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P. M. Thibado

Publications and source records attributed to P. M. Thibado.

At least 19 recordsLinked to original sources
arXiv2025

Charging capacitors using diodes at different temperatures. II Numerical studies

This study is presented in a series of two papers. The first paper is an analytical study. This is the second paper, and here we numerically study the thermal energy harvesting capability of two electronic circuits. The first circuit consists of a diode and capacitor in series. We solve the time-dependent Fokker-Planck equation and show the capacitor initially charges and then discharges to zero. The peak charge on the capacitor increases with temperature, capacitance, and diode quality. The second circuit has two current loops with one small capacitor, two storage capacitors, and two diodes wired in opposition. When the diodes are held at different temperatures we observe a non-zero steady-state charge is accumulated on both storage capacitors. The magnitude of the stored charges are nearly equal but the signs are opposite. When resistors are used in place of diodes there is no transient and no steady-state charge buildup. Numerical studies for the time-independent Fokker-Planck equation are presented and confirm the steady state charges.

cond-mat.stat-mech↗
arXiv2025

Charging capacitors using diodes at different temperatures. I Theor

Nonlinear elements in a rectifying circuit can be used to harvest energy from thermal fluctuations either steadily or transitorily. We study an energy harvesting system comprising a small variable capacitor (e.g., free standing graphene) wired to two diodes and two storage capacitors that may be kept at different temperatures (or at a single one) and use two current loops. The system reaches very rapidly a quasi stationary state with constant overall charge while the difference of the charges at the storage capacitors evolves much more slowly to its stationary value. In this paper, we extract an exponentially small factor out of the solution of the Fokker-Planck equation and use a Chapman-Enskog procedure to describe the long evolution of the marginal probability density for the charge difference, from the quasi stationary state to the final stationary state (thermal equilibrium for equal temperatures). The second paper of this series shows that the results of the perturbation procedure compare well with direct numerical simulations. For a specific form of the diodes' nonlinear mobilities, we can approximate the quasi stationary state by Gaussian functions and further study the evolution of the marginal probability density. The latter adopts the shape of a slowly expanding pulse (comprising left and right moving wave fronts whose fore edges become sharper as time elapses) in the space of charge differences that leaves the final stationary state behind it.

cond-mat.stat-mech↗
arXiv2023

Charging capacitors from thermal fluctuations using diodes

We theoretically consider a graphene ripple as a Brownian particle coupled to an energy storage circuit. When circuit and particle are at the same temperature, the second law forbids harvesting energy from the thermal motion of the Brownian particle, even if the circuit contains a rectifying diode. However, when the circuit contains a junction followed by two diodes wired in opposition, the approach to equilibrium may become ultraslow. Detailed balance is temporarily broken as current flows between the two diodes and charges storage capacitors. The energy harvested by each capacitor comes from the thermal bath of the diodes while the system obeys the first and second laws of thermodynamics.

cond-mat.mes-hall↗
arXiv2020

Fluctuation-induced current from freestanding graphene: toward nanoscale energy harvesting

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.

cond-mat.mes-hall↗
arXiv2016

Anomalous Dynamical Behavior of Freestanding Graphene Membranes

We report subnanometer, high-bandwidth measurements of the out-of-plane (vertical) motion of atoms in freestanding graphene using scanning tunneling microscopy. By tracking the vertical position over a long time period, a 1000-fold increase in the ability to measure space-time dynamics of atomically thin membranes is achieved over the current state-of-the-art imaging technologies. We observe that the vertical motion of a graphene membrane exhibits rare long-scale excursions characterized by both anomalous mean-squared displacements and Cauchy-Lorentz power law jump distributions.

cond-mat.mes-hall↗
arXiv2015

Reflection high-energy electron diffraction and scanning tunneling microscopy study of InP(001) surface reconstructions

The reconstructions of the InP(001) surface prepared by molecular beam epitaxy have been studied with in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). The growth chamber contains a highly accurate temperature measurement system and uses a solid-source, cracked phosphorus, valved effusion cell. Five InP(001) reconstructions are observed with RHEED by analyzing patterns in three principal directions. Under a fixed P2 flux, decreasing the substrate temperature gives the following reconstructions: c(2x8), (2x4), (2x1), (2x2), and c(4x4). In situ STM images reveal that only two of these reconstructions yields long-range periodicity in real space. InP(001) does not form the metal rich (4x2) reconstruction, which is surprising because the (4x2) reconstruction has been coined the universal surface reconstruction since all III-V(001) surfaces were thought to favor its formation.

cond-mat.mes-hall↗
arXiv2015

Time-evolution of the GaAs(0 0 1) pre-roughening process

The GaAs(0 0 1) surface is observed to evolve from being perfectly flat to a surface half covered with one-monolayer high spontaneously formed GaAs islands. The dynamics of this process are monitored with atomic-scale resolution using scanning tunneling microscopy. Surprisingly, pit formation dominates the early stages of island formation. Insight into the nucleation process is reported.

cond-mat.mes-hall↗
arXiv2015

Electron-beam evaporated cobalt films on molecular beam epitaxy prepared GaAs(001)

We have deposited Co films on the GaAs(001) surface by using an e-beam evaporation method. The thicknesses of the Co films are measured by using x-ray reflectivity and Rutherford backscattering. The magnetic properties of the films have been measured using superconducting quantum interference device. The magnetization of the films was found to decrease with increasing film thickness. The slight degradation of magnetic properties is attributed to increasing roughness on the Co surface or the Co/GaAs interface during the Co deposition.

cond-mat.mes-hall↗
arXiv2015

Electrical and optical studies of GaMnAs/GaAs(001) thin films grown by molecular beam epitaxy

GaMnAs/GaAs films were grown via molecular beam epitaxy using both low and high substrate temperatures. The films were investigated using Hall effect and photoluminescence (PL) measurements from 8 to 300 K. The carrier concentrations in the samples grown at a low substrate temperature are greater than those in the samples grown at a high substrate temperature. The PL spectra show a GaAs exciton peak, a peak involving a carbon acceptor, a substitutional Mn acceptor-related peak and an optical phonon-related peak.

cond-mat.mes-hall↗
arXiv2015

Atmospheric oxygen in Mn doped GaAs/GaAs(0 0 1) thin films grown by molecular beam epitaxy

Mn doped GaAs thin films were grown using molecular beam epitaxy at high and low substrate temperatures. The elemental concentration depth profiles in the thin films were determined by using Auger electron spectroscopy combined with ion etching. The Mn concentration is higher near the surface and then decreases with depth for films grown at high substrate temperatures. The Mn concentration profile is much more uniform when films are grown using a low substrate temperature. What was unexpectedly found are high levels of oxygen in the low substrate temperature grown thin films.

cond-mat.mes-hall↗
arXiv2015

Anomalous Mn depth profiles for GaMnAs/GaAs(001) thin films grown by molecular beam epitaxy

Mn concentration depth profiles in Mn-doped GaAs thin films grown at substrate temperatures of 580 and 250 °C using various Mn cell temperatures have been investigated with dynamic secondary ion mass spectrometry and Auger electron spectroscopy. When the samples are grown at a low substrate temperature of 250 °C, the Mn distributes uniformly. For the samples grown at a high substrate temperature of 580 °C, the concentration depth profiles are easily fitted with a temperature-dependent Fermi function only if the Mn concentration is above the solubility limit. However, when the Mn concentration is below the solubility limit, unexpected peaks are observed in the concentration depth profiles.

cond-mat.mes-hall↗
arXiv2015

Growth of and optical emission from GaMnAs thin films grown by molecular beam epitaxy

GaMnAs thin films with different Mn doping concentrations were grown via molecular beam epitaxy using a substrate temperature of 250 °C. The thin films were investigated using photoluminescence (PL) measurements from 8 to 300 K. Transitions involving Mn acceptors were identified and a binding energy of ~0.1 eV was found. A Mn doping concentration dependent PL spectrum was found to lend insight into the film quality at a local level. Temperature dependent PL studies show that the doping related emissions drop faster in energy than other peaks with increasing temperature, indicating that they are more sensitive to changes in the surrounding environment.

cond-mat.mes-hall↗
arXiv2015

Controlling Mn Depth Profiles in GaMnAs During High-Temperature Molecular Beam Epitaxial Growth

Mn-doped GaAs thin films were grown at a high substrate temperature of 580 C. During the growth process, the Mn cell temperature was ramped at different rates, resulting in a variety of different Mn concentration depth profile slopes, as measured using dynamic secondary ion mass spectrometry (SIMS). Results show that controlling the Mn deposition rate via temperature during molecular beam epitaxy (MBE) growth can mitigate the effect of Mn atoms diffusing toward the surface. Most importantly, the slope of the Mn concentration as a function of depth inside the sample can be tuned from negative to positive.

cond-mat.mes-hall↗
arXiv2015

Giant Surface Charge Density of Graphene Resolved From Scanning Tunneling Microscopy and First-Principles Theory

In this work, systematic constant-bias, variable-current scanning tunneling microscopy (STM) measurements and STM simulations from density-functional theory are made, yielding critical insights into the spatial structure of electrons in graphene. A foundational comparison is drawn between graphene and graphite, showing the surface charge density of graphene to be 300 percent that of graphite. Furthermore, simulated STM images reveal that high-current STM better resolves graphenes honeycomb bonding structure because of a retraction which occurs in the topmost dangling bond orbitals.

cond-mat.mes-hall↗
arXiv2015

Atomic Control of Strain in Freestanding Graphene

In this study, we describe a new experimental approach based on constant-current scanning tunneling spectroscopy to controllably and reversibly pull freestanding graphene membranes up to 35 nm from their equilibrium height. In addition, we present scanning tunneling microscopy (STM) images of freestanding graphene membranes with atomic resolution. Atomic-scale corrugation amplitudes 20 times larger than the STM electronic corrugation for graphene on a substrate were observed. The freestanding graphene membrane responds to a local attractive force created at the STM tip as a highly-conductive yet flexible grounding plane with an elastic restoring force. We indicate possible applications of our method in the controlled creation of pseudo-magnetic fields by strain on single-layer graphene.

cond-mat.mes-hall↗
arXiv2015

High-Percentage Success Method for Preparing and Pre-Evaluating Tungsten Tips for Atomic-Resolution Scanning Tunneling Microscopy

A custom double-lamella method is presented for electrochemically etching tungsten wire for use as tips in scanning tunneling microscopy (STM). For comparison, tips were also manufactured in-house using numerous conventional methods and examined using an optical microscope. Both sets of tips were used to obtain STM images of highly-oriented pyrolytic graphite, the quality of which varied. The clarity of the STM images was found to be correlated to the optically-measured cone angle of the STM tip, with larger cone angles consistently producing atomically resolved images. The custom etching procedure described allows one to create larger cone angles and consequently proved superior in reliably producing high-quality tips.

cond-mat.mes-hall↗
arXiv2015

A Pathway between Bernal and Rhombohedral Stacked Graphene Layers with Scanning Tunneling Microscopy

Horizontal shifts in the top layer of highly oriented pyrolytic graphite, induced by a scanning tunneling microscope (STM) tip, are presented. Excellent agreement is found between STM images and those simulated using density functional theory. First-principle calculations identify that the low-energy barrier direction of the top layer displacement is toward a structure where none of the carbon pz orbitals overlap, while the high-energy barrier direction is toward AA stacking. Each directional shift yields a real-space surface charge density similar to graphene; however the low-energy barrier direction requires only one bond length to convert ABA (Bernal) to ABC (rhombohedral).

cond-mat.mes-hall↗