arXiv subjects

A. MacKinnon

Publications and source records attributed to A. MacKinnon.

16 recordsLinked to original sources

arXiv2015

Origin of the 30 THz emission detected during the 2012 March 13 solar flare at 17:20 UT

Solar observations in the infrared domain can bring important clues on the response of the low solar atmosphere to primary energy released during flares. At present the infrared continuum has been detected at 30 THz (10 $μ$m) in only a few flares. In this work we present a detailed multi-frequency analysis of SOL2012-03-13, including observations at radio millimeter and sub-millimeter wavelengths, in hard X-rays (HXR), gamma-rays (GR), H-alpha, and white-light. HXR/GR spectral analysis shows that the event is a GR line flare and allows estimating the numbers of and energy contents in electrons, protons and alpha particles produced during the flare. The energy spectrum of the electrons producing the HXR/GR continuum is consistent with a broken power-law with an energy break at ~800 keV. It is shown that the high-energy part (above ~800 keV) of this distribution is responsible for the high-frequency radio emission (> 20 GHz) detected during the flare. By comparing the 30 THz emission expected from semi-empirical and time-independent models of the quiet and flare atmospheres, we find that most (~80%) of the observed 30 THz radiation can be attributed to thermal free-free emission of an optically-thin source. Using the F2 flare atmospheric model this thin source is found to be at temperatures T~8000 K and is located well above the minimum temperature region. We argue that the chromospheric heating, which results in 80% of the 30 THz excess radiation, can be due to energy deposition by non-thermal flare accelerated electrons, protons and alpha particles. The remaining 20% of the 30 THz excess emission is found to be radiated from an optically-thick atmospheric layer at T~5000 K, below the temperature minimum region, where direct heating by non-thermal particles is insufficient to account for the observed infrared radiation.

astro-ph.SR↗

arXiv2012

Electronic structure of magnetically modulated graphene

We present a theoretical study of the electronic structure of magnetically modulated graphene. We consider monolayer graphene in the presence of a perpendicular magnetic field and a unidirectional weak magnetic modulation. The density of states and the bandwidth of the Dirac electrons in this system are determined. We have found magnetic Weiss oscillations in the bandwidth and the density of states. These oscillations are out of phase and larger in amplitude than the ones in the electrically modulated graphene. In addition, these oscillations are in phase and smaller in amplitude to those of magnetically modulated standard electron gas system.

cond-mat.mes-hall↗

arXiv2012

Magnetocapacitance of a graphene monolayer

We present a theoretical study of magnetocapacitance in a graphene monolayer at finite temperature taking into account the effects of disorder. The density of states (DOS) and magnetocapacitance found for graphene are compared to those found in standard two dimensional electron gas (2DEG) systems. The magnetic oscillations in DOS and magnetocapacitance are found to be enhanced and much more robust with respect to temperature damping in monolayer graphene in comparison with a 2DEG. Furthermore, we find that there is a $π$ phase shift between magnetic oscillations in the two systems which can be attributed to Dirac electrons in graphene acquiring a Berry's phase as they traverse a closed path in a magnetic field.

cond-mat.mes-hall↗

arXiv2011

Turbulent cross-field transport of non-thermal electrons in coronal loops: theory and observations

A fundamental problem in astrophysics is the interaction between magnetic turbulence and charged particles. It is now possible to use \emph{Ramaty High Energy Solar Spectroscopic Imager (RHESSI)} observations of hard X-rays (HXR) emitted by electrons to identify the presence of turbulence and to estimate the magnitude of the magnetic field line diffusion coefficient at least in dense coronal flaring loops.} {We discuss the various possible regimes of cross-field transport of non-thermal electrons resulting from broadband magnetic turbulence in coronal loops. The importance of the Kubo number $K$ as a governing parameter is emphasized and results applicable in both the large and small Kubo number limits are collected.} {Generic models, based on concepts and insights developed in the statistical theory of transport, are applied to the coronal loops and to the interpretation of hard X-ray imaging data in solar flares. The role of trapping effects, which become important in the non-linear regime of transport, is taken into account in the interpretation of the data.} For this flaring solar loop, we constrain the ranges of parallel and perpendicular correlation lengths of turbulent magnetic fields and possible Kubo numbers. We show that a substantial amount of magnetic fluctuations with energy $\sim 1%$ (or more) of the background field can be inferred from the measurements of the magnetic diffusion coefficient inside thick-target coronal loops.

astro-ph.SR↗

arXiv2011

Temperature effects on magnetoplasmon spectrum of a weakly modulated graphene monolayer

In this work, we determine the effects of temperature on the magnetoplasmon spectrum of an electrically modulated graphene monolayer as well as the two-dimensional electron gas (2DEG). The intra-Landau-band magnetoplasmon spectrum within the Self Consistent Field (SCF) approach is investigated for both the aforementioned systems. Results obtained not only exhibit Shubnikov-de Hass (SdH) oscillations but also commensurability oscillations (Weiss oscillations). These oscillations are periodic as a function of inverse magnetic field. We find that both the magnetic oscillations, SdH and Weiss, have a greater amplitude and are more robust against temperature in graphene compared to conventional 2DEG. Furthermore, there is a $π$ phase shift between the magnetoplasmon oscillations in the two systems which can be attributed to Dirac electrons in graphene acquiring a Berry's phase as they traverse a closed path in a magnetic field.

cond-mat.mes-hall↗

arXiv2010

Current noise of a resonant tunnel junction coupled to a nanomechanical oscillator

We present a theoretical study of current noise of a resonant tunnel junction coupled to a nanomechanical oscillator within the non-equilibrium Green's function technique. An arbitrary voltage is applied to the tunnel junction and electrons in the leads are considered to be at zero temperature. The properties of the phonon distribution of the nanomechanical oscillator strongly coupled to the electrons on the dot are investigated using a non-perturbative approach. An analytical calculations and numerical results for the current-voltage, shot noise and the corresponding Fano factor as a function of applied bias show significant features of the nanomechanical oscillator coupling dynamics. This will provide useful insight for the design of experiments aimed at studying the quantum behavior of an oscillator.

cond-mat.mes-hall↗

arXiv2010

Time-dependent quantum transport in a resonant tunnel junction coupled to a nanomechanical oscillator

We present a theoretical study of time-dependent quantum transport in a resonant tunnel junction coupled to a nanomechanical oscillator within the non-equilibrium Green's function technique. An arbitrary voltage is applied to the tunnel junction and electrons in the leads are considered to be at zero temperature. The transient and the steady state behavior of the system is considered here in order to explore the quantum dynamics of the oscillator as a function of time. The properties of the phonon distribution of the nanomechnical oscillator strongly coupled to the electrons on the dot are investigated using a non-perturbative approach. We consider both the energy transferred from the electrons to the oscillator and the Fano factor as a function of time. We discuss the quantum dynamics of the nanomechanical oscillator in terms of pure and mixed states. We have found a significant difference between a quantum and a classical oscillator. In particular, the energy of a classical oscillator will always be dissipated by the electrons whereas the quantum oscillator remains in an excited state. This will provide useful insight for the design of experiments aimed at studying the quantum behavior of an oscillator.

cond-mat.mes-hall↗

arXiv2009

The Magnetoplasmon spectrum of a weakly modulated two-dimensional electron gas system

The magnetoplasmon spectrum of a magnetically modulated two-dimensional electron gas (MM2DEG) is investigated. We derive the inter and intra Landau band magnetoplasmon spectrum within the self consistent field approach. The derivation is performed at zero temperature as well as at finite temperature. Results are presented for the inter and intra Landau band magnetoplasmon spectrum as a function of the inverse magnetic field. As a result of magnetic modulation, magnetic Weiss oscillations are found to occur in the magnetoplasmon spectrum. Furthermore, our finite temperature theory facilitates analysis of effects of temperature on the magnetoplasmon spectrum. The results are compared with those obtained for an electrically modulated 2DEG system. In addition, we derive and discuss the effects of simultaneous electric and magnetic modulations on the magnetoplasmon spectrum of 2DEG when the modulations are in phase as well as when they are out of phase. Magnetic oscillations are affected by the relative phase of the two modulations and position of the oscillations depends on the relative strength of the two modulations in the former case while we find complete suppression of Weiss oscillations for particular relative strength of the modulations in the latter case.

cond-mat.mes-hall↗

arXiv2002

Transport via a quantum shuttle

We investigate the effect of a quantised vibrational mode on electron tunneling through a chain of three quantum dots. The outer dots are coupled to voltage leads, but the position of the central dot is not rigidly fixed. Motion of the central dot modulates the size of the tunneling barriers in opposite ways so that electron tunneling is correlated with the position of the oscillator. We treat the electronic part of the problem using a simple Coulomb-blockade picture, and model the vibration of the central dot as a quantum oscillator. We calculate the eigenspectrum of the system as a function of the energy level shift between the outer dots. Using a density matrix method, we include couplings to external leads and calculate the steady-state current through the device. The current shows marked resonances which correspond to avoided-level crossings in the eigenvalue spectrum. When the tunneling length of the electrons is of order the zero-point position uncertainty of the quantum oscillator, current far from the electronic resonance is dominated by electrons hopping on and off the central dot sequentially; the oscillator can be regarded as shuttling electrons across the system [L.Y. Gorelik, et al., Phys. Rev. Lett. 80, 4526 (1998)]. Damping of the oscillator can increase the current by preventing electrons from hopping `backwards'.

cond-mat.mes-hall↗

arXiv2002

Thermoelectric Properties of Disordered Systems

The electronic properties of disordered systems have been the subject of intense study for several decades. Thermoelectric properties, such as thermopower and thermal conductivity, have been relatively neglected. A long standing problem is represented by the sign of the thermoelectric power. In crystalline semiconductors this is related to the sign of the majority carriers, but in non-crystalline systems it is commonly observed to change sign at low temperatures. In spite of its apparent universality this change has been interpreted in a variety of ways in different systems. We have developed a Green's function recursion algorithm based on the Chester-Thelling-Kubo-Greenwood formula for calculating the kinetic coefficients on long strips or bars. From these we can deduce the electrical conductivity, the Seebeck and Peltier coefficients and the thermal conductivity, as well as the Lorenz number. We present initial results for 1D systems. We observe a Lorentzian distribution for the thermopower which is modified by the presence of inelastic scattering. This could give rise to non-negligible quantum fluctuations in macroscopic systems at low temperatures.

cond-mat.dis-nn↗

arXiv2000

Critical behavior of the thermoelectric transport properties in amorphous systems near the metal-insulator transition

The scaling behavior of the thermoelectric transport properties in disordered systems is studied in the energy region near the metal-insulator transition. Using an energy-dependent conductivity $σ$ obtained experimentally, we extend our linear-response-based transport calculations in the three-dimensional Anderson model of localization. Taking a dynamical scaling exponent $z$ in agreement with predictions from scaling theories, we show that the temperature-dependent $σ$, the thermoelectric power $S$, the thermal conductivity $K$ and the Lorenz number $L_0$ obey scaling.

cond-mat.dis-nn↗

arXiv2000

Behavior of the thermopower in amorphous materials at the metal-insulator transition

We study the thermal transport properties in three-dimensional disordered systems close to the metal-insulator transition within linear response. Using a suitable form for the energy-dependent conductivity $σ$, we show that the value of the dynamical scaling exponent for noninteracting disordered systems such as the Anderson model of localization can be reproduced. Furthermore, the values of the thermopower S have the right order of magnitude close to the transition as compared to the experimental results. A sign change in the thermoelectric power S - as is often observed in experiments - can also be modeled within the linear response formulation using modified experimental $σ$ data as input.

cond-mat.dis-nn↗

arXiv1996

Level Curvatures and Conductances: A Numerical Study of the Thouless Relation

The Thouless conjecture states that the average conductance of a disordered metallic sample in the diffusive regime can be related to the sensitivity of the sample's spectrum to a change in the boundary conditions. Here we present results of a direct numerical study of the conjecture for the Anderson model. They were obtained by calculating the Landauer-Büttiker conductance $g_L$ for a sample connected to perfect leads and the distribution of level curvatures for the same sample in an isolated ring geometry, when the ring is pierced by an Aharonov-Bohm flux. In the diffusive regime ($L\gg l_e$) the average conductance $g_L$ is proportional to the mean absolute curvature $< |c| >$: $ g_L = π<| c | > / Δ$, provided the system size $L$ is large enough, so that the contact resistance can be neglected. $l_e$ is the elastic mean free path, $Δ$ is the mean level spacing. When approaching the ballistic regime, the limitation of the conductance due to the contact resistance becomes essential and expresses itself in a deviation from the above proportionality. However, in both regimes and for all system sizes the same proportionality is recovered when the contact resistance is subtracted from the inverse conductance, showing that the ``curvatures measure the conductance in the bulk''. In the localized regime, the mean logarithm of the absolute curvature and the mean logarithm of the Landauer-Büttiker conductance are proportional.

cond-mat.mes-hall↗

arXiv1996

Incoherent tunnelling through two quantum dots with Coulomb interaction

The Ohmic conductance and current through two quantum dots in series is investigated for the case of incoherent tunnelling. A generalised master equation is employed to include the discrete nature of the energy levels. Regions of negative differential conductance can occur in the I-V characteristics. Transport is dominated by matching energy levels, even when they do not occur at the charge degeneracy points.

cond-mat↗

arXiv1996

Coherent tunnelling through two quantum dots with Coulomb interaction

The coherent conductance and current is calculated through two quantum dots using the Hubbard model for a single level per spin. The occurrence of negative differential conductance is demonstrated. The Ohmic conductance is calculated for dots with equally spaced levels. Transport is determined by matching energy levels, even when they do not occur at the charge degeneracy points.

cond-mat↗

arXiv1995

2-Dimensional Electron Gas in a Linearly Varying Magnetic Field. ``Quantisation'' of the Electron and Current Density

We have developed new methods to calculate dispersion curves (analytically in the simpler cases) from which we are able to derive the spatial distribution of electron and current densities. We investigate the case where the magnetic field varies linearly with position and the results provide useful insights into the properties of this and other field distributions. We consider spin as well as a confining electrostatic potential. We show that the electron and the current density exhibit a very rich structure related to the quantisation of the energy. Moreover there is a direct contribution to the current density due to the spin which could be of interest in relation to spin polarised current.

cond-mat↗