arXiv subjects

Haoyu Hu

Publications and source records attributed to Haoyu Hu.

At least 73 records · Page 4Linked to original sources

arXiv2024

Evidence of heavy fermion physics in the thermoelectric transport of magic angle twisted bilayer graphene

It has been recently postulated, that the strongly correlated flat bands of magicangle twisted bilayer graphene (MATBG) can host coexisting heavy and light carriers. While transport and spectroscopic measurements have shown hints of this behavior, a more direct experimental proof is still lacking. Here, we explore the thermoelectric response of MATBG through the photo-thermoelectric (PTE) effect in gate-defined MATBG pn-junctions. At low temperatures, we observe sign-preserving, fillingdependent oscillations of the Seebeck coefficient at non-zero integer fillings of the moiré lattice, which suggest the preponderance of one carrier type despite tuning the Fermi level from hole to electron doping of the correlated insulator. Furthermore, at higher temperatures, the thermoelectric response provides distinct evidence of the strong electron correlations in the unordered, normal state. We show that our observations are naturally accounted for by the interplay of light and long-lived and heavy and short-lived electron bands near the Fermi level at non-zero integer fillings. Our observations firmly establish the electron and hole asymmetry of the correlated gaps in MATBG, and shows excellent qualitative agreement with the recently developed topological heavy fermion model (THF).

cond-mat.mes-hall↗

arXiv2024

Dirac zeros in an orbital selective Mott phase: Green's function Berry curvature and flux quantization

How electronic topology develops in strongly correlated systems represents a fundamental challenge in the field of quantum materials. Recent studies have advanced the characterization and diagnosis of topology in Mott insulators whose underlying electronic structure is topologically nontrivial, through ``Green's function zeros". However, their counterparts in metallic systems have yet to be explored. Here, we address this problem in an orbital-selective Mott phase (OSMP), which is of extensive interest to a variety of strongly correlated systems with a short-range Coulomb repulsion. We demonstrate symmetry protected crossing of the zeros in an OSMP. Utilizing the concept of Green's function Berry curvature, we show that the zero crossing has a quantized Berry flux. The resulting notion of Dirac zeros provides a window into the largely hidden landscape of topological zeros in strongly correlated metallic systems and, moreover, opens up a means to diagnose strongly correlated topology in new materials classes.

cond-mat.str-el↗

arXiv2023

Pb$_9$Cu(PO4)$_6$(OH)$_2$: Phonon bands, Localized Flat Band Magnetism, Models, and Chemical Analysis

In a series of recent reports, doped lead apatite (LK-99) has been proposed as a candidate ambient temperature and pressure superconductor. However, from both an experimental and theoretical perspective, these claims are largely unsubstantiated. To this end, our synthesis and subsequent analysis of an LK-99 sample reveals a multiphase material that does not exhibit high-temperature superconductivity. We study the structure of this phase with single-crystal X-ray diffraction (SXRD) and find a structure consistent with doped $\text{Pb}_{10}(\text{PO}_4)_6(\text{OH})_2$. However, the material is transparent which rules out a superconducting nature. From ab initio defect formation energy calculations, we find that the material likely hosts $\text{OH}^-$ anions, rather than divalent $\text{O}^{2-}$ anions, within the hexagonal channels and that Cu substitution is highly thermodynamically disfavored. Phonon spectra on the equilibrium structures reveal numerous unstable phonon modes. Together, these calculations suggest it is doubtful that Cu enters the structure in meaningful concentrations, despite initial attempts to model LK-99 in this way. However for the sake of completeness, we perform ab initio calculations of the topology, quantum geometry, and Wannier function localization in the Cu-dominated flat bands of four separate doped structures. In all cases, we find they are atomically localized by irreps, Wilson loops, and the Fubini-Study metric. It is unlikely that such bands can support strong superfluidity, and instead are susceptible to ferromagnetism (or out-of-plane antiferromagnetism) at low temperatures, which we find in ab initio studies. In sum, $\text{Pb}_{9}\text{Cu}(\text{PO}_4)_6(\text{OH})_2$ could more likely be a magnet, rather than an ambient temperature and pressure superconductor.

cond-mat.supr-con↗

arXiv2023

Coupled topological flat and wide bands: Quasiparticle formation and destruction

Flat bands amplify correlation effects and are of extensive current interest. They provide a platform to explore both topology in correlated settings and correlation physics enriched by topology. Recent experiments in correlated kagome metals have found evidence for strange-metal behavior. A major theoretical challenge is to study the effect of local Coulomb repulsion when the band topology obstructs a real-space description. In a variant to the kagome lattice, we identify an orbital-selective Mott transition for the first time in any system of coupled topological flat and wide bands. This was made possible by the construction of exponentially localized and Kramers-doublet Wannier functions, which in turn leads to an effective Kondo lattice description. Our findings show how quasiparticles are formed in such coupled topological flat-wide band systems and, equally important, how they are destroyed. Our work provides a conceptual framework for the understanding of the existing and emerging strange-metal properties in kagome metals and beyond.

cond-mat.str-el↗

arXiv2023

Kagome Materials I: SG 191, ScV$_6$Sn$_6$. Flat Phonon Soft Modes and Unconventional CDW Formation: Microscopic and Effective Theory

Kagome Materials with flat bands exhibit wildly different physical properties depending on symmetry group, and electron number. For the case of ScV$_6$Sn$_6$ in space group 191, we investigate the existence of a charge density wave (CDW) at vector $\bar{K}=(\frac{1}{3},\frac{1}{3},\frac{1}{3})$ and its relationship with the phonon behavior. The experimental findings reveal a $\sim$95K CDW without nesting/peaks in the electron susceptibility at $\bar{K}$. Notably, ScV$_6$Sn$_6$ exhibits a collapsed phonon mode at $H=(\frac{1}{3},\frac{1}{3},\frac{1}{2})$ and an imaginary flat phonon band in the vicinity of $H$. The soft phonon is attributed to triangular Sn ($Sn^T$) mirror-even vibrations along the $z$-direction. We develop a simple force constant model to describe the entire soft phonon dispersion. By employing a new (Gaussian) approximation of the hopping parameter, we demonstrate the renormalization of the phonon frequency and the consequent collapse of the $H$ phonon. Additionally, we propose an effective model with two order parameters (OPs) to explain the appearance of the CDW at $\bar{K}$, which competes with the collapsed phonon at $H$. Through comparisons with experimental data, we show that the $H$ OP undergoes a second-order phase transition while exhibiting substantial fluctuations, ultimately inducing the first-order transition of the $\bar{K}$ OP. Furthermore, we extend our analysis to the similar compound YV$_6$Sn$_6$, which lacks a CDW phase, attributing this difference to the participation of the heavier Y atom in the out-of-plane phonon behavior. Our comprehensive study not only elucidates the CDW in ScV$_6$Sn$_6$ but also presents a significant advancement in modeling complex electronic systems, fostering collaborations between ab-initio simulations and analytical approaches.

cond-mat.str-el↗

arXiv2023

Cohomological boundedness for flat bundles on surfaces and applications

This paper explores the cohomological consequences of the existence of moduli spaces for flat bundles with bounded rank and irregularity at infinity and gives unconditional proofs. Namely, we prove the existence of a universal bound for the dimension of De Rham cohomology of flat bundles with bounded rank and irregularity on surfaces. In any dimension, we prove a Lefschetz recognition principle stating the existence of hyperplane sections distinguishing flat bundles with bounded rank and irregularity after restriction. We obtain in any dimension a universal bound for the degrees of the turning loci of flat bundles with bounded rank and irregularity. Along the way, we introduce a new operation on the group of b-divisors on a smooth surface (the partial discrepancy) and prove a closed formula for the characteristic cycles of flat bundles on surfaces in terms of the partial discrepancy of the irregularity b-divisor attached to any flat bundle by Kedlaya.

math.AG↗

arXiv2023

Observation of Kondo lattice and Kondo-enhanced anomalous Hall effect in an itinerant ferromagnet

The interplay between Kondo screening and magnetic interactions is central to comprehending the intricate phases in heavy-fermion compounds. However, the role of the itinerant magnetic order, which is driven by the conducting (c) electrons, has been largely uncharted in the context of heavy-fermion systems due to the scarcity of material candidates. Here we demonstrate the coexistence of the coherent Kondo screening and d-orbital ferromagnetism in material system La$_{1-x}$Ce$_x$Co$_2$As$_2$, through comprehensive thermodynamic and electrical transport measurements. Additionally, using angle-resolved photoemission spectroscopy (ARPES), we further observe the f-orbit-dominated bands near the Fermi level ($E_f$) and signatures of the f-c hybridization below the magnetic transition temperature, providing strong evidence of Kondo lattice state in the presence of ferromagnetic order. Remarkably, by changing the ratio of Ce/La, we observe a substantial enhancement of the anomalous Hall effect (AHE) in the Kondo lattice regime. The value of the Hall conductivity quantitatively matches with the first-principle calculation that optimized with our ARPES results and can be attributed to the large Berry curvature (BC) density engendered by the topological nodal rings composed of the Ce-4f and Co-3d orbitals at $E_f$. Our findings point to the realization of a new platform for exploring correlation-driven topological responses in a novel Kondo lattice environment.

cond-mat.str-el↗

arXiv2023

Kondo Lattice Model of Magic-Angle Twisted-Bilayer Graphene: Hund's Rule, Local-Moment Fluctuations, and Low-Energy Effective Theory

We apply a generalized Schrieffer-Wolff transformation to the extended Anderson-like topological heavy fermion (THF) model for the magic-angle $(θ=1.05^{\circ})$ twisted bilayer graphene (MATBLG) (Phys. Rev. Lett. 129, 047601 (2022)), to obtain its Kondo Lattice limit. In this limit localized $f$-electrons on a triangular lattice interact with topological conduction $c$-electrons. By solving the exact limit of the THF model, we show that the integer fillings $ν=0, \pm 1, \pm 2$ are controlled by the heavy $f$-electrons, while $ν= \pm 3$ is at the border of a phase transition between two $f$-electron fillings. For $ν=0, \pm 1, \pm 2$, we then calculate the RKKY interactions between the $f$-moments in the full model and analytically prove the SU(4) Hund's rule for the ground state which maintains that two $f$-electrons fill the same valley-spin flavor. Our (ferromagnetic interactions in the) spin model dramatically differ from the usual Heisenberg antiferromagnetic interactions expected at strong coupling. We show the ground state in some limits can be found exactly by employing a positive semidefinite "bond-operators" method. We then compute the excitation spectrum of the $f$-moments in the ordered ground state, prove the stability of the ground state favored by RKKY interactions, and discuss the properties of the Goldstone modes, the (reason for the accidental) degeneracy of (some of) the excitation modes, and the physics of their phase stiffness. We develop a low-energy effective theory for the $f$-moments and obtain analytic expressions for the dispersion of the collective modes. We discuss the relevance of our results to the spin-entropy experiments in twisted bilayer graphene.

cond-mat.str-el↗

arXiv2022

Extended Dynamical Mean Field Theory for Correlated Electron Models

An overarching question in strongly correlated electron systems is how the landscape of quantum phases emerges from electron correlations. The method of extended dynamical mean field theory (EDMFT) has been developed for clean lattice models of the correlated electrons. For such models, not only onsite Hubbard-like interactions are important, but so are intersite interactions. Importantly, the EDMFT method treats the interplay between the onsite and intersite interactions dynamically. It was initially formulated for models of the two-band Anderson-lattice type with intersite interactions, as well as for the one-band Hubbard type with intersite Heisenberg-like terms that are often called Hubbard-Heisenberg models. In the case of Kondo lattice models, the EDMFT method incorporates a dynamical competition between the local Kondo and intersite Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. In these models, the EDMFT-based analyses led to the notion of Kondo destruction, which has played a central role in the understanding of quantum critical heavy fermion metals. In this article, we summarize the EDMFT method, and survey its applications, particularly for Kondo/Anderson lattice models. We also discuss the prospect for further developing the EDMFT method, as well as for applying it to address the correlation physics in a variety of new settings. Among the latter are the orbital-selective Mott physics that arises both in iron-based superconductors and in frustrated bulk systems with topological flat bands.

cond-mat.str-el↗

arXiv2022

Physical Interaction: Reconstructing Hand-object Interactions with Physics

Single view-based reconstruction of hand-object interaction is challenging due to the severe observation missing caused by occlusions. This paper proposes a physics-based method to better solve the ambiguities in the reconstruction. It first proposes a force-based dynamic model of the in-hand object, which not only recovers the unobserved contacts but also solves for plausible contact forces. Next, a confidence-based slide prevention scheme is proposed, which combines both the kinematic confidences and the contact forces to jointly model static and sliding contact motion. Qualitative and quantitative experiments show that the proposed technique reconstructs both physically plausible and more accurate hand-object interaction and estimates plausible contact forces in real-time with a single RGBD sensor.

cs.CV↗

arXiv2022

Topological semimetals without quasiparticles

The interplay between interactions and topology in quantum materials is of extensive current interest. Strong correlations are known to be important for insulating topological states, as exemplified by the fractional quantum Hall effect. For the metallic case, whether and how they can drive topological states that have no free-electron counterparts is an open and pressing question. We introduce a general framework for lattice symmetries to constrain single-particle excitations even when they are not quasiparticles, and substantiate it in a periodic Anderson model with two channels of conduction electrons. We demonstrate that symmetry constrains correlation-induced emergent excitations to produce non-Fermi liquid topological phases. The loss of quasiparticles in these phases is manifested in a non-Fermi liquid form of spectral and transport properties, whereas its topological nature is characterized by surface states and valley and spin Hall conductivities. We also identify candidate materials to realize the proposed phases. Our work opens a door to a variety of non-Fermi liquid topological phases in a broad range of strongly correlated materials.

cond-mat.str-el↗

arXiv2022

Topological Semimetal driven by Strong Correlations and Crystalline Symmetry

Electron correlations amplify quantum fluctuations and, as such, they have been recognized as the origin of a rich landscape of quantum phases. Whether and how they lead to gapless topological states is an outstanding question, and a framework that allows for determining novel phases and identifying new materials is in pressing need. Here we advance a general approach, in which strong correlations cooperate with crystalline symmetry to drive gapless topological states. We test this materials design principle by exploring Kondo lattice models and materials whose space group symmetries may promote different kinds of electronic degeneracies, with a particular focus on square-net systems. Weyl-Kondo nodal-line semimetals -- with nodes pinned to the Fermi energy -- are identified. We describe how this approach can be applied to discover strongly correlated topological semimetals, identify three heavy fermion compounds as new candidates, provide first direct experimental evidence for our prediction in Ce$_2$Au$_3$In$_5$, and discuss how our approach may lead to many more. Our findings illustrate the potential of the proposed materials design principle to guide the search for new topological metals in a broad range of strongly correlated systems.

cond-mat.str-el↗

arXiv2022

Kondo destruction and fixed-point annihilation in a Bose-Fermi Kondo model

Quantum criticality that goes beyond the Landau framework of order-parameter fluctuations is playing a central role in elucidating the behavior of strange metals. A prominent case appears in Kondo lattice systems, which have been extensively analysed in terms of an effective Bose-Fermi Kondo model. Here, a spin is simultaneously coupled to conduction electron bands and gapless vector bosons that represent magnetic fluctuations. The Bose-Fermi Kondo model features interacting fixed points of Kondo destruction with such properties as dynamical Planckian ($\hbar ω/ k_{\rm B} T$) scaling and loss of quasiparticles. Here we carry out a renormalization-group analysis of the model with spin isotropy and identify pair-wise annihilations of the fixed points as the spectrum of the bosonic bath evolves. Our analysis not only provides an essentially complete understanding of the previous numerical results of an SU(2)-symmetric model, but also reveals a surprising feature of sequential fixed-point annihilation. Our results lay the foundation for the understanding of quantum criticality in spin-isotropic heavy-fermion metals as well as in doped Mott-Hubbard systems.

cond-mat.str-el↗

arXiv2022

Dynamical Planckian scaling of charge response at a particle-hole-asymmetric quantum critical point with Kondo destruction

Metallic quantum criticality is a central theme in a variety of strongly correlated systems. Recent experiments have raised the fundamental question of how the charge response can be singular in cases where the Landau framework of quantum criticality allows singularity only in the spin channel. Motivated by this emerging issue, we study the particle-hole-asymmetric regime of a Bose-Fermi Anderson model with power-law forms for both the bosonic bath spectrum and the fermionic band density of states. We realize a particle-hole-asymmetric quantum-critical state where quasiparticles are lost due to a critical destruction of Kondo screening, and demonstrate a dynamical Planckian scaling of the charge response. Implications for a new regime of heavy-fermion quantum criticality and for Mott-Hubbard systems are discussed.

cond-mat.str-el↗

arXiv2022

Characteristic cycle and wild ramification for nearby cycles of étale sheaves

In this article, we give a bound for the wild ramification of the monodromy action on the nearby cycles complex of a locally constant étale sheaf on the generic fiber of a smooth scheme over an equal characteristic trait in terms of Abbes and Saito's logarithmic ramification filtration. This provides a positive answer to the main conjecture in Isabel Leal's article "On the ramification of étale cohomology groups" for smooth morphisms in equal characteristic. We also study the ramification along vertical divisors of étale sheaves on relative curves and abelian schemes over a trait.

math.AG↗

arXiv2022

Semi-continuity of conductors, and ramification bound of nearby cycles

For a constructible étale sheaf on a smooth variety of positive characteristic ramified along an effective divisor, the largest slope in Abbes and Saito's ramification theory of the sheaf gives a divisor with rational coefficients called the conductor divisor. In this article, we prove decreasing properties of the conductor divisor after pull-backs. The main ingredient behind is the construction of étale sheaves with pure ramifications. As applications, we first prove a lower semi-continuity property for conductors of étale sheaves on relative curves in the equal characteristic case, which supplement Deligne and Laumon's lower semi-continuity property of Swan conductors and is also an $\ell$-adic analogue of André's semi-continuity result of Poincaré-Katz ranks for meromorphic connections on complex relative curves. Secondly, we give a ramification bound for the nearby cycle complex of an étale sheaf ramified along the special fiber of a regular scheme semi-stable over an equal characteristic henselian trait, which extends a main result in a joint work with Teyssier and answers a conjecture of Leal in a geometric situation.

math.AG↗

arXiv2022

Orbital-selective Mott phase as a dehybridization fixed point

Studies on the iron-based superconductors and related strongly correlated systems have focused attention on bad-metal normal state in proximity to antiferromagnetic order. An orbital-selective Mott phase (OSMP) has been extensively discussed as anchoring the orbital-selective correlation phenomena in this regime. Motivated by recent experiments, we advance the notion that an OSMP is synonymous to correlation-driven dehybridization. This idea is developed in terms of a competition between inter-orbital hopping and dynamical spatial spin correlations. Within effective models that arise from extended dynamical mean-field theory (EDMFT), and using a combination of continuous-time quantum Monte Carlo and analytical methods, we show how the OSMP emerges as a stable dehybridization fixed point. Concomitantly, the stability of the OSMP is demonstrated. Connections of this mechanism with partial localization-delocalization transition in other strongly correlated metals are discussed.

cond-mat.str-el↗

arXiv2022

Electron correlations and charge density wave in the topological kagome metal FeGe

Charge order in kagome metals is of extensive current interest. Recently, a charge density wave was discovered in the magnetic binary kagome metal FeGe. In analogy to its predecessor, the non-magnetic $A$V$_3$Sb$_5$ ($A$=K, Cs, Rb), the in-plane ordering occurs at the $M$ point. In contrast, however, the system manifestly shows effects of substantial correlations. Here we identify the topological bands crossing the Fermi energy (E$_F$) in FeGe and characterize the correlation-induced renormalization of these bands. We then derive a charge order from an effective model comprising topological kagome `flat' bands in the presence of a magnetic order. We demonstrate edge states as well as excess out-of-plane magnetic moment associated with the charge order; both are fingerprints of non-trivial band topology and consistent with recent experimental observations. Our results point to FeGe as an ideal platform to realize and elucidate correlated topological physics.

cond-mat.str-el↗