The generation of wave packets with well-defined momentum and energy distribution constitutes an experimental challenge in diverse areas of physics. The excitation of nontrivial states, on different lattice configurations, becomes mandatory nowadays to unveil new and exotic properties. In this work, we propose and demonstrate experimentally a straightforward technique to engineer on-chip wave packets by exploiting the radiation of an atomlike waveguide into a one-dimensional lattice. We show that the radiation modes generated after exciting this pseudoatom can be tuned at desired band energies by simply controlling the on-site properties, such that wave packets with precise quasimomentum can be generated. As a benchmark, we use the wave-packet generator for a precise excitation of the well-known topological edge states of a Su-Schrieffer-Heeger photonic lattice.

${\mathrm{MgCr}}_2{\mathrm{O}}_4$ is one of the best-known realizations of the pyrochlore-lattice Heisenberg antiferromagnet. The strong antiferromagnetic exchange interactions are perturbed by small further-neighbor exchanges such that this compound may in principle realize a spiral spin liquid (SSL) phase in the zero-temperature limit. However, a spin Jahn-Teller transition below $T_{\mathrm{N}}\approx 13$ K yields a complicated long-range magnetic order with multiple coexisting propagation vectors. We present neutron scattering and thermomagnetic measurements of ${\mathrm{MgCr}}_2{\mathrm{O}}_4$ samples under applied hydrostatic pressure up to $P=1.7$ GPa demonstrating the existence of multiple close-lying nearly degenerate magnetic ground states. We show that the application of hydrostatic pressure increases the ordering temperature by around 0.8 K per GPa and increases the bandwidth of the magnetic excitations by around 0.5 meV per GPa. We also evidence a strong tendency for the preferential occupation of a subset of magnetic domains under pressure. In particular, we show that the $\mathit{k}=(0,0,1)$ magnetic phase, which is almost negligible at ambient pressure, dramatically increases in spectral weight under pressure. This modifies the spectrum of magnetic excitations, which we interpret unambiguously as spin waves from multiple magnetic domains. Moreover, we report that the application of pressure reveals a feature in the magnetic susceptibility above the magnetostructural transition. We interpret this as the onset of a short-range ordered phase associated with $\mathit{k}=(0,0,1)$, previously not observed in magnetometry measurements.

By solving the renormalization of the $s\text{−}d$ interaction from magnetic impurities embedded in conventional superconductors at low concentration, we derive the macroscopic superconducting phase fluctuation and electromagnetic linear-response properties within the path-integral approach. It is found that there exist two superconducting phase modes, both exhibiting similar behaviors of the Nambu-Goldstone mode. The existence of two phase modes suggest that in addition to the conventional free Cooper pairs as in the BCS case, there emerges a small part of the localized Cooper pairs around magnetic impurities due to the quantum correlation by the $s\text{−}d$ interaction, acting as Josephson islands. The emerging impurity Shiba bands inside the superconducting gap then correspond to the excitation of the ground state of the localized Cooper pairs, associated with the breaking of these Cooper pairs. In the diamagnetic response, the state of the free Cooper pairs gives rise to the conventional real contribution in the generated supercurrent, whereas the one of the localized Cooper pairs results in an imaginary contribution, leading to the superconducting Friedel oscillation, i.e., oscillation in the decay of the vector potential in the Meissner effect. As for the optical absorption of a conventional superconductor lying in the anomalous-skin-effect region, it is found that besides the conventional interband transition of Bogoliubov quasiparticles as revealed by Mattis-Bardeen theory, there also exist the interband transition between the impurity Shiba bands as well as all interband transitions between Bogoliubov quasiparticle and impurity Shiba bands. These transitions exhibit clear and separate resonance characters, providing a feasible scheme for the experimental detection.

In this work, we investigate the tunneling characteristics of Majorana zero modes (MZMs) in vortex lattices based on scanning tunneling microscopy measurement. We find that zero bias conductance does not reach the quantized value owing to the coupling between the MZMs. On the contrary, the Fano factor measured in the high voltage regime reflects the local particle-hole asymmetry of the bound states and is insensitive to the energy splitting between them. We propose using spatially resolved Fano factor tomography as a tool to identify the existence of MZMs. In both cases of isolated MZM or MZMs forming bands, there is a spatially resolved Fano factor plateau at one in the vicinity of a vortex core, regardless of the tunneling parameter details, which is in stark contrast to other trivial bound states. These results reveal new tunneling properties of MZMs in vortex lattices and provide measurement tools for topological quantum devices.

We investigate quantum order by disorder in a frustrated spin nanotube formed by wrapping a $J_1\text{−}{J}_2$ Heisenberg model at ${45}^{\circ }$ around a cylinder. Using Schwinger boson theory and density matrix renormalization group (DMRG), we have computed the ground-state phase diagram to reveal a ${\mathbb{Z}}_2$ phase in which collinear spin stripes form a right- or left-handed helix around the nanotube. We have derived an analytic estimate for the critical ${\eta }_c=J_1/2J_2$ of the ${\mathbb{Z}}_2$-helical phase transition, which is in agreement with the DMRG results. By evaluating the entanglement spectrum and nonlocal string order parameters we discuss the topology of the ${\mathbb{Z}}_2$-helical phase.

We theoretically investigate the double-resonance Raman spectrum of monolayer graphene down to infrared laser excitation energies. By using first-principles density functional theory calculations, we improve upon previous theoretical predictions based on conical models or tight-binding approximations, and rigorously justify the evaluation of the electron-phonon enhancement found in Venanzi, Graziotto et al. [Phys. Rev. Lett. 130, 256901 (2023)]. We proceed to discuss the effects of such enhancement on the room-temperature graphene resistivity, hinting towards a possible reconciliation of theoretical and experimental discrepancies.

The measurements of anisotropic magnetoresistance (AMR), planar Hall effect (PHE) and temperature-dependent conductivity in materials with strong spin-orbit coupling yield valuable information about charge carrier scattering processes, localization effects, and band topology. Although electronic structure calculations establish the sesqui-chalcogenide $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ a topological insulator (TI), detailed measurements of AMR and PHE are valuable to address the manifestations of the band topology on charge carrier transport in this system. Here, we report on measurements of the longitudinal and Hall resistivity, ${\rho }_{xx}$ and ${\rho }_{xy}$, respectively, of the $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ films of varied crystallinity over a wide phase space of temperature ($T$), magnetic field (B), and the angle between B and charge current density (J). The films exhibit semiconducting or metallic behavior depending on their crystallinity. The epitaxial films on (0001) sapphire grown at $150{}^{\circ }\mathrm{C}$ are metallic with a hole carrier density $\left(n_h\right)$ and mobility $\left({\mu }_h\right)$ of $\sim {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$ and $\sim {10}^2{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, respectively, at room temperature. The conduction in the semiconducting film exhibits a Shklovskii-Efros (SE)-type variable range hopping (VRH) at very low temperature, with a transition to the Mott type VRH at $T\ge 30$ K. The SE-type VRH is characterized by a Coulomb gap of 0.3 meV and a localization length of ≈12 nm, which matches with the average crystallite size in these disordered films. While signatures of weak antilocalization are seen in the magnetoresistance (MR) of epitaxial films at $T\le 20$ K, the MR at $T$ > 20 K agrees with Kohler's rule when corrected for the temperature variation of carrier density. The epitaxial films are characterized by a negative AMR and PHE which varies quadratically with magnetic field, but the orbital plots of ${\rho }_{xy}$ vs ${\rho }_{xx}$ negate the presence of a chiral anomaly in transport. The amplitude of MR anisotropy for $100{}^{\circ }\mathrm{C}$ grown $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ film is $\approx 102\mathrm{n}\mathrm{\Omega }\mathrm{m}$ at 300 K, which is an order of magnitude larger than in $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3$ and potentially important for the development of AMR-based sensors.

We generalize the two-channel (Edwards) fermion-boson model describing quantum transport in a background medium to the more realistic case of dispersive bosons. Using the variational exact diagonalization technique, we numerically solve the extended model in a one-dimensional setting, for both downward and upward curved boson dispersion, and show that going away from the previous Einstein-boson assumption has profound consequences for the particle transport. Specifically, we analyze the ground-state and spectral properties and demonstrate the renormalization of the particle's coherent band, effective mass, photoemission spectra, and fermion-boson correlation functions due to the interplay of fluctuations and correlations in a dispersive environment.

Electron hydrodynamics gives rise to surprising correlated behaviors in which electrons cooperate to quench dissipation and reduce the electric fields needed to sustain the flow. Such collective free flows are usually expected at the hydrodynamic length scales exceeding the electron-electron scattering mean free path ${\ell }_{\mathrm{ee}}$. Here we predict that in two-dimensional electron gases, the collective free flows actually occur at distances much smaller than ${\ell }_{\mathrm{ee}}$, in a nominally ballistic regime. The sub-${\ell }_{\mathrm{ee}}$ free flows arise due to retroreflected holes originating from head-on quasiparticle collisions; the holes retrace the paths of impinging electrons and cancel out their potential. An exact solution, obtained in Corbino geometry, predicts potential strongly screened by the hole backflow. The screened potential is described by a fractional power law $r^{-5/3}$ over a wide range of $r$ values, from macroscales down to deep sub-${\ell }_{\mathrm{ee}}$ scales, and distinct non-Fermi-liquid temperature dependence.

Chalcogenide-based compounds are an important part of the family of layered materials, extensively studied for their two-dimensional properties. An interesting line of investigation relates to the evolution of their properties with hydrostatic pressure, which could lead to structural transitions and itinerant electronic behavior. Here, we investigate the evolution of a layered ternary compound GaGeTe as a function of pressure with x-ray diffraction, Raman and infrared spectroscopy, and ab initio calculations. The Ge layer retains a germanenelike vibration though enveloped in Ga and Te layers. We show experimental and theoretical evidence of metallization in two steps. At $\sim 6$ GPa Raman and infrared spectroscopic data undergo changes compatible with delocalized charge carriers. Calculations ascribe this to the Fermi-level crossing of a valence band. At $\sim 16$ GPa infrared transmission and Raman modes disappear and x-ray diffraction signals a structural transition to a phase which is metallic according to our calculations. We obtain consistent agreement between experiment and theory concerning the structural, vibrational, and electronic structure evolution with pressure.

Line degeneracies in the band structures have been largely explored and exhibit exotic phenomena, particularly in three-dimensional (3D) photonic crystals. The flat Landau levels are a generic feature of nodal ring semimetals when a magnetic field perpendicular to the nodal ring plane is applied. However, solid evidence for such effects is still absent in photonic nodal ring systems due to limitations in magneto-optical materials and structural complexity in 3D systems. In this paper, we propose a two-dimensional photonic structure that exhibits nodal rings protected by mirror symmetry, demonstrable through a simple two-band tight-binding model. An interacting term that breaks the mirror symmetry can open a photonic band gap, leading to the existence of Jackiw-Rebbi-like interface states. By introducing gradient deformation to the photonic crystal ribbon, we create a uniform pseudomagnetic field, further achieving flat Landau levels and gapless interface states. Additionally, we can dynamically adjust the width of the interface state by modifying the pseudomagnetic field strength, indicating a strategy for creating a waveguide with arbitrary widths. Our results offer a simple two-dimensional platform to study nodal ring physics and its magnetic responses in a pure dielectric system, potentially providing insights for manipulating electromagnetic waves through photonic crystals.

We study the three-component ${\varphi }^4$ model on the simple cubic lattice in the presence of a cubic perturbation. To this end, we perform Monte Carlo simulations in conjunction with a finite-size scaling analysis of the data. The analysis of the renormalization group (RG) flow of a dimensionless quantity provides us with the accurate estimate $Y_4-{\omega }_2=0.00081\left(7\right)$ for the difference of the RG eigenvalue $Y_4$ at the $\text{O}\left(3\right)$-symmetric fixed point and the correction exponent ${\omega }_2$ at the cubic fixed point. We determine an effective exponent ${\nu }_{\text{eff}}$ of the correlation length that depends on the strength of the breaking of the $\text{O}\left(3\right)$ symmetry. Field theory predicts that depending on the sign of the cubic perturbation, the RG flow is attracted by the cubic fixed point, or runs to an ever increasing amplitude, indicating a fluctuation-induced first-order phase transition. We demonstrate directly the first-order nature of the phase transition for a sufficiently strong breaking of the O(3) symmetry. We obtain accurate results for the latent heat, the correlation length in the disordered phase at the transition temperature, and the interface tension for interfaces between one of the ordered phases and the disordered phase. We study how these quantities scale with the RG flow, allowing quantitative predictions for weaker breaking of the $\text{O}\left(3\right)$ symmetry.

The Fe/MgO interface is a cornerstone of spintronics applications, where enhancing its perpendicular magnetic anisotropy (PMA) has been a formidable challenge. Recent studies have reported an increase in PMA by introducing an ultrathin LiF layer, hinting at the potential of fluorine atoms with strong electronegativity. However, the underlying cause of this enhancement, whether stemming from the strong electronegativity of fluorine atoms or improved lattice matching, remains uncertain. In this paper, to disentangle these two contributing factors, we introduce a NaF layer with suboptimal lattice matching at the Fe/MgO interface and investigate the magnetic anisotropy energy. We find that the interfacial PMA energy is enhanced by the insertion of a 0.1-nm-thick NaF layer but weakened by a thicker NaF layer insertion (0.2–1 nm). The observed PMA enhancement, even in an Fe/NaF interface with suboptimal lattice matching, highlights the essential role of Fe-F orbital hybridization. Our findings shall provide a foundational basis for dielectric layer design to strengthen the PMA of ultrathin ferromagnetic layers.

The pressure dependence of the magnetic ordering temperatures for lanthanide ferromagnets Er and Tm has been investigated in the pressure region of up to 30 GPa through ac magnetization measurements by employing a superconducting quantum interference device. Six $4f$ lanthanide ferromagnets, from Gd to Tm, commonly have successive structural transitions starting from a hexagonal close-packed (hcp) structure under pressure. The experiments reported here have been performed with high sensitivity to allow for a more comprehensive discussion of the magnetostructural correlation when compared to four other $4f$ ferromagnets, namely Gd, Tb, Dy, and Ho. Our results conclude that only Er exhibits the increase in ferromagnetic ordering temperature against initial compression among the six lanthanide ferromagnets. The ferromagnetic magnetization anomaly for Er reduces below the detection limit before the structural transformation from Sm-type to the dhcp structures, and that for Tm exhibits similar behavior before the structure transformation from the hcp to Sm-type structures. However, the helimagnetic magnetization anomalies of both Er and Tm remain stable at least within the Sm-type structure below 30 GPa.

Gray arsenic is a two-dimensional semimetal material that has attracted much attention due to its rich and fascinating properties such as ultrahigh carrier mobility and giant magnetoresistance properties. In this work, the structural and electrical properties of gray arsenic under high pressure have been systematically explored. Two structural phase transitions, A7 phase of gray arsenic to simple cubic phase and simple cubic phase to host-guest phase, were revealed at ∼28 and ∼35 GPa, respectively. And the transition to host-guest phase completed at ∼46 GPa. Superconductivity appeared after ∼35 GPa and the maximum of $T_c$, ∼5 K, was revealed in pure host-guest phase at ∼54 GPa, which is currently the highest recorded $T_c$ of arsenic. The anisotropic upper critical field of superconductivities revealed the presence of mixed phase and the relative crystal orientations of the high-pressure phases.

Hybridization between the localized $f$ electrons and the delocalized conduction electrons together with the crystal electric field (CEF) play a determinant role in governing the many-body ground state of a correlated-electron system. Here, we investigate the low-energy CEF states in ${\mathrm{CeAg}}_2{\mathrm{Ge}}_2$, a prototype Kondo-lattice antiferromagnet where Kondo correlation is found to exist within the antiferromagnetic phase. Using time-domain THz reflection spectroscopy, we show direct evidence of two low-energy CEF transitions at 0.6 THz (2.5 meV) and 2.1 THz (8.7 meV). The presence of low-frequency infrared-active phonon modes further manifests as a Fano-modified line shape of the 2.1 THz CEF conductivity peak. The temporal spectral weights obtained directly from the THz time traces, in addition, corroborate the corresponding CEF temperature scales of the compound.

This paper reports how a two-dimensional photonic crystal slab of ${\mathrm{TiO}}_2$ covered with liquid dye gain can generate lasing with vortex and linearly polarized output beams concurrently. The photonic crystal slab is formed by shallowly etching a ${\mathrm{TiO}}_2$ film, which can support multiple band-edge modes within the gain bandwidth of the chosen dye. We identify the lasing band edges by mapping out the band structure from correlated transmission and emission measurements. The experimental results are in good agreement with the simulated band structure. The vortex lasing is connected to a symmetry-protected bound state in the continuum mode, while the linearly polarized lasing is connected to a doubly degenerate symmetric band-edge mode. The involved band-edge modes are arising from hybridized first-order slab waveguide modes. The quality factor and spatial mode-gain overlap are analyzed to understand mode discrimination and gain competition in this lasing device.