Polycrystalline samples of $\mathrm{Ce}{\left({\mathrm{Cu}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{Ge}}_2$ were investigated by means of electrical resistivity $\rho \left(T\right)$, magnetic susceptibility $\chi \left(T\right)$, specific heat $C_p\left(T\right)$, and thermoelectric power $S\left(T\right)$ measurements. The long-range antiferromagnetic order, which set in at $T_N=4.1\mathrm{K}$ in ${\mathrm{CeCu}}_2{\mathrm{Ge}}_2$, is suppressed by non-isoelectronic cobalt doping at a critical value of the concentration $x_c=0.6$, accompanied by non-Fermi-liquid behavior inferred from the power-law dependence of heat capacity and susceptibility, i.e., $C\left(T\right)/T$ and $\chi \left(T\right)\propto T^{-1+\lambda }$, down to 0.4 K, along with a clear deviation from $T^2$ behavior of the electrical resistivity. However, we have not seen any superconducting phase in the quantum critical regime down to 0.4 K.

Theoretical analyses of the polariton optical parametric oscillator (OPO) regime often rely on a mean-field approach based on the complex Gross-Pitaevskii equations in a three-mode approximation, where only three momentum states, the signal, pump, and idler, are assumed to be significantly occupied. This approximation, however, lacks a constraint to uniquely determine the signal and idler momenta. In contrast, multimode numerical simulations and experiments show a unique momentum structure for the OPO states. In this work we show that an estimate for the signal momentum chosen by the system can be found from a simple analysis of the pump-only configuration. We use this estimate to investigate how the chosen signal momentum depends on the properties of the drive.

We investigate nonlinear magnetotransport in a Hall bar device made from a strained InGaAs/InP quantum well: a material system with attractive spintronic properties. From extensive maps of the longitudinal differential resistance ($r_{xx}$) as a function of current and magnetic (B) field, phase diagrams are generated for quantum Hall breakdown in the strong quantum Hall regime reaching filling factor $\nu =1$. By careful illumination, the electron sheet density ($n$) is incremented in small steps and this provides insight into how the transport characteristics evolve with $n$. We explore in depth the energetics of integer quantum Hall breakdown and provide a simple picture for the principal features in the $r_{xx}$ maps. A simple tunneling model that captures a number of the characteristic features is introduced. We present results on the quantum Hall transport diamonds, a spin-flip resonance at high current near $\nu =1$, instabilities at large B field and current, and a zero-current anomaly. In addition, parameters such as critical Hall electric fields and the exchange-enhanced $g$ factors for odd-filling factors including $\nu =1$ are extracted. A detailed examination is made of the B-field dependence of the critical current as determined by two different methods and compiled for different values of $n$. A simple rescaling procedure that allows the critical current data points obtained from $r_{xx}$ maxima for even filling to collapse onto a single curve is demonstrated. Exchange-enhanced $g$ factors for odd filling are extracted from the compiled data and are compared to those determined by conventional thermal-activation measurements. The exchange-enhanced $g$ factor is found to increase with decreasing $n$.

Topological phonon polaritons (TPhPs) are highly protected and localized edge modes that are capable of achieving a strong confinement of electromagnetic waves and immune to impurities and disorder. Here we realize TPhPs by constructing one-dimensional dimerized silicon carbide nanoparticle chains, which mimic the topological property of the well-known Su-Schrieffer-Heeger (SSH) model. We analytically calculate the complex band structure of such chains by taking all near-field and far-field dipole-dipole interactions into account. For longitudinal modes, we demonstrate that, despite the non-Hermiticity and breaking of the chiral symmetry, the band topology can be still characterized by the complex Zak phase, which is quantized and indicates a topological phase transition when the dimerization parameter $\beta$ changes from less than 0.5 to larger than 0.5, like the conventional Hermitian SSH model. By calculating the eigenmodes of a finite chain, we find such a dimerized chain with $\beta >0.5$ supports nontrivial topological eigenmodes localized over both of its edges, indicating the validity of the bulk-boundary correspondence. On the other hand, for transverse modes, we discover a topological phase transition by increasing the lattice constant, which is due to the presence of strong long-range far-field dipole-dipole interactions decaying with the distance $r$ as $1/r$ for an infinitely long chain. However, we surprisingly find the emergence of non-Hermitian skin effect in a finite chain, which leads to the breakdown of the bulk-boundary correspondence. Furthermore, by incorporating the effect of localized bulk eigenmodes and proposing a modified complex Zak phase for a finite lattice, we still recover the topological behavior of the conventional SSH model. Our comprehensive study provides profound implications to the fields of non-Hermitian topological physics and quantum mechanical models with long-range interactions. In addition to the theoretical analysis, we demonstrate the excitation of the topological phonon polaritons and show their enhancement to the photonic LDOS. These TPhPs offer an efficient tool for enhancing light-matter interaction in the mid-infrared.

Topological structural phases (TSPs) of nontrivial winding numbers are of long-standing fundamental relevance in the theory of phase transition. However, for decades, no TSPs have been discovered in technological ferroelectric bulk solids. Here we formulate a generally applicable scheme to create TSPs in ferroelectrics. The approach consists in selectively exciting phonon modes to produce the targeted TSPs and can be employed to generate TSPs that do not naturally exist in bulk materials. We further demonstrate the effectiveness of this approach by creating in bulk ${\mathrm{SrTiO}}_3$ three TSP structures, namely the flower phase, the tetragonal-like vortex phase, and the orthorhombiclike vortex phase. These topological phases lead to the discovery of interesting properties, which include (i) a marked difference in the excitation stiffness of different topological phases and (ii) the existence of symmetry breaking between vortex phases.

We theoretically investigate the creation of a magnetic moment in gold nanoparticles by circularly polarized laser light. To this end, we describe the collective electron dynamics in gold nanoparticles using a semiclassical approach based on a quantum hydrodynamic model that incorporates the principal quantum many-body and nonlocal effects, such as the electron spill-out, the Hartree potential, and the exchange and correlation effects. We use a variational approach to investigate the breathing and the dipole dynamics induced by an external electric field. We show that gold nanoparticles can build up a static magnetic moment through the interaction with a circularly polarized laser light at the localized surface plasmon (LSP) resonance. We analyze that the responsible physical mechanism is a plasmonic, orbital inverse Faraday effect, which can be understood from the time-averaged electron current that contains currents rotating on the nanoparticle's surface. The computed laser-induced magnetic moments are sizable, of about $0.35{\mu }_{\mathrm{B}}$/atom for a laser intensity of $45\times {10}^{10}{\mathrm{W}/\mathrm{cm}}^2$ at LSP resonance.

We report experiments demonstrating the spin to charge current conversion by means of the inverse spin Hall effect in ultrathin films of the metallic antiferromagnet $\mathrm{M}{\mathrm{n}}_2\mathrm{Au}$ at room temperature. The $\mathrm{M}{\mathrm{n}}_2\mathrm{Au}$ films, of different thicknesses, were grown by dc sputtering directly onto crystalline films of the ferrimagnetic insulator yttrium iron garnet (YIG). The spin currents are generated in the YIG film by two different processes, spin pumping effect (SPE) and spin Seebeck effect (SSE). In the SPE experiments we use microwave-driven ferromagnetic resonance of the YIG film to generate a spin current that is injected into the $\mathrm{M}{\mathrm{n}}_2\mathrm{Au}$, while in the SSE experiments the spin current is generated by a thermal gradient across the YIG film in the configuration of the longitudinal spin Seebeck effect. From these measurements, we obtain for $\mathrm{M}{\mathrm{n}}_2\mathrm{Au}$ at room temperature a spin-diffusion length, ${\lambda }_S$, of 1.6 nm and a spin Hall angle, ${\theta }_{\mathrm{SH}}\approx 0.04$, which is comparable to the value for platinum.

We fabricated epitaxial multilayers with Co and Pd atomic-layer insertion at Fe(001)/MgO(001) interface and systematically investigated voltage-controlled magnetic anisotropy (VCMA) and voltage-induced interfacial Dzyaloshinskii-Moriya interaction $\left(i\mathrm{DMI}\right)$ change. The VCMA and $i\mathrm{DMI}$ change was characterized by spin-wave spectroscopy. We found that the VCMA was increased by Co and Pd insertions to −180 fJ/Vm in the Fe/Co/Pd/MgO system. We also found a large voltage-induced $i\mathrm{DMI}$ change of 65 fJ/Vm. The postannealing dependence of the VCMA and $i\mathrm{DMI}$ change was also characterized to investigate the influence of the change in chemical ordering around the MgO interface.

Magnetic-field-induced semimetal-insulator phase transition in graphite has regained attention, although its mechanism is not fully understood. Recently, a study performed under the pulsed magnetic field discovered that this phase transition depends on thickness even in a relatively thick system of the order of 100 nm and suggested that the electronic state in the insulating phase has an order along the stacking direction. Here we report the thickness dependence observed under dc magnetic fields, which nicely reproduces the previous results obtained under the pulsed magnetic field. In order to look into the critical condition to control the phase transition, the effect of electrostatic gating is also studied in a field-effect transistor structure since it will introduce a spatial modulation along the stacking direction. Magnetoresistance, measured up to 35 T, is prominently enhanced by the gate voltage in spite of the fact that the underlying electronic state is not largely changed owing to the charge-screening effect. On the other hand, the critical magnetic field of the semimetal-insulator transition is found to be insensitive to gate voltages, whereas its thickness dependence is fairly confirmed. By applying positive gate voltages, a prominent oscillation pattern, periodic in magnetic field, becomes apparent, the origin of which is not clear at this stage. Although electrostatic control of the phase transition is not realized in this study, the findings of gate-voltage tunability will help determine the electronic state in the quantum limit in graphite.

The symmetry and the locality are the two major sources of various general theorems in quantum many-body systems. We demonstrate that, in gapped phases of a U(1) symmetric Hamiltonian with finite-range interactions, the bulk properties such as the expectation value of local operators, the ground-state energy and the excitation gap, and the static and low-frequency dynamical responses in general do not depend on the U(1) phase of the twisted boundary condition in the limit of the large system size. Specifically, their dependence on the twisted angle is exponentially suppressed with the linear dimension of the system. Our argument is solely based on the exponential decay of various types of equal-time correlation functions and does not assume any details of the Hamiltonian, meaning that the statement applies quite generally regardless of the dimensionality or the interaction strength of the system.

Incorporating heavy atoms into polymer chains represents an effective way to generate emissive triplets. Here we used magneto-optical emission spectroscopy up to 17.5 Tesla for studying the fine structure of the triplet exciton in a series of Pt-rich π-conjugated polymers with various intrachain Pt concentrations. We found that their phosphorescence emission band shows substantial field-induced circular polarization (FICPO) up to 50% with an unusual, nonmonotonic field dependence at cryogenic temperature. From the field-induced energy splitting between left and right circularly polarized phosphorescence we obtained the effective $g$ factor of triplet exciton varying in the range of $-0.13\text{−}0.85$, which depends on the Pt concentration in the polymer chains. The FICPO of triplet emission originates from the population difference in spin sublevels, which is determined by thermal equilibrium subjected to spin-orbit coupling (SOC), exchange, and Zeeman interactions. Surprisingly we also observed FICPO in the fluorescence emission that results from the singlet-triplet interaction caused by the strong SOC. From these results we extracted the various interaction parameters that describe the exciton fine structure in these Pt-contained compounds.

There are two competing pairing mechanisms for the superconductivity of doped Weyl semimetals, i.e., the internode Bardeen-Cooper-Schrieffer (BCS) pairing and the intranode Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing. To understand the edge excitations at the interface between the Weyl semimetal and the superconducting Weyl semimetal (WSM/SWSM) mediated by two different pairings, we study the energy dispersions and the density of states under a strong magnetic field. It is found that only the chiral zeroth Landau level exhibits a significant difference for the two pairings; the excitation spectra of higher Landau levels are insensitive to the way of pairings. In the vicinity of interface in the hybrid of WSM/SWSM, the spatial distributions of transverse current and the transverse conductance are independent of pairing mechanism. The pairing independence in the macroscopic conductance can be understood with the quantum effect of phase-coherent electron-hole states at the WSM/SWSM interface, which is responsible for the magnetically induced edge states supported by the Weyl Landau levels.

Monolayers of transition metal dichalcogenides (TMDCs) have unique optoelectronic properties. Density functional theory allows only for a limited description of the electronic excitation energies in these systems, while a more advanced treatment within many-body perturbation theory employing the $\mathit{GW}/\mathrm{BSE}$ approximation is often rather time consuming. Here, we show that the recently developed $\mathrm{LDA}+\mathit{GdW}$ approach provides an efficient and at the same time reliable description of one-particle energies, as well as optical properties including bound excitons in TMDCs. For five exemplary materials (${\mathrm{MoSe}}_2, {\mathrm{MoS}}_2, {\mathrm{WSe}}_2, {\mathrm{WS}}_2$, and ${\mathrm{ReSe}}_2$), we discuss the numerical convergence, in particular with respect to k-point sampling, and show that the $\mathit{GdW}/\mathrm{BSE}$ approximation gives results similar to common $\mathit{GW}/\mathrm{BSE}$ calculations. Such efficient approaches are essential to treat larger multilayer systems or defects.

Two-dimensional (2D) and three-dimensional (3D) nanowire networks are potential metastructures for nanoelectronics and thermoelectric applications. This new class of nanoarchitectured materials have interesting physical properties due to their low mass density and their high surface-to-volume ratio. Here, we report on thermal transport properties in 2D and 3D interconnected nanowire networks. The thermal conductivity of these networks decreases in increasing the distance between the nodes. This effect is much more pronounced in 3D networks due to increased porosity, surface-to-volume ratio, and the enhanced backscattering at 3D nodes compared to 2D nodes. We propose a model to estimate the thermal resistance related to the 2D and 3D interconnections in order to provide an analytic description of thermal conductivity of such nanowire networks; the latter is in good agreement with molecular dynamic results. The backscattering processes in the nodes increase the thermal resistance by a factor of 5 compared to simple nanowires.

A formalism is developed for the rigorous study of solvable fractional quantum Hall parent Hamiltonians with Landau-level mixing. The idea of organization through “generalized Pauli principles” is expanded to allow for root-level entanglement, giving rise to “entangled Pauli principles.” Through the latter, aspects of the effective field theory description become ingrained in exact microscopic solutions for a great wealth of phases for which no similar single Landau-level description is known. We discuss in detail braiding statistic, edge theory, and rigorous zero-mode counting for the Jain-221 state as derived from a microscopic Hamiltonian. The relevant root-level entanglement is found to feature an Affleck-Kennedy-Lieb-Tasaki (AKLT)-type matrix-product ground states structure associated with an emergent SU(2) symmetry.

We consider two-dimensional electron systems in zero magnetic field at fractional filling. For such systems a Lieb-Schultz-Mattis theorem applies, forbidding the existence of a trivial insulator. However, the theorem does not distinguish between bosonic and fermionic systems. In this Rapid Communication we argue that in the case of fermionic systems, the topological orders that are compatible with the microscopic constraints are in general different from the bosonic case. We find different results in the case of even and odd denominator fillings, with even denominator fillings deviating more strongly from the bosonic case. Parts of our results also hold in three dimensions.

We consider the extended half-filled Hubbard model on the honeycomb lattice for second nearest-neighbor interactions. Using a functional integral approach, we find that collective fluctuations suppress topological states and instead favor charge ordering, in agreement with previous numerical studies. However, we show that the critical point is not of the putative semimetal-Mott insulator variety. Due to the frustrated nature of the interactions, the ground state is described by a novel hidden metallic charge order with semi-Dirac excitations. We conjecture that this transition is not in the Gross-Neveu universality class.

We analyze the detailed structure of topological edge mode protection occurring in hexagonal quantum spin Hall (QSH) materials. We focus on bismuthene, antimonene, and arsenene on a SiC substrate, which, due to their large bulk gap, may offer new opportunities for room-temperature QSH applications. While time-reversal symmetry is responsible for the principal symmetry protected character of QSH states, the hexagonal edge terminations yield further aspects of crystal symmetry which affect the topological protection. We show that armchair QSH edge states remain gapless under an in-plane magnetic field in the direction along the edge, a hallmark of their topological crystalline protection. In contrast, an out-of-plane magnetic field opens a gap of the order of a few meV within realistic ranges of the parameters. We use these intriguing signatures of armchair QSH edge states to predict experimentally testable fingerprints of their additional topological crystalline character and their helicity emerging in tunneling spectroscopy and ballistic magnetotransport.

Weyl semimetals are three-dimensional analogs of graphene with pointlike Fermi surfaces. Their linear electronic dispersion leads to a window in the particle-hole excitation spectrum which allows for undamped propagation of collective excitations. We argue that interactions in Weyl semimetals generically lead to well-defined exciton modes. However, using a minimal model for interactions, we show that the exciton binding energy is exponentially small for weak interactions. This is due to effective two-dimensional character in the space of particle-hole pairs that are available for bound-state formation. This is ultimately a consequence of linear electronic dispersion in three dimensions. Nevertheless, intermediate interaction strengths can lead to sharp excitonic resonances. We demonstrate this in a model Weyl semimetal with broken time-reversal symmetry and Hubbard interactions. Using generalized random phase approximation analysis, we show that excitonic modes here carry spin. Excitons in Weyl semimetals have evoked interest as their condensation could lead to an axionic charge-density-wave order. However, we find that the leading instability corresponds to intravalley spin density wave order which shifts the Weyl points without opening a gap. Our results suggest interesting directions for experimental studies of three-dimensional Dirac systems.

${\mathrm{In}}_2{\mathrm{Se}}_3$ is a semiconductor material that can be stabilized in different crystal structures (at least one 3D and several 2D layered structures have been reported) with diverse electrical and optical properties. This feature has plagued its characterization over the years, with reported band gaps varying in an unacceptable range of 1 eV. Using first-principles calculations based on density functional theory and the HSE06 hybrid functional, we investigate the structural and electronic properties of four layered phases of ${\mathrm{In}}_2{\mathrm{Se}}_3$, addressing their relative stability and the nature of their fundamental band gaps, i.e., direct versus indirect. Our results show large disparities between fundamental and optical gaps. The absorption coefficients are found to be as high as those in direct-gap III-V semiconductors. The band alignment with respect to conventional semiconductors indicate a tendency to $n$-type conductivity, explaining recent experimental observations.