The origin of the spin glass (SG) phase in the well-known multiferroic $\mathrm{Pb}\left(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}\right){\mathrm{O}}_3$ compound remains controversial due to the complications introduced by the coexistence of SG and long-range ordered (LRO) antiferromagnetic (AFM) phases. We have addressed this controversy through a comprehensive study on a Pb-free system $\mathrm{Ca}\left(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}\right){\mathrm{O}}_3$ (CFN) which does not exhibit LRO AFM transition. The SG transition in CFN is confirmed by the appearance of a cusp in the temperature dependence of dc magnetization $M\left(T\right)$ with a SG freezing temperature $T_{\mathrm{f}}\sim 25\mathrm{K}$, and bifurcation of the zero-field-cooled and field-cooled magnetization $M\left(T\right)$ below the irreversibility temperature $T_{\mathrm{irr}}\sim 27\mathrm{K}$. Using ac susceptibility $\left[\chi \left(\omega ,T\right)\right]$ measurements, we show that the spin dynamics follows power/Vogel-Fulcher law-type critical dynamics which diverges at $T_{\mathrm{SG}}\sim 24\mathrm{K}$ with an attempt time ${\tau }_{\mathrm{o}}\sim {10}^{-6}\mathrm{s}$ suggesting cluster spin glass (CSG) behavior. The field dependence of $T_{\mathrm{f}}\left(H\right)$ and $T_{\mathrm{irr}}\left(H\right)$ is shown to follow the de Almeida–Thouless line which separates the ergodic and nonergodic phases in the $H-T$ plane and gives $T_{\mathrm{f}}\left(H=0\right)\sim 25\mathrm{K}$, which is in close agreement with $T_{\mathrm{SG}}$ obtained from $\chi \left(\omega ,T\right)$. The existence of the glassy phase below $T_{\mathrm{SG}}$ is further confirmed by the observation of slow nonexponential decay of thermoremanent magnetization with time, memory and rejuvenation effects, and unidirectional exchange-bias effect in the $M-H$ hysteresis loop of field-cooled samples. The neutron powder-diffraction patterns reveal the absence of any magnetic peak due to LRO AFM phase but show a broad diffuse peak due to the presence of $\sim 2-\mathrm{nm}$-size AFM spin clusters which are responsible for the CSG freezing in CFN.

We compare three distinct computational approaches based on first-principles calculations within density functional theory to explore the magnetic exchange and the Dzyaloshinskii-Moriya interactions (DMI) of a Co monolayer on Pt(111), namely, (i) the method of infinitesimal rotations of magnetic moments based on the Korringa-Kohn-Rostoker (KKR) Green function method, (ii) the generalized Bloch theorem applied to spiraling magnetic structures and (iii) supercell calculations with noncollinear magnetic moments, the latter two being based on the full-potential linearized augmented plane wave (FLAPW) method. In particular, we show that the magnetic interaction parameters entering micromagnetic models describing the long-wavelength deviations from the ferromagnetic state might be different from those calculated for fast rotating magnetic structures, as they are obtained by using (necessarily rather small) supercell or large spin-spiral wave vectors. In the micromagnetic limit, which we motivate to use by an analysis of the Fourier components of the domain-wall profile, we obtain consistent results for the spin stiffness and DMI spiralization using methods (i) and (ii). The calculated spin stiffness and Curie temperature determined by subsequent Monte Carlo simulations are considerably higher than estimated from the bulk properties of Co, a consequence of a significantly increased nearest-neighbor exchange interaction in the Co monolayer $(+50\%)$. The calculated results are carefully compared with the literature.

There has been a revival of interest in localization phenomena in quasiperiodic systems with a view to examining how they differ fundamentally from such phenomena in random systems. Motivated by this, we study transport in the quasiperiodic, one-dimensional Aubry-Andre model and its generalizations to two and three dimensions. We study the conductance of open systems, connected to leads, as well as the Thouless conductance, which measures the response of a closed system to boundary perturbations. We find that these conductances show signatures of a metal-insulator transition from an insulator, with localized states, to a metal, with extended states having (a) ballistic transport (one dimension), (b) superdiffusive transport (two dimensions), or (c) diffusive transport (three dimensions); precisely at the transition, the system displays subdiffusive critical states. We calculate the $\beta$ function $\beta \left(g\right)=dln\left(g\right)/dln\left(L\right)$ and show that, in one and two dimensions, single-parameter scaling is unable to describe the transition. Furthermore, the conductances show strong nonmonotonic variations with $L$ and an intricate structure of resonant peaks and subpeaks. In one dimension the positions of these peaks can be related precisely to the properties of the number that characterizes the quasiperiodicity of the potential; and the $L$ dependence of the Thouless conductance is multifractal. We find that, as dimension increases, this nonmonotonic dependence of $g$ on $L$ decreases and, in three dimensions, our results for $\beta \left(g\right)$ are reasonably well approximated by single-parameter scaling.

Recent studies confirmed that the long-searched-for phonon-mediated superconducting phase in graphene may be induced via lithium deposition. However, it is also suggested that lithium-decorated graphene (${\mathrm{LiC}}_6$) behaves as a canonical superconductor, despite the moderate electron-phonon coupling constant and low Fermi energy in this material. Herein, this issue is addressed within the isotropic Eliashberg formalism in the adiabatic and nonadiabatic regimes to account for the potentially pivotal effects hitherto not captured. The conducted analysis indicates signatures of the nonadiabatic superconductivity in ${\mathrm{LiC}}_6$ and the resulting effects on the selected thermodynamic properties. It is shown that although the critical temperature obtained in the nonadiabatic regime is similar to the earlier estimates ($\sim 6$ K), its value corresponds to the smaller magnitude of the Coulomb pseudopotential than previously expected. Moreover, the characteristic ratio of the pairing gap and the inverse temperature is found to notably exceed estimates of the BCS theory. In what follows, the obtained results attempt to present coherent interpretation of the superconducting phase in ${\mathrm{LiC}}_6$ and vital insight into its fundamental properties. Moreover, they allow us to draw potential directions for future research on low-dimensional superconductivity by reveling that many other graphene-related superconductors may be subject to nonadiabatic effects. This aspect appears to be essential for the design of new two-dimensional hexagonal superconductors aimed at relatively high critical temperature values.

Topological quantum field theory (TQFT) is a very powerful theoretical tool to study topological phases and phase transitions. In $2+1d$, it is well known that the Chern-Simons theory captures all the universal topological data of topological phases, e.g., quasiparticle braiding statistics, chiral central charge, and even provides us a deep insight for the nature of topological phase transitions. Recently, topological phases of quantum matter are also intensively studied in $3+1d$ and it has been shown that looplike excitation obeys the so-called three-loop-braiding statistics. In this paper, we will try to establish a TQFT framework to understand the quantum statistics of particle and looplike excitation in $3+1d$. We will focus on Abelian topological phases for simplicity, however, the general framework developed here is not limited to Abelian topological phases.

The generalized statistical dynamical theory of x-ray scattering by imperfect single crystals with randomly distributed Coulomb-type defects has been extended to characterize structure imperfections in the real multilayers of arbitrary thickness in Bragg diffraction geometry. The recurrence relations for the coherent amplitude reflection and transmission coefficients of such multilayers, which consist of any number of layers with constant strain and randomly distributed defects in each one, have been derived within the concept of the dynamical wave field, i.e., the so-called Ewald-Bethe-Laue approach, with rigorous accounting for boundary conditions at layer interfaces. The analytical expression for the differential dynamical diffuse component of the reflection coefficient of an imperfect multilayer system has been obtained as well. This expression establishes direct connection between the distribution of the diffuse scattering intensity in a momentum space and statistical characteristics of defects in each layer. In addition, the integrals from differential diffuse scattering intensity over the Ewald sphere and over vertical divergence have been found, which correspond to diffuse components in measurements of rocking curves and reciprocal space maps, respectively. The developed theory provides the dynamical description for the one- and two-dimensional angular distributions of the mutually consistent coherent and diffuse components of x-ray scattering intensities, which are measured by the high-resolution double- and triple-crystal diffractometers, respectively, from imperfect films, multilayer structures, superlattices, etc. Examples of simulated rocking curves for the imperfect superlattice with defects of several types and reciprocal space map for the ion-implanted sample of yttrium iron garnet film with defects are given and discussed.

Hybrid semiconductor/superconductor devices constitute an important platform for a wide range of applications, from quantum computing to topological-state-based architectures. Here, we demonstrate full modulation of the interference pattern in a superconducting interference device with two parallel islands of ballistic InAs quantum wells separated by a trench, by acting independently on two side gates. This so far unexplored geometry enables us to tune the device with high precision from a SQUID-like to a Fraunhofer-type behavior simply by electrostatic gating, without the need for an additional in-plane magnetic field. These measurements are successfully analyzed within a theoretical model of an extended tunnel Josephson junction, taking into account the focusing factor of the setup. The impact of these results on the design of novel devices is discussed.

We investigated the electronic structure of monolayer ${\mathrm{VTe}}_2$ grown on bilayer graphene by angle-resolved photoemission spectroscopy (ARPES). We found that monolayer ${\mathrm{VTe}}_2$ takes the octahedral $1T$ structure in contrast to the monoclinic one in the bulk, as evidenced by the good agreement in the Fermi surface topology between ARPES results and first-principles band calculations for octahedral monolayer $1T\text{−}{\mathrm{VTe}}_2$. We have revealed that monolayer $1T\text{−}{\mathrm{VTe}}_2$ at low temperatures is characterized by a metallic state whereas the nesting condition is better than that of isostructural monolayer ${\mathrm{VSe}}_2$ which undergoes a charge density wave (CDW) transition to an insulator at low temperatures. The present result suggests an importance of the Fermi surface topology for characterizing the CDW properties of monolayer transition-metal dichalcogenides.

${\mathrm{TiPO}}_4$ shows interesting structural and magnetic properties as temperature and pressure are varied, such as a spin-Peierls phase transition and the development of incommensurate modulations of the lattice. Recently, high-pressure experiments for ${\mathrm{TiPO}}_4$ reported two structural phases appearing at high pressures, the so-called phases IV and V [M. Bykov et al., Angew. Chem. Int. Ed. 55, 15053 (2016).]. The latter was shown to include the first example of fivefold O-coordinated P atoms in an inorganic phosphate compound. In this work, we characterize the electronic structure and other physical properties of these phases by means of ab initio calculations and investigate the structural transition. We find that the appearance of phases IV and V coincides with a collapse of the Mott insulating gap and quenching of magnetism in phase III as pressure is applied. Remarkably, our calculations show that in the high-pressure phase V, these features reappear, leading to an antiferromagnetic Mott insulating phase, with robust local moments.

We investigate a mesoscopic refrigerator based on chiral quantum Hall edge channels. We discuss a three-terminal cooling device in which charge transport occurs only between a pair of voltage-biased terminals. The third terminal, which is to be cooled, is set as a voltage probe with vanishing particle flux. This largely prevents the generation of direct Joule heating, which ensures a high coefficient of performance. Cooling operation is based on energy-dependent quantum transmissions. The latter are implemented with the aid of two tunable scattering resonances (quantum dots). To find the optimal performance point and the largest temperature difference created with our refrigerator, it is crucial to address the nonlinear regime of transport, accounting for electron-electron interaction effects. Our numerical simulations show that the maximal cooling power can be tuned with the quantum dot couplings and energy levels. Further, we provide analytical expressions within a weakly nonlinear scattering-matrix formalism which allow us to discuss the conditions for optimal cooling in terms of generalized thermopowers. Our results are important for the assessment of chiral conductors as promising candidates for efficient quantum refrigerators with low dissipation.

We model the change in the linear optical functions due to the propagation of a strain wave through a crystalline slab. Defects in the slab are introduced by arbitrarily displacing one of the atomic layers. The perturbing wave is assumed to be produced by laser light absorption; the properties of the propagating wave are explicitly related to the laser and material properties. We apply this model to a Si(111)($1\times 1$):H slab and find that the reflectance varies significantly at photon energies above 2.5 eV and that the disordered slab produces significant changes in the reflectance when compared to the unperturbed slab. We then induce a defect layer in the slab by uniformly translating an atomic layer. As the strain wave passes through the material, the behavior of the calculated reflectance difference spectrum changes according to the defect layer, allowing us to discern the depth of the defect.

Light-matter interactions in conventional nanophotonic structures typically lack directionality. For example, differently from microwave antenna systems, most optical emitters (e.g., excited atoms/molecules and simple nanoantennas) exhibit quasi-isotropic dipolar radiation patterns with low directivity. Furthermore, surface waves supported by conventional material substrates do not usually have a preferential direction of propagation, and their wavefront tends to spread as it propagates along the surface, unless the surface or the excitation is properly engineered and structured. In this article, we theoretically demonstrate the possibility of realizing unidirectional and diffractionless surface plasmon polariton modes on a nonreciprocal platform, namely, a gyrotropic magnetized plasma. Based on a rigorous Green's function approach, we provide a comprehensive and systematic analysis of all the available physical mechanisms that may bestow the system with directionality, both in the sense of one-way excitation of surface waves and in the sense of directive diffractionless propagation along the surface. The considered mechanisms include (i) the effect of strong and weak forms of nonreciprocity, (ii) the elliptic-like or hyperbolic-like topology of the modal dispersion surfaces, and (iii) the source polarization state, with the associated possibility of chiral surface-wave excitation governed by angular-momentum matching. We find that three-dimensional gyrotropic plasmonic platforms support a previously unnoticed wave-propagation regime that exhibit several of these physical mechanisms simultaneously, allowing us to theoretically demonstrate unidirectional surface plasmon polariton modes that propagate as a single ultranarrow diffractionless beam. We also assess the impact of dissipation and nonlocal effects. Our theoretical findings may enable a new generation of plasmonic structures and devices with highly directional response.

The electronic Hong-Ou-Mandel interferometer in the integer quantum Hall regime is an ideal system to probe the building up of quantum correlations between charge carriers and it has been proposed as a viable platform for quantum computing gates. Using a parallel implementation of the split-step Fourier method, we simulated the antibunching of two interacting fermionic wave packets impinging on a quantum point contact. Numerical results of the exact approach are compared with a simplified theoretical model based on one-dimensional scattering formalism. We show that, for strongly localized wave packets in a full-scale geometry, the Coulomb repulsion dominates over the exchange energy, this effect being strongly dependent on the energy broadening of the particles. We define analytically the spatial entanglement between the two regions of the quantum point contact, and obtain quantitatively its entanglement-generation capabilities.

The behavior of silicon carbide (SiC) under shock compression is of interest due to its applications as a high-strength ceramic and for general understanding of shock-induced polymorphism. Here we use the Matter in Extreme Conditions beamline of the Linac Coherent Light Source to carry out a series of time-resolved pump-probe x-ray diffraction measurements on SiC laser-shocked to as high as 206 GPa. Experiments on single crystals and polycrystals of different polytypes show a transformation from a low-pressure tetrahedral phase to the high-pressure rocksalt-type (B1) structure. We directly observe coexistence of the low- and high-pressure phases in a mixed-phase region and complete transformation to the B1 phase above 200 GPa. The densities measured by x-ray diffraction are in agreement with both continuum gas-gun studies and a theoretical B1 Hugoniot derived from static-compression data. Time-resolved measurements during shock loading and release reveal a large hysteresis upon unloading, with the B1 phase retained to as low as 5 GPa. The sample eventually reverts to a mixture of polytypes of the low-pressure phase at late times. Our study demonstrates that x-ray diffraction is an effective means to characterize the time-dependent structural response of materials undergoing shock-induced phase transformations at megabar pressures.

We present Raman spectroscopy measurements of the van der Waals bonded ferromagnet ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$, together with lattice dynamics. Four out of eight Raman active modes are observed and assigned, in agreement with numerical calculations. The energies and linewidths of the observed modes display an unconventional temperature dependence at about 150 and 220 K, followed by the nonmonotonic evolution of the Raman continuum. Whereas the former can be related to the magnetic phase transition, the origin of the latter anomaly remains an open question.

Inelastic scattering of photons is a promising technique to manipulate magnons but it suffers from weak intrinsic coupling. We theoretically discuss an idea to increase optomagnonic coupling in optical whispering gallery mode cavities by generalizing a previous analysis to include the exchange interaction. We predict that the optomagnonic coupling constant to surface magnons in yttrium iron garnet (YIG) spheres with radius $300\mu \mathrm{m}$ can be up to 40 times larger than that to the macrospin Kittel mode. Whereas this enhancement falls short of the requirements for magnon manipulation in pure YIG, nanostructuring and/or materials with larger magneto-optical constants can bridge this gap.

We present theoretically the thermal Hall effect of magnons in a ferromagnetic lattice with a Kekule-O coupling (KOC) modulation and a Dzyaloshinskii-Moriya interaction (DMI). Through a strain-based mechanism for inducing the KOC modulation, we identify four topological phases in terms of the KOC parameter and DMI strength. We calculate the thermal magnon Hall conductivity ${\kappa }^{xy}$ at low temperature in each of these phases. We predict an unconventional conductivity due to a nonzero Berry curvature emerging from band proximity effects in the topologically trivial phase. We find sign changes of ${\kappa }^{xy}$ as a function of the model parameters, associated with the local Berry curvature and occupation probability of the bulk bands. Throughout, ${\kappa }^{xy}$ can be easily tuned with external parameters such as the magnetic field and temperature.

We consider a thin film of superfluid ${}^3\mathrm{He}$ under conditions that stabilize the $A$ phase. We show that in the presence of a uniform superflow and an external magnetic field perpendicular to the film, the spin degrees of freedom develop a nonuniform, helical texture. Our prediction is robust and relies solely on Galilei invariance and other symmetries of ${}^3\mathrm{He}$, which induce a coupling of the orbital and spin degrees of freedom. The length scale of the helical order can be tuned by varying the velocity of the superflow and the magnetic field and may be in reach of near-future experiments.