Fluctuation-induced current from freestanding graphene
P. M. Thibado · P. Kumar · Surendra Singh · M. Ruiz-Garcia · A. Lasanta · L. L. Bonilla
Abstract and summary · read the original at the source
In one page
Paul Thibado’s group at the University of Arkansas put a scanning tunnelling microscope tip above a sheet of graphene suspended over a hole and watched it move. At room temperature the sheet never stops: its nanometre ripples flip between concave and convex, and the carbon atoms ride them. With the tip pulled back far enough that no electrons tunnel, that motion still pushes charge around the circuit — a displacement current, which the team routes through a pair of diodes so each flip drives current one way and the return flip the other. Rigid graphene on copper gives nothing, which rules out contamination and field emission. Power reaching one diode reached 40 picowatts. With Luis Bonilla and colleagues in Madrid they then model the ripple as a Brownian particle in a double-well potential wired to the diode circuit, and work the accounting through: the system sits at a single temperature in equilibrium, the graphene supplies exactly the power the load resistor dissipates, and the changing diode resistance lifts the output above Nyquist’s noise-power formula.
Why it matters hereChapter 6 wants devices that turn fluctuations straight into current, and this is the cleanest published example of one: no temperature difference across the device, no radioactive source, just a membrane that will not hold still and a circuit that rectifies it. Chapter 2 supplies the reason it will not hold still. The same group’s molecular-dynamics study of how those ripples invert is on this site at /library/stm-b278fb0be1.
What it claims
01At room temperature the out-of-plane fluctuations of freestanding graphene are about a hundred times larger than those of graphene supported on copper — 10 picoamps of tunnelling-current fluctuation against 0.1 picoamps — and extrapolated to zero tunnelling current the motion still contributes about 20 picoamps of displacement current.Figure 1, panels d and e, and the accompanying text
Published and peer-reviewed02With the tip withdrawn until no electrons tunnel, a DC bias of 15 or 45 volts produces a spiky, time-dependent current through diode 2, and the power dissipated in that diode reaches 40 picowatts; identically prepared rigid graphene produces no such current, which the authors take as confirmation that contamination and field emission are not the source.Figure 2, panels a to d, and the paragraph on backing the tip away
Published and peer-reviewed03Modelling the nearest ripple as a Brownian particle in a double-well potential coupled to the diode circuit by Langevin-Ito equations, the whole system reaches thermal equilibrium at one temperature and the rates of heat, work and entropy production go to zero — yet the time-averaged power dissipated in the resistor equals the time-averaged power supplied by the thermal bath, so electrical work is carried out on the load without violating the second law.Equations 1 to 8; Figure 3, panel a; summary paragraph
Published and peer-reviewed04The exact thermal power formula reduces to Nyquist’s noise power, temperature divided by the resistance-capacitance product, only if the resistance is constant; because the diode resistance changes with time the true result is a large power enhancement over Nyquist.Figure 3, panel b, and the text following Equation 8
Published and peer-reviewed05Because the power includes the Brownian motion of the graphene ripple and not only that of the electrons, the barrier-crossing rate of the double well introduces a new and much slower time scale: lowering that rate leaves the total power unchanged but redistributes it to very low frequencies, which the authors note adds technological value for harvesting.Figure 3, panel f, and the paragraph on the power spectral density
Published and peer-reviewed06Driving the system out of equilibrium — extra noise sources, or different temperatures in different parts of the circuit — would produce entropy and measurable deviations from detailed balance, and the authors name this as the next thing worth investigating in freestanding graphene.Closing paragraph of the summary section
What to watch
Read it · abstract
Abstract
At room temperature, micron-sized sheets of freestanding graphene are in constant motion even in the presence of an applied bias voltage. We quantify the out-of-plane movement by collecting the displacement current using a nearby small-area metal electrode and present a Langevin model for the motion coupled to a circuit containing diodes. The system reaches thermal equilibrium and the rates of heat, work, and entropy production tend quickly to zero. However, there is power generated by graphene which is equal to the power dissipated by the load resistor. The exact power formula is similar to Nyquist’s noise power formula, except that the rate of change of diode resistance significantly boosts the output power, and the movement of the graphene shifts the power spectrum to lower frequencies.
P. M. Thibado, P. Kumar and Surendra Singh, Department of Physics, University of Arkansas; M. Ruiz-Garcia, Department of Physics, University of Pennsylvania; A. Lasanta, Universidad Carlos III de Madrid and Universidad de Granada; L. L. Bonilla, G. Millán Institute, Universidad Carlos III de Madrid, and the Courant Institute, New York University. Physical Review E 102, 042101 (2020). Preprint: arXiv:2002.09947 [cond-mat.mes-hall].
(Abstract only. The scanning-tunnelling-microscope data, the Langevin-Ito derivation and the simulation results are at the source — see the rights note above for why the text is not reproduced here. The preprint is free to read at arXiv.)
The way in
https://doi.org/10.1103/physreve.102.042101Licence checked on the source itself: the arXiv posting 2002.09947, titled ‘Fluctuation-induced current from freestanding graphene: toward nanoscale energy harvesting’, carries the arXiv non-exclusive distribution licence and no Creative Commons statement, and the published version is under the APS default licence. This page therefore carries the summary, the claims and the authors’ own abstract, and sends the reader to the source. The preprint is free to read at arXiv.
How to cite it
P. M. Thibado, P. Kumar, Surendra Singh, M. Ruiz-Garcia, A. Lasanta, L. L. Bonilla (2020) Fluctuation-induced current from freestanding graphene. doi:10.1103/physreve.102.042101
Where it sits in the curriculum