arXiv subjects

Y. Tanaka

Publications and source records attributed to Y. Tanaka.

At least 109 records · Page 6Linked to original sources

arXiv2006

Charge transport in two dimensional electron gas/superconductor junctions with Rashba spin-orbit coupling

We have studied the tunneling conductance in two dimensional electron gas / insulator / superconductor junctions in the presence of Rashba spin-orbit coupling (RSOC). It is found that for low insulating barrier the tunneling conductance is suppressed by the RSOC while for high insulating barrier it is almost independent of the RSOC. We also find the reentrant behavior of the conductance at zero voltage as a function of RSOC for intermediate insulating barrier strength. The results are essentially different from those predicted in ferromagnet / superconductor junctions. The present derivation of the conductance is applicable to arbitrary velocity operator with off-diagonal components.

cond-mat.supr-con↗

arXiv2006

Spin-polarized proximity effect in superconducting junctions

We study spin dependent phonomena in superconducting junctions in both ballistic and diffusive regimes. For ballistic junctions we study both ferromagnet / $s$- and d-wave superconductor junctions and two dimensional electron gas / s-wave superconductor junctions with Rashba spin-orbit coupling. It is shown that the exchange field alway suppresses the conductance while the Rashba spin-orbit coupling can enhance it. In the latter part of the article we study the diffusive ferromagnet / insulator / $s$- and d-wave superconductor junctions, where the proximity effect can be enhanced by the exchange field in contrast to common belief. It is shown that the resonant proximity effect originating from the exchange field strongly influences the tunneling conductance and density of states.

cond-mat.supr-con↗

arXiv2006

Uniform Mixing of High-Tc Superconductivity and Antiferromagnetism on a Single CuO2 Plane in Hg-based Five-layered Cuprate

We report a site selective Cu-NMR study on under-doped Hg-based five-layered high-$T_{\rm c}$ cuprate HgBa2Ca4Cu5Oy with a Tc=72 K. Antiferromagnetism (AF) has been found to take place at TN=290 K, exhibiting a large antiferromagnetic moment of 0.67-0.69uB at three inner planes (IP's). This value is comparable to the values reported for non-doped cuprates, suggesting that the IP may be in a nearly non-doped regime. Most surprisingly, the AF order is also detected with M(OP)=0.1uB even at two outer planes (OP's) that are responsible for the onset of superconductivity (SC). The high-Tc SC at Tc = 72 K can uniformly coexist on a microscopic level with the AF at OP's. This is the first microscopic evidence for the uniform mixed phase of AF and SC on a single CuO2 plane in a simple environment without any vortex lattice and/or stripe order.

cond-mat.supr-con↗

arXiv2005

Resonant peak in the density of states in the normal metal / diffusive ferromagnet / superconductor junctions

The conditions for the formation of zero-energy peak in the density of states (DOS) in the normal metal / insulator / diffusive ferromagnet / insulator / s-wave superconductor (N/I/DF/I/S) junctions are studied by solving the Usadel equations. The DOS of the DF is calculated in various regimes for different magnitudes of the resistance, Thouless energy and the exchange field of the DF, as well as for various resistances of the insulating barriers. The conditions for the DOS peak are formulated for the cases of weak proximity effect (large resistance of the DF/S interface) and strong proximity effect (small resistance of the DF/S interface).

cond-mat.supr-con↗

arXiv2005

Intrinsically s-wave like property of triplet superconductors with spin-orbit coupling

We have studied a general property of unconventional superconductors with spin-orbit coupling by solving the Bogoliubov-de Gennes equation and found that the two of four eigenfunctions for triplet pairing coincide with those for s-wave pairing when a relation holds between pairing symmetry and spin-orbit coupling, indicative of an intrinsically s-wave like property of triplet superconductors. Applying the result, we have studied the tunneling conductance in normal metal / insulator / unconventional superconductor junctions with a candidate of pairing symmetry of CePt$_3$Si. The effect of Rashba spin-orbit coupling (RSOC) is taken into account. We have found that the RSOC induces a peak at the energy gap like s-wave superconductors in the tunneling spectrum.

cond-mat.supr-con↗

arXiv2005

Resonant proximity effect in normal metal / diffusive ferromagnet / superconductor junctions

Resonant proximity effect in the normal metal / insulator / diffusive ferromagnet / insulator / $s$- and d-wave superconductor (N/I/DF/I/S) junctions is studied for various regimes by solving the Usadel equation with the generalized boundary conditions. It is shown that the resonant proximity effect originating from the exchange field in DF layer strongly modifies the tunneling conductance and density of states.

cond-mat.supr-con↗

arXiv2005

Meissner effect in diffusive normal metal / d-wave superconductor junctions

The Meissner effect in diffusive normal metal / insulator / d-wave superconductor junctions is studied theoretically in the framework of the Usadel equation under the generalized boundary condition. The effect of midgap Andreev resonant states (MARS) formed at the interface of d-wave superconductor is taken into account. It is shown that the formation of MARS suppresses the susceptibility of the diffusive normal metal.

cond-mat.supr-con↗

arXiv2005

Theory of thermal and charge transport in diffusive normal metal / superconductor junctions

Thermal and charge transport in the diffusive normal metal(DN) / insulator / $s$-, $d$- and p-wave superconductor junctions are studied for various situations, where we have used the Usadel equation with Nazarov's generalized boundary condition. Thermal and electrical conductance of the junction and the Lorentz ratio are calculated by varying the magnitudes of the resistance, the Thouless energy and the magnetic scattering rate in DN, the transparency of the insulating barrier, and the angle between the normal to the interface and the crystal axis of d-wave superconductors or the angle between the normal to the interface and the lobe direction of the p-wave pair potential. New general expression is derived for the calculation of the thermal conductance. It is demonstrated that the proximity effect doesn't influence the thermal conductance while the mid gap Andreev resonant states suppress it. We have also discussed a possibility of distinguishing pairing symmetries based on the dependencies of the electrical and thermal conductance on temperatures.

cond-mat.supr-con↗

arXiv2005

Anomalous features of the proximity effect in triplet superconductors

Anomalous features of the proximity effect specific to diffusive normal metal / triplet superconductor (DN/TS) junctions are studied within the quasiclassical Green's function formalism. The pair amplitude $f_{N}$ as a function of energy $ε$ in DN, satisfies an anomalous relation, $% f_{N}(ε)=-f^{*}_{N}(-ε)$, contrary to that in singlet superconductor junctions case where $f_{N}(ε)=f^{*}_{N}(-% ε)$. Such an unusual $ε$ dependence is responsible for a zero energy peak in the local density of states and is a source of an anomalous penetration of the applied magnetic field into DN. % Our results are relevant to Sr$_{2}$RuO$_{4}$, which is considered to have a $p_{x}+ip_{y}$-wave symmetry.

cond-mat.supr-con↗

arXiv2005

Fish-tail Effect and Irreversibility Field of (Cu,C)Ba$_{2}$Ca$_{3}$Cu$_{4}$O$_{x}$-(LiF)$_{y}$ superconductor

Addition of (LiF)$_{y<0.15}$, and of proper amount of (AgO)$_{z=0.45-0.8}$ as oxidizing agent, to (Cu,C)Ba$_{2}$Ca$_{3}$Cu$_{4}$O$_{10+d}$ superconductor is useful to control and shift the doping characteristics (hole density and distribution, and level of disorder) into the region where the irreversible properties, i.e. fish-tail effect (FTE) and irreversibility field H$_{irr}$, are improved. Among notable effects are the development of the second magnetization peak with a higher amplitude J$_{c}$, max and the enhancement of H$_{irr}$ at high temperatures, above a certain value T* which depends on both y$_{LiF}$ and z$_{AgO}$. The best results are obtained for the sample with y$_{LiF}$=0.1 and z$_{AgO}$=0.73. This sample preserves its single phase Cu,C-1234 composition. The influence on the FTE and H$_{irr}$ of the interplay between doping characteristics, controlled by LiF and AgO content, is discussed.

cond-mat.supr-con↗

arXiv2005

Josephson interferometer in a ring topology as a symmetry prove of Sr_2RuO_4

The Josephson effect is theoretically studied in two types of SQUIDs consisting of $s$ wave superconductor and Sr$_2$RuO$_4$. Results show various response of the critical Josephson current to applied magnetic fields depending on the type of SQUID and on the pairing symmetries. In the case of a $p_x+ip_y$ wave symmetry, the critical current in a corner SQUID becomes an asymmetric function of magnetic fields near the critical temperatures. Our results well explain a recent experimental finding [Nelson et. al, Science \textbf{306}, 1151 (2004)]. We also discuss effects of chiral domains on the critical current.

cond-mat.supr-con↗

arXiv2004

Theory of proximity effect in normal metal/$d_{x^2-y^2}$-wave superconductor interface in the presence of subdominant components of the pair potentials

Superconducting proximity effect in normal metal (N) / $d_{x^2-y^2}$-wave superconductor (D) junctions in the presence of attractive interelectron potentials which can induce subdominant s-wave pair potentials both in N and D sides, is studied based on the quasiclassical Green's function theory, where spatial dependencies of the pair potentials are determined self-consistently. In the N/D junctions with orientational angle with $θ= 0$, the s-wave component is induced in the N side by the proximity effect only for high transparent case, where the induced s-wave components in both the N and D sides do not break the time reversal symmetry (TRS). For fully transparent case, the resulting local density of states has a very sharp zero-energy peak (ZEP), the origin of which is the sign change of the pair potentials felt by the quasiparticles between the s-wave component in the N side and $d_{x^2-y^2}$-wave dominant component in the D side through Andreev reflections. On the other hands, for $θ= π/4$, the subdominant s-wave component which breaks the TRS appears near the interface. Besides, for lower transparent cases, the subdominant imaginary s-wave component is also induced near the interface in the N side. The proximity induced s-wave component in the N side does not enhance the magnitude of the s-wave component of the pair potential which break the TRS in the D side. The resulting LDOS at the interface has the ZEP or its splitting depending on the transparency of the junction.

cond-mat.supr-con↗

arXiv2004

Influence of magnetic impurities on charge transport in diffusive-normal-metal / superconductor junctions

Charge transport in the diffusive normal metal (DN) / insulator / $s$- and $% d $-wave superconductor junctions is studied in the presence of magnetic impurities in DN in the framework of the quasiclassical Usadel equations with the generalized boundary conditions. The cases of $s$- and d-wave superconducting electrodes are considered. The junction conductance is calculated as a function of a bias voltage for various parameters of the DN metal: resistivity, Thouless energy, the magnetic impurity scattering rate and the transparency of the insulating barrier between DN and a superconductor. It is shown that the proximity effect is suppressed by magnetic impurity scattering in DN for any value of the barrier transparency. In low-transparent s-wave junctions this leads to the suppression of the normalized zero-bias conductance. In contrast to that, in high transparent junctions zero-bias conductance is enhanced by magnetic impurity scattering. The physical origin of this effect is discussed. For the d-wave junctions, the dependence on the misorientation angle $α$ between the interface normal and the crystal axis of a superconductor is studied. The zero-bias conductance peak is suppressed by the magnetic impurity scattering only for low transparent junctions with $α\sim 0$. In other cases the conductance of the d-wave junctions does not depend on the magnetic impurity scattering due to strong suppression of the proximity effect by the midgap Andreev resonant states.

cond-mat.supr-con↗

arXiv2004

Theory of enhanced proximity effect by mid gap Andreev resonant state in diffusive normal metal / triplet superconductor junctions

Enhanced proximity effect by mid gap Andreev resonant state (MARS) in a diffusive normal metal / insulator / triplet superconductor (DN/TS) junction is studied based on the Keldysh-Nambu quasiclassical Green's function formalism. By choosing a p-wave superconductor as a typical example of TS, conductance of the junction and the spatial variation of quasiparticle local density of states (LDOS) in DN are calculated as the function of the magnitudes of the resistance $R_{d}$, Thouless energy in DN and the transparency of the insulating barrier. The resulting conductance spectrum has a zero bias conductance peak (ZBCP) and LDOS has a zero energy peak (ZEP) except for $α= π/2$ $(0 \leq α\leq π/2)$, where $α$ denotes the angle between the lobe direction of the p-wave pair potential to the normal to the interface. These peaks are revealed to be suppressed by applying a magnetic field. When the magnitude of $R_{d}/R_{0}$ is sufficiently large, the total zero voltage resistance of the junction is almost independent of the $R_{d}$ for $α\neq π/2$. The extreme case is $α=0$, where total zero voltage resistance is always $R_{0}/2$. Based on these results, a crucial test to identify triplet pairing superconductors based on tunneling experiments is proposed.

cond-mat.supr-con↗

arXiv2004

Long-term spectral changes in the partial-covering candidate NLS1 1H 0707-495

We compare two XMM-Newton observations of the Narrow-Line Seyfert 1 galaxy 1H 0707-495 separated by two years, and discuss the results in terms of the partial-covering phenomenon. The second longer observation once again displays a sharp (< 200 eV) spectral drop above 7 keV; however, in contrast to the first observation, the edge depth and energy have changed significantly. In addition to changes in the edge parameters, the high-energy spectrum appears steeper. The changes in the high-energy continuum can be adequately explained in terms of a partial-covering absorber outflowing from the central region. The low-energy spectrum also shows significant long-term spectral variability, including: a substantial increase in the disc temperature; detection of a ~0.9 keV emission feature; and the presence of warm absorption that was also detected during the ASCA mission, but not seen during the first XMM-Newton observation. The large increase in disc temperature, and more modest rise in luminosity, can be understood if we consider the slim-disc model for 1H 0707-495. In addition, the higher disc luminosity could be the driving force behind the outflow scenario and the re-appearance of a warm medium during the second XMM-Newton observation.

astro-ph↗

arXiv2004

A search for cyclotron resonance features with INTEGRAL

We present an INTEGRAL observation of the Cen-Crux region in order to search the electron cyclotron resonance scattering features from the X-ray binary pulsars. During the AO1 200ks observation, we clearly detected 4 bright X-ray binaries, 1 Seyfert Galaxy, and 4 new sources in the field of view. Especially from GX301-2, the cyclotron resonance feature is detected at about 37 keV, and width of 3--4 keV. In addition, the depth of the resonance feature strongly depends on the X-ray luminosity. This is the first detection of luminosity dependence of the resonance depth. The cyclotron resonance feature is marginally detected from 1E1145.1-6141. Cen X-3 was very dim during the observation and poor statistics disable us to detect the resonance features.These are first INTEGRAL results of searching for the cyclotron resonance feature.

astro-ph↗

arXiv2004

X-ray reflection in the NLS1 1H 0707-495

We apply a reflection-dominated model to the second XMM-Newton observation of the Narrow Line Seyfert 1 galaxy 1H 0707-495. As in the first XMM-Newton observation a sharp spectral drop is detected with energy that has shifted from 7 keV to 7.5 keV in two years. The drop is interpreted in terms of relativistically blurred ionised reflection from the accretion disc, while the energy shift can be accounted for by changes in the ionisation state and, more importantly, emissivity profile on the disc. A flatter emissivity profile during the second higher flux observation reduces gravitational redshift effects, therefore shifting the edge to higher energy. Remarkably, ionised disc reflection and the associated power law continuum provide a reasonable description of the broadband spectrum, including the soft excess. Considering both observations, the spectral variability in 1H 0707-495 appears to be due to the interplay between these two spectral components. The reflection component in the second observation is significantly less variable than the power law. Changes of the emissivity profile, spectral shape and variability properties (such as the rms spectrum) within the two observations are all consistent with a recently proposed model in which relativistic effects in the very inner regions of the nucleus play a major role.

astro-ph↗

arXiv2004

Theory of charge transport in diffusive normal metal / unconventional singlet superconductor contacts

We analyze the transport properties of contacts between unconventional superconductor and normal diffusive metal in the framework of the extended circuit theory. We obtain a general boundary condition for the Keldysh-Nambu Green's functions at the interface that is valid for arbitrary transparencies of the interface. This allows us to investigate the voltage-dependent conductance (conductance spectrum) of a diffusive normal metal (DN)/ unconventional singlet superconductor junction in both ballistic and diffusive cases. For d-wave superconductor, we calculate conductance spectra numerically for different orientations of the junctions, resistances, Thouless energies in DN, and transparencies of the interface. We demonstrate that conductance spectra exhibit a variety of features including a $V$-shaped gap-like structure, zero bias conductance peak (ZBCP) and zero bias conductance dip (ZBCD). We show that two distinct mechanisms: (i) coherent Andreev reflection (CAR) in DN and (ii) formation of midgap Andreev bound state (MABS) at the interface of d-wave superconductors, are responsible for ZBCP, their relative importance being dependent on the angle $α$ between the interface normal and the crystal axis of d-wave superconductors. For $α=0$, the ZBCP is due to CAR in the junctions of low transparency with small Thouless energies, this is similar to the case of diffusive normal metal / insulator /s-wave superconductor junctions. With increase of $α$ from zero to $π/4$, the MABS contribution to ZBCP becomes more prominent and the effect of CAR is gradually suppressed. Such complex spectral features shall be observable in conductance spectra of realistic high-$T_c$ junctions at very low temperature.

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