Correlated Electron Systems Research Articles

Notes on quantum oscillation for Hatsugai–Kohmoto model

Motivated by the non-Fermi liquid (NFL) phase in solvable Hatsugai–Kohmoto (HK) model and ubiquitous quantum oscillation (QO) phenomena observed in strongly correlated electron systems, we have studied the QO in HK model in terms of a combination of analytical and numerical calculation. In the continuum limit, the analytical results indicate the existence of QO in NFL state and its properties can be described by Lifshitz–Kosevich-like formula. Furthermore, numerical calculations with Luttinger’s approximation on magnetic-field-dependent density of state, magnetization and particle’s density agree with the findings of analytical treatment. Although numerical simulation from exact diagonalization exhibits certain oscillation behavior, it is difficult to extract its oscillation period and amplitude. Therefore, more work (especially the large-scale numerical simulation) on this interesting issue is highly desirable and we expect the current HK model study to be helpful in understanding generic QO in correlated electron materials.

Material aspects of oxygen isotope effect studies in high-temperature superconductors

The article discusses issues related to the material aspects of the study of the oxygen isotope effect in High-temperature superconductors. These studies were initiated by Prof. K.A. Müller shortly after he and Dr. G. Bednorz discovered high-temperature superconductivity. Research on the isotope effect was carried out in the years 1991–1995, and then continued for various classes of superconductors and other strongly correlated electron systems. The paper presents methods of synthesis of complex oxide superconductors doped with isotopic oxygen-18. Methods for the determination of oxygen content and isotopic enrichment are also discussed.

Multiorbital exchange Hamiltonians: Derivation and examples

We present a detailed derivation of the effective Hamiltonian for a strongly correlated electron system involving both orbital and spin degrees of freedom. This problem is relevant to a wide class of materials containing ions with the orbitally degenerate ground state (Jahn–Teller ions). We treat in detail the case of double degenerate orbitals, obtain the Kugel–Khomskii model, and then pass to a more general multiorbital case. Based on the derived Hamiltonians, we analyze possible spin and orbital configurations, which can be rather nontrivial, especially in the multiorbital case. A special emphasis is put on to a specific example of the square lattice of Jahn–Teller ions typical of layered perovskites.

Magnetic transitions and excitonic order enhancement in spin crossover strongly correlated electron systems

The effects of exciton Bose condensation in strongly correlated spin crossover systems are considered within the effective Hamiltonian obtained from the two-orbital Hubbard–Kanamori model. The spectrum of collective excitations at various points of the temperature-crystal field phase diagram is calculated. The role of the electron–phonon interaction is discussed. A novel mechanism of excitonic and related magnetic order photoenhancement in strongly correlated spin crossover systems based on the massive collective phase mode appearance is demonstrated. In addition, we have shown that exciton and associated with it magnetic ordering can be photoinduced outside the exciton condensate phase due to the gap presence in the spectrum of single excitations.

Orthogonal Layers of Parallelism in Large-Scale Eigenvalue Computations

We address the communication overhead of distributed sparse matrix-(multiple)-vector multiplication in the context of large-scale eigensolvers, using filter diagonalization as an example. The basis of our study is a performance model, which includes a communication metric that is computed directly from the matrix sparsity pattern without running any code. The performance model quantifies to which extent scalability and parallel efficiency are lost due to communication overhead. To restore scalability, we identify two orthogonal layers of parallelism in the filter diagonalization technique. In the horizontal layer the rows of the sparse matrix are distributed across individual processes. In the vertical layer bundles of multiple vectors are distributed across separate process groups. An analysis in terms of the communication metric predicts that scalability can be restored if, and only if, one implements the two orthogonal layers of parallelism via different distributed vector layouts. Our theoretical analysis is corroborated by benchmarks for application matrices from quantum and solid state physics, road networks, and nonlinear programming. We finally demonstrate the benefits of using orthogonal layers of parallelism with two exemplary application cases—an exciton and a strongly correlated electron system—which incur either small or large communication overhead.

Friedel oscillation in non-Fermi liquid: lesson from exactly solvable Hatsugai–Kohmoto model

When non-magnetic impurity immerses in Fermi sea, a regular modulation of charge density around impurity will appear and such phenomena is called Friedel oscillation (FO). Although both Luttinger liquid and Landau Fermi liquid show such characteristic oscillation, FO in generic non-Fermi liquid (NFL) phase is still largely unknown. Here, we show that FO indeed exists in NFL state of an exactly solvable model, i.e. the Hatsugai–Kohmoto model which has been intensively explored in recent years. Combining T-matrix approximation and linear-response-theory, an interesting picture emerges, if two interaction-induced quasi-particles bands in NFL are partially occupied, FO in this situation is determined by a novel structure in momentum space, i.e. the ‘average Fermi surface’ (average over two quasi-particle Fermi surface), which highlights the inter-band particle-hole excitation. We hope our study here provides a counterintuitive example in which FO with Fermi surface coexists with NFL quasi-particle, and it may be useful to detect hidden ‘average Fermi surface’ structure in other correlated electron systems.

K. Alex Müller and superconductivity

The discovery of the high-temperature superconductivity in the cuprate is a remarkable case of success by thinking out-of-the-box. By abandoning the traditional path to seek solids with high-Tc, K. Alex Müller opened the door to the world of strongly correlated electron systems beyond magnetism. Following his example, we re-examine the role of electron correlation in shaping various properties of complex solids, including its possible role in the mechanism of superconductivity in the cuprates.

Impact of Cr–O hybridization in ACrO3 (A = La, Y): A theoretical investigation

Electronic properties of spin polarized antiferromagnetic ACrO3 (A = La, Y) are explored with Hubbard model using density functional theory (DFT). These two isostructural systems are investigated using the different Hubbard energy and analyzed the hybridization of chromium 3d orbitals and oxygen 2p orbitals and the change in energy bandgaps against the Hubbard energy. The bond length and bond angle affect significantly the orbital contributions of Cr-3d and O-2p electrons for both the system. We noticed that the Cr–O hybridization affects the orbital degeneracy and is substantiated with partial density of states. These results emphasize the contribution of Hubbard energy in correlated electron systems.

Enhancement of superconducting fluctuations in a correlated electron system coupled to the lattice

Enhancement of superconducting fluctuations in a correlated electron system coupled to the lattice

Quantum entanglement in an extended Hubbard model as evaluated from a spin concurrence measure

Quantum entanglement is a peculiar feature of quantum mechanics that turns out a powerful resource in quantum communication and information. For this reason, intense research has recently focused on the robustness of quantum entanglement also in strongly correlated electron systems. Here, we consider an extended Hubbard model on a dimer and evaluate the spin entanglement at finite temperature, under the action of an external magnetic field. We show how magnetic field and Coulomb interactions modify the spin concurrence, finding out that the magnetic field may act as a switch from a non-entangled to an entangled state.

Magnetic Susceptibility Study of Hole-Doped Organic Metal κ-(ET)<sub>4</sub>Hg<sub>3-δ</sub>Cl<sub>8, </sub>δ=22% and κ-(ET)<sub>4</sub>Hg<sub>3-δ</sub>Br<sub>8, </sub>δ=11%

The Non-Fermi-Liquid (NFL) state has been one of central issues in the strongly correlated electrons systems. This deviates from conventional Fermi Liquid (FL) behavior. In the hole-doped triangular lattice organic metal κ-(ET)4Hg3-δBr8, δ = 11% (κ-HgBr), the NFL state is observed as a linear temperature dependence of the resistivity which changed to the temperature square dependence behavior by pressure. The spin susceptibility dependence of the muon Knight shift, K(c), is not linear in the region below 50 K, unlike in other k-type organic superconductors. Furthermore, 13C-NMR study under pressure concluded that strong antiferromagnetic spin fluctuations (AFSF) contribute to the origin of NFL. To understand the underlying correlation of the enhanced AFSF and NFL state in k-HgBr, the K(c) plot study in the sister compound κ-(ET)4Hg3-δCl8, δ=22% (κ-HgCl) which shows metal to insulator transition at TMI ~ 20 K is desirable. In this study, we report a precise susceptibility measurement in both k-HgBr and k-HgCl. Furthermore, we summarize the c(T) data of k-HgBr and k-HgCl and discuss them from the viewpoint of the triangular and square lattice models.

Cryptomelane nanocrystals: Pseudosymmetry causes anisotropic peak broadening in Rietveld refinement

Cryptomelane (□2-xKx)[Mn8O16] is an octahedral molecular sieve with the hollandite framework, a strongly correlated electron system with tunnel structure. Rietveld refinement using powder X-ray diffraction patterns of a nanocrystalline sample was undertaken to assess both structural and morphological details which might be tuned for specific applications. The distribution of peak widths was highly anisotropic but successful refinement of the tetragonal structure could be achieved assuming both grain shape and microstrain anisotropy. The microstrain hypothesis appeared flawed from high uncertainties and correlations and was in conflict with the negative slope of a Williamson-Hall plot. As an alternative, lattice desymmetrization from tetragonal to monoclinic was considered. Comparison of calculated and observed hkl dependence of line broadening confirmed that the pseudosymmetry hypothesis was more appropriate, improving results for both structure and particle shape. This is the first time that lattice desymmetrization was used to explain anisotropic diffraction line broadening, a major issue in nanomaterials where peaks are intrinsically broad and the new model helps to abate correlations which afflict Rietveld refinement in these systems.

Taming Dyson-Schwinger Equations with Null States.

In quantum field theory, the Dyson-Schwinger equations are an infinite set of coupled equations relating n-point Green's functions in a self-consistent manner. They have found important applications in nonperturbative studies, ranging from quantum chromodynamics and hadron physics to strongly correlated electron systems. However, they are notoriously formidable to solve. One of the main obstacles is that a finite truncation of the infinite system is underdetermined. Recently, Bender etal. [Phys. Rev. Lett. 130, 101602 (2023)PRLTAO0031-900710.1103/PhysRevLett.130.101602] proposed to make use of the large-n asymptotic behaviors and successfully obtained accurate results in D=0 spacetime. At higher D, it seems more difficult to deduce the large-n behaviors. In this Letter, we propose another avenue in light of the null bootstrap. The underdetermined system is solved by imposing the null state condition. This approach can be extended to D>0 more readily. As concrete examples, we show that the cases of D=0 and D=1 indeed converge to the exact results for several Hermitian and non-Hermitian theories of the gϕ^{n} type, including the complex solutions.

A first-principles study of stabilities, magnetic, electronic, elastic and transport properties of the metal borides TM2B (TM = Mn, Fe, Co and Ni)

This work addresses the physical properties of the highly correlated electron systems; transition metal borides TM2B (TM = Mn, Fe, Co and Ni) and provides a solid foundation for resolving the longstanding debate concerning the stable magnetic phase of Mn2B. The generalized gradient approximation along with Hubbard U (GGA + U) approach handling the high correlated electron systems; a valuable insight into this article has been utilized in the domain of Density Functional Theory. The calculated lattice parameters are found consistent with the experiments. The optimization energies in different magnetic phases and magnetic susceptibilities reveal that Mn2B is antiferromagnetic (AFM) in nature. The replacement of the TM in these intermetallics; transform the AFM behavior of Mn2B in to ferromagnetic (Fe2B and Co2B) and then paramagnetic (Ni2B), respectively. Electronic properties, electrical resistivities and thermal electronic conductivities of these compounds confirm their metallic nature. The investigated elastic properties demonstrate the stability and show that Fe2B is harder and brittle among this series while the rest compounds are ductile. All these intermetallics are incompressible and possess high melting temperatures. Based on the above properties, it is expected that these compounds could be used in high temperature technology like furnaces, reactors, aerospace and other high temperature processing equipments.

Geometric Aspects of Nonlinear and Nonequilibrium Phenomena

We review recent developments in the research of nonlinear and nonequilibrium phenomena in solids focusing on their geometrical aspects. We start with introducing the basic concepts of geometrical phases of Bloch electrons and Floquet theory for periodically driven systems. Then we review recent attempts to engineer topological phases in nonequilibrium matters such as graphene, magnets, and superconductors irradiated with circularly polarized light. We next review a bulk photovoltaic effect of inversion broken materials focusing on the shift current response. The shift current is described with Berry connections and has a close relationship to the modern theory of polarization. We further review recent extensions of the shift current to correlated electron systems. Finally, we explain the geometric diabatic time evolution in the Zener tunneling process and its consequences on nonreciprocal transport.

Probing resonating valence bonds on a programmable germanium quantum simulator

Simulations using highly tunable quantum systems may enable investigations of condensed matter systems beyond the capabilities of classical computers. Quantum dots and donors in semiconductor technology define a natural approach to implement quantum simulation. Several material platforms have been used to study interacting charge states, while gallium arsenide has also been used to investigate spin evolution. However, decoherence remains a key challenge in simulating coherent quantum dynamics. Here, we introduce quantum simulation using hole spins in germanium quantum dots. We demonstrate extensive and coherent control enabling the tuning of multi-spin states in isolated, paired, and fully coupled quantum dots. We then focus on the simulation of resonating valence bonds and measure the evolution between singlet product states which remains coherent over many periods. Finally, we realize four-spin states with s-wave and d-wave symmetry. These results provide means to perform non-trivial and coherent simulations of correlated electron systems.

Data Analysis of Ab initio Effective Hamiltonians in Iron-Based Superconductors — Construction of Predictors for Superconducting Critical Temperature

High-temperature superconductivity occurs in strongly correlated materials such as copper oxides and iron-based superconductors. Numerous experimental and theoretical works have been done to identify the key parameters that induce high-temperature superconductivity. However, the key parameters governing the high-temperature superconductivity remain still unclear, which hamper the prediction of superconducting critical temperatures ($T_\text{c}$s) of strongly correlated materials. Here by using data-science techniques, we clarified how the microscopic parameters in the $ab$ $initio$ effective Hamiltonians correlate with the experimental $T_\text{c}$s in iron-based superconductors. We showed that a combination of microscopic parameters can characterize the compound-dependence of $T_\text{c}$ using the principal component analysis. We also constructed a linear regression model that reproduces the experimental $T_\text{c}$ from the microscopic parameters. Based on the regression model, we showed a way for increasing $T_\text{c}$ by changing the lattice parameters. The developed methodology opens a new field of materials informatics for strongly correlated electron systems.

Self-consistent optimization of the trial wave function within the constrained path auxiliary field quantum Monte Carlo method using mixed estimators

We propose a scheme to implement the self-consistent optimization of the trial wave function within the constrained path auxiliary field quantum Monte Carlo (CP-AFQMC) method in the framework of natural orbitals. In this scheme, a new trial wave function in the form of a Slater determinant is constructed from the CP-AFQMC results by diagonalizing the mixed estimator of the one-body reduced density matrix. We compare two ways (from real and mixed estimators in CP-AFQMC) to calculate the one-body reduced density matrix in the self-consistent process and study the ground state of a doped two-dimensional Hubbard model to test the accuracy of the two schemes. By comparing the local density, occupancy, and ground state energy we find the scheme in which a one-body reduced density matrix is calculated from the mixed estimator is computationally more efficient and provides more accurate results with less fluctuation. The local densities from the mixed estimator scheme agree well with the numerically exact values. This scheme provides a useful tool for studying strongly correlated electron systems.

True First-Order Surface Phase Transition without Nanoscale Phase Separation.

Nanoscale phase separation is common in many materials ranging from correlated electron systems to semiconductor surfaces undergoing phase transitions. On solid surfaces, nanoscale phase separations are known to occur over an extended temperature range during temperature-driven first-order surface phase transitions, precluding the true first-order transitions in thermodynamics. Here, we report the case of a surface phase transition very close to a true first-order transition. An array of indium wires on Si(111) undergoes a first-order charge-density-wave (CDW) transition with surprisingly little or no phase separation when prepared free of indium adatom impurities. The lack of phase separation was attributed to the small difference in strain with the substrate between the two competing normal and CDW phases. Indium adatom impurities cause phase separation and blur the transition, making it gradual and incomplete. These experimental observations provide insight into the surface phase transition at the nanoscale level.

Enhanced temperature coefficient of resistance in nanostructured Nd0.6Sr0.4MnO3 thin films

Manganite thin films are promising candidates for studying strongly correlated electron systems. Understanding the growth and morphology-driven changes in the physical properties of manganite thin films is vital for their applications in oxitronics. This work reports the morphological, structural, and electrical transport properties of nanostructured Nd0.6Sr0.4MnO3 thin films fabricated using the pulsed laser deposition technique. Scanning electron microscopy imaging of the thin films revealed two prominent surface morphologies: a granular and a crossed-nano-rod-type morphology. From X-ray diffraction and atomic force microscopy analysis, we found that the observed nanostructures resulted from altered growth modes on the terraced substrate surface. Furthermore, investigations on the electrical-transport properties of thin films revealed that the films with crossed-nano-rod morphology showed a sharp resistive transition near the metal-to-insulator transition. Also, an enhanced temperature coefficient of resistance (TCR) of up to one order of magnitude was observed compared to the films with granular morphology. Such enhancement in TCR% by tuning the morphology makes these thin films promising candidates for developing oxide-based temperature sensors and detectors.