arXiv subjects

Takahiro Sakurai

Publications and source records attributed to Takahiro Sakurai.

At least 19 recordsLinked to original sources

arXiv2025

Establishment of global phase coherence in a highly disordered fractal MgO/MgB2 nanocomposite: Roles of interface, morphology and defect

Recently, we have reported that a highly disordered fractal MgO/MgB2 nanocomposite exhibits bulk-like superconducting properties with isotropic pinning, showing an excellent phase-coherent capability irrespective of the low volume fraction (~30 vol. %) of MgB2 [Uchino et al., Phys. Rev. B 101, 035146 (2020); Teramachi et al,, Phys. Rev. B 108, 155146 (2023)]. Hence, this nanocomposite provides a useful experimental system to investigate the relationship between the structural disorder and the establishment of the superconducting phase coherence. In this work, we show from 3D focused ion beam scanning electron microscopy (FIB-SEM) data that in the nanocomposite, a complex MgO/MgB2 microstructure spreads isotropically throughout the sample with a constant fractal dimension of ~1.67. Atomic-resolution scanning transmission electron microscopy (STEM) has revealed that the MgO/MgB2 interfaces are atomically clean and free from amorphous grain boundaries, even leading to atomically coherent interfaces. Detailed ac susceptibility measurements have demonstrated a smooth crossover from an intragranular to an intergranular superconducting regime, giving evidence of the establishment of the critical state due to strong intergranular coupling just below the superconducting transition temperature. Also, spatially-resolved cathodoluminescence measurements have demonstrated that oxygen vacancies in the MgO-rich phase tend to aggregate near the MgO/MgB2 boundary regions, forming long channels of oxygen vacancies through the nanocomposite. These channels of oxygen vacancies will contribute to the long-range carrier transfer and the related Andreev reflection via coherent tunneling of charge carriers among the oxygen vacancy sites.

cond-mat.supr-con↗

arXiv2025

Successive Antiferromagnetic Transition in the Frustrated Compound CeMgIn

We report on the magnetic, transport, and thermal properties of the hexagonal ZrNiAl-type compound CeMgIn with Ce atoms forming a distorted kagome network. This compound exhibits successive antiferromagnetic transition at $T_\text{N1} =$ 2.1 K, $T_\text{N2} =$ 1.7 K, and possibly $T_\text{N3} =$ 1.3 K. The electrical resistivity exhibits a minimum at 11 K and a nonlogarithmic increase with decreasing temperature down to $T_\text{N2}$. We found that CeMgIn is the first ZrNiAl-type compound whose resistivity increase can be well explained by considering a model in which the electron-spin scattering is enhanced by the magnetic frustration and the Ruderman-Kittel-Kasuya-Yosida interaction. These results suggest that CeMgIn is a notable compound whose physical properties are strongly affected by geometrical frustration. Since the Sommerfeld coefficient is 97 mJ/mol K$^2$, CeMgIn is classified as a moderate heavy-fermion compound.

cond-mat.str-el↗

arXiv2024

Pressure-tuned quantum criticality in the large-$D$ antiferromagnet DTN

Strongly correlated spin systems can be driven to quantum critical points via various routes. In particular, gapped quantum antiferromagnets can undergo phase transitions into a magnetically ordered state with applied pressure or magnetic field, acting as tuning parameters. These transitions are characterized by $z=1$ or $z=2$ dynamical critical exponents, determined by the linear and quadratic low-energy dispersion of spin excitations, respectively. Employing high-frequency susceptibility and ultrasound techniques, we demonstrate that the tetragonal easy-plane quantum antiferromagnet NiCl$_{2}\cdot$4SC(NH$_2$)$_2$ (aka DTN) undergoes a spin-gap closure transition at about $4.2$ kbar, resulting in a pressure-induced magnetic ordering. The studies are complemented by high-pressure-electron spin-resonance measurements confirming the proposed scenario. Powder neutron diffraction measurements revealed that no lattice distortion occurs at this pressure and the high spin symmetry is preserved, establishing DTN as a perfect platform to investigate $z=1$ quantum critical phenomena. The experimental observations are supported by DMRG calculations, allowing us to quantitatively describe the pressure-driven evolution of critical fields and spin-Hamiltonian parameters in DTN.

cond-mat.str-el↗

arXiv2022

Strong phase coherence and vortex matter in a fractal system with proximity-induced superconductivity

The proximity effect in normal/superconductor heterostructures is an intriguing phenomenon in that the normal side takes on the properties of a superconductor with an induced gap. However, the structural and pinning properties of vortices inside the normal regions remain poorly understood. Here, we report structure and superconducting properties of a proximity-coupled Mg/MgO/MgB2 system with ~30 vol. % of superconducting MgB2, in which MgB2 nanograins are distributed in a fractal manner to form a proximity network with clean interfaces. Conductivity and magnetic measurements demonstrate that this proximity-coupled system acts as a fully phase coherent superconductor with isotropic pinning. Magneto-optical imaging also reveals a rather homogeneous flux density distribution with no apparent granularity. Furthermore, we observe quantized proximity vortices and their clustering behavior by scanning superconducting quantum interface device microscopy. These results show that in contrast to the case of conventional granular superconductors, the grain boundaries in the present sample carry high critical currents and have high vortex pinning efficiency, resulting in a robust phase coherent state irrespective of the low volume fraction of the MgB2 nanograins. This finding not only reveals the features of proximity-induced vortices, but also demonstrates an excellent phase-locked capability of the proximity-coupled fractal system.

cond-mat.supr-con↗

arXiv2021

Continuous control of classical-quantum crossover by external high pressure in the coupled chain compound CsCuCl$_3$

In solid materials, the parameters relevant to quantum effects, such as the spin quantum number, are basically determined and fixed at the chemical synthesis, which makes it challenging to control the amount of quantum correlations. We propose and demonstrate a method for active control of the classical-quantum crossover in magnetic insulators by applying external pressure. As a concrete example, we perform high-field, high-pressure measurements on CsCuCl$_3$, which has the structure of weakly-coupled spin chains. The magnetization process experiences a continuous evolution from the semi-classical realm to the highly-quantum regime with increasing pressure. Based on the idea of "squashing" the spin chains onto a plane, we characterize the change in the quantum correlations by the change in the value of the local spin quantum number of an effective two-dimensional model. This opens a way to access the tunable classical-quantum crossover of two-dimensional spin systems by using alternative systems of coupled-chain compounds.

cond-mat.str-el↗

arXiv2021

First ESR Detection of Higgs Amplitude Mode and Analysis with Extended Spin-Wave Theory in Dimer System KCuCl$_3$

KCuCl$_3$ is known to show a quantum phase transition from the disordered to antiferromagnetically ordered phases by applying pressure. There is a longitudinal excitation mode (Higgs amplitude mode) in the vicinity of the quantum critical point in the ordered phase. To detect the Higgs amplitude mode, high-pressure ESR measurements are performed in KCuCl$_3$. The experimental data are analyzed by the extended spin-wave theory on the basis of the vector spin chirality. We report the first ESR detection of the Higgs amplitude mode and the important role of the electric dipole described by the vector spin chirality.

cond-mat.str-el↗

arXiv2020

First-order phase transition to a nonmagnetic ground state in nonsymmorphic NbCrP

We report the discovery of a first-order phase transition at around 125 K in NbCrP, which is a nonsymmorphic crystal with Pnma space group. From the resistivity, magnetic susceptibility, and nuclear magnetic resonance measurements using the crystals made by the Sn-flux method, the high-temperature (HT) phase is characterized to be metallic with a non-negligible magnetic anisotropy. The low-temperature (LT) phase is also found to be a nonmagnetic metallic state with a crystal of lower symmetry. In the LT phase, the spin susceptibility is reduced by ~30 % from that in the HT phase, suggesting that the phase transition is triggered by the electronic instability. The possible origin of the phase transition in NbCrP is discussed based on the electronic structure by comparing with those in other nonsymmorphic compounds RuP and RuAs.

cond-mat.str-el↗

arXiv2020

Enhancement of the superconducting transition temperature of MgB2 by proximity effect of d0 ferromagnet

Heterostructures of superconducting and ferromagnetic materials are of fundamental interest because of the mutual interaction of antagonistic kinds of ordering at the S-F interface. Normally, the superconducting transition temperature Tc should be strongly suppressed at the S-F interface owing to the penetration of Cooper pairs into the ferromagnetic side. Nevertheless, constructive interactions between S and F orders have been suggested to occur via the modification of ferromagnetic order by the superconducting state. This may induce an inhomogeneous magnetic state, often called a cryptoferromagnetic state, and the relevant domain wall effect, which will lead to a local decrease of the pair-breaking parameter. However, the domain wall effect, even if it exists, is quite subtle from the experimental view point and is normally difficult to observe. Here we show that the defect-related d0 ferromagnetism in MgO and the superconductivity in MgB2 do not antagonize, but rather enhance the superconducting transition temperature Tc to any significant degree. We found in superconducting MgB2-d0 ferromagnetic MgO composites that the superconducting transition proceeds in two steps. The first at the S-F interface, between 110-120 K, then in the rest of the bulk at 39 K, which is the Tc of single phase MgB2 superconductor. Moreover, the additional transition emerges at 60 K at the S-F interface especially in the ferromagnetic side, showing a spin-glass-like magnetic state. Our findings reveal that the proximity effect in the superconductor-d0 ferromagnet heterostructures will provide the knowledge and basis to enhance the Tc value of the existing superconductors.

cond-mat.supr-con↗

arXiv2020

Spontaneous vortex state in a superconductor/ferromagnet nanocomposite

The mechanism of the interplay between superconductivity and magnetism is one of the intriguing and challenging problems in physics. Theory has predicted that the ferromagnetic order can coexist with the superconducting order in the form of a spontaneous vortex phase in which magnetic vortices nucleate in the absence of an external field. However, there has been no rigorous demonstration of spontaneous vortices by bulk magnetic measurements. Here we show the results of experimental observations of spontaneous vortices using a superconductor/ferromagnet fractal nanocomposite, in which superconducting MgB2 and ferromagnetic nanograins are dispersedly embedded in the normal matrix to realize the remote electromagnetic interaction and also to induce a long-range Josephson coupling. We found from bulk magnetization measurements that the sample with nonzero remanent magnetization exhibits the magnetic behaviors which are fully consistent with a spontaneous vortex scenario predicted theoretically for magnetic inclusions in a superconducting material. The resulting spontaneous vortex state is in equilibrium and coexists surprisingly with a Meissner state (complete shielding of an external magnetic field). The present observation not only reveals the evolution process of the spontaneous vortices in superconductor/ferromagnet hybrids, but it also sheds light on the role of the fractal disorder and structural heterogeneity on the vortex nucleation under the influence of Josephson superconducting currents.

cond-mat.supr-con↗

arXiv2019

Magnetic correlations in the pressure-induced superconductor CrAs investigated by $^{75}$As nuclear magnetic resonance

We report $^{75}$As-NMR results for CrAs under pressure, which shows superconductivity adjoining a helimagnetically ordered state. We successfully evaluated the Knight shift from the spectrum, which is strongly affected by the quadrupole interaction. The Knight shift shows the remarkable feature that the uniform spin susceptibility increases toward low temperatures in the paramagnetic state. This is in sharp contrast to CrAs at ambient pressure, and also to cuprates and Fe pnictides, where antiferromagnetic correlations are dominant. Superconductivity emerges in CrAs under unique magnetic correlations, which probably originate in the three-dimensional zigzag structure of its nonsymmorphic symmetry.

cond-mat.supr-con↗

arXiv2018

Superlattice formation lifting degeneracy protected by non-symmorphic symmetry through a metal-insulator transition in RuAs

The single crystal of RuAs obtained by Bi-flux method shows obvious successive metal-insulator transitions at T_MI1~255 K and T_MI2~195$ K. The X-ray diffraction measurement reveals a formation of superlattice of 3x3x3 of the original unit cell below T_MI2, accompanied by a change of the crystal system from the orthorhombic structure to the monoclinic one. Simple dimerization of the Ru ions is nor seen in the ground state. The multiple As sites observed in nuclear quadrupole resonance (NQR) spectrum also demonstrate the formation of the superlattice in the ground state, which is clarified to be nonmagnetic. The divergence in 1/T_1 at T_MI1 shows that a symmetry lowering by the metal-insulator transition is accompanied by strong critical fluctuations of some degrees of freedom. Using the structural parameters in the insulating state, the first principle calculation reproduces successfully the reasonable size of nuclear quadrupole frequencies for the multiple As sites, ensuring the high validity of the structural parameters. The calculation also gives a remarkable suppression in the density of states (DOS) near the Fermi level, although the gap opening is insufficient. A coupled modulation of the calculated Ru d electron numbers and the crystal structure proposes a formation of charge density wave (CDW) in RuAs. Some lacking factors remain, but it shows that a lifting of degeneracy protected by the non-symmorphic symmetry through the superlattice formation is a key ingredient for the metal-insulator transition in RuAs.

cond-mat.str-el↗

arXiv2018

Direct Observation of the Quantum Phase Transition of SrCu$_{2}$(BO$_{3}$)$_{2}$ by High-Pressure and Terahertz Electron Spin Resonance

High-pressure and high-field electron spin resonance (ESR) measurements have been performed on a single crystal of the orthogonal-dimer spin system SrCu$_{2}$(BO$_{3}$)$_{2}$. With frequencies below 1 THz, ESR signals associated with transitions from the singlet ground state to the one-triplet excited states and the two-triplet bound state were observed at pressures up to 2.1 GPa. We obtained directly the pressure dependence of the gap energies, finding a clear first-order phase transition at $P_{c}=1.85\pm0.05$ GPa. By comparing this pressure dependence with the calculated excitation energies obtained from an exact diagonalization, we determined the precise pressure dependence for inter- ($J'$) and intra-dimer ($J$) exchange interactions considering the Dzyaloshinski-Moriya interaction. Thus this system undergoes a first-order quantum phase transition from the dimer singlet phase to a plaquette singlet phase above the ratio $(J'/J)_{c}=0.660\pm0.003$.

cond-mat.str-el↗

arXiv2017

Macroscale three-dimensional proximity effect in disordered normal/superconductor nanocomposites

Recently, interest in Superconductor (S)-Normal (N) interfaces was renewed by the observation of exotic proximity effects in various systems, including S/semiconductor, S/ferromagnet, and S/topological insulator. In general, the proximity effect is enhanced in transparent weak links where coherent Andreev reflection is possible. Also, it is a common knowledge that the proximity effect is, by definition, is a localized phenomenon that can only be active in each S/N interface region. However, here we show that a three-dimensional (3D) macroscale proximity effect is realized in few-micrometer-thick MgO/Mg2Si/MgB2 nanocomposite layers with atomically smooth and clean heterointerfaces. We found from scanning superconducting quantum interference device (SQUID) microscopy measurements that a normal region of more than 100x100 square micrometers totally undergoes transition into a bulk-like superconducting state although the normal host originally contains less than ~10 vol % of superconducting MgB2 nanograins in a dispersed manner. In the proximity-induced superconducting region, vortex formation and annihilation processes as well as vortex-free Meissner regions were observed with respect to applied fields in a similar manner as Abrikosov vortices in type-II superconductors. Furthermore, we found that the induced superconducting layers exhibit an anisotropic magnetization behavior, in consistent with the formation of the large-scale superconducting coherence. This unusually extended proximity effect suggests that disorder-induced interaction and coupling of Andreev bound states, which are coherent superposition of time reversed electron hole pairs, is realized in the nanocomposite. Thus, the present results not only expand the limit of the proximity effect to bulk scales, but also provides a new route to obtain a proximity-induced superconducting state from disordered systems.

cond-mat.supr-con↗

arXiv2016

Dzyaloshinsky-Moriya interaction and the ground state in S=3/2 perfect kagome lattice antiferromagnet $\mathbf{KCr_3(OH)_6(SO_4)_2}$ (Cr-jarosite) studied by X-band and high-frequency ESR

A single crystal S=3/2 perfect kagome lattice antiferromagnet $\mathrm{KCr_3(OH)_6(SO_4)_2}$ (Cr-jarosite) has been studied by X-band and high-frequency electron spin resonance (ESR). The g-values perpendicular to the kagome plane (c-axis) and in the plane are determined to be $g_c=1.9704 \pm 0.0002$ and $g_ξ=1.9720 \pm 0.0003$, respectively, by high-frequency ESR observed at 265 K. Antiferromagnetic resonances (AFMR) with the antiferromagnetic gap of 120 GHz are observed at 1.9 K, which is below $T_N$=4.5 K. The analysis of AFMR modes by the conventional molecular field theory shows $d_p=0.27$ K and $d_z=0.07$ K, where $d_p$ and $d_z$ are in-plane and out-of-plane components of DM d vector, respectively. From these results and the estimated exchange interaction J=6.15 K by Okuta et al., the ground state of Cr-jarosite is discussed in connection with the Monte Carlo simulations result with classical Heisenberg spins on the kagome lattice by Elhajal et al. Finally, the angular dependence of linewidth and the lineshape observed at 296 K by X-band ESR show typical behavior of a two-dimensional Heisenberg antiferromagnet, suggesting a good two-dimensionality of Cr-jarosite.

cond-mat.str-el↗

arXiv2015

Frequency Extension to THz Range in High Pressure ESR System and Its Application to Shastry-Sutherland Model Compound SrCu$_{2}$(BO$_{3}$)$_{2}$

We have made a survey of ceramics for the inner parts of the transmission-type pressure cell to achieve the high pressure and the high transmission in THz range. By using the optimal combination of ZrO$_{2}$-based ceramic and Al$_{2}$O$_{3}$ ceramic, we have succeeded in obtaining the pressure up to 1.5 GPa and the frequency region up to 700 GHz simultaneously. We show the high-pressure ESR results of Shastry-Sutherland compound SrCu$_{2}$(BO$_{3}$)$_{2}$ as an application. We observed the direct ESR transition modes between the singlet ground state and the triplet excited states up to the pressure of 1.51 GPa successfully, and obtained the precise pressure dependence of the gap energy. The gap energy is directly proved to be suppressed by the pressure. Moreover, we found that the system approaches the quantum critical point with pressure by comparing the obtained data with the theory. This result also shows the usefulness of high-pressure ESR measurement in THz region to study quantum spin systems.

cond-mat.mtrl-sci↗

arXiv2014

NaCuMoO_4(OH) as a Candidate Frustrated J_1-J_2 Chain Quantum Magnet

In a frustrated J_1-J_2 chain with the nearest-neighbor ferromagnetic interaction J_1 and the next-nearest-neighbor antiferromagnetic interaction J_2, novel magnetic states such as a spin-nematic state are theoretically expected. However, they have been rarely examined in experiments because of the difficulty in obtaining suitable model compounds. We show here that the quasi-one-dimensional antiferromagnet NaCuMoO_4(OH), which comprises edge-sharing CuO_2 chains, is a good candidate J_1-J_2 chain antiferromagnet. The exchange interactions are estimated as J_1 = - 51 K and J_2 = 36 K by comparing the magnetic susceptibility, heat capacity, and magnetization data with the data obtained using calculations by the exact diagonalization method. High-field magnetization measurements at 1.3 K show a saturation above 26 T with little evidence of a spin nematic state expected just below the saturation field, which is probably due to smearing effects caused by thermal fluctuations and the polycrystalline nature of the sample.

cond-mat.str-el↗

arXiv2011

Computational wave optics library for C++: CWO++ library

Diffraction calculations, such as the angular spectrum method, and Fresnel diffractions, are used for calculating scalar light propagation. The calculations are used in wide-ranging optics fields: for example, computer generated holograms (CGHs), digital holography, diffractive optical elements, microscopy, image encryption and decryption, three-dimensional analysis for optical devices and so on. However, increasing demands made by large-scale diffraction calculations have rendered the computational power of recent computers insufficient. We have already developed a numerical library for diffraction calculations using a graphic processing unit (GPU), which was named the GWO library. However, this GWO library is not user-friendly, since it is based on C language and was also run only on a GPU. In this paper, we develop a new C++ class library for diffraction and CGH calculations, which is referred as to a CWO++ library, running on a CPU and GPU. We also describe the structure, performance, and usage examples of the CWO++ library.

physics.optics↗

arXiv2010

Anomalous Spin Dynamics observed by High Frequency ESR in Honeycomb Lattice Antiferromagnet InCu2/3V1/3O3

High-frequency ESR results on the S=1/2 Heisenberg hexagonal antiferromagnet InCu2/3V1/3O3 are reported. This compound appears to be a rare model substance for the honeycomb lattice antiferromagnet with very weak interlayer couplings. The high-temperature magnetic susceptibility can be interpreted by the S=1/2 honeycomb lattice antiferromagnet, and it shows a magnetic-order-like anomaly at TN=38 K. Although, the resonance field of our high-frequency ESR shows the typical behavior of the antiferromagnetic resonance, the linewidth of our high-frequency ESR continues to increase below TN, while it tends to decrease as the temperature in a conventional three-dimensional antiferromagnet decreases. In general, a honeycomb lattice antiferromagnet is expected to show a simple antiferromagnetic order similar to that of a square lattice antiferromagnet theoretically because both antiferromagnets are bipartite lattices. However, we suggest that the observed anomalous spin dynamics below TN is the peculiar feature of the honeycomb lattice antiferromagnet that is not observed in the square lattice antiferromagnet.

cond-mat.str-el↗