Electron Gas Research Articles

Neural-network quantum states for ultra-cold Fermi gases

Ultra-cold Fermi gases exhibit a rich array of quantum mechanical properties, including the transition from a fermionic superfluid Bardeen-Cooper-Schrieffer (BCS) state to a bosonic superfluid Bose-Einstein condensate (BEC). While these properties can be precisely probed experimentally, accurately describing them poses significant theoretical challenges due to strong pairing correlations and the non-perturbative nature of particle interactions. In this work, we introduce a Pfaffian-Jastrow neural-network quantum state featuring a message-passing architecture to efficiently capture pairing and backflow correlations. We benchmark our approach on existing Slater-Jastrow frameworks and state-of-the-art diffusion Monte Carlo methods, demonstrating a performance advantage and the scalability of our scheme. We show that transfer learning stabilizes the training process in the presence of strong, short-ranged interactions, and allows for an effective exploration of the BCS-BEC crossover region. Our findings highlight the potential of neural-network quantum states as a promising strategy for investigating ultra-cold Fermi gases.

Novel 2DEG System at the HfO2/STO Interface.

Multifunctional integration in a single device has always been a hot research topic, especially for contradictory phenomena, one of which is the coexistence of ferroelectricity and metallicity. The complex oxide heterostructures, as symmetric breaking systems, provide a great possibility to incorporate different properties. Moreover, finding a series of oxide heterostructures to achieve this goal remains as a challenge. Here, taking the advantage of different physical phenomena, we use H2 plasma to pretreat the SrTiO3 (STO) substrate and then fabricate HfO2/STO heterostructures with it. The novel, well-repeatable metallic two-dimensional electron gas (2DEG) is directly obtained at the heterointerfaces without any further complex procedures, while the obvious ferroelectric-like behavior and Rashba spin-orbit coupling are also observed. The understanding of the mechanism, as well as the modified facile preparation procedure, would be meaningful for further development of ferroelectric metal in complex oxide heterostructures.

Prospects for thermalization of microwave-shielded ultracold molecules

Toward more efficient schemes for achieving deeply degenerate molecular Fermi gases, we study anisotropic thermalization in dilute gases of microwave shielded polar molecular fermions. For collision energies above the threshold regime, we find that thermalization is suppressed due to a strong preference for forward scattering and a reduction in total cross section with energy, significantly reducing the efficiency of evaporative cooling. We perform close-coupling calculations on the effective potential energy surface derived by Deng [] to obtain accurate two-body elastic differential cross sections across a range of collision energies. We use Gaussian process regression to obtain a global representation of the differential cross section over a wide range of collision angles and energies. The route to equilibrium is then analyzed with cross-dimensional rethermalization experiments, quantified by a measure of collisional efficiency toward achieving thermalization. Published by the American Physical Society 2024

Machine learning methods for background potential estimation in 2DEGs

Two-dimensional electron gases (2DEGs) can show exceptional carrier mobility, making them promising candidates for future quantum technologies. However, impurities and defects can significantly degrade their performance, impacting transport, conductivity, and coherence times. We leverage scanning gate microscopy (SGM) and machine learning approaches to extract the potential landscape of 2DEGs from SGM data. We compare three techniques: image-to-image translation with generative adversarial networks (GANs), cellular neural networks (CNNs), and an evolutionary search algorithm. Notably, the evolutionary approach outperforms both alternatives in defect identification and analysis. This work clarifies the interaction between defects and 2DEG properties, demonstrating the potential of machine learning for understanding and manipulating quantum materials, facilitating advancements in quantum computing and nanoelectronics.

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The results indicate that to form 2DEG in the unstrained LaGaO3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document}/BaSnO3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} HS system, a minimum thickness of approximately 4 unit cells of LaGaO3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} film is necessary. An increased film thickness of LaGaO3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} is required to form the 2DEG for -3%-biaxially-strained HS system and the critical thickness is 3 unit cells for 3%-baxially-strained HS system, which is caused by the strain-induced change of the electrostatic potential in LaGaO3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} film. In addition, the biaxial strain plays an important role in tailoring the electrical properties of 2DEG in LaGaO3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document}/BaSnO3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} HS syestem. The interfacial charge carrier density, electron mobility and electrical conductivity can be optimized when a moderate tensile strain is applied on the BaSnO3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} substrate in the ab-plane.

Numerical simulation of Rashba spin–orbit coupling effect on quantum Hall resistivity within tight-binding approximation

A numerical simulation of the effect of Rashba spin–orbit coupling (RSO) on quantum transport properties of electrons confined to a two-dimensional electron gas (2DEG) in the integer quantum Hall regime is presented. The mechanism of quantization and the dependence of the Hall resistivity (HR) on the system built-in and external parameters are studied using the non-equilibrium Green’s function (NEGF) technique. Relevant Green’s functions are computed numerically using spectral decomposition method in combination with recursive techniques. The tight-binding-like representation of the system Hamiltonian including magnetic field is obtained within the finite difference method, using Dirichlet boundary conditions. Corresponding formula linking the Hall resistivity with the transmission function of the system has been derived analytically within the NEGF formalism in a general form, in the sense that the formula can also be used beyond the linear response limit and for nonzero temperatures. In particular, it was shown explicitly that in the presence of the RSO coupling the even- and odd plateaus emerging in the magnetic-field dependence of the quantum Hall resistivity are related to different physical mechanisms.

Decoherence and Momentum Relaxation in Fermi-Polaron Rabi Dynamics: A Kinetic Equation Approach.

Despite the paradigmatic nature of the Fermi-polaron model, the theoretical description of its nonlinear dynamics poses challenges. Here, we apply a quantum kinetic theory of driven polarons to recent experiments with ultracold atoms, where Rabi oscillations between a Fermi-polaron state and a noninteracting level were reported. The resulting equations separate decoherence from momentum relaxation, with the corresponding rates showing a different dependence on microscopic scattering processes and quasiparticle properties. We describe both the polaron ground state and the excited repulsive-polaron state and we find a good quantitative agreement between our predictions and the available experimental data without any fitting parameter. Our approach not only takes into account collisional phenomena, but also it can be used to study the different roles played by decoherence and the collisional integral in the strongly interacting highly imbalanced mixture of Fermi gases.

Improved nuclear mass formula with an additional term from the Fermi gas model

Improved nuclear mass formula with an additional term from the Fermi gas model

Ballistic to diffusive crossover in a weakly interacting Fermi gas

In the absence of disorder and interactions, fermions move coherently and their associated charge and energy exhibit ballistic spreading, even at finite energy density. In the presence of weak interactions and a finite energy density, fermion-fermion scattering leads to a crossover between early-time ballistic and late-time diffusive transport. The relevant crossover timescales and the transport coefficients are both functions of interaction strength, but the question of determining the precise functional dependence is likely impossible to answer exactly. In this work we develop a numerical method (fDAOE) which is powerful enough to provide an approximate answer to this question, and which is consistent with perturbative arguments in the limit of very weak interactions. Our algorithm, which adapts the existing dissipation-assisted operator evolution (DAOE) to fermions, is applicable to systems of interacting fermions at high temperatures. The algorithm approximates the exact dynamics by systematically discarding information from high n-point functions, and is tailored to capture noninteracting dynamics exactly. Applying our method to a microscopic model of interacting fermions, we numerically determine crossover timescales and diffusion constants for a wide range of interaction strengths. In the limit of weak interaction strength (Δ), we demonstrate that the crossover from ballistic to diffusive transport happens at a time tD∼1/Δ2 and that the diffusion constant similarly scales as D∼1/Δ2. We confirm that these scalings are consistent with a perturbative Fermi's golden rule calculation, and we provide a heuristic operator-spreading picture for the crossover between ballistic and diffusive transport. Published by the American Physical Society 2024

Two-dimensional electron gases in AlYN/GaN heterostructures grown by metal–organic chemical vapor deposition

Wurtzite AlN alloyed with group 3 elements Sc and Y boosts the performance of GaN-based high-electron-mobility transistors (HEMTs) significantly as they increase the spontaneous polarization of the barrier layer and, thus, enhance the charge carrier density ns in the two-dimensional electron gas (2DEG) formed at the interface with the GaN channel. The emerging nitride Al1−xYxN additionally features an a lattice parameter matching to that of GaN at x = 0.07–0.11, allowing for the growth of strain-free barriers. Here, we demonstrate the growth of Al1−xYxN/GaN heterostructures for HEMTs by metal–organic chemical vapor deposition for the first time. The effect of the Y concentrations on the 2DEG is investigated in a Y concentration range from 3% to 15%. At 8% Y, a record mobility of 3200 cm2/(Vs) was measured at a low temperature (7 K). Room and low-temperature ns was at 1–2 × 1013 cm−2. Al0.92Y0.08N barriers were coherently strained to the GaN channel for barrier thicknesses from 5 to 15 nm. Finally, the deposition of Al1−xYxN/GaN heterostructures deposited on 4″ 4H–SiC wafers had a room-temperature mobility close to 1400 cm2/(Vs). AlYN/GaN heterostructures may offer advantages over AlScN/GaN heterostructures not only for the lower price and higher abundance of the raw material but also in terms of electrical characteristics and may be more suitable for power amplifying applications due to increased electron mobility.

Static dielectric response and screening in solid state physics: Why dimensionality matters in dielectrics

Textbooks often present the phenomenon of screening within the Thomas–Fermi model for three-dimensional free electron gases, but obtaining the dielectric response function and screening potential for dielectric systems of reduced dimensionality is also of pedagogical interest. In this work, we introduce a simple approach to investigate static screening in dielectric systems in the presence of an impurity charge for different dimensionalities. This approach is applicable to semiconductors and insulators alike. We demonstrate that, in 3D systems, the macroscopic dielectric function in reciprocal space is a constant, while in 2D and 1D systems, it strongly depends on the momentum transferred to the electrons in the dielectric. Through the proposed dielectric screening model, one can also determine binding energies in a hydrogenic model that can be used to describe excitations in real semiconductor systems.

Emergent two-band conduction at Ti delta-doped LaAlO3/KTaO3 (111) heterointerface

Recently, several intriguing interfacial phenomena have been discovered at the KTaO3 (111)-based heterointerfaces, such as the two-dimensional electron gas, superconductivity, anisotropic conductivity, etc. However, the available techniques to systematically manipulate such interfacial states are quite limited. Here, we devise a delta-doping strategy by inserting a sub-nanometer Ti layer with distinct reducibilities to the LaAlO3/KTaO3 (111) interface. In the delta-doped samples, we observe an unforeseen two-band conduction, evidenced by the emergence of a new type of electron carrier with an order-of-magnitude enhancement of mobility (∼1800 cm2V−1s−1) than the original carriers. Moreover, the appearance of the high-mobility carriers causes a sharp transition between the non-Fermi-liquid superconducting state and the Fermi-liquid state with reduced spin–orbit scattering. Further evidence shows that the new type of carriers stems from another in-gap state with a shallower energy level compared to the original carriers. Our study broadens the spectrum of interfacial carrier manipulation by introducing an extra band/channel for carrier conduction, which not only opens up new possibilities in device applications but also shines a light on the underlying physics of interfacial superconductivity.

Ultrafast modulable 2DEG Huygens metasurface

Huygens metasurfaces have demonstrated remarkable potential in perfect transmission and precise wavefront modulation through the synergistic integration of electric resonance and magnetic resonance. However, prevailing active or reconfigurable Huygens metasurfaces, based on all-optical systems, encounter formidable challenges associated with the intricate control of bulk dielectric using laser equipment and the presence of residual thermal effects, leading to limitations in continuous modulation speeds. Here, we present an ultrafast electrically driven terahertz Huygens metasurface that comprises an artificial microstructure layer featuring a two-dimensional electron gas (2DEG) provided by an AlGaN/GaN heterojunction, as well as a passive microstructure layer. Through precise manipulation of the carrier concentration within the 2DEG layer, we effectively govern the current distribution on the metasurfaces, inducing variations in electromagnetic resonance modes to modulate terahertz waves. This modulation mechanism achieves high efficiency and contrast for terahertz wave manipulation. Experimental investigations demonstrate continuous modulation capabilities of up to 6 GHz, a modulation efficiency of 90%, a transmission of 91%, and a remarkable relative operating bandwidth of 55.5%. These significant advancements substantially enhance the performance of terahertz metasurface modulators. Importantly, our work not only enables efficient amplitude modulation but also introduces an approach for the development of high-speed and efficient intelligent transmissive metasurfaces.

Nonlinear helicity anomalies in the cyclotron resonance photoresistance of two-dimensional electron systems

Our studies of the cyclotron resonance (CR) photoresistance in GaAs-based two-dimensional electron systems (2DES) reveal an anomalously low sensitivity to the helicity of the incoming circularly polarized terahertz radiation. We find that this anomaly is strongly intensity dependent, and the ratio of the low-temperature photoresistance signals for the CR-active (CRA) and CR-inactive (CRI) polarities of magnetic field increases with lowering power, but, nevertheless, remains substantially lower than expected from conventional theory assuming interaction of the plane electromagnetic wave with the uniform 2DES. Our analysis shows that all data can be well described by the nonlinear CR-enhanced electron gas heating in both CRA and CRI regimes. This description, however, does not specify the source of anomalous CRI absorption that remains unclear. Published by the American Physical Society 2024

Erratum: Exact results for polaron and molecule in one-dimensional spin-1/2 Fermi gas [Phys. Rev. A 94 , 043645 (2016)

Erratum: Exact results for polaron and molecule in one-dimensional spin-1/2 Fermi gas [Phys. Rev. A <b>94</b> , 043645 (2016)

Competing conduction mechanisms for two-dimensional electron gas at LaTiO3/SrTiO3 heterointerfaces

In pursuit of understanding the mechanism of two-dimensional electron gas (2DEG), artificial heterostructures based on SrTiO3 have sparked a wealth of exciting experimental and theoretical results. However, to date, the physical origin of this phenomenon still remains controversial. In our endeavor to unravel the mystery, we have synthesized a series of LaTiO3+δ/SrTiO3 films by controlling growth temperature and oxygen pressure, respectively. X-ray absorption results identify the transition of Ti ions from Ti3+ to Ti4+. It is proved that electrical transport properties in the LaTiO3/SrTiO3 film are dominated by oxygen vacancies in SrTiO3. In addition, the metallic behavior observed in the amorphous LaTiO3/SrTiO3 film suggests contributions from growth-induced donor defects to 2DEG. Our work presents direct evidence of competing conduction mechanisms for the observed 2DEG at the LaTiO3/SrTiO3 interface and highlights the important role of oxygen diffusion from the substrate SrTiO3. Meanwhile, we emphasize the significance of LaTiO3 as an oxygen scale meter to investigate interfacial properties of oxide heterostructures.

Low-resistance TiAl3/Au ohmic contact and enhanced performance on AlGaN/GaN HEMT

The advent of an era characterized by intelligent, interconnected systems of AlGaN/GaN High Electron Mobility Transistors (HEMTs) electronic devices is underway. Within this context, the performance of ohmic contacts in HEMTs bears substantive influence on the electrical characteristics of power devices, notably impacting parameters such as on-resistance and output current. Therefore, the improvement of ohmic contacts attributes in AlGaN/GaN HEMT devices has perennially constituted a focal point of considerable industry. Based on the AlGaN/GaN heterostructure, a novel TiAl3/Au ohmic contact has been successfully developed for HEMTs. A low contact resistance of 0.23 Ω·mm (2.33 × 10-6 Ω·cm2) is obtained, which proposes a new interface contact for AlGaN/GaN heterojunction. Transmission electron microscope (TEM) illustrates that the contact mechanism for low resistance is direct contact with two-dimensional electron gas (2DEG) through the substitution of the TiN compound with a smaller area Au dominated penetration. Concurrently, in contrast to TiN, Au exhibits a diminished capacity for Ga dissolution, thereby substantiating its efficacy in preserving the structural integrity of the crystal lattice. In addition, the reduction of Al can impede the diffusion notably, further showing the smooth surface morphology of the electrode with atomic force microscope (AFM) and scanning electron microscopy (SEM) measurements. The ohmic contacts of HEMTs are individually fabricated with Ti/Al/Ni/Au and TiAl3/Au configurations. The HEMTs with TiAl3/Au ohmic contacts demonstrate superior DC characteristics.

Construction of Hierarchical Porous UiO-66-Br2@PS/DVB-Packed Columns by High Internal Phase Emulsion Strategy for Enhanced Separation of CF4/N2 and SF6/N2.

Recovery and separation of anthropogenic emissions of electronic specialty gases (F-gases, such as CF4 and SF6) from the semiconductor sector are of critical importance. In this work, the hierarchical porous UiO-66-Br2@PS/DVB-packed column was constructed by a high internal phase emulsions strategy. UiO-66-Br2@PS/DVB exhibits a superior selectivity of CF4/N2 (2.67) and SF6/N2 (3.34) predicted by the IAST due to the diffusion limitation in the micropore and the gas-framework affinity. Especially, UiO-66-Br2@PS/DVB showed significant CF4 and SF6 retention and enabled the successful separation of CF4/N2 and SF6/N2 with a resolution of 2.37 and 8.89, respectively, when used as a packed column in gas chromatography. Compared with the Porapak Q column, the HETP of the UiO-66-Br2@PS/DVB-packed column decreased and showed good reproducibility. This research not only offers a convenient method for fabricating a hierarchical porous MOF-packed column but also showcases the prospective utilization of MOFs for the separation of the F-gas/N2 mixture.

Onset of magnetization in the one-dimensional Fermi gas with attractive [formula omitted]-function potential

The magnetic field dependence of the ground state of the one-dimensional spin-1/2 Fermi gas is studied with emphasis on the onset of the magnetization. The Bethe Ansatz solution shows that the system has three phases, namely, spin–singlet bound states (preformed Cooper pairs), spin-polarized particles and a mixed phase of the above. For small fields the ground state consists of paired states. At the critical field the band of unpaired (spin-up) particles starts to get gradually populated following the square-root dependence of the van Hove singularity of the empty parabolic band. The filled unpaired states correspond to the rapidity interval −B0≤k≤B0. We show that the square of the magnetization and B02 are initially linear in the field. For the case of spin-3/2 the Bethe Ansatz solution for the ground state has four fundamental bands: The particles can be either unpaired or bound in states of two, three and four fermions. The onset of the population of the first three bands again follows the square-root dependence of the one-dimensional van Hove singularity of the respective band. Our results are compared with previous studies where the magnetization is linear in the field.

Внезонное поглощение электромагнитного излучения электронами с оптическим фононным участием в сверхрешетках квантовых точек

Physics Institute of Azerbaijan National Academy of Sciences In this paper, we investigate the absorption of electromagnetic radiation by a free electron gas interacting with lattice vibrations in a quantum dot superlattice. It is assumed that the electron gas in the quantum dot superlattice is limited by the anisotropic parabolic potential. The absorption of light by free carriers with the participation of phonons is calculated in the second order of the perturbation theory. When calculating the absorption coefficient, the matrix elements of the electron-photon and electron-phonon interactions (with polar and nonpolar optical phonons) are used. We can expect three possible transitions in the absorption coefficient for electron-polar phonon scattering: (1) a transition due to the size subband levels for only the x direction, (2) a transition due to the size subband levels for only the y direction and (3) a transition due to the size subband levels for both the x direction and the y direction. For electron-nonpolar phonon scattering we can expect оnly one possible transitions in the, the absorption coefficient: a transition due to the size subband levels for both the x direction and the y direction.