Anderson localization of two-dimensional noninteracting high-spin particle gases subject to random spin-orbit coupling and random on-site potential is numerically studied on square lattices. Employing the finite-size scaling of localization length, we show that the Anderson localization transition occurs for high-spin fermions and belongs to the same universality class as two-dimensional systems with time-reversal symmetry and spin-orbit interaction characterized by the universal critical exponent of $\nu \simeq 2.73$, while all states of disordered bosons are localized in the thermodynamic limit. These observations are commensurate with the scaling theory. Moreover, a strong finite-size effect is observed for high-spin particles, which can be attributed to the large spin degrees of freedom acting as an extra spin dimension in finite lattices. As a consequence, the system indeed behaves as a three-dimensional system and the Anderson localization transition can even happen for bosons with very large spins when the spin degrees of freedom is comparable to the lattice size.

Noncollinear magnetism can play an important role in multiferroic materials but is relatively understudied in oxide heterostructures compared to their bulk counterparts. Using variable temperature magnetometry and neutron diffraction, we demonstrate the presence of helical magnetic ordering in $\mathrm{CaM}{\mathrm{n}}_7{\mathrm{O}}_{12}$ and $\mathrm{C}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{M}{\mathrm{n}}_7{\mathrm{O}}_{12}$ (for $x$ up to 0.51) thin films. Consistent with bulk $\mathrm{C}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{M}{\mathrm{n}}_7{\mathrm{O}}_{12}$, the net magnetization increases with Sr doping. Neutron diffraction confirms that the helical magnetic structure remains incommensurate at all values of $x$, while the fundamental magnetic wavevector increases upon Sr substitution. This result demonstrates a chemical-based approach for tuning helical magnetism in quadruple perovskite films and enables future studies of strain and interfacial effects on helimagnetism in oxide heterostructures.

We study the damping of plasma waves in linear Josephson junction chains as well as in two capacitively coupled chains. In the parameter regime where the ground capacitance can be neglected, the theory of the antisymmetric mode in the double chain can be mapped onto the theory of a single chain. We consider two sources of relaxation: the scattering from quantum phase slips (QPS) and the interaction among plasmons related to the nonlinearity of the Josephson potential. The contribution to the relaxation rate $1/\tau$ from the nonlinearity scales with the fourth power of frequency $\omega$, while the phase-slip contribution behaves as a power law with a nonuniversal exponent. In the parameter regime where the charging energy related to the junction capacitance is much smaller than the Josephson energy, the amplitude of QPS is strongly suppressed. This makes the relaxation mechanism related to QPS efficient only at very low frequencies. As a result, for chains that are in the infrared limit on the insulating side of the superconductor-insulator transition, the quality factor $\omega \tau$ shows a strongly nonmonotonic dependence on frequency, as was observed in a recent experiment.

We report on our systematic attempts at finding local interactions for which the lowest-Landau-level projected composite-fermion wave functions are the unique zero energy ground states. For this purpose, we study in detail the simplest nontrivial system beyond the Laughlin states, namely, bosons at filling factor $\nu =2/3$, and identify local constraints among clusters of particles in the ground state. By explicit calculation, we show that no Hamiltonian up to (and including) four-particle interactions produces this state as the exact ground state and speculate that this remains true even when interaction terms involving a greater number of particles are included. Surprisingly, we can identify an interaction, which imposes an energetic penalty for a specific entangled configuration of four particles with relative angular momentum of $6\hslash$, that produces a unique zero energy solution (as we have confirmed for up to 12 particles). This state, referred to as the $\lambda$ state, is not identical to the projected composite-fermion state, but the following facts suggest that the two might be topologically equivalent: the two states have a high overlap, they have the same root partition, the quantum numbers for their neutral excitations are identical, and the quantum numbers for the quasiparticle excitations also match. On the quasihole side, we find that even though the quantum numbers of the lowest energy states agree with the prediction from the composite-fermion theory, these states are not separated from the others by a clearly identifiable gap. This prevents us from making a conclusive claim regarding the topological equivalence of the $\lambda$ state and the composite-fermion state. Our study illustrates how new candidate states can be identified from constraining selected many-particle configurations and it would be interesting to pursue their topological classification.

The transport distance of excitons in exciton-polariton systems has previously been assumed to be very small ($\lesssim 1\mu \mathrm{m}$). The sharp spatial profiles observed when generating polaritons by nonresonant optical excitation show that this assumption is generally true. In this paper, however, we show that the transport distances of excitons in two-dimensional planar cavity structures with even a slightly polaritonic character are much longer than expected ($\approx 20\mu \mathrm{m}$). Although this population of slightly polaritonic excitons is normally small compared to the total population of excitons, they can substantially outnumber the population of the polaritons at lower energies, leading to important implications for the tailoring of potential landscapes and the measurement of interactions between polaritons.

Two-dimensional (2D) or sheet materials have been recently recognized as fascinating materials for nonlinear photonics. Here, we develop a rigorous mathematical framework based on perturbation theory and temporal coupled-mode theory capable of analyzing third-order, ${\chi }^{\left(3\right)}$, multichannel nonlinear processes in resonant systems comprising 2D materials. The framework is applied to model degenerate four-wave mixing in a guided-wave graphene plasmon-polariton resonant structure, consisting of a standing-wave resonator directly coupled to access waveguides. The results obtained with the proposed framework are compared with full-wave finite-element simulations revealing excellent agreement. Aside from being accurate and efficient, our framework allows for selectively incorporating different nonlinear phenomena, identifying their unique impact on the nonlinear response and providing valuable physical insight. We are, thus, able to specify the optimal operating point leading to maximum conversion efficiency for the generated wave in a multiparameter space. In addition, we identify unstable operating regimes exhibiting optical bistability or limit cycles, thoroughly characterizing the component performance. Our framework enables the study of diverse multichannel phenomena (frequency generation, frequency mixing, and parametric amplification) in the thriving field of 2D material photonics, thus allowing for assessing the potential of these exciting materials for practical nonlinear applications.

We discuss the quasiparticle band structure of the antiferromagnetic phase of the planar Kondo lattice model with a half-filled conduction band, the so-called Kondo insulator. The band structure is obtained by the bond fermion technique, and good agreement is obtained with the single-particle spectral function obtained by the dynamical cluster approximation. In particular, various changes of the band structure with $J/t$ observed in the numerical spectra are reproduced qualitatively by the calculation. In the absence of Fermi surface nesting, we find a semimetallic phase for a sufficiently small exchange constant, and possible experimental consequences are discussed.

The optical properties of two-dimensional transition-metal dichalcogenide monolayers, such as ${\mathrm{MoS}}_2$ or ${\mathrm{WSe}}_2$ are dominated by excitons, Coulomb bound electron-hole pairs. Screening effects due to the presence of hexagonal-boron nitride (hBN) surrounding layers have been investigated by solving the Bethe-Salpeter equation on top of $GW$ wave functions in density functional theory calculations. We have calculated the dependence of both the quasiparticle gap and the binding energy of the neutral exciton ground-state $E_b$ as a function of the hBN-layer thickness. This paper demonstrates that the effects of screening at this level of theory are more short ranged than is widely believed. The encapsulation of a ${\mathrm{WSe}}_2$ monolayer by three sheets of hBN ($\sim 1\mathrm{nm}$) already yields a 20% decrease in $E_b$, whereas the maximal reduction is 27% for thick hBN. We have performed similar calculations in the case of a ${\mathrm{WSe}}_2$ monolayer deposited on stacked hBN layers. These results are compared to the recently proposed quantum electrostatic heterostructure approach.

The nontrivial band gap plays a critical role in quantum anomalous Hall (QAH) insulators. In this work, we propose that the intrinsic QAH phase with sizable band gaps up to 367 meV is achieved in two-dimensional hexagonal organometallic frameworks (HOMFs). Based on first-principles calculations and effective model analysis, we uncover that these large band gaps in transition metal based HOMFs are opened by strong spin-orbital coupling of the localized 4d or 5d electrons. Importantly, we reveal that Coulomb correlations can further significantly enhance the nontrivial band gaps. In addition, we suggest a possible avenue to grow these organometallic QAH insulators on a semiconducting SiC(111) substrate, and the topological features are perfectly preserved due to the van der Waals interaction. Our work shows that the correlation remarkably enhances the nontrivial band gaps, providing exotic candidates to realize the QAH states at high temperatures.

The multipole moment is an established concept of electrons in solids. Entanglement of spin, orbital, and sublattice degrees of freedom is described by the multipole moment, and spontaneous multipole order is a ubiquitous phenomenon in strongly correlated electron systems. In this paper, we present group-theoretical classification theory of multipole order in solids. Intriguing duality between the real-space and momentum-space properties is revealed for odd-parity multipole order which spontaneously breaks inversion symmetry. Electromagnetic responses in odd-parity multipole states are clarified on the basis of the classification theory. A direct relation between the multipole moment and the magnetoelectric effect, Edelstein effect, magnetopiezoelectric effect, and dichromatic electron transport is demonstrated. More than 110 odd-parity magnetic multipole materials are identified by the group-theoretical analysis. Combining the list of materials with the classification tables of multipole order, we predict emergent responses of the candidate materials.

We show that acoustic double Weyl points are observed in a square lattice by breaking inversion symmetry. We also show that the double Weyl points are protected by the $C_4$ rotation symmetry but are unaffected by translation symmetry along the $z$ direction. When $C_4$ rotation symmetry is broken, the double Weyl point will split into two single Weyl points in the x-y plane. Gapless surface states and backscattering immune properties are demonstrated in double and single Weyl systems. The topologically protected one-way propagation of sound waves is demonstrated experimentally. The acoustic Weyl points obtained in the easily fabricated square lattice structure will provide a platform to study the topological properties and lead to potential applications in acoustic devices.

Despite a well-ordered pyrochlore crystal structure and strong magnetic interactions between the ${\mathrm{Dy}}^{3+}$ or ${\mathrm{Ho}}^{3+}$ ions, no long-range magnetic order has been detected in the pyrochlore titanates ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. To explore the actual magnetic phase formed by cooling these materials, we measure their magnetization dynamics using toroidal, boundary-free magnetization transport techniques. We find that the dynamical magnetic susceptibility of both compounds has the same distinctive phenomenology, which is indistinguishable in form from that of the dielectric permittivity of dipolar glass-forming liquids. Moreover, ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ both exhibit microscopic magnetic relaxation times that increase along the super-Arrhenius trajectories analogous to those observed in glass-forming dipolar liquids. Thus, upon cooling below about 2 K, ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ both appear to enter the same magnetic state exhibiting the characteristics of a glass-forming spin liquid.

The ${\mathrm{Fe}}_5{\mathrm{PB}}_2$ compound offers tunable magnetic properties via the possibility of various combinations of substitutions on the Fe and P sites. Here, we present a combined computational and experimental study of the magnetic properties of (${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_5{\mathrm{PB}}_2$. Computationally, we are able to explore the full concentration range, while the real samples were only obtained for $0\le x\le 0.7$. The calculated magnetic moments, Curie temperatures, and magnetocrystalline anisotropy energies (MAEs) are found to decrease with increasing Co concentration. Co substitution allows for tuning the Curie temperature in a wide range of values, from about six hundred to zero kelvins. As the MAE depends on the electronic structure in the vicinity of the Fermi energy, the geometry of the Fermi surface of ${\mathrm{Fe}}_5{\mathrm{PB}}_2$ and the k-resolved contributions to the MAE are discussed. Low-temperature measurements of an effective anisotropy constant for a series of (${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_5{\mathrm{PB}}_2$ samples determined the highest value of 0.94 MJ ${\mathrm{m}}^{-3}$ for the terminal ${\mathrm{Fe}}_5{\mathrm{PB}}_2$ composition, which then decreases with increasing Co concentration, thus confirming the computational result that Co alloying of ${\mathrm{Fe}}_5{\mathrm{PB}}_2$ is not a good strategy to increase the MAE of the system. However, the relativistic version of the fixed spin moment method reveals that a reduction in the magnetic moment of ${\mathrm{Fe}}_5{\mathrm{PB}}_2$, by about 25%, produces a fourfold increase of the MAE. Furthermore, calculations for (${\mathrm{Fe}}_{0.95}{X}_{0.05}{)}_5{\mathrm{PB}}_2$ ($X=5d$ element) indicate that 5% doping of ${\mathrm{Fe}}_5{\mathrm{PB}}_2$ with W or Re should double the MAE. These are results of high interest for, e.g., permanent magnet applications, where a large MAE is crucial.

Quantum fluctuations in magnetic lattices can yield a quantum spin-liquid (QSL) state, where no long-range order appears even at zero temperature. The variety of mechanisms that can generate the spin-liquid state and the more exotic QSL state remain unclear, however. Here, we report a magnetic honeycomb system, $\mathrm{BaC}{\mathrm{o}}_2{\left({\mathrm{P}}_{1\text{−}x}{\mathrm{V}}_x\right)}_2{\mathrm{O}}_8$, in which the spin correlations can be tuned by the disorder, leading to different magnetic behaviors. At low $x$, the material has a spin-glass ground state that appears to be due to coexisting and competing correlations. We have found that an external magnetic field can introduce spin-liquid-like behavior for some members of the solid solution, testified by the magnetic and thermodynamic experiments. Our results suggest that structural geometry, chemical disorder, and external field may help enhance quantum fluctuations in magnetic honeycomb materials.

We study scattering of propagating microwave fields by a dc-voltage-biased Josephson junction. At subgap voltages, a small Josephson junction works merely as a nonlinear boundary that can absorb, amplify, and diversely convert propagating microwaves. In the leading-order perturbation theory of the Josephson coupling energy, the spectral density and quadrature fluctuations of scattered thermal and coherent radiation can be described in terms of the well-known $P\left(E\right)$ function. Applying this, we study how thermal and coherent radiation is absorbed and amplified in an Ohmic transmission line and in a circuit with a resonance frequency. We show when a coherent input can create a two-mode squeezed output. In addition, we evaluate scattering amplitudes between arbitrary photon-number (Fock) states, characterizing individual photon multiplication and absorption processes occurring at the junction.

We theoretically study a spin-orbit-coupled nanowire proximitized by a superconductor in the presence of an externally applied Zeeman field (“Majorana nanowire”) with zero-energy Majorana bound states localized at the two ends of the wire when the Zeeman spin splitting is large enough for the system to enter the topological phase. The specific physics of interest in the current work is the effect of having several tunnel probes attached to the wire along its length. Such tunnel probes should allow, as a matter of principle, one to observe both the predicted bulk superconducting gap closing and opening associated with the topological quantum phase transition as well as the Majorana bound states at the wire ends showing up as zero-bias conductance peaks, depending on which probes are used for the tunneling spectroscopy measurement. Because of the possible invasive nature of the tunnel probes, producing local potential fluctuations in the nanowire, we find the physical situation to be quite complex. In particular, depending on the details of the tunnel barrier operational at the probes, the Majorana nanowire could manifest additional low-energy Andreev bound states which will manifest their own almost-zero-bias peaks, complicating the interpretation of the tunneling data in multiprobe Majorana nanowires. We use two complementary microscopic models to simulate the probes, finding that the tunneling conductance spectrum depends rather sensitively on the details of the tunnel barriers at the probes, but in some situations it should be possible to observe the Majorana bound-state-induced zero-bias conductance peak at the wire ends along with the gap closing and opening features associated with the bulk topological quantum phase transition in the multiprobe Majorana nanowires. Our detailed numerical results indicate that such a system should also be capable of directly manifesting the nonlocal conductance correlations arising from Majorana bound states at the two ends of the nanowire. We apply our general analysis to simulate a recent multiprobe nanowire experiment commenting on the nature of the quasiparticle gaps likely controlling the experimental observations.

We study the electronic structure of the skutterudite ${\mathrm{Ce}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$, which is known to exhibit a charge density wave (CDW) transition, at temperature $T_{CDW}\sim 160$ K, coupled to a metal-to-metal transition. We use temperature dependent hard x-ray photoemission spectroscopy (HAXPES) and x-ray absorption spectroscopy (XAS) to investigate the occupied and unoccupied electronic states of ${\mathrm{Ce}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$. The Co $2p$ and Sn $3p$ core level spectra show small but finite shifts in binding energy positions across $T_{CDW}$ while Ce $3d$ core level spectra do not show any change across the transition. The Ce $M_{4,5}$-edge XAS spectrum compared with calculations indicate a typical trivalent ionic ${\mathrm{Ce}}^{3+}$ spectral shape, ruling out Kondo screening in ${\mathrm{Ce}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$. In contrast, the Co $L_{2,3}$-edge XAS spectrum compared with a calculated spectrum shows evidence for hybridization with neighboring Sn atoms in a trigonal prismatic co-ordination. Temperature dependent XAS across the Co $L_{2,3}$-edge shows a small shift across $T_{CDW}$, consistent with HAXPES results. Detailed XAS measurements as a function of temperature show that the spectral shifts occur with a hysteresis across $T_{CDW}$, indicative of a first-order transition. Valence band spectra show a normal Fermi edge above and below $T_{CDW}$. The Co $3d$ states are observed at a binding energy of $\sim 2$ eV while the Ce $4f$ states occur as a weak feature within 0.5 eV of the Fermi level. The results suggest an unusual CDW transition coupled to a metal-to-metal transition in ${\mathrm{Ce}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$.

We present a tight-binding calculation of a twisted bilayer graphene at magic angle $\theta \sim 1.{08}^{\circ }$, allowing for full, in- and out-of-plane, relaxation of the atomic positions. The resulting band structure displays, as usual, four narrow minibands around the neutrality point, well separated from all other bands after the lattice relaxation. A thorough analysis of the miniband Bloch functions reveals an emergent $D_6$ symmetry, despite the lack of any manifest point-group symmetry in the relaxed lattice. The Bloch functions at the $\mathrm{\Gamma }$ point are degenerate in pairs, reflecting the so-called valley degeneracy. Moreover, each of them is invariant under $C_{3z}$, i.e., transforming like a one-dimensional, in-plane symmetric irreducible representation of an “emergent” $D_6$ group. Out of plane, the lower doublet is even under $C_{2x}$, while the upper doublet is odd, which implies that at least eight Wannier orbitals, two $s$-like and two $p_z$-like ones for each of the supercell sublattices AB and BA, are necessary but probably not sufficient to describe the four minibands. This unexpected one-electron complexity is likely to play an important role in the still unexplained metal-insulator-superconductor phenomenology of this system.