arXiv subjects

Takuya Nomoto

Publications and source records attributed to Takuya Nomoto.

77 records · Page 5Linked to original sources

arXiv2016

Upper critical field and quantum oscillations in tetragonal superconducting FeS

The magnetoresistance and magnetic torque of FeS are measured in magnetic fields $B$ of up to 18 T down to a temperature of 0.03 K. The superconducting transition temperature is found to be $T_c$ = 4.1 K, and the anisotropy ratio of the upper critical field $B_{c2}$ at $T_c$ is estimated from the initial slopes to be $Γ(T_c)$ = 6.9. $B_{c2}(0)$ is estimated to be 2.2 and 0.36 T for $B \parallel ab$ and $c$, respectively. Quantum oscillations are observed in both the resistance and torque. Two frequencies $F$ = 0.15 and 0.20 kT are resolved and assigned to a quasi-two-dimensional Fermi surface cylinder. The carrier density and Sommerfeld coefficient associated with this cylinder are estimated to be 5.8 $\times$ 10$^{-3}$ carriers/Fe and 0.48 mJ/(K$^2$mol), respectively. Other Fermi surface pockets still remain to be found. Band-structure calculations are performed and compared to the experimental results.

cond-mat.supr-con↗

arXiv2016

Classification of multipole superconductivity in multi-orbital systems and its implications

Motivated by a growing interest in multi-orbital superconductors with spin-orbit interactions, we perform the group-theoretical classification of various unconventional superconductivity emerging in symmorphic $\rm O$, $\rm D_4$, and $\rm D_6$ space groups. The generalized Cooper pairs, which we here call "multipole" superconductivity, possess spin-orbital coupled (multipole) degrees of freedom, instead of the conventional spin singlet/triplet in single-orbital systems. From the classification, we obtain the following key consequences, which have been overlooked in the long history of research in this field: (1) A superconducting gap function with $\varGamma_9\otimes\varGamma_9$ in $\rm D_6$ possesses nontrivial momentum dependence, different from the usual spin 1/2 classification. (2) Unconventional gap structure can be realized in the BCS approximation of purely local (on-site) interactions irrespective of attractive/repulsive. It implies the emergence of an electron-phonon (e-ph) driven unconventional superconductivity. (3) Reflecting symmetry of orbital basis functions, there appear not symmetry-protected but inevitable line nodes/gap minima, and thus, anisotropic $s$-wave superconductivity can be naturally explained without any competitive fluctuations.

cond-mat.supr-con↗

arXiv2016

Exotic multi-gap structure in UPt$_3$ unveiled by the first-priniciples analysis

A heavy-fermion superconductor UPt$_3$ is a unique spin-triplet superconductor with multiple superconducting phases. Here we provide the first report on the first-principles analysis of the microscopic superconducting gap structure. We find that the promising gap structure is an unprecedented $E_{2u}$ state, which is completely different from the previous phenomenological $E_{2u}$ models. Our obtained $E_{2u}$ state has in-plane twofold vertical line nodes on small Fermi surfaces and point nodes with linear dispersion on a large Fermi surface. These peculiar features cannot be explained in the conventional spin $1/2$ representation, but is described by the group-theoretical representation of the Cooper pairs in the total angular momentum $j=5/2$ space. Our findings shed new light on the long-standing problems in the superconductivity of UPt$_3$

cond-mat.supr-con↗

arXiv2016

Evidence for chiral d-wave superconductivity in URu2Si2 from the field-angle variation of its specific heat

Low-energy quasiparticle (QP) excitations in the heavy-fermion superconductor URu$_2$Si$_2$ were investigated by specific-heat $C(T, H, ϕ, θ)$ measurements of a high-quality single crystal. The occurrence of QP excitations due to the Doppler-shift effect was detected regardless of the field direction in $C(H)$ of the present clean sample, which is in sharp contrast to a previous report. Furthermore, the polar-angle-dependent $C(θ)$ measured under a rotating magnetic field within the ac plane exhibits a shoulder-like anomaly at $θ\sim 45$ deg and a sharp dip at $θ= 90$ deg ($H \parallel a$) in the moderate-field region. These features are supported by theoretical analyses based on microscopic calculations assuming the gap symmetry of $k_z(k_x+ik_y)$, whose gap structure is characterized by a combination of a horizontal line node at the equator and point nodes at the poles. The present results have settled the previous controversy over the gap structure of URu$_2$Si$_2$ and have authenticated its chiral $d$-wave superconductivity.

cond-mat.supr-con↗

arXiv2013

Effect of magnetic criticality and Fermi-surface topology on the magnetic penetration depth

We investigate the effect of anti-ferromagnetic (AF) quantum criticality on the magnetic penetration depth $λ(T)$ in line-nodal superconductors, including the cuprates, the iron pnictides, and the heavy-fermion superconductors. The critical magnetic fluctuation renormalizes the current vertex and drastically enhances zero-temperature penetration depth $λ(0)$, which is more remarkable in the iron-pnictide case due to the Fermi-surface topology. Additional temperature ($T$) dependence of the current renormalization makes the expected $T$-linear behavior at low temperatures approaching to $T^{1.5}$ asymptotically. These anomalous behaviors are well consistent with experimental observations. We stress that $λ(T)$ is a good probe to detect the AF quantum critical point in the superconducting state.

cond-mat.supr-con↗