arXiv subjects

Dennis Huang

Publications and source records attributed to Dennis Huang.

14 recordsLinked to original sources

arXiv2026

Electronic tuning of the soft-phonon transport anomaly in Ta$_2$Ni(S$_x$Se$_{1-x}$)$_5$

Ta$_2$NiSe$_5$ continues to be investigated for its phase transition at $T_\textrm{c}$ = 326 K, where it develops both an electronic gap and a distortion of its Ta/Ni chains. One intriguing feature at $T_\textrm{c}$ seen in thermal transport is the giant anisotropic scattering of phonons moving perpendicular to the chains, which is apparently associated with the softening of a transverse acoustic phonon, but whose microscopic origin and significance demand clarification. By tuning the normal-state band overlap/gap with S substitution, we uncover a close connection between this soft-phonon transport anomaly and underlying electronic instabilities: When Ta$_2$Ni(S$_x$Se$_{1-x}$)$_5$ approaches a band insulator at high $x$, and signatures of the electronic transition are suppressed, the soft-phonon transport anomaly concomitantly vanishes. Our results establish the following picture for the Ta$_2$Ni(S$_x$Se$_{1-x}$)$_5$ family: Near the S end, a sole lattice instability gives rise to a weak structural transition with $T_\textrm{c}$ approaching 130 K. Near the Se end, additional electronic instabilities boost $T_\textrm{c}$ up to 326 K and amplify experimental signatures of the transition. The strong interaction between electrons, holes, and the lattice is manifested as a soft-phonon transport anomaly accompanied by electronic fluctuations, which include excitonic and hybridization-gap fluctuations.

cond-mat.str-el↗

arXiv2025

Substrate tuning of the structural and electronic transition in thin flakes of the excitonic insulator candidate Ta$_2$NiSe$_5$

Ta$_2$NiSe$_5$ continues to draw interest for its 326 K phase transition, whose dual electronic and structural nature reflects a complex interplay of electron-hole (excitonic) and electron-lattice interactions. Most studies that have attempted to decipher the relative importance of these interactions, particularly through charge transfer, have been limited to bulk samples. We utilized a thin-flake approach to modify the excitonic interactions in Ta$_2$NiSe$_5$ via an underlying film of Au. Using polarized Raman spectroscopy, we found that four layers of Ta$_2$NiSe$_5$ supported on conducting Au show a transition temperature that is both reduced by over 100 K and broadened due to an interfacial charge gradient effect, manifesting the presence of excitonic interactions. In contrast, four layers of Ta$_2$NiSe$_5$ supported on insulating Al$_2$O$_3$ show nearly bulk-like properties. We also report the development of an all-dry exfoliation and transfer protocol that generalizes substrate engineering for strongly correlated van der Waals materials.

cond-mat.str-el↗

arXiv2025

Visualizing the internal structure of the charge-density-wave state in CeSbTe

The collective reorganization of electrons into a charge density wave has long served as a textbook example of an ordered phase in condensed matter physics. Two-dimensional square lattices with $p$ electrons are well-suited to the realization of charge density waves, due to the anisotropy of the $p$ orbitals and the resulting one dimensionality of the electronic structure. In spite of a long history of study of charge density waves in square-lattice systems, few reports have recognized the significance of a hidden orbital degree of freedom. The degeneracy of $p_x$ and $p_y$ electrons may give rise to orbital patterns in real space that endow the charge density wave with additional broken symmetries or unusual order parameters. Here, we use scanning tunneling microscopy to visualize the internal structure of the charge-density-wave state of CeSbTe, which contains Sb square lattices with 5$p$ electrons. We image atomic-sized, anisotropic lobes of charge density with periodically modulating anisotropy, which we interpret in terms of a superposition of $p_x$ and $p_y$ bond density waves. Our results support the fact that delocalized $p$ orbitals can reorganize into emergent electronic states of matter.

cond-mat.mes-hall↗

arXiv2024

KV-Tandem -- a Modular Approach to Building High-Speed LSM Storage Engines

We present~\emph{KV-Tandem}, a modular architecture for building LSM-based storage engines on top of simple, non-ordered persistent key-value stores (KVSs). KV-Tandem enables advanced functionalities such as range queries and snapshot reads, while maintaining the native KVS performance for random reads and writes. Its modular design offers better performance trade-offs compared to previous KV-separation solutions, which struggle to decompose the monolithic LSM structure. Central to KV-Tandem is~\emph{LSM bypass} -- a novel algorithm that offers a fast path to basic operations while ensuring the correctness of advanced APIs. We implement KV-Tandem in \emph{XDP-Rocks}, a RocksDB-compatible storage engine that leverages the XDP KVS and incorporates practical design optimizations for real-world deployment. Through extensive microbenchmark and system-level comparisons, we demonstrate that XDP-Rocks achieves 3x to 4x performance improvements over RocksDB across various workloads. XDP-Rocks is already deployed in production, delivering significant operator cost savings consistent with these performance gains.

cs.DB↗

arXiv2024

Alternate cleavage structure and electronic inhomogeneity in Ca-doped YBa$_2$Cu$_3$O$_{7-\delta}$

YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) has favorable macroscopic superconducting properties of $T_\mathrm{c}$ up to 93 K and $H_{c2}$ up to 150 T. However, its nanoscale electronic structure remains mysterious because bulk-like electronic properties are not preserved near the surface of cleaved samples for easy access by local or surface-sensitive probes. It has been hypothesized that Ca-doping at the Y site could induce an alternate cleavage plane that mitigates this issue. We use scanning tunneling microscopy (STM) to study both Ca-free and 10% Ca-doped YBCO, and find that the Ca-doped samples do indeed cleave on an alternate plane, yielding a spatially-disordered partial (Y,Ca) layer. Our density functional theory calculations support the increased likelihood of this new cleavage plane in Ca-doped YBCO. On this surface, we image a superconducting gap with average value 24 $\pm$ 3 meV and characteristic length scale 1-2 nm, similar to Bi-based high-$T_\mathrm{c}$ cuprates, but the first map of gap inhomogeneity in the YBCO family.

cond-mat.supr-con↗

arXiv2021

Real-space visualization of quasiparticle dephasing near the Planckian limit in the Dirac line node material ZrSiS

Dirac line node (DLN) materials are topological semimetals wherein a set of symmetry protected crossing points forms a one-dimensional (1D) line in reciprocal space. Not only are the linearly dispersing bands expected to give rise to exceptional electronic properties, but the weak screening of the Coulomb interaction near the line node may enhance electronic correlations, produce new many-body ground states, or influence the quasiparticle lifetime. We investigate the quasiparticle dynamics in the DLN material ZrSiS via spectroscopic imaging scanning tunneling microscopy (SI-STM). By studying the spatial decay of quasiparticle interference patterns (QPI) from point scatterers, we were able to directly and selectively extract the phase coherence length $l_{\textrm{QPI}}$ and lifetime $τ_{\textrm{QPI}}$ for the bulk DLN excitations, which are dominated by inelastic electron-electron scattering. We find that the experimental $τ_{\textrm{QPI}}(E)$ values below $-$40 meV are very short, likely due to the stronger Coulomb interactions, and lie at the Planckian limit $\hbar/|E|$. Our results corroborate a growing body of experimental reports demonstrating unusual electronic correlation effects near a DLN.

cond-mat.str-el↗

arXiv2017

Monolayer FeSe on SrTiO$_3$

Epitaxial engineering of solid-state heterointerfaces is a leading avenue to realizing enhanced or novel electronic states of matter. As a recent example, bulk FeSe is an unconventional superconductor with a modest transition temperature ($T_c$) of 9 K. When a single atomic layer of FeSe is grown on SrTiO$_3$, however, its $T_c$ can skyrocket by an order of magnitude to 65 K or 109 K. Since this discovery in 2012, efforts to reproduce, understand, and extend these findings continue to draw both excitement and scrutiny. In this review, we first present a critical survey of experimental measurements performed using a wide range of techniques. We then turn to the open question of microscopic mechanisms of superconductivity. We examine contrasting indications for both phononic (conventional) and magnetic/orbital (unconventional) means of electron pairing, and speculations about whether they could work cooperatively to boost $T_c$ in a monolayer of FeSe.

cond-mat.supr-con↗

arXiv2017

Etching of Cr tips for scanning tunneling microscopy of cleavable oxides

We report a detailed three-step roadmap for the fabrication and characterization of bulk Cr tips for spin-polarized scanning tunneling microscopy. Our strategy uniquely circumvents the need for ultra-high vacuum preparation of clean surfaces or films. First, we demonstrate the role of $ex$-$situ$ electrochemical etch parameters on Cr tip apex geometry, using scanning electron micrographs of over 70 etched tips. Second, we describe the suitability of the $in$-$situ$ cleaved surface of the layered antiferromagnet La$_{1.4}$Sr$_{1.6}$Mn$_2$O$_7$ to evaluate the spin characteristics of the Cr tip, replacing the UHV-prepared test samples that have been used in prior studies. Third, we outline a statistical algorithm that can effectively delineate closely-spaced or irregular cleaved step edges, to maximize the accuracy of step height and spin-polarization measurements.

cond-mat.mtrl-sci↗

arXiv2016

Electric Field Tuning of Band Offsets in Transition Metal Dichalcogenides

We use first-principles calculations to investigate the band structure evolution of W$X_2$/Mo$X_2$ ($X$ = S, Se) heterobilayers under a perpendicular electric field. We characterize the extent to which the type-II band alignment in these compounds can be tuned or inverted electrostatically. Our results demonstrate two effects of stacking configuration. First, different stackings produce different net dipole moments, resulting in band offset variations that are larger than 0.1 eV. Second, based on symmetry constraints that depend on stacking, a perpendicular electric field may hybridize W$X_2$ and Mo$X_2$ bands that cross at the Brillouin Zone corner $K$. Our results suggest that external electric fields can be used to tune the physics of intralayer and interlayer excitons in heterobilayers of transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2016

Dumbbell Defects in FeSe Films: A Scanning Tunneling Microscopy and First-Principles Investigation

The properties of iron-based superconductors (Fe-SCs) can be varied dramatically with the introduction of dopants and atomic defects. As a pressing example, FeSe, parent phase of the highest-$T_c$ Fe-SC, exhibits prevalent defects with atomic-scale "dumbbell" signatures as imaged by scanning tunneling microscopy (STM). These defects spoil superconductivity when their concentration exceeds 2.5%. Resolving their chemical identity is prerequisite to applications such as nanoscale patterning of superconducting/nonsuperconducting regions in FeSe, as well as fundamental questions such as the mechanism of superconductivity and the path by which the defects destroy it. We use STM and density functional theory to characterize and identify the dumbbell defects. In contrast to previous speculations about Se adsorbates or substitutions, we find that an Fe-site vacancy is the most energetically favorable defect in Se-rich conditions, and reproduces our observed STM signature. Our calculations shed light more generally on the nature of Se capping, the removal of Fe vacancies via annealing, and their ordering into a $\sqrt{5}$$\times$$\sqrt{5}$ superstructure in FeSe and related alkali-doped compounds.

cond-mat.supr-con↗

arXiv2016

Bounds on Nanoscale Nematicity in Single-Layer FeSe/SrTiO$_3$

We use scanning tunneling microscopy (STM) and quasiparticle interference (QPI) imaging to investigate the low-energy orbital texture of single-layer FeSe/SrTiO$_3$. We develop a $T$-matrix model of multi-orbital QPI to disentangle scattering intensities from Fe $3d_{xz}$ and $3d_{yz}$ bands, enabling the use of STM as a nanoscale detection tool of nematicity. By sampling multiple spatial regions of a single-layer FeSe/SrTiO$_3$ film, we quantitatively exclude static $xz/yz$ orbital ordering with domain size larger than $δr^2$ = 20 nm $\times$ 20 nm, $xz/yz$ Fermi wave vector difference larger than $δk$ = 0.014 $π$, and energy splitting larger than $δE$ = 3.5 meV. The lack of detectable ordering pinned around defects places qualitative constraints on models of fluctuating nematicity.

cond-mat.supr-con↗

arXiv2015

Revealing the Empty-State Electronic Structure of Single-Unit-Cell FeSe/SrTiO$_{3}$

We use scanning tunneling spectroscopy to investigate the filled and empty electronic states of superconducting single-unit-cell FeSe deposited on SrTiO$_3$(001). We map the momentum-space band structure by combining quasiparticle interference imaging with decay length spectroscopy. In addition to quantifying the filled-state bands, we discover a $Γ$-centered electron pocket 75 meV above the Fermi energy. Our density functional theory calculations show the orbital nature of empty states at $Γ$ and suggest that the Se height is a key tuning parameter of their energies, with broad implications for electronic properties.

cond-mat.supr-con↗

arXiv2014

Nanoscale interplay of strain and doping in a high-temperature superconductor

The highest temperature superconductors are electronically inhomogeneous at the nanoscale, suggesting the existence of a local variable which could be harnessed to enhance the superconducting pairing. Here we report the relationship between local doping and local strain in the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$. We use scanning tunneling microscopy to discover that the crucial oxygen dopants are periodically distributed, in correlation with local strain. Our picoscale investigation of the intra-unit-cell positions of all oxygen dopants provides essential structural input for a complete microscopic theory.

cond-mat.str-el↗

arXiv2013

Nanoscale Surface Element Identification and Dopant Homogeneity in the High-$T_c$ Superconductor Pr$_x$Ca$_{1-x}$Fe$_2$As$_2$

We use scanning tunneling microscopy to determine the surface structure and dopant distribution in Pr$_x$Ca$_{1-x}$Fe$_2$As$_2$, the highest-$T_c$ member of the 122 family of iron-based superconductors. We identify the cleaved surface termination by mapping the local tunneling barrier height, related to the work function. We image the individual Pr dopants responsible for superconductivity, and show that they do not cluster, but in fact repel each other at short length scales. We therefore suggest that the low volume fraction high-$T_c$ superconducting phase is unlikely to originate from Pr inhomogeneity.

cond-mat.supr-con↗