Browsing by Author "Si, Qimiao"
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Item A Mott insulator continuously connected to iron pnictide superconductors(Springer Nature, 2016) Song, Yu; Yamani, Zahra; Cao, Chongde; Li, Yu; Zhang, Chenglin; Chen, Justin S.; Huang, Qingzhen; Wu, Hui; Tao, Jing; Zhu, Yimei; Tian, Wei; Chi, Songxue; Cao, Huibo; Huang, Yao-Bo; Dantz, Marcus; Schmitt, Thorsten; Yu, Rong; Nevidomskyy, Andriy H.; Morosan, Emilia; Si, Qimiao; Dai, Pengcheng; Rice Center for Quantum MaterialsIron-based superconductivity develops near an antiferromagnetic order and out of a bad-metal normal state, which has been interpreted as originating from a proximate Mott transition. Whether an actual Mott insulator can be realized in the phase diagram of the iron pnictides remains an open question. Here we use transport, transmission electron microscopy, X-ray absorption spectroscopy, resonant inelastic X-ray scattering and neutron scattering to demonstrate that NaFe1−xCuxAs near x≈0.5 exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators with the insulating behaviour persisting above the Néel temperature, indicative of a Mott insulator. On decreasing x from 0.5, the antiferromagnetic-ordered moment continuously decreases, yielding to superconductivity ∼x=0.05. Our discovery of a Mott-insulating state in NaFe1−xCuxAs thus makes it the only known Fe-based material, in which superconductivity can be smoothly connected to the Mott-insulating state, highlighting the important role of electron correlations in the high-Tc superconductivity.Item Andreev reflection and spin injection intod-wave,p-wave, ands-wave superconductors(2000) Merrill, Robert Louis; Si, QimiaoThe effect of spin injection into d-wave, p-wave, and s-wave superconductors is studied here. A theory is developed which treats the interplay between the bulk and boundary spin transport, as well as the interplay between pair and single-particle transport at the boundary. This theory is used to study the relationship between Andreev reflection and the linear-response spin-injection characteristics. Among the quantities of interest are, the amount of injected magnetization (m), the induced spin-dependent current (I s), and the induced boundary voltage (Vs). In general, Andreev reflection makes each of these three quantities depend on a different combination of the boundary and bulk contributions. The conditions are identified under which some of these quantities that depend solely on the bulk properties can be isolated. A correlation between the Andreev bound states and the spin-injection characteristics is established, which implies a strong directional dependence of spin injection into high-temperature superconductors and in p-wave superconductors.Item Anisotropic neutron spin resonance in underdoped superconducting NaFe1−xCoxAs(American Physical Society, 2014) Zhang, Chenglin; Song, Yu; Regnault, L.-P.; Su, Yixi; Enderle, M.; Kulda, J.; Tan, Guotai; Sims, Zachary C.; Egami, Takeshi; Si, Qimiao; Dai, PengchengWe use polarized inelastic neutron scattering (INS) to study spin excitations in superconducting NaFe0.985Co0.015As (C15) with static antiferromagnetic (AF) order along the a axis of the orthorhombic structure and NaFe0.935Co0.045As (C45) without AF order. In previous unpolarized INS work, spin excitations in C15 were found to have a dispersive sharp resonance near Er1=3.25 meV and a broad dispersionless mode at Er2=6 meV. Our neutron polarization analysis reveals that the dispersive resonance in C15 is highly anisotropic and polarized along the a and c axes, while the dispersionless mode is isotropic similar to that of C45. Since the a-axis polarized spin excitations of the anisotropic resonance appear below Tc, our data suggests that the itinerant electrons contributing to the magnetism are also coupled to the superconductivity.Item Antiferroquadrupolar and Ising-Nematic Orders of a Frustrated Bilinear-Biquadratic Heisenberg Model and Implications for the Magnetism of FeSe(American Physical Society, 2015) Yu, Rong; Si, QimiaoMotivated by the properties of the iron chalcogenides, we study the phase diagram of a generalized Heisenberg model with frustrated bilinear-biquadratic interactions on a square lattice. We identify zero-temperature phases with antiferroquadrupolar and Ising-nematic orders. The effects of quantum fluctuations and interlayer couplings are analyzed. We propose the Ising-nematic order as underlying the structural phase transition observed in the normal state of FeSe, and discuss the role of the Goldstone modes of the antiferroquadrupolar order for the dipolar magnetic fluctuations in this system. Our results provide a considerably broadened perspective on the overall magnetic phase diagram of the iron chalcogenides and pnictides, and are amenable to tests by new experiments.Item Cluster extended dynamical mean-field approach and unconventional superconductivity(American Physical Society, 2015) Pixley, J.H.; Cai, Ang; Si, QimiaoThe extended dynamical mean-field theory has played an important role in the study of quantum phase transitions in heavy-fermion systems. In order to incorporate the physics of unconventional superconductivity, we develop a cluster version of the extended dynamical mean-field theory. In this approach, we show how magnetic order and superconductivity develop as a result of intersite spin-exchange interactions, and analyze in some detail the form of correlation functions. We also discuss the methods that can be used to solve the dynamical equations associated with this approach. Finally, we consider different settings in which our approach can be applied, including the periodic Anderson model for heavy-fermion systems.Item Correlation-driven electronic reconstruction in FeTe1−xSex(Springer Nature, 2022) Huang, Jianwei; Yu, Rong; Xu, Zhijun; Zhu, Jian-Xin; Oh, Ji Seop; Jiang, Qianni; Wang, Meng; Wu, Han; Chen, Tong; Denlinger, Jonathan D.; Mo, Sung-Kwan; Hashimoto, Makoto; Michiardi, Matteo; Pedersen, Tor M.; Gorovikov, Sergey; Zhdanovich, Sergey; Damascelli, Andrea; Gu, Genda; Dai, Pengcheng; Chu, Jiun-Haw; Lu, Donghui; Si, Qimiao; Birgeneau, Robert J.; Yi, Ming; Rice Center for Quantum MaterialsElectronic correlation is of fundamental importance to high temperature superconductivity. While the low energy electronic states in cuprates are dominantly affected by correlation effects across the phase diagram, observation of correlation-driven changes in fermiology amongst the iron-based superconductors remains rare. Here we present experimental evidence for a correlation-driven reconstruction of the Fermi surface tuned independently by two orthogonal axes of temperature and Se/Te ratio in the iron chalcogenide family FeTe1−xSex. We demonstrate that this reconstruction is driven by the de-hybridization of a strongly renormalized dxy orbital with the remaining itinerant iron 3d orbitals in the emergence of an orbital-selective Mott phase. Our observations are further supported by our theoretical calculations to be salient spectroscopic signatures of such a non-thermal evolution from a strongly correlated metallic phase into an orbital-selective Mott phase in dxy as Se concentration is reduced.Item Coupled topological flat and wide bands: Quasiparticle formation and destruction(AAAS, 2023) Hu, Haoyu; Si, QimiaoFlat 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 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.Item Crossovers and critical scaling in the one-dimensional transverse-field Ising model(American Physical Society, 2018) Wu, Jianda; Zhu, Lijun; Si, Qimiao; Rice Center for Quantum MaterialsWe consider the scaling behavior of thermodynamic quantities in the one-dimensional transverse field Ising model near its quantum critical point (QCP). Our study has been motivated by the question about the thermodynamical signatures of this paradigmatic quantum critical system and, more generally, by the issue of how quantum criticality accumulates entropy. We find that the crossovers in the phase diagram of temperature and (the nonthermal control parameter) transverse field obey a general scaling ansatz, and so does the critical scaling behavior of the specific heat and magnetic expansion coefficient. Furthermore, the Grüneisen ratio diverges in a power-law way when the QCP is accessed as a function of the transverse field at zero temperature, which follows the prediction of quantum critical scaling. However, at the critical field, upon decreasing the temperature, the Grüneisen ratio approaches a constant instead of showing the expected divergence. We are able to understand this unusual result in terms of a peculiar form of the quantum critical scaling function for the free energy; the contribution to the Grüneisen ratio vanishes at the linear order in a suitable Taylor expansion of the scaling function. In spite of this special form of the scaling function, we show that the entropy is still maximized near the QCP, as expected from the general scaling argument. Our results establish the telltale thermodynamic signature of a transverse-field Ising chain, and will thus facilitate the experimental identification of this model quantum-critical system in real materials.Item Dynamic zero modes of Dirac fermions and competing singlet phases of antiferromagnetic order(American Physical Society, 2017) Goswami, Pallab; Si, QimiaoIn quantum spin systems, singlet phases often develop in the vicinity of an antiferromagnetic order. Typical settings for such problems arise when itinerant fermions are also present. In this paper, we develop a theoretical framework for addressing such competing orders in an itinerant system, described by Dirac fermions strongly coupled to an O(3) nonlinear sigma model. We focus on two spatial dimensions, where upon disordering the antiferromagnetic order by quantum fluctuations the singular tunneling events also known as (anti)hedgehogs can nucleate competing singlet orders in the paramagnetic phase. In the presence of an isolated hedgehog configuration of the nonlinear sigma model field, we show that the fermion determinant vanishes as the dynamic Euclidean Dirac operator supports fermion zero modes of definite chirality. This provides a topological mechanism for suppressing the tunneling events. Using the methodology of quantum chromodynamics, we evaluate the fermion determinant in the close proximity of magnetic quantum phase transition, when the antiferromagnetic order-parameter field can be described by a dilute gas of hedgehogs and antihedgehogs. We show how the precise nature of emergent singlet order is determined by the overlap between dynamic fermion zero modes of opposite chirality, localized on the hedgehogs and antihedgehogs. For a Kondo-Heisenberg model on the honeycomb lattice, we demonstrate the competition between spin Peierls order and Kondo singlet formation, thereby elucidating its global phase diagram. We also discuss other physical problems that can be addressed within this general framework.Item Electron Correlation and Spin Dynamics in Iron Pnictides and Chalcogenides(arXiv, 2012) Yu, Rong; Si, Qimiao; Goswami, Pallab; Abrahams, ElihuItem Electronic nematic correlations in the stress-free tetragonal state of BaFe2−xNixAs2(American Physical Society, 2015) Man, Haoran; Lu, Xingye; Chen, Justin S.; Zhang, Rui; Zhang, Wenliang; Luo, Huiqian; Kulda, J.; Ivanov, A.; Keller, T.; Morosan, Emilia; Si, Qimiao; Dai, PengchengWe use transport and neutron scattering to study electronic, structural, and magnetic properties of the electron-doped BaFe2−xNixAs2 iron pnictides in uniaxial-strained and external-stress-free detwinned states. Using a specially designed in situ mechanical detwinning device, we demonstrate that the in-plane resistivity anisotropy observed in the uniaxial-strained tetragonal state of BaFe2−xNixAs2 below a temperature T∗, previously identified as a signature of the electronic nematic phase, is also present in the stress-free tetragonal phase below T**(Item Emergent flat band and topological Kondo semimetal driven by orbital-selective correlations(Springer Nature, 2024) Chen, Lei; Xie, Fang; Sur, Shouvik; Hu, Haoyu; Paschen, Silke; Cano, Jennifer; Si, QimiaoFlat electronic bands are expected to show proportionally enhanced electron correlations, which may generate a plethora of novel quantum phases and unusual low-energy excitations. They are increasingly being pursued in d-electron-based systems with crystalline lattices that feature destructive electronic interference, where they are often topological. Such flat bands, though, are generically located far away from the Fermi energy, which limits their capacity to partake in the low-energy physics. Here we show that electron correlations produce emergent flat bands that are pinned to the Fermi energy. We demonstrate this effect within a Hubbard model, in the regime described by Wannier orbitals where an effective Kondo description arises through orbital-selective Mott correlations. Moreover, the correlation effect cooperates with symmetry constraints to produce a topological Kondo semimetal. Our results motivate a novel design principle for Weyl Kondo semimetals in a new setting, viz. d-electron-based materials on suitable crystal lattices, and uncover interconnections among seemingly disparate systems that may inspire fresh understandings and realizations of correlated topological effects in quantum materials and beyond.Item Energy dependence of the spin excitation anisotropy in uniaxial-strained BaFe1.9Ni0.1As2(American Physical Society, 2015) Song, Yu; Lu, Xingye; Abernathy, D.L.; Tam, David W.; Niedziela, J.L.; Tian, Wei; Luo, Huiqian; Si, Qimiao; Dai, PengchengWe use inelastic neutron scattering to study the temperature and energy dependence of the spin excitation anisotropy in uniaxial-strained electron-doped iron pnictide BaFe1.9Ni0.1As2 near optimal superconductivity (Tc=20K). Our work has been motivated by the observation of in-plane resistivity anisotropy in the paramagnetic tetragonal phase of electron-underdoped iron pnictides under uniaxial pressure, which has been attributed to a spin-driven Ising-nematic state or orbital ordering. Here we show that the spin excitation anisotropy, a signature of the spin-driven Ising-nematic phase, exists for energies below ∼60 meV in uniaxial-strained BaFe1.9Ni0.1As2. Since this energy scale is considerably larger than the energy splitting of the dxz and dyz bands of uniaxial-strained Ba(Fe1−xCox)2As2 near optimal superconductivity, spin Ising-nematic correlations are likely the driving force for the resistivity anisotropy and associated electronic nematic correlations.Item Evolution of Magnetic Double Helix and Quantum Criticality near a Dome of Superconductivity in CrAs(American Physical Society, 2018) Matsuda, M.; Lin, F.K.; Yu, R.; Cheng, J.-G.; Wu, W.; Sun, J.P.; Zhang, J.H.; Sun, P.J.; Matsubayashi, K.; Miyake, T.; Kato, T.; Yan, J.-Q.; Stone, M.B.; Si, Qimiao; Luo, J.L.; Uwatoko, Y.At ambient pressure, CrAs undergoes a first-order transition into a double-helical magnetic state at TN=265 K, which is accompanied by a structural transition. The recent discovery of pressure-induced superconductivity in CrAs makes it important to clarify the nature of quantum phase transitions out of the coupled structural/helimagnetic order in this system. Here, we show, via neutron diffraction on the single-crystal CrAs under hydrostatic pressure (P), that the combined order is suppressed at Pc≈10 kbar, near which bulk superconductivity develops with a maximal transition temperature Tc≈2 K. We further show that the coupled order is also completely suppressed by phosphorus doping in CrAs1−xPx at a critical xc≈0.05, above which inelastic neutron scattering evidenced persistent antiferromagnetic correlations, providing a possible link between magnetism and superconductivity. In line with the presence of antiferromagnetic fluctuations near Pc(xc), the A coefficient of the quadratic temperature dependence of resistivity exhibits a dramatic enhancement as P (x) approaches Pc(xc), around which ρ(T) has a non-Fermi-liquid form. Accordingly, the electronic specific-heat coefficient of CrAs1−xPx peaks around xc. These properties provide clear evidence for quantum criticality, which we interpret as originating from a nearly second-order helimagnetic quantum phase transition that is concomitant with a first-order structural transition. Our findings in CrAs highlight the distinct characteristics of quantum criticality in bad metals, thereby bringing out new insights into the physics of unconventional superconductivity such as those occurring in the high-Tc iron pnictides.Item Exploring aspects of nonequilibrium physics with quantum impurity problems(2014-04-22) Shashi, Aditya; Demler, Eugene; Si, Qimiao; Nevidomskyy, Andriy; Kono, JunichiroTraditionally the study of quantum mechanical ensembles was focused on the exploration of their equilibrium properties: the program has consisted of the classification of the quantum mechanical states of matter, and the identification of the striking phase transitions between them. On the other hand, questions about the out of equilibrium properties of quantum ensembles have largely remained academic until fairly recently. Particularly, the rapid technological progress in the field of atomic physics has enabled experimental demonstrations of nontrivial out of equilibrium phenomena which moreover are describable in terms of relatively simple theoretical models with a few parameters. Thus the time is ripe for a theoretical exploration of nonequilibrium physics. To this end, quantum impurity models offer a natural and simple starting point for studying nonequilibrium phenomena in the context of ultracold atoms, and pave the way toward the study of more complicated systems. I will discuss how the impurity-bath model offers a clean, simple realization of rich phenomenology including the dynamics of polaron formation as well as the orthogonality catastrophe, and can be engineered using dilute mixtures of cold atomic gases. Moreover I will demonstrate how impurity models are also embedded in the more complicated physics of the response of a one-dimensional system to an external perturbation, or a sudden local parameter change. Lastly, I will describe the approach to equilibrium of a more complicated system, the one dimensional Bose gas, following a sudden parameter change, and discuss some of the important questions which arise in this connection: does a quantum mechanical system thermalize? What is the appropriate asymptotic description of a nonequilibrium state? Does such a system retain a memory of its initial state?Item Field-induced long-range magnetic order in the spin-singlet ground-state system YbAl3C3: Neutron diffraction study(American Physical Society, 2013) Khalyavin, D.D.; Adroja, D.T.; Manuel, P.; Daoud-Aladine, A.; Kosaka, M.; Kondo, K.; McEwen, K.A.; Pixley, J.H.; Si, QimiaoThe 4f-electron system YbAl3C3 with a nonmagnetic spin-dimer ground state has been studied by neutron diffraction in an applied magnetic field. A long-range magnetic order involving both ferromagnetic and antiferromagnetic components has been revealed above the critical field HC∼6 T at temperature T=0.05 K. The magnetic structure indicates that the geometrical frustration of the prototype hexagonal lattice is not fully relieved in the low-temperature orthorhombic phase. The suppression of magnetic ordering by the remanent frustration is the key factor stabilizing the nonmagnetic singlet ground state in zero field. Temperature-dependent measurements in the applied field H=12 T revealed that the long-range ordering persists up to temperatures significantly higher than the spin gap, indicating that this phase is not directly related to the singlet-triplet excitation. Combining our neutron diffraction results with the previously published phase diagram, we support the existence of an intermediate disordered phase as the first excitation from the nonmagnetic singlet ground state. Based on our results, we propose YbAl3C3 as a material for studying the quantum phase transitions of heavy-fermion metals under the influence of geometrical frustration.Item Finite-Temperature Spin Dynamics in a Perturbed Quantum Critical Ising Chain with an E8 Symmetry(American Physical Society, 2014) Wu, Jianda; Kormos, Márton; Si, QimiaoA spectrum exhibiting E8 symmetry is expected to arise when a small longitudinal field is introduced in the transverse-field Ising chain at its quantum critical point. Evidence for this spectrum has recently come from neutron scattering measurements in cobalt niobate, a quasi-one-dimensional Ising ferromagnet. Unlike its zero-temperature counterpart, the finite-temperature dynamics of the model has not yet been determined. We study the dynamical spin structure factor of the model at low frequencies and nonzero temperatures, using the form factor method. Its frequency dependence is singular, but differs from the diffusion form. The temperature dependence of the nuclear magnetic resonance (NMR) relaxation rate has an activated form, whose prefactor we also determine. We propose NMR experiments as a means to further test the applicability of the E8 description for CoNb2O6.Item Fully gapped d-wave superconductivity in CeCu2Si2(National Academy of Sciences, 2018) Pang, Guiming; Smidman, Michael; Zhang, Jinglei; Jiao, Lin; Weng, Zongfa; Nica, Emilian M.; Chen, Ye; Jiang, Wenbing; Zhang, Yongjun; Xie, Wu; Jeevan, Hirale S.; Lee, Hanoh; Gegenwart, Philipp; Steglich, Frank; Si, Qimiao; Yuan, HuiqiuThe nature of the pairing symmetry of the first heavy fermion superconductor CeCu2Si2 has recently become the subject of controversy. While CeCu2Si2 was generally believed to be a d-wave superconductor, recent low-temperature specific heat measurements showed evidence for fully gapped superconductivity, contrary to the nodal behavior inferred from earlier results. Here, we report London penetration depth measurements, which also reveal fully gapped behavior at very low temperatures. To explain these seemingly conflicting results, we propose a fully gapped d+d band-mixing pairing state for CeCu2Si2, which yields very good fits to both the superfluid density and specific heat, as well as accounting for a sign change of the superconducting order parameter, as previously concluded from inelastic neutron scattering results.Item Giant spontaneous Hall effect in a nonmagnetic Weyl–Kondo semimetal(PNAS, 2021) Dzsaber, Sami; Yan, Xinlin; Taupin, Mathieu; Eguchi, Gaku; Prokofiev, Andrey; Shiroka, Toni; Blaha, Peter; Rubel, Oleg; Grefe, Sarah E.; Lai, Hsin-Hua; Si, Qimiao; Paschen, Silke; Rice Center for Quantum MaterialsNontrivial topology in condensed-matter systems enriches quantum states of matter to go beyond either the classification into metals and insulators in terms of conventional band theory or that of symmetry-broken phases by Landau’s order parameter framework. So far, focus has been on weakly interacting systems, and little is known about the limit of strong electron correlations. Heavy fermion systems are a highly versatile platform to explore this regime. Here we report the discovery of a giant spontaneous Hall effect in the Kondo semimetal Ce3Bi4Pd3 that is noncentrosymmetric but preserves time-reversal symmetry. We attribute this finding to Weyl nodes—singularities of the Berry curvature—that emerge in the immediate vicinity of the Fermi level due to the Kondo interaction. We stress that this phenomenon is distinct from the previously detected anomalous Hall effect in materials with broken time-reversal symmetry; instead, it manifests an extreme topological response that requires a beyond-perturbation-theory description of the previously proposed nonlinear Hall effect. The large magnitude of the effect in even tiny electric and zero magnetic fields as well as its robust bulk nature may aid the exploitation in topological quantum devices.Item Glide reflection symmetry, Brillouin zone folding, and superconducting pairing for the P4/nmm space group(American Physical Society, 2015) Nica, Emilian M.; Yu, Rong; Si, QimiaoMotivated by the studies of the superconducting pairing states in the iron-based superconductors, we analyze the effects of Brillouin zone folding procedure from a space-group symmetry perspective for a general class of materials with the P4/nmm space group. The Brillouin zone folding amounts to working with an effective 1-Fe unit cell, instead of the crystallographic 2-Fe unit cell. We show that the folding procedure can be justified by the validity of a glide reflection symmetry throughout the crystallographic Brillouin zone and by the existence of a minimal double degeneracy along the edges of the latter. We also demonstrate how the folding procedure fails when a local spin-orbit coupling is included although the latter does not break any of the space-group symmetries of the bare Hamiltonian. In light of these general symmetry considerations, we further discuss the implications of the glide reflection symmetry for the superconducting pairing in an effective multiorbital t−J1−J2 model. We find that, for spin-singlet pairing states, the P4/nmm space-group symmetry allows only even parity under the glide reflection and zero total momentum.