arXiv subjects

Hisatomo Harima

Publications and source records attributed to Hisatomo Harima.

50 records · Page 3Linked to original sources

arXiv2009

Resistivity and upper critical field in KFe$_2$As$_2$ single crystals

We report resistivity measurements performed on KFe$_2$As$_2$ single crystals down to $T$ = 0.3 K and in magnetic fields up to 17.5 T. The in-plane resistivity vs. $T$ curve has a convex shape down to $\sim$50 K and shows a $T^2$ dependence below $\sim$45 K. The ratio of the c-axis to in-plane resistivities is $\sim$10 at room temperature and $\sim$40 at 4.2 K. The superconducting upper critical field $B_{c2}(T)$ has been determined from the resistivity data: $B_{c2}^{ab}$(0) = 4.47 T and $B_{c2}^{c}$(0) = 1.25 T. The anisotropy parameter $Γ$ = $B_{c2}^{ab}$ / $B_{c2}^{c}$ increases with $T$ and is 6.8 at $T$ = $T_c$. The strong curvature of the $B_{c2}^{ab}(T)$ curve indicates the existence of strong spin paramagnetic effects.

cond-mat.supr-con↗

arXiv2009

Fermi surface in BaNi$_2$P$_2$

We report measurements of the de Haas-van Alphen (dHvA) oscillation and a band structure calculation for the pnictide superconductor BaNi$_2$P$_2$, which is isostructural to BaFe$_2$As$_2$, the mother compound of the iron-pnictide high-$T_c$ superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$. Six dHvA-frequency branches with frequencies up to $\sim$8 kT were observed, and they are in excellent agreement with results of the band-structure calculation. The determined Fermi surface is large, enclosing about one electron and hole per formula unit, and three-dimensional. This is in contrast to the small two-dimensional Fermi surface expected for the iron-pnictide high-$T_c$ superconductors. The mass enhancement is about two.

cond-mat.supr-con↗

arXiv2009

Field angle dependence of the zero-energy density of states in unconventional superconductors: analysis of the borocarbide superconductor YNi2B2C

We investigate the field-angle-dependent zero-energy density of states for YNi2B2C with using realistic Fermi surfaces obtained by band calculations. Both the 17th and 18th bands are taken into account. For calculating the oscillating density of states, we adopt the Kramer-Pesch approximation, which is found to improve accuracy in the oscillation amplitude. We show that superconducting gap structure determined by analyzing STM experiments is consistent with thermal transport and heat capacity measurements.

cond-mat.supr-con↗

arXiv2008

Electronic Structure of Superconducting FeSe Studied by High-Resolution Photoemission Spectroscopy

We have performed soft x-ray and ultrahigh-resolution laser-excited photoemission measurements on tetragonal FeSe, which was recently identified as a superconductor. Energy dependent study of valence band is compared to band structure calculations and yields a reasonable assignment of partial densities of states. However, the sharp peak near the Fermi level slightly deviates from the calculated energy position, giving rise to the necessity of self-energy correction. We have also performed ultrahigh-resolution laser photoemission experiment on FeSe and observed the suppression of intensity around the Fermi level upon cooling.

cond-mat.supr-con↗

arXiv2007

Positions of Point-Nodes in Borocarbide Superconductor YNi2B2C

To determine the superconducting gap function of YNi2B2C, we calculate the local density of states (LDOS) around a single vortex core with the use of Eilenberger theory and the band structure calculated by local density approximation assuming various gap structures with point-nodes at different positions. We also calculate the angular-dependent heat capacity in the vortex state on the basis of the Doppler-Shift method. Comparing our results with the STM/STS experiment, the angular-dependent heat capacity and thermal conductivity, we propose the gap-structure of YNi2B2C, which has the point-nodes and gap minima along <110>. Our gap-structure is consistent with all results of angular-resolved experiments.

cond-mat.supr-con↗

arXiv2007

A de Haas-van Alphen study of the filled skutterudite compounds PrOs$_4$As$_{12}$ and LaOs$_4$As$_{12}$

Comprehensive magnetic-field-orientation dependent studies of the susceptibility and de Haas-van Alphen effect have been carried out on single crystals of the filled skutterudites PrOs$_4$As$_{12}$ and LaOs$_4$As$_{12}$ using magnetic fields of up to 40~T. Several peaks are observed in the low-field susceptibility of PrOs$_4$As$_{12}$, corresponding to cascades of metamagnetic transitions separating the low-field antiferromagnetic and high-field paramagnetic metal (PMM) phases. The de Haas-van Alphen experiments show that the Fermi-surface topologies of PrOs$_4$As$_{12}$ in its PMM phase and LaOs$_4$As$_{12}$ are very similar. In addition, they are in reasonable agreement with the predictions of bandstructure calculations for LaOs$_4$As$_{12}$ on the PrOs$_4$As$_{12}$ lattice. Both observations suggest that the Pr 4$f$ electrons contribute little to the number of itinerant quasiparticles in the PMM phase. However, whilst the properties of LaOs$_4$As$_{12}$ suggest a conventional nonmagnetic Fermi liquid, the effects of direct exchange and electron correlations are detected in the PMM phase of PrOs$_4$As$_{12}$. For example, the quasiparticle effective masses in PrOs$_4$As$_{12}$ are found to decrease with increasing field, probably reflecting the gradual suppression of magnetic fluctuations associated with proximity to the low-temperature, low-field antiferromagnetic state.

cond-mat.str-el↗

arXiv2006

Chemical trends of superconducting properties in pyrochlore oxides

Chemical trends of fundamental superconducting parameters and normal-state properties are described for a family of pyrochlore oxide superconductors. Particularly, the change of Tc from 1.0 K for alpha-pyrochlore Cd2Re2O7 to 3.3 K (A = Cs), 6.3 K (Rb), and 9.6 K (K) for beta-pyrochlore AOs2O6 is discussed on the basis of the conventional BCS scheme. Enhanced Tc and anomalous features observed for KOs2O6 are ascribed to low-energy phonons probably coming from the rattling of the K cations.

cond-mat.supr-con↗

arXiv2006

Impurity Effect as a Probe for the Gap Function in the Filled Skutterudite Compound Superconductor PrOs$_{4}$Sb$_{12}$: Sb-NQR Study

We have carried out nuclear quadrupole resonance (NQR) measurements in the filled skutterudite compounds Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ ($x=0.1, 0.2$), in order to gain insights into the symmetry of the superconducting gap function. Upon replacing Os with Ru, the spin-lattice relaxation rate $1/T_1$ becomes proportional to temperature ($T$) at low $T$ far below $T_c$, and the magnitude of $(1/T_1T)_{low-T}$ increases with increasing Ru content. These results indicate that a finite density of states is induced at the Fermi level by the impurity, and thus suggest that there exist nodes in the gap function of PrOs$_{4}$Sb$_{12}$.

cond-mat.supr-con↗

arXiv2006

Effective Crystalline Electric Field Potential in a j-j Coupling Scheme

We propose an effective model on the basis of a $j$-$j$ coupling scheme to describe local $f$-electron states for realistic values of Coulomb interaction $U$ and spin-orbit coupling $λ$, for future development of microscopic theory of magnetism and superconductivity in $f^n$-electron systems, where $n$ is the number of local $f$ electrons. The effective model is systematically constructed by including the effect of a crystalline electric field (CEF) potential in the perturbation expansion in terms of $1/λ$. In this paper, we collect all the terms up to the first order of $1/λ$. Solving the effective model, we show the results of the CEF states for each case of $n$=2$\sim$5 with $O_{\rm h}$ symmetry in comparison with those of the Stevens Hamiltonian for the weak CEF. In particular, we carefully discuss the CEF energy levels in an intermediate coupling region with $λ/U$ in the order of 0.1 corresponding to actual $f$-electron materials between the $LS$ and $j$-$j$ coupling schemes. Note that the relevant energy scale of $U$ is the Hund's rule interaction. It is found that the CEF energy levels in the intermediate coupling region can be quantitatively reproduced by our modified $j$-$j$ coupling scheme, when we correctly take into account the corrections in the order of $1/λ$ in addition to the CEF terms and Coulomb interactions which remain in the limit of $λ$=$\infty$. As an application of the modified $j$-$j$ coupling scheme, we discuss the CEF energy levels of filled skutterudites with $T_{\rm h}$ symmetry.

cond-mat.str-el↗

arXiv2006

59Co-NMR Probe for Stepwise Magnetization and Magnetotransport in SrCo6O11 with Metallic Kagome Layer and Triangular Lattice with Local Moments

We report on novel magnetic and electronic properties of SrCo6O11 that exhibits a unique stepwise magnetization and its relevant magnetotransport phenomena investigated by the site-selective 59Co nuclear magnetic resonance (NMR) at zero and applied magnetic fields. This compound is composed of three Co sites in the unit cell, i.e., Co(1) in the metallic Kagome layer, a Co(2) dimerized pillar between the layers and Co(3) in the triangular lattice. Zero-field NMR spectra have revealed that large local moments at the Co(3) sites are magnetically ordered without any trace of bulk magnetization M at zero field. The field-swept NMR spectra show that the internal hyperfine field at the Co(1) site is derived from fully polarized moments Ms at the Co(3) sites in the "1"-plateau state at fields higher than 2.5 T, whereas it is partially cancelled out in the "1/3"-plateau state in which one-third of Ms is induced at intermediate fields once a small field is applied. It has been clarified from a microscopic point of view that the local moments at Co(3) site undergo a field-induced ferrimagnetic (up-up-down)-to-ferromagnetic (up-up-up) transition, which is consistent with the evidence obtained from the recent neutron diffraction experiment. The Co(1) Kagome layer and the dimerized pillar Co(2) site between the layers are of nonmagnetic origin, suggesting that the nearly quasi-2D metallic conductivity is dominated by nonmagnetic Co(1) and Co(2) sites. Consequently, unique magneto-transport phenomena observed in SrCo6O11 are demonstrated owing to the interaction between the conduction electrons at the Co(1) and Co(2) sites and the local moments at Co(3) sites.

cond-mat.str-el↗

arXiv2006

Direct observation of quasi-particle band in CeIrIn$_5$: Angle-resolved photoemission spectroscopy study

We have performed a high-resolution angle resolved Ce 4$d-4f$ resonant photoemission experiment on the heavy fermion superconductor CeIrIn$_5$. We have observed a quasi-particle band which has an energy dispersion of $\sim 30$ meV in the Ce 4$f$ on-resonance spectra. The result suggests that although the 4$f$ spectra are dominated by the localized/correlated character, the small itinerant component is responsible for the superconductivity in this compound.

cond-mat.str-el↗

arXiv2005

Cooperative Phenomenon of Ferromagnetism and Unconventional Superconductivity in UGe$_2$: A ^73^Ge-NQR Study under Pressure

We report on a cooperative phenomenon of ferromagnetism and unconventional superconductivity (SC) in UGe$_2$ through the measurements of $^{73}$Ge nuclear-quadrupole-resonance (NQR) under pressure ($P$). The NQR spectra evidenced phase separation into ferromagnetic and paramagnetic phases in the vicinity of $P_c\sim 1.5$ GPa, pointing to a first-order transition. The measurements of nuclear-spin-lattice-relaxation-rate $1/T_1$ revealed that SC emerges under the background of ferromagnetism, but not of the paramagnetic phase.

cond-mat.str-el↗

arXiv2003

Formation Mechanism of Hybridization Gap in Kondo Insulators based on a Realistic Band Model and Application to YbB$_{12}$

A new LDA+U band calculation is performed on the Kondo insulator material YbB$_{12}$ and an energy gap of about 0.001Ryd is obtained. Based on this, a simple tight-binding model with 5d$ε$ and 4f $Γ_8$ orbitals on Yb atoms and the nearest neighbor $σ$-bonds between them is constructed with a good agreement to the above the LDA+U calculation near the gap. The density of states is also calculated and the shape is found to be very asymmetric with respect to the gap. A formation mechanism of the gap is clarified for the first time in a realistic situation with the orbital degeneracies in both conduction bands and the f states. This model can be a useful starting point for incorporating the strong correlation effect, and for understanding all the thermal, thermoelectric, transport and magnetic properties of YbB$_{12}$.

cond-mat.str-el↗

arXiv2002

Energy bandstructures of MgB2 and the related compounds

Energy band structures are calculated for the new superconductor MgB$_2$ and the related compounds by using the LDA and an FLAPW method. It is found that the strong three dimensional network in low-lying $π$ bands brings about two dimensional $σ$ holes in MgB$_2$, which should be responsible for the superconductivity. The de Haas-van Alphen frequencies and the cyclotron masses are obtained for the Fermi surfaces. The $σ$ hole is not found in LiBC due to the less three dimensional network. MgB$_2$C$_2$ is found a semiconductor, but the top of the valence bands are similar to the $σ$ bands in MgB$_2$. The total energy calculations are also performed for Mg$_{1-x}$Al$_x$B$_2$ to investigate the structural phase instability, experimentally observed around $x=$10%.

cond-mat.supr-con↗