Temperature dependence of the band structure, as well as the spin-polarization on the topologically trivial insulator ${\mathrm{TlBiS}}_2$ and nontrivial insulator $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_2$, have been investigated by spin- and angle-resolved photoelectron spectroscopy. Despite the recent theoretical prediction [Phys. Rev. Lett. 117, 246401 (2016).], topological phase transition (trivial to nontrivial for ${\mathrm{TlBiS}}_2$, nontrivial to trivial for $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_2$) does not occur in the observed temperature ranges (from 50 to 400 K for ${\mathrm{TlBiS}}_2$, from 50 to 475 K for $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_2$). Our results indicate that the discrepancy between the theory and the experiment can be understood by considering the overestimation of lattice parameters and the effect of lattice expansion that is neglected in the previous theory. Although some spin-polarized states are observed in ${\mathrm{TlBiS}}_2$, it is probably due to the Rashba-like surface (resonant) states and not the topological surface states. On the other hand, the topological surface states in $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_2$ is very robust even at 400 K and the spin-polarized states can be used for the real spintronic devices in the large temperature range.

We have observed intensity enhancement in exciton photoluminescence and $G$-band Raman scattering of suspended pristine semiconducting carbon nanotubes (CNTs) under resonant excitation while preventing surrounding dielectric Coulomb screening and emission quenching effects. Photoluminescence excitation (PLE) microspectroscopy for the isolated CNTs shows that the PL and Raman scattering are simultaneously enhanced when the excitation photon energy is equal to the sum of the energies of the lowest-energy bright exciton and a longitudinal optical (LO) phonon. This simultaneous enhancement is attributed to a large density of states of the lowest-energy bright exciton strongly interacting with the LO phonon at the Γ point. The resonant excitation is a key to understanding the excitation and emission processes of CNTs and to designing optimal photonic nanostructures applicable to efficient CNT-based light-emitting devices.

Optical harmonic generation on excitons is found in ZnSe/BeTe quantum wells with type-II band alignment. Two experimental approaches with spectrally broad femtosecond laser pulses and spectrally narrow picosecond pulses are used for spectroscopic studies by means of second and third harmonic generation (SHG and THG). The SHG signal is symmetry-forbidden in the electric-dipole approximation for light propagation along the structure's growth axis, which is the [001] crystal axis, but it can be induced by an external magnetic field. The THG signal is detected at zero magnetic field and its intensity is field-independent. A group theory analysis of SHG and THG rotational anisotropy diagrams allows us to identify the involved excitation mechanisms.

Near-field radiative heat transfer between two bodies can exceed the far-field blackbody limitation predicted by Planck's law due to the evanescent waves tunneling or coupling of additional surface modes, which typically can occur only in TM modes for nonmagnetic materials. The Mie resonance may have the potential to enhance near-field radiation in both TM and TE modes according to the calculated results from effective medium theory. However, there is no exact solution to verify this to date. In this paper, we will give a rigorous numerical investigation of the role of Mie resonance in near-field radiative heat transfer. The framework of fluctuational electrodynamics that combines scattering matrix theory with the rigorous coupled wave analysis method is used to exactly compute the near-field radiative flux between Mie resonant dielectric cubic arrays. It shows that due to the excitation of Mie resonance, causing $\varepsilon /\mu$ near-pole effects, the radiative heat flux could be spectrally enhanced in TM/TE modes. The discrepancy between the effective medium theory and the exact method is also elucidated in detail. The findings will provide a way to control near-field thermal spectrum via all-dielectric metamaterials for emerging thermal technologies.

We present a methodical study of grazing-incidence small-angle x-ray scattering performed in situ during pulsed-laser deposition of Pt on sapphire substrates. From measured two-dimensional intensity distributions in reciprocal space we calculated horizontal and vertical intensity projections and compare them to numerical simulations. The structure of the Pt layers was described using a simple Monte-Carlo model and the Ornstein-Zernicke theory with the Percus-Yevick approximation, and the scattering process was simulated using distorted-wave Born approximation. The validity of the structure models as well as the effect of the indirect scattering processes are discussed. From the comparison of the measured and simulated data we determined the lateral and vertical sizes of surface islands, the surface coverage, the island coalescence, as well as the thickness of the wetting layer at the substrate surface. We studied the time evolution of these parameters and their dependence on the substrate temperature.

We provide experimental and theoretical evidence for the realization of the Peierls-type structurally distorted state in the quasi-one-dimensional superconductor ${\mathrm{Sc}}_3\mathrm{Co}{\mathrm{C}}_4$ by a phonon-softening mechanism. The transition from the high- to the final low-temperature phase below 80 K proceeds via an extended intermediate temperature regime between 80 and 150 K characterized by phonon-driven atom displacements. In support of the low-dimensional character of the title compound we find a highly anisotropic correlation length of these dynamic distortions.

The manganese tellurate ${\mathrm{Li}}_2{\mathrm{MnTeO}}_6$ consists of trigonal spin lattices made up of ${\mathrm{Mn}}^{4+}$ ($d^3$, $S=3/2$) ions. The magnetic properties of this compound were characterized by several experimental techniques, which include magnetic susceptibility, specific-heat, dielectric permittivity, electron-spin-resonance, nuclear magnetic resonance (NMR), and neutron powder-diffraction measurements, and by density functional theory calculations. The magnetic susceptibility $\chi \left(T\right)$ demonstrates very unusual behavior. It is described by the Curie-Weiss law at high temperature with Curie-Weiss temperature of $\mathrm{\Theta }=-74\mathrm{K}$ and exhibits no obvious anomaly indicative of a long-range magnetic ordering at low magnetic fields. At high magnetic fields, however, the character of $\chi \left(T\right)$ changes showing a maximum at about 9 K. That this maximum of $\chi \left(T\right)$ reflects the onset of an antiferromagnetic order was confirmed by specific-heat measurements, which exhibit a clear $\lambda$-type anomaly at $T_{\mathrm{N}}\approx 8.5\mathrm{K}$ even at zero magnetic field, and by ${}^7\mathrm{Li}$ NMR and dielectric permittivity measurements. The magnetic structure of ${\mathrm{Li}}_2{\mathrm{MnTeO}}_6$, determined by neutron powder-diffraction measurements at 1.6 K, is described by the ${120}^{\circ }$ noncollinear spin structure with the propagation vector $\mathit{k}=\left(1/3,1/3,0\right)$. Consistent with this finding, the spin-exchange interactions evaluated for ${\mathrm{Li}}_2{\mathrm{MnTeO}}_6$ by density functional calculations are dominated by the nearest-neighbor antiferromagnetic exchange within each triangular spin lattice. This spin lattice is strongly spin frustrated with $f=\left|\mathrm{\Theta }\right|/T_{\mathrm{N}}\approx 8$ and exhibits a two-dimensional magnetic character in a broad temperature range above $T_{\mathrm{N}}$.

Artificial ferromagnetic (FM)/nonmagnetic multilayers, with large enough FM thickness to prevent the dominance of interface anisotropies, offer a straightforward insight into the understanding and control of perpendicular standing spin wave (PSSW) modes. Here we present a study of the static and dynamic magnetic properties of ${\left[\mathrm{Co}\left(3.0\mathrm{nm}\right)/\mathrm{Au}\left(0.6\mathrm{nm}\right)\right]}_{1\le N\le 30}$ multilayers. Magnetometry reveals that the samples exhibit magnetization reversal properties typical of an effective single layer with weak perpendicular anisotropy, with the distinctive thickness-dependent magnetization reorientation transition from uniform in-plane to out-of-plane stripe domains at remanence. However, when such multilayer systems are out-of-plane saturated, the dynamic response reveals the existence of several different ferromagnetic resonances in the form of PSSW modes that strongly depend on the material modulation characteristics along the total thickness. These modes are induced by the layer stacking itself as the effective single layer model fails to describe the observed complex dynamics. In contrast to most systems considered in the past, described by a dynamic model of a single effectively homogeneous thick layer, the specific structures investigated here provide a unique platform for a large degree of tunability of the mode frequencies and amplitude profiles. We argue that the combination of periodic magnetic properties with vertical deformation gradients, arising from heteroepitaxial strain relaxation, converts the Au interlayer regions into a vertical regular array of magnetic pinning planes for the PSSW modes, which promotes the complex dynamics observed in this system.

We theoretically study superconducting islands based on semiconductor-nanowire Josephson junctions and take into account the presence of subgap quasiparticle excitations in the spectrum of the junction. Our method extends the standard model Hamiltonian for a superconducting charge qubit and replaces the Josephson potential by the Bogoliubov–de Gennes Hamiltonian of the nanowire junction, projected onto the relevant low-energy subgap subspace. This allows us to fully incorporate the coherent dynamics of subgap levels in the junction. The combined effect of spin-orbit coupling and Zeeman energy in the nanowires forming the junction triggers a topological transition, where the subgap levels evolve from finite-energy Andreev bound states into near-zero-energy Majorana bound states. The interplay between the microscopic energy scales governing the nanowire junction (the Josephson energy, the Majorana coupling, and the Majorana energy splitting), with the charging energy of the superconducting island, gives rise to a great variety of physical regimes. Based on this interplay of different energy scales, we fully characterize the microwave response of the junction, from the Cooper pair box to the transmon regimes, and show how the presence of Majoranas can be detected through distinct spectroscopic features. In split-junction geometries, the plasma mode couples to the phase-dispersing subgap levels resulting from Majorana hybridization via a Jaynes–Cummings-like interaction. As a consequence of this interaction, higher order plasma excitations in the junction inherit Majorana properties, including the $4\pi$ effect.

Photonic topological insulators (PTIs) have raised significant interest in recent years, in part because of their protected edge propagation. Most PTI realizations have been based on reciprocal platforms, with protection only from symmetry-preserving defects. Nonreciprocal PTIs protect against arbitrary disorder, but their implementation requires breaking reciprocity either through a magnetic bias or complex spatiotemporal modulation schemes. Here, we demonstrate that nonreciprocal topological protection can be achieved by optically pumping a nonlinear photonic crystal with a uniform bias field, drastically simplifying the implementation of quantum-Hall-like topological protection in silicon-compatible platforms for classical and quantum optics.

Ferropericlase (Fp), $\left({\mathrm{Mg}}_{1-x}{\mathrm{Fe}}_x\right)\mathrm{O}$, is the second most abundant phase in the Earth's lower mantle. At relevant pressure-temperature conditions, iron in Fp undergoes a high spin (HS), S = 2, to low spin (LS), S = 0, state change. The nature of this phenomenon is quite well understood now, but there are still basic questions regarding the structural stability and the existence of soft phonon modes during this iron state change. General theories exist to explain the volume reduction, the significant thermoelastic anomalies, and the broad nature of this HS-LS crossover. These theories make extensive use of the quasiharmonic approximation. Therefore, dynamical and structural stability is essential to their validity. Here, we investigate the vibrational spectrum of Fp throughout this spin crossover using ab initio density-functional theory $+U_{\mathrm{sc}}$ calculations. We address vibrational modes associated with isolated and (second-)nearest-neighbor iron ions undergoing the HS-LS state change. As expected, acoustic modes of this solid solution are resilient, while optical modes are the most affected. We show that there are no soft phonon modes across this HS-LS crossover, and Fp is dynamically stable at all relevant pressures.

We report large-scale molecular dynamics simulations of the glass formation from the liquid phase and homogeneous nucleation phenomena of pure zirconium. For this purpose, we have built a modified embedded atom model potential in order to reproduce relevant structural, dynamic, and thermodynamic properties from ab initio and experimental data near the melting point. By means of liquid-solid interface simulations, we show that this potential provides a thermodynamic melting temperature and densities of the solid and liquid state in good agreement with experiments. Using melt-quenching simulations with one million atoms, we determine the glass transition from the temperature evolution of the inherent structure energy as well as the nose of the time-temperature-transformation curve located in the deep undercooling regime. We identify the local structural origin of the glass-forming ability as a competition between bcc and fivefold polytetrahedral structures that may represent an impediment of rapid homogeneous nucleation at such high undercoolings. This suggests the ability of single elemental zirconium to form a glass from the melt with cooling rates of at least ${10}^{12}\mathrm{K}/\mathrm{s}$, compatible with modern experiments.

CrAs is a well-known helimagnet with the double-helix structure originating from the competition between the Dzyaloshinskii-Moriya interaction (DMI) and antiferromagnetic exchange interaction $J$. By resonant soft-x-ray scattering, we observe the magnetic peak (0 0 $q_m$) that emerges at the helical transition with $T_S\approx 267.5\mathrm{K}$. Intriguingly, the helimagnetic domains significantly shrink on cooling below $\sim 255\mathrm{K}$, opposite to the conventional thermal effect. The weakening of DMI on cooling is found to play a critical role here. It causes the helical wave vector to vary, ordered spins to rotate, and extra helimagnetic domain boundaries to form at local defects, thus leading to the anomalous shrinkage of helimagnetic domains. Our results indicate that the size of magnetic helical domains can be controlled by tuning DMI in certain helimagnets.

The mechanism of superconductivity in a purely interacting electron system has been one of the most challenging issues in condensed matter physics. In the BCS theory, the Landau's Fermi liquid is the ground state of weakly interacting fermions dictated by the fermion sign structure, and the superconducting instability only occurs when an additional pairing force is added. By contrast, as shown by density matrix renormalization group calculation, a quasi-one-dimensional superconducting state (specifically a Luther-Emery state) is an intrinsic ground state of the $t-J$ two-leg ladder and four-leg cylinder at finite doping. Such a Luther-Emery state with pairing can be directly turned into a Fermi-gas-like normal state without pairing by merely switching off the phase-string signs hidden in the model via two schemes, which reduce the sign structure to the trivial fermion signs associated with doped holes. It thus demonstrates a new pairing paradigm in a model-doped Mott insulator as due to the generic phase-string sign structure. Finally, as an example, the latter is explicitly shown to lead to a “stringlike” pairing force after adiabatically connecting to a strong anisotropic limit of the model.

A general and beautiful picture for the realization of topological insulators is that the mass term of the Dirac model has a nodal surface wrapping one Dirac point. We show that this geometric picture based on Dirac points can be generalized to extended band degeneracies with nontrivial topological charges. As the nontrivial topological charges force the extended band degeneracies to be created or annihilated in pairs, when the nodal surface of mass wraps one such extended band degeneracy, the resulting gapped phase must be topologically nontrivial since it cannot adiabatically be deformed into a topologically trivial atomic insulator without closing the energy gap. We use nodal lines which carry a nontrivial $Z_2$ monopole charge in three dimensions to illustrate the physics. Notably, because the wrapping surfaces for an extended band degeneracy are diverse, we find that this generalization unearths topological insulators with unconventional patterns of boundary states.

The isotope effects in x-ray absorption spectra of liquid water are studied by a many-body approach within electron-hole excitation theory. The molecular structures of both light and heavy water are modeled by path-integral molecular dynamics based on the advanced deep-learning technique. The neural network is trained on ab initio data obtained with SCAN density functional theory. The experimentally observed isotope effect in x-ray absorption spectra is reproduced semiquantitatively in theory. Compared to the spectrum in normal water, the blueshifted and less pronounced pre- and main-edge in heavy water reflect that the heavy water is more structured at short- and intermediate-range of the hydrogen-bond network. In contrast, the isotope effect on the spectrum is negligible at post-edge, which is consistent with the identical long-range ordering in both liquids as observed in the diffraction experiment.

Strongly correlated quantum systems often display universal behavior as, in certain regimes, their properties are found to be independent of the microscopic details of the underlying system. An example of such a situation is the Tomonaga-Luttinger liquid description of one-dimensional strongly correlated bosonic or fermionic systems. Here, we investigate how such a quantum liquid responds under dissipative dephasing dynamics and, in particular, we identify how the universal Tomonaga-Luttinger liquid properties melt away. Our study, based on adiabatic elimination, shows that dephasing first translates into the damping of the oscillations present in the density-density correlations, a behavior accompanied by a change in the Tomonaga-Luttinger liquid exponent. This first regime is followed by a second one characterized by the diffusive propagation of featureless correlations as expected for an infinite temperature state. We support these analytical predictions by numerically exact simulations carried out using a number-conserving implementation of the matrix product states algorithm adapted to open systems.

Using the density functional theory of electronic structure, we compute the anisotropic dielectric response of bulk black phosphorus subject to strain. Employing the obtained permittivity tensor, we solve Maxwell's equations and study the electromagnetic response of a layered structure comprising a film of black phosphorus stacked on a metallic substrate. Our results reveal that a small compressive or tensile strain, $\approx 4\%$, exerted either perpendicular or in the plane to the black phosphorus growth direction, efficiently controls the epsilon-near-zero response and allows perfect absorption tuning from low angle of the incident beam $\theta =0^{\circ }$ to high values $\theta \approx {90}^{\circ }$ while switching the energy flow direction. Incorporating the spatially inhomogeneous strain model, we also find that for certain thicknesses of the black phosphorus, near-perfect absorption can be achieved through controlled variations of the in-plane strain. These findings can serve as guidelines for designing largely tunable perfect electromagnetic wave absorber devices.

The relaxation time of a classical spin interacting with a large conduction-electron system is computed for a weak magnetic field, which initially drives the spin out of equilibrium. We trace the spin and the conduction-electron dynamics on a timescale which exceeds the characteristic electronic scale that is set by the inverse nearest-neighbor hopping by more than five orders of magnitude. This is achieved with a construction of absorbing boundary conditions, which employs a generalized Lindblad master-equation approach to couple the edge sites of the conduction-electron tight-binding model to an external bath. The failure of the standard Lindblad approach to absorbing boundaries is traced back to artificial excitations initially generated due to the coupling to the bath. This can be cured by introducing Lindblad parameter matrices and by fixing those matrices to perfectly suppress initial-state artifacts as well as reflections of physical excitations propagating to the system boundaries. Numerical results are presented and discussed for generic one-dimensional models of the electronic structure.