Spin dynamics of the square lattice Heisenberg antiferromagnet, ${\text{Ba}}_2{\text{MnGe}}_2{\text{O}}_7$, is studied by a combination of bulk measurements, neutron diffraction, and inelastic neutron-scattering techniques. Easy plane type antiferromagnetic order is identified at $T\le 4.0\text{K}$. The exchange interactions are estimated as $J_1=27.8\left(3\right)\mu \text{eV}$ and $J_2=1.0\left(1\right)\mu \text{eV}$, and the saturation field $H_{\text{C}}$ is 9.75 T. Magnetic excitation measurements with high experimental resolution setup by triple axis neutron spectrometer reveals the instability of one magnon excitation in the field range of $0.7H_{\text{C}}\lesssim H\lesssim 0.85H_{\text{C}}$.

Angle-resolved magneto-Raman scattering has been performed on spin-polarized two-dimensional electron gas embedded in ${\text{Cd}}_{1-x}{\text{Mn}}_x\text{Te}$ quantum wells to explore the intrinsic damping of propagating spin-wave modes with in plane momentum $q$. The damping rate $\eta$ follows a quadratic law $\eta ={\eta }_0+{\eta }_2{q}^2$ due to losses in the spin current driven by the magnetization in qualitative agreement with Phys. Rev. B 78, 020404(R) (2008). As a consequence, the propagation length of a spin wave in a conducting system has an intrinsic maximum of importance for spin-wave-based spintronics.

We use magnetic force microscopy (MFM) to measure the local penetration depth $\lambda$ in $\text{Ba}{\left({\text{Fe}}_{0.95}{\text{Co}}_{0.05}\right)}_2{\text{As}}_2$ single crystals and use scanning superconducting quantum interference device susceptometry to measure its temperature variation down to 0.4 K. We observe that superfluid density ${\rho }_s$ over the full temperature range is well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of $\lambda$, as well as obtain its temperature dependence and spatial homogeneity. We find ${\rho }_s$ to be uniform on the submicron scale despite the highly disordered vortex pinning.

Folded single-layer graphene forms a system of two decoupled monolayers being only a few angstroms apart. Using magnetotransport measurements we investigate the electronic properties of the two layers conducting in parallel. We show a method to obtain the mobilities for the individual layers despite them being jointly contacted. The mobilities in the upper layer are significantly larger than in the bottom one indicating weaker substrate influence. This is confirmed by larger transport and quantum scattering times in the top layer. Analyzing the temperature dependence of the Shubnikov-de Haas oscillations, effective masses and corresponding Fermi velocities are obtained yielding reduced values down to 66% in comparison to monolayers.

We report on single-pulse resonant magnetic scattering experiments using soft x-ray pulses generated by the free-electron laser FLASH at DESY. We could record a magnetic diffraction pattern from a Co/Pt multilayer sample at the $\text{Co}{M}_{2,3}$ edge with a single 30-fs-long FEL pulse. The analysis of the magnetic small-angle scattering signal for subsequent pulses indicates a threshold energy density below which there is no indication that the magnetic properties of the sample might be altered.

We investigate charge transport in two-dimensional ferromagnet/ferromagnet junction on a topological insulator. The conductance across the interface depends sensitively on the directions of the magnetizations of the two ferromagnets, showing anomalous behaviors compared with the conventional spin valve. This stems from the way how the wave functions connect between both sides. It is found that the conductance depends strongly on the in-plane direction of the magnetization. Moreover, in sharp contrast to the conventional magnetoresistance effect, the conductance at the parallel configuration can be much smaller than that at the antiparallel configuration.

We show that the Haldane phase of $S=1$ chains is characterized by a double degeneracy of the entanglement spectrum. The degeneracy is protected by a set of symmetries (either the dihedral group of $\pi$ rotations about two orthogonal axes, time-reversal symmetry, or bond centered inversion symmetry), and cannot be lifted unless either a phase boundary to another, “topologically trivial,” phase is crossed, or the symmetry is broken. More generally, these results offer a scheme to classify gapped phases of one-dimensional systems. Physically, the degeneracy of the entanglement spectrum can be observed by adiabatically weakening a bond to zero, which leaves the two disconnected halves of the system in a finitely entangled state.

We extend the recently developed real-time diagrammatic Monte Carlo method, in its hybridization expansion formulation, to the full Kadanoff-Baym-Keldysh contour. This allows us to study real-time dynamics in correlated impurity models starting from an arbitrary, even interacting, initial density matrix. As a proof of concept, we apply the algorithm to study the nonequilibrium dynamics after a local quantum quench in the Anderson impurity model. Being a completely general approach to real-time dynamics in quantum impurity models, it can be used as a solver for nonequilibrium dynamical mean field theory.

The temperature-dependent optical reflectivity and complex transmissivity of an epitaxially grown $\text{Ba}{\left({\text{Fe}}_{0.9}{\text{Co}}_{0.1}\right)}_2{\text{As}}_2$ thin film were measured over a wide frequency range $\left(4–35000{\text{cm}}^{-1}\right)$. The opening of the superconducting gap $2{\Delta }_0=\left(3.7\pm 0.3\right)\text{meV}$ is directly observed by vanishing optical conductivity at $30{\text{cm}}^{-1}$ for $T<T_c=20\text{K}$. While in this range the measured temperature- and frequency-dependent electrodynamic properties agree well with the BCS predictions of a nodeless order parameter, unexpectedly a strong quasiparticle absorption shows up below 1.5 meV. The spectral weight of the condensate $1.94\times {10}^7{\text{cm}}^{-2}$ corresponds to a penetration depth $\lambda =3600\text{Å}$.

We discuss a particular source of error in the numerical renormalization group (NRG) method for quantum impurity problems, which is related to a renormalization of impurity parameters due to the bath propagator. At any step of the NRG calculation, this renormalization is only partially taken into account, leading to systematic variation in the impurity parameters along the flow. This effect can cause qualitatively incorrect results when studying quantum-critical phenomena, as it leads to an implicit variation in the phase transition’s control parameter as function of the temperature and thus to an unphysical temperature dependence of the order-parameter mass. We demonstrate the mass-flow effect for bosonic impurity models with a power-law bath spectrum, $J\left(\omega \right)\propto {\omega }^s$, namely, the dissipative harmonic oscillator and the spin-boson model. We propose an extension of the NRG to correct the mass-flow error. Using this, we find unambiguous signatures of a Gaussian critical fixed point in the spin-boson model for $s<1/2$, consistent with mean-field behavior as expected from quantum-to-classical mapping.

The temperature and magnetic field dependence of neutron-scattering studies on the heavy fermion compound ${\text{Ce}}_3{\text{Pd}}_{20}{\text{Si}}_6$ are reported. Inelastic neutron scattering reveals two crystal-field excitations corresponding to the environments of the two distinct rare-earth locations within this cubic compound. Surprisingly, the ground states of the two individual Ce sites are inequivalent with the $4a$ site having a ${\Gamma }_7$ ground state, while the $8c$ site reveals a ${\Gamma }_8$ ground state. This is in contrast to the situation found in the analogous compound ${\text{Ce}}_3{\text{Pd}}_{20}{\text{Ge}}_6$, where the ${\Gamma }_8$ ground state is realized for both Ce sites. In addition, these results reveal an interaction between the $8c$ and $4a$ sites previously believed to be absent. The ground state was probed with uniaxial polarization analysis across the region in which field-induced quantum criticality has previously been determined. In zero applied magnetic field no long-range magnetic order could be determined. However, further evidence of field-induced quantum criticality is presented with the observation of diffuse scattering consistent with quantum fluctuations due to nearest-neighbor spin correlations between the two Ce sites.

We investigate the electronic structure of an endohedral fullerene, ${\text{Sc}}_3{\text{N@C}}_{80}$, chemisorbed on $\text{Cu}\left(110\right)\text{−}\left(2\times 1\right)\text{-O}$ surface by scanning tunneling microscopy and density-functional theory. Scanning tunneling microscopy and spectroscopy identify a series of delocalized atomlike superatom molecular orbitals (SAMOs) in the ${\text{Sc}}_3{\text{N@C}}_{80}$ and its aggregates. By contrast to ${\text{C}}_{60}$, the encapsulated ${\text{Sc}}_3\text{N}$ cluster in ${\text{Sc}}_3{\text{N@C}}_{80}$ distorts the nearly-spherical central potential of the carbon cage, imparting an asymmetric spatial distribution to the SAMOs. When ${\text{Sc}}_3{\text{N@C}}_{80}$ molecules form dimers and trimers, however, the strong intermolecular hybridization results in highly symmetric hybridized SAMOs with clear bonding and antibonding characteristics. The electronic-structure calculations on ${\text{Sc}}_3{\text{N@C}}_{80}$ and its aggregates confirm the existence of SAMOs and reproduce their hybridization as observed in the experiment.

Bosonization technique for one-dimensional fermions out of equilibrium is developed in the framework of the Keldysh action formalism. We first demonstrate how this approach is implemented for free fermions and for the problem of nonequilibrium Fermi edge singularity. We then employ the technique to study an interacting quantum wire attached to two electrodes with arbitrary energy distributions. The nonequilibrium electron Green’s functions, which can be measured via tunneling spectroscopy technique and carry the information about energy distribution, zero-bias anomaly, and dephasing, are expressed in terms of functional determinants of single-particle “counting” operators. The corresponding time-dependent scattering phase is found to be intrinsically related to “fractionalization” of electron-hole excitations in the tunneling process and at boundaries with leads. Results are generalized to the case of spinful particles as well to Green’s functions at different spatial points (relevant to the problem of dephasing in Luttinger liquid interferometers). For double-step distributions, the dephasing rates are oscillatory functions of the interaction strength.

The gap generation is studied in suspended clean graphene in the continuum model for quasiparticles with the Coulomb interaction. We solve the gap equation with the dynamical polarization function and show that, comparing to the case of the static polarization function, the critical coupling constant lowers to the value ${\alpha }_c=0.92$, which is close to that obtained in lattice Monte Carlo simulations. It is argued that additional short-range four-fermion interactions should be included in the continuum model to account for the lattice simulation results. We obtain the critical line in the plane of electromagnetic and four-fermion coupling constants and find a second-order phase transition separating zero gap and gapped phases with critical exponents close to those found in lattice calculations.

We studied the interaction of excitons with terahertz (THz) electric fields in ZnSe/ZnMgSSe multiple quantum wells with THz-pump and optical-probe spectroscopy. The incident THz pulse with a peak electric field of $\sim 70\text{kV}/\text{cm}$ induces strong spectral modulations in the heavy-hole and light-hole excitonic absorption resonances. The dependence of the excitonic absorption resonance energy on the THz field follows the Stark effect for smaller THz fields. For larger THz fields, the peak shift deviates from the Stark effect, which implies that the interaction enters the nonperturbative regime.

We present a computational scheme to study spin excitations in magnetic materials from first principles. The central quantity is the transverse spin susceptibility, from which the complete excitation spectrum, including single-particle spin-flip Stoner excitations and collective spin-wave modes, can be obtained. The susceptibility is derived from many-body perturbation theory and includes dynamic correlation through a summation over ladder diagrams that describe the coupling of electrons and holes with opposite spins. In contrast to earlier studies, we do not use a model potential with adjustable parameters for the electron-hole interaction but employ the random-phase approximation. To reduce the numerical cost for the calculation of the four-point scattering matrix we perform a projection onto maximally localized Wannier functions, which allows us to truncate the matrix efficiently by exploiting the short spatial range of electronic correlation in the partially filled $d$ or $f$ orbitals. Our implementation is based on the full-potential linearized augmented-plane-wave method. Starting from a ground-state calculation within the local-spin-density approximation (LSDA), we first analyze the matrix elements of the screened Coulomb potential in the Wannier basis for the $3d$ transition-metal series. In particular, we discuss the differences between a constrained nonmagnetic and a proper spin-polarized treatment for the ferromagnets Fe, Co, and Ni. The spectrum of single-particle and collective spin excitations in fcc Ni is then studied in detail. The calculated spin-wave dispersion is in good overall agreement with experimental data and contains both an acoustic and an optical branch for intermediate wave vectors along the [1 0 0] direction. In addition, we find evidence for a similar double-peak structure in the spectral function along the [1 1 1] direction. To investigate the influence of static correlation we finally consider $\text{LSDA}+U$ as an alternative starting point and show that, together with an improved description of the Fermi surface, it yields a more accurate quantitative value for the spin-wave stiffness constant, which is overestimated in the LSDA.

The effective vibrational density of states (VDOS) has been derived from inelastic neutron-scattering data, for isotopically substituted $\text{Si}{{}^{18}\text{O}}_2$ and $\text{Si}{{}^{16}\text{O}}_2$ glasses, to gain information about the relative contribution to the Si and O partial VDOS. This is a necessary point of comparison for vibrational mode analyses of molecular-dynamics models. The mass contrast has led to a measurable shift between vibrational mode frequencies in the effective VDOS of $\text{Si}{{}^{18}\text{O}}_2$ and $\text{Si}{{}^{16}\text{O}}_2$, which is well reproduced in an ab initio simulation. The vibrational band centered at 100.2 meV is confirmed to have significantly lower contribution from the oxygen partial VDOS, than the higher (150.3 and 135.8 meV) and lower energy bands (53.3 meV).

In a recent paper by the authors [I. S. Burmistrov and A. M. M. Pruisken, Phys. Rev. Lett. 101, 056801 (2008)] it was shown that single-electron devices (single-electron transistor or SET) display “macroscopic charge quantization” which is completely analogous to the quantum Hall effect observed on very different electron systems. In this investigation we present more detail on these findings. Based on the Ambegaokar-Eckern-Schön (AES) theory of the Coulomb blockade we introduce a general response theory that probes the sensitivity of SET to changes in the boundary conditions. This response theory defines a set of physical observables and we establish the contact with the standard results obtained from ordinary linear-response theory. The response parameters generally define the renormalization behavior of the SET in the entire regime from weak coupling with large values of the tunneling conductance all the way down to the strong-coupling phase where the system displays the Coulomb blockade. We introduce a general criterion for charge quantization that is analogous to the Thouless criterion for Anderson localization. We present the results of detailed computations on the weak-coupling side of the theory, i.e., both perturbative and nonperturbative (instantons). Based on an effective theory in terms of quantum spins we study the quantum critical behavior of the AES model on the strong-coupling side. Consequently, a unifying scaling diagram of the SET is obtained. This diagram displays all the super universal topological features of the $\theta$ angle concept that previously arose in the theory of the quantum Hall effect.

A model-independent Fourier-inversion method for imaging elemental profiles from multilayer and total-external reflection x-ray standing wave (XSW) data is developed for the purpose of understanding the assembly of atoms, ions, and molecules at well-defined interfaces in complex environments. The direct-method formalism is derived for the case of a long-period XSW generated by low-angle specular reflection in an attenuating overlayer medium. It is validated through comparison with simulated and experimental data to directly obtain an elemental distribution contained within the overlayer. We demonstrate this formalism by extracting the one-dimensional profile of Ti normal to the surface for a ${\text{TiO}}_2/\text{Si}/\text{Mo}$ trilayer deposited on a Si substrate using the $\text{Ti}K\alpha$ fluorescence yield measured in air and under an aqueous electrolyte. The model-independent results demonstrate reduced coherent fractions for the in situ results associated with an incoherency of the x-ray beam (which are attributed to fluorescence excitation by diffusely or incoherently scattered x-rays). The uniqueness and limitations of the approach are discussed.

We present an analysis of infrared optical and magneto-optical conductivity data for a range of underdoped cuprate superconductors including ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_y$ and ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$. In light of recent magneto-oscillation experiments which have been interpreted in terms of Fermi surface reconstruction in magnetic field, we search for far-infrared signatures of field-induced pockets of coherent quasiparticles. Analysis of the conductivity spectra in magnetic field reveals no sign of field-induced Drude-like response, as well as no evidence for modification of the pseudogap. By considering changes of the low-frequency spectral weight, we are able to place limits on the oscillator strength of coherent modes deriving from proposed field-induced pockets in the Fermi surface. In underdoped ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ we observe a complete suppression of the superfluid density but no evidence for a coherent contribution to the conductivity. Other limits are established for ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_y$. We further discuss these results in the context of cuprate systems in which stripe order can account for the observed nodal effects but does not lead to the development of antinodal pockets with long-lived quasiparticles.