Temperature-dependence of spin-polarized transport in ferromagnet / unconventional superconductor junctions

Tunneling conductance in ferromagnet / unconventional superconductor junctions is studied theoretically as a function of temperatures and spin-polarization in feromagnets. In d-wave superconductor junctions, the existence of a zero-energy Andreev bound state drastically affects the temperature-dependence of the zero-bias conductance (ZBC). In p-wave triplet superconductor junctions, numerical results show a wide variety in temperature-dependence of the ZBC depending on the direction of the magnetic moment in ferromagnets and the pairing symmetry in superconductors such as $p_{x}$, $p_{y}$ and $p_{x}+ip_{y}$-wave pair potential. The last one is a promising symmetry of Sr$_2$RuO$_4$. From these characteristic features in the conductance, we may obtain the information about the degree of spin-polarization in ferromagnets and the direction of the $d$-vector in triplet superconductors

A phenomenological theory of zero-energy Andreev resonant states

A conceptual consideration is given to a zero-energy state (ZES) at the surface of unconventional superconductors. The reflection coefficients in normal-metal / superconductor (NS) junctions are calculated based on a phenomenological description of the reflection processes of a quasiparticle. The phenomenological theory reveals the importance of the sign change in the pair potential for the formation of the ZES. The ZES is observed as the zero-bias conductance peak (ZBCP) in the differential conductance of NS junctions. The split of the ZBCP due to broken time-reversal symmetry states is naturally understood in the present theory. We also discuss effects of external magnetic fields on the ZBCP.Comment: 12 page

Collisional energy transfer in two-component plasmas

The friction in plasmas consisting of two species with different temperatures is discussed together with the consequent energy transfer. It is shown that the friction between the two species has no effect on the ion acoustic mode in a quasi-neutral plasma. Using the Poisson equation instead of the quasi-neutrality reveals the possibility for an instability driven by the collisional energy transfer. However, the different starting temperatures of the two species imply an evolving equilibrium. It is shown that the relaxation time of the equilibrium electron-ion plasma is, in fact, always shorter than the growth rate time, and the instability can thus never effectively take place. The results obtained here should contribute to the definite clarification of some contradictory results obtained in the past

Temperature-dependent photoemission spectral weight transfer and chemical potential shift in Pr1−x_{1-x}Cax_xMnO3_3 : Implications for charge density modulation

We have studied the temperature dependence of the photoemission spectra of Pr$_{1-x}$Ca$_x$MnO$_3$ (PCMO) with $x=0.25$, 0.3 and 0.5. For $x=0.3$ and 0.5, we observed a gap in the low-temperature CE-type charge-ordered (CO) phase and a pseudogap with a finite intensity at the Fermi level ($E_F$) in the high-temperature paramagnetic insulating (PI) phase. Within the CO phase, the spectral intensity near $E_F$ gradually increased with temperature. These observations are consistent with the results of Monte Carlo simulations on a model including charge ordering and ferromagnetic fluctuations [H. Aliaga {\it et al.} Phys. Rev. B {\bf 68}, 104405 (2003)]. For $x=0.25$, on the other hand, little temperature dependence was observed within the low-temperature ferromagnetic insulating (FI) phase and the intensity at $E_F$ remained low in the high-temperature PI phase. We attribute the difference in the temperature dependence near $E_F$ between the CO and FI phases to the different correlation lengths of orbital order between both phases. Furthermore, we observed a chemical potential shift with temperature due to the opening of the gap in the FI and CO phases. The doping dependent chemical potential shift was recovered at low temperatures, corresponding to the disappearance of the doping dependent change of the modulation wave vector. Spectral weight transfer with hole concentration was clearly observed at high temperatures but was suppressed at low temperatures. We attribute this observation to the fixed periodicity with hole doping in PCMO at low temperatures.Comment: 5pages, 7figure

PT-Symmetric Quantum Theory Defined in a Krein Space

We provide a mathematical framework for PT-symmetric quantum theory, which is applicable irrespective of whether a system is defined on R or a complex contour, whether PT symmetry is unbroken, and so on. The linear space in which PT-symmetric quantum theory is naturally defined is a Krein space constructed by introducing an indefinite metric into a Hilbert space composed of square integrable complex functions in a complex contour. We show that in this Krein space every PT-symmetric operator is P-Hermitian if and only if it has transposition symmetry as well, from which the characteristic properties of the PT-symmetric Hamiltonians found in the literature follow. Some possible ways to construct physical theories are discussed within the restriction to the class K(H).Comment: 8 pages, no figures; Refs. added, minor revisio