arXiv subjects

Y. Koike

Publications and source records attributed to Y. Koike.

At least 91 records · Page 5Linked to original sources

arXiv2004

Drastic enhancement of magnon thermal conductivity in the Bose-Einstein condensed state of TlCuCl$_3$

We have measured the thermal conductivity of a TlCuCl$_3$ single crystal in magnetic fields up to 14 T. It has been found that the temperature dependence of the thermal conductivity exhibits a sharp peak at 4 K in zero field, which is suppressed by the application of magnetic fields up to 7 T. The peak is concluded to be attributable to the enhancement of the thermal conductivity due to phonons because of the formation of a spin-gap state. In high magnetic fields above 7 T, on the other hand, another sharp peak appears around 4 K and this is enhanced with increasing magnetic field. This peak is regarded as being attributable to the enhancement of the thermal conductivity due to magnons and/or phonons because of the drastic extension of the mean free path of magnons and/or phonons in the Bose-Einstein condensed state.

cond-mat.str-el↗

arXiv2004

Single-crystal growth and dependences on the hole concentration and magnetic field of the magnetic ground state in the edge-sharing CuO$_2$ chain system Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$

We have succeeded in growing large-size single-crystals of Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$ with $0 \le x \le 1.67$ and measured the magnetic susceptibility, specific heat and magnetization curve, in order to study the magnetic ground state in the edge-sharing CuO$_2$ chain as a function of hole concentration and magnetic field. In $0 \le x \le 1.3$, it has been found that an antiferromagnetically ordered phase with the magnetic easy axis along the b-axis is stabilized and that a spin-flop transition occurs by the application of magnetic fields parallel to the b-axis. The antiferromagnetic transition temperature decreases with increasing $x$ and disappears around $x =$ 1.4. Alternatively, a spin-glass phase appears around $x = 1.5$. At $x = 1.67$ where the hole concentration is $\sim$ 1/3 per Cu, it appears that a spin-gap state is formed owing to the formation of spin-singlet pairs. No sign of the coexistence of an antiferromagnetically ordered state and a spin-gap one suggested in Ca$_{1-x}$CuO$_2$ has been found in Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$.

cond-mat.str-el↗

arXiv2004

Field-induced magnetic order in La$sub {2-x}$Sr$sub x$CuO$sub 4$ (x = 0.10, 0.115, 0.13) studied by in-plane thermal conductivity measurements

We have measured the thermal conductivity in the $ab$-plane of La$_{2-x}$Sr$_x$CuO$_4$ ($x =$ 0.10, 0.115, 0.13) in magnetic fields up to 14 T parallel to the c-axis and also parallel to the $ab$-plane. By the application of magnetic fields parallel to the c-axis, the thermal conductivity has been found to be suppressed at low temperatures below the temperature $T_κ$ which is located above the superconducting transition temperature and is almost independent of the magnitude of the magnetic field. The suppression is marked in $x =$ 0.10 and 0.13, while it is small in $x =$ 0.115. Furthermore, no suppression is observed in the 1 % Zn-substituted La$_{2-x}$Sr$_x$Cu$_{0.99}$Zn$_{0.01}$O$_4$ with $x =$ 0.115. Taking into account the experimental results that the temperature dependence of the relative reduction of the thermal conductivity is quite similar to the temperature dependence of the intensity of the incommensurate magnetic Bragg peak corresponding to the static stripe order and that the Zn substitution tends to stabilize the static order, it is concluded that the suppression of the thermal conductivity in magnetic fields is attributed to the development of the static stripe order. The present results suggest that the field-induced magnetic order in La$_{2-x}$Sr$_x$CuO$_4$ originates from the pinning of the dynamical stripes of spins and holes by vortex cores.

cond-mat.supr-con↗

arXiv2004

Muon-spin-relaxation and magnetic-susceptibility studies of effects of the magnetic impurity Ni on the Cu-spin dynamics and superconductivity in La_2-x_Sr_x_Cu_1-y_Ni_y_O_4_ with x = 0.13

Effects of the magnetic impurity Ni on the Cu-spin dynamics and superconductivity have been studied in La_2-x_Sr_x_Cu_1-y_Ni_y_O_4_ with x = 0.13 changing y finely up to 0.10. Compared with the case of the nonmagnetic impurity Zn, it has been found from the muon-spin-relaxation measurements that a large amount of Ni is required to stabilize a magnetic order of Cu spins. However, the evolution toward the stabilization of the magnetic order with increasing impurity concentration is qualitatively similar to each other. The area of the non-superconducting and slowly fluctuating or static region of Cu spins around Ni has been found to be smaller than that around Zn, suggesting that the pinning of rather long-ranged dynamical spin correlation such as the so-called dynamical stripe by Ni is weaker than that by Zn. This may be the reason why Zn destroys the superconductivity in the hole-doped high-T_c_ cuprates more markedly than Ni.

cond-mat.supr-con↗

arXiv2004

Muon-spin-relaxation and magnetic-susceptibility studies of the effects of nonmagnetic impurities on the Cu-spin dynamics and superconductivity in La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ around x = 0.115

Zero-field muon-spin-relaxation (muSR) and magnetic-susceptibility measurements have been carried out in La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ with x = 0.10, 0.115 and 0.13 changing the Zn-concentration y finely up to 0.10, with the aim to clarify effects of the nonmagnetic impurity Zn on the Cu-spin dynamics and superconductivity. The muSR measurements have revealed that, in each x, a magnetic order of Cu spins is observed at y ~ 0.0075, while it disappears and Cu spins turn into a fast fluctuating state for y > 0.03. From the magnetic-susceptibility measurements, on the other hand, it has been found that the volume fraction of the superconducting state rapidly decreases through the slight doping of Zn and that its y dependence corresponds to the y dependence of the volume fraction of the fast fluctuating region of Cu spins estimated from the muSR results. Both the rapid decrease of the volume fraction of the superconducting state and the formation of the magnetic order through the slight doping of Zn can be interpreted in terms of the development of the so-called "Swiss cheese" model. That is, it is concluded that Cu spins in a non-superconducting region around Zn exhibit slowing-down of the fluctuations or form an incoherent or coherent static magnetic order. Moreover, the formation of a non-superconducting region around Zn is considered to be due to the pinning of the dynamical spin correlation or dynamical stripe correlations.

cond-mat.supr-con↗

arXiv2004

Magnetization plateaux of S = 1/2 two-dimensional frustrated antiferromagnet Cs$_2$CuBr$_4$

The field induced magnetic phase transitions of Cs$_2$CuBr$_4$ were investigated by means of magnetization process and neutron scattering experiments. This system undergoes magnetic phase transition at Neél temperature $T_\mathrm{N}=1.4$ K at zero field, and exhibits the magnetization plateau at approximately one third of the saturation magnetization for the field directions $H\parallel b$ and $H\parallel c$. In the present study, additional symptom of the two-third magnetization plateau was found in the field derivative of the magnetization process. The magnetic structure was found to be incommensurate with the ordering vector $\boldsymbol{Q}=(0, 0.575, 0)$ at zero field. With increasing magnetic field parallel to the c-axis, the ordering vector increases continuously and is locked at $\boldsymbol{Q}=(0, 0.662, 0)$ in the plateau field range $13.1 \mathrm{T} < H < 14.4 \mathrm{T}$. This indicates that the collinear \textit{up-up-down} spin structure is stabilized by quantum fluctuation at the magnetization plateau.

cond-mat.str-el↗

arXiv2003

Complex Energy Method in 4-Body Faddeev-Yakubovsky Equations

The Complex Energy Method [Prog. Theor. Phys. {\bf 109}, 869L (2003)] is applied to the 4-body Faddeev-Yakubovsky equations in the 4-nucleon system. We obtain a well converged solution in all energy regions below and above the 4-nucleon break-up threshold.

nucl-th↗

arXiv2003

A new screening function for Coulomb renormalization

We introduce a new screening function which is useful for the few-body Coulomb scattering problem in ``screening and renormalization'' scheme. The new renormalization phase factor of the screening function is analytically shown. The Yukawa type of the screening potential has been used in several decades, we modify it to make more useful. As a concrete example, we compare the proton-proton scattering phase shifts calculated from these potentials. The numerical results document that high precision calculations of the renormalization are performed by the new screening function.

nucl-th↗

arXiv2003

1/8 anomaly in the excess-oxygen-doped La_1.8_Nd_0.2_Cu_1-y_Zn_y_O_4+δ_

We have found the 1/8 anomaly, namely, the anomalous suppression of superconductivity at p (the hole concentration per Cu) = 1/8 in the excess-oxygen-doped La_1.8_Nd_0.2_Cu_1-y_Zn_y_O_4+δ_, where the excess oxygen is doped by the electrochemical technique and the phase separation of the excess oxygen is suppressed. The 1/8 anomaly has become marked by the 1 % substitution of Zn for Cu. The muon-spin-relaxation measurements have revealed that the magnetic correlation develops at low temperatures below about 50 K in both Zn-free and 1 % Zn-substituted samples with p = 1/8. Clear precession of muon spins suggesting the formation of a long-range magnetic order has been observed below 1 K in the 1 % Zn-substituted sample with p = 1/8. These results are consistent with the stripe-pinning model.

cond-mat.supr-con↗

arXiv2003

Complex Energy Method for Scattering Processes

A method for solving few-body scattering equations is proposed and examined. The solution of the scattering equations at complex energies is analytically continued to get scattering T-matrix with real positive energy. Numerical examples document that the method works well for two-nucleon scattering and three-nucleon scattering, if the set of complex energies is properly chosen.

nucl-th↗

arXiv2002

Violation of Sum Rules for Twist-3 Parton Distributions

Sum rules for twist-3 distributions are reexamined. Integral relations between twist-3 and twist-2 parton distributions suggest the possibility for a $δ$-function at $x=0$. We confirm and clarify this result by constructing $h_L$ and $h_L^3$ (quark-gluon interaction dependent part of $h_L$) explicitly from their moments for a one-loop dressed massive quark. The physics of these results is illustrated by calculating $h_L(x,Q^2)$ using light-front time-ordered pQCD to ${O}(α_S)$ on a quark target.

hep-ph↗

arXiv2002

9Be: A gateway nucleus into heavier nuclei?

Level structure of 9Be is investigated calculating poles of the alpha alphar n three-body scattering amplitude in the complex energy plane. We find 10 levels, which make rotational bands in both positive and negative parities. All of them, except 7/2^+, have been observed experimentally and the corresponding positions and widths reasonably well matched. The 7/2^+ has a large width, and therefore has not been found, but may cause a noise in the experiment. No other levels are needed to explain the existing experimental observables below 11.28 MeV, where the 9th excited state exists. We definitely assign it 9/2^-.

nucl-th↗

arXiv2001

Violation of Sum Rules for Twist-3 Parton Distributions in QCD

hep-ph↗

arXiv2001

Crystal Growth, Transport Properties and Crystal Structure of the Single-Crystal La_{2-x}Ba_xCuO_4 with x=0.11

We have attempted the crystal growth by the traveling-solvent floating-zone (TSFZ) method of La_{2-x}Ba_xCuO_4 with x~1/8, where the superconductivity is strongly suppressed. Under flowing O_2-gas of a high pressure (4 bars), we have succeeded in growing single crystals of x=0.11 with 5 mm in diameter and 20 mm in length. It has been found that both in-plane and out-of-plane electrical resistivities of the single-crystal La_{2-x}Ba_xCuO_4 with x=0.11 exhibit a clear jump at ~53 K. The temperature corresponds to the structural phase transition temperature between the orthorhombic mid-temperature (OMT) and tetragonal low-temperature (TLT) phases, T_{d2}. It has also been found that both in-plane thermoelectric power and Hall coefficient drop rapidly at T_{d2} and decrease below T_{d2} with decreasing temperature. These results are quite similar to those observed in the single-crystal La_{1.6-x}Nd_{0.4}Sr_xCuO_4 with x~1/8, suggesting that the so-called static stripe order of holes and spins in the CuO_2 plane is formed below T_{d2} in La_{2-x}Ba_xCuO_4 with x~1/8 as well as in La_{1.6-x}Nd_{0.4}Sr_xCuO_4 with x~1/8.

cond-mat.supr-con↗

arXiv2001

Magnetic Properties of a Pressure-induced Superconductor UGe$_2$

We performed the DC-magnetization and neutron scattering experiments under pressure {\it P} for a pressure-induced superconductor UGe$_2$. We found that the magnetic moment is enhanced at a characteristic temperature {\it T}$^{*}$ in the ferromagnetic state, where {\it T}$^{*}$ is smaller than a Curie temperature {\it T}$_{\rm C}$. This enhancement becomes remarkable in the vicinity of {\it P}$_{\rm C}^{*}$ = 1.20 GPa, where {\it T}$^{*}$ becomes 0 K and the superconducting transition temperature {\it T}$_{\rm SC}$ shows a maximum. The characteristic temperature {\it T}$^{*}$, which decreases with increasing pressure, also depends on the magnetic field.

cond-mat.str-el↗

arXiv1998

Higher Twist Distribution Amplitudes of Vector Mesons in QCD: Formalism and Twist Three Distributions

We present a systematic study of twist three light-cone distribution amplitudes of vector mesons in QCD, which is based on conformal expansion and takes into account meson and quark mass corrections. A complete set of distribution amplitudes is constructed for rho, omega, K* and phi mesons, which satisfies all (exact) equations of motion and constraints from conformal expansion. Nonperturbative input parameters are calculated from QCD sum rules, including an update of SU(3) breaking corrections in the leading twist distributions.

hep-ph↗

arXiv1998

Renormalization of Chiral-Even Twist-3 Light-cone Wave Functions for Vector Mesons in QCD

We present the one-loop anomalous dimension matrices for the chiral-even twist-3 (nonsinglet) conformal operators, which govern the scale-dependence of the vector meson light-cone wave functions through the conformal expansion. It is clarified that the constraints from the charge-conjugation invariance and the chirality conservation allow only one independent anomalous dimension matrix for each conformal spin.

hep-ph↗

arXiv1997

Q^2 Evolution of Chiral-Odd Twist-3 Distributions h_L(x, Q^2) and e(x, Q^2) in Large-N_c QCD

We prove that the twist-3 chiral-odd parton distributions obey simple Gribov-Lipatov-Altarelli-Parisi evolution equations in the limit $N_c\to\infty$ and give analytic results for the corresponding anomalous dimensions. To this end we introduce an evolution equation for the corresponding three-particle twist-3 parton correlation functions and find an exact analytic solution. For large values of n (operator dimension) we are further able to collect all corrections subleading in N_c, so our final results are valid to $O(1/N_c^2\cdot \ln(n)/n)$ accuracy.

hep-ph↗