Publications (289)672.7 Total impact


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ABSTRACT: We construct tightbinding models of the layered nitride halide superconductor LixZrNCl by firstprinciples calculation. 14, 10, 8, and 4orbital models are constructed, all of which reproduce the Fermi surface by the firstprinciples calculation, and the band structure away from the Fermi level at various levels. On the basis of the tightbinding parameters in real space, it is found that bilayer coupling is relatively strong, suggesting that a singlelayer approximation may not be valid. Taking into account the onsite and nearestneighbor electronelectron interactions, and calculating the spin and charge susceptiblities for each model, we conclude that the 8 (= Zrd(x2y2); d(xy); Npx; p(y) x bilayer)orbital model is the minimal model for describing lowenergy physics.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the crucial but still unresolved issue for high$T_c$ cuprates, namely the identification of the parent compounds. While there has been intensive discussion of their insulating nature, all arguments rely strongly on, or are closely related to, the correlation strength of the materials. Clear understanding has been seriously hampered by the absence of a direct measure of this interaction, traditionally denoted by $U$. Here, we report a firstprinciples estimation of $U$ for several different types of cuprates. The $U$ values clearly increase as a function of the inverse bond distance between apical oxygen and copper. Our results show that the electrondoped cuprates are less correlated than their holedoped counterparts, which strongly supports the Slater picture rather than the Mott picture. Further, the $U$ values significantly vary even among the holedoped families. The correlation strengths of the Hgcuprates are noticeably weaker than that of La$_2$CuO$_4$ and may also be regarded as Slatertype insulators. Our results suggest that the strong correlation enough to induce Mott gap is not a prerequisite for the high$T_c$ superconductivity.  [Show abstract] [Hide abstract]
ABSTRACT: We perform a firstprinciples band calculation for quasitwodimensional organic superconductors β(BDATTP)2I3 and β(BDATTP)2SbF6. The firstprinciples band structures between the I3 and SbF6 salts are apparently different. We construct a tightbinding model for each material which accurately reproduces the firstprinciples band structure. The obtained transfer energies give the differences as follows: (i) larger dimerization in the I3 salt than the SbF6 salt, and (ii) different signs and directions of the interstacking transfer energies. To decompose the origin of the difference into the dimerization and the interstacking transfer energies, we adopt a simplified model by eliminating the dimerization effect and focus only on the difference caused by the interstacking transfer energies. From the analysis using the simplified model, we find that the difference of the band structure comes mainly from the strength of the dimerization. To compare the strength of the electron correlation having roots in the band structure, we calculate the physical properties originating from the effect of the electron correlation such as the spin susceptibility applying the twoparticle selfconsistent method. We find that the maximum value of the spin susceptibility for the I3 salt is larger than that of the SbF6 salt. Hypothetically decreasing the dimerization within the model of the I3 salt, the spin susceptibility takes almost the same value as that of the SbF6 salt for the same magnitude of the dimerization. We expect that the different ground state between the I3 and SbF6 salt mainly comes from the strength of the dimerization which is apparently masked in the band calculation along a particular k path. © 2015 American Physical Society.  [Show abstract] [Hide abstract]
ABSTRACT: We study antimony doping effects in the ironbased superconductor CaFe(Sb$_{x}$As$_{1x}$)$_{2}$ by using the firstprinciple calculation. The calculations reveal that the substitution of the doped antimony atom into As of the chainlike As layers is more stable than that in FeAs layers. This prediction can be checked by experiments. Our results suggest that doping homologous elements into the chainlike As layers existing only in novel 112 system is responsible for rising up the critical temperature. We discuss antimony doping effects on the electronic structure. It is found that the calculated band structures with and without the antimony doping are similar to each other within our framework.  [Show abstract] [Hide abstract]
ABSTRACT: We perform a first principles band calculation for quasitwodimensional organic superconductors $\beta$(BDATTP)$_{2}$I$_{3}$ and $\beta$(BDATTP)$_{2}$SbF$_{6}$. The first principles band structures between the I$_{3}$ and SbF$_{6}$ salts are apparently different. We construct a tightbinding model for each material which accurately reproduces the first principles band structure. The obtained transfer energies give the differences such as (i) larger dimerization in the I$_{3}$ salt than the SbF$_{6}$ salt, and (ii) different signs and directions of the interstacking transfer energies. To decompose the origin of the difference into the dimerization and the interstacking transfer energies, we adopt a simplified model by eliminating the dimerization effect and extract the difference caused by the interstacking transfer energies. From the analysis using the simplified model, we find that the difference of the band structure comes mainly from the strength of dimerization. To compare the strength of the electron correlation having roots in the band structure, we calculate the physical properties originated from the effect of the electron correlation such as the spin susceptibility applying two particle selfconsistent (TPSC) method. We find that the maximum value of the spin susceptibility of the I$_{3}$ salt is larger than that of the SbF$_{6}$ salt. Hypothetically decreasing the dimerization within the model of the I$_{3}$ salt, the spin susceptibility takes almost the same value as that of the SbF$_6$ salt for the same magnitude of the dimerization. We expect that the different ground state between the I$_{3}$ and SbF$_{6}$ salt mainly comes from the strength of the dimerization which is apparently masked in the band calculation along a particular $k$path.  [Show abstract] [Hide abstract]
ABSTRACT: Despite decades of progress, an understanding of unconventional superconductivity still remains elusive. An important open question is about the material dependence of the superconducting properties. Using the quasiparticle selfconsistent GW method, we reexamine the electronic structure of copper oxide highTc materials. We show that QSGW captures several important features, distinctive from the conventional LDA results. The energy level splitting between dx2y2 and d3z2r2 is signi?cantly enlarged and the van Hove singularity point is lowered. The calculated results compare better than LDA with recent experimental results from resonant inelastic xray scattering and angle resolved photoemission experiments. This agreement with the experiments supports the previously suggested twoband theory for the material dependence of the superconducting transition temperature, Tc.  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by recent experiments on isovalentdoped 1111 ironbased superconductors ${\text{LaFeAs}}_{1$${}x}{\text{P}}_{x}{\text{O}}_{1$${}y}{\text{F}}_{y}$ and the theoretical study that followed, we investigate, within the fiveorbital model, the correlation between spin fluctuations and the superconducting transition temperature, which exhibits a doubledome feature upon varying the FeAsFe bond angle. Around the first dome with higher ${T}_{c}$, the lowenergy spin fluctuation and ${T}_{c}$ are not tightly correlated because the finiteenergy spin fluctuation also contributes to superconductivity. On the other hand, the strength of the lowenergy spin fluctuation originating from the ${d}_{xz/yz}$ orbital is correlated with ${T}_{c}$ in the second dome with lower ${T}_{c}$. These calculation results are consistent with a recent NMR study, and hence strongly suggest that the pairing in ironbased superconductors is predominantly caused by multiorbital spin fluctuations.  [Show abstract] [Hide abstract]
ABSTRACT: Since the discovery of high Tc ironbased superconductors in early 2008, more than 15,000 papers have been published as a result of intensive research. This paper describes the current status of ironbased superconductors (IBSC) covering most uptodate research progress along with the some background research, focusing on materials (bulk and thin film) and pairing mechanism.  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by recent experiments on isovalentdoped 1111 ironbased superconductors LaFeAs$_{1{x}}$P$_{x}$O$_{1{y}}$F$_{y}$ and the theoretical study that followed, we investigate, within the five orbital model, the correlation between the spin fluctuation and the superconducting transition temperature, which exhibits a double dome feature upon varying the FeAsFe bond angle. Around the first dome with higher $T_c$, the low energy spin fluctuation and $T_c$ are not tightly correlated because the finite energy spin fluctuation also contributes to superconductivity. On the other hand, the strength of the lowenergy spin fluctuation originating from the $d_{xz/yz}$ orbital is correlated with $T_c$ in the second dome with lower $T_c$. These calculation results are consistent with recent NMR study, and hence strongly suggest that the pairing in the ironbased superconductors is predominantly caused by the multiorbital spin fluctuation.  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by recent experimental investigations of the isovalent doping ironbased superconductors LaFe(AsxP1x)O1yFy and NdFe(AsxP1x)O1yFy we theoretically study the correlation between the local lattice structure, the Fermi surface, the spin fluctuationmediated superconductivity, and the composition ratio. In the phosphides, the dXZ and dYZ orbitals barely hybridize around the Gamma point to give rise to two intersecting ellipse shape Fermi surfaces. As the arsenic content increases and the FeAsFe bond angle is reduced, the hybridization increases, so that the two bands are mixed to result in concentric inner and outer Fermi surfaces, and the orbital character gradually changes to dxz and dyz, where xy axes are rotated by 45 degrees from XY. This makes the orbital matching between the electron and hole Fermi surfaces better and enhances the spin fluctuation within the dxz/yz orbitals. On the other hand, the hybridization splits the two bands, resulting in a more dispersive inner band. Hence, there is a tradeoff between the density of states and the orbital matching, thereby locally maximizing the dxz/yz spin fluctuation and superconductivity in the intermediate regime of As/P ratio. The consistency with the experiment strongly indicate the importance of the spin fluctuation played in this series of superconductors.  [Show abstract] [Hide abstract]
ABSTRACT: Application of physical pressure on the cuprate superconductors often results in an enhancement of Tc. Motivated by this fact, we study the chemical pressure effect on the singlelayered La2CuO4 and HgBa2CuO4 starting from the twoorbital Hubbard model deduced from a firstprinciples calculation. It is shown that the chemical pressure effects induced by Lasite substitution in La2CuO4 or Hgsite substitution in HgBa2CuO4 are not expected to be effective for raising Tc.  [Show abstract] [Hide abstract]
ABSTRACT: We perform first principles band calculation of electron doped ironbased superconductors adopting the virtual crystal approximation. We find that when electrons are doped by element substitution in the blocking layer, the band structure near the Fermi level is affected due to the increase of the positive charge in the layer. On the other hand, when Fe in the conducting layer is substituted by Co, the band structure is barely affected. This difference should be a key factor in understanding the phase diagram of the heavily doped electron doped systems LnFeAsO1xHx.  [Show abstract] [Hide abstract]
ABSTRACT: In this review, we present a comprehensive overview of superconductivity in electrondoped metal nitride halides $M$N$X$ ($M$ = Ti, Zr, Hf; $X$ = Cl, Br, I) with layered crystal structure and twodimensional electronic states. The parent compounds are band insulators with no discernible longrange ordered state. Upon doping tiny amount of electrons, superconductivity emerges with several anomalous features beyond the conventional electronphonon mechanism, which stimulate theoretical investigations. We will discuss experimental and theoretical results reported thus far and compare the electrondoped layered nitride superconductors with other superconductors.  [Show abstract] [Hide abstract]
ABSTRACT: The electronic structure of nearly optimallydoped novel superconductor LaO$_{1x}$F$_x$BiS$_2$ (${\it x}$ = 0.46) was investigated using angleresolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level (${\it E}_{\rm F}$), which are well reproduced by first principles calculations when the spinorbit coupling is taken into account. The ARPES intensity map near ${\it E}_{\rm F}$ shows a squarelike distribution around the $\Gamma$(Z) point in addition to electronlike Fermi surface (FS) sheets around the X(R) point, indicating that FS of LaO$_{0.54}$F$_{0.46}$BiS$_2$ is in close proximity to the theoreticallypredicted topological change.  [Show abstract] [Hide abstract]
ABSTRACT: We study the Seebeck effect in the $\tau$type organic conductors, $\tau$(EDO$S$,$S$DMEDTTTF)$_{2}$(AuBr$_{2}$)$_{1+y}$ and $\tau$(P$S$,$S$DMEDTTTF)$_{2}$(AuBr$_{2}$)$_{1+y}$, where EDO$S$,$S$DMEDTTTF and P$S$,$S$DMEDTTTF are abbreviated as OOSS and NNSS, respectively, both experimentally and theoretically. Theoretically in particular, we perform firstprinciples band calculation for the two materials and construct a twoorbital model, on the basis of which we calculate the Seebeck coefficient. We show that the calculated temperature dependence of the Seebeck coefficient $S$ is semiquantitatively consistent with the experimental observation. In both materials, the absolute value of the Seebeck coefficient is maximum at a certain temperature, and this temperature is lower for NNSS than for OOSS. From a band structure viewpoint, we find that this can be traced back to the narrowness of the band gap between the upper and the lower puddingmold type bands. On the other hand, the Seebeck coefficient of NNSS in the low temperature regime steeply increases with increasing temperature, which is due to the narrowness of the upper band. These differences in thermoelectric properties demonstrate the effectiveness of controlling the band structure through molecular modification.  [Show abstract] [Hide abstract]
ABSTRACT: We present a theoretical understanding of the superconducting phase diagram of the electrondoped iron pnictides. We show that, besides the Fermi surface nesting, a peculiar motion of electrons, where the next nearest neighbor (diagonal) hoppings between iron sites dominate over the nearest neighbor ones, plays an important role in the enhancement of the spin fluctuation and thus superconductivity. In the highest T_{c} materials, the crossover between the Fermi surface nesting and this "prioritized diagonal motion" regime occurs smoothly with doping, while in relatively low T_{c} materials, the two regimes are separated and therefore results in a double dome T_{c} phase diagram. 
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ABSTRACT: We theoretically study the thermoelectric properties of electrondoped FeAs2 and holedoped PtSe2 from a bandstructure point of view using firstprinciples band calculations. The band structure of both materials has a peculiar band shape with a flat portion at the top (bottom) of the band, namely the puddingmoldtype band as found in Nax CoO2. The puddingmoldtype band has a quasionedimensional nature in FeAs2 and a quasitwodimensional nature in PtSe2. We study the origins of the puddingmoldtype band and find that the $ d_{z^2} $ orbital in FeAs2, and not only the p z but also the p x and the p y orbitals in PtSe2 play an important role in making the puddingmoldtype band. We calculate the Seebeck coefficients by the Boltzmann equation approach using a tightbinding model constructed from firstprinciples band calculations, finding values close to experimental observations. The present study shows the general efficiency of the puddingmoldtype band. We suggest that an efficient route towards obtaining good thermoelectric materials is to realize ideal puddingmoldtype bands by modification of lattice structures.
Publication Stats
5k  Citations  
672.70  Total Impact Points  
Top Journals
Institutions

20122015

Osaka University
 Department of Physics
Suika, Ōsaka, Japan


2013

Japan Science and Technology Agency (JST)
Edo, Tōkyō, Japan


20092013

Osaka ElectroCommunication University
Edo, Tokyo, Japan 
Tokyo Electron
Edo, Tōkyō, Japan


20002012

The University of ElectroCommunications
 Department of Applied Physics and Chemistry
Edo, Tokyo, Japan


19912006

National Institute of Police Science Japan
Tiba, Chiba, Japan


19902006

The University of Tokyo
 • College of Art and Science & Graduate School of Arts and Sciences
 • Department of Physics
Tōkyō, Japan


2003

Okayama University
Okayama, Okayama, Japan 
Nagoya University
 Department of Quantum Engineering
Nagoya, Aichi, Japan


1993

Miyazaki University
 Faculty of Engineering
Миядзаки, Miyazaki, Japan
