arXiv subjects

Wei Ku

Publications and source records attributed to Wei Ku.

At least 37 records · Page 2Linked to original sources

arXiv2023

Probing a Bose Metal via Electrons: Inescapable non-Fermi liquid scattering and pseudogap physics

Non-Fermi liquid behavior and pseudogap formation are among the most well-known examples of exotic spectral features observed in several strongly correlated materials such as the hole-doped cuprates, nickelates, iridates, ruthenates, ferropnictides, doped Mott organics, transition metal dichalcogenides, heavy fermions, d- and f- electron metals, etc. We demonstrate that these features are inevitable consequences when fermions couple to an unconventional Bose metal [1] mean field consisting of lower-dimensional coherence. Not only do we find both exotic phenomena, but also a host of other features that have been observed e.g. in the cuprates including nodal anti-nodal dichotomy and pseudogap asymmetry(symmetry) in momentum(real) space. Obtaining these exotic and heretofore mysterious phenomena via a mean field offers a simple, universal, and therefore widely applicable explanation for their ubiquitous empirical appearance. [1] A. Hegg, J. Hou, and W. Ku, Geometric frustration produces long-sought Bose metal phase of quantum matter, Proceedings of the National Academy of Sciences Nov 2021, 118 (46) e2100545118.

cond-mat.str-el↗

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

Strongly correlated doped hole carriers in the superconducting nickelates: Their location, local many-body state, and low-energy effective Hamiltonian

The families of high-temperature superconductors recently welcomed a new member: hole doped nickelate Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ with a $\sim$15K transition temperature. To understand its emergent low-energy behaviors and experimental properties, an immediate key question is whether the superconducting hole carriers reside in oxygen as in the cuprates, or in nickel as in most nickelates. We answer this crucial question via a ``(LDA+$U$)+ED'' scheme: deriving an effective interacting Hamiltonian of the hole carriers from density functional LDA+$U$ calculation, and studying its local many-body states via exact diagonalization. Surprisingly, distinct from the expected Ni$^{2+}$ spin-triplet state found in most nickelates, the local ground state of two holes is actually a Ni-O spin-singlet state with second hole greatly residing in oxygen. The emerged eV-scale model therefore resembles that of the cuprates, advocating further systematic experimental comparisons. Tracing the microscopic origin of this unexpected result to the lack of apical oxygen in this material, we proposed a route to increase superconducting temperature, and a possible quantum phase transition absent in the cuprates.

cond-mat.supr-con↗

arXiv2022

Proposal to improve Ni-based superconductors via enhanced charge transfer

Recently discovered superconductivity in hole-doped nickelate Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ has attracted intensive attention in the field. An immediate question is how to improve its superconducting properties. Guided by the key characteristics of electronic structures of the cuprates and the nickelates, we propose that nickel chalcogenides with a similar lattice structure should be a promising family of materials. Using NdNiS$_2$ as an example, through first-principle structural optimization and phonon calculation, we find this particular crystal structure a stable one. We justify our proposal by comparing with CaCuO$_2$ and NdNiO$_2$ with regard to strength of the charge-transfer characteristics and the trend in their low-energy many-body effective Hamiltonians of doped hole carriers. This analysis indicates that nickel chalcogenides host low-energy physics closer to that of the cuprates, with stronger magnetic interaction than the nickelates, and thus they deserve further experimental exploration. Our proposal also opens up the possibility of a wide range of parameter tuning through ligand substitution among chalcogenides, to further improve superconducting properties.

cond-mat.supr-con↗

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 Schrödinger 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↗

arXiv2022

Stronger quantum fluctuation with larger spins: Emergent magnetism in the pressurized high-temperature superconductor FeSe

A counter-intuitive enhancement of quantum fluctuation with larger spins, together with a few novel physical phenomena, is discovered in studying the recently observed emergent magnetism in high-temperature superconductor FeSe under pressure. Starting with experimental crystalline structure from our high-pressure X-ray refinement, we analyze theoretically the stability of the magnetically ordered state with a realistic spin-fermion model. We find surprisingly that in comparison with the magnetically ordered Fe-pnictides, the larger spins in FeSe suffer even stronger long-range quantum fluctuation that diminishes their ordering at ambient pressure. This "fail-to-order" quantum spin liquid state then develops into an ordered state above 1GPa due to weakened fluctuation accompanying the reduction of anion height and carrier density. The ordering further benefits from the ferro-orbital order and shows the observed enhancement around 1GPa. We further clarify the controversial nature of magnetism and its interplay with nematicity in FeSe in the same unified picture for all Fe-based superconductors. In addition, the versatile itinerant carriers produce interesting correlated metal behavior in a large region of phase space. Our study establishes a generic exceptional paradigm of stronger quantum fluctuation with larger spins that complements the standard knowledge of insulating magnetism.

cond-mat.str-el↗

arXiv2022

Mottness induced superfluid phase fluctuation with increased density

Recent observation of diminishing superfluid phase stiffness upon increasing carrier density in cuprate high-temperature superconductors is unexpected from the quantum density-phase conjugation of superfluidity. Here, through analytic estimation and verified via variational Monte Carlo calculation of an emergent Bose liquid, we point out that Mottness of the underlying carriers can cause a stronger phase fluctuation of the superfluid with increasing carrier density. This effect turns the expected density-increased phase stiffness into a dome shape, in good agreement with the recent observation. Specifically, the effective mass divergence due to "jamming" of the low-energy bosons reproduces the observed nonlinear relation between phase stiffness and transition temperature. Our results suggest a new paradigm, in which unconventional superconductivity in some strongly correlated materials is described by physics of bosonic superfluidity, as opposed to pairing-strength limited Cooper pairing.

cond-mat.supr-con↗

arXiv2021

Geometric frustration produces long-sought Bose metal phase of quantum matter

Two of the most prominent phases of bosonic matter are the superfluid with perfect flow and the insulator with no flow. A now decades-old mystery unexpectedly arose when experimental observations indicated that bosons could organize into the formation of an entirely different intervening third phase: the Bose metal with dissipative flow. The most viable theory for such a Bose metal to date invokes the use of the extrinsic property of impurity-based disorder; however, a generic intrinsic quantum Bose metal state is still lacking. We propose a universal homogeneous theory for a Bose metal in which geometric frustration confines the essential quantum coherence to a lower dimension. The result is a gapless insulator characterized by dissipative flow that vanishes in the low-energy limit. This failed insulator exemplifies a frustration-dominated regime that is only enhanced by additional scattering sources at low energy and therefore produces a Bose metal that thrives under realistic experimental conditions.

cond-mat.supr-con↗

arXiv2020

Fragility of the Kondo insulating gap against disorder: relevance to recent puzzles in topological Kondo insulators

Kondo insulators are strongly correlated system in which a clean insulating gap emerges only at very low temperature due to many-body effects involving localized $f$-electrons. However, certain Kondo insulators, like SmB$_6$ and Ce${_3}$Bi${_4}$Pt${_3}$, display metallic behaviors at extremely low temperatures, that have defied current understanding. Recent advances in topological effects in materials has raised the attention on the protected surface states in these "topological Kondo insulators" as a potential resolution to some of the puzzling behaviors. Here we resolve these puzzles via a different route, by showing that the emergent Kondo insulating scale is extremely vulnerable against moderate degree of disorder, such that the gap is filled with a small number of states. Therefore, the real samples are probably never truly insulating and this in turn compromises the essential building block of topological considerations. Our results suggest strongly that systems like the Slater insulators would be a more promising direction to extend the realm of topology to strongly correlated systems.

cond-mat.str-el↗

arXiv2019

Entropic elasticity and negative thermal expansion in a simple cubic crystal

While most solids expand when heated, some materials show the opposite behavior: negative thermal expansion (NTE). In polymers and biomolecules, NTE originates from the entropic elasticity of an ideal, freely-jointed chain. The origin of NTE in solids has been widely believed to be different. Our neutron scattering study of a simple cubic NTE material, ScF3, overturns this consensus. We observe that the correlation in the positions of the neighboring fluorine atoms rapidly fades on warming, indicating an uncorrelated thermal motion constrained by the rigid Sc-F bonds. This leads us to a quantitative theory of NTE in terms of entropic elasticity of a floppy network crystal, which is in remarkable agreement with experimental results. We thus reveal the formidable universality of the NTE phenomenon in soft and hard matter.

cond-mat.mtrl-sci↗

arXiv2019

Non-Fermi liquid scattering against an emergent Bose liquid: manifestations in the kink and other exotic quasiparticle behaviors in the normal-state cuprate superconductors

The normal state of cuprate superconductors exhibits many exotic behaviors qualitatively different from the Fermi liquid, the foundation of condensed matter physics. Here we demonstrate that non-Fermi liquid behaviors emerge naturally from scattering against an emergent Bose liquid. Particularly, we find a finite zero-energy scattering rate at low-temperature limit that grows linearly with respect to temperature, against clean fermions' generic non-dissipative characteristics. Surprisingly, three other seemingly unrelated experimental observations are also produced, including the well-studied "kink" in the quasi-particle dispersion, as well as the puzzling correspondences between the normal and superconducting state. Our findings provide a general route for fermionic systems to generate non-Fermi liquid behavior, and suggest strongly that by room temperature the doped holes in the cuprates have already formed an emergent Bose liquid of tightly bound pairs, whose low-temperature condensation gives unconventional superconductivity.

cond-mat.supr-con↗

arXiv2018

Hunting Down Magnetic Monopoles in 2D Topological Insulators

Contrary to the electric charge that generates the electric field, magnetic charge (namely magnetic monopoles) does not exist in the elementary electromagnetism. Consequently, magnetic flux lines only form loops and cannot have a source or a sink in nature. It is thus extraordinary to find that magnetic monopoles can be pictured conceptually in topological materials. Specifically in the 2D topological insulators, the topological invariant corresponds to the total flux of an effective magnetic field (the Berry curvature) over the reciprocal space.It is thus tempting to wrap the 2D reciprocal space into a compact manifold--a torus, and imagine the total flux to originate from magnetic monopoles inside the torus with a quantized total charge. However, such a physically appealing picture has not been realized quantitatively: other than their existence in a toy (actually misleading) picture, the properties of the magnetic monopoles remain unknown. Here, we will address this long-standing problem by hunting down the magnetic monopoles in the reciprocal $k$-space. We will show that a simple and physically useful picture will arrive upon analytically continuing the system to a third imaginary momentum space. We then illustrate the evolution of the magnetic monopoles across the topological phase transition and use it to provide natural explanations on: 1) discontinuous jump of integer topological invariants, 2) the semi-metallic nature on the phase boundary, and 3) how a change of global topology can be induced via a local change in reciprocal space.

cond-mat.mes-hall↗

arXiv2018

Transition between two metallic ferroelectric orders in multiferroic Ca$_3$Ru$_2$O$_7$, induced by magnetism-mediated orbital re-polarization

For the past decades, the low-temperature phase of Ca$_3$Ru$_2$O$_7$ below the 48K first-order phase transition remains a puzzle with controversial suggestions involving metallic ferroelectric, orbital or magnetic ordering. Through analysis of experimental lattice structure, density functional theory calculation, and effective model analysis, we propose that the 48K phase transition is a bond formation transition promoted by the magnetism mediated orbital re-polarization. Most interestingly, this transition is accompanied by a switch of \textit{two} metallic ferroelectric orders from a $xy+y$ symmetry to $xz+z$. Our study not only resolves a long-standing puzzle of this phase transition in this material, but also demonstrates perhaps the first example of transition between multiple emergent ferroelectric orders in bad metals, resulting from interplay between multiferroic orders.

cond-mat.str-el↗

arXiv2018

Electron and hole contributions to normal-state transport in the superconducting system Sn$_{1-x}$In$_x$Te

Indium-doped SnTe has been of interest because the system can exhibit both topological surface states and bulk superconductivity. While the enhancement of the superconducting transition temperature is established, the character of the electronic states induced by indium doping remains poorly understood. We report a study of magneto-transport in a series of Sn$_{1-x}$In$_x$Te single crystals with $0.1\le x \le 0.45$. From measurements of the Hall effect, we find that the dominant carrier type changes from hole-like to electron-like at $x\sim0.25$; one would expect electron-like carriers if the In ions have a valence of $+3$. For single crystals with $x = 0.45$, corresponding to the highest superconducting transition temperature, pronounced Shubnikov-de Haas oscillations are observed in the normal state. In measurements of magnetoresistance, we find evidence for weak anti-localization (WAL). We attribute both the quantum oscillations and the WAL to bulk Dirac-like hole pockets, previously observed in photoemission studies, which coexist with the dominant electron-like carriers.

cond-mat.supr-con↗

arXiv2018

Existence of electron and hole pockets and partial gap opening in the correlated semimetal Ca3Ru2O7

The electronic band structure of correlated Ca3Ru2O7 featuring an antiferromagnetic as well as a structural transition has been determined theoretically at high temperatures, which has led to the understanding of the remarkable properties of Ca3Ru2O7 such as the bulk spin valve effects. However, its band structure and Fermi surface (FS) below the structural transition have not been resolved even though a FS consisting of electron pockets was found experimentally. Here we report magneto electrical transport and thermoelectric measurements with the electric current and temper- ature gradient directed along a and b axes of an untwined single crystal of Ca3Ru2O7 respectively. The thermopower obtained along the two crystal axes were found to show opposite signs at low temperatures, demonstrating the presence of both electron and hole pockets on the FS. In addition, how the FS evolves across T* = 30 K at which a distinct transition from coherent to incoherent behavior occurs was also inferred - the Hall and Nernst coefficient results suggest a temperature and momentum dependent partial gap opening in Ca3Ru2O7 below the structural transition, with a pos- sible Lifshitz transition occurring at T*. The experimental demonstration of a correlated semimetal ground state in Ca3Ru2O7 calls for further theoretical studies of this remarkable material.

cond-mat.str-el↗

arXiv2017

Non-necessity of band inversion process in 2D topological insulators for bulk gapless states and topological phase transitions

In commonly employed models for 2D topological insulators, bulk gapless states are well known to form at the band inversion points where the degeneracy of the states is protected by symmetries. It is thus sometimes quite tempting to consider this feature, the occurrence of gapless states, a result of the band inversion process under protection of the symmetries. Similarly, the band inversion process might even be perceived as necessary to induce 2D topological phase transitions. To clarify these misleading perspectives, we propose a simple model with a flexible Chern number to demonstrate that the bulk gapless states emerge at the phase boundary of topological phase transitions, despite the absence of band inversion process. Furthermore, the bulk gapless states do not need to occur at the special $k$-points protected by symmetries. Given the significance of these fundamental \textit{conceptual} issues and their wide-spread influence, our clarification should generate strong general interests and significant impacts. Furthermore, the simplicity and flexibility of our general model with an arbitrary Chern number should prove useful in a wide range of future studies of topological states of matter.

cond-mat.mtrl-sci↗

arXiv2017

Pressure-induced melting of magnetic order and emergence of new quantum state in alpha-RuCl3

Here we report the observation of pressure-induced melting of antiferromagnetic (AFM) order and emergence of a new quantum state in the honeycomb-lattice halide alpha-RuCl3, a candidate compound in the proximity of quantum spin liquid state. Our high-pressure heat capacity measurements demonstrate that the AFM order smoothly melts away at a critical pressure (Pc) of 0.7 GPa. Intriguingly, the AFM transition temperature displays an increase upon applying pressure below the Pc, in stark contrast to usual phase diagrams, for example in pressurized parent compounds of unconventional superconductors. Furthermore, in the high-pressure phase an unusual steady of magnetoresistance is observed. These observations suggest that the high-pressure phase is in an exotic gapped quantum state which is robust against pressure up to ~140 GPa.

cond-mat.str-el↗