Chao Zhang

Strong nonlinear optical response in graphene in terahertz regime

Anthony Wright et al. — Mon, 31 Oct 2011 21:54:37 PDT

We demonstrate that within the model of massless Dirac fermions, graphene has a strong nonlinear optical response in the terahertz regime. It is found that the nonlinear contribution significantly alters both the single frequency and frequency tripled optical response at experimentally relevant field strengths. The optical activity of single layer graphene is significantly enhanced by nonlinear effects, and the frequency tripled response opens the gateway to photonic and optoelectronic device applications.

Enhanced optical conductivity of bilayer graphene nanoribbons in the terahertz regime

Anthony Wright et al. — Mon, 31 Oct 2011 21:54:35 PDT

We reveal that there exists a class of graphene structures (a sub-class of bilayer graphene nanoribbons) which has an exceptionally strong optical response in the terahertz (THz) and far infrared (FIR) regime. The peak conductance of THz/FIR active bilayer ribbons is around two orders of magnitude higher than the universal conductance of $\sigma_0=e^2/4\hbar$ observed in graphene sheets. The criterion for the THz/FIR active sub-class is a bilayer graphene nanoribbon with a one-dimensional massless Dirac Fermion energy dispersion near the $\Gamma$ point. Our results overcome a significant obstacle that hinders potential application of graphene in electronics and photonics.

Noise temperature spectrum of hot electrons in semiconductor superlattices

Chang Wang et al. — Mon, 31 Oct 2011 21:54:33 PDT

The small signal response and thermal noise spectra in miniband superlattice are investigated. The properties of hot electron differential mobility, velocity fluctuation, and noise temperature are determined around a stationary condition. The field and frequency dependent drift velocity, electron energy, effective mass, and electron temperature are obtained. At low frequencies, noise temperature increases rapidly with the electric field. Our calculated noise temperatures for miniband superlattice are in good agreement with the experimental results, with the sample thickness estimated to be around 4 um.

Multilayer thermionic cooling in semiconductor heterostructures

B. C. Lough et al. — Mon, 31 Oct 2011 21:54:31 PDT

The feasibility of using semiconductor heterostructures as cooling devices is currently being studied. Utilising thermionic emission is proposed because of its potentially high cooling power. Multilayer devices have higher efficiency than single layers due to reduced phonon transport, or increased thermal resistance. We have studied the behaviour of thermionic cooling in periodic barriers using 10-layer GaAs/AlxGal-xAs heterostructures. Two methods of measuring cooling and heating are currently being investigated. Micro-thermocouples show temperature changes, but may affect readings because of their finite thermal mass. Also being investigated is the use of a non-contact optical method that relies on the temperature dependence of features in the reflected spectrum of the devices. Preliminary measurements have been carried out on direct band-gap bulk materials such as GaAs and InP and show strong temperature dependence. Cooling, either relative or absolute, has not yet been observed in our first generation devices. Any such cooling may be masked by joule heating in the comparatively large substrate on which the devices are built. Simulations are currently being carried out to determine where the most power is being dissipated in the devices.

Stretching induced Hall current and conductance anisotropy in graphene

A R Wright et al. — Mon, 31 Oct 2011 21:54:29 PDT

We evaluate the effect of stretching on the optical conductance of a single layer graphene sheet. It is found that the low energy (Dirac regime) isotropy that leads to the `universal conductance' is lost. More significantly, due to the loss of C_3 symmetry, a non-zero Hall conductance emerges for stretching along chiral directions, reaching a maximum at a stretching angle of 45 degrees, and being as high a S_0 = e2/4h at van Hove singular point for bond angle changes of about 2 degrees.

Geomechanical/Geochemical Modeling Studies Conducted within the International DECOVALEX Project

J.T. Birkholzer et al. — Wed, 28 May 2008 12:24:12 PDT

Measurement of the atmospheric muon spectrum from 20 to 3000 GeV

P. Achard et al. — Wed, 28 May 2008 12:24:08 PDT

The absolute muon flux between 20 and 3000 GeV is measured with the L3 magnetic muon spectrometer for zenith angles ranging from 0degrees to 58degrees. Due to the large exposure of about 150 m(2) sr d, and the excellent momentum resolution of the L3 muon chambers, a precision of 2.3% at 150 GeV in the vertical direction is achieved. The ratio of positive to negative muons is studied between 20 and 500 GeV, and the average vertical muon charge ratio is found to be 1.285 +/- 0.003(stat.) +/-0.019(syst.). (C) 2004 Elsevier B.V. All rights reserved.

Thermal and electrical characteristics of a multilayer thermionic device

B. C. Lough et al. — Wed, 28 May 2008 12:24:00 PDT

We report our recent experimental and numerical investigation into the thermal and electrical transport in GaAs-AlGaAs semiconductor multilayer structures. Electrical and thermal conduction measurements were performed on multilayer structures to determine the temperature gradient across the sample. AuGe was used for top contact metallisation, and an InGa eutectic for bottom substrate contact. Metallisation contacts were also grown directly onto the substrate in order to compare results with and without the device included. By using a variable load resistor connected in series with the device, we can accurately determine the current-voltage characteristics of the device. Thus the power input can be obtained. The temperature distribution on the top and bottom substrate was measured with micro thermocouples. Since the cooling device is grown on an n-type semiconductor substrate the effects of joule heating in the substrate had to be considered. Treating the substrate as bulk material and calculating joule heating showed that this effect is negligible. Comparing experimental measurements of the device and of the substrate alone support this. The experimental I-V characteristics of the device differ significantly in shape from theoretical I-V characteristics. This may be due to that fact that space-charge effects are not included in the currently accepted model (Richardson's equation). Due to the small size of the devices and therefore very large electric fields, this effect may be important. Work is currently being carried out to modify the model. The devices studied so far have been made from undoped GaAs-AI/sub 0.07/Ga/sub 0.03/As heterostructures. For large cooling power it is a requirement that the conduction band of the layers be close to the Fermi level.

Measurement of the shadowing of high-energy cosmic rays by the Moon: A search for TeV-energy antiprotons

P. Achard et al. — Wed, 28 May 2008 12:23:56 PDT

The shadowing of high-energy cosmic rays by the Moon has been observed with a significance of 9.4 standard deviations with the L3 + C muon spectrometer at CERN. A significant effect of the Earth magnetic field is observed. Since no event deficit on the east side of the Moon has been observed, an upper limit at 90% confidence level on the antiproton to proton ratio of 0.11 is obtained for primary energies around 1 TeV. © 2005 Elsevier B.V. All rights reserved.

Exact formula for nondiagonal Green's functions in condensed-matter physics

T. Toyoda et al. — Wed, 28 May 2008 12:23:52 PDT

An exact formula for the nondiagonal part of the one-particle temperature Green’s function is obtained within the framework of the nonperturbative canonical formulation of the finite-temperature generalized Ward-Takahashi relation. It is shown that the Landau-level Green’s function for two-dimensional electrons has a nonvanishing nondiagonal part if the electron-electron interaction is included.

Optical absorption in terahertz-driven quantum wells

X. W. Mi et al. — Wed, 28 May 2008 12:23:49 PDT

The optical absorption spectra in a quantum well driven both by an intense terahertz (THz) and by an optical pulse are theoretically investigated within the theory of density matrix. We found that the optical absorption spectra and the splitting of the excitonic peaks splitting can be controlled by changing the THz field intensity and/or frequency. The Autler–Towns splitting is a result of the THz nonlinear dynamics of confined excitons, which is in agreement with the experiments. In addition, the dependence of the optical absorption on the quantum well width and the carrier density is also discussed.

Dynamic screening and collective excitation of an electron gas under intense terahertz radiation

C. Zhang — Wed, 28 May 2008 12:23:46 PDT

By using time-dependent wave functions for electrons under an intense laser, we calculated the chargedensity fluctuation of an electronic system under a weak probing potential. The dielectric function of the system as a function of the laser frequency and intensity is derived. The spectrum of the collective excitation is calculated. The spectrum exhibits the contribution of various multiphoton processes.

Optical spectrum of a two-dimensional hole gas in the presence of spin-orbit interaction

C. H. Yang et al. — Wed, 28 May 2008 12:23:43 PDT

We present a theoretical study on how the Rashba type of spin splitting affects the optical spectrum of a two-dimensional hole gas (2DHG) realized from a p-type GaAs/AlxGa1−xAs heterojunction. The optical conductivity is evaluated on the basis of the Kubo formula and a standard random-phase approximation for hole-hole interaction in different spin branches. It is found that similar to the case of a spin-split twodimensional electron gas (2DEG), the optical spectrum of a spin-split 2DHG depends strongly on two opticlike plasmon modes caused by collective excitation between two different spin branches. The position and width of the absorption spectrum relate directly to important spintronic coefficients. Thus, the spin splitting induced by the Rashba effect can be identified optically and important spintronic properties of a 2DHG can be measured via optical experiments. The results are also compared to those obtained for a spin-split 2DEG.

Two colour plasmon excitation in an electron-hole bilayer structure controlled by the spin-orbit interaction

C. H. Yang et al. — Wed, 28 May 2008 12:23:40 PDT

In recent years, the terahertz plasma effects in high-mobility electronic systems have attracted much attention theoretically and experimentally. Plasma excitation in the terahertz regime can be used for generation, detection, and frequency multiplication of terahertz radiation. A channel of a field-effect transistor with sufficiently high electron mobility can serve as a resonant cavity for the plasma oscillation. When the signal period is in the vicinity of the electron transit time, self-excitation of plasma oscillation can occur. If the typical plasma frequency is in the terahertz regime, the phenomenon can be used as a terahertz source.

Magnetoplasmon emission versus Landau-level scattering in resonant tunneling through double-barrier structures

C. Zhang et al. — Wed, 28 May 2008 12:23:37 PDT

We consider electron tunneling in a parallel magnetic field taking into account the electron-electron interaction. It is shown that a self-consistent treatment of the dynamical electron-electron interaction in resonant tunneling can lead to the proposed plasmon assisted resonant tunneling. Such magnetoplasmon assisted resonant tunneling gives rise to satellite peaks or shoulders in the tunneling current. At low temperatures, only magnetoplasmon emission processes contribute and satellites only appear on the high bias side of the main resonance. The mechanism proposed here may be used to study the magnetotunneling in high-mobility systems where disorder is at minimum.

Solid-state thermionics and thermoelectrics in the ballistic transport regime

T. E. Humphrey et al. — Wed, 28 May 2008 12:23:34 PDT

It is shown that the equations for electrical current in solid-state thermionic and thermoelectric devices converge for devices with a width equal to the mean free path of electrons, yielding a common expression for the intensive electronic efficiency in the two types of devices. This result is used to demonstrate that the material parameters for thermionic and thermoelectric refrigerators are equal, rather than differing by a multiplicative factor as previously thought.

Electronic efficiency in nanostructured thermionic and thermoelectric devices

M. F. O'Dwyer et al. — Wed, 28 May 2008 12:23:31 PDT

Advances in solid-state device design now allow the spectrum of transmitted electrons in thermionic and thermoelectric devices to be engineered in ways that were not previously possible. Here we show that the shape of the electron energy spectrum in these devices has a significant impact on their performance. We distinguish between traditional thermionic devices where electron momentum is filtered in the direction of transport only and a second type, in which the electron filtering occurs according to total electron momentum. Such “total momentum filtered” thermionic devices could potentially be implemented in, for example, quantum dot superlattices. It is shown that whilst total momentum filtered thermionic devices may achieve an efficiency equal to the Carnot value, traditional thermionic devices are limited to an efficiency below this. Our second main result is that the electronic efficiency of a device is not only improved by reducing the width of the transmission filter as has previously been shown, but also strongly depends on whether the transmission probability rises sharply from zero to full transmission. The benefit of increasing efficiency through a sharply rising transmission probability is that it can be achieved without sacrificing device power, in contrast to the use of a narrow transmission filter which can greatly reduce power. We show that devices that have a sharply rising transmission probability significantly outperform those that do not and that such transmission probabilities may be achieved with practical single and multibarrier devices. We discuss how the shape of the electron energy spectrum will also have an effect on the electronic efficiency of thermoelectric devices due to mathematical equivalences in the ballistic and diffusive formalisms. Finally, we present an experimental measure that might be used to provide an indication of the nature of the electron energy spectrum and the electronic efficiency of a ballistic device.

Light scattering from a periodically modulated two-dimensional electron gas with partially filled Landau levels

A. Brataas et al. — Wed, 28 May 2008 12:23:28 PDT

We study light scattering from a periodically modulated two-dimensional electron gas in a perpendicular magnetic field. If a subband is partially filled, the imaginary part of the dielectric function as a function of frequency contains additional discontinuities to the case of completely filled subbands. The positions of the discontinuities may be determined from the partial filling factor and the height of the discontinuity can be directly related to the modulation potential. The light-scattering cross section contains a peak which is absent for integer filling.

Response function of a two-dimensional electron gas in a unidirectional periodic potential

S. M. Stewart et al. — Wed, 28 May 2008 12:23:25 PDT

The dynamical density-response function of a periodically modulated two-dimensional electron gas under a perpendicular magnetic field is calculated within the random-phase approximation. It is found that the response function is not only broadened by the periodic potential, it also contains a series of subsingularities at the band edges. The maximum number of subsingularities is 2nmax, where nmax is the number of occupied Landau leveles (the last level may be partially filled). It is further proposed that light-scattering or electromagnetic absorption experiments can be performed to study these predicted structures.

Power generation with nanowire resonant tunneling thermoelectrics

M. F. O'Dwyer et al. — Wed, 28 May 2008 12:23:22 PDT

Nanowires have been predicted to yield significantly improved thermoelectric figures of merit. Here we describe the electron energy spectrum of a resonant tunneling heterostructure nanowire in detail and show how it may be used to realize energy filtering for thermoelectric electronic efficiencies approaching the Carnot limit. A proof-of-principle experiment is described that seeks to demonstrate such idealized electronic properties.