arXiv subjects

Y. Furukawa

Publications and source records attributed to Y. Furukawa.

At least 55 records · Page 3Linked to original sources

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↗

arXiv2018

Magnetic properties of the itinerant A-type antiferromagnet CaCo2P2 studied by 59Co and 31P NMR

$^{59}$Co and $^{31}$P nuclear magnetic resonance (NMR) measurements in external magnetic and zero magnetic fields have been performed to investigate the magnetic properties of the A-type antiferromagnetic (AFM) CaCo$_2$P$_2$. NMR data, especially, the nuclear spin lattice relaxation rates 1/$T_1$ exhibiting a clear peak, provide clear evidence for the AFM transition at a Néel temperature of $T_{\rm N}\sim$110~K. The magnetic fluctuations in the paramagnetic state were found to be three-dimensional ferromagnetic, suggesting ferromagnetic interaction between Co spins in the ${\it ab}$ plane characterize the spin correlations in the paramagnetic state. In the AFM state below $T_{\rm N}$, we have observed $^{59}$Co and $^{31}$P NMR signals under zero magnetic field. From $^{59}$Co NMR data, the ordered magnetic moments of Co are found to be in $ab$ plane and are estimated to be 0.35 $μ_{\rm B}$ at 4.2 K. Furthermore, the external field dependence of $^{59}$Co NMR spectrum in the AFM state suggests a very weak magnetic anisotropy of the Co ions and also provides microscopic evidence of canting the Co ordered moments along the external magnetic field directions. The magnetic state of the Co ions in CaCo$_2$P$_2$ is well explained by the local moment picture in the AFM state, although the system is metallic as seen by $1/T_1T$ = constant behavior.

cond-mat.str-el↗

arXiv2018

Ferromagnetic spin fluctuations in the filled skutterudite SrFe4As12 revealed by 75As NMR-NQR measurements

$^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been carried out to investigate the magnetic and electronic properties of the filled skutterudite metallic compound SrFe$_4$As$_{12}$. The temperature dependence of Knight shift $K$ determined by the NQR spectrum under a small magnetic field ($\le$ 0.5 T) shows the similar temperature dependence of the magnetic susceptibility $χ$ which exhibits a broad maximum at $T^\ast$ $\sim$ 50 K. The nuclear spin-lattice relaxation rate divided by temperature, 1/$T_1T$, increases with decreasing temperature and exhibits a broad maximum at $T$ $\sim$ 70 K, similar to the case of $χ$. The temperature dependence of $K$ and $1/T_1T$ is reasonably explained by a simple model where we assume a concave-shaped band structure near the Fermi energy. Based on a Korringa ratio analysis using the $T_1$ and $K$ data, ferromagnetic spin fluctuations are found to exist in SrFe$_4$As$_{12}$. These results indicate that SrFe$_4$As$_{12}$ can be characterized to be a metal with ferromagnetic correlations and also the peculiar band structure responsible for the suppression of $1/T_1T$ and $K$ at low temperatures.

cond-mat.str-el↗

arXiv2018

Hedgehog Spin-vortex Crystal Antiferromagnetic Quantum Criticality in CaK(Fe1-xNix)4As4 Revealed by NMR

Two ordering states, antiferromagnetism and nematicity, have been observed in most iron-based superconductors (SCs). In contrast to those SCs, the newly discovered SC CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ exhibits an antiferromagnetic (AFM) state, called hedgehog spin-vortex crystal structure, without nematic order, providing the opportunity for the investigation into the relationship between spin fluctuations and SC without any effects of nematic fluctuations. Our $^{75}$As nuclear magnetic resonance studies on CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ (0$\le x\le$ 0.049) revealed that CaKFe$_4$As$_4$ is located close to a hidden hedgehog SVC AFM quantum-critical point (QCP). The magnetic QCP without nematicity in CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ highlights the close connection of spin fluctuations and superconductivity in iron-based SCs. The advantage of stoichiometric composition also makes CaKFe$_4$As$_4$ an ideal platform for further detailed investigation of the relationship between magnetic QCP and superconductivity in iron-based SCs without disorder effects.

cond-mat.str-el↗

arXiv2018

Correlation between Superconductivity and Magnetic Fluctuations in Fe(Se1-xSx) Revealed by 77Se NMR

We present $^{77}$Se-NMR measurements on FeSe$_{1-x}$S$_x$ samples with sulfur content $x=0,9,15$ and $29\%$. Twinned nematic domains are observed in the NMR spectrum for all samples except $x=29\%$. The NMR spin-lattice relaxation rate shows that magnetic fluctuations are initially enhanced between $x=0\%$ and $x=9\%$, but are strongly suppressed for higher $x$ values. The observed behavior of the magnetic fluctuations parallels the superconducting transition temperature $T_c$ in these materials, providing strong evidence for the primary importance of magnetic fluctuations for superconductivity, despite the presence of nematic quantum criticality in this system.

cond-mat.supr-con↗

arXiv2018

Pressure dependence of coherence-incoherence crossover behavior in KFe2As2 observed by resistivity and 75As-NMR/NQR

We present the results of $^{75}$As nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and resistivity measurements in KFe$_2$As$_2$ under pressure ($p$). The temperature dependence of the NMR shift, nuclear spin-lattice relaxation time ($T_1$) and resistivity show a crossover between a high-temperature incoherent, local-moment behavior and a low-temperature coherent behavior at a crossover temperature ($T^*$). $T^*$ is found to increase monotonically with pressure, consistent with increasing hybridization between localized $3d$ orbital-derived bands with the itinerant electron bands. No anomaly in $T^*$ is seen at the critical pressure $p_{\rm c}=1.8$ GPa where a change of slope of the superconducting (SC) transition temperature $T_{\rm c}(p)$ has been observed. In contrast, $T_{\rm c}(p)$ seems to correlate with antiferromagnetic spin fluctuations in the normal state as measured by the NQR $1/T_1$ data, although such a correlation cannot be seen in the replacement effects of A in the AFe$_2$As$_2$ (A= K, Rb, Cs) family. In the superconducting state, two $T_1$ components are observed at low temperatures, suggesting the existence of two distinct local electronic environments. The temperature dependence of the short $T_{\rm 1s}$ indicates nearly gapless state below $T_{\rm c}$. On the other hand, the temperature dependence of the long component 1/$T_{\rm 1L}$ implies a large reduction in the density of states at the Fermi level due to the SC gap formation. These results suggest a real-space modulation of the local SC gap structure in KFe$_2$As$_2$ under pressure.

cond-mat.supr-con↗

arXiv2017

NMR Study of the New Magnetic Superconductor CaK(Fe$0.951Ni0.049)4As4: Microscopic Coexistence of Hedgehog Spin-vortex Crystal and Superconductivity

Coexistence of a new-type antiferromagnetic (AFM) state, the so-called hedgehog spin-vortex crystal (SVC), and superconductivity (SC) is evidenced by $^{75}$As nuclear magnetic resonance study on single-crystalline CaK(Fe$_{0.951}$Ni$_{0.049}$)$_4$As$_4$. The hedgehog SVC order is clearly demonstrated by the direct observation of the internal magnetic induction along the $c$ axis at the As1 site (close to K) and a zero net internal magnetic induction at the As2 site (close to Ca) below an AFM ordering temperature $T_{\rm N}$ $\sim$ 52 K. The nuclear spin-lattice relaxation rate 1/$T_1$ shows a distinct decrease below $T_{\rm c}$ $\sim$ 10 K, providing also unambiguous evidence for the microscopic coexistence. Furthermore, based on the analysis of the 1/$T_1$ data, the hedgehog SVC-type spin correlations are found to be enhanced below $T$ $\sim$ 150 K in the paramagnetic state. These results indicate the hedgehog SVC-type spin correlations play an important role for the appearance of SC in the new magnetic superconductor.

cond-mat.supr-con↗

arXiv2017

NMR evidence for static local nematicity and its cooperative interplay with low-energy magnetic fluctuations in FeSe under pressure

We present $^{77}$Se-NMR measurements on single-crystalline FeSe under pressures up to 2 GPa. Based on the observation of the splitting and broadening of the NMR spectrum due to structural twin domains, we discovered that static, local nematic ordering exists well above the bulk nematic ordering temperature, $T_{\rm s}$. The static, local nematic order and the low-energy stripe-type antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation rate measurements, are both insensitive to pressure application. These NMR results provide clear evidence for the microscopic cooperation between magnetism and local nematicity in FeSe.

cond-mat.str-el↗

arXiv2017

Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by 75As NMR

We report $^{75}$As nuclear magnetic resonance (NMR) studies on a new iron-based superconductor CaKFe$_4$As$_4$ with $T_{\rm c}$ = 35 K. $^{75}$As NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites close to the K and Ca layers, respectively, revealing that K and Ca layers are well ordered without site inversions. We found that nuclear quadrupole frequencies $ν_{\rm Q}$ of the As(1) and As(2) sites show an opposite temperature ($T$) dependence. Nearly $T$ independent behavior of the Knight shifts $K$ are observed in the normal state, and a sudden decrease in $K$ in the superconducting (SC) state clearly evidences spin-singlet Cooper pairs. $^{75}$As spin-lattice relaxation rates 1/$T_1$ show a power law $T$ dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wavevector ${\bf q}$ = ($π$, 0) or (0, $π$) in the single-iron Brillouin zone notation are revealed by 1/$T_1T$ and $K$ measurements. Such magnetic fluctuations are necessary to explain the observed temperature dependence of the $^{75}$As quadrupole frequencies, as evidenced by our first-principles calculations. In the SC state, 1/$T_1$ shows a rapid decrease below $T_{\rm c}$ without a Hebel-Slichter peak and decreases exponentially at low $T$, consistent with an $s^{\pm}$ nodeless two-gap superconductor.

cond-mat.supr-con↗

arXiv2017

Hedgehog spin-vortex crystal stabilized in a hole-doped iron-based superconductor

Magnetism is widely considered to be a key ingredient of unconventional superconductivity. In contrast to cuprate high-temperature superconductors, antiferromagnetism in Fe-based superconductors (FeSCs) is characterized by a pair of magnetic propagation vectors. Consequently, three different types of magnetic order are possible. Of theses, only stripe-type spin-density wave (SSDW) and spin-charge-density wave (SCDW) orders have been observed. A realization of the proposed spin-vortex crystal (SVC) order is noticeably absent. We report a magnetic phase consistent with the hedgehog variation of SVC order in Ni- and Co-doped CaKFe4As4 based on thermodynamic, transport, structural and local magnetic probes combined with symmetry analysis. The exotic SVC phase is stabilized by the reduced symmetry of the CaKFe4As4 structure. Our results suggest that the possible magnetic ground states in FeSCs have very similar energies, providing an enlarged configuration space for magnetic fluctuations to promote high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2017

NMR Determination of an Incommensurate Helical Antiferromagnetic Structure in EuCo2As2

We report $^{153}$Eu, $^{75}$As and $^{59}$Co nuclear magnetic resonance (NMR) results on EuCo$_2$As$_2$ single crystal. Observations of $^{153}$Eu and $^{75}$As NMR spectra in zero magnetic field at 4.3 K below an antiferromagnetic (AFM) ordering temperature $T_{\rm N}$ = 45 K and its external magnetic field dependence clearly evidence an incommensurate helical AFM structure in EuCo$_2$As$_2$. Furthermore, based on $^{59}$Co NMR data in both the paramagnetic and the incommensurate AFM states, we have determined the model-independent value of the AFM propagation vector ${\bf k}$ = (0, 0, 0.73 $\pm$ 0.07)2$π$/$c$ where $c$ is the $c$ lattice parameter. Thus the incommensurate helical AFM state was characterized by only NMR data with model-independent analyses, showing NMR to be a unique tool for determination of the spin structure in incommensurate helical AFMs.

cond-mat.str-el↗

arXiv2016

Coexistence of antiferromagnetic and ferromagnetic spin correlations in Ca(Fe1-xCox)2As2 revealed by 75As nuclear magnetic resonance

Recent nuclear magnetic resonance (NMR) measurements revealed the coexistence of stripe-type antiferromagnetic (AFM) and ferromagnetic (FM) spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors by a Korringa ratio analysis. Motivated by the NMR work, we investigate the possible existence of FM fluctuations in another iron pnictide superconducting family, Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$. We re-analyzed our previously reported data in terms of the Korringa ratio and found clear evidence for the coexistence of stripe-type AFM and FM spin correlations in the electron-doped CaFe$_2$As$_2$ system. These NMR data indicate that FM fluctuations exist in general in iron-pnictide superconducting families and thus must be included to capture the phenomenology of the iron pnictides.

cond-mat.supr-con↗

arXiv2016

Electronic properties of GeTe and Ag- or Sb-substituted GeTe studied by low temperature 125Te NMR

We have carried out $^{125}$Te nuclear magnetic resonance (NMR) in a wide temperature range of 1.5 -- 300 K to investigate electronic properties of Ge$_{50}$Te$_{50}$, Ag$_{2}$Ge$_{48}$Te$_{50}$ and Sb$_{2}$Ge$_{48}$Te$_{50}$ from a microscopic point of view. From the temperature dependence of NMR shift ($K$) and nuclear spin lattice relaxation rate (1/$T_1$), we found that two bands contribute to the physical properties of the materials. One band overlaps the Fermi level providing the metallic state where no strong electron correlations are revealed by Korringa analysis. The other band is separated from the Fermi level by an energy gap of $E_{\rm g}/k_{\rm B}$ $\sim$ 67 K, which gives rise to the semiconductor-like properties. First principle calculation revealed that the metallic band originates from the Ge vacancy while the semiconductor-like band may be related to the fine structure of the density of states near the Fermi level. Low temperature $^{125}$Te NMR data for the materials studied here clearly show that the Ag substitution increases hole concentration while Sb substitution decreases it.

cond-mat.mtrl-sci↗

arXiv2016

Quantum Spin Liquid State in the Disordered Triangular Lattice Sc2Ga2CuO7

We present microscopic magnetic properties of a two dimensional triangular lattice Sc2Ga2CuO7, consisting of single and double triangular Cu planes. A Curie-Weiss temperature theta_CW = --44 K and an antiferromagnetic (AFM) exchange interaction J/KB = 35 K between Cu2+ (S = 1/2) spins in the triangular bi-plane are obtained from the analysis of magnetic susceptibility data. The intrinsic magnetic susceptibility extracted from 71Ga NMR shift data displays the presence of AFM short range spin correlations and remains finite down to 50 mK suggesting an non-singlet ground state. The nuclear spin-lattice relaxation rate (1/T1) reveals a slowing down of Cu2+ spin fluctuations with decreasing T down to 100 mK. Magnetic specific heat (Cm) and 1/T1 exhibit a power law behavior at low temperatures implying gapless nature of the spin excitation spectrum. Absence of long range magnetic ordering down to ~ J/700, nonzero spin susceptibility at low T, and power law behavior of Cm and 1/T1 suggest a gapless quantum spin liquid (QSL) state. Our results demonstrate that persistent spin dynamics induced by frustration maintain a quantum-disordered state at T -> 0 in this triangular lattice antiferromagnet. This suggests that the low energy modes are dominated by spinon excitations in the QSL state due to randomness engendered by disorder and frustration.

cond-mat.str-el↗

arXiv2016

Volovik Effect and Fermi-Liquid Behavior in the s-Wave Superconductor CaPd2As2: 75As NMR-NQR Measurements

The electronic and magnetic properties of the collapsed-tetragonal CaPd$_2$As$_2$ superconductor (SC) with a transition temperature of 1.27 K have been investigated by $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. The temperature ($T$) dependence of the nuclear spin lattice relaxation rates (1/$T_1$) and the Knight shifts indicate the absence of magnetic correlations in the normal state. In the SC state, 1/$T_1$ measured by $^{75}$As NQR shows a clear Hebel-Slichter (HS) peak just below $T_{\rm c}$ and decreases exponentially at lower $T$, confirming a conventional $s$-wave SC. In addition, the Volovik effect, also known as Doppler shift effect, has been clearly evidenced by the observation of the suppression of the HS peak with applied magnetic field.

cond-mat.supr-con↗

arXiv2016

Spin Liquid State in the 3D Frustrated Antiferromagnet PbCuTe2O6: NMR and muSR Studies

PbCuTe2O6 is a rare example of a spin liquid candidate featuring a three dimensional magnetic lattice. Strong geometric frustration arises from the dominant antiferromagnetic interaction which generates a hyperkagome network of Cu2+ ions although additional interactions enhance the magnetic lattice connectivity. Through a combination of magnetization measurements and local probe investigation by NMR and muSR down to 20 mK, we provide a robust evidence for the absence of magnetic freezing in the ground state. The local spin susceptibility probed by the NMR shift hardly deviates from the macroscopic one down to 1 K pointing to a homogeneous magnetic system with a low defect concentration. The saturation of the NMR shift and the sublinear power law temperature (T) evolution of the 1/T1 NMR relaxation rate at low T point to a non-singlet ground state favoring a gapless fermionic description of the magnetic excitations. Below 1 K a pronounced slowing down of the spin dynamics is witnessed, which may signal a reconstruction of spinon Fermi surface. Nonetheless, the compound remains in a fluctuating spin liquid state down to the lowest temperature of the present investigation.

cond-mat.str-el↗

arXiv2015

Antiferromagnetic spin correlations and pseudogap-like behavior in Ca(Fe1-xCox)2As2 studied by 75As nuclear magnetic resonance and anisotropic resistivity

We report $^{75}$As nuclear magnetic resonance (NMR) measurements of single-crystalline Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($x$ = 0.023, 0.028, 0.033, and 0.059) annealed at 350~$^{\circ}$C for 7 days. From the observation of a characteristic shape of $^{75}$As NMR spectra in the stripe-type antiferromagnetic (AFM) state, as in the case of $x$ = 0 ($T_{\rm N}$ = 170 K), clear evidence for the commensurate AFM phase transition with the concomitant structural phase transition is observed in $x$ = 0.023 ($T_{\rm N}$ = 106 K) and $x$ = 0.028 ($T_{\rm N}$ = 53 K). Through the temperature dependence of the Knight shifts and the nuclear spin lattice relaxation rates (1/$T_1$), although stripe-type AFM spin fluctuations are realized in the paramagnetic state as in the case of other iron pnictide superconductors, we found a gradual decrease of the AFM spin fluctuations below a crossover temperature $T^*$ which was nearly independent of Co-substitution concentration, and is attributed to a pseudogap-like behavior in the spin excitation spectra of these systems. The $T^*$ feature finds correlation with features in the temperature-dependent inter-plane resistivity, $ρ_c(T)$, but not with the in-plane resistivity $ρ_a (T)$. The temperature evolution of anisotropic stripe-type AFM spin fluctuations are tracked in the paramagnetic and pseudogap phases by the 1/$T_1$ data measured under magnetic fields parallel and perpendicular to the $c$ axis. Based on our NMR data, we have added a pseudogap-like phase to the magnetic and electronic phase diagram of Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$.

cond-mat.supr-con↗

arXiv2015

Competing Magnetic Fluctuations in Iron Pnictide Superconductors: Role of Ferromagnetic Spin Correlations Revealed by NMR

In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic (AFM) and in-plane ferromagnetic (FM) wavevectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using $^{75}$As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of $T_{\rm c}$ and the shape of the superconducting dome in these and other iron-pnictide families.

cond-mat.supr-con↗