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H. Tou

Publications and source records attributed to H. Tou.

17 recordsLinked to original sources
arXiv2021

Magnetic properties of the itinerant ferromagnet LaCrGe3 under pressure studied by 139La NMR

$^{139}$La nuclear magnetic resonance (NMR) measurements under pressure ($p = 0-2.64$ GPa) have been carried out to investigate the static and dynamic magnetic properties of the itinerant ferromagnet LaCrGe$_3$. $^{139}$La-NMR spectra for all measured pressures in the ferromagnetically ordered state show a large shift due to the internal field induction $|$$B_{\rm int}$$|$ $\sim$ 4 T at the La site produced by Cr ordered moments. The change in $B_{\rm int}$ by less than 5\% with $p$ up to 2.64~GPa indicates that the Cr 3$d$ moments are robust under pressure. The temperature dependence of NMR shift and $B_{\rm int}$ suggest that the ferromagnetic order develops below $\sim$ 50~K under higher pressures in a magnetic field of $\sim$ 7.2 T. Based on the analysis of NMR data using the self-consistent-renormalization (SCR) theory, the spin fluctuations in the paramagnetic state well above $T_{\rm C}$ are revealed to be three dimensional ferromagnetic throughout the measured $p$ region.

cond-mat.str-el↗
arXiv2020

Helimagnetic Structure and Heavy-Fermion-Like Behavior in the Vicinity of the Quantum Critical Point in Mn$_3$P

Antiferromagnet Mn$_3$P with Neel temperature $T_N=30$ K is composed of Mn-tetrahedrons and zigzag chains formed by three inequivalent Mn sites. Due to the nearly frustrated lattice with many short Mn-Mn bonds, competition of the exchange interactions is expected. We here investigate the magnetic structure and physical properties including pressure effect in single crystals of this material, and reveal a complex yet well-ordered helimagnetic structure. The itinerant character of this materials is strong, and the ordered state with small magnetic moments is easily suppressed under pressure, exhibiting a quantum critical point at $\sim1.6$ GPa. The remarkable mass renormalization, even in the ordered state, and an incoherent-coherent crossover in the low-temperature region, characterize an unusual electronic state in Mn$_3$P, which is most likely effected by the underlying frustration effect.

cond-mat.str-el↗
arXiv2019

Magnetic fluctuations in the itinerant ferromagnet LaCrGe3 studied by 139La NMR

LaCrGe$_3$ is an itinerant ferromagnet with a Curie temperature of $T_{\rm c}$ = 85 K and exhibits an avoided ferromagnetic quantum critical point under pressure through a modulated antiferromagnetic phase as well as tri-critical wing structure in its temperature-pressure-magnetic field ($T$-$p$-$H$) phase diagram. In order to understand the static and dynamical magnetic properties of LaCrGe$_3$, we carried out $^{139}$La nuclear magnetic resonance (NMR) measurements. Based on the analysis of NMR data, using the self-consistent-renomalization (SCR) theory, the spin fluctuations in the paramagnetic state are revealed to be isotropic ferromagnetic and three dimensional (3D) in nature. Moreover, the system is found to follow the generalized Rhodes-Wohfarth relation which is expected in 3D itinerant ferromagnetic systems. As compared to other similar itinerant ferromagnets, the Cr 3$d$ electrons and their spin fluctuations are characterized to have a relatively high degree of localization in real space.

cond-mat.str-el↗
arXiv2019

Nonsaturating large magnetoresistance in the high carrier density nonsymmorphic metal CrP

The band structure of high carrier density metal CrP features an interesting crossing at the Y point of the Brillouin zone. The crossing, which is protected by the nonsymmorphic symmetry of the space group, results in a hybrid, semi-Dirac-like energy-momentum dispersion relation near Y. The linear energy-momentum dispersion relation along Y-$Γ$ is reminiscent of the observed band structure in several semimetallic extremely large magnetoresistance (XMR) materials. We have measured the transverse magnetoresistance of CrP up to 14 T at temperatures as low as $\sim$ 16 mK. Our data reveal a nonsaturating, quadratic magnetoresistance as well as the behaviour of the so-called `turn-on' temperature in the temperature dependence of resistivity. Despite the difference in the magnitude of the magnetoresistance and the fact that CrP is not a semimetal, these features are qualitatively similar to the observations reported for XMR materials. Thus, the high-field electrical transport studies of CrP offer the prospect of identifying the possible origin of the nonsaturating, quadratic magnetoresistance observed in a wide range of metals.

cond-mat.str-el↗
arXiv2016

Quasilinear quantum magnetoresistance in pressure-induced nonsymmorphic superconductor CrAs

In conventional metals, modification of electron trajectories under magnetic field gives rise to a magnetoresistance that varies quadratically at low field, followed by a saturation at high field for closed orbits on the Fermi surface. Deviations from the conventional behaviour, e.g. the observation of a linear magnetoresistance, or a non-saturating magnetoresistance, have been attributed to exotic electron scattering mechanisms. Recently, linear magnetoresistance has been observed in many Dirac materials, in which the electron-electron correlation is relatively weak. The strongly correlated helimagnet CrAs undergoes a quantum phase transition to a nonmagnetic superconductor under pressure. Near the magnetic instability, we observe a large and non-saturating quasilinear magnetoresistance from the upper critical field to 14 T at low temperatures. We show that the quasilinear magnetoresistance arises from an intricate interplay between a nontrivial band crossing protected by nonsymmorphic crystal symmetry and strong magnetic fluctuations

cond-mat.supr-con↗
arXiv2016

Emergent Weyl fermion excitations in TaP explored by 181-Ta quadrupole resonance

The $^{181}$Ta quadrupole resonance (NQR) technique has been utilized to investigate the microscopic magnetic properties of the Weyl semi-metal TaP. We found three zero-field NQR signals associated with the transition between the quadrupole split levels for Ta with $I$=7/2 nuclear spin. A quadrupole coupling constant, $ν_{\mathrm{Q}}$ =19.250 MHz, and an asymmetric parameter of the electric field gradient, $η$ = 0.423 were extracted, in good agreement with band structure calculations. In order to examine the magnetic excitations, the temperature dependence of the spin lattice relaxation rate (1/$T_{\mathrm{1}}T$) has been measured for the $f_{\mathrm{2}}$-line ($\pm$5/2 $\leftrightarrow$ $\pm$3/2 transition). We found that there exists two regimes with quite different relaxation processes. Above $T\text{*}$ $\approx$ 30 K, a pronounced (1/$T_{\mathrm{1}}T$) $\propto$ $T^{2}$ behavior was found, which is attributed to the magnetic excitations at the Weyl nodes with temperature dependent orbital hyperfine coupling. Below $T\text{*}$, the relaxation is mainly governed by Korringa process with 1/$T_{\mathrm{1}}T$ = constant, accompanied by an additional $T^{-1/2}$ type dependence to fit our experimental data. We show that Ta-NQR is a novel probe for the bulk Weyl fermions and their excitations.

cond-mat.mtrl-sci↗
arXiv2013

Pressure-Temperature-Magnetic Field Phase Diagram of Ferromagnetic Kondo Lattice CeRuPO

We report the temperature-pressure-magnetic field phase diagram made from electrical resistivity measurements for the ferromagnetic (FM) Kondo lattice CeRuPO. The ground state at zero field changes from the FM state to another state, which is suggested to be an antiferromagnetic (AFM) state, above ~0.7 GPa, and the magnetically ordered state is completely suppressed at ~2.8 GPa. In addition to the collapse of the AFM state under pressure and a magnetic field, a metamagnetic (MM) transition from a paramagnetic state to a polarized paramagnetic state appears. CeRuPO will give us a rich playground for understanding the mechanism of the MM transition under comparable FM and AFM correlations in the Kondo lattice.

cond-mat.str-el↗
arXiv2012

NMR Investigation of the iron-based superconductors Ca4(Mg,Ti)3Fe2As2O8-y and Ca5(Sc,Ti)4Fe2As2O11-y

$^{75}$As and $^{45}$Sc NMR measurements unravel the electronic state for Fe-based superconductors with perovskite-type blocking layers Ca$_4$(Mg,Ti)$_3$Fe$_2$As$_2$O$_{8-y}$ ($T_c^{onset}=47$ K) and Ca$_5$(Sc,Ti)$_4$Fe$_2$As$_2$O$_{11-y}$ ($T_c^{onset}=41$ K). In Ca$_5$(Sc,Ti)$_4$Fe$_2$As$_2$O$_{11-y}$, the nuclear spin relaxation rate $1/T_1$ shows pseudogap behavior below $\sim80$ K, suggesting that the electronic state is similar to that of LaFeAs(O,F) system with moderate electron doping. The presence of the pseudogap behavior gives an interpretation that the hole-like band (so-called $γ$ pocket) is located just below the Fermi level from the analogy to LaFeAs(O,F) system and the disappearance of the $γ$ pocket yields the suppression of the low-energy spin fluctuations. On the other hand, in Ca$_4$(Mg,Ti)$_3$Fe$_2$As$_2$O$_{8-y}$ satisfying the structural optimal condition for higher $T_c$ among the perovskite systems, the extrinsic contribution, which presumably originates in the Ti moment, is observed in $1/T_1T$; however, the moderate temperature dependence of $1/T_1T$ appears by its suppression under high magnetic field. In both systems, the high $T_c$ of $\sim40$ K is realized in the absence of the strong development of the low-energy spin fluctuations. The present results reveal that the structural optimization does not induce the strong development of the low-energy spin fluctuations. If we consider that superconductivity is mediated by spin fluctuations, the structural optimization is conjectured to provide a benefit to the development of the high-energy spin fluctuations irrespective to the low-energy part.

cond-mat.supr-con↗
arXiv2010

Anion height dependence of Tc for the Fe-based superconductor

We have established a plot of the anion height dependence of Tc for the typical Fe-based superconductors. The plot appeared a symmetric curve with a peak around 1.38 A. Both data at ambient pressure and under high pressure obeyed the unique curve. This plot will be one of the key strategies for both understanding the mechanism of Fe-based superconductivity and search for the new Fe-based superconductors with higher Tc.

cond-mat.supr-con↗
arXiv2009

Contrasting Pressure Effects in Sr2VFeAsO3 and Sr2ScFePO3

We report the resistivity measurements under pressure of two Fe-based superconductors with a thick perovskite oxide layer, Sr2VFeAsO3 and Sr2ScFePO3. The superconducting transition temperature Tc of Sr2VFeAsO3 markedly increases with increasing pressure. Its onset value, which was Tc{onset}=36.4 K at ambient pressure, increases to Tc{onset}=46.0 K at ~4 GPa, ensuring the potential of the "21113" system as a high-Tc material. However, the superconductivity of Sr2ScFePO3 is strongly suppressed under pressure. The Tc{onset} of ~16 K decreases to ~5 K at ~4 GPa, and the zero-resistance state is almost lost. We discuss the factor that induces this contrasting pressure effect.

cond-mat.supr-con↗
arXiv2009

Effect of Uniaxial Stress for Pressure-Induced Superconductor SrFe_2As_2

We report that the pressure-temperature phase diagram of single-crystalline SrFe$_2$As$_2$ is easily affected by the hydrostaticity of a pressure-transmitting medium. For all of the three mediums we used, superconductivity with zero resistance appears, accompanied by the suppression of an antiferromagnetic (orthorhombic) phase, but the critical pressure $P_c$ was found to depend on the type of medium. $P_c$ was estimated to be 4.4 GPa under almost hydrostatic condition, but it decreased to $3.4-3.7$ GPa with the use of the medium already solidified at room temperature. The uniaxial stress along the c-axis is suggested to aid in the suppression of the antiferromagnetic (orthorhombic) phase. The pressure effect of BaFe$_2$As$_2$ is also reported.

cond-mat.supr-con↗
arXiv2009

Precise Pressure Dependence of the Superconducting Transition Temperature of FeSe: Resistivity and ^77Se--NMR Study

We report the precise pressure dependence of FeSe from a resistivity measurement up to 4.15 GPa. Superconducting transition temperature (T_c) increases sensitively under pressure, but shows a plateau between 0.5-1.5 GPa. The maximum T_c, which is determined by zero resistance, is 21 K at approximately 3.5 GPa. The onset value reaches ~37 K at 4.15 GPa. We also measure the nuclear spin-lattice relaxation rate 1/T_1 under pressure using 77Se--NMR measurement. 1/T_1 shows that bulk superconductivity is realized in the zero-resistance state. The pressure dependence of 1/T_1T just above T_c shows a plateau as well as the pressure dependence of T_c, which gives clear evidence of the close relationship between 1/T_1T and T_c. Spin fluctuations are suggested to contribute to the mechanism of superconductivity.

cond-mat.supr-con↗
arXiv2008

Abrupt Emergence of Pressure-Induced Superconductivity of 34 K in SrFe2As2: A Resistivity Study under Pressure

We report resistivity measurement under pressure in single crystals of SrFe_2As_2, which is one of the parent materials of Fe-based superconductors. The structural and antiferromagnetic (AFM) transition of T_0 = 198 K at ambient pressure is suppressed under pressure, and the ordered phase disappears above P_c ~ 3.6-3.7 GPa. Superconductivity with a sharp transition appears accompanied by the suppression of the AFM state. T_c exhibits a maximum of 34.1 K, which is realized close to the phase boundary at P_c. This T_c is the highest among those of the stoichiometric Fe-based superconductors.

cond-mat.supr-con↗
arXiv2008

Evidence for Unconventional Superconductivity in Arsenic-Free Iron-Based Superconductor FeSe : A ^77Se-NMR Study

We report the results of $^{77}$Se--nuclear magnetic resonance (NMR) in $α$-FeSe, which exhibits a similar crystal structure to the LaFeAsO$_{1-x}$F$_x$ superconductor and shows superconductivity at 8 K. The nuclear-spin lattice relaxation rate $1/T_1$ shows $T^3$ behavior below the superconducting transition temperature $T_c$ without a coherence peak. The $T_1T=$ const. behavior, indicative of the Fermi liquid state, can be seen in a wide temperature range above $T_c$. The superconductivity in $α$-FeSe is also an unconventional one as well as LaFeAsO$_{1-x}$F$_x$ and related materials. The FeAs layer is not essential for the occurrence of the unconventional superconductivity.

cond-mat.supr-con↗
arXiv2007

Energy gap formation in a valence fluctuating compound CeIrSb probed by Sb NMR and NQR

Sb-NMR/NQR study has revealed a formation of a pseudogap at the Fermi level in the density of states in a valence fluctuating compound CeIrSb. The nuclear spin-lattice relaxation rate divided by temperature, 1/T_1T has a maximum around 300 K and decreases significantly as 1/T_1T ~ T^2, followed by a 1/T_1T = const. relation at low temperature. This temperature dependence of 1/T_1T is well reproduced by assuming a V-shaped energy gap with a residual density of states at the Fermi level. The size of energy gap for CeIrSb is estimated to be about 350 K, which is by one order of magnitude larger than those for the isostructural Kondo semiconductors CeRhSb and CeNiSn. Despite the large difference in the size of energy gap, CeIrSb, CeRhSb and CeNiSn are indicated to be classified into the same group revealing a V-shaped gap due to c-f hybridization. The temperature dependence of the Knight shift measured in a high magnetic field agrees with the formation of this pseudogap.

cond-mat.str-el↗
arXiv2006

Thermopower of CexR1-xB6 (R=La, Pr and Nd)

The thermopower, S, of CexR1-xB6 (R=La, Pr, Nd) was investigated. S with a positive sign shows a typical behavior observed in the Ce Kondo system, an increase with decreasing temperature at high temperatures and a maximum at low temperatures. The S values of all the systems at high temperatures are roughly linearly dependent on the Ce concentration, indicating the conservation of the single-impurity character of the Kondo effect in a wide x range. However, the maximum value of S, S_max, and the temperature, T_max, at which S_max is observed exhibit different x dependences between CexLa1-xB6 and CexR1-xB6 (R=Pr, Nd). In CexLa1-xB6, T_max, which is ~8 K in CeB6, decreases with decreasing x and converges to ~1 K in a very dilute alloy and S_max shows an increase below x ~ 0.1 after decreasing with decreasing x. In CexR1-xB6 (R=Pr, Nd), T_max shows a weak x dependence but S_max shows a roughly linear decrease in x. These results are discussed from the standpoint of the chemical pressure effect and the Ce-Ce interaction. S in the long-range ordered phase shows very different behavior between CexPr1-xB6 and CexNd1-xB6.

cond-mat.str-el↗