arXiv subjects

Wei Ku

Publications and source records attributed to Wei Ku.

At least 19 recordsLinked to original sources

arXiv2026

Effective Ionic Valence and Local Magnetic Moment in Kagome Superconductors

In order to understand the unexpected similarity and the correlated behavior in kagome superconductor families AV$_3$Sb$_5$ (A = K, Rb, Cs) and ATi$_3$Bi$_5$ (A = Rb, Cs), we investigate the Hartree-scale local electronic structure of these systems. Our result indicates that V and Ti ions are both of 2+ valence such that the corresponding itinerant carrier densities are similar, and the difference in electron count is instead reflected in their quantum fluctuating ionic magnetic moments. However, due to the frustrated lattice geometry of these materials, such local moments are difficult to experimentally observe via standard probes. For verification, we systematically introduce nonmagnetic Sn impurities to locally relieve the geometric frustration and experimentally demonstrate the existence of well-defined local magnetic moments via magnetic susceptibility and muon spin rotation or relaxation ($\mu$SR) measurements. All experiments discover a systematic increase of magnetic susceptibility upon increasing nonmagnetic impurity level. Our discovered ionic moments suggest a paradigm shift from the existing itinerant carrier-only picture to one incorporating strong correlation from local ionic spins. The associated interatomic and local-itinerant correlations offer a solid ground for the emergence of the observed rich correlated behavior in this new family of superconducting materials.

cond-mat.str-el↗

arXiv2026

Orbital dimerization-induced first-order structural phase transition: a case study in La$_3$Ni$_2$O$_7$

First-order structural phase transition is a common phenomenon in materials that qualitatively alters their physical properties. Yet, the abrupt first-order nature is usually unexplained by realistic computations, implying an omission of important physics in describing the electronic structure of the nearby stable phases. Using the recently discovered nickelate superconductors La$_3$Ni$_2$O$_7$ as a prototypical example, we demonstrate that such first-order nature is typically beyond intra-atomic correlation considered in state-of-the-art material computations. Instead, a full many-body treatment of low-energy active orbitals reveals a generic inter-atomic "orbital dimerization" mechanism of first-order structural phase transition, corresponding to abrupt energy reduction upon a spin-singlet bond formation. Such an inter-atomic correlation qualitatively changes not only the essential lattice bonding but also the characteristics of low-energy electronic properties across the transition. This strong mechanism and the developed computational framework are generally applicable to a wide variety of ionic materials, to produce valuable insights into atomic and electronic structures essential for their physical properties and functionalities.

cond-mat.str-el↗

arXiv2025

Emergent charge-2$e$ carriers well hidden from electronic band structures

Emergence of charge-2$e$ bosonic carriers as tightly bound electrons offer perhaps the simplest route to understand the non-Fermi liquid behaviors widely observed in functional materials. However, such scenarios are often discarded when electronic carriers are observed with well-defined energy-momentum dispersion. Here, using attractive Hubbard model as a representative example, we demonstrate the emergence of such 2$e$-carriers coexisting with residual electronic carriers through determinant quantum Monte Carlo computation of their propagators. Interesting, even already dominant in density, the emergent 2$e$-carriers appear to efficiently elude detection by the electronic spectral function, which still shows good quasi-particles with negligible mass enhancement. Nonetheless, above the superfluid temperature, the presence of 2$e$-carriers is revealed through a pseudogap in the electronic spectral function. Our results exemplify the risk of common practice in discarding strong correlation in materials solely based on observation of clean band structures with weakly enhanced mass. More importantly, our finding provides the microscopic foundation for scenarios of boson-fermion mixed liquid as effective descriptions for some of the correlated functional materials.

cond-mat.str-el↗

arXiv2025

A universal description of Mott insulators: Characterizing quantum phases beyond broken symmetries

Using Mott insulators as a prototypical example, we demonstrate a dynamics-based characterization of quantum phases of matter through a general N-body renormalization group framework. The essential "Mott-ness" turns out to be characterized by a change of size-scaling of the effective intra- momentum repulsions between long-lived emergent "eigen-particles" that encodes the dynamics of two-body bound states in the high-energy sector. This directly offers a universal characterization at long space-time scale for the corresponding class of Mott insulators through a uniform single occupation of all momenta, and otherwise Mott metals. This universal description naturally paves the way to topological Mott insulators and is straightforward to extend to bosonic Mott systems. More generally, this demonstration exemplifies a generic paradigm of characterizing quantum phases of matter through their distinct dynamics beyond broken symmetries.

cond-mat.str-el↗

arXiv2025

Charge order through crystallization of Frenkel excitons: realization in kagome metals

Charge order is a widely observed and representative example of spontaneous broken symmetries in quantum states of matter. Owing to the large intra-atomic Coulomb energy, the charge redistribution in such an order typically implies significant alteration of the electronic and lattice properties of materials. While the standard description of charge order, namely a "charge density wave" instability of the Fermi surface, has been broadly and successfully applied to good metals, its applicability to correlated ionic materials has been rather limited. Here, we propose an alternative general scenario of charge order - crystallization of long-lived Frenkel excitons - suitable for these ionic materials. We demonstrate this scenario on the recently discovered kagome superconductors and successfully reproduce all the characteristics of experimental observations on both local charge correlations and long-range ordering. The proposed generic scenario offers a long-sought understanding of charge order applicable to modern correlated functional materials.

cond-mat.str-el↗

arXiv2025

Bogoliubov quasi-particles in superconductors are integer-charged particles inapplicable for braiding quantum information

We present a rigorous proof that under a number-conserving Hamiltonian, one-body quasi-particles generally possess quantized charge and inertial mass identical to the bare particles. It follows that, Bogoliubov zero modes in the vortex (or on the edge) of superconductors $\textit{cannot}$ be their own anti-particles capable of braiding quantum information. As such, the heavily pursued Majorana zero mode-based route for quantum computation requires a serious re-consideration. This study further reveals the conceptual challenge in preparing and manipulating braid-able quantum states via physical thermalization or slow external fields. These profound results should reignite the long-standing quest for a number-conserving theory of superconductivity and superfluidity without fictitiously breaking global U(1) symmetry.

cond-mat.str-el↗

arXiv2025

2D ferroelectricity accompanying antiferro-orbital order in semi-metallic WTe$_2$

The first switchable electric polarization in metals was recently discovered in bilayer and trilayer WTe2. Strangely, despite the tininess of the ordered polarization, the ferroelectricity survives up to 350 K, rendering the mechanism of such ferroelectricity challenging for standard understandings. Here, via a density-functional-based multi-energy-scale analysis of the system's broken symmetries, we identify a weak out-of-plane ferroelectricity accompanying a strong in-plane antiferro-orbital order. This unusual low-energy correlation, which emerges from an antiferroelectric structure formed at much higher energy, naturally explains the above puzzling observation. This result reveals an unprecedented paradigm of electronic ferroelectricity generally applicable to 2D polar metals with ultrafast-switchable polarization ideal for the next-generation non-volatile memory and other devices.

cond-mat.mtrl-sci↗

arXiv2025

Pressure-induced trans-proximate correlation in La$_4$Ni$_3$O$_{10}$ and possible routes to enhance its superconductivity

We report an unexpected trans-proximate interlayer correlation (stronger correlation between disjoint layers than the adjacent ones) in the high-pressure phase of the recently discovered La$_4$Ni$_3$O$_{10}$ superconductors. Accompanied by an unusual pressure-induced fractionalization of Ni$^{2+}$ ionic spin from the standard spin-1 to spin-$\frac{1}{2}$, this trans-proximate correlation results from the emergence of a cross-layer trimer in our multi-energy-scale derivation of the electron dynamics. The resulting low-energy effective description resembles that of the cuprates and suggests a universal superconducting mechanism in all existing nickelate and cuprate superconductors. The rare trans-proximate correlation not only explains the weaker superconductivity in comparison with the related La$_3$Ni$_2$O$_7$ samples, but it also indicates a viable strategy to improve superconductivity in this trilayer nickelate by lowering layer symmetry. Such pressure-induced trans-proximate correlation is expected in many materials and examplifies the engineering of rich uncharted quantum states of matter through pressure.

cond-mat.supr-con↗

arXiv2024

Manipulable compact many-body localization and absence of superfluidity in geometrically frustrated systems

Geometric frustration is known to completely damage kinetic processes of some of the orbitals (and their associated quantum coherence) as to produce flat bands in the non-interacting systems. The impact of introducing additional interaction to the system in such frustrated systems is, however, a highly controversial issue. On the one hand, numerical studies on geometrically frustrated systems of hard-core boson (equivalent to a spin-1/2 systems) typically lead to glass or solid phases containing only local many-body coherence, indicating the persistence of the damage in quantum coherence. On the other, there continues to be noticeable claims of development of superfluidity that implies kinetic flow of particles. To resolve this apparent contradiction of great significance, we present a rigorous proof showing that density-density interaction is incapable of defeating the geometric frustration to allow propagation of those immobile particles, let alone sustaining a superfluidity. Instead, the frustrated systems develop many $\textit{compact}$ many-body localized states as "many-body scars" that do not thermalize, making them good candidates for storing $\textit{robust}$ and $\textit{manipulable}$ quantum information.

cond-mat.str-el↗

arXiv2024

`Interaction annealing' to determine effective quantized valence and orbital structure: an illustration with ferro-orbital order in WTe$_2$

Correlated materials are known to display qualitatively distinct emergent behaviors at low energy. Conveniently, upon absorbing rapid quantum fluctuations, these rich low-energy behaviors can always be effectively described by dressed particles with fully quantized charge, spin, and orbital structure. Such a powerful and simple description is, however, difficult to access through bare particles used in most many-body computations, especially when fluctuations are strong such as in $4d$ and $5d$ compounds. To decipher the dominant quantized structure, we propose an easy-to-implement `interaction annealing' approach that utilizes suppressed charge fluctuation through enhancing ionic charging energy. We establish its theoretical foundation using an exactly treated two-site Hubbard model as a generic example. We then demonstrate its applications with more affordable density functional calculations to a representative $3d$ Mott insulator La${_2}$CuO${_4}$ and a highly fluctuating $5d$ semi-metal WTe${_2}$. In the latter, it reveals an emergent local electronic structure that makes possible an unprecedented explanation of several experimental observations. Finally, we demonstrate the effectiveness of this approach in studying competing local electronic structures in functional materials.

cond-mat.str-el↗

arXiv2024

Universal low-temperature fluctuation of unconventional superconductors revealed: 'Smoking gun' leaves proper bosonic superfluidity the last theory standing

Low-temperature thermal fluctuations offer an essential window in characterizing the true nature of a quantum state of matter, a quintessential example being Fermi liquid theory. Here, we examine the leading thermal fluctuation of the superfluid density across numerous families ranging from relatively conventional to highly unconventional superconductors (MgB$_2$, bismuthates, doped buckyballs, heavy fermions, UTe$_2$, doped SrTiO$_3$, Chevrel clusters, intermetallics, organic superconductors, transition metal dichalcogenides, ruthenates, iron-pnictides, cuprates, and kagome metals). Amazingly, in all of them an unprecedented universal $T^3$ depletion materializes in the low-temperature superfluid density, even in the believed-to-be-conventional MgB$_2$. This reveals a new quantum superfluid state of matter and requires a necessary change of paradigm in describing modern superconductors. We demonstrate that such unorthodox yet generic behavior can be described by a strictly Galilean consistent theory of bosonic superfluidity hosting a long-lived 'true condensate'.

cond-mat.supr-con↗

arXiv2024

Showcasing the necessity of the principle of relative motion in physical statistics: Inconsistency of the `segmented Fermi surface'

The hunt for exotic properties in flowing systems is a popular and active field of study, and has recently gained renewed attention through claims such as a ``segmented Fermi surface'' in a superconducting system that hosts steady superflow of screening current driven by an external field. Apart from this excitement and the promise of hosting Majorana zero modes, claims such as this imply exotic gap-to-gapless quantum phase transitions merely through boost of inertial frames of observation, and challenge the very concept behind the principle of relative motion. Here, we first illustrate an obvious inescapable physical inconsistency of such claims concerning the flow velocity. Taking into account this basic principle from the beginning, we then demonstrate that a proper employment of physical statistics naturally reproduces the experimental observation without causing such a conceptual crisis. This example showcases the importance of strict adherence to the basic principle of relative motion in physical statistics, especially when pushing the frontiers of physics and technology.

cond-mat.supr-con↗

arXiv2024

Geometric inhibition of superflow in single-layer graphene suggests a staggered-flux superconductivity in bilayer and trilayer graphene

In great contrast to the numerous discoveries of superconductivity in layer-stacked graphene systems, the absence of superconductivity in the simplest and cleanest monolayer graphene remains a big puzzle. Here, through realistic computation of electronic structure, we identify a systematic trend that superconductivity appears to emerge only upon alteration of the low-energy electronic lattice from the underlying honeycomb atomic structure. We then demonstrate that this inhibition can result from from geometric frustration of the bond lattice that disables quantum phase coherence of the order parameter residing on it. In comparison, upon deviating from the honeycomb lattice, relief of geometric frustration allows robust superfluidity with non-trivial spatial structure. For the specific examples of bilayer and trilayer graphene under an external electric field, such bond centered order parameter would develop superfluidity with staggered flux that breaks the time-reversal symmetry. Our study also suggests the possible realization of the long-sought superconductivity in single-layer graphene via the application of uni-directional strain.

cond-mat.supr-con↗

arXiv2023

Relevance of long-range screening in Mott transition examined via a hydrogen lattice

The Mott transition, a metal-insulator transition due to strong electronic interaction, is observed in many materials without an accompanying change of system symmetry. An important open question in Mott's proposal is the role of long-range screening, whose drastic change across the quantum phase transition may self-consistently make the transition more abrupt, toward a first-order one. Here we investigate this effect in a model system of hydrogen atoms in a cubic lattice, using charge self-consistent dynamical mean-field theory that incorporates approximately the long-range interaction within the density functional treatment. We found that the system is well within the charge-transfer regime and that the charge-transfer gap intimately related to the Mott transition closes smoothly instead. This indicates that the long-range screening does not play an essential role in this prototypical example. This finding can be understood from the fact that the obtained insulating phase in this model system is driven by strong local interaction, and the transition is associated with the closing of charge-transfer gap. Contrary to Mott's length scale argument, such energetic competition between kinetic energy and local interaction is thus insensitive to long-range screening.

cond-mat.str-el↗

arXiv2023

Pressure Driven Fractionalization of Ionic Spins Results in Cupratelike High-$T_c$ Superconductivity in La$_3$Ni$_2$O$_7$

Beyond 14GPa of pressure, bi-layered La$_3$Ni$_2$O$_7$ was recently found to develop strong superconductivity above the liquid nitrogen boiling temperature. An immediate essential question is the pressure-induced qualitative change of electronic structure that enables the exciting high-temperature superconductivity. We investigate this timely question via a numerical multi-scale derivation of effective many-body physics. At the atomic scale, we first clarify that the system has a strong charge transfer nature with itinerant carriers residing mainly in the in-plane oxygen between spin-1 Ni$^{2+}$ ions. We then elucidate in eV- and sub-eV-scale the key physical effect of the applied pressure: It induces a cupratelike electronic structure through partially screening the Ni spin from 1 to 1/2. This suggests a high-temperature superconductivity in La$_3$Ni$_2$O$_7$ with microscopic mechanism and ($d$-wave) symmetry similar to that in the cuprates.

cond-mat.supr-con↗

arXiv2023

Chemical disorder induced electronic orders in correlated metals

In strongly correlated metals, long-range magnetic order is sometimes found only upon introduction of a minute amount of $disordered$ non-magnetic impurities to the unordered clean samples. To explain such anti-intuitive behavior, we propose a scenario of inducing electronic (magnetic, orbital, or charge) order via chemical disorder in systems with coexisting local moments and itinerant carriers. By disrupting the damaging long-range quantum fluctuation originating from the itinerant carriers, the electronic order preferred by the local moment can be re-established. We demonstrate this mechanism using a realistic spin-fermion model and show that the magnetic order can indeed be recovered as a result of enhanced disorder once the length scale of phase coherence of the itinerant carriers becomes shorter than a critical value. The proposed simple idea has a general applicability to strongly correlated metals, and it showcases the rich physics resulting from the interplay between mechanisms of multiple length scales.

cond-mat.str-el↗

arXiv2023

Quantum fluctuation of ferroelectric order in polar metals

Since its discovery a decade ago, "polar metallic phase" has ignited significant research interest, as it further functionalizes the switchable electric polarization of materials with additional transport capability, granting them great potential in next-generation electronic devices. The polar metallic phase is an unusual metallic phase of matter containing long-range ferroelectric (FE) order in the electronic and atomic structure. Distinct from the typical FE insulating phase, this phase spontaneously breaks the inversion symmetry but without global polarization. Unexpectedly, the FE order is found to be dramatically suppressed by carriers and destroyed at moderate ~10% carrier density. Here, we propose a general mechanism based on carrier-induced quantum fluctuations to explain this puzzling phenomenon. Basically, the quantum kinetic effect would drive the formation of polaronic quasi-particles made of the carriers and their surrounding dipoles. The disruption in dipolar directions can therefore weaken or even destroy the FE order. We demonstrate such polaron formation and the associated FE suppression via a simple model using exact diagonalization, perturbation, and quantum Monte Carlo approaches. This quantum mechanism also provides an intuitive picture for many puzzling experimental findings, thereby facilitating new designs of multifunctional FE electronic devices augmented with quantum effects.

cond-mat.mtrl-sci↗

arXiv2022

Gauge independent description of Aharonov-Bohm Effect

The Aharonov-Bohm (AB) effect is a pure quantum effect that implies a measurable phase shift in the wave function for a charged particle that encircles a magnetic flux located in a region \textit{inaccessible} to the particle. Classically, such a non-local effect appears to be impossible since the Lorentz force depends on only the magnetic field at the location of the particle. In quantum mechanics, the Hamiltonian, and thus the Schr\"odinger equation, has a local coupling between the current due to the particle, and the electromagnetic vector potential $\mathbf{A}$, which extends to the entire space beyond the region with finite magnetic field. This has sometimes been interpreted as meaning that in quantum mechanics $\mathbf{A}$ is in some sense more "fundamental" than $\mathbf {B}$ in spite of the former being gauge dependent, and thus unobservable. Here we shall, with a general proof followed by a few examples, demonstrate that the AB-effect can be fully accounted for by considering only the gauge invariant $\mathbf{B}$ field, as long as it is included as part of the quantum action of the entire isolated system. The price for the gauge invariant formulation is that we must give up locality -- the AB-phase for the particle will arise from the change in the action for the $\mathbf{B}$ field in the region inaccessible to the particle.

quant-ph↗