Journal Papers (79)
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51:Title: Weber B; Tan YHM; Mahapatra S; Watson TF; Ryu H; Rahman R; Hollenberg LCL; Klimeck G; Simmons MY, 2014, 'Spin blockade and exchange in Coulomb-confined silicon double quantum dots', Nature Nanotechnology, vol. 9, pp. 430 - 435Year : 2014
Publication Type: Journal Papers
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Abstract
Electron spins confined to phosphorus donors in silicon are promising candidates as qubits1 because of their long coherence times, exceeding seconds in isotopically purified bulk silicon2. With the recent demonstrations of initialization, readout3 and coherent manipulation4 of individual donor electron spins, the next challenge towards the realization of a Si:P donor-based quantum computer is the demonstration of exchange coupling1,5,6 in two tunnel-coupled phosphorus donors. Spin-to-charge conversion3,7 via Pauli spin blockade8,9, an essential ingredient for reading out individual spin states, is challenging in donor-based systems due to the inherently large donor charging energies (∼45 meV), requiring large electric fields (>1 MV m–1) to transfer both electron spins onto the same donor10. Here, in a carefully characterized double donor-dot device, we directly observe spin blockade of the first few electrons and measure the effective exchange interaction between electron spins in coupled Coulomb-confined systems.
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52:Title: Salfi J; Mol JA; Rahman R; Klimeck G; Simmons MY; Hollenberg LCL; Rogge S, 2014, 'Spatially resolving valley quantum interference of a donor in silicon', Nature Materials, vol. 13, pp. 605 - 610Year : 2014
Publication Type: Journal Papers
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Abstract
Electron and nuclear spins of donor ensembles in isotopically pure silicon experience a vacuum-like environment, giving them extraordinary coherence. However, in contrast to a real vacuum, electrons in silicon occupy quantum superpositions of valleys in momentum space. Addressable single-qubit and two-qubit operations in silicon require that qubits are placed near interfaces, modifying the valley degrees of freedom associated with these quantum superpositions and strongly influencing qubit relaxation and exchange processes. Yet to date, spectroscopic measurements have only probed wavefunctions indirectly, preventing direct experimental access to valley population, donor position and environment. Here we directly probe the probability density of single quantum states of individual subsurface donors, in real space and reciprocal space, using scanning tunnelling spectroscopy. We directly observe quantum mechanical valley interference patterns associated with linear superpositions of valleys in the donor ground state. The valley population is found to be within 5% of a bulk donor when 2.85 ± 0.45 nm from the interface, indicating that valley-perturbation-induced enhancement of spin relaxation will be negligible for depths greater than 3 nm. The observed valley interference will render two-qubit exchange gates sensitive to atomic-scale variations in positions of subsurface donors. Moreover, these results will also be of interest for emerging schemes proposing to encode information directly in valley polarization.
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53:Title: Pla JJ; Mohiyaddin FA; Tan KY; Dehollain JP; Rahman R; Klimeck G; Jamieson DN; Dzurak AS; Morello A, 2014, 'Coherent control of a single Si 29 nuclear spin qubit', Physical Review Letters, vol. 113Year : 2014
Publication Type: Journal Papers
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Abstract
Magnetic fluctuations caused by the nuclear spins of a host crystal are often the leading source of decoherence for many types of solid-state spin qubit. In group-IV semiconductor materials, the spin-bearing nuclei are sufficiently rare that it is possible to identify and control individual host nuclear spins. This Letter presents the first experimental detection and manipulation of a single 29 Si nuclear spin. The quantum nondemolition single-shot readout of the spin is demonstrated, and a Hahn echo measurement reveals a coherence time of T 2 = 6.3 ( 7 ) ms —in excellent agreement with bulk experiments. Atomistic modeling combined with extracted experimental parameters provides possible lattice sites for the 29 Si atom under investigation. These results demonstrate that single 29Si nuclear spins could serve as a valuable resource in a silicon spin-based quantum computer.
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54:Title: Hsueh Y-L; Büch H; Tan Y; Wang Y; Hollenberg LCL; Klimeck G; Simmons MY; Rahman R, 2014, 'Spin-Lattice Relaxation Times of Single Donors and Donor Clusters in Silicon', Physical Review Letters, vol. 113Year : 2014
Publication Type: Journal Papers
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Abstract
An atomistic method of calculating the spin-lattice relaxation times (T1) is presented for donors in silicon nanostructures comprising of millions of atoms. The method takes into account the full band structure of silicon including the spin-orbit interaction. The electron-phonon Hamiltonian, and hence, the deformation potential, is directly evaluated from the strain-dependent tight-binding Hamiltonian. The technique is applied to single donors and donor clusters in silicon, and explains the variation of T1 with the number of donors and electrons, as well as donor locations. Without any adjustable parameters, the relaxation rates in a magnetic field for both systems are found to vary as B5, in excellent quantitative agreement with experimental measurements. The results also show that by engineering electronic wave functions in nanostructures, T1 times can be varied by orders of magnitude.
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55:Title: Buch H; Mahapatra S; Rahman R; Morello A; Simmons MY, 2013, 'Spin readout and addressability of phosphorus-donor clusters in silicon', Nature communications, vol. 4, pp. Article number: 2017Year : 2013
Publication Type: Journal Papers
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Abstract
The spin states of an electron bound to a single phosphorus donor in silicon show remarkably long coherence and relaxation times, which makes them promising building blocks for the realization of a solid-state quantum computer. Here we demonstrate, by high-fidelity (93%) electrical spin readout, that a long relaxation time T1 of about 2 s, at B=1.2 T and T≈100 mK, is also characteristic of electronic spin states associated with a cluster of few phosphorus donors, suggesting their suitability as hosts for spin qubits. Owing to the difference in the hyperfine coupling, electronic spin transitions of such clusters can be sufficiently distinct from those of a single phosphorus donor. Our atomistic tight-binding calculations reveal that when neighbouring qubits are hosted by a single phosphorus atom and a cluster of two phosphorus donors, the difference in their electron spin resonance frequencies allows qubit rotations with error rates ≈10−4. These results provide a new approach to achieving individual qubit addressability.
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56:Title: Mohiyaddin FA; Rahman R; Kalra R; Klimeck ; Hollenberg ; Pla JJ; Dzurak AS; Morello A, 2013, 'Noninvasive spatial metrology of single-atom devices', Nano Letters, vol. 13, pp. 1903 - 1909Year : 2013
Publication Type: Journal Papers
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Abstract
The exact location of a single dopant atom in a nanostructure can influence or fully determine the functionality of highly scaled transistors or spin-based devices. We demonstrate here a noninvasive spatial metrology technique, based on the microscopic modeling of three electrical measurements on a single-atom (phosphorus in silicon) spin qubit device: hyperfine coupling, ground state energy, and capacitive coupling to nearby gates. This technique allows us to locate the qubit atom with a precision of ±2.5 nm in two directions and ±15 nm in the third direction, which represents a 1500-fold improvement with respect to the prefabrication statistics obtainable from the ion implantation parameters.
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57:Title: Tettamanzi G, 2013, 'Erratum: Lifetime-Enhanced Transport in Silicon due to Spin and Valley Blockade (vol 107, 136602, 2011)', Physical Review Letters, vol. 110, pp. 49901 - 049901Year : 2013
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58:Title: Nguyen KT; Lilly MP; Nielsen E; Bishop N; Rahman R; Young R; Wendt J; Dominguez J; Pluym T; Stevens J; Lu TM; Muller R; Carroll MS, 2013, 'Charge sensed Pauli blockade in a metal-oxide-semiconductor lateral double quantum dot', Nano Letters, vol. 13, pp. 5785 - 5790Year : 2013
Publication Type: Journal Papers
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Abstract
We report Pauli blockade in a multielectron silicon metal–oxide–semiconductor double quantum dot with an integrated charge sensor. The current is rectified up to a blockade energy of 0.18 ± 0.03 meV. The blockade energy is analogous to singlet–triplet splitting in a two electron double quantum dot. Built-in imbalances of tunnel rates in the MOS DQD obfuscate some edges of the bias triangles. A method to extract the bias triangles is described, and a numeric rate-equation simulation is used to understand the effect of tunneling imbalances and finite temperature on charge stability (honeycomb) diagram, in particular the identification of missing and shifting edges. A bound on relaxation time of the triplet-like state is also obtained from this measurement.
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59:Title: Witzel WM; Rahman R; Carroll MS, 2012, 'Nuclear spin induced decoherence of a quantum dot in Si confined at a SiGe interface: Decoherence dependence on 73Ge', Physical Review B - Condensed Matter and Materials Physics, vol. 85Year : 2012
Publication Type: Journal Papers
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Abstract
We theoretically study the nuclear spin induced decoherence of a quantum dot in Si that is confined at a SiGe interface. We calculate decoherence time dependence on 73 Ge in the barrier layer to evaluate the importance of Ge as well as Si enrichment for long decoherence times. We use atomistic tight-binding modeling for an accurate account of the electron wave function which is particularly important for determining the contact hyperfine interactions with the Ge nuclear spins. We find decoherence times due to Ge spins at natural concentrations to be milliseconds. This suggests that SiGe/Si quantum dot devices employing enriched Si will require enriched Ge as well in order to benefit from long coherence times. We provide a comparison of T 2 times for various fractions of nonzero spin isotopes of Si and Ge.
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60:Title: Neupane MR; Lake RK; Rahman R, 2012, 'Electronic states of Ge/Si nanocrystals with crescent-shaped Ge-cores', Journal of Applied Physics, vol. 112Year : 2012
Publication Type: Journal Papers
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Abstract
Ge/Si nanocrystals can serve as charge storage sites in a nanocrystal memory by providing a hole quantum-well in the Ge region. The electronic states of realistically shaped Ge/Si nanocrystals with crescent-shaped Ge-cores are calculated to determine the hole confinement energies, effective masses, barrier heights, and thermionic lifetimes. As the Ge crescent thickness increases from 1 nm to 3.5 nm, the hole confinement energy decreases from 0.52 to 0.28 eV, the barrier height to escape into the Si valence band increases from 0.25 to 0.51 eV, and the resulting thermionic hole lifetime increases from 10−9 to 10−5 s. The nanocrystals are modeled with an atomistic, 20-band sp3d5s* tight-binding model including spin-orbit coupling as implemented in NEMO3D. Geometry relaxation and strain are included using the valence-force-field model with Keating potentials.
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