arXiv subjects

K. Oka

Publications and source records attributed to K. Oka.

At least 19 recordsLinked to original sources

arXiv2026

Coupled structural and electronic evolution under pressure in CuIr2Se4, CuRh2S4, and CuRh2Se4

Spinel chalcogenides provide a platform for investigating the interplay among metallic, superconducting, and pressure-induced insulating states. Here, we combine synchrotron powder X-ray diffraction and electrical-resistivity measurements to investigate the pressure evolution of CuIr2Se4, CuRh2S4, and CuRh2Se4 over pressure ranges extending beyond those previously explored. High-pressure diffraction reveals closely related monoclinic supercells in all three compounds. For CuIr2Se4 and CuRh2S4, constrained profile fits based on structural models relaxed using density functional theory are compatible with Phase-IV-type bond-disproportionated structures, whereas the data for CuRh2Se4 establish a compatible monoclinic unit cell without resolving its atomic-scale ordering pattern. Insulating-like transport develops abruptly over a narrow pressure range in CuIr2Se4 but more gradually over broader pressure ranges in the Rh-based compounds, in close correspondence with their respective structural transformations. We also establish previously unreported bulk superconductivity in CuIr2Se4 at ambient pressure: zero resistance is attained at 0.29 K, and the accompanying ac diamagnetic response is consistent with nearly complete superconducting shielding. These results establish a close relationship between the formation of the high-pressure monoclinic phases and the evolution toward insulating transport, and demonstrate that transition-metal and chalcogen substitutions tune the characteristic pressure scales and the competition with superconductivity within a closely related structural framework.

cond-mat.mtrl-sci↗

arXiv2019

Highly dispersive magnons with spin-gap like features in the frustrated ferromagnetic S=1/2 chain compound Ca2Y2Cu5O10 detected by inelastic neutron scattering

We report inelastic neutron scattering experiments in Ca2Y2Cu5O10 and map out the full one magnon dispersion which extends up to a record value of 53 meV for frustrated ferromagnetic (FM) edge-sharing CuO2 chain (FFESC) cuprates. A homogeneous spin-1/2 chain model with a FM nearest-neighbor (NN), an antiferromagnetic (AFM) next-nearest-neighbor (NNN) inchain, and two diagonal AFM interchain couplings (ICs) analyzed within linear spin-wave theory (LSWT) reproduces well the observed strong dispersion along the chains and a weak one perpendicularly. The ratio R=|J_{a2}/J_{a1}| of the FM NN and the AFM NNN couplings is found as ~0.23, close to the critical point Rc=1/4 which separates ferromagnetically and antiferromagnetically correlated spiral magnetic ground states in single chains, whereas Rc>0.25 for coupled chains is considerably upshifted even for relatively weak IC. Although the measured dispersion can be described by homogeneous LSWT, the scattering intensity appears to be considerably reduced at ~11.5 and ~28 meV. The gap-like feature at 11.5 meV is attributed to magnon-phonon coupling whereas based on DMRG simulations of the dynamical structure factor the gap at 28 meV is considered to stem partly from quantum effects due to the AFM IC. Another contribution is ascribed to the intrinsic superstructure from the distorting incommensurate pattern of CaY cationic chains adjacent to the CuO2 ones. It gives rise to non-equivalent CuO4 units and Cu-O-Cu bond angles Phi and a resulting distribution of all exchange integrals. The J's fitted by homogeneous LSWT are regarded as average values. The record value of the FM NN integral J1=24 meV among FFESC cuprates can be explained by a non-universal Phi (not 90 deg.) and Cu-O bond length dependent anisotropic mean direct FM Cu-O exchange K_{pd}~120 meV. Enhanced K_{pd} values are also needed to compensate a significant AFM J_{dd} > ~6 meV.

cond-mat.str-el↗

arXiv2018

Probing ultrafast spin-relaxation and precession dynamics in a cuprate Mott insulator with 7-fs optical pulses

A charge excitation in a two-dimensional Mott insulator is strongly coupled with the surrounding spins, which is observed as magnetic-polaron formations of doped carriers and a magnon sideband in the Mott-gap transition spectrum. However, the dynamics related to the spin sector are difficult to measure. Here, we show that pump-probe reflection spectroscopy with 7-fs laser pulses can detect the optically induced spin dynamics in Nd$_2$CuO$_4$, a cuprate Mott insulator. The bleaching signal at the Mott-gap transition is enhanced at $\sim$18 fs, which corresponds to the spin-relaxation time in magnetic-polaron formations and is characterized by the exchange interaction. More importantly, ultrafast coherent oscillations appear in the time evolutions of the reflectivity changes, and their frequencies (1400-2700 cm$^{-1}$) are equal to the probe energy measured from the Mott-gap transition peak. These oscillations originate from interferences between charge excitations with two magnons and provide direct evidence for charge-spin coupling.

cond-mat.str-el↗

arXiv2014

Superconductivity in Noncentrosymmetric Iridium Silicide Li2IrSi3

The effects of lithium absorption on the crystal structure and electronic properties of IrSi3, a binary silicide with a noncentrosymmetric crystal structure, were studied. X-ray and neutron diffraction experiments revealed that hexagonal IrSi3 (space group P6_3mc) transforms into trigonal Li2IrSi3 (space group P31c) upon lithium absorption. The structure of Li2IrSi3 is found to consist of a planar kagome network of silicon atoms with Li and Ir spaced at unequal distances between the kagome layers, resulting in a polar structure along the c-axis. Li2IrSi3 exhibited type-II superconductivity with a transition temperature Tc of 3.8 K, displaying a structure type that no previous superconductors have been reported to have.

cond-mat.supr-con↗

arXiv2013

Muon spin spectroscopy in multiferroic (Cu,Zn)$_{3}$Mo$_{2}$O$_{9}$

We present the muon spin relaxation/rotation spectra in the multiferroic compound (Cu,Zn)$_{3}$Mo$_{2}$O$_{9}$. The parent material Cu$_{3}$Mo$_{2}$O$_{9}$ has a multiferroic phase below $T_{\rm N}$ = 8.0 K, where the canted antiferromagnetism and the ferroelectricity coexist. The asymmetry time spectra taken at RIKEN-RAL pulsed muon facility show a drastic change at $T_{\rm N}$. At low temperatures the weakly beating oscillation caused by the static internal magnetic fields in the antiferromagnetic phase was observed in Cu$_{3}$Mo$_{2}$O$_{9}$ and the lightly ($0.5\%$) Zn-doped sample. From the fitting of the oscillating term, we obtain the order parameter in these samples: ferromagnetic moment in two sublattices of antiferromagnet. In the heavily ($5.0\%$) Zn-doped sample, the muon-spin oscillation is rapidly damped. The frequency-domain spectrum of this sample suggests a formation of magnetic superstructure.

cond-mat.str-el↗

arXiv2013

Magnetic and Dielectric Properties in Multiferroic Cu3Mo2O9 under High Magnetic Fields

The magnetic and dielectric properties under high magnetic fields are studied in the single crystal of Cu3Mo2O9. This multiferroic compound has distorted tetrahedral spin chains. The effects of the quasi-one dimensionality and the geometrical spin frustration are expected to appear simultaneously. We measure the magnetoelectric current and the differential magnetization under the pulsed magnetic field up to 74 T. We also measure the electric polarization versus the electric field curve/loop under the static field up to 23 T. Dielectric properties change at the magnetic fields where the magnetization jumps are observed in the magnetization curve. Moreover, the magnetization plateaus are found at high magnetic fields.

cond-mat.str-el↗

arXiv2012

Ca2Y2Cu5O10: the first frustrated quasi-1D ferromagnet close to criticality

Ca2Y2Cu5O10 is build up from edge-shared CuO4 plaquettes forming spin chains. From inelastic neutron scattering data we extract an in-chain nearest neighbor exchange J1 approximately -170 K and the frustrating next neighbor J2 approximately 32 K interactions, both significantly larger than previous estimates. The ratio alpha= J2/J1 approximately 0.19 places the system very close to the critical point alpha_c=0.25 of the J1-J2 chain, but in the ferromagnetic regime. We establish that the vicinity to criticality only marginally affects the dispersion and coherence of the elementary spin-wave-like magnetic excitations, but instead results in a dramatic T-dependence of high-energy Zhang-Rice singlet excitation intensities.

cond-mat.str-el↗

arXiv2011

Magnetic and electric properties in the distorted tetrahedral spin chain system Cu3Mo2O9

We study the multiferroic properties in the distorted tetrahedral quasi-one dimensional spin system Cu$_3$Mo$_2$O$_9$, in which the effects of the low dimensionality and the magnetic frustration are expected to appear simultaneously. We clarify that the antiferromagnetic order is formed together with ferroelectric properties at $T_{\rm N}=7.9$ K under zero magnetic field and obtain the magnetic-field-temperature phase diagram by measuring dielectric constant and spontaneous electric polarization. It is found that the antiferromagnetic phase possesses a spontaneous electric polarization parallel to the c axis when the magnetic field $H$ is applied parallel to the a axis. On the other hand, there are three different ferroelectric phases in the antiferromagnetic phase for $H$ parallel to the c axis.

cond-mat.str-el↗

arXiv2009

Low-energy (< 10 meV) feature in the nodal electron self-energy and strong temperature dependence of the Fermi velocity in Bi(2)Sr(2)CaCu(2)O(8+delta)

Using low-photon energy angle-resolved photoemission (ARPES), we study the low-energy dispersion along the nodal (pi, pi) direction in Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) as a function of temperature. Less than 10 meV below the Fermi energy, the high-resolution data reveals a novel "kink"-like feature in the real part of the electron self-energy that is distinct from the larger well-known kink roughly 70 meV below E_F. This new kink is strongest below the superconducting critical temperature and weakens substantially as the temperature is raised. A corollary of this finding is that the Fermi velocity, as measured over this energy range, varies rapidly with temperature - increasing by almost 30% from 70 to 110 K.

cond-mat.supr-con↗

arXiv2008

Effect of Zn substitution for Cu on Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ near the hole concentration of 1/8 per Cu

A weakening of superconductivity upon substitution of Cu by Zn (0.5~1 %) is observed in a high-T_c cuprate, Ca_{2-x}Na_xCuO2Cl2, near the hole concentration of 1/8 per Cu. The superconducting transition temperature and its volume fraction, estimated by magnetic susceptibility, exhibit a sizable anomaly for x=0.12~0.14, where the slowing down of Cu spin fluctuations below 5 K is demonstrated by muon spin relaxation experiments. These observations are in close resemblance to other typical cuprates including YBa2Cu3O_{7-d}, and Bi2Sr2Ca_{1-x}Y_xCu2O_{8+d}, providing further evidence that Zn-induced "stripe" correlation is a universal feature of high-T_c cuprate superconductors common to that of La_{2-x}A_{x}CuO4 (A=Ba, Sr).

cond-mat.supr-con↗

arXiv2008

Mesoscopic Phase Coherence in a Quantum Spin Fluid

Mesoscopic quantum phase coherence is important because it improves the prospects for handling quantum degrees of freedom in technology. Here we show that the development of such coherence can be monitored using magnetic neutron scattering from a one-dimensional spin chain Y2BaNiO5, a quantum spin fluid where no classical, static magnetic order is present. In the cleanest samples, the quantum coherence length is 20 nm, almost an order of magnitude larger than the classical antiferromagnetic correlation length of 3 nm. We also demonstrate that the coherence length can be modified by static and thermally activated defects in a quantitatively predictable manner.

cond-mat.str-el↗

arXiv2005

3D Gasdynamic Modelling of the Changes in the Flow Structure During Transition From Quiescent to Active State in Symbiotic Stars

The results of 3D modelling of the flow structure in the classical symbiotic system Z~Andromedae are presented. Outbursts in systems of this type occur when the accretion rate exceeds the upper limit of the steady burning range. Therefore, in order to realize the transition from a quiescent to an active state it is necessary to find a mechanism able to sufficiently increase the accretion rate on a time scale typical to the duration of outburst development. Our calculations have confirmed the transition mechanism from quiescence to outburst in classic symbiotic systems suggested earlier on the basis of 2D calculations (Bisikalo et al, 2002). The analysis of our results have shown that for wind velocity of 20 km/s an accretion disc forms in the system. The accretion rate for the solution with the disc is ~22.5-25% of the mass loss rate of the donor, that is, ~4.5-5*10^(-8)Msun/yr for Z And. This value is in agreement with the steady burning range for white dwarf masses typically accepted for this system. When the wind velocity increases from 20 to 30 km/s the accretion disc is destroyed and the matter of the disc falls onto the accretor's surface. This process is followed by an approximately twofold accretion rate jump. The resulting accretion rate growth is sufficient for passing the upper limit of the steady burning range, thereby bringing the system into an active state. The time during which the accretion rate is above the steady burning value is in a very good agreement with observations. The analysis of the results presented here allows us to conclude that small variations in the donor's wind velocity can lead to the transition from the disc accretion to the wind accretion and, as a consequence, to the transition from quiescent to active state in classic symbiotic stars.

astro-ph↗

arXiv2005

Angular dependence of direct rf power absorption studies in Bi2Sr2CaCu2O8 single crystals

We have studied rf power dissipation in Bi2Sr2CaCu2O8 single crystals as a function of angle between the applied field and c-axis of the crystal by using a novel technique of measuring direct rf power absorption. We observed two peaks in power absorption as a function of temperature. The amplitude and position of the first peak (peak A) change with orientation and attributed to Josephson junctions decoupling. The second peak (peak B) appears just below Tc has been attributed to the vortex motion and its amplitude depends on the angle of orientation.

cond-mat.supr-con↗

arXiv2005

Laser ARPES, the sudden approximation, and quasiparticle-like peaks in Bi2Sr2CaCu2O8+delta

A new low photon energy regime of angle resolved photoemission spectroscopy is accessed with lasers and used to study the superconductor Bi2Sr2CaCu2O8+delta. The low energy increases bulk sensitivity, reduces background, and improves resolution. With this we observe spectral peaks which are sharp on the scale of their binding energy - the clearest evidence yet for quasiparticles in the normal state. Crucial aspects of the data such as the dispersion, superconducting gaps, and the bosonic coupling kink and associated weight transfer are robust to a possible breakdown of the sudden approximation.

cond-mat.supr-con↗

arXiv2005

Magnetic field dependent direct rf power absorption studies in Bi_2 Sr_2 CaCu_2 O_8 single crystals

Using a novel technique of measuring direct rf power dissipation, the temperature and magnetic field dependence of rf absorption in superconducting single crystals of Bi_2Sr_2CaCu_2O_8 is studied. An unexpected, larger than normal state, dissipation is observed below Tc, which exhibits peaks at magnetic field dependent temperatures. An explanation is provided for this observation in terms of the energy required to decouple the intrinsic Josephson junctions in the crystals.

cond-mat.supr-con↗

arXiv2004

Effects of hole-doping on the magnetic ground state and excitations in the edge-sharing CuO$_2$ chains of Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$

Neutron scattering experiments were performed on the undoped and hole-doped Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$, which consists of ferromagnetic edge-sharing CuO$_2$ chains. It was previously reported that in the undoped Ca$_2$Y$_2$Cu$_5$O$_{10}$ there is an anomalous broadening of spin-wave excitations along the chain, which is caused mainly by the antiferromagnetic interchain interactions [Matsuda $et$ $al.$, Phys. Rev. B 63, 180403(R) (2001)]. A systematic study of temperature and hole concentration dependencies of the magnetic excitations shows that the magnetic excitations are softened and broadened with increasing temperature or doping holes irrespective of $Q$ direction. The broadening is larger at higher $Q$. A characteristic feature is that hole-doping is much more effective to broaden the excitations along the chain. It is also suggested that the intrachain interaction does not change so much with increasing temperature or doping although the anisotropic interaction and the interchain interaction are reduced. In the spin-glass phase ($x$=1.5) and nearly disordered phase ($x$=1.67) the magnetic excitations are much broadened in energy and $Q$. It is suggested that the spin-glass phase originates from the antiferromagnetic clusters, which are caused by the hole disproportionation.

cond-mat.str-el↗

arXiv2004

Wind accretion in binary stars - I. Mass accretion ratio

Three-dimensional hydrodynamic calculations are performed in order to investigate mass transfer in a close binary system, in which one component undergoes mass loss through a wind. The mass ratio is assumed to be unity. The radius of the mass-losing star is taken to be about a quarter of the separation between the two stars. Calculations are performed for gases with a ratio of specific heats gamma=1.01 and 5/3. Mass loss is assumed to be thermally driven so that the other parameter is the sound speed of the gas on the mass-losing star. Here, we focus our attention on two features: flow patterns and mass accretion ratio, which we define as the ratio of the mass accretion rate onto the companion to the mass loss rate from the mass-losing primary star. We characterize the flow by the mean normal velocity of wind on the critical Roche surface of the mass-losing star, Vr. When Vr<0.4 A Omega, where A and Omega are the separation between the two stars and the angular orbital frequency of the binary, respectively, we obtain Roche-lobe over-flow (RLOF), while for Vr>0.7 A Omega we observe wind accretion. We find very complex flow patterns in between these two extreme cases. We derive an empirical formula of the mass accretion ratio in the low and in the high velocity regime.

astro-ph↗

arXiv2004

Gas flow in close binary star systems

We first present a summary of our numerical work on accretion discs in close binary systems. Our recent studies on numerical simulations of the surface flow on the mass-losing star in a close binary star is then reviewed.

astro-ph↗