Showing 1–50 of 107 results for author: Ku, W

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

Authors: Xingchen Shen, Wei Ku

Abstract: 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… ▽ More 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. △ Less

Submitted 13 March, 2026; originally announced March 2026.

Comments: 6 pages, 3 figures

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

Authors: Muhammad Gaffar, Wei Ku

Abstract: 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… ▽ More 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. △ Less

Submitted 27 May, 2026; v1 submitted 3 November, 2025; originally announced November 2025.

Comments: 6 pages, 4 figures

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

Authors: Matheus de Sousa, Zhiyu Fan, Wei Ku

Abstract: 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 boun… ▽ More 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. △ Less

Submitted 16 October, 2025; originally announced October 2025.

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

Authors: Ruoshi Jiang, Bartomeu Monserrat, Wei Ku

Abstract: 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… ▽ More 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. △ Less

Submitted 2 October, 2025; originally announced October 2025.

Comments: 14 pages, 4 figures, and 1 table

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

Authors: Zhiyu Fan, Wei Ku

Abstract: 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 zer… ▽ More 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. △ Less

Submitted 19 June, 2026; v1 submitted 11 September, 2025; originally announced September 2025.

Comments: 12 pages (supplementary included), 2 figures

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

Authors: Fangyuan Gu, Ruoshi Jiang, Wei Ku

Abstract: 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,… ▽ More 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. △ Less

Submitted 24 July, 2025; originally announced July 2025.

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

Authors: Ruoshi Jiang, Zhiyu Fan, Bartomeu Monserrat, Wei Ku

Abstract: 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 f… ▽ More 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. △ Less

Submitted 27 October, 2025; v1 submitted 19 May, 2025; originally announced May 2025.

Comments: 7 pages, 3 figures, 3 tables, and the Supplemental Materials

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

Authors: Xinyao Zhang, Matheus S. M. de Sousa, Xinyi Li, Anthony Hegg, Wei Ku

Abstract: 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 h… ▽ More 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. △ Less

Submitted 7 August, 2024; originally announced August 2024.

Comments: 6 pages, 4 figures

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

Authors: Ruoshi Jiang, Fangyuan Gu, Wei Ku

Abstract: 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 us… ▽ More 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. △ Less

Submitted 17 December, 2025; v1 submitted 3 July, 2024; originally announced July 2024.

Comments: 30 pages, 7 figures and 4 tables

Journal ref: npj Comput Mater 12, 39 (2026)

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

Authors: Anthony Hegg, Ruoshi Jiang, Jie Wang, Jinning Hou, Tao Zeng, Yucel Yildirim, Wei Ku

Abstract: 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, he… ▽ More 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'. △ Less

Submitted 26 June, 2024; v1 submitted 13 February, 2024; originally announced February 2024.

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

Authors: Wei Ku, Anthony Hegg

Abstract: 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 g… ▽ More 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. △ Less

Submitted 16 January, 2024; originally announced January 2024.

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

Authors: Xinyao Zhang, Ruoshi Jiang, Xingchen Shen, Xiaomo Huang, Qing-Dong Jiang, Wei Ku

Abstract: 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… ▽ More 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. △ Less

Submitted 8 January, 2024; originally announced January 2024.

Journal ref: Nano Letters 2024 24 (34), 10451-10457

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

Authors: Zi-Jian Lang, Sudeshna Sen, Pak Ki Henry Tsang, Kristjan Haule, Vladimir Dobrosavljević, Wei Ku

Abstract: 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 in… ▽ More 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. △ Less

Submitted 28 December, 2025; v1 submitted 27 August, 2023; originally announced August 2023.

Comments: 6pages, 6figures

Journal ref: Phys. Rev. B 112, 235158(2025)

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

Authors: Ruoshi Jiang, Jinning Hou, Zhiyu Fan, Zi-Jian Lang, Wei Ku

Abstract: 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 effe… ▽ More 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. △ Less

Submitted 20 March, 2024; v1 submitted 22 August, 2023; originally announced August 2023.

Comments: 12 pages with 11 figures and 3 tables

Journal ref: Phys. Rev. Lett. 132, 126503 (2024)

Chemical disorder induced electronic orders in correlated metals

Authors: Jinning Hou, Yuting Tan, Wei Ku

Abstract: 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.… ▽ More 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. △ Less

Submitted 2 March, 2023; originally announced March 2023.

Comments: 6+5 pages

Quantum fluctuation of ferroelectric order in polar metals

Authors: Fangyuan Gu, Jie Wang, Zi-Jian Lang, Wei Ku

Abstract: 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… ▽ More 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. △ Less

Submitted 21 February, 2023; originally announced February 2023.

Comments: 12 pages, 6 figures in total

Journal ref: npj Quantum Materials, volume 8, Article number: 49 (2023)

Variation of carrier density in semimetals via short-range correlation: A case study with nickelate NdNiO$_2$

Authors: Ruoshi Jiang, Zi-Jian Lang, Tom Berlijn, Wei Ku

Abstract: Carrier density is one of the key controlling factors of material properties, particularly in controlling the essential correlations in strongly correlated materials. Typically, carrier density is externally tuned by doping or gating and remains fixed below room temperature. Strangely, the carrier density in correlated semimetals is often found to vary sensitively against weak external controls su… ▽ More Carrier density is one of the key controlling factors of material properties, particularly in controlling the essential correlations in strongly correlated materials. Typically, carrier density is externally tuned by doping or gating and remains fixed below room temperature. Strangely, the carrier density in correlated semimetals is often found to vary sensitively against weak external controls such as temperature, magnetic field, and pressure. Here, we develop a realistic simulation scheme that incorporates interatomic noncollinear magnetic correlation without a long-range order. Using the recently discovered nickelate superconductor as an example, we demonstrate a rather generic low-energy mechanism that in semimetals short-range correlation can reversely modulate the carrier density as well. Such a mutual influence between correlation and carrier density provides an extra ingredient for sensitive bifurcating behavior. This special feature of correlated semimetals explains their versatile carrier density at low energy and opens up new possibilities of functionalizing these materials. △ Less

Submitted 31 October, 2023; v1 submitted 18 July, 2022; originally announced July 2022.

Journal ref: Phys. Rev. B 108, 155126 (2023)

Transport in the emergent Bose liquid: Bad metal, strange metal, and weak insulator, all in one system

Authors: Tao Zeng, Anthony Hegg, Long Zou, Shengtao Jiang, Wei Ku

Abstract: Non-saturating high-temperature resistivity ("bad metal"), T-linear low-temperature resistivity ("strange metal"), and a crossover to activation-free growth of the resistivity in the low-temperature limit ("weak insulator") are among the most exotic behaviors widely observed in many strongly correlated materials for decades that defy the standard Fermi liquid description of solids. Here we investi… ▽ More Non-saturating high-temperature resistivity ("bad metal"), T-linear low-temperature resistivity ("strange metal"), and a crossover to activation-free growth of the resistivity in the low-temperature limit ("weak insulator") are among the most exotic behaviors widely observed in many strongly correlated materials for decades that defy the standard Fermi liquid description of solids. Here we investigate these puzzling behaviors by computing temperature-dependent optical conductivity of an emergent Bose liquid and find that it reproduces all the unexplained features of the experiments, including a featureless continuum and a well-known mid-infrared peak. Amazingly and with physically intuitive mechanisms, the corresponding doping- and temperature-dependent resistivity displays the bad metal and strange metal simultaneously and sometimes weak insulating behaviors as well. The unification of all these non-Fermi liquid behaviors in a single model suggests that a new quantum state of matter, namely the emergent Bose liquid, will guide the development of the next generation of solid state physics. △ Less

Submitted 10 December, 2021; originally announced December 2021.

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

Authors: Xinlei Yue, Anthony Hegg, Xiang Li, Wei Ku

Abstract: 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 inevitab… ▽ More 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. △ Less

Submitted 9 March, 2023; v1 submitted 15 April, 2021; originally announced April 2021.

Comments: 9 pages, 6 figures; revised appendix, updated reference

Journal ref: New Journal of Physics, Volume 25, May 2023

Proposal to improve Ni-based superconductors via enhanced charge transfer

Authors: Zi-Jian Lang, Ruoshi Jiang, Wei Ku

Abstract: 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 fam… ▽ More 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. △ Less

Submitted 14 December, 2022; v1 submitted 26 January, 2021; originally announced January 2021.

Comments: 3 pages + 2 pages supplemental material

Journal ref: Phys. Rev. B 105,100501(2022)

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

Authors: Anthony Hegg, Jinning Hou, Wei Ku

Abstract: 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… ▽ More 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. △ Less

Submitted 22 November, 2021; v1 submitted 15 January, 2021; originally announced January 2021.

Comments: 19 pages, 5 figures, supplementary material included

Journal ref: Proceedings of the National Academy of Sciences, 118, e2100545118 (2021)

Nearly degenerate ground states of a checkerboard antiferromagnet and their bosonic interpretation

Authors: Haiyuan Zou, Fan Yang, Wei Ku

Abstract: The spin-$1/2$ model system with antiferromagnetic (AF) couplings on a $J_1$-$J_2$ checkerboard lattice, known as the planar pyrochlore model, is strongly frustrated and associated with a two-to-one dimensional crossover. Using the Projected Entangled Simplex States tensor network ansatz, we identify a large number of nearly degenerate states in the frustrated region ($J_1<J_2$). Specifically, we… ▽ More The spin-$1/2$ model system with antiferromagnetic (AF) couplings on a $J_1$-$J_2$ checkerboard lattice, known as the planar pyrochlore model, is strongly frustrated and associated with a two-to-one dimensional crossover. Using the Projected Entangled Simplex States tensor network ansatz, we identify a large number of nearly degenerate states in the frustrated region ($J_1<J_2$). Specifically, we find the long-sought crossed-dimer valence bond solid (VBS) state to be the ground state at $J_1\lesssim J_2$, while various 1D AF correlated states take over the rest. We verify the stability of the VBS state against nematic perturbation. The corresponding bosonic picture provides an intuitive understanding of the low-energy physics. Particularly, it predicts weaker VBS states in the easy-plane limit, which we confirm numerically. Our results clarify the most essential ground state properties of this interesting system and demonstrate the usefulness of bosonic picture in dealing with frustrated magnetism. △ Less

Submitted 24 September, 2024; v1 submitted 12 November, 2020; originally announced November 2020.

Comments: 16 pages, 4+13 figures

Journal ref: Sci. China Phys. Mech. Astron. 67, 217211 (2024)

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

Authors: Zi-Jian Lang, Ruoshi Jiang, Wei Ku

Abstract: 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$15K transition temperature. To understand its emergent low-energy behaviors and experimental properties, an immediate key question is whether the superconducting hole carriers reside in oxygen as in the cuprates, or in nickel as in most nickelates. We answe… ▽ More The families of high-temperature superconductors recently welcomed a new member: hole doped nickelate Nd$_{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. △ Less

Submitted 14 December, 2022; v1 submitted 30 April, 2020; originally announced May 2020.

Comments: 6 pages, 2 figures

Journal ref: Phys. Rev. B 103, 180502 (2021)

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

Authors: Yuting Tan, Tianyu Zhang, Tao Zou, A. M. dos Santos, Jin Hu, Dao-Xin Yao, Z. Q. Mao, Xianglin Ke, Wei Ku

Abstract: 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… ▽ More 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. △ Less

Submitted 21 September, 2022; v1 submitted 30 July, 2019; originally announced July 2019.

Comments: 7 pages, 4 figures

Journal ref: Phys. Rev. Research 4, 033115(2022)

Entropic elasticity and negative thermal expansion in a simple cubic crystal

Authors: David Wendt, Emil Bozin, Joerg Neuefeind, Katharine Page, Wei Ku, Limin Wang, Brent Fultz, Alexei Tkachenko, Igor Zaliznyak

Abstract: 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 th… ▽ More 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. △ Less

Submitted 12 June, 2019; originally announced June 2019.

Comments: 6 pages, 4 figures. Work reported at 2019 APS March meeting (http://meetings.aps.org/Meeting/MAR19/Session/B33.8)

Journal ref: Science Advances 5 (11), eaay2748 (2019)

Mottness induced superfluid phase fluctuation with increased density

Authors: Zi-Jian Lang, Fan Yang, Wei Ku

Abstract: 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 stronge… ▽ More 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. △ Less

Submitted 18 September, 2022; v1 submitted 28 February, 2019; originally announced February 2019.

Comments: 11 pages, 7 figures

Journal ref: New J. Phys. 24 093026(2022)

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

Authors: Zheting Jin, Wei Ku

Abstract: 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 form… ▽ More 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. △ Less

Submitted 12 September, 2018; originally announced September 2018.

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

Authors: Sudeshna Sen, N. S. Vidhyadhiraja, Eduardo Miranda, Vladimir Dobrosavljević, Wei Ku

Abstract: 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 mater… ▽ More 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. △ Less

Submitted 15 August, 2020; v1 submitted 30 July, 2018; originally announced July 2018.

Comments: Accepted in Physical Review Research

Journal ref: Phys. Rev. Research 2, 033370 (2020)

On the Nature of Localization in Ti doped Si

Authors: Yi Zhang, R. Nelson, K. -M. Tam, W. Ku, U. Yu, N. S. Vidhyadhiraja, H. Terletska, J. Moreno, M. Jarrell, T. Berlijn

Abstract: Intermediate band semiconductors hold the promise to significantly improve the efficiency of solar cells, but only if the intermediate impurity band is metallic. We apply a recently developed first principles method to investigate the origin of electron localization in Ti doped Si, a promising candidate for intermediate band solar cells. Although Anderson localization is often overlooked in the co… ▽ More Intermediate band semiconductors hold the promise to significantly improve the efficiency of solar cells, but only if the intermediate impurity band is metallic. We apply a recently developed first principles method to investigate the origin of electron localization in Ti doped Si, a promising candidate for intermediate band solar cells. Although Anderson localization is often overlooked in the context of intermediate band solar cells, our results show that in Ti doped Si it plays a more important role in the metal insulator transition than Mott localization. Implications for the theory of intermediate band solar cells are discussed. △ Less

Submitted 10 July, 2018; v1 submitted 14 May, 2018; originally announced May 2018.

Comments: 5 pages, 3 figures for the main text. 8 pages, 11 figures for the supplementary material

Journal ref: Phys. Rev. B 98, 174204 (2018)

Decoupled Pairing Amplitude and Electronic Coherence in Iron-Based Superconductors

Authors: H. Miao, W. H. Brito, Z. P. Yin, R. D. Zhong, G. D. Gu, P. D. Johnson, M. P. M. Dean, S. Choi, G. Kotliar, W. Ku, X. C. Wang, C. Q. Jin, S. -F. Wu, T. Qian, H. Ding

Abstract: Here we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi liquid phase (LiFeAs) and an incoherent bad-metal phase (FeTe0.55Se0.45). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum superconducting pairing ampl… ▽ More Here we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi liquid phase (LiFeAs) and an incoherent bad-metal phase (FeTe0.55Se0.45). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum superconducting pairing amplitude, which shows a universal scaling that is valid for all FeSCs. Our observation excludes pairing scenarios in the BCS and the BEC limit for FeSCs and calls for a universal strong coupling pairing mechanism for the FeSCs. △ Less

Submitted 20 April, 2018; originally announced April 2018.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 98, 020502 (2018)

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

Authors: Cheng Zhang, Xu-Gang He, Hang Chi, Ruidan Zhong, Wei Ku, Genda Gu, J. M. Tranquada, Qiang Li

Abstract: 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… ▽ More 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. △ Less

Submitted 9 August, 2018; v1 submitted 27 February, 2018; originally announced February 2018.

Comments: 8 pages, 7 figures; accepted version

Journal ref: Phys. Rev. B 98, 054503 (2018)

Hunting Down Magnetic Monopoles in 2D Topological Insulators

Authors: Wenjie Xi, Wei Ku

Abstract: 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 topolog… ▽ More 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. △ Less

Submitted 28 September, 2018; v1 submitted 15 February, 2018; originally announced February 2018.

Journal ref: Phys. Rev. B 100, 121201 (2019)

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

Authors: Hui Xing, Libin Wen, Chenyi Shen, Jiaming He, Xinxin Cai, Jin Peng, Shun Wang, Mingliang Tian, Zhu-An Xu, Wei Ku, Zhiqiang Mao, Ying Liu

Abstract: 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 t… ▽ More 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. △ Less

Submitted 27 January, 2018; originally announced January 2018.

Journal ref: Phys. Rev. B 97, 041113(R) (2018)

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

Authors: Shengtao Jiang, Long Zou, Wei Ku

Abstract: 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… ▽ More 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. △ Less

Submitted 1 March, 2019; v1 submitted 14 December, 2017; originally announced December 2017.

Journal ref: Phys. Rev. B 99, 104507 (2019)

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

Authors: Wenjie Xi, Wei Ku

Abstract: 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 proce… ▽ More 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. △ Less

Submitted 30 October, 2017; originally announced October 2017.

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

Authors: Zhe Wang, Jing Guo, F. F. Tafti, Anthony Hegg, Sudeshna Sen, Vladimir A Sidorov, Le Wang, Shu Cai, Wei Yi, Yazhou Zhou, Honghong Wang, Shan Zhang, Ke Yang, Aiguo Li, Xiaodong Li, Yanchun Li, Jing Liu, Youguo Shi, Wei Ku, Qi Wu, Robert J Cava, Liling Sun

Abstract: 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… ▽ More 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. △ Less

Submitted 24 September, 2017; v1 submitted 17 May, 2017; originally announced May 2017.

Comments: 20 pages, 4 figures

Journal ref: Phys. Rev. B 97, 245149 (2018)

Unravelling local spin polarization of Zhang-Rice singlet in lightly hole-doped cuprates using high-energy optical conductivity

Authors: Iman Santoso, Wei Ku, Tomonori Shirakawa, Gerd Neuber, Xinmao Yin, M. Enoki, Masaki Fujita, Ruixing Liang, T. Venkatesan, George A. Sawatzky, Aleksei Kotlov, Seiji Yunoki, Michael Rübhausen, Andrivo Rusydi

Abstract: Unrevealing local magnetic and electronic correlations in the vicinity of charge carriers is crucial in order to understand rich physical properties in correlated electron systems. Here, using high-energy optical conductivity (up to 35 eV) as a function of temperature and polarization, we observe a surprisingly strong spin polarization of the local spin singlet with enhanced ferromagnetic correlat… ▽ More Unrevealing local magnetic and electronic correlations in the vicinity of charge carriers is crucial in order to understand rich physical properties in correlated electron systems. Here, using high-energy optical conductivity (up to 35 eV) as a function of temperature and polarization, we observe a surprisingly strong spin polarization of the local spin singlet with enhanced ferromagnetic correlations between Cu spins near the doped holes in lightly hole-doped La$_{1.95}$Sr$_{0.05}$Cu$_{0.95}$Zn$_{0.05}$O$_{4}$. The changes of the local spin polarization manifest strongly in the temperature-dependent optical conductivity at ~7.2 eV, with an anomaly at the magnetic stripe phase (~25 K), accompanied by anomalous spectral-weight transfer in a broad energy range. Supported by theoretical calculations, we also assign high-energy optical transitions and their corresponding temperature dependence, particularly at ~2.5 ~8.7, ~9.7, ~11.3 and ~21.8 eV. Our result shows the importance of a strong mixture of spin singlet and triplet states in hole-doped cuprates and demonstrates a new strategy to probe local magnetic correlations using high- energy optical conductivity in correlated electron systems. △ Less

Submitted 8 April, 2017; originally announced April 2017.

Comments: 38 pages, 11 figures

Journal ref: Phys. Rev. B 95, 165108 (2017)

Indium substitution effect on the topological crystalline insulator family (Pb$_{1-x}$Sn$_{x}$)$_{1-y}$In$_{y}$Te: Topological and superconducting properties

Authors: Ruidan Zhong, John Schneeloch, Qiang Li, Wei Ku, John Tranquada, Genda Gu

Abstract: Topological crystalline insulators (TCIs) have been of great interest in the area of condensed matter physics. We investigated the effect of indium substitution on the crystal structure and transport properties in the TCI system (Pb$_{1-x}$Sn$_{x}$)$_{1-y}$In$_{y}$Te. For samples with a tin concentration $x\le50\%$, the low-temperature resisitivities show a dramatic variation as a function of indi… ▽ More Topological crystalline insulators (TCIs) have been of great interest in the area of condensed matter physics. We investigated the effect of indium substitution on the crystal structure and transport properties in the TCI system (Pb$_{1-x}$Sn$_{x}$)$_{1-y}$In$_{y}$Te. For samples with a tin concentration $x\le50\%$, the low-temperature resisitivities show a dramatic variation as a function of indium concentration: with up to ~2% indium doping the samples show weak-metallic behavior, similar to their parent compounds; with ~6% indium doping, samples have true bulk-insulating resistivity and present evidence for nontrivial topological surface states; with higher indium doping levels, superconductivity was observed, with a transition temperature, Tc, positively correlated to the indium concentration and reaching as high as 4.7 K. We address this issue from the view of bulk electronic structure modified by the indium-induced impurity level that pins the Fermi level. The current work summarizes the indium substitution effect on (Pb,Sn)Te, and discusses the topological and superconducting aspects, which can be provide guidance for future studies on this and related systems. △ Less

Submitted 28 March, 2017; originally announced March 2017.

Comments: 16 pages, 8 figures

Journal ref: Crystals 2017, 7, 55; doi:10.3390/cryst7020055

Non-rigid band shift and non-monotonic electronic structure changes upon doping in the normal state of the pnictide high temperature superconductor Ba2(Fe1-xCox)2As2

Authors: PPaolo Vilmercati, Sung-Kwan Mo, Alexei Fedorov, Michael McGuire, Athena Sefat, Brian Sales, David Mandrus, David J. Singh, Wei Ku, Steve Johnston, Norman Mannella

Abstract: We report systematic Angle Resolved Photoemission (ARPES) experiments using different photon polarizations and experimental geometries and find that the doping evolution of the normal state of Ba(Fe1-xCox)2As2 deviates significantly from the predictions of a rigid band model. The data reveal a non-monotonic dependence upon doping of key quantities such as band filling, bandwidth of the electron po… ▽ More We report systematic Angle Resolved Photoemission (ARPES) experiments using different photon polarizations and experimental geometries and find that the doping evolution of the normal state of Ba(Fe1-xCox)2As2 deviates significantly from the predictions of a rigid band model. The data reveal a non-monotonic dependence upon doping of key quantities such as band filling, bandwidth of the electron pocket, and quasiparticle coherence. Our analysis suggests that the observed phenomenology and the inapplicability of the rigid band model in Co-doped Ba122 are due to electronic correlations, and not to either the size of the impurity potential, or self-energy effects due to impurity scattering. Our findings indicate that the effects of doping in pnictides are much more complicated than currently believed. More generally, they indicate that a deep understanding of the evolution of the electronic properties of the normal state, which requires an understanding of the doping process, remains elusive even for the 122 iron-pnictides, which are viewed as the least correlated of the high-TC unconventional superconductors. △ Less

Submitted 12 October, 2016; originally announced October 2016.

Comments: Accepted in Phys. Rev. B

Journal ref: Phys. Rev. B 94, 195147 (2016)

Towards a quantitative description of tunneling conductance of superconductors: application to LiFeAs

Authors: A. Kreisel, R. Nelson, T. Berlijn, W. Ku, Ramakrishna Aluru, Shun Chi, Haibiao Zhou, Udai Raj Singh, Peter Wahl, Ruixing Liang, Walter N. Hardy, D. A. Bonn, P. J. Hirschfeld, Brian M. Andersen

Abstract: Since the discovery of iron-based superconductors, a number of theories have been put forward to explain the qualitative origin of pairing, but there have been few attempts to make quantitative, material-specific comparisons to experimental results. The spin-fluctuation theory of electronic pairing, based on first-principles electronic structure calculations, makes predictions for the superconduct… ▽ More Since the discovery of iron-based superconductors, a number of theories have been put forward to explain the qualitative origin of pairing, but there have been few attempts to make quantitative, material-specific comparisons to experimental results. The spin-fluctuation theory of electronic pairing, based on first-principles electronic structure calculations, makes predictions for the superconducting gap. Within the same framework, the surface wave functions may also be calculated, allowing, e.g., for detailed comparisons between theoretical results and measured scanning tunneling topographs and spectra. Here we present such a comparison between theory and experiment on the Fe-based superconductor LiFeAs. Results for the homogeneous surface as well as impurity states are presented as a benchmark test of the theory. For the homogeneous system, we argue that the maxima of topographic image intensity may be located at positions above either the As or Li atoms, depending on tip height and the setpoint current of the measurement. We further report the experimental observation of transitions between As and Li-registered lattices as functions of both tip height and setpoint bias, in agreement with this prediction. Next, we give a detailed comparison between the simulated scanning tunneling microscopy images of transition-metal defects with experiment. Finally, we discuss possible extensions of the current framework to obtain a theory with true predictive power for scanning tunneling microscopy in Fe-based systems. △ Less

Submitted 31 December, 2016; v1 submitted 3 October, 2016; originally announced October 2016.

Comments: 14 pages, 15 figures, published version

Report number: NBI CMT 2016

Journal ref: Phys. Rev. B 94, 224518 (2016)

Impact of Iron-site defects on Superconductivity in LiFeAs

Authors: Shun Chi, Ramakrishna Aluru, Udai Raj Singh, Ruixing Liang, Walter N. Hardy, D. A. Bonn, A. Kreisel, Brian M. Andersen, R. Nelson, T. Berlijn, W. Ku, P. J. Hirschfeld, Peter Wahl

Abstract: In conventional s-wave superconductors, only magnetic impurities exhibit impurity bound states, whereas for an s+- order parameter they can occur for both magnetic and non-magnetic impurities. Impurity bound states in superconductors can thus provide important insight into the order parameter. Here, we present a combined experimental and theoretical study of native and engineered iron-site defects… ▽ More In conventional s-wave superconductors, only magnetic impurities exhibit impurity bound states, whereas for an s+- order parameter they can occur for both magnetic and non-magnetic impurities. Impurity bound states in superconductors can thus provide important insight into the order parameter. Here, we present a combined experimental and theoretical study of native and engineered iron-site defects in LiFeAs. Detailed comparison of tunneling spectra measured on impurities with spin fluctuation theory reveals a continuous evolution from negligible impurity bound state features for weaker scattering potential to clearly detectable states for somewhat stronger scattering potentials. All bound states for these intermediate strength potentials are pinned at or close to the gap edge of the smaller gap, a phenomenon that we explain and ascribe to multi-orbital physics. △ Less

Submitted 11 July, 2016; originally announced July 2016.

Comments: 5 pages, 4 figures

Report number: NBI CMT 2016

Journal ref: Phys. Rev. B 94, 134515 (2016)

Generalized Multiband Typical Medium Dynamical Cluster Approximation: Application to (Ga,Mn)N

Authors: Yi Zhang, R. Nelson, Elisha Siddiqui, K. -M. Tam, U. Yu, T. Berlijn, W. Ku, N. S. Vidhyadhiraja, J. Moreno, M. Jarrell

Abstract: We generalize the multiband typical medium dynamical cluster approximation and the formalism introduced by Blackman, Esterling and Berk so that it can deal with localization in multiband disordered systems with both diagonal and off-diagonal disorder with complicated potentials. We also introduce a new ansatz for the angle resolved typical density of states that greatly improves the numerical stab… ▽ More We generalize the multiband typical medium dynamical cluster approximation and the formalism introduced by Blackman, Esterling and Berk so that it can deal with localization in multiband disordered systems with both diagonal and off-diagonal disorder with complicated potentials. We also introduce a new ansatz for the angle resolved typical density of states that greatly improves the numerical stability of the method while preserving the independence of scattering events at different frequencies. Starting from the first-principles effective Hamiltonian, we apply this method to the diluted magnetic semiconductor Ga$_{1-x}$Mn$_x$N, and find the impurity band is completely localized for Mn concentrations $x<0.03$ while for $0.03 <x<0.10$ the impurity band has delocalized states but the chemical potential resides at or above the mobility edge. So, the system is always insulating within the experimental compositional limit ($x\approx 0.10$) due to Anderson localization. However, for $0.03 <x<0.10$ hole doping could make the system metallic allowing double exchange mediated, or enhanced, ferromagnetism. The developed method is expected to have a large impact on first-principles studies of Anderson localization. △ Less

Submitted 13 December, 2016; v1 submitted 10 July, 2016; originally announced July 2016.

Comments: 9 pages, 5 figures

Journal ref: Phys. Rev. B 94, 224208 (2016)

"Forbidden" phonon: dynamical signature of bond symmetry breaking in the iron chalcogenides

Authors: David M. Fobes, Igor A. Zaliznyak, John M. Tranquada, Zhijun Xu, Genda Gu, Xu-Gang He, Wei Ku, Yang Zhao, Masaaki Matsuda, V. Ovidu Garlea, Barry Winn

Abstract: Investigation of the inelastic neutron scattering spectra in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ near a signature wave vector $\mathbf{Q} = (1,0,0)$ for the bond-order wave (BOW) formation of parent compound Fe$_{1+y}$Te [Phys. Rev. Lett. 112, 187202 (2014)] reveals an acoustic-phonon-like dispersion present in all structural phases. While a structural Bragg peak accompanies the mode in the low-temperatu… ▽ More Investigation of the inelastic neutron scattering spectra in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ near a signature wave vector $\mathbf{Q} = (1,0,0)$ for the bond-order wave (BOW) formation of parent compound Fe$_{1+y}$Te [Phys. Rev. Lett. 112, 187202 (2014)] reveals an acoustic-phonon-like dispersion present in all structural phases. While a structural Bragg peak accompanies the mode in the low-temperature phase of Fe$_{1+y}$Te, it is absent in the high-temperature tetragonal phase, where Bragg scattering at this $\mathbf{Q}$ is forbidden by symmetry. Notably, this mode is also observed in superconducting FeTe$_{0.55}$Se$_{0.45}$, where structural and magnetic transitions are suppressed, and no BOW has been observed. The presence of this "forbidden" phonon indicates that the lattice symmetry is dynamically or locally broken by magneto-orbital BOW fluctuations, which are strongly coupled to lattice in these materials. △ Less

Submitted 10 May, 2016; v1 submitted 19 September, 2015; originally announced September 2015.

Comments: 5 pages, 4 figures; supplement: 8 pages, 9 figures

Journal ref: Phys. Rev. B 94, 121103 (2016)

Study of multiband disordered systems using the typical medium dynamical cluster approximation

Authors: Yi Zhang, Hanna Terletska, C. Moore, Chinedu Ekuma, Ka-Ming Tam, Tom Berlijn, Wei Ku, Juana Moreno, Mark Jarrell

Abstract: We generalize the typical medium dynamical cluster approximation to multiband disordered systems. Using our extended formalism, we perform a systematic study of the non-local correlation effects induced by disorder on the density of states and the mobility edge of the three-dimensional two-band Anderson model. We include inter-band and intra-band hopping and an intra-band disorder potential. Our r… ▽ More We generalize the typical medium dynamical cluster approximation to multiband disordered systems. Using our extended formalism, we perform a systematic study of the non-local correlation effects induced by disorder on the density of states and the mobility edge of the three-dimensional two-band Anderson model. We include inter-band and intra-band hopping and an intra-band disorder potential. Our results are consistent with the ones obtained by the transfer matrix and the kernel polynomial methods. We apply the method to K$_x$Fe$_{2-y}$Se$_2$ with Fe vacancies. Despite the strong vacancy disorder and anisotropy, we find the material is not an Anderson insulator. Our results demonstrate the application of the typical medium dynamical cluster approximation method to study Anderson localization in real materials. △ Less

Submitted 17 September, 2015; v1 submitted 16 September, 2015; originally announced September 2015.

Comments: 10 pages, 8 figures

Journal ref: Phys. Rev. B 92, 205111 (2015)

Itinerancy enhanced quantum fluctuation of magnetic moments in iron-based superconductors

Authors: Yu-Ting Tam, Dao-Xin Yao, Wei Ku

Abstract: We investigate the influence of itinerant carriers on dynamics and fluctuation of local moments in Fe-based superconductors, via linear spin-wave analysis of a spin-fermion model containing both itinerant and local degrees of freedom. Surprisingly against the common lore, instead of enhancing the ($π$,0) order, itinerant carriers with well nested Fermi surfaces is found to induce significant amoun… ▽ More We investigate the influence of itinerant carriers on dynamics and fluctuation of local moments in Fe-based superconductors, via linear spin-wave analysis of a spin-fermion model containing both itinerant and local degrees of freedom. Surprisingly against the common lore, instead of enhancing the ($π$,0) order, itinerant carriers with well nested Fermi surfaces is found to induce significant amount of \textit{spatial} and temporal quantum fluctuation that leads to the observed small ordered moment. Interestingly, the underlying mechanism is shown to be intra-pocket nesting-associated long-range coupling, rather than the previously believed ferromagnetic double-exchange effect. This challenges the validity of ferromagnetically compensated first-neighbor coupling reported from short-range fitting to the experimental dispersion, which turns out to result instead from the ferro-orbital order that is also found instrumental in stabilizing the magnetic order. △ Less

Submitted 9 March, 2015; originally announced March 2015.

Comments: 6 pages, 3 figures, submitted to Phys. Rev. Lett. on 17 September 2014

Journal ref: Phys. Rev. Lett. 115, 117001 (2015)

Surface-state-dominated transport in crystals of the topological crystalline insulator In-doped Pb$_{1-x}$Sn$_x$Te

Authors: Ruidan Zhong, Xugang He, John A. Schneeloch, Cheng Zhang, Tiansheng Liu, Ivo Pletikosic, Qiang Li, Wei Ku, Tonica Valla, J. M. Tranquada, Genda Gu

Abstract: Three-dimensional topological insulators and topological crystalline insulators represent new quantum states of matter, which are predicted to have insulating bulk states and spin-momentum-locked gapless surface states. Experimentally, it has proven difficult to achieve the high bulk resistivity that would allow surface states to dominate the transport properties over a substantial temperature ran… ▽ More Three-dimensional topological insulators and topological crystalline insulators represent new quantum states of matter, which are predicted to have insulating bulk states and spin-momentum-locked gapless surface states. Experimentally, it has proven difficult to achieve the high bulk resistivity that would allow surface states to dominate the transport properties over a substantial temperature range. Here we report a series of indium-doped Pb$_{1-x}$Sn$_x$Te compounds that manifest huge bulk resistivities together with strong evidence of topological surface states, based on thickness-dependent transport studies and magnetoresistance measurements. For these bulk-insulating materials, the surface states determine the resistivity for temperatures approaching 30 K. △ Less

Submitted 9 February, 2015; originally announced February 2015.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. B 91, 195321 (2015)

What is the valence of Mn in Ga$_{1-x}$Mn$_x$N?

Authors: Ryky Nelson, Tom Berlijn, Juana Moreno, Mark Jarrell, Wei Ku

Abstract: We investigate the current debate on the Mn valence in Ga$_{1-x}$Mn$_x$N, a diluted magnetic semiconductor (DMSs) with a potentially high Curie temperature. From a first-principles Wannier-function analysis, we unambiguously find the Mn valence to be close to $2+$ ($d^5$), but in a mixed spin configuration with average magnetic moments of 4$μ_B$. By integrating out high-energy degrees of freedom d… ▽ More We investigate the current debate on the Mn valence in Ga$_{1-x}$Mn$_x$N, a diluted magnetic semiconductor (DMSs) with a potentially high Curie temperature. From a first-principles Wannier-function analysis, we unambiguously find the Mn valence to be close to $2+$ ($d^5$), but in a mixed spin configuration with average magnetic moments of 4$μ_B$. By integrating out high-energy degrees of freedom differently, we further derive for the first time from first-principles two low-energy pictures that reflect the intrinsic dual nature of the doped holes in the DMS: 1) an effective $d^4$ picture ideal for local physics, and 2) an effective $d^5$ picture suitable for extended properties. In the latter, our results further reveal a few novel physical effects, and pave the way for future realistic studies of magnetism. Our study not only resolves one of the outstanding key controversies of the field, but also exemplifies the general need for multiple effective descriptions to account for the rich low-energy physics in many-body systems in general. △ Less

Submitted 12 November, 2015; v1 submitted 23 December, 2014; originally announced December 2014.

Comments: 4 figures

Journal ref: Phys. Rev. Lett. 115, 197203 (2015)

Generic Symmetry Breaking Instability of Topological Insulators due to a Novel van Hove Singularity

Authors: Xu-Gang He, Xiaoxiang Xi, Wei Ku

Abstract: We point out that in the deep band-inverted state, topological insulators are generically vulnerable against symmetry breaking instability, due to a divergently large density of states of 1D-like exponent near the chemical potential. This feature at the band edge is associated with a novel van Hove singularity resulting from the development of a Mexican-hat band dispersion. We demonstrate this gen… ▽ More We point out that in the deep band-inverted state, topological insulators are generically vulnerable against symmetry breaking instability, due to a divergently large density of states of 1D-like exponent near the chemical potential. This feature at the band edge is associated with a novel van Hove singularity resulting from the development of a Mexican-hat band dispersion. We demonstrate this generic behavior via prototypical 2D and 3D models. This realization not only explains the existing experimental observations of additional phases, but also suggests a route to activate additional functionalities to topological insulators via ordering, particularly for the long-sought topological superconductivities. △ Less

Submitted 10 October, 2014; originally announced October 2014.

Bulk Signatures of Pressure-Induced Band Inversion and Topological Phase Transitions in Pb$_{1-x}$Sn$_x$Se

Authors: Xiaoxiang Xi, Xu-Gang He, Fen Guan, Zhenxian Liu, R. D. Zhong, J. A. Schneeloch, T. S. Liu, G. D. Gu, D. Xu, Z. Chen, X. G. Hong, Wei Ku, G. L. Carr

Abstract: The characteristics of topological insulators are manifested in both their surface and bulk properties, but the latter remain to be explored. Here we report bulk signatures of pressure-induced band inversion and topological phase transitions in Pb$_{1-x}$Sn$_x$Se ($x=$0.00, 0.15, and 0.23). The results of infrared measurements as a function of pressure indicate the closing and the reopening of the… ▽ More The characteristics of topological insulators are manifested in both their surface and bulk properties, but the latter remain to be explored. Here we report bulk signatures of pressure-induced band inversion and topological phase transitions in Pb$_{1-x}$Sn$_x$Se ($x=$0.00, 0.15, and 0.23). The results of infrared measurements as a function of pressure indicate the closing and the reopening of the band gap as well as a maximum in the free carrier spectral weight. The enhanced density of states near the band gap in the topological phase give rise to a steep interband absorption edge. The change of density of states also yields a maximum in the pressure dependence of the Fermi level. Thus our conclusive results provide a consistent picture of pressure-induced topological phase transitions and highlight the bulk origin of the novel properties in topological insulators. △ Less

Submitted 6 June, 2014; originally announced June 2014.

Journal ref: Phys. Rev. Lett. 113, 096401 (2014)

Orbital-Parity Distinct Superconducting Pairing Structures of Fe-based Superconductors under Glide Symmetry

Authors: Chia-Hui Lin, Chung-Pin Chou, Wei-Guo Yin, Wei Ku

Abstract: We investigate an unusual symmetry of Fe-based superconductors (FeSCs) and find novel superconducting pairing structures. FeSCs have a minimal translational unit cell composed of two Fe atoms due to the staggered positions of anions with respect to the Fe plane. We study the physical consequences of the additional glide symmetry that further reduces the unit cell to have only one Fe atoms. In the… ▽ More We investigate an unusual symmetry of Fe-based superconductors (FeSCs) and find novel superconducting pairing structures. FeSCs have a minimal translational unit cell composed of two Fe atoms due to the staggered positions of anions with respect to the Fe plane. We study the physical consequences of the additional glide symmetry that further reduces the unit cell to have only one Fe atoms. In the regular momentum space, it not only leads to a particular orbital parity separated spectral function but also dictates orbital parity distinct pairing structures. Furthermore, it produces accompanying Cooper pairs of $(π,π,0)$ momentum, which have a characteristic \textit{odd} form factor and break time reversal symmetry. Such novel pairing structures explain the unusual angular modulations of the superconducting gaps on the hole pockets in recent ARPES and STS experiments. △ Less

Submitted 14 March, 2014; originally announced March 2014.

Comments: 5 pages, 4 figures