We derive the lowest-temperature correction to the self-energy of a spin-up particle injected in a ferromagnetic background. The background is modeled with both Heisenberg and Ising Hamiltonians so that differences due to gapless versus gapped magnons can be understood. Beside the expected thermal broadening of the quasiparticle peak as it becomes a resonance inside a continuum, we also find that spectral weight is transferred to regions lying outside this continuum. We explain the origin of this spectral weight transfer and its low-temperature evolution.

Femtosecond laser spectroscopy is an important tool for investigating fundamental processes of light-matter interaction. Here, we report on transient optical anisotropy in the multiferroic hexagonal manganite YMnO${}_3$ excited by linearly or circularly polarized laser pulses with $\sim$17 fs and $\sim$34 fs duration. Spectral dependencies of photoinduced optical rotation and ellipticity at different temperatures are analyzed by considering ultrafast population and relaxation processes near the interband transitions from the hybridized O($2p$)-Mn($3d$) to the Mn($3d$) states at photon energies around 1.6 eV. A Raman coherence time of $\sim$10 fs between the excited ${\Gamma }_5|x\rangle$ and ${\Gamma }_5|y\rangle$ states and a relaxation time of $\sim$500 fs between the ${\Gamma }_5|x,y\rangle$ and ${\Gamma }_1|g\rangle$ states for the electronic transition ${\Gamma }_1\to {\Gamma }_5$ are determined in that way. The ultrashort Raman coherence time is related to the strong electron Coulomb interaction for the ${\Gamma }_5|x,y\rangle$ states.

Recently, monatomic chains on surfaces have been synthesized that show evidence of Luttinger liquid physics. The experimental data point to a dispersion along the chain with four Fermi points. Here we investigate a general low-energy effective Hamiltonian for such a two-band model where SU(2) spin symmetry is broken but time-reversal symmetry persists, as is expected due to the surface geometry. Spin-orbit coupling gives rise to an energy scale ${\varepsilon }_{\text{SO}}$ much smaller than the Fermi energy ${\varepsilon }_F$ and to spin-nonconserving scattering processes. We derive the generic phase diagram at zero temperature as well as an effective phase diagram at temperatures ${\varepsilon }_{\text{SO}}<T\ll {\varepsilon }_F$. For the part of the phase diagram where a Luttinger liquid is found to be stable, the density of states and the spectral function are calculated and discussed in relation to the experimental data.

Experimental studies have been performed of the nonequilibrium transitions in multistable cavity-polariton systems under 10 ps long pulsed excitation, i.e., within nearly the shortest time scale necessary for the multistability to be revealed in GaAs-based microcavities. The experiments were aimed to determine the ratio of the strengths of the pair interactions between polaritons with parallel and antiparallel spins, which also was the question raised in a number of recent studies. We show that in contrast to the cases of continuous-wave or nanosecond long pumping conditions, in the fast pulse regime the system is not affected by the excitonic reservoir. The interaction between polaritons with opposite spins is definitely attractive at a nearly zero detuning of exciton and photon modes; as a result, optical response can be essentially multistable at a linearly polarized excitation. With increasing pulse duration up to 100 ps, the reservoir can start manifesting itself in a reduction of the spin multistability.

The charge density relaxation propagator of a two-dimensional electron system, which is the slope of the imaginary part of the polarization function, exhibits singularities for bosonic momenta having the order of the spin-orbit momentum and depending on the momentum orientation. We have provided an intuitive understanding for this nonanalytic behavior in terms of the interchirality subband electronic transitions, induced by the combined action of Bychkov-Rashba (BR) and Dresselhaus (D) spin-orbit coupling. It is shown that the regular behavior of the relaxation propagator is recovered in the presence of only one BR or D spin-orbit field or for spin-orbit interaction with equal BR and D coupling strengths. This creates a new possibility to influence carrier relaxation properties by means of an applied electric field.

We demonstrate that, due to their spin-orbit interaction, carbon nanotube cross-junctions have attractive spin projective properties for transport. First, we show that the junction can be used as a versatile spin filter as a function of a backgate and a static external magnetic field. Switching between opposite spin filter directions can be achieved by small changes of the backgate potential, and a full polarization is generically obtained in an energy range close to the Dirac points. Second, we discuss how the spin filtering properties affect the noise correlators of entangled electron pairs, which allows us to obtain signatures of the type of entanglement that are different from the signatures in conventional semiconductor cross-junctions.

We derive an exact solution to the problem of spin snake states induced in a nonhomogeneous magnetic field by a combined action of the Rashba spin-orbit and Zeeman fields. The electron spin behavior as a function of the cyclotron orbit center position and an external homogeneous magnetic field was obtained. It is shown that in an antisymmetric magnetic field the electron spin in the snake states has only an in-plane projection, perpendicular to the magnetic interface, which vanishes at large positive momenta. Applying an external homogeneous magnetic field adds a finite out-of-plane spin component and simultaneously gaps out the spectral branches, which results in regular beating patterns of the spin current components.

Optimizing all three thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) simultaneously is a nontrivial task, especially for the Seebeck coefficient and electrical conductivity. Here, we demonstrate that chemically tuned rutile TiO${}_2$ sintered from N- and Nb-codoped nanoparticles possesses not only an enhanced power factor but also a decreased thermal conductivity. In samples produced under strongly reducing conditions, the electrical resistivity is decreased markedly to $3.96\times {10}^{-5}$ Ωm while maintaining a high Seebeck coefficient. In this manner, the power factor can be greatly improved to $9.87\times {10}^{-4}$ W m${}^{-1}$ K${}^{-2}$ at 785 °C with a significantly enhanced figure of merit $ZT$ of 0.35 at 700 °C. Theoretical modeling based on density functional theory suggests that enhanced asymmetrical carrier transport (i.e., with the minority carrier relatively localized on one side of the Fermi level but the majority carrier delocalized on the other) can be induced by locally distorting the crystal structure, since this imposes local perturbations on the intrinsic periodic potential field, which may be responsible for the improved power factor. The greatly improved thermoelectric behavior of chemically tuned rutile TiO${}_2$ provides a promising route to producing practical thermoelectric materials without the use of toxic heavy elements. Our findings also provide a promising new strategy for enhancing the thermoelectric properties of other thermoelectric materials.

Spin and density patterns of exciton-polariton condensates in strained planar cavities with a nonuniform energy landscape are shown to undergo fast nonequilibrium transformations under the conditions of pulsed resonant excitation. Maxima in the spatial distribution of condensate density shift from the potential humps to the traps, which is accompanied by a linear-to-circular polarization conversion at each space point. The transitions are explained within the framework of modified Gross-Pitaevskii equations with account taken for the spin-anisotropic polariton-polariton interaction and the spatial dependence of the polariton energy.

Structure and electronic properties of one-dimensional bimetallic In-Sn chains formed by codeposition on a Si(100)-2$\times$1 surface are studied experimentally by means of scanning tunneling microscopy (STM) and scanning tunneling spectroscopy and theoretically using density-functional theory. The codeposition of In with a small amount of Sn allows separation of various In-Sn structures and their identification in empty-state STM images. A 16$\times$2 supercell is employed to model an indium atomic chain in which one or two Sn atoms are embedded. This atomic model is used to identify unambiguously various In-Sn structures observed experimentally. At low Sn:In ratio the codeposition results in strongly preferential formation of isolated heterogeneous In-Sn dimers. The In-Sn dimer induces tilting of the neighboring homogeneous In-In dimer accompanied with a charge transfer. Consequently a localized state at Fermi level appears. These results contribute to a discussion on possible transport of electric charge along one-dimensional atomic chains of metals.

Measurements of low-temperature magnetotransport in lithographic wires of submicron widths fabricated from high-mobility AlGaSb/InAs/AlGaSb two-dimensional electron system heterostructures are presented. The dependence of the spin and phase coherence lengths on wire width and diffusion constant is investigated by analyzing the conductance in low applied magnetic fields with antilocalization models. Predominantly diffusive boundary scattering is deduced from the magnitude and wire width dependence of the conductance. Diffusive boundary scattering leads to a diffusion constant decreasing with wire width and hence allows the dependence of spin coherence on wire width and diffusion constant to be investigated concurrently. The spin coherence lengths are experimentally found to be proportional to the ratio of the diffusion constant to wire width. The phase coherence lengths follow Nyquist decoherence for low-dimensional wires.

The microscopic mechanism for Rashba-type band splitting is examined in detail. We show how an asymmetric charge distribution is formed when the local orbital angular momentum (OAM) and crystal momentum get interlocked due to surface effects. An electrostatic energy term in the Hamiltonian appears when such an OAM- and crystal-momentum-dependent asymmetric charge distribution is placed in an electric field produced by inversion-symmetry breaking. Analysis by using an effective Hamiltonian shows that, as the atomic spin-orbit coupling (SOC) strength increases from weak to strong, the originally OAM-quenched states evolve into well-defined chiral OAM states and then to states of total angular momentum $J$. In addition, the energy scale of the band splitting changes from the atomic SOC energy to electrostatic energy. To confirm the validity of the model, we study OAM and spin structures of the Au(111) system by using an effective Hamiltonian for the $d$-orbital case. As for the strong-SOC regime, we choose Bi${}_2$Te${}_2$Se as a prototype system. We performed circular dichroism angle-resolved photoemission spectroscopy experiments as well as first-principles calculations. We find that the effective model can explain various aspects of the spin and OAM structures of the system.

We present a theory of temperature-dependent photoemission which accurately describes phonon effects in soft and hard x-ray angle-resolved photoemission. Our approach is based on a fully relativistic one-step theory of photoemission that quantitatively reproduces the effects of phonon-assisted transitions beyond the usual $\mathbf{k}$-conserving dipole selection rules which lead to the so-called XPS limit in the hard x-ray and/or high temperature regime. Vibrational atomic displacements have been included using the coherent potential approximation in analogy to the treatment of disordered alloys. The applicability of this alloy analogy model is demonstrated by direct comparison to experimental soft x-ray data from W(110) showing very satisfying agreement.

We theoretically investigate phonon-photon interaction in cavities created in a phoxonic crystal slab constituted by a two-dimensional (2D) square array of holes in a silicon membrane. The structure without defects provides 2D band gaps for both electromagnetic and elastic waves. We consider two types of cavities, namely, an L3 cavity (a row of three holes is removed) and a cross-shape cavity, which both possess highly confined phononic and photonic localized modes suitable for enhancing their interaction. In our theoretical study, we take into account two mechanisms that contribute to optomechanical interaction, namely, the photoelastic and the interface motion effects. We show that, depending on the considered pair of photonic and phononic modes, the two mechanisms can have similar or very different magnitudes, and their contributions can be either in or out of phase. We find out that only acoustic modes with a specific symmetry are allowed to couple with photonic cavity modes. The coupling strength is quantified by two different methods. In the first method, we compute a direct estimation of coupling rates by overlap integrals, while in the second one, we analyze the temporal modulation of the resonant photonic frequency by the phonon-induced acoustic vibrational motion during one acoustic period. Interestingly, we obtain high optomechanical interaction, with the coupling rate reaching more than 2.4 MHz for some specific phonon-photon pairs.

We present results of self-consistent, full-potential electronic structure calculations for slabs of hexagonal graphite with stacking faults and for slabs with one displaced surface layer. There are two types of stacking faults, which differ qualitatively in their chemical bonding picture. We find that both types induce localized interface bands near the symmetry line $K$-$M$ in the Brillouin zone and a related peak in the local density of states (LDOS) very close to the Fermi energy, which should give rise to a dominating contribution of the interface bands to the local conductivity at the stacking faults. In contrast, a clean surface does not host any surface bands in the energy range of the $\pi$ and $\sigma$ bands, and the LDOS near the surface is even depleted. On the other hand, displacement of even one single surface layer induces a surface band near $K$-$M$. A special role is played by $p_z$-bonded dimers (directed perpendicular to the layers) in the vicinity of one type of stacking faults. They produce a half-filled pair of interface states or interface resonances. The formation energy of both types of stacking faults and the surface energy are estimated.

The low-temperature behavior of the magnetic insulator Cs${}_2$CuCl${}_4$ can be modeled by an anisotropic triangular lattice spin-$1/2$ Heisenberg antiferromagnet with two different exchange couplings $J$ and $J^{\prime }\approx J/3$. We show that in a wide range of magnetic fields the experimentally observed field dependence of the crossover temperature $T_c$ for spin-liquid behavior can be explained within a mean-field theory based on the representation of spin operators in terms of Majorana fermions. We also show that for small magnetic fields the specific heat and the spin susceptibility both exhibit a maximum as a function of temperature at $T_c=J/2$. In the spin-liquid regime, the Majorana fermions can only propagate along the direction of the strongest bond, in agreement with the dimensional reduction scenario advanced by Balents [Nature (London) 464, 199 (2010)].

Recent experiments suggest that, besides antiferromagnetic fluctuations, nematic fluctuations may contribute to the occurrence of superconductivity in iron pnictides. Motivated by this observation, we study superconductivity from nematic fluctuations in a minimal two-band model. The employed band parameters are appropriate for iron pnictides and lead to four pockets for the Fermi line. It is shown that low-energy, long-wavelength nematic fluctuations within the pockets give rise to strong-coupling superconductivity whereas the large momenta density fluctuations between pockets are rather irrelevant. The obtained transition temperatures are similar to those typically found in the pnictides and are rather robust against repulsive Coulomb interactions. The superconducting and nematic states coexist in a large region of the phase diagram.

Unconventional superconductivity induced by the magnetic moments in a conventional spin-singlet $s$-wave superconductor is theoretically studied. By choosing the spin directions of these moments, one can design spinless pairing states appearing within the $s$-wave superconducting energy gap. It is found that the helix spins produce a $p_x+p_y$-wave state while the skyrmion crystal configuration a $p_x+i{p}_y$-wave-like state. Nodes in the energy gap and the zero-energy flat band of Majorana edge states exist in the former case, while the chiral Majorana channels along edges of the sample and the zero-energy Majorana bound state at the core of the vortex appear in the latter case.

Biomedical applications of superparamagnetic iron oxide particles have been of interest for quite a number of years. Recent developments show that multifunctionality can be efficiently achieved using polymers to coat the particles and to provide anchoring elements to their surface. This leads to the formation of nanobeads with a reduced number of particles trapped by the polymeric structure. While the magnetothermic behavior of isolated nanoparticles has been a subject of interest over the past several years, multicore magnetic nanobeads have thus far not received the same attention. The influence of structural and magnetic properties in the hyperthermia performance of a series of magnetic fluids designed for biomedical purposes is studied here. The fluids are made of maghemite multicore polymeric beads, with variable nanoparticle size and hydrodynamic size, dispersed in a buffer solution. The specific loss power (SLP) was measured from 5 to 100 kHz with a field intensity of 21.8 kA/m. SLP increases with increasing magnetic core size, reaching 32 W/g Fe${}_2$O${}_3$ at 100 kHz for 16.2 nm. Within the framework of the linear response theory, a graphical construction is proposed to describe the interplay of both size distributions and magnetic properties in the heating performance of such fluids in a given frequency range. Furthermore, a numerical model is developed to calculate the spare contribution of Néel and Brown relaxation mechanisms to SLP, which gives a fair reproduction of the experimental data.

We explore the phase diagram and the low-energy physics of three Heisenberg antiferromagnets which, like the kagome lattice, are networks of corner-sharing triangles but contain two sets of inequivalent short-distance resonance loops. We use a combination of exact diagonalization, analytical strong-coupling theories, and resonating valence bond approaches, and scan through the ratio of the two inequivalent exchange couplings. In one limit, the lattices effectively become bipartite, while at the opposite limit heavily frustrated nets emerge. In between, competing tunneling processes result in short-ranged spin correlations, a manifold of low-lying singlets (which can be understood as localized bound states of magnetic excitations), and the stabilization of valence bond crystals with resonating building blocks.