arXiv subjects

Y. Onuki

Publications and source records attributed to Y. Onuki.

At least 91 records · Page 5Linked to original sources

arXiv2003

Giant Nernst effect in a Kondo lattice close to a quantum critical point

We present a study of Nernst and Seebeck coefficients of the heavy-fermion superconductor CeCoIn$_{5}$. Below 18 K, concomitant with a field-dependent Seebeck coefficient, a large sub-linear Nernst signal emerges with a magnitude drastically exceeding what is expected for a multi-band Fermi-liquid metal. In the mixed state, in contrast with all other superconductors studied before, this signal overwhelms the one associated with the motion of superconducting vortices. The results point to a hitherto unknown source of transverse thermoelectricity in strongly interacting electrons.

cond-mat.str-el↗

arXiv2003

Unconventional superconductivity and normal state properties of epsilon-iron at high pressure

Following the discovery of superconductivity in epsilon-iron, subsequent experiments hinted at non-Fermi liquid behaviour of the normal phase and sensitive dependence of the superconducting state on disorder, both signatures of unconventional pairing. We report further resistive measurements under pressure of samples of iron from multiple sources. The normal state resistivity of epsilon-iron varied as rho_0+AT^{5/3} at low temperature over the entire superconducting pressure domain. The superconductivity could be destroyed by mechanical work, and was restored by annealing, demonstrating sensitivity to the residual resistivity rho_0. There is a strong correlation between the rho_0 and A coefficients and the superconducting critical temperature T_c. Within the partial resistive transition there was a significant current dependence, with V(I)=a(I-I_0)+bI^2, with a >> b, possibly indicating flux-flow resistivity, even in the absence of an externally applied magnetic field.

cond-mat.supr-con↗

arXiv2003

Gapless Magnetic and Quasiparticle Excitations due to the Coexistence of Antiferromagnetism and Superconductivity in CeRhIn$_5$ : A study of $^{115}$In-NQR under Pressure

We report systematic measurements of ac-susceptibility, nuclear-quadrupole-resonance spectrum, and nuclear-spin-lattice-relaxation time ($T_1$) on the pressure ($P$)- induced heavy-fermion (HF) superconductor CeRhIn$_5$. The temperature ($T$) dependence of $1/T_1$ at $P$ = 1.6 GPa has revealed that antiferromagnetism (AFM) and superconductivity (SC) coexist microscopically, exhibiting the respective transition at $T_N = 2.8$ K and $T^{MF}_c$ = 0.9 K. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below $T_c^{onset} = 2$ K, but $T_c^{MF} = 0.9$ K, followed by a $T_1T$ = const law. These results point to the unconventional characteristics of SC coexisting with AFM. We highlight that both of the results deserve theoretical work on the gapless nature in low-lying excitation spectrum due to the coexistence of AFM and SC and the lack of the mean-field regime below $T_c^{onset} = 2$ K.

cond-mat.str-el↗

arXiv2003

Anisotropic spin fluctuations in heavy-fermion superconductor CeCoIn5: In-NQR and Co-NMR studies

We report In-NQR and Co-NMR experiments of CeCoIn$_5$ that undergoes a superconducting transition with a record high $T_{\rm c}$ = 2.3 K to date among heavy-fermion superconductors. At zero magnetic field, an anomalous temperature ($T$) dependence of nuclear spin-lattice relaxation rate $1/T_1$ of $^{115}$In is explained by the relation $1/T_1\propto T\cdotχ_Q(T)^{3/4}$ based on the anisotropic spin-fluctuations model in case of the proximity to an antiferromagnetic (AFM) quantum critical point (QCP). The novel behavior of $1/T_1\sim T^{1/4}$ over a wide $T$ range of $T_{\rm c} < T < 40$ K arises because the staggered susceptibility almost follows the Curie law $χ_Q(T)\propto 1/(T+θ)$ with $θ= 0.6$ K and hence $1/T_1\propto T/(T+0.6)^{3/4}\sim T^{1/4}$ for $θ< T$. We highlight that the behavior $1/T_1\sim T^{1/4}$ is due to the proximity to the anisotropic AFM QCP relevant with its layered structure, and is not associated with the AFM QCP for isotropic 3D systems. We have also found that the AFM spin fluctuations in CeCoIn$_5$ are suppressed by small magnetic field so that $θ=0.6$ K at $H$=0 increases to $θ= 2.5$ K at $H$ = 1.1 T, reinforcing that CeCoIn$_5$ is closely located at the QCP.

cond-mat.str-el↗

arXiv2003

Normal-state Hall Angle and Magnetoresistance in quasi-2D Heavy Fermion CeCoIn_5 near a Quantum Critical Point

The normal-state Hall effect and magnetoresisitance (MR) have been measured in the quasi-2D heavy fermion superconductor CeCoIn_5. In the non-Fermi liquid region where the reistivity rho_xx exhibits an almost perfect T-linear dependence, the Hall angle varies as cot theta_H propto T^2 and the MR displays a strong violation of Kohler's rule. We demonstrate a novel relation between the MR and the Hall conductivity, Delta rho_xx/rho_xx propto (sigma_xy rho_xx)^2. These results bear a striking resemblance to the normal-state properties of high-T_c cuprates, indicating universal transport properties in the presence of quasi-2D antiferromagnetic fluctuations near a quantum critical point.

cond-mat.supr-con↗

arXiv2002

Fermi surface of the heavy fermion superconductor PrOs$_{4}$Sb$_{12}$

We have investigated the de Haas-van Alphen effect in the Pr-based heavy fermion superconductor PrOs$_4$Sb$_{12}$.The topology of Fermi surface is close to the reference compound LaOs$_4$Sb$_{12}$ and well explained by the band structure calculation based on the FLAPW-LDA+U method, where the 4{\it f} electrons are localized. We have confirmed a highly enhanced cyclotron effective mass 2.4$\sim7.6m_{\rm 0}$ which is apparently large compared to the usual Pr-based compounds.

cond-mat.str-el↗

arXiv2002

^{115}In-NQR evidence for unconventional superconductivity in CeIn_3 under pressure

We report evidence for unconventional superconductivity in CeIn_3 at a pressure P = 2.65 GPa above critical pressure (P_c ~ 2.5 GPa) revealed by the measurements of nuclear-spin-lattice-relaxation time (T_1) and ac-susceptibility (ac-chi). Both the measurements of T_1 and ac-chi have pointed to a superconducting transition at T_c = 95 mK, which is much lower than an onset temperature T_{onset} = 0.15 K at zero resistance. The temperature dependence of 1/T_1 shows no coherence peak just below T_c, indicative of an unconventional nature for the superconductivity induced in CeIn_3.

cond-mat.str-el↗

arXiv2002

Superconductivity of epsilon-Fe: complete resistive transition

Last year, iron was reported to become superconducting at temperatures below 2K and pressures between 15 and 30 GPa. The evidence presented was a weak resistivity drop, suppressed by a magnetic field above 0.2 T, and a small Meissner signal. However, a compelling demonstration, such as the occurrence of zero resistance, was lacking. Here we report the measurement of a complete resistive transition at 22.2 GPa with an onset slightly above 2 K in two very pure samples of iron, of different origins. The superconductivity appears unusually sensitive to disorder, developing only when the electronic mean free path is above a threshold value, while the normal state resistivity is characteristic of a nearly ferromagnetic metal.

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

Pressure-induced Superconductivity in a Ferromagnet UGe$_2$ -- Resistivity Measurements in Magnetic Field --

The electrical resistivity measurements in the magnetic field are carried out on the pressure-induced superconductor UGe$_2$. The superconductivity is observed from 1.06 to 1.44 GPa. The upper critical field of $H_{C2}$ is anisotropic where $H_{C2}(T)$ exhibits positive curvature for $H//b$ and $c$-axis. The characteristic enhancement of $H_{C2}$ is reconfirmed for $H//a$-axis. In the temperature and field dependence of resistivity at $P > P_{C}$ where the ferromagnetic ordering disappears, it is observed that the application of the external field along the {\it a}-axis increases the coefficient of Fermi liquid behavior $AT^{2}$ correspondingly to the metamagnetic transition.

cond-mat↗

arXiv2001

Heat Capacity of the Pressure-induced Superconductivity in Itinerant Ferromagnet UGe$_2$

Recently co-existence of the ferromagnetism and superconductivity was reported in the high-pressure region (1.0-1.6 GPa) in UGe$_2$. We performed the heat capacity measurement on UGe$_2$ under high pressure. At 1.13 GPa, we found a peak corresponding to the superconducting transition. The superconducting temperature $T_{SC}$ and ${\itΔ} C/(γT_{SC})$ are 0.6 K and 0.25, respectively. The superconducting transition was also confirmed by the appearance of the Meissner effect in the {\it ac} susceptibility. From these results, we confirm a bulk nature of the superconductivity in UGe$_2$. The value of $C/T$ ($\sim$ 95 mJ/moleK$^2$) just above $T_{SC}$ at 1.15 GPa is as much as 3 times larger than that at ambient pressure, which indicates a large mass enhancement of quasiparticles under high pressure.

cond-mat.str-el↗

arXiv2001

Superconducting gap and pair breaking in CeRu2 studied by point contacts

The superconducting gap in a CeRu$_{2}$ single crystal is investigated by point contacts. BCS-like behavior of the gap $Δ$ in the temperature range below T$_{c}^{*}<$T$_{c}$, where T$_{c}$ is the critical temperature, is established, indicating the presence of a gapless superconductivity region (between T$_{c}^{*}$ and T$_{c}$). The pair-breaking effect of paramagnetic impurities, supposedly Ce ions, is taken into consideration using the Scalski-Betbeder-Matibet-Weiss approach based on Abrikosov-Gorkov theory. It allows us to recalculate the superconducting order parameter $Δ^α $ (in the presence of paramagnetic impurities) and the gap $Δ^{P}$ (in the pure case) for the single crystal and for the previously studied polycrystalline CeRu$_{2}$. The value 2$Δ^{P}$(0)$ \approx $2 meV, with 2$Δ^{P}$(0)$/$k$_{B}$T$_{c} \approx $3.75, is found in both cases, indicating that CeRu$_{2}$ is a ``moderate'' strong-coupling superconductor.

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↗

arXiv2001

Angular Position of Nodes in the Superconducting Gap of Quasi-2D Heavy-Fermion Superconductor CeCoIn_5

The thermal conductivity of the heavy-fermion superconductor CeCoIn_5 has been studied in a magnetic field rotating within the 2D planes. A clear fourfold symmetry of the thermal conductivity which is characteristic of a superconducting gap with nodes along the (+-pi,+-pi)-directions is resolved. The thermal conductivity measurement also reveals a first order transition at H_c2, indicating a Pauli limited superconducting state. These results indicate that the symmetry most likely belongs to d_{x^2-y^2}, implying that the anisotropic antiferromagnetic fluctuation is relevant to the superconductivity.

cond-mat.supr-con↗

arXiv2001

Unique Spin Dynamics and Unconventional Superconductivity in the Layered Heavy Fermion Compound CeIrIn_5:NQR Evidence

We report measurements of the ^{115}In nuclear spin-lattice relaxation rate (1/T_1) between T=0.09 K and 100 K in the new heavy fermion (HF) compound CeIrIn_5. At 0.4 K < T < 100 K, 1/T_1 is strongly T-dependent, which indicates that CeIrIn_5 is much more itinerant than known Ce-based HFs. We find that 1/T_1T, subtracting that for LaIrIn_5, follows a 1/(T+θ)^{3/4} variation with θ=8 K. We argue that this novel feature points to anisotropic, due to a layered crystal structure, spin fluctuations near a magnetic ordering. The bulk superconductivity sets in at 0.40 K below which the coherence peak is absent and 1/T_1 follows a T^3 variation, which suggests unconventional superconductivity with line-node gap.

cond-mat.supr-con↗

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

Superconducting Gap Structure of Spin-Triplet Superconductor Sr_2RuO_4 Studied by Thermal Conductivity

To clarify the superconducting gap structure of the spin-triplet superconductor Sr_2RuO_4, the in-plane thermal conductivity has been measured as a function of relative orientations of the thermal flow, the crystal axes, and a magnetic field rotating within the 2D RuO_2 planes. The in-plane variation of the thermal conductivity is incompatible with any model with line nodes vertical to the 2D planes and indicates the existence of horizontal nodes. These results place strong constraints on models that attempt to explain the mechanism of the triplet superconductivity.

cond-mat.supr-con↗