arXiv subjects

Yukio Tanaka

Publications and source records attributed to Yukio Tanaka.

At least 55 records · Page 3Linked to original sources

arXiv2024

A phenomenological theory of superconductor diodes

We study theoretically the superconductor diodes, where the magnitude of the critical current changes as the direction is reversed, in terms of a generalized Ginzburg-Landau model with the higher-order terms in the momentum of the order parameter. This theory is applied to Rashba spin-orbit coupled systems, where analytical relations between the nonreciprocal critical currents and the system parameters are achieved. Numerical calculations with mean-field theory are also obtained to study broader parameter regions. These results offer a rather general description and design principles of superconductor diodes.

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arXiv2023

Positive magnetoresistance in anapole superconductor junctions

The article presents a method to detect time-reversal symmetry breaking in non-centrosymmetric superconductors using only transport measurements. Specifically, if time-reversal symmetry is broken via a phase difference between singlet and triplet correlations, as in anapole superconductors, the conductance in SFN junctions is enhanced by increasing the exchange field strength in the ferromagnet. This is in sharp contrast with the negative magnetoresistance when using superconductors in which time reversal symmetry is preserved. Moreover, results show a large quadrupolar component of the magnetoresistance which is qualitatively different from the bipolar giant magnetoresistance in strong ferromagnets.

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arXiv2023

Particle-hole spectral asymmetry at the edge of multiorbital noncentrosymmetric superconductors

Bogoliubov quasiparticles are a coherent electron-hole quantum superposition which typically, for time-reversal symmetric superconductors, exhibit a spectral distribution with particle-hole symmetry. Here, we demonstrate that in two-dimensional noncentrosymmetric superconductors with multiorbital spin-triplet pairing the energy profile of the density of states at the edge can violate this paradigm. We show that the structure of Andreev reflections generally leads to pairing states made of configurations that are orbitally split due to the low degree of crystalline symmetry at the edge. The resulting pairing state has a mixed parity character in the orbital sector that, in the presence of reduced crystal symmetry at the edge, sets out a particle-hole asymmetric profile for the spectral function. These findings indicate a path to design asymmetric spectral functions at the edge of superconductors with orbital degrees of freedom and time-reversal symmetry. The emerging signatures can be exploited for the detection of spin-triplet pairing equipped with internal degrees of freedom.

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arXiv2023

Anomalous Josephson Coupling and High-Harmonics in Non-Centrosymmetric Superconductors with $S$-wave Spin-Triplet Pairing

We study the Josephson effects arising in junctions made of non-centrosymmetric superconductors with spin-triplet pairing having $s$-wave orbital-singlet symmetry. We demonstrate that the orbital dependent character of the spin-triplet order parameter determines its non-trivial texture in the momentum space due to the inversion symmetry breaking and spin-orbit interactions. The emergence of this pattern is responsible for the occurrence of an anomalous Josephson coupling and a dominance of high-harmonics in the current phase relation. Remarkably, due to the spin-orbital couplings, variations in the electronic structure across the heterostructure can generally turn the ground state of the junction from 0- to a generic value of the Josephson phase, thus realizing the so-called $φ$-junction. Hallmarks of the resulting Josephson behavior, apart from non-standard current-phase relation, are provided by an unconventional temperature and magnetic field dependence of the critical current. These findings indicate the path for the design of superconducting orbitronics devices and account for several observed anomalies of the supercurrent in oxide interface superconductors.

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arXiv2023

Tunable Josephson diode effect on the surface of topological insulators

The Josephson rectification effect, where the resistance is finite in one direction while zero in the other, has been recently realized experimentally. The resulting Josephson diode has many potential applications on superconducting devices, including quantum computers. Here, we theoretically show that a superconductor-normal metal-superconductor Josephson junction diode on the two-dimensional surface of a topological insulator has large tunability. The magnitude and sign of the diode quality factor strongly depend on the external magnetic field, gate voltage, and the length of the junction. Such rich properties stem from the interplay between different current-phase relations for the multiple transverse transport channels, and can be used for designing realistic superconducting diode devices.

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arXiv2023

Proximity effect of time-reversal symmetry broken non-centrosymmetric superconductors

In non-centrosymmetric superconductors the pair potential has both even-parity singlet and odd-parity triplet components. If time-reversal symmetry is broken, the superconducting phase of these components is not the same, for example in anapole superconductors. In this paper it is shown that breaking time-reversal symmetry by a phase difference between the two components significantly alters both the density of states and the conductance in s+helical p-wave superconductors. The density of states and conductance in s+chiral p-wave superconductors are less influenced by adding a phase difference because time reversal symmetry is already broken in the s+p-wave superconductor. The Tanaka-Nazarov boundary conditions are extended to 3D superconductors, allowing to investigate a greater variety of superconductors, such as B-W superconductors, in which the direction of the d-vector is parallel to the direction of momentum. The results are important for the determination of pair potentials in potentially time-reversal symmetry broken non-centrosymmetric superconductors.

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arXiv2023

Topological superconductivity in helical crystals

We study superconductivity and surface Andreev bound states in helical crystals. We consider the interlayer pairings along the helical hopping and investigate the surface local density of states on the (001) and zigzag surfaces for all the possible irreducible representations. There are three and four irreducible representations exhibiting the zero energy peaks in the local density of states at the (001) and zigzag surfaces of helical lattices, respectively. By calculating the one dimensional winging number, we show that these appearances of the zero energy peaks stem from the surface Andreev bound states.

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arXiv2023

Theory of Josephson current on a lattice model of grain boundary in $d$-wave superconductors

Identifying the origins of suppression of the critical current at grain boundaries of high-critical-temperature superconductors, such as cuprates and iron-based superconductors, is a crucial issue to be solved for future applications with polycrystalline materials. Although the dominant factor of current suppression might arise during material fabrication and/or processing, investigating it due to an internal phase change of the pair potential is an important issue in understanding the threshold of the critical current. In this paper, we study the Josephson current on a symmetric [001]-tilt grain boundary (GB) of a $d$-wave superconductor on a lattice model. In addition to the suppression of the maximum Josephson current associated with the internal phase change of the $d$-wave pair potential which has been predicted in continuum models, we find a unique phase interference effect due to folding of the Fermi surface in the lattice model. In particular, the resultant maximum Josephson current at low-tilting-angle regions tends to be suppressed more than that in preexisting theories. Because similar suppressions of the critical current at GBs have been reported in several experimental works, the present model can serve as a guide to clarify the complicated transport mechanism in GBs.

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arXiv2023

Phase-dependent Andreev molecules and superconducting gap closing in coherently coupled Josephson junctions

The Josephson junction (JJ) is an essential element of superconducting (SC) devices for both fundamental and applied physics. The short-range coherent coupling of two adjacent JJs forms the Andreev molecule states (AMSs), which will provide a new ingredient to engineer the SC transport in JJs and control the Andreev qubits. However, no experimental evidence of the AMSs in the coupled JJs has been reported. Here we provide the tunnel spectroscopic results of electrically controllable two planar JJs sharing one SC electrode. We discover that the coupled JJ results are highly modulated from the single JJ results, due to formation of the phase-dependent AMSs, meaning that the two JJs are coherently coupled. In addition, the superconducting gap closing due to the AMS formation is observed. Our results would help in understanding the microscopic mechanism of the coherent coupling and promoting the AMS physics to apply for research of the topological superconductivity and quantum information technology.

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arXiv2023

Generation of polarized spin-triplet Cooper pairings by magnetic barriers in superconducting junctions

We investigate the proximity effect in an s-wave superconductor/ferromagnetic metal with a Rashba spin-orbit coupling/diffusive normal metal junction and an s-wave superconductor/noncollinear magnetic metal/diffusive normal metal junction. We show the generation of equal spin-triplet pairings in the diffusive normal metal due to spin-flip scattering in the intermediate magnetic regions. The emergence of the spin-triplet odd-frequency Cooper pairings can generate a zero-energy peak in the quasiparticle density of states in the diffusive normal metal.

cond-mat.supr-con↗

arXiv2023

$η$-pairing on bipartite and non-bipartite lattices

The $η$-pairing is a type of Cooper pairing state in which the phase of the superconducting order parameter is aligned in a staggered manner, in contrast to the usual BCS superconductors with a spatially uniform phase. In this study, we search for a characteristic $η$-pairing state in a triangular lattice where a simple staggered alignment of the phase is not possible. As an example, we consider the attractive Hubbard model on both the square and triangular lattices under strong external Zeeman field. Using the mean-field approximation, we have identified several $η$-pairing states. Additionally, we have examined the electromagnetic stability of the pairing state by calculating the Meissner kernel. Odd-frequency pairing plays a crucial role in achieving diamagnetic response if the electrons experience a staggered superconducting phase during the propagation of current.

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arXiv2022

Theory of diode effect in d-wave superconductor junctions on the surface of topological insulator

Nonreciprocal responses of noncentrosymmetric quantum materials attract recent intensive interests, which is essential for the rectification function in diodes. A recent breakthrough is the discovery of superconducting diode effect. The principle to enlarge rectification effect is highly desired to guide the design of superconducting diode. Here, we study theoretically the Josephson junction S/FI/S (S: $d$-wave superconductor, FI: ferromagnetic insulator) on the surface of a topological insulator (TI). The simultaneous existence of $\sinφ$, $\cosφ$ and $\sin 2φ$ terms with almost the same order in Josephson current $I(φ)$ is essential to get larger values of $Q$ factor given by $Q=(I_{c}^{+} - \mid I_{c}^{-} \mid)/ (I_{c}^{+} + \mid I_{c}^{-} \mid)$ with $I_{c}^{+}={\rm max}(I(φ))$ and the negative one $I_{c}^{-}$ for macroscopic phase difference $φ$ of two superconductors on TI. We find that it can show a very large diode effect by tuning the crystal axes of $d$-wave superconductors and the magnetization of FI. The difference of the maximum Josephson currents $I_{c}$'s between the positive and negative directions can be about factor 2, where the current-phase relation is modified largely from the conventional one. The relevance of the zero energy Majorana bound states at the interface is also revealed. This result can pave a way to realize an efficient superconducting diode with low energy cost.

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arXiv2022

The supercurrent diode effect and nonreciprocal paraconductivity due to the chiral structure of nanotubes

The research interest in the supercurrent diode effect (SDE) has been growing. It has been found in various kinds of systems, in a large part of which it may be understood by combining spin-orbit coupling and Zeeman field. Here, we show that there exists another mechanism of generating SDE in chiral nanotubes that trap magnetic fluxes, without spin-orbit coupling or Zeeman field. We further show that the same generalized Ginzburg-Landau theory leads to nonreciprocal paraconductivity (NPC) near the transition temperature. The main features of both the SDE and the NPC are revealed by their parameter dependence. Our study suggests a new kind of platforms to explore nonreciprocal properties of superconducting materials. It also provides a theoretical link between the SDE and the NPC, which were often studied separately.

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arXiv2022

Odd-frequency pairs and anomalous proximity effect in nematic and chiral states of superconducting topological insulators

We investigate emergent odd-frequency pairs and proximity effect in nematic and chiral states of superconducting topological insulators (STIs), such as $M_x$Bi$_2$Se$_3$ ($M=$ Cu, Sr, Nb). The interplay of superconducting gap symmetry, the orbital degrees of freedom, and strong spin-orbit interaction generates a variety of odd-frequency pairs in the bulk and surface of STIs. The nematic and chiral states are the prototypes of topological superconductors with and without time-reversal symmetry, respectively. We find that the Fermi surface evolution from a closed spheroidal to an open cylindrical shape gives rise to the evolution of the emergent odd-frequency pairs and surface Andreev bound states (SABSs). In addition, spin polarization of odd-frequency pairs and SABSs stems from the non-unitary pairing in the chiral state. These evolution and spin polarization of odd-frequency pairs and SABSs can be captured by tunnel conductance spectroscopy. Furthermore, we study the anomalous proximity effect in various irreducible representations of STIs. The anomalous proximity effect is originally predicted in spin-triplet superconductor junctions without spin-orbit interaction; Odd-frequency spin-triplet $s$-wave pairs penetrate into diffusive normal metals (DN) and induce a pronounced zero-energy peak of the local density of states in the DN region. Here we demonstrate that contrary to the well-known results, the anomalous proximity effect in STIs is not immune to nonmagnetic impurities. The fragility is attributed to the fact that the proximitized odd-frequency even-parity pairs are admixtures of $s$-wave and non-$s$-wave pairs due to strong spin-orbit interaction inherent to the parent materials.

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arXiv2022

Surface State of Inter-orbital Pairing State in Sr$_2$RuO$_4$ Superconductor

We study the (001) surface state of a recently proposed $E_g$ symmetry inter-orbital-odd spin-triplet s-wave superconducting (SC) state in Sr$_2$RuO$_4$ (SRO). We confirm that this pair potential is transformed into a chiral $d$-wave pair potential and a pseudo-Zeeman field in the band basis for a low-energy range. Due to the chiral $d$-wave pair potential, the surface states appear near zero energy in the momentum range enclosed by the nodal lines of the chiral d-wave pair potential for each band at the (001) surface. Nevertheless, the pseudo-Zeeman field gives band splitting of the surface states, and its splitting energy is much smaller than the SC energy gap. The local density of states (LDOS) at the (001) surface of the SC state has a pronounced peak structure at zero energy due to the surface states near zero energy when the order of the resolution is lower than the splitting energy. This peak structure is robust under perturbations, such as an orbital Rashba coupling or an $E_u$ SC pair potential at the surface.

cond-mat.str-el↗

arXiv2022

Vortex supercurrent inversion by frequency-symmetry conversion of Cooper pairs

We theoretically demonstrate that the vortex supercurrent can be reversed by odd-frequency Cooper pairs accompanied by surface Andreev bound states. The surface of a three-dimensional superconductor pierced by a flux quantum is considered. We compare the vortex supercurrents near the surface of the spin-singlet $s$-wave and spin-triplet $p_z$-wave superconductors using quasiclassical Eilenberger theory, where the surface is perpendicular to the $z$ direction. We demonstrate that the vortex supercurrent near the surface of a $p_z$-wave superconductor is reversed compared to those far from the surface, whereas that of an $s$-wave superconductor is not. The splitting of the zero-energy states caused by the interference of the surface Andreev bound states and Caroli-de Gennes-Matricon modes is demonstrated.

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arXiv2022

Spin conductance in SNN junctions with non-centrosymmetric superconductors

An SNN-junction in which the superconducting potential is a mixture between s-wave and p-wave potentials is investigated using the Usadel equation equipped with Tanaka-Nazarov boundary conditions. The article provides several ways to distinguish between s + chiral and s + helical p-wave superconductors and a way to determine whether a superconductor has a mixed pair potential. Thus, it is of great importance in the determination of the pair potential of superconductors. It is shown that the different spin sectors satisfy independent equations and can thus be calculated separately even if the d-vector depends on the direction of momentum. This greatly simplifies the equations to be solved. It was found that a difference in conductance for sectors with opposite spins arises if both an s-wave and a p-wave component is present, even in the absence of a magnetic field. The results are confirmed by calculations in the ballistic regime. It is shown that the spin conductance for s + chiral p-wave and s + helical p-wave junctions is qualitatively similar. A setup containing two SN junctions is shown to give a clear difference between the two types of superconductivity.

cond-mat.supr-con↗

arXiv2022

Theory of proximity effect in $s+p$-wave superconductor junctions

We derive a boundary condition for the Nambu Keldysh Green's function in diffusive normal metal / unconventional superconductor junctions applicable for mixed parity pairing. Applying this theory to a 1d model of $s+p$-wave superconductor, we calculate LDOS in DN and charge conductance of DN / $s+p$-wave superconductor junctions. When the $s$-wave component of the pair potential is dominant, LDOS has a gap like structure at zero energy and the dominant pairing in DN is even-frequency spin-singlet $s$-wave. On the other hand, when the $p$-wave component is dominant, the resulting LDOS has a zero energy peak and the dominant pairing in DN is odd-frequency spin-triplet $s$-wave. We show the robustness of the quantization of the conductance when the magnitude of $p$-wave component of the pair potential is larger than that of $s$-wave one. These results show the robustness of the anomalous proximity effect specific to spin-triplet superconductor junctions.

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