Single crystals of $\mathrm{Gd}{\mathrm{Fe}}_2{\mathrm{Zn}}_{20-x}{\mathrm{Cd}}_x$ ($0.0<x<1.4$) were grown and characterized through structural, magnetic, and electronic properties using x-ray diffraction, field- and temperature-dependent magnetization, specific heat, ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy, and electron spin resonance (ESR). A negative chemical pressure effect is accompanied by an unexpected increase of $T_C$ from 86 to 96 K, together with a reduction of the magnetic effective moment and saturation magnetic moment, as evidenced by all of the experimental techniques. From the microscopic point of view, probing at the $4f$ electron level and the Fe nucleus has allowed the extraction of important information about the configuration and the effective role of the partial Cd substitution for Zn in this ferromagnetic system. Our ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy experiments show a negligible variation of the hyperfine field at the Fe site, and ESR experiments reveal an enhancement of the Korringa-type relaxation and a molecular field effect as Cd is incorporated. This complex behavior is assigned to a possible reconstruction of the Fermi surface and/or a new distribution of the $d$ type of conduction electrons in response to the negative chemical pressure, leading to an enhancement of the ferromagnetic transition temperature in a generalized Ruderman-Kittel-Kasuya-Yosida interaction scenario.

The pressure-induced change in the electronic structures of the superconductors ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Ag}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ has been measured with high-resolution x-ray absorption spectroscopy. As a common feature for these compounds, we find that pressure causes the broadening of the Se $4p$ band and an energy shift of the Bi $6s$ band above the Fermi level up to the pressure of the first structural transition. These results, corroborated by density functional theory calculations, correlate with an increase of the carrier density, the disappearance of the band gap, and the emergence of superconductivity. The electronic structure changes significantly at the pressure of the first structural transition, which may be a trigger of the emergence of superconductivity, while above the pressure of the first phase transition it does not change much even around the second phase transition pressure, corresponding to the nearly constant $T_{\mathrm{c}}$ above the pressure of the second structural transition.

First-principles density functional theory plus Bethe-Salpeter equation calculations are employed to investigate the electronic and excitonic properties of monolayer titanium trichalcogenide alloys $\mathrm{Ti}{\mathrm{S}}_{3-x}{\mathrm{Se}}_x$ ($x=1$ and 2). It is found that the band gap and exciton binding energy display asymmetric dependence on the substitution of Se for S. While the band gap can be significantly decreased as compared to that of pristine $\mathrm{Ti}{\mathrm{S}}_3$, the exciton binding energy varies slightly, regardless of the position and concentration of the Se substitution. A negative exciton formation energy is found when the central S atoms are replaced by Se atoms, suggesting a many-body ground state with spontaneous exciton condensation. Our work thus offers insight for engineering an excitonic insulator.

We study the spontaneous emission rate of a dipole emitter in $\mathcal{PT}$-symmetric environment of two coupled waveguides using the reciprocity approach generalized to nonorthogonal eigenmodes of non-Hermitian systems. Considering emission to the guided modes, we define and calculate the modal Purcell factor composed of contributions of independent and interfering nonorthogonal modes leading to the emergence of cross-mode terms in the Purcell factor. We reveal that the closed-form expression for the modal Purcell factor within the coupled mode theory slightly alters for the non-Hermitian coupled waveguide compared to the Hermitian case. It is true even near the exceptional point, where the eigenmodes coalesce and the Petermann factor goes to infinity. This result is fully confirmed by the numerical simulations of active and passive $\mathcal{PT}$-symmetric systems being the consequence of the mode nonorthogonality.

Resonance effects play an important role in the transport properties of low-dimensional systems. Here, we investigate the thermally driven spin-transport properties of ruthenium-terpyridine molecular devices, where the molecule is linked to graphene electrodes by the carbon atomic chains from two sides. The results show that an odd-even oscillating behavior of the transmission spectra and a perfect spin-filtering effect at the Fermi level are observed in the ruthenium-terpyridine molecular devices. Furthermore, the negative differential thermoelectric resistance more easily appears in the long even-numbered carbon atomic chains, and the sign of the thermally driven single-spin current can be tuned via the temperature. Such a behavior is attributed to the competition between the Breit-Wigner and Fano resonances in the vicinity of the Fermi level for the contributions of the thermally driven current. In brief, here we present an alternative to control the thermally driven single-spin current in molecular devices via quantum effect.

The simplest continuum model of a one-dimensional noninteracting superconducting fermionic symmetry-protected topological (SPT) phase is studied in great detail using analytical methods. In a first step, we present a full exact diagonalization of the fermionic Bogoliubov–de Gennes Hamiltonian for a system of finite length and with open boundaries. In particular, we derive exact analytical expressions for the Majorana zero modes emerging in the topologically nontrivial phase, revealing their spatial localization, their transformation properties under symmetry operations, and the exact finite-size energy splitting of the associated quasidegenerate ground states. We then proceed to analyze the model via exact operator bosonization in both open and closed geometries. In the closed wire geometry, we demonstrate fermion parity switching from twisting boundary conditions in the topologically nontrivial phase. For the open wire, on the other hand, we first take a semiclassical approach employing the Mathieu equation to study the two quasidegenerate ground states as well as their energy splitting at finite system sizes. We then finally derive the exact forms of the Majorana zero modes in the bosonic language using vertex-algebra techniques. These modes are verified to be in exact agreement with the results obtained from the fermionic description. The complementary viewpoints provided by the fermionic and bosonic formulations of the superconducting SPT phase are reconciled, allowing us to provide a complete and exact account of how Majorana zero modes manifest in a bosonized description of an SPT phase.

Boundaries constitute a rich playground for quantum many-body systems because they can lead to novel degrees of freedom such as protected boundary states in topological phases. Here, we study the ground state of integer quantum Hall systems in the presence of boundaries through the reduced density matrix of a spatial region. We work in the lowest Landau level and choose our region to intersect the boundary at arbitrary angles. The entanglement entropy (EE) contains a logarithmic contribution coming from the chiral edge modes, and matches the corresponding conformal field theory prediction. We uncover an additional contribution due to the boundary corners. We characterize the angle dependence of this boundary corner term, and compare it to the bulk corner EE. We further analyze the spatial structure of entanglement via the eigenstates associated with the reduced density matrix. The influence of the physical boundary and the region's geometry on the reduced density matrix is thus clarified. Finally, we test a bulk-boundary correspondence for the EE originally obtained for quantum critical systems such as conformal field theories in two spatial dimensions, and discuss the implications of our findings for other topological phases.

The layered van der Waals ferromagnetic ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ harbors an unconventional interplay between topology and magnetism, leading to a large anomalous Hall conductivity at low temperatures. Here, we investigate the temperature dependence of its charge dynamics and reveal that upon entering the ferromagnetic state at $T_C\sim 200$ K and further lowering the temperature there is the onset of a gradual spectral weight reshuffling from the mid-infrared range towards far- as well as near-infrared frequencies. This twofold spectral weight transfer indicates the important role of the Hund's coupling as primary source for electronic correlations and signals an incoherent-coherent crossover at low temperatures. Our findings also convey the electronic environment, based on nodal-line topological states, favoring the large anomalous Hall conductivity.

We report here the properties of single crystals of ${\mathrm{La}}_2{\mathrm{Ni}}_2\mathrm{In}$. Electrical resistivity and specific heat measurements concur with the results of density functional theory calculations, finding that ${\mathrm{La}}_2{\mathrm{Ni}}_2\mathrm{In}$ is a weakly correlated metal, where the Ni magnetism is almost completely quenched, leaving only a weak Stoner enhancement of the density of states. Superconductivity is observed at temperatures below 0.9 K. A detailed analysis of the field and temperature dependencies of the resistivity, magnetic susceptibility, and specific heat at the lowest temperatures reveals that ${\mathrm{La}}_2{\mathrm{Ni}}_2\mathrm{In}$ is a dirty type-II superconductor with likely $s$-wave gap symmetry. Nanoclusters of ferromagnetic inclusions significantly affect the subgap states resulting in a nonexponential temperature dependence of the specific heat $C\left(T\right)$ at $T\ll T_c$.

We extend the description of the nonequilibrium Gross-Pitaevskii equation for microcavity polaritons by including not only the kinetics of the nonresonantly pumped exciton reservoir but also that of the collective excitations—called bogolons—above the condensate. Quasiequilibrium Hartree-Fock-Bogoliubov-Popov self-energies are used to describe the condensate and the bogolons. With nonequilibrium Green functions, the quantum Boltzmann equation of the bogolons in a frame moving with the condensate, together with the coupled Gross-Pitaevskii equation can be derived. The resulting scattering rates contain vertex corrections in form of symmetrized products of the $u$ and $v$ pairing functions of all involved scattering states. Corresponding results have been derived for ultracold atoms by A. Griffin, T. Nikuni, and E. Zaremba [Bose-Condensed Gases at Finite Temperatures, 1st ed. (Cambridge University Press, Cambridge, New York, 2009)] Furthermore, we show that the results of the quantum kinetics also determine the luminescence spectrum including the ghost branch. The kinetics is solved numerically, the resulting condensate, the bogolon distribution, and the luminescence are calculated temporally and spatially resolved after the switch-on of a $\mathit{cw}$ nonresonant pump beam with a Gaussian profile. It is studied in detail to which extent the bogolons approach a local thermal equilibrium.

We study the electronic structure of GdNi and HoNi, which are magnetic materials with a Curie temperature $T_c=69$ and $T_c=36\mathrm{K}$, respectively. These materials are useful for magnetic refrigerator applications at low temperature as they exhibit a large magnetocaloric effect near $T_c$. We have used hard x-ray photoemission spectroscopy (HAXPES) to investigate the core-level and valence-band electronic states of GdNi and HoNi. HAXPES measurements of the Gd and Ho $3d, 4d, 4p, 5p$, and $4s$ core-level spectra have been compared with atomic multiplet calculations of ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Ho}}^{3+}$ ionic configurations. The good match between the experimental and calculated spectra clarify the important role of spin-orbit coupling, as well as Coulomb and exchange interactions in the intermediate-coupling scheme. The core-level spectra also show plasmons in addition to the atomic multiplets. The Gd and Ho $4s$ spectra show clear evidence of exchange splitting. The Ni $2p$ and $3s$ spectra of GdNi and HoNi show a correlation satellite at a binding energy of 7 eV above their main peaks. The Ni $2p$ and Ni $3s$ spectra could be reproduced using charge transfer multiplet calculations. Valence-band HAXPES of GdNi and HoNi shows that the Gd $4f$ and Ho $4f$ features are also consistent with atomic multiplets and occur at high binding energies away from the Fermi level. The Ni $3d$ density of states are spread from the Fermi level to about 3 eV binding energy. The results indicate a partially filled Ni $3d$ band and show that the charge transfer model is not valid for describing the electronic structure of GdNi and HoNi.

Fast and dynamical control of quantum phases is highly desired in the application of quantum phenomena in devices. On the experimental side, ultrafast laser pulses provide an ideal platform to induce femtosecond dynamics in a variety of materials. Here we show that a laser pulse driven heavy fermion system can be tuned to dynamically evolve into a phase, which is not present in equilibrium. Using the state-of-the-art time-dependent variational Monte Carlo method, we perform numerical simulations of realistic laser pulses applied to the Kondo lattice model. By tracking spin and charge degrees of freedom we identify a dynamical phase transition, within the partially Kondo screened phase, from a charge ordered into a metallic state, while preserving the spin order of the system. We propose using high-harmonic generation and nonequilibrium optical conductivity to identify the dynamical phase transition in an ultrafast experiment.

Multiferroics with intrinsic ferromagnetism and ferroelectricity are highly desired but rather rare, while most ferroelectric magnets are antiferromagnetic. A recent theoretical work [Tan et al., Phys. Rev. B 99, 195434 (2019)] predicted that oxyhalides $\mathrm{VO}{X}_2$ ($X$: halogen) monolayers are two-dimensional multiferroics by violating the empirical $d^0$ rule. Most interestingly, the member $\mathrm{VO}{\mathrm{I}}_2$ are predicted to exhibit spontaneous ferromagnetism and ferroelectricity. In this work, we extend the previous study on the structure and magnetism of $\mathrm{VO}{\mathrm{I}}_2$ monolayer by using density-functional theory and Monte Carlo simulation. The presence of the heavy element iodine with a strong spin-orbit coupling gives rise to an effective Dzyaloshinskii-Moriya interaction in the polar structure, which favors a short-period spiral magnetic structure.. Another interesting result is that the on-site Coulomb interaction can strongly suppress the polar distortion thus leading to a ferromagnetic metallic state. Therefore, the $\mathrm{VO}{\mathrm{I}}_2$ monolayer is either a ferroelectric insulator with spiral magnetism or a ferromagnetic metal, instead of a ferromagnetic ferroelectric system. Our study highlights the key physical role of the Dzyaloshinskii-Moriya interaction.

We theoretically study Josephson junctions with a transition metal dichalcogenide zigzag ribbon as a weak link. We demonstrate that the spatial profile of the supercurrent carried by the edge modes determines the critical current dependence on the perpendicular magnetic field. We explore this finding and analyze the impact of Zeeman interaction and the orbital effects of the magnetic field on the Andreev bound states energies. We show that the unequal Fermi velocities of the spin-opposite edge modes lead to an anomalous shift of the Andreev bound states in the presence of the magnetic field. This is manifested in a pronounced modification of the SQUID critical current oscillations when two opposite edges of the ribbon are conducting and can be exploited in order to reveal the anomalous phase shift of the Andreev bound states in a single Josephson junction device.

In this article we develop an analog to the SSH model in tight-binding chains of resonators and an innovative Hermitian matrix formulation to describe the topological phases induced by multiple scattering at subwavelength scales in one-dimensional structured and locally resonant metamaterial crystals. We first start from a set of coupled dipole equations capturing the nature of the wave-matter interactions, i.e., hybridization between locally dispersive resonances and a continuum as well as infinite long range multiple scattering coupling, to analytically derive a matrix operator $H_{\mathrm{MS}}$, which is found to be Hermitian when evaluated on propagative bands. This new operator straightforwardly highlights how the composition, structure, scattering resonance, and Bloch periodicity together set in the chain macroscopic properties, in particular its topology. We analytically confirm the existence of a structure based topological transition in chiral-symmetric biperiodic metamaterial crystal chains, characterized by a winding number. We further demonstrate that chiral symmetry breaking in metamaterial crystal chains prevents us from defining a proper topological invariant. We finally numerically confirm, in the microwave domain, the existence of the topological transition in a chiral symmetric metamaterial crystal chain through the study of topological interface modes and their robustness to disorder.

Two-dimensional (2D) materials with coexistence of ferromagnetism (FM) and ferroelectricity (FE) are rare. By using first-principles modeling, here we report that doping of magnetic transition metal (TM) atoms in FE monolayer ${\mathrm{In}}_2{\mathrm{Se}}_3$ could introduce giant local FM magnetic moments ($4{\mu }_{\mathrm{B}}$ per Mn atom). The exchange splitting energy, and the $C_{3\nu }$ crystal field together with the hybridization of the $\mathrm{Mn}\text{−}d$ and $\mathrm{Se}\text{−}p$ orbitals result in spin-polarized states near the Fermi surface. More interestingly, the asymmetric charge distribution of the Jahn-Teller ion ($\mathrm{M}{\mathrm{n}}^{3+}{d}^4$) further lift the twofold degeneracy of the Mn-d e$1^{*}$ states. This affects the band structure and enhances the FE polarization in monolayer ${\mathrm{In}}_2{\mathrm{Se}}_3$. Our work paves the way for tailoring FM in 2D FE via doping magnetic moments from TM atoms. The half-metallicity combined with the enhanced FE polarization make Mn-doped ${\mathrm{In}}_2{\mathrm{Se}}_3$ a candidate for potential applications in information technology and spintronic devices.

Motivated by recent experiments, we investigate the nonreciprocal magnetoelastic interaction between the surface acoustic phonons of dielectric nonmagnetic substrates and magnons of proximity nanomagnets. The magnetization dynamics exerts rotating forces at the edges of the nanomagnet that causes the nonreciprocal interaction with surface phonons due to its rotation-momentum locking. This coupling induces the nonreciprocity of the surface phonon transmission and a nearly complete phonon diode effect by several (tens of) magnetic nanowires of high (ordinary) magnetic quality. Phase-sensitive microwave transmission is also nonreciprocal that can pick up clear signals of the coherent phonons excited by magnetization dynamics. Nonreciprocal pumping of phonons by precessing magnetization is predicted using Landauer-Büttiker formalism.

We report inelastic neutron scattering measurements from our newly synthesized single crystals of the structurally metastable antiferromagnetic pyrochlore ${\mathrm{Yb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$. We determine the four symmetry-allowed nearest-neighbor anisotropic exchange parameters via fits to linear spin wave theory supplemented by fits of the high-temperature specific heat using the numerical linked-cluster expansion method. The exchange parameters so determined are strongly correlated to the values determined for the $g$-tensor components, as previously noted for the related Yb pyrochlore ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. To address this issue we directly determined the $g$ tensor from electron paramagnetic resonance of 1% Yb-doped ${\mathrm{Lu}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$, thus enabling an unambiguous determination of the exchange parameters. Our results show that ${\mathrm{Yb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ resides extremely close to the classical phase boundary between an antiferromagnetic ${\mathrm{\Gamma }}_5$ phase and a splayed ferromagnet phase. By juxtaposing our results with recent ones on ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, our work illustrates that the Yb pyrochlore oxides represent ideal systems for studying quantum magnets in close proximity to classical phase boundaries.

In this Rapid Communication, we investigate the symmetry of the phonon landscape of twisted kagome lattices across their duality boundary. The study is inspired by recent work by Fruchart et al. [Nature (London) 577, 636 (2020)], who specialized the notion of duality to the mechanistic problem of kagome lattices and linked it to the existence of duality transformations between configurations that are symmetrically located across a critical point in configuration space. Our first goal is to elucidate how the existence of matching phonon spectra between dual configurations manifest in terms of observable wave-field characteristics. To this end, we explore the possibility of aggregating dual kagome configurations into bidomain lattices that are geometrically heterogeneous but retain a dynamically homogeneous response. Our second objective is to extend the analysis to structural lattices of beams, which are representative of realistic cellular metamaterials. We show that, in this case, the symmetry of the phonon landscape across the duality boundary is broken, implying that the conditions for dual behavior do not merely depend on the geometry, but also on the dominant mechanisms of the cell, suggesting a dichotomy between geometric and functional duality.