We present a simpler derivation of the exact solution of a spin polaron in a ferromagnet and generalize it to complex lattices and/or longer-range exchange interactions. As a specific example, we analyze a two-dimensional ${\text{MnO}}_2$-like lattice (as in the ferromagnetic layers in ${\text{LaMnO}}_3$) and discuss the properties of the resulting spin polaron in various regimes. At strong couplings the solution is reminiscent of the Zhang-Rice singlet, however the electronic wave function involved in the singlet is dependent on the momentum of the singlet, and multiple bands may appear.

An important issue in the study of the iron-arsenic-based superconductors is the symmetry of the superconducting gap, a problem complicated by multiple gaps on different Fermi-surface sheets. Electronic Raman scattering is a flexible bulk probe which allows one in principle to determine gap magnitudes and test for gap nodes in different regions of the Brillouin zone by employing different photon polarization states. Here we calculate the clean Raman intensity for $A_{1g}$, $B_{1g}$, and $B_{2g}$ polarizations and discuss the peak structures and low-energy power laws which might be expected for several popular models of the superconducting gap in these systems.

The parent compounds of recently discovered iron-arsenide superconductors, $A{\text{Fe}}_2{\text{As}}_2$ with alkaline earth $A=\text{Ca},\text{Sr},\text{Ba}$, undergo simultaneous structural and magnetic phase transitions at a temperature $T_{SM}$. Using a combination of polarized light microscopy and spatially resolved high-energy synchrotron x-ray diffraction we show that the orthorhombic distortion leads to the formation of $45°$-type structural domains in all parent compounds. Domains penetrate through the sample thickness in the $c$ direction and are not affected by crystal imperfections such as growth terraces. The domains form regular stripe patterns in the plane with a characteristic dimension of $10–50\mu \text{m}$. The direction of the stripes is fixed with respect to the tetragonal (100) and (010) directions but can change by $90°$ on thermal cycling through the transition. This domain pattern may have profound implications for intrinsic disorder and anisotropy of iron arsenides.

We carried out scanning tunneling spectroscopy measurements on exfoliated monolayer graphene on ${\text{SiO}}_2$ to probe the correlation between its electronic and structural properties. Maps of the local density of states are characterized by electron and hole puddles that arise due to long-range intravalley scattering from intrinsic ripples in graphene and random-charged impurities. At low energy, we observe short-range intervalley scattering which we attribute to lattice defects. Our results demonstrate that the electronic properties of graphene are influenced by intrinsic ripples, defects, and the underlying ${\text{SiO}}_2$ substrate.

We report on results of our theoretical study of the $c$-axis infrared conductivity of bilayer high-$T_c$ cuprate superconductors using a microscopic model involving the bilayer-split (bonding and antibonding) bands. An emphasis is on the gauge invariance of the theory, which turns out to be essential for the physical understanding of the electrodynamics of these compounds. The description of the optical response involves local (intrabilayer and interbilayer) current densities and local conductivities. The local conductivities are obtained using a microscopic theory, where the quasiparticles of the two bands are coupled to spin fluctuations. The coupling leads to superconductivity and is described at the level of generalized Eliashberg theory. Also addressed is the simpler case of quasiparticles coupled by a separable and nonretarded interaction. The gauge invariance of the theory is achieved by including a suitable class of vertex corrections. The resulting response of the model is studied in detail and an interpretation of two superconductivity-induced peaks in the experimental data of the real part of the $c$-axis conductivity is proposed. The peak around $400{\text{cm}}^{-1}$ is attributed to a collective mode of the intrabilayer regions, which is an analog of the Bogolyubov-Anderson mode playing a crucial role in the theory of the longitudinal response of superconductors. For small values of the bilayer splitting, its nature is similar to that of the transverse plasmon of the phenomenological Josephson superlattice model. The peak around $1000{\text{cm}}^{-1}$ is interpreted as a pair-breaking feature that is related to the electronic coupling through the spacing layers separating the bilayers.

We present a study of the transport properties of Pt-C nanowires created by focused-ion-beam (FIB)-induced deposition. By means of the measurement of the resistance while the deposit is being performed, we observe a progressive decrease in the nanowire resistivity with thickness, changing from ${10}^8\mu \Omega \text{cm}$ for thickness $\sim 20\text{nm}$ to a lowest saturated value of $700\mu \Omega \text{cm}$ for thickness $>150\text{nm}$. Spectroscopy analysis indicates that this dependence on thickness is caused by a gradient in the metal-carbon ratio as the deposit is grown. We have fabricated nanowires in different ranges of resistivity and studied their conduction mechanism as a function of temperature. A metal-insulator transition as a function of the nanowire thickness is observed. The results will be discussed in terms of the Mott-Anderson theory for noncrystalline materials. An exponential decrease in the conductance with the electric field is found for the most resistive samples, a phenomenon understood by the theory of hopping in lightly doped semiconductors under strong electric fields. This work explains the important discrepancies found in the literature for Pt-C nanostructures grown by FIB and opens the possibility to tune the transport properties of this material by an appropriate selection of the growth parameters.

We studied trigonal Bi nanowires $\left(30\text{nm}<\text{diameter}<200\text{nm}\right)$ via low-temperature magnetotransport for fields up to 14 T in order to investigate the role of their surfaces. A two-dimensional behavior was expected; we found instead a three-dimensional behavior, with a rich spectrum of Landau levels in a nearly spherical Fermi surface. We show that recent observations of sharp peaks in the bulk-Bi Nernst thermopower near the 9 T quantum limit attributed to charge fractionalization, can be more plausibly interpreted in terms of surface states. Bismuth has a true quantum limit at around 70 T.

We take advantage of the connection between the free-carrier optical conductivity and the glue function in the normal state, to reconstruct from the infrared optical conductivity the glue spectrum of ten different high-$T_c$ cuprates revealing a robust peak in the 50–60 meV range and a broad continuum at higher energies for all measured charge-carrier concentrations and temperatures up to 290 K. We observe that the strong-coupling formalism accounts fully for the known strong temperature dependence of the optical spectra of the high-$T_c$ cuprates, except for strongly underdoped samples. We observe a correlation between the doping trend of the experimental glue spectra and the critical temperature. The data obtained on the overdoped side of the phase diagram conclusively exclude the electron-phonon coupling as the main source of superconducting pairing.

We present ${}^{13}\text{C}$-NMR studies of the pressure-temperature phase diagram of the quasi-two-dimensional organic system ${\text{EtMe}}_3\text{P}{\left[\text{Pd}{\left(\text{dmit}\right)}_2\right]}_2$, which has a spin-gapped Mott insulating ground state at ambient pressure. As the system is pressurized, the spin-gapped insulating ground state survives until the insulator-metal Mott transition, just beyond which a superconducting state appears. This demonstrates that the superconducting phase borders the spin-gapped insulating phase. The phase diagram contrasts with those of the large majority of other correlated-electron superconductors in which the superconducting phase borders a magnetically ordered phase. This implies the possibility that the present superconductivity has an exotic origin and nature.

Using first-principles methods, we investigate the structural and electronic properties of SiGe nanowires-based heterostructures, whose lattice contains the same number of Si and Ge atoms but arranged in a different manner. Our results demonstrate that the wires with a clear interface between Si and Ge regions not only form the most stable structures but show a strongly reduced quantum confinement effect. Moreover, we, with the inclusion of many-body effects, prove that these nanowires—under optical excitation—display a clear electron-hole separation property which can have relevant technological applications.

We present a study of the hard-core Bose-Hubbard model at zero temperature on an infinite square lattice using the infinite projected entangled pair state algorithm [J. Jordan, R. Orús, G. Vidal, F. Verstraete, and J. I. Cirac, Phys. Rev. Lett. 101, 250602 (2008)]. Throughout the whole phase diagram our values for the ground-state energy, particle density, and condensate fraction accurately reproduce those previously obtained by other methods. We also explore ground-state entanglement, compute two-point correlators, and conduct a fidelity-based analysis of the phase diagram. Furthermore, for illustrative purposes we simulate the response of the system when a perturbation is suddenly added to the Hamiltonian.

We propose graphite intercalation compounds (GICs) as a material system with precisely the same electronic properties as doped few layer graphene. Despite the fact that GICs have been around for the last four decades, this fact has gone unnoticed so far. Especially, we focus on the electronic energy bands of ${\text{KC}}_8$ which correspond to a doped graphene monolayer. We provide extensive theoretical and experimental evidence for this claim employing a combined angle-resolved photoemission and theory approach using tight-binding, standard density-functional theory and including electron-electron correlation on a $GW$ level. We observe a strong momentum-dependent kink in the quasiparticle dispersion at 166 meV highlighting electron-phonon coupling to an in-plane transversal optical phonon. These results are key for understanding both the unique electronic properties of doped graphene layers and superconductivity in ${\text{KC}}_8$.

Is it possible to increase $T_c$ by constructing cuprate heterostructures, which combine the high pairing energy of underdoped layers with the large carrier density of proximate overdoped layers? We investigate this question within a model bilayer system using an effective theory of the doped Mott insulator. Interestingly, the question hinges on the fundamental nature of the superconducting state in the underdoped regime. Within a plain slave boson mean-field theory, there is absolutely no enhancement of $T_c$. However, we do get a substantial enhancement for moderate interlayer tunneling when we use an effective low energy theory of the bilayer in which the effective quasiparticle charge in the underdoped regime is taken as an independent phenomenological parameter. We study the $T_c$ enhancement as a function of the doping level and the interlayer tunneling, and discuss possible connections to recent experiments by Yuli et al. [Phys. Rev. Lett. 101, 057005 (2008)]. Finally, we predict a unique paramagnetic reduction in the zero-temperature phase stiffness of coupled layers, which depends on the difference in the current carried by quasiparticles on the two types of layers as ${\left({\mathbf{J}}_1-{\mathbf{J}}_2\right)}^2$.

We study the critical charge dynamics of the superconducting to the normal-state transition for ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ (LSCO) thin films with a wide range of the Sr concentration by measuring the frequency-dependent excess parts of the complex microwave conductivity, which is induced by the superconducting fluctuations. We present a dynamic scaling analysis of the complex fluctuation conductivity, which includes the information on the universality class and the dimensionality of the critical charge dynamics as a function of the Sr concentration, the film thickness, and the magnetic field. In our previous study [H. Kitano et al., Phys. Rev. B 73, 092504 (2006)], the two-dimensional $\left(2\text{D}\right)\text{−}XY$ critical dynamics for underdoped LSCO and the three-dimensional $\left(3\text{D}\right)\text{−}XY$ critical dynamics for optimally doped LSCO were reported. In this study, we observed a two-dimensional unknown critical charge dynamics for overdoped thin films from $x=0.17$ to 0.20, which is clearly distinguished from the $2\text{D-}XY$ critical dynamics. Through the systematic measurements by changing the film thickness or by applying small magnetic field, it was confirmed that this unusual behavior, which is referred as 2D-“U” below, was not induced by the finite-size effect but was intrinsic to the overdoped LSCO. Thus, it was found that the critical behavior in the phase diagram of LSCO is classified into the following three types: (i) $2\text{D-}XY$ for underdoped region, (ii) $3\text{D-}XY$ for optimally doped region, and (iii) 2D-“U” for overdoped region. In other words, the dimensionality in the critical charge dynamics is changed twice with hole doping. We discuss possible origins of such anomalous dimensional crossovers with hole doping, including an interpretation based on the possible existence of a hidden quantum critical point near the optimally doped region.

Using time-evolution time-dependent density functional theory (TDDFT) within the adiabatic local-density approximation, we study the interactions between single electrons and molecular resonances at surfaces. Our system is a nitrogen molecule adsorbed on a ruthenium surface. The surface is modeled at two levels of approximation, first as a simple external potential and later as a 20-atom cluster. We perform a number of calculations on an electron hitting the adsorbed molecule from inside the surface and establish a picture, where the resonance is being probed by the hot electron. This enables us to extract the position of the resonance energy through a fitting procedure. It is demonstrated that with the model we can extract several properties of the system, such as the presence of resonance peaks, the time electrons stay on the molecule before returning to the surface when hitting a molecular resonance and the lowering of the resonance energy due to an image charge effect. Finally we apply the TDDFT procedure to only consider the decay of molecular excitations and find that it agrees quite well with the width of the projected density of Kohn-Sham states.

First-principles density-functional theory (DFT) calculations have been used to investigate the crystal structures, thermodynamic stability, and decomposition pathways of Li-Mg-Al-H hydrogen storage compounds. We find that the recently discovered $\text{LiMg}{\left({\text{AlH}}_4\right)}_3$ compound is stable with respect to solid-state decomposition into ${\text{LiAlH}}_4$ and $\text{Mg}{\left({\text{AlH}}_4\right)}_2$; however, we also find that $\text{LiMg}{\left({\text{AlH}}_4\right)}_3$ is unstable with respect to hydrogen release and decomposes exothermically into ${\text{LiMgAlH}}_6$, Al, and ${\text{H}}_2$ with a calculated $T=300\text{K}$ enthalpy of $-7.3\text{kJ}/\left(\text{mol}{\text{H}}_2\right)$, in excellent agreement with the weakly exothermic value of $-5\text{kJ}/\left(\text{mol}{\text{H}}_2\right)$ obtained from differential scanning calorimetry measurements [M. Mamatha et al., J. Alloys Compd. 407, 78 (2006)]. ${\text{LiMgAlH}}_6$ is a stable intermediate, which has two competing endothermic decomposition pathways for ${\text{H}}_2$ release: one going directly into the binary hydrides of Li and Mg and the other proceeding via the formation of an intermediate ${\text{Li}}_3{\text{AlH}}_6$ phase, with room-temperature enthalpies of $+18.6$ and $+16.6\text{kJ}/\left(\text{mol}{\text{H}}_2\right)$, respectively. Using database searching based on known crystal structures from the inorganic crystal structure database, we predict that the hypothetical ${\text{MgAlH}}_5$ compound should assume the orthorhombic ${\text{BaGaF}}_5$ prototype structure, in contrast to a previous DFT study of ${\text{MgAlH}}_5$, [A. Klaveness et al., Phys. Rev. B 73, 094122 (2006)]. However, the decomposition enthalpy of ${\text{MgAlH}}_5$ is only weakly endothermic, $+1.1\text{kJ}/\left(\text{mol}{\text{H}}_2\right)$, and therefore this compound is not expected to occur in the high-temperature decomposition sequence of Mg alanate. We also present a comprehensive investigation of the phonon spectra and vibrational thermodynamics of Li-Mg-Al-H compounds, finding that vibrations typically decrease reaction enthalpies by up to $10\text{kJ}/\text{mol}{\text{H}}_2$ at ambient temperatures and significantly lower reaction entropies.

We use frequency resolved magneto-optic Kerr effect to probe the spin dynamics and mode structure in 50–200-nm-diameter ${\text{Ni}}_{80}{\text{Fe}}_{20}$ nanomagnets ranging from 3 to 10 nm in thickness. We find that the intrinsic Gilbert damping parameter is largely unaffected by the nanopatterning process despite a large linewidth dependence on the size of the nanomagnets. In the larger nanomagnets, both end and center modes are observed. The linewidth of these two modes differ considerably, which is most likely the result of the sensitivity of the end mode to small variations and imperfection of the shape and edge materials. We show that this effect can be exploited as a means to separately characterize the magnetic properties of the nanomagnets as well as the size and shape variations within the array.

The spin stiffness ${\rho }_s$ of a two-dimensional (2D) Heisenberg antiferromagnet depends nonanalytically on external magnetic field. We demonstrate that the hydrodynamic relation between ${\rho }_s$, the uniform susceptibility $\chi$, and the spin-wave velocity $c$ is not violated by such a behavior because similar nonanalytic terms from all three quantities mutually cancel out. In this work, explicit expressions for the field-dependent spin stiffness and for the magnon velocity of the 2D square-lattice antiferromagnet are obtained by direct calculation to order $1/S$ and in the whole range of magnetic fields.

Pronounced anisotropy of magnetic properties and complex magnetic order of a new oxi-halide compound ${\text{Co}}_7{\left({\text{TeO}}_3\right)}_4{\text{Br}}_6$ has been investigated by powder and single-crystal neutron-diffraction, magnetization, and ac susceptibility techniques. Anisotropy of susceptibility extends far into the paramagnetic temperature range. A principal source of anisotropy are anisotropic properties of the involved octahedrally coordinated single ${\text{Co}}^{2+}$ ions, as confirmed by angular-overlap-model calculations presented in this work. Incommensurate antiferromagnetic order sets in at $T_N=34\text{K}$. The propagation vector is strongly temperature dependent reaching ${\mathbf{k}}_1=\left[0.9458\left(6\right),0,0.6026\left(5\right)\right]$ at 30 K. A transition to a ferrimagnetic structure with ${\mathbf{k}}_2=0$ takes place at $T_C=27\text{K}$. The magnetically ordered phase is characterized by very unusual anisotropy as well: while $M\text{−}H$ scans along the $b$ axis reveal spectacularly rectangular but otherwise standard ferromagnetic hysteresis loops, $M\text{−}H$ studies along other two principal axes are perfectly reversible, revealing very sharp spin-flop (or spin-flip) transitions, such as in a standard antiferromagnet (or metamagnet). Altogether, the observed magnetic phenomenology is interpreted as an evidence of competing magnetic interactions permeating the system, in particular of the single-ion anisotropy energy and the exchange interactions. Different coordinations of the ${\text{Co}}^{2+}$ ions involved in the low-symmetry $C2/c$ structure of ${\text{Co}}_7{\left({\text{TeO}}_3\right)}_4{\text{Br}}_6$ render the exchange-interaction network very complex by itself. Temperature-dependent changes in the magnetic structure, together with an abrupt emergence of a ferromagnetic component, are ascribed to continual spin reorientations described by a multicomponent, but yet unknown, spin Hamiltonian.

Spin splitting of the energy spectrum of single-layer graphene on Au/Ni(111) substrate has been recently reported. I show that eigenstates of spin-orbit coupled graphene are polarized in-plane and are perpendicular to electron momentum $\mathbf{k}$; the magnitude of spin polarization $\mathbf{S}$ vanishes when $k\to 0$. In a perpendicular magnetic field $\mathbf{B}$, $\mathbf{S}$ is parallel to $\mathbf{B}$, and two zero modes emerge in the Landau level spectrum. Singular $\mathbf{B}$ dependence of their magnetization suggests existence of a new variety of magnetic instability. They also manifest themselves in a new variety of unconventional quantum Hall effect.