arXiv subjects

Shunsaku Kitagawa

Publications and source records attributed to Shunsaku Kitagawa.

At least 55 records · Page 3Linked to original sources

arXiv2021

S-wave Superconductivity in the Dirac Line-nodal Material CaSb2

We performed 121/123Sb-nuclear quadrupole resonance (NQR) measurements on the superconducting (SC) line-nodal material CaSb2 in order to investigate electronic properties in the normal and SC states from a microscopic point of view. In the normal state, the nuclear spin-lattice relaxation rate 1/T1 for the Sb(1) site, which is responsible for the line-nodal parts, is approximately proportional to temperature, indicating the conventional Fermi liquid state. From comparison with band structure calculations, it is considered that the NQR properties related to the line-nodal character are hidden because the conventional behavior originating from Fermi-surface parts away from the nodes is dominant. In the SC state, a clear coherence peak just below the transition temperature and an exponential decrease at lower temperatures were observed in 1/T1. These results strongly suggest that conventional s-wave superconductivity with a full gap is realized in CaSb2.

cond-mat.supr-con↗

arXiv2021

Inhomogeneous Superconducting State Probed by $^{125}$Te NMR on UTe$_2

UTe$_2$ is a recently discovered promising candidate for a spin-triplet superconductor. In contrast to conventional spin-singlet superconductivity, spin-triplet superconductivity possesses spin and angular momentum degrees of freedom. To detect these degrees of freedom and obtain the solid evidence of spin-triplet superconductivity in UTe$_2$, we performed $^{125}$Te-NMR measurement. We previously reported that the shoulder signal appears in NMR spectra below the superconducting (SC) transition temperature $T_{\rm c}$ in $H \parallel b$, and that a slight decrease in the Knight shift along the $b$ and $c$ axes ($K_b$ and $K_c$, respectively) below $T_{\rm c}$ at a low magnetic field $H$. To clarify the origin of the shoulder signal and the trace of the decrease in $K_b$, we compared the $^{125}$Te-NMR spectra obtained when $H~\parallel~b$ and $H~\parallel~c$ and measured the $^{125}$Te-NMR spectra for $H~\parallel~b$ up to 14.5~T. The intensity of the shoulder signal observed for $H~\parallel~b$ has a maximum at $\sim 6$~T and vanishes above 10~T, although the superconductivity is confirmed by the $χ_{\rm AC}$ measurements, which can survive up to 14.5~T (maximum $H$ in the present measurement). Moreover, the decrease in $K_b$ in the SC state starts to be small around 7~T and almost zero at 12.5~T. This indicates that the SC spin state gradually changes with the application of $H$. Meanwhile, in $H~\parallel~c$, unexpected broadening without the shoulder signals was observed below $T_{\rm c}$ at 1~T, and this broadening was quickly suppressed with increasing $H$. We construct the $H$--$T$ phase diagram for $H~\parallel~b$ and $H~\parallel~c$ based on the NMR measurements and discuss possible SC states with the theoretical consideration. We suggest that the inhomogeneous SC state characterized by the broadening of the NMR spectrum originates from the spin degrees of freedom.

cond-mat.supr-con↗

arXiv2021

Anisotropic response of spin susceptibility in the superconducting state of UTe$_2$ probed with $^{125}$Te-NMR measurement

To investigate spin susceptibility in a superconducting (SC) state, we measured the $^{125}$Te-nuclear magnetic resonance (NMR) Knight shifts at magnetic fields ($H$) up to 6.5 T along the $b$ and $c$ axes of single-crystal UTe$_2$, a promising candidate for a spin-triplet superconductor. In the SC state, the Knight shifts along the $b$ and $c$ axes ($K_b$ and $K_c$, respectively) decreased slightly and the decrease in $K_b$ was almost constant up to 6.5 T. The reduction in $K_c$ decreased with increasing $H$, and $K_c$ was unchanged through the SC transition temperature at 5.5 T, excluding the possibility of spin-singlet pairing. Our results indicate that spin susceptibilities along the $b$ and $c$ axes slightly decrease in the SC state in low $H$, and the $H$ response of SC spin susceptibility is anisotropic in the $bc$ plane. We discuss the possible $d$-vector state within the spin-triplet scenario and suggest that the dominant $d$-vector component for the case of $H \parallel b$ changes above 13 T, where $T_{\rm c}$ increases with increasing $H$.

cond-mat.supr-con↗

arXiv2020

Magnetic-Field Dependence of Novel Gap Behavior Related to the Quantum-Size Effect

$^{195}$Pt-NMR measurements of Pt nanoparticles with a mean diameter of 4.0 nm were performed in a high magnetic field of approximately $μ_0 H = 23.3$ T to investigate the low-temperature electronic state of the nanoparticles. The characteristic temperature $T^*$, below which the nuclear spin-lattice relaxation rate $1/T_1$ deviates from the relaxation rate of the bulk, shows a magnetic-field dependence. This dependence supports the theoretical prediction of the appearance of discrete energy levels.

cond-mat.mtrl-sci↗

arXiv2020

Two qualitatively different superconducting phases under high pressure in single-crystalline CeNiGe$_{3}$

We have measured the temperature dependence of resistivity in single-crystalline CeNiGe$_{3}$ under hydrostatic pressure in order to establish the characteristic pressure-temperature phase diagram. The transition temperature to AFM-I phase $T_{\rm N1}$ = 5.5 K at ambient pressure initially increases with increasing pressure and has a maximum at $\sim$ 3.0 GPa. Above 2.3 GPa, a clear zero-resistivity is observed (SC-I phase) and this superconducting (SC) state coexists with AFM-I phase. The SC-I phase suddenly disappears at 3.7 GPa simultaneously with the appearance of an additional kink anomaly corresponding to the phase transition to AFM-II phase. The AFM-II phase is continuously suppressed with further increasing pressure and disappears at $\sim$ 6.5 GPa. In the narrow range near the critical pressure, an SC phase reappears (SC-II phase). A large initial slope of upper critical field $μ_0H_{\rm c2}$ and non-Fermi liquid behavior indicate that the SC-II phase is mediated by antiferromagnetic fluctuations. On the other hand, the robust coexistence of the SC-I phase and AFM-I phase is unusual on the contrary to superconductivity near a quantum critical point on most of heavy-fermion compounds.

cond-mat.supr-con↗

arXiv2020

Variation in Superconducting Symmetry against Pressure on Noncentrosymmetric Superconductor Cd$_2$Re$_2$O$_7$ Revealed by $^{185/187}$Re Nuclear Quadrupole Resonance

We performed $^{185/187}$Re nuclear quadrupole resonance (NQR) measurements under pressure to investigate the superconducting properties of noncentrosymmetric superconductor Cd$_{2}$Re$_{2}$O$_{7}$ under various crystal structures. The pressure dependence of superconducting transition temperature $T_{\rm c}$ determined through ac susceptibility measurements is consistent with the results of previous resistivity measurements [T. C. Kobayashi $et al$., J. Phys. Soc. Jpn. 80, 023715 (2011).]. Below 2.2 GPa, in the nuclear spin-lattice relaxation rate $1/T_{1}$, a clear coherence peak was observed just below $T_{\rm c}$, indicating conventional $s$-wave superconductivity. In contrast, the coherence peak disappears at 3.1 GPa, suggesting a change in superconducting symmetry to the $p$-wave dominant state against pressure.

cond-mat.supr-con↗

arXiv2020

NMR-based gap behavior related to the quantum size effect

We conducted$^{195}$Pt-nuclear magnetic resonance measurements on various-diameter Pt nanoparticles coated with polyvinylpyrrolidone in order to detect the quantum size effect and the discrete energy levels in the electron density of states, both of which were predicted by Kubo more than 50 years ago. We succeeded in separating the signals arising from the surface and interior regions and found that the nuclear spin-lattice relaxation rates in both regions show the metallic behavior at high temperatures. Surprisingly, the magnetic fluctuations in both regions exhibited anomalous behavior below the same temperature $T^*$, which points to a clear size dependence and is well scaled with $δ_\mathrm{Kubo}$. These results suggest that a size-tunable metal-insulator transition occurs in the Pt nanoparticles as a result of the Kubo effect.

cond-mat.mes-hall↗

arXiv2019

Superconducting Properties of Heavy Fermion UTe$_2$ Revealed by $^{125}$Te-Nuclear Magnetic Resonance

We have performed the $^{125}$Te-nuclear magnetic resonance (NMR) measurement in the field along the $b$ axis on the newly discovered superconductor UTe$_2$, which is a candidate of a spin-triplet superconductor. The nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ abruptly decreases below a superconducting (SC) transition temperature $T_c$ without showing a coherence peak, indicative of UTe$_2$ being an unconventional superconductor. It was found that the temperature dependence of $1/T_1T$ in the SC state cannot be understood by a single SC gap behavior but can be explained by a two SC gap model. The Knight shift, proportional to the spin susceptibility, decreases below $T_c$, but the magnitude of the decrease is much smaller than the decrease expected in the spin-singlet pairing. Rather, the small Knight-shift decrease as well as the absence of the Pauli-depairing effect can be interpreted by the spin triplet scenario.

cond-mat.supr-con↗

arXiv2019

Search for the field-induced magnetic instability around the upper critical field of superconductivity in H || c in CeCoIn5

We present nuclear spin-lattice relaxation rate (1/T1) at the Co site and ac-susceptibility results in the normal and superconducting (SC) states of CeCoIn5 for H || c near the SC upper critical field Hc2 above 0.1 K. At 4.2 T, 1/T1 rapidly decreases below the SC transition temperature, consistent with the previous reports. Although the field dependence of 1/T1T at 0.1 K shows a peak at 5.2 T above Hc2, the temperature dependence of 1/T1T at 5.2 T is independent of temperature below 0.2 K, showing a Fermi-liquid behavior. In addition, we found no NMR-spectrum broadening by the appearance of internal fields around Hc2 at 0.1 K. We could not detect any field-induced magnetic instability around Hc2 down to 0.1 K although the remarkable non-Fermi-liquid behavior towards Hc2 was observed in various physical quantities.

cond-mat.str-el↗

arXiv2019

Superconductivity at the Pressure-Induced Ferromagnetic Critical Region in UCoGe

The phase separation of the ferromagnetic (FM) and paramagnetic (PM) phases in the superconducting (SC) state of UCoGe at the FM critical region was investigated using $^{59}$Co nuclear quadrupole resonance (NQR) technique by taking advantage of its site-selective feature. The NQR measurements revealed that the first-order quantum phase transition occurs between the FM and the PM states. The nuclear spin-lattice relaxation rate $1/T_1$ exhibited a clear drop at the SC state in the PM phase, whereas it was not detected in the FM phase, which indicates that the superconductivity in the FM phase becomes weaker at the FM critical region due to the presence of the PM SC state. This result suggests that the SC condensation energy of the PM SC state is equal or larger than that of the FM SC state in this region. The pressure-temperature phase diagram of UCoGe was modified by taking the results from this study into account.

cond-mat.supr-con↗

arXiv2019

Universal relationship between low-energy antiferromagnetic fluctuations and superconductivity in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$

To identify the key parameter for optimal superconductivity in iron pnictides, we measured the $^{31}$P-NMR relaxation rate on BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ ($x = 0.22$ and 0.28) under pressure and compared the effects of chemical substitution and physical pressure. For $x = 0.22$, structural and antiferromagnetic (AFM) transition temperatures both show minimal changes with pressure up to 2.4~GPa, whereas the superconducting transition temperature $T_{\rm c}$ increases to twice its former value. In contrast, for $x=0.28$ near the AFM quantum critical point (QCP), the structural phase transition is quickly suppressed by pressure and $T_{\rm c}$ reaches a maximum. The analysis of the temperature-dependent nuclear relaxation rate indicates that these contrasting behaviors can be quantitatively explained by a single curve of the $T_{\rm c}$ dome as a function of Weiss temperature $θ$, which measures the distance to the QCP. Moreover, the $T_{\rm c}$-$θ$ curve under pressure precisely coincides with that with chemical substitution, which is indicative of the existence of a universal relationship between low-energy AFM fluctuations and superconductivity on BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$.

cond-mat.supr-con↗

arXiv2019

Spin-triplet superconductivity in the paramagnetic UCoGe under pressure studied by $^{59}$Co NMR

A $^{59}$Co nuclear magnetic resonance (NMR) measurement was performed on the single-crystalline ferromagnetic (FM) superconductor UCoGe under a pressure of 1.09 GPa, where the FM state is suppressed and superconductivity occurs in the paramagnetic (PM) state, to study the superconducting (SC) state in the PMstate. $^{59}$Co-NMR spectra became broader but hardly shifted across the SC transition temperature. The Knight-shift change determined from fitting the spectral peak with a Gaussian was much smaller than the spin part of the Knight shift; this is in good agreement with the spin-triplet pairing suggested from the large upper critical field. The spectrum broadening in the SC state cannot be attributed to the SC diamagnetic effect but is related to the properties of spin-triplet superconductivity. The origins of the broadening are discussed herein.

cond-mat.supr-con↗

arXiv2019

Oxidation annealing effects on the spin-glass-like magnetism and appearance of superconductivity in T*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ (0.14 $\leq x \leq$ 0.28)

We investigated the magnetism and superconductivity in as-sintered (AS) and oxidation annealed (OA) T*-type La$_{1-x/2}$Eu$_{1-x/2}$Sr$_x$CuO$_4$ (LESCO) with 0.14 $\leq x \leq$ 0.28 by the first comprehensive muon spin rotation/relaxation ($μ$SR), magnetic susceptibility, and electrical resistivity measurements. In OA superconducting samples, no evidence of magnetic order was observed, whereas AS semiconducting samples exhibited evidence of a disordered magnetic state in the measured temperature range between $\sim$4 K and $\sim$8 K. Therefore, the ground state in LESCO drastically varies with oxidation annealing and the magnetic phase competitively exists with the superconducting (SC) phase. The magnetic phase in the AS LESCO is quite robust against Sr doping, while the SC phase degrades with increasing $x$. A monotonous decrease of the SC transition temperature from 24.5 K in $x$ = 0.14 to 9.0 K in $x$ = 0.28 suggests the disappearance of the SC phase at $x$ $\sim$ 0.34. Furthermore, we clarified the simultaneous development of (quasi) static magnetism and the electrical resistivity at a low temperature in AS samples, suggesting the inducement of magnetism by the suppression of carrier mobility. The variation in magnetism due to annealing is discussed from a viewpoint of structural defects, which was previously reported from neutron diffraction measurements.

cond-mat.supr-con↗

arXiv2019

Magnetic Field Effect on s-wave Superconductor LaRu4P12 Studied by 31P-NMR

We have performed 31P-NMR measurements on the s-wave superconductor LaRu4P12 to investigate the magnetic field effect of the nuclear spin-lattice relaxation rate 1/T1 on a conventional full-gap superconductor. With increasing magnetic field, the Hebel-Slichter peak immediately below Tc in 1=T1 was suppressed, and the magnetic field dependence of 1/T1 at 0.8 K, well below Tc, was proportional to H2. These behaviors can be fully understood by the orbital pair-breaking effect in a single-band s-wave superconductor

cond-mat.supr-con↗

arXiv2019

125Te-NMR Study on a Single Crystal of Heavy Fermion Superconductor UTe2

We report 125Te-NMR studies on a newly discovered heavy fermion superconductor UTe2. Using a single crystal, we have measured the 125Te-NMR Knight shift K and spin-lattice relaxation rate 1/T1 for fields along the three orthorhombic crystal axes. The data confirm a moderate Ising anisotropy for both the static (K) and dynamical susceptibilities (1/T1) in the paramagnetic state above about 20 K. Around 20 K, however, we have observed a sudden loss of NMR spin-echo signal due to sudden enhancement of the NMR spin-spin relaxation rate 1/T2, when the field is applied along the easy axis of magnetization (=a axis). This behavior suggests the development of longitudinal magnetic fluctuations along the a axis at very low frequencies below 20 K.

cond-mat.str-el↗

arXiv2019

Modification of magnetic fluctuations by interfacial interactions in artificially engineered heavy-fermion superlattices

Recent progress in the fabrication techniques of superlattices (SLs) has made it possible to sandwich several-layer-thick block layers (BLs) of heavy-fermion superconductor CeCoIn5 between conventional-metal YbCoIn5 BLs or spin-density-wave-metal CeRhIn5 BLs of a similar thickness. However, the magnetic state in each BL, particularly at the interface, is not yet understood, as experimental techniques applicable to the SL system are limited. Here, we report measurements of 59Co nuclear magnetic resonance, which is a microscopic probe of the magnetic properties inside the target BLs. In the CeCoIn5/YbCoIn5 SL, the low-temperature magnetic fluctuations of the CeCoIn5 BL are weakened as expected from the Rashba spin-orbit effect. However, in the CeCoIn5/CeRhIn5 SL, the fluctuations show an anomalous enhancement below 6 K, highlighting the importance of the magnetic proximity effect occurring near a magnetic-ordering temperature TN ~ 3 K of the CeRhIn5 BL. We suggest that the magnetic properties of the BLs can be altered by the interfacial interaction, which is an alternative route to modify the magnetic properties.

cond-mat.str-el↗

arXiv2019

Enhancement of superconductivity by pressure-induced critical ferromagnetic fluctuations in UCoGe

A $^{59}$Co nuclear quadrupole resonance (NQR) was performed on a single-crystalline ferromagnetic (FM) superconductor UCoGe under pressure. The FM phase vanished at a critical pressure $P_c$, and the NQR spectrum just below $P_c$ showed phase separation of the FM and paramagnetic (PM) phases below Curie temperature $T_{\textrm{Curie}}$, suggesting first-order FM quantum phase transition (QPT). We found that the internal field was absent above $P_c$, but the superconductivity is almost unchanged. This result suggests the existence of the nonunitary to unitary transition of the superconductivity around $P_c$. Nuclear spin-lattice relaxation rate $1/T_1$ showed the FM critical fluctuations around $P_c$, which persist above $P_c$ and are clearly related to superconductivity in the PM phase. This FM QPT is understood to be a weak first order with critical fluctuations. $1/T_1$ sharply decreased in the superconducting (SC) state above $P_c$ with a single component, in contrast to the two-component $1/T_1$ in the FM SC state, indicating that the inhomogeneous SC state is a characteristic feature of the FM SC state in UCoGe.

cond-mat.str-el↗

arXiv2018

Nematic transition and highly two-dimensional superconductivity in BaTi$_2$Bi$_2$O revealed by $^{209}$Bi-nuclear magnetic resonance/nuclear quadrupole resonance measurements

In this Rapid Communication, a set of $^{209}$Bi-nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) measurements has been performed to investigate the physical properties of superconducting (SC) BaTi$_2$Bi$_2$O from a microscopic point of view. The NMR and NQR spectra at 5~K can be reproduced with a non-zero in-plane anisotropic parameter $η$, indicating the breaking of the in-plane four-fold symmetry at the Bi site without any magnetic order, i.e., `the electronic nematic state'. In the SC state, the nuclear spin-lattice relaxation rate divided by temperature, $1/T_1T$, does not change even below $T_{\rm c}$, while a clear SC transition was observed with a diamagnetic signal. This observation can be attributed to the strong two-dimensionality in BaTi$_2$Bi$_2$O. Comparing the NMR/NQR results among BaTi$_2$$Pn$$_2$O ($Pn$ = As, Sb, and Bi), it was found that the normal and SC properties of BaTi$_2$Bi$_2$O were considerably different from those of BaTi$_2$Sb$_2$O and BaTi$_2$As$_2$O, which might explain the two-dome structure of $T_{\rm c}$ in this system.

cond-mat.supr-con↗