Sharp Metal-Insulator Transition in a Random Solid

We have measured zero temperature metallic conductivities above and below Mott's minimum value σ_(MIN) in bulk crystals of P doped Si. Studies of lattice heating, electronic heating and macroscopic inhomogeneities support the finding that conductivities below σ_(MIN) increase by over 10 as the P density is increased by 1%, and that over a wider density range the data can be fit t o a scaling form with a characteristic length that tends to diverge with the same exponent (ν = 0.55±0.10) in the metal and insulator

Measurements of conductivity near the metal-insulator critical point

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

Giant dielectric constants at the approach to the insulator-metal transition

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

Critical Behavior of the Conductivity of Si:P at the Metal-Insulator Transition under Uniaxial Stress

We report new measurements of the electrical conductivity sigma of the canonical three-dimensional metal-insulator system Si:P under uniaxial stress S. The zero-temperature extrapolation of sigma(S,T -> 0) ~\S - S_c\^mu shows an unprecidentedly sharp onset of finite conductivity at S_c with an exponent mu = 1. The value of mu differs significantly from that of earlier stress-tuning results. Our data show dynamical sigma(S,T) scaling on both metallic and insulating sides, viz. sigma(S,T) = sigma_c(T) F(\S - S_cT^y) where sigma_c(T) is the conductivity at the critical stress S_c. We find y = 1/znu = 0.34 where nu is the correlation-length exponent and z the dynamic critical exponent.Comment: 5 pages, 4 figure

Charge Berezinskii-Kosterlitz-Thouless transition in superconducting NbTiN films

A half-century after the discovery of the superconductor-insulator transition (SIT), one of the fundamental predictions of the theory, the charge Berezinskii-Kosterlitz-Thouless (BKT) transition that is expected to occur at the insulating side of the SIT, has remained unobserved. The charge BKT transition is a phenomenon dual to the vortex BKT transition, which is at the heart of the very existence of two-dimensional superconductivity as a zero-resistance state appearing at finite temperatures. The dual picture points to the possibility of the existence of a superinsulating state endowed with zero conductance at finite temperature. Here, we report the observation of the charge BKT transition on the insulating side of the SIT, identified by the critical behavior of the resistance. We find that the critical temperature of the charge BKT transition depends on the magnetic field exhibiting first the fast growth and then passing through the maximum at fields much less than the upper critical field. Finally, we ascertain the effects of the finite electrostatic screening length and its divergence at the magnetic field-tuned approach to the superconductor-insulator transition.Comment: 9 pages, 6 figure

Conductivity Cusp in a Disordered Metal

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

Sharp Metal-Insulator Transition in a Random Solid

Zero-temperature metallic conductivities have been measured above and below Mott's minimum value σ_(min) in bulk crystals of P-doped Si. Conductivities below σ_(min) increase by over 10^3 as the density is raised by less than 1%, and do not rule out a discontinuous transition. However, over a wider density range the data can be fitted with a scaling form with a characteristic length that tends to diverge with the same exponent in the metal and insulator

Conductivity of Metallic Si:B near the Metal-Insulator Transition: Comparison between Unstressed and Uniaxially Stressed Samples

The low-temperature dc conductivities of barely metallic samples of p-type Si:B are compared for a series of samples with different dopant concentrations, n, in the absence of stress (cubic symmetry), and for a single sample driven from the metallic into the insulating phase by uniaxial compression, S. For all values of temperature and stress, the conductivity of the stressed sample collapses onto a single universal scaling curve. The scaling fit indicates that the conductivity of si:B is proportional to the square-root of T in the critical range. Our data yield a critical conductivity exponent of 1.6, considerably larger than the value reported in earlier experiments where the transition was crossed by varying the dopant concentration. The larger exponent is based on data in a narrow range of stress near the critical value within which scaling holds. We show explicitly that the temperature dependences of the conductivity of stressed and unstressed Si:B are different, suggesting that a direct comparison of the critical behavior and critical exponents for stress- tuned and concentration-tuned transitions may not be warranted

Critical Scaling of the Conductance in a Disordered Insulator

A critical scaling of the real and imaginary parts of the low-frequency ac conductance of insulating phosphorus-doped silicon near the metal-insulator transition has been observed. The results are interpreted as evidence of an electron glass, i.e., glasslike behavior, intimately connected with the scaling description of the transition, in which Coulomb interactions play a significant role