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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Acoustoelectric drag current in vanadium oxide films

Two different Mott insulator wires, vanadium dioxide and vanadium sesquioxide, were prepared on the piezoelectric LiNbO 3 substrates. Coupling of acoustic waves propagating in LiNbO 3 with free carriers in vanadium oxide gives rise to the acoustoelectric effect that manifests itself as the generation of direct electric current by the acoustic wave. According to a phenomenological model, the value of the effect strongly depends on the wires conductivity, which, for the vanadium-oxide films, changes by a few orders of magnitude. We demonstrated that this yields a significant enhancement of the direct current (DC) current generated in the wires at the metal–insulator transition temperatures. The sign of the generated DC voltage is different for excitations by surface and bulk acoustic wave modes, which may happen due to reverse wave propagation at the substrate surface. For each resonance mode, polarities of the generated DC signal are the same in both wires, despite the signs of charge carriers being different for these materials. It was shown that two complementary techniques (acoustoelectric and Hall effect measurements) yield opposite signs of charge carriers in VO 2 .

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Optical contrast analysis of α -RuCl 3 nanoflakes on oxidized silicon wafers

α-RuCl 3 , a narrow-band Mott insulator with a large work function, offers intriguing potential as a quantum material or as a charge acceptor for electrical contacts in van der Waals devices. In this work, we perform a systematic study of the optical reflection contrast of α-RuCl 3 nanoflakes on oxidized silicon wafers and estimate the accuracy of this imaging technique to assess the crystal thickness. Via spectroscopic micro-ellipsometry measurements, we characterize the wavelength-dependent complex refractive index of α-RuCl 3 nanoflakes of varying thickness in the visible and near-infrared. Building on these results, we simulate the optical contrast of α-RuCl 3 nanoflakes with thicknesses below 100 nm on SiO 2 /Si substrates under different illumination conditions. We compare the simulated optical contrast with experimental values extracted from optical microscopy images and obtain good agreement. Finally, we show that optical contrast imaging allows us to retrieve the thickness of the RuCl 3 nanoflakes exfoliated on an oxidized silicon substrate with a mean deviation of –0.2 nm for thicknesses below 100 nm with a standard deviation of only 1 nm. Our results demonstrate that optical contrast can be used as a non-invasive, fast, and reliable technique to estimate the α-RuCl 3 thickness.

2D materials↗
DOE OSTI2026

Bootstrap embedding for interacting electrons in phonon coherent-state mean field

Here, we develop a Fermi–Bose bootstrap embedding framework for the ground state of interacting electrons coupled to a phonon mean field. The method combines bootstrap embedding for correlated electrons with a self-consistent coherent-state mean-field treatment for phonons. This method models the interacting electron–phonon problem as a system of correlated electrons traveling in a self-consistently specified potential landscape, allowing for efficient treatment of large lattice systems. Convergence of the methods for fragment size and total system size is demonstrated for the one-dimensional Hubbard–Holstein model for up to 350 sites. Finite-size scaling is performed to extrapolate to the infinite system size. Benchmarking against the density matrix renormalization group for a small 8-site system at half- and quarter-filling shows an orders-of-magnitude runtime advantage. The comparison further reveals that the method performs best in regimes dominated by localization, such as the Mott insulating phase and the strong-coupling tiny polaron regime, where the local embedding ansatz is still valid. However, due to the mean-field treatment for phonons, we find limitations of our methods in the weakly coupled delocalized region and at the Peierls transition, where quantum phonon fluctuations and long-range kinetic correlations become substantial.

Islam, Shariful [North Carolina State University, ↗
DOE OSTI2020

Linear resistivity and Sachdev-Ye-Kitaev (SYK) spin liquid behavior in a quantum critical metal with spin-1/2 fermions

Significance In “Planckian metals,” electrons dissipate energy at the fastest possible rate allowed by the fundamental laws of quantum mechanics, resulting in a linear temperature dependence of their electrical resistivity. Although observed for a number of quantum materials, this phenomenon lacks a general theoretical understanding and is often considered as one of the prominent fundamental questions in condensed matter physics. Here, we show that Planckian dissipation and a behavior consistent with the “marginal Fermi liquid” phenomenology emerge in the quantum critical regime separating a Mott insulating spin glass and a Fermi liquid. By establishing this behavior in an explicit model solvable by state-of-the-art computational methods, our theory paves the way toward a deeper understanding of Planckian or “strange” metals.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Pseudogap in electron-doped cuprates: Strong correlation leading to band splitting

The pseudogap phenomena have been a long-standing mystery of the cuprate high-temperature superconductors. The pseudogap in the electron-doped cuprates has been attributed to band folding due to antiferromagnetic (AFM) long-range order or short-range correlation. We performed an angle-resolved photoemission spectroscopy study of the electron-doped cuprates Pr 1.3−x La 0.7 Ce x CuO 4 showing spin-glass, disordered AFM behaviors, and superconductivity at low temperatures and, by measurements with fine momentum cuts, found that the gap opens on the unfolded Fermi surface rather than the AFM Brillouin zone boundary. The gap did not show a node, following the full symmetry of the Brillouin zone, and its magnitude decreased from the zone-diagonal to (π,0) directions, opposite to the hole-doped case. These observations were reproduced by cluster dynamical-mean-field-theory calculation, which took into account electron correlation precisely within a (CuO 2 ) 4 cluster. In conclusion, the present experimental and theoretical results are consistent with the mechanism that electron or hole doping into a Mott insulator creates an in-gap band that is separated from the upper or lower Hubbard band by the pseudogap.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Gapless spin liquid and pair density wave of the Hubbard model on three-leg triangular cylinders

We study the ground state properties of the Hubbard model on three-leg triangular cylinders using large-scale density-matrix renormalization group simulations. At half-filling, we identify an intermediate gapless spin liquid phase, which has one gapless spin mode and algebraic spin–spin correlations but exponential decay scalar chiral–chiral correlations, between a metallic phase at weak coupling and Mott insulating dimer phase at strong interaction. Upon light doping the gapless spin liquid, the system exhibits power-law charge-density-wave (CDW) correlations but short-range single-particle, spin–spin, and chiral–chiral correlations. Similar to CDW correlations, the superconducting correlations also decay in power-law but oscillate in sign as a function of distance, which is consistent with the striped pair-density wave. When further doping the gapless spin liquid phase or doping the dimer order phase, another phase takes over, which has similar CDW correlations but all other correlations decay exponentially.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Single crystal growth and electronic structure of Rh-doped Sr 3 Ir 2 O 7

Ruddlesden-Popper iridate Sr 3 Ir 2 O 7 is a spin–orbit coupled Mott insulator. Hole doped Sr 3 Ir 2 O 7 provides an ideal platform to study the exotic quantum phenomena that occur near the metal–insulator transition (MIT) region. Rh substitution of Ir is an effective method to induce hole doping into Sr 3 Ir 2 O 7 . However, the highest doping level reported in Sr 3 (Ir 1− x Rh x ) 2 O 7 single crystals was only around 3%, which is far from the MIT region. In this paper, we report the successful growth of single crystals of Sr 3 (Ir 1− x Rh x ) 2 O 7 with a doping level of ~ 9%. The samples have been fully characterized, demonstrating the high quality of the single crystals. Transport measurements have been carried out, confirming the tendency of MIT in these samples. The electronic structure has also been examined by angle-resolved photoemission spectroscopy (ARPES) measurements. Our results establish a platform to investigate the heavily hole doped Sr 3 Ir 2 O 7 compound, which also provide new insights into the MIT with hole doping in this material system.

Physics↗
DOE OSTI2022

Circuit complexity near critical points

Here, we consider the Bose–Hubbard model in two and three spatial dimensions and numerically compute the quantum circuit complexity of the ground state in the Mott insulator and superfluid phases using a mean field approximation with additional quadratic fluctuations. After mapping to a qubit system, the result is given by the complexity associated with a Bogoliubov transformation applied to the reference state taken to be the mean field ground state. In particular, the complexity has peaks at the O(2) critical points where the system can be described by a relativistic quantum field theory. Given that we use a Gaussian approximation, near criticality the numerical results agree with a free field theory calculation. To go beyond the Gaussian approximation we use general scaling arguments that imply that, as we approach the critical point t → t c , there is a non-analytic behavior in the complexity c 2 (t) of the form |c 2 (t) – c 2 (t c )| ~ |t – t c | νd , up to possible logarithmic corrections. Here d is the number of spatial dimensions and ν is the usual critical exponent for the correlation length ξ ~ |t – t c | –ν . As a check, for d = 2 this agrees with the numerical computation if we use the Gaussian critical exponent $v=\frac{1}{2}$. Finally, using AdS/CFT methods, we study higher dimensional examples and confirm this scaling argument with non-Gaussian exponent ν for strongly interacting theories that have a gravity dual.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Hubbard-corrected oxide formation enthalpies without adjustable parameters

A density functional theory (DFT) approach to computing transition metal oxide heat of formation without adjustable parameters is presented. Different degrees of d-electron localization in oxides are treated within the DFT+U approach with site-dependent, first-principles Hubbard U-parameters obtained from linear response theory, and delocalized states in the metallic phases are treated without Hubbard corrections. Comparison of relative stabilities of these differently treated phases is enabled by a local d-electron density matrix-dependent model, which was found by genetic programming against experimental reference formation enthalpies. This mathematically simple model does not explicitly depend on the Hubbard-corrected ionic species and is shown to reproduce the heats of formation of the Mott insulators Ca 2 RuO 4 and Y 2 Ru 2 O 7 within ~3% of experimental results, where the experimental training data did not contain Ru oxides. This newly developed method thus absolves from the need for element-specific corrections fitted to experiments in existing Hubbard-corrected approaches to the prediction of reaction energies of transition metal oxides and metals. The absence of fitting parameters opens up here the possibility to predict relative thermodynamic stabilities and reaction energies involving d-states of varying degree of localization at transition metal oxide interfaces and defects, where site-dependent U-parameters will be particularly important and devising a fitting scheme against experimental data with predictive power would be exceedingly difficult.

transition metal oxides↗
DOE OSTI2026

High magnetic field response of superconductivity dome in quantum artificial high−⁢𝑇 𝐶 superlattices with variable geometry

It is known that cuprate artificial high-𝑇 𝐶 superlattices (AHTS) with period 𝑑, composed of quantum wells confining interface space charge in stoichiometric Mott insulator layers (𝑆), with thickness 𝐿, at the interface with overdoped normal metallic cuprate layers (𝑁) show a superconducting dome by tuning the geometric 𝐿 over 𝑑 ratio of the SNSN superlattice with the top predicted by quantum material design engineering quantum size effects. Here we report high-field magnetotransport measurements up to 41 Tesla of AHTS across the entire superconducting dome. The results show the universal upward-concave behavior of the temperature-dependent upper critical magnetic field in low-𝑇 𝐶 samples at the rising edge and drop edge of the dome, providing strong evidence consistent with two-band superconductivity in agreement with multigap theory used for quantum design of the SNSN superlattices. The measured superconducting coherence length demonstrates that atomic-scale engineering controls not only the critical temperature but also the intrinsic pair size at Fano-Feshbach resonances physics paving the way toward next-generation quantum devices and shedding light on unconventional superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Upper critical magnetic field and multiband superconductivity in artificial high-𝑇 𝑐 superlattices of nano quantum wells

Artificial high-T c superlattices (AHTS) composed of quantum building blocks with tunable superconducting critical temperature have been synthesized by engineering their nanoscale geometry using the Bianconi-Perali Valletta (BPV) two-gap superconductivity theory. These quantum heterostructures consist of quantum wells made of superconducting, modulation-doped Mott insulators (S), confined by a metallic (N) potential barrier. The lattice geometry has been carefully engineered to induce the predicted Fano-Feshbach shape resonance between the gaps, near a topological Lifshitz transition. Here, we validate the BPV theory by providing compelling experimental evidence that AHTS samples, at the peak of the superconducting dome, exhibit resonant two-band, two-gap superconductivity. This is demonstrated by measuring the temperature dependence of the upper critical magnetic field, μ 0 H c2 , in samples with superlattice periods 3.3 < d < 5.28 nm and L/d ratios close to the magic value 2/3 (where L is the thickness of the superconducting La 2 CuO 4 layer and d is the superlattice period). Here, the data reveal the predicted upward concavity in H c2 (T) and a characteristic kink in the coherence length as a function of temperature, confirming the predicted two-band superconductivity with Fermi velocity ratio ≈ 0.25 and significant pair-exchange term among the two condensates.

Heterostructures↗
DOE OSTI2020

Modeling the pseudogap metallic state in cuprates: Quantum disordered pair density wave

We present a way to quantum-disorder a pair density wave and propose it to be a candidate of the effective low-energy description of the pseudogap metal which may reveal itself in a sufficiently high magnetic field that suppresses the d-wave pairing. The ground state we construct is a small-pocket Fermi liquid with a bosonic Mott insulator in the density-wave-enlarged unit cell. At low energy, the charge density is mainly carried by charge 2e bosons, which develop a small insulating gap. As an intermediate step, we discuss the quantum disordering of a fully gapped superconductor and its excitation spectrum. A simplified 1D model, which we solve numerically, is used to illustrate the introduced concepts. Here, we discuss a number of experimental consequences. The interplay between the electron and the small-gap boson results in a step-function background in the electron spectral function which may be consistent with existing angle-resolved photoemission spectroscopy data. Optical excitation across the boson gap can explain the onset and the magnitude of the mid infrared absorption reported long ago.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Off-shell effective energy theory: A unified treatment of the Hubbard model from $\textit{d} = 1$ to $\textit{d = ∞}$

Here we propose an exact formalism, off-shell effective energy theory (OET), which provides a thermodynamic description of a generic quantum Hamiltonian. The OET is based on a partitioning of the Hamiltonian and a corresponding density matrix ansatz constructed from an off-shell extension of the equilibrium density matrix; and there are dual realizations based on a given partitioning. To approximate OET, we introduce the central point expansion (CPE), which is an expansion of the density matrix ansatz, and we renormalize the CPE using a standard expansion of the ground-state energy. In this work we showcase the OET for the one-band Hubbard model in $\textit{d}$ = 1 , 2, and $\infty$, using a partitioning between kinetic and potential energy, yielding two realizations denoted as $\mathcal{K}$ and $\mathcal{X}$. OET shows favorable agreement with exact or state-of-the-art results over all parameter space, and has a negligible computational cost. Physically, $\mathcal{K}$ describes the Fermi liquid, while $\mathcal{X}$ gives an analogous description of both the Luttinger liquid and the Mott insulator. Our approach should find broad applicability in lattice model Hamiltonians, in addition to real materials systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Magnetic phase transitions in quantum spin-orbital liquids

Here, we investigate the spin and orbital correlations of a superexchange model with spin $\textit{S}$ = 1 and orbital $\textit{L}$ = 1 relevant for $5d^4$ transition-metal Mott insulators, using exact diagonalization and density matrix renormalization group (DMRG). For spin-orbit coupling λ = 0, the orbitals are in an entangled state that is decoupled from the spins. We find two phases with increasing λ: (I) the $\textit{S}$2 phase with two peaks in the structure factor for λ ≤ λ$_{c1}$ ≈ 0.34$\textit{J}$ where $\textit{J}$ is the ferromagnetic exchange; and, (II) the $\textit{S}$1 phase for λ$_{c1}$ < λ ≤ λ$_{c2}$ ≈ 1.2$\textit{J}$ with emergent antiferromagnetic correlations. Both $\textit{S}$1 and $\textit{S}$2 phases are shown to exhibit power-law correlations, indicative of a gapless spectrum. Upon increasing λ > λ$_{c2}$ leads to a product state of local spin-orbital singlets that exhibit exponential decay of correlations, indicative of a gapped phase. We obtain insights into the phases from the well-known Uimin-Lai-Sutherland model in an external field that provides an approximate description of our model within mean-field theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

DFT Plus DMFT Study of Spin-Charge-Lattice Coupling in Covalnt LaCoO{siub 3}.

We study energetics and the nature of both homogeneous and mixed spin (MS) states in LaCoO3 incorporating structural changes of the crystal volume expansion and the Co-O bond disproportionation (BD) during the spin-state transition using the density functional theory plus dynamical mean field theory (DFT + DMFT) method. DFT + DMFT predicts that energetics of both excited spin states are almost the same while DFT + U calculations of the same structures energetically favor the MS states and produce various metastable solutions whose energetics depend sensitively on final spin states. Within DFT + DMFT, the homogeneous spin state in the expanded crystal volume shows the multiconfigurational nature with non-negligible occupancy probabilities of both high spin (HS) and low spin (LS) states along with d(6) and d(7) charge configurations indicating the dynamically fluctuating nature of spin and charge states due to the Co-O covalency. The nature of the MS state under the BD structure reveals that Co sites with the long Co-O bonds develop a Mott insulating state and favor HS with a d(6) configuration, while more covalent Co sites with the short Co-O bonds occupy more LS states with a d(7) configuration and behave as a band insulator, as a result, charge ordering is induced in the BD structure from the spin-state ordering. We also find that both energetics and electronic structure sensitively depend on the Co-O covalency effect, which can be tuned by changing the double counting potential and the resulting d occupancy (N-d), and N-d close to 6.7 is consistent with the nature of the spin-state transition. Our results show that structural changes during the spin-state transition can play an important role in understanding energetics and electronic structure of LaCoO3.

Park, Hyowon↗
DOE OSTI2020

Charge density waves on a half-filled decorated honeycomb lattice

Tight binding models like the Hubbard Hamiltonian are most often explored in the context of uniform intersite hopping $\textit{t}$. The electron-electron interactions, if sufficiently large compared to this translationally invariant $\textit{t}$, can give rise to ordered magnetic phases and Mott insulator transitions, especially at commensurate filling. The more complex situation of nonuniform $\textit{t}$ has been studied within a number of situations, perhaps most prominently in multiband geometries where there is a natural distinction of hopping between orbitals of different degree of overlap. In this paper we explore related questions arising from the interplay of multiple kinetic energy scales and electron-phonon interactions. Specifically, we use determinant quantum Monte Carlo (DQMC) to solve the half-filled Holstein Hamiltonian on a “decorated honeycomb lattice,” consisting of hexagons with internal hopping $\textit{t}$ coupled together by $\textit{t'}$. This modulation of the hopping introduces a gap in the Dirac spectrum and affects the nature of the topological phases. Here, we determine the range of $\textit{t/t'}$ values which support a charge density wave phase about the Dirac point of uniform hopping $\textit{t = t'}$, as well as the critical transition temperature $T_c$. The QMC simulations are compared with the results of mean field theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Pseudospin-lattice coupling and electric control of the square-lattice iridate Sr2IrO4

Sr 2 IrO 4 is an archetypal spin-orbit-coupled Mott insulator and has been extensively studied in part because of a wide range of predicted states. Limited experimental characterization of these states thus far brings to light the extraordinary susceptibility of the physical properties to the lattice, particularly, the Ir-O-Ir bond angle. Here, we report a microscopic rotation of the IrO 6 octahedra below 50 K measured by single crystal neutron diffraction. Overall, this sharp lattice anomaly provides keys to understanding the anomalous low-temperature physics and a direct confirmation of a crucial role that the Ir-O-Ir bond angle plays in determining the ground state. Indeed, as also demonstrated in this study, applied electric current readily weakens the antiferromagnetic order via the straightening of the Ir-O-Ir bond angle, highlighting that even slight change in the local structure can disproportionately affect the physical properties in the spin-orbit-coupled system.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Twinned nanostructure of VO 2 thin films grown on r-cut sapphire

Due to strong electron-phonon interactions, strain engineering is a powerful tool to control quantum critical phenomena in strongly correlated oxides. Exploring this possibility requires understanding the nanoscale structure of quantum materials and the role it plays in forming the strain landscape. In this work, we used a combination of x-ray nanoimaging and reciprocal space mapping to study the nanostructure of the archetypal Mott insulator VO 2 featuring an insulator-to-metal and structural phase transitions. We found that VO 2 thin films grown on r-cut sapphire consist of two intertwined crystal lattices, permanently inclined with respect to each other. This persistent pattern of twin domains stands out from the symmetry breaking induced by the structural phase transition and conceivably originates from the post-growth strain relaxation process. We propose a model explaining the formation of twin domains and the emergence of anisotropy in the film nanostructure. Our work suggests using miscut substrates to suppress either one or the other twin that can serve as a new tool to control strain in VO 2 films.

36 MATERIALS SCIENCE↗