We report on spinor polariton interactions in GaAs based microcavities. This investigation is carried out by means of heterodyne polarized pump-probe spectroscopy. We show the dependence of the energy renormalization of the lower and upper polariton resonances with cavity detuning for different polariton densities. We use the exciton-photon based Gross-Pitaevskii equation to model the experiment for both lower and upper polariton modes. The theoretical results reproduce qualitatively the experimental observations revealing the magnitude and sign of the parallel and antiparallel spin interaction strength. We evidence the strong influence of the biexciton resonance on the antiparallel spin polariton energy shift and provide the exciton-biexciton coupling constant. We derive our results in the lower polariton basis using the Gross-Pitaevskii equation, from which we express analytically the spinor polariton interactions and identify the clear role of the biexciton resonance.

It is known that for a topologically ordered state the area law for the entanglement entropy shows a negative universal additive constant contribution, $-\gamma$, called the topological entanglement entropy. We theoretically study the entanglement entropy of the two-dimensional Shastry-Sutherland quantum antiferromagnet using exact diagonalization on clusters of 16 and 24 spins. By utilizing the Kitaev-Preskill construction [A. Kitaev and J. Preskill, Phys. Rev. Lett. 96, 110404 (2006)] we extract a finite topological term, $-\gamma$, in the region of bond-strength parameter space corresponding to high geometrical frustration. Thus, we provide strong evidence for the existence of an exotic topologically ordered state and shed light on the nature of this model's strongly frustrated, and long controversial, intermediate phase.

The electric-field response of a one-dimensional ring of interacting fermions, where the interactions are described by the extended Hubbard model, is investigated. By using an accurate real-time propagation scheme based on the Chebyshev expansion of the evolution operator, we uncover various nonlinear regimes for a range of interaction parameters that allows modeling of metallic and insulating (either charge density wave or spin density wave insulators) rings. The metallic regime appears at the phase boundary between the two insulating phases and provides the opportunity to describe either weakly or strongly correlated metals. We find that the fidelity susceptibility of the ground state as a function of magnetic flux piercing the ring provides a very good measure of the short-time response. Even completely different interacting regimes behave in a similar manner at short time scales as long as the ground-state fidelity susceptibility is the same. Depending on the strength of the electric field we find various types of responses: persistent currents in the insulating phase, a dissipative regime, or damped Bloch-like oscillations with varying frequencies or even irregular in nature. Furthermore, we also consider the dimerization of the ring and describe the response of a correlated band insulator. In this case the distribution of the energy levels is more clustered and the Bloch-like oscillations become even more irregular.

We report the annealing-induced enhancement of ferromagnetism and nanoparticle formation in group-IV-based ferromagnetic semiconductor $\mathrm{GeFe}$. We successfully increase the Curie temperature of the $\mathrm{G}{\mathrm{e}}_{0.895}\mathrm{F}{\mathrm{e}}_{0.105}$ film up to 210 K while keeping a nearly single ferromagnetic phase when the annealing temperature is lower than 600 °C. In contrast, when it is annealed at 600 °C, single-crystal GeFe nanoparticles with stacking faults and twins, which have high Curie temperature up to room temperature, are formed in the film. We show that the inhomogeneity of the Fe concentration plays an essential role in determining the ferromagnetism in both cases. Although all the GeFe films show weak spin-glass-like behavior in the very low-temperature region (lower than $\sim 26$ K), which is insensitive to the annealing temperature, due to the nonuniform distribution of the Fe atoms, the ferromagnetism is much stronger than the spin glass and it dominates the system.

In this paper, we critically review the usual explanation of the blue shift in the temperature dependence of the light emission, which is commonly observed for polar GaInN/GaN quantum wells at intermediate temperatures. We demonstrate that this blue shift is not necessarily caused by a thermally induced change of occupation in an inhomogeneously broadened density of states. Instead, different energy dependencies of radiative and nonradiative lifetimes may lead to an energy dependence of the internal quantum efficiency strongly influencing the peak position of the luminescence: the piezoelectric fields within the quantum well induce an approximately exponential relationship between the electron-hole overlap matrix element for the radiative transition between the lowest quantized states and the respective transition energy. Via time-resolved photoluminescence spectroscopy, we observe a corresponding exponential energy dependence of radiative lifetimes at low temperatures and almost energy independent nonradiative lifetimes toward room temperature throughout the emission of polar single quantum wells. An analytical model is demonstrated, predicting a significant blue shift of several 10 meV for polar and a negligible shift for nonpolar GaInN/GaN quantum wells at the transition from high to low internal quantum efficiency due to the energy-dependent competition between radiative and nonradiative recombination processes. This model consistently explains the lack of a blue shift in nonpolar quantum wells grown on $m$-plane bulk GaN as well as a reduced characteristic temperature for the onset of the blue shift after artificial defect generation via argon ion implantation.

We report on the observation of magneto-oscillations of terahertz radiation induced photocurrent in HgTe/HgCdTe quantum wells of different widths, which are characterized by a Dirac-like, inverted, and normal parabolic band structure. The photocurrent data are accompanied by measurements of photoresistance (photoconductivity), radiation transmission, as well as magnetotransport. We develop a microscopic model of a cyclotron-resonance assisted photogalvanic effect, which describes main experimental findings. We demonstrate that the quantum oscillations of the photocurrent are caused by the crossing of Fermi level by Landau levels resulting in the oscillations of spin polarization and electron mobilities in spin subbands. Theory explains a photocurrent direction reversal with the variation of magnetic field observed in experiment. We describe the photoconductivity oscillations related with the thermal suppression of the Shubnikov–de Haas effect.

Due to the layered nature of graphite, the migration and interaction of point defects in the graphite crystal structure are highly anisotropic, and it is usually assumed that individual mobile lattice vacancies are confined to diffuse on a single plane. We present the results of ab initio calculations based on density functional theory which demonstrate that vacancies can, in fact, move between adjacent planes when they interact with one another via relatively low energy pathways, often with barriers of less than 1 eV. These interlayer transition mechanisms can significantly alter both the kinetics of point-defect aggregation and coalescence and also the resultant morphologies of multivacancy complexes that form as a result of migrating vacancies interacting.

We consider fully many-body-localized systems, i.e., isolated quantum systems where all the many-body eigenstates of the Hamiltonian are localized. We define a sense in which such systems are integrable, with localized conserved operators. These localized operators are interacting pseudospins, and the Hamiltonian is such that unitary time evolution produces dephasing but not “flips” of these pseudospins. As a result, an initial quantum state of a pseudospin can in principle be recovered via (pseudospin) echo procedures. We discuss how the exponentially decaying interactions between pseudospins lead to logarithmic-in-time spreading of entanglement starting from nonentangled initial states. These systems exhibit multiple different length scales that can be defined from exponential functions of distance; we suggest that some of these decay lengths diverge at the phase transition out of the fully many-body-localized phase while others remain finite.

We have synthesized the solid solutions of ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Ge}$ compounds with $x$ running from 0.0 to 0.9. Small-angle neutron scattering and magnetization measurements have shown that these compounds are ordered into the spin helix structure below the critical temperature $T_c$. The helix is transformed into the ferromagnet by application of the magnetic field above the critical value $H_{c2}$. It is shown that $T_c$ decreases smoothly with concentration $x$ from 280 K for FeGe to 0 for CoGe. The values of the helix wave vector $k_s$ and the critical field $H_{c2}$ depend strongly on concentration $x$, firstly, decreasing from pure FeGe to its minimum ($|{\mathbf{k}}_s|\to 0$, $H_{c2}\to 0$) at $x_c\approx 0.6$, and, then increasing again at higher $x$. Thus, we observe a transformation of the helix structure to the ferromagnet at $x\to x_c$ at zero field. We concluded that this transformation is caused by different signs of the spin helicity for the compounds with $x>x_c$ and $x<x_c$. We believe that the mechanism of the transformation is caused by the competition between the Dzyaloshinskii-Moriya interaction and the cubic anisotropy.

Nanopatterned FePt nanodots often exhibit low coercivity and a broad switching field distribution, which could arise due to edge damage during the patterning process causing a reduction in the $L{1}_0$ ordering required for a high magnetocrystalline anisotropy. Using an atomistic spin model, we study the magnetization reversal behavior of $L{1}_0$ FePt nanodots with soft magnetic edges. We show that reversal is initiated by nucleation for the whole range of edge widths studied. For narrow soft edges the individual nucleation events dominate reversal; for wider edges, multiple nucleation at the edge creates a circular domain wall at the interface which precedes complete reversal. Our simulations compare well with available analytical theories. The increased edge width further reduces and saturates the required nucleation field. The nucleation field and the activation volume manipulate the thermally induced switching field distribution. By control of the properties of dot edges using proper patterning methods, it should be possible to realize exchange spring bit-patterned media without additional soft layers.

Despite the fast development of computational material modeling, the theoretical description of macroscopic elastic properties of textured polycrystalline aggregates starting from basic principles remains a challenging task. In this study we use a supercell-based approach to obtain the elastic properties of a random solid solution cubic ${\mathrm{Zr}}_{1-x}{\mathrm{Al}}_x\mathrm{N}$ system as a function of the metallic sublattice composition and texture descriptors. The employed special quasirandom structures are optimized not only with respect to short-range-order parameters, but also to make the three cubic directions $\left[100\right]$, $\left[010\right]$, and $\left[001\right]$ as similar as possible. In this way, only a small spread of elastic constant tensor components is achieved and an optimum trade-off between modeling of chemical disorder and computational limits regarding the supercell size and calculational time is proposed. The single-crystal elastic constants are shown to vary smoothly with composition, yielding $x\approx 0.5$ an alloy constitution with an almost isotropic response. Consequently, polycrystals with this composition are suggested to have Young's modulus independent of the actual microstructure. This is indeed confirmed by explicit calculations of polycrystal elastic properties, both within the isotropic aggregate limit and with fiber textures with various orientations and sharpness. It turns out that for low AlN mole fractions, the spread of the possible Young's modulus data caused by the texture variation can be larger than 100 GPa. Consequently, our discussion of Young's modulus data of cubic ${\mathrm{Zr}}_{1-x}{\mathrm{Al}}_x\mathrm{N}$ contains also the evaluation of the texture typical for thin films.

The electronic structure of phase separated ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ has been studied by means of x-ray absorption spectroscopy (XAS) and photoemission spectroscopy. Both Fe $L_{2,3}$-edge XAS and Fe $2p$ photoemission spectra show a signature of the metallic phase that is embedded in the insulating texture. Valence-band spectra, measured with different excitation energies, reveal a finite density of states at the Fermi level driven by the Fe $3d$ orbitals. A combined analysis of the Se $L_{2,3}$-edge and the Se $K$-edge XAS provides further information on a significant density of states of Se $4p$ character near the Fermi level, consistent with a large hybridization of these states with the Fe $3d$ orbitals. The K $L_{2,3}$-edge XAS spectrum is also presented to show element specific partial density of states. The results are discussed in connection with the electronic phase separation in ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ driven by the Fe $3d$ states.

A series of states with different densities of stripes of Ir dimers is investigated using x-ray diffraction and density functional theory in layered nonmagnetic metal $\mathrm{IrT}{\mathrm{e}}_2$. With decreasing temperature, structures with and without inversion symmetry alternate. In noncentrosymmetric states, spin-orbit coupling splits the electronic energy bands into spin-polarized pairs. Factors affecting the stability of the observed dimerized states are established, and it is conjectured that an infinite series of alternating states with and without polarized bands is realized in $\mathrm{IrT}{\mathrm{e}}_2$. Switching dimerized states with different symmetries by changing temperature or strain enables control of band polarization, adding a new tool for spintronics and valleytronics research.

We present some physical properties of the filled skutterudite compound ${\mathrm{NdOs}}_4{\mathrm{As}}_{12}$, which exhibits a ferromagnetic transition at $T_{\mathrm{C}}\approx 1.1$ K, evidenced by distinct anomalies in susceptibility, electrical resistivity, and specific-heat $C\left(T\right)$ measurements. At $B=0$, the heat capacity displays a Schottky-like term with maximum values at about 0.93 K. Additionally, the $C\left(T\right)$ data show enhanced values in the low-temperature paramagnetic state. Upon small external fields $B\le 0.25$ T, a Schottky-like peak shifts above the ferromagnetic transition. Anomalous low-energy excitations of a quartet ground state of the ${\mathrm{Nd}}^{3+}$ multiplet are attributed to a lowering of the $T_h$ cubic point symmetry due to structural disorder rather than to the Zeeman splitting due to a molecular field. We expect that our findings will be also relevant for the study of exotic heavy-fermion behavior in filled skutterudites.

We performed a laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of the ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ (BaK) iron-based superconductor. We observed a robust low-binding energy (BE) kink structure (kink1) in the dispersion which is doping dependent whereby its energy peaks at the optimally doped level $\left(x\sim 0.4\right)$ and decreases towards the underdoped and overdoped sides. We attribute this kink to electron-mode coupling in good agreement with the inelastic neutron scattering (INS) and scanning tunneling microscopy (STM) results on the same compound where a similar bosonic mode associated with spin excitations was observed. The relation between the mode energy $\left(\Omega \right)$ and the SC transition temperature $\left(T_c\right)$ deduced from our laser ARPES data follow the universal relation deduced from INS and STM. In addition, we could resolve another kink at higher BE (kink2) showing less doping and temperature dependence compared to kink1 and which thus may be of different origin.

The ${\mathrm{He}}^{+}/\mathrm{Al}$ system is a very interesting projectile-surface combination which was thought initially as an example of a pure Auger neutralization mechanism. Then, because of the measured reionization explained by the antibonding interaction of the projectile state with the core target states, the resonant charge exchange with the band states was considered as another important contribution to the neutralization. Nevertheless, by only considering the neutralization to the ground state of helium, the measured ion survival probability is still overestimated. On the other hand, measurements of electron emission from an Al surface bombarded by He positive ions suggested the possibility of occupied excited states of helium due to the ion-surface collision. In this work, we also include the excited states of He within the time-dependent scattering process in which both neutralization mechanisms, resonant and Auger, are simultaneously contemplated. Our starting point is a multiorbital Anderson Hamiltonian projected over the selected space of ground and excited atomic configurations. An extra term related to the Auger mechanism is added to this Hamiltonian. A difference with previous works is that this approach includes the electron spin and, therefore, the spin fluctuation statistics in the charge-exchange process is correctly taken into account. We find a notable improvement in the agreement with the experiments and also that the interference between both mechanisms is not dramatic.

We study numerically the low-temperature electronic transport properties of a single-ion magnet with uniaxial and transverse spin anisotropies. We find clear signatures of a Kondo effect caused by the presence of a transverse (zero-field) anisotropy in the molecule. This Kondo effect has an SU(2) pseudospin character, associated with a doublet ground state of the isolated molecule, which results from the transverse anisotropy. Upon applying a transverse magnetic field to the single-ion magnet, we observe oscillations of the Kondo effect due to the presence of diabolical points (degeneracies) of the energy spectrum of the molecule caused by geometrical phase interference effects, similar to those observed in the quantum tunneling of multi-ion molecular nanomagnets. The field-induced lifting of the ground-state degeneracy competes with the interference modulation, resulting in some cases in a suppression of the Kondo peak.

While there are a number of models that tackle the problem of calculating friction forces on the atomic level, providing a completely parameter-free approach remains a challenge. Here we present a quasistatic model to obtain an approximation to the nanofrictional response of dry, wearless systems based on quantum-mechanical all-electron calculations. We propose a mechanism to allow dissipative sliding, which relies on atomic relaxations. We define two different ways of calculating the mean nanofriction force, both leading to an exponential friction-versus-load behavior for all sliding directions. Since our approach does not impose any limits on the lengths and directions of the sliding paths, we investigate arbitrary sliding directions for an fcc Cu(111) interface and detect two periodic paths that form the upper and lower bound of nanofriction. For long aperiodic paths, the friction force converges to a value in between these limits. For low loads, we retrieve the Derjaguin generalization of the Amontons-Coulomb kinetic friction law, which appears to be valid all the way down to the nanoscale. We observe a nonvanishing Derjaguin offset even for atomically flat surfaces in dry contact.

Recent years have seen a strong interest in topological effects within periodically driven systems. In this work, we explore topological effects in two closely related 2-dimensional driven systems described by Floquet operators possessing chiral symmetry (CS). Our numerical and analytical results suggest the following. First, the CS is associated with the existence of the anomalous counterpropagating (ACP) modes reported recently. Specifically, we show that a particular form of CS protects the ACP modes crossing the quasienergy band gap at $\pm \pi$. We also find that these modes are only present along selected boundaries, suggesting that they are a weak topological effect. Second, we find that CS can give rise to protected 0 and $\pi$ quasienergy modes, and that the number of these modes may increase without bound as we tune up certain system parameters. Like the ACP modes, these 0 and $\pi$ modes also appear only along selected boundaries and thus appear to be a weak topological effect. This work represents a detailed study of weak topological effects in periodically driven systems. Our findings add to the still-growing knowledge on driven topological systems.