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At least 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Spontaneous breaking of multipole symmetries

Multipole symmetries are of interest both as a window on fracton physics and as a crucial ingredient in realizing new universality classes for quantum dynamics. Here we address the question of whether and when multipole symmetries can be spontaneously broken, both in thermal equilibrium and at zero temperature. In this work, we derive generalized Mermin-Wagner arguments for the total or partial breaking of multipolar symmetry groups and generalized Imry-Ma arguments for the robustness of such multipolar symmetry breaking to disorder. We present both general results and explicit examples. Our results should be directly applicable to quantum dynamics with multipolar symmetries and also provide a useful stepping stone to understanding the robustness of fracton phases to thermal fluctuations, quantum fluctuations, and disorder.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Dipolar Bose-Hubbard model

We study a simple model of interacting bosons on a d-dimensional cubic lattice whose dynamics conserves both total boson number and total boson dipole moment. This model provides a simple framework in which several remarkable consequences of dipole conservation can be explored. As a function of chemical potential and hopping strength, the model can be tuned between gapped Mott insulating phases and various types of gapless condensates. The condensed phase realized at large hopping strengths, which we dub a Bose-Einstein insulator, is particularly interesting: despite having a Bose condensate, it is insulating, and despite being an insulator, it is compressible.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Pairing symmetry of twisted bilayer graphene: A phenomenological synthesis

One of the outstanding questions in the study of twisted bilayer graphene—from both experimental and theoretical points of view—is the nature of its superconducting phase. In this paper we perform a comprehensive synthesis of existing experiments, and argue that experimental constraints are strong enough to allow the structure of the superconducting order parameter to be nearly uniquely determined. In particular, we argue that the order parameter is nodal, and is formed from an admixture of spin-singlet and spin-triplet Cooper pairs. This argument is made on phenomenological grounds, without committing to any particular microscopic model of the superconductor. Existing data is insufficient to determine the orbital parity of the order parameter, which could be either p wave or d wave. We propose a way in which the measurement of Andreev edge states can be used to distinguish between the two.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Phase dynamics in an AC-driven multiterminal Josephson junction analog

In the presence of an AC drive, multiterminal Josephson junctions exhibit the inverse AC Josephson effect, where the oscillations of the superconducting phase of each junction can lock onto one another or onto the external drive. The competition between these different phase-locked states results in a complex array of quantized voltage plateaus whose stability strongly depend on the circuit parameters of the shunted junctions. This phase diagram cannot be explored with low-temperature transport experiments alone, given the breadth of the parameter space, so we present an easily tunable analog circuit whose dynamical properties emulate those of a three-terminal junction. We focus on the observation of the multiterminal inverse AC Josephson effect, and we discuss how to identify Shapiro steps associated with each of the three junctions as well as their quartet states. We only observe integer phase-locked states in strongly overdamped networks, but fractional Shapiro steps appear as well when the quality factor of the junctions increases. Finally, we discuss the role of transverse coupling in the synchronization of the junctions.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Dipole condensates in tilted Bose-Hubbard chains

Here we study the quantum phase diagram of a Bose-Hubbard chain whose dynamics conserves both a boson number and boson dipole moment, a situation which can arise in strongly tilted optical lattices. The conservation of the dipole moment has a dramatic effect on the phase diagram, which we analyze by combining a field theory analysis with DMRG simulations. In the thermodynamic limit, the phase diagram is dominated by various types of incompressible dipolar condensates. In finite-sized systems, however, it may be possible to stabilize a Bose-Einstein insulator: an exotic compressible phase which is insulating, despite the absence of a charge gap. We suggest several ways by which these exotic phases can be identified in near-term cold-atom experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Ion irradiation induced crystalline disorder accelerates interfacial phonon conversion and reduces thermal boundary resistance

Traditional understanding of the thermal boundary resistance (TBR) across solid-solid interfaces posits that the vibrational densities of states overlap between materials dictates interfacial energy transport, with phonon scattering occurring at the interface. Using atomistic simulations, we show a mechanism for control of TBR; point defects near an interface can lead to both short- and midrange disorder, accelerating the conversion of vibrational energy between bulk and interfacial modes, ultimately reducing the TBR. In conclusion, we experimentally demonstrate this reduction through ion irradiation of gallium nitride and subsequently measuring the TBR across Al/GaN interfaces.

Pfeifer, Thomas W.↗
DOE OSTI2025

Shock-driven amorphization and melting in Fe 2 O 3

We present measurements on Fe 2⁢ O 3 amorphization and melt under laser-driven shock compression up to 209(10) GPa via time-resolved in situ x-ray diffraction. At 122(3) GPa, a diffuse signal is observed indicating the presence of a noncrystalline phase. Structure factors have been extracted up to 182(6) GPa showing the presence of two well-defined peaks. A rapid change in the intensity ratio of the two peaks is identified between 145(12) and 151(12) GPa, indicative of a phase change. The noncrystalline diffuse scattering is consistent with shock amorphization of Fe 2 ⁢O 3 between 122(3) and 145(12) GPa, followed by an amorphous-to-liquid transition above 151(12) GPa. Upon release, a noncrystalline phase is observed alongside crystalline α–Fe 2⁢ O 3 . The extracted structure factor and pair distribution function of this release phase resemble those reported for Fe 2 ⁢O 3 melt at ambient pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

High-pressure structural behavior of silicon telluride Si2Te3 nanoplates

The high-pressure behavior of silicon telluride (Si2Te3), a two-dimensional (2D) layered material, was investigated using synchrotron x-ray powder diffraction in a diamond anvil cell to 11.5GPa coupled with first-principles theory. Si2Te3 undergoes a phase transition at <1GPa from a trigonal to a hexagonal crystal structure. At higher pressures (>8.5GPa), x-ray diffraction showed the appearance of new peaks possibly coincident with a new phase transition, though we suspect Si2Te3 retains a hexagonal structure. Density functional theory calculations of the band structure reveal metallization above 9.1GPa consistent with previous measurements of the Raman spectra and disappearance of color and transparency at pressure. The theoretical Raman spectra reproduce the prominent features of the experiment, though a deeper analysis suggests that the orientation of Si dimers dramatically influences the vibrational response. Given the complex structure of Si2Te3, simulation of the resulting high-pressure phase is complicated by disordered vacancies and the initial orientations of Si–Si dimers in the crushed layered phase.

Li, Bohan↗
DOE OSTI2025

Tunable atomically enhanced moiré Berry curvatures in twisted triple bilayer graphene

We report a twisted triple bilayer graphene platform consisting of three units of Bernal bilayer graphene consecutively twisted at 1.49° and 1.68°. We demonstrate the atomic reconstruction between the two competing moiré superlattices strongly enhances the Berry curvature of each moiré band insulator state, characterized by measured strong nonlocal valley Hall effect that sensitively depends on the intermoiré competition strength, tunable by manipulating the out-of-plane carrier distribution. Our study sheds light on the microscopic mechanism of atomic and electronic reconstruction in twisted multilayer systems, by systematically investigating transport signatures of moiré Berry curvature and its enhancement from moiré-of-moiré lattice reconstruction. In conclusion, we show that the reconstructed electronic band can be versatilely tuned by electrostatics, providing an approach toward engineering the band structure and its topology for a quantum material platform with designer electrical and optical properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Phonon-induced multicolor correlations in hBN single-photon emitters

Color centers in hexagonal boron nitride have shown enormous promise as single-photon sources, but a clear understanding of the electron-phonon interaction dynamics is critical to developing the material for quantum communications or quantum simulations. We demonstrate photon antibunching in the filtered auto- and cross-correlations g (2) lm (τ) between zero-, one-, and two-phonon replicas of defect luminescence. With no background correction, we observe a single-photon purity of g (2) (0) = 0.20 in a phonon replica and cross-spectral correlations of g (2) lm (0) = 0.18 between a phonon replica and the zero-phonon line. Furthermore, we combine a violation of the Cauchy-Schwarz inequality and a phenomenological model of the phonon-mediated emission cross section to distinguish a low quantum-efficiency defect from phonon replicas of a bright defect.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

The study of C-13(a, n)O-16 reactions in the Majorana Demonstrator calibration data

Neutron captures and delayed decays of reaction products are common sources of backgrounds in ultra-rare event searches. In this work, we studied 13C(a, n)1640 O reactions induced by a-particles emitted within the calibration sources of the Majorana Demonstrator. These sources are thorium-based calibration standards enclosed in carbon-rich materials. The reaction rate was estimated by using the 6129 keV ?-rays emitted from the excited 1643 O states that are populated when the incoming a-particles have enough energy. Thanks to the excellent energy performance of the Demonstrator’s germanium detectors, these characteristic photons can be clearly observed in the calibration data. Facilitated by Geant4 simulations, a comparison between the observed 6129 keV photon rates and predictions by a TALYS-based software was performed. The measurements and predictions were found to be consistent with each other, albeit with large statistical uncertainties. This agreement provides supports for further background predictions for (a, n)-reactions in future double-beta decay search efforts.

6 ≤ A ≤ 19↗
DOE OSTI2020

Application of dilaton chiral perturbation theory to N f = 8 , SU(3) spectral data

We extend dilaton chiral perturbation theory (dChPT) to include the taste splittings in the Nambu-Goldstone sector observed in lattice simulations of near-conformal theories with staggered fermions. We then apply dChPT to a recent simulation by the LSD collaboration of the SU(3) gauge theory with 8 fermions in the fundamental representation, which is believed to exhibit near-conformal behavior in the infrared, and in which a light singlet scalar state, nearly degenerate with the pions, has been found. We find that the mesonic sector of this theory can be successfully described by dChPT, including, in particular, the mesonic taste splittings found in the simulation. We confirm that current simulations of this theory are in the "large-mass" regime.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗