arXiv subjects

Alexander R. Hamilton

Publications and source records attributed to Alexander R. Hamilton.

At least 19 recordsLinked to original sources

arXiv2026

Harnessing Josephson-Shapiro physics to verify interlayer exciton superfluidity

Obtaining definitive evidence for zero-magnetic-field exciton superfluidity in electron-hole bilayers remains a longstanding challenge because the condensate is electrically neutral and its phase coherence is difficult to probe directly. We propose a direct test based on Shapiro steps in a Dayem-bridge excitonic Josephson junction. We predict clearly resolvable Shapiro plateaus in experimentally accessible current and voltage regimes for double-bilayer graphene and, in the low-density regime, double-layer transition-metal dichalcogenides. Moreover, by tuning the density across the BCS-BEC crossover we show that the Shapiro response acquires a distinct nonmonotonic evolution. This is determined by the nonmonotonic behavior of the healing length in the crossover from bosonic to fermionic excitations. Observation of these signatures would provide direct evidence of exciton superfluidity and establish exciton bilayers as a platform for neutral Josephson devices.

cond-mat.supr-con↗

arXiv2026

Electrical driving of hole spin states in planar silicon MOS device by g-matrix modulation

Hole spins in group IV quantum dots are a highly promising way to develop CMOS compatible spin qubits owing to their inherent spin-orbit coupling, which enables fast, coherent, and electrical spin control. However, spin-orbit coupling not only enables multiple spin-control mechanisms, but also exposes the qubits to charge noise. In this work, we perform a systematic study of the spin control mechanism in a planar silicon hole quantum dot. We use g-matrix formalism to discern contributions from the various spin driving mechanisms and identify regions where spins are less sensitive to charge noise. By mapping out the dependence of the Rabi frequency on the magnetic field orientation, we observe the largest Rabi frequency in the in-plane direction and the smallest Rabi frequency close to the out-of-plane direction. These results enhance the understanding of how different mechanisms contribute to spin driving within an industrially relevant architecture and aid in establishing the operating conditions for the rapid and coherent manipulation of hole qubits.

cond-mat.mes-hall↗

arXiv2025

Precision high-speed quantum logic with holes on a natural silicon foundry platform

Silicon spin qubits in gate-defined quantum dots leverage established semiconductor infrastructure and offer a scalable path toward transformative quantum technologies. Holes spins in silicon offer compact all-electrical control, whilst retaining all the salient features of a quantum dot qubit architecture. However, silicon hole spin qubits are not as advanced as electrons, due to increased susceptibility to disorder and more complex spin physics. Here we demonstrate single-qubit gate fidelities up to 99.8% and a two-qubit gate quality factor of 240, indicating a physical fidelity limit of 99.7%. These results represent the highest performance reported in natural silicon to date, made possible by fast qubit control, exchange pulsing, and industrial-grade fabrication. Notably, we achieve these results in a near-identical device as used for highly reproducible, high-fidelity electron spin qubits. With isotopic purification and device-level optimisations in the future, our hole spin qubits are poised to unlock a new operation regime for quantum CMOS architectures.

cond-mat.mes-hall↗

arXiv2025

A Gross-Pitaevskii theory for an excitonic incompressible Bose solid

We show that interlayer excitons in double-layer semiconductor heterostructures can form a Bose solid, which is an incompressible supersolid characterized by exactly one boson per lattice site. This exciton Bose solid would be the first realization of an incompressible supersolid, unlike the generally compressible cluster supersolids seen in dipolar quantum gases. Capturing its characteristics and associated emergent phenomena requires extending the Gross-Pitaevskii formalism to include strong two-particle correlations and exclude exciton self-interactions. We develop such a formalism, we apply it across experimentally accessible exciton densities and interlayer separations, and we show that it incorporates both superfluid and incompressible supersolid ground states. This extended framework allows us to determine the superfluid-supersolid transition and explore the low-temperature properties of the exciton supersolid across its complete parameter space.

cond-mat.quant-gas↗

arXiv2024

Transverse magnetic focusing in two-dimensional hole gases

Two-dimensional hole gases (2DHGs) have strong intrinsic spin-orbit coupling and could be used to build spin filters by utilising transverse magnetic focusing (TMF). However, with an increase in the spin degree of freedom, holes demonstrate significantly different behaviour to electrons in TMF experiments, making it difficult to interpret the results of these experiments. In this paper, we numerically model TMF in a 2DHG within a GaAs/Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As heterostructure. Our band structure calculations show that the heavy $(\langle J_{z} \rangle = \pm\frac{3}{2})$ and light $(\langle J_{z} \rangle = \pm\frac{1}{2})$ hole states in the valence band mix at finite $k$, and the heavy hole subbands which are spin-split due to the Rashba effect are not spin-polarised. This lack of spin polarisation casts doubt on the viability of spin filtering using TMF in 2DHGs within conventional GaAs/Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As heterostructures. We then calculate transport properties of the 2DHG with spin projection and offer a new perspective on interpreting and designing TMF experiments in 2DHGs.

cond-mat.mes-hall↗

arXiv2024

Probing g-tensor reproducibility and spin-orbit effects in planar silicon hole quantum dots

In this work, we probe the sensitivity of hole-spin properties to hole occupation number in a planar silicon double-quantum dot device fabricated on a 300 mm integrated platform. Using DC transport measurements, we investigate the g-tensor and spin-relaxation induced leakage current within the Pauli spin-blockade regime as a function of magnetic-field orientation at three different hole occupation numbers. We find the g-tensor and spin-leakage current to be highly anisotropic due to light-hole/heavy-hole mixing and spin-orbit mixing, but discover the anisotropies to be relatively insensitive to the dot hole number. Furthermore, we extract the dominant inter-dot spin-orbit coupling mechanism as surface Dresselhaus, with an in-plane orientation parallel to transport and magnitude $\boldsymbol{t_{SO}}$ $\approx$ 300 neV. Finally, we observe a strong correlation between the g-factor difference ($\delta$$\boldsymbol{g}$) between each dot and the spin-leakage current anisotropy, as a result of $\delta$$\boldsymbol{g}$ providing an additional spin-relaxation pathway, and should be considered. ]Our findings indicate that hole-spin devices are not as sensitive to precise operating conditions as anticipated. This has important implications optimizing spin control and readout based on magnetic-field direction, together with tuning large arrays of QDs as spin-qubits.

cond-mat.mes-hall↗

arXiv2024

Formation of artificial Fermi surfaces with a triangular superlattice on a conventional two dimensional electron gas

In nearly free electron theory the imposition of a periodic electrostatic potential on free electrons creates the bandstructure of a material, determined by the crystal lattice spacing and geometry. Imposing an artificially designed potential to the electrons confined in a GaAs quantum well makes it possible to engineer synthetic two-dimensional band structures, with electronic properties different from those in the host semiconductor. Here we report the fabrication and study of a tuneable triangular artificial lattice on a GaAs/AlGaAs heterostructure where it is possible to transform from the original GaAs bandstructure and Fermi surface to a new bandstructure with multiple artificial Fermi surfaces simply by altering a gate bias. For weak electrostatic potential modulation magnetotransport measurements reveal quantum oscillations from the GaAs two-dimensional Fermi surface, and classical oscillations due to these electrons scattering from the artificial lattice. Increasing the strength of the modulation reveals new quantum oscillations due to the formation of multiple artificial Fermi surfaces, and ultimately to new classical oscillations of the electrons from the artificial Fermi surface scattering from the superlattice modulation. These results show that low disorder gate-tuneable lateral superlattices can be used to form artificial two dimensional crystals with designer electronic properties.

cond-mat.mes-hall↗

arXiv2024

Artificial electrostatic crystals: a new platform for creating correlated quantum states

The electronic properties of solids are determined by the crystal structure and interactions between electrons, giving rise to a variety of collective phenomena including superconductivity, strange metals and correlated insulators. The mechanisms underpinning many of these collective phenomena remain unknown, driving interest in creating artificial crystals which replicate the system of interest while allowing precise control of key parameters. Here we demonstrate the formation of highly tunable artificial crystals by superimposing a periodic electrostatic potential on the 2D electron gas in an ultra-shallow (25 nm deep) GaAs quantum well. The 100 nm period artificial crystal is identified by the formation of a new bandstructure, different from the original cubic crystal and specific to the artificial triangular lattice: transport measurements show the Hall coefficient changing sign as the chemical potential sweeps through the artificial bands. Uniquely, the artificial bandstructure can be continuously tuned to form linear graphene-like and flat kagome-like bands in a single device. A strong insulating state is observed at half filling of the kagome flat band, which is not expected in the absence of strong interactions. This state, unique to the kagome lattice, is consistent with a loop-current Wigner insulator, which arises from long-range Coulomb interaction and delocalised electrons between neighbouring empty sites. The ability to continuously tune the bandstructure and access flat bands through electrical gating within a single device opens a new route to studying collective quantum states.

cond-mat.mes-hall↗

arXiv2022

Combining n-MOS Charge Sensing with p-MOS Silicon Hole Double Quantum Dots in a CMOS platform

Holes in silicon quantum dots are receiving significant attention due to their potential as fast, tunable, and scalable qubits in semiconductor quantum circuits. Despite this, challenges remain in this material system including difficulties using charge sensing to determine the number of holes in a quantum dot, and in controlling the coupling between adjacent quantum dots. In this work, we address these problems by fabricating an ambipolar complementary metal-oxide-semiconductor (CMOS) device using multilayer palladium gates. The device consists of an electron charge sensor adjacent to a hole double quantum dot. We demonstrate control of the spin state via electric dipole spin resonance (EDSR). We achieve smooth control of the inter-dot coupling rate over two orders of magnitude and use the charge sensor to perform spin-to-charge conversion to measure the hole singlet-triplet relaxation time of 11 μs for a known hole occupation. These results provide a path towards improving the quality and controllability of hole spin-qubits.

cond-mat.mes-hall↗

arXiv2022

P-type Ohmic contact to monolayer WSe2 field-effect transistors using high electron affinity amorphous MoO3

1 School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia 2 Australian Research Council Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), Monash University, Clayton, Victoria 3800, Australia 3 Department of Mechanical Engineering, Columbia University, New York, New York 10027, United States 4 Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States 5 School of Physics, the University of Melbourne, Melbourne, VIC 3010, Australia 6 Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan 7 International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan 8 School of Physics, University of New South Wales, 2052 Sydney, Australia 9 Center of Condensed Matter Sciences and Center of Atomic Initiative for New Material, National Taiwan University, Taipei 106, Taiwan 10 Monash Centre for Atomically Thin Materials, Monash University, Clayton, 3800, VIC, Australia

cond-mat.mes-hall↗

arXiv2022

Increased Phase Coherence Length in a Porous Topological Insulator

The surface area of Bi2Te3 thin films was increased by introducing nanoscale porosity. Temperature dependent resistivity and magnetotransport measurements were conducted both on as-grown and porous samples (23 and 70 nm). The longitudinal resistivity of the porous samples became more metallic, indicating the increased surface area resulted in transport that was more surface-like. Weak antilocalization (WAL) was present in all samples, and remarkably the phase coherence length doubled in the porous samples. This increase is likely due to the large Fermi velocity of the Dirac surface states. Our results show that the introduction of nanoporosity does not destroy the topological surface states but rather enhances them, making these nanostructured materials promising for low energy electronics, spintronics and thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2022

Chester supersolid of spatially indirect excitons in double-layer semiconductor heterostructures

A supersolid, a counter-intuitive quantum state in which a rigid lattice of particles flows without resistance, has to date not been unambiguously realised. Here we reveal a supersolid ground state of excitons in a double-layer semiconductor heterostructure over a wide range of layer separations outside the focus of recent experiments. This supersolid conforms to the original Chester supersolid with one exciton per supersolid site, as distinct from the alternative version reported in cold-atom systems of a periodic modulation of the superfluid density. We provide the phase diagram augmented by the supersolid. This new phase appears at layer separations much smaller than the predicted exciton normal solid, and it persists up to a solid--solid transition where the quantum phase coherence collapses. The ranges of layer separations and exciton densities in our phase diagram are well within reach of the current experimental capabilities.

cond-mat.mes-hall↗

arXiv2021

Geometric control of universal hydrodynamic flow in a two dimensional electron fluid

Fluid dynamics is one of the cornerstones of modern physics and has recently found applications in the transport of electrons in solids. In most solids electron transport is dominated by extrinsic factors, such as sample geometry and scattering from impurities. However in the hydrodynamic regime Coulomb interactions transform the electron motion from independent particles to the collective motion of a viscous `electron fluid'. The fluid viscosity is an intrinsic property of the electron system, determined solely by the electron-electron interactions. Resolving the universal intrinsic viscosity is challenging, as it only affects the resistance through interactions with the sample boundaries, whose roughness is not only unknown but also varies from device to device. Here we eliminate all unknown parameters by fabricating samples with smooth sidewalls to achieve the perfect slip boundary condition, which has been elusive both in molecular fluids and electronic systems. We engineer the device geometry to create viscous dissipation and reveal the true intrinsic hydrodynamic properties of a 2D system. We observe a clear transition from ballistic to hydrodynamic electron motion, driven by both temperature and magnetic field. We directly measure the viscosity and electron-electron scattering lifetime (the Fermi quasiparticle lifetime) over a wide temperature range without fitting parameters, and show they have a strong dependence on electron density that cannot be explained by conventional theories based on the Random Phase Approximation.

cond-mat.mes-hall↗

arXiv2020

Electron-hole superfluidity in strained Si/Ge type II heterojunctions

Excitons are promising candidates for generating superfluidity and Bose-Einstein Condensation (BEC) in solid state devices, but an enabling material platform with in-built bandstructure advantages and scaling compatibility with industrial semiconductor technology is lacking. Here we predict that spatially indirect excitons in a lattice-matched strained Si/Ge bilayer embedded into a germanium-rich SiGe crystal, would lead to observable mass-imbalanced electron-hole superfluidity and BEC. Holes would be confined in a compressively strained Ge quantum well and electrons in a lattice-matched tensile strained Si quantum well. We envision a device architecture that does not require an insulating barrier at the Si/Ge interface, since this interface offers a type II band alignment. Thus the electrons and holes can be kept very close but strictly separate, strengthening the electron-hole pairing attraction while preventing fast electron-hole recombination. The band alignment also allows a one-step procedure for making independent contacts to the electron and hole layers, overcoming a significant obstacle to device fabrication. We predict superfluidity at experimentally accessible temperatures of a few Kelvin and carrier densities up to $\sim 6\times 10^{10}$ cm$^{-2}$, while the large imbalance of the electron and hole effective masses can lead to exotic superfluid phases.

cond-mat.mes-hall↗

arXiv2020

Generating a topological anomalous Hall effect in a non-magnetic conductor

The ordinary Hall effect is driven by the Lorentz force, while its anomalous counterpart occurs in ferromagnets. Here we show that the Berry curvature monopole of non-magnetic 2D spin-3/2 holes leads to a novel Hall effect linear in an applied in-plane magnetic field B_x. There is no Lorentz force hence no ordinary Hall effect, while all disorder contributions vanish to leading order in B_x. This intrinsic phenomenon, which we term the anomalous planar Hall effect (APHE), provides a non-quantized footprint of topological transport directly accessible in p-type semiconductors.

cond-mat.mes-hall↗

arXiv2020

Experimental conditions for observation of electron-hole superfluidity in GaAs heterostructures

The experimental parameter ranges needed to generate superfluidity in optical and drag experiments in GaAs double quantum wells are determined, using a formalism that includes self-consistent screening of the Coulomb pairing interaction in the presence of the superfluid. The very different electron and hole masses in GaAs make this a particularly interesting system for superfluidity, with exotic superfluid phases predicted in the BCS-BEC crossover regime. We find that the density and temperature ranges for superfluidity cover the range for which optical experiments have observed indications of superfluidity, but that existing drag experiments lie outside the superfluid range. However we also show that for samples with low mobility with no macroscopically connected superfluidity, if the superfluidity survived in randomly distributed localized pockets, standard quantum capacitance measurements could detect these pockets.

cond-mat.supr-con↗

arXiv2017

Zero-Energy Modes from Coalescing Andreev States in a Two-Dimensional Semiconductor-Superconductor Hybrid Platform

We investigate zero-bias conductance peaks that arise from coalescing subgap Andreev states, consistent with emerging Majorana zero modes, in hybrid semiconductor-superconductor wires defined in a two-dimensional InAs/Al heterostructure using top-down lithography and gating. The measurements indicate a hard superconducting gap, ballistic tunneling contact, and in-plane critical fields up to $3$~T. Top-down lithography allows complex geometries, branched structures, and straightforward scaling to multicomponent devices compared to structures made from assembled nanowires.

cond-mat.mes-hall↗

arXiv2017

Strong influence of spin-orbit coupling on magnetotransport in two-dimensional hole systems

With a view to electrical spin manipulation and quantum computing applications, recent significant attention has been devoted to semiconductor hole systems, which have very strong spin-orbit interactions. However, experimentally measuring, identifying, and quantifying spin-orbit coupling effects in transport, such as electrically-induced spin polarizations and spin-Hall currents, are challenging. Here we show that the magnetotransport properties of two dimensional (2D) hole systems display strong signatures of the spin-orbit interaction. Specifically, the low-magnetic field Hall coefficient and longitudinal conductivity contain a contribution that is second order in the spin-orbit interaction coefficient and is non-linear in the carrier number density. We propose an appropriate experimental setup to probe these spin-orbit dependent magnetotransport properties, which will permit one to extract the spin-orbit coefficient directly from the magnetotransport.

cond-mat.mes-hall↗