arXiv subjects

J. A. Mol

Publications and source records attributed to J. A. Mol.

12 recordsLinked to original sources

arXiv2025

Landau Level Single-Electron Pumping

We present the first detailed study of the effect of a strong magnetic field on single-electron pumping in a device utilising a finger-gate split-gate configuration. In the quantum Hall regime, we demonstrate electron pumping from Landau levels in the leads, where the measurements exhibit pronounced oscillations in the lengths of the pumping plateaus with the magnetic field, reminiscent of Shubnikov-de Haas oscillations. This similarity indicates that the pumping process is dependent on the density of states of the 2D electron gas over a narrow energy window. Based on these observations, we develop a new theoretical description of the operation of single-electron pumps which for the first time allows for the determination of the physical parameters of the experiment; such as the capture energy of the electrons, the broadening of the quantised Landau levels in the leads, and the quantum lifetime of the electrons.

cond-mat.mes-hall↗

arXiv2022

Non-equilibrium thermodynamics in a single-molecule quantum system

Thermodynamic probes can be used to deduce microscopic internal dynamics of nanoscale quantum systems. Several direct entropy measurement protocols based on charge transport measurements have been proposed and experimentally applied to single-electron devices. To date, these methods have relied on (quasi-)equilibrium conditions between the nanoscale quantum system and its environment, which constitutes only a small subset of the experimental conditions available. In this paper, we establish a thermodynamic analysis method based on stochastic thermodynamics, that is valid far from equilibrium conditions, is applicable to a broad range of single-electron devices and allows us to find the difference in entropy between the charge states of the nanodevice, as well as a characteristic of any selection rules governing electron transfers. We apply this non-equilibrium entropy measurement protocol to a single-molecule device in which the internal dynamics can be described by a two-site Hubbard model.

cond-mat.mes-hall↗

arXiv2021

Electric-field-driven conductance switching in encapsulated graphene nanogaps

Feedback-controlled electric breakdown of graphene in air or vacuum is a well-established way of fabricating tunnel junctions, nanogaps, and quantum dots. We show that the method is equally applicable to encapsulated graphene constrictions fabricated using hydrogen silsesquioxane. The silica-like layer left by hydrogen silsesquioxane resist after electron-beam exposure remains intact after electric breakdown of the graphene. We explore the conductance switching behavior that is common in graphene nanostructures fabricated via feedback-controlled breakdown, and show that it can be attributed to atomic-scale fluctuations of graphene below the encapsulating layer. Our findings open up new ways of fabricating encapsulated room-temperature single-electron nanodevices and shed light on the underlying physical mechanism of conductance switching in these graphene nanodevices.

cond-mat.mes-hall↗

arXiv2021

A thermodynamic approach to measuring entropy in a few-electron nanodevice

The entropy of a system gives a powerful insight into its microscopic degrees of freedom, however standard experimental ways of measuring entropy through heat capacity are hard to apply in mesoscale and nanoscale systems, as they require the measurement of increasingly small amounts of heat. This problem calls for radically different measurement methods that do not suffer from decreasing accuracy with the decreasing size of the system. For nanoelectric devices in the state of Coulomb blockade, with only two energetically accessible charge states, two purely electric, size-independent methods of measuring the entropy difference between the charge states have been proposed: through transport properties and charge balance measurements. We suggest a self-consistent thermodynamic framework for the treatment of entropy in Coulomb-blocked electric nanodevices which incorporates both existing entropy measurement methods, generalises them, and expands to systems with arbitrarily complex microstates corresponding to each charge state.

cond-mat.mes-hall↗

arXiv2017

Charge-insensitive single-atom spin-orbit qubit in silicon

High fidelity entanglement of an on-chip array of spin qubits poses many challenges. Spin-orbit coupling (SOC) can ease some of these challenges by enabling long-ranged entanglement via electric dipole-dipole interactions, microwave photons, or phonons. However, SOC exposes conventional spin qubits to decoherence from electrical noise. Here we propose an acceptor-based spin-orbit qubit in silicon offering long-range entanglement at a sweet spot where the qubit is protected from electrical noise. The qubit relies on quadrupolar SOC with the interface and gate potentials. As required for surface codes, $10^5$ electrically mediated single-qubit and $10^4$ dipole-dipole mediated two-qubit gates are possible in the predicted spin lifetime. Moreover, circuit quantum electrodynamics with single spins is feasible, including dispersive readout, cavity-mediated entanglement, and spin-photon entanglement. An industrially relevant silicon-based platform is employed.

cond-mat.mes-hall↗

arXiv2016

Quantum Simulation of the Hubbard Model with Dopant Atoms in Silicon

In quantum simulation, many-body phenomena are probed in controllable quantum systems. Recently, simulation of Bose-Hubbard Hamiltonians using cold atoms revealed previously hidden local correlations. However, fermionic many-body Hubbard phenomena such as unconventional superconductivity and spin liquids are more difficult to simulate using cold atoms. To date the required single-site measurements and cooling remain problematic, while only ensemble measurements have been achieved. Here we simulate a two-site Hubbard Hamiltonian at low effective temperatures with single-site resolution using subsurface dopants in silicon. We measure quasiparticle tunneling maps of spin-resolved states with atomic resolution, finding interference processes from which the entanglement entropy and Hubbard interactions are quantified. Entanglement, determined by spin and orbital degrees of freedom, increases with increasing covalent bond length. We find separation-tunable Hubbard interaction strengths that are suitable for simulating strongly correlated phenomena in larger arrays of dopants, establishing dopants as a platform for quantum simulation of the Hubbard model.

cond-mat.mes-hall↗

arXiv2015

Spatially Resolving Valley Quantum Interference of a Donor in Silicon

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 only indirectly probe wavefunctions, 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 tunneling 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 $2.85\pm0.45$ nm from the interface, indicating that valley perturbation-induced enhancement of spin relaxation will be negligible for depths $>3$ 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 to emerging schemes proposing to encode information directly in valley polarization.

cond-mat.mes-hall↗

arXiv2015

Interface-induced heavy-hole/light-hole splitting of acceptors in silicon

The energy spectrum of spin-orbit coupled states of individual sub-surface boron acceptor dopants in silicon have been investigated using scanning tunneling spectroscopy (STS) at cryogenic temperatures. The spatially resolved tunnel spectra show two resonances which we ascribe to the heavy- and light-hole Kramers doublets. This type of broken degeneracy has recently been argued to be advantageous for the lifetime of acceptor-based qubits [Phys. Rev. B 88 064308 (2013)]. The depth dependent energy splitting between the heavy- and light-hole Kramers doublets is consistent with tight binding calculations, and is in excess of 1 meV for all acceptors within the experimentally accessible depth range (< 2 nm from the surface). These results will aid the development of tunable acceptor-based qubits in silicon with long coherence times and the possibility for electrical manipulation.

cond-mat.mes-hall↗

arXiv2013

Interplay between quantum confinement and dielectric mismatch for ultra-shallow dopants

Understanding the electronic properties of dopants near an interface is a critical challenge for nano-scale devices. We have determined the effect of dielectric mismatch and quantum confinement on the ionization energy of individual acceptors beneath a hydrogen passivated silicon (100) surface. Whilst dielectric mismatch between the vacuum and the silicon at the interface results in an image charge which enhances the binding energy of sub-surface acceptors, quantum confinement is shown to reduce the binding energy. Using scanning tunneling spectroscopy we measure resonant transport through the localized states of individual acceptors. Thermal broadening of the conductance peaks provides a direct measure for the absolute energy scale. Our data unambiguously demonstrates that these two independent effects compete with the result that the ionization energy is less than 5 meV lower than the bulk value for acceptors less than a Bohr radius from the interface.

cond-mat.mes-hall↗

arXiv2013

Comparative study of tight-binding and ab initio electronic structure calculations focused on magnetic anisotropy in ordered CoPt alloy

An empirical multiorbital (spd) tight binding (TB) model including magnetism and spin-orbit coupling is applied to calculations of magnetic anisotropy energy (MAE) in CoPt L1_0 structure. A realistic Slater-Koster parametrisation for single-element transition metals is adapted for the ordered binary alloy. Spin magnetic moment and density of states are calculated using a full-potential linearized augmented plane-wave (LAPW) ab initio method and our TB code with different variants of the interatomic parameters. Detailed mutual comparison of this data allows for determination of a subset of the compound TB parameters tuning of which improves the agreement of the TB and LAPW results. MAE calculated as a function of band filling using the refined parameters is in broad agreement with ab initio data for all valence states and in quantitative agreement with ab initio and experimental data for the natural band filling. Our work provides a practical basis for further studies of relativistic magnetotransport anisotropies by means of local Green's function formalism which is directly compatible with our TB approach.

cond-mat.mtrl-sci↗

arXiv2011

Balanced ternary addition using a gated silicon nanowire

We demonstrate the proof of principle for a ternary adder using silicon metal-on-insulator single electron transistors (SET). Gate dependent rectifying behavior of a single electron transistor results in a robust three-valued output as a function of the potential of the SET island. Mapping logical, ternary inputs to the three gates controlling the potential of the SET island allows us to perform complex, inherently ternary operations, on a single transistor.

cond-mat.mes-hall↗

arXiv2010

Drain current modulation in a nanoscale field-effect-transistor channel by single dopant implantation

We demonstrate single dopant implantation into the channel of a silicon nanoscale metal-oxide-semiconductor field-effect-transistor. This is achieved by monitoring the drain current modulation during ion irradiation. Deterministic doping is crucial for overcoming dopant number variability in present nanoscale devices and for exploiting single atom degrees of freedom. The two main ion stopping processes that induce drain current modulation are examined. We employ 500~keV He ions, in which electronic stopping is dominant, leading to discrete increases in drain current and 14~keV P dopants for which nuclear stopping is dominant leading to discrete decreases in drain current.

cond-mat.mtrl-sci↗