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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

High-temperature fractional quantum Hall state in the Floquet kagome flat band

A fractional quantum Hall effect (FQHE) has been predicted in a topological flat band (FB) by a single-particle band structure combined with phenomenological theory or solution of a many-body lattice Hamiltonian with fuzzy parameters. A long-standing roadblock toward the realization of a FB-FQHE is lacking the many-body solution of specific materials under realistic conditions. Here, we demonstrate a combined study of single-particle Floquet band theory with exact diagonalization (ED) of a many-body Hamiltonian. We show that a time-periodic circularly polarized laser inverts the sign of second-nearest-neighbor hopping in a kagome lattice and enhances spin-orbit coupling in one spin channel to produce a Floquet FB with a high flatness ratio of bandwidth over band gap, as exemplified in monolayer Pt 3 C 36 S 12 H 12 . The ED of the resultant Floquet-kagome lattice Hamiltonian gives a one-third-filling ground state with a laser-dependent excitation gap of a FQH state, up to an estimated temperature above 70 K. Our findings pave the way for exploring the alluding high-temperature FB-FQHE.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Hot-lines topology and the fate of the spin resonance mode in three-dimensional unconventional superconductors

In the quasi-two-dimensional (quasi-2D) copper- and iron-based superconductors, the onset of superconductivity is accompanied by a prominent peak in the magnetic spectrum at momenta close to the wave-vector of the nearby antiferromagnetic state. Such a peak is well described in terms of a spin resonance mode, i.e., a spin-1 exciton theoretically predicted for quasi-2D superconductors with a sign-changing gap. The same theories, however, indicate that such a resonance mode should be absent in a three-dimensional (3D) system with a spherical Fermi surface. This raises the question of the fate of the spin resonance mode in layered unconventional superconductors that are not strongly anisotropic, such as certain heavy-fermion compounds and potentially the newly discovered nickelate superconductor NdNiO 2 . Here, we use the random phase approximation to calculate the dynamical spin susceptibility of 3D superconductors with a d x 2 –y 2 -wave gap symmetry and corrugated cylindrical-like Fermi surfaces. By varying the out-of-plane hopping anisotropy t z /t, we demonstrate that the appearance of a spin resonance mode is determined by the topology of the hot lines, i.e., lines on the Fermi surface that are connected by the magnetic wave vector. For an in-plane antiferromagnetic wave vector, the hot lines undergo a topological transition from open lines to closed loops at a critical t z /t value. The closed hot lines cross the nodal superconducting lines, making the spin resonance mode overdamped and incoherent. In contrast, for an out-of-plane antiferromagnetic wave vector, the hot lines remain open and the spin resonance mode remains sharp. We discuss the experimental implications of our results for the out-of-plane dispersion of the spin resonance mode and, more generally, for inelastic neutron scattering experiments on unconventional superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Low-energy interband transition in the infrared response of the correlated metal SrVO 3 in the ultraclean limit

We studied the low-energy electronic response of the prototypical correlated metal SrVO 3 in the ultraclean and disordered limit using infrared spectroscopy and density functional theory plus dynamical mean field theory calculations (DFT+DMFT). A strong optical excitation at 70 meV is observed in the optical response of the ultraclean samples but is hidden by the low-energy Drude-like response from intraband excitations in the more disordered samples. DFT+DMFT calculations reveal that this optical excitation originates from interband transitions between the bands split by orbital off-diagonal hopping, which has often been ignored in cubic systems, such as SrVO 3 . A memory function analysis of the optical data shows that this interband transition can lead to deviations of optical self-energy from the expected Fermi-liquid behavior. Our findings demonstrate that analysis schemes employed to extract many-body effects from optical spectra may be oversimplified to study the true electronic ground state and that improvements in material quality can guide efforts to refine theoretical approaches.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Charge singlets and orbital-selective charge density wave transitions

The possibility of “orbitally selective Mott transitions” within a multiband Hubbard model, in which one orbital with large on-site electron-electron repulsion U 1 is insulating and another orbital, to which it is hybridized, with small U –1 , is metallic, is a problem of long-standing debate and investigation. In this paper we study an analogous phenomenon, the coexistence of metallic and insulating bands in a system of orbitals with different electron-phonon coupling. To this end, we examine two variants of the bilayer Holstein model: a uniform bilayer and a “Holstein-metal interface” where the electron-phonon coupling, λ, is zero in the “metallic” layer. In the uniform bilayer Holstein model, charge density wave (CDW) order dominates at small interlayer hybridization t 3 , but decreases and eventually vanishes as t 3 grows, providing a charge analog of singlet (spin liquid) physics. In the interface case, we show that CDW order penetrates into the metal layer and forms long-range CDW order at an intermediate ratio of inter- to intralayer hopping strengths, 1.4 ≲ t 3 /t ≲ 3.4. Furthermore, this is consistent with the occurrence of an “orbitally selective CDW” regime at weak t3 in which the layer with λ 1 ≠ 0 exhibits long-range charge order, but the “metallic layer” with λ –1 = 0, to which it is hybridized, does not.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Bilayer Hubbard model: Analysis based on the fermionic sign problem

The bilayer Hubbard model describes the antiferromagnet to spin singlet transition and, potentially, aspects of the physics of unconventional superconductors. Despite these important applications, significant aspects of its phase diagram in the interplane hopping t ⊥ versus on-site interaction U parameter space, at half filling, are largely in disagreement. Here we provide an analysis making use of the average sign of weights over the course of the importance sampling in quantum Monte Carlo simulations to resolve several central open questions. Specifically, this metric of the weights clarifies the finite-sized metallic regimes at small U. Furthermore, at strong interactions, it points to the existence of a crossover from a correlated to uncorrelated band insulator not yet explored in a variety of existing, unbiased numerical methods. Our paper demonstrates the versatility of using properties of the weights in quantum Monte Carlo simulations to reveal important physical characteristics of the models under study.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

$\textit{J}$ = 0 nonmagnetic insulating state in K 2 OsX 6 ($\textit{X}$ = F, Cl, and Br)

In 4$\textit{d}$ /5$\textit{d}$ transition-metal systems, many interesting physical properties arise from the interplay of bandwidth, electronic correlations, and spin-orbit interactions. Here, using ab initio density functional theory, we systematically study the double-perovskite-like system K 2 OsX6 ($\textit{X}$ = F, Cl, and Br) with a $5d^4$ electronic configuration. In this work, our main result is that the $\textit{J}$ = 0 nonmagnetic insulating state develops in this system, induced by strong spin-orbit coupling (SOC). Specifically, the well-separated OsX 6 octahedra lead to the cubic crystal-field limit and result in dramatically decreasing hoppings among nearest neighbor Os-Os sites. In this case, the three degenerate $t_{2g}$ orbitals are reconstructed into two “effective” $j_{\text{eff}}$ ($j_{\text{eff}}$ = 1/2 and $j_{\text{eff}}$ = 3/2 states) states separated by the strong SOC, opening a gap with four electrons occupying the$j_{\text{eff}}$ = 3/2 orbitals. Furthermore, the hybridization between the Os 5$\textit{d}$ orbitals and the $\textit{X}$ ($\textit{X}$ = F, Cl, and Br) $\textit{p}$ orbitals increases from F to Br, leading the electrons in K 2 OsF 6 to be more localized than in K 2 OsCl 6 and K 2 OsBr 6 , resulting in a smaller bandwidth for K 2 OsF 6 than in the Cl- or Br-cases. Our results provide guidance to experimentalists and theorists working on this interesting family of osmium halides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Finite-temperature tensor network study of the Hubbard model on an infinite square lattice

Here, the Hubbard model is a longstanding problem in the theory of strongly correlated electrons and a very active one in the experiments with ultracold fermionic atoms. Motivated by current and prospective quantum simulations, we apply a two-dimensional tensor network—an infinite projected entangled pair state—evolved in imaginary time by the neighborhood tensor update algorithm working directly in the thermodynamic limit. With U⁡(1)×U⁡(1) symmetry and the bond dimensions up to 29, we generate thermal states down to the temperature of 0.17 times the hopping rate. We obtain results for spin and charge correlators, unaffected by boundary effects. The spin correlators—measurable in prospective ultracold atoms experiments attempting to approach the thermodynamic limit—provide evidence of disruption of the antiferromagnetic background with mobile holes in a slightly doped Hubbard model. The charge correlators reveal the presence of hole-doublon pairs near half filling and signatures of hole-hole repulsion on doping. We also obtain specific heat in the slightly doped regime.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Thermal transport properties of IrSbSe

Here we report a thermal transport study of IrSbSe, which crystallizes in a noncentrosymmetric cubic structure with the P2 1 3 space group and shows a narrow-gap semiconducting behavior. The large discrepancy between the activation energy for conductivity [E ρ = 128(2) meV] and for thermopower [E S = 17.7 (9) meV] from 200 to 300 K indicates a polaronic transport mechanism. The electrical resistivity varies as exp (T 0 /T) 1/4 and thermopower varies as T 1/2 at low temperatures, indicating that it evolves into Mott variable-range hopping dominant conduction with decreasing temperature. IrSbSe shows a relatively low value of thermal conductivity (~1.65 W/Km) and thermopower of about 0.24 mV/K around 100 K, yet poor electrical conductivity. On the other hand, a high vacancy defect concentration on both Ir and Sb atomic sites of up to 15% suggests a high defect tolerance and points to the possibility of a future improvement of carrier density by chemical substitution or defect optimization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Effects of spin-orbit interaction and electron correlations in strontium titanate

Here, we show that the Bloch states in the conduction band of SrTiO 3 arise from the interplay between highly anisotropic hopping in subbands derived from the Ti t2g orbitals and spin-orbit coupling that mixes these orbitals. Because of the nearly flat-band characteristics for one of the principal axes, at sufficiently high doping these Bloch states become unstable with respect to electron interactions, resulting in Mott-like singlet correlations. These findings may be relevant to the anomalous electronic properties of SrTiO 3 , including its unusual superconductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Loop-current charge density wave driven by long-range Coulomb repulsion on the kagomé lattice

Recent experiments on vanadium-based nonmagnetic kagomé metals A⁢V 3 ⁢Sb 5 ⁢(A=K,Rb,Cs) revealed evidence for possible spontaneous time-reversal symmetry (TRS) breaking in the charge density wave (CDW) ordered state. The long-sought-after quantum order of loop currents has been suggested as a candidate for the TRS breaking state. However, a microscopic model for the emergence of the loop-current CDW due to electronic correlations is still lacking. Here, in this work, we calculate the susceptibility of the real and imaginary bond orders on the kagomé lattice near van Hove filling, and reveal the importance of next-nearest-neighbor Coulomb repulsion V 2 in triggering the instability toward imaginary bond ordered CDW. The concrete effective single-orbital t–V 1 –V 2 model on the kagomé lattice is then studied, where t and V 1 are the hopping and Coulomb repulsion on the nearest-neighbor bonds. We obtain the mean-field ground states, analyze their properties, and determine the phase diagram in the plane spanned by V 1 and V 2 at van Hove filling. The region dominated by V 1 is occupied by a 2⁢a 0 ×2⁢a 0 real CDW insulator with the inverse of Star-of-David (ISD) bond configuration. Increasing V 2 indeed drives a first-order transition from ISD to stabilized loop-current insulators that exhibit four possible current patterns of different topological properties, leading to orbital Chern insulators. We then extend these results away from van Hove filling and show that electron doping helps the stabilization of loop currents, and gives rise to doped orbital Chern insulators with emergent Chern Fermi pockets carrying large Berry curvature and orbital magnetic moment. Our findings provide a concrete model realization of the loop-current Chern metal at the mean-field level for the TRS breaking normal state of the kagomé superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Photoinduced enhancement of superconductivity in the plaquette Hubbard model

Real-time dynamics techniques have proven increasingly useful in theoretically and experimentally understanding strongly correlated systems. By employing unbiased time-resolved exact diagonalization, we study pump dynamics in the two-dimensional plaquette Hubbard model, where distinct hopping integrals t h and t' h are present within and between plaquettes. In the intermediate coupling regime, a significant enhancement of d-wave superconductivity is observed and compared with that obtained by simply examining expectation values with the eigenstates of the Hamiltonian. Furthermore, our work provides a further understanding of superconductivity in the Hubbard model, extends the description of the pairing amplitude to the frequency-anisotropy plane, and offers a promising approach for experimentally engineering emergent out-of-equilibrium states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Triangular lattice Hubbard model physics at intermediate temperatures

Moiré systems offer an exciting playground to study the many-body effects of strongly correlated electrons in regimes that are not easily accessible in conventional material settings. Motivated by a recent experiment on WSe 2 /WS 2 moiré bilayers [Y. Tang et al., Nature (London) 579, 353 (2020)], which realized a triangular superlattice with a small hopping (of approximately 10 Kelvin), with tunable density of holes, we explore the Hubbard model on the triangular lattice for intermediate temperatures t ≲ T < U. Employing finite-temperature Lanczos calculations and closely following the fitting protocols used in the experiment, we recover the observed trends in the reported Curie-Weiss temperature Θ with filling, using the reported interaction strength U/t=20. Here, we focus on the large increase of |Θ| on decreasing the density below half filling and the sign change of Θ at higher fillings, which signals the onset of ferromagnetism. The increase in |Θ| is also seen in the t–J model (the low-energy limit of the Hubbard model) in the intermediate temperature range, which we clarify is opposite to the trend in its high temperature limit. Differences between the low, intermediate, and high temperature regimes are discussed. Our numerical calculations also capture the crossover between short-range antiferromagnetic to ferromagnetic order in the intermediate temperature regime, a result broadly consistent with the experimental findings. We find that this behavior is a finite-temperature remnant of the underlying zero-temperature phase transition, which we explore with ground-state density matrix renormalization group calculations. We provide evidence of ferromagnetism characterized by weak (but robust) correlations that explain the small Θ seen in the experiment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Electronic transport and thermoelectricity in the selenospinel Cu 6—x Fe 4+x Sn 12 Se 32

Here, we report a study of selenospinel Cu 6—x Fe 4+x Sn 12 Se 32 (x = 0, 1, 2) single crystals, which crystallize in a cubic structure with the F$d$$\overline{3}$$m$ space group, and show typical semiconducting behavior. The large discrepancy between the activation energy for electrical conductivity E ρ (32.3–69.8 meV), and for thermopower E S (3.2–11.5 meV), indicates a polaronic transport mechanism between 350 and 50 K. With decreasing temperature, it evolves into variable-range hopping conduction. Furthermore, the heat capacity shows a hump around 25(5) K and diverges from the Debye T 3 law at low temperatures, indicating the observation of structural glassy features in these crystalline solids.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Enhancement of charge density wave correlations in a Holstein model with an anharmonic phonon potential

The Holstein Hamiltonian describes itinerant electrons whose site density couples to local phonon degrees of freedom. In the single-site limit, at half filling, the electron-phonon coupling results in a double-well structure for the lattice displacement, favoring empty or doubly occupied sites. In two dimensions and on a bipartite lattice in d ≥ 2, intersite hopping causes these doubly occupied and empty sites to alternate in a charge density wave (CDW) pattern when the temperature is lowered. Because a discrete symmetry is broken, this occurs in a conventional second-order transition at finite T cdw . In this paper, we investigate the effect of changing the phonon potential energy to one with an intrinsic double-well structure even in the absence of electron-phonon coupling. While this aids in the initial process of pair formation, the implications for subsequent CDW order are nontrivial. One expects that, when the electron-phonon coupling is too strong, the double wells become deep and the polaron mass gets large, an effect which reduces T cdw . Here, we show here the existence of regions of parameter space where the double-well potential, while aiding local pair formation, does so in a way which also substantially enhances long-range CDW order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Trends in electronic structures and s ± -wave pairing for the rare-earth series in bilayer nickelate superconductor R 3 Ni 2 O 7

The recent discovery of pressure-induced superconductivity in the bilayer La 3 Ni 2 O 7 (LNO) has opened a new platform for the study of unconventional superconductors. In this publication, we investigate theoretically the whole family of bilayer 327-type nickelates R 3 Ni 2 O 7 (R = rare-earth elements) under pressure. From La to Lu, the lattice constants and volume decrease, leading to enhanced in-plane and out-of-plane hoppings, resulting in an effectively reduced electronic correlation U / W. Furthermore, the Ni's t 2g states shift away from the e g states, while the crystal-field splitting between d 3z 2 -r 2 and d x 2 -y 2 is almost unchanged. In addition, six candidates were found to become stable in the Fmmm phase, with increasing values of critical pressure as the atomic number increases. Similar to the case of LNO, the s ± -wave pairing tendency dominates in all candidates, due to the nesting between the M = (π , π) and the X = (π , 0) and Y = (0 , π) points in the Brillouin zone. Then, T c is expected to decrease as the radius of rare-earth (RE) ions decreases. In conclusion, our results suggest that LNO is already the “optimal” candidate, with Ce a close competitor, among the whole of the RE bilayer nickelates, and to increase T c we suggest growing on special substrates with larger in-plane lattice spacings.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Singlet, triplet, and pair density wave superconductivity in the doped triangular-lattice moiré system

Recent experimental progress has established the twisted bilayer transition metal dichalcogenide (TMD) as a highly tunable platform for studying many-body physics. Particularly, the homobilayer TMDs under displacement field are believed to be described by a generalized triangular-lattice Hubbard model with a spin-dependent hopping phase θ. To explore the effects of θ on the system, we perform density matrix renormalization group calculations for the relevant triangular lattice t-J model. By changing θ at small hole doping, we obtain a region of quasi-long-range superconducting order coexisting with charge and spin density wave within 0 < θ < π/3. The superconductivity is composed of a dominant spin singlet d-wave and a subdominant triplet p-wave pairing. Intriguingly, the S z = ±1 triplet pairing components feature pair-density waves. In addition, we find a region of triplet superconductivity coexisting with charge-density wave and ferromagnetism within π/3 < θ < 2π/3, which is related to the former phase at smaller θ by a combined operation of spin-flip and gauge transformation. Furthermore, our findings provide insights and directions for experimental search for exotic superconductivity in twisted TMD systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Suppressed superexchange interactions in the cuprates by bond-stretching oxygen phonons

We study a multiorbital Hubbard–Su-Schrieffer-Heeger model for the one-dimensional (1D) corner-shared cuprates in the adiabatic and nonadiabatic limits using exact diagonalization and determinant quantum Monte Carlo. At half filling and in the adiabatic limit, lattice dimerization can be achieved only over a narrow range of couplings slightly below a critical coupling g c . Beyond this value, the sign of the effective hopping changes, and the lattice becomes unstable. Strong lattice fluctuations replace the dimerization state in the nonadiabatic case. We also examine the model's temperature-dependent uniform and dynamical magnetic susceptibilities and compare them to the results of an effective spin-1/2 Heisenberg model. In doing so, we demonstrate that lattice fluctuations induced by the e-ph interaction suppress the effective superexchange interaction when g < g c . Furthermore, our results elucidate the effect of bond-stretching phonons in the parent cuprate compounds in general and are particularly relevant to 1D cuprates, where strong e-ph interactions have recently been inferred.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Ground-state phase diagram and superconductivity of the doped Hubbard model on six-leg square cylinders

Here, we have studied the ground state properties of Hubbard model on long six-leg square cylinders with doped hole concentration per site 5.55%≤δ≤12.5% using density-matrix renormalization group. By keeping a large number of states for long system sizes, we find that the nature of the ground state is remarkably sensitive to the presence of next-nearest-neighbor electron hopping t'. In the positive t' side, we find a robust d-wave superconducting (SC) phase characterized by coexisting quasi-long-range SC and charge density wave (CDW) correlations. Without t' the ground state forms an insulating stripe phase with long-range CDW order but short-range spin-spin and SC correlations. In stark contrast to four-leg cylinders, our results show that the lightly doped Hubbard model on six-leg cylinders remains insulating in the negative t' side where the SC correlations decay exponentially with short correlation lengths. In the larger negative t' side, the doped holes form a novel holon Wigner crystal with one doped hole per emergent unit cell and short-range spin-spin correlations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗