Discovery of a Cooper-Pair Density Wave State in a Transition-Metal Dichalcogenide

To search for evidence that Cooper-pair density wave (PDW) states can occur in transition metal dichalcogenides (TMD) we use atomic-resolution scanned Josephson-tunneling microscopy (SJTM). Implementing an innovative SJTM technique, we detect and visualize a PDW state in the canonical TMD NbSe$_2$. Although its wavevectors are indistinguishable from those of the preexisting charge density wave (CDW) state, simultaneous atomic-scale imaging of the CDW and PDW demonstrates that their spatial arrangements are incongruent. By contrast, the PDW and the superconductive state are unmistakably coupled, as evidenced by their mutual decay into a superconducting vortex core. Despite the atomic-scale dissimilarity of simultaneous CDW and PDW images, large-scale visualization of their relative phase $\delta\Phi(r)$ yields a characteristic value $|\delta\Phi| \sim 2\pi/3$. This reveals an inter-state discommensuration between the CDW and PDW by one crystal unit cell, as the atomic-scale disjunction mechanism. Finally, because many TMDs sustain both CDW and superconducting states, the detection and imaging of a PDW in NbSe$_2$ presages abundant new PDW physics

Extending the first-order post-Newtonian scheme in multiple systems to the second-order contributions to light propagation

In this paper, we extend the first-order post-Newtonian scheme in multiple systems presented by Damour-Soffel-Xu to the second-order contribution to light propagation without changing the virtueof the scheme on the linear partial differential equations of the potential and vector potential. The spatial components of the metric are extended to second order level both in a global coordinates ($q_{ij}/ c^4$) and a local coordinates ($Q_{ab}/ c^4$). The equations of $q_{ij}$ (or $Q_{ab}$) are obtained from the field equations.The relationship between $q_{ij}$ and $Q_{ab}$ are presented in this paper also. In special case of the solar system (isotropic condition is applied ($q_{ij} = \delta_{ij} q$)), we obtain the solution of $q$. Finally, a further extension of the second-order contributions in the parametrized post-Newtonian formalism is discussed.Comment: Latex2e; 6 pages PS fil

Imaging Orbital-selective Quasiparticles in the Hund's Metal State of FeSe

Strong electronic correlations, emerging from the parent Mott insulator phase, are key to copper-based high temperature superconductivity (HTS). By contrast, the parent phase of iron-based HTS is never a correlated insulator. But this distinction may be deceptive because Fe has five active d-orbitals while Cu has only one. In theory, such orbital multiplicity can generate a Hund's Metal state, in which alignment of the Fe spins suppresses inter-orbital fluctuations producing orbitally selective strong correlations. The spectral weights $Z_m$ of quasiparticles associated with different Fe orbitals m should then be radically different. Here we use quasiparticle scattering interference resolved by orbital content to explore these predictions in FeSe. Signatures of strong, orbitally selective differences of quasiparticle $Z_m$ appear on all detectable bands over a wide energy range. Further, the quasiparticle interference amplitudes reveal that $Z_{xy}<Z_{xz}<<Z_{yz}$, consistent with earlier orbital-selective Cooper pairing studies. Thus, orbital-selective strong correlations dominate the parent state of iron-based HTS in FeSe.Comment: for movie M1, see http://www.physik.uni-leipzig.de/~kreisel/osqp/M1.mp4, for movie M2, see http://www.physik.uni-leipzig.de/~kreisel/osqp/M2.mp4, for movie M3, see http://www.physik.uni-leipzig.de/~kreisel/osqp/M3.mp4, for movie M4, see http://www.physik.uni-leipzig.de/~kreisel/osqp/M4.mp4, for movie M5, see http://www.physik.uni-leipzig.de/~kreisel/osqp/M5.mp

Quantized octupole acoustic topological insulator

The Berry phase associated with energy bands in crystals can lead to quantized quantities, such as the quantization of electric dipole polarization in an insulator, known as a one-dimensional (1D) topological insulator (TI) phase. Recent theories have generalized such quantization from dipole to higher multipole moments, giving rise to the discovery of multipole TIs, which exhibit a cascade hierarchy of multipole topology at boundaries of boundaries: A quantized octupole moment in the three-dimensional (3D) bulk can induce quantized quadrupole moments on its two-dimensional (2D) surfaces, which then produce quantized dipole moments along 1D hinges. The model of 2D quadrupole TI has been realized in various classical structures, exhibiting zero-dimensional (0D) in-gap corner states. Here we report on the realization of a quantized octupole TI on the platform of a 3D acoustic metamaterial. By direct acoustic measurement, we observe 0D corner states, 1D hinge states, 2D surface states, and 3D bulk states, as a consequence of the topological hierarchy from octupole moment to quadrupole and dipole moment. The critical conditions of forming a nontrivial octupole moment are further demonstrated by comparing with another two samples possessing a trivial octupole moment. Our work thus establishes the multipole topology and its full cascade hierarchy in 3D geometries

Severe dirac mass gap suppression in Sb 2 Te 3-based quantum anomalous Hall materials

The quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTIs) when a Dirac mass gap opens in the spectrum of the topological surface states (SSs). Unaccountably, although the mean mass gap can exceed 28 meV (or ∼320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using atomic-resolution Landau level spectroscopic imaging, we compare the electronic structure of the archetypal FMTI Cr0.08(Bi0.1Sb0.9)1.92Te3 to that of its nonmagnetic parent (Bi0.1Sb0.9)2Te3, to explore the cause. In (Bi0.1Sb0.9)2Te3, we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr0.08(Bi0.1Sb0.9)1.92Te3 with concurrent but uncorrelated Dirac mass gap disorder. These two classes of SS electronic disorder conspire to drastically suppress the minimum mass gap to below 100 μeV for nanoscale regions separated by <1 μm. This fundamentally limits the fully quantized anomalous Hall effect in Sb2Te3-based FMTI materials to very low temperatures