The layered perovskite cobaltite $R{\mathrm{BaCo}}_2{\mathrm{O}}_{5.5}$ ($R$: rare-earth elements) exhibits an abrupt temperature-induced metal-insulator transition (MIT) that has been attributed to spin-state ordering (SSO) of ${\mathrm{Co}}^{3+}$ ions. Here we investigate the hole-doping member of ${\mathrm{PrBaCo}}_2{\mathrm{O}}_{5.5+x}$ ($0\le x\le 0.24$) with multiple techniques. The analysis of crystal and magnetic structures by electron and neutron diffraction confirm the SSO in the insulating phase of undoped ${\mathrm{PrBaCo}}_2{\mathrm{O}}_{5.5}$, which is melted by increasing the temperature across the MIT. In addition, we discover that hole doping to ${\mathrm{PrBaCo}}_2{\mathrm{O}}_{5.5}$ also melts the SSO in conjunction with an insulator-metal transition. The experimental results from electron and neutron diffraction and soft-x-ray absorption spectroscopy (XAS) all lead to the conclusion that hole-doping-induced MIT occurs in the same manner as the temperature-induced MIT. Therefore, we propose a unified mechanism that dominates the temperature- and hole-doping-induced MIT in the ${\mathrm{PrBaCo}}_2{\mathrm{O}}_{5.5+x}$ system. Specifically, this mechanism involves symmetry breaking coupled with a SSO in the paramagnetic phase.

Nuclear spin relaxation is studied in $n$-GaAs thick layers and microcavity samples with different electron densities. We reveal that both in metallic samples where electrons are free and mobile, and in insulating samples where electrons are localized, nuclear spin relaxation is strongly enhanced at low magnetic fields. The origin of this effect could reside in the quadrupole interaction between nuclei and fluctuating electron charges, that has been proposed to govern nuclear spin dynamics at low magnetic fields in the insulating samples. The characteristic values of these magnetic fields are given by dipole-dipole interaction between nuclei in bulk samples, and are greatly enhanced in microcavities, presumably due to additional strain, inherent to microstructures and nanostructures.

The mechanisms contributing to experimental quality factors of short wavelength ($\lambda =440–480$ nm) III-nitride on silicon one-dimensional photonic crystal cavities were quantified. Fluctuations in fundamental and first-order cavity mode wavelength and quality factor were compared over sets of nominally identical cavities. Unlike at $\lambda =1.5\mu \mathrm{m}$, experimental quality factors were not limited by fabrication disorder modeled as smooth, normally distributed hole size and position variations; after ruling out absorption losses, additional scattering losses were found to predominate at short wavelengths. Experimental quality factors were sensitive to conformal deposition of few nanometer thin films on the photonic crystal surface, suggesting that the additional scattering losses were linked to the surface.

The origin of efficiency droop in state-of-the-art quality InGaN/GaN and GaN/AlGaN quantum wells (QWs) grown on various crystal planes is studied by means of time-resolved photoluminescence spectroscopy associated with a precise determination of the QW carrier density. In a first set of experiments, it is shown that a polar InGaN/GaN QW under nonresonant high optical excitation shows clear signatures of Auger loss mechanism and thus behaves quite differently compared to its binary based GaN/AlGaN QW counterpart, where no Auger signature is observed. In order to get rid of the impact of the built-in polarization field and illustrate the dominant role of carrier localization, similar experiments have been conducted on two $m$-plane InGaN/GaN QWs with similar In composition but a different degree of disorder. We demonstrate that carrier localization strongly enhances the Auger recombination process in nonpolar InGaN/GaN QWs. We also show that this effect may be further amplified by the presence of polarization fields on polar QWs. The relaxation of the $k$-selection rule during the Auger recombination process, resulting from QW potential disorder, can account for the enhancement of the efficiency droop in InGaN/GaN QWs.

We study a bipartite linear chain constituted by spherical metallic nanoparticles, where each nanoparticle supports a localized surface plasmon. The near-field dipolar interaction between the localized surface plasmons gives rise to collective plasmons, which are extended over the whole nanoparticle array. We derive analytically the spectrum and the eigenstates of the collective plasmonic excitations. At the edge of the Brillouin zone, the spectrum is of a pseudorelativistic nature similar to that present in the electronic band structure of polyacetylene. We find the effective Dirac Hamiltonian for the collective plasmons and show that the corresponding spinor eigenstates represent one-dimensional Dirac-like massive bosonic excitations. Therefore, the plasmonic lattice exhibits similar effects to those found for electrons in one-dimensional Dirac materials, such as the ability for transmission with highly suppressed backscattering due to Klein tunneling. We also show that the system is governed by a nontrivial Zak phase, which predicts the manifestation of edge states in the chain. When two dimerized chains with different topological phases are connected, we find the appearance of the bosonic version of a Jackiw-Rebbi midgap state. We further investigate the radiative and nonradiative lifetimes of the collective plasmonic excitations and comment on the challenges for experimental realization of the topological effects found theoretically.

The relationship between the universal conductance fluctuation and the weak localization effect in monolayer graphene is investigated. By comparing experimental results with the predictions of the weak localization theory for graphene, we find that the ratio of the elastic intervalley scattering time to the inelastic dephasing time varies in accordance with the conductance fluctuation; this is a clear evidence connecting the universal conductance fluctuation with the weak localization effect. We also find a series of scattering lengths that are related to the phase shifts caused by magnetic flux by Fourier analysis.

The influence of uniaxial single-ion anisotropy $-D{S}_z^2$ on the magnetic and thermal properties of Heisenberg antiferromagnets (AFMs) is investigated. The uniaxial anisotropy is treated exactly and the Heisenberg interactions are treated within unified molecular field theory (MFT) [Phys. Rev. B 91, 064427 (2015)], where thermodynamic variables are expressed in terms of directly measurable parameters. The properties of collinear AFMs with ordering along the $z$ axis ($D>0$) in applied field $H_z=0$ are calculated versus $D$ and temperature $T$, including the ordered moment $\mu$, the Néel temperature $T_{\mathrm{N}}$, the magnetic entropy, internal energy, heat capacity, and the anisotropic magnetic susceptibilities ${\chi }_{\parallel }$ and ${\chi }_{\perp }$ in the paramagnetic (PM) and AFM states. The high-field average magnetization per spin ${\mu }_z(H_z,D,T)$ is found, and the critical field $H_{\mathrm{c}}(D,T)$ is derived at which the second-order AFM to PM phase transition occurs. The magnetic properties of the spin-flop (SF) phase are calculated, including the zero-field properties $T_{\mathrm{N}}\left(D\right)$ and $\mu (D,T)$. The high-field ${\mu }_z(H_z,D,T)$ is determined, together with the associated spin-flop field $H_{\mathrm{SF}}(D,T)$ at which a second-order SF to PM phase transition occurs. The free energies of the AFM, SF, and PM phases are derived from which $H_z-T$ phase diagrams are constructed. For $f_J=-1$ and $-0.75$, where $f_J={\theta }_{\mathrm{p}J}/T_{\mathrm{N}J}$ and ${\theta }_{\mathrm{p}J}$ and $T_{\mathrm{N}J}$ are the Weiss temperature in the Curie-Weiss law and the Néel temperature due to exchange interactions alone, respectively, phase diagrams in the $H_z-T$ plane similar to previous results are obtained. However, for $f_J=0$ we find a topologically different phase diagram where a spin-flop bubble with PM and AFM boundaries occurs at finite $H_z$ and $T$. Also calculated are properties arising from a perpendicular magnetic field, including the perpendicular susceptibility ${\chi }_{\perp }(D,T)$, the associated effective torque at low fields arising from the $-D{S}_z^2$ term in the Hamiltonian, the high-field perpendicular magnetization ${\mu }_{\perp }$, and the perpendicular critical field $H_{\mathrm{c}\perp }$ at which the second-order AFM to PM phase transition occurs. In addition to the above results for $D>0$, the $T_{\mathrm{N}}\left(D\right)$ and ordered moment $\mu (T,D)$ for collinear AFM ordering along the $x$ axis with $D<0$ are determined. In order to compare the properties of the above spin systems with those of noninteracting systems with $-D{S}_z^2$ uniaxial anisotropy with either sign of $D$, Supplemental Material is provided in which results for the thermal and magnetic properties of such noninteracting spin systems are given.

The family of magnetic rare-earth pyrochlore oxides ${\mathrm{R}}_2{\mathrm{M}}_2{\mathrm{O}}_7$ plays host to a diverse array of exotic phenomena, driven by the interplay between geometrical frustration and spin-orbit interaction, which leads to anisotropy in both magnetic moments and their interactions. In this article we establish a general, symmetry-based theory of pyrochlore magnets with anisotropic exchange interactions. Starting from a very general model of nearest-neighbor exchange between Kramers ions, we find four distinct classical ordered states, all with $\mathbf{q}=0$, competing with a variety of spin liquids and unconventional forms of magnetic order. The finite-temperature phase diagram of this model is determined by Monte Carlo simulation, supported by classical spin-wave calculations. We pay particular attention to the region of parameter space relevant to the widely studied materials ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7, {\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, and ${\mathrm{Er}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$. We find that many of the most interesting properties of these materials can be traced back to the “accidental” degeneracies where phases with different symmetries meet. These include the ordered ground-state selection by fluctuations in ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, the dimensional reduction observed in ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, and the lack of reported magnetic order in ${\mathrm{Er}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$. We also discuss the application of this theory to other pyrochlore oxides.

We report an inelastic neutron scattering study of single crystals of (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}$)ODFeSe. Temperature-dependent low-energy spin excitations are observed near $\mathbf{Q}=(0.5,0.27,0.5)$ and equivalent wave vectors symmetrically surrounding $\mathbf{Q}=(0.5,0.5,0.5)$ in the 1-Fe Brillouin zone, consistent with a Fermi-surface-nesting description. The excitations are broadly distributed in energy, ranging from 16 to 35 meV. Upon cooling below the superconducting critical temperature ($T_{\mathrm{c}}$), a magnetic response below twice the superconducting gap $2{\mathrm{\Delta }}_{\mathrm{SC}}$ exhibits an abrupt enhancement, consistent with the notion of spin resonance, whereas the response at higher energies increases more gradually with only a weak anomaly at $T_{\mathrm{c}}$. Our results suggest that (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}$)ODFeSe might be on the verge of a crossover between different Cooper-pairing channels with distinct symmetries.

We consider a fermionic Hubbard chain with an additional next-to-nearest neighbor hopping term. We study the thermalization rates of the quasimomentum distribution function within a quantum Boltzmann equation approach. We find that the thermalization rates are proportional to the square of the next-to-nearest neighbor hopping: Even weak next-to-nearest neighbor hopping in addition to nearest neighbor hopping leads to thermalization in a two-particle scattering quantum Boltzmann equation in one dimension. We also investigate the temperature dependence of the thermalization rates, which away from half filling become exponentially small for small temperature of the final thermalized distribution.

We demonstrate the existence of exceptional points of degeneracy (EPDs) of periodic eigenstates in non-Hermitian coupled chains of dipolar scatterers. Guided modes supported by these structures can exhibit an EPD in their dispersion diagram at which two or more Bloch eigenstates coalesce, in both their eigenvectors and eigenvalues. We show the emergence of a second-order modal EPD associated with the parity-time $\left(\mathcal{P}\mathcal{T}\right)$ symmetry condition, at which each particle pair in the double chain exhibits balanced gain and loss. Furthermore, we also demonstrate a fourth-order EPD occurring at the band edge. Such a degeneracy condition was previously referred to as a degenerate band edge in lossless anisotropic photonic crystals. Here, we rigorously show it under the occurrence of gain and loss balance for a discrete guiding system. We identify a more general regime of gain and loss balance showing that $\mathcal{P}\mathcal{T}$ symmetry is not necessary to attain EPDs. Moreover, we investigate the degree of detuning of the EPD when the geometrical symmetry or balanced condition is broken. Furthermore, we demonstrate a realistic implementation of the EPD in a coupled chain made of pairs of plasmonic nanospheres and active core-shell nanospheres at optical frequencies. These findings open avenues toward superior light localization and transport with application to high-$Q$ resonators utilized in sensors, filters, low-threshold switching and lasing.

We report on spin- and angular-resolved photoemission spectroscopy using a high-resolution imaging spin filter based on a large Ir(001) crystal enhancing the effective figure of merit for spin detection by a factor of over ${10}^3$ compared to standard single-channel detectors. Furthermore, we review the spin filter preparation and its lifetime. The spin filter efficiency is mapped on a broad range of scattering energies and azimuthal angles. Large spin filter efficiencies are observed for the spin component perpendicular as well as parallel to the scattering plane depending on the azimuthal orientation of the spin filter crystal. A spin rotator capable of manipulating the spin direction prior to detection complements the measurement of three observables, thus allowing for a derivation of all three components of the spin polarization vector in multichannel spin polarimetry. The experimental results nicely agree with spin-polarized low-energy electron diffraction calculations based on a fully relativistic multiple scattering method in the framework of spin-polarized density functional theory.

We report on the multicaloric response of the ${\mathrm{Fe}}_{49}{\mathrm{Rh}}_{51}$ alloy under the combined application of hydrostatic pressure and magnetic field. Experimental data are complemented by a mean field model that takes into account the interplay between structural and magnetic degrees of freedom. A large multicaloric strength has been found for this alloy, and it is shown that a suitable combination of pressure and magnetic field enables the sign of the entropy change to be reversed and thus the multicaloric effect can be tuned from conventional to inverse. It is also shown that an extended temperature window for the multicaloric effect can be achieved by taking advantage of the coupling between structure and magnetism which enables a cross response of the alloy under the application of different external fields. Mean field calculations remarkably reproduce experimental results.

The Fano factor stability diagram of a $C_{3v}$ symmetric triangular quantum dot is analyzed for increasing electron fillings $N$. At low filling, conventional Poissonian and sub-Poissonian behavior is found. At larger filling, $N\ge 2$, a breaking of the electron-hole symmetry is manifested in super-Poissonian noise with a peculiar bias voltage dependence of the Fano factor at Coulomb and interference blockade. An analysis of the Fano map unravels a nontrivial electron-bunching mechanism arising from the presence of degenerate many-body states combined with orbital interference and Coulomb interactions. An expression for the associated dark states is provided for generic $N$.

We study the effects of strong coupling between a superconductor and a semiconductor nanowire on the creation of the Majorana bound states, when the quasiparticle dwell time in the normal part of the nanowire is much shorter than the inverse superconducting gap. This “short-junction” limit is relevant for the recent experiments using the epitaxially grown aluminum characterized by a transparent interface with the semiconductor and a small superconducting gap. We find that the small superconducting gap does not have a strong detrimental effect on the Majorana properties. Specifically, both the critical magnetic field required for creating a topological phase and the size of the Majorana bound states are independent of the superconducting gap. The critical magnetic field scales with the wire cross section, while the relative importance of the orbital and Zeeman effects of the magnetic field is controlled by the material parameters only: $g$ factor, effective electron mass, and the semiconductor-superconductor interface transparency.

We performed x-ray diffraction and electrical resistivity measurement up to pressures of 5 GPa and the first-principles calculations utilizing experimental structural parameters to investigate the pressure-induced topological phase transition in BiTeBr having a noncentrosymmetric layered structure (space group $P3m1)$. The $P3m1$ structure remains stable up to pressures of 5 GPa; the ratio of lattice constants $c/a$ has a minimum at pressures of 2.5–3 GPa. In the same range, the temperature dependence of resistivity changes from metallic to semiconducting at 3 GPa and has a plateau region between 50 and 150 K in the semiconducting state. Meanwhile, the pressure variation of band structure shows that the bulk band-gap energy closes at 2.9 GPa and re-opens at higher pressures. Furthermore, according to the Wilson loop analysis, the topological nature of electronic states in noncentrosymmetric BiTeBr at 0 and 5 GPa are explicitly revealed to be trivial and nontrivial, respectively. These results strongly suggest that pressure-induced topological phase transition in BiTeBr occurs at the pressures of 2.9 GPa.

Electron paramagnetic resonance (EPR) signatures, more specifically the elements of the electronic $g$ tensor, are calculated within density functional theory for hydrogenated Si(111), Si(001), Si(113), Si(114), $\mathrm{Si}\left(11\overline{2}\right)$, and Si(110) surfaces. Thereby both perturbation theory and a more sophisticated Berry phase technique are applied. Specific defects on different surface orientations are shown to reproduce the resonances at $\overline{g}=2.0043$ and $\overline{g}=2.0052$ measured for hydrogenated microcrystalline silicon: The latter value is argued here to originate from regions with low hydrogen coverage; the resonance at $\overline{g}=2.0043$ is shown to appear in positions with dihydride environment, where an H atom is directly bound to the silicon dangling-bond atoms. A third group of EPR signals with considerably larger $\overline{g}$ values between 2.006 and 2.009 is predicted for highly symmetric dangling bonds resembling single dangling-bond defects in silicon bulk material. As the exact value depends strongly on local strain, this type of defect can explain a less intense signal with large $g$ strain observed in microcrystalline as well as in amorphous material.

We present an input-output formalism describing a tunnel junction strongly coupled to its electromagnetic environment. We exploit it in order to investigate the dynamics of the radiation being emitted and scattered by the junction. We find that the nonlinearity imprinted in the electronic transport by a properly designed environment generates strongly squeezed radiation. Our results show that the interaction between a quantum conductor and electromagnetic fields can be exploited as a resource to design simple sources of nonclassical radiation.