenuation mechanism when compared with recent theories of anisotropic superconductors in the dirty limit.

We have studied the critical temperature T_c, upper critical field, and magnetotransport variation as a function of radiation damage in the low-T_c A15 superconductor Nb_3Ir. We find a nonmonotonic variation in Tc with disorder and analyze these results in terms of the competition between density-of-states effects and weak localization. Magnetoresistance measurements confirm the presence of electron interaction effects associated with weak localization.

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A tendency toward a cusp at zero temperature in the electrical conductivity of Si crystals doped with P is observed. It is found that, within the metallic state, decreasing P concentration enhances the cusp and then rapidly changes its sign as a pseudogap opens. Such a cusp has been predicted for a disordered metal in which Coulomb interactions dominate the scattering.

The physics of the metal-insulator transition has been explored for over thirty years now, but our understanding is still incomplete. As emphasized in the classic papers of Mott1 and Anderson2, electron correlation and disorder are essential ingredients in the phenomenon of electron localization. However, their exact manifestation in real systems remains a subject of active study and dispute.

The melting transition of Xe monolayers adsorbed on a single-crystal exfoliated graphite substrate has been studied by high-resolution synchrotron x-ray scattering. At temperatures slightly above the melting transition the fluid phase has a high degree of orientational order. The results are discussed in the context of current theories of two-dimensional melting including the effects of the substrate.

The behavior of electrons in a random potential has been a particularly rich problem for both theorists and experimentalists, as attested to by this collection of articles. In three-dimensional disordered systems there is the extra twist of a possible metal-insulator transition, going back to the classic papers by Mott1 and by Anderson2, which emphasize, respectively, the role of electron correlation and disorder in the phenomenon of electron localization. Most recent work3 has concentrated on the unusual properties of the disordered metal and the approach to the metal-insulator transition critical point from above. In fact, the nature of the disordered insulator near the critical point not only provides an equally good test of theories of the transition, but it is a fascinating entity unto itself. Under the proper circumstances, barely localized electrons can form glasses or even crystals, depending upon the relative importance of the disorder and the interactions. In this paper, I will briefly review results on the approach to the metal-insulator critical point from below, and summarize recent experiments on the electron glass in Si :P and the electron crystal in Hg1-xCdxTe.

We have performed magnetoresistance and Hall-resistance measurements on low-carrier-concentration n-type samples of Hg_(0.76)Cd_(0.24)Te at millikelvin temperatures. We observe an abrupt rise in the Hall resistance and magnetoresistance at a characteristic field H_c which is a significant function of temperature and which allows us to reject magnetic freezeout or localization by disorder as possible mechanisms. We believe our data provide compelling evidence for a model where the magnetic field induces localization of the electrons into a three-dimensional Wigner lattice.

We demonstrate substantial magnetic-field tuning of the metal-insulator transition in Ge:Sb for H < 20 kOe. From a fit to the conductivity in a series of samples at millikelvin temperatures in different fields to the critical form σ ~ (n/n_c(H) - 1)^μ, we find that μ ≈ 1 independent of H, but that n_c(H) increases more than 25% by H ~ 20 kOe. Moreover, n_c(H) - n_c(0) α H^(1/2), in agreement with a weak-disorder perturbative scaling approach, but in contrast to recent experimental results in Si:As.

We have measured the real and imaginary parts of the dielectric susceptibility of insulating samples of P-doped Si at millikelvin temperatures at 400 MHz using a resonant transmission cavity. We find that the real part is enhanced by more than two orders of magnitude over the isolated donor polarizability, and we determine the exponent which describes the critical divergence of the real part at the insulator-metal transition by fitting the temperature dependence of the corresponding imaginary part. The form of the observed divergence remains unexplained theoretically.

A high-resolution scan of the metal-insulator transition in Si:P at millikelvin temperatures has been obtained by applying uniaxial stress. A sharp, but continuous, metal-insulator transition is resolved, with conductivities below Mott's "minimum" value σ_M. These measurements join smoothly with previous low-resolution experiments, ruling out any discontinuity at σ_M. The reproducible critical behavior disagrees with predictions of existing scaling theories of localization.

We investigate how temperature affects transport through large networks of nonlinear conductances with distributed thresholds. In monolayers of weakly-coupled gold nanocrystals, quenched charge disorder produces a range of local thresholds for the onset of electron tunneling. Our measurements delineate two regimes separated by a cross-over temperature $T^*$. Up to $T^*$ the nonlinear zero-temperature shape of the current-voltage curves survives, but with a threshold voltage for conduction that decreases linearly with temperature. Above $T^*$ the threshold vanishes and the low-bias conductance increases rapidly with temperature. We develop a model that accounts for these findings and predicts $T^*$., 5 pages including 3 figures; replaced 3/30/04: minor changes; final version

Quantum states cohere and interfere. Atoms arranged imperfectly in a solid rarely display these properties. Here we demonstrate an exception in a disordered quantum magnet that divides itself into nearly isolated subsystems. We probe these coherent spin clusters by driving the system nonlinearly and measuring the resulting hole in the linear spectral response. The Fano shape of the hole encodes the incoherent lifetime as well as coherent mixing of the localized excitations. For the Ising magnet LiHo0.045Y0.955F4, the quality factor Q for spectral holes can be as high as 100,000. We tune the dynamics by sweeping the Fano mixing parameter q through zero via the ac pump amplitude as well as a dc transverse field. The zero crossing of q is associated with a dissipationless response at the drive frequency. Identifying localized two-level systems in a dense and disordered magnet advances the search for qubit platforms emerging from strongly interacting, many-body systems., The Ising magnet LiHo0.045Y0.955F4 allows experimental probes of disordered quantum dynamics. Silevitch et al. show that strongly-driven localized spin clusters form effectively decoupled two-level systems whose interactions and coherence can be tuned by external ac and dc magnetic fields.

We find low-temperature positive magnetoresistance independent of sample orientation in bulk, metallic samples of Si:P. The results are consistent with the Coulomb interactions picture including spin-splitting and show no evidence for the effects of localization.