Gap Symmetry of Superconductivity in UPd2Al3

The angle dependent thermal conductivity of the heavy-fermion superconductor UPd$_2$Al$_3$ in the vortex state was recently measured by Watanabe et al. Here we analyze this data from two perspectives: universal heat conduction and the angle-dependence. We conclude that the superconducting gap function $\Delta({\bf k})$ in UPd$_2$Al$_3$ has horizontal nodes and is given by $\Delta({\bf k}) =\Delta\cos(2\chi)$, with $\chi = ck_{z}$.Comment: 8 pages, 4 figures, improved figures in replacemen

Unconventional superconductors under rotating magnetic field II: thermal transport

We present a microscopic approach to the calculations of thermal conductivity in unconventional superconductors for a wide range of temperatures and magnetic fields. Our work employs the non-equilibrium Keldysh formulation of the quasiclassical theory. We solve the transport equations using a variation of the Brandt-Pesch-Tewordt (BPT) method, that accounts for the quasiparticle scattering on vortices. We focus on the dependence of the thermal conductivity on the direction of the field with the respect to the nodes of the order parameter, and discuss it in the context of experiments aiming to determine the shape of the gap from such anisotropy measurements. We consider quasi-two dimensional Fermi surfaces with vertical line nodes and use our analysis to establish the location of gap nodes in heavy fermion CeCoIn$_5$ and organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$.Comment: 17 pages, 13 figure

Enhanced quasiparticle heat conduction of the multigap superconductor Lu2Fe3Si5

The thermal transport measurements have been made on the Fe-based superconductor Lu2Fe3Si5 (Tc ~ 6 K) down to a very low temperature Tc/120. The field and temperature dependences of the thermal conductivity confirm the multigap superconductivity with fully opened gaps on the whole Fermi surfaces. In comparison to MgB2 as a typical example of the multigap superconductor in a p-electron system, Lu2Fe3Si5 reveals a remarkably enhanced quasiparticle heat conduction in the mixed state. The results can be interpreted as a consequence of the electronic correlations derived from Fe 3d-electrons.Comment: 5 pages, 4 figure

Effects of Fermi surface and superconducting gap structure in the field-rotational experiments: A possible explanation of the cusp-like singularity in YNi2_2B2_2C

We have studied the field-orientational dependence of zero-energy density of states (FODOS) for a series of systems with different Fermi surface and superconducting gap structures. Instead of phenomenological Doppler-shift method, we use an approximate analytical solution of Eilenberger equation together with self-consistent determination of order parameter and a variational treatment of vortex lattice. First, we compare zero-energy density of states (ZEDOS) when a magnetic field is applied in the nodal direction ($\nu_{node}(0)$) and in the antinodal direction ($\nu_{anti}(0)$), by taking account of the field-angle dependence of order parameter. As a result, we found that there exists a crossover magnetic field $H^*$ so that $\nu_{anti}(0) > \nu_{node}(0)$ for $H \nu_{anti}(0)$ for $H > H^*$, consistent with our previous analyses. Next, we showed that $H^*$ and the shape of FODOS are determined by contribution from the small part of Fermi surface where Fermi velocity is parallel to field-rotational plane. In particular, we found that $H^*$ is lowered and FODOS has broader minima, when a superconducting gap has point nodes, in contrast to the result of the Doppler-shift method. We also studied the effects of in-plane anisotropy of Fermi surface. We found that in-plane anisotropy of quasi-two dimensional Fermi surface sometimes becomes larger than the effects of Doppler-shift and can destroy the Doppler-shift predominant region. In particular, this tendency is strong in a multi-band system where superconducting coherence lengths are isotropic. Finally, we addressed the problem of cusp-like singularity in YNi$_2$B$_2$C and present a possible explanation of this phenomenon.Comment: 13pages, 23figure

Thermal conductivity through the quantum critical point in YbRh2Si2 at very low temperature

The thermal conductivity of YbRh2Si2 has been measured down to very low temperatures under field in the basal plane. An additional channel for heat transport appears below 30 mK, both in the antiferromagnetic and paramagnetic states, respectively below and above the critical field suppressing the magnetic order. This excludes antiferromagnetic magnons as the origin of this additional contribution to thermal conductivity. Moreover, this low temperature contribution prevails a definite conclusion on the validity or violation of the Wiedemann-Franz law at the field-induced quantum critical point. At high temperature in the paramagnetic state, the thermal conductivity is sensitive to ferromagnetic fluctuations, previously observed by NMR or neutron scattering and required for the occurrence of the sharp electronic spin resonance fracture.Comment: 11 pages + Supplementary Material

Spectral properties and geology of bright and dark material on dwarf planet Ceres

Variations and spatial distributions of bright and dark material on dwarf planet Ceres play a key role in understanding the processes that have led to its present surface composition. We define limits for bright and dark material in order to distinguish them consistently, based on the reflectance of the average surface using Dawn Framing Camera data. A systematic classification of four types of bright material is presented based on their spectral properties, composition, spatial distribution, and association with specific geomorphological features. We found obvious correlations of reflectance with spectral shape (slopes) and age; however, this is not unique throughout the bright spots. Although impact features show generally more extreme reflectance variations, several areas can only be understood in terms of inhomogeneous distribution of composition as inferred from Dawn Visible and Infrared Spectrometer data. Additional material with anomalous composition and spectral properties are rare. The identification of the composition and origin of the dark, particularly the darkest material, remains to be explored. The spectral properties and the morphology of the dark sites suggest an endogenic origin, but it is not clear whether they are more or less primitive surficial exposures or excavated subsurface but localized material. The reflectance, spectral properties, inferred composition, and geologic context collectively suggest that the bright and dark material tends to gradually change toward the average surface over time. This could be because of multiple processes, i.e., impact gardening/space weathering, and lateral mixing, including thermal and aqueous alteration, accompanied by changes in composition and physical properties such as grain size, surface temperature, and porosity (compaction).Comment: Meteoritics and Planetary Science; Dawn at Ceres special issu

Influence of gap structures to specific heat in oriented magnetic fields: Application to the orbital dependent superconductor, Sr2_2RuO4_4

We discuss influence of modulation of gap function and anisotropy of Fermi velocity to field angle dependences of upper critical field, $H_{c2}$, and specific heat, $C$, on the basis of the approximate analytic solution in the quasiclassical formalism. Using 4-fold modulation of the gap function and the Fermi velocity in the single-band model, we demonstrate field and temperature dependence of oscillatory amplitude of $H_{c2}$ and $C$. We apply the method to the effective two-band model to discuss the gap structure of Sr$_2$RuO$_4$, focusing on recent field angle-resolved experiments. It is shown that the gap structures with the intermediate magnitude of minima in $[100]$ direction for $\gamma$ band, and tiny minima of gaps in $[110]$ directions for $\alpha$ and $\beta$ bands give consistent behaviors with experiments. The interplay of the above two gaps also explains the anomalous temperature dependence of in-plane $H_{c2}$ anisotropy, where the opposite contribution from the passive $\alpha\beta$ band is pronounced near $T_c$.Comment: 7 pages, 11 figures in JPSJ forma

Temperature-dependent Ginzburg-Landau parameter

Taking into account both the orbital and the paramagnetic depairing effects we derive a simple analytic formula for the temperature dependence of the Ginzburg-Landau parameter valid in vicinity of field dependent critical temperature in a type-II superconductor.Comment: 3 pages, no figure

Point-contact spectroscopy of the borocarbide superconductor YNi2B2C

Point-contact (PC) spectroscopy measurements on YNi2B2C single crystals in the normal and superconducting (SC) state (T_c=15.4K) for the main crystallographic directions are reported. The PC study reveals the electron-phonon interaction (EPI) function with a dominant maximum around 12meV and a further weak structure (kink or shallow broad maximum) at higher energy at about 50meV. Other phonon maxima at 20, 24 and 32meV specified in the phonon DOS of YNi2B2C by neutron measurements [PRB, V.55, 9058 (1997)] are not resolved in the PC spectra pointing out to the main role of the low energy phonon modes in EPI. Directional study of the SC gap results in \Delta_[100]=1.5meV for the a- direction and \Delta_[001]=2.4meV along the c-axis which may point to anisotropic and/or multiband behavior. Noteworthy, the critical temperature T_c in all cases corresponds to that of bulk samples. The value 2\Delta_[001]/kT_c=3.6 is close to the BCS one of 3.52, and the temperature dependence \Delta(T) is BCS-like, while for the a-direction \Delta(T) deviates from mean-field BCS behavior above T_c/2. The directional variation in \Delta can be attributed to the multiband nature of the SC state in YNi2B2C predicted 10 years ago (PRL, V.80, 1730 (1998)).Comment: 3 figs, 2 pages, presented on M2S-HTSC Conference, July 9-14, 2006, Dresde

A Model of Direct Gauge Mediation

We present a simple model of gauge mediation (GM) which does not have a messenger sector or gauge singlet fields. The standard model gauge groups couple directly to the sector which breaks supersymmetry dynamically. This is the first phenomenologically viable example of this type in the literature. Despite the direct coupling, the model can preserve perturbative gauge unification. This is achieved by the inverted hierarchy mechanism which generates a large scalar expectation value compared to the size of supersymmetry breaking. There is no dangerous negative contribution to the squark, slepton masses due to two-loop renormalization group equation. The potentially non-universal supergravity contribution to the scalar masses can be suppressed enough to maintain the virtue of the gauge mediation. The model is completely chiral, and one does not need to forbid mass terms for the messenger fields by hand. Beyond the simplicity of the model, it possesses cosmologically desirable features compared to the original models of GM: an improved gravitino and string moduli cosmology. The Polonyi problem is back unlike in the original GM models, but is still much less serious than in hidden sector models.Comment: LaTeX, 12 page