Guided by a stylized experiment we develop a self-consistent anharmonic phonon concept for nonlinear lattices which allows for explicit “visualization.” The idea uses a small external driving force which excites the front particles in a nonlinear lattice slab and subsequently one monitors the excited wave evolution using molecular dynamics simulations. This allows for a simultaneous, direct determination of the existence of the phonon mean-free path with its corresponding anharmonic phonon wave number as a function of temperature. The concept for the mean-free path is very distinct from known prior approaches: the latter evaluate the mean-free path only indirectly, via using both a scale for for the phonon relaxation time and yet another one for the phonon velocity. Notably, the concept here is neither limited to small lattice nonlinearities nor to small frequencies. The scheme is tested for three strongly nonlinear lattices of timely current interest which either exhibit normal or anomalous heat transport.

The recently discovered spin Hall magnetoresistance effect electrically probes pure spin current flow across a ferrimagnetic insulator/normal metal bilayer interface. While usually the dc electrical resistance of the bilayer is measured as a function of the magnetization orientation in the magnetic insulator, here we present magnetoimpedance measurements using bias currents with frequencies up to several GHz. We find that the spin Hall magnetoresistance effect is frequency independent up to frequencies of 3 GHz, corroborating the assumption of a frequency-independent spin Hall angle. Our data therefore show that all interaction time constants relevant for the spin Hall magnetoresistance effect are shorter than about 50 ps. Therefore this technique should allow for the fast readout of the magnetization direction in magnetic insulator/normal metal bilayers.

The magnetic structure of the cubic helimagnet MnGe was studied by small-angle neutron scattering in a wide temperature range from 10 to 300 K. We show that MnGe undergoes a complex order-disorder phase transition covering more than 100 K above the critical temperature $T_N=(130\pm 2)$ K. Moreover, the helical structure is intrinsically unstable below $T_N$, since the profile of the reflection at $k_h\sim 2$ ${\mathrm{nm}}^{-1}$ associated with the spin helix can be described by the sum of Gaussian and Lorentzian contributions, indicating a stable helix and spin-helix fluctuations, respectively. The Gaussian behavior determines the system at low temperature, whereas the Lorentzian is negligible. With increasing temperature, however, the Lorentzian starts to contribute and dominates at $T_N$. The spin-helix fluctuations are accompanied by intensive spin excitations observed in small-angle neutron scattering as $Q$-independent scattering at $Q<k_h$. The integral intensities of both the spin excitations and spin fluctuations obey exactly the same temperature law proportional to $exp(T/T_a)$ with the activation temperature $T_a=(80\pm 1)$ K. These two contributions coexist above $T_N$, being well distinguished up to $T_{\mathrm{DM}}=150$ K, where they are intermixed with and transform into diffuse scattering at $T_{\mathrm{SR}}=(175\pm 5)$ K. Above $T_{\mathrm{SR}}$ the diffuse scattering is well described by a Gauss law, ascribed to static short-range ferromagnetic inhomogeneities with an inverse correlation length $\kappa \approx 2$ ${\mathrm{nm}}^{-1}$.

We present a study of granular Co-Pt multilayers by means of high-resolution transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS), SQUID-based magnetic measurements, anomalous Hall effect (AHE), and x-ray magnetic circular dichroism (XMCD). We describe these granular films as composed of particles with a pure cobalt core surrounded by an alloyed Co-Pt interface, embedded in a Pt matrix. The alloy between the Co and Pt in these granular films, prepared by room temperature sputter deposition, results from interdiffusion of the atoms. The presence of this alloy gives rise to a high perpendicular magnetic anisotropy (PMA) in the granular films, as consequence of the anisotropy of the orbital moment in the Co atoms in the alloy, and comparable to that of highly-ordered CoPt $L{1}_0$ alloy films. Their magnetic properties are those of ferromagnetically coupled particles, whose coupling is strongly temperature dependent: at low temperatures, the granular sample is ferromagnetic with a high coercive field; at intermediate temperatures the granular film behaves as an amorphous asperomagnet, with a coupling between the grains mediated by the polarized Pt, and at high temperatures, the sample has a superparamagnetic behavior. The coupling/decoupling between the grains in our Co-Pt granular films can be tailored by variation of the amount of Pt in the samples.

The effects of electron (Ir) and hole (Re) doping on the hybridization gap and antiferromagnetic order have been studied by magnetization, muon spin relaxation $\left({\mu }^{+}\mathrm{SR}\right)$, and inelastic neutron scattering on polycrystalline samples of $\mathrm{Ce}\left({\mathrm{Os}}_{1-x}{\mathrm{Ir}}_x\right){}_2{\mathrm{Al}}_{10}$ $(x=0.08$ and 0.15) and $\mathrm{Ce}\left({\mathrm{Os}}_{1-y}{\mathrm{Re}}_y\right){}_2{\mathrm{Al}}_{10}$ $\left(y=0.03\right)$. ${\mu }^{+}\mathrm{SR}$ spectra clearly reveal bulk magnetic ordering below 20 and 10 K for $x=0.08$ and 0.15 samples, respectively, with heavily damped oscillations of the muon asymmetry. Our important findings are that a small amount of electron doping (i) completely suppresses the inelastic magnetic excitations at low temperatures, which were observed at 11 meV in the undoped compound, and the magnetic response transforms into a broad quasielastic response, and (ii) surprisingly, the internal field at the corresponding muon site is remarkably enhanced by an order of magnitude compared with the parent compound. Moreover, a small amount (3% Re) of hole doping results in a significant reduction of the intensity of 11 meV peak and an increased $c-f$ hybridization, which is in agreement with the reduction of the Ce ordered moment seen through neutron diffraction and ${\mu }^{+}\mathrm{SR}$. The main origin of the observed doping effect is an extra $5d$ electron being carried by Ir and a hole carried by Re with respect to the Os atom. The absence of a spin gap/spin wave, despite a larger ordered state moment of Ce, in the electron-doped system cannot be explained based on a conventional theory of magnetism. Thus, the obtained results demonstrate a great sensitivity to the carrier doping and provide additional ways to study the anomalous magnetic properties in the $\mathrm{Ce}{T}_2{\mathrm{Al}}_{10}$ ($T=$ Fe, Ru, and Os).

We show that when spin and orbital angular momenta are entangled by spin-orbit coupling, this transforms a topological spin-triplet superfluid/superconductor state, such as ${}^3\text{He-}B$, into a topological $s^{\pm }$ state, with nontrivial gapless edge states. Similar to ${}^3\text{He-}B$, the $s^{\pm }$ state also minimizes on-site Coulomb repulsion for weak to moderate interactions. A phase transition into a topological $d$-wave state occurs for sufficiently strong spin-orbit coupling.

We suggest that an inhomogeneous (nonzero-momentum) paired phase can appear in the absence of an external magnetic field in a system with a predominant interband pairing and with separate Fermi-surface sheets. The Fermi wave vector mismatch which appears in such a situation can be compensated by nonzero center-of-mass momentum of the Cooper pairs, which can lead to a spontaneous appearance of the Fulde-Ferrell type of superconducting state. The idea is examined using a tight-binding model which emulates the holelike and the electronlike bands of iron-based superconductors. The state can appear for the cases of both spin-singlet and spin-triplet pairing channels.

Quasiparticle properties in the superconducting state are masked by the superfluid and are not directly accessible to infrared spectroscopy. We show how one can use a Kramers–Kronig transformation to separate the quasiparticle from superfluid response and extract intrinsic quasiparticle properties in the superconducting state. We also address the issue of a narrow quasiparticle peak observed in microwave measurements, and demonstrate how it can be combined with infrared measurements to obtain a unified picture of electrodynamic properties of cuprate superconductors.

Amorim et al. [Phys. Rev. B 89, 224307 (2014)] reported the theoretical investigation of quantum crystalline membranes. In this Comment we dismiss the validity of their calculations based on a “natural” estimation of the ultraviolet divergent contributions into correlation functions. We claim that such calculations give qualitatively the wrong results.

We reply to the preceding Comment by Kats and Lebedev [Phys. Rev. B 90, 176301 (2014)] about our paper [Phys. Rev. B 89, 224307 (2014)]. Kats and Lebedev criticize the validity of our calculation with the use of a Debye momentum as an ultraviolet regulator of the theory and the obtention of a $k^2$ term for the self-energy of out-of-plane modes. The arguments presented by Kats and Lebedev ignore basic facts about the theory of crystalline membranes. We address and counter argue the criticisms presented about our paper.

We investigate the Thomson-Onsager relation between the spin-dependent Seebeck and spin-dependent Peltier effect. To maintain identical device and measurement conditions we measure both effects in a single ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{Cu}/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanopillar spin valve device subjected to either an electrical or a thermal bias. In the low bias regime, we observe similar spin signals as well as background responses, as required by the Onsager reciprocity relation. However, at large biases, deviation from reciprocity occurs in the voltage-current relationships, dominated by nonlinear contributions of the temperature-dependent transport coefficients. By systematic modeling of these nonlinear thermoelectric effects and measuring higher-order thermoelectric responses for different applied biases, we identify the transition between the two regimes as the point at which Joule heating starts to dominate over Peltier heating. Our results signify the importance of local equilibrium (linearity) for the validity of this phenomenological reciprocity relation.

Recent experimental investigations of arrays of magnetic atoms deposited on top of a superconductor have opened a new chapter in the search for topological superconductivity. We generalize the microscopic model derived by Pientka et al. [Phys. Rev. B 88, 155420 (2013)] to accommodate the effects of finite supercurrent in the host material. Previously it was discovered that helical chains with nonplanar textures are plagued by a gapless phase. We show that by employing supercurrent it is possible to tune the chain from the gapless phase to the topological gapped phase. It is also possible to tune the chain between the trivial and the topological gapped phase, the size of which may be dramatically increased due to supercurrent. For planar textures supercurrent mainly contributes to proliferation of the gapless phase. Our predictions, which could be probed in scanning tunneling microscope experiments, are encouraging for the observation and manipulation of Majorana states.

A domain wall (DW) in a nanowire can propagate under a longitudinal magnetic field by emitting spin waves (SWs). We numerically investigated the properties of SWs emitted by the DW motion, such as frequency and wave number, and their relation with the DW motion. For a wire with a low transverse anisotropy and in a field above a critical value, a DW emits SWs to both sides (bow and stern), while it oscillates and propagates at a low average speed. For a wire with a high transverse anisotropy and in a weak field, the DW emits mostly stern waves, while the DW distorts itself and the DW center propagates forward like a drill at a relative high speed.

The ground state properties of an Ising chain with nearest ($J_1$) and next-nearest neighbor ($J_2$) interactions in a transverse field are investigated using the density matrix renormalization group and cluster mean-field theory methods. Its quantum phase diagram has two ordered regions (separated by a quantum disordered region), one of which has simple ferromagnetic or Néel order depending upon the sign of $J_1$, and the other is the double-staggered antiferromagnetic phase. The presence of the edge modes in these ordered phases is inferred by calculating the end-to-end spin-spin correlations on open chains. It is found that there occur four edge modes in the double-staggered phase, while the ferromagnetic and Néel phases support two edge modes, except very near $J_1=0$, where it seems they have four edge modes like the exact case at $J_1=0$ itself.

We study theoretically planar interfaces between two domains of superfluid ${}^3\mathrm{He}$-B. The structure of the B-B walls is determined on the scale of the superfluid condensation energy, and thus the domain walls have thickness on the order of the Ginzburg-Landau coherence length ${\xi }_{\mathrm{GL}}$. We study the stability and decay schemes of five inequivalent structures of such domain walls using a one-dimensional Ginzburg-Landau simulation. We find that only one of the structures is stable against small perturbations. We also argue that B-B interfaces could result from the adiabatic $A\to B$ transition and study textures at B-B interfaces. The B-B interface has a strong orienting effect on the spin-orbit rotation axis $\widehat{\mathit{n}}$ producing textures similar to those caused by external walls. We study the B-B interface in a parallel-plate geometry and find that the conservation of spin current sets an essential condition on the structure. The stable B-B interface gives rise to half-quantum circulation. The energies of bound quasiparticle excitations are studied in a simple model.

We use femtosecond resonant soft x-ray diffraction to measure the optically stimulated ultrafast changes of charge-density wave correlations in underdoped $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6.6}$. We find that when coherent interlayer transport is enhanced by optical excitation of the apical oxygen distortions, at least 50% of the in-plane charge-density wave order is melted. These results indicate that charge ordering and superconductivity may be competing up to the charge ordering transition temperature, with the latter becoming a hidden phase that is accessible only by nonlinear phonon excitation.

Where the doped holes reside in cuprate superconductors has crucial implications for the understanding of the mechanism responsible for their high-temperature superconductivity. It has been generally assumed that doped holes reside in hybridized $\mathrm{Cu}{d}_{x^2-y^2}\text{−}\mathrm{O}p\sigma$ orbitals in the ${\mathrm{CuO}}_2$ planes, based on results of density functional band-structure calculations. Instead, we propose that doped holes in the cuprates reside in $\mathrm{O}p\pi$ orbitals in the plane, perpendicular to the $\mathrm{Cu}\text{−}\mathrm{O}$ bond, that are raised to the Fermi energy through local orbital relaxation, that is not taken into account in band-structure calculations that place the bands associated with these orbitals well below the Fermi energy. We use a dynamic Hubbard model to incorporate the orbital relaxation degree of freedom and find in exact diagonalization of a small ${{\mathrm{Cu}}_4\mathrm{O}}_4$ cluster that holes will go to the $\mathrm{O}p\pi$ orbitals for relaxation energies comparable to what is expected from atomic properties of oxygen anions. The bandwidth of this band becomes significantly smaller than predicted by band-structure calculations due to the orbital relaxation effect. Within the theory of hole superconductivity the heavy hole carriers in this almost full band will pair and drive the system superconducting through lowering of their quantum kinetic energy.

We self-consistently study surface states of superconducting topological insulators. We clarify that, if a topologically trivial bulk $s$-wave pairing symmetry is realized, parity mixing of the pair potential near the surface is anomalously enhanced by surface Dirac fermions, opening an additional surface gap larger than the bulk one. In contrast to classical $s$-wave superconductors, the resulting surface density of state hosts an extra coherent peak at the induced gap besides a conventional peak at the bulk gap. We also find that no such extra peak appears for odd-parity superconductors with a cylindrical Fermi surface. Our calculation suggests that the simple U-shaped scanning tunneling microscope spectrum in ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ does not originate from $s$-wave superconductivity, but can be explained by odd-parity superconductivity with a cylindrical Fermi surface.