arXiv subjects

J. Akimitsu

Publications and source records attributed to J. Akimitsu.

71 records · Page 4Linked to original sources

arXiv2001

Manifestation of multiband optical properties of MgB$_2$

The optical conductivity of MgB$_2$ has been determined on a dense polycrystalline sample in the spectral range 6 meV - 4.6 eV using a combination of ellipsometric and normal incidence reflectivity measurements. $σ_{1}(ω)$ features a narrow Drude peak with anomalously small plasma frequency (1.4 eV) and a very broad "dome" structure, which comprises the bulk of the low-energy spectral weight. This fact can be reconciled with the results of band structure calculations by assuming that charge carriers from the 2D $σ$-bands and the 3D $π$-bands have principally different impurity scattering rates and negligible interband scattering. This also explains a surprisingly small correlation between the defect concentration and $T_c$, expected for a two-gap superconductor. The large 3D carrier scattering rate suggests their proximity to the localization limit.

cond-mat.supr-con↗

arXiv2001

Micro-Raman scattering investigation of MgB2 and RB2 (R=Al, Mn, Nb and Ti)

Phonon spectra have been investigated by micro-Raman scattering from 290 to 20K. New peaks at 500 and 674 cm-1 appear below 250K in MgB2. These peaks are explained by the Fermi resonance, where Raman-active E2g intereacts with the overtone of E1u due to anhamonicity. Raman spectra of the isostructural RB2 (R=Al, Mn, Nb and Ti) have been also measured at 290K. The line width of E2g phonon of the superconductor MgB2 and NbB2 shows the broader than that of the non-superconductors. It is found that the anharmonicity of phonons is important for the superconductivity for MgB2.

cond-mat.supr-con↗

arXiv2001

Bonding Nature in MgB2

The accurate charge density of MgB2 was observed at room temperature(R.T.) and 15K by the MEM(Maximum Entropy Method)/Rietveld analysis using synchrotron radiation powder data. The obtained charge density clearly revealed the covalent bonding feature of boron forming the 2D honeycomb network in the basal plane, on the other hand, Mg is found to be in divalent state. A subtle but clear charge concentration was found on boron 2D sheets at 15K, which should be relating to superconductivity.

cond-mat.mtrl-sci↗

arXiv2001

Optical measurements of the superconducting gap in MgB2

Far-infrared reflectivity studies on the polycrystalline intermetallic compound MgB_2 with a superconducting transition temperature T_c=39 K were performed at temperatures 20 K to 300 K. We observe a significant raise of the superconducting-to-normal state reflectivity ratio below 70 cm^{-1}, with a maximum at about 25-30 cm^{-1}, which gives a lower estimate of the superconducting gap of 2Δ(0)\approx3-4 meV.

cond-mat.supr-con↗

arXiv2001

High-Resolution Photoemission Study of MgB2

We have performed high-resolution photoemission spectroscopy on MgB2 and observed opening of a superconducting gap with a narrow coherent peak. We found that the superconducting gap is s-like with the gap value of 4.5 meV at 15 K. The temperature dependence (15 - 40 K) of gap value follows well the BCS form, suggesting that 2Delta/kBTc at T=0 is about 3. No pseudogap behavior is observed in the normal state. The present results strongly suggest that MgB2 is categorized into a phonon-mediated BCS superconductor in the weak-coupling regime.

cond-mat.str-el↗

arXiv2001

Compressibility of the MgB2 Superconductor

Considerable excitement has been caused recently by the discovery that the binary boride system with stoichiometry MgB2 is superconducting at the remarkably high temperature of 39 K (1). This potentially opens the way to even higher Tc values in a new family of superconductors with unexpectedly simple composition and structure. The simplicity in the electronic and crystal structures could allow the understanding of the physics of high-Tc superconductivity without the presence of the multitude of complicated features, associated with the cuprates. Synchrotron X-ray diffraction was used to measure the isothermal compressibility of MgB2, revealing a stiff tightly-packed incompressible solid with only moderate bonding anisotropy between intra- and inter-layer directions. These results, combined with the pressure evolution of the superconducting transition temperature, Tc establish its relation to the B and Mg bonding distances over a broad range of values.

cond-mat.supr-con↗

arXiv2001

Evidence for Strong-coupling S-wave Superconductivity in MgB2 :11B NMR Study

We have investigated a gap structure in a newly-discovered superconductor, MgB2 through the measurement of 11B nuclear spin-lattice relaxation rate, ^{11}(1/T_1). ^{11}(1/T_1) is proportional to the temperature (T) in the normal state, and decreases exponentially in the superconducting (SC) state, revealing a tiny coherence peak just below T_c. The T dependence of 1/T_1 in the SC state can be accounted for by an s-wave SC model with a large gap size of 2Δ/k_BT_c \sim 5 which suggests to be in a strong-coupling regime.

cond-mat.supr-con↗

arXiv2000

Hole distribution for (Sr,Ca,Y,La)_14 Cu_24 O_41 ladder compounds studied by x-ray absorption spectroscopy

The unoccupied electronic structure for the Sr_14Cu_24O_41 family of two-leg ladder compounds was investigated for different partial substitutions of Sr^2+ by Ca^2+, leaving the nominal hole count constant, and by Y^3+ or La^3+, reducing the nominal hole count from its full value of 6 per formula unit. Using polarization-dependent x-ray absorption spectroscopy on single crystals, hole states on both the chain and ladder sites could be studied. While for intermediate hole counts all holes reside on O sites of the chains, a partial hole occupation on the ladder sites in orbitals oriented along the legs is observed for the fully doped compound Sr_14Cu_24O_41. On substitution of Ca for Sr orbitals within the ladder planes but perpendicular to the legs receive some hole occupation as well.

cond-mat.str-el↗

arXiv2000

Quantum and classical dynamics in complex one-dimensional antiferromagnets

Of great recent interest in condensed matter physics are phenomena of coexistence of quantum and classical properties in the same material. Such duality occurs in certain mixed-spin antiferromagnets composed of quantum spin chains interacting through ``auxiliary'' magnetic ions. In this category are linear-chain rare earth nickelates that exhibit a seemingly paradoxical co-existence of long-range magnetic order and one-dimensional gapped quantum spin excitations at low temperatures. In the present paper we propose a unified physical description of these compounds, supported by recent neutron diffraction and inelastic scattering studies. Our basic concept is the effective separation between low-frequency classical and high-frequency quantum spin correlations. This interpretation enables experimental measurements of some previously inaccessible fundamental properties of quantum spin chains, and predicts new exotic magnetic excitations and mechanisms of long-range ordering in complex quantum magnets.

cond-mat.str-el↗

arXiv1999

The role of single-ion excitations in the mixed-spin quasi-1-dimensional quantum antiferromagnet Nd_2BaNiO_5

Inelastic neutron scattering experiments on Nd2BaNiO5 single-crystals and powder samples are used to study the dynamic coupling of 1-dimensional Haldane-gap excitations in the S=1 Ni2+ -chains to local crystal-field transitions, associated with the rare earth ions. Substantial interference between the two types of excitations is observed even in the 1-dimensional paramagnetic phase. Despite that, the results provide solid justification for the previously proposed ``static staggered field'' model for R2BaNiO5 nickelates. The observed behaviour is qualitatively explained by a simple chain-Random-Phase-Approximation (chain-RPA) model.

cond-mat.str-el↗

arXiv1999

Submillimeter Wave ESR Study of Spin Gap Excitations in CuGeO3

Transitions between the ground singlet state to the excited triplet state has been observed in CuGeO3 by means of submillimeter wave electron spin resonance. The strong absorption intensity shows the break down of the selection rule. The energy gap at zero field is evaluated to be 570 GHz(2.36 meV) and this value is nearly identical to the gap at the zone center observed by inelastic neutron scattering. The absorption intensity shows strong field orientation dependence but shows no significant dependence on magnetic field intensity. These features have been explained by considering the existence of Dzyaloshinsky-Moriya (DM) antisymmetric exchange interaction. The doping effect on this singlet-triplet excitation has been also studied. A drastic broadening of the absorption line is observed by the doping of only 0.5 % of Si.

cond-mat.str-el↗

arXiv1998

Polarized-neutron study of longitudinal Haldane-gap excitations in Nd$_2$BaNiO$_5$

Polarized and unpolarized inelastic neutron scattering is used to study Haldane-gap excitations in the mixed-spin linear-chain antiferromagnet Nd2BaNiO5. The longitudinal mode, polarized along the direction of ordered moments, is observed for the first time. The model of isolated Haldane chains in a static staggered exchange field, that is known to work very well for static properties and transverse spin excitations in R2BaNiO5 compounds, fails to explain new results for the longitudinal spin gap.

cond-mat.str-el↗

arXiv1998

Dopant-Bound Spinons in Cu_(1-x)Zn_xGeO_3

Polarized inelastic light scattering experiments on Cu_(1-x)Zn_xGeO_3 (0<= x <= 0.045) single crystals show for x<>0 a new distinct mode at nearly half the energy of the singlet response below the spin-Peierls transition. The temperature, magnetic field, polarization, and doping dependencies of this mode are similar to those of the singlet bound state. The data are interpreted in terms of a spinon-assisted light scattering process. Position and form of the peak provide strong evidence for the presence of dopant-bound spinons in Cu_(1-x)Zn_xGeO_3.

cond-mat.str-el↗

arXiv1998

Magnetic ordering, spin waves, and Haldane gap excitations in (Nd_x Y_{1-x})_2 Ba Ni O_5 linear-chain mixed-spin antiferromagnets

Linear-chain nickelates with the composition (Nd_x Y_{1-x})_2 Ba Ni O_5 (x=1, x=0.75, x=0.5, and x=0.25) are studied in a series of neutron scattering experiments. Powder diffraction is used to determine the temperature dependence of the magnetic structure in all four systems. Single-crystal inelastic neutron scattering is employed to investigate the temperature dependence of the Haldane-gap excitations and low-energy spin waves in the x=1 compound Nd_2 Ba Ni O_5. The results of these experiments are discussed in the context of the ``Haldane chain in a staggered field'' model for R_2 Ba Ni O_5 systems, and quantitative agreement with theory is obtained.

cond-mat.str-el↗

arXiv1998

An experimental measurement of the staggered magnetization curve for a Haldane spin chain

Long-range magnetic ordering in R_2 Ba Ni O_5 (R=magnetic rare earth) quasi-1-dimensional mixed-spin antiferromagnets is described by a simple mean-field model that is based on the intrinsic staggered magnetization function of isolated Haldane spin chains for the Ni-subsystem, and single-ion magnetization functions for the rare earth ions. The model is applied to new experimental results obtained in powder diffraction experiments on Nd_2 Ba Ni O_5 and Nd Y Ba Ni O_5, and to previously published diffraction data for Er_2 Ba Ni O_5. From this analysis we extract the bare staggered magnetization curve for Haldane spin chains in these compounds.

cond-mat.str-el↗

arXiv1997

Observation of three-magnon light scattering in CuGeO_3

Temperature and magnetic field dependent polarized Brillouin spectra of CuGeO_3 are reported. In addition to a bound singlet state at 30 cm^{-1}, a new feature has been observed at 18 cm^{-1}. This feature is interpreted in terms of a novel three-magnon light scattering process between excited triplet states.

cond-mat.str-el↗

arXiv1996

Temperature Dependence of the Spin-Peierls Energy Gap and Anomalous Line Shapes in CuGeO$_3$

Neutron scattering measurements on a large single crystal of CuGeO$_3$ have been used to determine the temperature-dependence of the spin-Peierls energy gap. While the power law behavior of the intensity of structural superlattice peaks is well fit by $I({\rm T})\propto ({\rm T}_c - {\rm T})^{2β}$ with an exponent of $β=0.33$, the exponent for the temperature dependence of the energy gap is significantly smaller than expected for conventional spin-Peierls materials. Usual scaling relations relate the energy gap to the superlattice reflection intensity as $Δ({\rm T}) \propto I^a$ with $a=1/3$; the present results suggest an exponent of $a\approx 1/6$ for CuGeO$_3$. Additional scattering cross-section is observed in constant-$q$ and constant-$E$ scans creating a long `tail' extending to higher energies relating to a proposed scattering continuum.

cond-mat↗