arXiv subjects

K. Ishida

Publications and source records attributed to K. Ishida.

101 records · Page 6Linked to original sources

arXiv2002

Spin dynamics in a structurally ordered non-Fermi liquid compound: YbRh_2Si_2

Muon spin relaxation (muSR) experiments have been carried out at low temperatures in the non-Fermi-liquid heavy-fermion compound YbRh_2Si_2. The longitudinal-field muSR relaxation function is exponential, indicative that the dynamic spin fluctuations are homogeneous. The relaxation rate 1/T_1 varies with applied field as H^{-y}, y = 1.0 \pm 0.1, which implies a scaling law of the form χ''(ω) \propto ω^{-y} f(ω/T), \lim_{x\to0} f(x) = x for the dynamic spin susceptibility.

cond-mat.str-el↗

arXiv2002

Evidence for High-frequency Phonon Mediated S-wave Superconductivity : 11B-NMR Study of Al-doped MgB2

We report $^{11}$B-NMR study on Al-doped MgB$_2$ that addresses a possible mechanism for a high superconducting (SC) transition temperature ($T_c$) of $\sim 40$ K in recently discovered MgB$_2$. The result of nuclear spin lattice relaxation rate $1/T_1$ in the SC state revealed that the size in the SC gap is not changed by substituting Al for Mg. The reduction on $T_c$ by Al-doping is shown to be due to the decrease of $N(E_F)$. According to the McMillan equation, the experimental relation between $T_c$ and the relative change in $N(E_F)$ allowed us to estimate a characteristic phonon frequency $ω\sim 700$ K and an electron-phonon coupling constant $λ\sim 0.87$. These results suggest that the high-$T_c$ superconductivity in MgB$_2$ is mediated by the strong electron-phonon coupling with high-frequency phonons.

cond-mat.supr-con↗

arXiv2002

Coexistence of antiferromagnetism and superconductivity in heavy-fermions systems

We report the novel pressure(P)-temperature(T) phase diagrams of antiferromagnetism (AF) and superconductivity (SC) in CeRhIn$_5$, CeIn$_3$ and CeCu$_2$Si$_2$ revealed by the NQR measurement. In the itinerant helical magnet CeRhIn$_5$, we found that the Néel temperature $T_N$ is reduced at $P \geq$ 1.23 GPa with an emergent pseudogap behavior. The coexistence of AF and SC is found in a narrow P range of 1.63 - 1.75 GPa, followed by the onset of SC with line-node gap over a wide P window 2.1 - 5 GPa. In CeIn$_3$, the localized magnetic character is robust against the application of pressure up to $P \sim$ 1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations. In CeCu$_2$Si$_2$, the SC and AF coexist on a microscopic level once its lattice parameter is expanded. We remark that the underlying marginal antiferromagnetic state is due to collective magnetic excitations in the superconducting state in CeCu$_2$Si$_2$. An interplay between AF and SC is discussed on the SO(5) scenario that unifies AF and SC. We suggest that the SC and AF in CeCu$_2$Si$_2$ have a common mechanism.

cond-mat.str-el↗

arXiv2001

Pressure-Temperature Phase Diagram of Antiferromagnetism and Superconductivity in CeRhIn5 and CeIn3 : In-NQR Study under Pressure

We report the novel pressure($P$) - temperature($T$) phase diagram of antiferromagnetism and superconductivity in CeRhIn$_5$ and CeIn$_3$ revealed by the $^{115}$In nuclear-spin-lattice-relaxation ($T_1$) measurement. In the itinerant magnet CeRhIn$_5$, we found that the Néel temperature $T_N$ is reduced at $P \geq$ 1.23 GPa with an emergent pseudogap behavior. In CeIn$_3$, the localized magnetic character is robust against the application of pressure up to $P \sim$ 1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations.

cond-mat.str-el↗

arXiv2001

Unexpected Metallic-like Behavior of the Resistance in the Dielectric Spin Density Wave State in (TMTSF)2PF6

We report unexpected features of the transport in the dielectric spin density wave (SDW) phase of the quasi one-dimensional compound (TMTSF)_2PF_6: the resistance exhibits a maximum and a subsequent strong drop as temperature decreases below approximately 2K. The maximum in R(T) is not caused by depinning or Joule heating of the SDW. The characteristic temperature of the R(T) maximum and the scaling behavior of the resistance at different magnetic fields B evidence that the non-monotonic R(T) dependence has an origin different from the one known for the quantum Hall effect region of the phase diagram. We also found that the borderline T_0(B,P) which divides the field induced SDW region of the P-B-T phase diagram into the hysteresis and non-hysteresis domains, terminates in the N=1 sub-phase; the borderline has thus no extension to the SDW N=0 phase.

cond-mat.mes-hall↗

arXiv2001

Pressure-induced anomalous magnetism and unconventional superconductivity in CeRhIn5 : 115In-NQR Study under Pressure

We report $^{115}$In nuclear-quadrupole-resonance (NQR) measurements of the pressure($P$)-induced superconductor CeRhIn$_5$ in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal field $H_{int}$ at the In site is substantially reduced from $H_{int}=1.75$ kOe at P=0 to 0.39 kOe at $P=1.23$ GPa, while the Néel temperature slightly changes with increasing $P$. This suggests that either the size in the ordered moment $M_{Q}(P)$ or the angle $θ(P)$ between the direction of $M_{Q}(P)$ and the tetragonal $c$ axis is extrapolated to zero at $P^*=1.6 \pm 0.1$ GPa at which a bulk SC transition is no longer emergent. In the SC state at $P=2.1$ GPa, the nuclear spin-lattice relaxation rate $^{115}(1/T_1)$ has revealed a $T^3$ dependence without the coherence peak just below $T_c$, giving evidence for the unconventional superconductivity. The dimensionality of the magnetic flutuations in the normal state are also discussed.

cond-mat.str-el↗

arXiv2001

Novel Phases in the Field Induced Spin Density Wave State in (TMTSF)_2PF_6

Magnetoresistance measurements on the quasi one-dimensional organic conductor (TMTSF)_2PF_6 performed in magnetic fields B up to 16T, temperatures T down to 0.12K and under pressures P up to 14kbar have revealed new phases on its P-B-T phase diagram. We found a new boundary which subdivides the field induced spin density wave (FISDW) phase diagram into two regions. We showed that a low-temperature region of the FISDW diagram is characterized by a hysteresis behavior typical for the first order transitions, as observed in a number of studies. In contrast to the common believe, in high temperature region of the FISDW phase diagram, the hysteresis and, hence, the first order transitions were found to disappear. Nevertheless, sharp changes in the resistivity slope are observed both in the low and high temperature domains indicating that the cascade of transitions between different subphases exists over all range of the FISDW state. We also found that the temperature dependence of the resistance (at a constant B) changes sign at about the same boundary. We compare these results with recent theoretical models.

cond-mat.str-el↗

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

Exotic Superconducting Phase in CeCu2Si2 Close to Antiferromagnetism : A Cu-NQR Study under Hydrostatic Pressure

We report Cu nuclear-quadrupole-resonance results under pressure on a homogeneous CeCu2Si2 (Tc = 0.65 K) that revealed critical magnetic fluctuations at the border to an antiferromagnetic phase. This exotic superconducting phase evolves into the typical heavy-fermion superconducting phase at minute pressures exceeding Pc ~ 0.2 GPa. The nuclear spin-lattice relaxation data in a pressure range 0.85 - 2.58 GPa are shown to be accountable by the SCR theory based on the nearly antiferromagnetic Fermi-liquid model. In 0 < P < Pc ~ 0.2 GPa, by contrast, we conclude that the exotic superconducting phase manifests itself under the unconventional normal state where antiferromagnetic waves propagate over a long range without any trace of antiferromagnetic order and thereby the heavy-fermion state breaks up.

cond-mat.str-el↗

arXiv2000

Coexistence of Superconductivity and Antiferromagnetism in Heavy-Fermion Superconductor CeCu_{2}(Si_{1-x}Ge_{x})_{2} Probed by Cu-NQR --A Test Case for the SO(5) Theory--

We report on the basis of Cu-NQR measurements that superconductivity (SC) and antiferromagnetism (AF) coexist on a microscopic level in CeCu_{2}(Si_{1-x}Ge_{x})_{2}, once a tiny amount of 1%Ge (x = 0.01) is substituted for Si. This coexistence arises because Ge substitution expands the unit-cell volume in nearly homogeneous CeCu2Si2 where the SC coexists with slowly fluctuating magnetic waves. We propose that the underlying exotic phases of SC and AF in either nearly homogeneous or slightly Ge substituted CeCu2Si2 are accountable based on the SO(5) theory that unifies the SC and AF. We suggest that the mechanism of the SC and AF is common in CeCu2Si2.

cond-mat.str-el↗

arXiv1998

NMR Study of MnSi under Pressure

The magnetic phase diagram $T_c(P)$ of the weak itinerant helimagnetic compound MnSi is reviewed upon a Nuclear Magnetic Resonance (NMR) experiment. We present a systematic study on the evolution of the NMR spin echo signal at $T=1.4$ K of the non-magnetic silicone sites, $^{29}$Si, up to 17.8 kbar. The pressure effect is interpreted as a weak variation of the local electronic spin polarization and the coexistence of magnetic and non magnetic Mn atoms under pressure. From the volume dependence of $f_0$ ($P$), we show notably that a local magnetic order remains above the critical pressure of $P_c=14.8$ kbar.

cond-mat.str-el↗