arXiv subjects

Kazutaka Kudo

Publications and source records attributed to Kazutaka Kudo.

49 records · Page 3Linked to original sources

arXiv2012

Large Seebeck effect in the electron-doped FeAs$_2$ driven by quasi one dimensional pudding-mold type band

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 $μ$V/K is reached at 300 K for 1% doping, and about $-$120 $μ$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.

cond-mat.mtrl-sci↗

arXiv2012

Superconductivity Induced by Bond Breaking in the Triangular Lattice of IrTe2

IrTe2, a layered compound with a triangular iridium lattice, exhibits a structural phase transition at approximately 250 K. This transition is characterized by the formation of Ir-Ir bonds along the b-axis. We found that the breaking of Ir-Ir bonds that occurs in Ir1-xPtxTe2 results in the appearance of a structural critical point in the T = 0 limit at xc = 0.035. Although both IrTe2 and PtTe2 are paramagnetic metals, superconductivity at Tc = 3.1 K is induced by the bond breaking in a narrow range of x > xc in Ir1-xPtxTe2. This result indicates that structural fluctuations can be involved in the emergence of superconductivity.

cond-mat.supr-con↗

arXiv2012

Iron-platinum-arsenide superconductors Ca10(PtnAs8)(Fe2-xPtxAs2)5

An overview of the crystal structures and physical properties of the recently discovered iron-platinum-arsenide superconductors, Ca10(PtnAs8)(Fe2-xPtxAs2)5 (n = 3 and 4), which have a superconducting transition temperature up to 38 K, is provided. The crystal structure consists of superconducting Fe2As2 layers alternating with platinum-arsenic layers, PtnAs8. The upper critical field Hc2, hydrostatic pressure dependence of superconducting transition temperature Tc, and normal-state magnetic susceptibility are reported.

cond-mat.supr-con↗

arXiv2012

Breakdown of Chemical Scaling for Pt-Doped CaFe2As2

We report a breakdown of the scaling of the antiferromagnetic transition temperature T_N on chemical doping for Pt-doped CaFe2As2. The substitution of Pt is ineffective in the reduction of antiferromagnetic ordering as well as in inducing superconductivity in Ca(Fe1-xPtx)2As2. The Pt-doped CaFe2As2 that does not exhibit superconductivity will, however, provide us an ideal opportunity to elucidate the role of chemical doping in the occurrence of superconductivity in iron-based materials.

cond-mat.supr-con↗

arXiv2012

Superconductivity in Pseudo-Binary Silicide SrNixSi2-x with AlB2-Type Structure

We demonstrate the emergence of superconductivity in pseudo-binary silicide SrNixSi2-x. The compound exhibits a structural phase transition from the cubic SrSi2-type structure (P4132) to the hexagonal AlB2-type structure (P6/mmm) upon substituting Ni for Si at approximately x = 0.1. The hexagonal structure is stabilized in the range of 0.1 < x < 0.7. The superconducting phase appears in the vicinity of the structural phase boundary. Ni acts as a nonmagnetic dopant, as confirmed by the Pauli paramagnetic behavior.

cond-mat.supr-con↗

arXiv2011

Interplay of Superconductivity and Fermi-Liquid Transport in Rh-Doped CaFe2As2 with Lattice-Collapse Transition

Ca(Fe$_{1-x}$Rh$_x$)$_2$As$_2$ undergoes successive phase transitions with increasing Rh doping in the $T$ $=$ 0 limit. The antiferromagnetic-metal phase with orthorhombic structure at 0.00 $\le$ $x$ $\le$ 0.020 is driven to a superconducting phase with uncollapsed-tetragonal (ucT) structure at 0.020 $\le$ $x$ $\le$ 0.024; a non-superconducting collapsed-tetragonal (cT) phase takes over at $x$ $\geq$ 0.024. The breakdown of Fermi-liquid transport is observed in the ucT phase above $T_{\rm c}$. In the adjacent cT phase, Fermi-liquid transport is restored along with a disappearance of superconductivity. This interplay of superconductivity and Fermi-liquid transport suggests the essential role of magnetic fluctuations in the emergence of superconductivity in doped CaFe$_2$As$_2$.

cond-mat.supr-con↗

arXiv2011

Superconductivity at 38 K in Iron-Based Compound with Platinum-Arsenide Layers Ca10(Pt4As8)(Fe2-xPtxAs2)5

We report superconductivity in novel iron-based compounds Ca10(PtnAs8)(Fe2-xPtxAs2)5 with n = 3 and 4. Both compounds crystallize in triclinic structures (space group P-1), in which Fe2As2 layers alternate with PtnAs8 spacer layers. Superconductivity with a transition temperature of 38 K is observed in the n = 4 compound with a Pt content of x ~ 0.36 in the Fe2As2 layers. The compound with n = 3 exhibits superconductivity at 13 K.

cond-mat.supr-con↗

arXiv2011

Superconductivity in the Honeycomb-Lattice Pnictide SrPtAs

We report superconductivity in the honeycomb-lattice pnictide SrPtAs with Tc = 2.4 K. To our knowledge, SrPtAs is the first superconducting pnictide with a honeycomb lattice structure. Our finding opens up new playground to develop pnictide superconductors with honeycomb lattices, in addition to square lattices known in iron-based superconductors.

cond-mat.supr-con↗

arXiv2010

Coexistence of Superconductivity and Charge Density Wave in SrPt2As2

SrPt2As2 is a novel arsenide superconductor, which crystallizes in the CaBe2Ge2-type structure as a different polymorphic form of the ThCr2Si2-type structure. SrPt2As2 exhibits a charge-density-wave (CDW) ordering at about 470 K and enters into a superconducting state at Tc = 5.2 K. The coexistence of superconductivity and CDW refers to Peierls instability with a moderately strong electron-phonon interaction. Thus SrPt2As2 can be viewed as a nonmagnetic analog of iron-based superconductors, such as doped BaFe2As2, in which superconductivity emerges in close proximity to spin-density-wave ordering.

cond-mat.supr-con↗

arXiv2010

Thermal Conductivity in the Bose-Einstein Condensed State of Triplons in the Bond-Alternating Spin-Chain System Pb2V3O9

In order to clarify the origin of the enhancement of the thermal conductivity in the Bose-Einstein Condensed (BEC) state of field-induced triplons, we have measured the thermal conductivity along the [101] direction parallel to spin-chains, $kappa_{\|[101]}$, and perpendicular to spin-chains, $kappa_{\perp[101]}$, of the S=1/2 bond-alternating spin-chain system Pb2V3O9 in magnetic fields up to 14 T. With increasing field at 3 K, it has been found that both $kappa_{\|[101]}$ and $kappa_{\perp[101]}$ are suppressed in the gapped normal state in low fields. In the BEC state of field-induced triplons in high fields, on the other hand, $kappa_{\|[101]}$ is enhanced with increasing field, while $kappa_{\perp[101]}$ is suppressed. That is, the thermal conductivity along the direction, where the magnetic interaction is strong, is markedly enhanced in the BEC state. Accordingly, our results suggest that the enhancement of $kappa_{\|[101]}$ in the BEC state is caused by the enhancement of the thermal conductivity due to triplons on the basis of the two-fluid model, as in the case of the superfluid state of liquid 4He.

cond-mat.str-el↗

arXiv2010

Superconductivity in SrFe2As2 with Pt Doping

We have synthesized polycrystalline samples of Pt-substituted SrFe2As2 and measured the temperature dependence of magnetization and electrical resistivity. We have observed the superconducting transition at Tc = 17 K with the maximum shielding volume fraction at x = 0.125 in Sr(Fe1-xPtx)2As2. It is found that the maximum Tc depends on the substituted element, so it is important to substitute various elements to explore new iron-based superconductors with higher Tc.

cond-mat.supr-con↗

arXiv2010

Pseudogap Phase Boundary in Overdoped Bi_2Sr_2CaCu_2O_8 Studied by Measuring Out-of-plane Resistivity under the Magnetic Fields

The characteristic pseudogap temperature T* in Bi2Sr2CaCu2O8 system has been systematically evaluated as a function of doping, especially focusing on its overdoped region, by measuring the out-of-plane resistivity under the magnetic fields. Overdoped samples have been prepared by annealing TSFZ-grown Bi2Sr2CaCu2O8 single crystals under the high oxygen pressures (990 kgf/cm2). At a zero field, the out-of-plane resistivity showed a metallic behavior down to Tc (= 62 K), while under the magnetic fields of over 3 T,it showed typical upturn behavior from around 65 K upon decreasing temperature. This result suggests that the pseudogap and superconductivity are different phenomena.

cond-mat.supr-con↗

arXiv2008

Evidence for Ballistic Thermal Conduction in the One-Dimensional S=1/2 Heisenberg Antiferromagnetic Spin System Sr2CuO3

We have measured the thermal conductivity of the one-dimensional (1D) S=1/2 Heisenberg antiferromagnetic spin system of Sr2Cu1-xPdxO3 single crystals including nonmagnetic impurities of Pd2+. It has been found that the mean free path of spinons along the 1D spin chain at low temperatures is very close to the average length of finite spin chains between spin defects estimated from the magnetic susceptibility measurements. This proves that the thermal conduction due to spinons at low temperatures in Sr2CuO3 is ballistic as theoretically expected [Zotos et al.: Phys. Rev. Lett. 55 (1997) 11029].

cond-mat.str-el↗