arXiv subjects

F. Green

Publications and source records attributed to F. Green.

10 recordsLinked to original sources

arXiv2006

Metallic Carbon Nanotubes as High-Current-Gain Transistors

Low-dimensional metallic transport is at the heart of modern high-performance transistors. While the best devices are currently based on III-V heterojunction quantum-well channels, we demonstrate that much greater performance gains reside in the unique properties of one-dimensional carbon nanotubes. Using recent experimental data, we show how the specific features of degenerate carrier kinetics in metallic nanotubes enable a novel class of quantum-confined signal detector whose current gain exceeds present transistor technology. We analyze the key interplay of degeneracy and scattering dynamics on the transconductance via a microscopically conserving quantum-kinetic description.

cond-mat.mtrl-sci↗

arXiv2004

Charge shelving and bias spectroscopy for the readout of a charge-qubit on the basis of superposition states

Charge-based qubits have been proposed as fundamental elements for quantum computers. One commonly proposed readout device is the single-electron transistor (SET). SETs can distinguish between localized charge states, but lack the sensitivity to directly distinguish superposition states, which have greatly enhanced coherence times compared with position states. We propose introducing a third dot, and exploiting energy dependent tunnelling from the qubit into this dot (bias spectroscopy) for pseudo-spin to charge conversion and superposition basis readout. We introduce an adiabatic fast passage-style charge pumping technique which enables efficient and robust readout via charge shelving, avoiding problems due to finite SET measurement time.

cond-mat.other↗

arXiv2003

Maximizing the Hilbert space for a finite number of distinguishable quantum states

We consider a quantum system with a finite number of distinguishable quantum states, which may be partitioned freely by a number of quantum particles, assumed to be maximally entangled. We show that if we partition the system into a number of qudits, then the Hilbert space dimension is maximized when each quantum particle is allowed to represent a qudit of order $e$. We demonstrate that the dimensionality of an entangled system, constrained by the total number quantum states, partitioned so as to maximize the number of qutrits will always exceed the dimensionality of other qudit partitioning. We then show that if we relax the requirement of partitioning the system into qudits, but instead let the particles exist in any given state, that the Hilbert space dimension is greatly increased.

quant-ph↗

arXiv2002

Excitonic Superconductivity in Charge Injected Organics

Transport of charge carriers can be controlled by doping through chemical and physical means. Unlike chemical doping, physical doping is carried out by a special technique through gate voltages in a field-effect transistor geometry. This technique keeps the carrier channels free from defects without complications from the crystalline structure and the dopant impurity sites. In this paper we discuss the occurrence of superconductivity at the interface of a device by an unconventional technique. We examine a dynamical pairing mechanism governed by the excitons in the active device. The pairing of charge carriers takes place when the system is in a nonequilibrium (driven) state. We discuss the physics of a plausible superconducting transition and suggest new experiments.

cond-mat.supr-con↗

arXiv2000

Shot Noise in Mesoscopic Quantum Systems

We examine critically the idea that fractional charges may carry electrical current in a conductor, much as do the normal electrons in a metallic Fermi liquid. We explore a range of issues that have gained the status of indispensability for analyzing conductance and shot noise in fractionally charged systems. For these fundamental transport problems, however, a truly microscopic understanding is not yet in sight.

cond-mat.mes-hall↗

arXiv2000

On the Complexity of Quantum ACC

For any $q > 1$, let $\MOD_q$ be a quantum gate that determines if the number of 1's in the input is divisible by $q$. We show that for any $q,t > 1$, $\MOD_q$ is equivalent to $\MOD_t$ (up to constant depth). Based on the case $q=2$, Moore \cite{moore99} has shown that quantum analogs of AC$^{(0)}$, ACC$[q]$, and ACC, denoted QAC$^{(0)}_{wf}$, QACC$[2]$, QACC respectively, define the same class of operators, leaving $q > 2$ as an open question. Our result resolves this question, proving that QAC$^{(0)}_{wf} =$ QACC$[q] = $ QACC for all $q$. We also develop techniques for proving upper bounds for QACC in terms of related language classes. We define classes of languages EQACC, NQACC and BQACC$_{\rats}$. We define a notion $\log$-planar QACC operators and show the appropriately restricted versions of EQACC and NQACC are contained in P/poly. We also define a notion of $\log$-gate restricted QACC operators and show the appropriately restricted versions of EQACC and NQACC are contained in TC$^{(0)}$. To do this last proof, we show that TC$^{(0)}$ can perform iterated addition and multiplication in certain field extensions. We also introduce the notion of a polynomial-size tensor graph and show that families of such graphs can encode the amplitudes resulting from apply an arbitrary QACC operator to an initial state.

quant-ph↗

arXiv1999

Kinetic Theory and Mesoscopic Noise

In the theory of noise processes for mesoscopic conductors, the relationship between shot noise and hot-electron noise is absolutely fundamental to understanding the underlying microscopic fluctuations. From the vantage point of orthodox microscopics and kinetics, their relation is a long way from being settled. Its resolution calls for the tools of many-body theory. We motivate the many-body approach to noise, review the analysis of conductance and current noise within linear diffusive theories, and discuss the smooth crossover between thermal noise and shot noise. We outline a kinetic theory of nonequilibrium fluctuations, and show how this completely orthodox approach directly negates drift-diffusive explanations of the crossover.

cond-mat.mes-hall↗

arXiv1999

Mesoscopic Noise Theory: Microscopics, or Phenomenology?

We argue, physically and formally, that existing diffusive models of noise yield inaccurate microscopic descriptions of nonequilibrium current fluctuations. The theoretical shortfall becomes pronounced in quantum-confined metallic systems, such as the two-dimensional electron gas. In such systems we propose a simple experimental test of mesoscopic validity for diffusive theory's central claim: the smooth crossover between Johnson-Nyquist and shot noise.

cond-mat.mes-hall↗

arXiv1999

High-field noise in degenerate and mesoscopic systems

We analyse high-field current fluctuations in metallic systems by direct mapping of the Fermi-liquid correlations to the semiclassical nonequilibrium state. We give three applications. First, for bulk conductors, we show that there is a unique nonequilibrium analogue to the fluctuation-dissipation theorem for thermal noise. With it, we calculate suppression of the excess hot-electron term by Pauli exclusion. Second, in the degenerate regime, we argue that shot noise and thermal noise are incommensurate. They cannot be connected by a smooth, universal interpolation formula. This follows from their contrasting responses to Coulomb screening. We propose an experiment to test this mismatch. Third, we carry out an exact model calculation of high-field shot noise in narrow mesoscopic wires. We show that a distinctive mode of suppression arises from the structure of the semiclassical Boltzmann equation in two and three dimensions. In one dimension such a mode does not exist.

cond-mat.mes-hall↗

arXiv1997

Nonequilibrium Noise in Metals at Mesoscopic Scales

We review a semiclassical theory of high-field noise in degenerate conductors, based on propagator solutions to the Boltzmann equation for the fluctuation distribution function. The theory provides a microscopic description of correlation-induced suppression of noise in quantum-confined systems, such as heterojunction devices. It is also capable of describing diffusive conductors in the mesoscopic regime. We discuss nonequilibrium thermal noise in a simple model of a mesoscopic wire.

cond-mat↗