Showing papers by "Myriam P. Sarachik published in 1999"

Scaling of the Conductivity with Temperature and Uniaxial Stress in Si:B at the Metal-Insulator Transition

Abstract: Using uniaxial stress S to tune Si:B through the metal-insulator transition at a critical value S{sub c} , we find the dc conductivity at low temperatures shows an excellent fit to the scaling form {sigma}(S,thinspT)=AT{sup x}f[(S{minus}S{sub c})/T{sup y}] on both sides of the transition. The scaling functions yield reliable determinations of the temperature dependence of the conductivity in the metallic and insulating phases in the critical region. {copyright} {ital 1998} {ital The American Physical Society }

Inelastic neutron scattering study of Mn12–acetate

Abstract: We report zero-field inelastic neutron scattering experiments on a deuterated powder sample of Mn12–acetate consisting of a large number of nominally identical spin-10 magnetic clusters. Our resolution enables us to see a series of peaks corresponding to transitions between the anisotropy levels within the spin-10 manifold. A fit to the spin Hamiltonian H=−DSz2−μBB⋅g⋅S−ASz4+C(S+4+S−4) yields an anisotropy constant D=(0.54±0.02) K and a fourth-order diagonal anisotropy coefficient A=(1.2±0.1)×10−3 K (the other terms being negligible). Performed in the absence of a magnetic field, our experiments do not involve the g values as fitting parameters, thereby yielding particularly reliable values of D and A.

Novel phenomena in dilute electron systems in two dimensions

Abstract: For the past two decades, all two-dimensional systems of electrons were believed to be insulating in the limit of zero temperature. Recent experiments provide evidence for an unexpected transition to a conducting phase at very low electron densities. The nature of this phase is not understood and is currently the focus of intense theoretical and experimental attention.

Low temperature magnetic hysteresis in Mn$_{12}$ acetate single crystals

Abstract: Precise magnetic hysteresis measurements of small single crystals of Mn$_{12}$ acetate of spin 10 have been conducted down to 0.4 K using a high sensitivity Hall magnetometer. At higher temperature (>1.6K) step-like changes in magnetization are observed at regularly spaced magnetic field intervals, as previously reported. However, on lowering the temperature the steps in magnetization shift to higher magnetic fields, initially gradually. These results are consistent with the presence of a second order uniaxial magnetic anisotropy, first observed by EPR spectroscopy, and thermally assisted tunnelling with tunnelling relaxation occurring from levels of progressively lower energy as the temperature is reduced. At lower temperature an abrupt shift in step positions is found. We suggest that this shift may be the first evidence of an abrupt, or first-order, transition between thermally assisted and pure quantum tunnelling, suggested by recent theory.

Response to parallel magnetic field of a dilute two-dimensional electron system across the metal-insulator transition

Abstract: The response to a parallel magnetic field of the very dilute insulating two-dimensional system of electrons in silicon metal-oxide-semiconductor field-effect transistors is dramatic and similar to that found on the conducting side of the metal-insulator transition: there is a large initial increase in resistivity with increasing field, followed by saturation to a value that is approximately constant above a characteristic magnetic field of about 1 T. This is unexpected behavior in an insulator that exhibits Efros-Shklovskii variable-range hopping in zero field, and appears to be a general feature of very dilute electron systems. {copyright} {ital 1999} {ital The American Physical Society}

Impurity resistivity of the double-donor system Si:P,Bi

Abstract: The electrical resistivity of the shallow double-donor system Si:P,Bi, prepared by ion implantation, was investigated in the temperature range from 1.7 to 300 K. Good agreement was obtained between the measured resistivities and resistivities calculated by a generalized Drude approach for the same temperatures and dopant concentrations. The critical impurity concentration for the metal-nonmetal transition for the double-doped Si:P,Bi system was found to lie between the critical concentrations of the two single-doped systems, Si:P and Si:Bi. @S0163-1829~99!11747-8#

Comment on ``Theory of Metal-Insulator Transitions in Gated Semiconductors''

Abstract: A Comment on the Letter by Boris L. Altshuler and Dmitrii L. Maslov, Phys.thinspthinspRev.thinspthinspLett.thinspthinsp{bold 82}, 145 (1999). The authors of the Letter offer a Reply. {copyright} {ital 1999} {ital The American Physical Society}

Hopping conduction in uniaxially stressed Si:B near the insulator-metal transition

Abstract: Using uniaxial stress to tune the critical density near that of the sample, we have studied in detail the low-temperature conductivity of p-type Si:B in the insulating phase very near the metal-insulator transition. For all values of temperature and stress, the conductivity collapses onto a single universal curve, {sigma}(S,T)=AT{sup 1/2}F[T{sup {asterisk}}(S)/T]. For large values of the argument, the scaling function F[T{sup {asterisk}}(S)/T] is well fit by exp[{minus}(T{sup {asterisk}}/T){sup 1/2}], the exponentially activated form associated with variable-range hopping when electron-electron interactions cause a soft Coulomb gap in the density of states at the Fermi energy. The temperature dependence of the prefactor, corresponding to the T dependence of the critical curve, has been determined reliably for this system, and is {proportional_to}T{sup 0.5}. We show explicitly that neglecting the prefactor leads to substantial errors in the determination of the T{sup {asterisk}}{close_quote}s and the critical exponents derived from them. The conductivity is not consistent with Mott variable-range hopping, exp[{minus}(T{sup {asterisk}}/T){sup 1/4}], in the critical region, nor does it obey this form for any range of the parameters. Instead, the conductivity of Si:B is well fit by {sigma}=AT{sup 1/2}hexp[{minus}(T{sup {asterisk}}/T){sup {alpha}}] for smaller argument of the scaling function, with {alpha}=0.31 related to the critical exponentsmore » of the system at the metal-insulator transition. {copyright} {ital 1999} {ital The American Physical Society}« less

Classical versus quantum effects in the b=0 conducting phase in two dimensions

Abstract: In a dilute two-dimensional electron system in silicon, we show that the temperature below which Shubnikov-de Haas oscillations become apparent is approximately the same as the temperature below which an exponential decrease in resistance is seen in B=0, suggesting that the anomalous behavior in zero field is observed only when the system is in a degenerate (quantum) state. The temperature dependence of the resistance is found to be qualitatively similar in B=0 and at integer Landau-level filling factors. [SO163-1829(99)50220-8].

Conductivity of metallic Si:B near the metal-insulator transition: Comparison between unstressed and uniaxially stressed samples

Abstract: 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, $\ensuremath{\sigma}(S,T)={\ensuremath{\sigma}}_{0}(\ensuremath{\Delta}{S/S}_{c}{)}^{\ensuremath{\mu}}G[{T/T}^{*}(S)],$ with ${T}^{*}\ensuremath{\propto}(\ensuremath{\Delta}{S)}^{z\ensuremath{ u}}.$ The scaling fit indicates that the conductivity of Si:B is $\ensuremath{\propto}{T}^{1/2}$ in the critical range. Our data yield a critical conductivity exponent $\ensuremath{\mu}=1.6,$ 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.

An Unexpected Conducting Phase in Two Dimensions

Abstract: Within the scaling theory of localization for noninteracting electrons, all two dimensional systems are insulating in the absence of a magnetic field when examined at sufficiently large length scales, or in the limit of zero temperature. Contrary to this two-decades-old expectation, recent experiments have shown there exists a conducting phase at low temperatures for low electron (hole) densities, a regime where electron-electron interaction energies are large compared to the Fermi energy. We briefly review experiments in silicon MOSFETs that provide evidence of an apparent conductor-insulator transition in two dimensions.