We investigate the properties of a frustrated spin-5/2 chain with next-nearest-neighbor two- and three-site interactions, with two questions in mind: the nature of the transition into the dimerized phase induced by the three-site interaction, and the possible presence of a critical floating phase at intermediate values of the next-nearest-neighbor interaction. We provide strong evidence that the continuous transition into the dimerized phase, which has been found to be generically in the Wess-Zumino-Witten $\mathrm{SU}{\left(2\right)}_{2S}$ universality class up to spin $S=2$, is $\mathrm{SU}{\left(2\right)}_5$ only at two isolated points of the phase diagram, and that it is $\mathrm{SU}{\left(2\right)}_3$ in between, in agreement with the presence of two relevant operators allowed by symmetry for $\mathrm{SU}{\left(2\right)}_5$, and with the conservation of the parity of the level index along the renormalization flow between $\mathrm{SU}{\left(2\right)}_k$ theories with different values of $k$. We also find that the dimerization induced by the next-nearest-neighbor interaction is a three step process, with first a small partially dimerized phase followed by a broad critical floating phase with incommensurate correlations before the fully dimerized phase is reached. Implications for the iron oxide ${\mathrm{Bi}}_3{\mathrm{FeMo}}_2{\mathrm{O}}_{12}$ are briefly discussed.

Bulk superconductivity in a topological semimetal is a first step towards realizing topological superconductors, which can host Majorana fermions allowing us to achieve quantum computing. Here, we report superconductivity and compensation of electrons and holes in single crystals of the nodal-line semimetal ${\mathrm{CaSb}}_2$. We characterize the superconducting state and find that Cooper pairs have moderate-weak coupling, and the superconducting transition in specific heat down to 0.22 K deviates from that of a BCS superconductor. The nonsaturating magnetoresistance and electron-hole compensation at low temperature are consistent with density functional theory (DFT) calculations showing nodal-line features. Furthermore, we observe de Haas-van Alphen (dHvA) oscillations consistent with a small Fermi surface in the semimetallic state of ${\mathrm{CaSb}}_2$. Our DFT calculations show that the two electron bands crossing the Fermi level are associated with Sb1 zig-zag chains, while the hole band is associated with Sb2 zig-zag chains. The Sb1 zig-zag chains form a distorted square net, which may relate the $M{\mathrm{Sb}}_2$ family to the well-known $M\mathrm{SbTe}$ square net semimetals. Realization of superconductivity and a compensated semimetal state in single crystals of ${\mathrm{CaSb}}_2$ establishes the diantimonide family as a candidate class of materials for achieving topological superconductivity.

Motivated by recent experiments on ABC-stacked rhombohedral trilayer graphene (RTG), which observed spin-valley symmetry breaking and superconductivity, we study instabilities of the RTG metallic state to symmetry-breaking orders. We find that interactions select the intervalley coherent order (IVC) as the preferred ordering channel over a wide range, whose theoretically determined phase boundaries agree well with experiments on both the hole- and electron-doped sides. The Fermi surfaces near van Hove singularities admit partial nesting between valleys, which promotes both intervalley superconductivity and IVC fluctuations. We investigate the interplay between these fluctuations and the Hunds (intervalley spin) interaction using a renormalization group approach. For antiferromagnetic Hund's coupling, intervalley pairing appears in the spin-singlet channel with enhanced $T_c$ that scales with the dimensionless coupling $g$ as $T_c\sim exp(-1/\sqrt{g})$, compared to the standard $exp(-1/g)$ scaling. In its simplest form, this scenario assumes a sign change in the Hund's coupling on increasing hole doping. On the other hand, the calculation incorporates breaking of the independent spin rotations between valleys from the start, and strongly selects spin-singlet over spin-triplet pairing, and naturally occurs in proximity to the IVC, consistent with observations.

We discuss a variety of many-body approaches, within effective-mass and $\mathbf{k}·\mathbf{p}$ envelope-function formalisms, for calculating correlated single excitons in semiconductor nanocrystals (NCs) to all orders in the electron-hole Coulomb interaction. These approaches are applied to NCs of the lead-halide perovskite ${\mathrm{CsPbBr}}_3$, which typically present excitons in intermediate confinement with physical observables often strongly renormalized by correlation (e.g., radiative decay rate enhanced by a factor of about seven relative to a mean-field approach, for a NC of edge length 11 nm). The many-body methods considered include the particle-hole Bethe-Salpeter equation, configuration interaction with single excitations, and the random-phase approximation with exchange (RPAE), which are shown to be closely related to each other but to treat $\mathbf{k}·\mathbf{p}$ corrections differently, with RPAE being the most complete method. The methods are applied to calculate the correlation energy, the radiative lifetime, and the long-range Coulomb contribution to the fine structure of the ground-state exciton. In the limit of large NC sizes, the numerical results are shown to agree well with analytical results for this limit, where these are known. Correlated excited states of the single exciton are used to calculate the one-photon absorption cross-section; the shape of the resulting cross-section curve (versus laser wavelength) at threshold and up to an excitation energy of about 1 eV is in good agreement with experimental cross sections. The equations for the methods are explicitly adapted to spherical symmetry (involving radial integrals and angular factors) and in this form permit a rapid computation for systems in intermediate confinement.

We investigate the role of the coupling between a spin-orbit coupled semiconductor nanowire and a conventional $s$-wave superconductor on the emergence of the topological superconducting phase with Majorana bound states in an applied magnetic field. We show that when the coupling is strong, the topological phase transition point is very sensitive to the size of the superconductor and in order to reach the topological phase a strong magnetic field is required, which can easily be detrimental to superconductivity. Moreover, the induced energy gap separating the Majorana bound states and other quasiparticles in the topological phase is substantially suppressed compared to the gap at zero field. In contrast, in the weak-coupling regime, we find that the situation is essentially the opposite, with the topological phase emerging at much lower magnetic fields and a sizable induced energy gap in the topological phase, which can also be controlled by the chemical potential of the superconductor. Furthermore, we show that the weak-coupling regime does not generally allow for the formation of topologically trivial zero-energy states at the wire end points, in stark contrast to the strong-coupling regime, where such states are found for a wide range of parameters. Our results thus put forward the weak-coupling regime as a promising route to mitigate the most unwanted problems present in nanowires for realizing topological superconductivity and Majorana bound states.

Doping graphene near the 1/4-filling to shift the extended flat band and van Hove singularity below $E_F$ has been highly desirable. Here we report the experimental observation of an extended flat band below $E_F$ in Li-intercalated graphene. Strong electron-phonon interaction is clearly identified by notable kinks in the band dispersion. Moreover, the evolution of the band structure upon Li intercalation shows that the extended flat band and the Kekulé order emerge simultaneously. Our work provides opportunities for investigating flat-band-related instabilities and its interplay with the Kekulé order.

Among the iron-based superconductors, LiFeAs is unrivaled in the simplicity of its crystal structure and phase diagram. However, our understanding of this canonical compound suffers from conflict between mutually incompatible descriptions of the material's electronic structure, as derived from contradictory interpretations of the photoemission record. Here we explore the challenge of interpretation in such experiments. By combining comprehensive photon energy- and polarization-dependent angle-resolved photoemission spectroscopy (ARPES) measurements with numerical simulations, we establish the providence of several contradictions in the present understanding of this and related materials. We identify a confluence of surface-related issues which have precluded unambiguous identification of both the number and the dimensionality of the Fermi-surface sheets. Ultimately, we arrive at a scenario which supports indications of topologically nontrivial states while also being incompatible with superconductivity as a spin-fluctuation-driven Fermi-surface instability.

The metallic compound FeP belongs to the class of materials that feature a complex noncollinear spin order driven by magnetic frustration. While its double-helix magnetic structure with a period ${\lambda }_{\text{s}}\approx 5c$, where $c$ is the lattice constant, was previously well determined, the relevant spin-spin interactions that lead to that ground state remain unknown. By performing extensive inelastic neutron scattering measurements, we obtained the spin-excitation spectra in a large part of the momentum-energy space. The spectra show that the magnons are gapped with a gap energy of $\sim 5\mathrm{meV}$. Despite the 3D crystal structure, the magnon modes display strongly anisotropic dispersions, revealing a quasi-one-dimensional character of the magnetic interactions in FeP. The physics of the material, however, is not determined by the dominating exchange, which is ferromagnetic. Instead, the weaker two-dimensional antiferromagnetic interactions between the rigid ferromagnetic spin chains drive the magnetic frustration. Using linear spin-wave theory, we were able to construct an effective Heisenberg Hamiltonian with an anisotropy term capable of reproducing the observed spectra. This enabled us to quantify the exchange interactions in FeP and determine the mechanism of its magnetic frustration.

We construct a class of edge theories for a family of fermionic Abelian topological phases with $K$ matrices of the form $K=\left(\begin{array}{cc}{k}_1& 0\\ 0& -k_2\end{array}\right)$, where $k_1,k_2>0$ are odd integers. Our edge theories are notable for two reasons: (i) they have finite-dimensional Hilbert spaces (for finite-sized systems) and (ii), depending on the values of $k_1,k_2$, some of the edge theories describe boundaries that cannot be gapped by any local interaction. The simplest example of such an ungappable boundary occurs for $(k_1,k_2)=(1,3)$, which is realized by the $\nu =2/3$ fractional quantum Hall state. We derive our edge theories by starting with the standard chiral boson edge theory, consisting of two counterpropagating chiral boson modes, and then introducing an array of pointlike impurity scatterers. We solve this impurity model exactly in the limit of infinitely strong impurity scattering, and we show that the energy spectrum consists of a gapped phonon spectrum together with a ground-state degeneracy that scales exponentially with the number of impurities. This ground-state subspace forms the Hilbert space for our edge theory. We believe that similar edge theories can be constructed for any Abelian topological phase with vanishing thermal Hall coefficient, ${\kappa }_H=0$.

It is widely known that the twisted bilayer graphene (TBG) shows flat bands at magic angles, which can be well described by the effective continuum model derived by Bistritzer and MacDonald (BM). We propose in this paper a similar twisted bilayer system but defined on the square lattice with $\pi$ flux per plaquette, and study its spectrum using the BM Hamiltonian with a mass term which is originated from the staggered potential. The basic difference between the TBG and the present model is simply rotational symmetry, $C_3$ versus $C_4$, as well as a mass term. Nevertheless, the feature of the flat bands is quite different: those of the TBG appear at magic angles only, while the present model shows many flat bands, which are reminiscent of Landau levels, quite stably at any angles even in the absence of a magnetic field other than $\pi$ flux, which keeps time reversal (TR) symmetry. Moreover, flat bands emerge in the mass gap of the Dirac spectrum, and each state composing these flat bands is well-localized at the position forming the moiré lattice. It turns out that the moiré potential serves as a periodic magnetic field, which can give energies smaller that the gap around moiré lattice positions. We derive a local Hamiltonian valid around the moiré lattice sites and show that it indeed reproduces the energies of the flat bands within the mass gap. Since these mid-gap states are localized at the moiré lattice, they form degenerate levels, which may be referred to as moiré Landau levels, although the mechanism of degeneracies are different from the conventional Landau levels. Interestingly, doubled fermions of the BH Hamiltonian associated with two layers have opposite charges when they couple with the effective moiré magnetic filed, which concern TR symmetry. This is a generic feature of the $C_4$-symmetric moiré system described by the BM Hamiltonian.

The topology of typical Chern insulators is rooted in the periodicity of the system along two directions of real space. In this paper, we depart from this standard concept and demonstrate that a generic non-Hermitian photonic waveguide periodic along a single direction of real space can be regarded as a subcomponent of an extended system with a synthetic dimension and with a nontrivial Chern topology. We show that the number of bands below a bandgap of a generic waveguide determines the gap Chern number of the extended system. It is theoretically and numerically demonstrated that in real space the gap Chern number gives the number of gapless Tamm state branches localized at the system boundary, when its geometry is continuously displaced by one lattice period. In the non-Hermitian case, the Tamm states connect different bands in the complex plane.

It has recently been established that quantum many-body scarring can prevent the thermalization of some isolated quantum systems starting from certain initial states. One of the first models to show this was the so-called PXP Hamiltonian, which was used to theoretically model an experiment on a chain of strongly interacting Rydberg atoms. A defining feature of the PXP Hamiltonian is a set of dynamical constraints that make certain states inaccessible to the dynamics. In this paper we construct a class of spin chain models that are parameterized by a discrete variable $\ell$ that controls the “strength” of a dynamical constraint. We show that by increasing $\ell$ the constraint becomes weaker in the sense that fewer states are excluded from the dynamics. The PXP Hamiltonian is a special case for $\ell =2$. By weakening the constraint to $\ell \ge 4$, however, we find a more extreme version of quantum scarring than in the PXP Hamiltonian with the number of scar states growing exponentially in the system size.

Nonlocal conductance spectroscopy in three-terminal topological superconductor hybrid setups has been proposed as a way to conclusively detect topological phase transitions involving Majorana zero modes. Topological entanglement entropy is yet another way to gauge nonlocality in connection with the bulk-boundary correspondence of a topological phase. We show that while both the entanglement entropy and the nonlocal conductance exhibit a clear topological phase-transition signature for long enough pristine nanowires, nonlocal conductance fails to signal a topological phase transition for shorter disordered wires. While recent experiments have indeed shown premature gap-closing signatures in the nonlocal conductance spectra, we demonstrate that the entanglement entropy can indeed signal a genuine transition, regardless of the constituent nonidealities in an experimental situation.

Optically-induced spin currents have proven to be useful in spintronics applications, allowing for sub-ps all-optical control of magnetization. However, the mechanism responsible for their generation is still heavily debated. Here we use the excitation of spin-current induced THz spin-waves in noncollinear bilayer structures to study optically-induced spin currents in the time domain. We measure a significant laser-fluence dependence of the spin-wave phase, which can quantitatively be explained assuming the spin current is proportional to the time derivative of the magnetization. Measurements of the absolute spin-wave phase, supported by theoretical calculations and micromagnetic simulations, suggest that angular momentum transfer via the s-d interaction in combination with ballistic interlayer transport is sufficient to fully explain spin-current generation and transport in our experiments. Finally, we show that the damping-like optical STT dominates THz spin-wave generation. Our findings suggest laser-induced demagnetization and spin-current generation share the same microscopic origin.

We report experimental signatures of topological transitions among the Weyl semimetal states of pyrochlore ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, where the Kondo coupling between the Ir topological electrons and the spin-ice-like orders of Pr moments plays a decisive role. The magnetic-field dependence of resistivity and the Hall conductivity exhibits a plateau and a sharp jump associated with a magnetic-field hysteresis, similar to a liquid-gas-like transition in a dipolar spin-ice system. Furthermore, the Kondo coupling is controlled by hydrostatic pressure, revealing that the field-induced displacement of Weyl points in the momentum space strongly depends on the respective electronic state as well as on the Kondo coupling strength. These observations pave a route toward the engineering of band topology in hybrid quantum materials with relativistic conduction electrons and localized magnetic moments.

Plasmonic crystals are an important backbone of modern photonics offering versatile control of light via manipulation of surface plasmon polaritons (SPPs). In particular, they sustain resonant light localization near the metal surface resulting in the enhancement of magneto-optical effects and nonlinear optical phenomena, the bridge of which is appealing for sensing and light routing. In this work we investigate the resonant SPP-driven enhancement of the transverse magneto-optical Kerr effect and the second harmonic generation under the conical diffraction of light in a one-dimensional magnetoplasmonic crystal (MPC) based on the combination of gold and ferromagnetic permalloy films. We demonstrate an extra way for the control over the magneto-optical effects in the MPC via the magnetic anisotropy of corrugated ferromagnetic film in the structure of the MPC.

Recent advances in vortex imaging allow for tracing the position of individual vortices with high resolution. Pushing an isolated vortex through a superconducting sample with the help of a controlled $dc$ transport current and measuring its local $ac$ response, the pinning energy landscape could be reconstructed along the vortex trajectory [L. Embon et al., Sci. Rep. 5, 7598 (2015)], providing unprecedented access to the mesoscopic, i.e., sample dependent, properties of a superconducting device. The controlled motion of objects through such tilted energy landscapes is fundamentally limited to those areas of the landscape developing local minima under appropriate tilt. We introduce the Hessian stability map and the Hessian character of a pinning landscape as quantities that help characterizing a pinning landscape, or short, pinscape. We determine the Hessian character, the area fraction admitting stable vortex positions, for various types of pinning potentials: Assemblies of cut parabolas, Lorentzian- and Gaussian-shaped traps, as well as a Gaussian random disordered energy landscape, with the latter providing a universal result of $(3-\sqrt{3})/6\approx 21\%$ of stable area. Our Hessian analysis sheds light on various aspects of mesoscopic vortex physics in the single vortex limit and opens interesting avenues in strong pinning theory.

The topological Hall effect (THE) originating from a real-space Berry phase is a significant transport signal for chiral spin textures and has been extensively investigated recently due to its potential applications in topological spintronics. Recently, chiral spin textures and THE were realized in heterointerfaces where spatial inversion symmetry is naturally broken. However, multichannel transport in heterointerfaces can mask the intrinsic THE associated with chiral spin textures. Here, we systematically investigate multichannel transport in two different kinds of van der Waals (vdW) heterointerfaces. In ferromagnetic-ferromagnetic (FM-FM) heterointerfaces with two opposite anomalous Hall effects, multichannel transport has mimicked both positive and negative THE without involving any topological charges, which is dubbed as an artificial THE. Moreover, artificial THEs were also observed in FM-metal heterointerfaces consisting of a single FM layer stacked onto a nodal-line semimetal; this was attributed to the presence of multichannel transport as well. Our findings provide an alternative explanation for THE-like features in heterointerfaces. They also reveal exotic multichannel transport properties in vdW heterointerfaces which could facilitate the development of multifunctional nanodevices based on vdW heterointerfaces.

The temperature profile around nano- and microscale heat sources on silicon substrate is technologically important for many integrated circuit applications. Here we present full-field thermal imaging of heater lines and rings with different sizes at temperatures ranging from 100 to 300 K. We show significant deviations compared to Fourier's law which are both size and geometry dependent. This can be explained by a hydrodynamic model for heat transport in silicon based on the Guyer and Krumhansl equation using ab initio calculated parameters within the kinetic collective model framework. One of these parameters, the nonlocal length, is shown to quantitatively determine the situations where nonFourier behavior occurs. This length scale is some orders of magnitude smaller than the longest phonon mean free paths. Ballistic phonons are shown not to manifest directly in these experiments, thus indicating the failure of the multiscale relaxation time approximation. Furthermore, we discuss the differences between Hamiltonian/microscopic and entropic/mesoscopic strategies to address nanoscale heat transport, and the relations between phonon dynamics and thermodynamics. Finally, the nonequilibrium phonon distribution function is used to determine conditions under which hydrodynamic modeling can be used.

We have measured the Seebeck and anomalous Nernst coefficients and corresponding transverse and longitudinal thermoelectric conductivities from 2 to 400 K in thin film (thickness $t\sim 10$ nm) ${\mathrm{Co}}_2{\mathrm{MnAl}}_x{\mathrm{Si}}_{1-x}$ $(0\le x\le 1)$ and ${\mathrm{Co}}_2\mathrm{FeAl}$ grown by molecular beam epitaxy (MBE). A large ($-14\mathrm{A}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K) anomalous component of the transverse thermoelectric conductivity is observed in ${\mathrm{Co}}_2\mathrm{MnAl}$, especially as contrasted to ${\mathrm{Co}}_2\mathrm{MnSi}$ (0.28 $\mathrm{A}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K). This enhancement is likely due to Weyl points close to the Fermi level of ${\mathrm{Co}}_2\mathrm{MnAl}$ which disappear as $x$ decreases.