arXiv subjects

S. Maki

Publications and source records attributed to S. Maki.

4 recordsLinked to original sources

arXiv2018

Anomalous ^{125}Te Nuclear Spin Relaxation Coincident with Charge Kondo Behavior in Superconducting Pb_{1-x}Tl_{x}Te

We report a ^{125}Te-NMR study of single-crystalline Pb_{1-x}Tl_{x}Te for x= 0 (undoped non-superconducting "parent"), 0.35 at% (doped but on the borderline of superconductivity), and 1.0 at% (superconducting, with a critical temperature Tc ~ 1.0 K). The Knight shift in the normal state is enhanced as x increases, corresponding to an increase in the average density of states (DOS). The NMR line-width also increases significantly with increasing x, indicative of a strong spatial variation in the local DOS surrounding each Tl dopant. Remarkably, for the superconducting composition, the ^{125}Te nuclear spin relaxation rate (1/T1T) for Te ions that are close to the Tl dopants is unexpectedly enhanced in the normal state below a characteristic temperature of ~10 K. This temperature coincides with the temperature below which the normal state resistivity experiences an upturn. Such a simultaneous upturn in both the resistivity and (1/T1T) is highly anomalous, and puts Pb_{1-x}Tl_{x}Te in a distinct new class of doped semiconductors. These observations provide microscopic evidence for dynamical charge fluctuations in the absence of paramagnetism, and are consistent with expectations for charge Kondo behavior associated with the Tl dopant ions. In contrast, such anomalies were not detected in the non-superconducting samples (x=0 and 0.35 at%), suggesting a connection between dynamical valence fluctuations and the occurrence of superconductivity in Pb_{1-x}Tl_{x}Te.

cond-mat.supr-con↗

arXiv2014

Quantum Criticality Based on Large Ising Spins: YbCo$_2$Ge$_4$ with New 1-2-4 Structure Type

We present a new type of quantum critical material YbCo$_2$Ge$_4$, having the largest quantum-critical pseudospin size ever. The YbCo$_2$Ge$_4$-type structure is new, forms in the orthorhombic $Cmcm$ system, and is related to the well-known ThCr$_2$Si$_2$ structure. Heavy rare earth (Tm,Yb,Lu, or Y) members are also possible to be grown. YbCo$_2$Ge$_4$ possesses the Ising-type ground-state doublet, namely the simplest ones of uniaxially up or down, $|\pm \sim 7/2\rangle$. It is clearly manifested through comprehensive resistivity, magnetization, specific heat, and NQR/NMR experiments. Large pseudospin state usually tends to order in simple magnetisms, or hard to be screened by Kondo effect. Therefore, the discovery of the quantum criticality of the fluctuating large spins opens a new door to new-material search and theoretical studies.

cond-mat.str-el↗

arXiv2011

Crystallographic and superconducting properties of the fully-gapped noncentrosymmetric 5d-electron superconductors CaMSi3 (M=Ir, Pt)

We report crystallographic, specific heat, transport, and magnetic properties of the recently discovered noncentrosymmetric 5d-electron superconductors CaIrSi3 (Tc = 3.6 K) and CaPtSi3 (Tc = 2.3 K). The specific heat suggests that these superconductors are fully gapped. The upper critical fields are less than 1 T, consistent with limitation by conventional orbital depairing. High, non-Pauli-limited μ0 Hc2 values, often taken as a key signature of novel noncentrosymmetric physics, are not observed in these materials because the high carrier masses required to suppress orbital depairing and reveal the violated Pauli limit are not present.

cond-mat.supr-con↗

arXiv2006

Structural transitions of monoolein bicontinuous cubic phase induced by inclusion of protein lysozyme solutions

Inclusion of protein lysozyme molecules in lipidic monoolein cubic phase induces a transition from a $\rm Pn\bar{3}m$ structure to $\rm Im\bar{3}m$ one. Small-angle X-ray scattering (SAXS) method with high intensity synchrotron radiation enabled us to follow closely the transition depending on the conditions of lysozyme solutions. We showed that concentrated lysozyme solutions induced the appearance of the $\rm Im\bar{3}m$ structure coexisting with the $\rm Pn\bar{3}m$ structure. From the relation between the lattice parameters of these two structures it was shown that they were related by the Bonnet transformation of underlying triply periodic minimal surfaces. We found that the transition also occurred at lower lysozyme concentration when NaCl induced attraction between lysozyme molecules. The origin of the transition was considered as a frustration in the cubic phase where lysozyme molecules were highly confined. A simple estimation of the frustration was given, which took into account of the translational entropy of lysozyme molecules. At the highest concentration of lysozyme and NaCl the $\rm Im\bar{3}m$ structure was found to disappear and left only the $\rm Pn\bar{3}m$ structure. This was probably either due to the crystallization or phase separation of lysozyme solutions ongoing microscopically, which absorbed lysozyme molecules from channels of the cubic phase and thus removed the frustration.

cond-mat.soft↗