arXiv subjects

D. Kikuchi

Publications and source records attributed to D. Kikuchi.

10 recordsLinked to original sources

arXiv2012

Spin Hall Magnetoresistance Induced by a Non-Equilibrium Proximity Effect

We report anisotropic magnetoresistance in Pt|Y3Fe5O12 bilayers. In spite of Y3Fe5O12 being a very good electrical insulator, the resistance of the Pt layer reflects its magnetization direction. The effect persists even when a Cu layer is inserted between Pt and Y3Fe5O12, excluding the contribution of induced equilibrium magnetization at the interface. Instead, we show that the effect originates from concerted actions of the direct and inverse spin Hall effects and therefore call it "spin Hall magnetoresistance."

cond-mat.mtrl-sci↗

arXiv2011

Suppression of time reversal symmetry breaking superconductivity in Pr(Os,Ru)_4Sb_12 and (Pr,La)Os_4Sb_12

Zero-field muon spin relaxation experiments have been carried out in the Pr(Os_{1-x}Ru_x)_4Sb_12 and Pr_{1-y}La_yOs_4Sb_12 alloy systems to investigate broken time-reversal symmetry (TRS) in the superconducting state, signaled by the onset of a spontaneous static local magnetic field B_s. In both alloy series B_s initially decreases linearly with solute concentration. Ru doping is considerably more efficient than La doping, with a ~50% faster initial decrease. The data suggest that broken TRS is suppressed for Ru concentration x >~ 0.6, but persists for essentially all La concentrations. Our data support a crystal-field excitonic Cooper pairing mechanism for TRS-breaking superconductivity.

cond-mat.supr-con↗

arXiv2010

Comment on "Pronounced Enhancement of the Lower Critical Field and Critical Current Deep in the Superconducting State of PrOs4Sb12"

Cichorek et al. [Phys. Rev. Lett. 94, 107002 (2005)] reported enhancements of the lower critical field Hc1(T) and critical current in superconducting PrOs4Sb12 below a transition temperature Tc3 ~ 0.6 K, and speculated that this reflects a transition between superconducting phases. Features have been observed near Tc3 in other properties, but not in the specific heat. We report muon spin rotation measurements of the penetration depth in the vortex state of PrOs4Sb12 near Hc1(T), that to high accuracy exhibit no anomaly Tc3 and therefore cast doubt on the putative phase transition.

cond-mat.supr-con↗

arXiv2008

123Sb-NQR study of unconventional superconductivity in the filled skutterudite heavy-fermion compound PrOs4Sb12 under high pressure up to P = 3.82 GPa

We report $^{123}$Sb nuclear quadrupole resonance (NQR) measurements of the filled skutterudite heavy-fermion superconductor PrOs$_4$Sb$_{12}$ under high pressure. The temperature dependence of NQR frequency and the spin-lattice relaxation rate $1/T_1$ indicate that the crystal-electric-field splitting $Δ_{\rm CEF}$ between the ground state $Γ_1$ singlet and the first excited state $Γ_4^{(2)}$ triplet decreases with increasing pressure. Ac-susceptibility measurements indicate that the superconducting transition temperature ($T_{\rm c}$) also decreases with increasing pressure. However, above $P$ $\sim$ 2 GPa, both $Δ_{\rm CEF}$ and $T_{\rm c}$ do not depend on external pressure up to $P$ = 3.82 GPa. These pressure dependences of $Δ_{\rm CEF}$ and $T_{\rm c}$ suggest an intimate relationship between quadrupole excitations associated with the $Γ_4^{(2)}$ level and unconventional superconductivity in PrOs$_4$Sb$_{12}$. In the superconducting state, 1/$T_1$ below $T_{\rm c}$ = 1.55 and 1.57 K at $P$ = 1.91 and 2.63 GPa shows a power-law temperature variations and are proportional to $T^5$ at temperatures considerably below $T_{\rm c}$. These data can be well fitted by the gap model $Δ(θ) = Δ_0\sinθ$ with $Δ_0$ = 3.08 $k_{\rm B}T_{\rm c}$ and 3.04 $k_{\rm B}T_{\rm c}$ for $P$ = 1.91 and 2.63 GPa, respectively. The results indicate there exists point nodes in the gap function.

cond-mat.supr-con↗

arXiv2008

Gap opening with ordering in PrFe4P12 studied by local tunneling spectroscopy

We present measurements of the local tunneling density of states in the low temperature ordered state of PrFe4P12. The temperature dependencies of the Fermi level density of states and of the integrated density of states at low bias voltages show anomalies at T=6.5 K, the onset of multipolar ordering as detected by specific heat and other macroscopic measurements. In the ordered phase, we find a local density of states with a V-shape form, indicating a partial gap opening over the Fermi surface. The size of the gap according to the tunneling spectra is about 2 meV.

cond-mat.str-el↗

arXiv2007

Muon spin relaxation and hyperfine-enhanced 141Pr nuclear spin dynamic in Pr(Os,Ru)4Sb12 and (Pr,La)Os4Sb12

Zero- and longitudinal-field muon spin relaxation (MuSR) experiments have been carried out in the alloy series Pr(Os1-xRux)4Sb12 and Pr1-yLayOs4Sb12 to elucidate the anomalous dynamic muon spin relaxation observed in these materials. The damping rate associated with this relaxation varies with temperature, applied magnetic field, and dopant concentrations x and y in a manner consistent with the ``hyperfine enhancement'' of 141Pr nuclear spins first discussed by Bleaney in 1973. This mechanism arises from Van Vleck-like admixture of magnetic Pr3+ crystalline-electric-field-split excited states into the nonmagnetic singlet ground state by the nuclear hyperfine coupling, thereby increasing the strengths of spin-spin interactions between 141Pr and muon spins and within the 141Pr spin system. We find qualitative agreement with this scenario, and conclude that electronic spin fluctuations are not directly involved in the dynamic muon spin relaxation.

cond-mat.str-el↗

arXiv2006

Multiband Superconductivity in Filled-Skutterudite Compounds (Pr_{1-x}La_{x})Os_{4}Sb_{12}: An Sb Nuclear-Quadruple-Resonance (NQR) Study

We report on Sb nuclear-quadrupole-resonance (NQR) study in filled-skutterudite compounds (Pr_1-xLa_x)Os_4Sb_12. The Sb-NQR spectra have split into two sets, arising from different Sb_12 cages containing either Pr or La, which enables us to measure two kinds of nuclear spin-lattice relaxation time T_1^Pr and T_1^La. In the normal state, the temperature (T) dependence of 1/T^Pr_1T showed almost the same behavior as that for PrOs_4Sb_12 regardless of. In contrast, 1/T^La_1T markedly decreases with increasing La concentration. In the superconducting state for x=0.05 and 0.2, 1/T_1^Pr exponentially decreases down to T=0.7 K with no coherence peak below T_c as well as in PrOs_4Sb_12. A remarkable finding is that the residual density of states at the Fermi level below T_c is induced by La substitution for Pr. These results are proposed to be understood in terms of a multiband-superconductivity model that assumes a full gap for part of the FS and the presence of point nodes for a small 4f^2-derived FS inherent in PrOs_4Sb_12. For x=0.8 and 1,1/T^La_1 exhibits a coherence peak and the nodeless energy gap characteristic for weak-coupling s-wave superconductors. With increasing Pr content, T_c and the energy gap increases. The novel strong-coupling superconductivity in PrOs_4Sb_12 is inferred to be mediated by the local interaction between 4f^2-derived crystal-electric-field states with the electric quadrupole degree of freedom and conduction electrons. This coupling causes a mass enhancement of quasi-particles for a part of FS and induces a small FS, which is responsible for point nodes in the superconducting gap function. Note that the small FS does not play any primary role for the strong-coupling superconductivity in PrOs_4Sb_12.

cond-mat.supr-con↗

arXiv2006

Drastic change in transport of entropy with quadrupolar ordering in PrFe$_{4}$P$_{12}$

The antiferroquadrupolar ordering of PrFe$_{4}$P$_{12}$ is explored by probing thermal and thermoelectric transport. The lattice thermal conductivity drastically increases with the ordering, as a consequence of a large drop in carrier concentration and a strong electron-phonon coupling. The low level of carrier density in the ordered state is confirmed by the anomalously large values of the Seebeck and Nernst coefficients. The results are reminiscent of URu$_{2}$Si$_{2}$ and suggest that both belong to the same class of aborted metal-insulator transitions. The magnitude of the Nernst coefficient, larger than in any other metal, indicates a new route for Ettingshaussen cooling at Kelvin temperatures.

cond-mat.str-el↗

arXiv2005

Evidence for s-wave superconductivity with antiferromagnetic fluctuations in filled skutterudite LaFe$_{4}$P$_{12}$: $^{139}$La and $^{31}$P-NMR studies

We have performed $^{139}$La,$^{31}$P-NMR studies on a filled skutterudite superconductor LaFe$_{4}$P$_{12}$ with a critical temperature $T_c=4.1$ K. In the normal state, the presence of antiferromagnetic (AFM) fluctuations at finite wave vectors is suggested from the relation between the nuclear spin-lattice relaxation rate $1/T_1$ and the Knight shift. In the superconducting (SC) state, the distinct coherence peak was observed just below $T_c$, and is easily suppressed by the applied field. An exponential decrease of $1/T_1$ was observed, suggestive of the isotropic superconducting gap with $2Δ/k_{\rm B}T_c =3.8$. Besides, we observed the decrease of the Knight shift, indicative of the singlet-pair formation below $T_c$. These results are clear evidence that LaFe$_{4}$P$_{12}$ is a rare s-wave superconductor with significant AFM fluctuations.

cond-mat.str-el↗

arXiv2004

Evolution of superconducting order in Pr(Os$_{1-x}$Ru$_{x}$)$_{4}$Sb$_{12}$

We report measurements of the magnetic penetration depth $λ$ in single crystals of Pr(Os$_{1-x}$Ru$_{x}$)$_{4}$Sb$_{12}$ down to 0.1 K. Both $λ$ and superfluid density $ρ_{s}$ exhibit an exponential behavior for the $x$$\geq$0.4 samples, going from weak ($x$=0.4,0.6), to moderate, coupling ($x$=0.8). For the $x$$\leq$0.2 samples, both $λ$ and $ρ_{s}$ vary as $T^{2}$ at low temperatures, but $ρ_{s}$ is s-wave-like at intermediate to high temperatures. Our data are consistent with a three-phase scenario, where a fully-gapped phase at $T_{c1}$ undergoes two transitions: first to an unconventional phase at $T_{c2}$$\lesssim$$T_{c1}$, then to a nodal low-$T$ phase at $T_{c3}$$<$$T_{c2}$, for small values of $x$.

cond-mat.supr-con↗