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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Topological origin of phase separation in hydrated gels

Depending on their composition, hydrated gels can be homogeneous or phase-separated, which, in turn, affects their dynamical and mechanical properties. However, the nature of the structural features, if any, that govern the propensity for a given gel to phase-separate remains largely unknown. Here, we argue that the propensity for hydrated gels to phase-separate is topological in nature. We employ reactive molecular dynamics simulations to model the early-age precipitation of calcium–alumino–silicate–hydrate (Csingle bondAsingle bondSsingle bondH) gels with varying compositions, i.e., (CaO) 1.7 (Al 2 O 3 ) x (SiO 2 ) 1 – x (H 2 O) 3.7 + x . By adopting topological constraint theory, we investigate the structural origin of phase separation in hydrated gels. We report the existence of a homogeneous-to-phase-separated transition, wherein Si-rich (x ≤ 0.10) Csingle bondAsingle bondSsingle bondH gels are homogeneous, whereas Al-rich (x > 0.10) Csingle bondAsingle bondSsingle bondH gels tend to phase-separate. Furthermore, we demonstrate that this transition is correlated to a topological flexible-to-rigid transition within the atomic network. Finally, we reveal that the propensity for topologically-overconstrained gels to phase-separate arises from the existence of some internal stress within their atomic network, which acts as an energy penalty that drives phase separation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Quantum simulation: From spin models to gauge-gravity correspondence

Quantum many-body systems with coherent and controllable interactions enable the realization of novel quantum materials, the quantum simulation of problems that cannot be simulated on classical computers, computational systems that are exponentially faster than existing classical algorithms, and the direct investigation of the conjectured duality between gravitational theories and quantum field theories. This project focusses on quantum phase transitions in strongly interacting many-body spin systems, and in particular on the dynamical evolution near the critical point and across the phase transition. The dynamics of a large quantum system containing more than 50 qubits is investigated in various critical regimes. In particular, the Kibble-Zurek transition has been characterized and critical exponents have been measured. Furthermore, quantum many-body scars have been observed and characterized for the first time. We are investigating the Kibble-Zurek mechanism in the quantum phase transition in one dimension from a disordered system to a system with ordering induced by Rydberg interactions between neighboring atoms. The Kibble-Zurek mechanism describes the formation of topological defects as a system evolves across a phase transitions, and is expected to apply to a large number of problems, including cosmology, where it should describe the structure of domain formation in the early universe.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Kibble-Zurek mechanism for nonequilibrium phase transitions in driven systems with quenched disorder

Abstract We show that the Kibble-Zurek mechanism applies to nonequilibrium phase transitions found in driven assemblies of superconducting vortices and colloids moving over quenched disorder where a transition occurs from a plastic disordered flowing state to a moving anisotropic crystal. We measure the density of topological defects as a function of quench rate through the nonequilibrium phase transition, and find that on the ordered side of the transition, the topological defect density ρ d scales as a power law, $${\rho }_{{{{{{{{\rm{d}}}}}}}}}\propto 1/{t}_{{{{{{{{\rm{q}}}}}}}}}^{\beta }$$ ρ d ∝ 1 / t q β , where t q is the quench time duration, consistent with the Kibble-Zurek mechanism. We show that scaling with the same exponent holds for varied strengths of quenched disorder and that the exponents fall in the directed percolation universality class. Our results suggest that the Kibble-Zurek mechanism can be applied to the broader class of systems that exhibit absorbing phase transitions.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2025

Classification of fragile topology enabled by matrix homotopy

Flat bands in twisted materials have attracted considerable attention due to the emergence of correlated phases that can be associated with the non-Wannier-representable nature of its single-particle states. Specifically, these bands can exhibit a class of topology that can be nullified by the addition of trivial bands, termed fragile topology, which has required an expansion of prior classification schemes. However, existing approaches for predicting fragile topology rely on momentum-space methods, e.g., Wilson loops, presenting a fundamental challenge for using fragile topology as a predictor of correlated phases in aperiodic systems, such as incommensurate twist angles in moiré materials. Here, we develop a ℤ 2 energy-resolved topological marker for classifying fragile phases using a system’s position-space description, enabling the direct classification of finite, disordered, and aperiodic materials. By translating the physical symmetries protecting the system’s fragile topological phase into matrix symmetries of the system’s Hamiltonian and position operators, we use matrix homotopy to construct our topological marker while simultaneously yielding a quantitative measure of topological robustness. We demonstrate our framework’s effectiveness in both a low-energy tight-binding model and a continuum photonic crystal model of 𝐶 2 ⁢𝒯-symmetric systems, and find that fragile topology can both persist under strong disorder and even exhibit disorder-induced reentrant phase transitions. Our photonic crystal results also demonstrate the robustness of fragile topology, and the applicability of our approach, to heterostructures lacking a bulk spectral gap. Overall, our framework serves as an efficient tool for elucidating fragile topology, offering guidance for the prediction and discovery of correlated phases in both crystalline and aperiodic materials.

Lee, Ki Young [Sandia National Laboratories (SNL-N↗
DOE OSTI2021

Electronic structure and topology across T c in the magnetic Weyl semimetal Co 3 Sn 2 S 2

Co 3 Sn 2 S 2 is a magnetic Weyl semimetal, in which ferromagnetic ordering at 177 K is predicted to stabilize Weyl points. We perform temperature and spatial dependent angle--resolved photoemission spectroscopy measurements through the Curie temperature (T c ), which show large band shifts and renormalization concomitant with the onset of magnetism. We argue that Co 3 Sn 2 S 2 evolves from a Mott ferromagnet below T c to a correlated metallic state above T c . To understand the magnetism, we derive a tight-binding model of Co-3d x 2 -y 2 orbitals on the kagome lattice. At the filling obtained by first-principles calculations, this model reproduces the ferromagnetic ground state, and results in the reduction of Coulomb interactions due to cluster effects. Using a disordered local moment simulation, we show how this reduced Hubbard U leads to a collapse of the bands across the magnetic transition, resulting in a correlated state which carries associated characteristic photoemission signatures that are distinct from those of a simple lifting of exchange splitting. Finally, the behavior of topology across T c is discussed in the context of this description of the magnetism.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

Three-dimensional dispersion in the type-II Dirac semimetals PtTe 2 and PdTe 2 revealed through circular dichroism in angle-resolved photoemission spectroscopy

PtTe 2 and PdTe 2 are among the first transition metal dichalcogenides that were predicted to host type-II Dirac fermions, exotic particles prohibited in free space. These materials are layered and air stable, which makes them top candidates for technological applications that take advantage of their anisotropic magnetotransport properties. Here, in this work, we provide a detailed characterization of the electronic structure of PtTe 2 and PdTe 2 using angle-resolved photoemission spectroscopy (ARPES) and density functional theory calculations, offering an alternative interpretation for one of the Dirac-like dispersions in these materials. Through the use of circularly polarized light, we report a different behavior of such dispersion in PdTe 2 compared to PtTe 2 , that we relate to a symmetry analysis of the dipole matrix element. Such analysis reveals a link between the observed circular dichroism and the different momentum-dependent terms in the dispersion of these two compounds, despite their close similarity in crystal structure. Additionally, our data show a clear difference in the circular dichroic signal for the type-II Dirac cones characteristic of these materials, compared to their topologically protected surface states. Our paper provides a useful reference for the ARPES characterization of other transition metal dichalcogenides with topological properties and illustrates the use of circular dichroism as a guide to identify the topological character and attributes of two otherwise equivalent band dispersions.

angle-resolved photoemission spectroscopy↗
DOE OSTI2024

Extraordinary phase transition revealed in a van der Waals antiferromagnet

While the surface-bulk correspondence has been ubiquitously shown in topological phases, the relationship between surface and bulk in Landau-like phases is much less explored. Theoretical investigations since 1970s for semi-infinite systems have predicted the possibility of the surface order emerging at a higher temperature than the bulk, clearly illustrating a counterintuitive situation and greatly enriching phase transitions. But experimental realizations of this prediction remain missing. Here, we demonstrate the higher-temperature surface and lower-temperature bulk phase transitions in CrSBr, a van der Waals (vdW) layered antiferromagnet. We leverage the surface sensitivity of electric dipole second harmonic generation (SHG) to resolve surface magnetism, the bulk nature of electric quadrupole SHG to probe bulk spin correlations, and their interference to capture the two magnetic domain states. Our density functional theory calculations show the suppression of ferromagnetic-antiferromagnetic competition at the surface is responsible for this enhanced surface magnetism. Our results not only show counterintuitive, richer phase transitions in vdW magnets, but also provide viable ways to enhance magnetism in their 2D form.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Spatially inhomogeneous superconductivity in UTe 2

The newly discovered superconductor UTe 2 is a strong contender for a topological spin-triplet state wherein a multicomponent order parameter arises from two nearly degenerate superconducting states. A key issue is whether both of these states intrinsically exist at ambient pressure. Through thermal expansion and calorimetry, we show that UTe 2 at ambient conditions exhibits two detectable transitions only in some samples, and the size of the thermal expansion jump at each transition varies when the measurement is performed in different regions of the sample. This result indicates that the two transitions arise from two spatially separated regions that are inhomogeneously mixed throughout the volume of the sample, each with a discrete superconducting transition temperature (T c ). Notably, samples with higher T c only show a single transition at ambient pressure. Above 0.3 GPa, however, two transitions are invariably observed in ac calorimetry. Our results not only point to a nearly vertical line (constant pressure) in the pressure-temperature phase diagram but also provide a consistent scenario for the sample dependence of UTe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

CsV 3 Sb 5 : A $\mathbb{Z}_2$ Topological Kagome Metal with a Superconducting Ground State

Recently discovered alongside its sister compounds KV 3 Sb 5 and RbV 3 Sb 5 , CsV 3 Sb 5 crystallizes with an ideal kagome network of vanadium and antimonene layers separated by alkali metal ions. This work presents the electronic properties of CsV 3 Sb 5 , demonstrating bulk superconductivity in single crystals with a T c = 2.5 K. The normal state electronic structure is studied via angle-resolved photoemission spectroscopy and density-functional theory, which categorize CsV 3 Sb 5 as a Z 2 topological metal. Multiple protected Dirac crossings are predicted in close proximity to the Fermi level (E F ), and signatures of normal state correlation effects are also suggested by a high-temperature charge density wavelike instability. Finally, the implications for the formation of unconventional superconductivity in this material are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

The magnetic, electronic, and light-induced topological properties in two-dimensional hexagonal FeX 2 (X = Cl, Br, I) monolayers

Using Floquet–Bloch theory, we propose to realize chiral topological phases in two-dimensional (2D) hexagonal FeX 2 (X = Cl, Br, I) monolayers under irradiation of circularly polarized light. Such 2D FeX 2 monolayers are predicted to be dynamically stable and exhibit both ferromagnetic and semiconducting properties. To capture the full topological physics of the magnetic semiconductor under periodic driving, we adopt ab initio Wannier-based tight-binding methods for the Floquet–Bloch bands, with the light-induced bandgap closings and openings being obtained as the light field strength increases. The calculations of slabs with open boundaries show the existence of chiral edge states. Interestingly, the topological transitions with branches of chiral edge states changing from zero to one and from one to two by tuning the light amplitude are obtained, showing that the topological Floquet phase of high Chern number can be induced in the present Floquet–Bloch systems.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Valley-Coherent Quantum Anomalous Hall State in AB-Stacked MoTe 2 / W S e 2 Bilayers

Moiré materials provide fertile ground for the correlated and topological quantum phenomena. Among them, the quantum anomalous Hall (QAH) effect, in which the Hall resistance is quantized even under zero magnetic field, is a direct manifestation of the intrinsic topological properties of a material and an appealing attribute for low-power electronics applications. The QAH effect has been observed in both graphene and transition metal dichalcogenide (TMD) moiré materials. It is thought to arise from the interaction-driven valley polarization of the narrow moiré bands. Here, we show that the newly discovered QAH state in AB-stacked MoTe 2 / W S e 2 moiré bilayers is not valley polarized but valley coherent. The layer- and helicity-resolved optical spectroscopy measurement reveals that the QAH ground state possesses spontaneous spin (valley) polarization aligned (antialigned) in two TMD layers. In addition, saturation of the out-of-plane spin polarization in both layers occurs only under high magnetic fields, supporting a canted spin texture. Our results call for a new mechanism for the QAH effect and highlight the potential of TMD moiré materials with strong electronic correlations and spin-orbit interactions for exotic topological states. Published by the American Physical Society 2024

2-dimensional systems↗
DOE OSTI2021

Fragile Topology and Flat-Band Superconductivity in the Strong-Coupling Regime

In flat bands, superconductivity can lead to surprising transport effects. The superfluid “mobility”, in the form of the superfluid weight D s , does not draw from the curvature of the band but has a purely band-geometric origin. In a mean-field description, a nonzero Chern number or fragile topology sets a lower bound for D s , which, via the Berezinskii-Kosterlitz-Thouless mechanism, might explain the relatively high superconducting transition temperature measured in magic-angle twisted bilayer graphene (MATBG). For fragile topology, relevant for the bilayer system, the fate of this bound for finite temperature and beyond the mean-field approximation remained, however, unclear. Here, we numerically use exact Monte Carlo simulations to study an attractive Hubbard model in flat bands with topological properties akin to those of MATBG. We find a superconducting phase transition with a critical temperature that scales linearly with the interaction strength. Then, we investigate the robustness of the superconducting state to the addition of trivial bands that may or may not trivialize the fragile topology. Our results substantiate the validity of the topological bound beyond the mean-field regime and further stress the importance of fragile topology for flat-band superconductivity.

2-dimensional systems↗
DOE OSTI2020

GaN‐based split phase transformer‐less PV inverter with auxiliary ZVT circuit

This paper explores performance enhancement of the common ground dynamic dc‐link (CGDL) inverter for single phase photovoltaic (PV) applications by a combination of gallium nitride (GaN) devices, split phase topology, coupled inductors, and zero voltage transition (ZVT) scheme. The CGDL inverter has the inherent advantage of minimised dc‐link capacitance and negligible leakage current due to the common ground configuration, but its reported efficiency was usually lower because of the higher dc‐link voltage used for the reduction of decoupling capacitance to a great extent. To solve the efficiency problem, in this study, a soft switching circuit is proposed for the first stage, while a coupled inductor integrated magnetics is incorporated in the second stage to reduce inductor loss, volume, and cost. Both of these topological improvements combined with the use of GaN devices facilitate in achieving high efficiency without compromising converter power density. Extensive experimental results are provided from a GaN based 1 kVA hardware prototype to demonstrate the superior performance of the CGDL inverter attaining a peak efficiency of 98.7% and a California Energy Commission efficiency of 98.5% at 75/50 kHz switching frequency.

Xia, Yinglai↗
DOE OSTI2024

High-pressure characterization of Ag 3 AuTe 2 : Implications for strain-induced band tuning

Recent band structure calculations have suggested the potential for band tuning in the chiral semiconductor Ag 3 AuTe 2 to zero upon application of negative strain. In this study, we report on the synthesis of polycrystalline Ag 3 AuTe 2 and investigate its transport and optical properties and mechanical compressibility. Transport measurements reveal the semiconducting behavior of Ag 3 AuTe 2 with high resistivity and an activation energy E a of 0.2 eV. The optical bandgap determined by diffuse reflectance measurements is about three times wider than the experimental E a ⁠. Despite the difference, both experimental gaps fall within the range of predicted bandgaps by our first-principles density functional theory (DFT) calculations employing the Perdew–Burke–Ernzerhof and modified Becke–Johnson methods. Furthermore, our DFT simulations predict a progressive narrowing of the bandgap under compressive strain, with a full closure expected at a strain of –4% relative to the lattice parameter. To evaluate the feasibility of gap tunability at such substantial strain, the high-pressure behavior of Ag 3 AuTe 2 was investigated by in situ high-pressure x-ray diffraction up to 47 GPa. Mechanical compression beyond 4% resulted in a pressure-induced structural transformation, indicating the possibility of substantial gap modulation under extreme compression conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Evolution of the structural transition in Mo 1–x W x Te 2

The composition dependence of the structural transition between the monoclinic 1T' and orthorhombic Td phases in the Mo 1–x W x Te 2 Weyl semimetal was investigated by elastic neutron scattering on single crystals up to x ≈ 0.54. First observed in MoTe 2 , the transition from T d to 1T' is accompanied by an intermediate pseudo-orthorhombic phase, T d *. Upon doping with W, the T d * phase vanishes by x ≈ 0.34. Above this concentration, a phase coexistence behavior with both T d and 1T' is observed instead. The interlayer in-plane positioning parameter δ, which relates to the 1T' β angle, decreases with temperature as well as with W substitution, likely due to strong anharmonicity in the interlayer interactions. As a result, the temperature width of the phase coexistence remains almost constant up to x ≈ 0.54, in contrast to the broadening reported under pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Microscopic Imaging Homogeneous and Single Phase Superfluid Density in $\mathrm{UTe_2}$

Odd-parity superconductor $\mathrm{UTe_2}$ shows spontaneous time-reversal symmetry breaking and multiple superconducting phases, which imply chiral superconductivity, but only in a subset of samples. Here, we microscopically observe a homogeneous superfluid density $n_s$ on the surface of $\mathrm{UTe_2}$ and an enhanced superconducting transition temperature near the edges. We also detect vortex-antivortex pairs even at zero magnetic field, indicating the existence of a hidden internal field. The temperature dependence of $n_s$, determined independent of sample geometry, does not support point nodes along the $b$ axis for a quasi-2D Fermi surface and provides no evidence for multiple phase transitions in $\mathrm{UTe_2}$.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Composite Fermi Liquid at Zero Magnetic Field in Twisted MoTe 2

Here, the pursuit of exotic phases of matter outside of the extreme conditions of a quantizing magnetic field is a long-standing quest of solid state physics. Recent experiments have observed spontaneous valley polarization and fractional Chern insulators in zero magnetic field in twisted bilayers of MoTe 2 , at partial filling of the topological valence band (ν =-2/3 and -3/5). We study the topological valence band at half filling, using exact diagonalization and density matrix renormalization group calculations. We discover a composite Fermi liquid (CFL) phase even at zero magnetic field that covers a large portion of the phase diagram near twist angle ~3.6°. The CFL is a non-Fermi liquid phase with metallic behavior despite the absence of Landau quasiparticles. We discuss experimental implications including the competition between the CFL and a Fermi liquid, which can be tuned with a displacement field. The topological valence band has excellent quantum geometry over a wide range of twist angles and a small bandwidth that is, remarkably, reduced by interactions. These key properties stabilize the exotic zero field quantum Hall phases. Finally, we present an optical signature involving “extinguished” optical responses that detects Chern bands with ideal quantum geometry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗