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ABSTRACT: Using the dx2-y2+dz2 two orbital model of the high Tc cuprates obtained from
the first-principle calculation, we show that the material dependence of the
Fermi surface shape can be understood by the degree of the mixture between the
dx2-y2$ and the dz2 orbitals. We explain, through investigating the
tightbinding hopping integrals, why some cuprates have square shaped Fermi
surface, while others have more rounded ones. From this viewpoint, we explain
the experimentally observed correlation between the curvature of the Fermi
surface and Tc.
05/2013;
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ABSTRACT: Recently, a very rich phase diagram has been obtained for an iron-based
superconductor Ca4Al2O6Fe2(As1-xPx)2. It has been revealed that nodeless (x=0)
and nodal (x = 1) superconductivity are separated by an antiferromagnetic
phase. Here we study the origin of this peculiar phase diagram using a five
orbital model constructed from first principles band calculation, and applying
the fluctuation exchange approximation assuming spin fluctuation mediated
pairing. Based on the calculation results, we propose a scenario where the
frustration in momentum space degrades superconductivity in the intermediate x
regime, while antiferromangetism takes place due to a very good nesting. In
order to see whether the present theoretical scenario is consistent with the
actual nature of the competition between superconductivity and
antiferromagnetism, we also perform hydrostatic pressure experiment for
Ca4Al2O6Fe2(As1-xPx)2. In the intermediate x regime where antiferromagnetism
occurs at ambient pressure, applying hydrostatic pressure smears out the
antiferromagnetic transition, but superconductivity does not take place. This
supports our scenario that superconductivity is suppressed by the momentum
space frustration in the intermediate x regime, apart from the presence of the
antiferromangnetism.
04/2013;
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ABSTRACT: We theoretically study the spin fluctuation and superconductivity in La1111
and Sm1111 iron-based superconductors for a wide range of electron doping. When
we take into account the band structure variation by electron doping, the hole
Fermi surface originating from the $d_{X^2-Y^2}$ orbital turns out to be robust
against electron doping, and this gives rise to large spin fluctuations and
consequently $s\pm$ pairing even in the heavily doped regime. The stable hole
Fermi surface is larger for Sm1111 than for La1111, which can be considered as
the origin of the apparent difference in the phase diagram.
04/2013;
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ABSTRACT: A potential thermoelectric material CuAlO$_2$ is theoretically studied. We
first construct a model Hamiltonian of CuAlO$_2$ based on the first principles
band calculation, and calculate the Seebeck coefficient. Then, we compare the
model with that of a well-known thermoelectric material Na$_x$CoO$_2$, and
discuss the similarities and the differences. It is found that the two
materials are similar from an electronic structure viewpoint in that they have
a peculiar pudding-mold type band shape, which is advantageous as
thermoelectric materials. There are however some differences, and we analyze
the origin of the difference from a microscopic viewpoint. The band shape of
CuAlO$_2$ is found to be even more ideal than that of Na$_x$CoO$_2$, and we
predict that once a significant amount of holes is doped in CuAlO$_2$,
thermoelectric properties (especially the power factor) even better than that
of Na$_x$CoO$_2$ can be expected.
04/2013;
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ABSTRACT: We investigate the thermoelectric propeties of the electron-doped FeAs$_2$
both experimentally and theoretically. Electrons are doped by partially
substituting Se for As, which leads to a metallic behavior in the resistivity.
A Seebeck coefficient of about $-$200 $\mu$V/K is reached at 300 K for 1%
doping, and about $-$120 $\mu$V/K even at 5% doping. The origin of this large
Seebeck coefficient despite the metallic conductivity is analyzed from a band
structure point of view. The first-principles band calculation reveals the
presence of a pudding-mold type band just above the band gap, somewhat similar
to Na$_x$CoO$_2$, but with a quasi-one-dimensional nature. We calculate the
Seebeck coefficient using a tightbinding model that correctly reproduces this
band structure, and this gives results close to the experimental observations.
The origin of this peculiar band shape is also discussed.
11/2012;
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ABSTRACT: We perform first principles band calculation of the newly discovered
superconductor LaO$_{1-x}$F$_x$BiS$_2$, and study the lattice structure and the
fluorine doping dependence of the gap between the valence and conduction bands.
We find that the distance between La and S as well as the fluorine doping
significantly affects the band gap. On the other hand, the four orbital model
of the BiS$_2$ layer shows that the lattice structure does not affect this
portion of the band. Still, the band gap can affect the carrier concentration
in the case of light electron doping, which in turn should affect the transport
properties.
11/2012;
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ABSTRACT: High-temperature cuprate superconductors have been known to exhibit
significant pressure effects. In order to fathom the origin of why and how Tc
is affected by pressure, we have recently studied the pressure effects on Tc
adoptig a model that contains two cupper d-orbitals derived from
first-principles band calculations, where the dz2 orbital is considere on top
of the usually considered dx2-y2 orbital. In that paper, we have identified two
origins for the Tc enhancement under hydrostatic pressure: (i) while at ambient
pressure the smaller the hybridization of other orbital components the higher
the Tc, an application of pressure acts to reduce the multiorbital mxing on the
Fermi surface, which we call the orbital distillation effects, and (ii) the
increase of the band width with pressure also contributes to the enhancement.
In the present paper, we further elabolrate the two points. As for point (i),
while the reduction of the apical oxygen height under pressure tends to
increase the dz2 mixture, hence to lower Tc, here we show that this effect is
strongly reduced in bi-layer materials due to the pyramidal coordination of
oxygen atoms. As for point (ii), we show that the enhancement of Tc due to the
increase in the band width is caused by the effect that the many-body
renormalization arising from the self-energy is reduced.
11/2012;
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ABSTRACT: The origin of the recently discovered large thermopower in hole-doped
PtSb$_2$ is theoretically analyzed based on a model constructed from first
principles band calculation. It is found that the valence band dispersion has
an overall flatness combined with some local ups and downs, which gives small
Fermi surfaces scattered over the entire Brillouin zone. The Seebeck
coefficient is calculated using this model, which gives good agreement with the
experiment. We conclude that the good thermoelectric property originates from
this "corrugated flat band", where the coexistence of large Seebeck coefficient
and large electric conductivity is generally expected.
07/2012;
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ABSTRACT: We construct minimal electronic models for a newly discovered superconductor
LaO$_{1-x}$F$_x$BiS$_2$ ($T_c=$ 10.6K) possessing BiS$_2$ layers based on first
principles band calculation. First, we obtain a model consisting of two Bi $6p$
and two S $3p$ orbitals, which give nearly electron-hole symmetric bands.
Further focusing on the bands that intersect the Fermi level, we obtain a model
with two $p$ orbitals. The two bands (per BiS$_2$ layer) have
quasi-one-dimensional character with a double minimum dispersion, which gives
good nesting of the Fermi surface. At around $x\sim 0.5$ the topology of the
Fermi surface changes, so that the density of states at the Fermi level becomes
large. Possible pairing states are discussed.
07/2012;
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ABSTRACT: Exotic superconductivity has often been discovered in materials with a
layered (two-dimensional) crystal structure. The low dimensionality can affect
the electronic structure and can realize high transition temperatures (Tc)
and/or unconventional superconductivity mechanisms. As standard examples, we
now have two types of high-Tc superconductors. The first group is the Cu-oxide
superconductors whose crystal structure is basically composed of a stacking of
spacer (blocking) layers and superconducting CuO2 layers.1-4 The second group
is the Fe-based superconductors which also possess a stacking structure of
spacer layers and superconducting Fe2An2 (An = P, As, Se, Te) layers.5-13 In
both systems, dramatic enhancements of Tc are achieved by optimizing the spacer
layer structure, for instance, a variety of composing elements, spacer
thickness, and carrier doping levels with respect to the superconducting
layers. In this respect, to realize higher-Tc superconductivity, other than
Cu-oxide and Fe-based superconductors, the discovery of a new prototype of
layered superconductors needs to be achieved. Here we show superconductivity in
a new bismuth-oxysulfide layered compound Bi4O4S3. Crystal structure analysis
indicates that this superconductor has a layered structure composed of stacking
of Bi4O4(SO4)1-x and Bi2S4 layers; the parent compound (x = 0) is Bi6O8S5. Band
calculation suggests that Bi4O4S3 (x = 0.5) is metallic while Bi6O8S5 (x = 0)
is a band insulator with Bi3+. Furthermore, the Fermi level for Bi4O4S3 is just
on the peak position of the partial density of states of the Bi 6p orbital
within the BiS2 layer. The BiS2 layer is a basic structure which provides
another universality class for layered superconducting family, and this opens
up a new field in the physics and chemistry of low-dimensional superconductors.
07/2012;
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ABSTRACT: In the present paper, we describe how the band structure and the Fermi
surface of the iron-based superconductors vary as the Fe-As-Fe bond angle
changes. We discuss how these Fermi surface configurations affect the
superconductivity mediated by spin fluctuations, and show that in several
situations, frustration in the sign of the gap function arises due to the
repulsive pairing interactions that requires sign change of the order
parameter. Such a frustration can result in nodes or very small gaps, and
generally works destructively against superconductivity. Conversely, we propose
that the optimal condition for superconductivity is realized for the Fermi
surface configuration that gives the least frustration while maximizing the
Fermi surface multiplicity. This is realized when there are three hole Fermi
surfaces, where two of them have $d_{XZ/YZ}$ orbital character and one has
$d_{X^2-Y^2}$ {\it for all $k_z$} in the three dimensional Brillouin zone.
Looking at the band structures of various iron-based superconductors, the
occurrence of such a "sweet spot" situation is limited to a narrow window.
04/2012;
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ABSTRACT: The origin of uniaxial and hydrostatic pressure effects on Tc in the single-layered cuprate superconductors is theoretically explored. A two-orbital model, derived from first principles and analyzed with the fluctuation exchange approximation gives axial-dependent pressure coefficients ∂Tc/∂Pa>0, ∂Tc/∂Pc<0, with a hydrostatic response ∂Tc/∂P>0 for both La214 and Hg1201 cuprates, in qualitative agreement with experiments. Physically, this is shown to come from a unified picture in which higher Tc is achieved with an “orbital distillation,” namely, the less the dx2−y2 main band is hybridized with the dz2 and 4s orbitals the higher the Tc. Some implications for obtaining higher Tc materials are discussed.
Phys. Rev. B. 03/2012; 86(13).
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ABSTRACT: We study the relation between the spin fluctuation and superconductivity in
an heavily hole doped end material KFe$_2$As$_2$. We construct a five orbital
model by approximately unfolding the Brillouin zone of the three dimensional
ten orbital model obtained from first principles calculation. By applying the
random phase approximation, we obtain the spin susceptibility and solve the
linearized Eliashberg equation. The incommensurate spin fluctuation observed
experimentally is understood as originating from interband interactions, where
the multiorbital nature of the band structure results in an electron-hole
asymmetry of the incommensurability in the whole iron-based superconductor
family. As for superconductivity, s-wave and d-wave pairings are found to be in
close competition, where the sign change in the gap function in the former is
driven by the incommensurate spin fluctuations. We raise several possible
explanations for the nodes in the superconducting gap of KFe$_2$As$_2$ observed
experimentally.
08/2011;
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ABSTRACT: We study the condition for optimizing superconductivity in the iron pnictides from the lattice structure point of view. Studying the band structure of the hypothetical lattice structure of LaFeAsO, the hole Fermi-surface multiplicity is found to be maximized around the Fe-As-Fe bond angle regime where the arsenic atoms form a regular tetrahedron. Superconductivity is optimized within this three hole Fermi-surface regime, thereby providing a natural explanation as to why Tc is optimized around the regular tetrahedron angle. On the other hand, the stoner factor of the antiferromagnetism has an overall tendency of increasing upon decreasing the bond angle, so the strength of the spin fluctuation and Tc is not necessarily correlated. Combining also the effect of the varying the Fe-As bond length, we provide a guiding principle for obtaining high Tc.
Phys. Rev. B. 07/2011; 84(2).
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ABSTRACT: We study the condition for optimizing superconductivity in the iron pnictides
from the lattice structure point of view. Studying the band structure of the
hypothetical lattice structure of LaFeAsO, the hole Fermi surface multiplicity
is found to be maximized around the Fe-As-Fe bond angle regime where the
arsenic atoms form a regular tetrahedron. Superconductivity is optimized within
this three hole Fermi surface regime, while the stoner factor of the
antiferromagnetism has an overall tendency of increasing upon decreasing the
bond angle. Combining also the effect of the varying the Fe-As bond length, we
provide a guiding principle for obtaining high $T_c$.
02/2011;
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ABSTRACT: We examine theoretically the superconducting state of
BaFe$_2$(As$_{1-x}$P$_x$)$_2$, an isovalent doping 122 iron pnictide
superconductor. We construct a three dimensional ten orbital model from first
principles band calculation, and investigate the superconducting gap within the
spin fluctuation mediated pairing mechanism. The gap is basically $s\pm$, where
the gap changes its sign between electron and hole Fermi surfaces, but three
dimensional nodal structures appear in the largely warped hole Fermi surface
having strong $Z^2/XZ/YZ$ orbital character. The present result, together with
our previous study on 1111 systems, explains the strong material dependence of
the superconducting gap in the iron pnictides.
10/2010;
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ABSTRACT: In order to explore the reason why the single-layered cuprates, La(2-x)(Sr/Ba)(x)CuO4 (T(c)≃40 K) and HgBa2CuO(4+δ) (T(c)≃90 K) have such a significant difference in T(c), we study a two-orbital model that incorporates the d(z2) orbital on top of the d(x2-y2) orbital. It is found, with the fluctuation exchange approximation, that the d(z2) orbital contribution to the Fermi surface, which is stronger in the La system, works against d-wave superconductivity, thereby dominating over the effect of the Fermi surface shape. The result resolves the long-standing contradiction between the theoretical results on Hubbard-type models and the experimental material dependence of T(c) in the cuprates.
Physical Review Letters 07/2010; 105(5):057003. · 7.37 Impact Factor
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ABSTRACT: We study the origin of the large Seebeck coefficient despite the metallic conductivity in the La-doped SrTiO$_3$ and Ba-doped KTaO$_3$. We calculate the band structure of SrTiO$_3$ and KTaO$_3$, from which the Seebeck coefficient is obtained using the Boltzmann's equation. We conclude that the multiplicity of the $t_{2g}$ bands in these materials is one major origin of the good thermoelectric property in that when compared at a fixed total number of doped electrons, the Seebeck coefficient and thus the power factor are larger in multiple band systems than in single band ones because the number of doped electron bands {\it per band} is smaller in the former. We also find that the second nearest neighbor hopping integral, which generally has negative values in these materials and works destructively against the Seebeck effect, is nearly similar between KTaO$_3$ and SrTiO$_3$ despite the larger band width in the former. This can be another factor favorable for thermopower in the Ba-doped KTaO$_3$. Comment: 6 pages, 4 figures
02/2010;
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ABSTRACT: Based on first-principles calculations, we study the origin of the large thermopower in Ni-doped LaRhO(3) and Mg-doped CuRhO(2). We calculate the band structure and construct the maximally localized Wannier functions from which a tight binding Hamiltonian is obtained. The Seebeck coefficient is calculated within the Boltzmann's equation approach using this effective Hamiltonian. For LaRhO(3), we find that the Seebeck coefficient remains nearly constant within a large hole concentration range, which is consistent with the experimental observation. For CuRhO(2), the overall temperature dependence of the calculated Seebeck coefficient is in excellent agreement with the experiment. The origin of the large thermopower is discussed.
Journal of Physics Condensed Matter 02/2009; 21(6):064223. · 2.55 Impact Factor
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ABSTRACT: For a newly discovered iron-based high T_{c} superconductor LaFeAsO1-xFx, we have constructed a minimal model, where inclusion of all five Fe d bands is found to be necessary. The random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin-fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended s-wave pairing, while d-wave pairing can also be another candidate.
Physical Review Letters 09/2008; 101(8):087004. · 7.37 Impact Factor
