The Meissner effect induced in a normal metal in close contact with a superconductor is known to weaken at very low temperatures, and to be restored by the application of a moderate magnetic field [Visani et al., Phys. Rev. Lett. 65, 1514 (1990)]. This reentrant behavior was recently interpreted as resulting from the presence of Kondo impurities in the normal metal, with calculated concentrations on the order of 100 ppm [Simons et al., Phys. Rev. B 86, 064509 (2012)]. We show here that in fact the presence of magnetic impurities in the normal metal at such concentrations is ruled out by the absence of a minimum of resistivity in the same samples down to 2 K.

The existence of Kondo impurities has recently been suggested as an explanation for the long-standing puzzle of paramagnetic reentrance in thick proximity systems. We show here that despite recent claims to the contrary such impurities may still be the source of this effect.

An electronic transmission line that contains an array of nonlinear elements (Josephson junctions) is studied theoretically. A continuous nonlinear wave equation describing the dynamics of the node flux along the transmission line is derived. It is shown that due to the nonlinearity of the system, a mixing process between four waves with different frequencies is possible. The mixing process can be utilized for amplification of weak signals due to the interaction with a strong pump wave. An analytical solution for the spatial evolution of the wave amplitudes is derived, and found to be in excellent agreement with the results of numerical computations. Simulations of realistic parameters show that the power gain can exceed 20 dB over a bandwidth of more than 2 GHz.

Magnetic flux dynamics in type-II superconductors is studied within the model of a viscous nonlinear diffusion of vortices for various sample geometries. We find that time dependence of magnetic moment relaxation after the field is switched off can be accurately approximated by $m\left(t\right)\propto 1-\sqrt{t/\tilde{\tau }}$ in the narrow initial time interval and by $m\left(t\right)\propto {(1+t/\tau )}^{-1}$ at later times before the flux creep sets in. The characteristic times $\tilde{\tau }$ and $\tau$ are proportional to the viscous drag coefficient $\eta$. Quantitative agreement with available experimental data is obtained for both conventional and high-temperature superconductors with $\eta$ exceeding by many orders of magnitude the Bardeen-Stephen coefficient for free vortices. Huge enhancement of the drag, as well as its exponential temperature dependence, indicates a strong influence of pinning centers on the flux diffusion. Notwithstanding the complexity of the vortex motion in the presence of pinning and thermal agitation, we argue that the initial relaxation of magnetization can still be considered as a viscous flux flow with an effective drag coefficient.

The thermal conductivity of iron-based superconductor CsFe${}_2$As${}_2$ single crystal ($T_c=1.81$ K) was measured down to 50 mK. A significant residual linear term ${\kappa }_0/T=1.27$ mW K${}^{-2}$ cm${}^{-1}$ is observed in zero magnetic field, which is about 1/10 of the normal-state value in upper critical field $H_{c2}$. In low magnetic field, ${\kappa }_0/T$ increases rapidly with field. The normalized ${\kappa }_0/T\left(H\right)$ curve for our CsFe${}_2$As${}_2$ (with residual resistivity ${\rho }_0=1.80$ $\mu \Omega$ cm) lies between the dirty KFe${}_2$As${}_2$ (with ${\rho }_0=3.32$ $\mu \Omega$ cm) and the clean KFe${}_2$As${}_2$ (with ${\rho }_0=0.21$ $\mu \Omega$ cm), which is consistent with its impurity level. These results strongly suggest a nodal superconducting gap in CsFe${}_2$As${}_2$, similar to its sister compound KFe${}_2$As${}_2$.

We study the finite temperature and magnetic-field phase diagram of electrons on the pyrochlore lattice subject to a local repulsion as a model for the pyrochlore iridates. We provide the most general symmetry-allowed Hamiltonian, including next-nearest-neighbor hopping, and relate it to a Slater-Koster-based Hamiltonian for the iridates. It captures Lifshitz and/or thermal transitions among several phases, such as metals, semimetals, topological insulators and Weyl semimetals, and gapped antiferromagnets with different orders. Our results on the charge conductivity, both dc and optical, Hall coefficient, magnetization, and susceptibility show good agreement with recent experiments and provide new predictions. As such, our effective model sheds light on the pyrochlore iridates in a unified way.

Nitrogen doping in graphene has important implications in graphene-based devices and catalysts. We have performed the density functional theory calculations to study the electronic structures of N-doped graphene with vacancies and Stone-Wales defect. Our results show that monovacancies in graphene act as hole dopants and that two substitutional N dopants are needed to compensate for the hole introduced by a monovacancy. On the other hand, divacancy does not produce any free carriers. Interestingly, a single N dopant at divacancy acts as an acceptor rather than a donor. The interference between native point defect and N dopant strongly modifies the role of N doping regarding the free carrier production in the bulk $\pi$ bands. For some of the defects and N dopant-defect complexes, localized defect $\pi$ states are partially occupied. Discussion on the possibility of spin polarization in such cases is given. We also present qualitative arguments on the electronic structures based on the local bond picture. We have analyzed the 1$s$-related x-ray photoemission and adsorption spectroscopy spectra of N dopants at vacancies and Stone-Wales defect in connection with the experimental ones. We also discuss characteristic scanning tunneling microscope (STM) images originating from the electronic and structural modifications by the N dopant-defect complexes. STM imaging for small negative bias voltage will provide important information about possible active sites for oxygen reduction reaction.

We investigated the structural transformations of the hexagonal phase of Ge${}_2$Sb${}_2$Te${}_5$ under pressure by means of ab initio molecular dynamics with a variable simulation cell. To overcome the enthalpy barriers between the different phases we used metadynamics techniques. We reproduced the hexagonal-to-$\mathrm{bcc}$ transformation under pressure found experimentally. The $\mathrm{bcc}$ phase retains a partial chemical order, as opposed to a second $\mathrm{bcc}$ phase we generated by pressuring the amorphous phase. This structural difference is suggested to be responsible for the memory effect uncovered experimentally, the $\mathrm{bcc}$ phase reverting to the amorphous or to the hexagonal phase upon decompression, depending on the type of precursor phase it originates from.

First-principles studies of iron oxyfluorides in the FeF${}_2$ rutile framework (FeO${}_x$F${}_{2-x}$, $0\le x\le 1$) are performed using density functional theory (DFT) in the general gradient approximation (GGA) with a Hubbard $U$ correction. Studies of O/F orderings reveal FeOF to be particularly stable compared to other FeO${}_x$F${}_{2-x}$ ($x\ne 1$) structures, where FeF${}_2$-FeOF mixing is not energetically favored. The band gap of FeF${}_2$ is found to decrease as oxygen is substituted into its structure. The GGA + $U$ electronic structure evolves from that of a Mott-Hubbard insulator ($x=0$) to a charge transfer semiconductor ($x=1$). Lithiation studies reveal that lithiation sites offering mixed O/F environments are the most stable. An insertion voltage plateau up to Li${}_{0.5}$FeOF on lithiation is found, in agreement with recent Li-ion battery experiments. The energetics of further lithiation with respect to conversion scenarios are discussed.

We investigate the properties of the nearest-neighbor singlet pairing and the emergence of $d$-wave superconductivity in the doped honeycomb lattice considering the limit of large interactions and the $t-J_1-J_2$ model. First, by applying a renormalized mean-field procedure as well as slave-boson theories which account for the proximity to the Mott-insulating state, we confirm the emergence of $d$-wave superconductivity, in agreement with earlier works. We show that a small but finite $J_2$ spin coupling between next-nearest neighbors stabilizes $d$-wave symmetry compared to the extended$s$-wave scenario. At small hole doping, to minimize the energy and to gap the whole Fermi surface or all the Dirac points, the superconducting ground state is characterized by a $d+id$ singlet pairing assigned to one valley and a $d-id$ singlet pairing to the other, which then preserves time-reversal symmetry. The slightly doped situation is distinct from the heavily doped case (around 3/8 and 5/8 filling) supporting a pure chiral $d+id$ symmetry and breaking time-reversal symmetry. Then, we apply the functional renormalization group and study in more detail the competition between antiferromagnetism and superconductivity in the vicinity of half filling. We discuss possible applications to strongly correlated compounds with copper hexagonal planes such as In${}_3$Cu${}_2$VO${}_9$. Our findings are also relevant to the understanding of exotic superfluidity with cold atoms.

We discuss heat transport through a Josephson tunnel junction under various bias conditions. We first derive the formula for the cooling power of the junction valid for arbitrary time dependence of the Josephson phase. Combining it with the classical equation of motion for the phase, we find the time-averaged cooling power as a function of bias current or bias voltage. We also find the noise of the heat current and, more generally, the full counting statistics of the heat transport through the junction. We separately consider the metastable superconducting branch of the current-voltage characteristics allowing quantum fluctuations of the phase in this case. This regime is experimentally attractive since the junction has low power dissipation, low impedance, and therefore may be used as a sensitive detector.

Using first-principles methods, we show that an applied perpendicular electric field $E$ breaks the inversion symmetry of AB-stacked bilayer graphene (BLG), thereby slightly mixing the two in-plane high-energy optical vibrations ($E_g$ and $E_u$ modes). The mixed amplitudes increase parabolically with respect to the field strength when $E<2.0$ V/nm, and then exhibit linear dependence when $E>2.0$ V/nm. In contrast, the mixing effect on the out-of-plane vibrations ($A_{1g}$ and $A_{2u}$ modes) is found to be much stronger, with the mixed amplitudes nearly an order of magnitude larger than those for the in-plane modes. For the two in-plane modes, we then calculate their phonon linewidths and frequency shifts as a function of the electric field as well as the Fermi level. Our results reveal delicate interplay between electrons and phonons in BLG, tunable by the applied fields and charge carrier densities.

Large-scale atomistic simulations using the reactive force field approach are implemented to investigate the thermomechanical properties of fluorinated graphene (FG). A set of parameters for the reactive force field potential optimized to reproduce key quantum mechanical properties of relevant carbon-fluorine cluster systems are presented. Molecular dynamics simulations are used to investigate the thermal rippling behavior of FG and its mechanical properties and compare them with graphene, graphane and a sheet of boron nitride. The mean square value of the height fluctuations $\langle h^2\rangle$ and the height-height correlation function $H\left(q\right)$ for different system sizes and temperatures show that FG is an unrippled system in contrast to the thermal rippling behavior of graphene. The effective Young's modulus of a flake of fluorinated graphene is obtained to be 273 N/m and 250 N/m for a flake of FG under uniaxial strain along armchair and zigzag directions, respectively.

We have studied the effects of substituting nonmagnetic Ga${}^{3+}$ for Mn${}^{3+}$ on dielectric and magnetic properties in multiferroic YMn${}_2$O${}_5$. Upon Ga substitution, whereby only the Mn${}^{3+}$ ions are selectively replaced, the commensurate magnetic phase eventually disappeared owing to the disconnection of the magnetic interaction between Mn${}^{3+}$ and Mn${}^{4+}$ ions. In contrast, the low-temperature incommensurate magnetic phase is hardly affected by Ga substitution. Precise crystal structure analyses of Ga-substituted YMn${}_2$O${}_5$ have revealed that the bonds between Mn ions along the $ab$ plane changed markedly with increasing Ga concentration, while those along the $c$ axis did not change at all. This indicates that the magnetic interactions along the $ab$ plane were altered dramatically, while those along the $c$ axis were robust against Ga substitution. From the dielectric and magnetic phase diagrams as a function of Ga substitution, we conclude that the electric polarization is predominantly induced by exchange-striction in the commensurate magnetic phase, and by an inverse Dzyaloshinskii-Moriya interaction in the incommensurate phase.

The spinon continues to be an elusive elementary excitation of frustrated antiferromagnets. To solidify evidence for its existence, we address the question of what will be the angle resolved photoemission spectroscopy (ARPES) signatures of single-crystal samples of herbertsmithite assuming it is described by the Dirac spin liquid state. In particular, we show that the electron spectral function will be linear in energy dependence near specific wave vectors and that this dependence is expected even after fluctuations of the mean-field values are taken into account. Observation of this unique signature in ARPES will provide very strong evidence of the existence of spinons in more than one dimensions.

We study the propagation and attenuation of zero sound and first sound in thin, arbitrarily polarized Fermi-liquid films. Following Khalatnikov and Abrikosov, we solve Landau's linearized kinetic equation in the relaxation-time approximation for a complex speed of sound. Analytic solutions are obtained in the hydrodynamic and ballistic limits for arbitrary polarization. By solving the collision integral in two dimensions, we find the well-known result that quasiparticle-quasiparticle collisions contribute to the collision frequencies $1/{\tau }_{\sigma }$, with a low-temperature term proportional to $T^2\ln (T_{F\sigma }/T)$, where $\sigma =\uparrow ,\downarrow$ is the spin state. If the films are adsorbed to a dynamic substrate, we find additional possible contributions to the collision frequency that come from quasiparticle-phonon interactions. We show, however, that for ${}^3$He thin films, the mismatch between possible maximum values of the Fermi velocity and the substrate speed of sound freezes this contribution out at usual experimental temperatures. Thus, we can conclude that zero sound propagates at absolute zero in this type of adsorbed Fermi-liquid film. By utilizing previous results for the Landau parameters of an arbitrarily polarized ${}^3$He film, we compute numerical solutions for the sound speeds and attenuation in the hydrodynamic and ballistic regimes, thereby studying the transition from first sound to zero sound as a function of temperature.

The cuprates contain a range of nanoscale phenomena that consist of both local density of states (LDOS) features and spatial excitations. These phenomena can be measured with spectroscopic imaging scanning tunneling microscopy, and the phenomena's disorder can be mapped out by fitting the data with a phenomenological model to the LDOS(E). The use of such a model allows a reduction of the complicated LDOS(r,E) data to key parameters for which doping and spatial dependence can be quantitatively determined. In this paper, we present a study of the nanometer-scale disorder in single-crystal, cryogenically cleaved samples of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+x}$ over a doping range of $p$ $=$ 0.19 to 0.06. The phenomenological model that we use to map out the disorder is the Tripartite model. This model has recently been successfully applied to the average LDOS(E), where its key energy scales have been shown to correspond to other observables. The application of the Tripartite model to the doping-dependent LDOS(r,E) data produces a series of energy scale maps that show a structured patchwork disorder of three energy scales. The spatial disorder structure is universal for all dopings and energy scales. It is independent of the oxygen dopants’ negative energy resonance. The interface between the different patches takes the form of a shortened lifetime pseudogap/superconducting gap state. The relationship between the energy scales and the spatial modulations of the dispersive quasiparticle interference (QPI), static q${}_1$${}^{*}$ modulation, and the pseudogap shows that the energy scales’ signatures in the LDOS(r,E) are tied to the onset and termination of these spatial excitations. The application of the Tripartite model allows the static q${}_1$${}^{*}$ modulation to have its local energy range measured and shows that its signature in the LDOS(E) is the kink, for which the total number of states is modulated by the wave vector of the q${}_1$${}^{*}$ wave vector. This analysis of both the LDOS(r,E) and the spatial modulations in q-space reveals a picture of a single underlying disordered parameter that determines both the LDOS(E) structure as well as the energy ranges of the QPI, q${}_1$${}^{*}$ modulation, and the pseudogap states. This underlying parameter represents a doping of chemical potential, if the patch is that of an infinite homogeneous superconductor.

We study chaotic scattering outside the wideband limit, as the Fermi energy $E_F$ approaches the band edges $E_B$ of a one-dimensional lattice embedding a scattering region of $M$ sites. We show that the delay-time and thermopower distributions differ near the edges from the universal expressions valid in the bulk. To obtain the asymptotic universal forms of these edge distributions, one must keep constant the energy distance $E_F-E_B$ measured in units of the same energy scale proportional to $M^{-1/3}$ which is used for rescaling the energy level spacings at the spectrum edges of large Gaussian matrices. In particular the delay time and the thermopower have the same universal edge distributions for arbitrary $M$ as those for an $M=2$ scatterer, which we obtain analytically.

We report on an ab initio strategy based on density functional theory to identify the muon sites. Two issues must be carefully addressed: muon delocalization about candidate interstitial sites and local structural relaxation of the atomic positions due to ${\mu }^{+}$-sample interaction. Here, we verify our strategy's validity focusing on two wide band gap materials, LiF and YF${}_3$, where both ${\mu }^{+}$ delocalization and crystal lattice relaxation play an important role in determining the ${\mu }^{+}$ stopping site positions.