arXiv subjects

Tomoo Yamamura

Publications and source records attributed to Tomoo Yamamura.

7 recordsLinked to original sources

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

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

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↗

arXiv2017

ScPd2Al3 New Polymorphic Phase in Al-Pd-Sc System

We have discovered a new compound of the composition ScPd2Al3 crystallizing in unknown structure type. Moreover, ScPd2Al3 reveals polymorphism. We have found an orthorhombic crystal structure at room temperature and a high temperature cubic phase. The polymorphic phases are separated by a reversible first order transition at 1053°C with a hysteresis of 19°C. ScPd2Al3 exists as a very stable intermetallic phase just in the vicinity of the icosahedral quasicrystal Tsai-type i-phase Al54Pd30Sc16.

cond-mat.mtrl-sci↗

arXiv2017

Switching of magnetic ground states across the UIr1-xRhxGe alloy system

We investigated the evolution of magnetism in the UIr1-xRhxGe system by the systematic study of high-quality single crystals. Lattice parameters of both parent compounds are very similar resulting in almost identical nearest interatomic uranium distance close to the Hill limit. We established the x-T phase diagram of the UIr1-xRhxGe system and found a discontinuous antiferromagnetic/ferromagnetic boundary at xcrit = 0.56 where a local minimum in ordering temperature and maximum of the Sommerfeld coefficient 175 mJ/mol K2 occurs in the UCoGe-URhGe-UIrGe system, signaling an increase in magnetic fluctuations. However, a quantum critical point is not realized because of the finite ordering temperature at xcrit. A magnon gap on the antiferromagnetic side abruptly suppresses magnetic fluctuations. We find a field-induced first order transition in the vicinity of the critical magnetic field along the b axis in the entire UIr1-xRhxGe system including the ferromagnetic region UCo0.6Rh0.4Ge - URhGe.

cond-mat.str-el↗

arXiv2016

Properties and Collapse of the Ferromagnetism in UCo1-xRuxAl Studied in Single Crystals

We have investigated the ferromagnetic (FM) phase which suddenly develops in UCo1-xRuxAl and is isolated by paramagnetic regions on both sides from the parent UCoAl and URuAl. For that purpose we have grown high quality single crystals with x = 0.62, 0.70, 0.74, 0.75 and 0.78. The properties of the FM phase have been investigated by microscopic and macroscopic methods. Polarized neutron diffraction on a single crystal with x = 0.62 revealed the gradual growth of the hybridization between U and T-site in the U - T plane with increasing x. Hybridization works here as a mediator of the strong indirect interaction, while the delocalized character of the 5 f states is still conserved. As a result very weak spontaneous magnetic moments are observed for all alloys with magnitude nearly in proportion to the TC for x < 0.62, while an enormous disproportion exists between them near xcrit.. The magnetization, specific heat, electrical resistivity, and Hall effect measurements confirmed that the FM transition is suppressed continuously at the critical concentration xcrit. = 0.77. Two quantum critical points are then expected on both sides of the FM dome. We propose a scenario that the order of the FM/PM transition differs at opposite boundaries of the FM dome. We conclude that both criticalities are influenced by disorder. Criticality on the UCoAl side has the character of a clean FM metal, while on the Ru rich side it has the character of a magnetically inhomogeneous system involving a Griffiths phase.

cond-mat.str-el↗

arXiv2014

Relationship between Ferromagnetic Criticality and the Enhancement of Superconductivity Induced by Transverse Magnetic Fields in UCoGe

We have performed 59Co NMR experiments on the ferromagnetic superconductor UCoGe under magnetic fields (H) along the a- and b- axes to investigate the relationship between ferromagnetic properties and superconductivity. The ferromagnetic ordering temperature TCurie is suppressed and the nuclear spin-lattice relaxation rate 1/T1 at 2 K is enhanced in H || b, although TCurie and 1/T1 are unchanged in H || a, indicating that the ferromagnetic criticality is induced only when H is applied along the b axis. We show the close relationship between the magnetic anisotropies and the superconducting ones reported by Aoki et al.: the superconductivity is gradually suppressed in H || a, but enhanced in H || b above 5 T. We strongly suggest that the enhancement of the superconductivity observed in H || b originates from the field induced ferromagnetic criticality, as pointed out by Aoki et al and Mineev.

cond-mat.str-el↗