arXiv subjects

Yuehua Su

Publications and source records attributed to Yuehua Su.

17 recordsLinked to original sources

arXiv2026

Coincidence detection techniques for direct measurement of many-body correlations in strongly correlated electron systems

Research on strongly correlated electron systems faces a fundamental challenge due to the complex nature of intrinsic many-body correlations. A key strategy to address this challenge lies in advancing experimental methods that can directly probe and elucidate the underlying many-body correlations. In this perspective article, we discuss the theoretically proposed coincidence detection techniques, which are designed to directly measure two-body correlations in various particle-particle and particle-hole channels, with momentum, energy, and/or spatial resolution. We also explore the prospects of these coincidence detection techniques for future theoretical and experimental developments. The successful implementation and refinement of these coincidence detection techniques promise to deliver powerful new approaches for unraveling long-standing puzzles in strongly correlated electron systems, such as the enigmatic mechanism of unconventional superconductivity and the long-sought quantum spin liquids. Furthermore, these coincidence detection techniques will offer powerful new methods to investigate novel phenomena like itinerant magnetism and electronic nematicity in quantum materials.

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arXiv2025

Post-experiment coincidence detection techniques for direct detection of two-body correlations

It is one challenge to develop experimental techniques for direct detection of the many-body correlations of strongly correlated electrons, which exhibit a variety of unsolved mysteries. In this article, we present a \textit{post-experiment} coincidence counting method and propose two \textit{post-experiment} coincidence detection techniques, \textit{post-experiment} coincidence angle-resolved photoemission spectroscopy (cARPES) and \textit{post-experiment} coincidence inelastic neutron scattering (cINS). By coincidence detection of two photoelectric processes or two neutron-scattering processes, the \textit{post-experiment} coincidence detection techniques can detect directly the two-body correlations of strongly correlated electrons in particle-particle channel or two-spin channel. The \textit{post-experiment} coincidence detection techniques can be implemented upon the \textit{pulse}-resolved angle-resolved photoemission spectroscopy (ARPES) or inelastic neutron scattering (INS) experimental apparatus with \textit{pulse} photon or neutron source. When implemented experimentally, they will be powerful techniques to study the highly esoteric high-temperature superconductivity and the highly coveted quantum spin liquids.

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arXiv2025

Coincidence double-tip scanning tunneling spectroscopy

The development of new experimental techniques for direct measurement of many-body correlations is crucial for unraveling the mysteries of strongly correlated electron systems. In this work, we propose a coincidence double-tip scanning tunneling spectroscopy (STS) that enables direct probing of spatially resolved dynamical two-body correlations of sample electrons. Unlike conventional single-tip scanning tunneling microscopy, the double-tip STS employs a double-tip scanning tunneling microscope (STM) equipped with two independently controlled tips, each biased at distinct voltages ($V_1$ and $V_2$). By simultaneously measuring the quantum tunneling currents $I_1(t)$ and $I_2(t)$ at locations $j_1$ and $j_2$, we obtain a coincidence tunneling current correlation $\overline{\langle I_1(t) I_2(t)\rangle}$. Differentiating this coincidence tunneling current correlation with respect to the two bias voltages yields a coincidence dynamical conductance. Through the development of a nonequilibrium theory, we demonstrate that this coincidence dynamical conductance is proportional to a contour-ordered second-order current correlation function. For the sample electrons in a nearly free Fermi liquid state, the coincidence dynamical conductance captures two correlated dynamical electron propagation processes: (i) from $j_1$ to $j_2$ (or vice versa) driven by $V_1$, and (ii) from $j_2$ to $j_1$ (or vice versa) driven by $V_2$. For the sample electrons in a superconducting state, additional propagation channels emerge from the superconducting condensate, coexisting with the above normal electron propagation processes. Thus, the coincidence double-tip STS provides direct access to spatially resolved dynamical two-body correlations, offering a powerful tool for investigating strongly correlated electron systems.

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arXiv2024

Microscopic Ampère current-current interaction

With the rapid development of modern measurement techniques, the energy resolution of $1 \, meV$ can now be easily obtained. Generally, the driving mechanisms of the physical, chemical or biological processes of the matters or the living organisms on Earth at about $1 \, meV$ energy scale are assumed to stem from the fundamental microscopic Coulomb interaction, its various reduced ones and the relativistic corrections. In this article, by using a path integral approach on a non-relativistic quantum electrodynamics theory, we show that there is another fundamental microscopic electromagnetic interaction at this energy scale, the microscopic Ampère current-current interaction. It has time-dependent dynamical feature and can be the driving interaction of the physical, chemical or biological processes at about $1\, meV$ energy scale. A new Ampère-type exchange spin interaction is also found with a magnitude about $10^{-4}$ of the well-known Heisenberg exchange spin interaction.

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arXiv2024

Coincidence detection probability of $(γ, 2e)$ photoemission measurement

In the study of strongly correlated electrons, one of the challenging core tasks is to develop the potential techniques for direct detection of the many-body correlations of strongly correlated electrons. The $(γ, 2e)$ photoemission technique has been developed to investigate the two-body correlations of the target correlated electrons. In this article, we will focus on this technique for the correlated electrons near the Fermi energy in condensed matter. The coincidence detection probability of the two emitted electrons in the $(γ, 2e)$ photoemission measurement is shown to be relevant to a two-body Bethe-Salpeter wave function, which describes the dynamical two-body correlations of the target correlated electrons near the Fermi energy. As the coincidence detection probability involves an electron-electron interaction matrix element, the arbitrary momentum and/or energy transfer due to this electron-electron interaction makes the $(γ, 2e)$ photoemission technique fail to reveal the inner-pair structures of the two-body Bethe-Salpeter wave function. However, the center-of-mass momentum and energy of the two-body Bethe-Salpeter wave function can be distinctly resolved. Thus, the $(γ, 2e)$ photoemission technique can provide the center-of-mass physics of the two-body correlations of the target correlated electrons. It will be one potential technique to study the center-of-mass physics of the Cooper pairs in superconductor.

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arXiv2023

Renormalization formalism for superconducting phase transition with inner-Cooper-pair dynamics

As charge carrier of the macroscopic superconductivity, the Cooper pair is a composite particle of two paired electrons, which has both center-of-mass and inner-pair degrees of freedom. In most cases, these two different degrees of freedom can be well described by the macroscopic Ginzburg-Landau theory and the microscopic Bardeen-Cooper-Schrieffer (BCS) theory, respectively. Near the superconducting phase transition where the Cooper pair is fragile and unstable because of the small binding energy, there are non-trivial couplings between these two different degrees of freedom due to such as finite energy and/or momentum transfer. The non-trivial couplings make the original derivation of the Ginzburg-Landau theory from the BCS theory fail in principle as where these two different degrees of freedom should not be decoupled. In this article, we will present a renormalization formalism for an extended Ginzburg-Landau action for the superconducting phase transition where there is finite energy transfer between the center-of-mass and the inner-pair degrees of freedom of Cooper pairs. This renormalization formalism will provide a theoretical tool to study the unusual dynamical effects of the inner-pair time-retarded physics on the superconducting phase transition.

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arXiv2021

Coincidence inelastic neutron scattering for detection of two-spin magnetic correlations

Inelastic neutron scattering (INS) is one powerful technique to study the low-energy single-spin dynamics of magnetic materials. A variety of quantum magnets show novel magnetic correlations such as quantum spin liquids. These novel magnetic correlations are beyond the direct detection of INS. In this paper we propose a coincidence technique, coincidence inelastic neutron scattering (cINS), which can detect the two-spin magnetic correlations of the magnetic materials. In cINS there are two neutron sources and two neutron detectors with an additional coincidence detector. Two neutrons from the two neutron sources are incident on the target magnetic material, and they are scattered by the electron spins of the magnetic material. The two scattered neutrons are detected by the two neutron detectors in coincidence with the coincidence probability described by a two-spin Bethe-Salpeter wave function. Since the two-spin Bethe-Salpeter wave function defines the momentum-resolved dynamical wave function with two spins excited, cINS can explicitly detect the two-spin magnetic correlations of the magnetic material. Thus, it can be introduced to study the various spin valence bond states of the quantum magnets.

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arXiv2020

Coincidence angle-resolved photoemission spectroscopy: Proposal for detection of two-particle correlations

The angle-resolved photoemission spectroscopy (ARPES) is one powerful experimental technique to study the electronic structure of materials. As many electron materials show unusual many-body correlations, the technique to detect directly these many-body correlations will play important roles in the study of their many-body physics. In this article, we propose a technique to detect directly the two-particle correlations, a coincidence ARPES (cARPES) where two incident photons excite two respective photoelectrons which are detected in coincidence. While the one-photon-absorption and one-photoelectron-emission ARPES provides the single-particle spectrum function, the proposed cARPES with two-photon absorption and two-photoelectron emission is relevant to a two-particle Bethe-Salpeter wave function. Examples of the coincidence detection probability of the cARPES for a free Fermi gas and a Bardeen-Cooper-Schrieffer (BCS) superconducting state are studied in detail. We also propose another two experimental techniques, a coincidence angle-resolved photoemission and inverse-photoemission spectroscopy (cARP/IPES) and a coincidence angle-resolved inverse-photoemission spectroscopy (cARIPES). As all of these proposed coincidence techniques can provide the two-particle frequency Bethe-Salpeter wave functions, they can show the momentum and energy dependent two-particle dynamical physics of the material electrons in the particle-particle or particle-hole channel. Thus, they can be introduced to study the Cooper-pair physics in the superconductor, the itinerant magnetism in the metallic ferromagnet/antiferromagnet, and the particle-hole pair physics in the metallic nematic state. Moreover, as the two-particle Bethe-Salpeter wave functions also involve the inner-pair dynamical physics, these proposed coincidence techniques can be used to study the inner-pair time-retarded physics.

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arXiv2017

Superconductivity driven by pairing of the coherent parts of the physical electrons

How the superconductivity in unconventional superconductors emerges from the diverse mother normal states is still a big puzzle. Whatever the mother normal states are the superconductivity is {\em normal} with BCS-like behaviours of the paired quasiparticles in condensation. To reconcile the diverse mother normal states and the normal superconductivity in unconventional superconductors, we revisit a proposal that the emergence of the low-energy coherent parts of the physical electrons, which survive from the interaction correlations, is an essential prerequisite for superconductivity. The superconductivity is driven by the pair condensation of these coherent parts of the physical electrons. Moreover the incoherent parts of the physical electrons can enhance the superconducting transition temperature $T_c$ although they are not in driving role in the emergence of the superconductivity. Some experimental responses of the coherent parts of the physical electrons are predicted.

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arXiv2017

Breakdown of Landau Fermi liquid theory: restrictions on the degrees of freedom of quantum electrons

One challenge in contemporary condensed matter physics is to understand unconventional electronic physics beyond the paradigm of Landau Fermi-liquid theory. Here, we present a perspective that posits that most such examples of unconventional electronic physics stem from restrictions on the degrees of freedom of quantum electrons in Landau Fermi liquids. Since the degrees of freedom are deeply connected to the system's symmetries and topology, these restrictions can thus be realized by external constraints or by interaction-driven processes via the following mechanisms: (i) symmetry breaking, (ii) new emergent symmetries, and (iii) nontrivial topology. Various examples of unconventional electronic physics beyond the reach of traditional Landau Fermi liquid theory are extensively investigated from this point of view. Our perspective yields basic pathways to study the breakdown of Landau Fermi liquids and also provides a guiding principle in the search for novel electronic systems and devices.

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arXiv2017

Driving force of the orbital-relevant electronic nematicity in Fe-based superconductors

The electronic nematic responses in Fe-based superconductors have been observed ubiquitously in various experimental probes. One novel nematic character is the d-wave {\it bond} orbital-relevant nematic charge order which was firstly proposed by symmetry analysis and then conformed by angle-resolved photoemission spectroscopy. In this paper, we present a mechanism that the driving force of the orbital-relevant nematic charge order is the reduction of the large Hubbard energy in the particle-hole charge channel by virtual hopping processes. This is one scenario from strong-coupling consideration. The same virtual hopping processes can lead to a super-exchange interaction for the spin magnetic order in the particle-hole spin channel and a pairing interaction for the superconducting order in the particle-particle channel. Thus the electronic nematic order, the spin magnetic order and the pairing superconducting order are intrinsically entangled and they can all stem from the same microscopic virtual hopping processes in reduction of the Hubbard energy. The electronic nematicity, the spin magnetism and the pairing superconductivity in unconventional superconductors are proposed to be unified within this mechanism.

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arXiv2016

Influence of orbital nematic order on spin responses in Fe-based superconductors

Electronic nematicity is ubiquitous in Fe-based superconductors, but what the primary nematic order is and how the various nematic phenomena correlate with each other are still elusive. In this manuscript we study the physical consequence of the orbital nematic order on the spin correlations. We find that the orbital nematic order can drive a significant spin nematicity and can enhance the integrated intensity of the spin fluctuations. Our study shows that the orbital nematic order has strong effect on the spin correlations and it can not be taken as an unimportant secondary effect of the nematic state in Fe-based superconductors.

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arXiv2016

Schwinger-Keldysh canonical formalism for electronic Raman scattering

Inelastic low-energy Raman and high-energy X-ray scatterings have made great progress in instrumentation to investigate the strong electronic correlations in matter. However, theoretical study of the relevant scattering spectrum is still a challenge. In this article, we present a Schwinger-Keldysh canonical perturbation formalism for the electronic Raman scattering, where all the resonant, non-resonant and mixed responses are considered uniformly. We show how to use this formalism to evaluate the cross section of the electronic Raman scattering off an one-band superconductor. All the two-photon scattering processes from electrons, the non-resonant charge density response, the elastic Rayleigh scattering, the fluorescence, the intrinsic energy-shift Raman scattering and the mixed response, are included. In the mean-field superconducting state, Cooper pairs contribute only to the non-resonant response. All the other responses are dominated by the single-particle excitations and are strongly suppressed due to the opening of the superconducting gap. Our formalism for the electronic Raman scattering can be easily extended to study the high-energy resonant inelastic X-ray scattering.

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arXiv2014

The form and the origin of the orbital ordering in the electronic nematic phase of the Iron-based superconductors

We investigated the form of the orbital ordering in the electronic nematic phase of the Iron-based superconductors by applying a group theoretical analysis on a realistic five-band model. We find the orbital order can be either of the inter-orbital s-wave form or the intra-orbital d-wave form. From the comparison with existing ARPES measurements of band splitting, we find the orbital ordering in the 122 system is dominated by an intra-orbital d-wave component, while that in the 111 system is dominated by an inter-orbital s-wave component. We find both forms of orbital order are strongly entangled with the nematicity in the spin correlation of the system. The condensation energy of the magnetic ordered phase is found to be significantly improved(by more than 20 percents) when the degeneracy between the $(π,0)$ and $(0,π)$ ordering pattern is lifted by the orbital order. We argue there should be large difference in both the scattering rate and the size of the possible pseudogap on the electron pocket around the $\mathrm{X}=(π,0)$ and $\mathrm{Y}=(0,π)$ point in the electronic nematic phase. We propose this as a possible origin for the observed nematicity in resistivity measurements.

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arXiv2013

Seeing the orbital ordering in Iron-based superconductors with magnetic anisotropy

The orbital fluctuation of the conduction electrons in the Iron-based superconductors is found to contribute significantly to the magnetic response of the system. With the use of a realistic five-band model and group theoretical analysis, we have determined the orbital magnetic susceptibility in such a multi-orbital system. At $n=6.1$, the in-plane orbital magnetic susceptibility is predicted to be about 10$μ_{\mathrm{B}}^{2}/\mathrm{eV}$, which is more than 2/3 of the observed total susceptibility around 200 K in 122 systems(of about 14$μ_{\mathrm{B}}^{2}/\mathrm{eV}$ or $4.5\times10^{-4}\mathrm{erg}/\mathrm{G}^{2}\mathrm{mol}_{\mathrm{AS}}$\cite{Klingeler}). We find the in-plane orbital magnetic response is sensitive to the breaking of the tetragonal symmetry in the orbital space. In particular, when the observed band splitting(between the $3d_{xz}$ and the $3d_{yz}$-dominated band) is used to estimate the strength of the symmetry breaking perturbation\cite{Shen}, a 4.5% modulation in the in-plane orbital magnetic susceptibility can be produced, making the latter a useful probe of the orbital ordering in such a multi-orbital system. As a by product, the theory also explains the large anisotropy between the in-plane and the out-of-plane magnetic response observed universally in susceptibility and NMR measurements.

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arXiv2011

Inter-layer Superconducting Pairing Induced c-axis Nodal Lines in Iron-based Superconductors

A layered superconductor with a full pairing energy gap can be driven into a nodal superconducting (SC) state by inter-layer pairing when the SC state becomes more quasi-3D. We propose that this mechanism is responsible for the observed nodal behavior in a class of iron-based SCs. We show that the intra- and inter-layer pairings generally compete and the gap nodes develop on one of the {\em hole} Fermi surface pockets as they become larger in the iron-pnictides. Our results provide a natural explanation of the c-axis gap modulations and gap nodes observed by angle resolved photoemission spectroscopy. Moreover, we predict that an anti-correlated c-axis gap modulations on the hole and electron pockets should be observable in the $S^{\pm}$-wave pairing state.

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arXiv2011

Orbital magnetic response and the anisotropy of magnetic susceptibility in the Iron-based superconductors

We propose that the orbital angular momentum of the conduction electrons in the Iron-based superconductors is activated in their low energy physics. Using a five-band tight-binding model derived from fitting the LDA band structure, we find that the orbital magnetic susceptibility of the conduction electrons in such a multi-orbital system is several times larger than the Pauli spin susceptibility and is comparable in magnitude to the observed total magnetic susceptibility. The orbital magnetic susceptibility in the Fe-As plane($χ^{x}_{L}$) is found to be larger than that perpendicular to the Fe-As plane($χ^{z}_{L}$) by a factor about two and the total magnetic susceptibility in the normal state can be fitted with formula $χ(T,θ)\approx χ_{s}(T)+χ_{L}(θ)$, where $χ_{s}(T)$ is the temperature dependent isotropic part due to spin and $χ_{L}(θ)$ is the temperature independent anisotropic part due to orbital. In the superconducting state, $χ^{x}_{L}$ is found to be significantly reduced as the pairing gap develops, while $χ^{z}_{L}$ is almost not affected by the superconducting transition. We argue the large anisotropy observed in the bulk magnetic susceptibility and the Knight shift in the Iron-based superconductors should be attributed to the orbital magnetic response of their conduction electrons.

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