arXiv subjects

Yutaka Itoh

Publications and source records attributed to Yutaka Itoh.

7 recordsLinked to original sources

arXiv2026

Nuclear spin-lattice relaxation rate near a two-dimensional antiferromagnetic quantum critical point

We study the effects of zero-point spin fluctuations on the nuclear spin-lattice relaxation rate 1/$T_1$ around a two-dimensional antiferromagnetic quantum critical point in the self-consistent renormalization theory. For a weak mode-mode coupling constant $y_1 < $ 0.1, the finite temperature behavior of 1/$T_1$ across the quantum critical point resembles that for only the thermal spin fluctuations. For a strong mode-mode coupling constant $y_1 > $ 0.1, 1/$T_1$ takes its local maximum as a function of temperature in the nearly antiferromagnetic state. Experimental observation of the local maximum in the $^{63}$Cu nuclear spin-lattice relaxation rate 1/$T_1$ of lightly-doped superconductors La$_{2-x}$Sr$_x$CuO$_4$ with $x$ = 0.06-0.10 is associated with the effects of the zero-point spin fluctuations. The two-dimensional magnetic quantum critical point and the role of the zero-point spin fluctuations are discussed in the electron spin dynamics of underdoped superconductors La$_{2-x}$Sr$_x$CuO$_4$.

cond-mat.str-el↗

arXiv2024

Quest for nuclear quadrupole relaxation in HgCa$_{2}$Ba$_{2}$Cu$_{3}$O$_{8+δ}$

We derived a simple reduced form from the exact solution of the mixed magnetic and quadrupolar nuclear spin--lattice relaxation function at the central transition line of a quadrupole-split NMR spectrum for the nuclear spin $I$ = 3/2. The reduced form of the exact solution could be extended to include inhomogeneous relaxation effects. From the application of the reduced form to the $^{63}$Cu nuclear spin--lattice relaxation curves in the high-$T_\mathrm{c}$ superconductors of $^{63}$Cu-enriched triple-layer HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$, we estimated the predominant magnetic relaxation rate $^{63}W_M$ and the upper limit of a weak nuclear electric quadrupole spin--lattice relaxation rate $^{63}W_Q$.

cond-mat.supr-con↗

arXiv2022

Nuclear spin-lattice relaxation studies of Cu$_{2}$O

We report the $^{63}$Cu and $^{65}$Cu nuclear spin-lattice relaxation rate measurements of cuprous oxide Cu$_2$O in a zero field Cu nuclear quadrupole resonance at $T$ = 77$-$325 K. From the detailed isotopic measurements of the relaxation rates, we successfully estimated a finite magnetic relaxation rate $^{63}W_M$ and a predominant nuclear quadrupole relaxation rate $^{63}W_Q$. $^{63}W_Q$ changed as $T^{2.1}$, while $^{63}W_M$ changed as $T^{1.6}$ or $T^β\mathrm{exp}(-\itΔ/T)$ with $β$ = 0.6(3) and $\itΔ$ = 190(62) K. The nuclear spin scattering process due to a non-degenerate Fermi gas was discussed as a possible candidate of the magnetic relaxation.

cond-mat.mtrl-sci↗

arXiv2015

Search for dark energy potentials in quintessence theory

The time evolution of the equation of state $w$ for quintessence models with a scalar field as dark energy is studied up to the third derivative ($d^3w/da^3$) with respect to the scale factor $a$, in order to predict the future observations and specify the scalar potential parameters with the observables. The third derivative of $w$ for general potential $V$ is derived and applied to several types of potentials. They are the inverse power-law ($V=M^{4+α}/Q^α$), the exponential ($V=M^4\exp{(βM/Q)}$), the mixed ( $V=M^{4+γ}\exp{(βM/Q)}/Q^γ$), the cosine ($V=M^4(\cos (Q/f)+1)$) and the Gaussian types ($V=M^4\exp(-Q^2/σ^2)$), which are prototypical potentials for the freezing and thawing models. If the parameter number for a potential form is $ n$, it is necessary to find at least for $n+2$ independent observations to identify the potential form and the evolution of the scalar field ($Q$ and $ \dot{Q} $). Such observations would be the values of $ Ω_Q, w, dw/da. \cdots $, and $ dw^n/da^n$. From these specific potentials, we can predict the $ n+1 $ and higher derivative of $w$ ; $ dw^{n+1}/da^{n+1}, \cdots$. Since four of the above mentioned potentials have two parameters, it is necessary to calculate the third derivative of $w$ for them to estimate the predict values. If they are tested observationally, it will be understood whether the dark energy could be described by the scalar field with this potential. At least it will satisfy the necessary conditions. Numerical analysis for $d^3w/da^3$ are made under some specified parameters in the investigated potentials, except the mixed one. It becomes possible to distinguish the potentials by the accurate observing $dw/da$ and $d^2w/da^2$ in some parameters.

astro-ph.CO↗

arXiv2015

Time variation of the Equation of State for Dark Energy

The time variation of the equation of state ($w_Q$) for the dark energy is analyzed by the current values of parameters $Ω_Q $, $w_Q $ and their time derivatives. In the future, detailed feature of the dark energy could be observed, so we have considered the second derivatives of $w_Q$ for two types of potential: One is an inverse power-law type ($V=M^{4+α}/Q^α$) and the other is an exponential one ($V=M^4\exp{(βM/Q)}$). The first derivative $dw_Q/da$ and the second derivative $d^2 w_Q/da^2$ for both potentials are derived. The first derivative is estimated by the observed two parameters $Δ=w_Q+1$ and $Ω_Q$, with the assuming for $Q_0$. In the limit $Δ\rightarrow 0$, the first derivative is null and, under the tracker approximation, the second derivative also becomes null. For the inverse power potential $V=M^{4+α}/Q^α$, the observed first and second derivatives are used to determine the potential parameter $M$ and $α$. For the potential of $V=M^4\exp{(βM/Q)}$, the second derivative is estimated by the observed parameters $Δ$, $Ω_Q$ and $dw_Q/da$.

astro-ph.CO↗

arXiv2010

Universal Phase Diagram and Reentrant Superconductivity of $Bi$layer Hydrated Na$_x$CoO$_2\cdot y$H$_2$O

We report duration dependence of superconductivity in a 43 \% humidity atmosphere and $^{59}$Co nuclear quadrupole resonance (NQR) of polycrystalline samples of $bi$layer hydrated Na$_x$CoO$_2 \cdot y$H$_2$O. We found a reentrant behavior of superconductivity with respect to the time duration. We obtained the universal phase diagram where two superconducting phases and an in-between magnetic phase are classified by $^{59}$Co NQR frequency. Zero field $^{59}$Co nuclear spin-lattice relaxation rates $1/T_1$ indicated a magnetic critical slowing down at 5 K in the magnetic phase, excluding a charge density wave ordering, and an enhancement at and just above $T_\mathrm{c}$ in a new superconducting phase, suggesting a coexistence of magnetic ordering.

cond-mat.supr-con↗

arXiv2005

Novel phase diagram of superconductor NaxCoO2-yH2O in a 75 % relative humidity

We succeeded in synthesizing the powder samples of bilayer-hydrate sodium cobalt oxide superconductors NaxCoO2-yH2O with Tc = 0 ~ 4.6 K by systematically changing the keeping duration in a 75 % relative humidity atmosphere after intercalation of water molecules. From the magnetic measurements, we found that the one-day duration sample does not show any superconductivity down to 1.8 K, and that the samples kept for 2 ~ 7 days show superconductivity, in which Tc increases up to 4.6 K with increasing the duration. Tc and the superconducting volume fraction are almost invariant between 7 days and 1month duration. The 59Co NQR spectra indicate a systematic change in the local charge distribution on the CoO2 plane with change in duration.

cond-mat.supr-con↗