Strong spin-momentum coupling in topological insulators give rise to topological surface states, protected against disorder scattering by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomenon in condensed matter physics such as the spin hall effect, but also lays the foundation for applications in quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly identify topological surface state from the bulk. We use terahertz time-domain spectroscopy to study the temperature-dependent optical behavior of a 23quintuple-thick film of bismuth selenide (Bi2Se3) allowing the deconvolution of the surface state response from the bulk. The signatures of the topological surface state at low temperatures (< 30 K) with the optical conductance of Bi2Se3 exhibiting a metallic behavior are observed. Measurement of carrier dynamics, obtain an optical mobility, exceeding 2000 cm 2 /V•s at 4 K, indicative of a surfacedominated response. A scattering lifetime of ~0.18 ps and a carrier density of 6×10 12 cm -2 at 4 K were obtained from the terahertz time-domain spectroscopy measurement. The terahertz conductance spectra reveal characteristic features at ~1.9 THz, attributed to the optical phonon mode, which becomes less prominent with falling temperature. The electrical transport measurements reveal weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a topological surface state. Device fabrication and electrical transport:Optical-lithography was used to define a microscale Hall bar of dimensions 1400 µm × 80 µm. This was followed by the deposition of 15 nm Ti and 90 nm Au and a standard lift-off process to obtain ohmic contacts to the mesa. The device was then packaged and measured in a He-3 cryostat with a base temperature of 300 mK. Hall measurements were obtained using an a.c lock-in four-terminal setup with an input current of 1 µA at a frequency of 133 Hz. ASSOCIATED CONTENTSupporting information: XRD and XRR measurements to characterize the quality, phase and thickness of the Bi2Se3 film; field dependence of the longitudinal and Hall resistance measurement at corresponding temperatures; THz time domain response of the (0001)-oriented sapphire substrate at different temperatures.

We report measurements of the temperature dependent conductivity in a silicon MOSFET 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 localisation 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.PACS numbers: 71.23. Cq, 71.55.Gs, 71.55.Jv, 72.15.Rn, 72.20.Ee, 72.80.Ng, 73.20.At, 73.40.Qv Since the invention of the silicon MOSFET, understanding the influence of impurities, especially sodium contamination, on device performance has been a priority and continues to provide a rich system for investigation by experimental and theoretical physicists alike. The electronic properties of sodium doped MOSFETs were first studied by Fowler and Hartstein 1,2 in the 1970s. They reported a single, broad peak in the subthreshold drain current against gate voltage and attributed it to the formation of an impurity band induced by the presence of sodium ions near the Si-SiO 2 interface. Further studies of narrow channel devices (∼ 100 nm) demonstrated a series of reproducible sharp peaks 3 , while later experiments found evidence for resonant tunneling between localised states in the channel. 4,5,6 For sufficiently low impurity concentrations, the overlap between neighbouring localized electron wavefunctions and consequently the hybridisation of their excited states is predicted to be reduced 7 , splitting the single impurity band observed at high concentrations into the ground and excited bands as modeled by Ghazali.8 Increasing the resistivity of the silicon substrate reduces the scattering from acceptors at the Si-SiO 2 interface, allowing the possibility for such a band splitting to be experimentally observed in the transport. In this paper, we will present evidence for the observation of two separate impurity bands with a soft gap, based on analysis of the temperature dependent conductivity below 20 K.The device we used is a MOSFET fabricated on a (100) oriented p-silicon wafer and was subsequently patterned in the circular Corbino geometry to eliminate Hall voltages and possible leakage paths. The effective gate channel length and interior width were respectively 1 µm and 346 µm. A high resistivity wafer (10 4 Ω.cm) provided a background concentration of less than 10 12 cm −3of boron corresponding to a mean distance between impurities of 1 µm. A 35 nm gate oxide was grown at 950• C in a dry, chlorine-free oxygen atmosphere. The phosphorous implanted and aluminium sputtered contacts were highly metallic and Ohmic at all temperatures investigated. Sodium ions were introduced onto the oxide surface by immersing the device in a 10 −7 N solution of high purity sodium chloride (99.999 %) in de-ionised water. The surface of the chip was dried with nitrogen gas and an aluminium gate s...

As semiconductor device dimensions are reduced to the nanometer scale, effects of high defect density surfaces on the transport properties become important to the extent that the metallic character that prevails in large and highly doped structures is lost and the use of quantum dots for charge sensing becomes complex. Here we have investigated the mechanism behind the detection of electron motion inside an electrically isolated double quantum dot that is capacitively coupled to a single electron transistor, both fabricated from highly phosphorous doped silicon wafers. Despite, the absence of a direct charge transfer between the detector and the double dot structure, an efficient detection is obtained. In particular, unusually large Coulomb peak shifts in gate voltage are observed. Results are explained in terms of charge rearrangement and the presence of inelastic cotunneling via states at the periphery of the single electron transistor dot.

Charge-based quantum computation can be attained through reliable control of single electrons in lead-less quantum systems. Single-charge transitions in electrically-isolated double quantum dots (DQD) realised in phosphorus-doped silicon can be detected via capacitively coupled single-electron tunnelling devices. By means of time-resolved measurements of the detector's conductance, we investigate the dots' occupancy statistics in temperature. We observe a significant reduction of the effective electron temperature in the DQD as compared to the temperature in the detector's leads. This sets promises to make isolated DQDs suitable platforms for long-coherence quantum computation.Quantum mechanical charge and spin states of electrons confined in semiconductor double quantum dots (DQD) 1-3 have recently attracted much interest, as they can be exploited to implement solid-state quantum computation. One key requirement to perform quantum logic operations is a long coherence time for the qubitembodying states. Among other system materials, silicon is particularly suited to retain spin-coherence for long time mainly due to the existence of a stable isotope ( 28 Si) without nuclear magnetic moment. 4 Another approach to mitigate the decoherence introduced by the interaction with the environment is the suppression of exchange processes with electrons in the reservoirs.5 This has proven to be beneficial if a charge-qubit implementation is to be preferred. Indeed, single-qubit operations have been successfully implemented in trench-isolated silicon double quantum dots, 6 where electrons are confined in a lead-less system and only capacitively coupled to gates.Here, we investigate the electronic occupation of phosphorus-doped isolated DQDs using a single-electron tunnelling device (SET) as a charge sensor. By means of time-resolved charge detection, the probability of occupation of each dot is evaluated as the control gate is swept across a degeneracy point (i.e. alignment of energy levels which leads to electron tunnelling). From these measurements we have extracted the electron temperature in the isolated quantum system and compared it with the one measured in the SET leads. The advantageous effects of a lead-decoupled system are quantitatively observed as a significant temperature reduction in the DQD.The top inset of Fig. 1(a) shows a scanning electron micrograph (SEM) image of a representative device. The use of silicon-on-insulator wafers ensures spatial confinement of electrons in the vertical direction; reactive ion etching is used to define deep trenches which produce confinement within the horizontal plane. The silicon active layer is doped with a concentration of phosphorus atoms of about 3×1019 cm −3 to provide the nanostructure with free carriers. From the density of ima) Electronic mail: ar446@cam.ac.uk planted donors, we estimate that a 40 nm diameter dot contains about 2000 electrons. Both electron-beam and optical lithography are employed to define the device pattern. Full details of the fabrication process,...

Preparing large-scale multi-partite entangled states of quantum bits in each physical form such as photons, atoms or electrons for each specific application area is a fundamental issue in quantum science and technologies. Here, we propose a setup based on Pauli spin blockade (PSB) for the preparation of large-scale W states of electrons in a double quantum dot (DQD). Within the proposed scheme, two W states of n and m electrons respectively can be fused by allowing each W state to transfer a single electron to each quantum dot. The presence or absence of PSB then determines whether the two states have fused or not, leading to the creation of a W state of n + m − 2 electrons in the successful case. Contrary to previous works based on quantum dots or nitrogen-vacancy centers in diamond, our proposal does not require any photon assistance. Therefore the ‘complex’ integration and tuning of an optical cavity is not a necessary prerequisite. We also show how to improve the success rate in our setup. Because requirements are based on currently available technology and well-known sensing techniques, our scheme can directly contribute to the advances in quantum technologies and, in particular in solid state systems.