arXiv subjects

S. Washburn

Publications and source records attributed to S. Washburn.

14 recordsLinked to original sources

arXiv2002

Torsional response and stiffening of individual multi-walled carbon nanotubes

We report on the characterization of torsional oscillators which use multi-walled carbon nanotubes as the spring elements. Through atomic-force-microscope force-distance measurements we are able to apply torsional strains to the nanotubes and measure their torsional spring constants and effective shear moduli. We find that the effective shear moduli cover a broad range, with the largest values near the theoretically predicted value. The data also suggest that the nanotubes are stiffened by repeated flexing.

cond-mat.mtrl-sci↗

arXiv2002

Effects of a Parallel Magnetic Field on the Metal-Insulator Transition in a Dilute Two-Dimensional Electron System

The temperature dependence of conductivity $σ(T)$ of a two-dimensional electron system in silicon has been studied in parallel magnetic fields B. At B=0, the system displays a metal-insulator transition at a critical electron density $n_c(0)$, and $dσ/dT >0$ in the metallic phase. At low fields ($B\lesssim 2$ T), $n_c$ increases as $n_c(B) - n_c(0) \propto B^β$ ($β\sim 1$), and the zero-temperature conductivity scales as $σ(n_s,B,T=0)/σ(n_s,0,0)=f(B^β/δ_n)$ (where $δ_n=(n_s-n_c(0))/n_c(0)$, and $n_s$ is electron density) as expected for a quantum phase transition. The metallic phase persists in fields of up to 18 T, consistent with the saturation of $n_c$ at high fields.

cond-mat.str-el↗

arXiv2000

Novel metallic behavior in two dimensions

Experiments on a sufficiently disordered two-dimensional (2D) electron system in silicon reveal a new and unexpected kind of metallic behavior, where the conductivity decreases as σ(n_s,T)=σ(n_s,T=0)+A(n_s)T^2 (n_s-carrier density) to a non-zero value as temperature T->0. In 2D, the existence of a metal with dσ/dT>0 is very surprising. In addition, a novel type of a metal-insulator transition obtains, which is unlike any known quantum phase transition in 2D.

cond-mat↗

arXiv2000

Effects of a parallel magnetic field on the novel metallic behavior in two dimensions

Magnetoconductance (MC) in a parallel magnetic field B has been measured in a two-dimensional electron system in Si, in the regime where the conductivity decreases as σ(n_s,T,B=0)=σ(n_s,T=0) + A(n_s)T^2 (n_s -- carrier density) to a non-zero value as temperature T->0. Very near the B=0 metal-insulator transition, there is a large initial drop in σwith increasing B, followed by a much weaker σ(B). At higher n_s, the initial drop of MC is less pronounced.

cond-mat.str-el↗

arXiv1999

In Situ Resistance Measurements of Strained Carbon Nanotubes

We investigate the response of multi-walled carbon nanotubes to mechanical strain applied with an Atomic Force Microscope (AFM) probe. We find that in some samples, changes in the contact resistance dominate the measured resistance change. In others, strain large enough to fracture the tube can be applied without a significant change in the contact resistance. In this case we observe that enough force is applied to break the tube without any change in resistance until the tube fails. We have also manipulated the ends of the broken tube back in contact with each other, re-establishing a finite resistance. We observe that in this broken configuration the resistance of the sample is tunable to values 15-350 kW greater than prior to breaking.

cond-mat.mes-hall↗

arXiv1999

Suppression of metallic behavior in two dimensions by spin flip scattering

We study the effect of the disorder on the metallic behavior of a two-dimensional electron system in silicon. The temperature dependence of conductivity $σ(T)$ was measured for different values of substrate bias, which changes both potential scattering and the concentration of disorder-induced local magnetic moments. We find that the latter has a much more profound effect on $dσ/dT$. In fact, the data suggest that in the limit of $T\to 0$ the metallic behavior, as characterized by $dσ/dT < 0$, is suppressed by an arbitrarily small amount of spin flip scattering by local magnetic moments.

cond-mat.str-el↗

arXiv1999

Phase diagram and validity of one-parameter scaling near the two-dimensional metal-insulator transition

We explore the scaling description for a two-dimensional metal-insulator transition (MIT) of electrons in silicon. Near the MIT, $β_{T}/p = (-1/p)d(\ln g)/d(\ln T)$ is universal (with $p$, a sample dependent exponent, determined separately; $g$--conductance, $T$--temperature). We obtain the characteristic temperatures $T_0$ and $T_1$ demarking respectively the quantum critical region and the regime of validity of single parameter scaling in the metallic phase, and show that $T_1$ vanishes as the transition is approached. For $T<T_1$, the scaling of the data requires a second parameter. Moreover, all of the data can be described with two-parameter scaling at all densities -- even far from the transition.

cond-mat.mes-hall↗

arXiv1999

Effect of Local Magnetic Moments on the Metallic Behavior in Two Dimensions

The temperature dependence of conductivity $σ(T)$ in the metallic phase of a two-dimensional electron system in silicon has been studied for different concentrations of local magnetic moments. The local moments have been induced by disorder, and their number was varied using substrate bias. The data suggest that in the limit of $T\to 0$ the metallic behavior, as characterized by $dσ/dT < 0$, is suppressed by an arbitrarily small amount of scattering by local magnetic moments.

cond-mat.dis-nn↗

arXiv1997

Conductance Fluctuations Near the Two-Dimensional Metal-Insulator Transition

Measurements of conductance $G$ on short, wide, high-mobility Si-MOSFETs reveal both a two-dimensional metal-insulator transition (MIT) at moderate temperatures (1 $<~ T <$ 4~K) and mesoscopic fluctuations of the conductance at low temperatures ($T~ <$ 1~K). Both were studied as a function of chemical potential (carrier concentration $n_s$) controlled by gate voltage ($V_g$) and magnetic field $B$ near the MIT. Fourier analysis of the low temperature fluctuations reveals several fluctuation scales in $V_g$ that vary non-monotonically near the MIT. At higher temperatures, $G(V_g,B)$ is similar to large FETs and exhibits a MIT. All of the observations support the suggestion that the MIT is driven by Coulomb interactions among the carriers.

cond-mat.str-el↗

arXiv1997

Metal-Insulator Transition in a Low-Mobility Two-Dimensional Electron System

We have varied the disorder in a two-dimensional electron system in silicon by applying substrate bias. When the disorder becomes sufficiently low, we observe the emergence of the metallic phase, and find evidence for a metal-insulator transition (MIT): the single-parameter scaling of conductivity with temperature near a critical electron density. We obtain the scaling function $β$, which determines the length (or temperature) dependence of the conductance. $β$ is smooth and monotonic, and linear in the logarithm of the conductance near the MIT, in agreement with the scaling theory for interacting systems.

cond-mat.str-el↗

arXiv1997

Comment on ``Electric Field Scaling at B=0 Metal-Insulator Transition in Two Dimensions''

In a recent Letter, Kravchenko et al. [cond-mat/9608101] have provided evidence for a metal-insulator transition (MIT) in a two-dimensional electron system (2DES) in Si metal-oxide-semiconductor field-effect transistors (MOSFETs). The transition observed in these samples occurs at relatively low electron densities $n_{s}\sim (1-2)\times 10^{11}cm^{-2}$ and high disorder $σ_{c}\sim e^{2}/2h$. We present evidence for a 2D MIT in a structure where the disorderis about two orders of magnitude weaker than in Si MOSFETs. The MIT occurs in the same range of $n_s$ Providing very strong evidence that the 2D MIT in Si-based devices is caused by electron-electron interactions.

cond-mat.str-el↗

arXiv1997

Minimum Metallic Mobility in a Two-Dimensional Electron Gas

We report the observation of a metal-insulator transition in a two-dimensional electron gas in silicon. By applying substrate bias, we have varied the mobility of our samples, and observed the creation of the metallic phase when the mobility was high enough ($μ~> 1 m^2/Vs$), consistent with the assertion that this transition is driven by electron-electron interactions. In a perpendicular magnetic field, the magnetoconductance is positive in the vicinity of the transition, but negative elsewhere. Our experiment suggests that such behavior results from a decrease of the spin-dependent part of the interaction in the vicinity of the transition.

cond-mat.str-el↗

arXiv1997

Mesoscopic Behavior Near a Two-Dimensional Metal-Insulator Transition

We study conductance fluctuations in a two-dimensional electron gas as a function of chemical potential (or gate voltage) from the strongly insulating to the metallic regime. Power spectra of the fluctuations decay with two distinct exponents (1/v_l and 1/v_h). For conductivity $σ\sim 0.1 e^{2}/h$, we find a third exponent (1/v_i) in the shortest samples, and non-monotonic dependence of v_i and v_l on σ. We study the dependence of v_i, v_l, v_h, and the variances of corresponding fluctuations on σ, sample size, and temperature. The anomalies near $σ\simeq 0.1 e^{2}/h$ indicate that the dielectric response and screening length are critically behaved, i.e. that Coulomb correlations dominate the physics.

cond-mat.mes-hall↗

arXiv1993

Correlations between Aharonov-Bohm effects and one-dimensional subband populations in GaAs/Al$_{x}$Ga$_{1-x}$As rings

The Aharonov-Bohm (AB) interference patterns in ring-shaped conductors are usually dominated by random features. The amplitude of the oscillations is random from sample to sample and from point to point on the magnetic field axis owing to random scattering of the electron trajectories by impurities within the wires. We report experiments on new devices made with wet etching and global gates, which have shown major progress towards removing the random features. In loops that exhibit ballistic conductance plateaux and cyclotron orbit trapping at $4.2K$, the random pattern of AB oscillations (observed for $T < 0.1K$) can be replaced by much more ordered one -- especially if only a few transverse modes are populated in the ring. The amplitude and shape of the oscillation envelope function change systematically as subbands are populated in the wires forming the loops. Mechanisms governing the AB effect in the ballistic regime are discussed. Correlation has been found between the $G(V_{g},B=0)$ staircase and the ``beating period" of the envelope functions. Quantum oscillations in $G(V_{g},B = 0)$ are consistent with direct interference of paths of unequal length. Both the correlations and the quantum oscillations in gate voltage are signatures of ballistic transport.

cond-mat↗