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We propose a system of real-space envelope function equations without fitting parameters for modeling the electronic spectrum and wave functions of a phosphorus donor atom embedded in silicon. The approach relies on the Burt-Foreman envelope function representation and leads to coupled effective-mass Schroedinger equations containing smooth effective potentials. These potentials result from the spatial filtering imposed on the exact potential energy matrix elements in the envelope function representation. The corresponding filter function is determined from the definition of the envelope function. The resulting effective potentials and the system of envelope functions jointly reproduce the valley-orbit coupling effect in the doped silicon. Including the valley-orbit coupling not only of the 1s, but also for 2s atomic orbitals, as well as static dielectric screening is found crucial to accurately reproduce experimental data. The measured binding energies are recovered with a maximum relative error of 1.53 %. The computed wave functions are in a good agreement with experimental measurements of the electron density provided by scanning tunneling microscopy.

Using the effective mass theory and the multi-valley envelope function representation, we have developed a theoretical framework for computing the single-electron electronic structure of several phosphorus donors interacting in an arbitrary geometrical configuration in silicon taking into account the valley-orbit coupling. The methodology is applied to three coupled phosphorus donors, arranged in a linear chain and in a triangle, and to six donors arranged in a regular hexagon. The results of the simulations evidence that the valley composition of the single-electron states strongly depends on the geometry of the dopant molecule and its orientation relative to the crystallographic axes of silicon. The electron binding energy of the triatomic linear molecules is larger than that of the diatomic molecule oriented along the same crystallographic axis, but the energy gap between the ground state and the first excited state is not significantly different for internuclear distances from 1.5 to 6.6 nm. Three donor atoms arranged in a triangle geometry have larger binding energies than a triatomic linear chain of dopants with the same internuclear distances. The planar donor molecules are characterized by a strong polarization in favor of the valleys oriented perpendicular to the plane of the molecule. The polarization increases with number of atoms forming the planar molecule.

Donor states in Si nanodevices can be strongly modified by nearby insulating barriers and metallic gates. We report here experimental results indicating a strong reduction in the charging energy of isolated As dopants in Si FinFETs relative to the bulk value. By studying the problem of two electrons bound to a shallow donor within the effective mass approach, we find that the measured small charging energy may be due to a combined effect of the insulator screening and the proximity of metallic gates., Comment: 7 pages, 6 figures

Adiabatic shuttling of single impurity bound electrons to gate-induced surface states in semiconductors has attracted much attention in recent times, mostly in the context of solid-state quantum computer architecture. A recent transport spectroscopy experiment for the first time was able to probe the Stark shifted spectrum of a single donor in silicon buried close to a gate. Here, we present the full theoretical model involving large-scale quantum mechanical simulations that was used to compute the Stark shifted donor states in order to interpret the experimental data. Use of atomistic tight-binding technique on a domain of over a million atoms helped not only to incorporate the full band structure of the host, but also to treat realistic device geometries and donor models, and to use a large enough basis set to capture any number of donor states. The method yields a quantitative description of the symmetry transition that the donor electron undergoes from a three-dimensional Coulomb confined state to a two-dimensional (2D) surface state as the electric field is ramped up adiabatically. In the intermediate field regime, the electron resides in a superposition between the atomic donor states and the 2D surface states. In addition to determining the effect of field and donor depth on the electronic structure, the model also provides a basis to distinguish between a phosphorus and an arsenic donor based on their Stark signature. The method also captures valley-orbit splitting in both the donor well and the interface well, a quantity critical to silicon qubits. The work concludes with a detailed analysis of the effects of screening on the donor spectrum.

We study the electrical transport through single As donors incorporated in the channel of a FinFET, i.e. a donor in a three-terminal geometry. By means of spectroscopic measurements in conjuction with a NEMO-3D model, we can identify the excited states and associate them with either the donors Coulomb potential, a triangular well at the interface or a hybridized combination of the two. The correspondence between the transport measurements, the theoretical model and the local environment provides an atomic understanding of actual gated donors in a nanostructure.

We observe a resonance in the conductance of silicon tunneling devices with a delta-doped barrier. The position of the resonance indicates that it arises from tunneling through the B^+ state of the boron atoms of the delta-layer. Since the emitter Fermi level in our devices is a field-independent reference energy, we are able to directly observe the diamagnetic shift of the B^+ level. This is contrary to the situation in magneto-optical spectroscopy, where the shift is absorbed in the measured ionization energy., Comment: submitted to PRB

We have theoretically studied the effect of an electric field on the energy levels of shallow donors and acceptors in silicon. An analysis of the electric field dependence of the lowest energy states in donors and acceptors is presented, taking the bandstructure into account. A description as hydrogen-like impurities was used for accurate computation of energy levels and lifetimes up to large (several MV/m) electric fields. All results are discussed in connection with atomic scale electronics and solid state quantum computation., Comment: 11 pages, 5 figures

We report the case of a 65-year-old man who developed multiple crusty ulcerative skin lesions on both lower extremities six months after liver transplantation. The causative pathogen was identified as Alternaria Infectoria, an opportunistic fungal agent. The patient was successfully treated with fluconazole for 27 weeks, with complete regression of the lesions. Due to the lack of well-designed clinical studies it is difficult to determine the best treatment course regarding solid organ transplant recipients presenting with invasive fungal infections. And for now, the clinician must lean upon case-reports or retrospective analyses to compose the most suited therapy for his patient. Based upon literature, it seems that the combination of a broad spectrum azole and reducing the dose of immunosuppressive drugs is the cornerstone of treating invasive fungal infections in solid organ transplant patients.

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In this thesis we study quantum transport in several one-dimensional systems with strong electronic interactions. The first chapter contains an introduction to the concepts treated throughout this thesis, such as the Aharonov-Bohm effect, the Kondo effect, the Fano effect and quantum state transfer. It also includes a brief historical introduction to these phenomena. The next three chapters discuss electronic transport in strongly interacting systems with a focus on the transport produced by the Kondo effect. The final chapter deals with spin quantum state transfer, where we analytically address the idea of having a one-dimensional spin chain as a quantum data bus in a quantum computer. In chapter 2 we model a system of coupled donors to explain experimental data of conductance, and use this model to investigate coherence and correlation effects. The two donors are strongly coupled to two leads in a parallel configuration embedded in a nano-wire field effect transistor. We then model the system as an Aharonov-Bohm ring with a strongly interacting quantum dot in each arm, and calculate the conductance in the middle of the Coulomb diamond when the system is in the Kondo regime. In the experimental data, interference was observed when a magnetic field was applied [Fig. 2.3(a)]. This interference shows a dependence on the Aharonov-Bohm phase picked up by electrons traversing the structure. This means that donors can be coupled coherently through a many-body state (the Kondo state). Calculations show the non-monotonic behavior of the conductance that was seen in the experimental data [Fig. 2.4(a)]. The conductance decreases since the Kondo effect is destroyed by the magnetic field, and at the same time an oscillatory behavior appears due to the magnetic phase picked up by the electrons going through the parallel structure. Our results improve the general understanding of possible interference effects in an atomic system, especially in the regime where strong interactions take over. Non-symmetric conductance resonances were observed in the data used in chapter 2 when the transport regime of one of the quantum dots changes from Coulomb blockade to sequential tunneling. We model this situation in chapter 3 and arrive at an analytical expression of conductance which we rewrite as a Fano equation. We demonstrate that the strongly interacting quantum dot creates a Kondo scattering channel which serves as a continuum and interferes with the resonant quantum dot, hence producing a non-symmetric Fano like shape in the conductance. Simulations were done using experimental parameters and good agreement with the data is found [Fig. 3.3]. Furthermore, we predict that even if the interacting channel is fully in the Kondo regime, we can use the magnetic flux to diminish its contribution by lowering the characteristic Kondo temperature (Kondo state broadening), producing an alteration in the electron’s path preference. The next challenge consisted of modeling a strongly-interacting chain of atoms, and study the impact of disorder on the Kondo conductance. In chapter 4 we model the energy levels of the quantum dots to be in the middle of the Coulomb blockade region without disorder. Transport calculations of the atomic chains show that in the weak disorder regime conductance drops with increasing disorder, which is surprising and not expected as the disorder is screened by the pinning of the Kondo state at the Fermi level. We demonstrate that the cause of this decrease is an induced non-screened disorder due to the local distribution of Kondo temperatures along the chain. We also show that weak disorder increases the Kondo temperature of a chain without disorder. We propose two experimental scanning tunneling microscopy setups where the impact of local Kondo temperatures can be observed [Fig. 4.5]. It has been reported that quantum state transfer (QST) can be achieved in a Heisenberg spin chain consisting of three spins. Then it might also be possible to achieve QST in longer spin chains if they can be modeled by an effective three-spin system during the complete quantum state transfer. This idea is formally discussed in chapter 5. We propose simple protocols to achieve quantum state transfer across a spin bus with high accuracy. We propose an effective toy model and apply our findings to a spin chain with a sender and a receiver qubit. We find that within the scope of the effective model the control of only the couplings of the spin bus to the sender and receiver qubits yield high fidelity. We also find an interesting high fidelity region that cannot be described by the effective toy model, and predict the high fidelities to be a consequence of a time-independent first excited state energy. We apply the socalled superadiabatic formalism, which makes the evolution 100% adiabatic, and find fidelities that are equal to one. We derive an approximate Hamiltonian containing parameters that correspond to physical (experimental) knobs, and demonstrate that this Hamiltonian improves the fidelity in both of the treated protocols [Fig. 5.9].

We report electronic transport on n-type silicon Single Electron Transistors (SETs) fabricated in Complementary Metal Oxide Semiconductor (CMOS) technology. The n-MOSSETs are built within a pre-industrial Fully Depleted Silicon On Insulator (FDSOI) technology with a silicon thickness down to 10 nm on 200 mm wafers. The nominal channel size of 20 $\times$ 20 nm$^{2}$ is obtained by employing electron beam lithography for active and gate levels patterning. The Coulomb blockade stability diagram is precisely resolved at 4.2 K and it exhibits large addition energies of tens of meV. The confinement of the electrons in the quantum dot has been modeled by using a Current Spin Density Functional Theory (CS-DFT) method. CMOS technology enables massive production of SETs for ultimate nanoelectronics and quantum variables based devices., Comment: 4 Figures