Electron Systems Research Articles

Recent Progress on Ultracold-atom Quantum Simulation of Fermi-Hubbard model

Fermi-Hubbard model is a fundamental lattice model for describing correlated electron systems in condensed matter physics, with a profound connection to high-temperature superconductivity. In recent years, quantum simulation with cold atoms has emerged as an important paradigm for studying the Fermi-Hubbard model, while advancements in quantum many-body computations have contributed to our understanding of its fundamental properties. Notably, a recent ultracold-atom experiment achieved the celebrated antiferromagnetic (AFM) phase transition in the three-dimensional (3D) Hubbard model, representing a crucial step in quantum simulation that establishes a foundation for exploring the connection between the quantum magnetism and high-temperature superconductivity. This paper reviews both the experimental and theoretical progress in studying the Fermi-Hubbard model, focusing primarily on 3D systems, discusses the history and current state of developments, and outlines future directions in this field.<br>The paper is organized as follows. We begin with a review of recent progress in observing AFM phase transitions in the 3D Hubbard model, focusing on an ultracold-atom experiment conducted by the research group at the University of Science and Technology of China (USTC). Next, we provide a theoretical introduction to the fundamental properties of the 3D Hubbard model. This includes a summary of prior theoretical studies, an overview of the current research landscape, and a discussion of unresolved or under-explored issues. In the third section, we discuss the quantum simulation of the Hubbard model using ultracold atoms in optical lattices, outlining the basic principle, historical developments and key challenges. The USTC team overcame these challenges with innovative techniques such as atom cooling, large-scale uniform box traps, and precise measurements of the AFM structure factor. Their work successfully confirmed the AFM phase transition via the critical scaling analysis. Finally, we highlight the significance of this achievement and discuss future research prospects for the 3D Hubbard model, including experimental studies on the doped regime and related theoretical benchmarks.

Electronic confinement induced quantum dot behavior in magic-angle twisted bilayer graphene.

Magic-angle twisted bilayer graphene (TBLG) has emerged as a versatile platform to explore correlated electron phases driven primarily by low-energy flat bands in moiré superlattices. While techniques for controlling the twist angle between graphene layers have spurred rapid experimental progress, understanding the effects of doping inhomogeneity on electronic transport in correlated electron systems remains challenging. In this work, we investigate the interplay of confinement and doping inhomogeneity on the electrical transport properties of TBLG by leveraging device dimensions and twist angles. We show that reducing device dimensions can magnify disorder potentials caused by doping inhomogeneity, resulting in pronounced carrier confinement. This phenomenon is evident in charge transport measurements, where the Coulomb blockade effect is observed. Temperature-dependent measurements reveal a large variation in the activation gap across the device. These findings highlight the critical role of doping inhomogeneity in TBLG and its significant impact on the transport properties of the system.

Quantum Monte Carlo study of the phase diagram of the two-dimensional uniform electron liquid

We present a study of spin-unpolarized and spin-polarized two-dimensional uniform electron liquids using variational and diffusion quantum Monte Carlo (VMC and DMC) methods with Slater-Jastrow-backflow trial wave functions. Ground-state VMC and DMC energies are obtained in the density range 1≤rs≤40. Single-particle and many-body finite-size errors are corrected using canonical-ensemble twist-averaged boundary conditions and extrapolation of twist-averaged energies to the thermodynamic limit of infinite system size. System-size-dependent errors in Slater-Jastrow-backflow DMC energies caused by partially converged VMC energy minimization calculations are discussed. We find that, for 1≤rs≤5, optimizing the backflow function at each twist lowers the twist-averaged DMC energy at finite system size. However, nonsystematic system-size-dependent effects remain in the DMC energies, which can be partially removed by extrapolation from multiple finite system sizes to infinite system size. The DMC energies in the thermodynamic limit are used to parametrize a local spin density approximation correlation functional for inhomogeneous electron systems. Our zero-temperature phase diagram shows a single transition from a paramagnetic fluid to a hexagonal Wigner crystal at rs=35(1), with no region of stability for a ferromagnetic fluid. Published by the American Physical Society 2024

Emergence and characterization of attraction in a dynamic Hubbard model

The Dynamic Hubbard Model (DHM) has been introduced as a framework for the electron-hole asymmetry observed in materials like high-temperature superconductors. In this study, we aim to understand the intricate dynamics of hole interactions, particularly focusing on the competition between on-site repulsion and hole-lattice coupling. Our results reveal a striking feature-an effective nearest-neighbor attraction between holes, even in the presence of the on-site repulsion typical of Mott insulators. Notably, the on-site repulsion not only coexists but also enhances the nearest-neighbor coupling. Our exploration of the DHM unravels unexpected and significant characteristics, enriching our understanding of hole interactions in correlated electron systems. The model’s capacity to capture both on-site repulsion and effective nearest-neighbor attraction offers a valuable perspective.

How the Piecewise-Linearity Requirement for the Density Affects Quantities in the Kohn-Sham System.

Kohn-Sham (KS) density functional theory (DFT) is an extremely popular, in-principle exact method, which can describe any many-electron system by introducing an auxiliary system of noninteracting electrons with the same density. When the number of electrons, N, changes continuously, taking on both integer and fractional values, the density has to be piecewise-linear, with respect to N. In this article, I explore how the piecewise-linearity property of the exact interacting density is reflected in the KS system. In particular, I suggest to express KS quantities using the two-point Taylor expansion in N and find how the expansion coefficients are restricted by the piecewise-linearity requirement. Focus is given to the total electron density, the KS subdensities, and the highest occupied (HOMO) orbital density. In addition to exact analytical results, common approximations for the HOMO, namely, the frozen and the linear regimes, are analyzed. A numerical investigation using various exchange-correlation approximations is performed to test the analytical findings. The outcomes of this work will help to remove density-driven errors in DFT calculations for open systems and ensembles.

Hidden Bose–Einstein Singularities in Correlated Electron Systems

Hidden Bose–Einstein Singularities in Correlated Electron Systems

On-surface synthesis of phthalocyanines with extended π-electron systems

Expanded phthalocyanines are a promising class of materials for optoelectronic applications, owing to their unique properties and versatile metal coordination reactivity. The expansion of their π-electron systems and resulting red-shifted absorption are of particular interest for achieving broader applications. Here, we report the on-surface synthesis of metallo-phthalocyanines with extended electron systems and an open-chain polycyanine from ortho-dicarbonitrile precursors on Ag(111) and Au(111), studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The larger 6,7-di(2-naphthyl)-2,3-naphthalenedicarbonitrile (NND) undergoes spontaneous cyclotetramerization on the Ag(111) surface forming the corresponding silver naphthalocyanines (Ag-NPc), contrasting previous reports where a partially aliphatic ortho-dicarbonitrile precursor formed polycyanine chains. In contrast, monolayers of the smaller 6,7-diphenyl-2,3-naphthalenedicarbonitrile (PND) form the corresponding naphthalocyanine only in the presence of co-adsorbed iron atoms (Fe-NPc). In the absence of iron, PND multilayers form polycyanine chains and Ag-NPc. NND and PND further differ in their reactivity due to the supramolecular behavior of their products. While the larger Ag-NPc aggregates to non-covalent one-dimensional ribbons, the smaller Fe-NPc forms an extended non-covalent two-dimensional network. Our study demonstrates the versatility of on-surface dinitrile tetramerization for the synthesis of π-extended cyclic phthalocyanines and their open-chain polycyanine counterparts.

Quantum Wire Coupled to Light

Experimental advances in cavity QED are raising the prospect of using light to probe quantum materials beyond the linear response regime. The capability to access quantum coherent phenomena would significantly advance the field. However, theoretical work on many-body systems coupled to light in the quantum coherent regime has been select. Here, we investigate the radiative properties of a finite-sized quantum wire in a microwave cavity. Examples of quantum wires include single-walled carbon nanotubes, a key experimental system in the field of nano-optics and plasmonics. We find that, for a variety of excited states, the repeated emission of photons results in the generation of many-body quantum entanglement. This leads to an increase in the rate at which subsequent photons are emitted, an example of Dicke superradiance. On the other hand, Pauli blocking tends to reduce this effect. Bosonization, the description of the excitations of a one-dimensional electron system as a gas of bosons, is found to be a powerful theoretical tool in this context. Its application means that many of our results generalize to wires with strong electron-electron interactions. The quantum wire thus represents a new platform to realize Dicke-model physics that does not rely on the various fine tunings necessary in traditional realizations involving many spatially isolated emitters. More broadly, this work demonstrates how quantum entanglement can be generated and measured in a many-body system. Published by the American Physical Society 2024

Influence of aqueous constituents on hexafluoropropylene oxide trimer acid (HFPO-TA) defluorination by UV/sulfite/iodide system

ABSTRACT HFPO-TA is an emerging alternative to traditional perfluoroalkyl substances (PFASs), which is characterized by its biotoxicity and persistence. The UV/sulfite/iodide photo-induced hydrated electrons system can effectively degrade HFPO-TA under mild conditions. However, the effects of water quality on this system need to be urgently investigated. This study explored the impact of common aqueous constituents (Cl−, HCO3−, PO43−, and HA) on the defluorination efficiency of HFPO-TA by the UV/sulfite/iodide system. Results indicated that low concentrations of Cl− (<1.0 mM), PO43− (<0.01 mM), and HA (<5.0 mg/L) have negligible impact on defluorination efficiency. However, as concentrations increase, these constituents can interact with photosensitizers or reactive species within the system, leading to a decrease in defluorination efficiency. HCO3−, in their various solution states, can compete with HFPO-TA for the hydrated electron (eaq−) or engage directly with the photosensitizer, resulting in a hindrance to the defluorination capabilities of the system. Furthermore, it was identified that the defluorination efficacy of the UV/sulfite/iodide for HFPO-TA in Xiaoqing River was diminished relative to that in deionized water, primarily due to the presence of Cl− and HCO3−. Adsorption/desorption or nanofiltration pretreatment using highly selective materials such as COFs can effectively mitigate this problem.

Anti-Poiseuille flow by spin Hall effect.

Hydrodynamics is known to emerge in electron flow when the electron-electron interaction dominates over the other momentum-nonconserving scatterings. The hydrodynamic equation that describes the electric current includes viscosity, extending beyond the Ohmic flow. The laminar flow of such a viscous electron fluid in a sample with finite width is referred to as the Poiseuille flow, where the flow velocity is maximum at the center and decreases towards the edges of the sample. In this paper, we show a unique viscous electron fluid arising in electron systems exhibiting the spin Hall effect (spin Hall systems), where the charge and spin currents are coupled. Such a viscous electron fluid emerges even in noninteracting electron systems, and the current density exhibits a minimum at the center of a flow and a maximum at the edges, i.e. an anti-Poiseuille flow realizing. We also find that the spin accumulation by the spin Hall effect is connected to the electric current vorticity in two-dimensional (2D) spin Hall systems. Furthermore, we propose a novel guiding principle to manipulate topological magnetic textures from the hydrodynamic viewpoint. By solving the hydrodynamic equation in a 2D spin Hall system with a cavity and employing micromagnetic simulations for an attached chiral magnetic insulator, we demonstrate that spin accumulation near the cavity's boundary leads to creating a magnetic skyrmion. Our research illuminates new aspects of electron hydrodynamics and spintronics, contributing significant insights to the fields.

Infrared Spectroscopy for Diagnosing Superlattice Minibands in Twisted Bilayer Graphene near the Magic Angle.

Twisted bilayer graphene (TBG) represents a highly tunable, strongly correlated electron system. However, understanding the single-particle band structure alone has been challenging due to a lack of spectroscopic measurements over a broad energy range. Here, we probe the band structure of TBG around the magic angle using infrared spectroscopy and reveal spectral features that originate from interband transitions. In combination with quantum transport, we connect spectral features over a broad energy range (10-700 meV) and track their evolution with the twist angle. We compare our data with calculations of the band structures obtained via the continuum model and find good agreement only when considering a variation of interlayer/intralayer tunneling parameters with the twist angle. Our analysis suggests that the magic angle also shifts due to lattice relaxation and is better defined for a wide angular range of 0.9-1.1°. Additionally, our measurements offer an optical fingerprint of the magic angle for screening heterostructures before nanofabrication.

Unconventional quantum oscillations and evidence of nonparabolic electronic states in quasi-two-dimensional electron system at complex oxide interfaces

The simultaneous occurrence of electric-field controlled superconductivity and spin-orbit interaction makes two-dimensional electron systems (2DES) constructed from perovskite transition metal oxides promising candidates for the next generation of spintronics and quantum computing. It is, however, essential to understand the electronic bands thoroughly and verify the predicted electronic states experimentally in these 2DES to advance technological applications. Here, we present insights into the electronic states of the 2DES at oxide interfaces through comprehensive investigations of Shubnikov–de Haas oscillations in three different systems: EuO/KTaO3, LaAlO3/SrTiO3, and amorphous-LaAlO3/KTaO3. To accurately resolve these oscillations, we conducted transport measurements in high magnetic fields up to 60 T and low temperatures down to 100 mK. For 2D confined electrons at these interfaces, we observed a progressive increase of oscillations frequency and cyclotron mass with the magnetic field. We interpret these universal and intriguing findings by considering the existence of nontrivial electronic bands, for which the E−k dispersion incorporates both linear and parabolic relations. In addition to providing experimental evidence for nonparabolic electronic states in KTaO3 and SrTiO3 2DES, the unconventional oscillations presented in this study establish a paradigm for quantum oscillations in 2DES based on perovskite transition metal oxides, where the oscillation frequencies in 1/B exhibit quadratic dependence on the magnetic field. Published by the American Physical Society 2024

Origin of the Suppression of Magnetic Order in MnSi under Hydrostatic Pressure.

We experimentally study the evolution of the magnetic moment m and exchange interaction J as a function of hydrostatic pressure in the zero-field helimagnetic phase of the strongly correlated electron system MnSi. The suppression of magnetic order at ≈1.5 GPa is shown to arise from the J collapse and not from a quantum fluctuations induced reduction of m. Our work provides benchmarks for first principles theories that are challenged by the presence of strong correlations and the possible role of Hund's coupling. In addition, our experimental data are consistent with a reorientation of the magnetic propagation wave vector recently evidenced above ≈1.2 GPa. This result calls for a thorough investigation of the crystal structure in this pressure range.

Interplay between the charge density wave phase and a pseudogap under antiferromagnetic correlations

In this study, we explore the impact of short-range antiferromagnetic correlations on the charge density wave (CDW) phase in strongly correlated electron systems exhibiting the pseudogap phenomenon. Our investigation employs an n-pole approximation to consider the repulsive Coulomb interaction (U) and antiferromagnetic correlations. Considering an one-band Hubbard model to account for the Coulomb interaction and a BCS-like model for the CDW order parameter, we observed that an increase in U enhances antiferromagnetic fluctuations, resulting in a flattened re-normalized band around the antinodal point (π,0). The pseudogap manifests itself in the band structure and density of states, prompting exploration of various U and occupation number values. Our findings indicate that antiferromagnetic correlations significantly influence the CDW state, as the Fermi surface is reconstructed within the ordered phase. Furthermore, we found a Lifshitz transition inside both the CDW phase and the normal state, with the latter preceding the onset of the pseudogap.

UOTe: Kondo-Interacting Topological Antiferromagnet in a Van der Waals Lattice.

Since the initial discovery of 2D van der Waals (vdW) materials, significant effort has been made to incorporate the three properties of magnetism, band structure topology, and strong electron correlations-to leverage emergent quantum phenomena and expand their potential applications. However, the discovery of a single vdW material that intrinsically hosts all three ingredients has remained an outstanding challenge. Here, the discovery of a Kondo-interacting topological antiferromagnet is reported in the vdW 5f electron system UOTe. It has a high antiferromagnetic (AFM) transition temperature of 150K, with a unique AFM configuration that breaks the combined parity and time reversal (PT) symmetry in an even number of layers while maintaining zero net magnetic moment. This angle-resolved photoemission spectroscopy (ARPES) measurements reveal Dirac bands near the Fermi level, which combined with the theoretical calculations demonstrate UOTe as an AFM Dirac semimetal. Within the AFM order, the presence of the Kondo interaction is observed, as evidenced by the emergence of a 5f flat band near the Fermi level below 100 K and hybridization between the Kondo band and the Dirac band. The density functional theory calculations in its bilayer form predict UOTe as a rare example of a fully-compensated AFM Cherninsulator.

Irradiated Coherent States in Magnetoresistance of 2D Electron Systems

Herein, a novel theoretical approach on the microwave‐induced resistance oscillations based on the coherent states of the quantum harmonic oscillator is reported. Within the theoretical model of microwave‐induced electron orbits, the coherent state expression for the quantum harmonic oscillator under a time‐dependent force (radiation) is calculated. Thus, it is found that the principle of minimum uncertainty of coherent states, involving time and energy, is the main responsible of photo‐oscillations and zero‐resistance states. Thus, essential experimental results of this remarkable effect are shed light (and explained) on. The origin of photo‐oscillations, their position with respect to the magnetic field, and their periodicity can be distinguished.

Similarity between oxygen evolution in photosystem II and oxygen reduction in cytochrome c oxidase via proton coupled electron transfers. A unified view of the oxygenic life from four electron oxidation–reduction reactions

Basic concepts and theoretical foundations of broken symmetry (BS) and post BS methods for strongly correlated electron systems (SCES) such as electron-transfer (ET) diradical, multi-center polyradicals with spin frustration are described systematically to elucidate structures, bonding and reactivity of the high-valent transition metal oxo bonds in metalloenzymes: photosystem II (PSII) and cytochrome c oxidase (CcO). BS hybrid DFT (HDFT) and DLPNO coupled-cluster (CC) SD(T0) computations are performed to elucidate electronic and spin states of CaMn4Ox cluster in the key step for oxygen evolution, namely S4 [S3 with Mn(IV) = O + Tyr161-O radical] state of PSII and PM [Fe(IV) = O + HO-Cu(II) + Tyr161-O radical] step for oxygen reduction in CcO. The cycle of water oxidation catalyzed by the CaMn4Ox cluster in PSII and the cycle of oxygen reduction catalyzed by the CuA-Fea-Fea3-CuB cluster in CcO are examined on the theoretical grounds, elucidating similar concerted and/or stepwise proton transfer coupled electron transfer (PT-ET) processes for the four-electron oxidation in PSII and four-electron reduction in CcO. Interplay between theory and experiments have revealed that three electrons in the metal sites and one electron in tyrosine radical site are characteristic for PT-ET in these biological redox reaction systems, indicating no necessity of harmful Mn(V) = O and Fe(V) = O bonds with strong oxyl-radical character. Implications of the computational results are discussed in relation to design of artificial systems consisted of earth abundant transition metals for water oxidation.Graphical abstract

Electron confinement-induced plasmonic breakdown in metals.

Plasmon resonance represents the collective oscillation of free electron gas density and enables enhanced light-matter interactions in nanoscale dimensions. Traditionally, the classical Drude model describes plasmonic excitation, wherein plasma frequency exhibits no spatial dispersion. Here, we show conclusive experimental evidence of the breakdown of plasmon resonance and a consequent metal-insulator transition in an ultrathin refractory plasmonic material, hafnium nitride (HfN). Epitaxial HfN thick films exhibit a low-loss and high-quality Drude-like plasmon resonance in the visible spectral range. However, as the film thickness is reduced to nanoscale dimensions, Coulomb interaction among electrons increases because of electron confinement, leading to the spatial dispersion of plasma frequency. With a further decrease in thickness, electrons lose their ability to shield the incident electric field, turning the medium into a dielectric. The observed metal-insulator transition might carry some signatures of Wigner crystallization and indicates that such transdimensional, between 2D and 3D, films can serve as a promising playground to study strongly correlated electron systems.

Quantum Hall effect in a CVD-grown oxide

Two-dimensional (2D) electron systems are promising for investigating correlated quantum phenomena. In particular, 2D oxides provide a platform that can host various quantum phases such as quantized Hall effect, superconductivity, or magnetism. The realization of such quantum phases in 2D oxides heavily relies on dedicated heterostructure growths. Here we show the integer quantum Hall effect achieved in chemical vapor deposition grown Bi2O2Se - a representative member of a more accessible oxide family. A single or few subband 2D electron system can be prepared in thin films of Bi2O2Se, where the film thickness acts as the key subband design parameter and the occupation is determined by the electric field effect. This oxide platform exhibits characteristic advantages in structural flexibility due to its layered nature, making it suitable for scalable growth. The unique small mass distinguishes Bi2O2Se from other high-mobility oxides, providing a new platform for exploring quantum Hall physics in 2D oxides.

A Quantitative Arrott Analysis Methodology for Magnetic Susceptibility of Microscale Ferromagnetic Nanoflakes.

Probing magnetic susceptibility of a microsized ferromagnet is a long-standing problem in condensed matter physics. Among various measuring methods for magnetic susceptibility including vibrating sample magnetometry and superconducting quantum interference device magnetometry, almost all require large-scale bulk samples or thick films. However, the quantitative measurement for magnetic susceptibility on a microscale nanoflake is a great challenge. Here, we demonstrate a new analysis method to quantitatively evaluate the magnetic susceptibility of a microscale ferromagnetic nanoflake. Based on the Arrott plot of magnetization isotherms obtained from anomalous Hall resistance, we achieve an in situ evaluation of the value of magnetic susceptibility of a microscale ferromagnetic Fe5GeTe2 nanoflake, identification of the out-of-plane and in-plane magnetization, and investigation of the magnetic anisotropy transition with quantifying critical exponents. Our method reveals critical information on magnetic phase transition in microscale ferromagnetic materials, providing deep insight into spin dynamics of correlated electron systems.