Theory of the proximity effect in junctions with unconventional superconductors

We present a general theory of the proximity effect in junctions between diffusive normal metals (DN) and unconventional superconductors in the framework of the quasiclassical Green's function formalism. Various possible symmetry classes in a superconductor are considered which are consistent with the Pauli principle: even-frequency spin-singlet even-parity (ESE) state, even-frequency spin-triplet odd-parity (ETO) state, odd-frequency spin-triplet even-parity (OTE) state and odd-frequency spin-singlet odd-parity (OSO) state. For each of the above four cases symmetry and spectral properties of the induced pair amplitude in the DN are determined. It is shown that the pair amplitude in a DN belongs respectively to an ESE, OTE, OTE and ESE pairing state.Comment: 5 pages with one figur

Stability of ferromagnetism in the Hubbard model on the kagom\'e lattice

The Hubbard model on the kagom\'e lattice has highly degenerate ground states (the flat lowest band) in the corresponding single-electron problem and exhibits the so-called flat-band ferromagnetism in the many-electron ground states as was found by Mielke. Here we study the model obtained by adding extra hopping terms to the above model. The lowest single-electron band becomes dispersive, and there is no band gap between the lowest band and the other band. We prove that, at half-filling of the lowest band, the ground states of this perturbed model remain saturated ferromagnetic if the lowest band is nearly flat.Comment: 4 pages, 1 figur

Optimal Output Feedback Architecture for Triangular LQG Problems

Distributed control problems under some specific information constraints can be formulated as (possibly infinite dimensional) convex optimization problems. The underlying motivation of this work is to develop an understanding of the optimal decision making architecture for such problems. In this paper, we particularly focus on the N-player triangular LQG problems and show that the optimal output feedback controllers have attractive state space realizations. The optimal controller can be synthesized using a set of stabilizing solutions to 2N linearly coupled algebraic Riccati equations, which turn out to be easily solvable under reasonable assumptions.Comment: To be presented at 2014 American Control Conferenc

Buckling of swelling gels

The patterns arising from the differential swelling of gels are investigated experimentally and theoretically as a model for the differential growth of living tissues. Two geometries are considered: a thin strip of soft gel clamped to a stiff gel, and a thin corona of soft gel clamped to a disk of stiff gel. When the structure is immersed in water, the soft gel swells and bends out of plane leading to a wavy periodic pattern which wavelength is measured. The linear stability of the flat state is studied in the framework of linear elasticity using the equations for thin plates. The flat state is shown to become unstable to oscillations above a critical swelling rate and the computed wavelengths are in quantitative agreement with the experiment

Uniaxial pressure dependencies of the phase boundary of TlCuCl_3

We present a thermal expansion and magnetostriction study of TlCuCl_3, which shows a magnetic-field induced transition from a spin gap phase to a Neel ordered phase. Using Ehrenfest relations we derive huge and strongly anisotropic uniaxial pressure dependencies of the respective phase boundary, e.g. the transition field changes by about $\pm 185$ GPa depending on the direction of uniaxial pressure.Comment: 2 pages, e figures; presented at SCES200

Theory of proximity effect in ferromagnet/superconductor heterostructures in the presence of spin dependent interfacial phase shift

We study the proximity effect and charge transport in ferromagnet (F)/superconductor (S) and S/F/I/F/S junctions (where I is insulator) by taking into account simultaneously exchange field in F and spin-dependent interfacial phase shifts (SDIPS) at the F/S interface. We solve the Usadel equations using extended Kupriyanov–Lukichev boundary conditions which include SDIPS, where spin-independent part of tunneling conductance GT and spin-dependent one Gφ coexist. The resulting local density of states (LDOS) in a ferromagnet depends both on the exchange energy Eex and Gφ/GT. We show that the magnitude of zero-temperature gap and the height of zero-energy LDOS have a non-monotonic dependence on Gφ/GT. We also calculate Josephson current in S/F/I/F/S junctions and show that crossover from 0-state to

Diffractive heavy pseudoscalar-meson productions by weak neutral currents

A first theoretical study for neutrino-induced diffractive productions of heavy pseudoscalar-mesons, \eta_c and \eta_b, off a nucleon is performed based on factorization formalism in QCD. We evaluate the forward diffractive production cross section in perturbative QCD in terms of the light-cone wave functions of Z boson and \eta_{c,b} mesons, and the gluon distribution of the nucleon. The diffractive production of \eta_c is governed by the axial vector coupling of the longitudinally polarized Z boson to Q\bar{Q} pair, and the resulting \eta_c production cross section is larger than the J/\psi one by one order of magnitude. The bottomonium \eta_b production, which shows up for higher beam energy, is also discussed.Comment: 5 pages with 3 embedded figures. Talk presented at the 15th International Spin Physics Symposium, Spin 2002, Brookhaven National Laboratory, September 9-14, 200

Anomalous Josephson effect in d-wave superconductor junctions on TI surface

We study Josephson effect of $d$-wave superconductor (DS)/ferromagnet insulator(FI)/DS junctions on a surface of topological insulator (TI). We calculate Josephson current $I\left(\varphi \right)$ for various orientations of the junctions where $\varphi$ is the macroscopic phase difference between two DSs. In certain configurations, we find anomalous current-phase relation $I(\varphi)=-I\left( -\varphi +\pi \right)$ with $2\pi$ periodicity. In the case where the first order Josephson coupling is absent without magnetization in FI, $I(\varphi)$ can be proportional to $\cos \varphi$. The magnitude of the obtained Josephson current is enhanced due to the zero energy states on the edge of DS on TI. Even if we introduce an $s$-wave component of pair potential in DS, we can still expect the anomalous current-phase relation in asymmetric DS junctions with $I\left( \varphi =0\right) \neq 0$. This can be used to probe the induced $d$-wave component of pair potential on TI surface in high-$T_{c}$ cuperate/TI hybrid structures.Comment: 6 pages, 4 figure