arXiv subjects

Qiangqiang Gu

Publications and source records attributed to Qiangqiang Gu.

At least 37 records · Page 2Linked to original sources

arXiv2023

Computational exploration of a viable route to Kitaev-quantum spin liquid phase in OsCl$_3$

In this computational study, we explore a viable route to access the Kitaev-Quantum Spin Liquid (QSL) state in recently synthesized monolayer of a so-called spin-orbit assisted Mott insulator OsCl$_3$. In addition to other magnetic ground states in different regions, the small $J_\text{H}$/$U$ region of our Hubbard $U$--Hund's $J_\text{H}$ quantum phase diagram, obtained by combining second-order perturbation and pseudo-Fermion renormalization group calculations, hosts Kitaev-QSL phase. Only Kitaev interaction of a smaller magnitude is obtained in this region. Negligibly small farther neighbor interactions appear as a distinct feature of monolayer \os, suggesting this material to be a better candidate for the exploration of possible Kitaev-QSL state than earlier proposed materials. Insights from our study might be useful to probe magnetic phase transitions by purportedly manipulating $U$ and $J_\text{H}$ with epitaxial strain in advanced crystal growth techniques.

cond-mat.str-el↗

arXiv2023

Observation of 1D Fermi arc states in Weyl semimetal TaAs

Fermi arcs on Weyl semimetals exhibit many exotic quantum phenomena. Usually considered on atomically-flat surfaces with approximate translation symmetry, Fermi arcs are rooted in peculiar topology of bulk Bloch bands of three-dimensional (3D) crystals. The fundamental question of whether a 1D Fermi arc can be probed remains unanswered. Such answer could significantly broaden potential applications of Weyl semimetals. Here, we report a direct observation of robust edge states on atomic-scale ledges in TaAs using low-temperature scanning tunneling microscopy/spectroscopy. Spectroscopic signatures and theoretical calculations reveal that the 1D Fermi arcs arise from the chiral Weyl points of bulk crystal. The crossover from 2D Fermi arcs to eventual complete localization on 1D edges was arrested experimentally on a sequence of surfaces. Our results demonstrate extreme robustness of the bulk-boundary correspondence, which offers topological protection for Fermi arcs, even in cases in which the boundaries are down to atomic-scale. The persistent 1D Fermi arcs can be profitably exploited in miniaturized quantum devices.

cond-mat.mes-hall↗

arXiv2023

Detection of a Pair Density Wave State in UTe$_2$

Spin-triplet topological superconductors should exhibit many unprecedented electronic properties including fractionalized electronic states relevant to quantum information processing. Although UTe$_2$ may embody such bulk topological superconductivity, its superconductive order-parameter $Δ(\mathbf{k})$ remains unknown. Many diverse forms for $Δ(\mathbf{k})$ are physically possible in such heavy fermion materials. Moreover, intertwined density waves of spin (SDW), charge (CDW) and pairs (PDW) may interpose, with the latter state exhibiting spatially modulating superconductive order-parameter $Δ(\mathbf{r})$, electron pair density and pairing energy-gap. Hence, the newly discovered CDW state in UTe$_2$ motivates the prospect that a PDW state may exist in this material. To search for it, we visualize the pairing energy-gap with $μ$$eV$-scale energy-resolution using superconductive STM tips. We detect three PDWs, each with peak-peak gap modulations circa 10 $μ$$eV$ and at incommensurate wavevectors $\mathbf{P}_{i=1,2,3}$ that are indistinguishable from the wavevectors $\mathbf{Q}_{i=1,2,3}$ of the prevenient CDW. Concurrent visualization of the UTe$_2$ superconductive PDWs and the non-superconductive CDWs reveals that every $\mathbf{P}_i$ : $\mathbf{Q}_i$ pair exhibits a relative phase $δϕ\approx π$. From these observations, and given UTe$_2$ as a spin-triplet superconductor, this PDW state should be a spin-triplet pair density wave. While such states do exist in superfluid $^{3}$He, for superconductors they are unprecedented.

cond-mat.supr-con↗

arXiv2022

Emergence of bond-dependent highly anisotropic magnetic interactions in Sr$_4$RhO$_6$: a theoretical study

The quantum spin liquid states as a natural ground state of the Kitaev model has led to a quest for new materials candidates hosting Kitaev physics. Yet, there are very few material candidates in this category. Using a combination of $ab$ $initio$ and model Hamiltonian methods, we propose that Ruddlesden-Popper compound Sr$_4$RhO$_6$ belongs to this category. With a tight-binding model and exact diagonalization approach, we show that despite substantial trigonal-like distortion, the electronic and magnetic properties of Sr$_4$RhO$_6$ can be well described in terms of pseudo-spin = 1/2 states. Magnetic interactions among pseudo-spins, estimated using the second-order perturbation method are highly bond-dependent anisotropic in nature with two particularly noticeable features, antiferromagnetic Kitaev and Dzyaloshinskii-Moriya interactions. The gaped spin-wave spectra of Sr$_4$RhO$_6$ obtained with linear spin-wave theory is consistent with the underlying magnetic frustration. Additional analysis of the role of individual or a particular combination of magnetic interactions reveals that the spin-wave spectra of Sr$_4$RhO$_6$ is a combined effect of the highly anisotropic interactions and a relatively simpler minimal model may not be plausible in the current case. The crucial insights about coupling between the local structural features and magnetic properties of Sr$_4$RhO$_6$ obtained in this study may be helpful for future studies belonging to this class.

cond-mat.str-el↗

arXiv2022

A computational method to estimate spin-orbital interaction strength in solid state systems

Spin-orbit coupling (SOC) drives interesting and non-trivial phenomena in solid state physics, ranging from topological to magnetic to transport properties. Thorough study of such phenomena often require effective models where SOC term is explicitly included. However, estimation of SOC strength for such models mostly depend on the spectroscopy experiments which can only provide a rough estimate. In this work, we provide a simple yet effective computational approach to estimate the on-site SOC strength using a combination of the $ab$ $initio$ and tight-binding calculations. We demonstrate the wider applicability and high sensitivity of our method considering materials with varying SOC strengths and the number of SOC active ions. The estimated SOC strengths agree well with the proposed values in literature lending support to our methodology. This simplistic approach can readily be applied to a wide range of materials.

cond-mat.mtrl-sci↗

arXiv2022

Neural network representation of electronic structure from $ab$ $initio$ molecular dynamics

Despite their rich information content, electronic structure data amassed at high volumes in $ab$ $initio$ molecular dynamics simulations are generally under-utilized. We introduce a transferable high-fidelity neural network representation of such data in the form of tight-binding Hamiltonians for crystalline materials. This predictive representation of $ab$ $initio$ electronic structure, combined with machine-learning boosted molecular dynamics, enables efficient and accurate electronic evolution and sampling. When applied to a one-dimension charge-density wave material, carbyne, we are able to compute the spectral function and optical conductivity in the canonical ensemble. The spectral functions evaluated during soliton-antisoliton pair annihilation process reveal significant renormalization of low-energy edge modes due to retarded electron-lattice coupling beyond the Born-Oppenheimer limit. The availability of an efficient and reusable surrogate model for the electronic structure dynamical system will enable calculating many interesting physical properties, paving way to previously inaccessible or challenging avenues in materials modeling.

cond-mat.mtrl-sci↗

arXiv2021

No observation of chiral flux current in the topological kagome metal CsV$_{3}$Sb$_{5}$

Compounds with kagome lattice usually host many exotic quantum states, including the quantum spin liquid, non-trivial topological Dirac bands and a strongly renormalized flat band, etc. Recently an interesting vanadium based kagome family $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, or Cs) was discovered, and these materials exhibit multiple interesting properties, including unconventional saddle-point driving charge density wave (CDW) state, superconductivity, etc. Furthermore, some experiments show anomalous Hall effect which inspires that there might be some chiral flux current states. Here we report scanning tunneling measurements by using spin-polarized tips. Although we have observed clearly the $2a_0\times2a_0$ CDW and $4a_0$ stripe orders, the well-designed experiments with refined spin-polarized tips do not reveal any trace of the chiral flux current phase in CsV$_3$Sb$_5$ within the limits of experimental accuracy. No observation of the local magnetic moment in our experiments may put an upper bound constraint on the magnitude of magnetic moments induced by the possible chiral loop current which has a time-reversal symmetry breaking along $c$-axis in CsV$_{3}$Sb$_{5}$.

cond-mat.str-el↗

arXiv2021

Superconductivity in nickel based 112 systems

Superconductivity has been discovered recently in nickel based 112 infinite thin films $R_{1-x}$A$_x$NiO$_2$ ($R$ = La, Nd, Pr and A = Sr, Ca). They are isostructural to the infinite-layer cuprate (Ca,Sr)CuO$_2$ and are supposed to have a formal Ni 3$d^9$ valence, thus providing a new platform to study the unconventional pairing mechanism of high-temperature superconductors. This important discovery immediately triggers a huge amount of innovative scientific curiosity in the field. In this paper, we try to give an overview of the recent research progress on the newly found superconducting nickelate systems, both from experimental and theoretical aspects. We will focus mainly on the electronic structures, magnetic excitations, phase diagrams, superconducting gaps and finally make some open discussions for possible pairing symmetries in Ni based 112 systems.

cond-mat.supr-con↗

arXiv2021

Worldline algorithm by oracle-guided variational autoregressive network

The variational autoregressive network is extended to the Euclidean path integral representation of quantum partition function. An essential challenge is adapting the sequential process of sample generation by an autoregressive network to a nonlocal constraint due to the periodic boundary condition in path integral. An add-on oracle is devised for this purpose, which accurately identifies and stalls unviable configurations as soon as they occur. The oracle enables rejection-free sampling conforming to the periodic boundary condition. As a demonstration, the oracle-guided autoregressive network is applied to obtain variational solutions of quantum spin chains at finite temperatures with relatively large system sizes and numbers of time slicing, and to efficiently compute thermodynamic quantities.

cond-mat.stat-mech↗

arXiv2021

Direct Visualization of a Static Incommensurate Antiferromagnetic Order by Suppressing the Superconducting Phase Coherence in Fe-doped Bi2Sr2CaCu2O8+delta

In cuprate superconductors, due to strong electronic correlations, there are multiple intertwined orders which either coexist or compete with superconductivity. Among them the antiferromagnetic (AF) order is the most prominent one. In the region where superconductivity sets in, the long-range AF order is destroyed. Yet the residual short-range AF fluctuations are present up to a much higher doping and their role in the emergence of the superconducting phase is still highly debated. Here, by using a spin polarized scanning tunneling microscope, for the first time, we directly visualize an emergent incommensurate AF order in the nearby region of Fe impurities embedded in the optimally doped Bi2Sr2CaCu2O8+δ (Bi2212). Remarkably the Fe impurities suppress the superconducting coherence peaks with the gapped feature intact, but pin down the ubiquitous short-range incommensurate AF order. Our work shows an intimate relation between antiferromagnetism and superconductivity.

cond-mat.supr-con↗

arXiv2020

Eightfold Fermionic Excitation in a Charge Density Wave Compound

Unconventional quasiparticle excitations in condensed matter systems have become one of the most important research frontiers. Beyond two- and fourfold degenerate Weyl and Dirac fermions, three-, six- and eightfold symmetry protected degeneracies have been predicted however remain challenging to realize in solid state materials. Here, charge density wave compound TaTe4 is proposed to hold eightfold fermionic excitation and Dirac point in energy bands. High quality TaTe4 single crystals are prepared, where the charge density wave is revealed by directly imaging the atomic structure and a pseudogap of about 45 meV on the surface. Shubnikov de-Haas oscillations of TaTe4 are consistent with band structure calculation. Scanning tunneling microscopy reveals atomic step edge states on the surface of TaTe4. This work uncovers that charge density wave is able to induce new topological phases and sheds new light on the novel excitations in condensed matter materials.

cond-mat.mtrl-sci↗

arXiv2020

Two superconducting components with different symmetries in Nd1-xSrxNiO2 films

The pairing mechanism in cuprates remains as one of the most challenging issues in the field of condensed matter physics. The unique 3d9 electron orbital of the Cu2+ ionic states in cuprates is supposed to be the major player for the occurrence of superconductivity. Recently, superconductivity at about 9-15 K was discovered in infinite layer thin films of nickelate Nd1-xSrxNiO2 (x=0.1-0.2) which is believed to have the similar 3d9 orbital electrons. The key issue concerned here is about the superconducting gap function. Here we report the first set data of single particle tunneling measurements on the superconducting nickelate thin films. We find predominantly two types of tunneling spectra, one shows a V-shape feature which can be fitted very well by a d-wave gap function with gap maximum of about 3.9 meV, another one exhibits a full gap of about 2.35 meV. Some spectra demonstrate mixed contributions of these two components. Our results suggest that the newly found Ni-based superconductors play as close analogs to cuprates, and thus demonstrate the commonality of unconventional superconductivity.

cond-mat.supr-con↗

arXiv2020

Twofold symmetry of proximity-induced superconductivity in Bi$_{2}$Te$_{3}$/Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ heterostructures revealed by scanning tunneling microscopy

We observe proximity-induced superconductivity in the \textit{in situ} prepared heterostructures constructed by topological insulator Bi$_{2}$Te$_{3}$ thin films and high-temperature cuprate superconductors Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$. The superconducting gap maximum is about 7.6 meV on the surface of Bi$_{2}$Te$_{3}$ thin films with a thickness of two quintuple layers, and the gap value decreases with an increase in the film thickness. Moreover, the quasiparticle interference data show a clear evidence of a twofold symmetric superconducting gap with gap minima along one pair of the principal crystalline axes of Bi$_{2}$Te$_{3}$. This gap form is consistent with the $Δ_{4y}$ notation of the topological superconductivity proposed in such systems. Our results provide fruitful information of the possible topological superconductivity induced by the proximity effect in high-temperature superconducting cuprates.

cond-mat.supr-con↗

arXiv2018

Berry Curvature Enhanced Nonlinear Photogalvanic Response of Type-II Weyl Cone

The experimental manifestation of topological effects in bulk materials under ambient conditions, especially those with practical applications, has attracted enormous research interest. Recent discovery of Weyl semimetal provides an ideal material platform for such endeavors. The Berry curvature in a Weyl semimetal becomes singular at the Weyl node, creating an effective magnetic monopole in the k-space. A pair of Weyl nodes carry quantized effective magnetic charges with opposite signs, and therefore, opposite chirality. Although Weyl-point-related signatures such as chiral anomaly and non-closing surface Fermi arcs have been detected through transport and ARPES measurements, direct experimental evidence of the effective k-space monopole of the Weyl nodes has so far been lacking. In this work, signatures of the singular topology in a type-II Weyl semimetal TaIrTe4 is revealed in the photo responses, which are shown to be directly related to the divergence of Berry curvature. As a result of the divergence of Berry curvature at the Weyl nodes, TaIrTe4 exhibits unusually large photo responsivity of 130.2 mA/W with 4-μm excitation in an unbiased field effect transistor at room temperature arising from the third-order nonlinear optical response. The room temperature mid-IR responsivity is approaching the performance of commercial HgCdTe detector operating at low temperature, making Type-II Weyl semimetal TaIrTe4 of practical importance in terms of photo sensing and solar energy harvesting. Furthermore, the high shift photocurrent response at the Weyl cones is found to enhance the circularly polarized galvanic response from Weyl cones with opposite chirality, which opens new experimental possibilities for studying and controlling the chiral polarization of Weyl Fermions through an in-plane DC electric field in addition to the optical helicities.

cond-mat.mtrl-sci↗

arXiv2018

Direct visualization of sign reversal s^+- superconducting gaps in FeTe0.55Se0.45

In many unconventional superconductors, the pairing of electrons is driven by the repulsive interaction, which leads to the sign reversal of superconducting gaps along the Fermi surfaces (FS) or between them. However, to measure this sign change is not easy and straightforward. It is known that, in superconductors with sign reversal gaps, non-magnetic impurities can break Cooper pairs leading to the quasiparticle density of states in the superconducting state. The standing waves of these quasiparticles will interfere each other leading to the quasiparticle interference (QPI) pattern which carries the phase message reflecting also the superconducting gap structure. Based on the recently proposed defect-bound-state QPI technique, we explore the applicability of this technique to a typical iron based superconductor FeTe$_{0.55}$Se$_{0.45}$ with roughly equivalent gap values on the electron and hole pockets connected by the wave vector q_2=(0,π). It is found that, on the negative energy side, with the energy slightly below the gap value, the phase reference quantity $|g(q,-E)|\cos(θ_{q,+E}-θ_{q,-E}) becomes negative and the amplitude is strongly enhanced with the scattering vector q_2, but that corresponding to the scattering between the electron-electron pockets, namely q_3=(π,π), keeps all positive. This is well consistent with the theoretical expectation of the s^+- pairing gap and thus serves as a direct visualization of the sign reversal gaps. This experimental observation is also supported by the theoretical calculations with the Fermi surface structure and s^+- pairing gap.

cond-mat.supr-con↗

arXiv2018

Directly visualizing the sign change of d-wave superconducting gap in Bi2Sr2CaCu2O8+δ by phase-referenced quasiparticle interference

The superconducting state is achieved by the condensation of Cooper pairs and is protected by the superconducting gap. The pairing interaction between the two electrons of a Cooper pair determines the superconducting gap function. Thus, it is very pivotal to detect the gap structure for understanding the mechanism of superconductivity. In cuprate superconductors, it has been well established that the superconducting gap may have a d-wave function Δ = Δ_0cos2θ. This gap function has an alternative sign change by every pi/2 in the momentum space when the in-plane azimuthal angle theta is scanned. It is very hard to visualize this sign change. Early experiments for recommending or proving this d-wave gap function were accomplished by the specially designed phase sensitive measurements based on the Josephson effect. Here we report the measurements of scanning tunneling spectroscopy in one of the model cuprate system Bi2Sr2CaCu2O8+δ and conduct the analysis of phase-referenced quasiparticle interference (QPI). Due to the unique quasiparticle excitations in the superconducting state of cuprate, we have seen the seven basic scattering vectors that connect each pair of the terminals of the banana-shaped contour of constant quasiparticle energy (CCE). The phase-referenced QPI clearly visualizes the sign change of the d-wave gap. Our results illustrate a very effective way for determining the sign change of unconventional superconductors.

cond-mat.supr-con↗

arXiv2018

Raman Signatures of Broken Inversion Symmetry and In-plane Anisotropy in Type-II Weyl Semimetal Candidate TaIrTe4

The layered ternary compound TaIrTe4 is an important candidate to host the recently predicted type-II Weyl Fermions. However, a direct and definitive proof of the absence of inversion symmetry in this material, a prerequisite for the existence of Weyl Fermions, has so far remained evasive. Herein, an unambiguous identification of the broken inversion symmetry in TaIrTe4 is established using angle-resolved polarized Raman spectroscopy. Combining with high-resolution transmission electron microscopy, we demonstrate an efficient and nondestructive recipe to determine the exact crystallographic orientation of TaIrTe4 crystals. Such technique could be extended to the fast identification and characterization of other type-II Weyl Fermions candidates. A surprisingly strong in-plane electrical anisotropy in TaIrTe4 thin flakes is also revealed, up to 200% at 10K, which is the strongest known electrical anisotropy for materials with comparable carrier density, notably in such good metals as copper and silver.

cond-mat.mtrl-sci↗

arXiv2018

Sign reversal superconducting gaps revealed by phase referenced quasi-particle interference of impurity induced bound states in (Li$_{1-x}$Fe$_x$)OHFe$_{1-y}$Zn$_y$Se

By measuring the spatial distribution of differential conductance near impurities on Fe sites, we have obtained the quasi-particle interference (QPI) patterns in the (Li$_{1-x}$Fe$_x$)OHFe$_{1-y}$Zn$_y$Se superconductor with only electron Fermi surfaces. By taking the Fourier transform on these patterns, we investigate the scattering features between the two circles of electron pockets formed by folding or hybridization. We treat the data by using the recent theoretical approach [arXiv:1710.09089] which is specially designed for the impurity induced bound states. It is found that the superconducting gap sign is reversed on the two electron pockets, which can be directly visualized by the phase-referenced QPI technique, indicating that the Cooper pairing is induced by the repulsive interaction. Our results further show that this method is also applicable for data measured for multiple impurities, which provides an easy and feasible way for detecting the gap function of unconventional superconductors.

cond-mat.supr-con↗