arXiv subjects

T. Nishikawa

Publications and source records attributed to T. Nishikawa.

14 recordsLinked to original sources

arXiv2021

Shallow NV centers augmented by exploiting n-type diamond

Creation of nitrogen-vacancy (NV) centers at the nanoscale surface region in diamond, while retaining their excellent spin and optical properties, is essential for applications in quantum technology. Here, we demonstrate the extension of the spin-coherence time ($\it{T}$${_2}$), the stabilization of the charge state, and an improvement of the creation yield of NV centers formed by the ion-implantation technique at a depth of $\sim$15 nm in phosphorus-doped n-type diamond. The longest $\it{T}$${_2}$ of about 580 $μ$s of a shallow NV center approaches the one in bulk diamond limited by the nuclear spins of natural abundant $^{13}$C. The averaged $\it{T}$${_2}$ in n-type diamond is over 1.7 times longer than that in pure non-doped diamond. Moreover, the stabilization of the charge state and the more than twofold improvement of the creation yield are confirmed. The enhancements for the shallow NV centers in an n-type diamond-semiconductor are significant for future integrated quantum devices.

cond-mat.mtrl-sci↗

arXiv2019

Evaluation of spin diffusion length and spin Hall angle of antiferromagnetic Weyl semimetal Mn$_3$Sn

Antiferromagnetic Weyl semimetal Mn$_3$Sn has shown to generate strong intrinsic anomalous Hall effect (AHE) at room temperature, due to large momentum-space Berry curvature from the time-reversal symmetry breaking electronic bands of the Kagome planes. This prompts us to investigate intrinsic spin Hall effect, a transverse phenomenon with identical origin as the intrinsic AHE. We report inverse spin Hall effect experiments in nanocrystalline Mn$_3$Sn nanowires at room temperature using spin absorption method which enables us to quantitatively derive both the spin diffusion length and the spin Hall angle in the same device. We observed clear absorption of the spin current in the Mn$_3$Sn nanowires when kept in contact with the spin transport channel of a lateral spin-valve device. We estimate spin diffusion length $λ_{s(Mn_3Sn)}$ $\sim$0.75 $\pm$0.67 nm from the comparison of spin signal of an identical reference lateral spin valve without Mn$_3$Sn nanowire. From inverse spin Hall measurements, we evaluate spin Hall angle $θ_{SH}$ $\sim$5.3 $\pm$ 2.4 $\%$ and spin Hall conductivity $σ_{SH}$ $\sim$46.9 $\pm$ 3.4 ($\hbar/e$) ($Ω$ cm)$^{-1}$. The estimated spin Hall conductivity agrees with both in sign and magnitude to the theoretically predicted intrinsic $σ_{SH}^{int}$ $\sim$36-96 ($\hbar/e$) ($Ω$ cm)$^{-1}$. We also observed anomalous Hall effect at room temperature in nano-Hall bars prepared at the same time as the spin Hall devices. Large anomalous Hall conductivity along with adequate spin Hall conductivity makes Mn$_3$Sn a promising material for ultrafast and ultrahigh-density spintronics devices.

cond-mat.mes-hall↗

arXiv2010

Possible quantum numbers of the pentaquark Θ^+(1540) in QCD sum rules

The QCD sum rule technique is employed to investigate pentaquark states with strangeness S = +1 and IJ^P = 0,1/2^\pm, 1,1/2^\pm, 0,3/2^\pm, 1,3/2^\pm. Throughout the calculation, emphasis is laid on the establishment of a valid Borel window, which corresponds to a region of the Borel mass, where the operator product expansion converges and the presumed ground state pole dominates the sum rules. Such a Borel window is achieved by constructing the sum rules from the difference of two independent correlators and by calculating the operator product expansion up to dimension 14. Furthermore, we discuss the possibility of the contamination of the sum rules by possible KN scattering states. As a result, we conclude that the 0,3/2^+ state seems to be the most probable candidate for the experimentally observed Θ^+(1540), while we also obtain states with 0,1/2^-, 1,1/2^-, 1,3/2^+ at somewhat higher mass regions.

hep-ph↗

arXiv2009

Spin-3/2 Pentaquark in QCD Sum Rules

The QCD sum rule method is formulated for the strangeness +1 pentaquark baryon with isospin I=0 and spin-parity J^P = 3/2^\pm. The spin-3/2 states are considered to be narrower than the spin-1/2 ones, and thus may provide a natural explanation for the experimentally observed narrow width of Θ^+. In order to obtain reliable results in QCD sum rule calculations, we stress the importance of establishing a wide Borel window, where convergence of the operator product expansion and sufficient low-mass strength of the spectral function are guaranteed. To this end, we employ the difference of two independent correlators so that the high-energy continuum contribution is suppressed. The stability of the physical quantities against the Borel mass is confirmed within the Borel window. It is found that the sum rule gives positive evidence for the (I, J^P) = (0, 3/2^+) state with a mass of about 1.4 \pm 0.2 GeV, while we cannot extract any evidence for the (0, 3/2^-) state.

hep-ph↗

arXiv2008

Exotic Quark Structure of Lambda(1405) in QCD Sum Rule

A new analysis is performed in QCD sum rule for the lightest negative parity baryon Lambda (1405). Mixings of three-quark and five-quark Fock components are taken into account. Terms containing up to dimension 12 condensates are computed in the operator product expansion. It is found that the sum rule gives much stronger coupling of Lambda* to the five-quark operator so that the five-quark components occupy about 90% of Lambda (1405).

hep-ph↗

arXiv2007

Mixings of 4-quark components in light non-singlet scalar mesons in QCD sum rules

Mixings of 4-quark components in the non-singlet scalar mesons are studied in the QCD sum rules. We propose a formulation to evaluate the cross correlators of q\bar q and qq\bar q \bar q operators and to define the mixings of different Fock states in the sum rule. It is applied to the non-singlet scalar mesons, a_0 and K_0^\ast. It is found that the 4-quark operators predict lower masses than the q\bar q operators and that the 4-quark states occupy about 70-90% of the lowest mass states.

hep-ph↗

arXiv2005

Axial vector tetraquark with S=+2

Possibility of an axial vector isoscalar tetraquark with $ud\bar{s}\bar{s}$ is discussed. If the pentaquark $Θ^+(1540)$ has the $(qq)_{\bar{3}}(qq)_{\bar{3}}\bar{q}$ configuration, the isoscalar $ud\bar{s}\bar{s}$($\vartheta^+$-meson) state with $J^P=1^+$ is expected to exist in the mass region lower than or close to the mass of $Θ^+(1540)$. Within a flux-tube quark model, a possible resonant state of $ud\bar{s}\bar{s}(J^{P}=1^{+})$ is suggested to appear around 1.4 GeV with the width ${\cal{O}}(20\sim 50)$ MeV. We propose that the $\vartheta^+$-meson is a good candidate for the tetraquark search, which would be observed in the $K^+K^+π^-$ decay channel.

hep-ph↗

arXiv2005

Prediction of Narrow $J^π=1/2^+$, $3/2^+$, and $3/2^-$ states of $Θ_+$ in a Quark model with Antisymmetrized Molecular Dynamics

The exotic baryon $Θ^+(uudd\bar{s})$ is studied with microscopic calculations in a quark model by using a method of antisymmetrized molecular dynamics. We predict that the lowest even state is $J^π=1/2^+(I=0)$ or $J^π=3/2^+(I=0)$, and the lowest odd state is $J^π=3/2^-(I=1)$. They nearly degenerate in the $uudd\bar{s}$ system. We discuss $K^+n$ decay widths and estimate them to be $Γ< 7$ for the $J^π=\{1/2^+,3/2^+\}$, and $Γ<1$ MeV for the $J^π=3/2^-$ state.

hep-ph↗

arXiv2004

Narrow Pentaquark States in a Quark Model with Antisymmetrized Molecular Dynamics

The exotic baryon $Θ^+(uudd\bar{s})$ is studied with microscopic calculations in a quark model by using a method of antisymmetrized molecular dynamics(AMD). We predict narrow states, $J^π=1/2^+(I=0)$, $J^π=3/2^+(I=0)$, and $J^π=3/2^-(I=1)$, which nearly degenerate in a low-energy region of the $uudd\bar{s}$ system. We discuss $NK$ decay widths and estimate them to be $Γ< 7$ for the $J^π=\{1/2^+,3/2^+\}$, and $Γ<1$ MeV for the $J^π=3/2^-$ state.

hep-ph↗

arXiv2004

Effects of polarized organosilane self-assembled monolayers on organic single-crystal field-effect transistors

The surface conductivity is measured by a four-probe technique for pentacene and rubrene single-crystals laminated on polarized and nearly unpolarized molecular monolayers with application of perpendicular electric fields. The polarization of the self-assembled monolayers (SAMs) shifts the threshold gate voltage, while maintaining a very low subthreshold swing of the single-crystal devices (0.11 V/decade). The results, excluding influences of parasitic contacts and grain boundaries, demonstrate SAM-induced nanoscale charge injection up to ~10^12 cm^-2 at the surface of the organic single crystals.

cond-mat.mtrl-sci↗

arXiv2004

Evolution of the electronic structure from electron-doped to hole-doped states in the two-dimensional Mott-Hubbard system La1.17-xPbxVS3.17

The filling-controlled metal-insulator transition (MIT) in a two-dimensional Mott-Hubbard system La1.17-xPbxVS3.17 has been studied by photoemission spectroscopy. With Pb substitution x, chemical potential mu abruptly jumps by ~ 0.07 eV between x=0.15 and 0.17, indicating that a charge gap is opened at x ~= 0.16 in agreement with the Mott insulating state of the d2 configuration. When holes or electrons are doped into the Mott insulator of x ~= 0.16, the gap is filled and the photoemission spectral weight at mu, rho(mu), gradually increases in a similar way to the electronic specific heat coefficient, although the spectral weight remains depressed around mu compared to that expected for a normal metal, showing a pseudogap behavior in the metallic samples. The observed behavior of varrho(mu)->0 for x->0.16 is contrasted with the usual picture that the electron effective mass of the Fermi-liquid system is enhanced towards the metal-insulator boundary. With increasing temperature, the gap or the pseudogap is rapidly filled up, and the spectra at T=300 K appears to be almost those of a normal metal. Near the metal-insulator boundary, the spectra around mu are consistent with the formation of a Coulomb gap, suggesting the influence of long-range Coulomb interaction under the structural disorder intrinsic to this system.

cond-mat.str-el↗

arXiv2003

Color Ferromagnetism of Quark Matter and Quantum Hall States of Gluons in SU(3) Gauge Theory

We show a possibility that a color ferromagnetic state exists in SU(3) gauge theory of quark matter with two flavors. Although the state involves three types of unstable modes of gluons, all of these modes are stabilized by forming a quantum Hall state of one of the modes. We also show that at large chemical potential, a color superconducting state (2SC) appears even in the ferromagnetic state. This is because Meissner effect by condensed anti-triplet quark pairs does not work on the magnetic field in the ferromagnetic state.

hep-ph↗

arXiv1999

A new approach to axial coupling constants in the QCD sum rule

We derive new QCD sum rules for the axial coupling constants by considering two-point correlation functions of the axial-vector currents in a one nucleon state. The QCD sum rules tell us that the axial coupling constants are expressed by nucleon matrix elements of quark and gluon operators which are related to the sigma terms and the moments of parton distribution functions. The results for the iso-vector axial coupling constants and the 8th component of the SU(3) octet are in good agreement with experiment.

hep-ph↗

arXiv1997

Electric Polarizabilities of Neutral Baryons in the QCD Sum Rule

We investigate the electric polarizabilities of neutral baryons using the method of QCD sum rules. The diagrams in the operator product expansion are taken into account up to dimension 6. We obtain different values of the polarizabilities among hyperons, the ordering of which is given $\barα_n>\barα_{Ξ^0}>\barα_Λ$. The polarizability of ${Σ^0}$, $\barα_{Σ^0}$, has possibly a very small value.

hep-ph↗