arXiv subjects

D. J. Paul

Publications and source records attributed to D. J. Paul.

12 recordsLinked to original sources

arXiv2022

A tuneable wavelength reference for chip-scale laser cooling

We demonstrate a tuneable, chip-scale wavelength reference to greatly reduce the complexity and volume of cold-atom sensors. A 1 mm optical path length micro-fabricated cell provides an atomic wavelength reference, with dynamic frequency control enabled by Zeeman shifting the atomic transition through the magnetic field generated by the printed circuit board (PCB) coils. The dynamic range of the laser frequency stabilization system is evaluated and used in conjunction with an improved generation of chip-scale cold atom platforms that traps 4 million 87Rb atoms. The scalability and component consolidation provide a key step forward in the miniaturization of cold atom sensors.

physics.atom-ph↗

arXiv2022

Invited Review: Micro-fabricated components for cold atom sensors

Laser cooled atoms have proven transformative for precision metrology, playing a pivotal role in state-of-the-art clocks and interferometers, and having the potential to provide a step-change in our modern technological capabilities. To successfully explore their full potential, laser cooling platforms must be translated from the laboratory environment and into portable, compact quantum sensors for deployment in practical applications. This transition requires the amalgamation of a wide range of components and expertise if an unambiguously chip-scale cold atom sensor is to be realized. We present recent developments in cold-atom sensor miniaturization, focusing on key components that enable laser cooling on the chip-scale. The design, fabrication and impact of the components on sensor scalability and performance will be discussed with an outlook to the next generation of chip-scale cold atom devices.

physics.atom-ph↗

arXiv2021

A Nineteen Day Earth Tide Measurement with a MEMS Gravimeter

The measurement of tiny variations in local gravity enables the observation of subterranean features. Gravimeters have historically been extremely expensive instruments, but usable gravity measurements have recently been conducted using MEMS (microelectromechanical systems) sensors. Such sensors are cheap to produce, since they rely on the same fabrication techniques used to produce mobile phone accelerometers. A significant challenge in the development of MEMS gravimeters is maintaining stability over long time periods, which is essential for long term monitoring applications. A standard way to demonstrate gravimeter stability and sensitivity is to measure the periodic elastic distortion of the Earth due to tidal forces - the Earth tides. Here we present a nineteen day measurement of the Earth tides, with a correlation coefficient to the theoretical signal of 0.979. The estimated bias instability of the proposed gravimeter is 8.18 microGal for an averaging time of ~400 s when considering the raw, uncompensated data. The bias instability extracted from the sensor electronic noise sits just under 2 mircoGal for an averaging time of ~200 s. After removing the long-term temperature and control electronics effects from the raw data, a linear drift of 268 microGal/day is observed in the data, which is among one of the best reported for a MEMS device. These results demonstrate that this MEMS gravimeter is capable of conducting long-therm time-lapse gravimetry, a functionality essential for applications such as volcanology.

physics.ins-det↗

arXiv2018

Room temperature operation of n-type Ge/SiGe terahertz quantum cascade lasers predicted by non-equilibrium Green's functions

n-type Ge/SiGe terahertz quantum cascade laser are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs QCL design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling operation up to room temperature. The better temperature robustness with respect to III-V is attributed to the much weaker interaction with optical phonons. The effect of lower interface quality is investigated and can be partly overcome by engineering smoother quantum confinement via multiple barrier heights.

cond-mat.mes-hall↗

arXiv2016

Disentangling nonradiative recombination processes in Ge micro-crystals on Si substrates

We address nonradiative recombination pathways by leveraging surface passivation and dislocation management in micron-scale arrays of Ge crystals grown on deeply patterned Si substrates. The time decay photoluminescence (PL) at cryogenic temperatures discloses carrier lifetimes approaching 45 ns in band-gap engineered Ge micro-crystals. This investigation provides compelling information about the competitive interplay between the radiative band-edge transitions and the trapping of carriers by dislocations and free surfaces. Furthermore, an in-depth analysis of the temperature dependence of the PL, combined with capacitance data and finite difference time domain modeling, demonstrates the effectiveness of GeO2 in passivating the surface of Ge and thus in enhancing the room temperature PL emission.

cond-mat.mtrl-sci↗

arXiv2008

Variation of the hopping exponent in disordered silicon MOSFETs

We observe a complex change in the hopping exponent value from 1/2 to 1/3 as a function of disorder strength and electron density in a sodium-doped silicon MOSFET. The disorder was varied by applying a gate voltage and thermally drifting the ions to different positions in the oxide. The same gate was then used at low temperature to modify the carrier concentration. Magnetoconductivity measurements are compatible with a change in transport mechanisms when either the disorder or the electron density is modified suggesting a possible transition from a Mott insulator to an Anderson insulator in these systems.

cond-mat.str-el↗

arXiv2007

Magnetoconductivity of Hubbard bands induced in Silicon MOSFETs

Sodium impurities are diffused electrically to the oxide-semiconductor interface of a silicon MOSFET to create an impurity band. At low temperature and at low electron density, the band is split into an upper and a lower sections under the influence of Coulomb interactions. We used magnetoconductivity measurements to provide evidence for the existence of Hubbard bands and determine the nature of the states in each band.

cond-mat.str-el↗

arXiv2007

Activation mechanisms in sodium-doped Silicon MOSFETs

We have studied the temperature dependence of the conductivity of a silicon MOSFET containing sodium ions in the oxide above 20 K. We find the impurity band resulting from the presence of charges at the silicon-oxide interface is split into a lower and an upper band. We have observed activation of electrons from the upper band to the conduction band edge as well as from the lower to the upper band. A possible explanation implying the presence of Hubbard bands is given.

cond-mat.str-el↗

arXiv2006

Evidence for multiple impurity bands in sodium-doped silicon MOSFETs

We report measurements of the temperature-dependent conductivity in a silicon metal-oxide-semiconductor field-effect transistor that contains sodium impurities in the oxide layer. We explain the variation of conductivity in terms of Coulomb interactions that are partially screened by the proximity of the metal gate. The study of the conductivity exponential prefactor and the localization length as a function of gate voltage have allowed us to determine the electronic density of states and has provided arguments for the presence of two distinct bands and a soft gap at low temperature.

cond-mat.str-el↗

arXiv2005

Transverse "resistance overshoot" in a Si/SiGe two-dimensional electron gas in the quantum Hall effect regime

We investigate the peculiarities of the "overshoot" phenomena in the transverse Hall resistance R_{xy} in Si/SiGe. Near the low magnetic field end of the quantum Hall effect plateaus, when the filling factor νapproaches an integer i, R_{xy} overshoots the normal plateau value h/ie^2. However, if magnetic field B increases further, R_{xy} decreases to its normal value. It is shown that in the investigated sample n-Si/Si_{0.7}Ge_{0.3}, overshoots exist for almost all ν. Existence of overshoot in R_{xy} observed in different materials and for different ν, where splitting of the adjacent Landau bands has different character, hints at the common origin of this effect. Comparison of the experimental curves R_{xy}(ν) for ν= 3 and ν= 5 with and without overshoot showed that this effect exist in the whole interval between plateaus, not only in the region where R_{xy} exceeds the normal plateau value.

cond-mat.mes-hall↗

arXiv2003

Longitudinal conductivity in Si/SiGe heterostructure at integer filling factors

We have investigated temperature dependence of the longitudinal conductivity $σ_{xx}$ at integer filling factors $ν=i$ for Si/SiGe heterostructure in the quantum Hall effect regime. It is shown that for odd $i$, when the Fermi level $E_{F}$ is situated between the valley-split levels, $Δσ_{xx}$ is determined by quantum corrections to conductivity caused by the electron-electron interaction: $Δσ_{xx}(T)\sim \ln T$. For even $i$, when $E_{F}$ is located between cyclotron-split levels or spin-split levels, $σ_{xx}\sim \exp[-Δ_{i}/T]$ for $i=6,10,12$ and $\sim \exp [-(T_{0i}/T)]^{1/2}$ for $i=4,8$. For further decrease of $T$, all dependences $σ_{xx}(T)$ tend to almost temperature-independent residual conductivity $σ_{i}(0)$. A possible mechanism for $σ_{i}(0)$ is discussed.

cond-mat.mes-hall↗

arXiv2001

The relative importance of electron-electron interactions compared to disorder in the two-dimensional "metallic" state

The effect of substrate bias and surface gate voltage on the low temperature resistivity of a Si-MOSFET is studied for electron concentrations where the resistivity increases with increasing temperature. This technique offers two degrees of freedom for controlling the electron concentration and the device mobility, thereby providing a means to evaluate the relative importance of electron-electron interactions and disorder in this so-called ``metallic'' regime. For temperatures well below the Fermi temperature, the data obey a scaling law where the disorder parameter ($k_{\rm{F}}l$), and not the concentration, appears explicitly. This suggests that interactions, although present, do not alter the Fermi-liquid properties of the system fundamentally. Furthermore, this experimental observation is reproduced in results of calculations based on temperature-dependent screening, in the context of Drude-Boltzmann theory.

cond-mat.mes-hall↗