arXiv subjects

H. Kuwahara

Publications and source records attributed to H. Kuwahara.

At least 19 recordsLinked to original sources

arXiv2026

Incommensurate Antiferromagnetic Order in the Fe-substituted Bi-2201 Cuprate in the Heavily Overdoped Regime

Elastic neutron scattering experiments showed incommensurate antiferromagnetic peaks in 5% Fe-substituted Bi-2201 cuprate in the non-superconducting heavily overdoped regime. The incommensurability delta~0.21 is comparable to that observed in Fe-substituted Bi-2201 in the overdoped regime. [Hiraka et al., Phys. Rev. B 81, 144501 (2010).] The magnetic correlation length is comparable between the overdoped and non-superconducting heavily overdoped regimes. It is plausible that incommensurate antiferromagnetic order is induced and stabilized by Fe in the heavily overdoped regime, which suggests a robust antiferromagnetic correlation beyond the superconducting dome in the phase diagram.

cond-mat.supr-con↗

arXiv2025

Cu-spin Correlation in the Electron-overdoped High-Tc Cuprate Thin Films of La_2-x_Ce_x_CuO_4_ Probed by Low-energy Muons

We investigated the Cu-spin correlation in the overdoped regime of the electron-doped high-Tc cuprate thin films of La_2-x_Ce_x_CuO_4_, changing the reduction condition from muon spin relaxation using low-energy muons. The Cu-spin correlation developed at low temperatures for optimally reduced films with x=0.13 as well as x=0.17 where the superconductivity was almost suppressed. These results are contrary to those observed in the hole-doped high-Tc cuprates where the development of the antiferromagnetic Cu-spin correlation disappears together with the suppression of superconductivity. The Cu-spin correlation developed at low temperatures in x=0.17 may be understood in terms of antiferromagnetism, but it may be related to a ferromagnetic order recently suggested in the nonsuperconducting heavily overdoped La$_2-x_Ce_x_CuO_4_ with x~0.18.

cond-mat.supr-con↗

arXiv2024

Field-induced electric polarization and elastic softening caused by parity-mixed $d$-$p$ hybridized states with electric multipoles in Ba$_2$CuGe$_2$O$_7$

We performed high-magnetic-field magnetization, polarization, and ultrasonic measurements in Ba$_2$CuGe$_2$O$_7$ to investigate field-induced multiferroic properties arising from a cross-correlation between electric dipoles and electric quadrupoles in addition to cross-correlation between magnetic dipoles and electric dipoles. Magnetization $M$ shows saturation behavior above 20 T for several magnetic field directions, however, electric polarization $P_c$ exhibits an increase, and elastic constants show a softening above 20 T. Based on quantum states with a crystalline electric field for the $D_{2d}$ point group and $d$-$p$ hybridization between Cu-$3d$ and O-$2p$ electrons, we confirmed that the matrix of an electric dipole $P_z$ was proportional to that of an electric quadrupole $O_{xy}$. Furthermore, considering the spin-orbit coupling of $3d$ electrons and the Zeeman effect, we showed that $P_z$ and $O_{xy}$ simultaneously exhibited field-induced responses. These findings indicate that the orbital degrees of freedom, in addition to the spin degrees of freedom, contribute to the high-field multiferroicity in Ba$_2$CuGe$_2$O$_7$.

cond-mat.str-el↗

arXiv2021

Magnetic Impurity Effects on Ferromagnetic Fluctuations in Heavily Overdoped (Bi,Pb)$_2$Sr$_2$Cu$_{1-y}$Fe$_y$O$_{6+δ}$ Cuprates

We investigated Fe-substitution effects on ferromagnetic fluctuations in the superconducting overdoped and non-superconducting heavily overdoped regimes of the Bi-2201 cuprates by the magnetization and electrical-resistivity measurements. It was found that the spin-glass state was induced at low temperatures by the Fe substitution. The Curie constant and the effective Bohr magneton, estimated from the magnetic susceptibility, as well as the dimensionality of the ferromagnetic fluctuations from the resistivity, suggest the enhancement of the ferromagnetic fluctuations owing to the Fe substitution. A ferromagnetic spin-cluster model is proposed in the heavily overdoped regime of Bi-2201, while an antiferromagnetic spin-cluster model has been proposed in the overdoped regime of Bi-2201 [Hiraka et al., Phys. Rev. B 81, 144501 (2010)].

cond-mat.supr-con↗

arXiv2014

Reduction of critical field for magnetic and orbital-ordering phase transition in impurity-substituted Nd$_{0.45}$Sr$_{0.55}$MnO$_3$ crystal

We have investigated the Mn-site substitution effect in Nd$_{0.45}$Sr$_{0.55}$MnO$_3$ single crystal, which has an $A$-type layered antiferromagnetic ($A$-AFM) phase with the 3$d_{x^2-y^2}$-type orbital-order. Substitution of Fe or Ga for Mn-site suppresses both the $A$-AFM order and competing ferromagnetic (FM) correlation whereas Cr substitution suppresses only the $A$-AFM order but reactivates the underlying FM correlation via double-exchange mechanism along the AFM coupled $c$-direction. In Nd$_{0.45}$Sr$_{0.55}$Mn$_{0.95}$Cr$_{0.05}$O$_3$, the $A$-AFM state with the orbital-order is changed into the orbital-disordered three-dimensional FM metallic state by applying magnetic field of $μ_0 H = 12$ T, which is much smaller than that of the parent compound Nd$_{0.45}$Sr$_{0.55}$MnO$_3$.

cond-mat.str-el↗

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

Multiferroic properties of an åkermanite Sr$_2$CoSi$_2$O$_7$ single crystal in high magnetic fields

The magnetic and dielectric properties of åkermanite Sr$_2$CoSi$_2$O$_7$ single crystals in high magnetic fields were investigated. We have observed finite induced electric polarization along the c axis in high fields, wherein all Co spins were forcibly aligned to the magnetic field direction. Existence of the induced polarization in the spin-polarized state accompanied with the finite slope in the magnetization curve suggests the possible role of the orbital angular momenta in the excited states as its microscopic origin. The emergence of the field-induced polarization without particular magnetic order can be regarded as the magnetoelectric effects of the second order from the symmetry point of view. A low magnetic field-driven electric polarization flip induced by a rotating field, even at room temperature, has been successfully demonstrated.

cond-mat.str-el↗

arXiv2009

$R$-site Randomness Effect in Double-Perovskite $R$BaMn$_{2}$O$_{6}$

We have investigated the $R$-site randomness effect of $R$/Ba-ordered $R$BaMn$_{2}$O$_{6}$ ($R$ = rare earth) by using Y$_{1-y}$La$_y$BaMn$_2$O$_6$ (0 (\leq) $y$ (\leq) 1) in which $R$ (Y,La) and Ba are regularly arranged while Y and La randomly occupy the $R$-site. YBaMn$_2$O$_6$ ($y$ = 0) undergoes charge/orbital ordering (CO) transition at $T_{\rm CO}$ = 500 K while LaBaMn$_{2}$O$_{6}$ ($y$ = 1) shows ferromagnetic metallic (FM) behavior below 350 K\@. In 0 (\leq) $y$ (\leq) 0.6, $T_{\rm CO}$ decreases with an increase in $y$. In 0.6 (\leq) $y$ (\leq) 0.8, $R$-site randomness causes strong phase separation tendencies among the FM, antiferromagnetic (AFM), and CO insulating (COI) states. The large magnetoresistance is observed in the phase separated region.

cond-mat.str-el↗

arXiv2009

Magnetoelectric Properties of (Ca$_{1-x}$Sr$_{x}$)$_2$CoSi$_2$O$_7$ Crystals

We have investigated the magnetoelectric properties of (Ca$_{1-x}$Sr$_{x}$)$_2$CoSi$_2$O$_7$ ($0\leq x\leq 1$) crystals with a quasi-two-dimensional structure. In Ca$_2$CoSi$_2$O$_7$ ($x=0$), a canted antiferromagnetic transition occurs at 5.6 K. The transition temperature $T_{\rm N}$ is increasing with increasing Sr concentration, and the rises of the magnetization and dielectric constant become larger. Since the dielectric constant shows large change at $T_{\rm N}$ and the magnetocapacitance effect is observed below $T_{\rm N}$, a coupling between the magnetism and dielectricity is strong in (Ca$_{1-x}$Sr$_{x}$)$_2$CoSi$_2$O$_7$. The positive magnetocapacitance is reduced by Sr substitution, and is not observed in $x\geq 0.5$. Namely, the compound of $x\geq 0.5$ does not show the magnetic-field-induced electric polarization. On the other hand, the negative magnetocapacitance is enhanced by Sr substitution.

cond-mat.str-el↗

arXiv2009

$A$-site substitution effect on physical properties of Sr$_3$Fe$_{2-x}$Co$_x$O$_{7-δ}$

We have investigated the $Ln^{3+}$-substitution ($Ln$ = lanthanoid) effect of a quasi two-dimensional ferromagnet Sr$_3$Fe$_{2-x}$Co$_x$O$_{7-δ}$ ($x$$ = $0.5). %$Ln^{3+}$-substitution creates antiferromagnetic insulating clusters around $Ln^{3+}$. With increasing $Ln^{3+}$-concentration, the ferromagnetism is gradually suppressed and the resistivity is increasing, which are ascribed to an increase in antiferromagnetic (AFM) clusters created by $Ln^{3+}$-substitution. In Sr$_{2.7}$Gd$_{0.3}$Fe$_{1.5}$Co$_{0.5}$O$_{7-δ}$, the magnetoresistance (MR) is enhanced by about 20 % compared with that of Sr$_3$Fe$_{1.5}$Co$_{0.5}$O$_{7-δ}$. Coexistence of ferromagnetic (FM) and AFM phases is essential for the enhancement of the MR\@. Applied magnetic fields align the FM clusters in the same direction, resulting in a reduction in the resistivity. A metamagnetic transition observed in the $Ln^{3+}$-doped samples also contributes to the enhancement of the MR.

cond-mat.str-el↗

arXiv2009

Gigantic magnetoelectric effect caused by magnetic-field-induced canted antiferromagnetic-paramagnetic transition in quasi-two-dimensional Ca$_2$CoSi$_2$O$_7$ crystal

We have investigated the magnetic and dielectric properties of Ca$_2$CoSi$_2$O$_7$ crystal. The dielectricity and magnetism of Ca$_2$CoSi$_2$O$_7$ are strongly coupled below a canted antiferromagnetic transition temperature ($T_{\rm N}$): Magnetic fields induce electric polarization below $T_{\rm N}$. Interestingly, the magnetic-field-induced electric polarization is detected even without poling electric fields. Below $T_{\rm N}$, a canted antiferromagnetic-paramagnetic transition is induced by magnetic fields. The large magnetocapacitance is observed around $T_{\rm N}$. The origin of the large magnetocapacitance is due to the magnetic-field-induced the canted antiferromagnetic-paramagnetic transition.

cond-mat.str-el↗

arXiv2009

Cr-doping effect on the orbital fluctuation of heavily doped Nd1-xSrxMnO3 (x ~ 0.625)

We have investigated the Cr-doping effect of Nd0.375Sr0.625MnO3 near the phase boundary between the x2-y2 and 3z2-r2 orbital ordered states, where a ferromagnetic correlation and concomitant large magnetoresistance are observed owing to orbital fluctuation. Cr-doping steeply suppresses the ferromagnetic correlation and magnetoresistance in Nd0.375Sr0.625Mn1-yCryO3 with 0 < y < 0.05, while they reappear in 0.05 < y < 0.10. Such a reentrant behavior implies that a phase boundary is located at y = 0.05, or a phase crossover occurs across y = 0.05.

cond-mat.str-el↗

arXiv2009

Persistent and Reversible Phase Control in GdMnO$_3$ near the Phase Boundary

We have investigated temperature and magnetic-field dependence of dielectric properties in the orthorhombic GdMnO$_3$ single crystal which is located near the phase boundary between the ferroelectric/spiral-antiferromagnetic phase and the paraelectric/$A$-type-antiferromagnetic one. In this compound, strong phase competition between these two phases results in a unique phase diagram with large temperature and magnetic-field hystereses. Based on the phase diagram, we have successfully demonstrated the persistent and reversible phase switching between them by application of magnetic fields.

cond-mat.str-el↗

arXiv2009

Magnetic and dielectric properties of A2CoSi2O7 (A=Ca, Sr, Ba) crystals

We have investigated the magnetic and dielectric properties of $A_2$CoSi$_2$O$_7$ ($A$=Ca, Sr, and Ba) crystals with a two-dimensional network of CoO$_4$ and SiO$_4$ tetrahedra connected with each other through the corners. In Ca$_2$CoSi$_2$O$_7$, a weak ferromagnetic transition occurs at 5.7 K, where the dielectric constant parallel to the c axis shows a concomitant anomaly. The large magnetocapacitance effect is observed below 5.7 K; $Δ\varepsilon (H)/\varepsilon (0) \equiv [\varepsilon (H)-\varepsilon (0)]/\varepsilon (0)$ reaches 13 % at 5.1 K\@. These results indicate a strong coupling between the magnetism and dielectricity in Ca$_2$CoSi$_2$O$_7$. Sr$_2$CoSi$_2$O$_7$ shows a similar magnetoelectric behavior to that of Ca$_2$CoSi$_2$O$_7$. In contrast, in Ba$_2$CoSi$_2$O$_7$, which has the different arrangement of SiO$_4$ and CoO$_4$ tetrahedra from that of Ca$_2$CoSi$_2$O$_7$, the magnetocapacitance is hardly observed. The key for the magnetocapacitance effect of $A_2$CoSi$_2$O$_7$ lies in the quasi-two-dimensional crystal structure.

cond-mat.str-el↗

arXiv2008

Chemical potential landscape in band filling and bandwidth-control of manganites: Photoemission spectroscopy measurements

We have studied the effects of band filling and bandwidth control on the chemical potential in perovskite manganites $R_{1-x}A_x$MnO$_3$ ($R$ : rare earth, $A$ : alkaline earth) by measurements of core-level photoemission spectra. A suppression of the doping-dependent chemical potential shift was observed in and around the CE-type charge-ordered composition range, indicating that there is charge self-organization such as stripe formation or its fluctuations. As a function of bandwidth, we observed a downward chemical potential shift with increasing bandwidth due to the reduction of the orthorhombic distortion. After subtracting the latter contribution, we found an upward chemical potential shift in the ferromagnetic metallic region $0.3<x<0.5$, which we attribute to the enhancement of double-exchange interaction involving the Jahn-Teller-split $e_g$ band.

cond-mat.str-el↗

arXiv2008

Comparative study of ordered and disordered Y1-xSrxCoO3-d

We have succeeded in preparing A-site ordered- and disordered-Y1/4Sr3/4CoO3-d with various oxygen deficiencies delta, and have made comparative study of their structural and physical properties. In the A-site ordered structure, oxygen vacancies order, and d = 0.34 sample shows a weak ferromagnetic transition beyond 300 K. On the other hand, in the A-site disordered structure, no oxygen vacancy ordering is observed, and d = 0.16 sample shows a ferromagnetic metallic transition around 150 K. A-site disordering destroys the orderings of oxygen-vacancies and orbitals, leading to the strong modification of the electronic phases.

cond-mat.str-el↗

arXiv2007

Chemical potential shift induced by double-exchange and polaronic effects in Nd_{1-x}Sr_xMnO_3

We have studied the chemical potential shift as a function of temperature in Nd$_{1-x}$Sr$_x$MnO$_3$ (NSMO) by measurements of core-level photoemission spectra. For ferromagnetic samples ($x=0.4$ and 0.45), we observed an unusually large upward chemical potential shift with decreasing temperature in the low-temperature region of the ferromagnetic metallic (FM) phase. This can be explained by the double-exchange (DE) mechanism if the $e_g$ band is split by dynamical/local Jahn-Teller effect. The shift was suppressed near the Curie temperature ($T_C$), which we attribute to the crossover from the DE to lattice-polaron regimes.

cond-mat.str-el↗