Conclusive evidence of order by disorder is scarce in real materials. Perhaps one of the strongest cases presented has been for the pyrochlore XY antiferromagnet ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, with the ground state selection proceeding by order by disorder induced through the effects of quantum fluctuations. This identification assumes the smallness of the effect of virtual crystal field fluctuations that could provide an alternative route to picking the ground state. Here we show that this order by virtual crystal field fluctuations is not only significant, but competitive with the effects of quantum fluctuations. Further, we argue that higher-multipolar interactions that are generically present in rare-earth magnets can dramatically enhance this effect. From a simplified bilinear-biquadratic model of these multipolar interactions, we show how the virtual crystal field fluctuations manifest in ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ using a combination of strong-coupling perturbation theory and the random-phase approximation. We find that the experimentally observed ${\psi }_2$ state is indeed selected and the experimentally measured excitation gap can be reproduced when the bilinear and biquadratic couplings are comparable while maintaining agreement with the entire experimental spin-wave excitation spectrum. Finally, we comment on possible tests of this scenario and discuss implications for other order-by-disorder candidates in rare-earth magnets.

Spin-spin correlations and microscopic origin of net magnetization in the spin-1 trimer chain compound ${\mathrm{CaNi}}_3{\mathrm{P}}_4{\mathrm{O}}_{14}$ have been investigated by powder neutron diffraction. The present study reveals a three-dimensional long-range magnetic ordering below 16 K where the magnetic structure consists of ferromagnetic trimers that are coupled ferromagnetically along the spin-chain direction $(b$ axis). The moment components along the $a$ and $c$ axes arrange antiferromagnetically. Our study establishes that the uncompensated moment components along the $b$ axis $\left(m_b\right)$ result in a net magnetization per unit cell. The magnetic structure, determined in the present study, is in agreement with the results of recent first-principles calculation; however, it is in contrast to a fascinating experimental prediction of ferrimagnetic ordering based on the periodicity of the exchange interactions in ${\mathrm{CaNi}}_3{\mathrm{P}}_4{\mathrm{O}}_{14}$. Our study also confirms the presence of broad diffuse magnetic scattering, due to one-dimensional short-range spin-spin correlations, over a wide temperature range below $\sim 50$ K down to a temperature well below the $T_c$. Total neutron scattering analysis by the reverse Monte Carlo (RMC) method reveals that the dominating spin-spin correlation above $T_c$ is ferromagnetic and along the $b$ axis. The nearest-neighbor spin-spin correlations along the $a$ and $c$ axes are found to be weakly antiferromagnetic. The nature of the trimer spin structure of the short-range ordered state (above $T_c)$ is similar to that of the 3D long-range ordered state (below $T_c)$. The present investigation of microscopic nature of the magnetic ground state also explains the condition required for the 1/3 magnetization plateau to be observed in the trimer spin chains. In spite of the $S=1$ trimer chain system, the present compound ${\mathrm{CaNi}}_3{\mathrm{P}}_4{\mathrm{O}}_{14}$ is found to be a good realization of a three-dimensional magnet below $T_c=16$ K with full ordered moment values of $\sim 2$ ${\mu }_B/{\mathrm{Ni}}^{2+}$ (1.98 and 1.$96{\mu }_B/{\mathrm{Ni}}^{2+}$ for two Ni sites, respectively) at 1.5 K.

Various Fe-vacancy orders have been reported in tetragonal $\beta -\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{Se}$ single crystals and nanowires/nanosheets, which are similar to those found in alkali-metal intercalated ${\mathrm{A}}_{1-x}\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$ superconductors. Here we report the in situ angle-resolved photoemission spectroscopy study of Fe-vacancy disordered and ordered phases in FeSe multilayer thin films grown by molecular beam epitaxy. Low-temperature annealed FeSe films are identified to be Fe-vacancy disordered phase and electron doped. A further long-time low-temperature anneal can change the Fe-vacancy disordered phase to the ordered phase, which is found to be semiconductor/insulator with $\sqrt{5}\times \sqrt{5}$ superstructure and can be reversely changed to the disordered phase with a high-temperature anneal. Our results reveal that the disorder-order transition in FeSe thin films can be simply tuned by a vacuum anneal and the FeSe thin films provide rich information to understand the complex electronic structure and superconductivity of FeSe.

We develop a quantum field theory for parametrically pumped polaritons using Keldysh Green's function techniques with which the occupations of the excitation spectra can be calculated. By considering the mean field and Gaussian fluctuations, we find that the highly nonequilibrium phase transition to the optical parametric oscillator regime is in some ways similar to equilibrium condensation. In particular, we show that this phase transition can be associated with an effective chemical potential, at which the system's bosonic distribution function diverges, and an effective temperature for low energy modes. As in equilibrium systems, the transition is achieved by tuning this effective chemical potential to the energy of the lowest normal mode. Since the nonequilibrium occupations of the modes are available, we determine experimentally observable properties such as the luminescence and absorption spectra.

The dynamics of spin-lattice relaxation in the magnetic ${\mathrm{Mn}}^{2+}$ ion system of (Zn,Mn)Se/(Zn,Be)Se quantum-well structures are studied using optical methods. Pronounced cusps are found in the giant Zeeman shift of the quantum-well exciton photoluminescence at specific magnetic fields below 10 T, when the Mn spin system is heated by photogenerated carriers. The spin-lattice relaxation time of the Mn ions is resonantly accelerated at the cusp magnetic fields. Our theoretical analysis demonstrates that a cusp occurs at a spin-level mixing of single ${\mathrm{Mn}}^{2+}$ ions and a quick-relaxing cluster of nearest-neighbor Mn ions, which can be described as intrinsic cross-relaxation resonance within the Mn spin system.

Topological classification in our previous paper [K. Shiozaki and M. Sato, Phys. Rev. B 90, 165114 (2014)] is extended to nonsymmorphic crystalline insulators and superconductors. Using the twisted equivariant $K$ theory, we complete the classification of topological crystalline insulators and superconductors in the presence of additional order-two nonsymmorphic space-group symmetries. The order-two nonsymmorphic space groups include half-lattice translation with ${\mathit{Z}}_2$ flip, glide, twofold screw, and their magnetic space groups. We find that the topological periodic table shows modulo-2 periodicity in the number of flipped coordinates under the order-two nonsymmorphic space group. It is pointed out that the nonsymmorphic space groups allow ${\mathbb{Z}}_2$ topological phases even in the absence of time-reversal and/or particle-hole symmetries. Furthermore, the coexistence of the nonsymmorphic space group with time-reversal and/or particle-hole symmetries provides novel ${\mathbb{Z}}_4$ topological phases, which have not been realized in ordinary topological insulators and superconductors. We present model Hamiltonians of these new topological phases and analytic expressions of the ${\mathbb{Z}}_2$ and ${\mathbb{Z}}_4$ topological invariants. The half-lattice translation with ${\mathit{Z}}_2$ spin flip and glide symmetry are compatible with the existence of boundaries, leading to topological surface gapless modes protected by the order-two nonsymmorphic symmetries. We also discuss unique features of these gapless surface modes.

We investigate the dynamics of a quantum emitter (quantum dot) placed in the vicinity of a flat metal surface. The dynamics is induced by the coupling between the emitter and surface plasmon-polaritons. The plasmon-polariton modes are described within a continuous media model with a nonlocal Lindhard-type dielectric response of the metal. The analytic solution of the dynamical equations is obtained in the rotating wave approximation. The results demonstrate a considerable influence of the nonlocality of the electromagnetic response and the Landau damping in the metal. In particular, the relaxation dynamics is characterized by two distinct times that may differ by large amounts as a consequence of the nonlocality of the response. It is also shown that one of the contributions to the relaxation can have a power-law long-time asymptote, leading to notable changes in the dynamical pattern.

Driven tunneling between graphene Landau levels is theoretically linked to the process of pair creation from vacuum, a prediction of quantum electrodynamics (QED). Landau levels are created by the presence of a strong, constant, quantizing magnetic field perpendicular to a graphene monolayer. Following the formal analogy between QED and the description of low-energy excitations in graphene, solutions of the fully interacting Dirac equation are used to compute electron-hole pair creation driven by a circularly or linearly polarized field. This is achieved via the coupled channel method, a numerical scheme for the solution of the time-dependent Dirac equation in the presence of bound states. The case of a monochromatic driving field is first considered, followed by the more realistic case of a pulsed excitation. We show that the pulse duration yields an experimental control parameter over the maximal pair yield. Orders of magnitude of the pair yield are given for experimentally achievable magnetic fields and laser intensities weak enough to preserve the Landau level structure.

This is part II in a series of two papers that introduce a general expression for the tracer diffusivity in complex, periodic energy landscapes with $M$ distinct hop rates in one-, two-, and three-dimensional diluted systems (low coverage, single-tracer limit). While Part I [Gosálvez et al., Phys. Rev. B 93, 075429 (2016)] focuses on the analysis of diffusion in systems where the end sites of the hops are located symmetrically with respect to the hop origins (symmetric hops), as encountered in many ideal surfaces and bulk materials, this report (Part II) presents a more general approach to determining the tracer diffusivity in systems where the end sites can be located asymmetrically with respect to the hop origins (asymmetric hops), as observed in reconstructed and/or chemically modified surfaces and/or bulk materials. The obtained diffusivity formulas for numerous systems are validated against kinetic Monte Carlo simulations and previously reported analytical expressions based on the continuous-time random walk (CTRW) method. The proposed method corrects some of the CTRW formulas and provides new expressions for difficult cases that have not been solved earlier. This demonstrates the ability of the proposed formalism to describe tracer diffusion.

We present an extensive study on the structure of oxygen adsorbates on Mo(112) by means of grazing scattering of fast hydrogen and helium atoms and low-energy electron diffraction. For projectile energies less than 2 keV, fast atom diffraction provides information on the surface unit cell and on adsorption sites for low coverages. In the classical scattering regime, we employed so-called triangulation techniques where for an azimuthal rotation of the target axial surface channels are identified. From comparison with computer simulation positions of surface atoms can be derived. Aside from the detection scheme of projectile-induced electron emission, we present details for a new variant of triangulation based on the detection of angular distributions of scattered particles. The different sensitivity of the methods to the topmost surface layers allows us to efficiently set up structural models for four adsorbate phases for which contradicting models exist in literature. The $c(4\times 2)$ phase is revealed to be one step in the formation of a missing-row reconstruction with $p(1\times 2)$ unit cell. Our studies demonstrate the potential of grazing scattering of fast atoms for quantitative structure analysis.

Limit-periodic structures are well ordered but nonperiodic, and hence have nontrivial vibrational modes. We study a ball and spring model with a limit-periodic pattern of spring stiffnesses and identify a set of extended modes with arbitrarily low participation ratios, a situation that appears to be unique to limit-periodic systems. The balls that oscillate with large amplitude in these modes live on periodic nets with arbitrarily large lattice constants. By studying periodic approximants to the limit-periodic structure, we present numerical evidence for the existence of such modes, and we give a heuristic explanation of their structure.

Τhe magnetoelectric $\mathrm{ZnC}{\mathrm{r}}_2\mathrm{S}{\mathrm{e}}_4$ spinel, with space group $Fd\overline{3}m$, undergoes a reversible first-order structural transition initiating at 17 GPa, as revealed by our high-pressure x-ray diffraction studies at room temperature. We tentatively assign the high-pressure modification to an $A\mathrm{M}{\mathrm{o}}_2{\mathrm{S}}_4$-type phase, a distorted variant of the monoclinic $\mathrm{C}{\mathrm{r}}_3{\mathrm{S}}_4$ structure. Furthermore, our Raman investigation provides evidence for a pressure-induced insulator-metal transition. Our density functional theory calculations successfully reproduce the structural transition. They indicate significant band gap and magnetic moment reduction accompanying the pressure-induced structural modification. We discuss our findings in conjunction with the available high-pressure results on other Cr-based chalcogenide spinels.

In this work, we discuss the mixing thermodynamics of cubic (B1) ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_x\mathrm{N}/\mathrm{TiN}\left(001\right)$ multilayers. We show that interfacial effects suppress the mixing enthalpy compared to bulk ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_x\mathrm{N}$. The strongest stabilization occurs for compositions in which the mixing enthalpy of bulk ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_x\mathrm{N}$ has its maximum. The effect is split into a strain and an interfacial (or chemical) contribution, and we show that both contributions are significant. An analysis of the local atomic structure reveals that the Ti atoms located in the interfacial layers relax significantly different from those in the other atomic layers of the multilayer. Considering the electronic structure of the studied system, we demonstrate that the lower Ti-site projected density of states at ${\epsilon }_{\mathrm{F}}$ in the ${\mathrm{Ti}}_{1-x}{\mathrm{Al}}_x\mathrm{N}/\mathrm{TiN}$ multilayers compared to the corresponding monolithic bulk explains a decreased tendency toward decomposition.

We report on the study of magnetic properties of the ${\mathrm{La}}_{1.5}{\mathrm{Ca}}_{0.5}{\mathrm{CoIrO}}_6$ double perovskite. Via ac magnetic susceptibility we have observed evidence of weak ferromagnetism and reentrant spin glass behavior on an antiferromagnetic matrix. Regarding the magnetic behavior as a function of temperature, we have found that the material displays up to three inversions of its magnetization, depending on the appropriate choice of the applied magnetic field. At low temperature, the material exhibits exchange bias effect when it is cooled in the presence of a magnetic field. Also, our results indicate that this effect may be observed even when the system is cooled at zero field. Supported by other measurements and also by electronic structure calculations, we discuss the magnetic reversals and spontaneous exchange bias effect in terms of magnetic phase separation and magnetic frustration of ${\mathrm{Ir}}^{4+}$ ions located between the antiferromagnetically coupled Co ions.

The origin of ferroelectric polarization in tetragonal tungsten-bronze- (TTB-) type oxide strontium barium niobate (SBN) is investigated using first-principles density functional calculations. We study in particular the relationship between the polarization and the cation and vacancy ordering on alkali-earth metal lattice sites. Lattice dynamical calculations for paraelectric structures demonstrate that all cation configurations that can be accommodated in a $1\times 1\times 2$ supercell result in a single unstable polar phonon, composed primarily of relative Nb-O displacements along the polar axis, as their dominant instability. The majority of the configurations also have a second octahedral tilt-mode instability which couples weakly to the polar mode. The existence of the tilt mode is strongly dependent on the local cation ordering, consistent with the fact that it is not found experimentally. Our results suggest that ferroelectricity in the SBN system is driven by a conventional second-order Jahn-Teller mechanism caused by the $d^0$ ${\mathrm{Nb}}^{5+}$ cations, and demonstrate the strong influence of the size of Sr and Ba on the lattice distortions associated with polarization and octahedral tilting. Finally, we suggest a mechanism for the relaxor behavior in Sr-rich SBN based on Sr displacement inside pentagonal channels in the TTB structure.

We consider the quantum conductance in a tight-binding chain with a locally applied potential which is oscillating in time. The steady state for such a driven impurity can be calculated exactly for any energy and applied potential using the Floquet formalism. The resulting transmission has a nontrivial, nonmonotonic behavior depending on incoming momentum, driving frequency, and the strength of the applied periodic potential. Hence there is an abundance of tuning possibilities, which allows finding the resonances of total reflection for any choice of incoming momentum and periodic potential. Remarkably, this implies that even for an arbitrarily small infinitesimal impurity potential it is always possible to find a resonance frequency at which there is a catastrophic breakdown of the transmission $T=0$. The points of zero transmission are closely related to the phenomenon of Fano resonances at dynamically created bound states in the continuum. The results are relevant for a variety of one-dimensional systems where local AC driving is possible, such as quantum nanodot arrays, ultracold gases in optical lattices, photonic crystals, or molecular electronics.

We report large magnetoresistance (MR) at low temperatures in single-crystalline nonmagnetic compounds ${\mathrm{TaAs}}_2$ and ${\mathrm{NbAs}}_2$. Both compounds exhibit parabolic-field-dependent MR larger than $5\times {10}^3$ in a magnetic field of 9 Tesla at 2 K. The MR starts to deviate from parabolic dependence above 10 T and intends to be saturated in 45 T for ${\mathrm{TaAs}}_2$ at 4.2 K. The Hall resistance measurements and band structure calculations reveal their compensated semimetal characteristics. Their large MR at low temperatures is ascribed to an effect for compensation of electrons and holes with large mobilities. After discussing the MR for different samples of ${\mathrm{TaAs}}_2$ and other semimetals, we found that the magnitudes of MR are strongly dependent on the samples' quality for different compounds.

The material ${\mathrm{USb}}_2$ is a correlated, moderately heavy-electron compound within the uranium dipnictide (${\mathrm{UX}}_2$) series. It is antiferromagnetic with a relatively high transition temperature $T_N=204\mathrm{K}$ and a large U-U separation. While the uranium atoms in the lighter dipnictides are considered to be localized, those of ${\mathrm{USb}}_2$ exhibit hybridization and itineracy, promoting uncertainty as to the continuity of the magnetic order within the ${\mathrm{UX}}_2$. We have explored the evolution of the magnetic order by employing magnetotransport measurements as a function of pressure and temperature. We find that the $T_N$ in ${\mathrm{USb}}_2$ is enhanced, moving towards that of its smaller sibling ${\mathrm{UAs}}_2$. But, long before reaching a $T_N$ as high as ${\mathrm{UAs}}_2$, the antiferromagnetism of ${\mathrm{USb}}_2$ is abruptly destroyed in favor of another magnetic ground state. We identify this pressure-induced ground state as being ferromagnetic based on the appearance of a strong anomalous Hall effect in the transverse resistance in magnetic field. With pressure, this emergent ferromagnetic state is suppressed and ultimately destroyed in favor of a non-Fermi-liquid ground state.

The effect of the circularly polarized laser field on quantum spin systems in the steady-state regime, in which relaxation plays the central role, has been studied. The dynamical mean-field-like theory predicts several general results for the behavior of the time-average magnetization caused by the laser field. The induced magnetization oscillates with the frequency of the laser field (while Rabi-like oscillations, which modulate the latter in the dynamical regime, are damped by the relaxation in the steady-state regime). At high frequencies, that magnetization is determined by the value to which the relaxation process is directed. At low frequencies the slope of that magnetization as a function of the frequency is determined by the strength of the laser field. The anisotropy determines the resonance behavior of the time-averaged magnetization in both the ferromagnetic and antiferromagnetic cases with nonzero magnetic anisotropy. Nonlinear effects (in the magnitude of the laser field) have been considered. The effect of the laser field on quantum spin systems is maximal in resonance, where the time-averaged magnetization, caused by the laser field, is changed essentially. Out of resonance the changes in the magnetization are relatively small. The resonance effect is caused by the nonzero magnetic anisotropy. The resonance frequency is small (proportional to the anisotropy value) for spin systems with ferromagnetic interactions and enhanced by exchange interactions in the spin systems with antiferromagnetic couplings. We show that it is worthwhile to study the laser-field-induced magnetization of quantum spin systems caused by the high-frequency laser field in the steady-state regime in “easy-axis” antiferromagnetic spin systems (e.g., in Ising-like antiferromagnetic spin-chain materials). The effects of the Dzyaloshinskii-Moriya interaction and the spin-frustration couplings (in the case of the zigzag spin chain) have been analyzed.

Quantum entanglement effects between the electronic spin and charge degrees of freedom are examined in an organic molecular solid, termed a dimer Mott-insulating system, in which molecular dimers are arranged in a crystal as fundamental units. A low energy effective model includes an antisymmetric exchange interaction, as one of the dominant magnetic interactions. This interaction favors a 90 deg spin configuration, and competes with the Heisenberg-type exchange interaction. Stabilities of the magnetic ordered phases are examined by using the spin-wave theory, as well as the Schwinger-boson theory. It is found that the spin-charge interaction promotes an instability of the long-range magnetic ordered state around a parameter region where two spin-spiral phases are merged. Implication for the quantum spin liquid state observed in $\kappa$-(BEDT-TTF)${}_2{\mathrm{Cu}}_2({\mathrm{CN})}_3$ is discussed.