Correlated Electron Systems Research Articles

Inhomogeneous Photosusceptibility of VO_{2} Films at the Nanoscale.

Pump-probe nano-optical experiments were used to study the light-induced insulator to metal transition (IMT) in thin films of vanadium dioxide (VO_{2}), a prototypical correlated electron system. We show that inhomogeneous optical contrast is prompted by spatially uniform photoexcitation, indicating an inhomogeneous photosusceptibility of VO_{2}. We locally characterize temperature and time dependent variations of the photoexcitation threshold necessary to induce the IMT on picosecond timescales with hundred nanometer spatial resolution. We separately measure the critical temperature T_{L}, where the IMT onsets and the local transient electronic nano-optical contrast at the nanoscale. Our data reveal variations in the photosusceptibility of VO_{2} within nanoscopic regions characterized by the same critical temperature T_{L} where metallic domains can first nucleate.

4f electron temperature driven ultrafast electron localization

Valence transitions in strongly correlated electron systems are caused by orbital hybridization and Coulomb interactions between localized and delocalized electrons. The transition can be triggered by changes in the electronic structure and is sensitive to temperature variations, applications of magnetic fields, and physical or chemical pressure. Launching the transition by photoelectric fields can directly excite the electronic states and thus provides an ideal platform to study the correlation among electrons on ultrafast timescales. The EuNi2(Si0.21Ge0.79)2 mixed-valence metal is an ideal material to investigate the valence transition of the Eu ions via the amplified orbital hybridization by the photoelectric field on subpicosecond timescales. A direct view on the 4f electron occupancy of the Eu ions is required to understand the microscopic origin of the transition. Here we probe the 4f electron states of EuNi2(Si0.21Ge0.79)2 at the sub-ps timescale after photoexcitation by x-ray absorption spectroscopy across the Eu M5-absorption edge. The observed spectral changes due to the excitation indicate a population change of total angular momentum multiplet states J = 0, 1, 2, and 3 of Eu3+ and the Eu2+ J = 7/2 multiplet state caused by an increase in 4f electron temperature that results in a 4f localization process. This electronic temperature increases combined with fluence-dependent screening accounts for the strongly nonlinear effective valence change. The data allow us to extract a time-dependent determination of an effective temperature of the 4f shell, which is also of great relevance in the understanding of metallic systems' properties, such as the ultrafast demagnetization of ferromagnetic rare-earth intermetallic and their all-optical magnetization switching. In addition, our results elucidate the energetics of charge fluctuations in valence-mixed electronic systems, which provide enriched knowledge regarding the role of valence transitions and orbital hybridization for quantum critical phenomena. Published by the American Physical Society 2024

Strongly correlated electron system NiWO4: A new family of materials for triboelectrics using inherent Coulombic repulsion

The strongly correlated electron system (SCES) insulators promise a wide range of intriguing physical phenomena arising from the exotic charge gap depending on the strength of the Coulombic repulsion between d and f orbital cations. The SCES exhibits polaron hopping conduction, which requires a higher activation energy for carrier conduction between nearest cations, indicating a strong insulating character with relatively high permittivity. Here, we demonstrate that the NiWO4 SCES insulator, which is formed by Coulombic repulsion between the nearest d orbital of Ni, proposes a suitable triboelectric material unlisted system from conventional metal oxides. The higher degree of freedom in NiWO4 by the lower symmetry of monoclinic structure and an additional selection of composition exhibits high relative permittivity (∼13.8) in a wide range of temperatures (up to 180 ℃). The optimized simple mixture of NiWO4 and PDMS (polydimethylsiloxane) for the negatively charged material to triboelectric nanogenerator (TENG) presents 1.78 times higher output compared to the pristine PDMS film-based TENG. NiWO4, considering the physical properties of the SCES determined by Coulombic repulsion strength, paves the way for a new option of TENG materials following the high permittivity.

Unraveling the origin of the Griffiths phase in Ho2NiMnO6: perspectives from linear and nonlinear AC susceptibility

In this study, we investigated the magnetic ordering and underlying mechanism of the Griffiths phase, observed in Ho2NiMnO6 through AC susceptibility measurements. Our results indicate that the transition around 86 K corresponds to a paramagnetic to ferromagnetic transition characterized by classical magnetic ordering. Notably, nonlinear AC susceptibility measurements revealed the existence of ferromagnetic clusters within a paramagnetic background well above the transition temperature, establishing this as the origin of the Griffiths-like phase within the Ni/Mn sublattice of Ho2NiMnO6. Our study on the Ho2NiMnO6 system provides insight into the intricate magnetic phenomena common to various other strongly correlated electron systems.

The Kondo scaling for spin thermocurrent in strongly correlated electron systems

One of the most important feature in the Kondo physics is the universal scaling behavior. In this study, we analyze the transport behavior of the spin thermocurrent driven by a small temperature bias and under a weak magnetic field. We conclude that the spin thermocurrent exhibits a universal scaling behavior, similar to the spin susceptibility. The validity of our conclusion is also checked by using the numerically exact hierarchical equations of motion approach. From our fitting result of the scaling function, it is found that the Kondo temperature can be determined by the maximum of the spin thermocurrent at T/TK=0.383 .

Deep learning bulk spacetime from boundary optical conductivity

We employ a deep learning method to deduce the bulk spacetime from boundary optical conductivity. We apply the neural ordinary differential equation technique, tailored for continuous functions such as the metric, to the typical class of holographic condensed matter models featuring broken translations: linear-axion models. We successfully extract the bulk metric from the boundary holographic optical conductivity. Furthermore, as an example for real material, we use experimental optical conductivity of UPd2Al3, a representative of heavy fermion metals in strongly correlated electron systems, and construct the corresponding bulk metric. To our knowledge, our work is the first illustration of deep learning bulk spacetime from boundary holographic or experimental conductivity data.

Chemically Tailored Semiconductor Moiré Superlattices of Janus Heterobilayers

Janus monolayers of transition metal dichalcogenides (TMDCs) are promising building blocks for moiré superlattices because of their built‐in electric field and clean fabrication process. In particular, Janus TMDC monolayers can be chemically converted from conventional TMDC monolayers by atomic substitution, enabling the direct formation of lattice‐mismatched heterobilayers from TMDC bilayers. However, the moiré superlattices of Janus heterobilayers have not been studied experimentally. Herein, this work reports the fabrication and characterization of semiconductor moiré superlattices in chemically tailored Janus heterobilayers. The MoSSe/MoSe2 (or WSSe/WSe2) Janus heterobilayers are prepared by replacing the top layer Se atoms with S atoms in MoSe2 (or WSe2) bilayer using H2 plasma treatment. Scanning transmission electron microscopy reveals that an average moiré period of about 14 nm formed due to lattice mismatch resulting from the chalcogen substitutions. The cryogenic photoluminescence spectra show sharp, near‐infrared emissions, which are attributed to excitons trapped by moiré potentials based on comparison with theoretical calculations. The Janus‐based heterostructures provide a long‐period moiré system with built‐in potential even for nontwisted heterobilayers, allowing the functionalization of confined and correlated electron systems.

Non-perturbative intertwining between spin and charge correlations: A ``smoking gun'' single-boson-exchange result

We study the microscopic mechanism controlling the interplay between local charge and local spin fluctuations in correlated electron systems via a thorough investigation of the generalized on-site charge susceptibility of several fundamental many-electron models, such as the Hubbard atom, the Anderson impurity model, and the Hubbard model. By decomposing the numerically determined generalized susceptibility in terms of physically transparent single-boson exchange processes, we unveil the microscopic mechanisms responsible for the breakdown of the self-consistent many-electron perturbation expansion. In particular, we unambiguously identify the origin of the significant suppression of its diagonal entries in (Matsubara) frequency space and the slight increase of the off-diagonal ones which cause the breakdown. The suppression effect on the diagonal elements originates directly from the electronic scattering on local magnetic moments, reflecting their increasingly longer lifetime as well as their enhanced effective coupling with the electrons. Instead, the slight and diffuse enhancement of the off-diagonal terms can be mostly ascribed to multiboson scattering processes. The strong intertwining between spin and charge sectors is partly weakened at the Kondo temperature due to a progressive reduction of the effective spin-fermion coupling of local magnetic fluctuations in the low frequency regime. Our analysis, thus, clarifies the precise mechanism through which the physical information is transferred between different scattering channels of interacting electron problems and highlights the pivotal role played by such an intertwining in the physics of correlated electrons beyond the perturbative regime.

Rigorous Screened Interactions for Realistic Correlated Electron Systems.

We derive a widely applicable first-principles approach for determining two-body, static effective interactions for low-energy Hamiltonians with quantitative accuracy. The algebraic construction rigorously conserves all instantaneous two-point correlation functions in a chosen model space at the level of the random phase approximation, improving upon the traditional uncontrolled static approximations. Applied to screened interactions within a quantum embedding framework, we demonstrate these faithfully describe the relaxation of local subspaces via downfolding high-energy physics in molecular systems, as well as enabling a systematically improvable description of the long-range plasmonic contributions in extended graphene.

Subnanometer Scale Mapping of Hydrogen Doping in Vanadium Dioxide.

Hydrogen donor doping of correlated electron systems such as vanadium dioxide (VO2) profoundly modifies the ground state properties. The electrical behavior of HxVO2 is strongly dependent on the hydrogen concentration; hence, atomic scale control of the doping process is necessary. It is however a nontrivial problem to quantitatively probe the hydrogen distribution in a solid matrix. As hydrogen transfers its sole electron to the material, the ionization mechanism is suppressed. In this study, a methodology mapping the doping distribution at subnanometer length scale is demonstrated across a HxVO2 thin film focusing on the oxygen-hydrogen bonds using electron energy loss spectroscopy (EELS) coupled with first-principles EELS calculations. The hydrogen distribution was revealed to be nonuniform along the growth direction and between different VO2 grains, calling for intricate hydrogenation mechanisms. Our study points to a powerful approach to quantitatively map dopant distribution in quantum materials relevant to energy and information sciences.

Hefei Advanced Light Facility: Empowering research of correlated electron systems

The Hefei Advanced Light Facility is the fourth-generation diffraction-limited storage ring light source, scheduled to begin operation in 2028. With its high-brightness and highly coherent X-rays, it will break through the current spatiotemporal resolution bottlenecks of X-ray techniques in studying correlated electron systems, providing crucial information for understanding the nature and microscopic origins of novel physical properties in these materials. This article introduces the main scientific goals and technical advantages of the Hefei Advanced Light Facility, focusing on the application perspectives of advanced technologies such as angle-resolved photoemission spectroscopy, magnetic circular dichroism, coherent X-ray scattering, and coherent X-ray imaging in researches of quantum materials and correlated electron systems. These techniques will enable the detailed analysis of the distribution and dynamics of electronic/spin/orbital states, reveal various novel quantum phenomena, and elucidate the fluctuations of order parameters in correlated electron systems. The completion of the Hefei Advanced Light Facility will provide advanced technical supports for decoding complex quantum states and non-equilibrium properties, ultimately promoting the application of quantum materials and correlated electron systems in frontier fields such as energy and information.

Re/Ir@Os-doping induced insulator-to-metal transition in Mott-insulator Ca2FeOsO6: octahedral distortion effects.

Regardless of several investigations to elucidate the ground state of the strongly correlated electron systems in doped Mott-insulators (MIs), the origin of the doping-induced insulator-to-metal transition (IMT) remains a crucial and debatable subject in solid-state physics. Herein, we explore the consequences of Re/Ir-doping at the Os-site (Re/Ir@Os), on the physical properties of the MI ferrimagnetic (FiM) Ca2FeOsO6 double perovskite oxides using density functional theory calculations. The doped structures' solidity is analyzed by computing the defect formation energies in terms of the dopant-rich situation, which confirms their growth credibility at ambient conditions along with mechanical and dynamical stabilities. Various FiM spin-ordering is taken into account in the doped structures to analyze the magnetic ground state, which is FiM-I/FiM-II in the Re/Ir@Os-doped system. Remarkably, an IMT is predicted in the Re/Ir@Os-doped structures, which is due to the admixture of the partially occupied 5d orbitals of these ions. The calculated partial spin magnetic moments (ms) of +4.12, -1.58, -0.75 and +0.88μB on the Fe, Os, Re, and Ir ions, endorse the +3, +5, +5, and +4 states having electronic configurations of t32g↑t02g↓e2g↑e0g↓, t32g↑t02g↓e0g↑e0g↓, t22g↑t02g↓e0g↑e0g↓, t32g↑t22g↓e0g↑e0g↓, respectively. The "+" and "-" signs on the ions ms values, lead the systems into various FiM magnetic ordering. Moreover, the estimated Curie temperature (TC) using the Heisenberg model in the pristine structure is 334 K, which is close to the experimentally observed value of 320 K along with a colossal uniaxial magneto crystalline anisotropy energy constant (K) of 2.95 × 107 erg cm-3 having the easy magnetic axis of the ac-plane ([101]). It is established that TC/K reduces and enhances to 298 K/1.33 × 107 erg cm-3 and 365 K/4.71 × 107 erg cm-3 for the Re@Os and Ir@Os-doped motif due to an increase and decrease in the octahedral distortions compared to that of the pristine system, respectively.

Reformation of La0.7Sr0.3MnO3 properties by using ZnO in La0.7Sr0.3MnO3-ZnO heterostructures grown on (001) oriented Si.

Study of the available density of states (DOS) close-to-zero bias for conduction in strongly correlated electron systems, such as half-metallic La0.7Sr0.3MnO3 (LSMO) and its heterostructures, is important for fundamental and application reasons. As the DOS is proportional to the differential conductance (dI/dV), the dI/dV of a 120 Å LSMO film and its reformation in LSMO/ZnO heterostructures was investigated for different ZnO thicknesses. Unlike in conventional metals, the dI/dV of LSMO exhibits a power-law dependent zero-bias anomaly, i.e., dI/dV ∝ Vm (m ∼ 1) near zero bias in the ferromagnetic metallic state at 10 K. The growth of ZnO on LSMO reforms the linear dI/dVvs. V of LSMO near zero bias to non-linear. The exponent 'm' becomes ∼0.5 for a higher ZnO thickness, revealing increased electron-electron interactions and suppression of Kondo-like, double and superexchange interactions, which are responsible for the depression of the DOS of LSMO near zero bias. In a magnetically disordered state, i.e., around the Curie temperature, ZnO reforms the linear V-shaped dI/dV vs. V of LSMO to parabolic U-shaped dI/dVvs.V and controls the electron concentrations in the t2g-orbitals of Mn realized from the DOS simulations. Additionally, ZnO introduces a peak in the dI/dV vs. V due to Fowler-Nordheim tunnelling, and the peak voltage can be tuned by varying the ZnO thickness or temperature from 300 K to 360 K. Such functions of ZnO yield major perspectives for novel applications in thin-film-based devices.

The Kondo Effect in CexLaLuScY (x = 0.05-1.0) High-Entropy Alloys.

In the search for electronic phenomena in high-entropy alloys (HEAs) that go beyond the independent-electron description, we have synthesized a series of hexagonal rare earth (RE)-based HEAs: CexLaLuScY (x = 0.05-1.0). The measurements of electrical resistivity, magnetic susceptibility and specific heat have shown that the CexLaLuScY HEAs exhibit the Kondo effect, which is of a single impurity type in the entire range of employed Ce concentrations despite the alloys being classified as dense (concentrated) Kondo systems. A comparison to other known dense Kondo systems has revealed that the Kondo effect in the CexLaLuScY HEAs behaves quite differently from the chemically ordered Kondo lattices but quite similar to the RE-containing magnetic metallic glasses and randomly chemically disordered Kondo lattices of the chemical formula RE1xRE21-xM (with RE1 being magnetic and RE2 being nonmagnetic). The main reason for the similarity between HEAs and the metallic glasses and chemically disordered Kondo lattices appears to be the absence of a periodic 4f sublattice in these systems, which prevents the formation of a coherent state between the 4f-scattering sites in the T→ 0 limit. The crystal-glass duality of HEAs does not bring conceptually new features to the Kondo effect that would not be already present in other disordered dense Kondo systems. This study broadens the classification of HEAs to correlated electron systems.

Dichotomy of heavy and light pairs of holes in the t−J model

A key step in unraveling the mysteries of materials exhibiting unconventional superconductivity is to understand the underlying pairing mechanism. While it is widely agreed upon that the pairing glue in many of these systems originates from antiferromagnetic spin correlations, a microscopic description of pairs of charge carriers remains lacking. Here we use state-of-the art numerical methods to probe the internal structure and dynamical properties of pairs of charge carriers in quantum antiferromagnets in four-legged cylinders. Exploiting the full momentum resolution in our simulations, we are able to distinguish two qualitatively different types of bound states: a highly mobile, meta-stable pair, which has a dispersion proportional to the hole hopping t, and a heavy pair, which can only move due to spin exchange processes and turns into a flat band in the Ising limit of the model. Understanding the pairing mechanism can on the one hand pave the way to boosting binding energies in related models, and on the other hand enable insights into the intricate competition of various phases of matter in strongly correlated electron systems.

Vanadium in strongly correlated electron system Ni1-xVxWO4: Paradoxically boosted deNOx reaction under SOx environment via modulating electron correlation

The persistent challenge of NOx selective catalytic reduction degradation in the presence of SOx has remained unresolved for decades. In this study, we report a paradoxical boosted denitration performance more than 20% under the SOx environment through controlling the charge state of V utilizing strongly correlated electron systems (SCES) in Ni1−xVxWO4. Substitutional doped V at the Ni site, and the computed charge gap from DFT confirms the SCES. Through the manipulation of the strong correlation between V and Ni, V preserves the between + 2–3 charge state in the presence of SOx. Furthermore, we achieved a stable oxidation/reduction cycle and higher denitration efficiency by altering the oxidation point of (NH4)HSO4-contaminated Ni1−xVxWO4. This results from deliberate decrease in Coulombic repulsion between Ni and V, improving electron transfer for catalytic performance. This revelation offers a resolution of exceptional denitration performance within SOx in industrial exhaust systems, exploiting the foundational principles of SCES.

Observation of an exotic insulator to insulator transition upon electron doping the Mott insulator CeMnAsO

A promising route to discover exotic electronic states in correlated electron systems is to vary the hole or electron doping away from a Mott insulating state. Important examples include quantum criticality and high-temperature superconductivity in cuprates. Here, we report the surprising discovery of a quantum insulating state upon electron doping the Mott insulator CeMnAsO, which emerges below a distinct critical transition temperature, TII. The insulator-insulator transition is accompanied by a significant reduction in electron mobility as well as a colossal Seebeck effect and slow dynamics due to decoupling of the electrons from the lattice phonons. The origin of the transition is tentatively interpreted in terms of many-body localization, which has not been observed previously in a solid-state material.

Evidence for nanosized magnetic clusters of Ce-ions in the archetypal heavy fermion metal CeB6

Fine details of crystal structure of archetypal CeB6 hexaboride with heavy fermions are studied at temperatures 85 and 500 K by precise X-ray diffraction technique. Small static Jahn-Teller distortions of a simple cubic lattice are observed at these temperatures, leading to emergence of (i) dynamic charge stripes along selected directions <110>, <100>, and <111> in the crystals in combination with (ii) vibrationally coupled pairs of Ce ions. Instead of a Currie-Weiss type behavior, the temperature dependence of magnetization M∼(T − TCrand)−0.8 with TCrand∼TQ∼3.3 K was deduced in a wide temperature range 5–800 K for various directions of external magnetic field, which indicates the Griffiths phase formation with nanosized clusters of magnetic ions in CeB6. Moreover, in contrast to the scenario of a single-ion Kondo lattice, when revealing the power-law behavior of the magnetic contribution ρm(T)∼T−0.4 to the resistivity in the range of 8–90 K, we conclude in favor of the regime of weak localization of charge carriers in CeB6. Fourier maps of electron density confirm the conclusion about the nanosized magnetic clusters of Ce ions in this archetypal strongly correlated electron system with unusual magnetic ground state.

A time-dependent momentum-resolved scattering approach to core-level spectroscopies

While new light sources allow for unprecedented resolution in experiments with X-rays, a theoretical understanding of the scattering cross-section is lacking. In the particular case of strongly correlated electron systems, numerical techniques are quite limited, since conventional approaches rely on calculating a response function (Kramers-Heisenberg formula) that is obtained from a perturbative analysis of scattering processes in the frequency domain. This requires a knowledge of a full set of eigenstates in order to account for all intermediate processes away from equilibrium, limiting the applicability to small tractable systems. In this work, we present an alternative paradigm, recasting the problem in the time domain and explicitly solving the time-dependent Schrödinger equation without the limitations of perturbation theory: a faithful simulation of the scattering processes taking place in actual experiments, including photons and core electrons. We show how this approach can yield the full time and momentum resolved Resonant Inelastic X-Ray Scattering (RIXS) spectrum of strongly interacting many-body systems. We demonstrate the formalism with an application to Mott insulating Hubbard chains using the time-dependent density matrix renormalization group method, which does not require a priory knowledge of the eigenstates and can solve very large systems with dozens of orbitals. This approach can readily be applied to systems out of equilibrium without modification and generalized to other spectroscopies.

Quasiparticle picture for high-harmonic generation in correlated electron systems

Quasiparticle picture for high-harmonic generation in correlated electron systems