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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
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DOE OSTI2020

Emergence of quasiparticles in a doped Mott insulator

How a Mott insulator develops into a weakly coupled metal upon doping is a central question to understanding various emergent correlated phenomena. To analyze this evolution and its connection to the high- T c cuprates, we study the single-particle spectrum for the doped Hubbard model using cluster perturbation theory on superclusters. Starting from extremely low doping, we identify a heavily renormalized quasiparticle dispersion that immediately develops across the Fermi level, and a weakening polaronic side band at higher binding energy. The quasiparticle spectral weight roughly grows at twice the rate of doping in the low doping regime, but this rate is halved at optimal doping. In the heavily doped regime, we find both strong electron-hole asymmetry and a persistent presence of Mott spectral features. Finally, we discuss the applicability of the single-band Hubbard model to describe the evolution of nodal spectra measured by angle-resolved photoemission spectroscopy (ARPES) on the single-layer cuprate La 2– x Sr x CuO 4 (0 ≤ x ≤ 0.15). This work benchmarks the predictive power of the Hubbard model for electronic properties of high- T c cuprates.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Four-Spin Terms and the Origin of the Chiral Spin Liquid in Mott Insulators on the Triangular Lattice

At strong repulsion, the triangular-lattice Hubbard model is described by s=1/2 spins with nearest-neighbor antiferromagnetic Heisenberg interactions and exhibits conventional 120° order. Using the infinite density matrix renormalization group and exact diagonalization, we study the effect of the additional four-spin interactions naturally generated from the underlying Mott-insulator physics of electrons as the repulsion decreases. Although these interactions have historically been connected with a gapless ground state with emergent spinon Fermi surface, we find that, at physically relevant parameters, they stabilize a chiral spin liquid (CSL) of Kalmeyer-Laughlin (KL) type, clarifying observations in recent studies of the Hubbard model. Finally, we present a self-consistent solution based on a mean-field rewriting of the interaction to obtain a Hamiltonian with similarities to the parent Hamiltonian of the KL state, providing a physical understanding for the origin of the CSL.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Quantum Phases of Transition Metal Dichalcogenide Moiré Systems

Moiré systems provide a rich platform for studies of strong correlation physics. Recent experiments on heterobilayer transition metal dichalcogenide Moiré systems are exciting in that they manifest a relatively simple model system of an extended Hubbard model on a triangular lattice. Inspired by the prospect of the hetero-transition metal dichalcogenide Moiré system’s potential as a solid-state-based quantum simulator, we explore the extended Hubbard model on the triangular lattice using the density matrix renormalization group. Specifically, we explore the two-dimensional phase space spanned by the key tuning parameters in the extended Hubbard model, namely, the kinetic energy strength and the further-range Coulomb interaction strengths. We find competition between Fermi fluid, chiral spin liquid, spin density wave, and charge order. In particular, our finding of the optimal further-range interaction for the chiral correlation presents a tantalizing possibility.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Topological order in Mott insulators

Here in this commemorative issue for Phil Anderson, I reproduce parts of a joint unpublished paper that first got me thinking about topological order in Mott insulators. With Phil, we explored aspects of the momentum distribution function and the non-classical rotational inertia in the Bose Hubbard model close to the quantum phase transition. My interest on this question continued, and later in the article I take up the question of topological order in the Kitaev Mott insulator on the honeycomb lattice with bond-dependent interactions in a finite magnetic field. Kitaev obtained the exact solution for zero field that demonstrated an exotic Z 2 quantum spin liquid phase with topological order. In a finite magnetic field, we discovered a new intermediate U(1) gapless quantum spin liquid phase, sandwiched between the known gapped Kitaev spin liquid and a polarized phase, that harbors fractionalized neutral fermionic excitations, dubbed spinons, that remarkably form a Fermi surface in a Mott insulator.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Doping Dependence of 2-Spinon Excitations in the Doped 1D Cuprate Ba 2 ⁢CuO 3+δ

Recent photoemission experiments on the quasi-one-dimensional Ba-based cuprates suggest that doped holes experience an attractive potential not captured using the simple Hubbard model. This observation has garnered significant attention due to its potential relevance to Cooper pair formation in high-T c cuprate superconductors. To scrutinize this assertion, we examined signatures of such an attractive potential in doped 1D cuprates Ba 2 ⁢CuO 3+δ by measuring the dispersion of the 2-spinon excitations using Cu L 3 -edge resonant inelastic x-ray scattering (RIXS). Upon doping, the 2-spinon excitations appear to weaken, with a shift of the minimal position corresponding to the nesting vector of the Fermi points, q F . Notably, we find that the energy scale of the 2-spinons near the Brillouin zone boundary is substantially softened compared to that predicted by the Hubbard model in one dimension. Finally, such a discrepancy implies missing ingredients, which lends support for the presence of an additional attractive potential between holes.

1-dimensional spin chains↗
DOE OSTI2023

The Wiedemann-Franz law in doped Mott insulators without quasiparticles

Many metallic quantum materials display anomalous transport phenomena that defy a Fermi liquid description. In this report we use numerical methods to calculate thermal and charge transport in the doped Hubbard model and observe a crossover separating high- and low-temperature behaviors. Distinct from the behavior at high temperatures, the Lorenz number L becomes weakly doping dependent and less sensitive to parameters at low temperatures. At the lowest numerically accessible temperatures, L roughly approaches the Wiedemann-Franz constant L, even in a doped Mott insulator that lacks well-defined quasiparticles. Decomposing the energy current operator indicates a compensation between kinetic and potential contributions, which may help to clarify the interpretation of transport experiments beyond Boltzmann theory in strongly correlated metals.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Topological electronic structure of YbMg 2 Bi 2 and CaMg 2 Bi 2

Zintl compounds have been extensively studied for their outstanding thermoelectric properties, but their electronic structure remains largely unexplored. Here, we present a detailed investigation of the electronic structure of the isostructural thermopower materials YbMg 2 Bi 2 and CaMg 2 Bi 2 using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). The ARPES results show a significantly smaller Fermi surface and Fermi velocity in CaMg 2 Bi 2 than in YbMg 2 Bi 2 . Our ARPES results also reveal that in the case of YbMg 2 Bi 2 , Yb- 4 f states reside well below the Fermi level and likely have a negligible impact on transport properties. To properly model the position of 4f-states, as well as the overall electronic structure, a Hubbard U at the Yb sites and spin-orbit coupling (SOC) have to be included in the DFT calculations. The theoretical results reveal that both materials belong to a Z 2 topological class and host topological surface states around EF. Due to the intrinsic hole doping, the topological states reside above the Fermi level, inaccessible by ARPES. Our results also suggest that in addition to SOC, vacancies and the resulting hole doping play an important role in the transport properties of these materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Sachdev-Ye-Kitaev models and beyond: Window into non-Fermi liquids

In this work is a review of the Sachdev-Ye-Kitaev (SYK) model of compressible quantum many-body systems without quasiparticle excitations, and its connections to various theoretical studies of non-Fermi liquids in condensed matter physics. The review is placed in the context of numerous experimental observations on correlated electron materials. Strong correlations in metals are often associated with their proximity to a Mott transition to an insulator created by the local Coulomb repulsion between the electrons. The phase diagrams of a number of models of such a local electronic correlation are explored, employing a dynamical mean-field theory in the presence of random spin exchange interactions. Numerical analyses and analytical solutions, using renormalization group methods and expansions in large spin degeneracy, lead to critical regions that display SYK physics. The models studied include the single-band Hubbard model, the t–J model, and the two-band Kondo-Heisenberg model in the presence of random spin exchange interactions. Also examined are non-Fermi liquids obtained by considering each SYK model with random four-fermion interactions to be a multiorbital atom, with the SYK atoms arranged in an infinite lattice. Connections are made to theories of sharp Fermi surfaces without any low-energy quasiparticles in the absence of spatial disorder, obtained by coupling a Fermi liquid to a gapless boson; a systematic large-N theory of such a critical Fermi surface, with SYK characteristics, is obtained by averaging over an ensemble of theories with random boson-fermion couplings. Finally, an overview of the links between the SYK model and quantum gravity is presented, and the review ends with an outlook on open questions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Orbital-selective correlations and renormalized electronic structure in LiFeAs

Multiorbital physics is important to both the correlation physics and topological behavior of quantum materials. LiFeAs is a prototype iron pnictide suitable for in-depth investigation of this issue. Its electronic structure is strikingly different from the prediction of the noninteracting description. Here, a multiorbital Hubbard model is studied using a U(1) slave-spin theory. We demonstrate a mechanism for a substantial change to the Fermi surface, namely, orbital selectivity of the energy-level renormalization cooperating with its counterpart in quasiparticle spectral weight. Using this effect, we show how the dominating features of the electronic structure in LiFeAs are understood by the local correlations alone. In conclusion, our results set the stage to understand the origins and nature of both the unconventional superconductivity and likely electronic topology in this prototype iron pnictide and, more generally, reveal a remarkable degree of universality out of the seemingly complex multiorbital building blocks across a broad range of strongly correlated superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Magnetic impurity as a local probe of the $U(1)$ quantum spin liquid with spinon Fermi surface

We solve the problem of a magnetic impurity coupled to a U(1) quantum spin liquid with a spinon Fermi surface, and we compute the impurity spectral function. Using the slave rotor mean-field approach combined with gauge field fluctuations, we find that a peak located at the top of the lower Hubbard band and one at the bottom of the upper Hubbard band can emerge in the impurity spectral function. The peaks at the Hubbard band edges arise from the gauge field fluctuation-induced spinon-chargon binding inside the spinon Kondo screening cloud of the magnetic impurity. For a magnetic impurity embedded in a Mott insulator, our findings suggest that the emergence of a pair of peaks at the Hubbard band edges in the impurity density of states spectra provides strong evidence that the host Mott insulator is a U(1) quantum spin liquid with a spinon Fermi surface.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Auxiliary field Quantum Monte Carlo for dilute neutrons on the lattice

We employ constrained path Auxiliary Field Quantum Monte Carlo (AFQMC) in the pursuit of studying physical nuclear systems using a lattice formalism. Since AFQMC has been widely used in the study of condensed-matter systems such as the Hubbard model, we benchmark our method against published results for both one- and two-dimensional Hubbard model calculations. We then turn our attention to cold atomic and nuclear systems. We use an onsite contact interaction that can be tuned in order to reproduce the known scattering length and effective range of a given interaction. Developing this machinery allows us to extend our calculations to study nuclear systems within a lattice formalism. In conclusion, we perform initial calculations for a range of nuclear systems from two- to few-body neutron systems.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Location and thermal evolution of the pseudogap due to spin fluctuations

We study pseudogap behavior in a metal near a spin density wave (SDW) instability due to thermal magnetic fluctuations. We consider the t–t' Hubbard model on a square lattice at a finite doping, at intermediate coupling strength, and analyze the thermal evolution of the electron spectral function between a SDW ordered state at low temperatures and a normal Fermi liquid at high temperatures. We argue that for proper description of the pseudogap one needs to sum up infinite series of diagrams for both the fermionic self-energy and the SDW order parameter in the SDW state or the magnetic correlation length in the paramagnetic state. We use the eikonal approach to sum up an infinite series of diagrammatic contributions from thermal fluctuations. Earlier studies found that in the SDW state the spectral function A k (ω) of a hot fermion at a finite T is exponentially small below the energy scale Δ(T), which scales with SDW order and vanishes at the ordering temperature T N , and has a hump at a larger frequency Δ PG , comparable to the zero-temperature SDW gap Δ(T = 0). We argue that the hump, which we associate with the pseudogap, survives in some T range above T N . We show that this range is split by regions of strong and weak pseudogap behavior. In the first region, Δ PG is weakly temperature dependent, despite the fact that it comes from thermal fluctuations. Such a behavior has been seen in numerical studies of the Hubbard model. We show that to obtain it one needs to go beyond the one-loop approximation and sum up the infinite series of diagrams. In the second regime, Δ PG decreases with increasing T and eventually vanishes. Here, we further argue that a magnetic pseudogap at a finite T emerges only if the ground state is magnetically ordered. We present the phase diagram and apply the results to high-T c cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Frustration- and doping-induced magnetism in a Fermi–Hubbard simulator

Geometrical frustration in strongly correlated systems can give rise to a plethora of novel ordered states and intriguing magnetic phases, such as quantum spin liquids. Promising candidate materials for such phases can be described by the Hubbard model on an anisotropic triangular lattice, a paradigmatic model capturing the interplay between strong correlations and magnetic frustration. However, the fate of frustrated magnetism in the presence of itinerant dopants remains unclear, as well as its connection to the doped phases of the square Hubbard model. Here we investigate the local spin order of a Hubbard model with controllable frustration and doping, using ultracold fermions in anisotropic optical lattices continuously tunable from a square to a triangular geometry. At half-filling and strong interactions U/t ≈ 9, we observe at the single-site level how frustration reduces the range of magnetic correlations and drives a transition from a collinear Néel antiferromagnet to a short-range correlated 120° spiral phase. Away from half-filling, the triangular limit shows enhanced antiferromagnetic correlations on the hole-doped side and a reversal to ferromagnetic correlations at particle dopings above 20%, hinting at the role of kinetic magnetism in frustrated systems. Furthermore, this work paves the way towards exploring possible chiral ordered or superconducting phases in triangular lattices and realizing t–t' square lattice Hubbard models that may be essential to describe superconductivity in cuprate materials.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Coupled Hubbard ladders at weak coupling: Pairing and spin excitations

The Hubbard model provides a simple framework in which one can study how certain aspects of the electronic structure of strongly interacting systems can be tuned to optimize the superconducting pairing correlations and how these changes affect the mechanisms giving rise to them. Here we use a weak-coupling random phase approximation to study a two-dimensional Hubbard model with a unidirectional modulation of the hopping amplitudes as the system evolves from the uniform square lattice to an array of weakly coupled two-leg ladders. We find that the pairing correlations retain their dominant d x 2 - y 2- wavelike structure and that they are significantly enhanced for a slightly modulated lattice. This enhancement is traced backed to an increase in the strength of the spin-fluctuation pairing interaction due to favorable Fermi surface nesting in the modulated system. We then use a random-phase approximation BCS framework to examine the evolution of the neutron resonance in the superconducting state. We find that it changes only weakly for moderate modulations, but breaks up into two distinct resonances at incommensurate wave vectors in the limit of weakly coupled ladders.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Emergence of p+ip superconductivity in two-dimensional doped Dirac systems

Searching for the $p + ip$ superconducting (SC) state has become a fascinating subject in condensed matter physics recently, as a dream application awaiting in topological quantum computation. Unfortunately, so far there is no universal principle for realizing $p + ip$ in generic solid-state systems. Here we report a theoretical discovery of a $p + ip$ SC ground state (coexisting with ferromagnetic order) in the honeycomb lattice Hubbard model in the extremely strong-coupling limit (e.g., infinite U) at low doping $(δ < 0.2)$, by using both the state-of-art Grassmann tensor product state approach and a continuum quantum field theory approach. Our discovery suggests a mechanism for the $p + ip$ SC state in generic strongly correlated systems based on spin-charge separation and the charge current–current coupling scenario, which opens a door towards experimental realization. The $p + ip$ SC state has an instability towards a potential non-Fermi liquid with a large but finite U . Nevertheless, by applying an in-plane Zeeman field, such a $p + ip$ SC state can be stabilized with finite U in a very wide range of doping. Relevant realistic materials are also proposed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Electronic spectra with paramagnon fractionalization in the single-band Hubbard model

We examine spectral properties of a recently proposed theory of the intermediate temperature pseudogap metal phase of the cuprates. We show that this theory can be obtained from the familiar paramagnon theory of nearly antiferromagnetic metals by fractionalizing the paramagnon into two “hidden” layers of S = 1 / 2 spins. The first hidden layer of spins hybridizes with the electrons as in a Kondo lattice heavy Fermi liquid, whereas the second hidden layer of spins forms a spin liquid with fractionalized spinon excitations. Here, we compute the imaginary part of the electronic self-energy induced by the spinon excitations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Adaptive Variational Quantum Computing Approaches for Green’s Functions and Nonlinear Susceptibilities

Here, we present and benchmark quantum computing approaches for calculating real-time single-particle Green’s functions and nonlinear susceptibilities of Hamiltonian systems. The approaches leverage adaptive variational quantum algorithms for state preparation and propagation. Using automatically generated compact circuits, the dynamical evolution is performed over sufficiently long times to achieve adequate frequency resolution of the response functions. We showcase accurate Green’s function calculations using a statevector simulator on classical hardware for Fermi-Hubbard chains of 4 and 6 sites, with maximal ansatz circuit depths of 65 and 424 layers, respectively, and for the molecule LiH with a maximal ansatz circuit depth of 81 layers. Additionally, we consider an antiferromagnetic quantum spin-1 model that incorporates the Dzyaloshinskii-Moriya interaction to illustrate calculations of the third-order nonlinear susceptibilities, which can be measured in two-dimensional coherent spectroscopy experiments. These results demonstrate that real-time approaches using adaptive parametrized circuits to evaluate linear and nonlinear response functions can be feasible with near-term quantum processors.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2024

Block Mott insulating state induced by next-nearest-neighbor hopping in the S = 3 2 zigzag chain BaCoTe 2 O 7

Quasi-one-dimensional correlated electronic multiorbital systems with either ladder or chain geometries continue attracting considerable interest due to their complex electronic phases arising from the interplay of the hopping matrix, the crystal-field splitting, the electronic correlations (Hubbard repulsion U and Hund coupling J H ), and strong quantum fluctuations. Recently, the intriguing cobalt zigzag chain system BaCoTe 2 ⁢ O 7 , with electronic density n=7, was prepared experimentally. Here, we systematically study the electronic and magnetic properties of this quasi-one-dimensional compound from the theoretical perspective. Based on first-principles density functional theory calculations, strongly anisotropic one-dimensional electronic Co 3⁢d bands were found near the Fermi level. By evaluating the relevant hopping amplitudes, we provide the magnitude and origin of the nearest-neighbor (NN) and next-nearest-neighbor (NNN) hopping matrices in BaCoTe 2 ⁢ O 7 . With this information, we constructed a three-orbital electronic Hubbard model for this zigzag chain system, and studied two cases: with only a NN hopping matrix, and with NN plus NNN hopping matrices. Introducing the Hubbard and Hund couplings and studying the model via the density matrix renormalization group method, we constructed the ground-state phase diagram. A robust staggered ↑ - ↓ - ↑ - ↓ antiferromagnetic (AFM) region was found when only the NN hopping matrix in the chain direction was employed. However, for the realistic case where the NNN hopping matrix is also included, the dominant state becomes instead a block AFM ↑ - ↑ - ↓ - ↓ order, in agreement with experiments. The system displays Mott insulator characteristics with three half-filled orbitals, when the block AFM order is stable. In conclusion, our results for BaCoTe 2 ⁢O 7 provide guidance to experimentalists and theorists working on this zigzag one-dimensional chain and related materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗