${}^{63,65}$Cu and ${}^{53}$Cr nuclear magnetic resonance spectra for CuCr${}_2$O${}_4$ were measured at various magnetic fields and temperatures. The microscopic evidence of orbital ordering in CuCr${}_2$O${}_4$ was obtained from a dipolar hyperfine field, NQR, and magnetic anisotropy analysis of the linewidth broadening of the Cu and Cr NMR spectra measured in the external magnetic field. The energy gap in the dispersion relation of the spin wave excitation was measured from the temperature dependence of the resonance frequency of Cu and Cr ions in CuCr${}_2$O${}_4$. The energy gap of the Cu ions is about 10 K ($\pm$ 5 K), and that of the Cr ions is about 40 K ($\pm$ 5 K). These values imply that the spin-orbit coupling of Cr ions is stronger than that of Cu ions related to the orbital ordering in CuCr${}_2$O${}_4$. The magnetic field dependence of the Cr NMR frequency shows that the angle between the Cr${}^{3+}$ magnetic moment and the Cu${}^{2+}$ magnetic moment is about 98${}^{\circ }$ ($\pm$ 2${}^{\circ }$).

We investigate a possibility to enhance the coherence time of electron spins in magnetic molecular complexes by application of the Carr-Purcell-Meiboom-Gill (CPMG) multiple microwave pulse sequence. Our theoretical analysis shows that the CPMG sequence can efficiently suppress the spin decoherence channel arising due to spectral diffusion induced by a random modulation of the hyperfine interaction which is an important source of the spin dephasing in molecular magnets. We confirm this by employing the CPMG protocol in pulse electron spin resonance experiments on model binuclear 1,2-diphosphacyclopentadienyl manganese complexes. We show that, compared to the standardly used two-pulse primary spin-echo technique, the CPMG experiment can boost the phase memory time up to one order of magnitude, bringing it to above 10 $\mu$s at low temperatures. This finding may be important for the implementation of quantum computation protocols on molecular magnets. We discuss a possible interesting analogy with the Zeno's paradox in quantum theory (the Zeno quantum effect), which could be implicit in the CPMG experiment.

We study the properties of the normal modes of a chain of Josephson junctions in the simultaneous presence of disorder and absorption. We consider the superconducting regime of small phase fluctuations and focus on the case where the effects of disorder and absorption can be treated additively. We analyze the frequency shift and the localization length of the modes. We also calculate the distribution of the frequency-dependent impedance of the chain. The distribution is Gaussian if the localization length is long compared to the absorption length, while it has a power-law tail in the opposite limit.

We present Raman scattering results on phonon polaritons in single crystals of $\alpha$-GaN. A detailed theoretical treatment of their dispersion and Raman scattering efficiency in wurtzite-type crystals is given. Pure symmetry polaritons (ordinary and extraordinary) are accessible in near-forward scattering geometry according to the theory. For this purpose, the experimental setup uses rectangular aperture windows in front of the entrance lens. Thus measurements with well defined wave-vector transfer can be realized. The observed dispersion curves and scattering efficiency results are compared with theoretical ones and are found to be in excellent agreement.

Thickness dependence of magnetic anisotropy energy (MAE) and spin polarization in Fe and Co multilayers on a Pt(001) substrate are investigated with ab initio techniques. As the thickness of the thin films is increased, an oscillatory behavior of the MAE of the systems is observed. For Fe multilayers, this even results in the periodic switching of the easy axis from out of plane to in plane. Capping the multilayers with Pt, in most cases, leads to a strong enhancement of the MAE. Our calculations give clear evidence that such oscillatory behavior of MAE can be attributed to spin-polarized quantum well states (QWS) existing in thicker multilayers. The spin polarization of the QWS is also shown to have a profound thickness dependence. Finally, we show that both MAE and spin polarization of the QWS are sensitive to external electric field and can be tailored therewith.

A hole in a two-dimensional Ising antiferromagnet was believed to be infinitely heavy due to the string of wrongly oriented spins it creates as it moves, which should trap it near its original location. Trugman showed that, in fact, the hole acquires a finite effective mass due to contributions from so-called Trugman loop processes, where the hole goes nearly twice around closed loops, first creating and then removing wrongly oriented spins, and ending up at a different lattice site. This generates effective second- and third-nearest-neighbor hopping terms which keep the quasiparticle on the sublattice it was created on. Here, we investigate the trapping of the quasiparticle near a single attractive nonmagnetic impurity placed at one lattice site. We consider the two cases with the quasiparticle and impurity being on the same versus on different sublattices. The main result is that even though the quasiparticle can not see the bare disorder in the latter case, the coupling to magnons generates an effective renormalized disorder on its own sublattice which is strong enough to lead to bound states, which however have a very different spectrum than when the quasiparticle and impurity are on the same sublattice.

We experimentally study the ghost critical field (GCF), a magnetic field scale for the suppression of superconducting fluctuations, using Hall effect and magnetoresistance measurements on a disordered superconducting thin film near its transition temperature $T_c$. We observe an increase in the Hall effect with a maximum in field that tracks the upper critical field below $T_c$, vanishes near $T_c$, and returns to higher fields above $T_c$. Such a maximum has been observed in studies of the Nernst effect and identified as the GCF. Magnetoresistance measurements near $T_c$ indicate quenching of superconducting fluctuations, agree with established theoretical descriptions, and allow us to extract the GCF and other parameters. Above $T_c$, the Hall peak field is quantitatively distinct from the GCF, and we contrast this finding with ongoing studies of the Nernst effect and superconducting fluctuations in unconventional and thin-film superconductors.

A first-principles approach, based on the real-space multiple scattering Green's function method, is presented for spin- and angle-resolved resonant photoemission from magnetic surfaces. It is applied to the Fe(010) valence band photoemission excited with circularly polarized x rays around the Fe $L_3$ absorption edge. When the photon energy is swept through the Fe $2p-3d$ resonance, the valence band spectra are strongly modified in terms of absolute and relative peak intensities, degree of spin polarization, and light polarization dependence. New peaks in the spin-polarized spectra are identified as spin-flip transitions induced by exchange decay of spin-mixed core holes. By comparison with single-atom and band-structure data, it is shown that both intra-atomic and multiple scattering effects strongly influence the spectra. We show how the different features linked to states of different orbital symmetry in the $d$ band are differently enhanced by the resonant effect. The appearance and origin of circular dichroism and spin polarization are analyzed for different geometries of light incidence and electron emission direction, providing guidelines for future experiments.

We used x-ray and neutron diffraction to determine the low-temperature structure of IrTe${}_2$. A structural modulation was observed with a wave vector of $\mathit{k}=(1/5,0,1/5)$ below $T_s\approx 285$ K, accompanied by a structural transition from a trigonal to a triclinic lattice. We also performed first-principles calculations for high-temperature and low-temperature structures, which elucidate the nature of the phase transition and the low-temperature structure. A local bonding instability associated with the Te $5p$ states is likely the origin of the structural phase transition in IrTe${}_2$.

We calculate the free energy of the quantum uniform electron gas for temperatures from near 0 to 100 times the Fermi energy, approaching the classical limit. An extension of the Vashista-Singwi theory to finite temperatures and a self-consistent compressibility sum rule is presented. Comparisons are made to other local-field correction methods, as well as recent quantum Monte Carlo simulation and classical map-based results. Accurate fits to the exchange-correlation free energy from both theory and simulation are given for future practical applications.

We have studied the optical conductivity of two-dimensional (2D) semiconducting transition metal dichalcogenides using ab initio density functional theory. We find that this class of materials presents large optical response due to the phenomenon of band nesting. The tendency towards band nesting is enhanced by the presence of van Hove singularities in the band structure of these materials. Given that 2D crystals are atomically thin and naturally transparent, our results show that it is possible to have strong photon-electron interactions even in 2D.

We compute Raman scattering intensities via the lowest-order coupling to the bosonic propagator associated with orbital nematic fluctuations in a minimal model for iron pnictides. The model consists of two bands on a square lattice exhibiting four Fermi pockets and a transition from a normal to a nematic state. It is shown that the orbital fluctuations produce in the $B_{1g}$ channel strong quasielastic light scattering around the nematic critical temperature $T_n$, both above and below $T_n$. This holds for the $A_{1g}$ symmetry only below $T_n$, whereas no low-energy scattering from orbital fluctuations is found in the $B_{2g}$ symmetry. Due to the nematic distortion, the electron pocket at the $X$ point may disappear at low temperatures. Such a Lifshitz transition causes in the $B_{2g}$ spectrum a large upward shift of spectral weight in the high-energy region, whereas no effect is seen in the other symmetries.

Quasi-one-dimensional organic conductors (TMT$C$F)${}_2$$X$ have various electric and magnetic properties. Although theoretical and experimental studies have suggested that (TMTTF)${}_2$Br has the properties of commensurate antiferromagnetism, details of the magnetic structure of this compound are unclear. Two types of antiferromagnetism are expected, one due to a localized electron and the other to the nesting of the Fermi surface. We therefore assessed the antiferromagnetic structure of (TMTTF)${}_2$Br using ${}^{13}$C NMR. Site assignment of the observed antiferromagnetic spectrum confirmed that there were two magnetic molecular sites, with staggered moments and amplitude of 0.11${\mu }_B$/molecule, as well as nonmagnetic molecular sites. A commensurate structure with antiferromagnetic ordering of ($\uparrow \circ \downarrow \circ$) along a one-dimensional chain would be expected as the freezing of antiferromagnetic fluctuations in the charge-ordered phase of the (TMTTF)${}_2$AsF${}_6$ salt. The presence of a nodal site is strongly suggestive of the nesting type antiferromagnetism. The fine structure of the antiferromagnetic spectrum suggests superlattice along the interchain direction. We could not observe line broadening due to the charge order above the antiferromagnetic transition.

We present a large class of three-dimensional spin models that possess topological order with stability against local perturbations, but are beyond description of topological quantum field theory. Conventional topological spin liquids, on a formal level, may be viewed as condensation of stringlike extended objects with discrete gauge symmetries, being at fixed points with continuous scale symmetries. In contrast, ground states of fractal spin liquids are condensation of highly fluctuating fractal objects with certain algebraic symmetries, corresponding to limit cycles under real-space renormalization group transformations which naturally arise from discrete scale symmetries of underlying fractal geometries. A particular class of three-dimensional models proposed in this paper may potentially saturate quantum information storage capacity for local spin systems.

Electric circuits involving Weyl semimetals possess unusual properties induced by the quantum anomaly. The chiral magnetic current in a Weyl semimetal subjected to magnetic field modifies the behavior of such circuits in a drastic way. We consider two explicit examples: (i) a circuit involving the “chiral battery” and (ii) a circuit that can be used as a “quantum amplifier” of magnetic field. The unique properties of these circuits stem from the chiral anomaly and may be utilized for creating “chiral electronic” devices.

The near-edge fine structure of an ionization edge in electron energy-loss spectroscopy provides bonding and elemental information. We investigate the fine structure of the O $K$ edge in SrTiO${}_3$, where the oxygen atoms have identical local bonding environments in three dimensions but differ in their two-dimensional projection. By removing the effects of elastic and thermal diffuse scattering in experimental data, we demonstrate a small difference between the O columns that are nonequivalent in projection, which was not evident before the removal of scattering of the probing electrons. We explore the robustness of this method to the uncertainty in various experimental parameters.

At half-filling of the fourfold degenerate Landau levels $\left|n\right|\ge 1$ in graphene, the ground states are spin-polarized quantum Hall states that support spin skyrmion excitations for $\left|n\right|=1,2,3$. Working in the Hartree-Fock approximation, we compute the excitation energy of an unbound spin skyrmion-antiskyrmion excitation as a function of the Zeeman coupling strength for these Landau levels. We find for both the bare and screened Coulomb interactions that the spin skyrmion-antiskyrmion excitation energy is lower than the excitation energy of an unbound spin-$1/2$ electron-hole pair in a finite range of Zeeman coupling in Landau levels $\left|n\right|=1,2,3$. This range decreases rapidly for increasing Landau level index and is extremely small for $\left|n\right|=3.$ For valley skyrmions, which should be present at $1/4$ and $3/4$ fillings of the Landau levels $\left|n\right|=1,2,3$, we show that screening corrections are more important in the latter case. It follows that an unbound valley skyrmion-antiskyrmion excitation has lower energy at $3/4$ filling than at $1/4.$ We compare our results with recent experiments on spin and valley skyrmion excitations in graphene.

Out of plane vibrations are suppressed in graphene layers placed on a substrate. These vibrations, in suspended samples, are relevant for the understanding of properties such as the electrical resistivity, the thermal expansion coefficient, and others. We use a general framework to study the properties of the out of plane mode in graphene on different substrates, taking into account the dynamics of the substrate. We discuss broadening of this mode and how it hybridizes with the substrate Rayleigh mode, comparing our model with experimental observations. We use the model to estimate the substrate induced changes in the thermal expansion coefficient and in the temperature dependence of the electrical resistivity.

The dynamics of vibrations localized in a monolayer of Pb on Si(111) is studied using density functional theory and molecular dynamics methods. Recently, a pump-probe experiment has given direct access to the relaxation dynamics of the vibrational excitations in Pb, revealing two distinct time scales which are separately related to the directions of the Pb vibrational motions [Ultramicroscopy 127, 2 (2013)]. We analyze the experimental findings theoretically using atomistic modeling. After excitation of two-dimensional optical phonons, equilibration between parallel and perpendicular modes is observed before dissipation into the Si substrate. Even after several nanoseconds, equilibration with the substrate is not achieved due to energy trapped in acoustic Pb modes.