Electron-electron Correlation Research Articles

Unveiling nodeless unconventional superconductivity proximate to honeycomb-vacancy ordering in the Ir-Sb binary system

Vacancies in solid-state physics are underexplored in materials with strong electron-electron correlations. Recent research on the Ir-Sb binary system revealed an extended buckled-honeycomb vacancy (BHV) order. Superconductivity arises by suppressing BHV ordering through high-pressure growth with excess Ir atoms or Rh substitution, yet the superconducting pairing nature remains unknown. To explore this, we conducted muon spin rotation experiments on Ir1−δSb (synthesized at 5.5 GPa, Tc = 4.2 K) and ambient pressure synthesized optimally Rh-doped Ir1−xRhxSb (x=0.3, Tc = 2.7 K). The exponential temperature dependence of the superfluid density suggests a fully gapped superconducting state exists in both samples. The ratio of Tc to the superfluid density resembles that of unconventional superconductors. A significant increase in the superfluid density in the high-pressure synthesized sample correlates with Tc, indicating that unconventional superconductivity is intrinsic to the Ir-Sb binary system. These findings, along with the dome-shaped phase diagram, highlight IrSb as the first unconventional superconducting parent phase with ordered vacancies, requiring further theoretical investigations.

Microenvironment Control over Electrocatalytic Activity of g-C3N4/2H-MoS2 Superlattice-like Heterostructures for Hydrogen Evolution.

Microenvironments in heterogeneous catalysis have been recognized as equally important as the types and amounts of active sites for regulating catalytic activity. Two-dimensional (2D) nanospaces between van der Waals (vdW) gaps of layered materials provide an ideal microenvironment to create novel functionalities. Here, we explore a facile method for fabricating g-C3N4/2H-MoS2 superlattice-like heterostructures based on thermochemical intercalation and polymerization reactions of formamide within enlarged vdW gaps of 2H-MoS2 nanosheets without any transfer process. DFT calculations demonstrate that the interlayer electron-electron correlations due to the intercalation effect of g-C3N4, rather than high-κ dielectric environments, lead to the improvement of intrinsic conductivity of 2H-MoS2 nanosheets. As the proof of concept in applications for the electrocatalysis field, the heterostructure for hydrogen electrochemical reaction (HER) exhibits high stability and catalytic activity in both acid and alkaline media, such as a quite low onset overpotential of 98 mV, a high exchange current density of 77.6 μA cm-2, and a small Tafel slope (52.9 mV dec-1) in an acid medium. The enhanced HER activity is attributed to the improved conductivity and nanoconfinement effect of 2D nanospaces that decrease the reaction activation energy and activate the inert basal planes.

Realizing one-dimensional moiré chains with strong electron localization in two-dimensional twisted bilayer WSe2

Two-dimensional (2D) moiré systems based on twisted bilayer graphene and transition metal dichalcogenides provide a promising platform to investigate emergent phenomena driven by strong electron-electron interactions in partially filled flat bands. A natural question arises: Is it possible to expand the 2D correlated moiré physics to one-dimensional (1D) that electron-electron correlation is expected to be further enhanced? This requires selectively doping of 1D moiré chain, which seems to be not within the grasp of today's technology. Therefore, an experimental demonstration of the 1D moiré chain with partially filled electronic states remains absent. Here, we show that we can introduce 1D boundaries, separating two regions with different twist angles, in twisted bilayer WSe2 (tWSe2) by using scanning tunneling microscopy (STM) and demonstrate that the electronic states of 1D moiré sites along the boundaries can be selectively filled. The strong localized charge states of correlated moiré electrons in the 1D moiré chain can be directly imaged and manipulated by combining a back-gate voltage with the STM bias voltage. Our results open the door for realizing new correlated electronic states of the 1D moiré chain in 2D systems.

Elucidating correlation-driven insulating state in an effective spin—½ antiferromagnet NdVO4

TheJeff= ½ state: a result of interplay of strong electronic correlations (U) with spin-orbit coupling (SOC) and crystal field splitting, offers a platform in the research of quantum materials. In this context, 4frare-earth based materials offer a fertile playground. Here, strong experimental and theoretical evidences for aJeff= ½ state is established in a three-dimensional spin system NdVO4. Magnetic measurements show the signatures of a SOC drivenJeff= ½ state along with the presence of antiferromagnetic (AFM) interaction between Nd3+moments, whereas, heat capacity reveals the presence of an AFM ordering around 0.8 K, within this state. An entropy of Rln2 (equivalent toJ= ½) is released around 4 K which implies the presence ofJeff= ½ state at low temperatures. Total energy calculations within the density functional theory (DFT) framework reflect the central role of SOC in driving the Nd3+ions to host such a state with AFM correlations between them, which is in agreement with experimental results. Further, DFT + SOC calculations with and without the inclusion ofU, points that electron-electron correlations give rise to the insulating state making NdVO4a potential candidate forU-driven correlated Mott insulator.

Theoretical Studies of the Electronic and Thermoelectric Properties of PrIn3 and NdIn3 in Cubic Phase

The electronic and thermoelectric properties of AB3 (A =Pr, Nd and B=In) materials (crystallizing in the cubic structure) having space group Pm-3m (221) are studied using B3PW91 hybrid functional through the Full-Potential Linear Augmented Plane Wave (FP-LAPW) approach within the framework of Density Functional Theory (DFT). The calculated lattice parameters for PrIn3 and NdIn3 are 4.5668 Å and 4.5539 Å respectively. Electron-electron correlation effect is due to the 4f orbitals present in these materials and therefore, with the use of B3PW91 hybrid functional, band structure and density of states are calculated. When analyzing electron charge density, these materials showed a stronger ionic character. Band structure and density of state analysis confirms the metallic nature of the materials. Using the semi-empirical Boltzmann approach implemented in the BoltzTraP code, the thermoelectric parameters, such as Seebeck coefficient, figure of merit, electrical conductivity per relaxation time, and electronic thermal conductivity per relaxation time as a function of chemical potential, were computed at 500 K temperature gradient. PrIn3 showed highest Seebeck coefficient value, 50.68 μV/K among these compounds. The peak value of electrical conductivity per relaxation time and electronic thermal conductivity per relaxation time among these compounds is calculated for NdIn3 is 2.43 x 1020 1/Ωms and 22.50×1014 W/mKs.

Many-Body Models for Chirality-Induced Spin Selectivity in Electron Transfer.

We present the first microscopic model for the chirality-induced spin selectivity effect in electron-transfer, in which the internal degrees of freedom of the chiral bridge are explicitly included. By exactly solving this model on short chiral chains we demonstrate that a sizable spin polarization on the acceptor arises from the interplay of coherent and incoherent dynamics, with strong electron-electron correlations yielding many-body states on the bridge as crucial ingredients. Moreover, we include the coherent and incoherent dynamics induced by interactions with vibrational modes and show that they can play an important role in determining the long-time polarized state probed in experiments.

Electronic States of Single Perovskite Quantum Dots in Weak and Strong Interaction Regimes: Implications in Electrically Pumped Quantum Emitters.

We investigate the effect of Coulomb interactions on the electronic states of a single perovskite quantum dot (PQD), CsPbBr3, through scanning tunneling microscopy/spectroscopy (STM/S). Under a weak interaction regime, where the time-averaged occupation of electrons in a PQD remains zero, the peaks observed in the differential tunneling conductance (dI/dV) spectrum correspond to the single-particle density of states (DOS) without any electron-electron correlation. However, with a shorter tunnel distance between the STM tip and PQD, additional electrons are trapped in the QD, leading to a strong interaction regime with well-defined electronic fine structures due to the lifting of spin degeneracy in the conduction bands. Interestingly, we observe that the strong Coulomb interaction can modify the spin-orbit coupling (SOC) strength in the PQDs. We have concluded that the energy levels under a strong electron-electron interaction regime are of utmost importance since they will be applicable to electrically pumped PQD-based single photon quantum emitters.

Low-loss plasmons in Weyl semimetals Mn3Sn

We report the optical reflectance measurement for Weyl semimetal Mn3Sn single crystal at room temperature. We observe a low-loss plasmon in the UV energy ranges. This low plasmonic loss is associated with a large electron scattering rate contributed by electron-electron interaction and scattering of conduction electrons by Mn atoms. The incorporation of more Mn atoms simultaneously adds more elastic scattering centers and increases the strength of electron-electron correlation which generates multiple plasmons with lower plasmonic loss in Mn3+xSn1-x. The UV tunable plasmonic properties in Mn3+xSn1-x have great potential for plasmonic and spintronic applications. We also discuss the plasmonic loss in Mn3Sn to the other conducting materials including correlated and non-correlated systems establishing the crucial role of electron correlation on the plasmonic loss. Our result suggests novel and effective strategies for obtaining lower plasmonic loss in the class of correlated materials.

Evolution from a charge-ordered insulator to a high-temperature superconductor in Bi2Sr2(Ca,Dy)Cu2O8+δ

How Cooper pairs form and condense has been the main challenge in the physics of copper-oxide high-temperature superconductors. Great efforts have been made in the ‘underdoped’ region of the phase diagram, through doping a Mott insulator or cooling a strange metal. However, there is still no consensus on how superconductivity emerges when electron-electron correlations dominate and the Fermi surface is missing. To address this issue, here we carry out high-resolution resonant inelastic X-ray scattering and scanning tunneling microscopy studies on prototype cuprates Bi2Sr2Ca0.6Dy0.4Cu2O8+δ near the onset of superconductivity, combining bulk and surface, momentum- and real-space information. We show that an incipient charge order exists in the antiferromagnetic regime down to 0.04 holes per CuO2 unit, entangled with a particle-hole asymmetric pseudogap. The charge order induces an intensity anomaly in the bond-buckling phonon branch, which exhibits an abrupt increase once the system enters the superconducting dome. Our results suggest that the Cooper pairs grow out of a charge-ordered insulating state, and then condense accompanied by an enhanced interplay between charge excitations and electron-phonon coupling.

Abundant electric-field tunable symmetry-broken states in twisted monolayer-bilayer graphene

Electron-electron correlations can lift the high degeneracies in strong correlated systems, resulting in various symmetry-broken states. Twisted monolayer-bilayer graphene (tMBG) is an especially rich system due to its low crystalline symmetry. Here, we report abundant electric-field tunable symmetry-broken states in tMBG. The ground state at half filling of the conduction flat band is spin- and valley-polarization dominated under positive and negative electric field, respectively, consistent with our theoretical calculations. In addition, we find a symmetry-broken Chern insulator emanating from 1.5 electrons per moiré unit with C = 3 emerges at high magnetic field in a negative electric field range. The C = 3 suggests that one and a half flavor-polarized Chern 2 bands within the same valley are filled, consistent with the valley-polarization-dominated half-filling state under negative electric field, while the fractional filling stems from a density-wave state held by enlarged unit cells containing two moiré units.

Antiferromagnetic phase transition in a 3D fermionic Hubbard model.

The fermionic Hubbard model (FHM)1 describes a wide range of physical phenomena resulting from strong electron-electron correlations, including conjectured mechanisms for unconventional superconductivity. Resolving its low-temperature physics is, however, challenging theoretically or numerically. Ultracold fermions in optical lattices2,3 provide a clean and well-controlled platform offering a path to simulate the FHM. Doping the antiferromagnetic ground state of a FHM simulator at half-filling is expected to yield various exotic phases, including stripe order4, pseudogap5, and d-wave superfluid6, offering valuable insights into high-temperature superconductivity7-9. Although the observation of antiferromagnetic correlations over short10 and extended distances11 has been obtained, the antiferromagnetic phase has yet to be realized as it requires sufficiently low temperatures in a large and uniform quantum simulator. Here we report the observation of the antiferromagnetic phase transition in a three-dimensional fermionic Hubbard system comprising lithium-6 atoms in a uniform optical lattice with approximately 800,000 sites. When the interaction strength, temperature and doping concentration are finely tuned to approach their respective critical values, a sharp increase in the spin structure factor is observed. These observations can be well described by a power-law divergence, with a critical exponent of 1.396 from the Heisenberg universality class12. At half-filling and with optimal interaction strength, the measured spin structure factor reaches 123(8), signifying the establishment of an antiferromagnetic phase. Our results provide opportunities for exploring the low-temperature phase diagram of the FHM.

Large-scale multiconfiguration Dirac-Hartree-Fock calculations of atomic data, Landé gj -factors and hyperfine constants of the neutral Rn-211 isotope

We computed the atomic data of ns[K]J ( n = 7, 8, 9), np[K]J ( n = 7, 8, 9), nd[K]J ( n = 6, 7, 8, 9), and nf[K]J ( n = 5, 6, 7, 8) and 5g[K]J autoionized Rydberg spectra of the neutral radon-211 isotope. The computations are carried out by using the Multiconfiguration Dirac-Hartree-Fock method (MCDHF) together with the relativistic configuration (RCI) approach. The convergence of the obtained results is monitored by incorporating relativistic electron-electron correlation, the Breit-interaction effect, self-energy, and vacuum-polarization corrections. As a result, we acquired the energy levels, transition rates, oscillator strengths, Landé gJ -factors and magnetic dipole hyperfine constants. The transition parameters between the levels belonging to different series are computed for all allowed electric dipole transition selection rules. The present results are in very good agreement with other experimental and theoretical data available in the literature database. The present data could be useful in predicting spectral lines of radon via astrophysics and stellar spectroscopy.

Second-order spectral line shift comparisons

The second-order spectral line width formulae from the projection operator and kinetic theory methods were recently compared. It was shown that a systematic expansion of the projection operator width expression including initial correlations formally agrees with the second-order kinetic theory result. It is now shown that the second-order dynamic shifts are also formally the same. The static shifts, however, differ due to an ad hoc treatment of electron-electron correlations in the projection operator method. The approximation is necessary in order to screen the radiator-electron interactions. The differences, however, are expected to be small. The results suggest using the rigorous and more compact second-order width and shift expressions from the kinetic theory method as the starting point for spectral line shape calculations. At line center, however, the projection operator second-order expression for the width and shift simplifies and reduces to the kinetic theory result.

Performance analysis of LaFeO3 perovskite solar cells: A theoretical and experimental study

Lanthanum ferrite (LaFeO3) is a potential candidate for Perovskite Solar Cell (PSC) application. The transition metal oxide has strong electron-electron correlation which results in an increase in quantum efficiency (QE) of PSC. In this investigation, we reported synthesis of LaFeO3 perovskite material, simulation of LaFeO3-based PSC followed by experimental validation. The sol-gel method was employed to prepare nanoparticles of LaFeO3 for PSC application. The prepared perovskite material is analyzed by XRD, SEM, EDS, UV–Vis and FTIR. The photovoltaic (PV) application of synthesized LaFeO3 material is studied by SCAPS-1D. The LaFeO3-based PSC is simulated with various ETLs such as TiO2, PCBM, IGZO, In2O3, C60, ZnO, SnO2, WS2 and HTLs of Cu2O, CuInSe2, Spiro-OMeTAD, P3HT, D-PBTTT-14, V2O5, CFTS, CBTS. The simulation outcomes of 64 distinct configurations deduced the eight-best performing PSCs. For these eight PSCs, impact of thickness of perovskite absorber layer, operating temperature, defect density, series and shunt resistance, work function of back contact, J-V characteristics and QE curve has been studied. We have achieved theoretical maximum efficiency of 13.03 % for the configuration of FTO/WS2/LaFeO3/CBTS/Au. The optimized structure has experimentally obtained maximum efficiency of 0.79 %. This shows that it is promising material for future PV application.

Cooper minima in high-Z atoms: effects of correlation and relativity on np photoionization

Photoionization dipole transition matrix elements pass through a zero or attain a minimum that leaves imprints on photoionization parameters like the cross-section, angular distribution asymmetry parameter, phase shift, and photoionization time delay. This minimum is commonly known as the ‘Cooper minimum’ (CM). The CM, in general, is strongly affected by relativistic and correlation effects. Previous works investigated CM in the 6p and 5p subshell photoionization up to Z = 100 using the single-particle Dirac-Slater (DS) method. The present work extends the earlier work to Z up to 120 using more accurate methods; Dirac–Hartree–Fock (DHF) which includes the relativistic effects and exchange correlations, and the relativistic random phase approximation (RRPA) which includes both initial and final state electron-electron correlations along with relativistic effects. In addition to the study of photoionization from the 6p and 5p subshells, the 4p subshell has also been investigated in the present work. To demonstrate the prominent effects in the high-Z atoms, Rn (Z = 86), Ra (Z = 88), No (Z = 102), Cn (Z = 112), Og (Z = 118), and Ubn (Z = 120) are investigated.

Electron transport through degenerate electron level in single-molecular junction in the presence of electron-vibrational coupling and attractive electron-electron correlations

Electron transport through a molecular junction consisted of a single molecule coupled to macroscopic leads is studied in the nonequilibrium regime. The molecule is modeled as a degenerate energy level with an electron-vibrational interaction and attractive electron-electron correlation which lifts the degeneracy. The level occupancy and differential conductance are calculated in the antiadiabatic regime with an account of molecule-lead coupling at low temperature and finite voltage. The method of transport spectroscopy made it possible to determine through which of the split levels tunneling takes place depending on the applied voltages and to estimate the current through these levels. The cases of a negative differential conductance are discussed. The influence of the electron-vibrational coupling of different strength on electron tunneling through 2- and 4-fold degenerate level on the molecule in the presence of the attractive electron-electron interaction is analyzed.

Gate-Tunable Antiferromagnetic Chern Insulator in Twisted Bilayer Transition Metal Dichalcogenides.

A series of recent experimental works on twisted MoTe_{2} homobilayers have unveiled an abundance of exotic states in this system. Valley-polarized quantum anomalous Hall states have been identified at hole doping of ν=-1, and the fractional quantum anomalous Hall effect is observed at ν=-2/3 and ν=-3/5. In this Letter, we investigate the electronic properties of AA-stacked twisted bilayer MoTe_{2} at ν=-2 by k-space Hartree-Fock calculations. We identify a series of phases, among which a noteworthy phase is the antiferromagnetic Chern insulator, stabilized by an external electric field. We attribute the existence of this Chern insulator to an antiferromagnetic instability at a topological phase transition between the quantum spin hall phase and a band insulator phase. Our research proposes the potential of realizing a Chern insulator beyond ν=-1, and contributes fresh perspectives on the interplay between band topology and electron-electron correlations in moiré superlattices.

Coherent charge hopping suppresses photoexcited small polarons in ErFeO3 by antiadiabatic formation mechanism.

Polarons are prevalent in condensed matter systems with strong electron-phonon coupling. The adiabaticity of the polaron relates to its transport properties and spatial extent. To date, only adiabatic small polaron formation has been measured following photoexcitation. The lattice reorganization energy is large enough that the first electron-optical phonon scattering event creates a small polaron without requiring substantial carrier thermalization. We measure that frustrating the iron-centered octahedra in the rare-earth orthoferrite ErFeO3 leads to antiadiabatic polaron formation. Coherent charge hopping between neighboring Fe3+─Fe2+ sites is measured with transient extreme ultraviolet spectroscopy and lasts several picoseconds before the polaron forms. The resulting small polaron formation time is an order of magnitude longer than previous measurements and indicates a shallow potential well, even in the excited state. The results emphasize the importance of considering dynamic electron-electron correlations, not just electron-phonon-induced lattice changes, for small polarons for transport, catalysis, and photoexcited applications.

Quasi-2D spin-Peierls transition through interstitial anionic electrons in K(NH3)2

Electron–phonon interactions and electron–electron correlations represent two crucial facets of condensed matter physics. For instance, in a half-filled spin-1/2 anti-ferromagnetic chain, the lattice dimerization induced by electron-nucleus interaction can be intensified by onsite Coulomb repulsion, resulting in a spin-Peierls state. Through first-principles calculations and crystal structure prediction methods, we have identified that under mild pressures, potassium and ammonia can form stable compounds: R3¯m K(NH3)2, Pm3¯m K(NH3)2, and Cm K2(NH3)3. Our predictions suggest that the R3¯m K(NH3)2 exhibits electride characteristics, marked by the formation of interstitial anionic electrons (IAEs) in the interlayer space. These IAEs are arranged in quasi-two-dimensional triangular arrays. With increasing pressure, the electronic van-Hove singularity shifts toward the Fermi level, resulting in an augmented density of states and the onset of both Peierls and magnetic instabilities. Analyzing these instabilities, we determine that the ground state of the R3¯m K(NH3)2 is the dimerized P21/m phase with zigzag-type anti-ferromagnetic IAEs. This state can be described by the triangular-lattice antiferromagnetic Heisenberg model with modulated magnetic interactions. Furthermore, we unveil the coexistence and positive interplay between magnetic and Peierls instability, constituting a scenario of spin-Peierls instability unprecedented in realistic 2D materials, particularly involving IAEs. This work provides valuable insights into the coupling of IAEs with the adjacent lattice and their spin correlations in quantum materials.

Variational Lang-Firsov Approach Plus Møller-Plesset Perturbation Theory with Applications to Ab Initio Polariton Chemistry.

We apply the Lang-Firsov (LF) transformation to electron-boson coupled Hamiltonians and variationally optimize the transformation parameters and molecular orbital coefficients to determine the ground state. Møller-Plesset (MP-n, with n = 2 and 4) perturbation theory is then applied on top of the optimized LF mean-field state to improve the description of electron-electron and electron-boson correlations. The method (LF-MP) is applied to several electron-boson coupled systems, including the Hubbard-Holstein model, diatomic molecule dissociation (H2, HF), and the modification of proton transfer reactions (malonaldehyde and aminopropenal) via the formation of polaritons in an optical cavity. We show that with a correction for the electron-electron correlation, the method gives quantitatively accurate energies comparable to that by exact diagonalization or coupled-cluster theory. The effects of multiple photon modes, spin polarization, and the comparison to the coherent state MP theory are also discussed.