Polarized Raman scattering measurements on $\mathrm{Ir}{\mathrm{Te}}_2$ single crystals carried out over the 15–640 K temperature range, and across the structural phase transition, reveal different insights regarding the crystal symmetry. In the high temperature regime three Raman active modes are observed at all of the studied temperatures above the structural phase transition, rather than two as predicted by the factor group analysis for the assumed $P\overline{3}m1$ symmetry. This indicates that the actual symmetry of the high temperature phase is lower than previously thought. The observation of an additional $E_g$ mode at high temperature can be explained by doubling of the original trigonal unit cell along the $c$ axis and within the $P\overline{3}c1$ symmetry. In the low temperature regime (below 245 K) the other Raman modes appear as a consequence of the symmetry lowering phase transition and the corresponding increase of the primitive cell. All of the modes observed below the phase transition temperature can be assigned within the monoclinic crystal symmetry. The temperature dependence of the Raman active phonons in both phases is mainly driven by anharmonicity effects. The results call for reconsideration of the crystallographic phases of $\mathrm{Ir}{\mathrm{Te}}_2$.

We carried out a systematic study of magnetic order and magnetic interlayer coupling in Fe/(Ga,Mn)As bilayers using superconducting quantum interference device magnetometry, polarized neutron reflectometry, element-specific x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and x-ray specular reflectivity. Our results clearly show that Fe/(Ga,Mn)As bilayers are strongly exchange coupled at the interface. However, contrary to recent reports in the literature, we observe a ferromagnetic rather than antiferromagnetic coupling between the magnetic moments of the Mn ions and the Fe layer. It is interesting in this context that the surface region of the (Ga,Mn)As layer that is in direct contact with the Fe film displays a nearly identical coercivity to that of Fe (indicating perfect ferromagnetic coupling of that region), while the bulk of the (Ga,Mn)As layer, which is more weakly ferromagnetically coupled with Fe, shows a significantly smaller coercive field.

Employing the time differential perturbed angular distribution (TDPAD) technique, we have measured the local susceptibility (${\chi }_{\text{loc}}$) of isolated Fe probe atoms implanted in nanocrystalline $\alpha$-Ta as well as $\beta$-Ta host matrices. While Fe dopants in bulk Ta and $\beta$-Ta exhibit no magnetic response, the temperature dependence of ${\chi }_{\text{loc}}$ in nanocrystalline $\alpha$-Ta reveals the emergence of a local magnetic moment on Fe below a critical size of $\approx$8 nm. Using ab initio electronic structure calculations, we show that the moment formation in nanocrystalline $\alpha$-Ta occurs due to Stoner enhancement arising from an unusual size-dependent lattice expansion that results in a shift in the Fermi level.

We calculate the pair correlation function and the order parameter correlation function, which probe, respectively, the intrapair and interpair correlations of a Fermi gas with attractive interparticle interaction, in terms of a diagrammatic approach as a function of coupling throughout the BCS-Bose-Einstein condensation (BEC) crossover and of temperature, both in the superfluid and normal phase across the critical temperature $T_c$. Several physical quantities are obtained from this calculation, including the pair coherence and healing lengths, the Tan's contact, the crossover temperature $T^{*}$ below which interpair correlations begin to build up in the normal phase, and the signature for the disappearance of the underlying Fermi surface which tends to survive in spite of pairing correlations. A connection is also made with recent experimental data on the temperature dependence of the normal coherence length as extracted from the proximity effect measured in high-temperature (cuprate) superconductors.

We study a two-terminal electronic conductance through an AB${}_2$ ring which is an example of the family of itinerant geometrically frustrated electronic systems. These systems are characterized by the existence of localized states with nodes in the probability density. We show that such states lead to distinct features in the conductance. For zero magnetic flux, the localized states act as a filter of the zero frequency conductance peak, if the contact sites have hopping probability to sites which are not nodes of the localized states. For finite flux, and in a chosen orthonormal basis, the localized states have extensions ranging from two unit cells to the complete ring, except for very particular values of magnetic flux. The conductance exhibits a zero frequency peak with a dip which is a distinct fingerprint of the variable extension of these localized states.

We propose a comprehensive scheme for the efficient design of graded optical metasurfaces capable of rerouting the impinging energy at will in a flexible way. We show that carefully designed conjoined optical nanoelements may be used as basic building blocks to arbitrarily shape the reflected phase front, while providing large controllability and low loss over deeply subwavelength thicknesses. The metasurface elements are designed using transmission line concepts combined with nanocircuit theory, and they can be realized with conventional lithographic techniques. Based on these concepts, we put forward a fast and accurate analytical model to design the optical analog of reflectarrays for light bending, steering, and focusing. The proposed designs show large efficiency over a broad angular spectrum, accompanied by broad bandwidths of operation. Our technique may lead to significant advances in the field of planarized nanophotonics and light manipulation over a surface, with potential applications in light trapping and efficient photonic couplers.

We calculate the acoustic phonon-assisted exciton spin relaxation via spin-orbit coupling in single self-assembled ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{As/GaAs}$ quantum dots using an atomistic empirical pseudopotential method. We show that the transition rate from bright to dark exciton states is zero under Hartree-Fock approximation. The exciton spin relaxation time obtained from sophisticated configuration interaction calculations is approximately 15–55 $\mu$s in pure InAs/GaAs QDs and even longer in alloy dots. These results are more than three orders of magnitude longer than previous theoretical and experimental results (a few ns), but agree with more recent experiments which suggest that excitons have long spin-relaxation times ($>$1 $\mu$s).

The valley physics in the two-dimensional graphenelike system has been put forward recently, and one of the key issues is to produce valley currents. We propose in this work to realize the topological valley resonance effect in graphene by using the time-dependent lattice vibration of optical phonon modes, which can pump out a noiseless and quantized valley current flowing into graphene leads. This optimal topological valley pump is protected by a nonzero valley Chern number and is robust against perturbations that could even break the time-reversal symmetry and valley conservation. The electrical measurement of the pumped valley currents is also discussed.

Nanogranular metal composites, consisting of immiscible metallic and insulating phases deposited on a substrate, are characterized by two distinct electronic transport regimes depending on the relative amount of the metallic phase. At sufficiently large metallic loadings, granular metals behave as percolating systems with a well-defined critical concentration above which macroscopic clusters of physically connected conductive particles span the entire sample. Below the critical loading, granular metal films are in the dielectric regime, where current can flow throughout the composite only via hopping or tunneling processes between isolated nanosized particles or clusters. In this case transport is intrinsically nonpercolative in the sense that no critical concentration can be identified for the onset of transport. It is shown here that, although being very different in nature, these two regimes can be described by treating percolation and hopping on equal footing. By considering general features of the microstructure and of the electrical connectedness, the concentration dependence of the dc conductivity of several nanogranular metal films is reproduced to high accuracy within an effective-medium approach. In particular, fits to published experimental data enable us to extract the values of microscopic parameters that govern the percolation and tunneling regimes, explaining thus the transport properties observed in nanogranular metal films.

We analyze the heat transport in an ion chain that is confined in a strongly anisotropic Paul trap. To drive a heat current across the chain different pairs of counterpropagating laser beams are applied to the ions on the edges. The lasers behave as heat reservoirs operating at different temperatures, and a nonequilibrium heat flow can be sustained. The control of the spatial distribution of the ions in the chain by variation of the trapping frequencies makes ion chains an ideal testbed to study heat transport properties in finite open systems of low dimensionality with tunable nonlinearities. We explore heat transport across a structural phase transition between the linear and zigzag configurations, identifying the condition for optimal heat transport.

The magnetic-domain resonance was studied for varying closure-domain structures in ${\mathrm{Co}}_{40}$${\mathrm{Fe}}_{40}$${\mathrm{B}}_{20}$ thin-film elements. A domain-shape anisotropy model was introduced, by which a significant increase of the acoustic-domain resonance frequency for decreasing domain width is predicted. We show that magnetic flux closure further increases the domain-width effect on the acoustic-domain resonance frequency. Furthermore, the domain resonance frequency is very sensitive to the effective width of the magnetic-domain walls due to dynamic dipolar interaction. The latter is quantified by calculating the dynamic wall width dependent dipolar fields. Domain-wall transitions in external magnetic fields manifest in either smooth or sudden resonance frequency changes, depending on the involved domain-wall types. Quantitative agreement between experimental and modeled data is obtained. Measuring the acoustic-domain resonance in small excitation fields therefore displays a method to analyze domain-wall instabilities and domain-wall interactions.

We analyze the effect of weak disorder on the competition between antiferromagnetic order and superconductivity in a model for iron-based superconductors. Under the assumption of an approximate particle-hole symmetry, we show that conventional $s^{++}$ superconductivity cannot be realized in the case of coexisting magnetic and superconductive orders, observed experimentally at intermediate doping levels. This result holds for arbitrary impurity concentrations, and, in particular, in the clean limit. The inclusion of disorder further amplifies the phase competition between itinerant antiferromagnetism and conventional superconductivity. In addition, we analyze the effect of disorder on the characteristic length scales of the two order parameters, and find that in a disordered sample the staggered moment fluctuates on shorter scales than the superconductive order parameter, even if both length scales are the same in the clean limit.

We report on the temperature dependence of the recently discovered spin Hall magnetoresistance in a yttrium iron garnet (YIG)/platinum (Pt) thin film. The YIG/Pt layers are an ideal choice as the combination of an insulating magnetic material and the high spin-orbit interaction in Pt gives a relatively large magnetoresistance and no electrical conduction occurs in the YIG. The temperature dependence of the magnetoresistance was measured between 1.4 K and 280 K from which the temperature dependence of the spin diffusion length in Pt has been extracted. We found that the best agreement between our data and the recently published [Chen et al., Phys. Rev. B 87, 144411 (2013)] theory of the spin Hall magnetoresistance is given by an assumed Elliot-Yafet mechanism of spin relaxation with temperature-independent spin Hall angle and spin mixing conductance. The best estimate for the spin diffusion length returns values between 0.57 and 3.85 nm.

A nonlinear thermodynamic formalism based on Ginzburg-Landau-Devonshire theory is developed to describe the total free energy density in (001)-oriented, compositionally graded, and monodomain ferroelectric films including the relative contributions and importance of flexoelectric, gradient, and depolarization energy terms. The effects of these energies on the evolution of the spontaneous polarization, dielectric permittivity, and the pyroelectric coefficient as a function of position throughout the film thickness, temperature, and epitaxial strain state are explored. In general, the presence of a compositional gradient and the three energy terms tend to stabilize a polar, ferroelectric state even in compositions that should be paraelectric in the bulk. Flexoelectric effects produce large built-in fields which diminish the temperature dependence of the polarization and susceptibilities. Gradient energy terms, here used to describe short-scale correlation between dipoles, have minimal impact on the polarization and susceptibilities. Finally, depolarization energy significantly impacts the temperature and strain dependence, as well as the magnitude, of the susceptibilities. This approach provides guidance on how to more accurately model compositionally graded films and presents experimental approaches that could enable differentiation and determination of the constitutive coefficients of interest.

Structural excitations of model Lennard-Jones glass systems are investigated using the activation-relaxation technique (ART$n$), which explores the potential energy landscape around a local minimum energy configuration by converging to a nearby saddle-point configuration. Performing ART$n$ results in a distribution of barrier energies that is single-peaked for well-relaxed samples. The present work characterizes such atomic-scale excitations in terms of their local structure and environment. It is found that, at zero applied stress, many of the identified events consist of chainlike excitations that can either be extended or ringlike in their geometry. The location and barrier energy of these saddle-point structures are found to correlate with the type of atom involved, and with spatial regions that have low Kelvin eigenshear moduli and are close to the excess free volume within the configuration. Such correlations are, however, weak and more generally the identified local structural excitations are seen to exist throughout the model glass sample. The work concludes with a discussion within the framework of $\alpha$ and $\beta$ relaxation processes that are known to occur in the undercooled liquid regime.

Ultrathin metallic ferromagnets on substrates with strong spin-orbit coupling can exhibit induced chiral interactions of the Dzyaloshinskii-Moriya (DM) form. For systems with perpendicular anisotropy, the presence of DM interactions has important consequences for current-driven domain-wall motion and underpins possible spintronic applications involving skyrmions. We show theoretically how spin textures driven by the DM interaction allow nonreciprocal channeling of spin waves, leading to measurable features in magnetic wires, dots, and domain walls. Our results provide methods for detecting induced DM interactions in metallic multilayers and controlling spin-wave propagation in ultrathin nanostructures.

We develop a theory of a direct, continuous quantum phase transition between a bosonic Laughlin fractional quantum Hall state and a superfluid, generalizing the Mott insulator to superfluid phase diagram of bosons to allow for the breaking of time-reversal symmetry. The direct transition can be protected by a spatial symmetry, and the critical theory is a pair of Dirac fermion fields coupled to an emergent Chern-Simons gauge field. The transition may be achieved in optical traps of ultracold atoms by starting with a $\nu =1/2$ bosonic Laughlin state and tuning an appropriate periodic potential to change the topology of the composite fermion band structure.

Self-heating effects are investigated in ultrascaled gate-all-around silicon nanowire field-effect transistors (NWFETs) using a full-band and atomistic quantum transport simulator where electron and phonon transport are fully coupled. The nonequilibrium Green's function formalism is used for that purpose, within a nearest-neighbor $s{p}^3{d}^5{s}^{*}$ tight-binding basis for electrons and a modified valence-force-field model for phonons. Electron-phonon and phonon-electron interactions are taken into account through specific scattering self-energies treated in the self-consistent Born approximation. The electron and phonon systems are driven out of equilibrium; energy is exchanged between them while the total energy current remains conserved. This gives rise to local variations of the lattice temperature and the formation of hot spots. The resulting self-heating effects strongly increase the electron-phonon scattering strength and lead to a significant reduction of the ON-current in the considered ultrascaled Si NWFET with a diameter of 3 nm and a length of 45 nm. At the same time, the lattice temperature exhibits a maximum close to the drain contact of the transistor.