David. A. Ritchie

Sensitive terahertz (THz) detectors are essential for all THz applications. The recently unveiled in-plane photoelectric effect is a novel, quantum mechanism enabling terahertz detection with high responsivities in 2D electron gases (2DEGs). Here, we investigate the influence of transverse confinement in 2DEGs on the in-plane photoelectric response by measuring AlGaAs/GaAs-based photoelectric tunable-step THz detectors with symmetric antennas but different shapes of the underlying 2DEG. Despite fully symmetric antennas, the geometric channel asymmetry notably shifts the gate voltage dependence of the photoresponse. Our findings underscore the highly localized nature of the in-plane photoelectric effect as a quantum photodetection mechanism. The results show how tailored shapes of 2DEG channels influence the quantum-mechanical photocurrent arising along the electron channel direction by modifying the number of subbands corresponding to electron motion perpendicular to the channel. Our work shows that quantum effects influence photodetection phenomena not only along the direction of electron flow but also in the transverse, perpendicular direction—even in degenerate 2DEGs and on length scales of several microns. This provides a path to structural engineering of quantum light–matter interactions in 2DEGs.

In this work, we demonstrate the fabrication of a device to investigate electron–phonon coupling in quantum systems on a suspended island, together with a systematic study of submicrometer AuGeNi ohmic contacts required for its implementation. The island is separated from the substrate but electrically connected to the rest of the surrounding structure with four micrometer- or submicrometer-sized ohmic contacts and metal leads. The contacts are characterized on planar test structures and subsequently integrated into suspended devices. Square contacts with areas ranging from 1600 down to 0.0056 μm2 are fabricated in both surface-deposited and recessed geometries contacting a 20 nm GaAs quantum well, 50 nm below the surface, in a 120 nm thick GaAs/AlGaAs heterostructure. Two-point DC current–voltage (I–V) measurements at 77 and 3.8 K show that double annealing the contacts reliably produces ohmic behavior across all footprint sizes. Analysis of the area dependence of resistance reveals a critical area of (0.80 ± 0.01) μm2, corresponding to a transfer length (0.894 ± 0.013) μm and specific contact resistivities in the range of (0.5–2.3) × 10−6 Ω cm2, demonstrating that robust ohmic performance can be maintained at deep submicrometer dimensions. We further examine electrochemical etching effects in GaAs/AlGaAs and develop a fabrication pathway that avoids electrochemical damage, enabling the realization of a fully suspended Hall cross isolated from the phonon bath. This provides a platform for future studies of electron–phonon interactions and many-body-localization effects in semiconductor heterostructures.

Spontaneous spin polarization in quasi-one-dimensional (1D) electron systems is inherently fragile, often masked by structural disorder or potential anharmonicity. Here we report the deterministic stabilization of an exchange-driven spin gap, manifesting as a robust 0.5 ( 2 e 2 / h ) conductance plateau at zero magnetic field in a mesoscopic bilayer cavity. We demonstrate that this many-body phase is governed by a dual-parity protection mechanism. The spectral isolation of the tunnel-coupled bilayer energetically protects the ground state, while the symmetric harmonicity of the lateral cavity geometry shields the 1D van Hove singularity against transverse mode mixing. Breaking this spatial parity via asymmetric gating or localized central-gate screening violates the Stoner criterion for 1D ferromagnetism, quenching the 0.5 ( 2 e 2 / h ) state and restoring spin-degenerate transport. Our findings establish electrostatic spatial parity as a macroscopic, tunable lever for engineering correlated quantum phases in mesoscopic architectures.

The electronic properties of solids are determined by their crystal structure and electron interactions, giving rise to phenomena such as superconductivity, strange metals and correlated insulators. Many of these effects remain poorly understood, motivating efforts to create artificial crystals that mimic real materials while allowing controlled tuning of key parameters. Cold atoms in optical lattices offer flexibility but cannot reproduce the long-range Coulomb interactions and hopping present in solids. Solid-state systems naturally support these features, although they suffer from tunability and flexibility issues. Here we demonstrate a highly tunable artificial crystal formed by superimposing a periodic electrostatic potential onto a two-dimensional electron gas in a shallow GaAs quantum well. This engineered lattice exhibits a band structure characteristic of the artificial triangular lattice, distinct from that of the underlying cubic crystal. Electronic transport measurements show a sign change in the Hall coefficient as the chemical potential sweeps through the artificial bands. The band structure can be continuously tuned to realize linear graphene-like and flat kagome-like bands within a single device. A strong insulating state emerges at half filling of the kagome flat band, consistent with interaction-driven behaviour. This tunability provides an opportunity to explore correlated quantum states in a controlled setting.

We present measurements of the cooling length ℓE for hot electrons in a GaAs-based high-mobility two-dimensional electron gas. The thermal measurements are performed on a long 60 μm-wide channel, which is Joule-heated at one end, along which there are three similar hot-electron thermocouples, spaced 30 μm apart. The thermocouples measure an exponentially decaying temperature profile with a characteristic length ℓE, which decreases from 23 to 16 μm as the lattice temperature increases from 1.8 to 5 K. From a simple one-dimensional model of heat diffusion, we measure an inelastic scattering time, which decreases from τi≈360 to 180 ps. The measured τi has a magnitude and temperature dependence consistent with acoustic phonon scattering times. We discuss how the sample design can be varied for further thermal investigations. Knowledge of the temperature profile and its gradient will prove useful in measurements of the thermal conductivity and the Nernst effect.

Historically, in measurements of electron transport in disordered two-dimensional systems, an Arrhenius Hall carrier density has never been observed alongside an Arrhenius conductivity, when the Fermi level is below a mobility edge. This has long been an issue with respect to claiming observation of transport via activation to a mobility edge. In this work, an Arrhenius conductivity and Arrhenius Hall carrier density have been observed alongside one another in such a system. Measurements were made of a two-dimensional electron gas hosted in a gated GaAs/Al0.33Ga0.67As heterostructure. Furthermore, in the regime of Arrhenius conductivity and Arrhenius carrier density, the mobility is shown to be independent of the position of the Fermi level below the mobility edge. A transition between carrier density and mobility dominating the resistivity temperature dependence has been observed, as the Fermi level is varied.

We present a detailed study of the effect of a strong perpendicular magnetic field on single-electron pumping in a device utilising a finger-gate split-gate configuration, characterised by high pumping precision. In the quantum Hall regime, we demonstrate electron pumping from Landau levels in the leads, where the measurements exhibit pronounced fluctuations in the lengths of the pumping plateaus with magnetic field, reminiscent of Shubnikov–de Haas oscillations. In order to explain this similarity, we introduce a new physical model for the dynamics of the operation of a single-electron pump. We show that the pumping process is dependent on the density of states of the 2D electron gas in the leads within a single narrow energy window. Applying our model to experimental data allows for the determination of physical parameters of the pump and its operation; 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.

Effective control of terahertz radiation requires fast and efficient modulators with a large modulation depth—a challenge that is often tackled by using metamaterials. Metamaterial-based active modulators can be created by placing graphene as a tuneable element shunting regions of high electric field confinement in metamaterials. However, in this common approach, the graphene is used as a variable resistor, and the modulation is achieved by resistive damping of the resonance. In combination with the finite conductivity of graphene due to its gapless nature, achieving 100% modulation depth using this approach remains challenging. Here, we embed nanoscale graphene capacitors within the gaps of the metamaterial resonators, and thus switch from a resistive damping to a capacitive tuning of the resonance. We further expand the optical modulation range by device excitation from its substrate side. As a result, we demonstrate terahertz modulators with over four orders of magnitude modulation depth (45.7 dB at 1.68 THz and 40.1 dB at 2.15 THz), and a reconfiguration speed of 30 MHz. These tuneable capacitance modulators are electrically controlled solid-state devices enabling unity modulation with graphene conductivities below 0.7 mS. The demonstrated approach can be applied to enhance modulation performance of any metamaterial-based modulator with a 2D electron gas. Our results open up new frontiers in the area of terahertz communications, real-time imaging, and wave-optical analogue computing.

We present experimental results on electron transport in a bilayer electron system that consists of two uncoupled two-dimensional electron gases (2DEGs) formed in a double quantum well within a GaAs/AlGaAs heterostructure. Our measurements of the capacitance between the top gate and the 2DEGs as a function of varying magnetic fields at total integer filling factors reveal critical insights into the net interlayer charge transfer required for the layers to maintain equal electrochemical potentials. In instances where both layers operate in the quantum Hall effect regime, the magnetocapacitance demonstrates anticoercive hysteresis. In particular, the change in electron density linked to this hysteresis significantly surpasses that associated with equilibrium interlayer charge transfer, indicating that hysteresis and interlayer charge transfer arise from distinct phenomena. Furthermore, at high amplitudes of the alternating gate voltage, we observe a nonlinear change in magnetocapacitance, indicating a breakdown of the quantum Hall effect.

Luttinger liquids occupy a notable place in physics as one of the most understood classes of quantum many-body systems. The experimental mission of measuring its main prediction, power laws in observable quantities, has already produced a body of exponents in different semiconductor and metallic structures. Here, we combine tunneling spectroscopy with density-dependent transport measurements in the same quantum wires over more than two orders of magnitude in temperature to very low electron temperatures down to ∼40 mK. This reveals that, when the second 1D subband becomes populated, the temperature dependence splits into two ranges with different exponents in the power-law dependence of the conductance, both dominated by the finite-size effect of the end-tunneling process. This result demonstrates the importance of measuring the Luttinger parameters as well as the number of modes independently through spectroscopy in addition to the transport exponent in the characterization of Luttinger liquids. This opens a pathway to unambiguous interpretation of the exponents observed in quantum wires.