Quantum Transport Research Articles

Intrinsic performance limits of extremely scaled field-effect transistors based on MX2 (M = {Zr, Hf}, X = {S, Se}) nanoribbons

We investigate the MX2 (M = {Hf, Zr}, X = {S, Se}) transition metal dichalcogenides patterned into armchair (AC) and zigzag (ZZ) nanoribbons (NRs) as potential channel materials in future logic field-effect devices. Ab initio quantum transport simulations are employed to assess the electronic, transport, and ballistic field-effect transistor (FET) properties of devices with such quasi-one-dimensional channels. We report a non-monotonic scaling behavior of MX2NR properties due to strong quantum confinement effects, which is reflected in a strong dependence of the ON-state current (ION) of MX2NR FETs on the nanoribbon configuration. The ∼2 nm-wide HfSe2 and ZrSe2 AC-PFETs have the highest ION of up to 2.6 mA/μm at 10 nA/μm OFF-state current. Surprisingly, MX2NR ZZ-NFETs exhibit a current increase of up to 70% when channel width is scaled down, with ION reaching 2.2 mA/μm in ∼2 nm-wide devices. The high ON-state performance is a direct consequence of high carrier injection velocity, which is explained by analyzing the band structure, transmission, and density of states. We demonstrate that nanostructured MX2 materials can be promising candidates for future logic transistors based on multi-nanowire architectures.

High-efficiency photocatalyst and high-response ultraviolet photodetector based on the Ga2SSe@GaN heterojunctions

The ultraviolet photodetector based on the photo-galvanic effect (PGE) has garnered extensive interest due to its capability of achieving photo-response without bias voltage. In this research, the catalytic characteristics of the Ga2SSe@GaN heterojunctions are calculated using first-principles calculations and the photocurrent is obtained through quantum transport simulations. Both the Ga2SSe/GaN and GaN/Ga2SSe heterojunctions exhibit the S-type band alignment with the stable built-in electric field. The Ga2SSe/GaN heterojunction demonstrates the high solar-to-hydrogen efficiency and low overpotentials, exhibiting excellent catalytic properties. Both the Ga2SSe/GaN and GaN/Ga2SSe heterojunctions have high electron mobilities. The photodetector based on the Ga2SSe/GaN heterojunction exhibits high photo-response in ultraviolet area. Our research corroborates that the Ga2SSe@GaN heterojunctions hold potential application in the fields of photocatalytic, electrocatalytic and photodetector.

Computation of biological conductance with Liouville quantum master equation

Recent experiments have revealed that single proteins can display high conductivity, which stays finite for low temperatures, decays slowly with distance, and exhibits a rich spatial structure featuring highly conducting and strongly insulating domains. Here, we intruduce a new formula by combining the density matrix of the Liouville-Master Equation simulating quantum transport in nanoscale devices, and the phenomenological model of electronic conductance through molecules, that can account for the observed distance- and temperature dependence of conductance in proteins. We demonstrate its efficacy on experimentally highly conductive extracellular cytochrome nanowires, which are good candidates to illustrate our new approach by calculating and visualizing their electronic wiring, given the interest in the arrangement of their conducting and insulating parts. As proteins and protein nanowires exhibit significant potential for diverse applications, including energy production and sensing, our computational technique can accelerate the design of nano-bioelectronic devices.

Anisotropic quantum transport in ZrSiS probed by high-field torque magnetometry

Anisotropic quantum transport in ZrSiS probed by high-field torque magnetometry

Self-similarity of quantum transport in graphene using electrostatic gate and substrate

Abstract A particular design for multibarrier structure in graphene, yielding a self-similar transport response, is proposed. The potential profile is based on rectangular wells and barriers, generated according independent n-th order scaling laws for their heights and widths. The barriers are constructed by means of two distinct approaches (electrostatic or substrate). Dirac equation and transfer matrix approach are used to calculate transmission properties which, in turn, allow to evaluate the conductance via Landauer-Büttiker formalism. It is found that self-similarity with determined scaling rules between n-th and (n + 1)-th generations of transport properties appears when the order of generating laws is equal or greater than n = 7. Our proposal would be the first in which the self-similarity property is transferred from geometry to the spectrum, and consequently, to the transport properties of a quantum heterostructure. Possible ways of practical realization for the proposed structures are commented.

Giant tunneling resistance and robust switching behavior in ferroelectric tunnel junctions of WS2/Ga2O3 heterostructures: The influence of metal–semiconductor contacts

The ferroelectric tunneling junctions (FTJs) are widely recognized as one of the non-volatile memories with significant potential. Ferroelectricity usually fades away as materials are thinned down below a critical value, and this problem is particularly acute in the case of shrinking device sizes, thus attracting attention to two-dimensional ferroelectric materials (2DFEMs). In this work, we designed 2D ferroelectric Ga2O3-based FTJs with out-of-plane polarization, and the influence of metal–semiconductor contact in the electrode region on the system is considered. Here, using density functional theory combined with the non-equilibrium Green's function approach to quantum transport calculations, we demonstrate robust ferroelectric polarization-controlled switching behavior between metallic and semiconducting states in Ga2O3/WS2 ferroelectric heterostructures. The potential barrier of the metal–semiconductor contact in the electrode region is lower than that of the intrinsic material, thereby resulting in an increased probability of electron tunneling. Our results reveal the crucial role of 2DFEMs in the construction of FTJs and highlight the significant impact of electrode contact types on performance. This provides a promising approach for developing high-density ferroelectric memories based on 2D ferroelectric semiconductor heterostructures.

Unified diagrammatic approach to quantum transport in few-level junctions for bosonic and fermionic reservoirs: Application to the quantum Rabi model

Unified diagrammatic approach to quantum transport in few-level junctions for bosonic and fermionic reservoirs: Application to the quantum Rabi model

Interaction Effects on the Dynamical Anderson Metal-Insulator Transition Using Kicked Quantum Gases.

Understanding the interplay of interaction and disorder in quantum transport poses long-standing scientific challenges for theory and experiment. While highly controlled ultracold atomic platforms combining atomic interactions with spatially disordered lattices have led to remarkable advances, the extension of such controlled studies to phenomena in high-dimensional disordered systems, such as the three-dimensional Anderson metal-insulator transition has been limited. Kicked quantum gases provide an alternate experimental platform that captures the Anderson model in momentum space and features dynamical localization as the analog of Anderson localization. Here, we utilize a momentum space lattice platform using quasiperiodically kicked ultracold atomic gases to experimentally investigate interaction effects on the three-dimensional dynamical Anderson metal-insulator transition. We observe interaction-driven subdiffusion and a divergence of delocalization onset time on approaching the phase boundary. Mean-field numerical simulations show qualitative agreement with experimental observations, but with significant quantitative deviations.

Quantum transport of charged fermions in curved graphene super lattice

The study investigates the fascinating phenomenon of quantum transport in a single-layer curved graphene superlattice, considering out-of-plane deformations. By employing the Dirac equation in curved space, the research explores the intricate interplay between geometry and quantum mechanics, revealing how structural deformations affect electronic transport properties. The focus is on two periodic deformation examples, providing valuable insights into the influence of curvature-induced modifications on the electronic band structure and highlighting the enhanced transmission coefficients that arise. The periodic deformations serve as a mechanism for tuning and optimizing transmission properties by carefully adjusting deformation parameters. Numerical simulations underscore the substantial impact of structural parameters on quantum transport. Notably, the length of the curved graphene region significantly influences the transmission coefficients. Additionally, the study identifies the incident angle and barrier number as crucial factors in shaping the system's response to external stimuli. By manipulating the incident angle and fermion energy, researchers can modulate transmission properties and control the system's behavior for specific applications.

RealTimeTransport: An open-source C++ library for quantum transport simulations in the strong coupling regime.

The description of quantum transport in the strong system-reservoir coupling regime poses a significant theoretical and computational challenge that demands specialized tools for accurate analysis. RealTimeTransport is a new open-source C++ library that enables the computation of both stationary and transient transport observables for generic quantum systems connected to metallic reservoirs. It computes the Nakajima-Zwanzig memory kernels for both dynamics and transport in real-time, going beyond traditional expansions in the bare system-reservoir couplings. Currently, several methods are available as follows: (i) A renormalized perturbation theory in leading and next-to-leading order, which avoids the low-temperature breakdown that limits the traditional theory. (ii) Starting from this well-behaved reference solution, a two- and three-loop, self-consistent renormalization-group transformation of the memory kernels is implemented. This allows refined quantitative predictions even in the presence of many body resonances, such as the Kondo enhancement of cotunneling. This paper provides an overview of the theory, the architecture of RealTimeTransport, and practical demonstrations of the currently implemented methods. In particular, we analyze the stationary transport through a serial double quantum dot and showcase for the T = 0 interacting Anderson model the complete time-development of single-electron tunneling (SET), cotunneling-assisted SET, and inelastic cotunneling resonances throughout the entire gate-bias stability diagram. We discuss the range of applicability of the implemented methods and benchmark them against other advanced approaches.

Exploring Aromaticity Effects on Electronic Transport in Cyclo[n]carbon Single-Molecule Junctions.

Cyclo[n]carbon (Cn) is one member of the all-carbon allotrope family with potential applications in next-generation electronic devices. By employing first-principles quantum transport calculations, we have investigated the electronic transport properties of single-molecule junctions of Cn, with n = 14, 16, 18, and 20, connected to two bulk gold electrodes, uncovering notable distinctions arising from the varying aromaticities. For the doubly aromatic C14 and C18 molecules, slightly deformed complexes at the singlet state arise after bonding with one Au atom at each side; in contrast, the reduced energy gaps between the highest occupied and the lowest unoccupied molecular orbitals due to the orbital reordering observed in the doubly anti-aromatic C16 and C20 molecules lead to heavily deformed asymmetric complexes at the triplet state. Consequently, spin-unpolarized transmission functions are obtained for the Au-C14/18-Au junctions, while spin-polarized transmission appears in the Au-C16/20-Au junctions. Furthermore, the asymmetric in-plane π-type hybrid molecular orbitals of the Au-C16/20-Au junctions contribute to two broad but low transmission peaks far away from the Fermi level (Ef), while the out-of-plane π-type hybrid molecular orbitals dominate two sharp transmission peaks that are adjacent to Ef, thus resulting in much lower transmission coefficients at Ef compared to those of the Au-C14/18-Au junctions. Our findings are helpful for the design and application of future cyclo[n]carbon-based molecular electronic devices.

GPU-Enabled Exascale Quantum Transport Modeling of Carbon Nanotube Devices

The work will describe our efforts to develop an exascale electrostatic-quantum transport framework, Quantum-eXstatic (https://github.com/AMReX-Microelectronics/eXstatic), currently supporting the modeling of carbon nanotube field-effect transistors (CNTFETs). The framework is capable of modeling realistic three-dimensional structures, efficiently, using CPU/GPU heterogeneous architectures of state-of-the-art supercomputers. It is built on top of the AMReX library, which is a GPU-enabled software library containing functionality for writing particle-mesh applications on structured grids. The Quantum-eXstatic framework comprises three major components: the electrostatic module, the quantum transport module, and the part that self-consistently couples the two modules. The electrostatic module using a finite-volume multigrid solver to compute the electrostatic potential induced by charges on the surface of carbon nanotubes, as well as by source, drain, and gate terminals, which can be modeled using an embedded boundary approach to allow for intricate shapes. The quantum transport module uses the Nonequilibrium Green's Function (NEGF) method to model induced charge. Currently, it supports coherent (ballistic) transport, mode-space approximation for subbands of nanotube, contacts modeled as semi-infinite leads, and Hamiltonian representation using the tight-binding approximation. Special features are being included for accurate modeling of long nanotubes, such as adaptive refinement of integration contours to resolve van Hove singularities in long channels. Particular attention is paid to overlap computations with communication to optimize GPU performance. The self-consistency between the two modules is achieved using Broyden's modified second algorithm, which is parallelized using CPUs and GPUs. Preliminary scaling studies show that the code exhibits ~77% weak-scaling efficiency on 512 NVIDIA A100 GPUs of Perlmutter supercomputer for modeling gate-all-around CNTFETs with a channel length of 13.5 microns at equilibrium conditions. This case involved computations of electrostatic potential in 14.5 billion computational cells and Green’s function computation for a Hamiltonian of size 131,072 x 131,072. For problems of this large size, the code takes about 5 min to reach self-consistency. Figure 1

Nonlinear Charge Transport and Excitable Phenomena in Semiconductor Superlattices.

Semiconductor superlattices are periodic nanostructures consisting of epitaxially grown quantum wells and barriers. For thick barriers, the quantum wells are weakly coupled and the main transport mechanism is a sequential resonant tunneling of electrons between wells. We review quantum transport in these materials, and the rate equations for electron densities, currents, and the self-consistent electric potential or field. Depending on superlattice configuration, doping density, temperature, voltage bias, and other parameters, superlattices behave as excitable systems, and can respond to abrupt dc bias changes by large transients involving charge density waves before arriving at a stable stationary state. For other parameters, the superlattices may have self-sustained oscillations of the current through them. These oscillations are due to repeated triggering and recycling of charge density waves, and can be periodic in time, quasiperiodic, and chaotic. Modifying the superlattice configuration, it is possible to attain robust chaos due to wave dynamics. External noise of appropriate strength can generate time-periodic current oscillations when the superlattice is in a stable stationary state without noise, which is called the coherence resonance. In turn, these oscillations can resonate with a periodic signal in the presence of sufficient noise, thereby displaying a stochastic resonance. These properties can be exploited to design and build many devices. Here, we describe detectors of weak signals by using coherence and stochastic resonance and fast generators of true random sequences useful for safe communications and storage.

Integration of Artificial Intelligence and Quantum Transport toward Stereoselective Identification of Carbohydrate Isomers.

Detection of stereoisomers of carbohydrates with molecular resolution, a challenging goal analysts desire to achieve, is key to the full development of glycosciences. Despite the promise that analytical techniques made, including widely used nuclear magnetic resonance and mass spectrometry, high throughput de novo carbohydrate sequencing remains an unsolved issue. Notably, while next-generation sequencing technologies are readily available for DNA and proteins, they are conspicuously absent for carbohydrates due to the immense stereochemical and structural complexity inherent in these molecules. In this work, we report a novel computational technique that employs quantum tunneling coupled with artificial intelligence to detect complex carbohydrate anomers and stereoisomers with excellent sensitivity. The quantum tunneling footprints of carbohydrate isomers show high distinguishability with an in-depth analysis of underlying chemistry. Our findings open up a new route for carbohydrate sensing, which can be seamlessly integrated with next-generation sequencing technology for real-time analysis.

Probing Site-Resolved Current in Strongly Interacting Superconducting Circuit Lattices.

Transport measurements are fundamental for understanding condensed matter phenomena, from superconductivity to the fractional quantum Hall effect. Analogously, they can be powerful tools for probing synthetic quantum matter in quantum simulators. Here we demonstrate the measurement of insitu particle current in a superconducting circuit lattice and apply it to study transport in both coherent and bath-coupled lattices. Our method utilizes controlled tunneling in a double-well potential to map current to on-site density, revealing site-resolved current and current statistics. We prepare a strongly interacting Bose-Hubbard lattice at different lattice fillings, and observe the change in current statistics as the many-body states transition from superfluid to Mott insulator. Furthermore, we explore nonequilibrium current dynamics by coupling the lattice to engineered driven-dissipative baths that serve as tunable particle source and drain. We observe steady-state current in discrete conduction channels and interaction-assisted transport. These results establish a versatile platform to investigate microscopic quantum transport in superconducting circuits.

Weak localization and magnetoresistance phenomena of hydrogen-annealed WO3 films

We explore the electronic transport properties of hydrogenated epitaxial WO3 films. Ionized hydrogen not only generates an electron carrier but also acts as an impurity center, providing a useful approach to investigate a macroscopic quantum transport phenomenon of weak localization. Temperature dependent-resistivity and anomalous magnetoresistance results are analyzed based on Mott's variable range hopping and weak localization. From the model studies, the phase coherence length is estimated to have a temperature dependence of T −1.6 and is determined to vary from ∼22 nm at 80 K to ∼180 nm at 20 K. The length at 20 K is of a similar order of magnitude of film thickness, which requires a dimensional crossover between three-dimensional and two-dimensional localization.

Anomalous quantum transport in fractal lattices

Fractal lattices are self-similar structures with repeated patterns on different scales. Quantum transport through such structures is subtle due to the possible co-existence of localized and extended states. Here, we study the dynamical properties of two fractal lattices, the Sierpiński gasket and the Sierpiński carpet. While the gasket exhibits sub-diffusive behavior, sub-ballistic transport occurs in the carpet. We show that the different dynamical behavior is in line with qualitative differences of the systems’ spectral properties. Specifically, in contrast to the Sierpiński carpet, the Sierpiński gasket exhibits an inverse power-law behavior of the level spacing distribution. As a possible technological application, we discuss a memory effect in the Sierpiński gasket which allows to read off the phase information of an initial state from the spatial distribution after long evolution times. We also show that interpolating between fractal and regular lattices allows for flexible tuning between different transport regimes.

From stochastic Hamiltonian to quantum simulation: exploring memory effects in exciton dynamics

The unraveling of open quantum system dynamics in terms of stochastic quantum trajectories offers a picture of open system dynamics that consistently considers memory effects stemming from the finite correlation time of environment fluctuations. These fluctuations significantly influence the coherence and energy transport properties of excitonic systems. When their correlation time is comparable to the timescale of the Hamiltonian evolution, it leads to the departure of open system dynamics from the Markovian limit. In this work, we leverage the unraveling of exciton dynamics through stochastic Hamiltonian propagators to design quantum circuits that simulate exciton transport, capturing finite memory effects. In addition to enabling the synthesis of parametrizable quantum circuits, stochastic unitary propagators provide a transparent framework for investigating non-Markovian effects on exciton transport. Our analysis reveals a nuanced relationship between environment correlation time and transport efficiency, identifying a regime of ‘memory-assisted’ quantum transport where time-correlated fluctuations allow the system to reach higher efficiency. However, this property is not universal and can only be realized in conjunction with specific features of the system Hamiltonian.

High-performance negative differential resistance characteristics in homoepitaxial AlN/GaN double-barrier resonant tunneling diodes

In this work, high-performance negative differential resistance (NDR) characteristics are demonstrated in homoepitaxial AlN/GaN double-barrier resonant tunneling diodes (RTDs). The devices are grown by plasma-assisted MBE on bulk GaN substrates and exhibit robust and repeatable NDR at RT. A high peak current density of 183 kA cm−2 is simultaneously demonstrated with a large peak-to-valley current ratio of 2.07, mainly benefiting from the significantly reduced dislocation density and improved hyper-abrupt heterointerfaces in the active region, which boosts the electron quantum transport in the resonant tunneling cavity. The achievement shows the promising potential to enhance the oscillation frequency and output power of GaN-based RTD oscillators, imperative for next-generation high-power solid-state compact terahertz oscillators application.

Millikelvin confocal microscope with free-space access and high-frequency electrical control.

Cryogenic confocal microscopy is a powerful method for studying solid state quantum devices such as single photon sources and optically controlled qubits. While the vast majority of such studies have been conducted at temperatures of a few Kelvin, experiments involving fragile quantum effects often require lower operating temperatures. To also allow for electrical dynamic control, microwave connectivity is required. For polarization-sensitive studies, free space optical access is advantageous compared to fiber coupling. Here we present a confocal microscope in a dilution refrigerator providing all the above features at temperatures below 100 mK. The installed high frequency cabling meets the requirements for state-of-the-art spin qubit experiments. As another unique advantage of our system, the sample fitting inside a large puck can be exchanged while keeping the cryostat cold with minimal realignment. Assessing the performance of the instrument, we demonstrate confocal imaging, sub-nanosecond modulation of the emission wavelength of a suitable sample, and an electron temperature of 76 mK. While the instrument was constructed primarily with the development of optical interfaces to electrically controlled qubits in mind, it can be used for many experiments involving quantum transport, solid state quantum optics, and microwave-optical transducers.