Electric-field dependent g-factor anisotropy in Ge-Si core-shell nanowire quantum dots

We present angle-dependent measurements of the effective g-factor g* in a Ge-Si core-shell nanowire quantum dot. g* is found to be maximum when the magnetic field is pointing perpendicular to both the nanowire and the electric field induced by local gates. Alignment of the magnetic field with the electric field reduces g* significantly. g* is almost completely quenched when the magnetic field is aligned with the nanowire axis. These findings confirm recent calculations, where the obtained anisotropy is attributed to a Rashba-type spin-orbit interaction induced by heavy-hole light-hole mixing. In principle, this facilitates manipulation of spin-orbit qubits by means of a continuous high-frequency electric field

Highly tuneable hole quantum dots in Ge-Si core-shell nanowires

We define single quantum dots of lengths varying from 60 nm up to nearly half a micron in Ge-Si core-shell nanowires. The charging energies scale inversely with the quantum dot length between 18 and 4 meV. Subsequently, we split up a long dot into a double quantum dot with a separate control over the tunnel couplings and the electrochemical potential of each dot. Both single and double quantum dot configurations prove to be very stable and show excellent control over the electrostatic environment of the dots, making this system a highly versatile platform for spin-based quantum computing

Anisotropic Pauli spin blockade in hole quantum dots

We present measurements on gate-defined double quantum dots in Ge-Si core-shell nanowires, which we tune to a regime with visible shell filling in both dots. We observe a Pauli spin blockade and can assign the measured leakage current at low magnetic fields to spin-flip cotunneling, for which we measure a strong anisotropy related to an anisotropic g-factor. At higher magnetic fields we see signatures for leakage current caused by spin-orbit coupling between (1,1)-singlet and (2,0)-triplet states. Taking into account these anisotropic spin-flip mechanisms, we can choose the magnetic field direction with the longest spin lifetime for improved spin-orbit qubits

RNA secondary structure design

We consider the inverse-folding problem for RNA secondary structures: for a given (pseudo-knot-free) secondary structure find a sequence that has that structure as its ground state. If such a sequence exists, the structure is called designable. We implemented a branch-and-bound algorithm that is able to do an exhaustive search within the sequence space, i.e., gives an exact answer whether such a sequence exists. The bound required by the branch-and-bound algorithm are calculated by a dynamic programming algorithm. We consider different alphabet sizes and an ensemble of random structures, which we want to design. We find that for two letters almost none of these structures are designable. The designability improves for the three-letter case, but still a significant fraction of structures is undesignable. This changes when we look at the natural four-letter case with two pairs of complementary bases: undesignable structures are the exception, although they still exist. Finally, we also study the relation between designability and the algorithmic complexity of the branch-and-bound algorithm. Within the ensemble of structures, a high average degree of undesignability is correlated to a long time to prove that a given structure is (un-)designable. In the four-letter case, where the designability is high everywhere, the algorithmic complexity is highest in the region of naturally occurring RNA.Comment: 11 pages, 10 figure

Single-hole tunneling through a two-dimensional hole gas in intrinsic silicon

In this letter we report single-hole tunneling through a quantum dot in a two-dimensional hole gas, situated in a narrow-channel field-effect transistor in intrinsic silicon. Two layers of aluminum gate electrodes are defined on Si/SiO$_2$ using electron-beam lithography. Fabrication and subsequent electrical characterization of different devices yield reproducible results, such as typical MOSFET turn-on and pinch-off characteristics. Additionally, linear transport measurements at 4 K result in regularly spaced Coulomb oscillations, corresponding to single-hole tunneling through individual Coulomb islands. These Coulomb peaks are visible over a broad range in gate voltage, indicating very stable device operation. Energy spectroscopy measurements show closed Coulomb diamonds with single-hole charging energies of 5--10 meV, and lines of increased conductance as a result of resonant tunneling through additional available hole states.Comment: 4 pages, 4 figures. This article has been submitted to Applied Physics Letter

A fabrication guide for planar silicon quantum dot heterostructures

We describe important considerations to create top-down fabricated planar quantum dots in silicon, often not discussed in detail in literature. The subtle interplay between intrinsic material properties, interfaces and fabrication processes plays a crucial role in the formation of electrostatically defined quantum dots. Processes such as oxidation, physical vapor deposition and atomic-layer deposition must be tailored in order to prevent unwanted side effects such as defects, disorder and dewetting. In two directly related manuscripts written in parallel we use techniques described in this work to create depletion-mode quantum dots in intrinsic silicon, and low-disorder silicon quantum dots defined with palladium gates. While we discuss three different planar gate structures, the general principles also apply to 0D and 1D systems, such as self-assembled islands and nanowires.Comment: Accepted for publication in Nanotechnology. 31 pages, 12 figure

A conceptual model of the groundwater contribution to streamflow during drought in the Afon Fathew catchment, Wales

In 2022 BGS was commissioned by Dŵr Cymru Welsh Water (DCWW) to undertake desk and field investigations to develop a conceptual understanding of the contribution of groundwater to streamflow during drought in the Afon Fathew, Wales. This report details the findings of these investigations. In addition to a desk study, two field visits were completed to survey water features in the catchment, take samples for groundwater residence time indicators, and undertake a passive seismic (Tromino) geophysical survey. The results of the desk study and field visits were combined with flow accretion profile data to develop a conceptual model of groundwater flow to the Afon Fathew during drought, described herein. The Fathew is underlain by a bedrock of silty mudstones which are traditionally considered to be poor aquifers. In the Fathew catchment there is evidence from boreholes for local-scale groundwater flow in the bedrock within fractures and other discontinuities. An upper weathered layer, in combination with faulting and folding patterns, is likely to control the geometry and magnitude of bedrock groundwater flow systems and the location of springs. The residence time indicator data suggest that groundwater in the bedrock is over 40 years old. Estimated discharge from bedrock springs (< 2 l/s, 0.17 Ml/day) is very small relative to the total flow in the Fathew and tributary inflows. The Tromino has shown the superficial deposits in the catchment to be highly heterogeneous in the valley bottom. Changes in the likely permeability and areal extent of the superficial deposits going down the valley bottom correspond to changes in river flows in the Fathew based on the accretion profiles. The Fathew and its tributaries are losing over well drained alluvial gravels, and gaining over low permeability lacustrine and clay-ey alluvial deposits. The Fathew is likely to be hydraulically isolated from the Dysynni catchment. 60% of low flow inflows to the Fathew are coming directly from upland tributary inflows. where very limited superficial deposits are present. In these upland settings during dry periods it is likely that the majority of discharge is coming from baseflow from bedrock. Baseflow support to the Fathew during drought periods can be conceptualised as a two-phase system: (1) Discharge from the superficial deposits to the river, particularly associated with the down-catchment variability in the permeability and thickness of the deposits, (2) Discharge from the weathered bedrock aquifer into the river, from both springs and tributary inflows. The contribution of these two processes is likely to vary as drought conditions develop. Moreover, flows in springs and tributaries may contribute to downstream storage within the superficial deposits, which may complicate the deconvolution of the Fathew river flow hydrograph into different flow components. This temporal sequencing requires further investigation. Further work such as groundwater and surface water monitoring during dry periods and electrical resistivity tomography may be beneficial to constrain these uncertainties